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Methane as a Source of Energy
How the "Greenhouse" Gas can be Used as Biogas
by Jochen Mailander

Methane is the third most common of the trace gases contributing to the man-made greenhouse effect. One way of reducing methane emissions is to use the gas as a source of energy. Because methane is a constituent of biogas, which is a highly attractive regenerative energy source, not least from an economic point of view. In Germany, as in most other countries, biogas is still a marginal technology, although it is very widespread in some parts of south-east Asia. China and India in particular are traditional biogas countries which have for decades been making a meaningful contribution to reducing the methane component of the greenhouse effect.

Methane (CH4) is a gas which occurs in nature as a result of all anaerobic fermentation processes, i.e. the decomposition of vegetable substances. It occurs in small amounts as a trace gas in the atmosphere. The principal natural sources of methane are the world's great marshlands and swamplands. As a result of human interference with nature - e.g. agriculture, mining, burning of biomass and fossile fuels, refuse dumps methane production ha . been increased so much that for about 200 years now it has been a dangerous "greenhouse" gas.

The greenhouse effect

The natural greenhouse effect produces a 33ncrease in the temperature of the earth's surface. It is essential for human and animal life, since without it the mean temperature of the earth would be -18 However, the additional man-made greenhouse effect, which has now been increasing continuously for a hundred years, is a cause for concern. Until now, it has resulted in a measurable rise of about 0.7n the mean temperature of the earth. The forecasts for the coming decades are "hot": an increase in global warming by 1.5o 4.5the most probable range) by the year 2030, assuming an unchanged increase in emissions.

The greenhouse effect is a result of thermal re-radiation of trace gases. The natural greenhouse effect is primarily due to water vapour (62%) and CO2 (21%), plus natural emissions of methane and nitrous oxides. The man-made greenhouse effect is caused by emissions of the five principal trace gases (see table 1).

Since the various measures proposed for reducing emissions were controversial, were only adopted reluctantly and had no effect, concentrations of trace gases in the atmosphere continued to rise. Different retention times and delays in the effect of the molecules in the atmosphere (CO2 100 years, CFCs 50-100 years methane 10 years) have a cumulative effect. One CFC molecule has the same greenhouse effect as approx. 15,000 CO2 molecules; methane has the same greenhouse effect as 30 CO2 molecules.

Typical examples of measures that are hard to enforce and have so tar had practically no impact on the greenhouse effect include

- control of slash-and-burn and deforestation;

- reduction of emissions from burning of fossil fuels, e.g. in vehicle engines

- calling a global halt to the use of CFCs and substitutes for them (a reduction which will have an impact on ozone levels is foreseeable).

Table 1: Trace gases so far responsible for the man-made greenhouse effect

CO2 (carbon dioxide)

50%

= 0.35° C

CFCs (chlorofluorocarbons)

17% (increasing->25%)

= 0.12° C

CH4 (methane)

19% (13%)

= 0.13° C

O3 (tropospheric ozone)

8%

= 0.06° C

N2O (nitrous oxide)

4%

= 0.03° C

Methane emissions

Methane emissions have a not inconsiderable impact on the earth's climate. The effects achieved by reducing methane missions are comparatively short-term, since a methane molecule survives no more than ten years in the atmosphere. In the last 200 years, the methane content of the atmosphere has more than doubled and present methane emissions are running at a rate of around 500 million tonnes a year. Of this, 400 million tonnes are man-made; that is, in addition to the 100 million tonnes produced by natural processes (e.g. decomposition in swamplands) or by termites and woodworm, around 400 million tonnes more, produced by human activity, find their way into the atmosphere and thus contribute directly to global warming.

The extent to which individual sources of methane contribute to the greenhouse effect is still partially based on estimates (see table 2):

One striking figure is the high level of methane emissions resulting from rice cultivation. At present there is little chance of reducing this. However, methane production can be reduced in agriculture (animal husbandry), in the burning of biomass, in refuse dumps and in "natural gas loss" situations, by consistently using methane as a fuel. In addition to reducing ecologically destructive emissions, this will also help to preserve energy supplies and protect the environment, assuming that the methane-containing biogas IS used as a substitute for coal or oil.

Table 2: Man-made sources of methane

Sources

Methane

Percentage of effect of methane

(mill. tonnes/ year)


Man-made greenhouse

Paddy fields (wet rice cultivation)

130

37%

Fermentation in stomachs of ruminants, agriculture/animal husbandry (liquid manure)

75

21.5%

Burning of biomass(slash-and-burn, fossil fuels)

40

11.5%

Refuse dumps

40

11.5%

Coal mining

35

10%

Natural gas loss during extraction and distribution

30

8.5%

Biogas

Biogas is a mixture of gases with a methane (H4) content of 50% to 80%. It also contains CO2 (20-50%) and traces of H2S and other gases. Depending on where it originates, it is termed landfill gas, sewage gas, marsh gas, fire-damp or pit gas, fermentation or manure gas, and sometimes also natural gas. The general term biogas is attributable to the fact that this gas mixture is the result of anaerobic fermentation of biomass.

Energy can be obtained from biogas by controlled combustion in the following ways:

- direct utilization for cooking, heating and refrigeration;

- power generation and waste heat utilization via gas engines (spark-ignition engines, Stirling engines) and block-type thermal power stations;

- district heating networks served by large-scale linked biogas plants or landfill gas plants, combined with block-type thermal power stations.

With minor modifications, all appliances designed for natural gas can be run on biogas.

A number of eminently practical concepts for using biogas as a fuel have already been put into practice in sewage plants, refuse dumps and in agriculture.

1. Utilization of sewage gas in sewage plants

During aerobic wastewater purification in a sewage plant, sewage sludge is produced. This is anaerobically fermented in a digesting chamber. In Germany, biogas has been produced in sewage plant digesting chambers since the early 1930s, and there are now about 200 digester gas plants in the country. Worldwide, there are thousands of digester gas plants producing biogas, which is used in particular for domestic purposes. In some sewage plants the arising biogas is only burned off.

Increasingly, both in developing and in industrialized countries, industrial and domestic/communal sewage is anaerobically fermented, purified directly in a biogas plant.

Table 3: Agricultural biogas plants in the world

China

4.5 million

Caribbean

240

India

1.2 million

Morocco

240

Korea

30,000

Ivory Coast

56

Nepal

6,000

Nicaragua

40

Kenya

320

Tunisia

36

Tanzania

260

Bolivia

34

EC (without FRG)

640 agricultural

200 industrial


Germany

160 agricultural

80 industrial


Switzerland

140 agricultural



However, methane emissions from organic sewage and the resulting sewage sludge are low compared with the methane sources listed in Table 2. For operators of such sewage plants, therefore, there is less incentive to exploit the energy content of the sewage. This is why the majority of the sewage plants in the world have no facilities for utilizing sewage gas.

2. Utilization of landfill gas on refuse dumps

In refuse dumps a methane-containing gas is produced when the oxygen supply is insufficient. This gas normally penetrates the covering and prevents vegetation from growing for years. By providing bore-holes and laying a mesh-like drainage system, the methane can be extracted, collected and used as a fuel. In block-type thermal power stations it is burnt to generate electricity. The waste heat is also utilized. In Germany there are meanwhile about 80 sanitary landfill gas plants (compared with 20 at the beginning of 1986). Some 560 million m3 of landfill gas are burnt in these plants every year, i.e. a 20 per cent utilization rate. Landfill gas utilization is still relatively unknown outside Germany, and there are correspondingly fewer landfill gas plants in other countries. The technology is mature, however, and plants can be installed at existing landfill sites. This clearly illustrates the future need for action and financing - in particular for the industrialized countries as the principal waste producers.

About 8 to 18 per cent of the methane in the earth's atmosphere originates in refuse dumps. Forty million tonnes of methane emissions per year worldwide compare with about 120,000 tonnes per year used in a controlled manner to generate energy in German biogas plants.

3. Utilization of biogas in agriculture

Biogas plants used in agriculture vary widely in both design and size. The majority of them are installed on fairly small farms in developing and threshold countries, where a simple, proven technology is often used. Besides farm plants there are village community plants and plants with complex processes for industry. The most suitable input for fermentation is cattle and pig excrement (liquid manure). In addition, practically any non-wood-containing organic material can be fermented (e.g. fats, gras, harvest residues, organic domestic waste, organically contaminated waste water). About 6 million of the biogas plants in the world are primarily agricultural (see table 3).

Since the mid- 1970s, the Deutsche Gesellschaft fur Technische Zusammenarbeit (GTZ) has promoted the establishment of about 2,000 biogas plants in developing countries on behalf of the Federal Ministry for Economic Development (BMZ).

Worldwide, agricultural biogas plants currently use approximately 2 billion m3 of biogas per year. The total of around 75 million tonnes of methane per year generated by agriculture contrast with about I million tonnes produced and utilized in biogas plants. However, it must be borne in mind that a high percentage of this methane cannot be used (e.g. belch gas produced by ruminants).

There are many good reasons for burning biogas in preference to coal, oil, natural gas or wood. The emissions of pollutants and CO2 are around five per cent lower than those from fossil fuels. Using biogas as a fuel also means that fewer trees are destroyed by deforestation and slash-and-burn. Ecological fertilizer can be made from the digested sludge, ground water contamination is reduced, and energy supplies can be decentralized. For example, if biogas was used on a wider scale instead of coal and oil, and the existing potential (20 million biogas plants by the year 2000), CO2 emissions from burning fossil fuels could be reduced by up to 1 per cent a year.

Huge untapped potential

If methane emissions from agriculture and refuse dumps are compared with the quantities of biogas so far utilized as a source of energy, the following picture emerges:

A certain reduction in methane emissions has already been achieved, in particular due to the high number of agricultural biogas plants. Worldwide, in agriculture/animal husbandry, this reduction is probably around 2-3%. However, the reduction in emissions from refuse dumps is only about I -2% (in Germany 20%). The comparison shows that up to now, far more has been done in agriculture worldwide than in waste management.

A huge biogas potential remains untapped. This is due for one thing to a lack of money, and for another to problems in improving and extending existing ecological concepts in energy generation, solid waste and sewage management, and agriculture. Anaerobic digestion of waste and sewage is regarded as the technology of the future. GTZ's Biogas Extension Programme played a significant role in pointing the way ahead. With regard to landfill gas technology, broader-based application of existing know-how would be a step in the right direction.

Canada: Rotting Reservoirs

Electricity from big hydrostations may not be such an environmentally friendly source of power after all.

A new study in Canada has found that some hydroelectric reservoirs give off as much carbon dioxide and methane - the two most important causes of the man-made greenhouse effect - as coal-fired power stations producing a similar amount of electricity.

The problem is the forests, soils and peat bogs flooded by the reservoirs. Once these are flooded, they decompose releasing the gases.

A study of one hydroelectric reservoir, Cedar Lake in Manitoba, revealed a likely release of about one kilogram of carbon dioxide for every kilowatt-hour of electricity produced by the reservoir's power station.

This emission was similar to the greenhouse effect of electricity generated by coal-fired plants and would persist for at least 50 years.

To date, emissions from hydroelectric reservoirs have not been taken into account in assessments of national emissions of greenhouse gases.

(Source: New Scientist, July 93)