Kinetics and Mechanisms of NO(x) - Char Reduction. Page: 17 of 70
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examined by CO2. The suggestion is that the smallest micropores, those primarily
responsible for CO2 area are disappearing due to pore enlargement. The C02 results are
most sensitive this loss. Since these smallest micropores are filled in a manner not entirely
consistent with BET theory, the implication is that the true reactive surface in non-
molecular sieving pores is not well tracked by CO2.
Quantitatively the differences between gasification agents as revealed by C02
isotherms are small; NO and 02 develop slightly higher surface areas, and even in the pore
diffusion limited reaction with NO the C02 surface area evolution is similar. There is
clearly loss with burn-off of small microporosity, as revealed by CO2 , even under Zone II
conditions in NO. This cannot necessarily be used to infer that reaction is occurring in these
micropores, however. The percentage loss of area is modest, so it is possible that some
micropores are reacting and others not. The loss might also be attributable to coalescence
processes.
The CO2 pore volume development curves do not drastically differ in shape from
the surface area curves, see Figure 7. This again suggest that C02 cannot condense in
mesopores. The surface area and pore volume revealed by C02 are largely attributable to
condensation in small micropores. It is, however, notable that the Zone II NO char shows
the slowest loss of volume in the small micropores - again consistent with lack of
enlargement in this range of porosity. Quantitatively NO and 02 gasification grow pore
volumes more effectively than does C02 gasification. Interestingly, there is almost no
difference in pore volume evolutions between CO2 gasification in Zone I and NO
gasification in Zone II. This suggests a possibly limited utilization by CO2 of micropores
even in the kinetically controlled regime.
3.1.4 Changes in adsorption isotherms during gasification
Figure 8 shows the adsorption isotherms of nitrogen on Wyodak coal char at
different levels of burn-off in NO at 911 K. It was observed that the equilibration times17
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Suurerg, E.M.; Lilly, W.D. & Aarna, I. Kinetics and Mechanisms of NO(x) - Char Reduction., report, December 31, 1997; Providence, Rhode Island. (https://digital.library.unt.edu/ark:/67531/metadc690356/m1/17/: accessed April 23, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.