Investigation into role of CO₂ in two-stage pyrolysis of spent coffee grounds

As a way of improving process efficiency of pyrolysis of waste biomass, the effect of carbon dioxide (CO₂) on pyrolysis of spent coffee grounds (SCGs) was examined using a two-stage pyrolysis reactor consisting of a region with increasing temperature and an isothermal region. It was experimentally validated that CO₂ accelerates thermal cracking of organic compounds formed during the pyrolysis of SCGs. The expedited thermal cracking attributed to employing CO₂ in pyrolysis of SCGs led to changing pyrolytic products in gas, liquid, and solid phases. The production of gaseous carbon monoxide was increased when using CO₂ as the pyrolysis medium. In liquid pyrolytic products, the formation of phenolic compounds was hindered in the CO₂-assited pyrolysis. Biochar morphology (solid pyrolytic product) was also changed with different pyrolysis environments. This study shows that CO₂ can help improve applicability of pyrolysis of waste biomass by modifying three phase pyrolytic products in a two-stage pyrolyzer.