Synthesis gas from sewage sludge:

Energy supplier for industrial firing processes

The KOPF SynGas process converts dried sewage sludge granulate into synthesis gas. At the same time, the phosphorus-rich ash contained in the granulate is separated. This makes it possible to use sewage sludge as an additional or main fuel for energy purposes while complying with the phosphorus recovery obligation that will apply from 2029.


“The gasification of sewage sludge using the SynGas process is a particularly interesting approach that enables the future use of sewage sludge as a fuel in energy-intensive industrial processes,” describes Dr Alexander Neagos, CEO of KOPF SynGas GmbH & Co. KG. By generating a combustible synthesis gas, temperatures of up to 1500 degrees centigrade can be reached. As a CO2-neutral energy source, sewage sludge can thus replace cost- and CO2-intensive primary energy sources such as natural gas or lignite. This is particularly interesting in cement production and, for example, also in lime production.

To produce synthesis gas from sewage sludge, it is needed in dried form with a maximum residual water content of 15 percent. By using external, regenerative heat sources such as waste heat or solar heat, CO2-neutral synthesis gas is available.

The thermal treatment of the dried sludge takes place under reducing conditions at =0.25. The core of the process is the stationary fluidised bed, in which the combustible synthesis gas is produced in an autothermal process at temperatures between 850 and 900 degrees centigrade with the addition of preheated air. Depending on the application, the synthesis gas can be coarsely dedusted in a cyclone, cooled to 400 degrees centigrade in an air preheater and dedusted with the help of a ceramic fine filter.

Conversion of the energy contained in the sewage sludge into a thermal (Hh) and chemical (Hchem) energy flow. External sources (Qext) such as waste heat and solar heat are used for drying.

Important area of application: Cement industry
An important area is the use of fuel gas modules in cement production. Current practice in cement plants is to add the dried sewage sludge either via the main burner into the rotary kiln or as an additional fuel into the calciner. When using the fuel gas module, the fine filter can be dispensed with in this case. By using a cyclone downstream of the gasification process, the synthesis gas produced is freed from a large part of the remaining dust. Like the ash, this dust contains a high proportion of phosphorus and can be mixed with the ash for further processing.

The remaining hot synthesis gas is then fired in the cement plant directly or after possible cooling for the purpose of heat recuperation. This simple approach enables the selective use of the chemically bound energy contained in the sewage sludge in the cement production process and, at the same time, the separation of the valuable ash. The obligation to recover phosphorus prescribed in the amended Waste Sewage Sludge Ordinance from 2029 is thus fulfilled. The dust, which tends to be more heavily contaminated with heavy metals due to the large specific surface area, is not separated in the cyclone and is fed to the cement kiln by remaining in the synthesis gas.

Application of the fuel gas module in cement production.

Using existing infrastructure
“The synthesis gas can either be introduced in the rotary kiln via a satellite pipe to be installed, ensuring the quality of the cement production process,” says Alexander Neagos. The simpler variant is to use it in a calciner. “When introducing it into the rotary kiln, it is important to note that the required temperatures of around 2000 degrees Celsius cannot be reached by burning syngas alone. This means that the substitution rate of primary energy or high-calorific substitute fuels by syngas is limited to about 15 per cent,” Neagos tells us. However, by adapting the gasification process from air to steam-oxygen gasification, it is possible to significantly increase the calorific value of the syngas and thus achieve much higher substitution rates. Irrespective of the injection location to be chosen, the existing infrastructure at the cement plant can be used, which represents a significant advantage over sewage sludge mono-incineration plants that have to be newly built.

In addition to the application in classic cement plants, the production of calcined clays for composite cements represents a particularly interesting option for coupling with a fuel gas module. Calcined clays can replace up to 30 per cent of the classic clinker in composite cements, as they themselves have similar properties. When calcining the clays, however, far less CO2 is emitted compared to the classic clinker burner. However, the temperature of 750 to 850 degrees centigrade at which calcination takes place is particularly advantageous for coupling with a fuel gas module. This enables a very high substitution rate of up to 100 percent with synthesis gas from sewage sludge. The use of the gas is also conceivable in waste-fired power plants, lime kilns and brickworks.

Possible industrial sectors for the use of synthesis gas from sewage sludge

Industrial sector Specific process requirements Constructional requirements Legal provisions Maximum replacement rate of primary energy with syngas
Cement High combustion temperatures (clinker temperature 1450 °C) at the rotary kiln Additional satellite burner rotary kiln, new hot gas burner calciner Sewage sludge firing via the main burner is common practice. Permit according to 17. BImschV usually available, to be extended ~15-20 %
Calcined clay Combustion temperature < 900 °C, use of obsolete cement plants possible Analogue cement Analogue cement Up to 100 %
Waste-to-energy plants Raising the combustion chamber temperature > 850 °C in part-load operation Additional hot gas burner Co-combustion is common practice Up to 100 % of the stub firing
Lime kilns Kiln temperatures ~1200 °C, direct desulphurisation through the lime in the process, high primary energy substitution possible Adaptation of current burner and air supply, additional flue gas cleaning Permit required as co-incineration plant, 17th BImSchV Up to 80 %
Brickworks No impurities in the synthesis gas, transient process control Adaptation of current burner and air supply, additional flue gas cleaning Permit required as co-incineration plant, 17th BImSchV Up to 60 %

Ash as a raw material for phosphorus recovery
“For a holistic concept of sewage sludge utilisation in industrial combustion plants, ash utilisation must also be considered. KOPF SynGas is already working with several partners on a solution to further utilise the ash economically,” Alexander Neagos continues. For this purpose, ash from the process of an incineration plant in Koblenz is fed to the test plants for ash treatment. According to Neagos, when utilising synthesis gas from sewage sludge as a substitute fuel, a corresponding ash utiliser must be involved as a third party. Already today, such a plant for synthesis gas production can be installed without much effort, for example on a cement plant site, without having to landfill the ash.

The generation of synthesis gas offers a flexible use of sewage sludge as an energy source. “The residual material from wastewater treatment thus becomes part of the solution to one of the central challenges of our time: the resource-saving and sustainable use of energy sources,” Neagos sums up. The combination of an energy-intensive process and gas generation from sewage sludge means that existing infrastructure can be used to continue using sewage sludge as a substitute fuel and to return the valuable resource phosphorus to the nutrient cycle.

Fuel gas module for generating synthesis gas from sewage sludge with optional recuperation via an air preheater and fine dust removal.

Comparison of the combustion of lignite with that of synthesis gas from sewage sludge.