Kinetics Of Low Rank Coal Pyrolysis And Co-Pyrolysis With Biomass And Deep Processing Of Pyrolysis-derived Products

China is rich in coal resource,but the proportion of low rank coal is high.The efficient and clean utilization of low rank coal is now becoming the focus of the government and researchers.Based on the characteristics and the structure of the low rank coal,the integrated utilization of medium-low temperature pyrolysis is one of the efficient and economical ways for the conversion and utilization of it.The research on the efficient utilization of low rank coal pyrolysis and deep processing of pyrolysis products was carried out in this paper.The quantitative description of coal pyrolysis are expected to help to understand the physical and chemical phenomena occurred in a coal particle in the process of pyrolysis since it is the first step and also the most important step in the reactor design and amplification.Firstly,a comprehensive and systematic study on the fundamental pyrolysis behaviors of a single coal particle was performed in this study.The kinetic parameters in DAEM were obtained by model fitting and the isoconversional method was employed to provide the initial values.Based on the obtained kinetic parameters,a comprehensive single particle model coupling with heat transfer and reaction was established to characterize the detailed chemical and physical phenomena occurred within a coal particle in a fluidized bed.The simulation results indicated that the heat transfer and pyrolysis reaction were strongly coupled with each other.As such,for a large particle,the inherent heat-transfer resistance weakened the heating-up rate,extended the time to the final temperature and thus prolonged the devolatilization time.For a coal particle with D=3 mm,more than 300 K temperature difference was found in the particle and the reaction followed the shrinking core mechanism,resulting in a complete pyrolysis time as long as 6.2 s,which was far longer than the pyrolysis time(0.35 s)of coal particle with D=0.3 mm following the volume reaction mechanism.Such a particle model should be very helpful for the industrial reactor simulation.In order to further improve the simulation accuracy of low rank coal pyrolysis dynamics and reflect the nature of the low rank coal pyrolysis,a three-Gaussian-DAEM-reaction model(3-DAEM)was firstly applied for the pyrolysis kinetics study of four low-rank coals based on their non-isothermal thermogravimetric data.The obtained TG,DTG and DDTG curves indicated that the pyrolysis processes of different coals can be divided into three stages,which mainly correspond to the breakage of weak bonds,the primary pyrolysis and a further gas production from the char condensation and cross-linking reactions.The curve fitting results of DTG curves by 3 sub-curves using peakfit software also proved the above.Thereafter,the TGA data were processed using 3-DAEM to extract the kinetic parameters for the pyrolysis process.The mean activation energies(Eo)in three stages for four low-rank coals with different standard deviations(?E)were obtained.The results indicated that the modelling results using obtained kinetic parameters well matched over the entire range of experimental data.It provided a good methodology to study the pyrolysis process for understanding the pyrolysis mechanism,and the obtained kinetics parameters can be further used for process simulation of coal pyrolysis.Secondly,because the coal is a non-renewable energy resources,based on the similarity of low rank coal and biomass,the co-pyrolysis of low rank coal and biomass was studied in this paper.The co-pyrolysis behavior of two low rank coals(Inner Mongolia Xinghe coal,Xiao longtan coal)and three common biomass(Straw,Sunflower,Apple tree branches)was performed by non-isothermal thermogravimetric analysis and fixed bed experiments.The results show that whether the co-pyrolysis of biomass and coal exists synergistic effects was closely related to the type and the adding amount of biomass.In our study,among them,the effect of co-pyrolysis of Inner Mongolia Xinghe coal and Apple tree branches at the mixing proportion 2:1 was the most obvious coupling.The existence of synergistic effect was further proved by analysis of co-pyrolysis products in fixed bed.The analysis results of tar component show that the oxygenated compounds in tar increased and the amount of hydrocarbons decreased,that is,co-pyrolysis can increase the yield of tar but it cannot improve the quality of tar.Thus,it can be concluded that for a particular coal,looking for the suitable biomass is of great significance.Low rank coal pyrolysis or co-pyrolysis of coal with biomass can produce a large amount of char.Therefore,the study of char is also the foundation of clean,effective and high value-added utilization of coal.The gasfication of char with steam was carried out in a visual fluidized bed reactors.The results show that the introduction of O2 in the reactor system significantly improved the reactor temperature,increased dry gas yield,carbon conversion efficiency and changed the product distribution.The maximum LHV of the produced gas and H2 yield was obtained at ER of 0.2,too small or too large ER was unfavorable for fuel gas quality in the fluidized bed reactor.The introduction of steam promoted carbon conversion efficiency and H2 yield.In addition,the results of steam gasification of char by using 10%K2CO3 show that adding 10%K2CO3 catalyst in the char by impregnation method was beneficial to not only the main reaction,but also to the water gas reaction,thus significantly improved the H2 content in the gas product.Finally,upgrading of light tar obtained from pyrolysis was also performed in this study.Deoxygenation of pyrolysis tar is an important way to effectively use the tar.Since the pyrolysis tar contains high levels of fatty acids,which have low calorific value and corrosivity to the equipment,one of light tar components,formic acid,was chosen as the tar model compound and used for the catalytic experimental study over catalyst MoC1-X-?-Mo2N which was synthesized by soybean and ammonium molybdate.The catalyst Soy-Mo(0.1)prepared at a carbonization temperature of 750 °C,the weight ratio of the soybean powder to Mo precursor of 1:0.1 showed the best catalytic activity among those as-synthesized catalysts.Even at a temperature as low as 110 °C,HCOOH conversion reached above 80%with a 100%H2 selectivity and a long-term stability.XRD results and XPS data indicated that the optimal catalyst Soy-Mo(0.1)was composed of a catalytic a-Mo2C phase and an assistant y-Mo2N phase,which contributed a synergistic effect for its excellent performance to the formic acid decomposition.In addition,the presence of non-ignorable K in the catalyst,the uniform dispersion of molybdenum species,and the high basicity should be also responsible for the high activity and stability of catalyst Soy-Mo(0.1).Compared with these non-precious metal heterogeneous catalysts reported at present,the catalysts developed in this study had not only much higher catalytic activity at lower temperature but also higher H2 selectivity.Besides,the preparation of such a catalyst is simple and easy to scale up in practical applications.This work provides a good way for further deep processing of tar.