Feedstock for anaerobic digestion can come from municipal solid waste, waste water treatment, livestock operations. The process results in methane containing gas mixture, a.k.a., bio-gas, that can be burned.
Where sufficiently large streams of putrescent waste are available, Yanmar Energy System suggests highly efficient, bio-gas-powered, micro-co-generation. These co-generation plants can work on raw, unprocessed bio-gas if there is a sufficiently high enough concentration (60-70%) of methane.
Bio-gas-fired electric generation is a source of carbon emissions In this case because bio-gas is comprised partially of carbon dioxide.
|Carbon dioxide, CO2||25-50|
|Hydrogen sulphide, H2S||0-3|
A more environmentally friendly approach, advocated by the IPCC, is the CO2 that is removed from the bio-gas is either used or stored rather than released into the atmosphere.
As previously noted adequate upgrade of the bio-gas is crucial in transportation applications. Moisture, sulfur compounds, organo-silicon compounds (siloxanes) and other impurities in bio-gas or LFG (Land Fill Gas) are usually removed because they can cause problems in gas handling equipment and damage engines and emissions controls.
Combined heat and power from LFG (Land Fill Gas) is more controversial than some other methods of converting waste to energy because the gas contains trace components. In addition to heavy hydrocarbons (both aliphatic and aromatics such as benzene), bio-methane can contain chlorinated hydrocarbons (e.g., dioxin, furans, and PCBs (Poly Chlorinated Bi-phenyls) that are toxic and carcinogenic.
Cow power is a less hazardous form of methane reforming that LFG. Methane reforming, either from landfills or from cow or pig waste also is controversial because of the odor with lack of stringent control on release.
Information from the Biopact team, other than stating the obvious, i.e., bio-gas-powered, micro-co-generation plants manufactured by Yanmar are more efficient when the bioigas is upgraded to meet gas quality specifications (More Fuel == More Energy == Greater Efficiency, duh), there was an absence of any discussion about gas-cleaning. It would seem that such processing is other than part of the Yanmar package.
- Nuclear is cheap, reliable, accounts for about 20% of our current usage, is heavily regulated, is part of an inefficient production and delivery system, and has unresolved problems with waste.
- Renewable [Energy Sources] are more expensive, have less pollution, account for very little of our current usage, can be installed quickly in smaller applications, are becoming increasingly efficient, and do not mesh well with the current grid system.
- Coal is cheap and reliable, but costs a great deal in terms of environmental and human costs.
- Gas co-generation is on the increase, but will not be indefinitely because of fuel costs and potential shortages. It also is not a perfect match for the current grid.
Another condition, appropriate to other forms of micro-co-generation as well, is how well the local public utility welcomes such distributed generation. The model promoted by Yanmar Energy System consists of multiple, small generators sited close to the customer load.
While such an approach potentially can mean a more reliable, local supply of electricity, another centralized approach now being tested in Denmark and elsewhere throughout the European Union is to use co-digestion units, which are capable of treating different types of waste at the same time, principally liquid and solid manures mixed with diverse organic waste.
Research Centre Foulum is a new, experimental, bio-gas complex operated by the Faculty of Agricultural Sciences at Aarhus University. Between the four reactors, the facility can handle approximately 29,000 tonnes slurry and 2,000 tonnes biomass from the barns and fields. Experimentation is afforded by each reactor with their own holding tanks as well as a dosage system for adding different feeds of solid material such as leftover animal feed, deep straw manure, energy crops and other biomass materials.
While co-digestion is being promoted as a more energy efficient process, in 2005 the main bio-gas sources exploited were rubbish dumps according to European Commission Directorate-General for Energy and Transport. Such sites located in member countries produced 2961,4 kilo-tonnes of oil equivalent. Other less significant sites production came from sewage purification plants (898 ktoe).
Such output comes mainly from large operations, whereas Yanmar Energy System is targeting smaller operations. Some examples of suitable sites includes:
- Livestock production facilities,
- Small household and municipal waste treatment facilities
- Food processing industry
The niche for bio-gas-powered, micro-co-generation would seem to be where there is a sufficient amount of feedstock continuously available and the means to accommodate anaerobic digestion, yet not well-suited to a more centralized approach.
Depending upon how critical it is to have a dependable supply of heat and / or power, there may be need for the site also to have access to a supplementary supply. Unfortunately, supply of natural gas from a utility is susceptible to interruption due to untoward events.
The Underwriters Laboratory (UL)-rated Capstone MicroTurbineTM Model 330 produces 25-30 kW of electricity and can be configured for co-generation.
Skilled personnel need to be available to operate the co-generation plant. Several different technologies could form the basis of a Micro-CHP system, to include:
- Internal combustion engines
- Stirling engines
- Steam engines
- Fuel cells
At present, the largest deployment of micro-CHP is in Japan, over 50,000 units, most incorporate the MCHP engine generator unit produced by Honda. The Honda MCHP unit is a high-endurance, internal combustion engine. The MCHP unit is know for its extremely low noise output and solid-state grid interface. freewatt is a MCHP unit now available in some parts of the United States.
The quality of bio-gas is more stringent for a gas turbine or fuel cell. Also the initial outlay is much more expensive. Generally, they are considered to be more efficient than other approaches when comparisons are made with the same quality bio-methane.
Two of the above, the Stirling and steam engine, because they are heat engines can make use of raw, unprocessed bio-gas. An advantage of heat engines is that other sources of heat can be combined with combustion of the bio-gas. Nevertheless, an advantage of anaerobic digestion is that auxiliary heat can be used to encourage growth of organisms important to production.