A pilot-scale condensing waste heat exchanger

This paper presents a calculation algorithm, design assumptions and results of studies concerning a flue gas/water heat exchanger with the condensation of water vapor contained in flue gas from the combustion of brown coal. The algorithm was used for design calculations of a pilot-scale heat exchanger with capacity of 380/312 kW. A cross-counter flow heat exchanger with capacity of 312 kW and coils made of PFA (perfluoroalkoxy polymer) was designed and installed. Waste heat is recovered from flue gas produced by a pulverized brown coal fired subcritical steam boiler operated in a power unit with capacity of 370 MWe. The heat exchanger was theoretically divided into a non-condensing part (sensible heat recovery) and a part with the condensation of water vapor contained in flue gas (recovery of sensible and latent heat). The point of the division is the temperature of flue gas in the stream core (higher than near the pipe wall) where the condensation of water vapor occurs on the pipe surface. The heat transfer in the non-condensing part was calculated using the same formulas as for the economizer in a pulverized-fuel boiler, while the calculations of the heat and mass transfer in the condensing part were performed using the VDI algorithm. The results of the thermal calculations and the geometry of the heat exchanger together with the place of installation of the entire test rig are presented. The results of the calculation are then compared with the test results. Good correlation was achieved between the test results and the assumptions and results of the design calculations. Calculations for full scale exchanger for 370 MWe brown coal fired power unit showed a 1.18% net efficiency increase with improving wet flue gas desulphurization process (EUR 3.7 million annual savings of fuel consumption and CO2 emission).