Research On Key Issues Of Lignite Pyrolysis-based Staged Conversion Polygeneration Technology

Lignite is one of the most important coal resources in China. Currently, most lignite is directly used for combustion, which has relatively low efficiency and gives rise to serious pollution. Lignite pyrolysis-based staged conversion polygeneration technology is a clean and efficient lignite staged utilization technology, which extracts the reactive volatile from lignite with coal pyrolysis technology. Liquid and gaseous fuels produced in the technology will help to relieve the shortage of oil and gas in China. At present, lignite pyrolysis-based staged conversion polygeneration technologies in China are still in the research and development (R&D) phase. There are still some key issues in R&D and industrialization processes to be solved, i.e., lignite pyrolysis characteristics and its mechanism, technologies integration and optimization in the polygeneration systems, etc. According the literature review, the researches on the influences of CO, CO2, and CH4atmosphere on lignite pyrolysis is limited and arguable, and there are no reliable theoretical researches on the building, optimization, and feasibility of lignite pyrolysis-based staged conversion polygeneration system. Therefore, simulation and experimental studies have been conducted in this thesis, aimed to provide reliable information and data for the industrialization of lignite pyrolysis-based polygeneration technology.Xiaolongtan lignite pyrolysis under the atmosphere of N2, CO, CO2, and CH4were carried out on both slow-heating and fast-heating fixed bed experimental systems. The yields and characteristics of pyrolysis products in the atmosphere of CO, CO2, and CH4were analyzed and compared with those under nitrogen to study the influences of CO, CO2, and CH4on lignite pyrolysis. The effects of atmosphere concentration, temperature, inherent minerals, heating rate, and holding time were also investigated.The results indicated that, during lignite pyrolysis under CO-containing atmospheres, CO in the atmosphere involves in water gas shift reaction at500-600℃and disproportionation reaction at700-800℃, which result in higher char, CO2, and H2yields, and lower CO and water yields. The abundant Fe-based inherent minerals in the lignite act as catalysts of these reactions. At elevated temperatures, the influences of CO disproportionation reaction on tar and C1-C3 gases yields strongly depend on heating rate:1)under low heating rate, most tar and C1-C3gases have been released before700℃, which means the release of tar and C1-C3gases takes place ahead of the disproportionation of CO, therefore, tar and C1-C3gases yields are not affected by CO atmosphere at higher temperatures.2) under high heating rate, the release of tar and C1-C3gases lasts to elevated temperatures, which means the release of tar and C1-C3gases overlaps CO disproportionation. The deposited carbon on char surface blocks pores of char particles, and thus suppresses the diffusion of tar and C1-C3gases. Before700℃, CO atmosphere facilitates the release of polycyclic aromatic hydrocarbons (PAHs) and aliphatics, and improves the quality of tar. After700℃, the occurrence of CO disproportionation reaction suppresses the yields of every group of tar under fast heating condition.Results of lignite pyrolysis under CO2atmosphere showed that, the presence of CO2improves water and tar yields, but suppresses the release of H2and C1-C3gases. Below600℃, CO2shows no effects on char and CO yields, but after600℃, the occurrence of CO2-char gasification increases CO yield, decreases char yield, and promotes the release of carbon in the lignite and char combustion. Inherent minerals play the role of catalyst in char-CO2gasification. Compared the results under fast and slow heating rate, it is obvious that the influences of CO2on products yields of lignite pyrolysis are similar under different heating rate. The influence of gasification reaction on tar yield and characteristics at elevated temperatures is related to heating rate. Under slow heating condition, tar release is completed ahead of the occurrence of gasification, therefore, gasification shows minor effects on tar yield and characteristics at elevated temperatures. The promotion of tar yield under slow heating rate is due to the improvement of light aromatics, phenols and aliphatics yields. Under fast heating condition, CO2gasification overlaps tar release. CO2gasification improves the porosity of char particles, and thus improves the yield of heavy tar fraction, which should be responsible for the increase in tar yield at elevated temperature under fast heating rate.Compared the results under N2and CH4-containing atmospheres, it can be seen that CH4atmosphere shows insignificant effects on CO and CO2yields under different atmosphere. Cracking of CH4occurs at temperatures higher than600℃into H2and carbon by the catalysis of organic matter in the coal, which improves hydrogen and char yields, decreases carbon conversion, and has negative effects on char combustion. Furthermore, CH4cracking has insignificant effects on tar and C1--C3yields under slow heating rate. Comparatively, under fast heating rate, free radicals generated during methane cracking process at elevated temperatures promotes the stabilization of PAHs, heavy fraction and free radical groups, and thus improves tar and C1～C3yields.A2x300MWe liquid fuels and electricity-oriented lignite staged conversion polygeneration system was established. This system employs the following technical scheme:1) lignite is pyrolyzed in a fluidized bed pyrolyzer;2) char is combusted in conventional subcritical circulating fluidized bed boiler and producing high pressure steam for electricity generation;3) part of H2is extracted from coal gas and used for hydrofining tar into oil;4) coal gas is firstly synthesized into methanol, and the tail gas is further utilized in a gas turbine combine cycle (GTCC) to produce electricity. Also, steady models of this polygeneration system and a conventional subcritical coal-fired power plant were established and simulated using Aspen Plus. Technical and economic parameters, i.e., system efficiency, fixed capital, internal rate of return (IRR), and payback period, of polygeneration system and conventional power plant were calculated and compared. The uncertainty of the market fluctuation on the polygeneration system was also studied. Results revealed that, the system efficiency of the polygeneration system reaches43.20%,10%points higher than that of the power plant. Although the capital investment of the polygeneration system is higher than that of the power plant, it makes annual profit three times higher than that of conventional power plant based on the economic data in2011.The IRR and payback period of the polygeneration system reach24.07%and5.29years, respectively, better than conventional power plant. The results of uncertainty analysis showed that polygeneration system is more flexible than conventional power plant to withstand wild market fluctuation. The research results will provide reliable data for the establishment and commercialization of lignite pyrolysis-based staged conversion polygeneration system.