Simultaneous production of CH4-rich syngas and high-quality tar from lignite by the coupling of noncatalytic/catalytic pyrolysis and gasification in a pressurized integrated fluidized bed

Highlights: • An integrated fluidized bed consists of a transport bed pyrolyzer and a fluidized bed gasifier. • Coupling pyrolysis and gasification of lignite benefits CH₄ and tar production. • The contributions to CH₄ from pyrolysis and gasification are relevant to pressure and Ca(OH)₂ catalyst. • Pressure and Ca(OH)₂ catalyst contribute to tar upgrading. - Abstract: An integrated fluidized bed (IFB) reactor with a two-stage configuration has been developed to process small-size coal particles for the simultaneous production of CH₄-rich syngas and high-quality tar, in which coal pyrolysis occurred in an upper transport bed (TB) within an atmosphere mixing steam and syngas generated by steam-oxygen gasification of coal or char in a lower fluidized bed (FB). The coupling effect was investigated by combining non-catalytic/catalytic pyrolysis and gasification in a pressurized IFB. Having TB pyrolysis obviously raised CH₄ yield (also its content in producer gas) and tar yield. The respective contributions to CH₄ formation from the TB pyrolysis and FB gasification were mainly relevant to operating pressure and Ca(OH)₂ catalyst. Raising pressure facilitated CH₄ formation. When lignite was treated without catalyst, the contribution from TB pyrolysis was greater than that from FB gasification as hydropyrolysis was intensified by pressure in H₂-rich gas atmosphere from the FB gasifier. Adding catalyst into lignite reversed their contributions. The FB catalytic gasification formed more CH₄ because Ca(OH)₂ functioned well as a catalyst for CH₄ formation in pressurized gasification. With elevated pressure or/and the addition of Ca(OH)₂, pyrolysis tar yield decreased in different degrees but its quality was improved. With the combination effect of pressure and Ca(OH)₂, producer gas from the tested IFB reactor was highly rich in CH₄ to reach 10.8 vol%, with as well a high H₂/CO ratio of 2.3 that is suitable for making SNG. The obtained tar had its light tar content as high as 95 wt%.