Natural Gas, More Polluting than Coal? Only According to the IPCC. A Note from Cementafriend
METHANE is the major component of natural gas (>94 percent) and coal seam gas, which are claimed to be good clean fuels on the basis they have a lower carbon content than coal or oil.
Is this the same methane from the backsides of farting cows (or to a greater extent from the mouth of ruminants) that is considered to be a bad greenhouse gas and 21 times more potent than carbon dioxide? And the same methane that when escaping as a fugitive (lost through leakage) gas from coal mining is considered bad?
This is contradictory.
In the following note I consider how potent methane actually is as a greenhouse gas and then compare energy equivalents per heat absorbed all in carbon dioxide equivalents.
Methane (CH4) only absorbs significant radiation from the earth’s surface at around 288K, in the small range of 7.4-7.8 micron. By eye the absorption is less than one tenth that of CO2, see the Diagram (double click on the image for a larger view).
Yet we are continually told that methane is 21 times more potent as a greenhouse gas than CO2.
This is what the IPCC tells us.
I have contacted various respected climate scientists (on both sides of the AGW debate). But no one has been able or wished to provide a definite answer.
So I have made my own assessment.
When one burns CH4 in air the chemical reaction is:
CH4 + 2O2 > CO2 + 2H2O.
That is, methane combusts to form carbon dioxide and water vapour.
Water vapour absorbs IR close to 100 percent in the range 4.5 to 8.0 micron (completely overlapping CH4), raising from zero at about 12.5 micron to close to 100 percent at about 16 micron and then 100 percent above 16 micron into the microwave range, see the Diagram.
CO2 is only a significant IR absorber in the range 14 to 15.5 micron (with a peak at 14.8 micron) but there is an overlap with water vapour.
From a visual inspection of the amount of radiation transmitted, Diagram 1, it can be justified that the e-m absorption of water vapour in the range 4 to 40 micron is at least 10 times that of CO2.
So, CH4 equivalent IR absorption = (1* CO2 + 2*10*CO2) =21*CO2
In other words, that figure of 21 times more potent is actually calculated in water vapour equivalent.
So we have established that methane, the major component of natural gas, when it is burnt produces quantities of greenhouse gases. But it is called a green fuel. Why?
It is common practice for energy supply companies to give the energy content on a gross basis but in reference texts such as Perry’s Chemical Engineering Handbook the heats of formation and combustion energies are given for net and gross energy.
The gross energy includes the heat of condensation of water (2.3 GJ/t H2O) which in combustion processes is not available for heat transfer. To compare fuels, only the net heat should be used.
Reference values from Perry’s Chemical Engineering Handbook for the net heat of combustion are Hydrogen (H2) =120 GJ/t, Methane (CH4) =50 GJ/t, Ethane (C2H6) = 47.5 GJ/t, Carbon (C) =32.8 GJ/t, Carbon monoxide (CO) = 10.1 GJ/t
For a typical black coal with an ash content by weight of 15 percent and a delivered moisture of 7 percent and a typical natural gas with 94 percent methane, 2 percent ethane equivalent, and 4 percent CO2 by volume the following applies:
So, if we apply the IPCC methodology for methane as a greenhouse gas, to methane as a fuel, you more than double the emissions for the same energy when natural gas is used instead of coal.
Now let’s go back to the fugitive methane. In Perry’s Chemical Engineering Handbook the following can be found:-
a) ignition temperature in air =650C
b) lower limit of flammability (% gas in mixture)= 5
c) higher limit of flammability (% gas in mixture)= 15
It should be very clear that methane released into the air does not burn but to summarize this point for methane in the atmosphere a) at 1,7ppm it is below the flammability limit and b) everywhere in the atmosphere is below the ignition point (this even applies at points of high emission such as a swamp, landfill vent etc)
The level of methane in the air has been measured since at least 1980 and data from ice-cores is also available showing increases in CH4 up to the year 2000 and then levelling and possibly declining at approximately 1730 ppb (1.73ppm) (see Tom Quirk in Energy and Environment).
The existence of CH4 in the atmosphere is proof that CH4 does not burn.
There is a natural cycle for methane with sources and sinks. It is slightly soluble in water (oceans) and is absorbed by some plants and algae and bacteria. It can be oxidised in lower atmosphere by ozone (produced by lightning, electrical arcing such as welding, and breakdown of NOx emission by sunlight) such as
CH4 + O3 > CH3OH (methanol) + O2.
The methanol and other –OH radical compounds are highly soluble in water. This is part of the natural cycle. The removal of ozone in the lower atmosphere has health benefits.
In summary, if one is concerned about greenhouse gases then the statement that natural gas or coal seam gas (methane CH4) is a friendlier fuel than coal (i.e. good), and the statement, that fugitive methane (CH4) from coal mining or animal emissions is bad, are both false.
1. Perry’s Chemical Engineering Handbook 4th Edition & 7Th Edition
2. Tom Quirk has an article in Energy & Environment July 2010 (abstract http://multiscience.metapress.com/content/m7337203x121g1hh/?p=42ddd03a121f46138f01ccf97183c9ff&pi=4)
3. The source of the diagram http://en.wikipedia.org/wiki/Absorption_band
My superannuation fund has shares in Origin Energy which produces, distributes and burns (in power stations) natural gas and coal seam gas & shares in BHP-Billiton which produces oil, gas and coal – I am pragmatic.
Chart from comment 3 below- my thanks Sunsettommy