Frequently asked questions

Which NOx reduction rate can be achieved with SCR?

High reduction rates can be achieved at low NH3 slip. Less than 100 mg/Nm³ (approx. 50 ppmv) are possible. Thus the reduction rate depends mainly on the NOx inlet level. A cement kiln can emit strong fluctuations in NOx. For low NOx clean gas concentrations, the control of the ammonia injection is essential to avoid overstoichiometric injection.

What is the achievable lifetime of the catalyst?

In cement applications, the longest catalyst lifetime achieved to date is a total of 4 years (3 years after the first regeneration).  This regeneration process restores deactivations caused by poisons and coatings.  After treatment, catalyst performance is brought back to almost new but at a more attractive cost.

It is not yet known how many regenerations are possible in cement applications.  In power plants, the longest catalyst lifetime was 22 years with 4 regenerations.  Experience from power plants shows an average of 3-4 regenerations.  In biomass incinerators, a maximum of 6 regenerations has already been achieved.

Cement applications require particular attention to thallium when compared to other SCR applications.  Thallium is a strong catalyst poison and needs special consideration and experience which CemCat brings to the discussion.

Does CemCat supply only high-dust systems?

CemCat has a preference for high-dust applications. The breakthrough for this concept has been achieved with a reliable and efficient dust cleaning process running in 2 industrial plants.

The semi-dust concept with a hot temperature ESP upstream of the catalyst is also available. ELEX has significant experience in the supply of this special type of ESP. Reductions in dust cleaning are offset by the ESP power consumption. In 2 pilot plants of this type, strong thallium poisoning has been observed.

Additionally, in specialized cases an SCR system downstream of the kiln baghouse may be the best solution for a small number of plants. The required reheating of the flue gas to SCR temperature can be done with a heat-shuttling system, transfering heat from a clinker cooling mid-tapping to the SCR. Polysius' know-how in clinker coolers and ELEX's references in waste incineration plants are a solid basis for this tail-end concept.

What are the most important points for SCR in cement?

The key points are

  1. Dust cleaning in high-dust and semi-dust: The dust in cement applications is much finer and more sticky than in other industries. In high-dust areas, the dust concentration is also 3-5 times higher than in power plants. It contains anhydrite and clays, which both tend to stick to the catalyst and are difficult to clean.
  2. Operating costs:  These are split into reagent consumption (ammonia or urea) and power consumption.
  3. Catalyst lifetime and catalyst deactivation: A larger number of known catalyst poisons from other industries do not play a role in cement, like alkalis, phosphorous, arsenic and chromium. They are bound to calcium or to other species and are not harmful. However, thallium is a strong catalyst poison present in cement flue gas and kiln dust more than in other applications.

CemCat has achieved progress in all these fields and has adapted general SCR technology to a tailor-made cement SCR

  1. CemCat's dust cleaning has provided reliable operation under very tough conditions at 400-410°C, up to 100% alternative fuels with high water concentrations in the flue gas.
  2. The cost of the reagent is dominant. SCR consumption is much lower compared to SNCR, since SCR is running close to 1 ppm NOx per 1 ppm NH3 where the molar ratio is equal to reduction ratio. SNCR systems running especially with high molar ratios can be costly. The second largest cost is power consumption, which is dominated by the fan and the dust cleaning. With CemCat's dust cleaning, the (additional) fan power is very low and the compressed air consumption has been reduced drastically. The smallest contributor to operating costs is catalyst costs.
  3. The experience of 5 years high-dust operation shows that the deactivation of the catalyst is moderate. A continuous operation of 3 years has been achieved before a regeneration was required. There is no rapid deactiviation in high-dust. However, in semi-dust pilot plants a rapid thallium deactivation has been observed after a few thousand hours. The high-dust content seems to provide large alternative thallium condensation surfaces and thereby protects the catalyst.
Why is SCR called Multi-Pollutant Control?

SCR reduces NOx with ammonia to nitrogen and water by shifting the normal temperature level of the reaction from approx. 800-1100°C to  170 - 420°C. The active component, which is V2O5 has an oxidizing influence on VOCs, including ethane, SO2, dioxins and furans and also elemental mercury.

Methane is not oxidized, because it is too stable. The possible reduction rate of THCs depends therefore on the methane concentration in the flue gas. The other THCs (NMTHC) can be reduced by similar rates as NOx. Dioxins and furans have the same behavior, because they are chlorinated VOCs. The sizing of the catalyst determines the oxidation rate.

If sufficient halides, i.e. in cement primarily HCl , is present, elemental Hg is converted to HgCl2 or other mercury halides. They are called oxidized species and can be adsorbed much easier than the elemental. The SCR catalyst is a performance enhancer for a downstream collection technology like a scrubber or ACI. It does not provide direct mercury removal.


What are the operating costs?

Operating costs are from ammonia consumption, power cost and catalyst cost. Reducing NOx from 1000 mg/Nm³ to 200 mg/Nm³ (both at 10% O2, dry) with a high dust system, the ammonia cost contributes approx. 60%, the power approx. 25% and catalyst costs approx. 15%. Assuming European prices for ammonia and power, this 20% lower than for an SNCR system. 

Is there a preference for urea or aqueous ammonia?

The SCR performance is the same with both reagents. In a combination of SNCR with SCR there is a preference for aqueous ammonia due to its superior performance in the SNCR part.