Ind. Eng. Chem. Res. 1997, 36, 523-529
523
Thermogravimetric Studies on the Global Kinetics of Carbon Gasification in Nitrous Oxide Hsisheng Teng,* Hung-Chi Lin, and Ya-Sheng Hsieh Department of Chemical Engineering, Chung Yuan Christian University, Chung-Li 32023, Taiwan
The global kinetics of the carbon gasification in N2O were determined with a thermogravimetric system. The carbon was a phenol-formaldehyde resin char of low impurity levels, and the burnoff range of the carbon in this study was 0-45%. The temperature range studied was 673-923 K, and the partial pressure of N2O was varied from 10.1 to 101 kPa. The kinetics can be divided into two temperature regimes. The low-temperature regime (748 K) is exhibited by an activation energy, which is also an increasing function of the burn-off level, ranging from 170 to 230 kJ/mol. The reaction order with respect to the N2O partial pressure is not constant. The rate of gasification increases with the N2O pressure to a maximum and then begins to decrease with a further increase of the pressure. Introduction The mechanism of carbon gasification in O2, CO2, and H2O is not yet well understood in many respects (Laurendeau, 1978; Smith, 1982). The reactions of carbons with N2O are even less well characterized. Very few studies have addressed the gasification of carbon in N2O in a comprehensive way. However, the fact that N2O formed during combustion can be heterogeneously reduced by carbonaceous residues produced in situ is known (De Soete, 1990; Wo´jtowicz et al., 1993; Rodriguez-Mirasol et al., 1994; Gulyurtlu et al., 1994; Åmand and Leckner, 1994). There have been a limited number of studies of the N2O-carbon reaction (see Table 1). Heterogeneous reactions of N2O with carbon can reduce N2O to N2 and form CO and CO2 gaseous products (Shah, 1929). According to Table 1, the reaction order with respect to N2O pressure for this N2Ocarbon reaction was reported to be between zero and unity. The reported activation energies are not consistent and vary between 40 and 280 kJ/mol. One proposed mechanism, which was accepted by most of the researchers, for the reaction of carbon with N2O is as follows (Smith et al., 1957):
N2O + C T C(O) + N2
(R1)
N2O + C(O) T CO2 + N2
(R2)
where C(O) is a stable carbon-oxygen surface complex. The N2O is able to scavenge oxygen atoms from the carbon-oxygen surface complexes formed by its reaction with these sites (Rodriguez-Mirasol et al., 1994). This reaction model was proposed partially based on the fact that N2 and CO2 were the main products of the reaction. However, it has been reported by other studies (Shah, 1929; Rodriguez-Mirasol et al., 1994) that the formation of CO in this reaction cannot be neglected. Basically, this model has its shortcoming in terms of failing to explain the formation CO. An alternative mechanism is somewhat more elaborate (De Soete, 1992): * To whom correspondence should be addressed. Telephone: 886-3-456-3171 ext. 4124. Fax: 886-3-456-3160. Email:
[email protected]. S0888-5885(96)00582-9 CCC: $14.00
N2O + C f N2 + C(O)
(R3)
C(O) f CO + free site
(R4)
2C(O) f CO2 + C + free site
(R5)
CO + C(O) f CO2 + C
(R6)
N2O + C(O) f N2 + CO2 + free site
(R7)
This reaction model shows the formation of CO through (R4), which is usually an important step in oxidizing carbon gasification by O2, CO2, H2O (Laurendeau, 1978), or NO (Suuberg et al., 1990; Teng et al., 1992). According to the above studies, there is general agreement that the first step is chemisorption of N2O. It probably involves addition of N2O in an “O-down” configuration, followed by release of N2 and formation of carbonoxygen surface complexes. Since it has been found that there was little nitrogen fixed on the carbon surface in the reaction of carbon gasification by N2O, the quantity of N2 produced could be a measure of the N2O decomposed (Shah, 1929; Madley and Strickland-Constable, 1953). However, almost all of the kinetic data of the N2O-carbon reaction reported by the previous studies, as shown in Table 1, were determined by directly measuring the rate of N2O decomposition. There was only one study (Degroot and Richards, 1991) that reported the rate of this reaction based on the loss of carbon during gasification. In that study (Degroot and Richards, 1991) the rate of carbon loss for the N2O-carbon reaction was simply measured at a temperature of 873 K and an N2O pressure of 10 kPa, and, thus, no kinetic parameters, such as activation energy and reaction order, could be determined based on their data. However, it is worth noticing that the study (Degroot and Richards, 1991) reported that the reactivity of carbon in N2O is lower than that in NO, whereas the results from other studies (De Soete, 1990; Rodriguez-Mirasol et al., 1994) demonstrated that N2O is more readily reduced on char surface than NO at temperatures around 873 K. Further investigation to obtain a better understanding of the N2O-carbon reaction will provide a means by which they can be evaluated with respect to the reduction of N2O emissions from coal-fired combustion © 1997 American Chemical Society
524 Ind. Eng. Chem. Res., Vol. 36, No. 3, 1997 Table 1. Kinetic Parameters for the N2O-Carbon Reaction carbon
reactor
temp (K)
PN2O (kPa)
E (kJ/mol) react. order
charcoal
static reaction bulb
551-653
1-52
134
1a
charcoal graphon Cedar Grove bit. char Prosper bit. char Eschweiler bit. char Norit RX act. carbon DE53 lignite chair Gardanne subbit. char Daw-Mill bit. char Blanzy anthr. char Norit RX act. carbon
circulated batch reactor circulated batch reactor fixed bed fixed bed fixed bed packed bed fixed bed fixed bed fixed bed fixed bed fixed bed
673-873 673-923 700-1250 700-1250 700-1250 640-720 673-1223 673-1223 673-1223 673-1223 673-1223
0.26-46 0.17-50 0.008 0.008 0.008 0.0035-0.0045 0.019-0.19 0.019-0.19 0.019-0.19 0.019-0.19 0.019-0.19
176 284 116 101 83.1 96 ( 13 77 ( 23 68 ( 27 77 ( 17 66 ( 9 77 ( 23
1a 1a
0.59 ( 0.43 0.58 ( 0.59 0.59 ( 0.34 0.61 ( 0.27 0.59 ( 0.43
reference Madley and StricklandConstable (1953) Smith et al. (1957) Smith et al. (1957) De Soete (1990) De Soete (1990) De Soete (1990) Wo´jtowicz et al. (1991) Rodriguez-Mirasol et al. (1994) Rodriguez-Mirasol et al. (1994) Rodriguez-Mirasol et al. (1994) Rodriguez-Mirasol et al. (1994) Rodriguez-Mirasol et al. (1994)
a The reaction is first-order with respect to N O pressure during the course of a batch experiment; however, the first-order rate constant 2 decreases with the initial N2O pressure.
systems. It might also serve to indicate where this chemistry can be applied to other postcombustion N2O reduction processes. This paper presents some results of a new study of the N2O-carbon gasification reaction, in which the gasification is performed in a thermogravimetric system. The global kinetic data determined in this study are based on the rate of carbon consumption through gasification by N2O. The interest in the global kinetics of the N2O-carbon reaction is due to the fact that global kinetics are looked at to offer a clue to the key mechanistic steps in the overall gasification process. Experimental Section The carbon samples used in the present study were derived from phenol-formaldehyde resins. These resins have structural features similar to those in coals but contain fewer catalytic impurities. These can be controlled to very low levels in the synthesis process (Suuberg et al., 1988). The resin char was prepared by pyrolysis of the phenol-formaldehyde resin in a helium environment at 1223 K for 2 h, then ground, and sieved to the desired particle size (210-297 µm). The measurements of reactivity of the chars were performed in a Perkin-Elmer TGA 7 thermogravimetric analyzer (TGA). A sample of char (15-30 mg) was suspended in a sample pan in the heated zone of the TGA, and the temperature in the vicinity of the sample was measured by a small thermocouple probe (type K). During the course of an experiment, N2O diluted in helium was continuosuly passed into the TGA. The total flow rate used in this study was 140 cm3/min. The N2O partial pressure used for gasification was between 10.1 and 101 kPa. Since the amount of char sample was small and the N2O flow rate was high, the fraction of N2O consumed during reaction was negligible, and, therefore, the N2O partial pressure in reaction was considered to be the same as that in the inlet stream. Reactivity was determined from the weight loss of char with time. The temperature range of this gasification study was between 673 and 923 K. External masstransfer limitations have been determined to be insignificant in the range of reaction rates of interest here; this is confirmed by the fact that the reaction rate was not affected by increasing the gas flow rate at the highest temperature studied. Specific surface areas and porosities of the samples were determined by gas adsorption. An automated adsorption apparatus (Micromeritics, ASAP 2000) was employed for these measurements. Adsorption of N2, as a probe species, was performed at 77 K. Before any such analysis the sample was degassed at 300 °C in a
Figure 1. Adsorption isotherms of N2 on the resin chars with different extents of burn-off in N2O (reaction temperature, 673923 K; N2O partial pressure, 10.1-101 kPa).
vacuum at about 10-3 Torr. Surface areas and micropore volumes of the samples were determined from the application of the BET and Dubinin-Radushkevich (DR) equations, respectively, to the adsorption isotherms at relative pressures between 0.06 and 0.2. Results and Discussion Variation of Surface Structures of the Char with Burn-Off Level. Adsorption isotherms of N2 on the resin chars, with different extents of burn-off, are shown in Figure 1. The isotherms are typical of microporous carbons (type I); i.e., the knees of the isotherms are sharp and the plateaus are fairly horizontal. The adsorption capacity of the resin char increases upon gasification. It can also be seen from Figure 1 that at low levels of burn-off the resin char is mainly microporous, and as the gasification proceeds there is a widening of the porosity by an increase in supermicroporosity, as inferred from the opening of the knee of the isotherm (Mun˜oz-Guillena et al., 1992). In type I isotherms (Lowell and Shields, 1991), the amount of N2 adsorbed at pressures near unity corresponds to the total amount adsorbed at both micropores (filled at low relative pressures) and mesopores (filled by capillary condensation at pressures above 0.2), and, consequently, the subtraction of the micropore volume
Ind. Eng. Chem. Res., Vol. 36, No. 3, 1997 525
(from the D-R equation) from the total amount (determined at p/p0 ) 0.98 in this case) will provide the volume of the mesopore (Rodrı´guez-Reinoso et al., 1995). The adsorption isotherms are employed to deduce the BET surface area and the micropore and mesopore volumes. The average pore diameter can be determined according to the surface area and total pore volume (the sum of the micropore and mesopore volumes), if the pores are assumed to be parallel and cylindrical. This parameter is useful when comparing the porous texture of the resin char. The data of the surface structures at various extents of burn-off in N2O, regardless of the gasification conditions, are collected and shown in Table 2. Surface areas of these resin chars have been reported to vary rapidly with the extents of burn-off in O2 (Suuberg et al., 1989). The variations of surface areas must generally be taken into account for reactivity analysis. One can observe from Table 2 that the surface area and pore volume generally increases upon gasification. This is due to enlargening (deepening or widening) of the original pores and opening of the closed pores (Wigmans, 1989; Walker and Almagro, 1995). Since the surface area and pore volume shown in Table 2 are obtained from chars gasified at different temperatures (673-923 K) and N2O partial pressures (10.1-101 kPa) to determine the kinetic parameters, it appears that the surface structures of the resin char are simply functions of the burnoff level in N2O and the gasification conditions have little impact on the development of surface area. This is also true in the case of the resin char gasification in O2 (Suuberg et al., 1988). However, the surface area development in N2O is relatively slow, in comparison with that in O2 (Suuberg et al., 1989). This aspect has been explored, and the results will be presented elsewhere. Table 2 shows that the contribution to the total pore volume of the low burn-off (748 K) is characterized by an activation energy of 195 kJ/mol, whereas the low temperature regime (
