The effectiveness of a system that removes NOx  from the emission gasses of a coal-fired power plant may drop more rapidly when biomass is added to the coal, a studie of KEMA concludes. This means that the catalyst in the DeNOx system must be replaced more frequently. However, these extra investments are usually more than
compensated by the extra revenue
generated by the addition of biomass. 

It is expected that several years from now all coal-fired plants in the Netherlands will be equipped with a DeNOx system as the result of stricter legislation. In the Netherlands there are currently two such systems in operation that remove these acid-forming chemicals from emission gasses. Concurrently, limitation of CO2 emissions has a high priority on the environmental agenda. The use of biomass as a supplementary fuel is one of the means to achieve this. These two measures conflict with one another however.

Waiting
The heart of the most delicate component of the DeNOx system is the catalyst. The catalyst functions optimally at temperatures found just after the burning and before the cleansing of the emission gasses. Through the addition of biomass fuels, particles are added to the emission gasses that contaminate the catalyst. It would therefore be more desirable to wait until after the cleansing to remove the NOx. By then, however, the temperature has dropped so far that the emission gasses would have to be reheated, which is extremely expensive. 

Reduction of effectiveness
Over the course of a year-and-a-half KEMA has studied the reduction of activity in various catalysts used in the Maasvlaktecentrale coal-fired plant. This plant does not yet have a DeNOx system, but approximately 10 percent of the fuel burned consists of secondary fuels, including biomass. The results were compared with those of DeNOx systems from similar plants and then extrapolated. With 'clean' burning conditions, the activity appears to drop slowly but steadily so that the catalyst must be replaced after 5 to 10 years. With the addition of 10 percent biomass fuel, the reduction of activity is 10 percent greater, which means that replacement or reactivation is required sooner. This more rapid depreciation represents an additional expense for the electricity producer. On the other hand, reduced emission of CO2 also brings in extra money. 

In light of the current developments, this accelerated depreciation appears to be economically justifiable for businesses. The exact financial implications will be determined once the last measurements have been completed later this year.