Simulation and Process Development of Moving-bed Coal Pyrolysis by Circulating Ash Heat Carrier

Author:Zhang Ya Qing

Supervisor:liang peng


Degree Year:2018





At present,coal is mainly used for direct combustion,which has problems such as low utilization efficiency and serious environmental pollution.The circulating fluidized bed(CFB)combustion/coal pyrolysis polygeneration technology uses circulating ash as solid heat carrier for the low-temperature pyrolysis of coal.Gas and tar products are obtained in pyrolyzer;pyrolysis char product and circulating ash returns to CFB boiler to provide heat and electricity.Therefore,heat,electricity,oil and gas can be realized in this technology.In this paper,some technical problems and basic scientific laws in the coal pyrolysis by solid heat carrier are studied.The effects of operation parameters such as different placements of coal and ash,ash/coal ratios,ash compositions,particle sizes and heating rates on the secondary catalytic of pyrolysis volatiles.Double stage experiments of coal combustion and pyrolysis are carried out.The influence of pyrolysis conditions and minerals on the morphology and distribution of sulfur in pyrolysis products as well as sulfur release in combustion process of semicoke are studied.Considering kinetics of coal pyrolysis and heat transfer mechanisms in the moving bed,a two-dimensional mathematical model of coal pyrolysis by circulating ash heat carrier is established.The model of C.Tien et used to calculate filtration efficiency of the lab-scale continuous granular bed.The effects of filter medium characteristics,filter medium diameter,bed height and filtration velocity on the dust collecting efficiency are predicted.The optimized process conditions,stability,regeneration and influence of dust deposition on catalyst activity were investigated when nickel-based catalysts are used as filter medium.Besides,the grading condensation of coal tar were preliminarily explored.Through the study of the above aspects,the main results are as follows:1.The increasing mass ratio of ash to coal and Fe2O3 content in ash,decreasing ash size,and lower rank coal can improve the secondary catalytic effect of coal volatiles.Circulating ash can increase the yields of total pyrolysis gas,CO2,and C2+,however decrease H2 yield.Although the increasing circulating ash makes lower tar yield,the quality of tar is improved.Circulating ash has similar secondary catalytic effect on the volatiles produced by slow and fast pyrolysis of coal.The components with carbon numbers of≥26 in tar can be upgraded,and then results in the growing yields of≤C25 components.In addition,circulating ash not only has a positive effect on cracking of oxygen-,nitrogen-and sulfur-containing bonds in asphalt,but also can catalyze olefin polymerization and improve tar saturation.2.The sulfur migrates to the gas phase increases with the increase of pyrolysis temperature and the decrease of coal particle size.The mass ratios of quartz sand to coal and heating rate have little influence on sulfur distribution.In devolatilization process of coal,CaO can not only react with sulfur compounds in gas phase,but also play a certain effect on the removal of sulfur in tar.Fe2O3 mainly affects the migration of sulfur in gas phase to solid phase,and the metallic oxide additives such as MgO,NaOH and KOH do not show obvious sulfur fixation effect.Both CaO and Fe2O3 additives inhibite the decomposition of sulphate sulfur during coal pyrolysis and SO2 release during the combustion of solid phase products.The inhibition effect of CaO was stronger than that of Fe2O3.From the view point of pyrolysis/combustion grading conversion,the sulfur fixing effect of CaO is lower than that of Fe2O3 in pyrolysis stage,but in combustion stage,the sulfur retention product CaS is more stable than FeS which it is fixed in ash.3.The pyrolysis mathematical model can not only predict the temperature distribution of pyrolysis gas,circulating ash and coal particle along the bed height,but also predict the escape of total volatiles and main volatile products.The devolatilization process has a transient delay compared with the heating process.The decreasing diameter of coal and ash,increasing mass ratio of ash to coal,moving bed velocity,preheating temperature of coal,and initial temperature of ash can accelerate pyrolysis devolatilization process.The escape of volatile products is mainly concentrated at 0.08~0.24m of bed height.The reasonable design of residence time in industrial moving bed by solid heat carrier is 3~4min.The model can predict the devolatilization behavior of different coal species.Because of the high pyrolysis activation energy of bituminous coal,its theoretical bed height required for devolatilization is higher than that of lignite.4.Dust with a particle size of 0~20μm is difficult to be captured by granular bed.From the perspective of whole polygeneration process,semicoke as a filter medium can be directly discharged into CFB furnace as a boiler fuel after discharging from the dust collector,so that regeneration process of the filter medium is omitted.Because dust adsorbed on tar is easily adhered to the surface of filter medium,the prediction value of filtration efficiency is slightly lower than the experimental value.The model prediction results of initial filtration efficiency of C.Tien can be used as a basis for selecting filter medium and designing parameters of granular bed.The activity,stability,and regeneration performance of nickel-based catalysts are greatly improved when impregnate with Ce/Zr additives.When simulating the real dust-contained volatiles,inert components and active metal oxides in dust will cover some active sites on the catalyst surface.After the introduction of dust,the decreasing order of the ratio of C/Al on catalyst surface is SiO2<Blank<MgO<Fe2O3,which is in accordance with gas yields.5.The grading condensation of coal pyrolysis volatiles is a very complex process of physical transformation,which is closely related to the condensation temperature and volatiles composition.The fraction may still be condensed when volatiles temperature is greater than the dew point of a certain fraction.In the fraction condensation of tar,asphalt in tar can reach 90.99wt%,the highest content of light oil can reach 82.21wt%.With the decreasing of outlet temperature of volatiles,light oil,phenol oil,and naphthalene oil gradually increase in the first stage,while alsphalt gradually reduces.When the volatile outlet temperature of stages 1,2,3,and 4 are 360,230,1 70,and 0℃,respectively,components with the highest relative content are anthracene,3,5-xylenol,naphthalene,and toluene.With the stage decreasing of condenser temperature,the relative content of mono aromatic hydrocarbons increase from 8.05wt%to 54.00wt%,and the relative content of polycyclic aromatic hydrocarbons decrease from 52.41wt%to 15.22wt%.