Biogas Plant Used in Energy Production
Biogas is generated by the breakdown of organic matter by anaerobic bacteria and is used in energy production. Biogas differs from natural gas in that it is a renewable energy source produced biologically through anaerobic digestion rather than a fossil fuel produced by geological processes. Biogas is primarily composed of 40-70% methane, 30-60% carbon dioxide, and trace amounts of nitrogen, hydrogen, and carbon monoxide. It occurs naturally in compost heaps, as swamp gas, and as a result of enteric fermentation in cattle and other ruminants. Biogas can also be produced in anaerobic digesters in a controlled plant environment.
Product Details
Almost all forms of biodegradable organic material can be used to produce biogas. However, manure, sewage sludge, municipal waste, agricultural waste and energy crops are the most common feedstock.
Biogas occurs in 3 stages:
1 – Hydrolysis , 2 – Acidification, 3 – Methanogenesis
Hydrolysis
In the first step (hydrolysis), the organic matter is enzymolyzed externally by extracellular enzymes (cellulose, amylase, protease and lipase) of microorganisms. Bacteria decompose the long chains of the complex carbohydrates, proteins and lipids into shorter parts. For example, polysaccharides are converted into monosaccharides. Proteins are split into peptides and amino acids.
Acidification
Acid-producing bacteria, involved in the second step, convert the intermediates of fermenting bacteria into acetic acid (CH3COOH), hydrogen (H2) and carbon dioxide (CO2). These bacteria are facultatively anaerobic and can grow under acid conditions. To produce acetic acid, they need oxygen and carbon. For this, they use the oxygen dissolved in the solution or bounded-oxygen. Hereby, the acid-producing bacteria create an anaerobic condition which is essential for the methane producing microorganisms. Moreover, they reduce the compounds with a low molecular weight into alcohols, organic acids, amino acids, carbon dioxide, hydrogen sulfide and traces of methane. From a chemical standpoint, this process is partially endergonic (i.e. only possible with energy input), since bacteria alone are not capable of sustaining that type of reaction.
Methanogenesis
Methane-producing bacteria, involved in the third step, decompose compounds with a low molecular weight. For example, they utilize hydrogen, carbon dioxide and acetic acid to form methane and carbon dioxide. Under natural conditions, methane producing microorganisms occur to the extent that anaerobic conditions are provided, e.g. under water (for example in marine sediments), in ruminant stomachs and in marshes. They are obligatory anaerobic and very sensitive to environmental changes. In contrast to the acidogenic and acetogenic bacteria, the methanogenic bacteria belong to the archaebacteria genus, i.e. to a group of bacteria with a very heterogeneous morphology and a number of common biochemical and molecular-biological properties that distinguish them from all other bacteria. The main difference lies in the makeup of the bacteria's cell walls.
Thermal Value of Biogas:
The amount of heat supplied by 1 m3 of biogas is 4,700-5,700 kcal / m3.
1 m3 biogas is equivalent to:
- 0.62 liters of kerosene
- 1.46 kg of charcoal
- 3.47 kg of wood
- 0.43 kg of butane gas
- 12.3 kg of turf
- 4.70 kWh of electrical energy
- 0.666 liters of diesel fuel
- 0.75 liters of gasoline
- 0.25 m3 of propane equivalent fuel
Things to Pay Attention
- There should be no oxygen in the fermenter (production tank-digester).
- Animal wastes that have received antibiotics should not be inserted into the production tank.
- Organic waste with detergents should not be inserted into the production tank,
- A sufficient amount of nitrogen should be present for the formation and growth of new bacteria in the environment.
- Acidity in the production tank should be pH = 7.0 – 7.6.
- Organic acid concentration should be around 1,500 mg / liter.
- Temperature in the digester should be constant at between 35°C to 56°C.
- Environment should be dark. No light should enter the digester.
- Optimum water content in the digester is 90%. In no case should this be less than 50%.
- There should be sufficient amount of organic material to feed methane bacteria.
Facility Design
Biogas plants are constructed using different technologies according to the planned purpose. The classification of biogas plants, as per their digester capacity, is as follows:
- Family type: 6 - 12 m3
- Farm type: 50 - 100 / 150 m3
- Village type: 100 - 200 m3
- Industrial scale plants: 1,000 - 10,000 m3
Family type biogas plants are widely used in China, especially in rural areas. In most plants, other than family type small ones, heating of the environment in which biogas is formed (digester) is required for optimum biogas production. In the production of biogas, the ambient temperature should be around 35°C. In order to provide heat control in biogas plants, the most practical and widely used system is hot water coils installed inside the digester.
It is very important to correctly identify:
- Quantity of raw materials
- Type and properties of raw materials
- Heating requirements
- Mixing requirements
- Type of materials and equipment to be used
- Location of the plant
- Construction and insulation of the digesters
- Heating and operating conditions of the facility
- Storage and distribution of biogas
- Transportation of biogas
- Utilization of digestate
Plant Main Components
Preparation and Feeding Unit
Feed preparation enables the input of the organic matter / wastes to be brought to the desired size. Mix quantities of each material is also established in order to achieve the optimum “recipe”. The resulting feed is loaded in the digesters at the designed quantity and intervals by the automated feeding system.
Fermenters
In industrial biogas plants, the size and number of fermenters vary depending on the plant capacity and the hydraulic waiting time. Fermenters can be constructed from concrete or steel. In most facilities, the top of the gas tank is usually equipped with an elastic gas collection cover on the fermenter. The mixers are placed in the fermenter so that the material in the fermenter can be properly mixed. In addition, fermenters are well insulated to minimize heat loss and auto-controlled heating coils are placed in the fermenters to keep the interior temperature of the fermenter within the desired range.
Cogeneration Unit
The Cogeneration Unit is the part where the produced biogas is converted into electricity and heat. Biogas taken from the fermenter contains 50-60% CH4, 30-40% CO2, 500-2500 ppm H2S, and very small amounts of other gases and moisture. H2S in the content of biogas, combined with moisture in the gas engine, pipelines and so on causes corrosion of mechanical parts. Before the biogas is used in the gas engine, H2S and moisture in its’ content must be separated. Heat and electricity are obtained by using H2S and moisture in the separated biogas gas engines. Part of the heat generated is utilized in the heating of the materials in the fermenter, the remainder being utilized to provide heating for other facilities in the complex. While most of the electricity produced is supplied to the electricity grid, some is used to operate the equipment in the facility.
Organic Fertilizer Factory
The Digestate remaining at the end of biogas process is pumped to separators. After separation, the solids phase is sent to the Dry Organic Fertilizer Plant where it is dried to 25-30% moisture levels, before being granulated, pelleted or powdered, then hygienized before being packaged for sale. The liquid phase (3-6% solid matter) is pumped to the Liquid Organic Fertilizer Plant to be further processed to produce 30% solid matter liquid fertilizers with high humic-fulvic acid content, & packaged for sale
Benefits of Biogas
Well-functioning biogas systems can yield a whole range of benefits for their users, the society and the environment in general
- production of energy (heat, light, electricity);
- transformation of organic waste into high quality fertilizer;
- reduction of volume of disposed waste products;
- improvement of hygienic conditions through reduction of pathogens, worm eggs and flies;
- encouragement of better sanitation;
- reduction of workload, mainly for women, in firewood collection and cooking;
- environmental advantages through protection of soil, water, air and woodland vegetation;
- micro-economical benefits through energy and fertilizer substitution, additional income sources and increasing yields of animal husbandry and agriculture;
- macro-economical benefits through decentralized energy generation, import substitution and environmental protection
Usage Areas
Heating
Biogas can be combusted to produce heat. When biogas is mixed with air with a 1/7 ratio, complete combustion occurs. For heating purposes, biogas can be used in gas-fired furnaces and hobs. Biogas can be used in stoves operated with liquefied petroleum gas by adjusting the nozzle diameters. When used in biogas stoves, a chimney system is required to allow any hydrogen sulfide gas to vent to the atmosphere.
Electricity only
Electricity generation is a relatively straightforward use for biogas, and it can be the most profitable. Biogas requires minimal investment in cleaning and upgrading. Electricity is easier to transport than heat and supply is easily measured.
Electricity storage, however, is not simple and connecting to the electricity network is costly.
Combined Heat and Power (CHP)
Combined heat and power (CHP) is the simultaneous production of useable heat and electricity. As the process of AD requires some heat it is suited to CHP and this is currently the most popular option for most plants. Whilst coal and gas-fired power stations have an efficiency of around 34% and 55% respectively, CHP plants can achieve overall efficiencies in excess of 80% at the point of use.
The ratio of heat to power varies dependent on the scale and technology, but typically 35-40% is converted to electricity, 40-45% to heat and the balance lost due to inefficiencies at various stages of the process. This typically equates to over 2kWh electricity and 2.5kWh heat per cubic meter, at 60% methane.
A generator producing electricity from a biogas plant can be connected to a transmission network, distribution system or even to the cables owned by the end customer.
Biomethane Injection
Biogas can be upgraded to biomethane and injected into a gas grid. This can be the national high pressure gas transmission grid or a local low pressure gas distribution network. To be used in the gas grid biogas needs to be cleansed of impurities, dried and upgraded to higher methane content (> 95%) so that it resembles the qualities of natural gas.
Biomethane for Transport
Biogas can be cleaned to remove impurities and upgraded to pure biomethane. It can then be used as a renewable transport fuel in vehicles designed to run on compressed natural gas (CNG) or liquefied natural gas (LNG).
Organic Fertilizers
While there are suitable inorganic substitutes for the nutrients nitrogen, potassium and phosphorous from organic fertilizer, there is no artificial substitute for other substances such as protein, cellulose, lignin, etc. They all contribute to increasing a soil's permeability and hygroscopicity while preventing erosion and improving agricultural conditions in general. Organic substances also constitute the basis for the development of the microorganisms responsible for converting soil nutrients into a form that can be readily incorporated by plants.
Due to the decomposition and breakdown of parts of its organic content, digested sludge provides fast-acting nutrients that easily enter into the soil composition, thus becoming immediately available to the plants. They simultaneously serve as primary nutrients for the development of soil organisms, e.g. the replenishment of microorganisms lost through exposure to air in the course of spreading the sludge over the fields. They also nourish actinomycetes (ray fungi) that act as organic digesting specialists in the digested sludge. (Preconditions: adequate aeration and moderate moisture).
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