Biogas is renewable energy stored in organic materials, such as plant matter and animal waste, known as biomass. Sources of biomass fuel include agricultural residue, food waste, sewage, energy crops, landfills and animal waste. Anaerobic digestion is the process that occurs when bacteria decompose organic materials in the absence of oxygen to generate biogas.
Biogas is primarily composed of methane and carbon dioxide with smaller amounts of other gases such as hydrogen, nitrogen or carbon monoxide which are often present in the gas mixture. Usually the mixed gas is saturated with water vapor, and may contain trace impurities such as hydrogen sulphide (H2S) and ammonia as well as dust or dirt particles. For biogas to be used as a fuel, most of these impurities must be removed as they may cause corrosion, deposits and damage to equipment.
There are a number of benefits in drying biogas to a low dewpoint before combustion in a combined heat and power (CHP) engine.
The six major benefits of cooling to a low dewpoint include:
- Increased CHP engine efficiency
- Prevention of corrosion
- Reduction of engine oil contamination
- Increased service life of adsorbents
- Partial removal of impurities such as H2S, ammonia and siloxanes
- Complies with technical instruction of all major gas engine suppliers
It is therefore essential to use a cooling system which has been specifically designed to produce low dewpoints and operate in aggressive ambient conditions such as those experienced in biogas applications. Read more about the benefits of cooling to a low dewpoint in this white paper.
Anaerobic digestion process
The anaerobic digestion process (see Fig. 1) as defined by the Anaerobic Digestion and Biogas Association is the simple, natural breakdown of organic matter into carbon dioxide, methane and water, by two groups of microorganisms, bacteria and archaea. Since many of these are intolerant to oxygen, this process is known as anaerobic. At the end of the process we have a mixture of methane and carbon dioxide gases (biogas), water and some organic material (digestate).
Fig. 1. Anaerobic Digestion Process supplied by Anaerobic Digestion and Biogas Association website
Biogas as an alternative fuel
The characteristics of biogas are comparable to natural gas. The energy content is defined by the concentration of methane. For biogas to be used as a fuel, most of the impurities have to be removed as they may cause corrosion, deposits and damage to equipment. Impurities to be removed are water vapor, dust and dirt particles, and gaseous constituents. These gaseous constituents include hydrogen sulphide, CO2, halogen compounds (chlorides, fluorides), siloxanes and aromatic compounds.
It is generally accepted that a reduction in water content is beneficial to the CHP system, however traditional methods such as condensate traps and underground pipe work cannot achieve low dewpoints which therefore reduces the benefit of removing water. For underground pipe work to have any real cooling effect, long runs of pipe are necessary which is often impractical, expensive and difficult to maintain and service.
It is also common to use ‘air conditioning’ type chillers for biogas cooling but these units are not designed to produce low temperature water and either result in higher gas dewpoints or end up operating well outside their design limits resulting in higher energy consumption and reduced service life. A cooling system should be used that is designed for biogas applications.
During the 2013 UK Anaerobic Digestion and Biogas Exhibition, Steven Scott, Market Development Manager - Alternative Energies for Parker's Hiross Zander Division / domnick hunter Filtration & Separation Division presented on selecting the right technology for CHP engines.
Deciding which CHP technology to choose can be a difficult decision. A range of top technology providers discussed the issues such as
- Cost, efficiency, and environmental impact;
- Factors that may influence decisions;
- What technology is available; and
- Examples of how it is operating in practice.
Other experts presenting at the conference included:
- Mark Moreton, NIFES Consulting Group
- Ian Farr, Technical Sales Engineer, Edina UK
- Ian Burgess, Sales and Project Management, SEVA Energie AG
- Andy Cooper, Sales Engineer, Clarke Energy
For your review, here is Steven Scott's presentation from the exhibition.
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