Abstract This work mechanistically investigated the influence of CO 2 in co-pyrolysis of sub-bituminous coal and main constituents of ligno-cellulosic biomass (cellulose and hemicellulose). Our experimental work identified the crucial role of CO 2 in co-pyrolysis of coal and biomass. For example, CO 2 not only enhanced the thermal cracking behavior of VOCs evolved from the thermal degradation of a carbonaceous solid sample ( i . e ., sub-bituminous coal, cellulose, and xylan) via blocking the addition reaction, but also directly reacted with VOCs and CO 2 . The genuine effects induced by CO 2 led to a significant reduction of condensable hydrocarbons ( i . e ., tar), which directly lead to a significant enhancement of syngas production and modification of ratio of CO to H 2 : the ratio of CO to H 2 was increased approximately ∼1200% at 680 °C in pyrolysis of coal in the CO 2 environment and the ratio of CO to H 2 was adjustable by means of using a different amount of CO 2 during the pyrolysis process of carbonaceous samples. Furthermore, the identified role of CO 2 would be applicable to the in-situ air pollution control in various industrial applications, such as steelworks. Lastly, the identified role of CO 2 in pyrolysis will be applied in the gasification process since pyrolysis is the intermediate step for the gasification process.