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 for biogas power generation.
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.
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 that has been specifically designed to produce low dewpoints and operate in aggressive ambient conditions such as those experienced in biogas applications.
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
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 pipework cannot achieve low dewpoints which therefore reduces the benefit of removing water. For underground pipework 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.
Article contributed by the Filtration teaam at Gas Separation and Filtration Division Europe, Parker Hannifin