Freisetzung von toxischem HCN bei der Stickoxidreduktion mittels NH₃‐SCR in mager betriebenen Erdgasmotoren

Abstract

Die Reduzierung der Emissionen von Treibhausgasen und gesundheitsgefährdenden Schadstoffen ist eines der wichtigsten Ziele unserer Industriegesellschaft. Erdgasmotoren sind insbesondere bei magerem Betrieb attraktiv, da sie weniger CO2 ausstoßen als Verbrennungsmotoren, die mit flüssigen Kraftstoffen betrieben werden. Sie benötigen allerdings ein katalytisches Kontrollsystem, um Stickoxide (NOx) aus dem Abgas zu entfernen. Hier haben wir das Zusammenspiel der aktuellen Technologie zur Stickoxidreduktion, die selektive katalytische Reduktion (SCR) mittels NH3, und Formaldehyd, einer schädlichen Emission in Erdgasmotoren, untersucht. Bei der NOx Entfernung werden beachtliche Mengen an toxischem Cyanwasserstoff (HCN) gebildet. Alle getesteten Katalysatoren wandeln Formaldehyd teilweise in HCOOH und CO um. Zusätzlich werden Sekundäremissionen von HCN über die katalytische Reaktion von Formaldehyd und dessen Oxidationsintermediaten mit NH3 beobachtet. Da die HCN Emissionen mit den derzeitigen Komponenten nicht effizient in schadstofffreie Gase umgewandelt werden können, ist die Entwicklung von Katalysatoren mit einer gesteigerten Oxidationsaktivität nötig, um dieses kritische Problem zu lösen.

Impact of unintentionally formed CH2O in oxygenated fuel exhausts on DeNOx-SCR at different NO2/NOx ratios under close to real conditions

“…[11][12][13] Currently, the most efficient method of reducing NO x from engine exhaust gases is selective catalytic reduction (SCR) technology, in which the nitrogen oxides from the engine react with ammonia as a reducing agent on a zeolite or TiO 2 , V 2 O 5 and WO 3 -based catalyst. [14][15][16] The main SCR reactions are distinguished as follows: eqn (1) is referred to the standard SCR reaction and largely occurs at temperatures around 250 °C and above. 17 Eqn (2) is called the fast SCR reaction and takes place in a temperature range between 170-300 °C.…”

“…One compound, which can also be unintentionally formed during combustion of oxygenated and alternative fuels or methane, is formaldehyde (CH 2 O). 15,[22][23][24][25] This is a relevant matter of concern because of its toxicity and potential carcinogenicity. 26 A part of the formed CH 2 O can already be oxidized in an oxidation catalyst upstream of the SCR catalyst.…”

“…[27][28][29][30][31][32] Recent studies on this topic revealed that the presence of CH 2 O in the exhaust gas can also lead to the formation of hydrogen cyanide (HCN) under SCR-relevant conditions. 14,15,33,34 In 2020, Zengel et al proposed that HCN formation from CH 2 O on various self-produced SCR materials occurs via a formate pathway, which has already been published from studies on the use of ammonium formate as a reducing agent precursor. 15,35 For these investigations, NO 2 /NO x ratios of 0 and 50% were used at a gas hourly space velocity (GHSV) of 100 000 h −1 .…”

“…14,15,33,34 In 2020, Zengel et al proposed that HCN formation from CH 2 O on various self-produced SCR materials occurs via a formate pathway, which has already been published from studies on the use of ammonium formate as a reducing agent precursor. 15,35 For these investigations, NO 2 /NO x ratios of 0 and 50% were used at a gas hourly space velocity (GHSV) of 100 000 h −1 . 15 Also in 2020, Ngo et al showed investigations on CH 2 O on a commercial V 2 O 5 -WO 3 /TiO 2 catalyst using a GHSV of 70 000 h −1 .…”

“…15,35 For these investigations, NO 2 /NO x ratios of 0 and 50% were used at a gas hourly space velocity (GHSV) of 100 000 h −1 . 15 Also in 2020, Ngo et al showed investigations on CH 2 O on a commercial V 2 O 5 -WO 3 /TiO 2 catalyst using a GHSV of 70 000 h −1 . 14 These authors also reported the formation of small amounts of hexamethylenetetramine (HMT) on their catalyst.…”