Many farms in Europe are using energy crops which are processed in digesters to produce biogas. In most cases the main crop used is corn silage. To generate significantly more methane than seasonal corn supplies and/or manure alone, digester designs in Europe are increasingly being modified to accept multiple substrates, from grease to corn silage to food waste.
Manure is a well known potential and largely free feedstock, but has a low energy content since cows have already substantially digested the substrate, so adding high-energy value materials produces more methane. Cattle slurry is about 12% dry matter and produces about 25 cubic metres of biogas per tonne, but broiler manure would be two to four times more efficient. Unfortunately, about half of the energy goes heating the dung and there will be additional losses if the dung is not fresh, but it is still worthwhile.
The biogas is produced is usually burned to generate renewable energy. When used for heat only, the biogas is burned in a modified gas boiler to provide heat energy to heat the digester and for 먹튀검증 export. When used for electricity generation a gas engine is used, and the cooling water is best used for a Combined Heat and Power (CHP) use. This means heating greenhouses, or piping the hot water to a local factory for use in manufacturing.
The digester is kept at an optimum temperature of between 90-95 degrees F. About 40,000 cubic feet of methane gas is produced per day in an average sized AD unit.
The recent costings have been completed in which such digesters are expected to bring in at least $20,000 per year, although this would need verification, it seems to be a substantial sum.
Digesters produce a mixture of gases, primarily methane and carbon dioxide, and a nutrient-rich slurry or ‘liquor’.
However, some industries use the term to refer to a completely different type of digester. In papermaking the digester is a special piece of machinery used in the process. Its purpose is to cook small wood chips for several hours in order to soften them.
For a normal digestor the gasholder is equipped with a gas outlet, while it is also equipped with an overflow pipe to lead the sludge out into a drainage pit. Disposal of the products of digestion is a subject in itself.
The solid waste output from a digester can possibly be used instead of sawdust for animal bedding, and is also sold as mulch. Many other uses have been suggested, and these are too numerous to discuss here.
Utility companies are increasingly required to accept biogas power into the local power supply grid, and to provide smart meters that tell the digester plant operator exactly how much electricity is being produced or consumed at any given time and at what payment rate/cost, enabling operators to plan and balance their use and supply input times for when the power is needed most in the power grid during peak usage times when it is most expensive/commands the highest value to the digester plant operator. Operators suggest that this can add up to something like 15 percent on annual electricity revenues.
At one site the digester reportedly assumed to transform 83% of the biodegradable material into gas. The biogas there is produced at a high purity of 65% methane, and its heat of combustion is 5,720 kcal cubic metres at standard conditions. A suspended-pipe hot-water heat exchanger in the digester is used to heat the digester to between 35 degrees and 40 degrees Centigrade.
The water that remains at the top of the digester is decanted out of the digester through a floating drainpipe. This water may be returned directly to an oxidation ditch for further treatment. The digester is heated, and the temperature is raised so that it continues to the desired temperature.
This type of circulation is continued until the liquor leaving the top of the digester is within the target temperature (within the mesophilic and thermophilic range). Then in some plants types the circulation is terminated and the heat content of the heating section is restored by circulating the liquor therein through a heat exchanger to raise the temperature of the liquor.
Digesters are currently seeing a rapid rise in popularity and the reason for this is the lower operational (energy) costs when compared with biological aeration due to the avoidance of aeration and the use of the methane produced as an energy source. The rising price of oil is of course the biggest factor that has increased popularity. Also, research has found that anaerobic systems can be developed that have the ability to operate at much higher loadings than previously thought possible, raising the profit to be made from these systems