Vietnam's Power Development Plan 7A authorized many new coal power plants projects, implying an increase of greenhouse gases emissions from 90 MtCO2eq/year today to 360 MtCO2eq/year in 2030. How could co-firing technology-that is the partial substitution of coal by biomass-contributes to mitigate that problem? In this study, we assess the costs and potentials of co-firing rice residues in present and planned coal power plants in Vietnam using a spatially explicit optimization model: BeWhere, adapted as recursive annual dynamic. We found that, the cost of CO2 emissions is the key parameter determining at what level the technology is used. A cost of CO2 emissions of 8 $/tCO2 mobilizes the maximum technical potential of the rice straw and husk domestic resource, with an annual emission reduction of 28 MtCO2eq/year by 2030. At this level, biomass co-firing contributes to an 8% emission reduction in the coal power sector with the abatement cost of 137 Million USD.