Anaerobic digestion has a natural place on the farm, not just as a process within a cows stomach, but as part of a waste management system, the enhancement of nutrient recycling, and more recently as a renewable energy. Biogas plants are common place on farms in Germany and Austria, where farmers have taken advantage of long term guaranteed electricity prices and soft loans. These farmers are now farming for biogas rather than farming for food and they have simply switched from feeding maize to the cow and instead feed it to their digesters.
As with all anaerobic digesters the economic viability of a farm biogas plant is very dependent on the type and quantity of feedstock and the utilisation of the by-products, biogas and bio-fertiliser. The biogas can be used to produce to provide heat and electricity for which renewable obligation certifications can be claimed, or alternatively it could be scrubbed and compressed to be used as a vehicle fuel. The digestate is a valuable bio-fertiliser and soil conditioner which contains all the nutrients that were present within the original feedstock.
Every farm is different to the next, which also applies to biogas plants which is why we like to deal with each project on an individual basis, designing the plant to the feedstock and specifications required. There is no question that slurry is a very good feedstock for a healthy anaerobic digestion system, however this is not the best producer of biogas, as the animal has already digested the readily available material resulting in the production of its own biogas! We would therefore recommend that to get the best return from your digester, even on large slurry based systems, co-digestion is considered with fresh undigested feedstocks. This may be the addition of wet energy crops, such as grass silage, wholecrop cereals, or maize. Glycerine is also a very valuable as a co-digestion product within a biogas plant which is currently widely available from bio-diesel producers.
You may also consider bringing in food waste to digest for which a gate fee could be demanded. There is legalisation that needs to be met when importing food waste with can increase capital and operating costs, but these can be offset by high gate fees.
Marches Biogas is currently designing Anaerobic Digestion Systems ranging from 12 kWe to 500 kWe
Marches Biogas Continuously Stirred Tank Anaerobic Digestion System
This is a conventional above ground steel tank with fixed roof and separate double membrane gas holder.
The anaerobic digester, is an above-ground tank with a fixed pitched roof, insulated externally with 100mm of mineral wool and protected with plastic-coated steel panels. The digester tank is fitted with a level controller to continuously record the level, a high-level electrode for emergency protection, a temperature transducer to continuously record temperature, a gas pressure/vacuum relief valve and an emergency relief valve.
There is no mechanical equipment within the digester tank, the digester is heated by circulating its contents by means of a pump through an external concentric-tube heat exchanger. Hot water is circulated through the heat exchanger by a central-heating pump.
The digester is mixed by circulating biogas from the roof of the digester through 36 diffusers cast into the concrete base. This is carried out by two independent mixer sets, each comprising a gas compressor and an 18-port rotary valve each located adjacent to the digester tank.
The biogas flow is measured at the gas consumers from the double membrane gas holder, capacity 100m3 via low level condensate traps. The gas holder is inflated by an air blower and its pressure is maintained at a constant level of 20mbar by a control valve.
The digester is discharged by a dedicated pump which withdraws digestate from the base of the digester. The digester discharge will run once an hour, the amount of digestate discharged and the time between each discharge is variable and can be adjusted by the operator.
Marches Biogas Semi Plug Flow Anaerobic Digestion System
Marches Biogas has designed a semi-plug flow digester that can be installed below or above ground depending on ground conditions. The concept of the design is to provide a simple robust solution for on farm installations, the tank is constructed from concrete and is oblong in shape with a central baffle wall that allows the feedstock to flow from end to end without bypassing, it has a fixed roof on two thirds of the roof area and a flexible roof membrane on the remainder. The membrane acts as the biogas storage. The fixed roof includes a gas pressure/vacuum relief valve and an emergency liquid relief valve.
The digester is fed in two ways. First, liquid feedstock is pumped by a lobe pump once per hour. Second, solid feedstock is augered directly once per hour.
The digester is mixed by the recirculation of biogas through a series of seventy two (72) nozzles cast into the concrete base ensuring even temperature distribution and preventing stratification; each nozzle is mixed sequentially.
The digester is maintained at a constant temperature of 42oC by the circulation of hot water through an internal heat exchanger. The hot water for the heat exchanger is provided by the CHP unit during normal operating conditions.
There is no mechanical equipment within the digester tank apart from the heat exchanger.
The digester is designed to be fed and discharged by pumps; there is also a sealable overflow weir. This design has the benefits that there is no requirement to gain access to an overflow weir which might be blocked and that the digester can be operated at variable level and therefore variable retention times. Included within the digester is liquid level control, digester temperature control and gas holder level control.
Marches Biogas Plug and Play Anaerobic Digestion System
Marches Biogas has designed a plug and play digester that can be installed below or above ground depending on ground conditions as a single unit or fixed together to increase digestion capacity. The concept of the design is to provide a simple robust solution for an on farm slurry based installation. The digester is a cylindrical GRP insulated tank with a volume of 100m3 to 200m3 that is manufactured as a complete unit at a manufacturing facility, delivered directly to site and lifted onto a preformed concrete base.
The digester tank is fitted with a level controller to continuously record the level, a high-level electrode for emergency protection, a temperature transducer to continuously record temperature, a gas pressure/vacuum relief valve and an emergency relief valve.
The digester is mixed by the recirculation of biogas through a series of eighteen (18) nozzles fixed to the floor of the digester ensuring even temperature distribution and preventing stratification; each nozzle is mixed sequentially.
The digester temperature is maintained by the circulation of hot water through an internal heat exchanger. Hot water is circulated through the heat exchanger by a central-heating pump.
There is no mechanical equipment within the digester tank apart from the heat exchanger.
The digester is discharged by a dedicated pump which withdraws digestate from the base of the digester, the slurry output is measured via an inline flow meter. The digester discharge will run once an hour, the amount of digestate discharged and the time between each discharge is variable and can be adjusted by the operator.
The biogas flow is measured at the gas consumers from the double membrane gas holder, capacity 100m3 via low level condensate traps. The gas holder is inflated by an air blower and its pressure is maintained at a constant level of 20mbar by a control valve.
Marches Biogas
Anaerobic digestion
Unit 4 Lower Barns Business Park,
Ludlow,
Shropshire,
SY8 4DS
01584 879666
Russell Mulliner
07977 141776
email