Methanization of industrial biowastes in order to reduce pollution while producing energy.
The SGN fixed-film process has been implemented on an industrial scale in Guadeloupe (French West Indies) for methanization of sugarcane molasses stillage, after validation at a pilot facility.
Erection began in November 1985 and was completed at the end of April 1986.
Nominal capacity was approached at the end of the first distillation campaign, after 175 days.
Throughout the capacity increase period, the digester was fed continuously and the feed rate increased regularly. During the first months, sodium hydroxide was added continuously.
A biogas output of 370 Nm3/h was reached during the first weeks of the second distillation campaign in 1987. The biogas generated could be burnt after a few weeks in the boiler.
The total annual methane production is about 720 000 Nm3. Based on a net calorific value of 8.56 x 10(6) cal/m3 for methane, this is equal to 620 TOE.
Energy saving is important in the production of ethanol fuels by reducing the external energy consumption of distilleries. The biogas generated by the SGN process, which is equivalent in energy terms to 1.8 to 2 kg of steam per litre of pure alcohol, can satisfy most or all the energy needs of a distillery when combined with improvements in distillation technologies.
The Bonne Mere industrial facility is located at Sainte-Rose in Guadeloupe. The effluent to be treated is canesugar molasses stillage generated by the distillation columns and previously discharged as waste.
The outlet temperature of the stillage is approx. 90-95 deg. C.
The distillery operates continuously, 138 hours per week and 150 days per year on average, providing a daily alcohol production of 26,000 liters
Anaerobic treatment of the effluent is performed in a fixed-film digester whose sizing is based on the results of pilot plant tests conducted in 1983.
The patented SGN process uses the PVC-made FLOCOR packing. The surface action is significant (230 m2/m3) and allows a maximum of biomass to be fixed in the digester. The down-flow concept is used, the effluent being fed through a distribution pot at the top of the digester. The liquid is thus circulated from the top to bottom.
This technique has many advantages:
- the gas-liquid countercurrent ensures optimum agitation throughout the digester,
- the free sludges are removed together with the effluent, which noticeably simplifies excess sludge extraction.
- clogging risks are sharply reduced by the down-flow, but also by the high void fraction in the FLOCOR R packing (approx 95%).
Temperature is controlled to maintain the digester at a set point of 37 deg. C.
The gas recovered is sent to the boiler by a booster.
Biogas can also be recycled using a recycle compressor in order to control the quantity of biomass present in the digester and prevent any clogging.
The methanization facility is designed to eliminate most of the soluble carbonated pollution in stillage : up to 90% reduction of BOD5 (biodegradable part) and about 65% redcution of COD.