Steam power plant thermodynamic efficiency

In this chapter, we explore how the exergy concept can be used in the analysis of energy conversion processes. We provide a brief overview of commonly used technologies and analyze the thermodynamic efficiency of (1) coal and gas combustion, (2) a simple steam power plant, (3) gas turbine, and (4) combined cycle and cogeneration. At the end of this chapter, we summarize our findings with some concluding remarks. 

The second application of availability analysis is used to evaluate the nature and magnitude of thermodynamic irreversibilities in a methane reformer plant coupled to a high-temperature nuclear reactor. It is shown that a combination of thermal histograms and availability concepts are helpful not only in evaluating the net impact of irreversibilities in various chemical process steps on the steam power plant, but, more importantly, 1n suggesting process modifications that could improve the overall efficiency by avoiding unnecessary entropy production. 

Example 4.10 Thermodynamic efficiency in a power plant A steam power plant produces 65 MW electricity with an efficiency of 70%. It uses steam (stream 1) at 8200 kPa and 550°C. The discharged stream (stream 2) is at 75 kPa. If the expansion in the turbine is adiabatic, and the surroundings are at 298.15 K, determine ... 

Rankine Cycle - The thermodynamic cycle that Is an Ideal standard for comparing performance of heat-engines, steam power plants, steam turbines, and heat pump systems that use a condensable vapor as the working fluid efficiency is measured as work done divided by sensible heat supplied. 

Simplified the net efficiency of coal-fired steam power plants can be written as product of the partial efficiencies of the ideal thermodynamic cycle J/th.id, steam generator rjsc (boiler efficiency), steam turbine j/x, generator j/g, and the equivalent... 

Modern steam power plants have a thermodynamic efficiency of approxi-mat( h 40%. For a nuclear system to achieve comparable efficiencies, the working fluid will have to have a reactor outlet temperature in the neighborhood of 500°C. The LIMFR is one of the new types of nuclear reactors having this desirable characteristic. Thus, it is one of the few with potentialities for producing power competitive with the best of the present steam systems. 

If all the heat absorbed were converted into work, the efficiency would be 1, or 100 percent. If none of the heat absorbed was converted into work, the efficiency would be 0. The first law of thermodynamics limits the efficiency of any heat engine to 1 but does not prevent an efficiency of 1. The efficiency of practical heat engines is always less than 1. For example, the efficiency of a large steam turbine in an electric power plant is about 0.5, which is considerably more efficient than the typical 0.35 efficiency of an auto engine. When two objects at different temperatures are m... 

This remarkable result shows that the efficiency of a Carnot engine is simply related to the ratio of the two absolute temperatures used in the cycle. In normal applications in a power plant, the cold temperature is around room temperature T = 300 K while the hot temperature in a power plant is around T = fiOO K, and thus has an efficiency of 0.5, or 50 percent. This is approximately the maximum efficiency of a typical power plant. The heated steam in a power plant is used to drive a turbine and some such arrangement is used in most heat engines. A Carnot engine operating between 600 K and 300 K must be inefficient, only approximately 50 percent of the heat being converted to work, or the second law of thermodynamics would be violated. The actual efficiency of heat engines must be lower than the Carnot efficiency because they use different thermodynamic cycles and the processes are not reversible. 

The fuel gas from biomass gasifiers can be used to operate gas turbines for local power generation. A gas-turbine power station is similar to a steam plant except that instead of using heat from the burning fuel to produce steam to drive the turbine, it is driven directly by the hot combustion gases. Increasing the temperature in this way improves the thermodynamic efficiency, but in order not to corrode or foul the turbine blades the gases must be very clean which is why many gas-turbine plants use natural gas. 

A conventional power plant fired by fossil fuels converts the chemical energy of combustion of the fuel first to heat, which is used to raise steam, which in turn is used to drive the turbines that turn the electrical generators. Quite apart from the mechanical and thermal energy losses in this sequence, the maximum thermodynamic efficiency e for any heat engine is limited by the relative temperatures of the heat source (That) and heat sink (Tcoid) ... 

Power generation plants such as the steam plant, the gas turbine plant, and combined cycle plants require the combustion of a fossil fuel. Now, combustion is a chemical reaction of fuel with an oxidant (usually oxygen), and it makes sense to examine the combustion process more closely and analyze its thermodynamic efficiency. This means that we will examine the furnace/combustor of Figures 9.8, 9.10, and 9.12. We will examine coal and gas combustion at the level needed for thermodynamic analysis, after discussing some commonly used coal combustion processes. 

More efficient coal utilization can be realized with combined power plant cycles. For instance, the post combustion gases of a conventional combustor or an advanced MHD system can be further utilized to drive a gas or steam turbine. However, the sustained durability of downstream turbine or heat exchanger components requires minimal transport of corrosive fuel impurities. Control of mineral-derived impurities is also required for environmental protection. For the special case of open cycle-coal fired MHD systems, the thermodynamic activity of potassium is much higher in the seeded combustion gas (plasma) than in common coal minerals and slags. This results in the loss of plasma seed by slag absorption and is of critical concern to the economic feasibility of MHD. 

Combined-Cycle Power Plant - A power plant that uses two thermodynamic cycles to achieve higher overall system efficiency e.g. the heat from a gas-fired combustion turbine is used to generate steam for heating or to operate a steam turbine to generate additional electricity. 

A Rankine power plant using steam operates between pressures 6o bar and 1.013 bar. The turbine expands steam from 500 °C, 60 bar, to 1.013 bar. The liquid at the exit of the condenser is saturated. The efficiency of the turbine and of the pump is 75%. Calculate the energy balances, entropy generation, and thermodynamic efficiency. What are... 

Kehlhofer, Rolf, et al. Combined-Cycle Gas and Steam Turbine Power Plants. 3d ed. Tulsa, Okla. Penn Well, 2009. Application of gas turbine as part of cutting-edge powergeneration technology. Presents concepts, components, applications, operations of these plants. Each chapter has illustrative figures. Peng, William W. Fundamentals of Turbomachinery. Hoboken, N.J. John Wiley Sons, 2008. Chapter 8 discusses gas turbines and covers thermodynamics, design, efficiency, and performance, as well as applications illustrated. 

Imagine you are sitting in a meeting when the vice president asks, "Should we use an aerial cooler or colder seawater to condense the power plant turbine exhaust steam " Harry, the chief engineer, offers to run through the relative efficiency of the turbine for the two cases using his computer model and report back after lunch. You, the junior staff engineer, look up into space and after a few moments state, "Based on the Second Law of Thermodynamics I ve calculated an enhanced efficiency of 16.1 percent for the cooler seawater case." I always add that extra decimal for dramatic effect. 

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