Concentration of CO2 in the Atmosphere


By George Harvey

Farm biogas installation near Neuerkerode, Lower Saxony, Germany. Photo by Elmschrat.

Farm biogas installation near Neuerkerode, Lower Saxony, Germany. Photo by Elmschrat.

Methane is a powerful greenhouse gas, more than twenty times as bad as carbon dioxide. It also stays in the atmosphere for a long time, once released. This means it is better to burn it, even if no benefit is derived from the heat, than to let it go into the air as it is.

Nevertheless, methane has many uses, and that can make it valuable. It is the main constituent in natural gas, which is rapidly replacing the coal used in the United States. Since methane is much cleaner and releases less carbon dioxide than coal per unit of energy produced, it provides some improvement. This does mean it is perfect, just that it is better than coal. Also, we have to bear in mind that some natural gas comes from fracking, which may have very bad side effects.

Methane can be derived from sources other than natural gas. It is produced when bacteria break down cellulose, which is found in the cell walls of plants and makes up about a third of the weight of wood or straw. Since methane from such sources has carbon atoms that were recently part of the atmosphere, burning them and releasing the carbon dioxide they produce back to the atmosphere is considered carbon-neutral.

The bacteria that make methane are ubiquitous. They live in the stomachs of cattle, assisting their digestion, but making them belch enough to be a major source of greenhouse gasses.

The bacteria also work on materials in landfills, where they decompose such biomass as waste food, waste paper, and wood. This is why it is very important that landfills be covered to capture the gas coming out. The methane captured this way is used as fuel – or if there is not enough of it for such use, it is burned so carbon dioxide is released instead of methane.

The same kinds of bacteria live in biodigesters, where they decompose agricultural and food waste, or even municipal waste, to produce biogas. Biomethane is usually burned for energy and heat. It turns out to be one of the least expensive sources of electricity we have right now.

Methane can also be synthesized. It is relatively easy to capture carbon dioxide from the emissions of natural gas power plants, and only a bit more difficult to capture it from coal-burning plants. Meanwhile, excess power on the grid, which can be purchased at very low wholesale prices, can be used to make hydrogen. The carbon dioxide and the hydrogen can be combined with catalysis. This requires some heat, which can be captured as waste heat from the power plants, and some pressure, for which we can use more of the low-cost excess power.

Once we use low-priced power to synthesize methane, it is easy to store for use at peak demand times, when the price of power is high. Though the efficiency of the process is lower than that of just using natural gas, it can be more profitable and less polluting than using natural gas all the time.

The process of making methane from carbon dioxide was invented in 1913. It has not been used much as it is expensive compared to the cheap oil that was available in the 20th century. Now, when power is more costly, it is being tested commercially as a power source.

Methane can also be used as a feedstock for the production a number of important chemicals. A process dating to the 1930s can catalyze methane into any of a variety of chemicals, including propane, butane, octane, and so on. Gasoline, diesel oil, and home heating oil can be made in this way.

One set of products methane can be used to make is plastics. The plastics can be nearly identical drop-in replacements for such materials as polypropylene, polyethylene, or polystyrene. They can be made to be recycled or biodegradable.

Ultimately, there are two important questions about a supply of methane. First, is it a fossil fuel, or does it comes from biological activity that removes carbon from the air? Second, is it released as methane, or as some relatively benign product? If it comes from the ground, it will be a greenhouse gas. If it comes from the air, it can have an effect that is neutral at worst, or possibly even beneficial.

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