Natural Gas Overview — Why is Methane a Clean Fuel?

Introduction to Methane

What we call natural gas is mostly the chemical compound methane (95% or more; the rest is ethane or longer carbon chains). Methane, which comes out of the ground as a gas, is produced when microorganisms known as methanogens feed on organic matter in environments with little or no oxygen. It is abundant, seeping out of your garbage, landfills, and swamps. Also, everywhere you find oil, you find methane, usually in a pocket above the oil deposit. This methane emanated from the same organic material (dead plants and animals) that produced the oil. Methane can be captured where it naturally occurs or produced in a controlled environment like an anaerobic digester. After it is captured or produced, it is cleaned (by removing carbon dioxide and other liquids); compressed to a higher pressure; odorized (it’s odorless and lethal in high doses in its natural state); and piped into our homes, power plants, and factories for heat and power.

Clean Combustion

Methane, like all fossil fuels, can be combusted (reacted with oxygen) to form energy and water. In fact, a large and growing part of our electricity supply comes from methane. It is the simplest fossil fuel — a single carbon atom with four hydrogen atoms, or CH4. Compare that to diesel fuel, which is a soup of long-chained carbons with sulfur and other molecules attached.

The basic methane combustion reaction is:

CH4 (methane) + 2 O2 (oxygen) = CO2 (carbon dioxide) + 2 H20 (water) + energy

Because of its simplicity and lack of additional compounds, methane is the cleanest of the fossil fuels to combust. When we say cleanest, though, we often mean different things. In terms of the production of carbon dioxide (i.e., the major greenhouse gas), methane has the lowest density, meaning we get more energy per unit of carbon dioxide than we do with other fuels. It releases 29% less carbon than oil, 43% less than coal, and 20-30% less lifecycle carbon than oil when used as a transportation fuel. In addition, unlike other fuels, methane combustion results in basically no NOx (nitrous oxide), SOx (sulfur dioxide), or particulate matter being released into the atmosphere. These gases are all dangerous to our health and regulated under the Clean Air Act.

Fossil fuels and their energy density:

natural gas (51.6 kJ/g) > petroleum (43.6 kJ/g) > coal (39.3 kJ/g) > ethanol (27.3 kJ/g) > wood (16.1 kJ/g)

It should be noted that methane, by itself, when released into the atmosphere, is a potent greenhouse gas. It captures heat [70 times] better and thus, by weight, is 70 times as dangerous as carbon dioxide. This is why it’s so important to flare methane to ensure that it is completely combusted into carbon dioxide. It also means that it is critical that the infrastructure to transport methane — drilling sites, pipes, and tanks — minimizes any leakage into the air. Otherwise, the benefit of transitioning from coal to natural gas (in terms of greenhouse gases) would quickly be lost.

The Age of Methane

The increased use of methane in the US, predominately replacing coal, has stabilized if not actually lowered our level of emissions, and the retirement of old coal power plants is eliminating one of the country’s largest polluters.

Energy transitions typically move from a lower density fuel to a higher density fuel. We moved from wood to coal to oil, and now methane is creeping up, passing coal to become the second largest source of energy in the US. While denser, it is also a gas, which presents logistical issues for transport and storage.  Nonetheless, I think we will eventually make the transition from oil to methane, at least in the US, before renewables ultimately take over in the second half of this century. And when they do, it will be because technically they are a better fuel — i.e., more energy dense.

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Global Dynamics

As discussed, methane, being a gas, presents a transportation challenge. The US has a vast network of pipelines, and Russia pipes compressed natural gas into Europe and China from their vast reserves. However, in order to physically move natural gas, as opposed to transferring it via pipeline, you need to cool it to a liquid (at -260 degrees Fahrenheit), at which point it becomes liquified natural gas, or LNG (in it’s compressed form, it’s called compressed natural gas, or not surprisingly, CNG). Once liquified, it can then be transported via ship.

Less than 10 years ago, the US built import terminals to import LNG from abroad. More recently, however, with the discovery of new drilling techniques (i.e., fracking), those same terminals have been re-configured as export terminals as the US is now one of the world’s leading producers of natural gas, along with Russia and Qatar. However, the difficultly and, thus, cost to move it has created a huge pricing disparity around the world. For example, natural gas is routinely under $5/MMBTU in the US, while it can be as much as $20/MMBTU in Japan or China. This has put a huge amount of pressure on producers to export to Asia to satisfy growing demand, as well as on the Asian countries to produce more gas themselves through a combination of the gasification of coal and importing drilling technologies from the US. Just 10 years ago, the US was scared of running out of fuel, but now we find ourselves with an abundance; Asia, by importing fracking technology, could very well find itself in a similar situation.

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