Sunday, May 11, 2008

Coal is Dirtier than Gas.

There is much discussion currently concerning the relative merits of coal or natural gas as a fuel. It is debatable just how much extractable coal is in the earth, and there are estimates ranging from10 trillion tonnes down to around 0.5 trillion tonnes. To put this into perspective it is thought there is around 1 trillion barrels of readily extractable crude oil, or around 0.15 trillion tonnes, and a comparable amount of natural gas that will be recovered. Coal, then still looks like a good bet in terms of its quantity.

The heat of combustion of methane (which is what natural gas is mostly) is 891 kJ/mol, which amounts to 1 x 10^6 (g/tonne)/16 (g/mol) x 891 x 10^3 = 55.69 GJ/tonne. This can be compared with around 28 GJ/tonne for coal (the figure varies according to the nature of the coal, but this is a fair estimate).

When methane burns the process can be expressed as: CH4 + 2O2 --> CO2 + 2H2O

and for coal (being largely carbon) as: C + O2 --> CO2

Thus each tonne of methane yields 44/16 = 2.75 tonnes of CO2, while each tonne of coal yields 44/12 = 3.67 tonnes of CO2.

But since less heat is obtained per tonne of coal than per tonne of methane, we need to burn more coal to get the same amount of heat from it, and so a relative CO2 yield per unit of heat can be derived:

3.67/2.75 x 55.69 GJ/tonne/28 GJ/tonne = 2.65, or over two and a half times as much.

Either way, using natural gas or coal, produces a lot of CO2. For example, a typical 1 GW power plant burns 3 million tonnes of coal per year or half that amount of natural gas, and produces around 11 million or 4 million tonnes of CO2, respectively. For oil-fired stations (which exist mostly in Asia but are being phased-out and converted to coal), approximate thermal values of 42 GJ/tonne are often quoted, and there are some at 45 GJ/tonne, depending on the exact nature of the oil. For comparison, a value may be obtained for n-octane (a reasonable model for oil-based refined fuel) of 47.87 GJ/tonne.

We burn around 7 billion tonnes of "carbon" annually, which ends-up as 26 billion tonnes of CO2 - of which only about half is removed by natural processes, including photosynthesis. Thus the atmospheric concentration of the gas can only increase, unless we were to reduce fossil fuel use by around 50%.

People talk much about renewables, since they are ideally sustainable and non-polluting. i.e. They don't consume irreplaceable reserves like coal and gas and oil, or contribute to greenhouse-gas emissions, but matching the amount of them that we get through to sustain our current quality of life, by renewables is a long way off, if it can be done at all. Localised energy production, e.g. through CHP and micro-hydro generation might come some way to providing for relatively small communities, if the dearth of oil and hence transportation fuel forces civilization to take this course, but generating enough electricity to run power utilities on the scale of conventional power stations and the national grid will prove extremely challenging.

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