Energy-Saving Analysis And Operation Optimization Of High Back-Pressure Heating System In Combined Heat And Power Unit

In a typical coal-fired combined heat and power(CHP)unit,the exhaust steam from the steam turbine is condensed in the condenser and release heat to the ambient,known as the waste heat of exhaust steam,accounting over 30%of the total energy consumption,which results in a large amount of energy loss.There are three concepts of the waste heat recovery of exhaust steam in CHP units:absorption heat pumps(AHP)and absorption heat exchangers(AHE)and high back-pressure(HBP)heating.The HBP heating technology is promising to be widely exploited irn coal-fired CHP units due to its relatively low investment and concise system,particularly in retrofitting existing power plants for district heating.Through raising the back-pressure of steam turbine and directly heating the supply-water of the primary heating network with relatively high temperature exhaust steam,the HBP heating system could reduce the waste heat of exhaust steam and promote the overall efficiency of the CHP unit.Based on a 300 MW coal-fired CHP unit,the thermal performance of the HBP heating system was analyzed and assessed in this paper.And on the grounds of the first and second laws of thermodynamics,the detailed energy-saving mechanism of the HBP heating concept was synthetically explored by the analyses of energy flows and graphical exergy.The results indicated that the HBP design improved the thermal efficiency of the unit by 5.97%(absolute value)and cut down the standard coal consumption rate by 23.52 g/(kW·h),attributing to the exhaust steam recovery efficiency of 57%.In the HBP heating configuration,the exhaust steam energy loss could be remarkably reduced and the a lot of extraction steam could be saved to do work in the low-pressure cylinder,contributing to raising the exergy efficiency of the heating process from 55.07%to 72.46%,which implied that the energy utilization of the HBP heating process is more rational.In this paper,the impacts of the primary parameters(unit generation load,unit heating load,supply&return-water temperatures and turbine back-pressure)on the performance of the HBP-CHP unit were also examined and optimization suggestions were put forward.The results indicated that when the unit generation load fell,the unit heating load went up and the return&supply-water temperatures droped,the HBP heating system performed better and conduced to saving more fuel.If the turbine back-pressure was below the theoretical lowest saturation pressure needed by completely heating the supply-water in the exhaust steam-water heat exchanger(ET-SWHE),the increase of the back-pressure raised the unit energy efficiency.But the unit thermal efficiency would fall owing to the back-pressure increase when the back-pressure exceeded the lowest requested saturation pressure.Besides,the heat and power coupling characteristic of the HBP-CHP unit was studied comparing with the conventional CHP unit.