Gasification-based clean conversions and utilizations of coal, oil shale and biomass

Abstract

Gasification-based conversion of solid fuels into syngas for power and chemical product generations is regarded as the cleanest and most efficient thermal process. The objectives of this study is to have an in depth understanding of gasification (and co-gasification) of coal, oil shale and biomass in terms of its kinetics, thermodynamics, economics and environmental impacts through the combination of lab experiments and simulations. To summarize, the work carried out in this study can be divided into the following three parts. The first part focused on coal pyrolysis and gasification. Firstly, an isothermal CO2 gasification of four coal chars prepared via two different heating regimes, i.e., conventional and microwave pyrolysis, was carried out by thermogravimetric analysis. Results showed that the microwave induced char had the superior thermodynamic performances due to the greater C/H mass ratio and more ordered carbon structure. Secondly, the gasification performances of coal and its corresponding macerals (i.e. vitrinite, liptinite and inertinite) as well as the interactions among macerals under typical gasification conditions were investigated. It was found the cold gas efficiency was changed in the order of liptinite > vitrinite > inertinite. The relationships between synergistic coefficients of gasification indicators were correlated well with maceral contents. The increase of gasification temperature was found promoting the synergistic coefficients slightly, whilst at an oxygen-to-coal mass ratio of 0.8 and a steam-to-coal mass ratio of 0.8, the highest synergistic coefficients were obtained. Thirdly, the distributions and speciation of nine hazardous heavy metals (i.e., Ba, Co, Cr, Cu, Mn, Ni, Pb, V, and Zn) in coal and gasification slags collected from two opposed multi-burners gasifiers were quantified. The morphology of fine slags appeared fragmentized, small spheres and covered with fine floccules, whilst coarse slags were vitreous, angular and less porous than that of fine slags. The elements of Cr, Cu, Ni, V and Zn were mainly in residual fractions (48.8 -82.6 wt%) of the coal samples, while almost all heavy metals were principally bonded with residual fractions (42.3 -94.8 wt%) in gasification slags. The second part focused on thermal co-processing of coal and oil shale via combustion and gasification at laboratory scale. The thermal behaviours of co-combustion of Qinghai (QH) coal and Fushun (FS) oil shale were evaluated. The results indicated that the ignition index and burnout index of the blends reached maximum for 10% of FS. The increase of heating rates promoted the combustion performances. Significant synergistic interactions were observed in the temperature range of 410 - 480 oC. Besides, Flynn–Wall–Ozawa and Kissinger–Akahira–Sunose models were employed to derive the activation energy and the lowest apparent activation energy was found to be 64.1 kJ/mol at 10% blending of oil shale. Pre-exponential factors and reaction mechanism functions were investigated using an integral master-plots method. The slagging and fouling tendencies were alleviated with the addition of oil shale in combustion. On the other hand, a clean and effective utilization of inert oil shale semi-coke by co-gasification with coal was conducted under CO2 atmosphere using a non-isothermal thermogravimetric analyser within the temperature range of 25 - 1050 ℃. A back propagation neural network optimized by genetic algorithm (GA-BPNN) was applied to predict gasification mass loss curves at various heating rates, blending ratios and gasification temperatures. The GA-BPNN model was validated effectively using the experimental data, and the shrinking core kinetic model was found to be a better fit than the volumetric model. The last part focused on process simulations of gasification plants for olefins production using renewable biomass. An indirect steam gasification of biomass to olefins (IDBTO) coupled with CO2 utilization process was proposed. Comparisons of IDBTO and direct oxygen-steam gasification of biomass to olefins (DBTO) were performed in terms of their energy and exergy efficiencies, net CO2 emissions, and economics. The results indicated that the olefins yield, energy and exergy efficiencies of IDBTO were about 19%, 49% and 44%, respectively, which were 2%, 8% and 7% higher than those of the DBTO process. Meanwhile, the quantitative economic performances (net present value and internal rate of return) of IDBTO were superior than that of the DBTO process.

Date of Award	14 Nov 2020
Original language	English
Awarding Institution	Univerisity of Nottingham
Supervisor	Tao Wu (Supervisor), Mengxia Xu (Supervisor) & Cheng gong Sun (Supervisor)