Life cycle assessment of an energy-system with a superheated steam dryer integrated in a local district heat and power plant.

Life cycle assessment (LCA) is a method for analysing and assessing the environmental impact of a material, product or service throughout the entire life cycle. In this study 100 GWh heat is to be demanded by a local heat district. A mixture of coal and wet biofuel is frequently used as fuel for steam generation (Case I). A conversion of the mixed fuel to dried biofuel is proposed. In the district it is also estimated that it is possible for 4000 private houses to convert from oil to wood pellets. It is proposed that a sustainable solution to the actual problem is to combine heat and power production together with an in improvement in the quality of wood residues and manufacture of pellets. It is also proposed that a steam dryer is integrated to the system (Case 2). Most of the heat from the drying process is used to the municipal heating networks. In this study the environmental impact of the two cases is examined with LCA. Different valuation methods shows that Case 2 is an improvement over Case 1, but there is diversity in the magnitudes of environmental impact in the comparison of the cases. The differences depend particularly on how the emissions of CO2, NOx and hydrocarbons are estimated. The impact of the organic compounds from the exhaust gas during the drying is estimated as low in all of the three used methods.