Gasification of sludge is carried out on sewage sludges with energy production or pyrolysis of the thermally dried residues, under a non-oxidizing atmosphere. It can be a means of alternative renewalble fuel production and it could in part be a solution to the environmental problems that landfilling or conventional combustion could create.

The present work which also focuses on combustion and pyrolysis of cotton gin residues in Greece, is another example of alternative ways of energy production. All these methods are using biomass as an energy source, and the use of this as an energy source is expected to grow rapidly.

Recently, gasifier operation has been put on a sound thermodynamic footing by a number of technology provider companies. It has been shown that, apart from its deleterious impact on thermal efficiency, the presence of water in mechanically-dewatered fuel (containing ~ 32 wt% solids) does not lead to significant particle break-up compared with dried fuel.

Gasification has been demonstrated using partial oxidation in three stages before final combustion. When doing this it has been shown that the NOx was reduced from several thousand ppm to around 25 ppm. This is a truly significant reduction in favour of gasification.

Gasification in fluidized bed reactors and usually takes place in a fluidized bed formed above the slag bath and constituted by the dried sewage sludge or waste materials, the solid fuel, the oxygen-containing gas and the gasification gas. The gas produced in the gasifier can be used for power generation or as a reducing gas for iron ore, or any one of a large selection of uses.

Gasifiers can frequently handle high fouling fuels without excessive slagging/fouling due to the lower temperatures at which they can operate in comparison with direct combustion units. Waste fuel gasification generally involves heating fuel in an oxygen-starved environment to produce a medium or low calorific gas.

Gasification of sewage sludge for heat and power generation in combined heat and power (CHP) applications is an attractive concept that provides an environmentally acceptable, efficient, and economically viable means of generating energy from a waste disposal problem. The final solid residues are pathogen-free but may contain toxic elements such as barium, copper, mercury, lead, and zinc at levels that could make their disposal to landfills costly as well as environmentally questionnable.

However, these materials would also be produced by incineration, and gasification makes the overall sustainable benfits so much more positive when the usefulness of gasification syngas is considered.

Fuel gases from gasifires can be cleaned and then burned in internal combustion engines for the generation of electricity and process heat, or stored and used as feedstock for other chemical production.

Wastes can even be co-processed with coal to yield gas and liquid fuels or chemical feedstocks. Specific approaches discussed include: gasification of sewage sludge, straw, wood wastes, and plastics; liquefaction of paper and plastic wastes, papermaking wastes, oils, greases, waste tyres and waste-derived liquids; pyrolysis of plastics; and economic studies.

Wastes selected included dewatered sewage sludges, loaded rotary hearth furnace cokes, and processed packaging plastics. A number of plant modifications may need to be made to accommodate these feedstocks.