Direct Use of Biogas

Biogas can be produced at household level or at large-scale by anaerobic digestion of organic material, usually animal dung, human excreta and crop residue (see also anaerobic digestion). Small-scale biogas reactors (see also small-scale anaerobic digestion, biogas settlers, anaerobic baffled reactors, UASB reactors, digestion of organic wastes e.g. from kitchen, etc.) can provide a cheap and sustainable source of fuel for lighting, cooling, cooking or electricity. The produced biogas fuel can either be used directly (what this factsheet is about) or it can be transformed into electricity (see also the conversion of biogas to electricity at grande or small scale).

Household energy demand varies greatly and is influenced by cooking and eating habits (i.e., hard grains and maize may require substantial cooking times, and, therefore, more energy compared to cooking fresh vegetables and meat).

Biogas is a mixture essentially comprising mostly methane (CH₄, around 55-75%) but also contains carbon dioxide (CO₂), around 25-30%), varying quantities of water (H₂O) and hydrogen sulphide (H₂S). Other compounds can also be found, especially in waste dump biogas: ammonia (NH₃), hydrogen (H₂), nitrogen (N₂) and carbon monoxide (CO). Methane is the valuable component under the aspect of using biogas fuel. Biogas has an average methane content of 55-75%, which implies an energy content of 6-6.5 kWh/m³. 

The (thermal) energy available from the methane contained in biogas is about 6 to 8 kWh/m3. This corresponds to half a litre of diesel oil and 5.5 kg of firewood. Gas demand can be defined on the basis of energy previously consumed. For example, for a family having previousely consumed 1 kg firewood , this roughly corresponds to 200 L biogas, 1 kg dried cow dung corresponds to 100 L biogas and 1 kg charcoal corresponds to 500 L biogas . 1 kg of human faeces generates about 50 litres of biogas, 1 kg of cattle dung delivers 40 litres of biogas, and 1 kg of chicken droppings generates about 70 litres of biogas (NWP 2006) .

Gas consumption for cooking per person and per meal is between 150 and 300 L biogas. Approximately 30-40 L biogas is required to cook one litre of water, 120-140 L for 0.5 kg rice and 160-190 L for 0.5 kg vegetables.

Tests in Nepal and Tanzania have shown that the consumption rate of a household biogas stove is about 300-400 L/h. However, this depends on the stove design and the methane content of the biogas.

The following consumption rates in litres per hour (L/h) can be assumed for the use of biogas:

• household burners: 200-450 L/h
• industrial burners: 1000-3000 L/h
• refrigerator (100 L) depending on outside temperature: 30-75 L/h
• gas lamp, equivalent to a 60 W bulb: 120-150 L/h
• biogas/dieselengine per bhp: 420 L/h
• generation of 1 kWh of electricity with biogas/diesel mixture: 700 L/h
• plastics moulding press (15 g, 100 units) with biogas/diesel mixture: 140 L/h

To use the energy of the biogas, biogas appliances, such as gas stoves or gas lamps are required. Both special biogas appliancesor pressure-kerosene and LPG (Liquefied Petroleum Gas) equipment are adapted. Compared to other gases, biogas needs less air for combustion. Therefore, conventional gas appliances need to be modified when they are used for biogas combustion (e.g., larger gas jets and burner holes).

The distance through which the gas must travel should be minimized since losses and leakages may occur. Drip valves should be installed for the drainage of condensed water, which accumulates at the lowest points of the gas pipe.

For direct use, the following applying conditions should be kept:

• The main prerequisite of biogas use is the availability of specially designed biogas burners or modified consumer appliances. 
• In some cases, especially at larger scale, further treatment or conditioning of biogas is necessary before it is ready to use. This treatment aims to remove water, hydrogen sulphide or carbon dioxide from the raw gas.
• Safety measures are needed, especially to reduce the risk of explosion in case of leakages.

In general, users enjoy cooking with biogas as it can immediately be switched on and off (as compared to wood and coal). Also, it burns without smoke, and, thus, does not lead to indoor air pollution.  Cooking indoors over open fires, and lighting with kerosene, gives dangerous exposure to air pollutants and a high risk of fire, particularly for women and young children who spend much of their time indoors. Smoke inhalation in third world kitchens is a major cause of eye disease, respiratory illness and premature death. In addition, women and girls have the drudgery of collecting fuelwood, which typically takes several hours each day.

Furthermore the demand for fuelwood substantially exceeds the rate of regrowth, and this is leading to degradation of the land, forests and essentially damage to vital watersheds. Unsustainable use of fuelwood adds carbon dioxide to the atmosphere.

The biogas plants can replace nearly all the use of fuelwood, and make cooking easier, cleaner and safer. Thus, biogas contributes to the protection of the environment.

An average small-scale biogas plant can save up to 4.7 tonnes of carbon dioxide emissions per year (ASHDEN 2005). In fact, the contribution of a methane molecule to the greenhouse effect is 21 times greater than that of a carbon dioxide molecule (SUSANA 2009). Therefore burning methane, even though producing CO₂, reduces its impact on the environment (see also SSWM and Climate Change). Biogas generated from faeces may not be appropriate in all cultural contexts. Assuming that the biogas plant is well-constructed, operated and maintained (e.g. water is drained), the risk of leaks, explosions or any other threats to human health is negligible.

Biogas is usually fully saturated with water vapour, which leads to condensation. To prevent blocking and corrosion, the accumulated water has to be periodically emptied from the installed water traps. The gas pipelines, fittings and appliances must be regularly monitored by trained personnel.

When using biogas for an engine, it is necessary to first reduce the hydrogen sulphide because it forms corrosive acids when combined with condensing water.

The reduction of the carbon-dioxide content requires additional operational and financial efforts. As CO₂ "scrubbing" is not necessary when biogas is used for cooking, it is rarely advisable in developing countries.