Secondary controls post combustion techniques

4.6.1 Selective Catalytic Reduction (SCR)

SCR controls emissions of nitrogen oxides within the waste gases of boilers through catalytic reduction reactions. Ammonia vapour is injected into the flue gas stream at the optimum temperature range for reaction, typically between 300 - 400 °C (normally the temperature at the economiser outlet). The gases are then passed over a catalyst promoting a reaction between the NOx of the flue gas and the ammonia producing nitrogen and water vapour. The type of catalyst required is highly dependent on the exhaust conditions of the boiler and the constituents of the flue gas.

SCR options include "high dust" where the catalyst is positioned before the precipitators at higher gas temperatures and "tail end" where the installation takes flue gas after the dust has been removed but at lower temperatures. Investment and process costs differ significantly between the two positions.

Retrofitting SCR to existing UK plants is seen to be problematical and optimum extraction rates for this technology are unlikely to be attained. Each site is different but all are space restricted, limiting the size of catalyst structure and requiring ductwork to be extended outside the main building. In addition existing pressure and temperatures of gas flows and the ductwork arrangements on site will differ from those specified on an integrated "new build" site.

SCR remains unproven under flexible plant operation due to its requirement for a narrow catalyst operating temperature window and there is a reluctance to apply SCR to mid-merit or peaking plants. Although coal plants have seen a recent resurgence in production volumes, SCR is prone to greater uncertainty within the UK market where coal plants have generally been considered to be more marginal, operating mid-merit and at the peaks.

SNCR selectively reduces NOx by injection of ammonia or urea as a reagent into the boiler. The product of this reaction is molecular nitrogen, carbon dioxide and water. The optimum temperature range for the reaction is quite specific, although broader than for SCR, being between 900 and 1100ºC. Above this temperature range the NOxreduction efficiency is greatly reduced and below the minimum temperature excessive "ammonia slip" occurs.

The EA (2011) suggested that combustion optimisation coupled with SNCR appeared more attractive than an SCR retrofit on the basis of cost, although it is not clear whether such a solution would achieve adequate NOx reduction.

4.6.3 Rotating Over Fire Air (ROFA)

In a ROFA system 25 to 40 per cent of furnace air is injected into the upper furnace through asymmetrically placed air nozzles. This increases turbulence and mixing in the furnace improving temperature and species distribution and particle burnout in the upper furnace. As a result the formation of laminar flow is prevented. The furnace is used more effectively in the combustion process and the maximum temperature in the combustion zone can be reduced due to increased heat transfer. As the combustion air is mixed more effectively less excess air is needed and thus the amount of NOx produced is reduced (Coombs, 2004). ROFA can be used in conjunction with SNCR or SCR.

4.6.4 Rotamix

Rotamix is an SNCR and sorbent injection system and is used in conjunction with and downstream of ROFA. By injecting pre-ROFA the system takes advantage of the highly kinetic environment created by ROFA to mix the chemical reagents with the combustion products in the furnace. The Rotamix system adapts to changes in load and temperature in the furnace, ensuring that reducing chemicals are only introduced to the furnace when the temperature is favourable for pollution reduction. This reduces chemical consumption and slippage and increases the reaction efficiency (Coombs, 2004). Rotamix is used in conjunction with SNCR and ROFA.

4.6.5 Hybrid (SNCR and SCR)

Hybrid SNCR and SCR involves the processing of the exhaust gas through SNCR followed by SCR through the processes previously described. Hybrid has benefits over SCR in that the catalyst volumes can be decreased resulting in a lower capital cost investment and reduced space requirements.

Hybrid SCR/SNCR is technically feasible but to the authors’ knowledge has not been implemented on any plant. There are a number of uncertainties associated with the hybrid approach due to its first of a kind nature, however as both technologies are applied at different points in the cycle they are essentially independent, which reduces the likelihood of any problems with applying both in sequence. There may be

technical issues to be dealt with such as unexpected impacts on flexibility or possible issues with slippage of ammonia, however as both technologies have been

successfully retrofitted to coal plant separately it is unlikely that any such problems would be serious enough to render the hybrid unworkable. SNCR has not yet been applied at full scale in coal plant. One possible problem is that contractors may be unwilling to implement a cheaper option when a more expensive option is available and is more proven, however this is a challenge that faces every new technology and can be overcome through e.g. competitive bidding processes.

In a Hybrid plant SNCR would be applied by injecting reagent into the boiler furnace, reducing the NOx concentration at the furnace outlet and SCR would be applied to the boiler flue gases by introducing a new bypass section of ductwork cantilevered outside the rear of the boiler, which contains the catalyst. This could be achieved by building an SCR reactor with only 2/3 of the catalyst required for a full-size SCR. This hybrid option would be cheaper in capital cost to a full SCR and would have minimal impact on plant operation. Alternatively the catalyst required could be installed in existing ductwork which would be cheaper than a full size SCR, making the hybrid option significantly cheaper in terms of capital cost.