Total-Site Energy Optimization And Exergy Evaluation For Associated-Gas Combined Cycle System

In an iron and steel enterprise, associated energies that were uneven levels of quality, with wide varieties in kinds, were scattered. Being confined to conventional recycling technology and method, different types of associated energies were recovered individually, and their use efficiency was lower. In view of this, firstly, in the dissertation, various-quality associated energies were integrated and optimized from the point of view on total-site level. Secondly, exergy evaluations from component level for a associated-gas combined cycle system and a combined cycle system coupled with chemical-looping combustion were conducted. Finally, some key technologies of engineering application for associated-gas combined cycle system were elaborated.The main contents in the dissertation are as follows:To start with, the theoretical research on total-site energy optimization using a total-site profile (TSP) analysis based on pinch technology was performed. The pinch technology that was commonly used in chemical industry and network optimization of heat exchanger was tentatively applied to the utilizing of waste heat. First, taking coke oven quench (CDQ) unit as an example, how to select utilities for heaters and coolers, and how to extract data from heat exchange equipments in a steel plant were described. Then, as an illustration of the method that how to optimize the employment of waste heat, three cases including current case, targeting case and demand case were compared for a large steel plant, which shows that22.456MW of power generation could be increased after optimization. Next, for a blast-furnace-gas-fired combined cycle system and a hot-waste-gas power generation system, integrated schemes of the two systems were discussed based on TSP.After that, a associated-gas combined cycle system in ironmaking process was proposed, and exergy, exergoeconomic and exergoenvironmental evaluations of the system were conducted, respectively. Based on the theory of energy, exergy and energy level balances, thermodynamic performances of the common surplus-gas gas-steam combined cycle system and the associtated-gas combined cycle system were analyzed. Results show that under the same conditions of initial parameters of surplus gas and thermodynamic parameters on steam side, energy and exergy efficiencies of the system after integration improve by3.00%and1.18%, respectively, than that of the system before integration, and energy difference decreases by27.28%. In accordance with structural theory of thermoeconomics, three exergoeconomic indexes of each component, and the influences of performance parameters, purchase cost and fuel cost on the unitary cost of production were investigated, and also the effects of exergy destruction of each component on the environment were examined. Results comparing with the current three power generation systems show that for the system proposed, the cost of unitary output power was lower than others’, and the output power of unitary fuel higher.In addition, a combined cycle system coupled with chemical-looping combustion was proposed, and exergy, exergoeconomic and exergoenvironmental evaluations were carried out, respectively. After dividing exergy destruction into unavoidable and avoidable parts, and endogenous and exogenous parts, on the basis of structural theory of thermoecnomics, the performance evaluation of the combined cycle system coupled with chemical-looping combustion was studied from three aspects including exergy, exergoeconomics and exergoenvironmental impact, in order to find appropriate trade-offs between efficiency enhancement and investment cost and environmental impact reduction at the component level. And transformation sequence of components was also given according to the analysis results. In contrast with the associated-gas combined cycle system, we find that the output power of unitary fuel for the system proposed was16%higher, and the environmental impact of unitary output power about5.2%lower.Last but not the least, from the point of view on engineering application, some key technologies of actual operation for associated-gas combined cycle system were expounded, and the analysis of exergy destructions of gas turbine, heat recovery steam generator and supplementary-steam steam turbine was conducted with the help of energy utilization diagram (EUD). According to the fluctuation of surplus gas, the selection of gas turbine suited low-heat-value gas was finished, and the influences of pressure ratio and inlet temperature of air compressor and the low heat value of blast furnace gas on net output power and exhaust gas temperature of gas turbine were also set forth. In the light of the distribution law of sensible heat of sintering waste heat, the design of gravity-heat-pipe steam generator fitted low-temperature sensible heat, and some influence factors including the velocity of wind side at the inlet in the flue gas side of heat-pipe steam generator, the outer diameter and the length of evaporation section of heat pipe, for exergy dissipation number and entransy dissipation number were analyzed. The feasible study of regulation and control strategy for supplementary-steam steam turbine was performed, the steam pressure and flow rate in the supplementary-steam point could be kept stable by endothermic and exothermic processes of steam accumulator and control and regulation system of live steam using505E digital regulator.