Thanks to the development of filter media that is suitable for higher exhaust gas temperatures and the growing trend towards the use of secondary fuels, bag filter plants have eclipsed virtually every other kind of system when it comes to today's kiln dedusting applications, with some electrostatic precipitators being converted into bag filters.
Because of their size, kiln filters are the ideal application for EMC technology, allowing it to tap into their full potential for using energy efficiently and making operating cost savings. Additionally, EMC filters operate with a stable level of differential pressure – that is, without fluctuations – and are not affected by the kind of temporary overloading that occurs when secondary fuels are added, in kiln sections or during switchover processes, for instance. As a result, this application is guaranteed to offer maximum plant availability during continuous operation. This unique technology uses bag lengths of up to 10 m, something which has positive repercussions for investment costs too. Even bag lengths of 12 m and more can be used.
EMC filter technology from Scheuch has revolutionised dedusting in the cement industry and set new standards in bag length, pressure loss, cleaning pressure and bag service life. The results point to a clear reduction in life cycle costs (LCC). Thanks to its patented status, EMC is unique and is deemed to be the Best Available Technology (BAT) for process filters – even when compared with electrostatic and reverse-air filters. More than 200 EMC filter plants installed within the first ten years of this technology outline its pre-eminent status worldwide.
The Scheuch IMPULS filter is a high-capacity filtering separator that delivers outstanding cleaning performance combined with an excellent degree of separation. It is used for the dry separation of solid matter, primarily in the extraction of contaminants from machinery and the workplace, as a process filter in material recycling, or for conveyed material separation purposes.
Thanks to the specially shaped twin nozzles on the jet pipe, the pulsed free jet pulls along an envelope of clean gas from the primary compressed air as it makes its way to the injector. The two gas flows are mixed in the injector by means of pulse exchange and, at the same time, experience a significant increase in pressure. This ensures highly efficient cleaning throughout the bag length and filter bags that require cleaning less frequently.
Proven in practice for decades, the IMPULS cleaning system has become a byword for low operating costs:
A plant's ability to demonstrate a high level of safety and reliability is essentially dictated by its key components. The radial fan is one of the components at the heart of every extraction, dedusting, exhaust and flue gas purification, and pneumatic conveying plant. It helps to meet customer requirements with a level of quality that guarantees fault-free operation and excellent performance between maintenance intervals.
When it comes to ensuring availability and operational safety in dedusting plants, conveying mechanisms are just as crucial as fans and filter systems.
All conveying mechanisms can also be used in explosive, dust-laden atmospheres as defined in Directive 94/9/EC (ATEX).
In the cement industry, rotary valves are used as continuous discharge or dosage devices with slight negative pressure/overpressure for air exclusion purposes. The advantages of rotary valves lie in their comparatively low construction size combined with a high, speed-dependent conveying output, their ability to be used in a wide range of temperatures and with various pressure differences, and the relatively low energy costs associated with them.
Tube-type and trough screws are used to continuously transport bulk materials, at throughput volumes of 2 to 500 m³/h and with screw diameters between 200 and 1000 mm.
The longer the material conveying path and the higher the quantity of material that needs to be transported, the more sensible a choice high-pressure systems become. The main components of these are a high-efficiency rotary blower plus a rotary valve and an injector. Together, they ensure efficient conveying even when dealing with significant material quantities. High-pressure systems are ideal for use with conveying quantities of 1 to 100 t/h, conveying systems of up to 1500 m in length, and nominal widths between 88 and 500 mm.
Material loading stations consist of a rotary valve with an injector directly attached to it. The rotary valve loads the material into the pneumatic conveying system in a controlled manner. If it needs to handle items that cause wear, the system can also be lined with special materials.
Some years ago, virtually all kiln and clinker cooler filters – as well as some cement mill filters – were the electrostatic precipitator (ESP) type. As these have become unable to live up to growing requirements relating to emission limit values as well as operational safety and availability, however, electrostatic precipitators across the world are steadily being converted into bag filters. Compared with electrostatic precipitators, these ensure significantly lower clean gas dust loads and a consistent degree of separation.
The result is that almost every customer today plumps for the bag filter variety when installing new kiln filters.
Electrostatic precipitator housings are generally very tall in comparison to bag filters, making them ideal for conversion to EMC filters with bags which, over time, have grown to as much as 12 m in length. Excellent use can be made of the available space without the need to extend the housing – saving costs and cutting down on assembly time as conversion can usually be carried out during a scheduled inspection.
Thanks to Scheuch's comprehensive modular system comprising a range of filter head widths and bag lengths, it is possible to find exactly the right module for any housing dimension and thus maintain the required filter area. During the process of planning the engineering work for the conversion, a CFD flow analysis is performed where necessary in order to ensure a uniform inflow for all filter bags.
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 a high-dust circuit, the exhaust gas from the cement kiln exits the preheater at a temperature of 300 to 380 °C – an ideal level for catalytic nitrogen oxide reduction. Depending on the requirements of the application, it is possible to install several layers of catalytic converters in a reactor as a means of adhering to the required emission limit values. The exhaust gas containing dust (dust content up to 200 g/Nm³) is guided vertically from top to bottom. To prevent the ducts from clogging, each catalytic converter layer is cleaned during operation by an efficient cleaning concept.
Using the temperature level removes the need to heat the exhaust gas and, as a result, has a positive impact on operating costs.
Scheuch has developed this semi-dust design 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 catalytic converter – 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 catalytic converter system as an option, then ensures that the maximum filter inlet temperature for the EMC bag filter is adhered to.
The majority of the exhaust gas heat in a cement plant is used for drying raw materials and fuel, and exits the furnace filter with a temperature of 100 to 250 °C. Generally, the temperature of dedusted exhaust gases is too low to allow degradative reactions to take place in the catalytic converter: between 200 and 300 °C is required for this. When heat exchangers are used, some of the energy that is needed for heating purposes can be recovered. During normal operation, it is only necessary to compensate for the temperature loss of the heat exchanger 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 catalytic converter volumes can be used thanks to the dust-free exhaust gas. Thanks to this absence of dust pollution, the catalytic converters can be expected to offer a long service life.
The DeCONOx process combines regenerative thermal oxidation (RTO) with a low-dust SCR – two proven technologies in a single system.
DeCONOx makes it possible to reduce nitrogen oxides and organic carbon compounds simultaneously. NOx levels are depleted by means of a catalytic reduction process that uses ammonia, while carbon compounds are reduced by combustion (thermal oxidation in a combustion chamber). The heat that is produced in the afterburning process covers at least some of the thermal energy requirements of the denitrification that is needed to return the flue gas to the catalytic converter's necessary operating temperature. As a result, the expenditure associated with energy in the DeCONOx process is much lower than in a low-dust SCR.
We offer in-house-developed adsorption systems that can be used for purifying exhaust gases in applications where energy from contaminated primary or secondary fuels is being recycled.
During the dry sorption process, an absorbent – generally referred to as an additive – in the form of a finely disperse solid is dispersed in the exhaust gas flow. The air pollutants in the exhaust gas are separated at the inner and outer interphase of the additive by means of chemisorption and physisorption. A downstream deduster separates the resulting reaction products from the exhaust gas flow; ideally, this device should take the form of a fabric filter. The process of removing contaminants begins as soon as the additive is incorporated.