Co-firing biomass with coal seems a relatively low cost solution to reduce greenhouse gases (GHG) and is also an effective way of taking advantage of the high thermal efficiency of large coal fired boilers. Existing coal-fired power stations can quickly be modified for biomass co-firing achieving considerable levels of renewable generation at low capital costs and low commercial risk. However, certain biomass properties may require the retrofitting of the power plants that were originally designed to operate on a particular bituminous coal. These modifications may decrease the boiler reliability and as result reduce availability and increase operation and maintenance costs. To understand these undesirable effects and determine the optimum way to co-fire biomass in IEC's 5000 MW coal-fired boilers we studied the effect of biomass co-firing on the capacity, heat transfer surfaces, firing systems, pulverizers, fans and airheaters. We evaluated two biomass alternatives-pelletized biomass and torrefied-biomass (bio-coal) - co-fired with bituminous coals in 575 MW tangentially-fired and 550 MW opposite wall burner boilers. Boiler performance, emission and pulverizer self consumption were discussed. Considering all above aspects, we concluded that the most cost effective alternative is co-firing bio-coal with coal.