Latent heat recovery from oxygen-combustion flue gas

Abstract

The most part of energy losses in a boiler is due to the heat released by the exhaust flue gas to atmosphere. The released heat consists of sensible and latent parts. As a next generation boiler, an oxygen combustion boiler was planned and developed in Japan. The steam mass concentration of the oxygen-combustion flue gas was approximately 25% which value is higher than that of the air-combustion flue gas. The latent heat highly included in the flue gas is very important resource. Based on the previous basic studies, a prediction method was proposed for the design of heat exchanger to recover the latent heat in the oxygen-combustion flue gas. The one-dimensional heat and mass balance calculation was conducted along the flow direction of flue gas in the heat exchanger. For the condensation of steam on heat transfer tubes, the modified Sherwood number taking account of the mass absorption effect on the wall was proposed. The heat and mass transfer on tubes was evaluated with the modified analogy correlation and the thermal resistance of the condensate film on tubes could be neglected. In the calculation, it was possible that the gas temperature merged with the dew point which was the saturation temperature corresponding to the partial pressure of steam in the flue gas. When the gas temperature decreased below the dew point, the condensation of steam in the flue gas took place and the released latent heat increased the gas temperature until the gas temperature coincided with the dew point. The thermal-hydraulic behavior was experimentally studied in a heat exchanger for the latent heat recovery from a flue gas generated with the combustion of oxygen and A-oil. The parametric study varying the flue gas and feed water flow rate was conducted. The experimental results agreed well with the one-dimensional prediction proposed in this study.