The combined cycle gas turbine CCGT

In this chapter, a short review of the thermodynamics of CCGTs is given. However, the author recommends readers to refer to two books which deal with combined plants in greater detail [1,2]. 

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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. 

In recent years there has been a rapid growth in the number of combined heat and power (CHP) and combined cycle gas turbine (CCGT) plants, driven mainly by gas turbines using natural gas, sometimes with liquid fuel available as stand-by. Governments have encouraged the construction of these plants, as their efficiency is high and they produce less carbon dioxide than conventional coal and oil-burning power stations. However, they present some hazards, as gas turbines are noisy and are therefore usually enclosed. 

The required oxygen is produced in an air-separation plant. The electricity for the air-separation plant is provided by a natural-gas fuelled combined-cycle gas turbine (CCGT) power plant. 

In contrast, the yield of electricity from photovoltaics (PV) is about 337 500 kWh per ha and year, even if it is assumed that the area of the PV panels cover about one-third of the total plant area. If PV electricity were converted to liquid hydrogen (LH2), stored and then converted back to electricity by a combined cycle gas turbine (CCGT, efficiency 57.5%) about 104000 kWh electricity could be generated per ha and year. This yield is still more than three times the yield of the best biomass pathway (upper end of bandwidth electricity from biogas via large gas engine). 

The main drivers for the utilization of natural gas in power sector is twofold 1) emission reduction by shifting fuel from oil and/or coal to gas, and 2) the possibility of introducing combined cycle gas turbine (CCGT) technology with high electric efficiency (58 %). 

Most phase I NAPs provide for NE allocations based on a general emission rate and predicted activity level. For example in The Netherlands (NL), new entrants are allocated allowances based on projected output or fixed cap factor multiplied by uniform emission rate in line with that of a combined-cycle gas turbine (CCGT). In France, Germany and Poland, C02-intensive power generators, such as coal-fired installations, receive the highest number of allowances per kW installed. The literature highlights the risk that NE provisions can create distortions (Harrison and Radov, 2002). In order to illustrate how these rules can impact electricity prices and C02 emissions in our GB simulations, we focus on two approaches one based on a uniform benchmark and one based on a fuel-specific benchmark. In both cases the forecast capacity factor of new entrants is fixed at 60%. 

A transfer provision was initially drafted for new plants which had been built to replace old plants. If the operators complied with the requirement of the three-month (without any further examination) to the two-year period (if the respective verifications could be provided) from the closure of the old plant and the opening of the new plant, the allocation to the old plants would be transferred to the new plants without restrictions, provided that the new plants produced comparable products. In addition to the transfer rule - which was considered as a standard - there should be a rule that provides additional new plants (as well as plant expansions) a free allocation on the basis of the planned capacity use and an emission benchmark. The emission benchmark should be orientated towards BAT procedure. Above all, it should not be differentiated by fuel or technologies. As far as electricity production is concerned, a benchmark orientated towards a modern natural gas-fired combined cycle gas turbine (CCGT) power plant (365 g CCh/kWh) was scheduled. The allocation calculated in this way should be subject to an ex-post adjustment in the course of the following years. Thus, a special procedure was scheduled for CHP plants. The allocation should be determined with the help of the benchmark for electricity generation as well as the benchmark for heat generation. 

As can be seen from Table 6, the projected cost of electricity for the GT-MHR is about 31/MWh. Tliis cost can be compared to tlie projected cost of electricity from an Advanced Combustion Turbine (Combined Cycle Gas Turbine, CCGT) [16, Table 38]. (The analysis in this section follows [17].)... 

In a combined cycle power plant (CCPP), or combined cycle gas turbine (CCGT) plant, a gas turbine generator produces electricity and the waste heat is used to make steam for generating additional electricity via a steam turbine this last step enhances the efficiency for electricity generation to about 60%, because the temperature difference between the input and output heat levels is higher leading to an increase in the Carnot efficiency. Most modem power plants in Europe and in North America are of this type. 

The potential for an explosion inside a gas turbine combined heat and power (CHP) plant or a combined cycle gas turbine (CCGT) plant enclosure from a fuel leak has been widely addressed in the industry and reviewed at a number of seminars. Enron, the Teesside Power Station operators, contracted a study into the explosion potential inside the gas turbine (GT) enclostu-e and to recommend solutions to achieve an acceptable level of risk. 

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. 

Table 4. Cost estimates of alternative mitigation technologies in the power generation sector compared to baseline pulverized coal-fired power plant and natural gas Combined Cycle with Gas Turbine (CCGT) power stations and the potential reductions in C02 emissions to 2020 [14]...

The interaction between the gas turbine plant and the steam cycle is complex, and has been the subject of much detailed work by many authors [5-8]. A detailed account of some of these parametric studies can be found in Ref. [1], and hence they are not discussed here. Instead, we first illustrate how the efficiency of the simplest CCGT plant may be calculated. Subsequently, we summarise the important features of the more complex combined cycles. 

Even for this simplest CCGT plant, iterations on such a calculation are required, with various values of p, in order to meet the requirements set on T, the steam turbine entry temperature, and 7s (the calculated value of 7s has to be such that the dewpoint temperature of the gas (7jp) is below the economiser water entry temperature (7b) and that may not be achievable). But with the ratio /i satisfactorily determined, the work output from the lower cycle Wl can be estimated and the combined plant efficiency obtained from... 

After allowing for the performance penalties arising from the CO2 removal, Lozza and Chiesa estimated an efficiency of 46.1%, for a maximum gas turbine temperature of 1641 K and a pressure ratio of 15 (compared with the basic CCGT plant efficiency of 56.1%). They concluded that the plant cannot compete, in terms of electricity price, with a semi-closed combined cycle with CO2 removal (Cycle A2). 

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