Emission of Toxic HCN During NO_x Removal by Ammonia SCR in the Exhaust of Lean‐Burn Natural Gas Engines

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.

The Impact of Pressure and Hydrocarbons on NOx Abatement over Cu- and Fe-Zeolites at Pre-Turbocharger Position

“…In addition to the blockage of the surface sites, the effect of competing reactions should be considered as well. Thus, the formation of high HCN concentrations suggests a simultaneous reaction of NH 3 with HCHO, which is a propylene oxidation byproduct [36]. The enhancement of pressure also promotes the NO x conversion for Cu-SSZ-13 and, as expected, compensates the negative impact of C 3 H 6 presence.…”

“…The only notable difference was a sudden formation of significant amounts of hydrogen cyanide (29 ppm HCN at 500 • C, Table 2). Recent studies have shown that HCN can be formed by the reaction of NH 3 and HCHO over conventional SCR catalysts [36][37][38]. Since the conversion of propylene over Fe-ZSM-5 leads to a large amount of HCHO, this is most probably directly involved in HCN formation.…”

“…As the temperature rises, the negative effect increases until an activity loss of 12% is reached at 500 • C. Due to its larger pores, ZSM-5 is known to be more susceptible to hydrocarbon poisoning and coking [41,42]. Another reason for the loss of activity could be the HCN formation, which involves parasitic NH 3 consumption in a reaction with HCHO [36]. However, this negative effect appears to be compensated by elevated pressures.…”

Lean-Burn Natural Gas Engines: Challenges and Concepts for an Efficient Exhaust Gas Aftertreatment System

“…Future exhaust gas aftertreatment systems may be amended by implementing a state-of-the-art urea-based SCR system for NO x control in addition to the oxidation catalyst. This necessitates a careful choice of both, engine operation parameters for controlling engine-out emissions and catalyst formulation for minimizing secondary emissions, for instance since HCHO and NH 3 can form highly toxic HCN over several SCR catalysts [32,33]. However, as this approach is already well-established in the context of lean-burn diesel engines, the technological hurdles for extending the overall system are expected to be moderate, allowing a fast realization and transfer into serial production.…”

Au-Modified Pd catalyst exhibits improved activity and stability for NO direct decomposition

“…7,8 The formation of highly toxic HCN was even reported, formed from NH 3 and formaldehyde, molecules that are typically present in the exhaust gases of NH 3 -SCR system fitted to natural gas engines. 9 NO decomposition to N 2 and O 2 is strongly favoured below 450°C, as the equilibrium concentration of NO x (= NO + NO 2 + N 2 O) is lower than 1 ppm when starting with a reaction mixture comprising 200 ppm NO + 10% O 2 in N 2 (Fig. 1).…”