Ind. Eng. Chem. Res. 1993,32, 1747-1753
1747
Rate and Mechanism of Black Liquor Char Gasification with C02 at Elevated Pressures William J. Frederick' Department of Chemical Engineering, Oregon State University, Gleeson 103, Corvallis, Oregon 97331
Kaj J. Wig+ and Mikko M. Hupa Department of Chemical Engineering, Abo Akademi University, Lemminkliisenkatu 14-18B, SF-20520 Turku, Finland
The kinetics of sodium-catalyzed C02 gasification of carbon at elevated pressures were investigated using a char, produced from spent kraft pulping liquor, in which the catalyst is distributed more uniformly in the carbon matrix than in alkali-metal-impregnatedcarbons. Gasification measurements were carried out in a pressurized thermobalance at 700 "C, 0.1-25 bar C 0 2 partial pressures, and 0-6 bar CO partial pressures. The rate of gasification was an order of magnitude or more higher than the rate of gasification of potassium-impregnated activated carbon and several orders of magnitude greater than for alkali-metal-impregnatedcoal chars. It was less than first order in C02, strongly inhibited by CO, and strongly temperature dependent. The gasification kinetics followed the same mechanism as reported previously for potassium-catalyzed C02 gasification of activated carbon in spite of large differences in the carbon source, catalyst type, and loading. The rate of gasification initially increased with increasing carbon conversion, but decreased with increasing conversion over most of the range of conversion. The change in rate with carbon conversion is apparently related to changes in the number of active carbon sites and catalyst sites. There was no loss of sodium from the char during gasification.
Introduction Spent pulping (black) liquor from the chemical conversion of wood to papermaking fiber is an important fuel in paper manufacturing countries. In Finland it contributes 9% of the nonnuclear energy production, in the US. more than 1% ,and worldwide 80 OOO TJ per year. These fuels contain the inorganic pulping chemicals along with the dissolved biomass separated from the fibers during pulping. They are burned in large, specially designed recovery boilers to recover the pulping chemicals and energy from the dissolved wood. Gasification processes as alternatives to conventional black liquor combustion are currently under development in North America, Finland, and Sweden. Pressurized gasification and combined cycle power generation would improve the electrical energy production per unit of black liquor solids by a factor of 2 (McKeough and Fogelholm, 1991)and could reduce capital costs and gaseous emissions. Under gasification conditions, black liquor pyrolyzes rapidly but a significant fraction of the carbon remains as char (Frederick et al., 1991). Char gasification is the slow step in converting black liquor to combustible gases. Published data on black liquor char gasification kinetics is limited, and prior to our studies (Frederick and Hupa, 1991) only data at atmospheric pressure had been published (Georg and Cameron, 1986;Li and van Heiningen, 1990a,1991; Lee and Nichols, 1993). The results indicate that black liquor chars are orders of magnitude more reactive than other alkali-metal-impregnatedchars, possibly due to the very uniform distribution of the alkali metal catalyst (Li and van Heiningen, 1988). Table I compares gasification rates for typical chars. The kinetics of alkali-metal-catalyzed C02 gasification of carbon have been investigatedby impregnating a carbon
* Author to whom correspondence should be addressed.
Department of Chemical Engineering, Oregon State University, Gleeson 103,Corvallis, OR 97331. + Current address:
or char with potassium salts; gasificationrates are typically reported for catalyst loadings of 1-4 mol of alkali metal/ kg of carbon (Austin and Walker, 1963; McKee, 1982; Kapteijn and Moulijn, 1983; Spiro et al., 1983; Sams and Shadman, 1983;Cerfontain et al., 1987). With alkali metal impregnation, the gasification rate increases with increasing loading at low catalyst loadings,but there is amaximum beyond which the gasificationrate levels off and eventually decreases with increased catalyst loading (Ratcliffe, 1980; Sams and Shadman, 1983). With chars from spent pulping liquors, the rate of gasification increases with increasing sodium content up to far higher catalyst loadings than for alkali-metal-impregnated carbonaceous materials (van Heiningen et al., 1992). When carbon or char is impregnated with a salt, the catalyst is located mainly at the surface and is not well dispersed in the carbon matrix (Li and van Heiningen, 1988, 1990a). In this study, we have investigated the gasification kinetics using a carbonaceouschar with a very uniform mixture of sodium and carbon. The char was obtained by pyrolyzing black liquor from the kraft pulping process. The char precursor contained sodium phenolates and carboxylic acids that are believed to be the active catalytic moieties in carbon gasification with C02 (Sams and Shadman, 1986),and the sodium is distributed very uniformly, far more so than observed with more typical alkali-metal-impregnatedcarbons (Li and van Heiningen, 1988, 1990a).
Experimental Section Materials. The char used in this study was produced from black liquor from a kraft pulp millin the southeastern US., using a drop tube furnace (0.1 m by 3.7 m) operated under pyrolysis conditions at 900 OC and atmospheric pressure. The equipment and procedure are described in detail by Clay et al. (1987). Table I1shows the composition of the precursor liquor and the resulting char. The Na/C ratio is 26 mol of sodiumlkg of carbon. This is typical of
0 1993 American Chemical Society 0888-5885/93/2632-1747~04.00/0
1748 Ind. Eng. Chem. Res., Vol. 32, No. 8, 1993 Table I. Typical Alkali-Metal-CatalyzedGasification Rates at 700 "C and Atmospheric Pressure for Various Carbonaceous Materials initial loading carbon catalyst mol/kg atom ratio rate, l/min C02, bar CO, bar temp,' O C source black liquor chars Na 26 0.31 2.0 x 10-2 0.2 0.04 700 b 39 0.47 1.0 x lo-' 0.2 0.05 725 C 0.019 3.0 X 10-9 0.2 0 activated carbon K 1.6 777 d 0.050 2.0 x 10-9 coal char Na 4.2 1.0 0 700 e 0.096 2.5 X 10-9 K 8.0 0.15 0 800 f 0.0096 1.5 X lP 1.0 0 graphite K 0.8 727 g electrode carbon K 1.6 0.019 8.1 X 10" 0.2 0 727 h a Temperature at which data was taken. For this comparison, the rates were adjusted to 700 "C using an activation energy of 205 kJ/mol. This work. e Li and van Heininaen, 1990a. Kapteiin - - and Moulijn, 1983. e Spiro et al., 1983. f Sams and Shadman, 1983. McKee, 1982. Austin and Walker, 1963. ~~
Table 11. Composition of the Char Used in This Study and the Spent Pulping Liquor from Which It Was Made w t % in component liquor char C 36.7 31.2 H 3.3 2.0 Na 18.3 19.0 K 0.9 1.4 5.4 4.0 S c1 0.4 0.8 0 and unaccounted 35.0 41.6
black liquor char but is very high compared with ratios of 1-4 typically used in investigations of alkali-metalcarbonate-catalyzed gasification of coal char and other carbonaceous materials (Austin and Walker, 1963;McKee, 1982;Kapteijn and Moulijn, 1983; Spiro et al., 1983; Sams and Shadman, 1983; Cerfontain et al., 1987). Experimental Procedure. Gasificationrate data were obtained using a pressurized thermobalance capable of operating at 200-1050 OC and 1-100 bar pressure. The thermobalance per se consists of a microbalance, a sample chamber, and a high-temperature reaction zone, all contained within a pressure chamber (Miihlen and Sulimma, 1987; Iisa and Hupa, 1991). A cylindrical sample holder containing 100 mg of crushed char particles (
