Overview

The selective catalytic reduction (SCR) process is based on the reaction of ammonia or urea with nitrogen oxides (NOx) on a catalyst to remove NOx from flue gas. Ammonia water is injected into the upstream flue gas duct nozzles. The nitrogen oxides present in the flue gas are converted into nitrogen and water vapour at the catalytic surface. The catalyst itself with its internal catalytic modules is a maintenance-free black box.

The versatility of the SCR process reflects the fact that the catalytic reactor can either be installed in a high-dust arrangement downstream boiler (front end) or a low-dust (tail end) arrangement between the initial flue gas cleaning system (dedusting and desulphurisation) and the stack - either for a temperature range of 240° - 300°C (high temperature) or 180° - 200°C (low temperature). Low temperature requires a periodic reactivation of the catalyst surfaces by heating the gases to 320°C with an extreme burner.

Ammonia water, pure ammonia, liquid or solid urea can be utilised as reactants, depending on the technical, economical and safety requirements.

Application

Wherever low NOx-emissions without the possibility of ammonia-slip removal is requested, the SCR-technology has to be applied.

The positioning of the DeNOx/SCR system within the flue gas cleaning process depends on the energy-recovery concept of the EfW-plant and on the general required flue gas treatment concept.

The tail end or low-dust configuration has proved to be highly effective for use at EfW 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 costs and operating costs. The low-dust arrangement requires reheating of the flue gas upstream of the SCR inlet by the heat-exchanger and/or boiler.

Maximum reduction of nitrogen oxides contaminants: 70 to 95%

Example of Tail End Catalyst

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 highspecific surface
• Combined reduction of dioxins and furans or CO
• Re-heating necessary