| Oxy-steam combustion(or O2/H2 O oxy-combustion) is considered to be the next generation novel oxy-fuel combustion technology, in which the pulverized coal combusts in the mixed oxidizer of pure oxygen and steam. Under the oxy-steam combustion, the furnace outlet flue gas mainly consists of steam and CO2, and the flue gas with high contents of CO2 can be obtained by the flue gas condensation. Different from the conventional O2/CO2 oxy-combustion, the furnace temperature under oxy-steam combustion is adjusted by the recycled condensed water in the flue gas. Due to the absence of the recycled flue gas, the enrichment of the impurity gases in the boiler can be avoided, and the coupling effect of the boiler system is also decreased. But in order to demonstrate the application prospect of the oxy-steam combustion, it is necessary to make a preliminary comparison between the novel oxy-steam combustion and the conventional O2/CO2 oxy-combustion, and the efficiency and exergy analysis can be used as a tool to make the evaluation for the economic performance and the system difference.In this paper, one 600 MW pulverized-coal-fired power plant is selected to be retrofitted to oxy-steam combustion and O2/CO2 oxy-combustion respectively by the Aspen Plus. Referring to the methods adopted by Rodewald during the comparison between the O2/CO2 oxy-combustion and air-fired power plant, the same boundary conditions are also used when modeling the oxy-steam combustion and O2/CO2 oxy-combustion systems in this paper. The same boundary conditions mean the same coal feed rate, the same oxygen consumption, and the same adiabatic flame temperature(AFT). During the modeling, the conventional air-fired power plant system is not considered, and the O2/CO2 oxy-combustion system is modeled by referring to the classic research achievements. The AFT calculated from the O2/CO2 oxy-combustion model serves as a base for the determination of the amount of the steam injected into the oxy-steam combustion furnace. After the modeling and simulation, the detailed exergy analysis is implemented on these two kinds of oxy-combustion systems, and some feasible optimization methods are proposed based on the exergy analysis.Simulation results indicate that on the same boundary conditions, the mass flow of the main steam and reheated steam in the oxy-steam combustion system must be reduced by 0.84%. The comparative efficiency analyses reveal that the net thermal and exergy efficiencies of the oxy-steam combustion system are 0.16% and 0.153% lower than those of the O2/CO2 oxy-combustion system. The marked increase of the exergy destruction happening in the oxy-steam combustion furnace is the fundamental cause of the decrease of the exergy efficiency. The optimization is mainly carried on the oxy-steam combustion furnace and the air preheater(APH) where the exergy efficiency decreases significantly. The optimization results indicate that when the amount of the steam injected into the oxy-steam combustion furnace is decreased to 9.309kmol/s or the steam temperature is increased to 134?, the amounts of the main steam and reheated steam in the oxy-steam combustion system will be increased to be equal to those in the O2/CO2 oxy-combustion system. However, these two optimization methods will cause the respective temperature increase of 135.8K and 10 k as far as the AFT of the oxy-steam combustion system is concerned. |
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