The reaction rate of Powder River Basin coal gasification in CO2 and steam was measured using a rapid response, real-time gas analysis system. The gasification rate is dependent on the char structure, which is affected by the heating rate and pressure during pyrolysis. In our experiments, unlike much prior work, we approximated the pyrolysis and gasification conditions in an industrial fluidized bed gasifier.
The measurements were made using a pressurized reactor that combines features of drop tube and fixed bed reactors. A quartz frit at the center of the reactor prevents solids from passing through the reactor, providing sufficient reaction time for complete gasification. One gram of pre-dried coal is pre-loaded in a pressurized two-valve inlet system. When coal is dropped from the top of the reactor, rapid heating and fast pyrolysis of the coal sample is intended to approximate the injection of ambient temperature coal into the fluidized bed gasifier. Compared to char produced by slowly heating coal, the rapidly pyrolyzed char has a more porous structure that dramatically reduces the time required to gasify the sample. A quadrupole mass spectrometer with a customized sampling system is used to continuously measure concentrations of product gases. The differentially pumped sampling system rapidly responds to changes in gas composition.
CO2 and steam gasification experiments were done from 833oC to 975oC and from 1 atm to 12 atm using argon as inert sweep gas. In this temperature range, the overall reaction rate is expected to be controlled by the surface reaction rate. Experiments that tested the effect of particle size and inert gas partial pressure showed that most of rate measurements were under regime I (surface reaction limited), while some tests at the highest temperature were affected by mass transfer restrictions. Every experimental condition was repeated two to three times to ensure reproducible results.
Reaction rates and carbon conversions were calculated from product gas composition versus time data, which was measured using a quadrupole mass spectrometer. Results for the effects of temperature and pressure, and pyrolysis conditions are presented. For both CO2 and steam gasification, the gasification rate increases with increasing temperature. Steam gasification showed higher activity than CO2 gasification at the same temperature and pressure. The partial pressure of CO2 showed little effect on CO2 gasification rates. The random pore model closely fit the experimental results and fitting parameters are listed. The activation energy for CO2 gasification is 176.16 kg/mol. Char morphology was investigated using scanning electron microscopy, and surface areas were determined by N2 and CO2 absorption. The effect of pyrolysis temperature on char properties is shown. Additional experiments are planned, including additional gasification experiments in steam, gasification in mixtures of CO2 and steam and the inhibition of gasification by the CO and H2 reaction products.
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