Application of NOx Adsorber to Diesel Depollution: Performances and Durability

Abstract

To ensure overall optimization of engine performance (NOx, particulates, efficiency), the use of a NOx after-treatment system appears necessary to meet the future Euro IV emissions standards and after. Among the known means, the NOx trap should offer an efficiency of the same order than the reduction by urea, without having the same constraints. The trap operates on the basis of the alternation of operating phases with a lean mixture during which NOx is stored and rich phases during which nitrates are destored and treated. Important aspect of the technology, NOx adsorbers are very sensitive to sulfur poisoning and future fuel standards are unlikely to be sufficient to prevent the system from requiring periodic desulfation procedures. The present paper presents results obtained on a light duty application, on a one hand, on a heavy duty case on a second hand. In the light duty front, the purpose of the work presented first, was to study the effects of low fuel sulfur content such as 50 ppm and 10 ppm on the NOx adsorber efficiency for a Diesel application. Through this study, the influence of the substrate cell geometry has also been assessed. The use of a 10 ppm sulfur fuel is not enough to maintain, at a high level, the NOx adsorber performance during a 40 000 km aging test. The desulfation criterion (efficiency loss of 30%) is reached after the first 16 000 km. However, the desulfation operation is not enough to recover the initial catalyst performance and the poisoning velocity increases as the catalyst ages. The hexagonal cell substrate catalyst is less sensitive to sulfur poisoning than a square cell substrate catalyst so that its desulfation frequency is much lower. With a 10 ppm sulfur fuel, 4 desulfation operations were achieved with a hexagonal cell catalyst as opposed to 11 with a square cell catalyst during the 40 000 km aging test. In the heavy duty application, the investigation, using a single-cylinder engine, demonstrated that the best NOx emission/fuel consumption trade-off are obtained with destorage richer than stoichiometry. These destorage conditions could be obtained in various configurations (injection timing, EGR, post-injection). The various adjustments tested show the advantage of producing a large amount of carbon monoxide to obtain fast destorage and efficient treatment of the trapped nitrates. With a method like this, the extra fuel consumption resulting from the succession of rich/lean phases is around 3% for an emission objective of NOx < 3. 5 g/kW·h. By mounting a second trap in parallel, an oxidation catalyst, or using EGR, this result can be significantly improved. The gradual storage of sulfur contained in fuel limits the trapping capacity of the NOx system, necessitating periodic desulfation. Various configurations have been tested to purge the sulfates. Mass spectrometer exhaust gas analysis makes it possible to quantify COS, H2S and SO2 emissions during this phase. Nevertheless, the tests have shown the limits of the possibilities in a single cylinder installation.