Award nomination for thermal treatment

An innovative gasification process from German manufacturer, Kopf, has won a nomination for the European Environmental Press (EEP) Awards for technology at Pollutec in Paris in November. Kopf executive Eberhard Kipp describes how sewage sludge is converted into energy, pollutants are eliminated and a reusable by-product produced at a pilot plant at Balingen, Germany.

Sewage sludge is contaminated with many types of pollutants including bacteria, viruses and hormones, along with everything from heavy metals to dioxin. It is certainly problematic to distribute such a cocktail on farm-land.

EU pollution prevention regulations will have the effect of making the thermal treatment of sludge inevitable. The inherent material and energetic value of the sludge must be put to use in accordance with today’s ecological standards.

Kopf’s process is intended for installation at medium-sized wastewater treatment plants (WWTPs). It is a stationary fluidised-bed process using air as the gasification medium. The feedstock dried-sludge granulate, with 70 to 90% DS, is conveyed via the gas cooler and a dosing device into the gasification reactor.

The fluidised-bed principle ensures an optimal intermixture of the reaction partners in the whole reaction space, ensuring a fast, uniform and complete reaction. Typical gasification reactions take place at about 850°C with large organic molecules being cracked into simple gaseous compounds. Inorganic constituents remain as an inert granulate. The process results in a flowable product with no scorification.

Additives such as limestone are not required. The granulate flows from the overflow of the fluidised bed via a cooling screw into a closed container for further use.

The gasification air is fed to the reactor from below. Inside, oxidation and gasification reactions run in parallel. After dedusting in a cyclone, the gas temperature drops to about 600°C in the recuperator, where at the same time gasification air is pre-heated.

The gas cooler combines the functions of a quench and of a filter. Water is sprayed in at the top to cool down the gas instantaneously, to below 150°C. Thus, the much feared de-novo synthesis of dioxins is effectively avoided.

Cooling down condenses small amount of oils and tars in the lower part of the gas cooler, they are adsorbed at the surface of the sewage sludge granules. The product gas is dedusted by a special filter and then cooled to below the ambient tem-pera-ture.

The resulting condensate is fed back to the sewage works. Gas-drying improves both the ignition performance and the effective heating value of the gas. The product gas is essentially free of pollutants, it contains the combustible components hydrogen, car-bon monoxide and methane although composition will depend on the sludge analysis.

As a rule of thumb, 1kg of sewage sludge DS will generate 1.6m3 of gas with an LHV of 1 kWh/m3. The gas can be used in a gas engine, or to fuel a steam or thermo-oil boiler.

The solid product of gasification is completely inert mineral granulate containing high levels of potassium and phos-phorus. There is a wide range of possibilities for its use, such as, filler material for ditches, stabilising material for bodies of existing waste dumps or as admixture in asphalt-making. Analysis also points to a potential application as a sub-layer for recultivation projects.

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