| With the increasing shortage of global fossil fuels and the great challenge of environmental protection,how to further improve the efficiency of coal-fired power plants becomes a concerned and urgent problem to be solved.Compared with a single reheat ultra-supercritical unit,the double reheat ultra-supercritical unit shows more significant economic and environmental performances.Therefore,the high-parameter ultra-supercritical coal-fired power plants with double reheat technology will be more attractive in the future.In the view of this,this paper focused on the system optimization and cascade energy utilization of the double reheat ultra-supercritical coal-fired power plants.Firstly,according to the regenerative system of single reheat ultra-supercritical unit,a double reheat ultra-supercritical unit with eight-stage regenerative heaters was presented.Based on the energy and exergy analysis,a double reheat ultra-supercritical unit with ten-stage regenerative heaters was put forward to improve the energy utilization level of regenerative system.The reheat steam pressure was analyzed and optimized for the double reheat ultra-supercritical unit with eight-stage regenerative heaters and ten-stage regenerative heaters respectively.The reason of the efficiency improvement of the double reheat ultra-supercritical unit with optimized reheat steam pressure is revealed by the exergy analysis.Based on the principle of “energy cascade utilization” of the regenerative system,the energy grade analysis of the extracted steam in the double reheat ultra-supercritical unit with ten-stage regenerative heaters was analyzed,and two extracted steam energy cascade utilization systems are proposed,namely the outer steam cooler system and the regenerative turbine system.Lastly,considering the characteristics of tower-type boiler and double reheat ultra-supercritical unit,a new partially-underground tower-type boiler design was proposed for double reheat ultra-supercritical unit.The new design could significantly reduce the boiler height and steam pipeline length,leading to better thermodynamic and economic performances.To be specific,Based on the first law of thermodynamics and the second law of thermodynamics,the energy balance model and exergy balance model of thermal power plant were established,which would provide the theoretical basis for s ystem optimization and reheat steam parameter analysis of double reheat ultra-supercritical unit.According to the regenerative system of single reheat ultra-supercritical unit,a double reheat ultra-supercritical unit with eight-stage regenerative heaters was presented.The results of thermal performance analysis show that the heat consumption rate of the double reheat ultra-supercritical unit with eight-stage regenerative heaters was 297.4k J/k Wh lower than that of the single reheat ultra-supercritical unit,and the power generation efficiency was increased by 1.72 percentage points.The exergy analysis revealed that the exergy destruction of the regenerative heaters of double reheat ultra-supercritical unit with eight-stage regenerative heaters was large and the energy utilization level was low.In order to reduce the exergy destruction of the regenerative system,the double reheat ultra-supercritical unit with ten-stage regenerative heaters was proposed.Compared with the double reheat ultra-supercritical unit with eight-stage regenerative heaters,the heat consumption rate of the double reheat ultra-supercritical unit with ten-stage regenerative heaters was reduced by 54.5k J/k Wh,the power generation efficiency was increased by 0.33 percentage points.Besides,the exergy destruction of the regenerative system in the double reheat ultra-supercritical unit with ten-stage regenerative heaters was reduced.Compared with a single reheat ultra-supercritical unit,another reheat process was increased in double reheat ultra-supercritical unit,and thus,the reheat steam pressure selection was significant the thermal performance of the double reheat ultra-supercritical unit.The reheat steam pressure was analyzed and optimized for the double reheat ultra-supercritical unit with eight-stage regenerative heaters and ten-stage regenerative heaters respectively.The power generation efficiency of double reheat unit with eight-stage regenerative heaters achieves the peak point when the ratio of the first reheat steam pressure to the live steam pressure is 0.4 and the ratio of the second reheat steam pressure to the first reheat steam pressure is 0.3.However,the power generation efficiency of double reheat unit with ten-stage regenerative heaters achieves the peak point when the ratio of the first reheat steam pressure to the live steam pressure is 0.5 and the ratio of the second reheat steam pressure to the first reheat steam pressure is 0.3.The superheat degree of the extracted steam in regenerative system after the reheater can reach more than 200 oC,therefore,the energy grade of the extracted steam in double reheat ultra-supercritical unit was high,causing lower energy utilization level of the heat transfer process in the regenerative heater.Based on the principle of “energy cascade utilization” of the regenerative system,two system integration methods with outer steam coolers and regenerative turbine are proposed to optimize extracted steam waste heat utilization.In addition,the further discussion of extracted steam energy cascade utilization was also studied on the single reheat ultra-supercritical units.Comparative study of thermodynamic analysis of different systems under low different load conditions was conducted.The results indicated that the energy saving effect of the regenerative turbine system was better than that of outer steam cooler systems under high load condition.When the load decreases,the energy saving effect of the regenerative turbine system and outer steam cooler system would decrease and increase respectively.Considering the characteristics of tower-type boiler in the double reheat ultra-supercritical unit,a new partially-underground tower-type boiler design was proposed for double reheat ultra-supercritical unit,which significantly reduced the boiler height above-ground and saved the steel used to support the suspension.Besides,the new partially-underground tower-type boiler design could shorten the length of the live steam and reheat steam pipeline,which would effectively reduce the pressure drop and investment of the live steam and reheat steam pipeline and enhance the thermal and economic performance of the double reheat unit.Thermodynamic and economic analyses were quantitatively conducted,the results showed that the pressure drop of the live steam pipeline in the partially-underground tower-type boiler design was reduced by 3.7bar compared with that of the conventional tower-type boiler design,the pressure drop of the first and second reheat steam pipeline was reduced by 0.3bar and 0.6 bar respectively.This brought an increase in power generation efficiency of 0.22 percentage points and a reduction i n heat consumption rate of 36.0kJ / k Wh.Considering both the reduction of investment cost and the improvement of efficiency,the cost of electricity of the double reheat unit with new partially-underground tower-type boiler design was reduced from 49.96$/MWh to 49.30$/MWh. |
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