Znd. Eng. Chem. Res. 1991,30, 1594-1601
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GENERALRESEARCH Kinetics of Gasification of Black Liquor Char by Steam Jian Li and Adriaan R. P. van Heiningen* Department of Chemical Engineering, McGill University, Pulp and Paper Research Institute of Canada, Montreal, PQ, Canada H3A 2A7
The steam gasification kinetics of kraft black liquor char were studied in a thermogravimetric analysis reactor. The effect of steam and hydrogen concentration on gasification rate can be described by Langmuir-Hinshelwood type kinetics. An activation energy of 210 kJ/mol was obtained. Methane formation was negligible, and H2S was the major gaseous sulfur-containing product obtained over the temperature range studied, 873-973 K. The COPconcentration was higher than calculated for the water-shift reaction a t equilibrium. A gasification mechanism is proposed whereby COPis one of the primary gasification products.
Introduction Black liquor is the spent liquor resulting from digestion of wood in a solution of NaOH and Na2S. The dried product, black liquor solids (BLS), consists of approximately 50% dissolved wood and 50% inorganics. Black liquor char (BLC), the pyrolysis product, consists mainly of a mixture of carbon, Na2C03,Na2S, and Na2S04. In a conventional chemical recovery furnace, concentrated black liquor is introduced as a spray. After drying, pyrolysis, and partial combustion, the droplets fall onto a char bed at the bottom of the furnace where the remaining carbon is gasified and inorganic oxyaulfur compounds are reduced to sodium sulfide. Because of the complexity, corrosive nature, and heterogeneous character of the reactor system, present knowledge of all these reactions is far from complete. Steam gasification of black liquor char, for example, has never been studied. Not only will information about steam gasification kinetics lead to a better understanding of the conventional furnace, it is also needed for several proposed alternative kraft recovery processes (Andersson and Warnquist, 1989; Durai-Swamy et al., 1989; FallavoUita et al., 1987) which incorporate gasification as one of the key steps. Because the chemical composition of BLC and alkalimetal carbonate impregnated chars are similar, extensive studies on catalytic gasification of the latter chars serve as a useful reference. Alkali-metal salts are among the oldest known additives that markedly increase the rate of steam gasification of carbonaceous materials. Since then numerous studies have been performed, and the general effectiveness of alkali metals as catalysts for gasification is well established. Steam and C02gasification catalyzed by alkali-metal salts are similar in a number of aspects, such as increased reactivity with better catalyst dispersion (Mims et al., 1982), higher alkali metal/carbon ratio (Wigmans, 1982), and order of catalytic activity of alkali metals (Cs > Rb > K > Na > Li) (Moulijn and Kapteijn, 1987). The present state of knowledge of alkali-metalcatalyzed carbon gasification by C02was summarized recently by Moulijn and Kapteijn (1987). Detailed reaction mechanisms are given by Cerfontain et al. (1987), Moulijn and Kapteijn (1987), and Sams and Shadman (1986).
Like alkali-metal-catalyzed gasification, it is expected that steam gasification of BLC would have many similarities with C02gasification of BLC. The C02gasification study of BLC performed by the present authors has shown that the rate is 1or 3 orders of magnitude higher than that of cod char, respectively, with or without impregnation of alkali-metal carbonate (Li and van Heiningen, 1989a, 1990a; Li, 1989). The extremely high reactivity of BLC is explained by very fine dispersion of Na2C09 in the carbon matrix as a result of formation of the char from a liquid precursor. This was confirmed by a comparative SEM-EDS study of the sodium distribution on the surface of BLC and activated carbon impregnated with Na2C09. In the present investigation, the reaction system is first analyzed thermodynamically. Then the gasification rate of BLC is studied experimentally as a function of H20 and Hzconcentration, temperature, and carbon conversion. The equilibrium constant of the water-shift reaction is determined experimentally and compared to that at thermodynamic equilibrium.
Experimental Section Experimental System. A schematic picture of the experimental system, a standard thennobalance from Cahn with auxiliaries, is shown in Figure 1. Steam was generated by injecting deionized water with a calibrated syringe pump in a heated tube. The exhaust gas was dried by bubbling through 96% H8O4and then fed to IR analyzers for CO and C02 content. Furnace temperature, sample weight, and CO and C02concentrations were continuously recorded by a computer. The dry gas was also analyzed for other fixed gases and sulfurous gases by two gas chromatographs with a thermal conductivity and flame photometric detector, respectively. The solid gasification residues were analyzed by an ion chromatograph with a conductivity and electrochemical detector for sulfate, sulfite, thiosulfate, carbonate, sodium, and sulfide content. Material. Black liquor was obtained by cooking Black Spruce wood chips with an aqueous solution of NazS and NaOH (active alkali charge, 18% as Na20 on oven-dry wood; sulfidity, 30%) at 170 OC to 49% pulp yield. The pulp Kappa number (measure of residual lignin content)
0888-5885J 91/ 2630-1594$02.50/0 0 1991 American Chemical Society
Ind. Eng. Chem. Res., Vol. 30,No. 7,1991 1696 Table I. Chemical and Elemental Analysis of Black Liquor Solids and Chars Chemical Analysis, w t % [SO,*] [S2-I [s03'-1 [co32-1 7.2
