Combustion is used for the disposal of sewage sludge, although the PE or lime-conditioned press cakes are – properly speaking – not suited for a combustion process. Pressed sewage sludge material has a water content of more than 60%.
Therefore, combustion works only following previous drying and/or with adequate support fire. However, in any case, energy consumption is so high that it is completely unrealistic to use sewage sludge combustion for a net power gain for appreciable power generation.
The situation is totally different with sewage sludge conversion. Naturally, here again 60% of the water content cannot be utilised as such, but in a reform step downstream in the conversion process proper, up to 40% of the water contained is refined to water gas which is used for integrated sludge drying.
The dried organic part contained in the sludge is then converted to strong gas and converted to current by means of gas and steam turbines. Since no thermal surplus is released with this concept, a power/heat coupling device is not provided.
For a net energy gain as high as possible it is decisive whether the converted sewage sludge is fermented sludge which naturally contains less carbon and thus provides less energy, or if it also contains fresh sludge ratios. In addition, also PE conditioning versus Ca addition leads to an increase of the energy yield.
- The low temperatures of the conversion process do not lead to a reduction of heavy metals in ash/slag, therefore easy dumping of residual substances is ensured.
- The organic residual ratios in ash are below 5%, thus, dumping EU-wide is possible.
- Notably cheaper plant operation results from the continuously cheaper operating costs due to lower staff requirement, sludge drying due to exhaust heat and no additional support fire.
The conversion process facilitates a notably higher efficiency factor of current conversion due to the combination of gas and steam turbines. On the other hand, current sewage sludge combustion is only possible with steam turbines at a notably lower factor of efficiency. Due to the low exhaust heat production in the conversion process, there is a much lower and even negligible energy loss compared with the combustion process.