The producers of natural gas claim that it is the most "ecological" of all fossil fuels. If examined as fuel at the point of use, this may be true. However, this is very untrue if studied holistically and is, probably, the worst of all forms of fossil fuels. The purpose of this essay is to demonstrate that natural gas is a very heavy contributor to greenhouse gas emissions in many ways, probably even worse than coal, and its use should be curtailed to a minimum.
Natural gas (NG) is produced by the anaerobic decomposition of living matter. It was so named because one can see bubbles of it rising from the sediment of stagnant pools or the "will-o'-the-wisp" over marshland. Chemically, it is methane. It can also be collected from composting. However, probably more than 99% of the so-called natural gas that is used throughout the world as a fuel is anything but natural today, although its origin may have been 100 million years ago. It is extracted by man from deep in the earth's crust. As such, it is a fossil fuel, just as much as coal or oil. It is a fossil fuel which is 75% carbon.
Theoretically, whether it powers a fuel cell, is burnt in a gas ring, in an internal combustion engine, in a gas turbine, in a power station or in a central heating boiler, the result is the same. It reacts with oxygen in the air to form carbon dioxide and water vapour:
CH4 + 2O2 > CO2 + 2H2O
This reaction is very exothermic, that is, it produces a large amount of heat. In other words, the chemical energy stored in the methane is converted into thermal energy that may be used to cook a pan of spaghetti, drive a car or generate electricity etc.
Assuming the combustion is complete, the only apparent pollutant produced is the carbon dioxide, but, for every kilogram of methane that is burnt, 2.74 kg of carbon dioxide is produced, yes, nearly 2Â¾ times as much. Carbon dioxide is the most prevalent greenhouse gas, responsible for climate change.
Unfortunately, it is not as simple as that. There are four factors which make matters worse:
This where a large part of the crunch lies. Methane in the atmosphere is a powerful greenhouse gas. In fact, it is between 20 and 50 times worse than carbon dioxide, depending on what are called the free hydroxyl radicals that are present in the atmosphere, which are very variable (low concentrations of OH radicals decompose methane more slowly than high concentrations). Let us assume, for ease of argument, that the global average Global Warming Potential is 35, although it is likely to be higher in desert country and in polar regions, both with considerable NG production. (I have noted that some web sites authored by vested interests in NG cite the figure of 21, a figure which would be typical of a hot, very humid climate such as in a tropical rain forest. This low figure is not realistic in real life, except in a few relatively minor producing countries, such as Malaysia, Indonesia and Brunei.)
It is therefore clear that allowing NG to be emitted is far worse for climate change than burning it.
Natural gas, as it comes out of the ground, is very variable in composition, depending on the location. An average composition, synthesised from many sources throughout the world, and which I'll use for further discussion, is:
NG comes from both ad hoc gas wells, but also as a boil-off from crude oil (bubbling, much like the gas in soda water), the latter with more heavier HCs.
As I mentioned earlier, commercial NG is usually 95 to 99 per cent methane, averaging about 98 percent, the other 2 per cent being mostly ethane with traces of all the other gases. This implies that the NG must be purified before it is distributed.
There are many processes used for NG purification. A typical process line may include:
A purification plant is an important infrastructure and one plant may serve many wells over a considerable area, with a spider's web of small bore pipework. This is often cast iron pipes with flanged joints, notable for leaks. At the wellhead, there is a "tree" for initial separation of gross impurities, including sand, by purging them out with the gas.
The unpurified gas may be very corrosive, especially from "sour" gas wells with high water vapour and sulfur content. This means the lifetime of the pipework to the purification plant is limited and it must be regularly inspected for leaks.
Once purified, the gas has to be distributed to the end user. This is done by either liquefaction (LNG) or compressed by pipeline (CNG). Of course, the liquefied gas is eventually returned to gaseous state and compressed.
LNG is produced by refrigeration down to -163 Â°C at atmospheric pressure. It is then stored in large, double walled, well-insulated, spherical or cylindrical tanks in high-nickel steel, rather like enormous Thermos flasks. These tanks are not pressure vessels and have to be vented by pressure relief valves at, typically, 300 hPa, so that there is no risk of damage as the contents heat up, no matter how good the insulation.
The liquefaction process itself is done in two stages, initially a pre-cooling in a propane refrigeration circuit and then in a mixed gas one. It is quite a complex process requiring a great deal of energy. This is often supplied by gas turbines using the gas vented from the storage tanks at the liquefaction plant and raw methane. The resultant liquid-phase methane has a volumetric ratio of 1:593 compared to gas-phase methane. Unfortunately, I have not been able to obtain figures for the emissions or gas consumption (energy) at liquefaction plants but they are far from negligible.
Methane gas is easily converted to CNG. Three kinds of energy source for compressors are used: gas turbines running on NG, reciprocating engines running on NG and electric motors. Pressures up to 240 bars are sometimes used for bottled methane, but most pipelines run at 15 to 100 bars.
Some leakage is almost inevitable with compressors, especially as they age. Much maintenance is required to keep emissions to a minimum.
Most major pipelines are constructed of rolled sheet pipes with a welded seam and with sections welded together. When new, these are almost perfectly leak-free. However, they are generally buried at depths of typically 1.5 - 3 metres and the steel can corrode either from within or without. Corrosion is minimised by treatment with a coal tar coating, but this does not last for ever and leaks do develop over time.
Compressors are placed every 50 - 150 km along a pipeline, and isolating valves at 10 - 30 km intervals. Small leakages occur at every valve, through the stuffing and flange gaskets.
Pipelines need to regularly checked and maintained. Leaks are detected by portable gas detectors along the ground over the pipes. However, the greatest emissions are made when "pigging" a pipeline. A section of pipeline is isolated by closing the valves at each end and unscrewing the flanges. A very high-tech robot, nicknamed a "pig" is introduced into the pipe and sent from one end of the section to the other, examining the internal surface for weld problems, corrosion or leaks and transmitting the information back to an analytical computer. Obviously, this vents the gas in the section. After "pigging", the section has to be purged of air, before it can be put back in service and this, too, also involves considerable emissions. It is rare to either collect or flare the gas in the sections.
Small pipelines, particularly distribution pipelines in cities, are often relatively small bore flanged cast iron pipes, especially in older quarters. The leaks at the flanges are often aggravated by vibration from heavy traffic. Newer ones may be extruded steel from a punched blank, with welded joints, but street stop cocks are inevitably flanged. Some household distribution systems use welded plastic pipes, which are inevitably slightly porous.
It is difficult to obtain precise figures of emissions. Global estimates vary between 25 - 70 teragrams where 1 Tg = 1012 g or 1 million tonnes. This represents about 3 - 9 percent of all NG extraction. For convenience, I'll assume an average of 5%, although the precise figure cannot be substantiated. This represents about 40 Tg of methane, which will have the same climate change effect as about 1,400 million tonnes of carbon dioxide, a far from negligible quantity. The figure of 40 Tg is probably conservative as one major source cites 45 Tg (Tetlow-Smith, 1995). For comparison, the total weight of carbon added to the atmosphere annually from the combustion of fossil fuels is estimated at 7,000 million tonnes, of which 2,000 Mtonnes are naturally sequestered, mainly in the oceans.
The emissions can be divided into those:
The biggest studies of emissions have been carried out in the USA, with some 6 - 7 Tg/year, representing about 1.4 - 2 percent of NG consumption. However, this figure excludes unburnt gas emissions at the users' premises, drilling emissions and those due to gas produced and transported outside the USA but consumed in the country. As imports of LNG were over 15 percent of all NG consumption in the USA in 2002 and are expected to top over 20 percent in 2005, this is not a negligible factor considering that shipping LNG over several days to a few weeks from liquefaction to regasification will involve considerable emissions. It is therefore expected that the holistic figure of emissions due to all gas consumed in the USA will be as high as 4 to 5 percent. In many other countries, especially those with poor equipment and antiquated pipelines, the percentage of emissions would be worse.
Domestic emissions in the USA, excluding well-drilling and extraterritorial emissions of gas consumed in the USA and also excluding emissions due to incomplete combustion at users' premises, have been given as follows:
From the above data, an approximation of the effect that the use of NG will have in the climate change equation may be made (figures rounded off):
Assume that the user consumes, in a given length of time, 100 kg of NG, of which 98 percent is combusted. This will produce 98 x 2.74 = 268 kg CO2 + 2 kg methane = 70 kg equivalent CO2 = 338 total
Assume that 5 kg of methane has been emitted from the well-head to consumer system = 175 kg eq. CO2 = 513 cumulative total.
Assume that the energy requirement for the transport (compressor, liquefaction etc.) has consumed 5 kg of NG, totally combusted = 14 kg CO2= 527 cumulative total
Assume that 110 kg of gas is required from the purification plant and the input gas from the well-head is 85% methane and the conversion efficiency of the purification plant is 90%. 144 kg of gas is required for the process, of which 14 kg is converted to CO2 from the process = 38 kg CO2 = 565 cumulative total
Assume 144 kg of gas produces 11 kg of ethane, 1.5 kg of butane and 1 kg of propane, which are all subsequently converted to CO2 (burnt or decomposed), producing respectively 16, 1 and 1 kg CO2 = 583 cumulative total of equivalent CO2
Assume 144 kg of gas produces 3 kg of CO2 = 586 cumulative total of equivalent CO2
For comparison, burning 100 kg of pure carbon would produce 367 kg of CO2 Best Welsh anthracite coal is 91 percent carbon and 7 percent hydrocarbons. It would need about 115 kg of anthracite to equal 100 kg of methane in terms of usable heat produced (the equivalence is difficult to calculate as the difference in temperature of combustion makes losses non-equivalent). This would produce about 445 kg of CO2 or about 25 percent less than methane. However, this comparison is not strictly fair, because the carbon dioxide produced during the mining and transport of the anthracite has not been calculated in. Nevertheless, even if we add an extra, say, 15 percent for this, natural gas produces more greenhouse gas than coal when viewed holistically.
Natural gas is not the least polluting of fossil fuels, as the large oil producers would have us believe. In terms of purity, it is good, but greenhouse gas emissions are holistically very high from its use. It has been proposed as a substitute for petrol in internal combustion engines, but it is believed that this will increase greenhouse gases, especially as the disconnection of pressure hoses at filling stations will inevitably release raw methane into the atmosphere.
Many approximations have been made in these calculations, but these have been made conservatively and in good faith. Unfortunately, accurate data permitting a better calculation are not available.
I conclude that the use of natural gas would be better curtailed if we are to improve our record for greenhouse gas emissions. As a final word, methane concentrations in the atmosphere have increased to 2.25 times the pre-industrial level, compared to only 1.3 times for carbon dioxide, entirely due to man-made causes. As the atmospheric residency time of methane is only a small fraction that of carbon dioxide, even with low hydroxyl radical concentration, cutting emissions would have a much faster effect on reducing climate change effects than cutting down on other fossil fuel combustion.
Tetlow-Smith, A. (1995) Environmental factors affecting global atmospheric methane concentrations. Prog. Phys. Geog. 19, 336-50
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