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2.4 STEAM PROCESSES

2.4.1.1 Typical elements of a steam cycle

The process of generating electricity from steam comprises four parts: a heating subsystem (fuel to produce the steam), a steam subsystem (boiler and steam delivery system), a steam turbine (Figure 2.17), and a condenser (for condensation of the used steam).

Source:[ 138, NWS 2001 ]

Figure 2.17: Steam turbine of a coal-fired power plant

Heat for the system is usually provided by the combustion of coal, natural gas, biomass or oil. The fuel is conveyed into the boiler’s furnace. The boilers generate steam in the pressurised vessel in small boilers or in a water-wall tube system (Figure 2.19) in modern utility and

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industrial boilers. Additional elements within or associated with the boiler, such as a superheater, reheater, economiser and air heater, improve the boiler efficiency.

Residues from the combustion process include flue-gases and, when coal, biomass or oil is used as fuel, ash.

High-temperature, high-pressure steam is generated in the boiler and then enters the steam turbine, as shown schematically in Figure 2.18. At the other end of the steam turbine is the condenser, which is kept at a low temperature and pressure. Steam flowing from the high- pressure boiler to the low-pressure condenser drives the turbine blades, which powers the electric generator.

Source:[ 139, Cortés et al. 2000 ]

Figure 2.18: Schematic of an ideal combustion Rankine steam cycle

Steam expands as it works, hence the turbine is wider at the end where the steam exits. The theoretical thermal efficiency of the unit depends on the gradient between the high pressure and temperature in the boiler and the low temperature and pressure in the condenser.

Low-pressure steam exiting the turbine enters the condenser shell and is condensed on the condenser tubes. The condenser tubes are maintained at a low temperature by the flow of cooling medium. The condenser is necessary for efficient operation by providing a low-pressure sink for the exhausted steam. As the steam is cooled to condensate, the condensate is transported by the boiler feed-water system back to the boiler, where it is reused. Being a low- volume incompressible liquid, the condensate water can be efficiently pumped back into the high-pressure boiler.

A constant sufficient flow of low-temperature cooling medium in the condenser tubes is required to keep the condenser shell (steam side) at a proper pressure to ensure efficient electricity generation. Through the condensing process, the cooling medium is warmed. If the cooling system is an open or a once-through system, this warm water is released back to the source water body. In a closed system, the warm medium is cooled by recirculation through cooling towers, lakes or ponds, where the heat is released into the air by evaporation and/or sensible heat transfer. If a recirculating cooling system is used, only a small amount of make-up water is required to offset losses by evaporation and the cooling tower blowdown, which must be discharged periodically to control the build-up of solids. Compared to a once-through system, a recirculated system uses about one twentieth of the water. [ 131, EPA 1997 ]

2.4.1.1.1 Boiler

In general, three types of boilers are commonly used: natural circulation, forced circulation, and once-through boilers.

Figure 2.19 indicates the major differences between the natural circulation concept and the once-through boiler concept.

Source:[ 140, Siemens 2000 ]

Figure 2.19: The natural circulation and once-through boiler concepts

In natural circulation boilers, the density difference between the water/steam mixture at the outlet (top) and the steam/water at the inlet (bottom) is used to generate a natural circulation. In forced circulation boilers, additionally to the density difference, circulation is supported by circulating pumps. In once-through boilers, the water flow is determined by the feed pump, and the water is evaporated during one single passage. The advantages of the once-through boiler are:

 steam generation is possible at any pressure;

 highest achievable efficiency is possible with supercritical steam parameters;

 high plant efficiency, even with part loads;

 short start-up times;

 sliding-pressure operation with high load transients;

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Boiler components

The boiler or steam generator is a combination of the following components:

Economiser: The feed water is heated in the economiser to a temperature 10 ºC below the saturation point. The economiser is the first heat exchanger of the boiler collecting heat from the lower temperature flue-gas at the exit of the boiler.

Evaporator: In the combustion chamber, the energy of the fuel is released and transferred across the boiler and heat-exchanger walls to the water/steam circuit. The heated water is then evaporated in the boiler evaporator to at least saturated steam for subcritical pressure water/steam conditions, or to superheated steam for supercritical conditions. Usually the evaporator tubes constitute the combustion chamber walls and are aligned in a vertical or a spiral arrangement. Modern plants work with supercritical water/steam pressure, i.e. a pressure above the critical point in the water-steam diagram. At supercritical pressure, the conversion occurs without a phase transition, so the evaporation energy is zero and only a peak in heat capacity represents the change in the continuous fluid.

Superheater: The superheater uses the highest temperature flue-gas area of the boiler to produce superheated steam. Usually the superheater stage consists of several heat exchangers with an injection in between. This injection controls the live steam temperature. Live steam

temperatures are in the range of 540–570 ºC for supercritical units and in the range of about 600

ºC for ultra-supercritical units. Superheated steam has a temperature significantly above the pressure-dependent condensation temperature. Such temperatures are necessary to facilitate the high pressure drop in the steam turbine and thus avoid condensation during the expansion of steam in the high-pressure steam turbine. Part of this expanded steam is bled off and used to transfer heat to the feed water.

Reheater: The bulk of the steam is reheated by the flue-gas in the reheater systems to extract further work and to achieve a higher efficiency in the subsequent medium-pressure steam turbine. The hot reheat steam temperature is controlled, e.g. by means of water injection or burner tilt. Reheating is typically used at large power plants. Very large units can also have another superheating stage to further enhance power output.

2.4.1.1.2 Steam turbine

In the steam turbine, the thermal energy of the steam is converted to mechanical work (i.e. turbine shaft rotation). This occurs between the steam inlet point and the condenser, with the steam expansion being used as the driving force. The steam expansion is coupled with a pressure drop and with an adiabatic decrease of the steam temperature. During this adiabatic steam expansion, the temperature of the steam decreases in association with a pressure drop from about 180–300 bar to 0.03 bar for modern LCPs. For larger plants, due to the large difference in pressure, steam expansion is normally effected in three stages: high-pressure (HP), intermediate-pressure (IP) and low-pressure (LP) stages of steam turbines. In most cases, these steps allow the steam to be reheated in reheaters before re-entering the next lowest pressure steps in the steam turbine. For smaller plants, two stages are applied (IP and LP).

2.4.1.1.3 Condenser

Finally, in the condenser located downstream of the low-pressure section of the turbine, steam is condensed back to water (condensate). After expansion in the steam turbine, some condensation and kinetic energy remains in the steam and is not transferable to mechanical energy. Efficient condensation systems allow a reduction in the pressure of the steam turbine to well below atmospheric pressure (vacuum of down to 0.03 bar, depending on the cooling medium temperature and the cooling water mass flow), which is desirable to extract the maximum amount of mechanical energy. This maximises the extraction of mechanical energy from the expansion of steam in the turbine. At CHP plants the condenser can be cooled with district

heating water. With this application the condensing pressure is higher but the steam condensing heat can be utilised for the heating of buildings or industrial processes.

2.4.1.1.4 Cooling system

Cooling techniques are applied to remove the condensation energy from the steam, i.e. the thermodynamically unusable energy of the process. For some detailed information on cooling techniques, refer to the ICS BREF.

The effect of choice of cooling technology on overall plant efficiency is of importance at combustion plants. This leads to the following ordering of cooling system technologies: once- through cooling, natural draught tower-cooled recirculating systems, mechanical draught tower- cooled recirculating systems (including hybrid systems) and dry cooling systems, e.g. air-cooled condensers. Where sufficient volumes of water are available, wet cooling systems result in the highest plant efficiency and electrical output under nearly all conditions. In this case, the environmental impacts of water discharged to the receiving water body have to be considered. Recirculating tower-cooled systems offer slightly lower thermal efficiencies than those achievable with once-through cooling but may be used at locations where water availability is limited or the impact of entrainment and impingement and thermal discharges needs to be reduced. Dry cooling technologies are limited to developments at locations with very restricted water resources or with particular environmental concerns related to water use.