Exploratory Experimental Study On Coal Partial Gasification In Circulating Fluidized Bed

Because of the mild technical characteristics of low temperature and low pressure in the partial gasification of circulating fluidized bed(CFB),the production of tar is inevitable.The existence of tar will cause many problems,so there are strict regulations on tar content of syngas.The tar content of gasification power generation system is required to be 20-50 mg/Nm~3,and that of internal combustion engine system is required to be less than 10 mg/Nm~3.When the temperature is reduced to a certain degree,for example,250°C,the heavy components in tar will condense quickly,and it is easy to bond with water,coke powder,ash and other substances,block pipes and valves,corrode metal equipment,cause poisoning of downstream catalysts,and bring secondary pollution problems such as phenol water,which seriously affects the stable operation of the gasification system.At the same time,because of the instability and complex composition of tar,the nature of tar from different sources is very different,tar has become an important factor restricting the development of industry.Most of the researches on tar catalytic removal are based on the small fixed bed with external heating,which lacks the experimental data of fluidized char and real gasification process.Due to the limitation of two-stage reactor,there is no research on catalytic cracking of different tar content.In this work,the real tar was added to the reduction zone of the CFB char gasifier,and the catalytic conversion of tar occurred in the reducing atmosphere.In this experiment,the pyrolysis and catalytic conversion of tar in char were studied to explore the catalytic conversion characteristics of tar.Compared with the catalytic conversion of tar,the effective gas conversion rate(EGCR)of pyrolysis was lower,and there was no CO2 produced in this process.When the temperature increased from 850 to 950°C,the EGCR increased from 60.84%to 94.26%.The increase of tar content will aggravate the production of carbon deposition,which will cover the active sites of char and lead to the decrease of its catalytic activity.There were great differences in the reaction routes of tar pyrolysis and catalytic conversion,which revealed the reason why the carbon deposition of tar pyrolysis was greatly reduced in catalytic conversion.At 950°C,the appearance of char increased the EGCR from 42.00%to 94.26%,which indicated that fluidized char could inhibit the condensation reaction and carbon deposition of tar,thus improving the EGCR,which proved the feasibility of using char catalyst to remove tar.The quantitative experimental study on the catalytic conversion characteristics of the fluidized-bed char was carried out.The catalytic conversion characteristics of the fluidized-bed char to tar under different reaction conditions(temperature,char particle size,steam)were determined.The three-phase products of gas,solid and liquid were analyzed in detail.(1)The efficiency of tar conversion(ETC)increased from 91.66%to 97.92%with temperature(850 to 950°C).With the increase of temperature,the increment of ETC decreased,which was due to the change of catalytic performance of char at different temperatures.That made the proportion of tar involved in thermal cracking and heterogeneous reforming changed.At the same time,the content of heavy polycyclic aromatic hydrocarbons in residual tar decreased from 850 to 900°C,and increased from 900 to 950°C.Under the experimental conditions,the catalytic conversion performance of char showed certain selectivity to different tar components.The temperature sensitive components included indene and anthracene,while the temperature insensitive components included naphthalene and biphenylene.(2)The catalytic conversion performance of char depended on the change of particle size to change the structure of char.For the three kinds of fluidized char,the0.1-0.5 mm one had the highest ETC,98.89%.It could be seen from the analysis of tar components that the three kinds of char with different particle sizes had significant conversion effect on heavy polycyclic aromatic hydrocarbons.At the same time,the conversion rate of each component in tar was related to the particle size of char.Under 0.1-0.5 mm,the best conversion effect was obtained,which could significantly reduce the number of components and the content of polycyclic aromatic hydrocarbons in the tar,and made the residual tar convert to lighter aromatics;under0-0.1 mm,the ETC is the worst.(3)After adding the steam,the ETC of was significantly improved.Under the test conditions(at 900°C),the ETC increased from 96.70%to 99.81%after 2 kg/h steam was added,and the tar content in the syngas decreased from 530 mg/Nm~3 to 32mg/Nm~3.After feeding steam,the pore structure of char was obviously developed,and the pore distribution was also improved.At the same time,the content of active metal on the surface has been improved to some extent,which reflected the improvement of catalytic performance.On the other hand,the content of light aromatic hydrocarbons and other hydrocarbons increased,while the content of polycyclic aromatic hydrocarbons decreased,and a variety of polycyclic aromatic hydrocarbons were completely transformed.