CO2 gasification characteristics of nascent pyrolyzed particles from coals and oil shale

Abstract

Summary In this work, the co-pyrolysis characteristics of oil shale with two typical coals, bitumite and lignite, and the co-gasification characteristics of the mixture pyrolyzed fuels were studied via thermo-gravimetric analysis. The individual fuels and mixture fuels were first pyrolysis in N2 atmosphere to specified temperature (450, 550, and 620 °C) at the heating rate of 20, 30 and 40 °C/min, respectively, and then maintained at the given temperature for 20 min before converted to CO2 ambient to conduct the CO2 gasification tests. The kinetic behavior and effects of both fuel types and pyrolysis temperature were investigated. The shoulder peak at around 550 °C observed in the derivative of weight loss derivative thermogravimetry analysis (DTG) curve during the pyrolysis of oil shale has confirmed the existence of specific reactions of oil shale at around 550 °C that leads to a sharp trough in the differential curves of co-pyrolysis with coals and the unusual change in activation energies of gasification. In isothermal pyrolysis stage, oil shale lost its vast majority of organic matters at the temperature lower than 550 °C. The escape of pyrolysis gas and liquids in the coals is much harder than that in oil shale. The interaction between oil shale and bitumite was too weak to discriminate both in the pyrolysis and CO2 gasification process. The variation of the particle surface structure caused by the releasing of volatile gases is strongly affected by the reaction rate and temperature. Quick volatile decomposition and gas releasing lead to the increase of surface area, decrease of the average pore diameter as well as the uniformization of the pore structure, while the higher temperature results in the blockade and merging of fine pores. The two factors lead to the greatest mass loss rate in the pyrolyzed particles obtained at 550 °C in temperature programmed CO2 gasification stage. Two model-free methods, Friedman method and Flynn–Wall–Ozawa method, were used to extract kinetic parameters from the experimentally determined pyrolyzed fuel conversions. The volatile contend has a significant influence on the fixed carbon conversion during the partially pyrolyzed particles' CO2 gasification. In this study, significant interactions existed in co-thermal utilization, both pyrolysis and CO2 gasification, of oil shale and lignite. It is therefore surmised that co-gasification of pyrolyzed lignite and oil shale may represent a feasible, practical route to high-efficiency utilization of these fuels. Copyright © 2017 John Wiley & Sons, Ltd.