HCN formation during NOx removal by NH3-SCR in the exhaust of natural gas engines

Deniz  Zengel, Karlsruhe Institute of Technology (KIT), Institute of Chemical Technology and Polymer Chemistry (ITCP)

Deniz Zengel, Pirmin Koch, Bentolhoda Torkashvand, Jan-Dierk Grunwaldt, Maria Casapu, Olaf Deutschmann

 

Due to global warming, the search for alternative fuels that produce less greenhouse gas has been intense over the last decades. One promising candidate to reduce CO₂ emissions significantly is natural gas due to its high H/C ratio. Especially lean natural gas engines emit, due to their high efficiency, less CO₂ compared to combustion engines operated with liquid fuels. Although natural gas has good combustion properties and can be combusted with almost no formation of soot, additional catalysts are necessary to reduce emissions of unconverted CH₄, NO_x and by-products from partial oxidation. With respect to the NO_x abatement, the selective catalytic reduction (SCR) with ammonia (NH₃) is currently the most efficient aftertreatment technology. However, the SCR performance is highly sensitive to the gas composition and further toxic byproducts can be formed [1]. For natural gas engines, the most problematic by-product of incomplete methane combustion is formaldehyde (HCHO), which is known to be carcinogenic and mutagenic. Since complete suppression of HCHO is  difficult to achieve due to diffusion limitations to the catalyst surface [2], uncovering the influence of HCHO on the NH₃-SCR activity of the NO_x removal system is highly important.

Here we report a systematic investigation on the influence of HCHO on common SCR catalysts: Fe-ZSM-5, Fe-BEA, V₂O₅-WO₃/TiO₂ and Cu-SSZ-13. The catalysts were prepared and coated on cordierite honeycombs, which were tested with model gas mixtures consisting of 0/350 ppm NO, 0/350 ppm NH₃, 0/80 ppm HCHO, 10% O₂ and 12% H₂O in N₂ and a GHSV of 100,000 h^-1 in a plug-flow test bench between 150-550 °C. The resulting gas composition was characterized on-line by an FTIR spectrometer. In general, the catalytic activity tests of all SCR catalysts showed that HCHO conversion increases and NO_x conversion decreases when HCHO is present in the SCR reaction mixture. At the same time, an overconsumption of NH₃ was observed (about 10% higher compared to NO_x conversion), which was found to react with formaldehyde and lead to the emission of HCN. Of particular concern is the observation that more than 50% of HCHO may be converted to HCN [3]. Additional tests with gas mixtures containing only NH₃ or NO and formaldehyde showed that HCN is only formed in the presence of NH₃. DRIFTS further elucidated a mechanism involving formic acid and formamide as intermediates, the latter subsequently decomposes to HCN and H₂O. Significant emissions of HCN could be observed for all four tested catalysts over a broad temperature range, with Cu-SSZ-13 as the only exception at high temperature. Hence, by testing a variety of different catalysts, we could show that the formation of HCN emissions in presence of small amounts of formaldehyde is a serious issue for all typical SCR systems. Since HCN is a very toxic compound, it is of great importance to comprehensively understand its formation and to identify catalytic solutions to minimize such emissions.

 

References

[1] L. Zheng, M. Casapu, M. Stehle, O. Deutschmann, J.-D. Grunwaldt, Selective Catalytic Reduction of NO_x with Ammonia and Hydrocarbon Oxidation Over V₂O₅–MoO₃/TiO₂ and V₂O₅–WO₃/TiO₂ SCR Catalysts, Top. Catal., 62 (2019) 129-139.

[2] B. Torkashvand, L. Maier, M. Hettel, T. Schedlbauer, J.-D. Grunwaldt, O. Deutschmann, On the challenges and constrains of ultra-low emission limits: Formaldehyde oxidation in catalytic sinusoidal-shaped channels, Chem. Eng. Sci., 195 (2019) 841-850.

[3] D. Zengel, P. Koch, B. Torkashvand, J.-D. Grunwaldt, M. Casapu, O. Deutschmann, Emission of toxic HCN during NO_x removal by ammonia SCR in the exhaust of lean‐burn natural gas engines, Angew. Chem., (2020). DOI: https://doi.org/10.1002/anie.202003670.