Information about Biogas

Biogas - The Perfect Energy Source

Biogas is a renewable energy source with many different production pathways and various excellent opportunities to use.

Biogas typically refers to a gas produced by the anaerobic digestion or fermentation of organic matter including manure, sewage sludge, municipal solid waste, biodegradable waste, energy crops or any other biodegradable feedstock. Biogas is comprised primarily of methane and carbon dioxide.

One main advantage of biogas is the waste reduction potential. Biogas production by anaerobic digestion is popular for treating biodegradable waste because valuable fuel can be produced while destroying disease-causing pathogens and reducing the volume of disposed waste products.

Biogas burns more cleanly than coal, and emits less carbon dioxide per unit of energy. The carbon in biogas was recently extracted from the atmosphere by photosynthetic plants. Releasing it back into the atmosphere adds less total atmospheric carbon than burning fossil fuels.

Thus, biogas production kills two birds with one stone: it reduces waste and produces energy. In addition, the residues from the digestation process can be used as high quality fertilizer. This closes the nutrient cycle.

Therefore, biogas is a perfect energy source including many benefits!

Biogas - The Feedstock

Feedstock production for biogas is very diverse, ranging from livestock waste, manure, to harvest surplus. Also, wastewater sludge, municipal solid wastes and organic wastes from households can be used as feedstock. Recently, dedicated energy crops are more and more used as feedstock source for biogas production. Finally, biogas can be collected from landfill sites.

One main advantage of methane production is the ability to use so-called “wet biomass” as feedstock source. Wet biomass can not be used for the production of other biofuels such as biodiesel or biomethane. Examples for wet biomass are sewage sludge, manure from dairy and swine farms as well as residues from food processing. They all are characterized by moisture contents of more than 60–70 %.

The use of waste materials is not only excellent suitability for biogas production it also creates some additional benefits. Thus, it contributes to reduce animal wastes and odors. Digestion effectively eliminates environmental hazards, such as overproduction of liquid manure. Therefore biogas production is an excellent way for livestock farmers to comply with increasing national and European regulations of animal wastes. In addition it destroys disease-causing pathogens existing in waste materials. Nevertheless, using animal feedstock can be critical as well. For instance anaerobic degradation of poultry excrements with high contents of organic nitrogen produce high concentrations of ammonium. Furthermore, new economical and ecological solutions for the treatment of animal by-products are required due to the BSE-crisis. However, it is often the combination of environmental, economical and legal reasons that motivates farmers to use digester technology for waste treatment.

Apart from waste materials, suitable feedstock also includes dedicated energy crops. The suitability of energy crops for biogas production was received through improvements in the fermentation process. The main disadvantage of energy crops when compared to waste materials is their need for additional agricultural land. Nevertheless, energy crops for biogas production have several advantages which make them very promising for the future. One main advantage is the production of considerably high yields of energy crops even when they are cultivated extensively. Chemical fertilizers and pesticides are not required or only in small amounts. Damaged and uneatable harvests resulting from unfavorable growing and weather conditions, as well as from pest contaminations are suitable for biogas production, too. In addition, cultivations do not have to become fully ripe, since the whole plant can be used for biogas production. Harvests do not have to be dried.

Biogas - The Fermentation Process

Biogas is produced by means of anaerobic digestion. Organic matter is broken down by microbiological activity and in the absence of air. Symbiotic groups of bacteria have different functions at different stages of the digestion process in order to break down complex organic materials.

There are four basic types of microorganisms involved. Hydrolytic bacteria break down complex organic wastes into sugars and amino acids. Fermentative bacteria then convert those products into organic acids. Acidogenic microorganisms convert the acids into hydrogen, carbon dioxide and acetate. Finally, the methanogenic bacteria produce biogas from acetic acid, hydrogen and carbon dioxide.

Bacteria are sensitive to temperature, which plays an important role in the digestion process. In order to promote bacterial activity, temperatures of at least 20° C are required. Generally, higher temperatures shorten processing time and reduce the required volume of the digester tank by 25 % to 40 %. Regarding the temperature, bacteria of anaerobic digestion can be divided into psychrophile (25 °C), mesophile (32 – 38 °C) and termophile (42 – 55 °C) bacteria. The choice of the process temperature depends on the feedstock and of the utilized digester type. Thus, digesters have to be heated in colder climates in order to encourage the bacteria to carry out their function.

Digestion time ranges from a couple of weeks to a couple of months depending on feedstock and digester type as well as on the digestion temperature.

Biogas - The Procuction Technology

The common technology for biogas production is the digestion of feedstock in specially designed digesters. These must be strong enough to withstand the buildup of pressure and must provide anaerobic conditions for the bacteria inside.

Today, there are many different technologies and digester types available. Generally, the size of biogas plants can vary from a small household system to large commercial plants of several thousand cubic meters. Digester size also influences logistics and vice versa. Therefore, they are often built near the source of the feedstock.

The water content of substrate influences the design and type of digester. One of the most common classifications regarding the water content of the substrate is the classification into wet digestion which is fed with dry mass contents lower than 15 % and into dry digestion which is fed with dry mass content between 20 and 40 %. Wet digestion usually is applied to manure and sewage sludge, whereas dry digestion is often applied to the fermentation of energy crops.

Furthermore, digesters can be classified by the number of process steps. Single-stage and two-stage digesters are the most common technologies today. Single-stage digesters are characterized by no special separation of different process steps (hydrolysis, acidification, methanisation). All process steps are conducted in one single digester.

Digesters can be also classified regarding filling procedure and filling interval. Digester types include following technologies:

• Batch type: the digester is filled at once, the feedstock digests and subsequently the whole system is emptied
• Continuously expanding type: firstly, the digester is filled up to 1/3, then it is continuously filled until it is full and finally the digester is emptied
• Continuously flow type: the digester is initially filled completely, then the feedstock is continuously added and digested material is continuously removed
• Pug flow type: the feedstock is added regularly at one end and overflows the other end
• Contact type: this is a continuous type, but a support medium is provided for the bacteria

Biogas - The Characteristics

The product of digestion is Biogas, a combination of methane and carbon dioxide, typically in the ratio of 6:4 (55-80 % methane). In addition, there are small quantities of hydrogen sulfide and other trace gases.

Biogas - The Use

Biogas containing methane is a valuable product of anaerobic digestion which can be utilised in the production of renewable energy. Biogas can be used for generating electricity and heat. It can also be burned directly for cooking, heating, lighting and process heat. Furthermore, even pilot fuel cells installations are operated with by biogas.

In addition, biogas can be cleaned and upgraded to natural gas quality. The final product is biomethane, which has methane content between 95 and 100%. This biomethane is suitable for all natural gas applications: for instance, it can be fed into the natural gas grid or used for transport in vehicles.

Author: Dominik Rutz ©

Pictures: Dominik Rutz ©