Operation & Maintenance of Power Plants
Fig. 1. Waste Energy Company Amsterdam.
Optimising waste energy efficiency
Johan van der Kamp and Hans van Essen report on how reheaters were crucial to highly-efficient incinerator.
T
he Afval Energie Bedrijf (AEB) in Amsterdam, Te Netherlands, has been processing waste for almost a century. Te company aims for maximum environmental efficiency, in the sense
that waste does not simply remain waste, but is converted into energy and basic materials. Te waste is incinerated and the energy released is used to generate electricity, while the remainder of the energy released becomes usable heat. As many valuable materials as possible are reclaimed from the ash residue. To produce electricity from the available combustion energy, the well-known cycle of steam boiler, steam turbine (connected to the generator), condenser and feed water pump is used. It is also commonly known that the higher the pressure and temperature of the steam supplied to the turbine and/or the lower the pressure and temperature of the steam leaving the turbine, the higher the conversion efficiency is of fuel into electricity. In a steam turbine not only the pressure of the steam decreases, the temperature also drops increasingly, creating ‘wet’ steam; an increasing amount of condensate forms in the steam. If the water content in the steam becomes too high, it will damage the turbine. As a result, the condensate in the steam ultimately determines the achievable outlet pressure, and thus to a large extent, the shaft power.
When the steam entering the turbine is superheated
steam (dry steam), the moment that condensate formation starts is delayed, and the steam pressure at the outlet of the turbine may be reduced. Te result is a higher shaft power.
Moisture content
Te design goal is not that the moisture content of the exhaust steam determines the pressure at the turbine outlet, but the temperature of the condenser cooling medium, usually the outside air or cooling water. Tere is sufficient cooling water at AEB, so in principle a very deep vacuum could be achieved at the outlet of the turbine. But then extreme superheating of the boiler steam would be required. However, the many corrosive constituents in the
boiler’s combustion gases limit this temperature, since the already very expensive materials would corrode too quickly. To break this circle of limited conversion rate, AEB opted for a two-stage turbine, with reheating the steam between the two stages. Te steam – moderately superheated – flows through the first (high-pressure) stage of the steam turbine, up to a location where, approximately, condensation would start. Here, a connection has been created in the turbine housing for the steam to exit. Tis steam is then extremely superheated in a re-heater and will enter the second (low-pressure) stage of the steam turbine as superheated steam. A very deep vacuum can now be achieved in this second stage. Using a two-stage
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