Lowering the limit

Jürgen Lauer, BWF Envirotec, Germany, reveals a new approach to dust and NOx control in the cement industry, as emissions limits continue to tighten.

Introduction
It is a fact that emission limits in the cement industry have become and will continue to get more stringent. Although there are some differences, depending on what part of the world one looks at, the limits are undoubtedly heading downwards. A direct implication of this is that the dust control activities of cement manufacturers will have to be improved globally.

Dust control technologies
In cement plants across the world, cyclones, ESPs, and baghouses, either alone or in combination, are widely used dust control technologies, each with its own benefits. The well-known and established ESP technology is capable of handling dust emission limits of 5 – 10 mg/Nm3. ESPs in the cement industry can be operated up to a service temperature of around 450˚C, so there is no cooling of the gas needed for kiln exit gases and those of the clinker cooler. In the case of bypass filters, depending on the kiln exit temperature, cooling of the gases with air or water may be required. One of the most compelling arguments for an ESP installation is that ESPs are very easy to operate. Moreover, maintenance is relatively simple and the cost reasonable, due to fewer components involved as compared to a baghouse, for example. On the other hand, the space requirements for an ESP filter are huge and, if one wants to lower the dust emissions even further, the filter becomes very large, and the consumption of electricity very extensive. The extension of an ESP to meet new emission limits could imply higher fixed costs and increased operating costs. In most cases, the option of expanding the ESP filter is not available, as space is often the limiting factor in existing facilities. However, depending on the desired lower emission targets, ESP technology may not even be able to reach those new levels.
 

Finding other options
Time and again, changes in regulations and lower emission limits have forced the industry to look for other solutions. Lower emissions, in the range of about 3 – 5 mg/Nm3, can be achieved with baghouses, which also have a much smaller footprint than ESPs. No wonder that one of the industry’s responses to the challenge was the conversion of an existing ESP installation, either in full or in part, into a baghouse with a fabric filter. This type of filter conversion has become common practice in the cement industry and, with new limits in place, many more retrofits are expected worldwide in the coming years. However, the right implementation is crucial to reaping the benefits of such a retrofit. Some of the problems encountered can be traced back to fundamental differences between the two technologies. Two of these basic differences are the direction of flow of the flue gases and the operating temperature.
An ESP filter requires a horizontal flow of the flue gases going through the collecting plates. In a baghouse, the flue gases go through the vertically hanging bags. Therefore, in a baghouse filter, the gas flow should be vertical.
An ESP installation can be operated at approximately 450˚C whereas, in the case of a baghouse, the temperature is limited by the kind of filter media used. Maximum continuous operating temperatures for fabric filters are 250 – 260˚C; cooling of the flue gases is therefore required. The actual filter media may be a fabric cloth made of either a needle felt or a woven fiber glass. Both fabrics could be equipped with an expanded polytetrafluorethylene (ePTFE) membrane material. Due to the very small pore size of the membrane (1 – 2 μm), lower emission rates of about 3 – 5 mg/Nm3 can be achieved. The crystallite melting point of PTFE material is 327˚C and a potential active continuous service temperature of 288˚C seems possible. However, practical continuous filtration operating temperatures are between a maximum of 250 – 260˚C. In order to protect the fabric filtration media, valuable heat energy has to be wasted due to the cooling of the flue gas. In many cases, where cooling is done by air, about 30 – 50% of the air going through a fabric filter baghouse is the air required for cooling the flue gas to a desired temperature.

Cooling of flue gas can be avoided, if the filter medium can withstand higher temperatures, presenting a number of opportunities:

- The volume of air can be reduced, which saves electricity costs on the fan motor.
- Increased production capacity may become possible without having to scale up the ID fan capacity.
- The clean gases are higher in temperature and therefore do not need to be heated for potential SCR NOX reduction treatment. This will save on fuel consumption and therefore on cost.
- The thermal energy from the clean hot gases can be reused as thermal energy for drying raw material or coal. Those clean hot gases could also be used to generate electricity.


Rigid filter elements
Well-established in glass manufacturing, but new to the cement industry, are rigid filter elements. BWF Envirotec’s tradename for this product is Pyrotex® KE. Very low dust emissions of less than 1 mg/Nm3 are achievable. These filter elements are made out of calcium-magnesium-silicate fibres, which are non-carcinogenic and bio-soluble. Those fibres are safe for human health.
The materials used for the manufacture of Pyrotex® KE filter elements can withstand high temperatures, making the elements thermally stable up to a continuous operating temperature of 850˚C. No cooling of flue gases is required and no thermal heat energy will be wasted. About 1 MJ heat energy could be saved per 10 t of clinker when replacing an ESP kiln filter with a Pyrotex® KE filter.
The BWF Envirotec Pyrotex® KE filter elements are of a low density, which manifests itself in a relatively lightweight construction. Due to this low‑-density construction, the air permeability of Pyrotex® KE filter elements is similar to that of fibre glass with membrane material. A low differential pressure goes along with the high air permeability.
The first successful commercial installation of Pyrotex® KE elements in a clinker cooler filter, with operating performance far exceeding the design parameters, especially a highly favourable differential pressure, has paved the way for more to come. In this application, the cleaning pressure is about 2.0 – 2.5 bar with a resulting differential pressure over the entire filter of 10 – 12 mbar. Due to its excellent air permeability, the plant is experiencing only one full cleaning cycle per day. Furthermore, significant savings in compressed air can be achieved and an extended life of the Pyrotex® KE elements can be expected.
The Pyrotex® KE elements are available in 60 and 150 mm dia. The top collar design can be built as V or T-shape. Traditionally, the V-shaped collar has been established in the glass industry. However, more and more T-shaped designs are being requested. This will make a replacement of regular filter bags much easier. The elements are available in various lengths, with the longest element available as a single 4.5 m piece. All elements longer than 4.5 m are designed modularly and put together onsite. 8 m elements are in the testing phase, while 6 m elements are already available for commercial installations.
In addition to a dust control performance of ≤1 mg/Nm3, the filter elements can be equipped with a catalytic converter material for SCR NOX abatement. When compared to a traditional SNCR, SCR NOX reduction can take place at a lower temperature of 200 – 450˚C, due to the use of the catalyst.
BWF Envirotec offers four different catalysts that will work within that temperature window. When selecting a specific catalyst, the characteristics of the process gases will have to be reviewed and considered.
The Pyrotex® KE elements provide a single solution for dust emissions below 1 mg/Nm3. Combined with a catalytic converter, Pyrotex® KE elements can also take care of other gaseous emissions, especially NOX. Due to the ability of operating at elevated temperatures, even SOX reduction with calcium hydroxide (Ca(OH)2) can be optimised. The optimum temperature for SOX reduction with Ca(OH)2 is 350˚C, which is above the temperature that fabric filter media can handle; Pyrotex® KE elements, however, can. Since dust collection and gaseous emissions control steps can now be handled with a single dust control unit, the investment and operating cost for this type of filter will reach a new economical efficiency.
It is BWF Envirotec’s experience that each process requires a tailored solution, especially when it comes to NOX reduction with SCR catalytic systems. The savings in heat energy will also be reviewed on an individual basis. In some cases, it is beneficial to eliminate the cooling of the flue gases as the clean, hot gases are often used for material drying. When applying the Pyrotex® KE technology to hot cement clinker filtration, the clean, hot gases are used as combustion air for a different thermal process. An OEM specialising in cogeneration of electricity in a cement plant claims that the most economical approach doing this will start with a kiln size of at least 5000 tpd of clinker.
It seems obvious that with more and more stringent emission requirements different new technologies and solutions will enter the market.

About BWF Envirotec:
BWF Envirotec is the international market leader in the field of filter media for industrial filtration. The product portfolio ranges from needlona® brand filter media, including the PM-Tec® membrane product line to Pyrotex® KE ceramic filter elements. BWF Envirotec filter media find applications in the aluminium, cement and steel industry, in power plants and waste incineration plants, among others, or also in the food industry.

BWF Envirotec was the first manufacturer in the world to introduce needle felts as a filter medium in 1968. With its head office in Offingen/Bavaria and production plants in Germany, China, the USA, Italy, Turkey, Russia, India and Austria, in addition to a sales network in more than 50 other countries, BWF Envirotec guarantees economically viable solution concepts and technical services in line with the market.

BWF Envirotec is part of the BWF Group. With more than 1400 employees throughout the world, the company operates in the field of industrial dedusting, production of technical felt materials, wool felts and in plastics technology.