From a global warming standpoint, is burning coal worse than burning other fossil fuels?

Dear Cecil:

I've heard various pundits say that coal-generated electricity contributes disproportionately to global warming. Why is it any worse than electricity generated from any other carbon source? It would seem to me the oxidation of carbon to carbon dioxide creates the same CO2 and the same energy to generate electricity, and so electricity generated from natural gas, oil, or coal would contribute equally to global warming.

Cecil replies:

There are three aspects to how burning coal for electrical generation can contribute more to CO2 production than burning other fossil fuels, such as oil or natural gas.

The first aspect is looking at the fuels in terms of the amount of CO2 produced per unit of energy released. I’ll talk in terms of British Thermal Units of energy (BTU), since regardless of the units used the proportional CO2 generation between the fuels is constant. In terms of the raw amount of carbon contained in the fuel, coal generally has more per BTU produced than any other fuel, followed closely by oil, and then by natural gas. This is just a matter of chemistry – on a per-BTU basis oil and natural gas have more hydrogen atoms to burn, and fewer carbon atoms to burn, than coal. Let me share with you a chart with some examples of fossil fuels, examining their composition on what we call an “ultimate analysis” basis. Note that these values are just typical values – fuel quality can vary considerably among coals, and there could easily be a +/- 10 percent range in carbon and hydrogen content for any general type of coal (and much greater ranges in other coal quality data.)

Fossil Fuel Ultimate Analysis and CO2 Production

Percentages Anthracite Coal
Pennsylvania)
Bituminous Coal Sub-bituminous Coal
Carbon, % 84.2 72.3 51.0
Hydrogen, % 3.0 4.6 3.5
Nitrogen, % 0.9 1.5 0.6
Sulfur, % 1.3 1.8 0.3
Ash, % 7.5 7.0 5.3
Water, % 1.9 6.3 27.3
Oxygen, % 1.2 6.5 12.0
Heat Content, BTU/lb 14,000 13,000 8,800
CO2 Production, Pounds/Pound Fuel 3.09 2.65 1.87
CO2 Production, Pounds/MBTU Fuel 221 204 213
Percentages Lignite Coal #6 Fuel Oil Natural Gas
Carbon, % 38.5 85.7 72.0
Hydrogen, % 2.6 10.5 23.8
Nitrogen, % 0.6 0.4 3.1
Sulfur, % 0.9 2.8 0.0
Ash, % 18.0 0.1 0.0
Water, % 31.0 0.1 0.0
Oxygen, % 8.4 0.4 1.1
Heat Content, BTU/lb 6,400 18,266 22,900
CO2 Production, Pounds/Pound Fuel 1.41 3.14 2.64
CO2 Production, Pounds/MBTU Fuel 221 172 115

(Note: Anthracite coal is specified as “Pennsylvania” because low-BTU anthracite coals, such as Korean and Chinese, would change the calculations…this is probably more information than you want to hear, but it’s important given our international audience).

You can clearly see that in terms of a CO2 produced per MBTU of energy released, coal is much higher than oil or natural gas. The second aspect of how burning coal is different from other fossil fuels deals with how efficiently the heat from burning the fuels can be turned into electricity. In this case burning coal is generally less efficient because of the most common technologies used for power production in the world. Almost all coal power plants in the world run off what’s known as the Rankine cycle, where they burn the coal to heat steam in a boiler, which then is used to generate electricity by turning a steam turbine. While some oil and gas power plants are also based on the Rankine cycle, they are not very common. Most newer power plants burning oil or natural gas use what’s known as gas turbines, which are essentially stationary jet engines. A more advanced type of gas turbine, called a combined-cycle gas turbine, couples a gas turbine with a steam plant running off of its exhaust, and has the best efficiency of all. Gas turbines are much more efficient than Rankine cycle steam plants, with the most advanced models having efficiencies as high as 55-60%[5] for turning the energy in the fuel into usable electricity. Some typical fossil fuel power plant conversion efficiencies are listed below, keeping in mind that there is a wide range of power plants and fuels which lead to some variation in these numbers.

General Ranges for Power Plant Efficiency

Fuel Heat to Electricity
Conversion Efficiency, %
Old Coal Power Plant 15-20
Typical Coal Power Plant 20-35
New Coal Power Plant 35-40
Simple-cycle Gas Turbine (Oil or Gas Fuel) 32-40
Combined-cycle Gas Turbine (Oil or Gas Fuel) 45-55

[5]

So if you combine the aspects of fuel carbon content per BTU with generating plant efficiency, you can see why coal power produces much more CO2 per kWh generated than natural gas or oil. The final aspect of how coal is responsible for more CO2 production addresses the sheer amount of electricity generated by coal, relative to other fossil fuels. In the United States, coal is responsible for about 48%-55% of our annual electric generation, depending on the year, the economy, and other factors. On a world-wide basis this figure is about 40%-45%, again depending on the same factors. A comparison table below shows how the other fuels stack up against coal in terms of electrical generation.

Sources of Electrical Generation, United States and the World

United States World
Coal 48.58 41.62
Oil 1.58 5.78
Natural Gas 21.47 19.65
Nuclear 19.39 15.03
Renewables 8.45 18.50
Other 0.53 -0.6

[2 pg. 226 for US Energy, 6 pg. 62 for World Energy] (Note: rounding from independent sources may make the “Other” category negative.)

One final note – we’ve focused on electrical power generation, because coal isn’t really useful nowadays for transportation or residential use. But just as an interesting noteworthy fact, if we look at the total energy picture, and not just electrical power generation, in terms of CO2 contribution to the atmosphere in the United States it is actually petroleum use – from our cars, trucks, and airplanes – that generates more CO2, not burning coal.[1 pg. 13] ~ [B]Hi Cecil! I’ve been reading your web at some time, first I want to say sorry for my bad English I’m from Indonesia. I’ve been wanting to know how many barrels of oil produce energy equivalent with 1 tons of coal. I know it’s a little bit much of maths, and crude oil itself can be made to different products, and as for coal they have their own different calories. I just want to know how expensive the oil in terms of energy produced compares to the coal in lets say US/BTU ? or US$/Joule? — [email]offlimit@hotmail.com[/email] [/B] Well, as you correctly noted the heat content of coal will vary quite a bit depending upon the type of coal. Let’s just have me cut to the chase and list the equivalent heat content of a US and metric ton of various different types of coal, in terms of barrels of oil. Note that I’m going make some assumptions here: * All energy contents will be on a higher heating value (also known as gross heat content) basis. (note – should I explain this, or even just delete the comment? It makes a bit of a difference to the calculations). * When we speak of “oil” we mean “residual fuel oil”, which will be assumed to have an energy content of 6.287 million BTU/barrel. Note that lighter distillate fuel oil would have an energy content of 5.825 million BTU/barrel.[2 pg. 359] * Anthracite (Pennsylvania) is assumed to have an energy content of 14,000 Btu/lb, bituminous coal 12,500 Btu/lb, Indonesian sub-bituminous coal 9,300 Btu/lb, and lignite coal 6,000 Btu/lb.

Barrels of Oil Equivalent of Various Coals

US (short) ton Metric ton
Anthracite Coal (Pennsylvania) 4.45 4.91
Bituminous Coal 3.98 4.38
Sub-Bituminous Coal (Indonesian Adaro) 2.96 3.26
Lignite Coal 1.91 2.10

In terms of cost, the situation becomes much more complicated. Both coal and oil cost is a function of a large number of factors, including the initial production cost, transportation cost, handling and other loading/unloading costs, fees and taxes, and even things like union health care funds. In addition to this, oil futures, contracts, and hedging issues can mean that one power plant might pay $50 per barrel for oil, and another next door might pay $75 per barrel. And in the case of coal, where the power plants are often a captive consumer due to poor rail line routing, some of the costs are really “whatever the market will bear”, which further complicates things. I worked for a power plant once which was buying coal for years at $70 per ton delivered cost, and as soon as the power plant hired me to start seriously evaluating bids for other coal supplies, the mine and railroad got together and said in effect “oh, did we say $60 per ton? We meant $50 per ton. Now let’s all go out for a nice round of golf and a steak dinner at our private resort.” That difference of just $10 per ton resulted in a net difference of millions of dollars per year for that plant. In late 2008 I visited a couple of power plants that receive coal by truck from the same group of local mines – the two power plants are so close, they have the same exit from the highway, and easily within sight of each other. Yet one pays $45 per ton, and the other pays $65 per ton. Why the difference? It’s not really clear, but it’s all economic-based, rather than te

Send questions to Cecil via cecil@straightdope.com.

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