Originally Posted by
Bushmiller
I received an email the other day passed on by a friend. The email contained thoughts on coal fired power stations from somebody purporting to have spent twenty five years in the power industry. This was a statement that I picked up on.
"First coal fired power stations do NOT send 60 to 70% of the energy up the chimney. The boilers of modern power station are 96% efficient and the exhaust heat is captured by the economisers and reheaters that heat the air and water before entering the boilers."
The very slight amount exiting the stack is moist as in condensation and CO2. There is virtually no fly ash because this is removed by the precipitators or bagging plant that are 99.98% efficient. The 4% lost is heat through boiler wall convection."
I have to correct some aspects of this statement.
A thermal fired generating plant comprises three main components: The boiler, the turbine and the generator. The boiler and the generator are very efficient with figures certainly in the high nineties (it doesn't really matter exactly what it is, but it is good), but the turbine or more precisely the condenser is not so efficient and brings the overall efficiency of the power plant down to somewhere between 30% for the older designs and approaching 40% for the most modern installations. This means that 60% or more of the energy produced by the boiler does indeed go up the chimney as it is not converted to electricity. Compare this to the 25% efficiency of you house's slow combustion heater (not that it produces electricity) and a diabolical % of an open fire. In the power plant this loss of efficiency comes from the requirement to condense the steam back to water to be reused. The heat loss occurs in the transition from steam to water (technically referred to as the "latent heat of evaporation", although in this instance it is the reverse process back to water).
Modern power stations attempt to minimise these losses by a number of techniques. The air heater, which is a slow rotating device to warm incoming air to the boiler, is sited in the flue gas path as is the economiser, which pre heats the water entering the boiler. At our plant in Millmerran, water entering the boiler at this point is already about 300°C (still water because it is under high pressure) immediately after the economiser.
Modern plants also incorporate feed heaters where steam is bled off from the turbine at different points and used to pre heat water on it's way to the economiser and subsequently the boiler. All this heating takes place before the water enters the boiler. This takes advantage of the heat without the need to cool it in the condenser. However, there is a limit to how much of this you can do as our primary objective is to drive the turbine. I believe at Millmerran, which has a supercritical boiler, the efficiency is around 38%.
The Reheater, mentioned in the statement above, takes the steam from the exhaust of the HP (high pressure) cylinder of the turbine and returns it to the boiler to gain further heat (at a lower pressure of course) before continuing it's journey through the IP (intermediate pressure) and LP (low pressure) cylinders. Consequently with an efficiency of 38%, 62% of the generated energy is indeed "wasted" in going up the chimney.
