Natural gas: a cleaner burning alternative

ABCs of fossil fuel that produces less GHG emissions than oil or coal



Seen for many years as the poor relation to high-value liquid fuels such as gasoline and diesel, some people now think natural gas has become the fuel of choice in the age of greenhouse gas concerns and of heightened awareness of the dangers of air pollution.

New technical breakthroughs in the production of natural gas from vast deposits of a gas-holding but impervious type of rock known as shale have revolutionized the natural gas supply situation, creating the possibility of plentiful energy supplies for many years to come in North America. In addition, plans for gas pipelines from the northern regions of Canada and Alaska offer the possibility of future North American gas supplies from major Arctic oil and gas fields.

There also is a burgeoning worldwide trade in liquefied natural gas, or LNG, from huge gas fields in places such as Qatar and Australia, and countries such as China are substantially growing their use of natural gas as a fuel. Russia has major natural gas resources, and there are major gas fields offshore northwestern Europe.

But just how clean is natural gas as a fuel, and why do some people see this particular material as a useful transition fuel into a future of more renewable energy sources?

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The chemistry of fossil fuels

Apart from the fact that there are plentiful supplies of natural gas around the world, the appeal of the fuel really derives from its chemistry.

All fossil fuels, including natural gas, contain some combination of carbon and hydrogen, either in the form of nearly pure carbon, as in coal, or in the form of compounds called hydrocarbons in the case of liquid or gaseous fuels. And when these fuels ignite they generate energy by burning carbon to carbon dioxide and hydrogen to water vapor.

Carbon dioxide is a greenhouse gas that traps heat in the Earth’s atmosphere.

Water vapor also is a greenhouse gas, thought to be primarily responsible for maintaining global temperatures at high enough levels to maintain life as we know it.

But whereas there is potential for reducing the amount of carbon dioxide emitted into the atmosphere, water vapor cycles into and out of the atmosphere much more quickly than does carbon dioxide, with the amount of water vapor in the air determined by weather conditions rather than human activity.

Most climate scientists believe that the long-term buildup of human-produced carbon dioxide, if not brought under control, will lead to dangerously elevated global temperatures. But different forms of fossil fuel contain different proportions of carbon and hydrogen and, thus, produce different quantities of carbon dioxide for the same amount of output energy.

Natural gas is mostly methane

Natural gas essentially consists of methane, a compound with molecules containing one carbon atom and four hydrogen atoms. Liquid fuels such as gasoline, on the other hand, contain complex hydrocarbons, with fewer hydrogen atoms per carbon atom than methane. For example, octane, a component of gasoline, contains less than two hydrogen atoms for every carbon atom. Methane’s relatively high hydrogen content causes the exhaust from the burning of natural gas to contain a lower proportion of carbon dioxide relative to water vapor than the exhaust from the burning of other fossil fuels.

That lower proportion of carbon dioxide in the exhaust from natural gas usage gives the fuel its advantage in the battle to manage carbon dioxide emissions. In addition, with natural gas tending to contain few impurities, the exhaust from burning this particular fuel typically contains very low levels of pollutants such as nitrogen oxides.

Coal is carbon intensive

Coal, by comparison, is the most carbon-intensive of the fuels—the exhaust gas stream from fully burned coal consists predominantly of carbon dioxide. And, depending on what impurities the coal contains, the burning of coal typically produces varying amount of other gases, potentially including sulfur dioxide and various oxides of nitrogen. Some coal contains mercury. And if the coal is not burned at high efficiency, the resulting smoke contains waste carbon particles that form “black carbon,” another climate-warming agent in the atmosphere.

There are modern technologies for scrubbing pollutants from the exhaust from coal burning, although these technologies add to the construction and operation costs of, say, a coal-fired power station.

On the other hand, even natural gas does produce a significant quantity of carbon dioxide when burned, so that unless the exhaust carbon dioxide can be captured and stored in some way, the use of natural gas will slow down rather than stop the buildup of carbon dioxide in the atmosphere.

And there is another issue with natural gas: Methane, the gas’s main constituent, is itself a powerful greenhouse gas, with a global warming potency of perhaps 20 times that of carbon dioxide, depending on the time span over which people estimate its effects.

So, the careful handling of natural gas during production, transportation and use of the fuel is essential, to minimize spillage and thus prevent the inadvertent release of the gas into the atmosphere.

Need to consider the full life cycle

However, when evaluating the relative environmental impacts of different fuels, it’s also important to consider the fuel’s full life cycle, including the finding and development of fuel sources; producing the fuel; and transporting the fuel to wherever it is being used. The production of coal, for example, involves the use of energy for coal mining and for the transportation of the coal—perhaps by railroad, or perhaps by barge or oil-fueled cargo vessel.

In the case of natural gas, the drilling of gas wells has some carbon footprint, as does the shipping of the gas by pipeline or in the form of LNG. A gas pipeline, for example, requires compressors, typically fueled by gas, to push the gas through the line. The production of gas from shale, in addition to requiring a relatively large number of wells, requires energy for the fracturing of the underground shale using high-pressure water.

But, unlike gasoline and diesel fuel, the production of natural gas does not require an energy-consuming refinery.

The comparison of a fuel such as natural gas with renewable energy sources also requires full life-cycle analysis. For example, the production of corn ethanol requires energy for the operation of agricultural machinery; fertilizers, perhaps produced from natural gas; and the energy required for the extraction of alcohol from fermented corn. Solar cells require energy for their manufacture. And so on.

Then there are less tangible considerations, such as the relative impact on surface land of, say, a wind farm compared with a gas field.

So, figuring out the relative environmental impacts of different fuels can become a complex and sometimes uncertain exercise involving many different factors. And as well as encompassing full life cycle environmental impacts, those factors need to include cost comparisons between different ways of minimizing undesirable emissions—it could turn out, for example, to be more cost effective to remove pollutants from the exhaust from a cheaply produced fuel than to use an expensive fuel that does not require so much pollutant handling.

But there does seem to be a widely-held view that natural gas, with its relatively benign exhaust products and ready availability, will play an important role in mankind’s future energy mix, at least in the mid-term.

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