Research On Coal Partial Gasification-Based Staged Conversion Technology Co-Producing Char Gas Tar In One Fluidized Bed Reactor

The staged conversion technology based on coal partial gasification has become one of the best ways to efficiently and comprehensively utilize coal.Based on this,the coal partial gasification coproducing char,gas,and tar technology has been proposed by Zhejiang University.This process aims to obtain char,tar,and gas in one fluidized bed reactor.The char,recognized as a new material manufacturing clean briquette,can be applied to replace small-scale industrial or residential coal for achieving clean combustion.The tar can be utilized as liquid fuels after purifying and processing to alleviate China’s strong foreign dependence on petroleum.The gas can be flexibly utilized according to its components.Herein,the experimental study of coal partial gasification with air is carried out on a bench-scale atmospheric fluidized bed reactor,which provides a basis for further commercial units’design,development,and operation.The experimental gasification system consists of a fluidized bed reactor,gas feeding device,coal feeding device,gas-solid separation device,induced draft fan,and DCS control system.The gasifier furnace,surrounded by electric heating wires,has an inner diameter of 80 mm at the bottom and 120mm at the upper,with a height of 2600 mm.Firstly,the experiment is carried out with the char obtained from the partial gasification of typical bituminous coal as bed material at the partial gasification temperature below 800℃.The feasibility of the process of coal partial gasification co-producing char,gas,and tar is verified.Based on this,experimental studies on the effects of different reaction conditions on the coal partial gasification characteristics are carried out.The effects of（ⅰ）equivalence ratio（0.06,0.08,0.10,0.11,0.13,corresponding to partial gasification temperature 625℃,643℃,695℃,736℃,762℃,respectively）,（ⅱ）raw coal particle size（0.3-0.9 mm,0.9-2 mm）,and（ⅲ）coal properties（Gansu Huating bituminous coal,Xinjiang Hami long flame coal）on the yield of coal partial gasification products and the characteristics of each product are investigated employing quartz sand as the bed material.Firstly,the experimental results under different equivalence ratios show that as the equivalence ratio increases from 0.06 to 0.13,the share of coal oxidation reaction keeps rising due to the increment of oxygen content in the furnace,which could release more heat to instigate an increment in gasification temperature from 625℃to 762℃.The lower calorific value of gas reaches the maximum value,6.14 MJ/Nm³,at a temperature of 625℃.Higher gasification temperature leads to the continuous decrement in char yield,increment in carbon conversion rate and coal conversion rate.The characteristics of char are performed by a specific surface area and mesoporous analyzer,Raman spectroscopy,Fourier Infrared spectroscopy,and thermogravimetric analyzer.The results show that at temperatures below 640°C,increasing the equivalent ratio will promote the formation of oxygen-containing functional groups and aromatic compounds in the char,and disorganize the microstructures of char.At high temperatures,the combustion performance of char gets worse,whereas the pore structures in char become abundant.The tar yield reaches the maximum value at the gasification temperature of around 736℃.Moreover,from the analysis of tar column chromatography,it was found that the non-hydrocarbon and asphaltene content account for 77.33-87.81%of the total amount of tar,resulting in a more difficult tar processing.Secondly,the experimental results of partial gasification of Gansu Huating coal with different particle sizes（0.3-0.9 mm and 0.9-2 mm）under gasification temperatures of 625°C,695°C,736°C,and 762°C are analyzed,for clarifying the effects of particle size of raw coal.The trend of reaction temperature affecting the variation of partial gasification product yield of raw coal with different particle sizes is similar.There is a slightly lower gas yield and higher lower calorific value of gas,obtained from partial gasification with 0.9-2 mm Huating coal,than that of 0.3-0.9 mm Huating coal.The lower calorific value of gas reaches as high as 6.70 MJ/Nm³ obtained from larger particle size Huating coal partial gasification at the temperature of 625℃.Compared with the 0.3-0.9 mm Huating coal,the 0.9-2 mm Huating coal obtains a higher char yield with higher fixed carbon content,while the volatile content is higher in the early stage of the reaction and then decreases significantly with the increment of temperature.Moreover,it’s found that the aromatic ring condensation is higher in the char obtained by large particle size coal partial gasification,while the carboxyl cleavage is lower,resulting in lower CO₂ generation in the gas.The combustion characteristics of the char are performed by the thermogravimetric analyzer.The char obtained by0.9-2 mm Huating coal partial gasification has a higher combustion rate after ignition and better combustion stability.The tar yield reaches the maximum at the temperature of around 736℃.Meanwhile,the tar yield obtained from 0.3-0.9 mm Huating coal partial gasification is higher under the four reaction conditions than that of 0.9-2 mm Huating coal.From the results of tar chromatography column,it is found that the raw coal particle size has less effect on the distribution of tar components,but the high temperature promotes the cracking of heavy components in the tar of raw coal products with 0.3-0.9 mm particle size higher than that of 0.9-2 mm raw coal.It is attributed to the less sufficient contact between 0.9-2 mm raw coal and gasification agent,which is not conducive to tar cracking.Thirdly,the different types of raw coal（Gansu Huating bituminous coal and Xinjiang Hami long-flame coal）are adopted for partial gasification experiments.The variation of gas,char,and tar yields and components,structural characteristics,combustion characteristics of char,and the tar components are explored under the partial gasification temperatures of 625°C,643°C,695°C,736°C,and 762°C.With the increase of the gasification temperature,the variation trend of gas components yield is similar,but the combustible components in the gas obtained from the Hami coal partial gasification are higher compared with that of Huating coal.The lower calorific value of gas reaches as high as 9.45 MJ/Nm³ obtained from Hami coal partial gasification.The char yield gradually decreases as increasing the gasification temperature.Compared with Huating coal,the fixed carbon content of char from Hami coal is higher,and the volatile content is less.Moreover,the specific surface area of the char obtained from Hami coal partial gasification is larger.The combustion performance of the char obtained from Hami coal partial gasification is better,indicating that the combustion performance of char not only depends on its fraction but is also closely related to its pore structure.The tar yield,obtained from Hami coal partial gasification,reaches a maximum value of 11.99%at 695°C and is higher than that of Huating coal.Meanwhile,there are some differences in the distribution of tar fractions obtained from partial gasification of different coal types.Compared with Huating coal,there are more light fractions in the tar obtained from Hami coal,accounting for 35.93-42.86%.In sum,the feasibility of the process of coal partial gasification co-producing char,gas,and tar is verified through the experimental study of coal fluidized bed partial gasification.（ⅰ）The yields of gas,char,and tar,（ⅱ）the pore structures,carbon morphology,surface functional groups,and combustion characteristics of char,and（ⅲ）the composition and precipitation characteristics in tar are obtained during the partial gasification temperature range of 625-762°C.The differences in the effects of（ⅰ）air equivalent ratio and gasification temperature,（ⅱ）raw coal particle size,and（ⅲ）coal type on the distribution and characteristics of coal partial gasification products are investigated to provide a theoretical basis for the practical commercial process design.