James G. Speight - Encyclopedia of Renewable Energy

Biogas normally consists of 50 to 60% methane and has a variable composition and energy content (Btu/ft 3) depending upon the source ( Table B-12).

Table B-12Energy content of various gases.

Biogas is produced by means of a process known as anaerobic digestion – a process in which organic matter is broken down by microbiological activity in the absence of air. Biogas is generated from concentrations of sewage or manure. These are usually in the form of slurry comprised mostly of water (almost 95%). The slurry is fed into a digester, this input can be continuous (usually the case with sewage) or in batches. The digestion continues from approximately 10 days up to weeks. The temperature in the digester should be kept at 35°C (95°F), and although the digestion itself produces heat, in colder climates, heat may be necessary.

There are two common technologies for the production of biogas. The first involves the fermentation of human and/or animal waste in specially designed digesters:

The second is a more recently developed technology for capturing methane from municipal waste landfill sites. The scale of simple biogas plants can vary from a small household system to large commercial plants. The digestion of animal and human waste yields several benefits: (i) the production of methane for use as a fuel and (ii) the waste is reduced to slurry which has a high nutrient content which makes an ideal fertilizer, and in some cases, (iii) the fertilizer is the main product from the digester and the biogas is merely a by-product. During the digestion process, bacteria in the manure are killed, which is a great benefit to environmental health.

Two popular simple designs of digester have been developed: (i) the floating cover digester and (ii) fixed dome digester. The digestion process is the same in both digesters, but the gas collection method is different in each. In the floating cover digester, the water-sealed cover of the digester is capable of rising as gas is produced and acting as a storage chamber, whereas the fixed dome digester has a lower gas storage capacity and requires good sealing if gas leakage is to be prevented.

The waste fed into the digester undergoes digestion in the digestion chamber and the temperature of the process is quite critical – methane-producing bacteria operate most efficiently at temperatures between 30 and 40°C (86 to 104°F) or 50 and 60°C (122 to 140°F), and in colder climates, heat may have to be added to the chamber to encourage the bacteria to carry out their function. The product is a combination of methane and carbon dioxide, typically in the ratio of 6:4. Digestion time ranges from a week to months depending on the feedstock and the digestion temperature. The residual slurry is removed at the outlet and can be used as a fertilizer.

The composition of biogas varies depending upon the composition of the waste material in the landfill and the anaerobic digestion process. Landfill gas typically has methane concentrations around 50%. Advanced waste treatment technologies can produce biogas with 55 to 75% methane; often, air is introduced (5% by volume) for microbiological desulfurization. For example, the constituents (by volume) of biogas generally are methane (50 to 75%), carbon dioxide (25 to 50%), nitrogen (0 to 10%), hydrogen (0 to 1%), hydrogen sulfide (0 to 3%), and oxygen (0 to 2%).

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 clean to reach pipeline quality. Water (H 2O), hydrogen sulfide (H 2S), 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 co-fired with natural gas to improve combustion. Biogas cleaned up to pipeline quality is called renewable natural gas and can be used in any application in which natural gas is used.

Current technology allows the gas to be recovered using sealed vessels and therefore available for heating, electrical generation, mechanical power, and so forth. Biogas can be retrieved from garbage or mechanical biological treatment waste processing systems. The solid by-product, digestate, can be used as a biofuel or a fertilizer. Like natural gas, biogas has a low volumetric energy density compared to liquid biofuels, but it can be purified to a natural gas equivalent and further compressed for use as a transportation fuel, substituting for natural gas. Methane is also suitable for use in fuel cell generators. Biogas is often made from wastes, but is also made from biomass energy feedstocks. Landfill gas cannot be distributed through utility natural gas pipelines unless it is cleaned up to less than 3% v/v carbon dioxide and several parts per million hydrogen sulfide, because these chemicals corrode the pipelines.

See also: Aerobic Digester, Anaerobic Digester, Landfill Gas.

Biogas upgrading principally refers to removal of the carbon dioxide from the biogas to increase energy density and decrease needed storage volumes of finished product. Moisture, sulfur compounds, organo-silicon compounds (siloxanes), and other impurities in biogas or fuel gas are usually removed as well to meet gas quality specifications, and because they can cause problems in gas handling equipment and damage engines and emissions controls. Biomethane from upgraded biogas or landfill gas can be compressed or liquefied and used as a fuel in compressed natural gas (CNG) or liquefied natural gas (LNG) vehicles.

Processes available for separating carbon dioxide from the methane in biogas include adsorption methods such as scrubbing with water, Selexol (polyethylene glycol ether), and amines, pressure swing absorption (PSA), membrane separation, and cryogenic separation. However, each biogas project is unique it can be a challenge to determine the best gas upgrading technology for the given situation. Digester biogas can have varying levels of carbon dioxide (CO 2) and elevated hydrogen sulfide (H 2S) to address. Landfill gas and gas from covered lagoon digesters can have elevated nitrogen (N 2) and oxygen (O 2) levels, and landfills and municipal wastewater treatment plant (WWTP) digesters have siloxanes that need to be handled.

Typically, biogas is usually fully saturated with water vapor and typically has from 40 to 60% methane (CH 4) and 40 to 60% v/v carbon dioxide (CO 2). Thus, the choice of an appropriate technology or combination of technologies to upgrade the gas from these modest methane levels up to 99% v/v methane can be challenging. The main treatment goal of gas upgrading projects is to get the carbon dioxide removed from the biogas stream to an acceptable level, typically on the order of with 1 to 2% v/v carbon dioxide. Actual specifications will vary based on end use and the specific requirements provided by the gas utility.

Selecting an upgrading system that can reliably meet the methane-oxygen-carbon dioxide-hydrogen sulfide specifications is critical for a successful relationship with one’s gas off-taker. At times, such as at a new project site, technologies may need to be selected without having extensive biogas characterization data. Therefore, selecting a biogas upgrading system that can handle varying gas quality and quantity and stay within specifications is essential.