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Session: Poster Session and Sponsors Exhibition
Session starts: Thursday 22 September, 14:00

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Katja Kruit (TU Delft)
Diego Acevedo ()
Bert van Dorp ()
Christine van Zuijlen ()

Abstract:
Combined heat and power (CHP) is an interesting application in which low-grade waste heat from combustion can be used to drive a turbine to produce electrical power. This article describes a simple model in which combustion of biomass (i.e. demolition wood) in a bubbling fluidized bed (BFB) is combined with an Organic Rankine Cycle (ORC) of 800 kWthermal operating on toluene. The advantage of using an organic fluid is that the turbine can efficiently operate at lower temperatures than with steam. Toluene as a working fluid, furthermore, has an extremely low global warming potential. The thermal input of the fluidized bed combustor is 1.1 MW (HHV based) and complete combustion is assumed with an excess air ratio of 20% at 1 bar and 850 °C. Furthermore, the system is designed in such a way that the temperature of the freeboard zone does not exceed 350 °C in view of the stability of the thermal oil. For optimal efficiency of the 800kW ORC system and under particular assumptions of fuel utilization and heat exchanger efficiency, a fuel flow rate of 230 kg/h (with a moisture content of 9.1% for demolition wood) and an air flow rate of 1600 kg/h lead to a flue gas flow rate of 1800 kg/h. A model is set up to calculate the heat capacity and flow rates of the flue gas, and it is found that the inlet air must be preheated to an optimum of 133 °C. Part of the stack heat lost could be used to preheat the air to the optimum temperature, therefore increasing the system efficiency. Heat losses through the reactor walls are neglected. Heat released during combustion is transferred to the ORC via heat exchangers on the bed and the freeboard section. The two separate flows are combined to a single flow to transfer the heat to the ORC generator. The heat exchanger system is assumed to have an overall efficiency of 85% and a heat transfer coefficient of 400 W/m²K for the bed heat exchanger, and 50 W/m²K for the freeboard heat exchanger. A heat exchanging area of 3.3 m² for the bed heat exchanger, and 53.4 m² for the freeboard heat exchanger is sufficient to transport the heat, due to the temperature difference between the freeboard entrance and the thermal oil. This leads to a total flow rate of 3300 kg/h for the thermal oil. The model estimates a boiler thermal efficiency of 80.7% and electricity is produced with a system efficiency of 10.2%, which is in accordance with other studies [2]. If the heat from the flue gas is used to preheat the incoming air, the thermal efficiency could be raised up to 84%. A MATLAB model is implemented in which the efficiency and other parameters can be easily changed to obtain more precise temperature requirements and fluid flows. Key performance factors of the BFB and the ORC are identified and discussed. A considerable degree of uncertainty is found on the heat exchanger system efficiency. A change in the heat exchanger efficiency of +/- 5% can yield a difference in preheat temperature of 60-100 °C. Also, a cost benefit analysis of the cost of heat exchangers vs. efficiency of the complete system should be performed. A more sophisticated model can be designed to produce more a more accurate result. References: [1] P. Basu, Combustion and Gasification in Fluidized Beds, Taylor & Francis Group LLC, 2006. [2] A.A. Khan, Combustion and co-combustion of biomass in a bubbling fluidized bed boiler, Faculty 3mE, Delft University of Technology, Delft, 2007, p. 200.