The co-utilization of coal and biomass in large power units is considered in many countries (e.g. Poland) as fast and effective way of increasing renewable energy share in the fuel mix. Such a method of biomass use is especially suitable for power systems where solid fuels (hard coal, lignite) are dominating. On the other hand, the admixture of usually wet biomass to the main fuel impacts the steam boiler efficiency and durability. Moreover, the production of electricity generates emission not only by the direct combustion, but also during externally-connected processes, like e.g. preparation and transport of both renewable and fossil fuels. Considering abovementioned aspects, the legitimacy of biomass co-utilization in large, basically coal-fired, power units should be carefully analyzed. The main goal of the presented study is therefore the assessment of energy efficiency and CO2 emission due to biomass use as a secondary fuel in large, basically coal-fired power units. Two methods of fossil and renewable fuel coupling have been analyzed: direct biomass and coal co-combustion by mixing them before coal mills in classical pulverized-fuel unit, as well as, biomass gasification followed by co-combustion of syngas with coal in the same steam boiler. Both systems have been modeled mathematically to determine the mass and energy fluxes crossing their boundaries. Models were prepared using Aspen Plus software. The main assessment factor used for comparison of two biomass utilization methods is cumulative CO2 emission calculated per unit of produced electricity
