NOx catalytic removal at the exhaust of lean-burn natural gas heavy-duty engines

The present paper analyses the possibility to control NOx emissions of natural gas lean-burn engines by catalytic after-treatment. The proposed approach consists in realising the reduction of NOx by methane present in the exhaust on novel catalytic systems based on the zeolite Ferrierite. A sample of Ferrierite modified with cobalt by ion exchange (Co=3.5%) showed interesting activity and selectivity in laboratory tests (40% of NOx reduction to N2 at 470 °C and space velocity 20000 h-1), but very low performance at the engine exhaust was obtained (8% of NOx reduction to N2 at 570 °C and space velocity 20000 h-1). Experiments in real conditions evidenced a strong inhibiting effect of water on NOx catalytic reduction. The CH4/NOx ratio did not significantly affect the catalyst activity at the engine exhaust.