Energy & Fuels 2000, 14, 83-88
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Carbon Gasification of Kraft Black Liquor Solids in the Presence of TiO2 in a Fluidized Bed Le Zeng* Environmental Technologies, Alberta Research Council, Vegreville, AB T9C 1H6, Canada
Adriaan R. P. van Heiningen Department of Chemical Engineering, University of Maine, Orono, Maine 04469-5737 Received March 5, 1999
Atmospheric pyrolysis and gasification of kraft black liquor solids (KBLS) in the presence of TiO2 was conducted in a pilot fluidized-bed system operated at 700-900 °C. The effects of temperature, air ratio, and TiO2 particle size on the product gas composition and yields were investigated. The major components of the product gases are carbon monoxide, carbon dioxide, hydrogen, methane, and ethylene. The residual organic and inorganic carbon (carbonate) of the bed solids was determined. The heating value of the product gas and its behavior with operating conditions was also determined. It was found that addition of TiO2 to the gasifier had little influence on the carbon gasification of KBLS. The results suggest that for practical KBLS gasification, the temperature should be as high as possible (but less than ∼ 900 °C), and that the air ratio be controlled at a value of 0.3-0.4.
Introduction Kraft black liquor gasification is presently considered by all boiler manufacturers as a promising alternative technique for chemical recovery of kraft black liquor (KBL) and generation of steam and electricity. The potential advantages over conventional recovery include higher electrical energy production, partial or complete separation of the sodium and sulfur streams, reduction or elimination of the smelt-water explosion hazard, and reduced capital cost.1,2 However, the major incentive for development of a gasification process is that pressurization allows incorporation of the integrated gasification combined cycle (IGCC) technology so that the powerto-steam ratio can be increased by a factor of 2.5 compared to that of state-of-the-art conventional recovery with cogeneration.3 The gasification processes have been classified as either high-temperature or low-temperature depending on whether the reactor is operated, respectively, above or below the melting point (∼ 710 °C) of the inorganic salt mixture which remains after gasification. The advantages of a low-temperature process are the absence of a molten phase (and thus the smelt-water explosion hazard) and its higher power-to-steam ratio. Disadvantages, however, are the slow carbon gasification and sulfate reduction rates. High-temperature processes do not have these kinetic limitations. How* Author to whom correspondence should be addressed. (1) Grace, T. M.; Timmer, W. M. A comparison of Alternative Black Liquor Recovery Technologies. Proc. 1995 TAPPI/CPPA Intl. Chem. Recovery Conf., Toronto, 1995; pp B269-275. (2) Finchem, J. K. Black Liquor Gasification Research Yields Recovery Options for Future. Pulp Paper 1995, 49-59. (3) McKeough, P.; Fogelholm, C.-J. Proc. Intl. Symp. Energy Environ., Atlanta, 1991; pp 197-205.
ever, the elevated temperatures result in smelt formation and high rates of sodium emission leading to potential material and gas cleaning problems. These disadvantages can be eliminated or minimized by addition of TiO2 in the gasifier. Sodium carbonate in black liquor reacts with TiO2 to form high-melting (Tm g 960 °C) sodium titanates, thereby preventing smelt formation and minimizing sodium emission.4 An additional advantage is that the amphoteric character of TiO2 allows hydrolysis of some of the sodium titanates into NaOH and a TiO2-containing solid phase, which can be recycled, so that the fossil fuel consuming lime causticizing cycle may be eliminated. One of the key objectives of KBL gasification is to convert the organics into a low BTU gasification gas. Although a large number of studies have been reported on the topic of carbon conversion during black liquor pyrolysis and gasification,5-22 so far no experimental data has been presented for pyrolysis and gasification of KBL in the presence of TiO2. The objective of this (4) Backman, R.; Salmenoja, K. Equilibrium Behaviour of Sodium, Sulfur and Chlorine in Pressurized Black Liquor Gasification with Addition of Titanium Dioxide. Paperi Ja Puu 76 (5), 1994, 320-325. (5) Kelleher, E. G. Feasibility Study: Black Liquor Gasification and Use of the Products in Combined-cycle Cogeneration. Tappi J. 1984, 4, 114. (6) Kohl, A. L. Black Liquor Gasification. Can. J. Chem. Eng. 1986, 64 (4), 299. (7) Kelleher, E. G.; Kohl, A. L. Black Liquor Gasification Technology. Chemical Engineering Technology in Forest Products Processing, Vol. 2; AICHE (Forest Products Division), 1988; pp 40-45. (8) Fallavollita, J. A. Kraft Chemical Recovery in A Fluidized Bed, Ph.D. Thesis, McGill University, Montreal, 1992. (9) Li, J.; van Heiningen, A. R. P. Kinetics of CO2 Gasification of Fast Pyrolysis Black Liquor Char. I&EC Res. 1990, 29 (9), 1776. (10) Li, J.; van Heiningen, A. R. P. Kinetics of Gasification of Black Liquor Char by Steam. I & EC Res. 1991, 30 (7), 1594. (11) Frederick, W. J.; Hupa, M. Gasification of Black Liquor Char with CO2 at Elevated Pressures. Tappi J. 1991, 7, 177.
10.1021/ef9900381 CCC: $19.00 © 2000 American Chemical Society Published on Web 11/20/1999
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Figure 1. A schematic diagram of the pilot fluidized-bed system.
study was to obtain such data in a fluidized bed of TiO2 at conditions leading to direct causticization of the sodium salts in KBL by TiO2. Experimental Section Fluidized-Bed Setup. This study was conducted in a pilot fluidized-bed facility. A schematic diagram of the pilot plant is shown in Figure 1. It includes the gas supply system (units 1-6), the kraft black liquor solids (KBLS) delivery system (unit 7), the fluidized-bed reactor with a solids sampler (units 8-9), and the down-stream gas treatment system (unit 10-18). The fluidized-bed gasifier has an internal diameter (ID) of 0.1 m and a length of 0.58 m in the reaction zone, and 0.2 m ID in (12) Whitty, W.; Frederick, W. J.; Hupa, M. Gasification of Black Liquor Char with H2O at Elevated Pressures. Proc. 1992 TAPPI/CPPA Intl. Chem. Recovery Conf., 1992, p 627. (13) Goerg, K. A.; Cameron, J. H. A Kinetic Study of Kraft Char Gasification with CO2. Chemical Engineering Technology in Forest Products Processing, Vol. 2; AIChE (Forest Products Division), 1988; pp 46-52. (14) Li, J.; van Heiningen, A. R. P. Reaction Kinetics of Gasification of Black Liquor Char. Can. J. Chem. Eng. 1989, 67 (8), p 693. (15) Van Heiningen, A. R. P.; Arpiainen, V. T.; Ale´n, R. Effect of Liquor Type and Pyrolysis Rate on the Steam Gasification Reactivities of Black Liquors. 1992 TAPPI/CPPA Intl. Chem. Recovery Conf., Seattle, 1992, pp 641-649. (16) Feuerstein, D. L.; Thomas, J. F.; Brink, D. L. Malodorous Products from the Combustion of Kraft Black Liquor. I. Pyrolysis and Combustion Aspects. Tappi 1967, 50 (6), 258-262. (17) Brink, D. L.; Thomas, J. F.; Jones, K. H. Malodorous Products from the Combustion of Kraft Black Liquor. III. A Rationale for Controlling Odors. Tappi 1970, 53 (5), 837-843. (18) Bhattacharya, P. H.; Vidyasekara, P.; Kunzru, D. Pyrolysis of Black Liquor Solids. Ind. Eng. Chem. Proc. Dec. Dev. 1986, 25 (2), 420426. (19) Gairns. S. A.; Kubes, G. J.; van Heiningen, A. R. P. New Insights into TRS Gas Formation during Pyrolysis of Black Liquor. Paper 104d; 1994 AIChE Spring National Meeting, Atlanta, April 1994. (20) Sricharoenchaikul, V.; Frederick, W. J.; Grace, T. M. Thermal Conversion of Tar to Light Gases During Black Liquor Pyrolysis. Proc. 1995 TAPPI/CPPA Intl. Chem. Recovery Conf., Toronto, Book A, 1995; p 209. (21) Brink, D. L. Pyrolysis-Gasification-Combustion: A Process for Utilization of Plant Material. Appl. Polym. Symp. 28, 1976, pp 13771391. (22) Frederick, W. J.; Backman, R.; Hupa, M. Pressurized Gasification of Spent Pulping Liquors: Thermodynamic and Kinetic Constraints. TAPPI/CPPA Intl. Chem. Recovery Conf., Seattle, 1992; pp 617-625.
Table 1. Analysis of Dry Kraft Black Liquor Solids elemental composition, wt % C H O Na S Cl K total
32.3 3.76 35.8 18.2 2.97 0.655 3.04 96.7
moisture heating value
4.28 13210 kJ/kg
the freeboard zone with a total height of 1.22 m. A detailed description of the system can be found elsewhere.23 Materials. The dry KBLS were produced from the hardwood weak kraft black liquor by a pulsed combustion drying technique. The solids particle size is significantly smaller than 75 µm. The elemental compositions of the KBLS is shown in Table 1. The TiO2 is R-Granular containing 98% rutile, obtained from Tioxide, Ville Saint-Laurent, Que´bec. Different sieve fractions ranging from 75 to 250 µm were used as the bed material in the experiments. Experimental Procedure. The pyrolysis/gasification experiments were in a semibatch mode. TiO2 particles (1.2-1.4 kg) were initially loaded in the fluidized bed, and KBLS were continuously injected into the bed. The fluidizing gas, a mixture of air and N2, was added at a rate of about 13 L/min (STP) to obtain a fluidization velocity of approximately 0.11 m/s at the operating conditions. The air ratio was controlled by changing the mixture of N2 and air fed to the reactor. The air ratio in this study is defined as the fraction of air required for complete combustion of KBLS. The feed rate of KBLS and the reactor temperature were kept constant as much as possible during each experiment. Different experiments were performed with a KBLS feed rate varying from 0.5 to 0.75 kg/h and temperatures ranging from 700 to 900 °C. During the experiment, the TiO2 reacted with Na in KBLS to form sodium titanates which remained in the bed, while the product gases were withdrawn continuously and analyzed for CO, CO2, CH4, H2, and O2 as well as sulfurous gases by gas chromatography. (23) Zeng, L.; van Heiningen, A. R. P. Pilot Fluidized-Bed Testing of Kraft Black Liquor Gasification and Its Direct Causticization with TiO2. JPPS 1997, 23 (11), 511-516.
Carbon Gasification of Kraft Black Liquor Solids
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After each experiment, the bed residual solids (BRS) were analyzed for total char carbon and carbonate content. To account for the changes in KBLS feed rate and fluidizing gas flow rate during a run, the mean gas yield (YiAVE) was calculated as
YiAVE )
FtCiAVE FKBLS
(1)
where YiAVE is the mean gas yield of component i, (mol/kg KBLS); CiAVE is the time-weighted mean concentration of gas component i, (mol/mol); Ft is the mean total product gas flow rate, (mol/min); and FKBLS is the KBLS feed rate, (kg KBLS/ min).
Pyrolysis and Gasification Gas Composition Organic carbon in KBLS can be converted into CO, CO2, CH4, and higher hydrocarbons by gasification and pyrolysis reactions. When TiO2 is present in the gasifier, the direct causticization reactions 2 and 3 also take place, leading to further release of CO2.
Figure 2. Effect of temperature on gasification gas composition at air ratio of 0.389 ( 0.078, TiO2 particle size of 105177 µm, and KBLS feed rate of 8.0-12.0 g/min.
Na2CO3 + 3TiO2 ) Na2O‚3TiO2 + CO2
0.95-1.17% for CH4, and 0.10-0.18% for C2H4. It was also verified that the superficial velocity (0.1-0.2 m/s) had little influence on the gasification of KBLS at the present conditions.
(2)
7Na2CO3 + 5(Na2O‚3TiO2) ) 3(4Na2O‚5TiO2) + 7CO2 (3) For a full discussion of direct causticization of KBLS, see refs 23-25. The gas components detected at a percentage level during both pyrolysis and gasification in this study were CO, CO2, CH4, C2H4, H2, O2, and N2. The composition of the fixed gases generated during pyrolysis of KBLS provides insight into the structures of the lignocellulosic material.26 For example, CO2 is generated from carboxyl groups, H2O from hydroxyl groups, hydrocarbon gases from aliphatic and methoxy groups, and CO from weakly bound oxygen groups such as aldehydes. Later in the pyrolysis process, the rearrangement of the lignin subunits permits H2 to be evolved from aromatic hydrogen. Also, additional CO is released from tightly bound oxygen functionalities, such as diaryl ethers and phenols. Many other light gases were also detected during black liquor pyrolysis by different researchers,16-20 such as acetylene, formaldehyde, acetaldehyde, acetone, methanol, propylene, butadiene, and some carbon-containing sulfur species. However, these species were not detected in the present study, probably because either their concentrations in the gasification gas were too low or because they decomposed as a result of the longer residence time at high temperatures in our fluidized bed. Sulfur-containing gases are also formed during pyrolysis and gasification of KBLS. They include hydrogen sulfide, carbonyl sulfide, mercaptans, and organic sulfides and disulfides. Because the concentration of the carbon-sulfur compounds in the product gas of the present experiments was typically about a few hundred ppm, i.e., a level which is 50-100 times lower than that of the fixed gases, the accuracy of carbon balance calculations is not affected by deleting them. In the present study, the average gas concentrations for the whole range of gasification experiments, are 5.1-14.9% for CO2, 6.9-17.7% for CO, 5.2-8.3% for H2, 0.95-1.35% for CH4, and 0.08-0.26% for C2H4; while for pyrolysis experiments, they were 2.4-4.0% for CO2, 5.8-13.1% for CO, 7.1-8.9% for H2, (24) Pels, J. R.; Zeng, L.; van Heiningen, A. R. P. Direct Causticization of Kraft Black Liquor with TiO2 in a Fluidized bed - Indentification and Analysis of Sodium Titanates. JPPS 1997, 23 (12), 549554. (25) Zeng, L.; Pels, J. R.; van Heiningen, A. R. P. Direct Causticization of Kraft Black Liquor Solids with TiO2 in a Fluidized Bed. Tappi J., submitted. (26) Avni, E.; et al. Mathematical Modelling of Lignin Pyrolysis. Fuel 1985, 64 (11), 1495-1501.
Results and Discussion Effect of Temperature on Gas Composition. A series of experiments were performed at different operating temperatures but constant air ratio and KBLS feed rate. Figure 2 shows the effect of temperature on the mean gas yield. It can be seen that the major gases are CO2, CO, and H2. With increasing temperature, the yield of CO2 decreases and that of CO increases, while that of H2 does not change significantly. The CH4 yield is about 1 order of magnitude smaller than that of these three gases, and is essentially constant over the whole temperature range. The yield of C2H4 is again only onetenth of that of CH4 and also nearly independent of temperature. By comparison, the results of steam gasification of KBLS in a similar fluidized bed by Fallovollita8 showed CO and CO2 yields at 700 °C of 5 and 12 mol/kg KBLS, respectively, which are close to those of the present study (CO yield of 7 mol/kg KBLS and CO2 yield of 15 mol/kg KBLS at 700 °C). On the other hand, the present H2 yield of 6 mol/kg KBLS is much lower than his value of 45 mol/kg KBLS, most likely because steam gasification results in additional H2 formation. A number of pyrolysis runs were also performed at different temperatures. The results are shown in Figure 3. It can be seen that the yield of CO2 decreases, while that of CO increases with increase in temperatures, as was found for air gasification. The yield of CO is similar to that of gasification, but the CO2 yield during pyrolysis is much lower due to lack of oxygen, so that the CO/ CO2 molar ratio resulting from pyrolysis of KBLS is significantly higher than that produced during gasification. The H2 yield displays a slight decrease with increasing temperature. The CH4 and C2H4 yields are at the same low levels as those for gasification. By comparison, Brink et al.17 found that for pyrolysis of KBL between 700 and 900 °C the yield of CO increased from 1.2 to 4 mol/kg KBLS, and the yield of CO2 from 5 to 7.5 mol/kg KBLS, while the yield of H2 changed from 10 to 15 mol/kg KBLS. In the same temperature range
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Figure 3. Effect of temperature on major gases from pyrolysis of KBLS at TiO2 particle size of 125-149 µm, and KBLS feed rate of 9.0-12.0 g/min.
for pyrolysis of a relatively low solids content KBL (55% water content), Brink21 also reported that the CO yield increased from about 2.6 to 8 mol/kg KBLS, CO2 from 8 to 12 mol/kg KBLS, while H2 increased from 15 to 28 mol/kg KBLS. The KBLS pyrolysis experiment of Fallavollita8 showed that the CO, CO2, and H2 yields at 700 °C were 9, 2.3, and 14 mol/kg KBLS, respectively, which are reasonably close to those of the present pyrolysis experiment at 732 °C. In each of these reference pyrolysis studies,17,21 the CO and CO2 production were significantly smaller and higher, respectively, than those obtained with the present fluidized bed. These differences can be attributed to the use of black liquor with a high water content in the reference studies, compared to the dry black liquor solids used in this work, since water causes CO conversion to CO2 via the water-shift reaction. Effect of Air Ratio on Gas Composition. The gasification gas composition is a strong function of the air ratio. The preferred practical air ratio is obtained at autothermic operation of the gasifier. For KBL it has been shown22 that autothermic operation occurs at an air ratio of about 0.35. It was also shown that an air ratio larger than about 0.3 is required in order to achieve complete gasification of the organic carbon in kraft black liquor. Therefore, a number of experiments were conducted at air ratios varying from zero to about 0.5 at a temperature of around 850 °C. The effect of air ratio on gas composition is shown in Figure 4. It can be seen that increasing air ratio significantly increases the CO2 yield, and slightly decreases the yields of CO and H2. This implies that with increasing air ratio, CO and H2 are converted into CO2 and H2O. Therefore, the CO/ CO2 molar ratio drops quickly with increasing air ratio. The yields of CH4 and C2H4 are again much smaller and independent of the air ratio. This suggests that CH4 and C2H4 are formed during pyrolysis and are relatively inert when the air ratio is increased. Effect of TiO2 Particle Size on Gas Composition. The particle size of the bed solids is critically important in fluidization processes, since it affects the fluidization characteristics. In addition the size of the TiO2 bed solids is important for the direct causticization reaction kinetics, since the sodium salts of KBLS must penetrate the solids before reaction.
Zeng and van Heiningen
Figure 4. Effect of air ratio on major gases from gasification of KBLS at 853 ( 4 °C and KBLS feed rate of 8.0-12.0 g/min except for pyrolysis runs (1 at 833 °C and 2 at 871 °C).
Figure 5. Effect of TiO2 particle size on major gases from gasification of KBLS at 853 ( 4 °C, air ratio of 0.356 ( 0.05, and KBLS feed rate of 9.2-12.0 g/min. Table 2. Residual Carbon in BRS from Pyrolysis of KBLSa pyrolysis temperature (°C)
carbonate (g C/kg C in KBLS)
organic carbon (g C/kg C in KBLS)
732 836 871
7.21 3.45 2.23
288 128 96
a At bed TiO particle size of 125-149 µm and KBLS feed rate 2 of 9.0-12.0 g/min.
Five ranges of TiO2 particle size were used in the present experiments: 75-88 µm, 105-125 µm, 125149 µm, 149-177 µm, and 177-250 µm. The gas yield results are shown in Figure 5 as a function of the mean particle sizes used. It can be seen that there is no significant change in gas yield except for that of CO at the smallest TiO2 particle size of 75-88 µm. Therefore it can be concluded that the TiO2 particle size over the range of 105 to 250 µm does not have much influence on the gasification of KBLS at the present conditions. Residual Organic and Inorganic Carbon Yields. For each bed residual solids (BRS) sample, total carbon and inorganic carbonate were measured, and the organic carbon was calculated by difference. Table 2 shows the organic carbon and inorganic carbon (CO32-) content
Carbon Gasification of Kraft Black Liquor Solids
Energy & Fuels, Vol. 14, No. 1, 2000 87 Table 3. Carbon Product Distribution as a Function of Temperature component (wt % as C)
706 °C 751 °C 801 °C 853 °C 896 °C λ ) 0.47a λ ) 0.43a λ ) 0.38a λ ) 0.33a λ ) 0.39a
CO2 CO CH4 + C2H4 total C in BRS total C in dust mass closure a
55.9 25.9 5.4 0.3 11.2 -1.3
45.2 43.4 4.7 0.5 7.7 +1.5
38.3 47.8 5.0 0.4 8.0 -0.5
30.7 54.3 9.4 0.3 5.0 -0.3
30.8 59.6 6.3 0.2 3.1 0.0
λ ) air ratio.
Table 4. Carbon Product Distribution as a Function of Air Ratio(λ)a component (wt % as C)
Figure 6. Organic and inorganic carbon yield of BRS for all KBLS gasification experiments at air ratio of 0.24-0.51, TiO2 particle size of 105-250 µm, and KBLS feed rate of 8.0-12.0 g/min.
CO2 CO CH4 + C2H4 total C in BRS total C in dust mass closure a
of the BRS for three pyrolysis experiments. It can be seen that both the residual organic carbon and carbonate decrease linearly with increasing pyrolysis temperature, and that the carbonate represents an amount of carbon about 40 times smaller than that of the organic carbon. The decrease in organic carbon with increasing temperature agrees with the results obtained by smallscale pyrolysis experiments over the same temperature range.27 The present carbonate content, however, is much lower than that reported by McKeough et al.27 because of the direct causticization, reactions 2 and 3. For pyrolysis experiments with KBLS in a similar fluidized-bed system with alumina particles as bed materials8 it was found that about 70% of the total carbon, i.e., sum of inorganic and organic carbon, in KBLS remained in the char at 500-600 °C, and about 41% at 700 °C, i.e., values which are consistent with the temperature trend of the present data. Figure 6 shows the organic carbon and carbonate yields for the gasification runs. As expected, the carbon yield in BRS was much lower for gasification than that for pyrolysis, with inorganic carbon yield of 0.1-0.2% (inorganic C/C input from KBLS) and organic carbon yield of 0.0-0.4% (organic C/C input from KBLS). The scatter of the data is too large to identify any possible effect of temperature and air ratio. These yields indicate that carbon gasification is essentially complete for all experiments between 700 and 900 °C. Carbon Product Mass Distribution. The carbon product distribution is another piece of important information for the commercial implementation of KBL gasification. During pyrolysis and gasification of KBLS, the carbon products are distributed over three streams: gas products, bed residual solids, and elutriated dust. Table 3 shows the carbon product mass distribution as a function of temperature for gasification experiments performed at an air ratio of approximately 0.4. Similarly, in Table 4 the carbon product distribution is presented for runs performed at 853 °C but different air ratios. The results in Table 3 show that more than (27) McKeough, P. J.; et al. The Release of Carbon, Sodium and Sulfur During Rapid Pyrolysis of Black Liquor. 1995 TAPPI/CPPA Intl. Chem. Recovery Conf., Toronto, Book A, 1995; pp 217-225.
λ ) 0.0 λ ) 0.31 λ ) 0.33 λ ) 0.43 λ ) 0.51 15.0 46.9 5.8 13.1 18.8 -0.4
28.1 48.7 12.6 0.3 12.4 +2.0
30.7 54.3 9.4 0.3 5.0 -0.3
39.7 52.3 5.8 0.2 4.4 +2.4
44.1 41.0 5.7 0.1 2.6 -6.5
T ) 853 ( 4 °C except for the pyrolysis run at 833 °C.
90% of the carbon in KBLS is released as a gas, while the percentage of carbon which remains in the BRS is small and not a function of temperature. The percentage of carbon captured as dust in the filters is significant (3-11%) and decreases with increasing temperature. With increasing air ratio in Table 4, the percentage of carbon in KBLS released as CO2 increases continuously, while that released as CO shows a maximum at an air ratio of about 0.33. The percentage in the form of CH4 and C2H4 also shows a maximum at a similar air ratio. As expected, the carbon collected in the dust increases with decreasing air ratio, and reaches a high level of 18.8% at pyrolysis conditions. The relatively high percentage of carbon in the dust for the present experiments can be explained as follows: the density of the pyrolyzed KBLS is very low (,0.1 g/cm3) and its particle size is very small so that they are easily elutriated from the fluidized bed. Evidence for this is that the dust released accidentally from the filters would “float” in the air. Significant gasification of the elutriated carbon particles did not take place because of the relatively low temperature (only 400500 °C) in the free-board of the fluid bed. It was reported that fast pyrolysis of the small droplet KBL was complete in less than 1 s at temperatures above 700 °C,19,20 while about 2-3 min was needed for steam gasification of the rapid pyrolyzed kraft chars at 700 °C.15 Since the residence time in the reaction zone of the fluidized bed for the present gasification and pyrolysis experiments is about 3-5 s, the present results suggest that the time in the reaction zone is not sufficient to fully gasify all the organic carbon of KBLS. However, it can be expected that the dust problem can be minimized in an industrial fluidized bed by increasing the free-board temperature. Heating Value of Gasification Gas. The heating value of a gasification gas is perhaps the most important characteristic for design purposes and energy efficiency calculations. The major combustible gases produced by pyrolysis or/and gasification of KBLS are carbon monoxide, hydrogen, methane, and ethylene. Among those gases, C2H4 has the highest heating value but unfortunately its concentration is too small to contribute much
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normalized heating value of the gasification gas increases nearly 1.8 times due to the 2.5-fold increase in CO yield. Figure 8 shows the heating value as a function of air ratio. It indicates a maximum normalized heating value at an air ratio of about 0.25-0.40. The explanation for the existence of a maximum is that at a lower air ratio the carbon in KBLS is not completely gasified, while, at a higher air ratio more CO2 is formed mostly due to combustion of CO by oxygen. The reason that the heating value of the pyrolysis gas is close to that of a gasification gas at an air ratio of around 0.4, even though for pyrolysis not all organic carbon is gasified, is that heat is supplied externally to the reactor under pyrolysis conditions, while gasification of KBLS at an air ratio of about 0.35 is autothermic.22 Figure 7. Normalized higher heating value of gasification gas as a function of temperature at air ratio of 0.389 ( 0.078, KBLS feed rate of 8.0-12.0 g/min, and TiO2 particle size of 105-177 µm.
Figure 8. Normalized higher heating value of gasification gas as a function of air ratio of 853 ( 4 °C and KBLS feed rate of 8.0-12.0 except for pyrolysis runs (1 at 833 °C and 2 at 871 °C).
to the total heating value of the present gasification gas. The concentration of CH4 is normally 10 times smaller than that of CO, but since its heating value is more than three times larger than that of CO it is an important combustible component in the gasification gas. On the basis of average gas concentrations, the higher heating value of the product gas can be calculated. To compare the heating value for different experiments a normalized higher heating value based on the gas yield and expressed in kJ/g KBLS was used. Figure 7 shows the normalized heating value as a function of temperature. It is clear that by increasing the reaction temperature from 700 to 900 °C, the
Conclusions The major gases generated during pyrolysis and gasification of KBLS in the presence of TiO2 particles in a fluidized bed are CO, CO2, H2, and CH4 plus a minor amount of C2H4. The amount of organic carbon plus inorganic carbonate remaining in the BRS is less than 0.5% of the total carbon in KBLS under the gasification conditions. This indicates that gasification of organic carbon of KBLS is essentially complete over the entire temperature range of 700-900 °C. The product gas composition, mainly CO and CO2, changed significantly with temperature. Higher temperatures are beneficial for increasing the CO/CO2 molar ratio and thus the heating value of the product gas. The product gas composition is also significantly affected by the air ratio. Complete gasification of organic carbon can be achieved at an air ratio as low as 0.24. The maximum heating value of the product gas is obtained at an air ratio of 0.3-0.4. The experimental results also show that the presence of TiO2 particles in the present range of 75 to 250 µm does not significantly influence the carbon conversion, but reduces the amount of inorganic carbonate due to the direct causticization reactions. The practical implication for kraft black liquor gasification is that the air ratio should be controlled at 0.3-0.4 in order to maximize the heating value of the product gas. Acknowledgment. Financial support provided by NSERC and ABB Combustion Engineering is gratefully acknowledged. The authors also thank Dr. Jan Pels, Chris Pembroke, Jamie Eastwood, Sherri Griffin, Kerrie-Ann Noble, and Troy Thurlow for assistance of the pilot plant operation. EF9900381
