Waste heat?

I thought this was a great piece this week by Glenn Platt on the anatomy of the Australian electricity network. I just wanted to expand on one point that I’ve been meaning to write about for a while.

“A typical coal-fired power station loses (or wastes) almost 70% of the energy that goes into it, when converting the energy in coal to electricity, and up to a further 10% is lost during the transmission and distribution stage. An old-fashioned light bulb then loses 98% of this energy to make light.”

These numbers are all of similar magnitude to what I have heard, except no one really uses incandescent lights anymore do they? What many people are surprised by in these figures is the 70% waste out of a coal fired power station. Surely there is something we can do with all that wasted energy?

The range of coal plant efficiencies in Australia is approximately 25% for the oldest plants to about 35% for the newest. The old plants could be improved with new technology, but the cost of retrofitting is so high that it is often cheaper to build a new plant. Or just keep operating the old one.

The theoretical maximum efficiency of any system that uses heat to create electricity is governed by the Carnot relationship, or even better the Chambadal-Novikov efficiency. Leaving out the maths, both of these relationships state that a conversion process is more efficient when the hot part is hotter and the cold part is colder. In a power station the hot part is the water in the coal boiler and is usually somewhere between 500 and 600 degrees Celsius. This temperature is limited by the materials available and their cost. An old unit might use copper or stainless steel, where a newer plant would be more exotic materials like titanium alloys. The material needs to be able to tolerate the 1000 degrees and more in the burning coal stream and not corrode due to the steam passing through it. When a steam tube fails it leaks steam into the boiler and once enough go there’s no point running the boiler any more. Shutting down and restarting a coal boiler is a two day job and so they don’t want tubes to fail.

The cold part is essentially ambient temperature, but power stations do a few cunning things with water to create a cold vacuum on the turbine and maintain efficiency. The huge curved concrete cooling towers we associate with power stations, both nuclear and coal, are one of the solutions to keeping water cool. This technology has generally been surpassed and newer plants favour fan forced cooling.

So the maximum heat in the steam tubes governs maximum efficiency, and materials science governs the maximum temperature.

There are however reasons beyond this that plants do not extract every last joule from their coal. Coal is essentially pure carbon, burning which gives CO2. But buried alongside the coal are other elements and compounds, some of which hinder the combustion process while others combust to give other damaging compounds. Water occurs in quite high concentrations in brown coal, as high as 60% in some places. Water, somewhat obviously, hinders the combustion process because some of the energy that should have been heating the steam pipes is actually heating the water in the coal. There are also sulphur compounds, which combust to give SO2, sulphur dioxide, one of the compounds responsible for acid rain. Acid rain forms when water and SO2 mix, and since there is water in the exhaust stream these can combine and form acid in the smoke stack, which causes big problems. To stop this happening they keep the stack temperature elevated, around 130 and above, to ensure that acid does not precipitate in their stack.

Others have regulatory restrictions placed on them which mandate minimum stack temperatures. I know of one plant whose state regulatory authority mandates an exit gas temperature above 150 degrees. This higher temperature helps the exhaust gas rise faster to be dispersed in the jet stream.

Coal plants aren’t running as efficiently as they theoretically could be, but there are good technical and economic reasons why they aren’t.

There aren’t many waste-heat opportunities in the electricity sector that haven’t been exploited to some degree. The best example of this ruthless efficiency is the combined cycle gas turbine. In this system a gas turbine, essentially a plane engine, burns gas and generates electricity. Carnot does apply here, but since the combustion is happening inside the engine the theoretical maximum efficiency is much higher. Practically though, an ‘open cycle’ gas turbine achieves somewhere around 40% efficiency. Gas turbines are mostly trying to convert kinetic energy, expanding gas, into electricity, so the gas that comes out is roaring hot. To capture this heat combined cycle gas turbines employ a steam boiler in the gas stream which also drives a turbine and creates electricity. This arrangement pushes the efficiency up to 60% and higher.

These efficiencies are possible with coal, but it’s difficult and currently cutting edge technology. In these proposed systems the coal is liquefied and sprayed into an internal combustion engine, conceptually similar to a diesel engine. In this case the hot bit is inside the cylinder, with no need to heat water, so the maximum possible efficiency is much higher, over 50% some have suggested. But this is hard to do; coal has all sorts of things in it that you would not want inside your tractor motor  and removing them costs energy.

It’s getting harder to see why anyone bothers burning the stuff at all.

 

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About evcricket

Extreme gardener, engineer and bird nerd. View all posts by evcricket

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