Biogas is a fuel produced by the degradation of organic matter by microorganisms in the absence of oxygen and consists of the breakdown of substances contained in plants, sludge, and animal by-products. Biogas systems turn waste into a resource.
Biogas systems make it possible to produce biogas from;
- Agricultural biomass (corn, sorghum, sunflower, straw, cereal flour, agricultural waste in general),
- Livestock manure (cattle slurry, cattle manure, pig slurry, goat manure, sheep manure, poultry manure,
- Other biomass produced from processing waste (sewage sludge, fruit and vegetable residues, agro-food residues, table scraps, slaughterhouse waste, fish processing residues, blood, serum).
Biogas and Biomethane.
The different biogas plants are distinguished according to the matrix used i.e. from livestock manure and/or agricultural biomass, or from the organic fraction from waste. All biomethane and biogas plants must be carefully designed to ensure high safety, high reliability, and low management costs.
Biogas can be refined and purified for production of biomethane (99% methane) which has conditions and characteristics of use corresponding to those of methane, can be stored, transported or placed in the natural gas network. The different biogas plants are distinguished according to the matrix used i.e. from zootechnical waste or agricultural biomass, or from the organic waste fraction.
The production of electricity and heat from biomass represents a new and high source of income, thanks to the ever-increasing need for energy from renewable sources. The energy produced is considered green energy because it is produced with low environmental impact.
The main types of biogas digester technologies for small to medium sized units are fixed dome, floating dome, and tubular polybag. The technology for large sized biogas digesters is a concrete active bioreactor with balloon gas holders.
Common types of biogas digesters.
The most common types of biogas digester technologies are briefly described below. They are categorised as continuous wet digestion systems. This means they operate with daily input of fresh feedstock (“continuous”) into the same active bioreactor volume and are fed with a feedstock with high water content or by adding water. The waste material is a slurry and flows like a fluid material. The active bioreactor is always full (steady state system). In other words when fresh material enters the active bioreactor, digestate leaves the active bioreactor.
A fixed-dome plant includes a cylindrical rigid active bioreactor with a rigid gas-holder dome, a feedstock inlet, and an outlet area also called expansion chamber. The gas produced in the digester is stored in the upper part of the unit (in the gas holder dome). With a closed outlet gas valve, gas pressure inside the dome increases with time, pushing the digestate into the expansion chamber. When the gas valve is open for gas utilisation, gas pressure drops, and a proportional amount of slurry flows back from the expansion chamber into the active bioreactor. Typically, such a biogas unit is constructed underground, protecting the active bioreactor from low temperatures at night and during cold seasons. Given that the gasholder dome undergoes pressure, this system needs experienced and skilled construction to ensure gas tightness. Also, given its underground construction, it should be considered as permanent infrastructure.
A floating-drum biogas plant consists of a cylindrical active bioreactor and a movable, floating gasholder (drum). The produced gas collects in the gas drum, which rises or falls again, depending on the amount of gas produced and used. The rising and falling drum, thus, provides a helpful visual indicator of the gas production and available gas quantity. The weight of the gas drum creates gas pressure. To increase gas pressure, weights can be added on top of the gasholder.
A tubular polybag digester consists of a longitudinal shaped heat-sealed, weather resistant plastic or rubber bag that serves as an active bioreactor and gas holder. The gas is stored in the upper part of the bag, and the inlet and outlet are attached directly to the bag at each end. As a result of the longitudinal shape, no short-circuiting occurs and slurry flows through the biogas unit in a plug-flow manner. Gas pressure can be increased by placing weights on the bag while taking care not to damage it. The benefit of this technology is that it can be constructed at low cost by standardised prefabrication. However, the plastic balloon is quite fragile and susceptible to mechanical damage and has a relatively short life span of 2-5 years. To avoid damage to and deterioration of the bag, it is also important to protect the bag from direct solar radiation with a roof. Additionally, it is recommended that a wire-mesh fence protect it against damage by animals.
Concrete digesters with balloon gasholders are typically used for large scale systems, although the technologies described above can also be used for large units. Concrete and balloon designs are typically aboveground and have different units serving different purposes. With this technology, mixing pits are often an additional component. These allow feedstocks to be mixed and homogenised before they are fed into the digester. The mixing pits are equipped with propellers for mixing and/or chopping the feedstock and often a pump is used to transport the feedstock into the digester. The active bioreactor itself is made of concrete or steel and often insulated. Large active bioreactors are almost always stirred by slow rotating paddles, rotors, or injected biogas. The gas holder is usually made of flexible material. It can be placed either directly above the active bioreactor to act as a balloon plant or in a separate ‘gas-bag’.