Oxy-MILD (Oxygen Moderate and Intense Low-Oxygen Dilution) combustion is one of the most promising technologies for the mitigation of CO2 emissions from coal-fired furnaces, benefiting from its good performance in flame-temperature controlling and NOx reduction. Under oxy-MILD mode, the combustion or co-firing of biomass (CO2-neutral) can achieve "negative CO2 emissions". In this paper, oxy-MILD biomass co-firing is numerically studied by CFD modeling for the IFRF furnace NO.1, where Guasare coal and Olive waste are co-fired under air-MILD and oxy-MILD conditions, respectively. The effects of biomass co-firing ratio (0-30%, energy basis) and atmosphere on the temperature and heat flux distribution, and NOx emissions are discussed. The modeling results show that under MILD combustion mode, both oxy-combustion and biomass co-firing can generate a more moderate temperature distribution and lower NOx emissions than air-combustion and coal combustion, respectively. When biomass co-firing ratio increases from 0% to 30%, under oxy-MILD combustion mode, the peak temperature linearly decreases by 28 K and the NOx emissions decrease by 141 ppm; while under air-MILD combustion mode, the peak temperature increases by 15 K and the NOx emissions decrease by only 73 ppm. This modeling work suggests that oxy-MILD biomass co-firing is a more promising technology to achieve "negative CO2 emissions" in coal combustion, with lower furnace temperatures as well as NOx emissions. (C) 2019 Elsevier Ltd. All rights reserved.