VA STAHL/Linz/Austria
Selective Catalytic Reduction (SCR)
Process
The selective catalytic reduction (SCR) process is a reaction of ammonia with nitrogen oxides on a catalyst. The nitrogen oxides present in the flue gas are converted into nitrogen and water vapour.
The versatility of the SCR process reflects the fact that the catalytic reactor can either be installed in a high-dust arrangement between an economiser and an air preheater, or a low-dust (tailend) arrangement between the initial flue gas cleaning system (dedusting and desulphurisation) and the stack. Ammonia water, pure ammonia, liquid or solid urea can be utilised as reagents, depending on the technical, economic and safety requirements.
Applications
The positioning of the DeNOx/SCR system within the flue gas cleaning process depends on the upstream plant and the fuel involved. At power stations (coal, gas or oil-fired), the SCR reactor is usually installed in a high-dust arrangement where particular attention must be paid to avoidance of ash deposits on the catalyst. AE&E’s proprietary ash screens prevent deposition of large ash particles.
The tail-end or low-dust configuration has proved highly effective for use at Energy-from-Waste or biomass-fired plants. In this configuration, the catalytic converter is only exposed to minimum concentrations of catalyst poisons, leading to a longer catalyst life.
Key factors in decisions between the two potential configurations are overall plant investment cost and operating cost. The low-dust arrangement requires reheating of the flue gas upstream of the SCR inlet, which adds to cost.
Customisation is aided by the fact that AE&E has a large database containing information on the catalyst lifetimes of a variety of fuels. This enables us to minimise the catalyst volume for every application.
Key features high-dust configuration
- Reduced catalyst lifetime
- Clogging or build-up of deposits possible due to high dust content
- Lower space requirement
- Potential use as a hybrid catalyst after SNCR
- Lower invest and operating cost
- No flue gas reheating necessary
Key features low-dust configuration
- Higher catalyst lifetime
- No clogging or build-up of deposits
- Lower catalyst volume due to high specific surface
- Combined reduction of dioxins and furans or CO
- Re-heating necessary