Control of Automotive Sulfate Emissions

Abstract

<div class="htmlview paragraph">A study has been made of potential methods for controlling SO<sub>4</sub>= emissions from oxidation catalyst-equipped vehicles. The methods considered included operating condition and catalyst changes, as well as the use of a vehicle trap for SO<sub>4</sub>=. Emissions of SO<sub>4</sub>= from non-catalyst cars were also measured.</div> <div class="htmlview paragraph">The only engine operating variable we found to significantly lower SO<sub>4</sub>= emission was exhaust gas O<sub>2</sub> level. Limiting air pump use reduced SO<sub>4</sub>= emissions by factors of 5 to 7 over the FTP, and by factors of 2 to more than 10 at 96 km/h. Some increase in CO and HC emissions was observed when the greatest SO<sub>4</sub>= reductions were achieved, but it appears that properly modulated carburetion could overcome this problem. Limited excess air shows great promise as a means of minimizing SO<sub>4</sub>= emissions. Use of a three-way or oxidation catalyst system with closed-loop fuel metering control achieved very low levels of SO<sub>4</sub>=, while simultaneously controlling CO and HC, at the stoichiometric A/F ratio and slightly leaner than stoichiometric.</div> <div class="htmlview paragraph">Pelleted catalysts emitted lower amounts of SO<sub>4</sub>= during low speed operation than did monoliths, because of their greater storage capacity for sulfur oxides. However, during subsequent high speed operation, the sulfur oxides were driven off, resulting in higher SO<sub>4</sub>= emissions from pellets than from monoliths. There appears to be no clear cut overall choice between the two catalyst configurations as far as SO<sub>4</sub>= emissions are concerned.</div> <div class="htmlview paragraph">Catalyst age was found to be an important factor, Catalysts with 40,000 km of service gave about 65% less SO<sub>4</sub>= emissions over the FTP, and 25-40% less at 96 km/h, compared to relatively fresh catalysts. These results show that emission inventories which are predicted from tests with relatively fresh catalysts will tend to overestimate the buildup of atmospheric sulfate from catalyst-equipped vehicles, since these cars would have an average age greater than the test vehicles.</div> <div class="htmlview paragraph">Pt catalysts gave somewhat higher SO<sub>4</sub>= emissions than Pt-Pd catalysts, while concentration of Pt-Pd on the catalyst had no effect in the range studied. Gasoline powered vehicles without catalysts produced only negligible quantities of SO<sub>4</sub>=. A Diesel powered vehicle gave higher than expected SO<sub>4</sub>= emissions, but high carbonaceous particulate emissions may have caused inaccurate measurements.</div> <div class="htmlview paragraph">Vehicle SO<sub>4</sub>= traps containing CaO-based pellets achieved almost complete SO<sub>4</sub>= removal from the exhaust for 40,000 km, but pressure drop through the trap became excessive as the sorbent swelled with age. Efforts to lower the pressure drop resulted in poorer SO<sub>4</sub>= removal and capacity. Additional work is necessary to achieve satisfactory properties for all three parameters. Suggestions for further investigation are discussed.</div>