Forschungs- & Entwicklungsinformationsdienst der Gemeinschaft - CORDIS

Wastes gasification at small scale for power production

The fixed bed downdraft gasifier technology was shown to be technically feasible option for producing a low quality, combustible gas from thermally dried sewage sludge for small-scale applications. To minimise potential operational problems, co-firing sewage sludge in blends with other fuels, such as a lingo-cellulosic wastes, was recommended. Optimum gasification conditions for undigested sewage sludge were obtained at an equivalence ratio of 0.47 and 21% of sewage sludge in the blend. At these conditions the lower heating value of the product gas was 5MJ/Nm3 and the cold gas efficiency (CGE) reached 88%.

For digested sewage sludge, optimum conditions were achieved at an equivalence ratio of around 0.64 and 57% of sewage sludge in the blend, where a CGE of 83% was attained. During gasifier operation, some slag formation was observed on the grate and around the throat, which hindered continuous solid flow through the gasifier and led to a decrease in the product gas quality.

The slag formation was attributed to high temperatures (>1100 degrees Celsius) attained in the oxidation zone of the gasifier bed. Decreasing the airflow rate to the gasifier will effectively reduce the temperature in the oxidising zone; however, this impacts adversely on gas quality, solid to gas conversion and tar production. A comprise for the equivalence ratio value was identified for minimising slag formation on the grate whilst achieving good fuel gas quality and solid to gas conversions. A wet gas cleaning system, comprising cyclone + water scrubbing + gas drying, was shown to be efficient for product gas cleaning prior to use in an internal combustion (IC) engine but may lead to disposal problems for the waste water.

The use of product gas, generated from a fixed bed downdraft gasifier firing blends of sewage sludge-woody wastes, as a partial replacement fuel for a diesel engine was shown to be technically feasible option. Operation in dual fuel mode allowed a reduction in diesel fuel of up to 87%, however, useful power output decreased as the proportion of sludge/wood derived product gas was increased.

A high proportion of the energy was lost to the coolant, although customising the combustion chamber design for duel fuel operation should lead to improvements in overall efficiency. The exhaust gas temperatures are also higher for dual fuel operation than for pure diesel operation, so a higher level of thermal stress may be expected in the engine. The gasifier product gas is cleaned in a series of cyclones, wet scrubbers and cooling systems prior to the use in the diesel engine and contains relatively low levels of tars, moisture, alkali metals and soluble nitrogen and sulphur compounds. Subsequently NOx and SO2 levels in the exhaust gas were low when the engine was operated in dual fuel mode. CO levels, however, increased and for a commercial plant installation of an oxidation catalyst may be required for operation at low loads.

An overall efficiency of the system (gasifier and engine-generator) of 19% was achieved when the engine was run at 23kWe, the sewage sludge content of the gasifier feedstock was 43% and the energy content of the engine fuel was supplied by 90% of the sludge/wood product gas.

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Universidad de Zaragoza
c/ Maria de Luna 3
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