Primary measures are aimed at preventing the formation of NOx by making technical improvements to the combustion process at the point of creation, by influencing the key mechanisms of formation, the oxygen supply, flame and/or combustion temperature, and the flue gas dwell time at high temperatures. A significant reduction in NOx emissions has already been achieved in this way. Investigations currently in progress are establishing how, and to what extent, additional measures aimed at improving the flow of air, for example, will be able to achieve an even further reduction in the future.

NOx levels can also be reduced via secondary measures, i.e. by adding ammonia or urea to the flue gas. Various technologies are used depending on the fuel and flue gas composition, and also the combustion method:

• Selective non-catalytic reduction (SNCR) and
• Selective catalytic reduction (SCR)

In particular, RWE is investigating whether nitrogen oxide emissions in flue gas from lignite-fired power stations can be reduced further though the use of DeNOx catalysts (SCR). The technical, financial and environmental constraints on this method, however, impose limits on its application.

Lignite contains sand, which is not altered by the combustion process and has a similar action in the flue gas to the grit in a sandblaster. This could damage or even destroy catalysts through abrasion in a relatively short space of time. In addition, sodium (a component of table salt), which is contained in the coal in very small volumes, can react with the catalyst material and impede its function. For the appropriate tests, RWE has constructed a 30-metre high stand to test full-scale catalysts from various manufacturers and other applications under defined conditions for their effectiveness in reducing NOx, catalyst deactivation caused by sodium, and potential abrasion.

As part of a trial programme, flue gas was fed through three one-tonne catalyst modules one after the other. During the testing activities, ammonia was added at the upper end of the test stand, and this reacted with the nitrogen oxides at the catalysts, converting these oxides into harmless nitrogen and water vapour. A dedicated fan and a heat exchanger control the flow speed and temperature of the flue gas.

As expected, the test results from the DeNOx catalyst test stand to date confirm the fears of extensive abrasion and progressive deactivation, and thus demonstrate – despite a good initial reduction in NOx – the limitations on the use of catalysts in lignite power stations.