Concerns about CO2 emissions have caused renewed interest in biomass electricity in Ireland. A low-investment-cost option is the firing of locally grown willow in retrofitted Irish peat plants. Various options for such a biomass energy system were evaluated. All steps in the supply chain were integrated in a model and optimised economically. Retrofitting of existing peat plant was compared with building new biomass combustion and gasification plants. All conversion technologies considered are able to co-fire biomass and peat. The study focused on possibilities in the short term. To reflect uncertainties, all costs were presented in ranges. Neither agricultural subsidies nor possible CO2 taxes were included. The lowest cost retrofit option with a proven technology was the conversion of unit 3 of the Lanesborough peat plant into a bubbling fluidised bed. The willow costs at the plant gate ranged between 4.4 and $15/GJ(LHV) and the kW h costs between 7.5 and 21 cent/kW h. The not yet proven options of gasification and the retrofit into a whole-tree energy plant showed slightly lower costs. The large ranges in the costs were mainly caused by the difference between the low and high estimation of the willow yields and the farmer's annual income. It can be concluded that in the lowest cost estimate, willow firing in retrofitted Irish peat plants has about the same cost as peat firing. ($4.3/GJ(LHV) and 7.4 cent/kW h) and could therefore be a promising option to reduce CO2 emissions in Ireland. (C) 1997 Elsevier Science Ltd.