End products from anaerobic digestion process are biogas, digestate – which is a nutrient-rich soil-like substance that can be used as a fertilizer- and water.
Biogas is a mixture of biomethane and carbon dioxide, with appr. 60 – 65% of methane and appr. 35 – 40% of CO2.
Biogas can be utilised as such in heat and power production. However, when used as vehicle fuel or fed into gas grid, it must be purified into biomethane. The term used for increasing the concentration of biomethane up to required 97% – 99%, is upgrading.
The upgrading technology typically used in our plant deliveries is based on permeable membranes. Membrane technology is a separation process in which semi permeable membrane system is used to separate different components and impurities of gas mixture. Different gas molecules have different permeability in the membrane material. Small molecules (CO2, H2, H2S) have ability to penetrate the membrane more rapidly that larger molecules – ie. methane molecule.
Membrane technology is energy efficient, the separation result is excellent and the method is environmentally friendly. Membrane technology is well suited for different climate conditions, especially for Nordic. It is also possible to separate the CO2 component form biogas mixture for further utilisation.
With mebrane technology it is possible to utilise the carbon dioxide in the biogas mixture as valuable product. CO2 can be used ingreen houses, food or beverage production or in industrial applications. Finding the synergies in different processes is our forte – as an example – we have developed a process for utilising the CO2 in pulp mill operations.
Carbon dioxide produced can be compressed into containers and carried to other location for further utilisation.
The biomass fed into bioreactors is turning into soil like material – digestate – in the anaerobic digestion process. Depending on the reactor type – the solid content of the material is between 8-20%. The digestate is rich with various nutrients – escpecially nitrogen and phosphorus. Phosphorus and other mineral nutrients remain in the solid part of the digestate as the nitrogen is transferred mostly into liquid form and is solubel in the water phase.
The digestate can be utilised as such as fertilizer – or it can be mechanically dried. In this case the solid fertilizer and and nutrient water can be used separately in farming. The solid part can be further dried thermally and made into pellets that are easy to store and use.
The nutrient water can be used as such or the valuable nutrient component, nitrogen , can be separated in concentration or stripping process.
The fertilizer products acquired in biogas plant are economically affordable, ecological and sustainable solution. Using these products, the nutrients in the feedstock are recycled and global resources are saved as there is less need to use inorganic fertilizers.
Some feedstock materials, like waste streams and side-products of pulp and paper production, contain high amounts of ligning. Ligning is relatively difficult material to digestate unless energy consuming pretreatment methods are used in biogas production. Nevertheless, even side products and waste water sludges from pulp and paper mills produce significant amounts of biogas and are a viable resource to be used in biogas production. The digestate contains some lignin that has high heat capacity. In some cases this digestate is excellent biobased solid fuel material, that can be utilised for energy production when thermally dried and pelletized.
We have developed production process for utilisation of these lignin based digestate materials for pulp and paper industry. Biopellet for energy production is a good option when the digestate is not fulfilling the fertilizer regulations due to it’s origin and high cadmium amount.
Other usage models
Our aim is always to find the best synergies between the biogas plant and surrounding infrastructure. One alternative model for using the nitrogen rich reject waters from biogas plant is to use those as chemical replacement in pulp mill’s wastewater treatment plant. Typically nitrogen containing commercial chemicals (urea) are needed at the waste water treatment plant. When reject water from biogas plant is either led or transported to the pulp mill, significant savings in chemical usage can be achieved – and the total nitrogen load for environment is decreased.
We have experience in designing these systems for paper and pulp mills as pioneers with EPAD Forest process.