The CAMBI Process
CAMBI Anaerobic Digestion Process: MSW Anaerobic Digestion with hydrolysis and single or two stage digestion
 CAMBI’s process is designed for Municipal Solid Waste, and for sludge treatment, and it features an initial hydrolysis stage utilizing high pressures and temperatures to accelerate the process and to optimize the total proportion of the organic material within the feedstock which is converted to biogas.
CAMBI has undertaken substantial research, development, design and building of the both the hydrolyser and digestion plant.
The use of two stage digestion also increases conversion of the biodegradable fraction, making this technology on of the most sophisticated on the market.
The stages which we understand take place in a CAMBI MSW AD Plant are as follows:
- MSW reception;
- shredding;
- wet separation which entails separating the biodegradable material from the inert materials, in water;
- thermal hydrolysis
- one- or two-stage anaerobic digestion (we understand that single stage is used for smaller plants, two stage for larger plants).
The biogas produced powers a gas engine generator unit, and this is normally fitted with waste heat recovery. However, the gas can be cleaned and compressed as a fuel (LNG) for (for example) the town’s buses.
In many applications it is assumed that remaining waste heat is available for CHP (Combined Heat and Power) use, after the plant meets its own parasitic demand for steam and heat to provide the demands from the both hydolyser and digesters to maintain a suitably high temperature.
The advantages of this process over conventional anaerobic digestion is firstly the use of the thermal hydrolyser and secondly the use of a two-stage fermentation or digestion.
In a conventional digester a complex range of micro-organisms slowly undertake this process; in the Cambi process it occurs rapidly as a result of heating the material under pressure to around 150°C. This results in substantially more of the biodegradable material being made available for conversion into gas. This of course is a very admirable result, as long as the net additional gas energy production at equals or exceeds the greater parasitic load from the hydrolysis stage. Critics would also point to the need to justify the additional capital and running cost for the hydrolyser.
Of course the mass of the waste requiring disposal at the end of the process will also be reduced by the additional conversion provided by the hydrolysis stage, so any costs of the disposal of the (liquid and fibrous) end products will be lower.
There may be another benefit of the hydrolysis stage as well though. That is that although the liquid can be used/sold as a fertiliser, as in most AD process, the fibrous material after hydrolysis is likely to be much more inert and can possibly be used for such uses as production of fibreboard.
Cambi offer two designs for the digesters. The conventional one-digester is used at Lillehammer; the alternative two-digesters in series system is offered on larger installations such as the 60,000Te design described in this case study. The decision between one- or two-digester(s) is based on digester sizes and hence costs. The two-vessel design provides better control of the digestion process, with a consequential increase in efficiency.
As is normal from fermentation stages the gas produced from the organisms which provide methanogenesis in the digester, is typically about 60 per cent methane, with the remainder mostly being carbon dioxide.
This gas is burnt in a spark ignition engine driving a generator. Steam is raised in a waste heat boiler using the hot exhaust gas from the engine. Surplus steam can be supplied for heating or other process use. Additionally waste heat at up to 95°C can be provided from the engine-cooling jacket. In the absence of any use being available for the heat from the cooling jacket, this heat may have to be sent to a cooling tower.
The hydrolysis stage ensures that sufficient sterilisation of the feedstock is achieved such that this process readily complies with the Animal By-Products Regulations, for the prevention of spread of infections via the products of the process. This is considered a big plus point for this process, whereas some AD processes (eg mesophilic) need an additional pasteurisation stage after digestion, it is understood that the CAMI process does not.
There is more information at the CAMBI web site, and at the UK Environment Agency web site here.
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