Anaerobic digestion, also called biological gasification is a natural process by which a consortium of bacteria breaks down organic matter in the complete absence of oxygen to produce biogas.
In the anaerobic digestion process, the organic fraction of municipal solid waste gets converted to biogas, a predominantly gaseous mixture of roughly 60% methane and 40% carbon dioxide, and also containing trace amounts of particulate matter and other gases, including hydrogen sulphide. The digested slurry is typically a partially stabilized organic material in a nutrient rich liquid fraction. This can be separated further into a solid component that can be used as a soil conditioner (or utilized as compost) after proper curing and drying, and a nutrient-rich liquid effluent which can be sprayed on agricultural land or gardens as a bio-fertilizer.
The removal of contaminants from raw biogas is one of the key steps to be considered when planning biogas upgrade and processing systems. The type of clean up and the acceptable amounts of contaminants that can be tolerated downstream depend on the type of application to which the gas is intended to be used for.
The key contaminants in raw biogas exiting from the digester are hydrogen sulphide, carbon dioxide, particulates and entrained oil and water droplets. The removal of hydrogen sulphide can be achieved through a number of physical, chemical or biological processes. Biogas is typically saturated with water vapour when exiting the digester and usually needs to be dried prior to utilization (especially before injection into the gas grid or use as a vehicle fuel).
Though not strictly a contaminant, the removal of carbon dioxide is done especially for biogas utilization as a vehicle fuel or upgrade to pipeline quality gas. Since it is non-combustible and reduces the energy density of the product gas, removal of carbon dioxide is mandatory for upgrade to pipeline quality gas or for use as a vehicle fuel in order to standardize the gas. The removal of carbon dioxide is not required for combusting biogas in a boiler or for use in an engine generator set (or combined heat and power plant) for electricity production.
The dust and particulate matter present in biogas can cause damage to mechanical parts in downstream applications. These are removed using cyclones and/or coalescing filters. Small mesh filters (typically 2 – 5 microns mesh size) also have the additional advantage of removing water and oil droplets.
The methane fraction in biogas has a high calorific value and can be utilized as a source of energy production. There are a number of ways to exploit and utilize the energy content of methane in biogas. The direct combustion of biogas in a boiler to produce hot water or steam is the most efficient pathway to harness its calorific value in terms of energy conversion.
Electricity production, typically using an internal combustion engine (modified to run on biogas) and an electricity generator is the most widely followed application worldwide. Biogas can also be ‘cleaned’ and ‘upgraded’ by scrubbing out the hydrogen sulphide and removing carbon dioxide, moisture and particulates to yield a higher calorific value gas. This upgraded gas can in turn be compressed and blended with natural gas in the pipeline or used as a clean burning vehicular or cooking fuel as a substitute for compressed natural gas / propane.
Choosing the right option for biogas utilization can be complex and should be an iterative process. It is essential to understand the technological options available, the physical, chemical and combustion characteristics of the fuel, and the associated systems needed for transportation, clean-up, storage, and compression.