Over the years, more and more fabric-based filtration systems have been used to remove particulate matter from the hot exhaust gases from rotary cement kilns, clinker coolers and lime kilns.
Depending on the material quality, the use of modern filter bags is limited to maximum operating temperatures of 120° to 260° C (250-500 °F), making it necessary to cool these gases before beginning the filtration process. Exceeding these temperatures can lead to permanent damage or even the destruction of the filter bag material.
Using a mass cooler makes it possible to "cap" short-term temperature peaks in exhaust gases. In turn, this opens up advantages such as the ability to use less expensive filter media.
Temperature peaks occur during extraction in metallurgical processes such as charging and tapping, or when exothermic substances are added to melts. It is particularly necessary to avoid temperature peaks during the ejection of coke from coke batteries; in this situation, the mass cooler also functions as a heat storage device. The heat energy stored when the temperature peak occurs is subsequently returned to the exhaust gas, which protects downstream plant components from corrosion caused by undershooting of the dew point.
The plate stacks incorporated into mass coolers have been calculated to produce optimum thermal characteristics. Depending on the required cooling output, the design focuses on either free-standing coolers or individual mass cooler packets in extraction lines.
As an alternative to an gas conditioning tower, gas can also be cooled using a system that injects fresh air. This design is equipped with a mixing chamber where the fresh air is blown in and added to the process gas. The fresh air mixes with the hot exhaust gas, thus reducing its temperature. Compared with gas conditioning towers, this solution incurs lower construction costs; however, since it also involves a higher volume flow, the filter plant must have larger dimensions.
Gas conditioning towers are primarily found in the cement industry and are used as a direct means of cooling hot process gas from the preheating tower to the filter inlet temperature.
The exhaust gas temperature provides the driving force for the gas conditioning process. Therefore, assuming similar heat output levels, a gas conditioning tower generally has to have much larger dimensions in low temperatures than in high temperatures. The cooling water is atomised in the Scheuch gas conditioning towers using either single-medium nozzles or spraying compressed air (i.e. twin fluid nozzles). Ideally, the exhaust gas should flow vertically through the system; depending on the application or planning requirements, this can take place from top to bottom (top-down) or from bottom to top (bottom-up).
In heat exchangers, heat from exhaust gas is transferred to cooling air. The most common type of heat exchanger is the air/air type, which in most cases is operated in cross-flows or cross-counterflows. It is the ideal choice for dry exhaust gases. The gas to be cooled is led inside the pipe by inflow hoods, and ambient air is used to cool it in the external cross-flow by means of axial fans. Air/air heat exchangers (one or several) can be used on both the exhaust gas side and the cooling air side, depending on what the application requires.
The primary purpose of exhaust gas cooling is to adapt the gas temperatures in line with whichever filter media are available at an economically viable level (i.e. the defined filter inlet temperature) and, as a result, to reduce the exhaust gas volume flows that are to be cleaned. Therefore, the initial focus is on economic measures.
Since significant quantities of energy are released during the process of cooling process exhaust gases, however, we also consider how to make our systems more environmentally friendly and take advantage of regenerative waste heat utilisation in our concepts. As an example, cooling 100,000 Nm³/h of dry air by 100°C corresponds to a heat output of approximately 3.5 megawatts [MW].