Use of Biomass as a Sustainable and Green Fuel with Alkali-Resistant DeNOx Catalysts based on Sulfated or Tungstated Zirconia

Use of biomass as an alternative to fossil fuels has achieved increasing interest since it does not contribute to CO2 accumulation in the atmosphere. Over the past 10-15 years, heat and electricity production from biomass has increased to almost 7% of all energy supply in the European Union and is expected to increase further.

The by far most efficient use of solid bio-resources in energy production is combustion in combined biomass and coal or oil-fired power plants. However, in such applications nitrogen oxides are inevitably present in the flue gases. Selective catalytic reduction (SCR) of NO with ammonia as reductant is the most common method to eliminate NOx from flue gases in stationary sources.

Even though biofuels are considered as environmentally benign fuels, the reactions occurring inside the boilers during biomass combustion tend to be more “dirty”. Indeed, traditional V2O5-WO3-TiO2 SCR catalysts exhibit significant deactivation within relatively short time. The main reason for this deactivation is the presence of a high amount of potassium compounds (up to 2 wt.% in straw), which acts as a poison for the catalyst.

There exist different strategies to regenerate deactivated catalysts, for example, washing with dilute sulfuric acid enables complete restoration of the catalyst activity. The major disadvantage of such procedure is that the power plant has to be stopped in order to remove the deactivated catalyst. Another more efficient way to overcome the deactivation problem could be the use of alternative catalysts that are more resistant towards poisoning with potassium.

Since deactivation with alkali additives generally occurs due to strong acid-base interaction with the V-OH sites responsible for the activated ammonia adsorption, one possible way to increase catalyst resistance to alkaline poisons is the use of supports with highly acidic properties, which would interact stronger with potassium than the vanadium species.

Among those, sulfated and tungstated zirconica appears very attractive, since their surface acidity can be tuned in a wide range by varying the preparation procedure, WOX or sulfur content and calcination temperature. Besides that, zirconia-based catalysts have more than twice as high pouring density than traditional titania-supported catalysts leading to high catalyst loading in industrial facilities.

In the present work, vanadia-based catalysts supported on traditional, sulfated, and tungstated zirconia were prepared and tested. The influence of potassium additives on the acidity and activity was studied and the results were compared with traditional V2O5-WO3/TiO2 catalyst. Resistance of the catalysts towards poisoning with potassium was found to depend dramatically on the crystallinity and surface acidity of the support used.

Better resistance of the samples based on sulfated and tungstated zirconia seems to be connected with the fact that a significant part of the potassium on the surface of the catalyst preferentially interact with strong acid sites of the support thus preventing vanadium species from deactivation and leaving them available for the catalytic cycle.