Introduction – Biogas is a mixture of about 50 – 75% methane and 25 – 50% CO2 and other trace contaminants. The gas is created from anaerobically decaying organic matter, such as manure and plant material. As the gas is man-made, it is differentiated from natural gas.
Biogas is produced by means of a process known as anaerobic digestion. It is a process whereby organic matter is broken down by microbiological activity and, as the name suggests, it is a process which takes place in the absence of air. It is a phenomenon that occurs naturally at the bottom of ponds and marshes and gives rise to marsh gas or methane, which is a combustible gas.
There are two common human-made technologies for obtaining biogas, the first (which is more widespread) is the fermentation of human and/or animal waste in specially designed digesters. The second is a more recently developed technology for capturing methane from municipal waste landfill sites. The scale of simple biogas plants can vary from a small household system to large commercial plants of several thousand cubic metres. Large commercial biodigesters (that is, fed with animal feaces from animal husbandry farms) are typically a lot more efficient than domestic biodigesters, as manure from cows (and similar animals) contains a lot more volatile solids than from humans. Also, there is the issue of the small quantity of feaces and plant matter available in domestic systems, meaning that a lot less gas is produced making it a less obvious method for certain purposes as producing electricity.
Application – Jinan biogas engines are specifically designed to operate on different types of biogas. These gas engines are linked to an alternator in order to produce electricity at high efficiency. High efficiency electricity production enables the end user to maximise the electrical output from the biogas and hence optimise the economic performance of the anaerobic digestion plant. The Jinan biogas generators can also be deplored in CHP technologies as well where biologically-derived gases can be utilised in biogas engines to generate renewable power via cogeneration in the form of electricity and heat. The electricity can be used to power the surrounding equipment or exported to the national grid.
For anaerobic digestion plants that are using a CHP engine, there are two key types of heat:
- Low grade heat from the cooling circuits of the gas engine, typically available as hot water on a 70/90°C flow/return basis.
- High grade heat as engine exhaust gas (typically ~450°C).
The low grade heat is typically used to heat the digester tanks to the optimum temperature for the biological system. Mesophilic anaerobic digesters typically operate at 35-40°C. Thermophilic anaerobic digesters typically operate at a higher temperature between 49-60°C and hence have a higher heating requirement.
High temperature exhaust gas heat can either be used directly into a drier, waste heat boiler or organic rankine cycle unit. Alternatively it can be converted into hot water using a shell and tube exhaust gas heat exchanger to supplement the heat from the engine cooling systems.
Waste heat boilers produce steam typically at 8-15bar. Driers may be useful to reduce the moisture content of the digestate to assist in reducing transportation costs. Organic rankine cycle turbines are able to convert surplus waste heat into additional electrical output.
The heat from the CHP engine can also be used to drive an absorption chiller to give a source of cooling, converting the system to a trigeneration plant.
BENEFITS
- Production of renewable power through combined heat and power cogeneration
- Disposal of problematic wastes
- Diversion of waste from landfill
- Production of a low-carbon fertiliser
- Avoidance of landfill gas escape and reduction in carbon emissions
- During the digestion process bacteria in the manure are killed, which is a great benefit to environmental health.