Catalytic CO₂ Gasification Of A Sub-bituminous Coal From Powder River Basin

It is well know that coal is readily available in Power River basin(PRB) in Wyoming, which was widely used for power generation. Due to the upcoming strict CO2 emission regulations by Environmental Protection Agency(EPA), CO2 generated in coal utilization becomes a heavy burden for coal enterprises. Supposed that CO2 was used as gasifying agent in coal gasification, with CO product used for synthesis of O-containing chemicals, CO2 would no longer be a burnden, but be beneficial to enterprises. The objective of this work is to study the CO2 gasification of a PRB coal in atmosphere from 700 oC to 900 oC in the presence of Na2CO3 and FeCO3, which were widely available in nature, thus provide a way for CO2 utiliation. The effects of Na and Fe on coal pyrolysis and coal gasification with CO2 or CO2 +H2O were studied in this work.TGA,on-line Micro GC and ultimate analysis were employed to analyze the effect of Na and Fe on the mass loss rate, generation rates of gas products and solid products in pyrolysis, respectively. Results show that the mass change of residues was not significant, and the difference is in the range of ~2 wt.% in the presence of Na and Fe; with Na and Fe, the peak of mass loss rate at 430 oC was weakened, while a new peak appeared at around 650 oC(Na) / 700 oC(Fe). The possible reason is that Na and Fe could bond with the free radicals formed in coal pyrolysis, thus prevented them from volatilization, while the bonds would be broken at higher temperatures and the free radicals were released. Na promoted the H2 generation in pyrolysis, make the generation rate peak appear earlier, while Fe did not show the same performance; both of Na and Fe inhibited the formation of CH4; both of them inhibited the CO formation in the initial stage while did not show the effect in the later stage, the difference is that Na makes the CO generation rate peak appear earlier in the later stage; both of them show significant inhibiting effect on the CO2 formation, Fe is more prominent. The addition of Na and Fe increased the carbon content in solid product to some degree, while the increase became unconspicuous with increased pyrolysis temperature; both of them promoted the release of hydrogen from solid product; Na accelerated the release of oxygen before 700 oC, and then show inhibiting effect on it, while Fe showed the adverse performance; both of them inhibited the surfur release in the coal pyrolysis. XRD analyses found that part of Na existed as sodium aluminosilicate, while Fe was found as metallic Fe after pyrolysis to 700 oC.Grain model(GM), Integrated model(IM) and Random pore model(RPM) were employed to analyze the catalytic gasification kinetics with Na, and the Na species during CO2 gasification were examined by XRD. Results show that Na species observed by XRD were sodium aluminosilicate and sodium calcium silcate. The Na ratio in their compositions changed with gasification time and temperature. The Na ratio in aluminosilicates/ calcium silcates obtained after pyrolysis or after gasifcation was higher than that obtained during the gasification; the Na ratio in aluminosilicates/ calcium silcates obtained at low gasification temperatures was less than that obtained at high temperatures. In the kinetic data fitting, integrated model and Random pore model performed better that Grain model did. The fitting results for data at 850 oCand 900 oC were worse than those at other temperatures. The possible reason is that the Na mobility alleviated the decrease tendency of gasification rate with the incease of carbon conversion, which the models did not considered. The kinetic analyses show that the apparent activation energies of the coal-CO2 gasificationwithout and with 3 wt.% Na are ~91 kJ/mol and ~64 kJ/mol, respectively, a 30% decrease. Thus, Na2CO3 is a promising catalyst for the PRB coal-CO2 gasification.SEM/EDS analyses and kinetic analyses by Integrated model(IM), Random pore model(RPM) were employed to analyze the catalytic performance of Fe, and the Fe species during CO2 gasification were examined by XRD and M?ssbauerspectroscopy. Results show that Fe had significant catalytic effect on the char particles that it adhered to. Some Fe combined with other elements to form irregular spherules. EDS images indicate that the Fe distribution during the coal gasification is not very even. Compared to Random pore model, Integrated model performed better in the kinetic data fitting. Results show that the activation energy was 92.7 kJ/mol for the untreated coal, which decreased to 58.3 kJ/mol for the coal with 3 wt% Fe. FeCO3 was shown to have asignificant catalytic effect on the PRB coal gasification with CO2. During the gasification, metallic Fe and Fe3 C formed in coal pyrolysis underwent a gradual oxidation process in the CO2 atmophere: Fe/ Fe3C→ Fe O → Fe3O4. In the initial stage, metallic Fe and Fe3 C was supposed to be the catalytic Fe species, while in the later stage, the catalytic Fe species changed to FeO and Fe3O4.The effect of H2O/CO2 mole ratio on gas production and carbon conversion were studied. The catalytic effect of Na and Fe on the gasification with the CO2\H2O mole ratio of 3:7 were evaluated. The Na distribution and status were analyzed by SEM/EDS. Results show that H2 yields rose with the increase of H2O/CO2 mole ratio, while CO yield dropped down. With the addition of Na and Fe, the H2 yield at each gasification temperature went up, and CO yiled had an adverse tendency. The largest difference appeared at 800 oC, and the smallest one appeared at 900 oC. Na distribution in gasification is not even, as part of Na combined with Si to form particles with high sodium concentration. Two forms of Na were found in the coal ash. Some sodium existed as oxides; the others existed in the form of aluminosilicates/calcium silcates. The iron was observed as oxides in the coal ash.