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

Article abstract of DOI:10.1002/anie.202003670

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 NO_x removal. Using state-of-the-art technologies for selective catalytic reduction (SCR) of NO_x 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.

Full text of DOI:10.1002/anie.202003670

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Accepted Article
Title: Emission of toxic HCN during NOx removal by ammonia SCR in
the exhaust of lean-burn natural gas engines
Authors: Deniz Zengel, Pirmin Koch, Bentolhoda Torkashvand, Jan-
Dierk Grunwaldt, Maria Casapu, and Olaf Deutschmann
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202003670
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Emission of toxic HCN during NOx removal by ammonia SCR in
the exhaust of lean-burn natural gas engines
Deniz Zengel, Pirmin Koch, Bentolhoda Torkashvand, Jan-Dierk Grunwaldt, Maria Casapu,* and Olaf
Deutschmann*
[*]
Deniz Zengel, Pirmin Koch, Dr. Bentolhoda Torkashvand, Prof. Dr. Jan-Dierk Grunwaldt, Dr. Maria Casapu, Prof. Dr. Olaf Deutschmann
Institute for Chemical Technology and Polymer Chemistry
Karlsruhe Institute of Technology
Engesserstr. 20, 76131 Karlsruhe, Germany
E-mail: maria.casapu@kit.edu (M.C.), deutschmann@kit.edu (O.D)
Supporting information (SI) for this article is given via a link at the end of the document
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 over typical NOx removal catalysts. 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.
Considering that 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.
complete combustion, CO2 and water, harmful gases such as
nitrogen oxides (NOx), carbon monoxide (CO), volatile organic
compounds (VOC), light hydrocarbons including unburnt methane
(CH4) as well as carbonyl intermediates formed during partial
oxidation of methane need to be removed.[5] CO, NOx, and
hydrocarbons (in total) belong to the group of regulated emissions.
Regulations on specific hydrocarbon species such as
formaldehyde (HCHO) and non-methane hydrocarbons (NMHC)
have started to be introduced for some applications in various
regions of the world. Emissions standards will continuously
advance and also include other combustion products that are
known for their toxicity and greenhouse impact.[6]
Depending on the air-fuel ratio, a natural gas (NG) combustion
engine can be operated under stoichiometric and lean (excess of
oxygen) conditions, with the last one showing an improved
thermal efficiency and therefore less fuel consumption. Even
though the concentration of CO, HC and especially NOx
emissions in the exhaust stream is higher for the stoichiometric
engines, the removal of all three pollutant classes can be
efficiently achieved over a conventional three-way catalyst. To
comply with the tightened NOx emission limits, the exhaust
aftertreatment system of the lean-burn NG engines requires the
application of a NOx reduction catalyst.[4] In this respect, the
selective catalytic reduction (SCR) of NOx with ammonia is
currently the most efficient aftertreatment technology, using ion-
exchanged zeolites or vanadium-based catalyst formulations. The
NOx-removal catalyst is typically exposed to a lean gas mixture
containing nitrogen oxides and small amounts of unreacted
components (methane slip) or oxidation by-products as pollutants.
Among them, formaldehyde emissions formed due to incomplete
combustion and partial oxidation of methane in the hot exhaust
stream require special consideration, as formaldehyde is known
as a potential carcinogenic compound regulated since 2014.[5b, 7]
As shown by recent studies,[8] fresh noble metal-based oxidation
catalysts are able to significantly convert formaldehyde. However,
The growing global awareness towards climate change has led to
the introduction of alternative fuels able to reduce the net
greenhouse gas emissions. In addition to fuel-ethanol blends,
liquid petroleum gas and biodiesel, natural gas has shown to be
one of the most promising candidates for reducing up to 20% the
anthropogenic CO2 emissions per produced energy unit.[1] This
benefit is due to the high H/C ratio of methane, the major
component of natural gas (up to 97%). The growing interest in
natural gas as fuel is also boosted by the possibility to produce
methane from CO2-neutral sources such as biomass and even
more important from wind and solar derived electricity by the
Power to Gas (PtG) technology, which is a combination of
electrolysis of steam and subsequent methanation.[2] In contrast
to the diesel and gasoline powered engines, the combustion
process of methane is also 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 particularly for
decreasing local air pollution. As a consequence, the number of
natural gas fueled vehicles is expected to increase,[3] as also
predicted by the energy transition trends. However, natural gas
engines still require a catalytic exhaust-gas aftertreatment
system.[4] In addition to the ultimate chemical products of
when using more complex gas mixtures[8a, 8b] or upon catalyst
9]
ageing[8a,
(i.e. SO2 poisoning or field aging) the activity
significantly decreases, particularly at low temperatures.
Furthermore, complete conversion is virtually impossible to
achieve at high gas hourly space velocity with a typical catalyst
length due to the low diffusion rate of formaldehyde from the gas
phase to the catalyst surface, especially at low concentrations.[9-
10]
1
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Figure 1. (A) Comparison of NOx and NH3 conversion over Fe-ZSM-5 during standard SCR (350 ppm NO, 350 ppm NH3, 12% H2O, 10% O2 in N2) with and without
80 ppm HCHO. (B) HCHO conversion and product selectivity towards CO, HCN, and HCOOH.
When evaluating the impact of formaldehyde presence on the
NOx removal performance of a series of conventionally applied
SCR catalysts for the exhaust aftertreatment of lean-burn NG
engines, we identified the formation of the highly toxic hydrogen
10% O2 and N2 balance. This formaldehyde concentration of
80 ppm was selected based on direct engine measurements[16]
and also to ensure a high accuracy of the measured values for
the different gaseous products of formaldehyde oxidation. More
cyanide (HCN) over the catalyst bed during the NH3-SCR process. details on the catalyst preparation and testing procedure are
It is well known that the exposure to over 300 ppm HCN in air kills
within several minutes and thirty minutes exposure to 135 ppm
HCN in air can be lethal.[11] Up to now, HCN emissions have been
encountered predominantly in mining industry, metallurgical
plants and biomass burning.[12] At much lower concentration,
hydrogen cyanide was found also in the exhaust of gasoline and
diesel vehicles, directly formed during fossil fuel combustion, SCR
of NOx with hydrocarbons,[13] dehydration of methanamide
(intermediate/side-product during NH3 generation from
ammonium formate) over NH3-SCR catalysts[14] or for
malfunctioning three-way catalysts.[15] However, to the best of our
knowledge, the formation of HCN has never been reported for
natural gas engines, especially as a result of a catalytic reaction
between formaldehyde and ammonia.
In order to obtain a complete overview on the commercially
available NH3-SCR catalyst technologies, four different catalysts
have been used in our study: 1.3% Fe-ZSM-5, 1.4% Fe-BEA,
1.7% Cu-SSZ-13 and 2% V2O5, 9% WO3/TiO2. The catalytic tests
were performed with catalyst coated honeycombs at typical
technical conditions, i.e., a gas hourly space velocity (GHSV) of
100,000 h-1 using a synthetic SCR gas mixture of 0/175/350 ppm
NO, 0/175 ppm NO2, 0/350 ppm NH3, 0/80 ppm HCHO, 12% H2O,
provided in the electronic support information. The results
depicted in Fig. 1 illustrate the impact of formaldehyde presence
in the gas stream on the standard NOx conversion for the
Fe-ZSM-5 catalyst. A slightly increased NH3 consumption is
observed above 250 °C simultaneously with the decrease of NOx
reduction (Fig. 1A). During this process, HCHO is gradually
converted to CO and HCN, reaching 90% conversion at 550 °C
(Fig. 1B). The selectivity towards hydrogen cyanide increases
with temperature up to 50% at 400 °C, followed by a decrease to
only 20% at 550 °C. The oxidation process over the Fe-ZSM-5
catalyst leads also to high CO emissions, with 75% selectivity at
the highest investigated temperature. In addition, small traces of
formic acid were measured at low temperatures (Fig. 1B).
Considering that at low temperatures NH3 is known to directly
react with aldehydes to form amines,[17] we cannot exclude the
formation also of such compounds below 300 °C,[18] which would
close the carbon balance at these temperatures. This reaction is
also suggested by the slightly higher HCHO conversion at 150 °C
vs. 200 °C (Fig. S6). Moreover, the formation of CO2 in this
temperature range is unlikely since CO conversion onset on
Fe-ZSM-5 is only observed above 350 °C (Fig. S8).
Figure 2. Simultaneous oxidation of (A) NO and HCHO or (B) NH3 and HCHO over Fe-ZSM-5. Comparison of conversion and product selectivity with and without
HCHO in a gas mixture consisting of 350 ppm NO/NH3, 0-80 ppm HCHO, 12% H2O, 10% O2 in N2.
2
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As already indicated by the NH3 overconsumption relative to the
NO conversion (Fig. 1A), the formation of HCN seems to be
directly linked to a reaction between HCHO or its oxidation
intermediates/by-products and NH3. Since under standard SCR
conditions no gas phase reactions leading to hydrogen cyanide
could be observed during empty reactor tests (Fig. S3), the HCN
production obviously is a consequence of HCHO reactions on the
SCR catalyst. In contrast to previous studies in literature, which
reported the formation of HCN by the reduction of NO with CO[15a]
or other hydrocarbons,[13b-d] our results demonstrate a similar
selectivity trend towards HCN formation but in this case due to the
reaction between HCHO and NH3 (Fig. 2A vs. Fig. 2B). Thus, by
comparing the NO oxidation (Fig. 2A) in presence and absence of
HCHO it could be observed that the conversion of HCHO is
competing with the oxidation of NO for active sites, and therefore
results in a decreased NO oxidation activity. However, no
emissions of HCN could be measured. Indeed, only significant CO
emissions and HCOOH traces were detected during
formaldehyde-only or formaldehyde and NO oxidation on
Fe-ZSM-5 (Fig. 2A, Fig. S5). Also, for a stream containing CO and
the standard SCR gas mixture, no secondary emissions were
observed (Fig. S8), suggesting that not CO but an oxidation
intermediate of HCHO is responsible for the formation of HCN.
In case of NH3 oxidation (Fig. 2B) in presence of HCHO, the
oxidation of NH3 is enhanced up to 550 °C. Simultaneously, the
conversion of formaldehyde increased compared to the NO
oxidation profile in the same temperature window. This increment
in HCHO conversion could be directly linked to the formation of
HCN. Hence, a possible mechanism for hydrogen cyanide
formation from HCHO during NH3-SCR could involve the
oxidation to formate, followed by conversion to an amide
intermediate (Scheme 1). In a next step, formamide decomposes
to CO and NH3 or is dehydrated to HCN, the last reaction being
more probable.[19, 14] The observed formation of HCOOH (Figs. 1B,
2 and SI) at low temperatures supports this alternative reaction
path. Furthermore, it could be also linked to CO generation by
dehydration, as observed for zeolite-based catalysts.[20] The
impeding of the complete formaldehyde conversion to CO2 over
Fe-ZSM-5, which could be formed by CO or HCOOH oxidation[21]
(Scheme 1), could be explained by the lack of redox active sites
since the reoxidation of Fe2+ to Fe3+ is known to be a rate-
determining step during the SCR reaction,[22] and in the present
case is further inhibited by CO presence.
Figure 3. In-situ DRIFTS spectra of Fe-ZSM-5 at 150 °C after exposure to NH3
(150 ppm NH3 in N2, blue line), HCHO + O2 (25 ppm HCHO, 5% O2 in N2, black
line) and HCHO + O2 + NH3 (25 ppm HCHO, 150 ppm NH3, 5% O2 in N2, red
line) and subsequent flushing in N2.
This path involving the conversion of formic acid to formamide via
reaction with NH3, as depicted in Scheme 1, is supported also by
the DRIFTS measurements during HCHO and NH3 co-adsorption
on Fe-ZSM-5 at 150 °C (more details in SI). The DRIFT spectrum
of NH3 adsorbed on Fe-ZSM-5 show, for the spectral region
reported here, the appearance of a main band around 1450 cm-1.
This is in agreement with previous studies,[23] indicating NH3
+
adsorption as NH4 ions at the Brønsted acid sites. HCHO
adsorption resulted in a dominant band around 1580 cm-1,
previously attributed to the formation of formates at the Al or Fe
sites of Fe-ZSM-5.[24] The formation of formate on the Fe species
is supported also by the studies of Viertelhaus et al.[25] and of
Johnson et al.[26] on Fe(II) and Fe(III) formate complexes, with
characteristic bands between 1586 - 1625 cm-1 due to asymmetric
stretching frequencies of CO or OCO groups. The weaker bands
appearing at 1321 cm-1, 1348 cm-1, 1369 cm-1 and 1402 cm-1 can
be as well attributed to symmetric stretching in formates.[25, 27]
When dosing a combined gas mixture of NH3, HCHO and O2 on
Fe-ZSM-5 additional bands were observed at 1666 cm-1, 1678
cm-1, 1691 cm-1, 1708 cm-1 and 1726 cm-1. With a minor or no shift,
the most intense band at 1691 cm-1 was claimed by several
studies as the fingerprint of adsorbed formamide.[28]
Scheme 1. Suggested mechanism for HCN formation on state-of-the art catalysts for selective catalytic reduction of NOx with NH3 under standard SCR conditions.
Dotted arrows indicate less favoured pathways.
3
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Table 1: Formed emissions (ppm values) at two different temperatures in the
at 500 °C (Fig. 4 and Table 1). Only the Cu-SSZ-13 sample shows
a significantly different emissions profile and a pronounced drop
presence of 80 ppm HCHO.
of the low-temperature SCR activity (Figs.
4 and S16).
Std SCR
250 °C
Fast SCR
250 °C
Std SCR
500 °C
Fast SCR
500 °C
Nonetheless, due to its superior low temperature performance
(37% formaldehyde oxidation at 250 °C in comparison to only 6%
conversion measured for Fe-ZSM-5) the absolute HCN and CO
emissions values are larger in this case, with a higher HCN share.
This behavior is especially problematic since already at typical
catalyst working temperatures around 250 °C significant amounts
of HCN are formed. However, solely the Cu-SSZ-13 catalysts
converts formaldehyde to CO and CO2 above 400 °C (about 70%
CO2 selectivity, Figs. 4 and S16-S17), as no hydrogen cyanide
could be detected.
CO
HCN
2
CO
HCN
10
4
CO
HCN
19
18
0
CO
46
33
21
36
HCN
14
14
0
Fe-ZSM-5
Fe-BEA
Cu-SSZ-13
VWTi
1
1
1
2
1
1
1
2
40
25
17
29
2
19
5
30
4
27
17
Since the exhaust gas aftertreatment system of a lean-burn NG
engine contains also an oxidation catalyst, NO oxidation to NO2
is an expected reaction.[4] Hence, we investigated also the impact
of NO2 presence on the secondary emissions profile by testing the
NH3-SCR catalysts under fast SCR conditions (NO:NO2 = 1) for
all four catalysts. For this gas mixture, slight formaldehyde and
NH3 oxidation were measured above 500 °C as gas phase
reactions (Fig. S4). During the catalytic reaction, the influence of
NO2 is significantly different depending on the catalyst formulation.
Although the fast SCR reaction leads to a higher NOx conversion
in comparison to the standard SCR conditions, formaldehyde
oxidation is not always positively affected. A comparison of CO
and HCN emissions in ppm values under fast SCR conditions for
250 °C and 500 °C is shown in Table 1 for all four catalysts. An
improvement of the HCHO oxidation activity was recorded for Fe-
ZSM-5, Fe-BEA and Cu-SSZ-13 (Figs. S7, S11 and S17, about
30% at 250 °C for Cu-SSZ-13) while a slight decrease of the low
temperature performance was observed for the V-based catalyst
(Fig. S14). Concurrently, in comparison to the standard SCR
conditions higher HCN emissions were measured for both Fe-
exchanged zeolite catalysts and Cu-SSZ-13 at low temperatures
but lower ones for the VWTi catalyst. At high temperatures, NO2
presence resulted in slightly decreased HCN concentrations and
increased CO emissions for all samples. This difference is
probably due to decomposition of NO2 to NO with generation of
active oxygen radicals that oxidize formaldehyde in the gas phase,
as demonstrated by the empty reactor test (Fig. S4). NO2 could
also help to faster reoxidize the Fe2+, Cu+ or V4+- active centers,[30]
in this way promoting HCN conversion. Among the different
catalyst formulations, the Cu-SSZ-13 sample seems to be the less
problematic under both, standard and fast SCR conditions, since
hydrogen cyanide is formed only in the low temperature range.
Nonetheless, the high HCN emissions measured in this narrow
temperature (Table 1) window are equally critical, considering the
low catalytic efficiency of the proposed HCN removal catalysts at
these temperatures.[15b, 31]
Further characteristic bands of formamide adsorption were also
reported at lower or higher wavenumbers and were assigned to
NH, NH2, CH or CO groups stretching on a-Fe2O3,[29]
[28c]
Fe2O3/SiO2
and amorphous silica.[28d] These bands could be
only partially identified in our study due to the overlap with other
adsorbed species, particularly with formates. Hence, together
with the detection of gaseous formic acid at low temperatures (Fig.
2), the appearance of the bands characteristic for formates and
formamide adsorption (Fig. 3) clearly demonstrate the formation
of these intermediate products of HCN emissions, supporting the
mechanism suggested in Scheme 1.
Figure 4. HCHO conversion and yield of toxic byproducts during standard SCR
of NOx with NH3 in presence of 80 ppm HCHO at 250 °C (plain columns) and
500 °C (cross-striped columns). The difference in yield (grey area in the bar
graph) mainly corresponds to CO2 formation.
With small variations, the generation of HCN and CO secondary
emissions during NH3-SCR reaction in the presence of
formaldehyde was uncovered also for all the other investigated
catalysts. Table 1 reports the measured HCN and CO emissions
(ppm values) at 250 °C and 500 °C for the four catalysts
investigated in this study. For the same temperatures, the HCHO
conversion and the HCN, CO and HCOOH yields are shown in
Fig. 4 (the difference to 100% yield mainly corresponds to CO2).
As in the case of Fe-ZSM-5, Fe-BEA shows a similar share of
selectivity for CO and HCN at low and high temperatures (Figs. 4
and S11). In comparison with the iron zeolites, higher HCN
emissions were produced on V-based sample over the whole
temperature range, resulting in a maximum emission of 27 ppm
All in all, the poor activity of noble metal-based catalysts to oxidize
formaldehyde at low temperatures to CO2 under realistic reaction
conditions[8a, 8b, 9] (i.e. long-term run and SO2 presence) and also
the practically impossible complete conversion of formaldehyde
even at high temperatures due to the too low diffusion rate,[10]
result in an inevitable exposure of the NOx-removal catalysts to
HCHO emissions. In this context, this study uncovers the
formation of HCN as a potential major hazard during the
application of conventional NH3-SCR catalysts for NOx removal in
the exhaust of NG engines. Although such catalysts are
commercially applied and considered highly efficient for reducing
nitrogen oxides emissions, the presence of methane oxidation
4
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Liu, Y. Zhai, H. Ariga, N. Yi, Y. Liu, K. Asakura, M. Flytzani‐
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byproducts such as formaldehyde in the exhaust stream can lead
to a very significant formation of the highly toxic hydrogen cyanide.
In the worst case, we detected 30 ppm of HCN downstream of a
Cu-SSZ-13 SCR catalyst at 200-250 °C under fast SCR
conditions. In the high temperature regime and standard SCR
conditions, about 27 ppm HCN were produced over a VWTi
sample from 80 ppm HCHO dosed at the catalyst bed inlet. In
order to remove HCN emissions, different materials have been
proposed in literature,[15b, 31-32] some of them showing promising
activity at high temperatures. However, on Pt-based catalysts,
which are typically present in the exhaust aftertreatment system
to remove the potential NH3 slip emissions after the SCR catalyst,
HCN is either converted with high selectivity to N2O and NOx or is
only poorly oxidized at low temperatures.[15b, 31] Hence, without a
feasible removal catalyst the high HCN yield as measured in this
study under NG engine aftertreatment conditions represents a
strong challenge for the state-of-the-art NH3-SCR catalysts and
requires adequate measures to be taken. This is crucial especially
when considering the increasing share of natural gas fueled cars,
as predicted by the scenarios of the energy transition.
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Keywords: Formaldehyde • Ammonia • Hydrogen cyanide •
[24]
[25]
[26]
[27]
[28]
Selective catalytic reduction •
[1]
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10.1002/anie.202003670
Angewandte Chemie International Edition
COMMUNICATION
Entry for the Table of Contents
HCN formation over common SCR catalysts: Natural gas
engines emit less CO2 in comparison to common liquid fuel
combustion engines and are therefore an attractive alternative.
Nevertheless, they still require emission control devices for NOx
removal. Since HCHO is present in the exhaust of natural gas
engines, this study was focused on the interplay of HCHO and
other compounds of an SCR gas mixture and revealed the
formation of HCN over a broad variety of common NH3-SCR
catalysts.
6
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