Aldehydes and ketones in engine exhaust emissions

109

Review Paper

Aldehydes and ketones in engine exhaust emissions • -a reVIew T Wagner, Dipl lng, MSAE and M L Wyszynski, MEng, PhD, MSAE, MSIMP School of Manufacturing and Mechanical Engineering, University of Birmingham

Aldehydes ~nd ke~ones i~ engine exhaust gas.es are receiving i~creased attention and are beginning to be subject to special legislation due t? their carclnt?f?emc and ?zone format.lO~ potential. This paper gives an overview of their properties as well as of the basic che~lstry and conditIOns .of their formatIOn In Internal combustion engines. Extensive research on the effects of engine operation and fu,eliing parame.ters IS revlewe4 with specific references to gasoline, diesel, natural gas and methanol fuelled engines. This is accompamed by the review of the studies of the performance of exhaust .catalytic converters with respect to aldehydes. Aldehyde detection and measurement methods are summarized and analysed from the pOint of vieW of their applicability to exhaust gas analysis. Key words: aldehyde, measurement, engine, emissions, fuels, formation

NOTATION

AQIRP

BTDC CAS CFR

CI mOLA

DNPH ECD EI

ETBE FTIR

FTP

GC-FID

HPLC IMR

LEV

aut%il air quality improvement research programme (joint programme of major car manufacturers and oil industries in the United States) before top dead centre chemical abstracts service (reference number of chemical components) co-operative fuels research [used here as name of standard engine (US origin)] chemical ionization (an ion activation method used with mass spectrometer) diode laser spectroscopy (analytical method based on a semi-conductor laser, applied for engine emission analysis in this paper) 2,4-dinitrophenylhydrazine (exhaust aldehyde analysis test, named after the main reagent in the analytical procedure) electrical conductance detector (detector for gas analysis) electron impact (an ion activation method used with mass spectrometer) ethyltertiarybutylether (fuel additive) Fourier transform infrared spectrometry (analysis system, used for engine emission analysis in this paper) Federal test procedure (exhaust emission test procedure used commonly in the United States and widely documented) gas chromatograph-flame ionization detector (analysis system, used for engine emission analysis in this paper) \ high-performance liquid chromatography (analysis system, used for aldehyde measurement in this paper) ion molecule reaction (an ion activation method used with mass spectrometer) low emission vehicle (class of vehicles according to their exhaust emissions as

The MS was received on 11 November 1994 and was accepted Jor publication on 1 May 1995.

006094 © IMechE 1996

M85

MTBE MBTH

MIR

MS

m/z NMHC NMOG

TLEV

TLV ULEV

ZEV

defined in United States/California legislation) methanol/gasoline fuel [methanol/ gasoline blend (here 85 per cent methanol by weight)] methyltertiarybutylether (blending component for fuels) 3-methyl-2-benzothiazolonehydrazone hydrochloride (exhaust aldehyde analysis test, named after the main reagent in the analytical procedure) maximum incremental reactivity (reactivity factors of emissions components regarding ozone formation potential) mass spectrometry (analysis system, used for engine emission analysis in this paper) mass-{;harge ratio (applied to describe ions in mass spectrometry) non-methane hydrocarbons (group of total hydrocarbons with methane removed) non-methane organic gases [group of total organic gases with methane removed, in other words NMHC + oxygen-containing materials (aldehydes, ketones and alcohols)] transitional low emission vehicle (class of vehicles according to their exhaust emissions as defined in United States/California legislation) threshold limit value (concentration limit value of a chemical component, depends on legislation) ultra-low emission vehicle (class of vehicles according to their exhaust emissions as defined in United States/ California legislation) zero emission vehicle (class of vehicles according to their exhaust emissions as defined in United States/California legislation equivalence ratio [stoichiometric air-fuel ratio divided by actual air-fuel ratio (¢ < I for lean mixtures)] Proc InSln Mech Engrs Vol 210

T WAGNER AND M L WYSZYNSKI

110

1 INTRODUCTION

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The conditions of aldehyde formation in engines and the effect of aldehydes on human health have been the subject of intensive research. Although a study of the direct effect of aldehydes on human health in urban conditions (1) did not yield conclusive results, even under worst case scenarios (when full present German Federal Health Office threshold limits of formaldehyde concentrations applicable to workplace atmospheres, that is 0.1 ppm, were applied), there are distinct fears regarding the carcinogenic properties and strong ozoneforming potential of aldehydes. The threshold limits might thus be changed in the future. This is already demonstrated by the stricter emissions standards for California, shown in Table 1 (2). Furthermore, there is a growing trend to partly replace the fuels derived from crude oil with synthetic fuels such as methanol, ethanol and ethers. These components have large oxygen content, which may lead to higher aldehyde emissions. Therefore, this paper reviews the research in aldehyde emissions from engine exhaust when using different fuels (gasoline, diesel, natural gas, gasoline/methanol blends). The review begins with a brief presentation of aldehyde and ketone properties, environmental and health effects (Section 2), followed by the chemistry of their formation (Section 3). Next, Section 4 discusses the effects of engine designs, modes of operation as well as fuel formulation (gasoline, diesel, natural gas, methanol) on aldehyde emissions. Also, the effect of catalytic converters is discussed. Finally, aldehyde sampling and analysis methods, applicable to exhaust gases are discussed in Section 5. 2 PROPERTIES OF ALDEHYDES AND KETONES

Aldehydes and ketones are partially oxygenated organic compounds. Aldehydes are characterized by the HC=O group, while ketones are characterized by the RC=O group. The name aldehyde is derived from alcohol dehydrogenatus. The properties of the most common aldehydes/ketones in engine exhaust emissions are presented in Table 2 (3) and their chemical formulae are shown in Figure 1. 2.1 Environmental effect The most abundant aldehyde in engine exhaust, formaldehyde, is regarded as a suspected carcinogen by the Deutsche Forschungsgemeinschaft (German Research Society) and by the American Conference of Government Industrial Hygienists. The present threshold limits for aldehydes and ketones are given in Table 2. Their Table 1 California clean air requirements for gasoline fuelled passenger cars (gfmile) (2)

I

Standard category

CO

NMOG

NO,

Aldehydes

1991 regulation 1993 regulation TLEV (1994) LEV (1997) ULEV (1997) ZEV (1998)

7.0 3.4 3.4 3.4 1.7 0

0.390· 0.250· 0.125 0.075 0.040 0

0.4 0.4 0.4 0.2 0.2 0

0.015 0.Q15 0.008 0

• The regulations of 1991 and 1993 are in terms of non· methane hydrocar. bons (NMHC). Note: Based on the US FTP 75 test procedure, 50000 miles durability. Part D : Journal of Automobile Engineering

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Fig. 1 Chemical formulae of selected aldehydes/ ketones

direct effects on human health include also nausea, headaches, coughing and irritation of eyes, nose and throat (4). Another effect on the environment, the ozone formation potential, causes attention. Formaldehyde is a very reactive organic chemical with a high tendency to form ozone by photochemical oxidation. The widely accepted method to classify and to compare the effect on ozone formation of an exhaust component is the maximum incremental reactivity (MIR), developed by Carter and Lowi (5). Aldehydes have very high MIR values, representing their high ozone-forming potential, compared with other non methane organic gases (NMOG), as shown in Table 3. 3 ALDEHYDE FORMATION

Since neither the fuels nor air contain aldehydes or ketones (in significant concentrations compared to engine exhaust), any aldehydes and ketones appearing in exhaust gases are formed in the engine and exhaust system. In the following, the basic aldehyde formation reactions are presented. Important carriers of the chain reaction in the combustion of hydrocarbons are the alkyl radicals (R .), which are formed by cleavages of C-C or C- H bonds of hydrocarbons. At high temperatures, dehydrogenation by oxygen and the presence of other combustion related radicals influence this cleavage process. Important for the formation of formaldehyde is the methyl radical (CH3·)' Tsatsaronis (6) named six reactions at methane combustion. It is assumed that these reactions also take place in the combustion of other © IMechE 1996

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Table 2 Properties of aldehydes/ketones (3) Molecular weight Number Aldehydes I 2 3 4 5 6 7 8

IUPAC name

Synonym

Type

C number

Formula

CAS number

kg/mol

Density kg/m 3

Melting point °C

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°C

mg/m 3

ppm (ml/m3)

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