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.
Does CemCat supply only high-dust systems?
CemCat has a preference for high-dust applications. The breakthrough for this concept was achieved with a reliable and efficient dust cleaning process running in several industrial plants.
The semi-dust concept with a hot temperature ESP upstream of the catalyst is also available. 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 specialised cases a 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. thyssenkrupp Industrial Solutions’ know-how in clinker coolers is a solid basis for this tail-end concept.
What are the most important points for SCR in cement?
The key points are:
- 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.
- Operating costs: These are split into reagent consumption (ammonia or urea), power consumption and costs for catalysts.
- Catalyst lifetime and catalyst deactivation: A larger number of known catalyst poisons from other industries also play a role in cement, like alkalis, phosphor, arsenic and chromium. Furthermore, thallium is a strong catalyst poison present in cement flue gas and kiln dust more than in other applications.
CemCat has made progress in all these fields and has adapted general SCR technology to a tailor-made cement SCR.
- CemCat’s dust cleaning has proven itself in the most demanding operating conditions of temperatures up to 410°C, up to nearly 100% alternative fuels and high water concentrations in the flue gas.
- The cost of the reagent is dominant. SCR consumption is much lower compared to SNCR, since SCR runs close to 1 ppm NOx per 1 ppm NH₃ where the molar ratio is equal to the 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 could be significantly reduced. The smallest contributor to operating costs is catalyst costs.
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 270-370°C. The active component, which is V₂O₅ has an oxidising influence on VOCs, including ethane, SO₂, dioxins and furans and also elemental mercury.
Methane is not oxidised, 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 significantly reduced by similar rates. Dioxins and furans have the same behaviour, because they are chlorinated VOCs. The sizing of the catalyst determines the oxidation rate.
If sufficient halides, i.e. in cement primarily HCl , are 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.
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.