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Case Study Co-Fermentation

Co-fermentation at farm-level – The Netherlands

Co-fermentation is a microbiological process whereby organic matter is fermented under anaerobic conditions by methane-forming bacteria. The resulting methane rich biogas is burned in a CHP (combined heat and power unit) to produce electricity and heat. This energy can be used at location or delivered to others, for example an energy company. The fermented organic matter can be used as an organic fertilizer.

Principle

All manure from the farm is brought via a manure-pump into the fermentor (digestor tank). In the fermentor the manure is stirred with the help of electric mixers. The manure remains in the fermentor for a minimum of 30 days. To enhance biogas output other organic matter can be added to the mixture in the fermentor, in which case the process is defined as co-fermentation. Such feedstocks to the process can include manure, corn, leftover feed from the farm, waste vegetables, fruit and municipal organic waste.

From the fermentor the mixture (manure + co-product) is piped to a second tank, here the fermented organic waste (digestate) cools and the remaining methane is drawn-off. After the second fermentor the digestate can be stored in a normal manure silo or be spread directly onto the fields.

The methane gas released during this process is stored above the fermentor under the membrane cover specially designed for this purpose. In this gas-storage space a small amount of air is added to the biogas in order to remove the sulphur. The biogas resulting from this process is converted (unless it is to be burned directly for heating purposes) into electricity and heat via a gas engine with connected generator (CHP unit). The heat produced is used partly to create warmth for the fermentation process. The remaining heat can be stored in a warm-water buffer or else used directly for heating or cooling purposes. The electricity generated can be used for private consumption else sold to the national electricity grid.

In recent years subsidies have been given in the Netherlands and elsewhere to encourage the production of sustainable energy from biomass, which makes the fermentation of biomass financially attractive providing long-term secure cash-flows.

Environmental benefits

Methane has a greenhouse effect 21 times stronger than CO2. By converting methane into CO2 and water (by means of incineration through the biogas engine (CHP unit)) the emissions from the breakdown of the organic matter is greatly reduced, with positive effects for the environment. AD facilities are closed systems and thus emissions or odours from the system can only stem from any open feedstock silos and CO2 exhaust from the CHP unit.

The sustainable energy produced also contributes to a reduced consumption of fossil fuels by normal power plants. Moreover, the residue (digestate) is high in nitrogen, potassium and phosphorus and thus can be used as a high quality organic fertilizer. In addition this digestate or bio-fertiliser has good nitrogen fixing properties, is high in minerals which can be easily absorbed by plants and high in organic matter thus benefiting the soil structure.

Subsidy

Long-term fixed price subsidies as well as capital grants are available for the construction and operation of AD facilities. These enhance the financial returns that may be generated from AD facilities and provide a floor to the revenue generated from the production and sale of electricity.