RUB » Lehrstuhl

Circulating Fluidized Bed Combustor

To investigate the influence of operating parameters and fuel characteristics on the formation of gaseous pollutants, a Circulating Fluidised Bed Combustor (CFBC) with a thermal capacity of 100 kW was established. The central element of the fluidised bed combustor is the combustion chamber, which is lined with a three-layer non-abrasive refractory concrete. The overall height of the combustion chamber including wind box and cover plate adds up to about 6 m. The gas velocity varies between 1.8 and 2.5 m/s. Residence times up to 3 s are possible. To obtain fluid dynamics similar to a large scale CFBC, sand with a medium-sized diameter of 65 µm is used as bed material. The cyclone for the precipitation of elutriated bedmaterial is connected to a compensator, flanged to the third steel tube segment. The drop tube of the cyclone is connected to a siphon, that dams the separated bed material to prevent short circuit flow of the combustion air through the drop tube and the cyclone. Two nozzles at the bottom of the siphon ensure a constant supply of bed ash over the brick lined conveyor shoot into the combustion chamber.

After particle removal inside the cyclone the flue gas is cooled by a conventional heat exchanger. Subsequently, the remaining fly ash is removed by a fibrous bag filter. For taking fly ash samples the exhaust gases may be bypassed through an additional filter. This filter allows taking representative samples in a simple way for each experiment.

For feeding solid materials, e.g. coal, quartz and limestone, a spiral feeder system connected to the conveyor shoot is used. The facility is equipped with a special metering system for feeding materials like textile dusts. Additionally, the facility is equipped with a heated metering system for liquid fuels like heavy oil.

Primary air, staging air and recirculated flue gas, respectively, are supplied from independent fans and preheaters. It is possible to preheat these gases up to 430 °C in order to vary local temperatures inside the combustion chamber. The secondary air may be added at five different positions providing variable residence times within different reaction zones. Fuel rich conditions of the primary zone are adjustable by admixing recirculated flue gas without affecting the coal feeding rate and the fluidisation conditions. Two cooling pipes, one dipped into the combustion chamber and the other into the siphon, are used for heat extraction. The desired temperature are adjustable by varying the insertion depth of the cooling pipes.

In order to minimise pollutant emissions in combustion experiments the following operating parameters of the experimental facility may be varied independently over a wide range: - The residence times in the different reaction zones are influenced by admixing secondary air at five different positions; - The primary to secondary air ratio may be manipulated to achieve different overall fuel/air ratios; - Within the primary zone fuel rich conditions can be reached by admixing recirculated flue gas, without affecting the fluidisation condition of the bed material; - The desired combustion chamber temperature is varied through the insertion depth of the cooling pipes, without affecting the coal feeding rate and the fluidisation conditions; - Local combustion chamber temperatures are influenced by preheating the air or flue gas.

Pollutant concentrations detected on the pilot plant level correspond well to those measured in power plants. Due to the shorter residence times � compared to large scale facilities � the CO-emissions are slightly higher.