Co-pyrolysis coupled with torrefaction enhances hydrocarbons production from rice straw and oil sludge: The effect of torrefaction on co-pyrolysis synergistic behaviors

[Display omitted] •Oil sludge (OS) addition improved oil quality of rice straw (RS) co-pyrolysis.•Alkanes and olefins in oil increased while oxygenates generation was inhibited.•Torrefied RS also enhanced alkane production due to re-polymerization of hydrocarbon intermediates.•Co-pyrolysis mechanisms were proposed focusing on synergetic interactions. Pyrolysis of biomass offers a promising pathway to alleviate the depletion of fossil energy. Nonetheless, the high oxygen content in biomass-derived pyrolysis oil makes bio-oil a low-quality by-product and limits its commercial application. On the other hand, oil sludge (OS) is hazardous waste from the petroleum industry and is difficult for combustion treatment due to its high ash content. In this work, torrefaction of rice straw (RS) and its co-pyrolysis with oil sludge (OS) was performed to enhance the production of hydrocarbons. Influences of torrefaction on altering the synergistic effects during co-pyrolysis were investigated. Intensified torrefaction (from 200 to 300 °C) of RS gradually resulted in lower volatiles, higher ash content, low crystallinity, enhanced surface aromaticity, and improved bio-oil quality. Consequently, pyrolysis of torrefied RS (TS) yielded more char at the expense of volatiles. The addition of OS into the co-pyrolysis of RS promoted char conversion efficiency and yielded a positive synergistic effect on gas production by 0.97–5.40%. The co-pyrolysis significantly enhanced the formation of alkanes (11.22–23.84%) and olefins (2.33–4.48%), while suppressing the generation of oxygenates (13.71–26.54%) in oil. On the other hand, severe torrefaction of RS shifted the main temperature range of decomposition close to that of OS, leading to an intensive radical reaction between OS-derived hydrocarbon radicals and lignin derivatives evolved from TS. For example, a positive synergistic effect on oil generation was observed when OS was blended into TS obtained at 250 °C (3.22% increase). However, intensified torrefaction weakened the synergistic formation of hydrocarbons, although the re-polymerization of hydrocarbon intermediates from OS and alkyl radicals from TS contributed to alkane production, especially heavy-weight straight-chain alkanes. Moreover, torrefaction promoted the product energy yield and the total energy efficiency of individual pyrolysis of RS and co-pyrolysis of RS and OS. By comparisons between RS and TS focusing on synergetic interactions, this work proposes co-pyrolys