700°C High Ultra Supercritical (HUSC) technology is taken into account as a more efficient clean coal-fired power generation technology which can achieve higher efficiency and less CO2 emission. With the increase of the main steam and reheat steam temperature, the temperature of regenerative extraction increased accordingly. This not only means higher investment cost and higher unreliability of power plant, but also leads to a great reduction of energy grade efficiency. To solve the above-mentioned problem, we introduce a novel system, called echelon cycle system (EC system). In EC system, a BEST (Backpressure Extraction Steam Turbine) is added, which provides power for feed-water pump and steam for feed-water heaters. The steam source of high temperature regenerative extractions is switched from main turbine to BEST, and the steam source of BEST is cold-reheat. Hence the highest regenerative extraction steam temperature decreased accordingly. EC system has been demonstrated to be a more efficient system by exergy theory[1] and energy grade theory. Three types of EC system are proposed in this paper. Thermal performance calculation of these three types of EC system under rated-load condition and part-load condition is carried out to evaluate and compare the economy of system. In order to obtain a more appropriate thermodynamic system solution, safety and restriction should also be given sufficient consideration. Meanwhile, the matrix solution method for energy grade efficiency of EC system is derived in this paper. Finally, energy grade theory is used to analyze how different schemes cause different hate rate profits.
700°C HUSC technology is considered as the next generation of more efficiently coal-fired power generation technology, the heat rate of which can be reduced by more than 8% on the basis of current ultra-supercritical units. That means there is a huge energy saving benefits. With the main steam / reheat steam temperature increasing from 600°C / 620 °C to 700°C/ 720°C, the temperature of extraction steam increases dramatically, especially the first extraction stage after reheater, the temperature of which will increase to 630 ∼ 650 °C. That means a substantial increase in the cost of the initial investment because of the nickel-based material being used in extraction pipe and heaters. With EC system, the extraction steam temperature is reduced sharply because the high temperature extraction steam is moved from the main turbine to a small parallel extraction turbine and the steam source of the small extraction turbine is from the cold reheater. So the highest extraction steam temperature will not exceed 500 °C, and the high temperature risk of heat recovery system will be eliminated completely. In this paper, exergy theory is introduced to analyze the cycle efficiency of the new thermodynamic system and the conventional one. In order to obtain a better 700 °C high ultra-supercritical thermodynamic system solution, GA method is used to optimize the regenerative system parameters to lower the overall heat consumption. The exergy theory is also used to analyze the reason why optimal solution can bring economic benefits. Finally, the feasibility of the entire system project will be analyzed.
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