Basic knowledge on biogas formation
When organic matter (biomass) is degraded in the absence of oxygen (anaerobic) by microbiological processes, various gases are formed. This gas mixture produced by anaerobic digestion is also referred to as biogas. Digestate is a by-product of the fermentation process. The digestate which results from the anaerobic digestion process is a decomposed substrate, rich in macro and micro nutrients and therefore suitable to be used as plant fertilizer.
Anaerobic digestion is perfectly eligible for agricultural activities since energy crops (e.g. maize, whole grain crops.), organic residues (e.g. manure), side products (e.g. fruit pomace, oil seed leftover) and organic wastes are efficient substrates available or produced on farms.
Biogas consists essentially of methane (CH4) and carbon dioxide (CO2) and additionally of hydrogen (H2), hydrogen sulphide (H2S), ammonia (NH3) and other trace gases. The biogas composition is mainly influenced by the substrates used for digestion and the fermentation processes itself. The type of substrate or substrate mixture primarily determined the biogas condition.
The process of biogas formation basically runs in four microbiological steps which are temporally parallel. For a smooth process the individual degradation phases have to be optimally balanced to the requirements of the bacteria involved (e.g. pH-value, temperature). The four steps are: Hydrolisis, Acidification, Acetic acid formation and Methane formation.
In the first step the "hydrolysis", the substrate, which is composed of complex compounds (like carbohydrates, proteins and fats), will be cleaved by exo-enzymes in more simple organic compounds (e.g. amino acids, sugars, fatty acids).
The intermediates formed are further broken down in the second step, the so-called "acidification" (acidogenese), through acid-forming bacteria to short-chain fatty acids (acetic, propionic and butyric acid) and carbon dioxide and hydrogen and small amounts of lactic acid and alcohols.
In the third phase, the so-called "acetic acid formation (acetogenesis), the products of acidification will be implemented mainly to acetic acid, hydrogen and carbon dioxide. The acetic acid is formed from organic acids. When this process step is disturbed, an enrichment of acids will occur, because only the methanogens can degrade the acetic acids.
In the last step of biogas generation, the so-called "methanogenesis", bacteria produce biogas over two pathways; from acetic acid and hydrogen and carbon dioxide.
Depending on the design and operation of the biogas plant as well as the used biomasses, different environmental conditions are required for optimal activity of the microbes. Therefore, following parameters, among others, should be kept in mind:
the oxygen input into the fermenter (or digester or bio-digester) should not be too high
the temperature in the digester should be matched to the involved microorganisms (e.g. at mesophilic operation: 37-42 oC)
the pH-value of the substrate in the fermenter should range between pH 6.5 and 8 (at single-phase processes)
the digester should be balanced in terms of macro and micro nutrients
In general, the operating conditions of a biogas plant should be kept as constant as possible. Especially important is the feed-in of substrate. Some typical mistakes concerning the plant feeding are:
feeding substrate continuously over a long period of time substrate is to irregularly supplied
fast switching of substrates with different composition/quality or
feeding to much substrate after a "feeding pause" (e.g. due to technical problems).
The rate of gas production and the process itself is very sensible and can easily be inhibited. Inhibitors may decrease already in small amounts the degradation rate and gas production or lead to a complete standstill at toxic levels. For example, antibiotics can enter the digester via the manure. Even in small amounts antibiotics, disinfectants or solvents, herbicides or salts of heavy metals can inhibit the degradation process in the digester.
Production of Biogas
In agricultural livestock farms cost free substrates like manure, fodder residues and wastes incur. Manure only has a low energy density, because of the high water content and the rather low specific gas yield. This makes it less transport-worthy and economically unattractive at long transportation distances. In order to exploit the potential, inexpensive and easy to operate plants in low (power) capacity range are now available.
Slurry is hydraulically easy to handle. With a high proportion of manure in the digester, also hydraulically challenging substrates, such as grass or solid manure, can be used relatively easy in a small biogas plant. The adjustment of technology, used substrates and kind of operation thereby decides about the quality of plant operation and the achievable biogas yields. The substrates used ultimately determine the needed appropriate technology and their interpretation, such as cutting units, sizing of pipes, pumps, gas treatment, gas storage and Combined Heat & Power (CHP) unit.
Among the general technical requirements for a pure manure digestion, the heat balance can be a critical point – particularly during winter time. Especially at long periods of cold weather, the heat supply for the plant and external heat consumers, such as stables and residential buildings, might be difficult.
The market shows a considerable breadth of different technical solutions for biogas plants, incl. wet fermentation and solid-state fermentation systems. Depending on the type of substrate (e.g. fodder leftovers, litter or grass as a co-substrate) and local conditions (e.g. necessary construction for slurry storage), the technical suitability and benefits of each concept should be examined carefully and if possible with support of a neutral consultant.
Utilization of biogas
The biogas produced is versatile. Mostly it is used in combined heat and power plant-units (CHP) on site. The electricity generated is fed into the public grid or used on site for self-consumption. The internal consumption of the biogas plant can be covered either by the power grid or from the CHP.
In addition to the electricity produced, a CHP also provides heat energy from exhaust and engine cooling. A portion of the generated CHP heat is used to heat up the digester. However, some heat generated is for any other use available.
As engines for combined heat and power units, spark-ignition or pilot injection gas engines are normally used. Spark-ignition engines (gas engine) are specially designed for gas operation and can be operated with methane content in the biogas from about 45 % and above. They have a gas mixer and a spark ignition ignites the gas mixture.
The material terms of the digestate by spreading it on farmland in the immediate surroundings of the biogas plant is still the most (cost) effective variant to deal with the fermentation residues. The project development should clarify the question whether there is sufficient arable land for spreading available or if a guaranteed purchaser for the incurred digestate exists. Only when these options are not given, it makes sense to deal with the processing of the digestate.
With processing and treatment of digestate, the pressure on the local rental market for arable land can be reduced, the transportability of the nutrients from the digestate can be increased, a potential excess of nutrients in the region can be defused and storage and application costs might be saved.
In addition, the marketability of liquid and spreadable fertilizers is increased and, not least, reduced environmental impact by avoiding volatile air and atmospheric pollutants.
The digestate processing begins with the separation of liquid and solid fraction in preparation for the subsequent, mostly mechanical or thermal processes.
The separation is performed mechanically using centrifuges or screw presses. For simple applications such as the separation for the production of thin liquid recirculated, no further processing steps of the digestate are required.
With processing / treatment of the solid phase, a high-quality fertilizer can be produced. The removal of the water allows economically transport of the fraction over longer distances.
Drying process (like belt dryer, thrust reversing dryer, fluid bed dryer, screw presses) remove the water in the digestate by overflowing it with a hot air stream. As heat source in this case, the surplus heat from CHP can be used. In addition to the removal of water, the digestate will be sanitized and pasteurized by using the CHP heat.
Beside the direct use of the liquid phase as liquid fertilizer it is possible, to continue the treatment process (full treatment) to achieve a quality of the liquid phase which allows the direct discharge into receiving waters. The effort, requirements and costs for this full treatment process are usually high.