Combined Power Cycles

In a combined power cycle operation, clean (sulfur- and particulate-free) gas is burned with air in the combustor at elevated pressure. The gas is either low or medium heat-value, depending on the method of gasification. 

Chen, J. and Wu, C., General performance characteristics of a N-stage endo-reversible combined power cycle system at maximum specific power output. Energy Conversion and Management, 37(9), 1401-1406, 1996. 

Today the circulating fluidized bed (CFB) has become the dominating design for combustors operated at atmospheric pressure. Pressurized circulating fluidized bed combustors are under development for combined power cycle applications, but so far no clear advantages have been revealed yet. For this reason the existing commercial pressurized fluid bed systems are bubbling beds. 

Buckwalter, R. and A. Walters. 1989. Selection of coal slurry pipeline technologies for gasification combined power cycle plants. In Proceedings of the 14th International Conference on Coal and Slurry Technology. Washington, DC The Coal and Slurry Association. 

FIGURE 14.9 A combined power cycle system in which energy is used at several levels the combustion of fuel in a turbine linked to a generator produces electricity. The heat from this turbine raises steam in a boiler that can drive a steam turbine linked to another generator, and the exhaust steam from this turbine can be used to heat buildings (district heating). Steam from the boiler can also be used for process heat in manufacturing. 

Describe a combined power cycle. How may it be tied with district heating ... 

An important field of study for power plants is that of the combinedplant [ 1 ]. A broad definition of the combined power plant (Fig. 1.5) is one in which a higher (upper or topping) thermodynamic cycle produces power, but part or all of its heat rejection is used in supplying heat to a lower or bottoming cycle. The upper plant is frequently an open circuit gas turbine while the lower plant is a closed circuit steam turbine together they form a combined cycle gas turbine (CCGT) plant. 

The most developed and commonly used combined power plant involves a combination of open circuit gas turbine and a closed cycle (steam turbine), the so-called CCGT. Many different combinations of gas turbine and steam turbine plant have been proposed. Seippel and Bereuter [3] provided a wide-ranging review of possible propo.sed plants, but essentially there are two main types of CCGT. 

Thermal power plant is more commonly associated with very large central power stations. The capital cost for thermal power plant, in terms of cost per installed kilowatt of electrical generating capacity, rises sharply for outputs of less than some 15 MW. It is for this reason that thermal power plant is not usually considered for industrial applications unless it is the combined cycle or combined heat and power modes. However, for cases where the fuel is of very low cost (for example, a waste product from a process such as wood waste), then the thermal power plant, depending on output, can offer an excellent choice, as its higher initial capital cost can be offset against lower running costs. This section introduces the thermal power cycle for electrical generation only. 

The cycles considered so far in this chapter are power cycles. However, there are applications in which Rankine cycles are used for the combined supply of power and process heat. The heat may be used as process steam for industrial processes, or steam to heat water for central or district heating. This type of combined heat and power plant is called cogeneration. A schematic cogeneration plant is illustrated in Fig. 5.19. A different schematic cogeneration plant is illustrated in Fig. 5.20. 

Power consumers in the acrylonitrile process are the air compressors, as well as the compressors of the refrigeration units for HCN condensation. These can be largely covered by HP steam produced in the reactor in combined heat and power cycles. 

These examples demonstrate that the valorization of renewable raw materials should go beyond the production of biofuels, to added-value chemicals and products, as well as to energy generation, by combined heat and power cycles. 

Morita H etal, 2001, The Influence of Operating Temperature on the Efficiency of Combined Fuel Cell and Power Cycle. Journal of the Electrochemical Society, 148, A1051. 

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. 

The characteristics of the hydrothermal resource determine the power cycle of the geothermal power plant. A resource that produces dry steam uses a direct steam cycle. A power plant for a liquid-dominated resource with a temperature above 165°C typically uses a flash steam cycle. For liquid-dominated resources with temperatures below 165°C, a binary cycle is the best choice for power generation. Power plants on liquid-dominated resources often benefit from combined cycles, using both flash and binary energy-conversion cycles. 

The scheme of Figure 8 is obviously more attractive if one needs high level heat, available to the heat-transfer surfaces shown in reactor A, as well as need for H2. This scheme would permit production of byproduct H2 in future power stations which use some combination incorporating pressurized fuel gasification, fuel-gas cleanup, and an advanced power cycle. 

Another solution is given in Fig. 7.3 (Dimian, 1996). This time the heat integration considers a more global viewpoint based on site integration . Excess heat available at high temperature is exported to the utility system. The heat needed to drive the distillation columns is imported from the steam network, at a temperature level compatible with the site policy. Exported energy as high-pressure steam is more valuable, and can be used to produce electricity in a combined heat and power cycle. Therefore, heat recovery is more efficient if treated as a plantwide problem. 

Examine the efficiency of a combined heat power cycle that should supply the load required in the Example 10.3. Consider superheated steam at 60 bar and 540 C. The stream for heating is extracted at a 50% ratio. Assume ideal Rankine cycle with steam exit pressure at 3 bar. 

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