In this article, a co-gasification system blending coal and biomass is examined in the Aspen Plus environment in terms of energy conversion efficiency (ECE) and exergy efficiency (EE). Although the percentage of raw wood (RW), torrefied wood (TW) and coal in steam co-gasification is one of the most important parameters that affect the gasification process, the addition of CO2 could effectively improve ECE and EE while the steam-to-carbon ratio (S/C) is adjusted at the carbon boundary point (CBP). The optimum operating conditions such as S/C and CO2 supply ratio, are determined by solving a series of constrained optimization algorithms for maximizing ECE and EE. A combined heat and power (CHP) system using the maximum waste heat recovery and Rankine cycle is illustrated to assess performance in terms of power generation and CO2 utilization. The results show that the total power generation by feeding the TW-based fuel blend of 40 wt% TW and 60 wt% coal is increased by 8.43%, as compared to that of the RW-based fuel blend of 40 wt% RW and 60 wt% coal. Compared with 100 wt% coal fuel, the TW-based fuel can significantly reduce CO2 specific emission by 38.23%. (C) 2015 Elsevier Ltd. All rights reserved.