Product Distribution And Characterization Of The Low-rank Coal Pyrolysis In Fluidized Bed

Low-rank coal is abundant in China, which accounts for about50%of thenational total coal reserves. The low-rank coal is an ideal chemical and metallurgicalmaterial because of its high volatile and chemical reactivity. Currently low-rank coalis commonly combusted and gasified which doesn’t take full advantage of its highvolatile. This study is regarding to low-rank coal pyrolysis to upgrade the quality anddevoted to investigating reaction conditions and product distribution for low-rank coalpyrolysis in an experimental integrated fluidized bed to get high-quality char (calledLantan in China), tar and gas. Meanwhile, the mechanism of improving the quality oftar and gas using the multi-stage bed pyrolysis of coal is also investigated, the resultsof which provide the basic data for establishing the new technology of the low-rankcoal pyrolysis and gasification. The main research contents and conclusions are asfollowing two parts:The product distribution and characterization in integrated fluidized bed forlow-rank coal pyrolysis and gasification were studied in the first part. The influencesof excess air ratio (ER), gasification temperature in fluidized bed, pyrolysistemperature in transport bed, residence time and coal feeding position on productdistribution and properties in integrated fluidized bed were investigated to aim atrevealing the requirement conditions to produce high quality char product andco-produce tar and gas. The following results were obtained.(1) Products distribution:The yield of char and tar decreased while the gas yield significantly increased byelevating ER. That is because oxygen can react with carbon, tar and combustible gasto produce CO2, CO and H2O. The high-temperature of fluidized bed promoted thesecondary cracking of tar and the char in transported bed had the catalytic effect on tar,both of which led the of yield of H2and CH4increasing. The yield of tar firstlyincreased and then decreased with the transport bed temperature elevating while theyield of char decreased and the gas yield increased. As the residence time increasing,secondary cracking of tar became obviously, which increased the yield of gas.(2)Process conditions: When the fluidized bed temperature is850℃(900℃and950℃), residence time is180s, excess air ratio is0.15(0.2), the fixed carbon content inthe char collecting at the overflow of the fluidized bed was all above82%.(3) Tarcomposition: Light tar fractions first increased and then decreased with ER elevating,while the maximum value was reached at ER=0.15. Both long residence time and high temperature were helpful for heavy tar fractions cracked into light tar fractions.The mechanism of improvement on the quality of tar and gas for multi-stage bedpyrolysis of coal was studied in the second part. A two-stage fixed bed reactor wasused to simulate the three-stage bed pyrolysis of coal for implementing the pyrolysissuccessively at low, medium and high temperatures of450℃,650℃and900℃.The interaction of pyrolysis reactions between two adjacent stages, i.e. between thelow-and medium-temperature pyrolysis in the upper two stages and between themedium-and high-temperature pyrolysis in the lower two stages, was investigated toreveal the mechanism of improvement on the quality of tar and gas for multi-stagebed pyrolysis of coal. It was found that the interaction between the low-andmedium-temperature pyrolysis increased the light fractions in tar (boiling pointsbelow360℃) and CH4content in gas, while it decreased the H2content in the gas.This indicates that the tar quality improvement via the interaction between the low-and medium-temperature pyrolysis in the upper two stages is mainly due to the in-situupgrading of tar catalyzed by char. The interaction between the medium-andhigh-temperature pyrolysis increased the tar yield and light tar fractions, while itelevated the CH4and CO contents and decreased the H2content in the pyrolysis gas.These show that it is the H2-rich atmosphere that improved the yield and quality of tarby the interaction between the medium-and high-temperature pyrolysis. As aconsequence, the multi-stage bed coal pyrolysis realized mainly the in-situ upgradingof tar under catalysis of char and the H2-rich atmosphere of pyrolysis that can improvethe quality of the pyrolysis oil and gas.