Biogas Production by Anaerobic Digestion

Natural Energy News Items Government Policy Business Strategy Your Help

Biogas Production by Anaerobic Digestion – Natural Energy

Anaerobic digestion is a natural process which takes place in the absence of oxygen. Organic material is digested by bacteria in a closed reactor vessel and biogas is produced. This controlled digestion process is normally accelerated by increasing the reactor temperature into the mesophilic range (normally between 30-37ºC), or into the thermophilic range (normally between 55-65ºC). The decomposition of organic material consists of the following basic processes (Dohanyos et al., 2000).

Figure - The main degradation pathways in anaerobic digestion

Since the anaerobic digestion process is usually carried out in a single reactor vessel the processes described above run concurrently. Biogas consists of 45-85 % methane (CH4) and 15-45 % carbon dioxide (CO2), with the exact proportions depending on the production conditions and processing techniques. In addition, hydrogen sulphide (H2S), ammonia (NH3) and nitrogen gas (N2) may be present in small amounts. Biogas is normally saturated with water vapour. Confusingly, artificially produced methane, made for example from wood products by a process called thermal gasification, is sometimes also called biogas. This is also a renewable source of methane, but it is not discussed further on this website. A volume of biogas is normally expressed in ‘normal cubic meters’ (Nm³). This is the volume of gas at 0 ºC and standard atmospheric pressure. The energy value is expressed in joule (J) or watt hour (Wh). Pure methane has an energy value of 9.81 kWh/Nm³ (9810 Wh/Nm³). The energy value of biogas varies between 4.5 and 8.5 kWh/m³, depending on the relative amounts of methane, carbon dioxide and other gases present. Both methane and carbon dioxide are odourless. If raw biogas smells, it is usually due to the presence of sulphur compounds. Biogas may ignite at concentrations of about 5-20 % in air, depending on the methane concentration. Methane is lighter than air, whereas carbon dioxide is heavier. This is considered to be advantageous from a safety point of view, since methane easily rises and is quickly diluted by the air.

Biogas can be used for all applications suitable for natural gas. Not all gas appliances require the same gas standards. There is a considerable difference between the requirements of stationary biogas applications and fuel gas or pipeline quality. Boilers do not have a high gas quality requirement. Gas pressure usually has to be around 8 to 25 mBar. Hydrogen sulphide is a very corrosive component of biogas. In boilers it oxidises to form sulphuric acid which can dissolve the metal parts of the heat exchangers and chimneys. In internal combustion engines, hydrogen sulphide reacts with copper alloys and rapidly destroys the bearings and other engine parts. Removal of hydrogen sulphide is therefore a prerequisite for safe biogas utilisation.

Several methods are used, the most common being a reaction with iron salts. More recently a technique using small streams of air bubbled through the top layer of digested slurry in the digester has been used to promote growth of Thiobacillus spp. These oxidise hydrogen sulphide to elemental sulphur which is retained within the treated slurry.

The utilisation of biogas in internal combustion engines (gas engines) is a long established and extremely reliable technology. Thousands of engines are operated on sewage works, landfill sites and biogas installations.

Caption: MSP Jim Wallace enjoys a cup of coffee boiled from electricity generated by biogas with Heat and Power Ltd Chairman Colin Risbridger.

The utilisation of biogas as vehicle fuel uses the same engine and vehicle configuration as natural gas. Worldwide there are more than 3 million natural gas vehicles and about 10,000 biogas driven cars and buses, demonstrating that the vehicle configuration is not a problem for use of biogas as vehicle fuel. However, the gas quality demands are strict so the raw biogas from a digester or a landfill has to be upgraded. A demonstration project is currently underway with C Ris Energy on Westray producing biogas for road fuel. There are a number of compounds which have to be removed from biogas where they are present. Upgrading of biogas is an important cost factor in the production of fuel gas and Heat and Power Limited plan to retail some biogas as road fuel in 2007.

In smaller anaerobic digestion/biogas plants, the gas is usually purified and burned in boilers to help maintain the digestion process temperature. Hot water heats the digester to mesophilic or thermophilic temperatures and can also be used locally for space or process heating. In larger units, the gas can be used by internal combustion engines or gas turbines to produce electricity using an electricity generator. Heat developed during electricity generation can also be used to heat the digester.

Livestock excrements (pig, cattle and poultry) are substrates with a high percentage of biologically degradable organic compounds. They contain all the nutrients required for anaerobic digestion organisms. Waste from cattle and sheep even contain the methanogenic bacteria required in the AD process. The highest proportion of organic matter biodegradable by AD is in poultry waste (65%), with less in pig waste (50%) and even less (25-40%) in the waste of cattle where the material has already undergone methanogenesis in the ruminant stomachs. Odour from livestock slurry is substantially decreased by anaerobic digestion. Compounds associated with offensive odours, including volatile fatty acids (VFA) and small but potent molecules of mercaptans, are degraded into methane and carbon dioxide by anaerobic bacteria during the process. Volatile fatty acids may be reduced by 93% and phenols and pcresol virtually eliminated. The remaining odour in the treated material is caused by low residual concentrations of VFA’s and by hydrogen sulphide.

Figure – Schematic of anaerobic digestion system for treatment of livestock slurry.

The characteristics of the final treated stabilised digestate are the result of the complex anaerobic decomposition. Liquid feedstock stabilised by anaerobic digestion has almost the same volume as the raw feedstock, but the solids content is reduced by up to 50% and the digested feedstock odour is substantially decreased. The extent of pathogen inactivation in the digestate depends largely on the treatment temperature and the process arrangements. The anaerobic digestion decreases the C:N ratio and increases the concentration of immediately accessible plant nutrients. As shown in figure above, Heat and Power Limited intends to separate the liquor from the fibrous matter. The latter would normally be composted, and could be sold commercially if there is insufficient local demand. The liquor would normally be sprayed on to pastureland as liquid fertiliser, but could also be used as a nutrient medium for horticulture or aquaculture.