Research On Integration Mechanism And Performance Analysis Of Coal-fired Power Generation System Assisted By Solar Tower Technology

At present,coal is still the main energy source to generate electricity in China,has led to an increase in the carbon dioxide level in the earth’s atmosphere and other pollatant emissions,resulting in serious environmental problems.China’s "13th Five-Year Plan" requires the optimization of energy structure and further realizing energy conversion and emission reduction.At present,China’s new and existing coal-fired power units have basically achieved high parameters and large capacity,and their system efficiency and coal consumption rate have reached or approached the world’s advanced level.Due to technical factors such as materials,it is difficult to further increase the parameters and the capacity of the power system.Therefore,the energy-saving effect is limited.What’s more,as an intermittent energy source,solar energy requires a large-scale energy storage system so that it can achieve stable power output.If unstable solar energy is coupled with a coal-fired power system,the requirements for further reducing the coal consumption rate of the power system and expanding the share of renewable energy power generation can be met at the same time.In this paper,the solar tower aided coal-fired power generation system is studied.The main research work and conclusions are as follows:Firstly,the mathematical model of the solar field subsystem,the boiler subsystem and the turbine and regenerative subsystem are established for the solar tower aided coal-fired power generation system.By introducing safety distance optimization variable and radial spacing optimization variables,the established heliostat field can be optimized for the different heliostat field locations,direct normal irradiance(DNI)and solar multiples and the heat output of the solar field can be calculated.The detailed mathematical model of the boiler can be used to study the change of the inlet and outlet temperature of flue gas and working fluid for heating surfaces after the introduction of solar energy.The the energy and exergy destruction of the different heating surfsces can be investigated.The established mathematical model of the turbine and regenerative system can calculate the power output and solar generating power when the system is under off-design conditions.Secondly,according to the temperature range of the heating surfaces and the working fluid flow process,the heat load on the heating surface is mainly divided into three categories:evapration section,superheating section and reheating section.Therefore,three integration schemes of solar tower technology and coal-fired power generation system are proposed.Scheme I is using solar energy to replace the heat load of the superheating section;Scheme II is using solar energy to replace the heat load of the evapation and superating section;Scheme III is using solar energy to replace the heat load of evapation,superating and reheating section.The thermodynamic analysis and exergy analysis are carried out for all the three schemes under the "fuel-saving"and "power-boosting" modes.For the heating surfaces in the boiler,the exergy loss in the furnace accounts for more than 55%of the exergy loss of the entire boiler.In the solar field,the main exergy loss occurs in the heliostat field.The exergy loss in the boiler and solar field accounts for more than 88%of the total exergy loss of the power system.The comparative analysis of different schemes shows that the Scheme II is a better option.Compared with the coal-fired power system,the coal consumption rate of the "fuel-saving" mode is reduced by 8.92 g/kWh and the coal consumption rate of the "power-boosting" mode is reduced by 8.89 g/kWh.Thirdly,under the premise of maintaining the parameters of the main steam and reheat steam unchanged and without changing the internal heating surfaces of the boiler,the maxmium solar energy can be absorbed by the boiler under different loads are studied.The influences of solar load on the boiler efficiency,system efficiency,and coal consumption rate are explored.The effects of solar multiples and thermal storage capacity on the daily performance of the soalr tower aided coal-fired power generation system are investigated.A case study of a 600 MW supercritical unit shows that,for 100%load,75%load and 50%load,the maxmium solar energy can be absorbed by the boiler are 76.4,54.2 and 23.0 MW,respectively;the corresponding coal saving rates are 13.53,12.81 and 8.22 g/kWh,respectively;the corresponding solar generating power are 32.3,23.1 and 9.6 MW,respectively.With the increase in solar load,boiler efficiency,system efficiency and CO2 emission rate are gradually decreased while solar generating power has a linear growth trend.As the solar multiple and thermal storage capacity increase,the coal consumption of the power generation system is gradually reduced and then remains unchanged.Finally,the annual operation characteristics of the soalr tower aided coal-fired power generation system are studied.The effects of different DNI radiation intensities on the annual operational performance of the solar tower aided coal-fired power generation system are compared and analyzed.With the aim of economic performance,the thermal storage capacity of the power system is optimized and the sensitivity analysis of some important economic parameters is carried out.A case study of a 600 MW supercritical unit shows that,compared with the coal-fired system,the saving coal comsumption of high-,medium-and low-DNI systems are 25,500,19,900 and 14,200 tons per year,respectively;the corresponding annual solar generating power are 100.72,79.18 and 65.13 GWh,respectively;the corresponding LCOEs are 6.37,6.40 and 6.41 cent/kWh,respectively.The optimal thermal storage capacities of the high-,medium-and low-DNI systems are 2.76 hours,1.43 hours and 0.33 hours,respectively,and their corresponding LCOEs are 6.34,6.35 and 6.36 cent/kWh,respectively.Sensitivity analysis shows that with the maturity of solar tower technology,the cost of solar field will be further reduced,and the advantages of solar tower aide coal-fired power generation system will be more significant.