Bioenergy from Waste
EBRI has a remit to bring the benefits of bioenergy produced from waste to the West Midlands region and across the UK.
Bioenergy technology has been established for some time. It provides a renewable, green energy solution that is carbon neutral. If waste is used, as in the innovative EBRI process, then it also offers an effective waste management solution.
The conversion of waste into bioenergy and biochar (a form of charcoal) requires considerable technical expertise. However, the basics of the process are fairly straightforward. The backbone of this innovative solution is pyrolysis which is the breakdown of biological waste, through heat and pressure, into useful by-products.
The conversion cycle has four main stages:
Some waste needs to be pre-processed before the pyrolysis stage. This may be as simple as separating out the different types of waste, e.g. municipal waste would need to be sorted before being processed. Some waste, such as sewage, needs more preparation.
Sewage is first processed by anaerobic digestion. An anaerobe is a microorganism that does not require oxygen to metabolise food. Anaerobic digestion occurs when microorganisms feed on the sewage in an oxygen free environment. These microorganisms convert the sewage into three main products:
- Biogas (a combination of methane and carbon dioxide)
- Cake (the remaining residue)
The biogas can be used in a gas engine to produce power, while the cake is used as biomass (feedstock) for the bioenergy reactor.
Anaerobic digestion is a well-established process. The first anaerobic digester was built in 1859 by a leper colony in India. It is used extensively for energy in developing countries. The high capital costs and low process efficiencies have restricted the adoption of anaerobic digestion in developed countries. However, there a number of biogas plants (the name given to anaerobic digestion units which are used to generate energy) in the UK. Given the pressure to take environmental measures and the use of cake as a pyrolysis feedstock, these biogas plants will become increasingly popular in the West Midlands.
Note: Pyrolysis of the residue (cake) from the anaerobic digestion process will extract the same amount of energy again.
Pyrolysis (a combination of the Greek words "pyr" meaning "fire" and "lysis" - "separating") is the breakdown of organic material at elevated temperatures in an oxygen-free environment. This process has been used since ancient times to turn wood into charcoal.
In this innovative solution the processed waste is fed into the reactor. Here it is subjected to elevated temperatures (over 430°C) and pressure. The exact conditions depend on the waste being processed.
The end result of the pyrolysis of biological waste is a mix of:
- Biochar (a type of charcoal)
- Liquids (a combination of oils and water)
- Vapours (a range of gases depending on the biomass being processed)
Biochar makes an excellent fertiliser and will lock away the carbon for thousands of years in the soil. It can also be used as a fuel, as a substitute for coke and it can be used in the production of bio-hydrogen.
Gasification refers to the production of syngas (a mixture of hydrogen and carbon monoxide). At this stage the vapours are taken from the pyrolysis process and the various gasses it contains are separated out. In the case of contaminated waste, some of the separated gasses may be sold to the chemical industry.
The end product of this stage is syngas which is used for power generation.
4. Power generation
The final phase is to use the syngas as fuel for a Combined Heat and Power (CHP) generator. These generators produce electricity and useful heat in a single process. They are very efficient; a traditional coal-fired generator has an average efficiency of 34% while CHP plants work up to an efficiency level of around 80%.
CHP generators are a well-established technology. Given the high cost of energy along with environmental targets, they are set to increase in popularity.