Emission of Toxic HCN During NOx Removal by Ammonia SCR in the Exhaust of Lean-Burn Natural Gas Engines.

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

doi:10.1002/anie.202003670

Deniz Zengel, Jan-Dierk Grunwaldt + Show 4 more

Open Access

https://doi.org/10.1002/anie.202003670

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Abstract

Reducing greenhouse gas and pollutant emissions is one of the most stringent priorities of our society to minimize their dramatic effects on health and environment. Natural gas (NG) engines, in particular at lean conditions, emit less CO2 in comparison to combustion engines operated with liquid fuels but NG engines still require emission control devices for NOx removal. Using state‐of‐the‐art technologies for selective catalytic reduction (SCR) of NOx with NH3, we evaluated the interplay of the reducing agent NH3 and formaldehyde, which is always present in the exhaust of NG engines. Our results show that a significant amount of highly toxic hydrogen cyanide (HCN) is formed. All catalysts tested partially convert formaldehyde to HCOOH and CO. Additionally, they form secondary emissions of HCN due to catalytic reactions of formaldehyde and its oxidation intermediates with NH3. With the present components of the exhaust gas aftertreatment system the HCN emissions are not efficiently converted to non‐polluting gases. The development of more advanced catalyst formulations with improved oxidation activity is mandatory to solve this novel critical issue.

Highlights

Reducing greenhouse gas and pollutant emissions is one of the most stringent priorities of our society to minimize their dramatic effects on health and environment
In contrast to diesel and gasoline powered engines, the combustion process of methane is almost free of particulate matter (PM) emissions due to the absence of long hydrocarbon chains in the fuel, which is regarded as a positive aspect for decreasing local air pollution
When evaluating the impact of formaldehyde presence on the nitrogen oxides (NOx) removal performance of a series of conventionally applied selective catalytic reduction (SCR) catalysts for the exhaust aftertreatment of leanburn Natural gas (NG) engines, we identified the formation of the highly toxic hydrogen cyanide (HCN) over the catalyst bed during the NH3-SCR process

Summary

Introduction

Reducing greenhouse gas and pollutant emissions is one of the most stringent priorities of our society to minimize their dramatic effects on health and environment. Since under standard SCR conditions no gas phase reactions leading to hydrogen cyanide could be observed during empty reactor tests (Figure S3), the HCN production obviously is a consequence of HCHO reactions on the SCR catalyst.

Results

Conclusion