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September 29, 2017 Pietro Aresta

FLSmidth Airtech examines catalytic filtration as an adaptable tool for solving specific emissions challenges.

This is a defining year for catalytic filtration technologies in the cement industry. Cement producers have been looking forward to this technology with great interest, mainly driven by ever-stricter regulations on gaseous hazardous pollutants.

The cement industry in Germany is just one example of a region soon to face stricter NOX and NH3 emission limits. In January 2019, plants will be required to fall within daily averages of 200 mg/Nm3 (dry,10% O2) of NOX. Short-period peaks will be tolerated, but 30 min. average values may never exceed 400 mg/Nm3 (dry, 10% O2). For ammonia (NH3), limits will be 30 and 60 mg/Nm3 (dry, 10% O2) respectively.

Conversion rates vs. NH3 to NOX ratio in full-scale bypass CFF.

Importantly, ammonia emission limits will not be considered an addition to the baseline, which consists of the ammonia naturally released by the raw materials and alternative fuels (AFs) in the preheater tower. The limits will, in fact, apply to the ammonia concentration in the stack. This has a dramatic consequence for many plants, where the use of certain AFs, or the chemistry of the raw materials from the quarry, can give very high baseline level values.

CAPEX + 5 years OPEX comparison of different technologies.

Unfortunately, this problem cannot be solved simply by optimising the selective non-catalytic reduction (SNCR) with high-efficiency techniques because ammonia is released downstream, at higher stages of the preheater tower. On the other hand, one proven, highly efficient means to achieve this is with catalytic filtration. FLSmidth has chosen Haldor Topsøe, already active in
the industrial deNOX market, as its catalysis science partner for this development.

The FLSmidth CataFlex® catalytic filter bags consist of up to three layers of woven fiberglass. Each layer has a catalytic material embedded in the fabric with a formula that promotes chemical reactions to remove pollutants and convert them into harmless species, such as water, nitrogen, or carbon dioxide, together with dust, in a one-step process. This technology can be used for high‑efficiency removal of compounds, including NOX, NH3, volatile organic
compounds (VOCs) or, with a special formulation, even carbon monoxide. A multilayer concept allows a customised solution and, to achieve even lower emission rates, the solution is ready to be upgraded without the need for a technology shift, making the solution more cost effective.
This solution also allows the partial separation of the deNOX reaction from the SNCR downstream in the catalytic fabric filter (CFF). This reduces ammonia consumption, not only because ammonia from the raw material is used, but also because of the catalytic reaction’s higher efficiency (in terms of stoichiometric NOX to NH3 ratio) compared with the non-catalytic reaction.

Catalytic filtration can be implemented in different solutions at a cement plant to achieve NOX reduction targets. Depending on the efficiency of SNCR, the ammonia baseline, and the plant configuration, catalytic filtration can enable the most effective CAPEX and OPEX solution in two main ways, as discussed below.

Converting the kiln or raw mill fabric filter into a catalytic fabric filter
This solution involves replacing ordinary bags with catalytic filter bags and does not require major plant layout modifications. It can be used as a ‘NH3 slip killer’ when the SNCR system reduces NOX emissions to below, or slightly above, the necessary limits. In such cases, the catalytic filter bags reduce NH3 emissions to below the required limits, also removing approximately the same molar quantity of NOX.

A recent installation of a single layer catalytic at a German cement plant has yielded positive results, where the mechanical filtration process (differential pressure and dust emissions) and the catalytic performances (NOX, NH3 removal rates) have been outstanding from day one. This solution, combined with an existing SNCR, optimised the plant’s overall ammonia usage. Promoting catalytic conversion over SNCR can lead to OPEX benefits: it is estimated that savings in the range of d0.5/t clinker can be achieved by using the NH3 released by the raw material, as well as introducing a more efficient (catalytic) NOX conversion method.

If the current temperature level is not sufficient to allow a catalytic conversion process, as it can happen if the requirement is a full deNOX solution instead of a ‘NH3 slip killer’ solution, there are several means to achieve this by using simple, more equipment-heavy solutions. When the available heat in the system reaches the temperatures required by the catalytic reaction, modification only requires the replacement of the bags and adjustment of the heat balance. Such cases can be implemented very easily; for example, the above mentioned fabric filter required only three days to be retrofitted, mainly due to the timing of the bag replacement.

Gases from the bypass do not pass through the preheater tower’s SNCR-active zones, so they must be dealt with separately, if they contribute significantly to the combined NOX stack emission. In such cases, converting the bypass filter (ESP or FF) into a catalytic fabric filter, together with the SNCR for the preheater/calciner gases, can be a very cost-effective solution.

The bypass system offers advantages that make this a viable, relevant solution: it already has a cooling system, delivers stable flow and enables gases to be cooled to the optimum temperatures for catalytic filtration. Design precautions should, however, be taken, as high SO2 levels could make this solution more difficult to implement. This is because the combination
of ammonia injection and excessive SO2 concentration in the gas may lead to the formation of ammonium bisulphate (ABS), which reduces, and in the worst casesprevents, the removal of catalytic NOX.

A catalytic fabric filter (CFF) conversion of a bypass fabric filter with two-layer FLSmidth CataFlex® catalytic filter bags was commissioned at another German cement plant in October 2015. In this case, ammonia was not injected into the bypass continuously, but several tests were performed over different time periods. These tests have shown good conversion rates, with mechanical properties in line with the highest fabric filter quality standards. Satisfactory deNOX conversion rates were obtained at two different temperature levels (220˚C and 240˚C) with two-layer catalytic bags. Conversions above 80% were easily achieved, keeping ammonia slip below 10 mg/Nm3 dry at 10% O2 level (Figure 1).

A cost-effective solution
For plant owners who need to deal with environmental regulation emission limits, the process unit layout is likely to become more complicated, since different units are required to handle specific pollutants. It is quite possible that each unit would need to be based on a different technology to handle the various process conditions, such as large temperature ranges. The system may also require interlocks between units, and more instrumentation and controls. This greater complexity is also likely to reduce total plant availability, because of the need to rely on the availability of individual process devices.

These factors, combined with additional maintenance and spare parts storage requirements, have a significant cost implication on cement plant operations. Converting an air pollution control system into a catalytic filtration system is a cost-effective way of meeting these challenges. In addition to relatively fast and simple installation, many plant operators are
familiar with filtration technology, which reduces the need for training.

Cost comparisons
Figure 2 compares five different cost scenarios: the first three show the different applications of CataFlex® in a cement kiln, respectively, as a NH3 slip removal system, bypass retrofit, and full deNOx system, compared with high and low dust SCR. Even though a complete analysis is difficult, due to significant differences in the plants’ layout and emission requirements, CataFlex® represents an attractive solution from both a technical and commercial point of view.

Catalytic filtration technology comes of age
As a multilayer catalytic filter bag solution, the system can be tailored to process requirements, delivering optimised OPEX and CAPEX investments. This enables effective asset management planning to cope with future emissions requirements. A pre-existing one-layer CataFlex® bag system can, for example, be quickly and easily modified into a two-layer system, which can help reduce OPEX costs.

Although catalytic filtration may be seen as new to the cement production industry, it is already a mature technology, with several successful installations demonstrating highly satisfactory performance. It is important to see catalytic filtration not just as equipment, but as a solution for solving specific emissions challenges, encompassing flexible technology that is easily adaptable to different plants and process scenarios.

* This article was originally published in World Cement, June 2017.

Pietro Aresta

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