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, NOx and by-products from partial oxidation. With respect to the NOx 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 . 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 , uncovering the influence of HCHO on the NH3-SCR activity of the NOx 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 NOx 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 NOx 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 . 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.
 L. Zheng, M. Casapu, M. Stehle, O. Deutschmann, J.-D. Grunwaldt, Selective Catalytic Reduction of NOx with Ammonia and Hydrocarbon Oxidation Over V2O5–MoO3/TiO2 and V2O5–WO3/TiO2 SCR Catalysts, Top. Catal., 62 (2019) 129-139.
 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.
 D. Zengel, P. Koch, B. Torkashvand, J.-D. Grunwaldt, M. Casapu, O. Deutschmann, Emission of toxic HCN during NOx removal by ammonia SCR in the exhaust of lean‐burn natural gas engines, Angew. Chem., (2020). DOI: https://doi.org/10.1002/anie.202003670.