Research On Utilization And Thermal Economy Of Cold End Waste Heat And Power Units

Energy is an important resource for the survival of every country.But with the increasing economic development,the demand for energy is also increasing.For conventional extraction steam heating units,the exhaust steam contains a lot of waste heat.If it is directly discharged into the environment,it will cause a large amount of waste heat loss.Therefore,in recent years,people have paid more attention to the development and utilization of energy cascade and waste heat treatment.In view of the abundant waste heat and waste heat resources of cogeneration units in my country,the waste heat at the cold end can be recovered in different ways.This paper takes a 315MW extraction steam cogeneration unit as an example.The unit is modeled by Ebsilon simulation software,and the models of high back pressure heating and absorption heat pump heating are constructed respectively.On the basis of the absorption heat pump heating system,the condenser is changed to a double-sided operation mode,and the back pressure is appropriately increased.The heating network water is heated in two stages in the condenser and the heat pump respectively.It was transformed into a new type of cold-end waste heat utilization heating system.Then the accuracy and applicability of the thermal system are verified by analyzing the relevant parameters.Based on the first and second laws of thermodynamics.From the perspective of thermal efficiency and exergy efficiency,the thermal performance of extraction steam heating,high back pressure heating,absorption heat pump heating and new cold-end waste heat utilization heating systems are compared under different heat loads.It is concluded that when the new cold-end waste heat utilization heating system is adopted,with the increase of the water supply temperature of the heating network,compared with the other three heating methods,it has greater advantages in thermal efficiency,exergy efficiency and unit consumption.However,it is not suitable for working conditions with low water supply temperature.When the return water temperature is 47°C,the minimum water supply temperature shall not be lower than 78°C.Secondly,the analysis shows that there is an optimal back pressure of 23k Pa when the new cold end waste heat utilization heating system is running.And since the concentration of the dilute solution of lithium bromide in the absorption heat pump cannot be lower than 50%,the back pressure that can be obtained has an upper limit.When the water temperature rise of the heating network is small,and the upper limit cannot obtain the optimal back pressure,try to ensure the maximum back pressure within the allowable range.Under the same water supply temperature,appropriately reducing the return water temperature can ensure a larger heating load.It is beneficial to improve the COP of the heat pump and the overall thermal performance of the system.When the return water temperature drops to 40°C,the COP can reach 1.71.For a 2×300MW units,four schemes are established when both units are in operation and different heating methods are adopted.Among them,Schemes 3 and 4 are based on the use of cold-end waste heat recovery technology for both units.The maximum heating capacity has been significantly improved,reaching 762.841MW,which can meet the maximum heating area of 13.6113 million m~2.According to different environmental conditions and heating status,China is divided into highly cold areas and moderately cold areas.According to the peak heating period in different areas,I calculate and analyze the relatively best scheme of thermal performance parameters in each area.It is concluded that the fourth scheme is the best in highly cold areas.The total recoverable waste heat is 1.9227 million GJ,equivalent to the standard coal saved is 7.13×10~4t.And the third scheme is the best in moderately cold areas.The total waste heat that can be recovered is 1.1671 million GJ,equivalent to 4.33×10~4t of standard coal.Through the research of this thesis,the optimization principle of the new system is further obtained.And it provides a certain reference value for the recovery and utilization of the waste heat at the cold end of the cogeneration unit.