Objective
The aim of the project was to demonstrate the technical and economic viability at a full scale level of the production of biogas, with a concomitant production of compost, from source separated biowaste by means of the DRANCO (DRy ANaerobic COmposting) process. The purpose was also to demonstrate the innovative aspects of DRANCO, such as the dry fermentation with high loading rates and short retention times, the thermophilic operation, the stationary fermentation, the low space requirements and the limited odour emission. Finally, to be technically and economically successful, the aim was to produce 1.178.000 Nm3 of biogas per year.
The annual gross energy production amounts to 650 TOE while the payback time is estimated at approx. 7 years.
The installation as a whole worked better than anticipated. The initial goal was to treat 40 ton of biowaste per day. Within 2 months after biological start-up, the installation could treat this amount. After 6 months of full-scale operation, the amount of biowaste could be increased to peaks of 60 ton per day. This also resulted in a higher and more constant biogas production.
The performance of the high speed drum was better than expected. Disposable diapers were opened in such a way that the plastic sheet remained fairly intact and were sieved off. In this way only a very small fraction of plastic ended up in the dosing unit. The performance of the refining sieve was a little worse than expected, so that the percentage of overflow was higher than foreseen. This was a result of the bigger amount of waste which was processed. However, the overall effect could be minimized by recycling this overflow from time to time. In June '93 a centrifuge was installed to treat the waste watercoming from the press. Before, the press water was filtered by the nearly landfill, and further treated together with the leachate. The performance of the centrifuge was very good with respect to suspended solids and COD removal.
The project originally proposed to the CEC was anticipated to be a technical and economic success if more than 883,400 Nm3 of biogas was produced during a 1 year period. The actual project was constructed with an 800 m3 digester volume instead of a 600 m3 digester volume, so that 1.178.000 Nm3 of biogas should be produced on the same basis. Based on the results for the last months, it is clear that the installations will have a capacity of 12000 ton per year. The biogas production is roughly 100 Nm3/ton, so that a total of 1200.000 Nm3 of biogas can be expected, slightly higher than the amount needed to be regarded as a technical and economic success. The tipping fee is 56 ECU/ton. However, this will become considerably less once the installation has been paid off. With regard to the energetic balance, roughly 40% of the biogas is used for operating the plant. Only 17% of the biogas is converted into electricity and sold, because of the fact that no adequate compensation is given in Belgium for the electricity, if reinjected into the power grid nights and weekends. The rest of the biogas (43%) is burned by a flare.
A second DRANCO plant was ordered by the regional waste authorities of the city of Salzburg. The installation is designed for 20000 ton/year and is in operation since December 1, 1993. Theoretically, if HALF of the population would be connected to source separated collection of biowaste, composting of the biowaste would require 1250 similar DRANCO plants as the demonstration plant throughout the EEC.
INNOVATIVE TECHNOLOGY
The fermentation occurs under solid-state or "dry" conditions at exceptionally high loading rates and short retention times (20 days). The DRANCO digester concept allows operation between 25 to 40% total solids, while the bacteria are functioning as efficiently as in a completely mixed reactor. Due to the high concentration of degrading material and the same bacterial activity, the biogas yield per unit volume and per day is high and amounts to approx. 4 Nm3 biogas/m3xday.
The digestion process occurs at thermophilic operating conditions (50-55 deg. C) and yields a residue which is thoroughly stabilized and hygienically safe. Potential pathogens are killed off because of the high temperatures in combination with the anaerobic conditions.
The fermentation is a "stationary" fermentation. This means that, apart from feeding and removal of residue, there is no internal mixing or agitation in the digester which works at very low pressure. The stationary aspects makes the process simple and reliable, without coming any excessive wear of the digester.
Other interesting features of DRANCO are the low space requirement, application to relatively wet substrates without the need for structural bulking material, limited odour control, easy treatment of non-recyclable paper and production of a high hygienic quality compost.
CONTEXT IN WHICH THE TECHNOLOGY IS OPERATING
The DRANCO plant was designed for treating 10500 ton of the organic fraction of household refuse. Biowaste is obtained from a source separated collection and consists of kitchen waste, garden waste and non-recyclable paper. Biowaste is first comminuted in a homogenizing drum, sieved over 38 mm and then dosed to the fermentor, where it is mixed with digested residue and steam. During the thermophilic fermentation, biogas is produced. Part of the biogas is converted into electricity for operating the plant and for delivery to the power grid. The rest is burned by a flare. After three weeks digested residue is dewatered by a press, refined over a 12 mm screen and aerated during 2 weeks, to produce a high quality compost (Humotex). Wherever possible, the DRANCO plant is automatically controlled. The basis for the control and measurement system is formed by a PLC and two PC's.
ECONOMIC ASPECTS
The cost to produce energy is covered by the tipping fee for treating waste. The DRANCO process does not require fossil energy. The net amount of energy produced by anaerobic digestion amounts to 130 kWh/ton waste.
Topic(s)
Call for proposal
Data not availableFunding Scheme
DEM - Demonstration contractsCoordinator
9000 Gent
Belgium