Fundamental Studies on the Pulverized Coal Pyrolysis and Gasification Based on Y-type Entrained Flow Gasifier

Author:Fan Jun Feng

Supervisor:tian yuan yu


Degree Year:2017





The existing entrained flow gasifier has many disadvantages,such as the unreasonable distribution of flow field,short residence time of solid,small adjustment range of operation load,etc.In order to solve the above shortcomings and improve the efficiency of gasifier,a new type of coal gasifier named Y-type coal water slurry gasifier was developed on the basis of coal water slurry fluidized bed.The single channel foam nozzle in the top of furnace adopted a downward injection structure.The single channel foam nozzle in the side of furnace adopted a micro swirl horizontal jet structure.The impingement zone and recirculation zone of the gasifier was optimized.Therefore,the Y-type coal water entrained flow gasifier had not only the advantages of high efficiency,but also decreased the wear rate of the nozzle,overcame the disadvantages of unreasonable flow field in the furnace,increased the residence time of coal slurry coke in the furnace,promoted the turbulent flow in the furnace,the heat transfer,mass transfer and reaction of coal slurry and gasification agent more fully.The pyrolysis was the first step of gasification reaction process,this study selected five kinds coal with different coal rank and their chemical family composition to carry out the high-temperature fast pyrolysis experiment.Combined with thermogravimetry-online infrared(TG-FTIR)and fast pyrolysis coupled with gas chromatography time-of-flight mass spectrometry(Py-GC/TOF-MS),the compositions of light volatiles were online analyzed.The Friedman model and Flynne-Walle-Ozawa power(FWO)model were used to carry out the kinetic research of different coal species in the high-temperature fast pyrolysis.To find out the most possible reaction mechanism,Coats-Redfem single reaction kinetics model was used to analyze pre-exponential factor and activation energy of the reaction.To explore the gasification process of pyrolysis products,the volatile products from the gasification process of char were classified into alkanes,cycloalkanes,aromatics,sulfur compounds,chlorinated compounds,and nitrogen compounds.Fluent simulation of Y-type coal water slurry gasifier was performed to study flow field and reaction transformation.Combined with the experimental data,analysis and optimization of the structure of Y-type coal water slurry gasifier followed with high fitting degree results was obtained.The optimum operating parameters were determined.Through the above research,the main conclusions were as follows:1.The smaller the particle size of Jining bound bituminous coal was,the more difficult the pyrolysis weight loss was,and the more detrimental to the escaping of volatile products(methane,C2+,ethylene and light aromatics).The pyrolysis process of Yichun bituminous coal with high volatile showed a trend contrary to that of Jining bituminous coal.The study found that the heating rate was exponential positive related with Tin,Tmax,and Tf.The values of Rmax and Di increased monotonically with the heating rate.The oxygen-containing products such as CO2 and CO exhibited three escaping stages.The formation of methane and light aromatics was divided into two stages,while C2+ alkanes and ethylene contain only one escape phase.Pyrolysis kinetics results showed that the Friedman and FWO models could well describe the pyrolysis process of Jining coal with different particle sizes,and the apparent activation energy increaseed with the pyrolysis reaction.The most suitable reaction mechanism fitted by Coats-Redfem kinetic method was two-order chemical reaction.The results of Model free method and Coats-Redfern method indicated the more fine-grained composition of Jining coal showed greater reaction energy barrier.2.The formation of volatile substances in the rapid pyrolysis of Jining coal and its seven chemical family groups followed the trend of saturation fraction>aromatics>pectin>colloidal resin>asphaltene>carbene>raw coal>coke.The relative contents of alkanes,phenols,ketones and ethers in the fast pyrolysis volatiles of Jining coal decreased with the increase of pyrolysis temperature.Alkanes were mainly composed of C3 to C19 with linear and branched structures,in which the content of propane was highest.Naphthenic hydrocarbon/unsaturated aliphatic hydrocarbon obtained from different temperatures had the carbon number in the range of 3-18.Aromatic compounds mainly distributed in the carbon number of 6-15.Phenolic compounds mainly distributed in the carbon number of 6-10.The saturation fraction was alkanes and alkenes/alkynes in the pyrolysis products.The main contribution of aromatics was the generation of aromatic hydrocarbon.The pyrolysis products of pectin were mainly composed of ketone,while the heavy resin could generate most naphthenic/cyclic olefin.Ether species had highest content in the pyrolysis of asphaltene.Nitrogenous compounds in pyrolysis of carbene had the highest relative content.Phenolic species content generated from the fast pyrolysis of coke showed the highest.3.The increasing heating rate of coke preparation increased the porosity of coke,but when the preparation temperature was higher than 900℃.The results of SEM showed that Huainan coal coke was rich in pore structure,its structure was loose and the surface contained many pores and fissures.The pore structure of Yichun coal coke was scarce and its surface structure was compact.In addition,the XRD results showed that the higher the temperature of the coke was,the higher the packing height and the dimension of the crystallite structure were.The steam gasification reactivity of coal coke was in the order of Huainan>Liupanshui>Yichun>Kuangou.When gasification at low temperature,with the increasing final temperature of coke,the activity of gasification decreased,while the gasification at 1000℃ showed the opposite trend.Volume model,unreacted condensation model and random pore model could describe the gasification process of coal coke in different extent,but the reaction model had the highest degree of compliance.With the increase of pyrolysis final temperature,the apparent activation energy of the gasification reaction increased gradually.4.The simulation results of Fluent showed that the Y-type entrained flow gasifier had a relative uniform temperature distribution,a large proportion of the high temperature range and a higher effective gas(H2+CO)content.For the velocity distribution of pulverized coal gasifier,the fine coal particles coupled with the gasification agent stream were introduced into gasifier through the nozzle.Then the axial velocity and the swirl intensity increased,the turbulence of material impact zone was intensified,the diffusion was strengthened,the heat and mass transfer were intensified.The internal temperature distribution of furnace was caused by the severe combustion gasification reaction of pulverized coal and gasification agent.High temperature region was mainly in the boundary of the jet and logistics impact.The region had eddy current region and swirling jet region,which made the furnace temperature distribution uniform,increased the gasification furnace effective reaction volume,and prolonged the residence time of coke particles.For the concentration distribution of synthetic gas components in gasification furnace,CH4 and CO2 were in the main jet area.CH4 was mainly produced during coal pyrolysis,CO2 were mainly produced by the intense combustion reaction.With the decrease of gasifier bed height,the concentrations of CH4 and CO2 decreased dramatically,and the CH4 concentration near the gasification furnace outlet was basically zero,CO2 about 6 vol%.The concentrations of CO and H2 at the outlet of nozzle were relatively low,with the decrease of bed height of the gasifier,the concentration of CO and H2 increased rapidly.This was due to the intense gasification reactions of pulverized coal particles and H2O to generate a large number of CO and H2.Because the boiler swirl and eddy current in the furnace contributed the more uniform concentration of CO and H2.The concentration of CO and H2 increased along the gasifier height and remain unchanged when it reached a certain value.