Mechanism of cellulose pyrolysis and tar decomposition in a fixed bed reactor

Cellulose pyrolysis was studied in a fixed bed reactor at 500~900 ℃; the tar and gaseous products were analyzed by GC-MS and gas chromatograph, respectively. The results showed that the yield of gaseous products increases and that of tar and char decreases with the increase of pyrolysis temperature; the yield of CO, CH₄ and H₂ is increased significantly, while the yield of CO₂ is almost not altered. Tar is generated by the secondary reactions, while the non-condensable gases are generated by both the primary and secondary pyrolysis products. The mechanism of tar decomposition was considered through calculation with Gaussian 09, which suggests that cellulose is cleaved into cellulose monomer at the beginning of pyrolysis process. The hydroxy functional groups are removed from cellulose monomer preferentially, and then the intermediates are reformed into tar. With the increase of temperature, ethers, alcohols, acids and other compounds in tar are decomposed into free radicals; the amounts of alkenes, alkynes and non-condensable gases formed are then increased because of the reformation of free radicals at high temperature.