Over the last 30 years, CFB combustion technology has developed rapidly in China as coal is the principal energy resource and most coal in China is of low quality. However, conventional CFB combustion has not achieved high efficiency i. By the end of , the total capacity of coal-fired power was GW. Over the last decade, the need for improved efficiency has driven many companies to adopt SC and ultra-supercritical USC steam parameter design and operation for new PC boilers.
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The most obvious way to avoid throttling losses therefore is to stop operating the boiler at constant pressure! Instead, try to match the stop valve pressure to that existing inside the turbine at any given load. Since the turbine internal pressure varies linearly with load, this means that the boiler pressure must vary with load similarly. This is called. If the boiler pressure is matched to the pressure inside the turbine, then there are no valve throttling losses to worry about!
While sliding pressure is beneficial for the turbine, it can cause difficulties for the boiler. A change in the boiling point can change the conditions in each zone. The heat transfer coefficient in each zone depends upon the pressure. As the pressure falls, the heat transfer coefficient reduces. This means that the steam may not reach the correct temperature.
Also, if heat is not carried away by the steam, the boiler tubes will run hotter and may suffer damage. Any change in pressure causes a change in steam density and so alters the steam velocities and heat transfer rate in each zone.
Pressure and temperature cause the boiler tubes to expand. If conditions change, the tubes will move. The tube supports must be capable of accommodating this movement. The expansion movements must not lead to adverse stresses. The ability to use sliding pressure operation is determined by the boiler Boilers can be designed to accommodate sliding pressure.
Below this range, the boiler is operated at a fixed pressure. This achieves an acceptable result because large units are normally operated at high load for economic reasons.
Consequently, the governor valves throttle slightly. The offset is provided so that the unit can respond quickly to a sudden increase in load demand simply by pulling the valves wide open. This produces a faster load response than raising the boiler firing rate alone. The throttling margin is agreed during the tendering phase and then fixed.
The throttling margin means that the full potential gain of sliding pressure is not achieved. Nevertheless, most of the throttling losses which would otherwise occur are recovered. Temperature changes occur in the boiler and in the turbine during load changes.
These can cause thermal stresses in thick walled components. These are especially high in the turbine during constant-pressure operation. They therefore limit the maximum load transient for the unit. By contrast, in sliding pressure operation, the temperature changes are in the evaporator section. However, the resulting thermal stresses are not limiting in the Once through boiler due to its thermo elastic design. In fixed pressure operation , temperature change in the turbine when load changes, while in sliding-pressure operation ,they change in the boiler The enthalpy increase in the boiler for preheating, evaporation and pressure.
However, pressure is proportional to output in sliding pressure operation In a uniformly heated tube, the transitions from preheat to evaporation and from evaporation to superheat shift automatically with load such that the main steam temperature always remains constant.
At rated and relatively high loads the boiler is operated as a purely once through type. At partial loads, however, the boiler is operated by sliding the pressure as a function of load.
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