Chemical and Heat Recovery in The Paper Industry [PDF]

Zitiervorschau

The Babcock & Wilcox Company

Chapter 28 Chemical and Heat Recovery in the Paper Industry

In the United States (U.S.), the forest products industry is the third largest industrial consumer of energy, accounting for more than 11% of the total U.S. manufacturing energy expenditures. In 2002, 57% of the pulp and paper industry relied on cogeneration for their electric power requirements. Approximately one-half of the steam and power consumed by this industry is generated from fuels that are byproducts of the pulping process. The main source of self-generated fuel is the spent pulping liquor, followed by wood and bark. The energy required to produce pulp and paper products has been significantly reduced. Process improvements have allowed U.S. pulp and paper manufacturers to reduce energy consumption to 2.66 × 1012 Btu (2806.5 × 1012 J), a significant reduction. Pulp and paper mill electric power requirements have increased disproportionately to process steam requirements. This factor, coupled with steadily rising fuel costs, has led to the greater cycle efficiencies afforded by higher steam pressures and temperatures in paper mill boilers. The increased value of steam has produced a demand for more reliable and efficient heat and chemical recovery boilers. The heat value of the spent pulping liquor solids is a reliable fuel source for producing steam for power generation and process use. A large portion of the steam required for the pulp mills is produced in highly specialized heat and chemical recovery boilers. The balance of the steam demand is supplied by boilers designed to burn coal, oil, natural gas and biomass.

ing its name from the use of sodium sulfate (Na2SO4) as the makeup chemical. The paper produced from this process was originally so strong in comparison with alternative processes that it was given the name kraft, which is the Swedish and German translation for strong. Kraft is an alkaline pulping process, as is the soda process which derives its name from the use of sodium carbonate, Na2CO3 (soda ash), as the makeup chemical. The soda process has limited use in the U.S. and is more prominent in countries pulping nonwood fiber. Recovery of chemicals and the production of steam from waste liquor are well established in the kraft and soda processes. The soda process accounts for less than 1% of alkaline pulp production and its importance is now largely historic.

Kraft pulping and recovery process Kraft process The kraft process flow diagram (Fig. 2) shows the typical relationship of the recovery boiler to the overall pulp and paper mill.1 The kraft process starts with feeding wood chips, or alternatively a nonwood fibrous material, to the digester. Chips are cooked under pressure in a steam heated aqueous solution of USA 31%

All Other Countries 30%

Major pulping processes The U.S. and Canada have the highest combined consumption of paper and paperboard in the world (Fig. 1), consuming 105.6 million tons each year. With a base of more than 800 pulp, paper and paperboard mills, the U.S. and Canada are also the leader in the production of paper and paperboard. North America accounts for 32% of the total world output; pulp production is nearly 43%. Total pulp production in the U.S. is divided among the following principal processes: 85% chemical, groundwood and thermomechanical; 6% semi-chemical; and 9% mechanical pulping. The dominant North America pulping process is the sulfate process, derivSteam 41 / Chemical and Heat Recovery in the Paper Industry

Canada 2% Italy 3% France 3% United Kingdom 4% Germany 6%

Peoples Republic of China 11% Japan 10%

Fig. 1 World paper and board consumption by country, 2000.

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The Babcock & Wilcox Company

Fig. 2 Kraft process diagram.

sodium hydroxide (NaOH) and sodium sulfide (Na2S) known as white liquor or cooking liquor. Cooking can take place in continuous or batch digesters. After cooking, pulp is separated from the residual liquor in a process known as brown stock washing. The most common method features a countercurrent series of vacuum drum washers which displace the liquor with minimum dilution. Following washing, the pulp is screened and cleaned to remove knots and shives and to produce fiber for use in the final pulp and paper products. The black liquor rinsed from the pulp in the washers is an aqueous solution containing wood lignins, organic material, and inorganic compounds oxidized in the cooking process. Typically, the combined organic and inorganic mixture is present at a 13 to 17% concentration of solids in weak black liquor. The kraft cycle processes this black liquor through a series of operations, including evaporation, combustion of organic materials, reduction of the spent inorganic compounds, and reconstitution of the white liquor. The physical and chemical changes in the unit operations are shown in Fig. 3.2 The unique recovery boiler furnace was developed for combusting the black liquor organic material while reducing the oxidized inorganic material in a pile, or bed, supported by the furnace floor. The molten inorganic chemicals or smelt in the bed are discharged to a tank and dissolved to form green liquor. Green liquor active chemicals are Na2CO3 and Na2S. Green liquor contains unburned carbon and inorganic impurities from the smelt, mostly calcium and iron compounds, and this insoluble material, or dregs, must be removed through clarification. This operation is basically settling of sediment and decantation of

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Fig. 3 Kraft process cycle.

Steam 41 / Chemical and Heat Recovery in the Paper Industry

The Babcock & Wilcox Company clear green liquor that can be pumped to the slaker. The dregs are pumped out of the clarifier as a concentrated slurry. Normal operation is to water wash the dregs before landfill disposal. The water wash liquid containing the recovered sodium chemical is known as weak wash. The sodium chemicals are recovered by using the weak wash to dissolve the smelt in the dissolving tank. Clarified green liquor and lime (CaO) are continuously fed to a slaker where high temperature and agitation promote rapid slaking of the CaO into calcium hydroxide (Ca(OH)2). The liquor from the slaker flows to a series of agitated tanks that allow the relatively slow causticizing reaction to be carried to completion. The function of the causticizing plant is to convert sodium carbonate into active NaOH. The calcium carbonate (CaCO3) formed in the conversion reaction precipitates in the causticizing operation to form a suspended lime mud. The causticizing product must be clarified to remove the CaCO3 precipitate and produce a clear white liquor for cooking. This clarification is carried out by either settling and decanting, in a manner similar to green liquor clarification, or by using pressure filters. In pressure filtration, the white liquor is filtered through a medium to provide a separation of clear white liquor from the lime mud. The lime mud is then washed to remove sodium chemicals that can lead to increased kiln emissions and clinkering, and further filtered to obtain the desired consistency for feed to the kiln. The lime kiln calcines the washed lime mud feed into reburned lime. Calcination is the chemical breakdown with heat of the CaCO3 into active lime and carbon dioxide (CO2). The calcined lime is then slaked as previously described. The reactions occurring in the solids cycle operations are as follows: Slaking: CaO + water = Ca(OH)2 + heat Causticizing: Ca(OH)2 + Na2CO3 = CaCO3 + 2NaOH Calcination: CaCO3 + heat = CaO + CO2 The combination of these process steps is referred to as recausticization. In parallel with the reduction of sulfur compounds to form smelt, energy is released in the recovery furnace as the black liquor organic compounds are combusted. This combustion energy is used in the process recovery boiler to produce steam from feedwater. The steam can be introduced to a turbine generator to supply a large portion of the energy demand of the pulp and paper mill. Steam extracted from the turbine at low pressure is used for process requirements such as cooking wood chips, evaporation, recovery furnace air heating, and drying the pulp or paper products.

Rated capacity of a recovery unit The capacity of a pulp mill is based on the daily tons of pulp produced. The primary objectives of a recovery boiler are to reclaim chemicals for reuse and to generate steam by burning the black liquor residue. Accordingly, the capacity of the recovery boiler should be based on its ability to burn or process the dry solSteam 41 / Chemical and Heat Recovery in the Paper Industry

ids contained in the recovered liquor. Because the proper measure of recovery boiler capacity is the heat input to the furnace, The Babcock & Wilcox Company (B&W) has established a 24 hour heat input unit of 19,800,000 Btu (20,890 MJ). This unit, known as a B&W-Btu ton, corresponds to the heat input from 3000 lb (1361 kg) of solids (approximately equivalent to one ton of pulp produced) having a heating value of 6600 Btu/lb (15,352 kJ/kg) of solids. These were averages for the typical black liquor solids generated from a ton of kraft pulp production when this unit was originally defined. The black liquor solids produced from modern operations generally are characterized by considerable variation in the quantity of the solids per ton of pulp product and somewhat lower heating values. For this reason, a more common rating term applied to current recovery boilers is the amount of dry solids processed over a given period of time (hour or day). The recovery boiler is a heat input machine, and as such, The B&W-Btu rating recognizes the true indication of the unit’s design capacity to process that energy input. The nominal size of a B&W kraft recovery boiler can be determined by application of a simple formula as follows: Nominal size =

A × B × C , B&W-Btu tons (1) 19, 800, 000

where A = dry solids recovered, lb/t of pulp B = pulp output of mill, t/24 h C = heating value of dry solids, Btu/lb and 19,800,000 is the product of 3000 lb/t and 6600 Btu/lb. Today’s recovery furnace is conservatively designed for a heat release rate (heat input rate divided by furnace plan area) of approximately 0.90 × 106 Btu/h ft2 (2.84 MW/m2 ). This heat release has increased over time, but was always kept below 1.0 × 106 Btu/h ft2 (3.15 MW/m2 ). Although new recovery boiler furnaces are sized for a low heat release rate, there are many recovery boilers that have experienced successful capacity increases at much higher heat release rates. Mills historically have increased pulp production making the recovery boiler a limiting factor at the mill. In order to maintain increased pulp production, the recovery boiler has been called upon to process an ever increasing amount of black liquor solids. Today it is common for a recovery boiler to successfully process solids that result in heat release rates up to 1.25 × 106 Btu/h ft 2 (3.94 MW/m 2 ). This has been achieved through improvements to the combustion air and liquor delivery systems, sootblower systems, reduction of chlorine in the as-fired liquor, and changes to convection pass arrangements. There are several criteria commonly used to evaluate the potential success of capacity increases for recovery boilers. In addition to the heat release rate, the furnace exit gas temperature, superheater exit gas temperature, flue gas velocity, and furnace volume are important criteria. Depending on the extent of the

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The Babcock & Wilcox Company capacity increase, most of these criteria can be met with changes to the original design. For example, flue gas temperatures can be reduced with furnace screen and superheater changes. Furnace volume has been modified by expanding the furnace forward, relocating the furnace front wall forward and making the side walls wider thus providing a greater furnace volume for higher rates of liquor solids. The criteria B&W uses to predict recovery boiler performance is based on operating experience. Some of this has changed over time as equipment improvements have affected operation.

Process flows through the recovery boiler The kraft process recovery boiler is similar in many respects to a conventional fossil fuel-fired boiler. The concentrated black liquor fuel is introduced into the furnace along with combustion air. Inside the furnace, the residual water is evaporated and the organic material is combusted. The inorganic portion of the black liquor solids is recovered as molten smelt. Most of the sulfur is in the reduced form of Na2S and most of the remaining sodium is Na2CO3. The requirement to recover sulfur in a reduced state is the most unique aspect of recovery boiler design. Fig. 4 illustrates a typical modern recovery boiler. Combustion air is introduced into the furnace at staged elevations: primary, secondary, tertiary, and at times, quaternary. One-fourth to one-half of the air enters at the primary level near the furnace floor. The balance is staged at the secondary, tertiary and quaternary levels. Heavy black liquor (solids greater than 60%) is fed to the furnace through multiple burners between the secondary and tertiary air levels. The gases generated by the black liquor combustion rise out of the furnace and flow across convection heat transfer surface. Superheater surface is arranged at the entrance to the convection pass, followed by steam generating surface and finally the economizer. In designs featuring direct contact evaporators, the flue gas may flow from the boiler bank to the evaporator with no economizer surface provided, or a relatively small economizer may be required. Feedwater enters the recovery boiler at the bottom of the first pass economizer. Heated water from the second pass economizer is discharged into the steam drum. From the drum, saturated water is routed through pipe downcomers to lower furnace enclosure wall headers and the boiler bank. From these steam generating circuits, the steam-water mixture is returned by natural circulation to the steam drum where the mixture is separated. From the drum, steam-free water is again returned to the furnace and boiler bank circuits, and water-free steam is directed to the superheater. After flowing through the superheater sections, the steam leaves the recovery boiler and is typically piped to a turbine-generator.

to the feedwater in generating steam and can be expressed as:
, Btu/h ( J / s ) Output = ( H s − H fw ) m

(2)

where H s = enthalpy of steam leaving superheater, Btu/ lb (J/kg) Hfw = enthalpy of entering feedwater, Btu/lb (J/kg) m
= steam or water flow rate, lb/h (kg/s) Boiler water is frequently withdrawn from the steam drum as blowdown to maintain steam purity. Steam may also be withdrawn prior to the final superheater stage for use in sootblowing. In these instances, the output expression must be corrected to account for the energy leaving the boiler prior to the superheater outlet.

Boiler thermal performance The thermal efficiency of a recovery boiler is defined as the ratio of energy output to energy input. The boiler output is a measure of the energy transferred 28-4

Fig. 4 Typical modern recovery boiler.

Steam 41 / Chemical and Heat Recovery in the Paper Industry

The Babcock & Wilcox Company The portion of the input energy available to generate steam can be determined by calculating a steady-state heat and material balance around the boiler. Because steady-state output must equal the input less energy losses, boiler efficiency can also be expressed as: Boiler efficiency =

Output Input − Losses = (3) Input Input

Fig. 5 illustrates the major streams crossing the heat and material balance boundaries. The total heat input can be calculated by summing the chemical and thermal energy contained in the streams entering the boundary. The total losses are then calculated by summing the heat losses due to endothermic reactions occurring within the boiler and the thermal energy losses of the exiting streams. In practice, it is not feasible to precisely measure all streams entering and leaving the system boundaries. An unaccounted for heat loss and a manufacturer’s margin are added to the total losses to correct the calculated efficiency for these limitations. The gross heating value or chemical energy of black liquor is determined by combusting a black liquor sample with an excess of oxidant, under pressure, in a bomb calorimeter. Under these laboratory conditions, the combustion products predominantly exist as CO 2 , H 2O, Na 2 CO 3 , Na 2 SO 4 and sodium chloride (NaCl). A key process in black liquor combustion is the reclamation of sodium compounds in a reduced state. The reduction reactions occurring in the recovery furnace result in different combustion products than those resulting from the bomb calorimeter procedure. These endothermic reactions account for a portion of the black liquor heating value that is not available in the recovery furnace to generate steam. To accurately determine recovery boiler efficiency, the bomb calorimeter gross heating value must be corrected for the heats of reaction of these different combustion products.

Fig. 6 Determination of black liquor heat of reaction correction.

The heat of reaction correction is the difference between the standard heat of formation of the bomb products and the heat of formation of the furnace products. Application of the heats of formation to determine a reaction correction is illustrated for kraft liquor in Fig. 6. Step 1 This is the gross heating value of the black liquor sample determined in the bomb calorimeter. Step 2 From the quantitative analysis of the fully oxidized bomb calorimeter compounds, the heat required to convert these products to their elemental state can be calculated from the standard heats of formation of the compounds from their elements. Step 3 Similarly, from the quantity of each chemical compound present in the furnace combustion products, the heat of formation for the actual furnace products can be calculated.

Fig. 5 Plant heat balance diagram. (See also Table 2.)

Steam 41 / Chemical and Heat Recovery in the Paper Industry

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The Babcock & Wilcox Company The difference between Step 2 and Step 3 is the heat of reaction correction. Sulfur dioxide (SO2 ) and Na2S are the most significant recovery furnace combustion products that differ from those formed under bomb calorimeter conditions. The heat of reaction correction for Na2S is calculated as follows: Na 2 SO4 = 2Na + S + 2O2 (Step 2)

(4)

2Na + S = Na 2S (Step 3)

The calculation is simplified by combining Steps 2 and 3 and using standard heats of formation:

Na 2 SO4 = Na2 S + 2O2 ∆Hf (Na2S) ∆Hf0 (O2) ∆Hf0 (Na2SO4) Heat of reaction correction 0

= = = = =

(5)

89.2 kcal/gmole 0.0 −330.9 −241.7 kcal/gmole −5550.0 Btu/lb Na2S (−12,909.0 kJ/kg)

Similarly, the heat of reaction correction for sulfur dioxide can be determined from standard heats of formation from the bomb calorimeter combustion products:

Na 2 SO4 + CO2 = SO2 + Na2 CO3 + ∆Hf0 (SO2) ∆Hf0 (Na2CO3) ∆Hf0 (O2 ) ∆Hf0 (Na2SO4 ) ∆Hf0 (CO2 ) Heat of reaction correction

= = = = =

1

2

O2

(6)

71.0 kcal/gmole 270.3 0.0 −330.9 −94.1

= −83.7 kcal/gmole = −2360.0 Btu/lb SO2 (−5489.0 kJ/kg)

In actual furnace operations, there is a variety of partially reduced, partially oxidized combustion products. However, accounting only for the presence of SO2 and Na2S in correcting the bomb calorimeter gross heating value closely approximates black liquor combustion in a recovery furnace. Salt cake makeup and other additives to the black liquor are treated in a manner similar to the heat of reaction correction in calculating recovery boiler efficiency. The heat of formation or gross heating value of the Na2SO4 salt cake is accounted for as a contribution to the total system energy input. The subsequent reduction of Na2SO4 to Na2S and O2 is then taken as a heat loss. The black liquor elemental analysis and gross heating value are used to determine the chemical and thermal performance of the recovery boiler. A typical black liquor analysis is presented in Table 1. Table 2 lists the inputs and losses for a recovery unit firing 250,000 lb/h (31.5 kg/s) dry solids at 70% black liquor concentration based on the composition and heating value given in Table 1. Industry practice is to express the various heat losses as a percentage of the total heat input, also shown in Table 2. The system boundaries for the heat and material balance are shown diagrammatically in Fig. 5. 28-6

Table 1 Black Liquor Analysis Dry solids, % by wt Nitrogen (N) Sodium (Na) Sulfur (S) Hydrogen (H2 ) Carbon (C) Oxygen (O2 ) Inerts Potassium (K) Chlorine (Cl) Total solids

0.10 19.77 4.36 3.86 35.14 34.74 0.30 1.31 0.42 100.00

Solids gross heating value = 5985 Btu/lb (13,921 kJ/kg)

The black liquor gross heating value is the predominant energy input to the recovery boiler system. The balance of the input is the sum of the sensible heats contributed by those process streams entering the boiler above a base reference temperature. The black liquor is typically preheated to 230 to 270F (110 to 132C) prior to firing. The majority of the combustion air (primary and secondary) is also generally preheated to promote stable furnace conditions. The heat of reaction correction is expressed as a heat loss due to the endothermic reduction reactions in calculating recovery boiler efficiency. To determine this heat loss, the fraction of sodium and sulfur converted to Na2S, Na2SO4 and SO2 must be calculated from the chemical analysis of the smelt and flue gas leaving the recovery boiler. For the example presented in Table 2, 0.099 lb of Na2S is formed for each pound of black liquor solids entering the recovery boiler. The heat of reaction correction or heat loss associated with the formation of Na2S is calculated as follows: 0.099 lb Na 2S 5550 Btu 250, 000 lb solids × × lb solids lb Na2S h = 137.36 × 106 Btu/h

(7)

In addition, 0.0017 lb of SO2 is formed in the reduction of Na2SO4 to Na2CO3: 0.0017 lb SO2 2360 Btu 250, 000 lb solids × × lb solids lb SO2 h = 1.0 × 106 Btu/h

(8)

The heat loss due to the reduction reactions is the sum of these heat of reaction corrections: Reduction reaction heat loss =137.36 × 106 + 1.0 × 106 =138.36 × 106 Btu/h In addition to the heat of reaction correction and the heat loss attributed to reducing salt cake makeup, energy is lost from the boiler in the form of sensible heat. Heat is also lost through water vaporization and through the molten smelt. Typically, smelt leaving the recovery furnace represents 532 Btu/lb (1237 kJ/kg) of heat consumed to melt the smelt and raise its temSteam 41 / Chemical and Heat Recovery in the Paper Industry

The Babcock & Wilcox Company

Table 2 Material and Energy Balances for a Recovery Boiler Firing 250,000 lb/h Dry Solids at 70% Liquor Concentration Material balance: Entering combustion air Entering infiltration air Entering black liquor Total in

= = =

1,176,620 lb/h 20,550 lb/h 357,143 lb/h 1,554,313 lb/h

Smelt leaving Wet gas leaving Particulate leaving Total out

= = =

111,200 lb/h 1,442,290 lb/h 823 lb/h 1,554,313 lb/h

Energy balance:

106 Btu/h

% Total

Chemical heat in liquor Sensible heat in liquor Sensible heat in air Input

= = =

1496.25 37.47 49.59 1583.31

94.50 2.37 3.13 100.00

Sensible heat in dry gas Moisture from air Moisture from hydrogen Moisture from liquor Reduction reactions Heat in smelt Radiation Unaccounted for and manufacturer's margin Losses

= = = = = = =

101.49 2.37 104.82 129.20 138.36 71.25 4.75

6.41 0.15 6.62 8.16 8.74 4.50 0.30

=

15.83 568.07

1.00 35.88

Boiler efficiency = Input − Losses 1583.31 − 568.04 = = 64.12% Input 1583.31 Output = Efficiency x Input = 64.12 6 x 1583.31 x 106 = 1015.22 x 10 Btu/h 100 Steam flow = Output 1015.22 x 106 = 847,970 lb/h = (1444.16 − 246.93) Hs − Hfw

perature to a nominal 1550F (843C). The balance of the heat losses are determined in a manner similar to those for conventional power boilers. (See Chapter 22.) The contribution to boiler efficiency offered by the remaining streams crossing the recovery unit heat and material balance is primarily established by their sensible heat content at the temperature at which they cross the system boundary. The minimum temperature of the flue gas leaving the boiler is selected to minimize corrosion. The heat transfer surface arrangement and thermodynamic considerations then dictate the economic limit for the flue gas exit temperature, typically 350 to 400F (177 to 204C). The gross heating value of a given black liquor sample is strongly influenced by its carbon content. As this content increases, the heating value typically increases, as illustrated in Fig. 7. An increased liquor heating value also generally corresponds to an increased hydrogen content, with a corresponding decrease in inorganic sodium and sulfur contents. These Steam 41 / Chemical and Heat Recovery in the Paper Industry

factors result in an increased quantity of theoretical air (see Chapter 10) required to combust the black liquor. The overall trends can be summarized as follows: 1. Carbon (and hydrogen) content increases with increasing heating value. 2. Inorganic sodium and sulfur contents decrease with increasing heating value. 3. Theoretical air increases with increasing carbon and hydrogen contents. 4. Theoretical air increases with increasing heating value. These trends can be used as quick checks on laboratory results for a given black liquor sample chemical analysis and gross heating value.

Black liquor as a fuel Black liquor Black liquor is a complex mixture of inorganic and organic solids partially dissolved in an aqueous solution. Heavy or strong black liquor introduced to the recovery furnace ranges from 60 to 80% solids by weight. The organic fraction of the solids is principally derived from the hemicellulose and the lignin removed from the cellulose strands of the wood chips. The solids’ inorganic fraction is primarily Na 2 CO 3 , sodium hydrosulfide (NaHS) and oxidized sulfur compounds. Black liquor also contains various chemical elements which enter the process with the wood, as impurities in makeup limestone and salt cake, and as contaminants in makeup water. These elements include potassium, chlorine, aluminum, iron, silicon, manganese, magnesium and phosphorous. The waste stream from a chlorine dioxide generator in a bleached mill can also contribute NaCl. Potassium and chlorine directly impact the recovery boiler design and operation if they are present in the black liquor in sufficient quantities. It is not uncommon for many other waste streams and effluents from the mill to be added to the black liquor. This may include soap, various brines or other effluents from the bleach plant, or a variety of other streams. These may significantly affect the chemical composition or heating value of the black liquor fuel, which can have major impacts on performance, operations, emissions and/or cleanability of the recovery boiler. Black liquor is sprayed into the furnace as coarse droplets which fall to the floor in a dry and partially combusted state to form a char bed. The mounded bed

Fig. 7 Black liquor high heating value as a function of carbon content in dry solids.

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The Babcock & Wilcox Company

Reduction efficiency = Na 2 S + NaHS × 100%, Na 2O (9) Total sodium sulfur compounds A common industry simplification is as follows:

Reduction efficiency = Na 2 S × 100%, Na 2O Na 2 S + Na2 SO4

(10)

The industry practice is to express the compounds in the equations as the equivalent weight of Na2O.

Emissions The black liquor combustion process is never theoretically complete. This results in small concentrations of unburned combustibles, typically carbon monoxide (CO), organic and sulfur compounds, and hydrogen sulfide (H2S), being discharged to the atmosphere. The volatile organic compounds, or VOC, are generally expressed in terms of equivalent methane (CH4) and are sometimes more specifically referred to as nonmethane volatile organic compounds (NMVOC). H2S and sulfur-bearing organic compounds such as mercaptans are grouped together as total reduced sulfur (TRS). Trace amounts of SO2 also exist in addition to TRS-bound sulfur. As in most combustion processes, nitrogen oxides (NOx) are present and are expressed in terms of equivalent nitrogen oxide (NO2). Black liquor combustion also creates particulate matter. The modern recovery boiler achieves effective NOx control by staged air combustion, control of excess air, and a uniform distribution of the black liquor through multiple burners. A recovery furnace inherently produces lower NOx emissions compared to fossil fuel boilers. Burning 68 to 75% solids concentration black liquor, NOx levels would normally be expected to be below 100 ppm. Recovery boilers can often be upgraded in capacity with little or no increase in NOx emissions, as air port sizes and locations can be modified to allow adequate control of operations, and emissions, when increasing solids input. Numerical modeling of the existing air system using actual operating data provides the engineer with accurate predictions of NOx and particulate emissions before hardware is installed. SO2 emissions are a function of the sulfidity of the smelt. Fig. 8 shows the general variation of SO2 with 28-8

350 Lower Solids Black Liquor (70% Solids)

50

0 14

18

26

22

30

34

38

42

Smelt Sulfidity, %

Na2S Na2S + Na2CO3 + NaOH Expressed as Na2O in the Smelt

Where Smelt Sulfidity is Percentage Ratio (weight, %) of:

Fig. 8 Sulfur dioxide emissions.

sulfidity. An environmental benefit of increased black liquor concentrations fired in recovery boilers is a reduction in SO2 emissions. Concentrations of TRS in the combustion gases leaving a modern boiler are readily controlled below 5 ppm, as the H2S and volatile organic sulfide compounds are oxidized in the high temperature furnace. VOC and CO emissions can be controlled by proper furnace design and operation. VOC can be maintained below approximately 80 ppm (current Environmental Protection Agency/EPA limit) while CO emissions are controllable to less than 500 ppm. A hot furnace and thorough mixing of combustion air with the generated volatiles are essential in minimizing VOC, TRS and CO emissions. Particulate is removed from the combustion gases in a high efficiency electrostatic precipitator, which can control the stack discharge particulate to the current EPA limit of 0.044 gr/dscf.

Ash Ash accumulations on heat transfer surfaces create an insulating barrier that reduces heat transfer to the boiler tubes. Consequently, as the ash deposits build and heat transfer decreases, steam outlet temperatures decay and the flue gases retain higher temperatures as they pass through the boiler surfaces. Higher flue gas temperatures lead to more ash in its molten phase being carried further back in the boiler convection pass where it adds to already present accumulations. As ash deposits grow, they also begin restricting gas flow through the boiler, plugging the gas passes, and eventually increasing the furnace draft loss to inoperable levels. The characteristics of ash from black liquor combustion impact the design of the process recovery boiler. Approximately 45% by weight of the dry, as-fired solids is inorganic ash. The majority of these inorganics are removed from the furnace as Na2S and Na2CO3 in Steam 41 / Chemical and Heat Recovery in the Paper Industry

The Babcock & Wilcox Company

Steam 41 / Chemical and Heat Recovery in the Paper Industry

stack. More fuming than that sufficient to capture the sulfur causes the excess alkali to be converted to carbonate in the ash. Less fuming than that sufficient to capture sulfur causes excess sulfur to be released as SO2, and chlorine to be released as HCl in the stack gas, rather than being converted to chloride in the ash. As potassium (K) has a higher vapor pressure than sodium (Na), the fume contains a higher ratio of K/ Na than that found in smelt or black liquor. This is referred to as potassium enrichment. Chlorides are also found at higher concentrations in fume. Potassium and chlorides can contribute to severe plugging in the recovery boiler convection surfaces. Ash fouling and gas path plugging within recovery boilers are directly related to the melting properties of the ash in the boiler. Chloride and potassium concentrations within the liquor cycle are the most significant factors affecting ash melting points. High chloride concentrations are the result of high chloride sources such as in the wood supply or makeup chemicals. Environmental improvements within the last decade have reduced chemical losses throughout the system, and increased various dead-load chemicals, including chlorides, within the liquor stream. One of the main factors in determining ash deposition rates is the ash stickiness.3 Ash stickiness is a function of the amount of liquid phase present in the ash, that is dependent upon ash chloride content and temperature. A simplified relationship between the ash chloride content and the sticky temperature is shown in Fig. 9. This graph may be used to illustrate the anticipated effects of elevated ash chloride levels in general terms. The conditions of this particular graph are at a 5% potassium molar ratio. Notice that the sticky band on this graph covers the widest temperature range from 5 to 10% molar chloride levels and would be in a narrow range at higher and lower chloride levels. Ash enrichment by chloride reduces the ash melting point and increases the deposit sintering rate. The characteristic sticky temperature has been defined as the temperature where 15% of the alkali salt mixture is liquid. The presence of K in combination with chlorides further reduces the sticky temperature of depos850 5 Mole% K/(Na+K)

Radical Deformation

800 750

Temperature, C

the molten smelt. A significant amount of ash is present as particulate entrained in the existing flue gases. Generally, about 8% by weight of the entering black liquor dry solids leaves the furnace as ash. Ash is generally categorized as fume or carryover. Carryover consists of char particles and black liquor droplets that are swept away from the char bed and liquor spray by the upward flue gas flow. Entrainment occurs when small particles caught in the furnace gases are not of sufficient size, shape or density to fall back into the furnace. Entrainment results in combustion of black liquor in the upper furnace which affects temperature and ash deposit properties. Entrainment of smelt and char materials is a major cause of convection surface plugging. Once entrained, the black liquor carryover droplet follows the gas flow. When complete particle burnout occurs, the entrained droplet can settle out of the gas flow as a smelt bead. Otherwise, the partially combusted particles form sparklers that deposit on tubes as char, then continue to burn and yield a smelt deposit. At low loads with lower furnace gas flow rates, entrained droplets have time to burn out, and only small smelt droplets show up as carryover. As load is increased, larger drops can be entrained by the correspondingly increased gas flow. The particles can then include small smelt droplets and large char particles. Carryover can be controlled by furnace size and by proper design and operation of the firing and combustion air systems. Fume consists of volatile sodium compounds and potassium compounds rising into the convection sections of recovery boilers. These volatiles condense into submicron particles that deposit onto the superheater, boiler bank and economizer surfaces. Fume particles in kraft recovery boilers are usually 0.25 to 1.0 microns in diameter and consist primarily of Na2SO4 and a lower content of Na2CO3. Fume also contains potassium and chloride salts. The much larger carryover particles, typically greater than 100 microns, are easily distinguishable from the submicron fume particles. Fume and carryover ash are also different in their chemical analyses. Carryover is similar in composition to the smelt (see comments above). Fume is mostly Na2SO4 and is enriched in potassium and chloride relative to their concentration in the smelt. Fume can contribute to deposit formation and plugging in the convection heat transfer sections of the boiler, particularly if allowed to sinter and harden. Fume particles are also the predominant source of particulate emissions from recovery boiler stacks. A third category of ash is represented by intermediate size particles (ISP), a class of particles between carryover (> 100 microns) and submicron fume, which are produced during combustion of black liquor droplets and the char bed. While ISPs are abundant, exist everywhere, and are potentially important to fouling in recovery boilers, the mechanisms by which these particles are formed are not yet well understood. Char bed temperature controls the fuming rate. A rate just sufficient to capture the sulfur released during combustion should be established. This minimizes the dust load to the precipitator and the SO2 to the

Slagging

700 Sticky

650 Not Sticky

600 550

First Melting

500 0

2

4

6

Fig. 9 Sticky temperature.4

8

10

12

14

16

18

20

CI/(Na+K) Mole %

28-9

The Babcock & Wilcox Company its. The recovery boiler should be designed to reduce the gas temperature entering the boiler bank to below the ash sticky temperature to avoid bank plugging. Decreasing the Cl level in the black liquor can also decrease the plugging tendency of the resulting ash. The successful reduction of chloride levels has been achieved by ash purging. When purging ash (see Ash system), ash is removed periodically to maintain chloride levels at a specified target. For the most part, maintaining a chloride level in the precipitator ash of 1.5% or less has proven to be successful in reducing fouling and plugging within recovery boilers.

Recovery boiler design evolution The kraft recovery process evolved in Danzing, Germany approximately 25 years after the soda process was developed in the United Kingdom in 1853. In 1907, the kraft recovery process was introduced in North America. From its inception, a variety of furnace types competed for a successful commercial design, including rotary and stationary furnaces. During the late 1920s and early 1930s, significant design developments were achieved by G.H. Tomlinson, working in conjunction with B&W engineers. The first Tomlinson recovery boiler was supplied by B&W Canada in 1929, at the Canada Paper Company’s Windsor Mills, Quebec plant (Fig. 10). This black liquor recovery boiler had refractory furnace walls that proved costly to maintain. The steam generated with the refractory furnace was also much less than that theoretically possible. Tomlinson decided that the black liquor recovery furnace should be completely water-cooled, with tube sections forming an integral part of the furnace. This new concept boiler, designed

Fig. 10 First Tomlinson recovery boiler.

28-10

in cooperation with B&W, was installed at Windsor Mills in 1934. The water-cooled design was a complete success, and the boiler operated until 1988. The first Tomlinson recovery boilers in the U.S. were two 90 B&W-Btu t/day units sold to the Southern Kraft Corporation in Panama City, Florida in 1935. The Tomlinson design evolved with a technique of spraying black liquor onto the furnace walls. The liquor is dehydrated in flight and on the furnace walls, where pyrolysis begins with the release of volatile combustibles and organically bound sodium and sulfur. As the liquor mass builds on the furnace walls, its weight eventually causes it to break off and fall to the hearth. There, pyrolysis is completed and the char is burned, providing the heat and carbon required in the reduction reaction. By the end of World War II, the recovery boiler design (Fig. 11) had evolved to the general two-drum arrangement that represented B&W’s standard product until the mid-1980s. Retractable sootblowers using steam as a medium eliminated hand lancing in the 1940s; this significant development made large recovery boiler designs practical.

Fig. 11 General two-drum arrangement of the 1940s.

Steam 41 / Chemical and Heat Recovery in the Paper Industry

The Babcock & Wilcox Company

Wall and floor construction By 1946, furnace wall construction had evolved from tube and refractory designs to a completely water-cooled furnace enclosure, using flat plate studs to close the space between tubes and to minimize smelt corrosion and the resultant smelt leaks. The flat stud design was superseded in 1963, with membrane tube construction where the gas-tight seal is along the plane of the wall rather than formed by casing behind the wall. The 1963 furnace wall construction had 3 in. (76 mm) outside diameter (OD) tubes on 4 in. (102 mm) centers. The advantages of this construction included less air infiltration, reduced refractory maintenance, and a completely gas-tight unit. The design used cylindrical pin studs for corrosion protection of the tubes in the reducing zone of the lower furnace. The pin studs held solidified smelt, forming a barrier to the corrosive furnace environment. The current construction calls for 64 half-inch (13 mm) diameter studs per linear foot (0.3 m) of tubing. The lower furnace design continued to evolve in the 1980s from the traditional pin stud arrangement to the use of composite or bimetallic tubes. The composite tubes are comprised of an outer protective layer of AISI 304L stainless steel and an inner core layer of standard American Society of Testing and Materials (ASTM) A 210 Grade A1 carbon steel. The composite tube inner and outer components are metallurgically bonded. The outer layer of austenitic stainless steel, which is also used to cover the furnace side of the carbon steel membrane bar, protects the core carbon steel material from furnace corrosion. Not long after the introduction of 304L composite tubes, issues with sodium hydroxide attack of the stainless layer near air ports, and cracking of the 304L layer and tube-to-membrane weld, were discovered. The cracking and corrosion have typically been specific to the floor and primary air ports. Considerable investigation has occurred with other materials identified as probable substitutes for 304L. Differential expansion of the two layers has been identified as an issue. Table 3 shows the coefficients of expansion and tensile strength for different materials commonly used to protect recovery boiler lower furnace tube surfaces. Several methods are used for lower furnace corro-

Carbon Steel with Pin Studs (900 psig/6.2 MPa)

Expansion coefficient Mean to 700F (371C) x 10−6 (in./in. F)

7.59

9.69

8.3

7.5

Thermal Conductivity at 700F (371C) (Btu/h ft2 F)

320

142

115

117

56

86

132

Ultimate Tensile Strength ksi at 1000F (538C)

Enhanced Tube Materials or Bare Carbon Steel

Coextruded 304L (>900 psig/6.2 MPa)

Table 3

Property

sion protection. Lower furnace protection should be determined from criteria such as steam pressure, the history of furnace corrosion, capital budget, and maintenance expectations. The list of common means of protection includes chromized carbon steel tubes, chromized pin studs, carbon steel pin studs, metallic spray coatings, high density pin studs, 304L, Alloy 825 and Alloy 625 composite tubes, and weld overlay of carbon steel tubes. Normally, carbon steel tubes are used below 900 psig (6.2 MPa) and tubes with an alloy outer surface are used above this pressure. A new or rebuilt high pressure recovery boiler may have several different tubes in the lower furnace (see Fig. 12). The floor may be carbon steel with pin studs. To provide additional circulation margin in the floor, and protect against extreme heat absorption upset conditions, the use of multi-lead ribbed (MLR) tubing has become common, and has become the norm in carbon steel floors regardless of whether the floor is sloped or flat. It is also common for carbon steel floors to use tubes with an alloy outer surface for the first 3 ft (0.9 m) from each side wall, as these tubes can have accelerated corrosion rates over the remainder of the floor due to the continued exposure to molten smelt. The vertical walls from the floor to an elevation above the primary air port elevation would normally be 825 composite or 625 weld overlay. Above this elevation, up to approximately 3 to 15 ft (0.9 to 5 m) above the tertiary air ports (or quaternary zone), 304L can be used to reduce costs. Materials continue to be studied and evaluated in the laboratory and in operating recovery boilers for corrosion protection.

Steam 41 / Chemical and Heat Recovery in the Paper Industry

Carbon Steel with Pin Studs (0.75 in. (19.05 mm)

Pendant

Fig. 18 Superheater arrangements.

28-17

The Babcock & Wilcox Company retractable sootblowers. Trough hoppers are attached to the economizer casing to collect dislodged ash deposits. The economizer surface area is set to achieve a final gas outlet temperature approximately 100F (56C) higher than the feedwater temperature. Although it is possible to achieve an exit gas temperature closer to that of the feedwater, the decreased temperature differential results in substantially increased surface requirements for small improvements in end temperature. In addition to this thermodynamic limitation, concern for cold end corrosion generally establishes a minimum gas exit temperature around 350F (177C). The minimum recommended temperature of the feedwater entering the economizer is 275F (135C) for corrosion protection of the tube surface. With special considerations, the feedwater entering the economizer can be designed for as low as 250F (121C).

Fig. 19 Boiler bank isometric.

a trough hopper connected to the bank enclosure. Tube section inlet headers are widely spaced and vertically staggered to facilitate ash dropping into the hopper. Impact-type particle deposition on the boiler bank tubes is less likely to occur with the gas longflow orientation. As a result, the allowable gas velocity in the downflow portion of the bank can be increased. To improve heat transfer, longitudinal fins are welded to the front and back of each tube. Fins are tapered at the ends and welded to the tube on both sides. The welds are terminated by wrapping around the end of the fin. This combination of welding technique and tapered ends assures minimal stress concentration at fin termination for fins as large as 1.5 in. (38 mm).

Economizer The boiler bank surface area is typically set to achieve a nominal exit gas temperature of about 800F (427C). This temperature maintains a reasonable differential with the saturated steam temperature [610F (321C) for a 1650 psig (11.4 MPa) drum pressure] and allows the use of carbon steel casing to enclose the downstream economizer banks. The modular economizer has vertical finned tubes arranged in multiple sections with upward water flow and downward longflow of gas (Fig. 20). The common arrangement features two banks. The flue gas enters at the upper end and discharges at the lower end of each bank. Gas flows down the length of the bank to provide good cleanability. As in the boiler bank, a central cavity dimensioned for personnel access accommodates fully 28-18

Emergency shutdown system An emergency shutdown procedure for black liquor recovery boilers has been adopted by the Black Liquor Recovery Boiler Advisory Committee (BLRBAC) in the U.S. An immediate emergency shutdown must be performed whenever water enters the furnace and can not be stopped immediately, or when there is evidence of a leak in the furnace setting pressure parts. The boiler must be drained as rapidly as possible to a level 8 ft (2.4 m) above the mid point of the furnace floor. An auxiliary fuel explosion can occur when an accumulated combustible mixture is ignited within the confined spaces of the furnace and/or the associated boiler passes, duct work and fans which convey the combustion gases to the stack. A furnace explosion will result from ignition of this accumulation if the quantity of the combustion mixture and the proportion of air to fuel are within the explosive limit of the fuel involved. The magnitude and intensity of the explosion will depend upon both the quantity of accumulated combustibles and the proportion of air in the mixture at the moment of ignition. Contacting molten smelt with water can also result in a very powerful explosion. The mechanism for a smelt-water explosion is keyed to the contact of water with hot liquid smelt. Rapid water vaporization causes the propagation of a physical detonation or shock wave. In the design and operation of black liquor recovery boilers, every effort is made to exclude water from any source from getting to the furnace, or introducing liquor at less than 58% solids. For example, furnace attachment details are designed to prevent external tube loads, which can lead to stress assisted corrosion.

Recovery boiler auxiliary systems Black liquor evaporation The high black liquor solids concentration required for efficient burning is achieved by evaporating water from the weak black liquor. Large amounts of water can be economically evaporated by multiple effect evaporation. A multiple effect evaporator consists of a series of evaporator bodies, or effects, operating at different pressures.5 Typically, low or medium presSteam 41 / Chemical and Heat Recovery in the Paper Industry

The Babcock & Wilcox Company vessel with the flue gas admitted through a tangential inlet near the conical bottom. The gas flows in a whirling helical path to the cylinder’s top and leaves through a concentric re-entrant outlet. Black liquor is sprayed across the gas inlet to obtain contact with the gas. The liquor droplets mix intimately with the high velocity gas and are centrifugally forced to the cylinder wall. Recirculated liquor flowing down the cylinder wall carries the droplets and any dust or fumes from the gas to the conical bottom, out through the drain, and into an integral sump tank. Sufficient liquor from the sump tank is recirculated to the nozzles at the top of the evaporator to keep the interior wall wet, preventing ash accumulation or localized drying. In the cascade evaporator, horizontally spaced tubular elements are supported between two circular side plates to form a wheel that is partially submerged in a liquor pool contained in the lower evaporator housing. The wetted tubes are slowly rotated into the gas stream. As the tubes rise above the liquor bath, the surface coated with black liquor contacts the gas stream flowing through the wheel.

Fig. 20 Economizer isometric.

sure steam is utilized in the first evaporator effect and then the vapor from one body becomes the steam supply to the next, operating at a lower pressure. Modern evaporator systems integrate a concentrator into the flow sequence to achieve the final liquor concentration. As a general rule, each pound of water evaporated from the weak liquor results in one additional pound of high pressure steam generation in the recovery boiler. For many years, direct contact evaporation was the technology used to achieve firing solids to the recovery boiler, and may still be necessary to evaporate liquor from some special fiber sources. In the direct contact evaporator, liquor and flue gas are brought together, heat is transferred from flue gases to the liquor, and mass transfer of liquor water vapor to the gas occurs across the liquor-gas interface. Adequate liquor surface must be provided for the heat and mass transfer. The gas contact acidifies the liquor by absorbing CO2 and SO2, which decreases the solubility of the dissolved solids and requires continuous agitation. The acidification also results in the release of malodorous compounds into the flue gas, a negative aspect of direct contact evaporation. There are two types of direct contact evaporators used in the recovery unit, cyclone and cascade. The cyclone evaporator (Fig. 21) is a vertical, cylindrical Steam 41 / Chemical and Heat Recovery in the Paper Industry

Black liquor oxidation When a direct contact evaporator is used, odor can be reduced by oxidation of sulfur compounds in the liquor before introduction to the evaporator. The oxidation stabilizes the sulfide compounds to preclude their reaction with flue gas in the evaporator and the consequent release of reduced sulfur gases. Oxidation can effectively reduce, but does not eliminate, discharge of these reduced sulfur gases. The direct contact evaporator is the prime source of odor. Odor is generated in direct contact evaporators when the hot combustion gases strip hydrogen sulfide gas from the black liquor: 2NaHS + CO2 + H2O = Na 2 CO3 + 2H2 S

(11)

Fig. 21 Cyclone evaporator.

28-19

The Babcock & Wilcox Company Oxidation stabilizes the black liquor sulfur by converting it to thiosulfate:

2NaHS + 2O2 = Na 2 S2 O3 + H2O

(12)

Black liquor oxidation involves high capital and operating costs; the oxidation step also robs the liquor of heating value. Modern recovery facilities incorporate multiple effect evaporators, and the low odor design, which eliminates the need for a direct contact evaporator.

Black liquor system Recovery boilers are operated primarily to recover pulping chemicals. This objective is best realized by maintaining steady-state operation. Recovery boilers are base loaded at a selected black liquor feed flow or heat input, in contrast to power boiler applications where fuel flow is varied in response to demand for steam generation. The black liquor solids concentration can vary with the rate that recirculated ash sheds from heat transfer surfaces, the rate it is collected in the precipitator, and the rate it is returned to the black liquor stream system. Considerable fluctuations can occur, depending on which surface is being cleaned by sootblowers, the frequency of sootblower operation, and the rapping sequence of precipitator collection surfaces. The black liquor system design should provide uniform dispersion of the ash into the liquor to minimize the fluctuation of solids at the burner. This is increasingly important as the liquor solids concentration is increased into the regime where burners are maintained in a fixed position and liquor drying is in-flight (not on the walls). A fluctuation in ash flow would change the solids concentration by several percent, significantly impacting the characteristics of the liquor. A heater is used to adjust the black liquor temperature to that required for optimum combustion and minimum liquor droplet entrainment in the gas stream. The black liquor is typically heated in a tubeand-shell heat exchanger, using low pressure steam on the shell side and black liquor on the tube side. There are two general categories of heater designs that can be operated with minimum scaling of the heat transfer surface. The first uses a conventional heat exchanger with once-through flow of liquor at high velocity in the tubes. The tubes have polished surfaces to inhibit scale formation. This heat exchanger is satisfactory for long periods of operation without cleaning. The second approach is to recirculate liquor through a standard heat exchanger with stainless steel tube surface to maintain high velocities that inhibit scale formation. This approach also permits long periods of operation without cleaning. As the liquor solids concentration increases above 70%, storage can become a problem. Moreover, it becomes increasingly difficult to blend recycled ash into the highly concentrated liquor. Recycled ash can be returned to an intermediate concentration liquor stream (of about 65% concentration) prior to final evaporation in a concentrator. In this type of arrangement, liquor is routed to the recovery furnace from an evaporator system product flash tank. The as-fired 28-20

liquor temperature is established by controlling the flash tank operating pressure. There are two designs of liquor burners, the oscillator and the limited vertical sweep (LVS). Oscillators are generally applied to liquors of lower heating values or percent solids where longer droplet drying time is necessary. LVS burners are best used for higher percent solids and heating value liquors where inflight, suspension drying of the droplets works well. Both types of burners utilize a nozzle splash plate to produce a sheet spray of coarse droplets. The oscillator sprays the black liquor on the furnace walls, where it is dehydrated and falls to the char bed. The oscillator burners, located in the center of the furnace wall between the secondary and tertiary air ports, are continuously rotated and oscillated, spraying liquor in a figure eight pattern to cover a wide band of the walls above the hearth. Oscillators work well on smaller units, or with a wide range of liquor characteristics. With LVS burners (Fig. 22), black liquor is sprayed into the furnace for in-flight drying and devolatization of the combustible gas stream rising from the char bed. The objective of the LVS burner is to minimize the liquor on the wall. The LVS gun is normally used in a fixed position, but can also sweep vertically to burn low solids liquor or those with poor burning characteristics. LVS burners are normally applied to larger sized, heavier loaded units. The temperature and pressure of atomized liquor directly impact recovery furnace operations. Lower temperature and pressure generally create a larger

Fig. 22 Limited vertical sweep burner.

Steam 41 / Chemical and Heat Recovery in the Paper Industry

The Babcock & Wilcox Company particle or droplet of atomized liquor. This minimizes the entrainment of liquor in the combustion gases passing to the heat absorbing surfaces. Where wall drying is carried out, large liquor droplets maximize the liquor sprayed on the wall and minimize in-flight drying. For oscillator firing, liquor at approximately 230F (110C) and 30 psig (207 kPa) generally provides the most satisfactory operation. For LVS burners, liquor with a slightly lower pressure of 25 psig (172 kPa) and higher temperature of 250F (121C) provides acceptable results. As the liquor sprayed on the walls builds, it eventually falls to the char hearth. The majority of the char falling from the wall is deposited in front of the primary air ports, requiring 30 to 50% of the total air to be introduced through the primary ports. In-flight drying deposits a minimum of char in the primary air zone around the periphery of the unit, as all the drying and a majority of the devolatization are in-flight over the furnace area. Consequently, less primary air flow is required to keep the char from in front of the primary air ports. Because the higher liquor solids concentration dictated by in-flight drying translates into less water evaporated in the furnace, a greater fraction of the combustion heat released is available to maintain bed temperatures. Less primary air is therefore required when burning liquor in-flight in contrast to the lower concentration liquors encountered in oscillator burner applications.

Ash system The sodium compounds entrained in the flue gas originate from fume generation and liquor droplet carryover from the lower furnace. The resultant ash drops out of the flue gas stream and is collected in trough hoppers located below the boiler and economizer modules. The electrostatic precipitator removes nearly all the remaining ash. The majority of the entrained ash is Na2SO4, and is commonly referred to as salt cake ash, or simply, salt cake. The ash collected in the trough hoppers and precipitator must be returned to the black liquor to recover its significant sodium and sulfur contents. This is accomplished by mixing the ash into the liquor in a specially designed tank. The salt cake ash is transferred to the mix tank through a wet ash sluice system or a drag chain ash conveyor system. With a dry ash system (Fig. 23), mechanical drag chain conveyors are bolted to the bottom of the boiler and economizer trough hoppers, which extend across the full unit width. The drag chain conveyors are equipped with heat treated, high alloy forged link chains to support and convey the flights. The conveyors discharge through rotary seal valves into a collection conveyor. The collection conveyor discharges to the mix tank where the ash is uniformly mixed with the liquor. Drag chain conveyors are also provided across the floor of the electrostatic precipitator, beneath the collecting surfaces. The conveyors discharge to mechanical combining conveyors, which in turn discharge the salt cake ash through rotary valves into the mix tank. Frequently, the dry ash system is arranged with two Steam 41 / Chemical and Heat Recovery in the Paper Industry

Fig. 23 Dry ash system.

mix tanks, one serving the boiler and economizer hopper ash system and one dedicated to the precipitator. As discussed previously, successful reductions of chloride and potassium levels in the ash and black liquor have been achieved through precipitator ash purging. Ash can be removed periodically to maintain chloride or potassium concentrations at a specified level. An ash chute with a rotary valve is added to the back end of an existing precipitator ash transfer conveyer. When ash purging is desired, the ash conveyer is reversed and the ash falls through the chute into a sluice tank. In a wet ash removal system, black liquor is circulated through the hoppers to sluice the collected ash. The sluice discharges directly from the hoppers through large pipes to the mix tank. However, these pipes can become plugged with ash and overflow liquor which create safety and cleanliness problems. This has led to wider acceptance of the dry ash mechanical conveyor system. However, the mechanical conveyors generally require more maintenance than the wet ash sluice designs. The mix tank (Fig. 24) includes a mechanically scraped screen to assure that all material which passes to the fuel pumps is small enough to readily pass through the burner nozzles. Scraping is provided by flights on a low horsepower, slow moving agitator. The tank is designed to receive salt cake from the generating bank, economizer or precipitator hoppers, mixing it with the incoming black liquor.

Combustion air system B&W’s advanced air management system provides combustion air at three, and sometimes four, elevations of the furnace: primary, secondary, tertiary, and potentially quaternary (Fig. 25). The use of at least three levels allows performance optimization of the respective furnace zones – the lower furnace reducing zone, the intermediate liquor drying zone, and the upper furnace burnout or combustion completion zone. A quaternary air level may also be applied above the tertiary zone for additional NOx or particulate control. 28-21

The Babcock & Wilcox Company

Fig. 24 Salt cake mix tank.

The primary air flow quantity must be sufficient to produce stable combustion and to provide the hot reducing zone for the molten smelt. Increasing primary air flow beyond the level required to achieve these objectives increases the amount of Na2S re-oxidized to Na2SO4. The balance of the air at or less than the stoichiometric requirement is introduced above the char bed at the secondary level to control the rate at which the liquor dries and the volatiles combust, and to minimize the formation of NOx. The additional air required to complete combustion is introduced at the tertiary level (and quaternary if supplied). The air system may be arranged as a single-fan, twofan (one primary and one secondary/tertiary/quaternary) or three-fan system. On larger systems, additional provisions are made to bias the air flow between respective air levels, from side to side on the primary and secondary air, or from front to rear on the tertiary air. This gives additional flexibility to the operator. The primary air ports are arranged on all four furnace walls about 3 ft (0.9 m) above the floor. Air is introduced at a low velocity and 3 to 4 in. wg (0.75 to 1.0 kPa) static pressure which prevents it from penetrating the bed. The air lifts the carbon char in front of the port back onto the bed and maintains ignition. Approximately 40% of the total air is admitted at the secondary zone. The air is introduced at the pressure and velocity needed to penetrate the furnace 4 to 6 ft (1.2 to 1.8 m) above the primary air ports. A velocity damper is used on each port. Proper secondary air mixing with the gases rising from the char bed results in volatile combustion generating the heat required for in-flight liquor drying. This secondary zone typically exhibits the highest temperatures in the recovery furnace. The quantity of secondary air is dictated by the amount of burning required

28-22

to dry liquor and control bed height, and decreases as the black liquor solids concentration increases. The balance of the air is admitted at ambient temperature through the tertiary air ports located above the liquor guns, and through quaternary ports, if supplied. A velocity damper is again used on each port. The tertiary (and quaternary) flow increases proportionately as the solids increase. The secondary air ports are normally arranged on the longest furnace wall, generally the side walls, as the furnace is deeper than it is wide. In contrast, the tertiary air ports are arranged on the front and rear walls. Buildup of smelt and char that restricts the air port openings can cause a performance deterioration. A reduction in port area affects air pressure at the port or air flow through the port, depending on which is being controlled. When air flow is controlled, the effect of plugged ports is to increase air pressure and push the bed farther from the wall. If pressure is controlled, the effect is a resultant decrease in air flow. This results in less effective burning of the primary zone with a decrease in furnace temperature and increased emissions. Primary air port openings are generally cleaned (rodded) every two hours to maintain proper air distribution and bed height. Automatic port rodders provide continuous cleaning to maintain a constant flow area. The automatic port rodding system stabilizes lower furnace combustion for maximum thermal efficiency and is vital in achieving low emissions. Secondary and tertiary air ports also require periodic rodding, although the plugging that occurs at these ports is generally less severe than at the primary air ports. In the secondary and tertiary ports, the rodding equipment can be integrated with the velocity control damper. This is necessary to provide synchronization of the damper and rodder drives.

Carbon Burnout, NO / Particulate Reduction

Quaternary Air (0-10%) Liquor Firing Equipment

Tertiary Air (10-20%)

Devolatilization and Drying Zone Chemical Reduction Zone (Maximum Na So Na S)

Secondary Air ( 40%) Primary Air ( 40%)

Fig. 25 Advanced air management system.

Steam 41 / Chemical and Heat Recovery in the Paper Industry

The Babcock & Wilcox Company To further enhance stability in the lower furnace, the primary and secondary air is preheated in a steam coil air heater. Low pressure steam, normally 50 to 60 psig (345 to 414 kPa) is used for preheating. Additional preheating is provided by steam at 150 to 165 psig (1034 to 1138 kPa), typically achieving a 300F (149C) combustion air temperature. When burning liquors with a low heating value or from a nonwood fiber, air should be preheated to about 400F (204C). Water coil air heaters, sometimes used within the feedwater circuit around the economizer, may also be used to provide these air temperatures.

Flue gas system Combustion gas exiting the economizer is routed through flues to the electrostatic precipitator and induced draft fan before discharge through a stack to the atmosphere. The induced draft fan speed controls the pressure inside the furnace. The fan is normally located after the precipitator, allowing the fan to operate in the cleaner gas. A two-chamber precipitator is normally used. Each chamber is equipped with isolation gates or dampers and a dedicated induced draft fan discharging to a common stack. Flue gas exiting the economizer is divided into two flues, which route the gases to the precipitator’s two chambers. Each chamber typically has sufficient capacity to operate the recovery boiler at 70% load, corresponding to a stable black liquor firing mode. When designing the precipitator chamber, the expected gas flow rate should include that of the sootblower steam and the increased excess air under which the boiler would operate at a reduced rating. The maximum permissible particulate discharge rate then establishes the electrostatic precipitator size. Cleaning system Ash entrainment in furnace gases is affected by gas velocity, air distribution and liquor properties. The design of all recovery unit heating surfaces should include sootblowers using steam as the cleaning medium. Gas temperatures must be calculated to make certain that velocities and tube spacings are compatible with the sootblowers. High levels of chloride and potassium in the black liquor may require greater cleaning frequency. As unit overload is increased, additional entrainment of ash and sublimated sodium compounds also leads to more frequent water washing. This term describes an offline cleaning procedure which uses high pressure water to eliminate excessive deposits or buildups on the boiler’s convection surfaces. In addition to excess quantities of flue gas ash, velocities and temperatures throughout the unit are increased, making ash deposits more difficult to remove. Auxiliary fuel system The primary objective of the recovery boiler is to process black liquor. However, the unit can fire auxiliary fuel, usually natural gas and/or fuel oil. This fuel is fired through specially designed burners, arranged at the secondary air level. These burners raise steam Steam 41 / Chemical and Heat Recovery in the Paper Industry

pressure during startup, sustain ignition while building a char bed, stabilize the furnace during upset conditions, carry load while operating as a power boiler, and burn out the char bed when shutting down. In some installations, the recovery boiler must be able to generate full load steam flow and temperature on auxiliary fuel. These applications typically arise in mills that cogenerate electricity and/or have limited power boiler steam generating capability. Increased auxiliary fuel capacity can be accommodated by adding auxiliary burners above the secondary air level (typically at or above the tertiary air level). Upper level burners allow combination black liquor and auxiliary fuel firing with minimal interference with lower furnace operations. This is not possible when operating the secondary level auxiliary burners. Also, the upper level burners can provide higher steam temperatures at lower loads during startup, which can be important in mills operating high steam temperature turbine-generators. The auxiliary fuel burners are in a very hostile environment that compromises burner reliability and increases burner maintenance. B&W has always utilized rectangular shaped bent tube openings for burners to reduce accumulations of salt cake in the burner windbox and to keep the damaging furnace radiant heat away from vulnerable burner and lighter components. The oil atomizer or gas pipe elements are manually retracted when not in use. They would then be manually inserted when needed to start the burners. The LM2100 auxiliary burner (Fig. 26) is designed to retract the burner and lighter elements into the windbox and insert them again to the same location using locally activated air cylinders. In addition to protection from radiant heat, the use of air cylinders also keeps the fuel elements at the correct position when inserted. The LM2100 improves burner reliability and maintenance. The LM2100 is standard design for new recovery boilers and can be retrofit to existing recovery boilers of any manufacturer.

Non-condensable gases/waste streams Within the typical pulp and paper mill, there are numerous liquid and gaseous waste streams that are subject to stringent environmental disposal regulations. These waste streams are commonly incinerated on site. One simple and effective incineration method is to introduce these streams into an operating boiler. Gaseous streams are typically characterized as noncondensable gas (NCG) or stripper off gas (SOG). NCG is a mixture of sulfur compounds and hydrocarbons mixed with air and is either dilute (DNCG) or concentrated (CNCG). These terms refer to the concentration of active compounds within the NCG. If the percentage of sulfur compounds and hydrocarbons is below the lower explosion limit (LEL) in air, the NCG is considered dilute, is essentially foul air, and is relatively easy to handle. It does, however, present minor challenges. It can be corrosive and is unpleasant to smell. If the percentage of active compounds is above the upper explosion limit (UEL) in air, the NCG is considered concentrated. CNCG should be treated as a corrosive fuel stream. These streams require added precautions. 28-23

The Babcock & Wilcox Company

Fig. 26 B&W’s LM2100® burner.

The explosion limits for some representative gases found in NCG are shown in Table 4. The generally accepted LEL for mixtures of gases in air is 2% by volume, and the generally accepted UEL is 50%. SOG is usually a mixture of methanol in air, and is typically handled like CNCG. These gases require special safety precautions for collection and handling. Their escape must be prevented in and around the boiler building for personnel and equipment protection, and condensate must not be allowed to enter the furnace. For CNCG gases, additional precautions are required to avoid fire and explosion in equipment and transport systems. Positive ignition systems are required for gases entering the furnace. It is generally preferred that these non-condensable gases be burned in mill power boilers, as these units can more easily include the appropriate safety equipment. One disadvantage to using power boilers is the requirement to capture the SO2 generated by combustion of the sulfur compounds, unless the boiler is equipped with a scrubber. For recovery boilers, the explosion potential due to tube failures does not make this type of boiler a good application to introduce foreign streams. BLRBAC does not encourage the introduction of waste streams into recovery boilers. It has, however, become more commonplace in recent years to utilize recovery boilers for this disposal, as mills are

Table 4 NCG Gases Explosion Limits Constituent Gas Turpentine Methanol Hydrogen Sulfide Methyl Mercaptain Dimethyl Sulfide Dimethyl Disulfide

28-24

Lower Explosion Limit (LEL) % Volume in Air

Upper Explosion Limit (UEL) % Volume in Air

0.8 7.3 4.3 3.9 2.2 1.1

Not established 36.0 45.0 21.8 19.7 16.1

required to meet more stringent regulations. Recovery boilers can have the ability to capture the SO2 from NCG combustion. Significant caution and special system design are necessary to safely burn NCG in a recovery boiler. DNCGs can often be mixed with the combustion air for the tertiary air port level on a recovery boiler, or for the stoker on a power boiler. The primary concern is to assure adequate main fuel, usually equal to or above 50% of load, to assure burnout of the gases. To avoid odor problems, the ducts, windboxes and other air transport components must be air tight. Corrosion in the feed ducts can be reduced by mixing the gases with hot air or heating the mixture. For power boilers, if the sulfur input is high, boiler exit corrosion and emissions can increase. The corrosion rate can be reduced by upgrading alloys and/or coatings or by increasing the gas temperature to stay above the dew point. For CNCG gases, transport must avoid air leakage that could move the mixture into the explosive range. Typically, these gases are fired with a support combustion source, such as an igniter in close proximity, to assure burnout. Stainless steel piping is used to avoid corrosion. One stream explicitly excluded from any boiler incineration consideration is terpenes (turpentine vapor). The UEL of these streams is not defined, and terpenes have been the most prevalent causes of explosions in non-condensable gas collection, transport and combustion systems. For either gas, provision must be made to avoid slugs of liquid entering the furnace. System design uses flame propagation arresters to avoid explosions, and safety interlocks must be integrated into the existing controls. For any type of waste gas, it is important that a complete analysis of the fuel parameters (constituents, amounts, moisture, etc.) be understood so that the appropriate systems and safeguards can be designed. It is then possible to determine the proper furnace temperatures and residence times required to assure complete combustion. There are also several liquid waste streams with disposal requirements. These include soap, tall oil, methanol, spent acid and secondary sludge. These are normally blended and combusted with the black liquor fuel. Under specific circumstances, they may be fired in a separate, dedicated burner. Turpentine, another liquid stream, is extremely unpredictable and generally restricted from either of the above methods. Handling and combusting recommendations and cautions can be found in BLRBAC guidelines.

Smelt spout system Smelt exits the furnace through specially designed openings at the low point of the floor and is conveyed to the dissolving tank in a sloped water-cooled trough, or smelt spout (Fig. 27). The spout is bolted to the furnace wall mounting box. Most original spout designs had the machined face of the spout positioned against the outside of the furnace wall tubes surrounding the opening. While many of this arrangement are still in operation, the fully insertable spout has become common. This type of spout inserts directly into the spout opening, where the spout taps the smelt directly from Steam 41 / Chemical and Heat Recovery in the Paper Industry

The Babcock & Wilcox Company

Fig. 27 Integral smelt spout hood with shatter jet assembly.

the furnace and no smelt contacts the spout opening tube surface. This has helped alleviate significant spout opening wear and deterioration. Insertable spouts use a round-shaped trough. For noninserted spouts, the trough is V-shaped to correspond to the V-shaped bottom of the wall opening. Either spout design is constructed of a double wall carbon steel trough, with a continuous flow of cooling water passing between the inner and outer walls. Given the explosive nature of smelt-water reactions and the extreme temperature of the molten smelt, the spout must receive adequate cooling water. A dedicated cooling system with built in redundancy and multiple sources of backup water assures system reliability (Fig. 28). This system may either be a pressurized cooling system, or more commonly a gravity flow, induced vacuum system where the water in the smelt spout is at less than atmospheric pressure. The cooling water is treated in a dedicated, closed cycle to minimize scale-forming contaminants.

Green liquor system The smelt is dissolved in green liquor in an agitated tank. Green liquor is withdrawn from the tank at a controlled density and the volume replaced with weak wash. The entering smelt stream must be finely dispersed, or shattered, to control the smelt-water reaction by rapid dissolution of the smelt into the green liquor. Excessively large smelt particles entering the tank can lead to explosions. The outer surface of a large smelt particle cools quickly as it contacts the green liquor, forming an outer shell around its hot core. As the shrinking forces build, the particle shell explodes, exposing the hot core to water and resulting in the sudden release of steam. To eliminate large smelt particles from reaching the dissolving tank contents, Steam 41 / Chemical and Heat Recovery in the Paper Industry

steam shatter jets are located above the smelt stream to disperse the flow into the tank. The smelt spout discharge is enclosed by a hood (Fig. 27). The steam shatter jets are mounted on the top of the hood, with the nozzle angle adjusted to direct the jet at the smelt stream cascading off the spout end. The periphery of the hood is equipped with wash headers to flush the hood walls and prevent smelt buildup. A door is provided in the hood for operator access to inspect and to rod the spout and opening. The dissolving tank is of heavy construction and is equipped for agitation. One or more agitators, either side or top entering, are generally used, including an emergency backup system, such as steam nozzles. A stainless steel band is commonly used in the carbon steel tank at the liquor level for corrosion protection, while the floor is protected with steel grating or poured refractory. As the smelt is cooled and dissolved into the green liquor, large quantities of steam are released. The steam vapors are pulled through an oversized atmospheric vent on the tank to quickly relieve pressure in the event of a surge or explosion. Smelt particles and green liquor droplets are entrained in the steamair mixture vented from the tank. These vented gases also contain H2S that must be removed prior to discharge to the atmosphere. Typically, a vent stack scrubber (Fig. 29) is used to reduce H2S emissions and to trap entrained particulates. Weak wash is used as the scrubbing medium to take advantage of its residual NaOH, which absorbs the malodorous gases.

Alternate processes and liquors Soda process With few exceptions, the requirements and features of the kraft recovery boiler apply to the soda recovery boiler. Because sulfur is not present in the soda pro-

Fig. 28 Smelt spout cooling system.

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The Babcock & Wilcox Company The liquor is generally characterized by high viscosity and high silica content. The high viscosity limits the level at which liquor can be concentrated in multiple effect evaporators, and direct contact evaporators are generally required. Higher combustion air temperatures are used to compensate for the low liquor concentrations while allowing flexibility in adjusting the firing conditions.

Fig. 29 Vent stack scrubber.

cess, there is no sulfur reduction in the recovery furnace. Soda ash (Na2CO3) is added to the recovered green liquor. The ash collected from the boiler hoppers and the electrostatic precipitator is in the form of Na2CO3 and can be added directly to the green liquor in the dissolving tank. A salt cake mix tank is not required. In the furnace, the soda liquor does not form a suitably reactive char for burning in a bed. It is finely atomized and sprayed into the furnace by multiple soda liquor burners. The fine spray dehydrates in flight and combustion takes place largely in suspension in an oxidizing atmosphere. Combustion air is admitted through primary and secondary air ports around the furnace periphery, with the hottest part of the furnace just above the hearth. The Na2CO3 collects in molten form on the hearth and discharges through the smelt spouts. The Na2CO3 smelt has a higher melting point than the kraft process smelt. This makes the soda smelt more difficult to tap from the furnace and shatter. Auxiliary fuel burners can be located low in the side walls, close to the spout wall, to keep the smelt hot for easier tapping.

Nonwood fiber liquor Many countries do not have adequate forest resources for pulp production. As a result, alternative fiber sources are used, such as bamboo, sugar cane bagasse, reeds and straw. Black liquor from these fibrous materials requires special consideration in recovery system design. 28-26

Sulfite process Pulp produced by the sulfite process is divided into two broad categories, semi-chemical and chemical. Semi-chemical pulp requires mechanical fiberizing of the wood chips after cooking. Chemical pulp is largely manufactured by the acid sulfite and bisulfite processes, which differ from the alkaline processes in that an acid liquor is used to cook the wood chips. The acid sulfite process is characterized by an initial cooking liquor pH of 1 to 2. The bisulfite process operates with a pH of 2 to 6. Cooking liquor used in a neutral sulfite process for manufacture of semi-chemical pulps has an initial pH of 6 to 10. Spent sulfite liquor, separated from chemical and semi-chemical pulp and containing the residual cooking chemicals and dissolved constituents of the wood, is evaporated and burned, and the chemicals are recovered in a system particular to each base. Sulfite process pulp mills normally use one of four basic chemicals for digestion of wood chips to derive a large spectrum of pulp products: sodium, calcium, magnesium or ammonium. The principal differences in the sulfite waste liquor of the four bases are the physical properties of the base and the products of combustion. Sodium This system reconstitutes the base in a form suitable for reuse in the pulping cycle. The sodium-base liquor may be burned alone or in combination with black liquor in a kraft recovery unit. The base chemical is recovered as a smelt of Na2S with some Na2CO3 which would be a suitable makeup chemical for a nearby kraft mill. The combination of sulfite liquor and kraft liquor, referred to as cross recovery, is common to pulp mill operations. The proportion of sulfite liquor in cross recovery is generally the equivalent of the sodium makeup requirement of the kraft mill cycle. Calcium Calcium-base liquor is concentrated and burned in specially designed furnaces. The furnace and boiler design applicable to magnesium-base liquor combustion can also be applied to calcium. This process is largely historic today due to its unacceptable effluents. Magnesium With the magnesium base, a simple system is available for recovery of heat and total chemicals. This is due to the chemical and physical properties of the base. The spent liquor is burned at elevated temperatures in a controlled oxidizing atmosphere, and the base is recovered in the form of an active magnesium oxide (MgO) ash. The oxide can be readily recombined in a simple secondary system with the SO2 produced in combustion, thereby reconstituting the cooking acid for pulping. Industrial interest in the improved pulp from a variety of wood species stimulated the development of pulping techniques using a magnesium base. The two major pulping procedures, whereby a variety of pulps can be produced, are magnesium acid sulfite and Steam 41 / Chemical and Heat Recovery in the Paper Industry

The Babcock & Wilcox Company bisulfite, or magnefite. The basic magnesium recovery system is appropriate for each of the pulping processes. The heavy liquor is fired with steam-atomizing burners located in opposite furnace walls (see Fig. 30). The combustion products of the liquor’s sulfur and magnesium are discharged from the furnace in the gas stream as sulfur dioxide and solid particles of MgO ash. Most of the MgO is removed from the gas stream in a mechanical collector or electrostatic precipitator and is then slaked to magnesium hydroxide, or Mg(OH)2. In the complex secondary recovery system, the SO2 is recovered by reaction with the Mg(OH)2 to produce a magnesium bisulfite acid in an absorption system. This acid is passed through a fortification or bisulfiting system and is fortified with makeup SO2. The finished cooking acid is filtered and placed in storage for reuse in the digester. Ammonium Ammonium-base liquor is the ideal fuel for producing a low ash combustion product. Burning can be accomplished in a simple recovery boiler. The ammonia decomposes on burning to nitrogen and hydrogen, the latter oxidizing to water vapor and thus destroying the base. Concentrated ammonium sulfite liquor is burned in a furnace and recovery system similar to magnesium-base liquor. SO2 produced in combustion can be absorbed in a secondary system to yield cooking acid for pulping. The SO2 reacts in an absorption system with an anhydrous or aqueous ammonium makeup chemical to produce ammonium bisulfite acid. In a neutral sulfite semi-chemical plant, the absorption system produces cooking liquor consisting essentially of ammonium sulfite.

BLG, integrated with combined cycle power production, may offer increased cycle efficiencies and electricity production. Process flexibility Gasification technology can return sulfur to the pulping process in a variety of forms (sulfide, elemental sulfur, polysulfide, and/or sulfite). This provides opportunities to produce pulp with substantially improved yields and properties. Availability The fouling of convection pass surfaces can reduce recovery boiler availability. BLG may provide a greater availability in that heat transfer surface fouling in either a power boiler or gas turbine heat recovery steam generator might be lower. Intermediate cleanup equipment removes sulfur compounds and alkali fume from the product gas. Safety Smelt-water reactions are largely unpredictable and continue to pose a risk when operating kraft recovery boilers. Low-temperature BLG produces no molten smelt, eliminating the potential of smelt-water reactions, and high-temperature BLG at elevated pressure is expected to significantly reduce the likelihood and resultant force of an explosion.

Black liquor gasification Advanced technologies – gasifier development A new technology being investigated for the combustion/reduction of black liquor (and thus replacing the process recovery boiler) is black liquor gasification (BLG). This technology involves the partial oxidation of the black liquor to produce a fuel gas that can be burned in a gas turbine/electric generator or fired in a power boiler. In the process, sulfur and sodium compounds are recovered from the fuel gas, before it is sent to the turbine or boiler, for regeneration of the pulping chemicals. Potential BLG advantages The pulp and paper process requires high capital investment in plant equipment, high internally generated energy consumption, and high purchased power consumption. BLG may have the potential to favorably impact mill energy efficiency, environmental performance, operating costs and pulp yields. Energy efficiency The amount of electricity pulp mills need to import varies considerably from mill to mill and is dependant on the power/steam cycle used and on the type of pulp produced. Electrical loads have increased relative to steam loads as more electricity is needed to operate recycling, mechanical pulping, and pollution control equipment. Most mills import a significant portion of the mill’s electricity demand, and Steam 41 / Chemical and Heat Recovery in the Paper Industry

Fig. 30 MgO recovery boiler.

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The Babcock & Wilcox Company Emissions BLG processes require that sulfur and particulate emissions be removed from the fuel gas. As a result, overall emissions from a BLG-based process are expected to be low.

BLG processes Black liquor gasification technologies can be classified by operating temperature. Low-temperature BLG is operated below the first melting temperature of the inorganic salts, typically 1112 to 1292F (600 to 700C), to produce a solid inorganic ash. High-temperature BLG is operated above the final melting temperature of the inorganic salts, typically 1652 to 1832F (900 to 1000C), to produce a molten smelt. Black liquor gasifiers have utilized either the fluidized-bed (for low-temperature BLG) or entrained-flow designs (for high-temperature, molten-phase designs), as described in Chapter 18. Typical BLG processes are shown in Fig. 31. Low temperature gasifiers operate at 1292F (700C) or lower so that the organics leave as dry solids. This type of gasification process is normally conducted at atmospheric pressure and produces a product gas of approximately 200 to 300 Btu/DSCF (11.8 MJ/Nm3 ). Minimal gas cleanup is required, and the product gas is generally fired to offset auxiliary fuel in a power boiler. This allows additional black liquor processing capacity, provides a useful product gas for auxiliary fuel use, and is well suited for incremental capacity additions at existing mills. High temperature gasifiers generally operate at 1742F (950C) or higher and produce a product gas of approximately 85 to 100 Btu/DSCF (3.9 MJ/Nm3 ) and a molten smelt of inorganic chemicals. High-temperature BLG can be pressurized and integrated with a combined cycle system for power generation. Product gas cleanup systems are required for the removal/reduction of sulfur and particulate emissions before the gas turbine. These systems can also separate the sul-

Product Gas to Process or Power Boiler

600-700C Black Liquor

Gasifier

fur compounds from the product gas and return them to the pulping process.

Status and development needs Atmospheric pressure BLG has had the most development to date. Various concepts, both high and low temperature, have been in design and pilot stages since the early 1990s. The first fully commercial process is an air-blown, high-temperature entrained flow gasifier with a processing capacity of 15 t/h of dry solids. This process provides incremental recovery capacity with product gas burned in a boiler. High-temperature BLG with combined-cycle power generation is in development. Various pilot work is being done for pressurized, oxygen-blown entrainedflow reactors and large-scale commercial units are contemplated for the near future. Black liquor gasification is a promising technology, but key issues must be resolved before full commercialization. While current Tomlinson technology is capital intensive, early BLG installations are expected to be quite expensive. Capital costs must be reduced, positive economics must result and the overall economic case must be strengthened for BLG to provide superior returns. Specific gasifier design and process integration issues exist, including gasifier materials selection, refractory corrosion, product gas cleanup equipment, tar formation, sulfur removal, and an increased causticizing load. Many activities are underway within the industry to investigate and resolve these and other issues. An important aspect of the overall process and economics is the effect of integrating higher yield pulping technologies into the BLG cycle (split sulfidity, polysulfide, etc.), as these can have significant positive impact on overall economics. They, too, are capital intensive and will take some development, but integrating them into the process is the key to providing acceptable, positive economic returns.

Steam O2

Gasifier

Black Liquor

Gas Cooling / Heat Recovery

900-1000C

Green Liquor

Raw Gas

Sulfur Chemicals

Gas Cleanup

Gas Cooling

Clean Fuel Gas Steam

Bed Solids Particulate / Gas Cleanup

H 2S Absorber

Green Liquor

Sodium Sulfide

HRSG

Electricity

Gas Turbine Low-Temperature, Atmospheric Black Liquor Gasification

Feedwater

Flue Gas

High-Temperature, Pressurized Combined Cycle Black Liquor Gasification6

Fig. 31 Black liquor gasification processes.

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The Babcock & Wilcox Company

References 1. Adapted from Hough, G.W., “Chemical Recovery in the Alkaline Pulping Processes,” Technical Association of the Pulp and Paper Industry, Inc., (TAPPI), p. 197, 1985. 2. Adapted from Smook, G.A., Handbook for Pulp and Paper Technologists, Joint Textbook Committee of the Paper Industry TAPPI/CPPA, p. 69, 1986. 3. Wiggins, D., et al., “Liquor Cycle Chloride Control Restores Recovery Boiler Availability,” TAPPI Engineering Conference, Atlanta, Georgia, Sept. 17-21, 2000.

Steam 41 / Chemical and Heat Recovery in the Paper Industry

4. Tran, H.N., “Kraft Recovery Boiler Plugging and Prevention,” Notes from the Tappi Kraft Recovery Short Course, pp. 209-218, Orlando, Florida, January, 1992. 5. Adapted from Whitney, R.P., Chemical Recovery in Alkaline Pulping Processes, TAPPI, Monograph Series No. 32, p. 40, 1968. 6. Taken from Air Products & Chemicals, Inc. presentation at the American Institute of Chemical Engineers (AIChE) Annual Meeting, Miami Beach, Florida, November 15-20, 1998.

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The Babcock & Wilcox Company

Pulp and paper facility.

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Steam 41 / Chemical and Heat Recovery in the Paper Industry