gas and energy

Methane - a greenhouse gas, a deadly gas and a green energy source

Dr. Dror Bar-Nir

Rice fields in the Mekong Delta in Vietnam. Photo: Yael Givon (February 2006)

On June 8, 2007, in the town of Heiligendam in Germany, the leaders of the eight most industrialized and richest countries in the world (the G8) declared their agreement to set a non-binding goal of cutting greenhouse gas emissions (see box) by 8% by 50. This cut is, of course, intended to slow down the rate of global warming. We will dedicate this column to the hydrocarbon methane (CH2050), which is an almost exclusive product of the activity of special archaea (Archaea - ancient bacteria), called methanogens.

The "greenhouse gases" are so called because they act as a partial barrier against the emission of heat radiation from the Earth into space. If it weren't for greenhouse gases, the temperature on Earth would be an average of 40ºC lower than it is today, and life as we know it would not exist.

The main greenhouse gas is water vapor, which is responsible for about 95% of the greenhouse effect. The second most important greenhouse gas is carbon dioxide (CO2), which is responsible for only 3.6% of the greenhouse effect. Methane contributes 0.4%, while nitrogen oxide (N2O) contributes about 1%.

We humans have no real effect on the amount of water vapor. On the other hand, any significant change in the amounts of the last three gases (to which the G8 leaders were referring) may cause an increase in the average temperature of the Earth, which has very serious consequences - sea level rise, storms, floods, droughts, diseases, and more .

The reason for the "marginal" contribution of methane gas to the greenhouse effect is its relatively small amount in the atmosphere. If you check the contribution of one molecule of methane to the greenhouse effect, it contributes 22 times more than a molecule of carbon dioxide.

Methane, then, is a hydrocarbon. The hydrocarbons, as their name indicates, are molecules composed of carbon and hydrogen atoms only. It is customary to distinguish between light and gaseous hydrocarbons and heavier hydrocarbons. Some of the light hydrocarbons are methane, ethane (C2H6), propane (C3H8) and butane (C4H10), the last two of which are main components of cooking gas (also known as LPG - condensed hydrocarbon gas). The heavier hydrocarbons are, among other things, the components of all types of petroleum-based fuels. All hydrocarbons are "high energy" substances and very flammable, and the smallest ones are even explosive.


A pipeline for collecting methane at the Hiria site; photo: Yigal Moorg

Methane, which is the lighter hydrocarbon, is emitted into the atmosphere from natural and man-made sources, and is an odorless gas.

The vast majority of methane on Earth originates from the activity (both in the distant, geological past, and today) of a special group of non-aerial archaeons, most of which cannot survive in an atmosphere containing oxygen. These archaeons use molecular hydrogen, H2, to oxidize their carbon source, which could be carbon dioxide (CO2), acetic acid (CH3COOH), or another substance. Because of the methane they produce, these archaeons are called methanogens, and the process as a whole is called methanogenesis.

A relatively small amount of methane is emitted following volcanic activity. Methane was also discovered in the planets Jupiter, Saturn, Uranus and Neptune, and even in Titan (one of Jupiter's moons). Until it is proven otherwise, it is assumed that all the methane in the atmosphere of these planets originates from volcanic activity (although today some claim otherwise).

Other, aerial bacteria, called methanotrophs, can utilize methane as a source of carbon and energy, and they break it down, in a series of steps, into carbon dioxide and water.

The main natural source of methane gas is bacterial activity at the bottom of the extensive wetlands in the tropical and subtropical regions of the Earth. That's why methane is also called "swamp gas". The methanogenic bacteria in these swamps break down organic compounds - the remains of animals and plants that sink into the water - and emit 200-120 million tons of methane gas per year. Another significant source, related to human activity, is the extensive rice fields in East Asia, the methanogens at the bottom of which emit 120-70 million tons of methane per year.

Another important source of methane is the digestive tracts of many vegetarian animals, which contain methanogenic archaeons in their intestines alongside other microorganisms, which help them break down the plant materials lignin and cellulose. The methanogens consume the hydrogen gas produced in the fermentation processes of other bacteria, and emit methane. Termites, which inhabit forests and other tropical areas, are considered to be the main producers of methane among animals living in the wild (with a multi-year average of about 25 million tons of methane). The vegetarian mammals in nature also emit methane, but the largest amounts of methane are emitted by the millions of vegetarian mammals that we humans raise - the herds of cattle (100-80 million tons of methane per year). The methane is emitted from the digestive system of the vegetarian mammals mainly through the mouth - in hiccups ("gurgling"). A small amount of methane is also emitted from our digestive systems, humans, together with hydrogen and other gases, but with us it is emitted through the anus (in what is known in Hebrew as a flatulence).

At the bottom of the oceans, especially near the coasts (continental shelf area), at a temperature of 4C, as a result of the activity of methanogens for millions of years, methane hydrates (a methane molecule bound to several water molecules) accumulate - at this temperature they are in a solid state (ice-like). These hydrates emit, in their current state, between 2 and 4 million tons of methane per year. As a result of this activity, dissolved methane also accumulates in the ocean waters, part of which is emitted naturally (about 20 million tons per year). Methane emissions from ocean water reach 24 million tons per year.

With the warming of the earth, the water of the oceans will also warm, and then the emission of dissolved methane from the hydrates may increase.

Methane also accumulates at the bottom of freshwater lakes, just as it accumulates at the bottom of the ocean. But the accumulation of dissolved methane in freshwater lakes, in geologically active areas, could be dangerous. Rapid heating of the water as a result of contact with boiling lava can cause a sudden release of the dissolved methane as a cloud of methane into the atmosphere. Since methane is a flammable gas, a spark can cause it to ignite and the damage, especially in populated areas, can be enormous. Lake Kivu, for example, on the border between Rwanda and the Democratic Republic of Congo, is in a very populated area - about two million people live around the lake. It is estimated that about 55 billion cubic meters of dissolved methane are dissolved inside the lake. A sudden warming of the bottom of the lake, as a result of volcanic activity, will cause a sudden release of all the methane and its explosion. According to geological evidence, once every thousand years or so, the extinction of all the animals and plants around the lake occurs. Is this a ticking bomb?


a forest pool in Hadera; The last remnant of the swamps of the Sharon region; photo: Yohai Gotvin (Tulii www.tiuli.com)

The coal mines and the oil reserves exploited by man are also a source of methane emissions. In oil wells, methane is emitted in natural gas (which contains about 85% methane), and in coal mines, gas containing about 40% methane is emitted. In the oil wells they burn all the gas that is released; In the coal mines the situation is more problematic. The gas accumulates in closed places and sometimes explodes and causes disasters. In November 2006, in the Halemba mine in Poland, 23 people were killed in a methane gas explosion. In March 2007, in Russia, in the Ulyanovskaya mine near the city of Novokuznetsk, a methane explosion killed 74 miners. In China, in 2005, over 3,000 miners were killed in gas explosions in mines.

Another source of methane is landfills. About 70 million tons of methane are released from these landfills every year.
Reducing methane emissions associated with human activity

To reduce methane emissions resulting from human activity, and thus reduce the greenhouse effect and air pollution, the following steps are taken:

In East Asia, they are trying to increase the rice yield per unit area. In this way, it will be possible to reduce the growing areas and the methane emissions that occur in them.

At Hohenheim University in Stuttgart, Germany, Winifred Drochner's research group is developing "pills" intended for vegetarian mammals, called bolus. Combined with a special menu, these pills will reduce the amount of methane emitted by the growing herds of cattle and sheep, and will allow their metabolism to utilize some of the energy that would have been wasted with the emitted methane. The researchers intend to change the composition of the population of microorganisms in the digestive system - to reduce the amount of hydrogen-emitting bacteria, and subsequently the methanogenic archaea, and to increase the amount of bacteria that produce sugars and other available substances.

And last but not least and green. At some waste sites, the methane emitted from the waste is collected in pipes, and transported to a central site to be used as a green fuel (compared to oil burning). The collected methane can be burned in a controlled manner and used to generate electricity, or stored at a nearby gas station, which will be reached by cars equipped with an engine that allows a choice between using gasoline and using methane. There are no such cars in Israel yet, but in the landfills in Hevron, Bhiriya and Ether Dudaim they collect the accumulated methane for the purpose of electricity generation. The members of Kibbutz Evron, who were the pioneers in the field in Israel, disconnected from the electricity company in 2002, and all of Evron's electricity consumption is sourced from the kibbutz's landfill.

Dr. Dror Bar-Nir teaches microbiology and cell biology at the Open University.

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