Dry Anaerobic Conversion: The Dranco Process
Summarised from a paper by W.Six and L.De Baere, Organic Waste Systems N.V., Gent, Belgium (DoE 1990)
The DRANCO process has been developed for the conversion of solid organic wastes, specifically the organic fraction of municipal solid waste (MSW), to energy and a humus-like final product, called Humotex.
The DRANCO process can be compared to landfill gas production, accelerated by a factor 1000.
The following operating information has been reported: Operating temperature in the digester is 55'C and the total solids concentration is ca. 32%. The mean gas production rate is 3.33 Nm3 CH4/m3reactor.day.
The gas production process is finalised in 3 weeks. The Humotex is free of pathogens.
Low concentrations of heavy metals can only be obtained through separate collection of garbage.
The DRANCO process is then also focussing on the digestion of source separated waste as vegetable, fruit, garden and non recyclable paper waste.
A full scale DRANCO-plant is elaborated treating 25.000 ton per year of vegetable, fruit, garden and non-recyclable paper waste.
The organic fraction of municipal solid waste can be composted aerobically, but there is a growing interest in anaerobic fermentation of the organic fraction of municipal solid waste.
The DRANCO process utilises a dry anaerobic fermentation technique in order to produce both biogas and a stable humus-like final product, called Humotex.
Substrate is introduced at a total solids (TS) content of 25-40 % without further dilution with water. In this way problems of phase-separation and crust formation are avoided.
Since the high solids concentration proved to have no effect on the efficiency or the rate of the microbial conversions, the overall efficiency of a reactor is very high in comparison with a conventional low solids reactor.
Per unit of reactor volume about 5 to 8 times more biogas is produced for a given time interval.
A second characteristic of the DRANCO process is the temperature of the reactor. Whereas a dry or solid fermentation can take place at both mesophilic (30 -40 *C) and thermophilic (50 - 60 'C) temperatures, the Dranco process utilizes a thermophilic operation since this improves the rate of bioconversion and the overall energetic efficiency considerably.
The DRANCO technology is specifically suited to handle the organic fraction of MSW.
If mixed garbage is collected, the first steps in the Dranco process will consist of a size reduction and a separation of the different fractions in the waste stream.
If source separated waste is collected these front-end processes can be largely simplified.
The DRANCO process itself will consume 30 to 50% of the produced electricity. The digested residue is subsequently dewatered to 60% TS by means of a screw press. filter press.
The filter press liquid can be further treated by an evaporation plant. The cakes of the filter press are mixed with the cakes of the screw press and the dried sludge of the evaporator.
The Humotex is stabilized and free of pathogens. Concentrations of heavy metals depend on the incoming substrate. Low concentrations of heavy metals can only be obtained through separate collection of garbage.
Other sources list the features of the process as follows:
- Vertical plug flow design
- Waste at 35-40% TS shredded and sieved (22 mm)
- Waste fed in at top of reactor with recycled digestate at a ratio 1:6
- Steam injected to 55 0C (thermophilic process)
- No internal mixing, unloaded at base by helical screw
- Retention time 2-3 weeks
- Little or no liquid effluent.
Examples have been built in Switzerland, Germany, Italy, Austria and Spain and comprise the following categories:
More than 20 full-scale Dranco plants (as of January 2011) based on:
- food/biowaste: 12 plants (since 1992)
- residual/mixed waste: 7 plants (since 1997)
- energy crops: 1 plant (since 2006)
The process is dry anaerobic digestion, which is described as providing:
- high solids digestion
- high loading rate and biogas productivity
- small digester volumes
- phase separation in the digester
- crust formation or sedimentation
Physical features include:
- vertical digester with conical outlet
- extraction at the lowest point
- accumulation in the digester
- low surface are requirement
- one-stage digestion giving reduced reduced plant complexity.
Other features described are: