part 1: overview of cogeneration and its status in asia - Fire

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State of art review of cogeneration 17 Recuperators are used to increase the power output of gas turbine cogeneration systems if the heat demands are low. The recuperator is in fact only a heat exchanger that is employed to heat the air leaving the compressor. The exhaust stream from the turbine is passed through the recuperator before going into the HRSG so that a part of the energy contained in turbine exhaust is utilized in the recuperator. The gas turbine cogeneration system with recuperator is sometimes known as the heat exchange cycle. 2.4 Steam Turbines Steam turbines are the most commonly employed prime movers for cogeneration applications, particularly in industries and for district heating. The technology is well proven in sugar and paper mills having demand for both electricity and large quantity of steam at high and low pressures. Some steam turbine manufacturers are over 100 years old and have products ranging from a few kW to 80 MW. However, turbines below two MW may be uneconomical except where the fuel has no commercial value. A cogeneration system using a backpressure steam turbine (see Figure 1.2) consists of boiler, turbine, heat exchanger and pump. In the steam turbine, the incoming high pressure steam is expanded to a lower pressure level, converting the thermal energy of high pressure steam to kinetic energy through nozzles and then to mechanical power through rotating blades. Thermal energy of the turbine exhaust steam is then transferred to another fluid, water, air, etc., in a heat exchanger, providing heat to the processes. For instance, the air heated by heat exchanger can be used to dry products in food processing industries. Depending on the pressure (or temperature) levels at which process steam is required, backpressure steam turbines can have different configurations. The most common types of backpressure steam turbines are shown in Figure 2.4. In extraction and double extraction backpressure turbines, some amount of steam is extracted from the turbine after being expanded to a certain pressure level. The extracted steam meets the heat demands at pressure levels higher than the exhaust pressure of the steam turbine. The backpressure steam turbine has a higher heat to power ratio and higher overall efficiency. Furthermore, back pressure turbine cogeneration systems need less auxiliary equipment than condensing systems, leading to lower initial investment costs. The extraction condensing turbines have higher power to heat ratio in comparison with backpressure turbines. Although condensing systems need more auxiliary equipment such as the condenser and cooling towers, better matching of electrical power and heat demand can be obtained where electricity demand is much higher than the steam demand and the load patterns are highly fluctuating. In the reheat cycle, steam is extracted from the turbine and reheated in the boiler during the expansion process. Reheat cycles improve the overall thermal efficiency and eliminate any moisture that may form as the steam pressure and temperature are lowered in the turbine. Steam turbines may also include a regenerative cycle where the steam is extracted from the turbine and used to preheat the boiler feedwater. The efficiency of a backpressure steam turbine cogeneration system is the highest. In cases where 100 per cent backpressure exhaust steam is used, the only inefficiencies are gear drive and electric generator losses, and the inefficiency of steam generation. Therefore, with an efficient boiler, the overall thermal efficiency of the system could reach as much as 90 per cent.
18 Part I: Overview of cogeneration and its status in Asia High pressure steam Extracted steam Exhaust steam (i) Simple backpressure (ii) Extraction backpressure (iii) Double extraction backpressure Figure 2.4 Different configurations for back pressure steam turbines The overall thermal efficiency of an extraction condensing turbine cogeneration system is lower than that of back pressure turbine system, basically because the exhaust heat cannot be utilized (it is normally lost in the cooling water circuit). However, extraction condensing cogeneration systems have higher electricity generation efficiencies. The techniques available for energy generation from fossil fuels are well established. In order to make greater use of alternative fuels, efforts have been made to take the specificity of fuel characteristics into account in order to overcome the technological constraints. The physical properties of agro-industrial residues vary considerably and can affect the conversion efficiency. Some areas where technological progresses have been made include fuel handling, combustion system and pollution abatement equipment. Fuel handling and transformation is important for appropriate functioning of the installation. Handling biomass residues depends mainly on the fuel granulometry and moisture content. Coarse residues can be transformed into homogeneous mass by crushing and chipping. Reduction of the moisture content by drying represents two main advantages: increases in the fuel heating value, and decrease in the fuel losses through fermentation during storage. Suitable technologies are available in the market to cover the handling, drying and storage requirements of different types of fuels. The selection of combustion system using alternative fuels depends on parameters such as the size of the unit, energy required, fuel characteristics, etc. Though grate-fired systems (Dutch-oven type or spreader-stokers) have been widely used because of the flexibility they offer, suspension burners and fluidized-bed combustors are emerging as relevant technologies because of their high conversion efficiencies and improved performance in meeting the environmental constraints. In suspension burners, ash is dragged out with the exhaust gases or it falls to the furnace bottom. Fluidized-bed combustors control the combustion better and make use of an inert material capable of absorbing energy, thus maximizing the heat transfer from the fuel. These units are capable of burning fuels with very low calorific values. Modern designs of furnaces offer staging combustion and good control of air-fuel ratio.
Page 1: PART 1: OVERVIEW OF COGENERATION AN
Page 4 and 5: 4 Part I: Overview of cogeneration
Page 12 and 13: State of art review of cogeneration
Page 36 and 37: Policy framework for promoting coge
Page 46 and 47: Cogeneration in Asia today 49 1.1 I
Page 48 and 49: Cogeneration in Asia today 51 4,000
Page 50 and 51: Cogeneration in Asia today 53 1.3 T
Page 52 and 53: Cogeneration in Asia today 55 1.3.3
Page 58 and 59: Cogeneration in Asia today 61 1.7 R
Page 60 and 61: Cogeneration in Asia today 63 Every
Page 62 and 63: Cogeneration in Asia today 65 Cogen
Page 64 and 65: 68 Part II: Cogeneration experience
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70 Part II: Cogeneration experience
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Cogeneration experiences around the
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PART 3: SUMMARY OF COUNTRY STUDIES
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98 Part3: Summary of country studie
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Sample case study in a pulp and pap
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Summary of country study - Banglade
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Summary of country study - Viet Nam
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