Modeling And Optimization Of Combined Heat And Power System Coupling With Steam Ejector

With the acceleration of urbanization process,central heating demand has grown exponentially in China.As the most important centralized heating method,combined heat and power(CHP)can reduce cold source loss and increase the overall efficiency in thermal power plants.There are mainly three heating modes:extraction steam heating,high back pressure heating and absorption heat pumps heating.Generally,the exergy loss of extraction steam heating is large and the investment and maintenance cost of absorption heat pumps are high.However,the high back pressure heating has small exergy loss and it conforms the principle of energy cascade utilization,which is the focus of current heating retrofits.In order to solve the problem of low recovery efficiency of exhaust steam caused by high return water temperature in high back-pressure heating process,a 2×300MW "high back pressure-ejector-extraction steam" three-stage heating mode is proposed in this paper.The primary steam source of the ejector is steam extraction,and the secondary steam source of the ejector is exhaust steam of high back pressure unit.The network water is heated by the exhaust steam of high-back pressure unit,the mixed steam of the ejector,the steam extraction respectively.In this paper,Ebsilon software is used for the modeling of the mentioned cogeneration system,and the heating load characteristics and the off-design characteristics of a heating network are obtained by mathematical calculations.According to "Constant Pressure Mixing(CPM)" One-Dimensional Model,a Matlab program is established and the optimal compression ratio of the ejector is 1.8 when the exhaust steam is maximally recovered.Based on this,the geometries of the ejector are fully designed according to the ESDU standards.The performance curves of the adjustable ejector are obtained by numerical simulation when the primary steam pressure,the secondary steam pressure and the Blockage Area Ratio(BAR)change.By the combination the performance curves of the adjustable ejector and temperature rise curves of network water,the optimal operation strategy of the adjustable ejector is obtained.The new system is compared with steam extraction-high back pressure cogeneration system,which found that when the supply water temperature is lower than 68?,the ejector should not come into use.When the supply water temperature is between 68? and 82?,the blockage area ratio of the ejector should be adjust to reach an optimal operating state.When the supply water temperature is higher than 82?,the ejector is under design condition.Under this circumstance,the exhaust steam recovery efficiency of the high back pressure rises about 20%and the coal consumption rate of the whole power plant is down about 9g/kW h.Finally,a technical and economic analysis of the new system is evaluated in this paper.Compared with the conventional steam extraction-high back pressure cogeneration system,the new system decreases fuel cost about 7.47 million yuan in each heating period and the dynamic payout period of the investment is only 2.9 years.