Biogas Energy in a Nutshell Overview
Anaerobic digestion is a process that occurs in nature and produces biogas as a by-product. Biogas typically contains between 60 and 70 percent methane.
Anaerobic digestion involves bacteria that require an environment that is void of oxygen to survive. Converting organic waste to methane gas by anaerobic digestion can be considered a two-step process. The first step involves a group of anaerobic bacteria – referred to as the acid formers – that produces organic acids as a by-product of the initial organic degradation. The second step involves a group of bacteria – known as the methane formers – that breaks down the organic acids and produces methane as a by-product of the degradation of the organic acids.
This methane content makes biogas an excellent source of renewable energy to replace natural gas and other fossil fuels. Anaerobic treatment has been historically used to biologically stabilise high-strength wastes at a low cost. In many cases, the biogas has not been used as an energy resource. Rather, it has been burnt in a flare and discharged to the atmosphere. Concerns over the environment and rising costs for energy have caused a resurgence of interest in anaerobic treatment and a new interest in using biogas produced during this treatment of organic wastes.
Anaerobic Treatment Technologies
Many different types of anaerobic treatment technologies are available, and new advancements in anaerobic treatment are evolving as interest in the technology increases. The technologies vary in terms of the length of time required for treating the wastes; the size, configuration and complexity of the reaction vessel; and the operating temperature. Anaerobic treatment systems range from relatively large, simple plug flow covered lagoons to high-rate, two-stage fluidised bed systems. They also include hybrid systems that combine several technological innovations to reduce the size of the treatment vessel and to increase the treatment efficiency of the system.
The technology of anaerobic manure digestion has the potential to accomplish the following:
Greatly reduce odour levels during manure processing, creating a relatively odour-free end product (closed vessel processing confines odourous compounds which are converted to other chemicals). Digested manure, which could be subsequently applied to land, would not have more odour associated with it than would composted manure.
Reduce pathogen levels in the final products. Anaerobic digestion greatly reduces pathogen levels. Additional pre- or post-digester technologies can ensure pathogen-free end products.
Conserve nutrients - more than 90% of nutrients entering anaerobic digesters are conserved through the digestion process. By conserving nitrogen during digestion, the N: P ratio of the treated manure is more favourable for plant growth. Reducing the demand for additional mineral nitrogen helps decrease the use of natural gas for production of new mineral nitrogen, as well as reduces greenhouse gas emissions associated with nitrogen fertilizer production.
Reduce greenhouse gas [ghg] emissions - Since anaerobic digestion operates in a closed system, substantial reductions in greenhouse gas emissions (methane, nitrous oxide) are achieved. Ammonia losses, while not of direct ghg concern, are also reduced. Ammonia is now on the list of toxic substances under the Canadian Environmental Protection Act (CEPA).
Co-generation and Energy Independence - Anaerobic digesters produce methane which can be captured for supplying energy (heat, electricity) for the operation, thereby achieving substantial cost recovery. With the increasing privatisation of power generation utilities, the “net cost“ of power delivered to the farm is rapidly increasing, and energy independence for the farmer is rapidly becoming an attractive option. Often the delivered cost of electrical power will be at least double the original generating costs. The decision to co-generate electricity should primarily be driven by the savings for meeting the farm’s power requirements, rather than based on potential sales of excess power back to the grid, if allowed. Net metering laws are now starting to be implemented in numerous American jurisdictions, which may further improve the economics for co-generation. In Canada, there are some jurisdictions now considering buying back “green energy” at a premium, to encourage the production of green power. Unlike some other intermittent green sources of power (e.g. wind or solar power), co-generation anaerobic digesters have the advantage of continuous capacity, meaning that electricity can be generated 24 hr/day, seven days/week. If premiums were to be paid on a demand basis, biogas could be stored and additional electricity could be generated during peak demand periods (7-9 AM; 5-9 PM).
The final products of anaerobic digestion are quite homogenous and are more predictable as sources of plant nutrients since they are in a more mineral form (50% of carbon is converted to biogas - methane & CO2). If there is treated manure in excess of land base requirements, this homogenous product lends itself well to further processing for off-farm “value-added“ products (adding supplemental nutrients can make this a valuable “organic fertiliser”). If the end product is dried and pelletised, it can be stored, transported and applied with existing fertiliser application equipment. Recycling of livestock nutrients back to cereal production sources should be an essential part of any long-term agricultural sustainability plan.
How It Works
The digestion process takes place in a warmed, sealed airless container (the digester) which creates the ideal conditions for the bacteria to ferment the organic material in oxygen-free conditions. The digestion tank needs to be warmed and mixed thoroughly to create the ideal conditions for the bacteria to convert organic matter into biogas (a mixture of carbon dioxide, methane and small amounts of other gases). There are two types of AD process:
Mesophilic digestion: The digester is heated to 30 - 35oC and the feedstock remains in the digester typically for 15-30 days. Mesophilic digestion tends to be more robust and tolerant than the thermophilic process, but gas production is less, larger digestion tanks are required and sanitisation, if required, is a separate process stage.
Thermophilic digestion: The digester is heated to 55oC and the residence time is typically 12-14 days. Thermophilic digestion systems offer higher methane production, faster throughput, better pathogen and virus ‘kill’, but require more expensive technology, greater energy input and a higher degree of operation and monitoring.
During this process 30-60% of the digestible solids are converted into biogas. This gas must be burned, and can be used to generate heat or electricity or both.