In a high-dust circuit, the exhaust gas from the cement kiln exits the preheater at a temperature of 300–380 °C, which is the ideal temperature range for catalytic denitrification. Depending on the specific requirements, multiple catalyst layers can be integrated into a single reactor to allow the required emissions level to be maintained. The dust-laden exhaust gas (dust content up to 200 g/Nm³) is conveyed vertically downwards. To prevent the channels from becoming clogged, each catalyst layer is cleaned during operation by means of an efficient cleaning concept. By taking advantage of the temperature level, heating of the exhaust gas is unnecessary, which has a positive effect on operating costs.

Scheuch has developed the semi-dust circuit on the basis of experience and findings gained from high-dust and low-dust circuits operated in parallel to one another in pilot plants. The process involves installing a pre-separator in the form of a dry electrostatic precipitator for temperatures up to 400 °C upstream of the catalyst – and directly downstream of the heat exchanger, as is the case in high-dust circuits. A gas conditioning tower, which can be integrated into the catalyst system as an option, then ensures that the maximum filter inlet temperature for the emc bag filter is adhered to.

A significant proportion of the exhaust gas heat from a cement plant is used for drying raw materials and fuels, and leaves the kiln filter at a temperature of 100–120 °C. The temperature of the dedusted exhaust gas is usually too low for breakdown to take place in the catalyst. For breakdown to occur, temperatures of 220–300 °C are required. With heat exchangers, some of the energy that must be used for heating can be recovered. In normal operation, only the temperature loss of the heat exchanger needs to be compensated using an external energy source (heat displacement from other exhaust gas flows, natural gas burners). The advantage of this circuit type is that relatively low catalyst volumes can be used thanks to the dust-free exhaust gas. As a result of this absence of dust pollution, the catalyst can be expected to offer a long service life.