12th International Symposium on District Heating and Cooling

The <strong>12th</strong> <strong>International</strong> <strong>Symposium</strong> on District Heating and Cooling,September 5 th to September 7 th , 2010, Tallinn, Estoniapart load using the thermal power plant simulatorProsim. Performance data of the CHP plant and fuelinput specification is derived from [4] and [8],respectively and given in table 1. The higher heatingvalue (HHV) is calculated by the simulation software.Table 1: Base Plant – Input Specification and Performanceat Design LoadSimulation model input data - design loadWood FuelUltimate analysis C 50.64 O 42.22H 6.10 N 0.16Ash 0.8 S 0.08Moisture 50 % HHV 18.8 MJ/kgFuel input26 MWSteam CycleHigh pressure steam 60 bars Condenser pressure 0.69 bar510°CPlant PerformanceDistrict heat output 16.5 MW Electrical efficiency η el 0.243Power output 6.3 MW Power to heat ration α 0.381Fig. 1: CHP Plant – Base CaseCHP Plant – Integrated Wood PyrolysisThe modified CHP plant is illustrated in Fig. 2. In orderto provide heat for the pyrolysis process, the heat mustbe extracted from the flue gases leaving the fluidizedbed reactor (numbered 3 in Fig.2) boiler at 850 °C. Therequired amount of flue gas is split off (18) after thefluidized bed reactor. As in the FZK process, those fluegases are thought to heat up sand to 550 °C (whichwould provide the heat for the pyrolysis process bycooling down to 450 °C) [6]. The flue gas thereby isestimated to cool down to 480 °C. The flue gas is thenmixed back (20) into the main flow before theeconomizer. The heat extraction needed for biomassfast pyrolysis process is modelled by help of anadditional evaporator (19). 90% of the biomass energyon a lower heating value base will form pyrolysis slurrywhereas 10% accrues as pyrolysis gas. The energycarried by the pyrolysis gas reduces the biomass fuelinput as explained above (―Wood Pyrolysis Model‖).CHP plant – Integrated Steam DryingThe dryer is modelled as a steam tube dryer. Payingattention to the retrofit situation, live steam is extracted,throttled to 10 bars and further cooled to 190 °C byspraying in the saturated water leaving the dryer. Dryingof biomass to low moisture contents requirestemperatures far above the saturation temperature at agiven pressure due to the hygroscopic properties ofbiomass. Heat consumption of the dryer has beenestimated to 2750 kJ/kg water evaporated [9]. Woodand hot flue gases are led in the dryer (24). If heat isavailable from the flue gases, those are cooled down to120 °C and together with the fully condensing steamprovide the heat needed for the drying process. Driedwood leaves the dryer at wet bulb temperature. For thedrying process live steam is extracted (21), throttled(22) to 10 bars and further cooled to 190 °C by sprayingin condensate (23) leaving the dryer. The dryercondensate is throttled to 2 bars (26) and send to thefeedwater tank (15). Flue gas temperatures of 120 °C170