Thoughts on Curbing Carbon Emissions
After being pressed hard by public opinion, the Harper government has decided to do something about climate change. Instead of a simple, broad and easy to understand attack on the problem, they have focused on coal-fired power stations.
As I understand it, once they have reached the end of their normal service life, a coal-fired power station will have to be decommissioned. They can no longer be refurbished to extend their working life. That means old, dirty. inefficient power plants will gradually disappear – with 2025 looking like the last gasp for the most modern units.
Alberta, where coal generation dominates, the utility responsible has said that they will begin to convert their power plants to natural gas. That will improve emissions, but probably have no effect on efficiency. Most people are probably not aware of the fact that most North American coal-fired power plants only convert about 30% of the energy in their fuel into electricity. The rest of the energy – 70% – is used to heat Canada . . .
This standard of performance has pretty-well remained constant for half a century. Coal-fired power plant designs with efficiencies of 45% first appeared around then, but they were more costly to build. Why bother if coal is cheap? In Europe, many coal-fired power plants have been built near urban areas or industrial sites where they are able to sell their waste heat. By so doing, they achieve an overall efficiency of 80%. Their electrical output may still represent just 30% of the energy in their coal fuel. However, by putting their waste heat to good use, the society they serve burns considerably less fuel overall.
This is called Combined Heat and Power (CHP). You can only do this if the client for heat is located within one or two kilometers of the power plant – you cannot move heat very far without losing much of it! This also means that the generating station needs to be relatively small. A really large power plant like our Nanticoke on Lake Erie produces such a lot of waste heat that you could never hope to sell it, let alone locate enough industry close enough to the heat source.
The City of Barrie has had a CHP plant for about 15 years. Instead of running on coal, it burns methane from the anaerobic digestion of City sewage. Several years ago this (rather small) power plant generated about $120,000 of electricity. Its waste heat is used to warm the sewage to speed the digestion process. That year it was estimated that the plant produced enough heat that the City would have had to burn $180,000 worth of natural gas to this job! Canadian pulp and paper mills often build their own CHP plants where they burn bark and small branches. They generate enough electricity to power their operation, plus a surplus to feed into the grid. The waste heat is used to heat and partially digest the cellulose out of their wood.
Europe has had CHP plants for a very long time. Some of these began as district heating schemes in which a part of the downtown was heated by a large furnace instead of each building having its own small furnace. It did not take long to realize that such an installation could also generate electricity!
Returning to the plan of converting the power station from coal to natural gas, if you simply change the fuel, the efficiency of the device will remain exactly the same. To improve efficiency, you need to make better use of the natural gas. One way to achieve this would be to burn it in a gas turbine. This is a stationary version of the engine which powers a commercial aircraft. On its own, such an engine will convert ~45% of the energy in the fuel into electricity. However, you can do considerably better than that by using the still hot exhaust gases to raise steam for a second turbine – a steam turbine. Finally, the now considerably cooled exhaust might be able to perform some more work. CHP power plants routinely capture at least 60% of the energy in the fuel – and even more if a use can be found nearby for their waste heat.
Since Alberta was first to announce their plans, it is interesting to think about a HUGE customer for just the low-grade waste heat a generating plant puts out. I am thinking of Alberta’s famous (infamous?) Tar Sands! This deposit of tarry sand is really “kerogen” or incompletely “cooked” petroleum. The Tar Sands are 20% semi-solid hydrocarbon (the tar) and 80% sand. The hydrocarbon can be melted out of the sand by heating it to around 80oC. That, in fact, is more or less the temperature of the waste heat coming from a thermal power plant.
So, if Alberta were to build new electricity generating plants near Fort McMurray, their waste heat could be used to heat the tar sands. At present, natural gas from the Mackenzie River Valley is used for this purpose. If the Tar Sand operations were converted to use waste heat, Alberta might be entitled to call their synthetic petroleum “environmentally friendly”.
Imagine that!
Another Idea
If you Google “Chena Hot Springs”, you can read a story of a small town near the very centre of Alaska. Heating for the entire town was supplied by a hot spring. The townsfolk thought they should be able to generate electricity as well, so advertised for a company to offer to do this. Only one company applied. This was the industrial division of Carrier. The Carrier folk thought they could modify one of the industrial chillers they sell for making ice at hockey rinks or cooling freezer plants.
Basically, chiller consists of a compressor which takes a gas and applies enough pressure to turn it into a liquid. Applying pressure heats the gas, and converting the gas to a liquid creates even more heat. Once this hot liquid is cooled (in the condenser coil), it is allowed to spray into another coil. The spray evaporates in the coil, cooling it. This is the evaporator coil, and where freezing takes place. The cool gas is then picked up by the compressor and pressurised for another circuit. This is the way your refrigerator and air conditioner work.
Carrier modified their unit so that the compressor operated like a turbine, and the motor which normally drove the compressor now became a generator.
The problem with Chena Town’s hot spring is that it isn’t very hot. In fact, it comes out of the ground at 80oC. If you wish to generate steam to spin a turbine, you would like your hot spring to be at least 150oC. But Carrier were not trying to produce steam. They were producing a vapour from the synthetic heat transfer fluid in their chiller. And 80oC was more than enough to create a high pressure vapour. At the other end of the modified chiller they used cold water from a small stream to condense the vapour back into liquid.
In the end, Chena now pays about $0.08/kWh for their electricity, which is about ¼ of the price they used to pay to run their diesel generator. This is now their standby power plant. The $0.08/kWh includes paying the interest on the loan they took out to buy their new Carrier generator. Here in Ontario, we are paying about $0.06/kWh for our electricity + the cost of distributing it around the province. In fact, because Chena has only a tiny municipal grid, their electricity is actually cheaper than ours!
But why should we care? We don’t have a convenient hot spring nearby from which we can extract energy.
Actually, we do – we simply don’t realize it. Each of our thermal power stations (coal or nuclear) discharge heat into the environment. The discharge temperature could easily be 80oC. Even if such a scheme captured only half of the waste heat from a conventional thermal power plant, that would boost its overall efficiency from 30% to at least 60%. In other words, we could double our electricity generating capacity from the same amount of fuel. Even if the original fuel was coal, and burned in a polluting plant, the actual volume of pollution could be halved.
And at $0.08/kWh, this electricity would be far cheaper than the power from any kind of generating plant we could build today. A straight coal-fired power plant, built with today’s dollars, would produce electricity at around $0.12/kWh, while a nuclear power plant would come in at about double that sum! That’s why the Ontario Government balked at commissioning two new nuclear generating stations several months ago – plain, old fashioned “sticker shock.”!
Instead, we could use all those artificial “hot springs” . . . That’s got to be at least as good as a “Fake Lake”. We could have built quite a few of these for the $1.2billion the twin summits are costing us.
And they would have lasted a lot longer than 3 days . . .
Imagine that!
Peter Bursztyn,
Barrie