The Deep And Efficient Utilization Of Waste Heat From Low-temperature Flue Gas In The Coal-fired Boiler

The coal-fired boiler produces heat by burning coal to produce steam or hot water. But not all the heat can be effective transformation, there will be a lot of energy lost through the flue gas. The temperature of coal-fired utility boiler exhaust is between 120℃～140℃ and the temperature of coal-fired industrial boiler exhaust is between 150℃～204℃. The characteristics of the energy is not high, but the emissions is big. If we can effectively recycle coal-fired boiler flue gas waste heat, it has important meanings to improve the utilization efficiency of coal-fired boiler, reduce the coal consumption, reduces pollutant emission.For the characteristics of the facts that the temperature of coal-boiler flue gas is low and the coal-boiler flue gas contains water vapor and acid vapor which can produce acid corrosion, a cyclic evaporation Organic Rankine Cycle (ORC) system was proposed which is more suitable for recoverying the heat of coal-boiler flue gas. The cyclic evaporation ORC system is an ORC system which introduces the circulating evaporator. Circulating evaporator can make the preheating and evaporation process in traditional dry-evaporator separation to segmentally deal with the acid corrosion. Meanwhile, the influence of the circulating evaporator to a one-stage evaporation ORC system and two-stage evaporation ORC system was researched from the perspective of thermodynamics and thermal economics. In order to further improve the performance of two-stage cycle evaporation ORC system, the high flux heat exchange tube was introduced into two-stage cycle evaporation ORC system, and the convective boiling heat transfer characteristics of R123 in high flux tube was explored. At the same time, the change of high pressure stage evaporator’s heat transfer area after introducing high flux heat exchange tube to the system was compared according to the experimental data. The main conclusions are as follows:1. The ORC system which used circulating evaporator has more advantages than the ORC system which used dry-evaporator. And compared with one-stage cycle evaporation ORC system, the performance of two-stage cycle evaporation ORC system is better.2. Under the condition of laboratory, the convective boiling heat transfer coefficient in high flux tube for R123 is about 1.99 times of the light tube. As a benchmark, assuming that the convective boiling heat transfer coefficient in high flux tube for R123 is 1.99 times of the light tube, the heat transfer area of the high pressure stage evaporator after introducing high flux heat exchange tube to the system can reduce about 3%-4%,which is not only beneficial to control the tube wall temperature of the high pressure stage evaporator higher than acid dew point,but also conducive to reducing the cost of two-stage cycle evaporation ORC system.