Natural gas, called “the prince of hydrocarbons” by some, is an increasingly important fuel source in the world energy system. Easy to transport, easy to use, cheap and relatively clean, it is an excellent way to improve our energy system in the short run. While a vast improvement over coal and oil, it is not a sustainable solution to global warming, air pollution, and resource depletion. But advanced electric generation technology like combined-cycle gas turbines and fuel cells show that natural gas will be an essential way to reduce the environmental impacts of electricity.
Formation, Discovery, and Production of Natural Gas
Like petroleum, natural gas is a product of decomposed organic material. Ancient plants and animals, trapped in bogs and under water, broke down without the presence of oxygen. As they were covered with sediment, they became trapped. Gas and oil are held in porous rocks, like sandstone, under a cap of impermeable rock.
The discovery and production of natural gas are similar in many ways to that of oil. Gas is often found mixed in with oil, or floating on top of underground reservoirs of oil. Seismic methods are used to detect rock formations likely to contain oil and gas. Drills are dug, and the gas and oil are extracted. Gas provides much of the natural pressure that drives oil to the surface.
In the past, gas was not considered a useful product, and transportation pipelines were not adequate to pipe it to markets. As a result, gas was simply burned off at the well, in huge flares. How much was lost is unknown.
Also unknown is how much gas remains in the ground. Natural gas reserves are dwarfed by coal reserves in the U.S. but are still expected to be able to supply the nation for 60 years or more. What makes estimating difficult is that new supplies are still being discovered. Worldwide this is even truer. The former Soviet Union is anticipated to have huge supplies, up to ten times as much as the US, especially in Siberia. Major finds have been found recently in Indonesia, Mexico, North Africa, and other places. Estimates of worldwide gas reserves range from 120 to 175 years of supply, but some predict that with improved extraction methods, it could be three times higher.
The Uses of Natural Gas
In the early 20th century, gas was used to lighthouses and buildings, known as Gaslight. Most of this gas was manufactured from coal and oil. People soon realized that gas pumped out of the ground, “natural” gas, was cheaper and cleaner. The problem was getting it from the oil fields to the cities.
A network of pipelines was put together in the 1950s and 1960s, finally covering most of the nation by the 1980s. There are currently over a million miles of gas lines in the US, with more under construction.
In 1995, the US used about 20 trillion cubic feet of natural gas, up 14 percent from five years earlier. Almost half of this was used by industry, a quarter by the residential sector, and the rest divided evenly between commercial users and electric utilities. Natural gas is used by industry mostly for heat, for combined heat and electricity (called “cogeneration”) and as an input for plastics, chemicals, and fertilizer. Gas is used in homes for space and water heating and cooking. Nearly two-thirds of American homes are heated with gas. The best home furnaces are over 90 percent efficient at extracting heat from the gas.
There is increasing interest by utilities in using gas to generate electricity. Gas turbines, derived from jet engines, use the hot gases from fuel combustion directly, rather than using the heat to make steam, like in coal plants. The most efficient designs, called “combined cycle” turbines, are two turbines hooked together. After the hot gas is used to drive the jet engine turbine, it is used to boil water into steam, which then drives a steam turbine. Combined cycle turbines can be over 50 percent efficient at converting gas into electricity, compared to about 33 percent for steam turbines.
Although natural gas is a fossil fuel and so is made up mostly of carbon, global warming emissions from gas are much less than coal or oil. Compared to coal, gas produces 43 percent fewer carbon emissions for each unit of energy produced, and 30 percent less than oil. Gas also produces no solid waste, unlike the massive amounts of ash from a coal plant, and very little sulfur dioxide and particulate emissions.
On the other hand, the combustion of gas still produces nitrogen oxides, a cause of smog and acid rain. Natural gas is a powerful greenhouse gas with lower carbon emissions. Natural gas (methane) is much more effective than carbon dioxide at trapping heat in the atmosphere, 58 times more effective on a pound-for-pound basis. Methane concentrations have increased eight times faster than carbon dioxide, doubling since the beginning of the industrial age. Natural gas use has accounted for about 10 percent of all global warming emissions.
The Future of Natural Gas
The market for gas continues to expand rapidly. In 1995, gas added one million new customers in the US. As utilities anticipate electric utility restructuring, they are buying more gas turbines as well as more gas — gas sales to electric utilities rose by seven percent between 1994 and 1995. This growth is due to new gas turbines, which are cheaper to install than coal plants and easier to site and get permits for, and the currently low cost of gas.
Provided gas prices stay low; this trend will continue. Even a better technology for converting gas to electricity is rapidly approaching the market, the fuel cell. Fuel cells convert gas directly into power without combustion. A molecule of natural gas is made up of carbon and hydrogen. When the hydrogen is segregated from the carbon and filled into a fuel cell, it fuses with oxygen to produce water, electricity, and heat. The carbon is released as carbon dioxide in smaller quantities than from gas turbines.
Fuel cells are highly efficient, converting about 60% of the energy in gas into electricity. They are completely silent and can be made in a wide range of sizes, small enough to power a car and large enough to provide electricity, heat and hot water to apartment buildings and factories.
Some people see natural gas-powered fuel cells as a critical bridge to an energy system run entirely on hydrogen. Hydrogen doesn’t occur on its own; instead, it bonds with oxygen to make water. Water can be split using electricity, and the hydrogen captured and stored for later use. By using renewable electric generators like solar and wind, hydrogen could be created with no emissions. Also, large conventional power plants like nuclear and coal plants, could run at peak output and thus peak efficiency around the clock, producing electricity when it is needed and hydrogen during the off-hours.
The hydrogen would then be shipped through pipelines just like natural gas and used in fuel cells. Hydrogen would be better than natural gas because no carbon emissions are produced and because hydrogen is an undepletable resource.
For a pure hydrogen economy to become a reality, we will likely have to run out of natural gas first. Natural gas is much cheaper because we pull it out of the ground. Also, converting electricity into hydrogen and then back into power will result in losses of energy. There are fewer losses when the electricity from wind and solar plants is used directly rather than converting it to hydrogen and back again. Long distance power lines can deliver electricity faster and with less loss of energy than the hydrogen conversion process. Other storage options, like batteries and pumped hydroelectric systems, will be a source of competition.
Another important source of natural gas in the future may be gas made from biomass. In China, India and the developing world, small farmers collect animal manure in a vat and capture the methane given off when the droppings decay. In the US, we do the same with landfills and sewage treatment plants. As biomass (in the form of newspapers, cardboard, and other trash) decays in landfills, methane is produced. Environmental regulations require landfill and sewage treatment gas to be captured to prevent air pollution and global warming emissions. The captured gas can be burned for heat and power. An experiment underway in Connecticut is using landfill gas in fuel cells. Biogas can be made directly from plants too. But like hydrogen, biogas must compete with the lower cost of readily available natural gas.
As the use of natural gas increases, it will become a more important source of greenhouse gases. Provided supplies hold out; we could be getting much of our energy from gas. It will be essential to use it in the cleanest way possible, in fuel cells, to reduce global warming impacts and to make it last as long as possible.