Modelling and analyses of heat exchangers in a biomass boiler plant

Abstract

SUMMARYA boiler plant is presented, in which the fuel is dried before combustion in a silo with air. The drying air is heated in a recuperative heat exchanger by the heat of flue gases. Hot air is then blown through the bed of fuel in the drying silo, while the fuel dries and the air cools down and becomes humidified. Heat of the moist exhaust air of the silo is recovered for the drying air and combustion air by a recuperative heat exchanger. Modelling of the thermal behaviour of the plant helps in understanding complex interdependencies of the two heat exchangers, the boiler and the dryer. The models of the heat exchangers and applications in analysing the boiler system are described in this paper. Calculating the combinations of extreme operational conditions gives the input data needed in comparing different types of heat exchangers, dimensioning the heat transfer area, choosing the control strategy and selecting the operating parameters and set-values of the control system. Results of verification measurements and practical operation at a 40 kW th pilot plant and a 500 kW th demonstration plant are also discussed. Using engineering correlation formulas for heat and mass transfer, an adequate accuracy between the model and the measurements was achieved.Fouling was detected to be a major problem with the flue gas heat exchanger. However, in absence of condensation, the increase of a fouling layer with respect to time was observed to be low. Fouling was also a problem with the drying exhaust gas heat exchanger, but after the installation of a simple dust collector, a reasonable cleaning period was achieved. A mixed-flow configuration was found to be the most appropriate for the flue gas heat exchanger. In order to avoid condensation of the flue gas the drying exhaust gas heat exchanger is indispensable in Finnish climate in the considered system. In addition to this, it decreases the need of fuel. A parallel-flow type was found the most appropriate as the drying exhaust gas heat exchanger.

Design and Evaluation of the Lab-Scale Shell and Tube Heat Exchanger (STHE) for Poultry Litter to Energy Production

“…Flue gas from biomass combustion was used to evaluate other types of heat exchangers. Yrjölä and Paavilainen [24] studied the use of heat of flue gas from biomass combustion to heat air in a recuperative heat exchanger and the use of heated air to dry the fuels in a drying silo. Stehlik et al [25] investigated the performance of a heat exchanger (e.g., air pre-heater, plate heat exchanger, radiation recuperative exchanger) during the biomass combustion process.…”