Tail-end solutions for efficient NOx abatement in biomass and waste incineration plants have been developed.
The share of biomass and waste incineration in our energy supply is growing steadily. This is a key factor in the ongoing transition away from fossil fuels. However, these feedstocks come with new challenges. The project presents strategies for post-combustion removal of nitrogen oxides (NOx) which is a health and environmental concern.
Formation of NOx is inevitable during high temperature combustion processes in air. NOx emissions cause acid rain, contribute to smog formation, induce respiratory diseases, and contribute to depletion of the ozone layer. Therefore, catalytic removal of NOx is installed in modern power plants. The deNOx unit is usually placed in a very exposed position as the first emission control unit. However, this is not ideal when biomass and municipal waste are used, as these feedstocks contain levels of catalyst poisons such as potassium and hydrochloric acid. This problem must be solved, as it hinders a more widespread use of alternative fuels.
The project approach is to aim for a tail-end de-NOx unit. At tail-end, the level of potential catalyst poisons is expected to be insignificant, because the gas has passed through a wet scrubber. Furthermore, a tail-end unit is typically more easily installed in existing power plants.
The focus is on ionic liquids (ILs), which have been shown as possible effective and selective absorbers. Due to the high viscosity of ILs it is feasible to disperse them onto a porous support in so-called supported ionic liquid phase (SILP) materials. The ILs facilitate oxidation of NOx to nitric acid, HNO3, by oxygen and water present in the exhaust. HNO3 is retained in the IL until it is released by heating the IL above 130 °C.
Dimensioning of an absorber unit for retrofitting into a cargo ship was performed. It was found, that in order for the SILP absorbers to become viable for marine applications, the absorption capacity had to be increased by a factor of 30-50.
Further, hollow-sphere silica (HS) was applied for the purpose. SILP formulations utilizing the HS-support material performed significantly better than other SILP formulations. An improvement with a factor of 4-6 was found when the SILP materials were compared volumetrically. At this level of efficiency, NOx absorption and oxidation may be applied for specialized purposes like nitric acid production. Presently, however, for large scale application in flue gas cleaning the absorption strategy would need improvement by at least a factor of 10.
Another highly promising approach for tail-end deNOx is to utilize the efficient oxidation of NO over the IL based material to higher nitrogen oxides and acids (NO2, HNO2, HNO3) to boost the rate of known deNOx catalyst technology. This effect can be sustained in steady state at low temperature and at significantly higher volumetric rates than those found in the literature. Also, unlike other low-temperature oxidation methods, this method is not affected negatively by the presence of water in the flue gas. Thus, the requirements needed for large scale installations are within reach.
In conclusion, the results are very encouraging. Tail-end NOx abatement seems more attractive than ever. This will allow for retrofitting of deNOx units in existing plants and consequently for much more efficient NOx abatement in biomass and waste incineration plants. 
Illustration:
Proposed catalytic cycle for the absorption and oxidation of NO to HNO3 by [BMIM][NO3]. Blue species are reactants, red species are the products, and black species are intermediates.