The production of biogas from agricultural crops and crop residues has the potential to become a very significant source of renewable energy. So far, biomethane has been on the fringe of the booming bio-energy market, with wind, biodiesel and bio-ethanol being the more visible and larger contributors to the rising tide of alternative energy sources. 

 

Until a couple of years ago, biomethane was being generated in small quantities on farms, in industrial and municipal wastewater treatment plants, in landfills and in digesters for organics derived from municipal solid waste. However, in recent years the production of biomethane has become increasingly large-scale.  By 2008, for example, the installed capacity of biogas plants in Germany reached more than 1300 MW in a total of more than 3800 biogas plants.  Also, the number of plants treating the organic fraction of household waste in Europe has grown from 3 in 1990,  to 62 in 2000,  to up to more than 170 plants that will be installed by the end of 2010.  The digestion capacity of more than 5.000.000 ton per year can handle almost 3% of the OFMSW produced in Europe by 2010.  This may seem little but represents 20 to 30% of the biological treatment capacity for organics derived from household waste.

 

But biogas does not have to be converted into renewable electricity per se. Biogas can also be cleaned and upgraded into biomethane and injected into the natural gas network or even used as a vehicle fuel. Both of these applications are being implemented on a full-scale and rapidly gaining interest. Injection into the natural gas pipeline can provide transportation to a user with a large heat consumption, where the biomethane is converted into electricity and the waste heat from the engines can be used continuously. In this way, an ideal CHP-application is created, while heat is otherwise often lost or used inefficiently at the location of the biogas plant. Use as a vehicle fuel after upgrading is an immediate potential in countries like Italy and Germany, which have an important network of gas stations for cars running on natural gas. Already there are numerous gas stations in Germany and the Netherlands offering biomethane. More than 30 models of cars running on pure natural gas or both gasoline and natural gas/biomethane can be purchased on the European market.

 

The biggest reason however, why biogas should be given a lot of consideration, is the fact that it can generate a large net quantity of bio-energy per ha cultivated. Much larger amounts of bio-energy can be produced per hectare of land utilized in the form of biogas than compared to the net energy yield for bio-ethanol and biodiesel. Production of biomethane will power a car three to four times further than using the same land for production of bio-ethanol and biodiesel. Almost all of the biomass is converted to biogas through the digestion process. Also the quality of the crops can be rather low so that energy crops can be harvested before they are mature. This enables the farmer to grow two crops per year on the same field, increasing energy yield immediately by another 20 to 30%. Additionally, digesters can be constructed more locally, within a transportation radius of 5 to 10km for the feedstock, while transportation for central biodiesel and bio-ethanol plants can be a large negative factor in the overall net energy yield. Digestion can also utilize a wide variety of feedstocks and crop residues.

Neither has biogas technology development been stagnant. New systems are being developed that are more suitable for the digestion of energy crops and crop residues. Dry fermentation of crops offers great potential for the production of methane as a renewable source of energy. So far, dry fermentation technology has been mostly limited to the digestion of organic fractions derived from municipal solid waste. Source separated organics (biowaste), the organic fraction of residual waste and the organic fraction of mixed municipal waste are being successfully treated by means of dry anaerobic fermentation. There are more than 80 “dry” anaerobic digestion plants treating organics derived from municipal solid waste in Europe, and more are being constructed. Some plants are almost 20 years old.

 

The application of dry fermentation to agricultural energy crops and crop residues has been limited.  Most of the agricultural plants are designed to treat liquid manure with a smaller proportion of energy crops, such as maize, being added. However, many farms do not produce liquid manure and if energy production of agricultural crops is going to play a significant role in the production of renewable energy, then digesters will need to run mixtures that do not contain a significant proportion of liquid manure. 

 

The technology in this regard is clearly proven.  In 2006, for example, a first continuous dry digestion plant of energy crops - using the Dranco-Farm process - was taken into operation in Nüstedt, Germany. This plant is converting silaged whole plants of maize, sunflowers and rye together with silaged grass and a small amount of solid manure. Nominal operation of the plant was reached within two months of start-up and the proper electrical consumption for the plant amounted to 5%, leaving a net electrical yield of 95% to be put into the power utility grid.  The plant can treat about 18.000 ton per year of agricultural crops, and has an electric generating capacity of 750 kW. 

 

To conclude, the production of biogas is fast becoming an important source of bio-energy in Germany due to the widespread digestion of crops, both complete plants and crop residues. The most important driving force is the fact that 3 to 4 times more net energy can be produced per hectare cultivated. Agricultural crops can be converted to biogas for up to 90% of the biomass, while also the plants do not need to be matured and 2 crops per year are feasible in most regions. Anaerobic dry digestion is an efficient way of producing biogas from crops, without the need for the addition of liquid manure or water. Thus, dry anaerobic digestion may possibly contribute to the rapid increase of biogas as a source of bio-energy.