The Selective Reduction of NOx to harmless Nitrogen (N2) and Water (H2O) using Ammonia (NH3) or Urea injection is a convenient and proven technique.
If combined at temperatures around 1000°C, the reaction is fast enough not to require a catalyst to speed up the action. This is called Selective Non-Catalytic Reduction (SNCR) and the predominant reaction is:
4NO + 4NH3+ O2 => 4N2+ 6H2O
At lower temperatures (typically <420°C), a catalyst must be used to speed up the action. This is called Selective Catalytic Reduction (SCR) and the predominant reactions are: 4NO+4NH3+O2 – 4N2+6H2O and 6NO2 +8NH3 – 7N2+12H2O
Continuous analysis of NOx pre-reduction is often required to measure the concentration generated by the combustion process. This may be required to provide feed-forward to the reductant injection system and also to allow the effectiveness of the reduction process to be assessed.
Continuous analysis of NOx post-reduction is required to allow the effectiveness of the reduction process to be assessed. It may also be beneficial to measure NH3 slippage at this stage.
Where sulphur-bearing fuels are used, other unfortunate reactions make conventional continuous gas analysis very difficult. Below 300°C, Ammonia and SO3 react to form solids in the form of Ammonium Bisulphate (ABS) and Ammonium Sulphate salts as follows:
NH3 + SO3 + H2O – (NH4)HSO4 and 2NH3 + SO3 + H2O – (NH4)2SO4
When a catalyst is used (SCR), this salt deposition can be accelerated as many catalysts encourage the formation of SO3 as follows:
2SO2 + O2 – 2SO3
It is important, therefore, that process gases are maintained at temperatures higher than the Ammonium Bisulphate salt deposition phase to prevent clogging of the catalyst.
As measurements in both SNCR and SCR must be made at these elevated temperatures, and often before any significant solids have been removed from the gas stream, the major problems for reliable continuous gas analysis are:
• Long-term continuous removal of high levels of solids from the sample prior to analysis
• Long-term continuous gas conditioning prior to analysis without corruption
• Long-term continuous removal of the ABS salts prior to analysis
All of these issues are resolved using the CODEL GCEM-40E analyser where a sample is continuously extracted from the duct via a multiple-filtered probe with pre-shield and automatic reverse blow-back system.
The sampled gas is maintained at ~180°C (well above any condensate dew-point) and transported for hot gas analysis via heated sample lines. In-line salt deposition chambers trap significant ABS salts before they can enter the analyser.
It is worth noting that conventional extractive systems require the sampled gas to be cleaned and dried to a very high standard prior to analysis invariably resulting in an extremely high maintenance demand. Such elaborate pre-conditioning is not required by the CODEL GCEM-40E analyser ensuring maximum availability even under these arduous conditions.