Energy and exergy analysis of a biomass power plant

Abstract

The fulfilment of energy targets regarding the use of renewable energy has been a strong incentive to the development of dedicated power plants using biomass. Because these facilities operate with low value residues, this orientation has also received a strong incentive by the quest to reduce the amount of fuel in the woods and lowering the risk of fire. This issue is a major problem in dry climates. Although the number of plants planned for Portugal was very large, the actual number of units in operation is small. The first plant to enter production in Portugal was the thermal power plant of Mortágua. In this paper the energy and exergy analysis in this plant is presented. The objective of this work is to develop an analysis for the energy conversion in the plant. This is based on parameters that were retrieved during its normal operation and include: biomass and air flow rate, temperature measurements of the flue gases throughout the facility including the economizer, the two superheaters and flue gas composition. The elemental analysis of the biomass resulting from a blend of various sources was measured in addition to its moisture (ranging between 20 and 45% depending on the sources). The analysis was carried out for a fuel batch with a moisture of 35%. The plant was operated at three loads (full load, 80% and 65% load) and from the energy/mass balances the overall efficiency was calculated to be approximately 26%. The data were also used to perform a second law analysis of the plant which enables the calculation of the exergy destruction in the various components of the facility, including the external irreversibility. It was observed that the combustion chamber is the prime contributor to the overall loss of efficiency. The exergy efficiency was found approximately 22%.