How much do you know about natural gas? iStockphoto/Thinkstock

In 2010, a West Virginia coal mine collapse and a massive oil spill in the Gulf have renewed people's interest in other energy sources, like natural gas. Natural gas is a fossil fuel in liquid form that can be used as an energy source. When it's burned, it releases 25 percent fewer greenhouse gases and pollutants (sulfur, carbon, nitrogen) into the atmosphere than burning oil does. Natural gas is primarily methane gas (anywhere from 70 to 90 percent), but it also contains trace amounts of other usable gases, such as ethane, propane, butane and nitrogen.

Natural gas is found in subterranean reservoirs, often near oil deposits. It's refined and transferred via pipelines for use. But is natural gas sustainable? Will there be enough of it left for future generations? Or can we make our own?

Three Kinds of Gas

The primary ingredient in natural gas is methane. There are three types of methane:

• Thermogenic methane: This methane is created when sediment and mud pressurize the putrefied remains of plants and animals over millions of years. This happens at high temperatures — hotter than it takes to make oil — and it takes place 1 to 2 miles (1.6 to 3.2 kilometers) beneath the surface of the Earth.
• Abiogenic methane: Over the course of millions of years, hydrogen-rich gases and carbon molecules rise to the surface. They combine into the proper molecular proportion to create abiogenic methane (one carbon atom to four hydrogen atoms), which then settles into large, subterranean deposits.
• Biogenic methane: Microorganisms called methanogens feed on decaying organic matter. What goes in must come out, and the fecal matter excreted by the methanogens is methane. Methanogens live wherever there is organic material to eat. That includes landfills and the intestines of cows.

Are they sustainable?

Of these three types of methane gas, thermogenic and abiogenic are not renewable, insomuch as we don't know how many more dinosaurs or molecules are left to putrefy below the Earth's surface. Also, drilling in the Earth to reap this resource is extremely expensive.

On the other hand, biogenic methane is sustainable. The microorganisms that create this type of methane are simply doing what comes naturally. The United States Department of Agriculture has organized more than 100 projects since 2003 to collect biomethane from cow manure. All that manure saved 8 million gallons of oil. In fact, most biofuel in the United States was obtained from cow manure.

In landfills, the power of garbage decomposition can be harnessed to reap natural gas, since the methanogens that eat the organic garbage in dumps produce methane. As long as humans (and cows) keep producing organic, biogenic methane, or biomethane, it will always be renewable.

What about algae?

Microorganisms are at the forefront of futuristic energy generation. In 2009, the Pacific Northwest National Laboratory, which is part of the U.S. Department of Energy, developed a process called catalytic hydothermal gasification. This process mines huge amounts of natural gas, or biomethane, out of algae. What's even more Earth-friendly about this process is that the carbon dioxide byproduct from burning this biofuel can be recycled. It's used to feed the algae.