Flue gas condensation of solid fuel fired heating plant

Abstract

In the Czech Republic, increasing trend exists in utilization of biomass as a fuel in heating and power plants. This is preferred solution by EU Climate plans, and it is connected with some economic benefits (e.g. green bonuses, guaranteed purchase price), on the other hand the combustion of fossil fuels is penalized (EU ETS – Emission Trading System). There are many types of biomass with different parameters but one of the most discussed are wooden resides because of its quantity. There are big differences between quality parameters, especially in moisture content, which is decreasing the LHV. There are some technologies which can decrease moisture. Dryer technologies could be simple so-lution, but final decreasing of moisture is quite low. More effective is application of flue gas condensation. This technology is well known for gas-fired boilers but nowa-days is still more often build by new solid fuels-fired plants. This deals with design of condensation technology for existing heating plant in Mladá Boleslav. The fuel mixture is based on wood residues (70 %) and pelletized plant biomass (30 %). The calculation was done for three boilers for soild fuels – two same CFBs (steam production 100 t·h-1) and one BFB (steam production 80 t·h-1). Moisture content was calculated for two cases of wooden residues with moisture content 35 and 50 %. System of condensation include three step water scrubber, heat ex-changer, heat pump and humidifier of combustion air. The final designed output of unit for BFB is 12.7 MW (19 MW for each CFBs), but from these the output of heat pump is 5 MW (7.5 MW). The source of heat for heating pumps is steam, which can be used in current heater, so the final net output from condensation is 7.7 MW (11.5 MW). These parameters are only for 50 % of moisture content in wooden residues. The application of these system is not cost-effective for moisture content of fuel around 35 %. It is possible to build this technology for 50 % of fuel moisture content, but technology will not raise the temperature parameters of hot water. There are two differences between Mladá Boleslav heating plant and Finnish Vuosaari power plant in Helsinky, where the similar unit is already built. First of them is moisture content of fuel more than 50 %. Second one is temperature of hot water system 60 °C, however in Mladá Boleslav is at least 80 °C, sometimes it could be more than 110 °C. The decreasing of this temperature is problem because the most of heating systems were designed by current standards with temperature 80 °C. The only possible solution is to build two steps scrubber and the waste heat utilize as preheater of hot water.