| Ultra-supercritical double reheat power generation technology is an important representative of highly efficient clean coal-fired power generation technology,which can significantly improve the energy utilization efficiency of coal-fired power generation.Based on currently available steel materials,Ultra-supercritical double reheat power generation technology will be one of the inevitable choices for China’s current coal-fired power generation technology to achieve higher efficiency,however,it also increases the complexity of the power generation system.Ultra-supercritical double reheat unit is more complex in design and more stringent in operation,the rules of energy utilization and transformation are also different from traditional ones.Ultra-supercritical double reheat technology still lacks in-depth understanding of the design,the operation,and especially,the essential rules of the energy utilization and transformation in the process of ultra-supercritical double reheat.Therefore,it is necessary to carry out in-depth research on ultra-supercritical double reheat units from multiple aspects.Combined with the characteristics of the double reheat unit system,this paper deeply studied the thermal equilibrium analysis method and the exergy analysis method.A thermal equilibrium analysis method and exergy analysis method model and the calculation method for the ultra-supercritical secondary reheating process were established.At the same time,a1000 MW ultra-supercritical double reheat unit was considered as the research object,the model construction and simulation calculation of the boiler system and steam turbine system were carried out respectively.The reliability of the model was verified by comparing the simulation results with the design values.The calculation and analysis of the thermal equilibrium for the actual operating conditions of 1000 MW ultra-supercritical double reheat units were performed.The results show that the 1000 MW ultra-supercritical double reheat unit can ensure higher boiler efficiency during actual operation.The main source of heat loss is exhaust heat loss,followed by mechanical incomplete combustion heat loss.The thermal efficiency of each cylinder in the double reheat unit is higher than95%,in which the super-high-pressure cylinder has the lowest thermal efficiency while the low-pressure cylinder has the highest thermal efficiency.Compared with the 1000 MW ultra-supercritical single reheat unit boiler system,the double reheat boiler is more efficient under the same operation conditions,mainly because of the mechanical incomplete combustion loss and the ash slag physical heat loss all decrease with the number of boiler reheat increasing;The furnace of the double reheat boiler adopts more reasonable measures in the organization and manner of combustion,which guaranteed more complete combustion of fuel.The thermal efficiency of each cylinder of the 1000 MW ultra-supercritical double reheat turbine is higher than that of the single reheat turbine especially the efficiency of the low-pressure cylinder.This is mainly due to the fact that the double reheat can effectively increase the inlet parameters of the low-pressure cylinder Inproves the effective use of lower grade energy effectively.The performance of a 1000 MW ultra-supercritical double reheat turbine unit was analyzed using the established exergy analysis method.The results show that,the low-pressure cylinder has the largest exergy loss while the high-pressure cylinder has the smallest;The exergy losses of the regenerative heating system mainly exist in the #1,#3high pressure heaters and the #6,#7 low pressure heaters;Condenser exergy loss shows a significant downward trend with decreasing load.Compared with the same type of1000 MW ultra-supercritical single reheat turbine system,since the double reheat turbine adds a super-high-pressure cylinder,the main steam mass flow rate decreases,and the reheat steam flow increases,resulting in a large exergy loss in double reheat unit steam turbine.The comparative analysis of the different types of 1000 MW ultra-supercritical double reheat turbine systems shows that the work volume of each cylinder of the two turbine systems has the same trend as the exergy efficiency,but the low pressure regenerative heating system of the original unit and the comparison unit’s high-pressure regenerative heating systems all have their own advantages.Based on this,the optimal arrangement of turbines with an external steam cooler has been proposed to reduce the superheat of the steam and reheat heater terminal difference,thus reducing the irreversible loss of work and improving the efficiency of the system.The performance of the 1000 MW ultra-supercritical double reheat boiler system was further analyzed using the exergy analysis method.The performance of the 1000 MW ultra-supercritical single reheat boiler system and the performance of different types of1000 MW ultra-supercritical double reheat boiler systems were also comparatively studied.The results show that with the reduction of the boiler load,the overall boiler exergy loss and exergy efficiency are reduced.The energy loss of the double reheat boiler mainly comes from the exergy loss caused by the irreversible process of fuel combustion in the furnace and the heat exchange temperature difference between the high-temperature flue gas and each heated surface in the heat exchange process,which account for more than97% of the total exergy loss of the boiler.Compared with the same type of 1000 MW ultra-supercritical single reheat boiler system,the secondary reheat boiler system has lower exergy loss and improved exergy efficiency.Although the secondary reheat boiler adds one heat exchange system in the furnace,it will make the system more complicated and increase the cost.Compared with different types of 1000 MW ultra-supercritical double reheat boiler systems shows that,because of the flue gas recirculation loop and the corresponding heating surface layout used in the comparison unit,the exergy loss is lower,the energy of the boiler fuel and the flue gas can be more effectively utilized,the exergy efficiency is higher,and the performance is better;However,the flue gas recirculation also complicates the boiler system and operation,and increases the investment cost.Therefore,comprehensive assessment and consideration are needed when apply such technologies in ultra-supercritical double reheat boiler systems.In summary,the performance of the 1000 MW ultra-supercritical double reheat unit was analyzed in depth through the thermal equilibrium analysis method and the exergy analysis method,the key heat loss and exergy loss of the 1000 MW ultra-supercritical double reheat unit under different loads and different types of operating conditions were explored.The performance of the 1000 MW ultra-supercritical double reheat unit was also compared with of the 1000 MW ultra-supercritical single reheat unit.The essential rules of energy utilization and transformation in the ultra-supercritical double reheat unit were obtained.The research results can provide important scientific basis for improving the energy utilization potential of the ultra-supercritical double reheat power generation technology and guiding the optimization design and operation of the ultra-supercritical double reheat unit. |
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