What Is Biogas?

People have been using biogas for centuries: for instance in the 18th century biogas was drawn from the underground sewer pipes in London and burned in street lamps, which were known as "gaslights" and in many parts of the world biogas is used to heat and light homes, to cook and even to fuel buses. It is now also collected from large-scale sources such as landfills and from animal dejections from large-scale farms.

Biogas is a combination of several gases which are produced by the biological process of decomposition of any organic matter by bacteria in absence of oxygen (anaerobic situation). The main flammable component of biogas is methane, or CH4. Although it is in itself a notorious 'Greenhouse' gas - with 21 times the greenhouse warming potential of carbon dioxide (CO2) - when used as fuel it gives off only circa half as much CO2 for a given fuel value. It is therefore an ecological fuel.

I. Biogas Production

1. Biogas from Landfills
A landfill is a carefully designed structure built into or on top of the ground in which garbage is isolated from the surrounding environment (groundwater, air, rain). This isolation is accomplished with a bottom HDPE liner and daily covering of soil.

How it works

As the trash is put into and compacted into a landfill, air is gradually squeezed out of the landfill due to the weight of the mass of accumulated matherila – landfills could be up 100meters deep – therefore there is little oxygen left (anaerobic situation) and little moisture. Under these conditions, bacteria decompose the organic matters contained in the landfill, producing biogas.

2. Biogas from Digesters
The production method used is called 'bio-digestion', which is an artificially-maintained version of what naturally goes at the bottom of certain ponds or in cows’ stomachs. For industrial-scale biogas production digesters are essentially sealable containers of a size sufficient to supply the amount of gas needed. The gas thus generated is then 'scrubbed', stored, pressure-regulated and piped to the machinery using it.

How it works

There are two basic types of digester; either 'batch' or 'continuous'. They can be feed from a variety of natural waste products, including animal manure. There are also two 'flavors' of digesters: mesophilic, which operate at around 'blood-heat' or 38°C, give or take 10°C and thermophilic, work at hot-water temperatures of around 60°C.

Batch Digesters operate on a single charge until they are exhausted. At the end of the digestion cycle, the Batch Digester is emptied, cleaned, recharged and restarted for a new cycle then left until done. Batch digesters have the quality of predictability because once started, they are not disturbed or interrupted, but they need to be assembled in arrays.

Continuous-Feed Digesters have increments of organic waste added and subtracted on a daily basis to provide an ongoing replenishment of charge materials and water. Continuous-feed digester systems have lower capital costs (you only need one digester) but they do require close monitoring of feedstock.

3. Coal mines
Methane trapped in coal deposits and in the surrounding coal seam strata is released during normal mining operations in both underground and surface mines. Deregulation of energy markets and changes to the oil and gas exploration licensing regime have opened up the potential to capture and utilize Coal Bed Methane (CBM) and Coal Mine Methane (CMM).

How it works

It is widely acknowledged that CBM/CMM can provide a valuable source of clean energy. The growth potential for coal mine methane energy applications is considerable, given that operators face a major challenge to consistently meet international and local emission standards. In addition, handling of the coal after mining results in methane emissions. Methane is captured through pipes bored directly into the coal –ore face or scrubbed from the coal mine air through filters.

II. Biogas Uses

Biogas contains a large proportion of carbon dioxide along with water vapor, ammonia, hydrogen sulphide and a few traces of other gases which are insignificant for practical purposes, hence it needs to be preprocessed by ‘scrubbing’ to remove as much as practicable of the corrosive gases which combine with the water vapor to form acids and hence corrode all metal parts of the gas system.

The Technology

Biogas has excellent anti-detonation properties - effectively a high octane rating - and can be used to power petrol engines or as 95% of the fuel for a diesel.

  • Power Generation: by using biogas or CMM/CBM methane in endothermic gas engines to spin alternators, it is possible to generate renewable energy electrical power (whilst at the same time reducing substantially the venting of greenhouse gases into the atmosphere. One tone of organic waste will produce between 100 m3 and 160 m3 of biogas per day. This is approximately equivalent to between 65 and 95 liters of petrol. A cogeneration plant can produce 170 kWh of electricity and 340 kWh of heat in the form of water at 70°C from this. Approximately 10 % - 20 % of this energy is used for the plant itself. The electricity from renewable sources fed into the national gird and a district heating scheme can use the heat produced in nearby buildings or in industrial processes.
  • Compressed Biogas as Vehicle Fuel: It is not always possible to use the biogas produced in a cogeneration plant. Finding customers for the heat produced can be a problem in summer, for instance. Therefore by scrubbing, compressing and bottling it in special tanks, biogas can be used to fuel Biogas-powered vehicles, which have extremely good exhaust emission figures, approximately half of those produced by an equivalent petrol-driven vehicle.


Environmental Aspects

Biogas is an ideal and abundant source of renewable energy. This is because the production of biogas depends on the supply of organic matter, which usually grows back each year or is a wasted by product of organic waste deposited on landfills or a wasted (and dangerous) by products of coal-mining activities. By comparison, the natural gas used in most of our homes or to generate electricity in large-scale power-plants is not considered a form of renewable energy as it is of hydrocarbon origin. These resources do not "grow back" in a time scale that is meaningful for humans.