Solar pyrolysis of cotton stalks: Combined effects of torrefaction pretreatment and HZSM-5 zeolite on the bio-fuels upgradation

Abstract

Solar pyrolysis of biomass is a novel method for bio-fuels production. In this study, cotton stalks were first subjected to solar torrefaction at 200–300 °C, and then subjected to solar catalytic pyrolysis at 550 °C. The effects of torrefaction deoxygenation pretreatment and HZSM-5 zeolite on the bio-fuels (gas, biochar, and bio-oil) upgradation was investigated. Results showed that the elements of cotton stalks removed via torrefaction follow the order of deoxygenation > dehydrogenation > decarbonization. Torrefaction pretreatment has a remarkably influence on the subsequent catalytic pyrolysis. The bio-oil and gas yields were considerably reduced while the biochar yield increased with increasing torrefaction temperature. The primary pyrolytic volatiles cracked under the HZSM-5 catalyst, followed by a series of deoxygenation and aromatization reactions to form H2O, CO2, CO, alkanes, olefins, and aromatic hydrocarbons. Consequently, the oxygen-containing functional groups were removed to generate hydrocarbons that were rich in bio-oil, meanwhile the selective yield of benzene, toluene, and xylene (BTX) increased remarkably. Moreover, the torrefaction pretreatment inhibited the formation of coke deposited on the catalyst and promoted the carbon yield in the biochar. Considering the benefits of electricity savings, the approach proposed herein is a potential way to improve the competitiveness of biomass-derived products.