DIESEL PARTICULATE EMISSION CONTROL

This paper reviews the emission control of particulates from diesel ex haust gases. The efficiency and exhaust emissions of diesel engines wi ll be compared with those of otto engines (petrol engines). The format ion of particulates (or ''soot''), one of the main nuisances of diesel exhaust gases, will be briefly outlined. The effects of various emiss ion components on human health and the environment will be described, and subsequently the emission standards for particulates and for NOx, which have been introduced worldwide, will be summarized. Possible mea sures for reducing exhaust emissions of particulates and NOx will be d iscussed, such as the use of alternative fuels, modifications to the e ngine and the use of aftertreatment devices. It will be made clear tha t aftertreatment devices may become necessary as diesel emission stand ards become more stringent, in spite of important progress in the othe r fields of reducing exhaust emissions. Selective catalytic reduction via hydrocarbons, ammonia or urea, a possible aftertreatment method fo r NOx emission control, will be discussed briefly. Filters for collect ing particulates from diesel exhaust gases will be examined in more de tail and aftertreatment control systems for particulate removal will b e reviewed. These can be divided into (i) non-catalytic filter based s ystems which use burners and electric heaters to burn the soot once it has been collected on the filter, (ii) catalytic filter-based systems which consist of filters with a catalyst coating, or filters used in combination with catalytically active precursor compounds added to the diesel fuel; and (iii) catalytic non-filter-based systems in which ga seous hydrocarbons, carbon monoxide and part of the hydrocarbon fracti on of the particulates are oxidized in the exhaust gases. Finally, rec ent trends in diesel particulate emission control will be discussed, i ndicating the growing importance of the catalytic solutions: the fast introduction of non-filter-based catalysts for diesel engines and the possible application of filters in combination with catalytically acti ve precursor compounds added to diesel fuel.