# Biogas

File:Biogas-Linienbus.jpg
Biogas-bus in Bern, Switzerland

Biogas typically refers to a (biofuel) gas produced by the anaerobic digestion or fermentation of organic matter including manure, sewage sludge, municipal solid waste, biodegradable waste or any other biodegradable feedstock, under anaerobic conditions. Biogas is comprised primarily of methane and carbon dioxide.

Depending on where it is produced, biogas is also called:

Biogas containing methane is a valuable by-product of anaerobic digestion which can be utilised in the production of renewable energy [1].Biogas can be used as a vehicle fuel or for generating electricity. It can also be burned directly for cooking, heating, lighting, process heat and absorption refrigeration.

## Biogas and anaerobic digestion

Biogas production by anaerobic digestion is popular for treating biodegradable waste because valuable fuel can be produced while destroying disease-causing pathogens and reducing the volume of disposed waste products. It burns more cleanly than coal, and emits less carbon dioxide per unit of energy. The harvesting of biogas is an important part of waste management because methane is a greenhouse gas with a greater global warming potential than carbon dioxide. The carbon in biogas was generally recently extracted from the atmosphere by photosynthetic plants, so releasing it back into the atmosphere adds less total atmospheric carbon than burning fossil fuels.

Recently, developed countries have been making increasing use of biogas generated from both wastewater and landfill sites or produced by mechanical biological treatment systems for municipal waste. High energy prices and increases in subsidies for electricity from renewable sources (such as renewables obligation certificates) and drivers such as the EU Landfill Directive have led to much greater use of biogas sources.

## Landfill gas

Electricity from biogas (GWh)[2]
Country 2006 2005
Germany 7 338 4 708
UK 4 997 4 690
Italy 1 234 1 198
Spain 675 620
Greece 579 179
France 501 483
Austria 410 70
Netherlands 286 286
Denmark 285 275
Poland 241 175
Belgium 237 240
Czech Republic 175 161
Ireland 108 106
Sweden 54 54
Portugal 33 35
Luxembourg 33 27
Slovenia 32 32
Hungary 22 25
Finland 22 22
Estonia 7 7
Slovakia 4 4
Malta 0 0
EU (GWh) 17 272 13 397
Biogas in EU 2006 (GWh)[2]
Country Total Landfill Sludge Other
Germany 22 370 6 670 4 300 11 400
UK 19 720 17 620 2 100 0
Italy 4 110 3 610 10 490
Spain 3 890 2 930 660 300
France 2 640 1 720 870 50
Netherlands 1 380 450 590 340
Austria 1 370 130 40 1 200
Denmark 1 100 170 270 660
Poland 1 090 320 770 10
Belgium 970 590 290 90
Greece 810 630 180 0
Finland 740 590 150 0
Czech Republic 700 300 360 40
Ireland 400 290 60 50
Sweden 390 130 250 10
Hungary 120 0 90 40
Portugal 110 0 0 110
Luxembourg 100 0 0 100
Slovenia 100 80 10 10
Slovakia 60 0 50 10
Estonia 10 10 0 0
Malta 0 0 0 0
EU (GWh) 62 200 36 250 11 050 14 900

Landfill gas is produced from organic waste disposed of in landfill. The waste is covered and compressed mechanically and by the pressure of higher levels. As conditions become anaerobic the organic waste is broken down and landfill gas is produced. This gas builds up and is slowly released into the atmosphere. This is hazardous for three key reasons:

### Biogas composition

The composition of biogas varies depending upon the origin of the anaerobic digestion process. Landfill gas typically has methane concentrations around 50%. Advanced waste treatment technologies can produce biogas with 55-75%CH4 [3].

Typical composition of biogas[4]
Matter %
Methane, CH4 50-75
Carbon dioxide, CO2 25-50
Nitrogen, N2 0-10*
Hydrogen, H2 0-1
Hydrogen sulphide, H2S 0-3
Oxygen, O2 0-2*

*often 5 % of air is introduced for microbiological desulphurisation

### Siloxanes and gas engines

In some cases, biogas from landfills and sewage treatment contains siloxanes. During combustion of biogas containing siloxanes, silicon is released and can combine with free oxygen or various other elements in the combustion gas. Deposits are formed containing mostly silica (${\displaystyle SiO_{2}}$) or silicates (${\displaystyle Si_{x}O_{y}}$) in general, but can also contain calcium, sulphur, zinc, phosphor… as indicated by the analysis piston scrapings from biogas-fired engines. These (mostly white) deposits can ultimately build to a surface thickness of several millimetres and are difficult to remove by chemical or mechanical means.

In internal combustion engines deposits on pistons and cylinder heads are extremely abrasive and even a small amount is sufficient to cause enough damage to the engine to require a complete overhaul at 5,000 h or less of operation. The damage is similar to that caused by carbon build up during light load running of diesel engines. Deposits on the turbine of the turbocharger will eventually reduce the charger’s efficiency.

Luckily, simply cooling the gas to roughly -4 C is suficaint to remove siloxanes due to condensantion.

Stirling engines are more resistant against siloxanes, though deposits on the tubes of the heat exchanger will reduce the efficiency.[5][6]

## Biogas to natural gas

If biogas is cleaned up sufficiently, biogas has the same characteristics as natural gas. In this instance the producer of the biogas can utilize the local gas distribution networks. The gas must be very clean to reach pipeline quality. Water (H2O), hydrogen sulfide (H2S) and particulates are removed if present at high levels or if the gas is to be completely cleaned. Carbon dioxide is less frequently removed, but it must also be separated to achieve pipeline quality gas. If the gas is to be used without extensively cleaning, it is sometimes cofired with natural gas to improve combustion. Biogas cleaned up to pipeline quality is called renewable natural gas or biomethane.

### Applications of renewable natural gas

In this form the gas can be now used in any application that natural gas is used for. Such applications include distribution via the natural gas grid, electricity production, space heating, water heating and process heating. If compressed, it can replace compressed natural gas for use in vehicles, where it can fuel an internal combustion engine or fuel cells.

#### Cooking

Gober gas is a biogas generated out of cow dung. In India, gober gas is generated at the countless number of micro plants (an estimated more than 2 million) attached to households. The gober gas plant is basically an airtight circular pit made of concrete with a pipe connection. The manure is directed to the pit (usually directed from the cattle shed). The pit is then filled with a required quantity of water (usually waste water). The gas pipe is connected to the kitchen fire place through control valves. The flammable methane gas generated out of this is practically odorless and smokeless. The residue left after the extraction of the gas is used as biofertiliser. Owing to its simplicity in implementation and use of cheap raw materials in the villages, it is often quoted as one of the most environmentally sound energy source for the rural needs.

#### Railway transport

A biogas-powered train has been in service in Sweden since 2005 [7].

## Landfill gas legislation

### United States

In the United States, because landfill gas contains these VOCs, the United States Clean Air Act and Title 40 of the Code of Federal Regulations (CFR) requires landfill owners to estimate the quantity of non-methane organic compounds (NMOCs) emitted. If the estimated NMOC emissions exceeds 50 tonnes per year the landfill owner is required to collect the landfill gas and treat it to remove the entrained NMOCs. Treatment of the landfill gas is usually by combustion. Because of the remoteness of landfill sites it is sometimes not economically feasible to produce electricity from the gas.