Experimental Study On Selective Catalytic Reduction Of NO_X With Dimethyl Ether

As an excellent alternative fuel, Dimethyl ether achieves a significant effect on high combustion efficiency and low harmful emissions. The hydrocarbon (HC) and carbon monoxide (CO) emissions of Dimethyl ether (DME) engine are much lower than that of diesel engine, and particulate matter (PM) emission is zero. In order to meet increasingly stringent standards, NO_X need to be further reduced. DME is not only an alternative fuel, but also can be used as reductant for catalytic reduction of NO_X. With the support of National Natural Science Foundation, this study launched Selective Catalytic Reduction (SCR) technology research, which provides a theoretical basis for the realization of DME-SCR NO_X on dimethyl ether engine. The main contents include:1 Set up a sample simulation test bench, on which selective catalytic reduction (SCR) NO_X by Dimethyl-ether (DME) over MoO₃/γ-Al₂O₃ catalyst was experimental studied in this paper. The influence of the reaction temperatures, the MoO₃ loadings, the oxygen concentrations, and the molar ratios of DME/NO were investigated respectively. The results showed that MoO₃/γ-Al₂O₃ catalyst displays a good catalytic activity in middle or high temperature (300℃⁵50℃), and the NO_X conversion ratio increases at the beginning and then decreases as the temperature increasing, and the highest activity is observed between 300℃and 350℃. With the increasing MoO₃ loading, the maximum NO_X conversion shows a decreasing trend, and the 3wt.% MoO₃ loading can get a satisfying catalytic effect.The NO_X conversion ratio climbs up and then declines as the oxygen concentration increasing, and the best catalyst activity appears when oxygen content is about 8%. The highest NO_X conversion ratio increases with the increasing DME/NO molar ratio. The NO_X conversion ratio rises up to the peak 60.5% when the DME/NO molar ratio is 3, which can maintain about 20% after 500℃. The carbon products of DME-SCR reaction are mainly CO and CO₂, a small amount of methanol and trace formaldehyde.2 In order to expand the low temperature window of DME-SCR reaction, selective catalytic reduction (SCR) NO_X by Dimethyl-ether (DME) over Ag/γ-Al₂O₃ catalyst was experimental studied in this paper. The catalyst was prepared by using impregnation method, and the influence of the reaction temperatures, the Ag loadings, the oxygen concentrations, NO_X source and the molar ratios of DME/NO were investigated respectively. The results showed that Ag/γ-Al₂O₃ displays favorable activity at low temperature. In presence of O2 the catalyst displays active even at 200℃, and the NO_X conversion ratio increases at the beginning and then decreases as the temperature increasing, and the highest activity is observed between 250℃and 300℃. In the absence of O2, however, higher conversion ratios appear after 300℃, and the NO_X conversion ratio climbs up and then declines as the oxygen concentration increasing, and the best catalyst activity appears when oxygen content is about 8%. The Ag loading of about 3wt.% can get best catalyst activity, and the DME/NO molar ratio for the highest NO_X conversion is 2 over Ag/γ-Al₂O₃. The influence of NO_X source on NO_X conversion is very small.3 In order to acquire good efficiency in the whole temperature range (200℃⁵50℃), selective catalytic reduction (SCR) NO_X by Dimethyl-ether (DME) over composite catalyst 3wt.%Ag–3wt.% MoO₃/γ-Al₂O₃ was experimental studied in this paper. The composite catalyst has good overall performance, and can meet the need of possessing catalytic activity at low and high temperature conditions, which displays favorable activity especially between 200℃and 400℃. The NO_X conversion ratio increases at the beginning and then decreases as the temperature increasing, which reaches the maximum of 60.5% at 300℃and can maintain about 15% after 400℃. NO2 is more easily to be reduced than NO when the reducing agent is DME over the composite catalyst. In the absence of O2, the composite catalyst displays worse activity at low temperature, but higher NO_X conversion ratios appear after 450℃. Repeatability tests also verify that the catalyst has good durability.In this research, in-depth reaction mechanism and properties of reducing NO_X by DME-SCR have been studied, which will lay a good foundation for the application of selective catalytic reduction of NO_X with Dimethyl-ether on DME engine.