Model MBT-AD - Mechanical Biological Treatment Systems
Mechanical Biological Treatment systems with Anaerobic Digestion (MBT-AD) plants are designed to process mixed household waste as well as commercial and industrial wastes. MBT-AD is a waste treatment technology that processes waste mechanically in order to separate recyclable elements from the organic component of the waste. The organic component is then treated biologically converted into biogas, a renewable fuel, and a solid digestate to use as fertilizer.
Anaerobic digestion is a 4-stages biological and chemical process. Many microorganisms are involved. But the different bacteria prefer a different environment (pH-value, temperature, organic acid concentration, etc) where they can work in a most efficient way.
Therefore, FBI has developed the partial-aerobic hydrolysis pretreatment technology. This means that the first stage of the digestion process, the hydrolysis process, is now a separate process in an extra tank. In that tank, air is injected because only aerobic bacteria can produce the enzyme cellulase. Cellulase can open cellulose and hemi-cellulose from fibrous materials (like grass, straw, green waste, etc.) and turn them into sugar (that is later turned into biogas).
Thus, more kind of biomass can be digested. Like wheat and corn straw in China. Instead of burning it which results in a high air pollution, FBI's biogas plants can digest the straw and turn it into biogas and fertilizer.
Typical FBI Chinese biogas plant running with pure corn and wheat straw. The Chinese plants have 1 to 6 hydrolysis tanks (350 m3 each) and up to 8 digesters (up to 8’700 m3 each).
Typical FBI German biogas plant running with manure and corn silage. The German plants have 1 hydrolysis tank (180 m3 each) and up to 3 digesters (up to 2’500 m3 each).
Higher biogas yields
Fibrous organic material with high cellulose and hemi-cellulose contents are digested and turned into sugar and thus more biogas can be produced. Cellulase could be added daily to the fermentation process to increase the gas yield but this is too expensive. The FBI hydrolysis pretreatment allows cellulase forming bacteria (strictly aerobic) to reproduce themselves and thus to produce continuously cellulase, almost for free. So, for fibrous plants (grass, reed, etc.) and dry fibrous biomass (straw, manure with straw, bagasse, etc.) much more biogas can be produced by the digestion of cellulose and hemicellulose.
Higher methane concentration
In the hydrolysis unit most biological processes for the formation of CO2 take place. The CO2 leaves the unit with the exhaust air. Thus, the biogas has a 3-5% higher methane concentration than with a 1-stage fermentation process. This lets CHPs run smoother and helps saving cost on biomethane upgrading plants.
Stable fermentation process
With the separation of the sensitive methane forming process from the acidification process the whole fermentation process runs much more stable.
Less digester volume
Due to the much better environment conditions for the methane bacteria less hydraulic retention time (HRT) is necessary. With the FBI hydrolysis pretreatment process an HRT of 20 to 25 days is enough. Thus, digester volumes can be reduced or more biogas can be produced with the same volume.
Trouble-free hydraulic processes
With the FBI hydrolysis pretreatment technology most of fibrous materials are being liquified and thus pumping and agitating are easier and there are no more floating layers in the digesters.
Laoling, China. Biggest Asian biogas plant using FBI’s hydrolysis technology to digest corn and wheat straw. Plant size: 5.5 MW