By Kevin Widenmaier, P.Eng.
COAL: FOSSIL WITH A FUTUREEngineering
For most people, coal brings images of chuffing steam engines or a smoggy Victorian London. The lumpy black rock seems to be a remnant of the past, something that fueled the industrial revolution but ...
For most people, coal brings images of chuffing steam engines or a smoggy Victorian London. The lumpy black rock seems to be a remnant of the past, something that fueled the industrial revolution but is hardly relevant today. In fact, not only has coal continued to be a major player in this century, but it is also likely to expand its role as a major energy source for the next 100 years. Today, coal provides 38% of the world’s electrical generation, and 57% in the U.S. In Alberta, the figure is much higher, with about 75% of electricity in the grid coming from coal. Government and industry predictions are that the fossil fuel will continue to play a pivotal role in our future power needs, and not just for power production.
The chief advantages of coal are that it is cheap and abundant. Coal represents fully 91% of the world’s conventional hydrocarbon reserves and it is expected to last for at least another 200 years. Oil and gas are expected to last less than 100 years.
Although coal is much more widely dispersed than either oil or gas, Canada and the U.S. have one quarter of the world’s reserves, making North America to some degree the “OPEC of coal.” Almost half the area of Alberta has coal deposits, and even though we have been using the fuel in Canada since the last century, large coal fields, especially in the Far North, remain virtually unexplored.
Coal is so important to Canada that recently the Geological Survey completed the first step in a major characterization of Canadian coal fields. This ongoing project involved nearly 40,000 boreholes from coast to coast in order to document our reserves. This information is integrated into a software tool that allows users to see 3-D visualizations of coal beds and drill “virtual boreholes” across Canada (see http://spans.gscc.nrcan.gc.ca)
The high quality coals that are used extensively in raw steel production represent a significant export for Canada and have a high value. The lower quality coal that is used to produce power at the mine mouth, on the other hand, is insensitive to market prices. Consequently, the fuel price for such a power plant is low (approximately $0.75/GJ for coal compared to $6-$15/GJ for gas), and does not vary significantly over the 30-year life of a facility. Given its wide availability and low cost, therefore, coal is uniquely positioned to provide the energy needs of the future.
Getting to the dirt
Since the days when houses and factories belched out smoke from their chimneys and coal dust cast a black pall over industrialized cities, people have thought of coal as a dirty fuel.
It is true that coal inherently contains many contaminants including sulphur, nitrogen, ash and water. It is not unusual for coal in the as-mined condition to be 50% non-combustible (mostly ash and water). And even though most coal contains less than 3% sulphur, sulphur oxides (SOx) are a major pollution concern, being linked to acid rain.
However, current technologies coupled with strategic combustion and fuel choices, enable emissions of SOx, nitrogen oxides (NOx) and particulates (the most significant emissions from coal) to be far less than they were even 15 years ago. Furthermore, advances in plant efficiency mean that power that took eight kilograms of coal to produce in 1902, now requires only one kilogram.
As world population grows, pressure is mounting to make fossil-fuelled power plants reduce emissions further. New legislation in the U.S. targets mercury emissions, and controls on greenhouse gas emissions such as carbon dioxide (CO2) loom on the horizon. Though less than 20% of worldwide CO2 emissions come from coal, these tend to be concentrated in “point sources” such as power plants, making them easy targets for legislation.
Retrofit systems are being developed to control emissions by adding scrubbers to plant stacks and chemically removing the offending substances. However, these tail-end technologies can significantly reduce a plant’s efficiency and output, and they cost hundreds of millions of dollars.
Beyond these end-of-stack solutions, the next step is to move from the traditional pulverized coal-fired boilers to newer, more efficient technologies. The efficiency of power generation can be increased by raising operating temperatures and pressures, which in turn reduces the emissions per unit of electricity produced. Advanced materials such as high temperature chrome steels or nickel-based super alloys in the boiler allow for supercritical steam conditions far hotter than are achieved in most plants today. Methods already exist to create a “stackless” power plant, which would generate no emissions at all.
Going to gas
A different approach entirely — gasification — is required to realize the full advantages of coal. Gasification is a potentially high-value use for the material and can be likened to refining or upgrading oil. The coal is oxidized with small amounts of air or oxygen under high pressure to break down the volatile components. This process can be optimized to produce SNG (substitute natural gas), a high heating value gas. Or, it can be made to produce syngas, a combination of hydrogen, carbon monoxide and other chemicals.
Gasifiers are especially appropriate for co-production applications. In this scenario, a coal gasifier is used to make hydrogen, carbon monoxide and steam as feedstock to a refinery. Some syngas is used in a gas turbine combined cycle to make electricity and steam. When the refinery is offline or downrated, the plant can sell the electricity. When the plant is at full production, the electrical generation offsets refinery demands, and the steam is used in the process. Further treatments and refinements can be used to produce high value transportation fuels and anything that can be derived from crude oil. The process has been used for years in South Africa to make both gasoline and oil.
Coal gasification is an inherently low emission technology, allowing it to meet new regulations, and is particularly suited to the capture of CO2. Since the gasification process can produce a concentrated stream of high-pressure CO2 before combustion, it is much easier to capture than afterwards. Removing the carbon before burning a fuel is much more economical than trying to remove CO2 from the flue gas, where it might make up as little as 3% of the low pressure exhaust. In fact, considering CO2 capture, a gasification plant coupled with a gas turbine is more economical that a conventional gas turbine combined cycle at gas prices above about US $4 per GJ.
Coal gasification is well suited to meet the U.S. Department of Energy’s Vision 21 Plant, a program to promote a virtually pollution-free, co-production plant, making electricity and chemical feedstocks at a high efficiency. As well, there has been much talk lately about the upcoming “hydrogen economy” that will replace today’s fossil fuel energy resources. High-efficiency fuel cells will convert the carbon-free hydrogen into electricity. What people don’t realize is that most hydrogen produced today comes from natural gas and is used within refineries. Why convert a high value resource like natural gas into a low value commodity like electricity, even in a fuel cell? Any potential future hydrogen economy relies on finding a “cheap” source of hydrogen. Gasification allows the conversion of a relatively low-value commodity, coal, into a high-value, transportable one, hydrogen.
The negative image associated with coal has meant that the industry has kept a relatively low profile in the past, to the point where most people don’t know that a large portion of the world’s electricity comes from burning the material. However, coal’s wide availability and long-term reserves, along with new technologies that broaden its usefulness while making it more environmentally benign, mean we shall continue to rely on this resource. It is ironic that coal — the fuel of the industrial revolution — is poised to power the world of tomorrow.
Kevin Widenmaier, P.Eng. is a research engineer with TransAlta in Calgary who has been a plant engineer at several th
ermal power plants. This article is based in part on a presentation made at the Canadian Institute of Energy September 2000 luncheon in Calgary (www.CIECalgary.com).