Stirling CHP for sewage plants

General description for sewage plants

Due to anaerobic fermentation during wastewater treatment, sewage gas is generated. During external combustion on the Stirling engine the sewage gas is turned into electricity and heat.

Process description for sewage plants

The sewage plant Niederfrohna can serve 40,000 residents and is used to 85% of this capacity. It cleans the communal and industrial wastewater biologically according to the SBR procedure (Sequencing Batch Reactor). The sludge generated is anaerobically utilised. The sewage gas generated contains between 50 and 60% methane. The sewage gas from the sewage plant Niederfrohna is used at source to generate electricity and heat with a Stirling CHP.

On its way from the gas storage to the Stirling CHP it only passes through a biological desulphurisation and two condensate separators. The sewage gas is not purified with an activated carbon filter.
Very high temperatures are generated in the combustion chamber during the combustion of the sewage gas, which are transmitted on to the heater of the Stirling engine. At the same time the part of the engine that has to be kept cool, is cooled by the connected heating coil. The electricity generated according to the Stirling principle of about 35 kWel is used to cover the energy demand of the sewage plant.  

The combustion air is preheated according to the counter-current principle in the combustion chamber and the waste gasses reach a medium temperature of about 425°C at the exit of the combustion chamber. A downstream waste gas heat exchanger cools the waste gasses further down to about 100°C. The cooling of the Stirling engine, the combustion chamber as well as of the waste gasses generates up to 120 kW of thermic power.
The heat is mainly used for the mesophilic run fermenter and also for heating the company building.

The operation of the Stirling CHP is laid out for a base load protection. Due to the high efficiency, even in partial load operation, the Stirling CHP can be run for 24 hours, 7 days a week.
The control of the gas range, regulation of the combustion air as well as the whole plant operation is done automatically. The Stirling CHP is integrated into the guidance system of the whole plant.