Removal of NOx after Biomass Combustion

While in coal- and gas-fired power plants, technology for NOx removal is mature, this is not the case for biomass fired power plants. This use of biomass has increased strongly over the latest decade and is expected to increase even further in the near future. The thesis investigates several selective catalytic reduction (SCR) solutions of relevance to NOx reduction after biomass combustion. These solutions are also relevant in relation to flue gases from waste incineration.

At coal- and gas-fired power plants NOx is usually removed by SCR using a vanadia-tungstatinania (VWT) catalyst and ammonia (NH3) as reductant. However, flue gas from both biomass fired power generation and from waste incineration contains large amounts of potassium, which deactivates the VWT catalyst rapidly. This is known as alkali poisoning.

Commonly, the catalyst is placed at the so-called tail-end position, just before the stack. The advantage here is that the flue gas is very clean, so the catalyst has a long life-time. However, tail-end placement usually requires costly reheating of the flue gas. Thus, there is a need for a different type of catalyst, which is either able to sustain the potassium levels, or can function at the lower tail-end temperatures without need for flue gas heating.

For the first approach, a catalytic process with hydrocarbons as reductants was developed, but proved unable to sustain potassium better than the traditional VWT catalyst based process.

For the second approach, a number of catalysts were tested, and especially one of these proved able to function at significantly lower temperatures than the best commercial VWT catalysts. As a patent application has been filed on this catalyst, only limited information can be disclosed.

The catalyst under patenting is a binary, supported transition metal catalyst. At 10 % water concentration the catalyst performs at the same level at 150 °C as does the commercial VWT at 220 °C. However, data from biomass combustion practice show that water content may actually come as high as 20 % at which strong inhibition occurs at 150 °C, which indicates that it will be needed to operate the catalyst at 180° C. If used at low temperature and high water concentrations, the catalyst needs to be made more hydrophobic by e.g. coating with polymers. Also, the catalyst can probably only be used in SO2 free applications.

Furthermore, the catalyst might have potential for removal of volatile organic compounds (VOC) due to its high chemi-sorbed surface oxygen.