General Introduction about What Biogas Is
Biogas refers to a mixture of methane and carbon dioxide. It is formed when bacteria decompose in an oxygen-free environment (also known as “anaerobic digestion”). As the raw materials are constantly reproduced, this biogas is considered a renewable source of energy. The composition of biogas is methane around 60-70%, carbon dioxide 30-40%, and with traces of hydrogen, nitrogen, and hydrogen sulphide.
The various research evidence shows there is raising interest in biogas use as a fuel for vehicle applications, domestic cooking, and in many other areas. Driving factors toward the use of biogas include taxes and regulation on waste disposal, the rising need for renewable sources, and the need to improve the quality of the air.
Biogas – Availability of Resource and Production Technology
Biogas is generated under anaerobic conditions. Sewage waste, wet manure slurries from agriculture, dry manures from animal beddings (FYM, or farm yard manure), food processing waste, wastes from commercial operations, waste from household kitchens, and even from the garden, are all resources for the production of biogas. Per day, a family needs at least one cubic meter of biogas just to meet their demand for cooking.
The three main elements in the anaerobic fermentation process are pre-digestion treatment, digestion, and post digestion treatment.
The goal of the pre-digestion treatment process is to get the raw materials into a suitable form for digesting. This includes the removal of inorganic materials, maceration or shedding with an aim to reduce the material size, and adding water and other related liquids. Different feed stocks are mixed together, and some products like animal by-products need sterilization or pasteurization to reduce pathogens.
In this process, the materials are naturally digested or degraded by bacteria in sealed airtight containers with an absence of oxygen. A range of characteristics describe the different digester types.
Low or high solid digesters – These treat the material that is in liquid state with 15-20% solids. These low solid digesters need continuous mixing. High solid digesters usually hold the material with 20-40% solids and this process does not have any internal mixing.
Single or multi-stage systems – The digestion process has many different phases. The important phase involves hydrolysis or acidogenesis. In this stage, material is broken into organic acids and then methanogenesis, where the methane is produced, takes place. Single digesters take in all stages of digestion in a single container, but the multi-stage digesters takes up the process in different containers.
Mesophilic and thermophilic systems – The mesophilic system works at 35oC and need less external heating. This produces less gas and the retention times taken to get the material digested are around 15-30 days. The thermophilic system usually operates at 55o C and needs more heating energy. This produces more gas, is harder to control, and the retention times are only 12-15 days.
Continuous or batch flow systems – As the name clearly suggests, some systems work in a batch mode in which the materials are processed in one go and left for the full retention time. These tend to be smaller systems, whereas the continuous system is a larger system that gradually feeds material and draws gas from the digestate at the same time.
The post-digestion treatment involves upgrading or cleaning the raw biogas to make it into a suitable form to use in vehicles and for domestic cooking.
After the production of raw gas from the digestion process, there is need for a process that upgrades the raw biogas to natural gas quality in order to be used in vehicles. This process involves the removal of carbon dioxide, hydrogen sulphide, ammonia, and water (as well as sometimes other trace compounds). The end result should be product gas that has methane at 95-98% by volume. This is generally referred as “biomethane.”
Carbon dioxide removal – For an efficient use of biogas as fuel for the vehicle, it must to be enhanced in methane. This can be obtained when the carbon dioxide is removed; moreover this provides a consistent quality in gas with regard to energy value. At present four methods are in practice, but the most common practices are water scrubber technology and pressure-swing-absorption technology.
Hydrogen sulphide removal – This has to be removed in order to avoid corrosion in engines, gas storage tanks, and compressors. Based on the research report of Rob Pilling of NSCA, air-oxygen dosing in the biogas and iron chloride dosing to the digester slurry are the most suitable techniques to achieve this for small-scale operations.
For the water removal process, gas drying technology is preferred.
Purified and enriched biogas can be stored in bottles after liquefaction. However, unlike commercially available bottled gas, liquefying methane is quite difficult. It can be liquefied at the temperature and pressure of 162o C and 1500 kg/cm2. However methane can be filled in cylinders at pressures and temperatures other than in liquid form. 28 m3 of biogas can be stored in a 0.2 m3 steel cylinder that is available commonly for filling with carbon dioxide, oxygen, etc.
Gas fuel can be stored in the vehicle at two basic forms: either in compressed or liquefied form. Compressed Natural Gas (CNG) is common in practice for fuelling vehicles. Liquefied Natural gas (LNG) for vehicle fuel is cooled and compressed to become liquid and stored in high pressure tanks.
Manufacturing and production cost
A case study report of Berglund, LTH estimated the cost, for biogas produced from poultry manure, as it is dried, scrubbed, compressed, and stored at a pressure of 4 bars in a 200 liter steel tank in Belgium.
- Farm scale plants with a capacity of some 3,000 tons/year of input – £100,000 to £200,000, with £2,000 operating costs
- Community scale digesters treating waste from several farms range from £500,000 for 10,000 tons/year plant to £5 million for a 1-200,000 tons/year plant, with operating costs between £30,000 and £500,000 per year
- Large scale plant treating municipal waste range from £3 million for a 5,000 tons/year plant to £12 million for a 100,000 tons/year plant, with operating costs between £100,000 and £900,000 per year
- A plant operator can sell his biogas at a forecourt price equivalent to fossil CNG and the effective price of the biogas would be 33 p/m3 (excluding fuel duty and VAT31). The value of gas sales from 1 ton of feedstock material is then £66.24.
Biogas is a safe, cheap, and reliable source of energy with endless uses. Through the process of producing the resource a nutrient rich fertilizer is produced that is safe in the environment. This energy source has a bright future ahead of it as it clears out the problems of both pollution and diminishing oil supplies
Biogas Production process
Hagen M, Polman E. 2001. Adding gas from biomass to the gas grid. Final report submitted to Danish Gas Agency. pp 26–47
M. Berglund, LTH 2006, Biogas Production from a Systems Analytical Perspective.
The committee on agriculture, Nutrition, and Forestry, United States Senate (2006)
M. Nilsson, KTH 2001, LCA och MKB for produktion av biogas.