Biogas Production Systems

Biogas Production

Anaerobic digestion is the decomposition of wet and green biomass through bacterial action in the absence of oxygen to produce a mixed gas output of methane and carbon dioxide known as ‘biogas’, which can then be used as a substitute for fossil fuels. Both liquid and solid wastes or green crops can be digested to produce biogas. The resulting mixture of methane and carbon dioxide are both greenhouse gases.

The natural decomposition of organic wastes in the absence of oxygen (anaerobic decomposition) by bacteria also occurs on the bottom of lakes and wetlands indicated by gas bubbles rising. This is the reason why hydropower installations generate a major source of methane, one of the major greenhouse gases, when the surrounding land area is first flooded and the vegetation decomposes over fairly long periods of time.

The breakdown of the organic materials involves a number of biological steps, each involving a well defined class of bacteria that absorb energy from the gradually decomposing biomass which is finally converted to methane, carbon dioxide and water. The process can be encouraged by placing the organic material in large airtight tanks known as digesters, and the biogas produced captured for use. As a result, odours are removed and the pollution potential of the waste is reduced. Biogas can be burnt directly in thermal applications displacing natural gas in cooking and space heating, or used as fuel in internal combustion engines to generate electricity.

For further information on Waste to Energy and Biomass technologies, see Technologies.

Biogas Production Installations in Australia

One example of an Australian biogas installation is Berrybank farm, located at Windermere, west of Ballarat. Berrybank Farm is home to 15,000 pigs, fed in an intensive feedlot farm, operated by Melville Charles. The large number of pigs at Berrybank farm produce the same quantity of effluent as a city of 40,000 people - almost two-thirds as much as the City of Ballarat. To address the disposal issue, Melville Charles has installed an anaerobic digester. Berrybank’s recycling system cost approximately $2 million to install, which was repaid in five years through sales of the products and efficiency savings (Victoria Museum, 1999). Products of the recycling system are; 7 tonnes of fertiliser, large amounts of mineralised and recycled water, and 1,700 cubic metres of biogas per day that is used in a cogeneration plant to generate 2900 kW of electricity daily (The University of Ballarat, 2004). The electricity is used on the farm and fed into the electricity grid, the mineralised water to irrigate crops, and the remaining solids is used as potting mix, fertiliser, compost and worm food (Victoria Museum, 1999).

Figure 1 A component of the biogas installation at Berrybank farm
(© Museum Victoria Australia 1999).

Another Australian biogas project undertaken by Agricycle Pty Ltd in Western Australia was supported by a grant from the Waste Management and Recycling Fund to establish a facility for converting chicken manure into energy.

Digesters range in size from around 1m3 for a small household unit to as large as 2000m3 for a large commercial installation. In Western Australia, the Water Corporation operates wastewater digesters at its Woodman Point operation, which separates sludge from the water.

Figure 2 Egg-shaped anaerobic digesters at Woodman Point rated at 1.8 MW
(courtesy of BG&E Consulting Engineers).

At Woodman Point the sludge is digested in one of two 38-metre tall anaerobic digesters. Biogas produced by the digester is used on-site to provide electricity, and excess power is sold to the electricity retailer, Western Power Corporation. What is left after the digestion is complete and the gas is extracted is a sludge. This biosolid is dried then sold as a soil conditioner and fertilizer to the agricultural and landscaping industries.

Benefits

A significant benefit of such systems is that it removes and converts wastes into useful and valuable products such as fertilisers and electricity. The removal of waste has several advantages, such as preventing eutrification of waterways, contamination of drinking water and stopping toxic chemicals from entering the wider environment. In addition the feedstock is derived from biological organisms themselves (or their waste) and therefore provides a renewable supply of biodigester feedstock. In addition this technology can be successfully used in developing countries to improve community sanitation and provide a sustainable supply of much needed energy for cooking and a myriad of other uses. There has been a large increase in small biodigester installations in many regions of the world due to their simplicity, effectiveness and practicality in solving many waste issues, as well as a general lack of sustainable domestic energy supply issues.

Constraints

Limitations of incorporating biodigesters tends to revolve around cost effectiveness and limited scales of operation. For example, the larger the amount of waste that is produced, the larger the amount of biogas that can be produced. Therefore for a relatively small farm with cows in a paddock, the impractical nature of collecting dung as feedstock for the biodigester becomes apparent. However with higher concentrations of animals in a small space the collection of wastes becomes technically easier. While the cost of a system may appear to be the most obvious barrier to implementation, this is not always the case. Connecting your system to the electricity grid involves technical and practical challenges as the system must meet the local standards and safety requirements. Operating, maintaining and servicing biodigesters is not a simple task and the costs and time involved may be another barrier to implementing such a technology.

Resource Assessment & System Design

Through the Information Portal, RISE provides networking services to assist people researching various renewable energy applications. Other quality institutions also provide many excellent decision-making and capacity building tools and software freely available for downloading. By complementing the applications section in the Information Portal with these tools and programs, RISE has provided an excellent collection of resources that are directly applicable for a sustainable solution to your own energy needs.

In this section, some highly regarded decision making tools and software programs are summarised below;

RETScreen

“The RETScreen International Clean Energy Decision Support Centre seeks to build the capacity of planners, decision-makers and industry to implement renewable energy and energy efficiency projects. This objective is achieved by: developing decision-making tools that reduce the cost of pre-feasibility studies; disseminating knowledge to help people make better decisions; and by training people to better analyse the technical and financial viability of possible projects.”

Visit the RETScreen International Clean Energy Decision Support Centre

These RETScreen files contain a collection of project case studies, including assignments, worked-out solutions (RETScreen Software Analysis) and information about how the projects fared in the real world. This document includes a background of energy technology and it provides algorithms for Project Models. In addition there are many case studies that provide succinct details on various renewable installations including system descriptions, lessons learned and many other important and useful information.

Areas that are included are;

Wind Energy	Small Hydro
Photovoltaics	Refrigeration
Biomass Heating	Solar Air Heating
Solar Water Heating	Passive Solar Heating
Ground-Source Heat Pumps	Combined Heat & Power

HOMER

“HOMER is a computer model that simplifies the task of evaluating design options for both off-grid and grid-connected power systems for remote, stand-alone, and distributed generation (DG) applications. HOMER's optimization and sensitivity analysis algorithms allow you to evaluate the economic and technical feasibility of a large number of technology options and to account for variation in technology costs and energy resource availability. HOMER models both conventional and renewable energy technologies.”

Visit the HOMER Optimisation Model for Distributed Power Page

HOMER allows up to three independent generation technologies to be included in the simulation model. Each generation technology can be a different size, cost, and fuel, or they can all be alike. HOMER dispatches the generators in an economically optimal way, meaning that each hour it chooses the generator (or combination of generators) that can meet the load and operating reserve requirements at least cost. It considers replacement, O&M, and fuel cost when making its dispatch decisions, as well as the value (if any) of the waste heat recovered from each generator. Homer allows an extremely large range of system configurations to be simulated. You can download and use the full version of HOMER for free. You must be a registered user to download the software. When you install HOMER, you automatically receive a free six-month license, which you can renew for free an unlimited number of times.

Sources/Systems incorporated in HOMER are:

Solar Photovoltaics	Battery Banks
Wind Turbines	Hydrogen Storage
Run-of-river Hydro Power	Daily Load Profiles with Seasonal Variation
Electricity Utility Grid	Defferable Loads (Water Pumping, Refridgeration)
Fuel Cells	Thermal Space Heating
Microturbines	Thermal Crop Drying
Generator: Diesel, Gasoline, Biogas, Alaternative and Custom Fuels, Cofired	Efficiency Measures

BIODIGESTER AIDG

Another particularly good resource for biodigesters is the Appropriate Infrastructure Development Group (AIDG's) pdf resources on Biodigesters.

The topics include;

Biodigester Effluent and Duckweed	Biodigester Effluent and Duckweed 2
Biodigester Impact on Women	Biodigester in Philippines
Biodigester Installation Manual	Biodigester in Integrated Farming
Masonry Biodigester	Polyethylene Biodigester
Producing Methane Gas from Effluent	Retention Time in Biodigester
Sizing Biodigesters	Tubular Plastic Digester
Water Decontamination

Conclusion

There is a large potential for the application of biogas technologies to provide sustainable power supplies for distributed generation. The methane captured in these technologies is converted to energy, carbon dioxide and water. However as a greenhouse gas, methane is much more potent than carbon dioxide molecule for molecule. Therefore burning the methane for useful work before it is released into the atmosphere is beneficial to reducing anthropogenic climate change directly, while displacing some other fuel that may be used to perform the useful work. Unfortunately the energy supplies that humans currently use to provide energy services are mostly derived from unsustainable sources, such as coal, oil, gas, and radioactive isotopes. The development of biogas technology makes social, environmental and economic sense and it is being applied in many areas of the developing and industrialised nations to complement other renewable energy supplies.