Biomethanisation Plant for Farms
The principle layout of a farm-scale agricultural biomethanisation plant is quite simple. Manure is collected in a reservoir nearby the digester. From this reservoir the digester is fed by a pump. The digester itself is a gas-tight completely sealed tank out of steel or concrete. It is insulated because there must be a fixed optimal temperature inside it for the microorganisms. This temperature can be either mesophilic with about 35° C or thermophilic with about 55° C. Inside the digester there is an agitator. This agitator is responsible for complete mixing of digester`s contents.
The operator must ensure that there is no possibility for the development of swimming-layers and/or sedimentation. Additionally, the microorganisms must be supplied with all necessary nutrients. Fresh manure has to be fed to the digester several times a day, in small quantities. The average hydraulic retention time of the manure inside the digester is - depending on the substrate - between 10 and 20 days. During this time the organic substances in the manure are metabolized by the microorganisms.
There are two different outputs: biogas and digested substrate. The latter is stored in a standard manure storage tank. In Canada it is most common to use it as a fertilizer because of it high ammonia concentration (NH4). The biogas is stored in a gas storage tank. Here all irregularities in quality and quantity will be equalized. From the gas storage tank there is continuous supply to a gas or diesel gas engine. Here heat and electricity is produced. Initially, the farmer used to use the electricity produced on his own farm. He saved money because he needed electricity from outside sources. Surplus electricity can be sold to the nearest public utility company - . Especially in such situations where there is no (or few) consumer on the farm that need electricity, the farmer earns money by selling the electricity from renewable energy ‚biogas`.
The heat produced can be used for heating the stables, the farm-house, drying of grain and many other useful purposes. In general, about 10 to 30% of the electricity and heat produced have to be used for direct consumption by the biogas plant itself. Surplus electricity and heat are used for the financing of the biogas plant. Normal times for writing-off are 4 to 8 years. Of course, there are other ways to utilise biogas. For example, all the biogas can be used for heat production: steam or hot water. This is attractive if there is need for large amounts of steam. Or the heat can be used to power refrigeration - this can be employed, for example, for cooling stored fresh milk in a dairy.
Cofermentation Biogas Plant
To improve economy of biogas plants some owners decide not to digest manure alone but to take other organic substances - so-called coferments - as well. Typical coferments are fats, market wastes, spice residues, residues from food industry and many similar substances. The operator of the cofermentation biogas plant increases his earnings in two ways: first, just by taking the coferments from the person who wants to dispose them and second, through higher biogas production. Cofermentation biogas plants are generally (much) larger then farm-scale biogas plants. Sometimes they still conform to the agricultural standards but most often to the industrial ones.
In Europe many large-scale cofermentation biogas plants have been constructed at a location central to several (large) farms. All these surrounding farms deliver their manure to the plant. Additionally coferments are delivered. The standard ratio is about 3:1 to 2:1 for manure and coferments.
The manure from all the surrounding farms is delivered by trucks or pumped to the cofermentation biogas plant. The coferments are delivered by truck. These trucks are unloaded in sumps which are normally closed to reduce odour emissions and opened only for adding the coferments. For further reduction of odours sometimes all deliveries are made within a closed hall. At first the coferments are ground, hygienized and mixed with manure. Hygienisation is most often performed at 70° C for one hour with a maximum particle size of 1 cm.
The homogenisation with manure is performed in a mixing tank with strong agitators. After this pretreatment all the organics are pumped into the digester. Normally large tanks are constructed out of coated steel. Coatings are either enamel or epoxy. Most tanks are bolted together. Standard digestion volumes of cofermentation biogas plants range from 500 m³ to several thousand m³. Mixing is sometimes done by a centrally located mixer on top of the roof, sometimes by submersible stirrers. The biogas produced is used in gas or diesel gas engines. Power can be several MW.
Large-scale cofermentation biogas plants have emergency flares in case the engine(s) is (are) not in operation and biogas has to be burnt. The gas system may include a blower, condensate trap, desulfurisation and so on. Everything is controlled by a gas system control unit. Digested manure is pumped into a standard manure storage tank.
An ever-increasing number of these tanks are covered with a roof to collect as much biogas as possible. Although the gas production inside manure storage tank is not large, it is worthwhile to collect some of the roof incorporate biogas storage membranes.
All large-scale cofermentation biogas plants are controlled by an overall process control system. There are many devices for measurement and safety purposes. At night everything is run automatically, during the day there are operators on site - especially for repair and maintenance reasons and for taking the coferments and manure. Large-scale cofermentation biogas plants are constructed for one reason only: to make a profit. Therefore, the plant must operate day and night. The investment costs may be as high as several million dollars. Depending on the input substrates the pretreatment has to be engineered and constructed. Indeed, depending on the input substrate and the pretreatment, the hydraulic retention time may vary. This has direct impact on the digester volume.
Therefore, a lot of expert information is needed to construct such a large-scale cofermentation biogas plant. BIO-METHA and their partners offer this know-how.
