Optimum design of two-pressure heat recovery boiler and analysis (3E) of combined cycle with steam injection into the combustion chamber in class gas turbines (V94.2-LM6000-PG9351FA)

Master's Thesis of Mechanical Engineering

Energy Conversion

Abstract

In recent years, due to the importance of finding energy, global warming and environmental pollution and its production resources, and the growing need of various industries for various forms of energy as well as the large volume of its consumers around the world. It was felt to provide patterns to optimize energy consumption and production.

Using exergy analysis, economic exergy and calculation of Nox and Co production rate of the combined cycle were evaluated. The results show that the combustion chamber has the most exergy destruction (MW 145) and the heat recovery boiler after that brings the most cost of exergy destruction to the cycle. In investigating the effects of the environment, the amount of production of these pollutants increases with the operation of the combined cycle power plant in partial loads. As the ambient temperature increases, the amount of Nox production will increase, but the amount of Co production will decrease. In the studies related to the effects of Co2 production, the results showed that the use of the combined cycle has a significant role in reducing global warming. In this part, by changing the fuel, the sensitivity of the production of greenhouse gases along with the optimization of the entire combined cycle was done.

Also, in this work, considering the effect of the environmental conditions on the gas turbine in the combined cycle, the design results of the thermal recovery boiler in ISO mode and the conditions of the Damavand power plant near Tehran were investigated and the design of the recovery boiler was validated with the results of this power plant in this area. The results show a sharp reduction in net power from 237 to 207 MW in the combined cycle power plant. As a result, considering that one of the basic problems in combined cycles is the inability to produce maximum power in site conditions and the lack of preparation in power generation for the power grid. As a result, considering the problems ahead in this work, increasing the power in the downstream cycle by reducing the pressure drop on the gas side on the gas turbine and increasing the power by injecting steam into the combustion chamber and in the upstream cycle by increasing the steam production due to the change in the pinch and approach temperature without injecting additional fuel in the combined cycle is considered. In the existing cost function, in addition to the cost of producing environmental effects, the cost of HRSG construction, its exergy destruction is considered. The optimization with the objective functions, where the weight coefficients of the two cost functions of the price, the inverse of the exergy and thermal efficiencies of the entire cycle are considered, shows that the power can be increased by a maximum of 2 megawatts of the net production power so that the costs are greatly reduced. Also, the combined cycle was optimized in relative loads and the amount of injection and optimal cycle parameters were calculated for loads of 100%, 75% and 50%, respectively, the optimal value of X=s/f parameter (steam to fuel ratio) is equal to 20%, 21% and 19%, respectively.  

Key words: combined cycle, heat recovery boiler, exergy analysis, environmental effects, optimization, injection

Chapter 1. Introduction

According to the advantages of the combined cycle, the number and power of this type of power plants is surpassing other types of power plants. In the most common of these cycles, the Brayton gas turbine cycle is the upper cycle of the Rankine steam turbine. The resulting combined cycle has higher thermal efficiency and power than any of the cycles that work alone. Rankine cycles have the advantage that their reverse work ratio is much lower than that of Brayton cycles. Because in steam power plants, the liquid that the pump moves has a small specific volume, while the specific volume of the steam that flows in the turbine is several times larger. Therefore, the work output from the steam turbine is much more than the input work to the pump and the return work ratio is very small, but in gas power plants, the working fluid (usually air) is condensed in a gaseous state, which has a high specific volume, as a result, a significant part of the work output of the gas turbine is consumed by the compressor, and the gas power plant produces less work per unit volume of the working fluid.In contrast to the low critical temperature of water (which is the most common working fluid in steam cycles) and the limitation of the maximum allowable metallurgical temperature in steam power plants, it has caused real gas turbine cycles to work at significantly higher temperatures than steam cycles. The maximum temperature of the working fluid at the turbine inlet for steam power plants is around 540 to 650, while the same temperature in gas power plants is around 1100 to 1650 k [1]. Therefore, gas turbine cycles have the ability to create higher thermal efficiency due to the higher average temperature in the heat input process. The weakness of these cycles is that the working fluid leaves the gas turbine at very high temperatures (around 500). This defect causes that the potential of receiving heat at high temperatures, which is the property of these cycles, is not used properly and the efficiency of these cycles is lower than steam cycles. A regenerator can be used to utilize the energy of the exhaust gases and improve the efficiency of gas cycles (usually Brayton), but it should be noted that the use of the regenerator only increases the efficiency and does not increase the output power. In fact, due to the higher pressure drop that the heat recovery boiler [1] imposes on the cycle, its use reduces the pressure ratio of the turbine and thus reduces the net output power by a few percent. It should be noted that if the regenerator is used, the optimal pressure ratio that leads to the maximum thermal efficiency tends to smaller values.

1-1. An introduction to exergy, economic exergy

In 1983, the authors coined the term economic exergy for more clarity and unambiguous characteristic of combining exergy analysis with economic analysis. For the first time, Trebus and Elsir expressed the concept of thermoeconomics. Studies related to cost in 1988 were presented by Ketas [2] and Zargut in the conferences of the American Society of Mechanical Engineers, and Moran [3] has done a lot of research in the field of exergy analysis. Fiaschi and Manfreda [4] in their analysis for the semi-closed gas turbine cycle showed that water injection and water recovery are the most important sources of exergy loss and together they account for 80% of the total irreversibilities in the cycle. They also checked the efficiency of the second law for the cycle when no heat was injected in the cycle until the time of complete injection and concluded that the maximum irreversibility can be observed in the cycle in complete injection. 

The idea of ??exergy economy was proposed by Keenan in 1932. He used the concept of exergy to divide the cost according to the electric power and steam produced in the power plant. He points out that from the economic point of view electricity and steam are comparable according to the useful work they do and not according to their energy. An article presented by Benedict in 1949, the economic value of exergy destruction and its use to optimize the air separator system was taken into consideration. Later, exergy research was continued by Tribus and Evans from the University of California in Los Angeles, USA, and then by Ebert and Galligoli at the University of Wisconsin in Madison, USA. Berman and Schmidt in 1980 used attribution of economic value to exergy destruction to optimize feed water preheaters. they did In 1982, Franser, Claudetiz and using the works of Tribus and Evans applied exergy economics in the design of heat exchangers. In 1983, they proposed the term economic exergy for the economic value of exergy instead of the term thermodynamic economy. Many scientists have suggested that the thermodynamic performance of the process is better done by performing an exergy analysis because the exergy analysis seems to be more useful to provide better insight. Dinser [2] and Moslem [3] [5] analyzed the Rankine cycle with Reheater and the changes of energy efficiency and exergy under different operating conditions (for example, temperature and pressure) Boiler) were investigated. Rosen [4] and Dinser studied the heating of industrial processes with steam through exergy analysis and concluded that exergy analysis should be considered as the main tool in the optimization of processes where large amounts of steam are used in energy centers. Ketas [2], Moran and Shapiro [6] analyzed exergy for the combined cycle. They found that There are exergy losses in each section.