| Thermal power generation is one of the most efficient forms of utilization of coal resources.Subjected to the laws of thermodynamics and environmental conditions,the energy efficiency of thermal power units is only about 40%,resulting in a serious waste of energy.The combined heat and power(CHP)unit not only generates electricity and heats,but also effectively reduces the loss of cold sources and significantly increases the energy utilization efficiency.The coal-fired generating units using cogeneration technology can theoretically increase the energy utilization efficiency by more than 80%.In northern China,especially in the northwestern region,there is a shortage of water and rich in coal and has a very large heat demand in the cold season.Therefore,the air-cooled combined heat and power unit has strong adaptability in northern China.The traditional extraction heating unit,due to there is a big temperature difference between extraction temperature and return heat network,results in a great loss.The unit with high back-pressure heating mode improves the exhaust pressure of steam turbine and the temperature of exhaust steam,and heats the thermal network water by exhaust steam of the turbine so as to achieve the purpose of recovering the waste heat of exhausted steam and saving high-quality steam from steam turbine extraction.However,the parameters of heat network water heated by exhaust steam are lower,so that the heating distance is limited.And when the outdoor temperature is low,it can not meet the needs of heat users.In view of the above problems,the high back pressure-extraction combined heating method with stage heating can be adopted.The exhaust steam of steam turbine is firstly used to heat the heat net water to a certain temperature,and then the intermediate pressure cylinder steam is used to heat the heatnet water to a higher temperature to achieve the purpose of cascade heating,increasing the heating distance,ensuring heating parameters,reducing the heat loss.There are three heating modes available for units retrofitted with high back pressure-extraction steam:high back-pressure heating mode,extraction-back pressure heating mode,extraction heating mode.High back pressure heating mode only uses the exhaust steam of low pressure cylinder as the heat source to heat the network water;extraction-high back pressure heating mode firstly uses the exhaust steam of low pressure cylinder to heat the water backwater to preliminarily heat,and extraction steam of intermediate pressure cylinder is to let the network water further heated to required water temperature;extraction heating mode is currently used by most of the cogeneration units,not repeat them here.For the heating unit,on the one hand,the change of ambient temperature affects the heating load;meanwhile,the heating condenser and hot-wire heater as the heat source also have a very complicated coupling relationship in the working condition of the unit.Taking into account the importance of winter heating to protect people’s livelihood,a considerable number of units are operated in the "heat and power" mode.Therefore,studing on direct air-cooled extraction-high back heating unit characteristics and the optimal heat load distribution ratio between heating condenser at different ambient temperatures and comparing energy consumption under different heating modes are essential.Based on the flow simulation software Ebsilon and the equation solving software EES,a heat transfer and heat supply model of a direct air-cooled extraction-high back pressure heating unit is established.According to the different heat load conditions,the optimum heat load distribution ratio between heating condenser and peak heater under different ambient temperatures is explored and the energy consumption of various heating modes under different ambient temperatures is compared.Hope to provide reference for power plant operation. |
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