Structure Optimization And System Performance Analysis Of The Steam Ejector For A CHP System

With the development of urbanization and the explosion of heating demands,highefficiency heating in the combined heat and power(CHP)system can significantly improve energy use efficiency and increase the proportion of clean energy,which is key to achieving carbon peaking and carbon neutrality in China.The traditional extraction condensing CHP units used high-quality energy in a low manner and had a great loss of cold source.Although the heat pumps or exchangers could be applied to recover the waste heat of the exhaust steam,their performance is suboptimal in variable operation conditions.The high back pressure(HBP)technology is a straightforward solution to substantially enlarge the system heating capacity and improve energy use efficiency.But in the actual operation,the exhaust steam couldn’t be fully utilized due to the influence of the heat supply and return water temperature and heating load.Therefore,this paper proposed a three-stage heating CHP system integrated with a steam ejector to sufficiently recover the waste heat of the exhaust steam.The return water in the heat network was heated by the exhaust steam,the ejector outlet steam,and extraction steam in a cascade configuration,which can substantially reduce the heating exergy loss and improve the system efficiency.Steam ejectors are non-standard components.The EBSILON software was applied to establish the system model accor-ding to the thermodynamic parameters of the case unit.The design parameters of the ejector were determined by aiming at the maximum system heating capacity.Three optimized methods including the gas dynamics function method,empirical coefficient method,and optimized thermodynamic method were used to design the ejector.And the performance and the internal flow characteristics of the ejectors in different methods were compared and analyzed by numerical simulation.The results showed that the ejector designed with the optimized thermodynamic method yielded the largest heating capacity of 212MW,corresponding to system heating capacity and heating exergy efficiency of 779MW and 71.0%,respectively,thus offering greater strengths for designing ejectors in the CHP cascade heating systems.The original designed ejector was further optimized by adjusting the strructure of the mixing chamber based on the exergy analysis.The ejector entrainment ratio maximally increased by 11.4%and 6.4%through optimizing the inlet angle and the length of the contraction section of the mixing chamber.The optimal diam eter of the constant section mixing chamber was the design value of the optimized therm odynamic method.A steady simulation model was built for the three-stage heating system integrated with the optimized ejector based on EBSIL.ON software.The thermodynamic performance of the HBP system and the three-stage heating system under design condition s and variable condition s were compared and analyzed by simulation.In the design condition,compared with the HBP system,the recovery rate of exhaust steam and system heating capacity of the three-stage heating system had a significant increment of 14.8%and 62.64MW,with a decrease of 13.85g/kWh in the standard coal consumption rate for electricity supply.Meanwhile,the average temperature of the heat source dropped from 108.8℃ to 99.0℃and the total heating exergy loss reduced by 9.8MW.The thermodynamic performance of the three-stage heating system was still better than that of the HBP system within a wide operating range when the water supply and return temperature of the heat network,heating load,and electricity generation load changed.The steam ejector didn’t need to be put into operation when the HBP system could fully utilize the exhaust steam.Finally,the energy-saving effect of the three-stage heating system was calculated for a complete heating period of the case region.The average standard coal consumption rate of the novel system in the heating period is 2.85g/kWh lower than that of the HBP system,which can save 7,010 tons of standard coal in the whole heating period.The technoeconomic analysis was carried out for the investment and renovation of the three-stage heating system.The net present value of the project is 30.51 million yuan and the dynamic investment payback period is only 4.2 years,which was feasible for investment.