Temperatures diesel catalysts

Desulfurization will become mandatory when oxidizing catalysts are installed on the exhaust systems of diesel engines. At high temperatures this catalyst accelerates the oxidation of SO2 to SO3 and causes an increase in the weight of particulate emissions if the diesel fuel has not been desulfurized. As an illustrative example, Figure 5.22 shows that starting from a catalyst temperature of 400°C, the quantity of particulates increases very rapidly with the sulfur content. 

The second method used to reduce exliaust emissions incorporates postcombustion devices in the form of soot and/or ceramic catalytic converters. Some catalysts currently employ zeolite-based hydrocarbon-trapping materials acting as molecular sieves that can adsorb hydrocarbons at low temperatures and release them at high temperatures, when the catalyst operates with higher efficiency. Advances have been made in soot reduction through adoption of soot filters that chemically convert CO and unburned hydrocarbons into harmless CO, and water vapor, while trapping carbon particles in their ceramic honeycomb walls. Both soot filters and diesel catalysts remove more than 80 percent of carbon particulates from the exliatist, and reduce by more than 90 percent emissions of CO and hydrocarbons. 

The main causes of the deactivation of diesel catalysts are poisoning by lubrication oil additives (phosphorus), and by SOx, and the hydrothermal instability. The SCR by HC is less sensitive to SOx than the NO decomposition. The Cu-based catalysts are slightly inhibited by water vapor and SOx, and suffer deactivation at elevated temperature. Noble metal catalysts such as Pt-MFI undergo low deactivation under practical conditions, are active at temperatures below 573 K but the major and undesired reduction product is N20 (56). 

DOCs must function in a demanding environment. Although diesel exhaust temperatures are well below those in auto exhaust (200- 00 C vs. 300-1000°C), diesel catalysts must contend with solids, liquids, and gases (not just gases) and deposits of noncombustible additives from lubricating oil. This last contains zinc, phosphorus, antimony, calcium, magnesium, and other contaminants that can shorten catalyst life below that mandated. Contamination also can come from sulfur dioxide in the exhaust. 

A wide variety of fuels such as diesel, jet fuel, and NATO F-76 have been reformed using adiabatic prereformers.103 Since the prereforming process is generally carried out at low temperatures, the catalyst must be highly active. Industries... 

Figure 1 5. Conversion of carbon monoxide, gaseous hydrocarbons and sulfur dioxide reached over a diesel catalyst with and without measures to suppress the formation of sulfates, as a function of the exhaust gas temperature (monolith catalyst with 62 cells cm dedicated diesel washcoat formulations with platinum at a loading of 1.76 g I" diesel engine test bench light-off test at a space velocity of 120000 N1 F h diesel engine bench aging procedure for 100 h at a catalyst inlet temperature of 773 K).

Figure 106. Conversion of carbon monoxide, gaseous hydrocarbons and sulfur dioxide reached over diesel oxidation catalysts as a function of the preeious metal formulation at equimolar loading (monolith catalyst with 62 cells cm - dedicated diesel washeoat formulations with platinum, palladium and rhodium at an equimolar loading of 8 mmol I, fresh model gas test at a gas temperature at catalyst inlet of 723 K a space velocity 50000Nil h model gas simulates the exhaust gas composition of an IDl passenger car diesel engine at medium load and speed and contains 100 vol. ppm SO2). Reprinted with permission from ref. [68], C 1991 Society of Automotive Engineers, Inc.

Likewise, Fig. 112 shows the conversion efficiency for diesel particulate matter reached in the US transient cycle with a diesel catalyst mounted on a heavy duty engine. Table 29 shows the conversion for CO, HC and particulate matter in the European 13-mode test performed with a heavy duty engine for a fresh and for an engine-aged catalyst. The catalyst performance at operating conditions, reflecting the lowest and the highest exhaust gas temperature in this 13-mode test, is also reported. 

Figure 113. Conversion of nitrogen oxides and gaseous hydrocarbons reached over different NO.v-reduction catalyst formulations, as a function of the exhaust gas temperature (monolith catalyst with 62 cells cm dedicated NO t-reduction catalyst formulations with zeolites and with different active components, after laboratory oven-aging in air at a temperature of 1023 K for 16 hours light-off test in a model gas reactor at a space velocity of 50000 N11 h model gas simulates the exhaust gas composition of an IDl/NA passenger car diesel engine at medium load and speed, except for the hydrocarbon concentration, which was increased to reach yHc yNO, 3 1 (mol mol)).

The bed material consisted of a mixture of the powder sample and quartz sand in order to obtain a constant space velocity (25000 h ) for all tested catalysts. The gas composition used in the experiments was 10% O2,405 ppm NO and 911 ppm C3H6, balanced with Ar to yield a total flow of 420 ml/min. The samples were initially reduced in 5000 ppm H2 at 400°C for 15 min and stabilised in the reaction mixture at 525°C for 1 h. The samples were then cooled down to room temperature under an Ar flow. At this temperature, the catalyst was exposed to the reaction mixture under 15 min before starting the heating ramp up to 525°C, at a constant rate of 6°C/min. The steady-state experiments were performed by subsequently lowering the temperature in steps of 50°C, starting from the final ramp temperature and the products were analysed after approximately 90 min. In order to facilitate the interpretation of the flow reactor and FTIR results the model gas was simplified by omitting H O and SO2 (which would have been present if a diesel exhaust was used). 

Diesel fuel was found to volatilize approximately 50°C lower than lube oil. Tliis was accompanied by a corresponding sliift to lower combustion temperatures with Catalyst "C" (Figure. 9). Fuel conversion with Catalyst "C was typically 80% by mass balance, with CO2 the only noncondensable product. In contrast to the onset of fuel combustion shifting to lower temperatures over Catalyst "C", the onset of fuel combustion over alumina was at higher temperature and occurred at almost the same temperature as did lube oil combustion. 

Allied Signal Ib, fully catalytic Recuperative single-shaft 50 kW Diesel or JET Proprietary washcoat on Coming EX-22 substrate NR + Pre-heated air from heat exchanger, no pilot-flame, low outlet temperature prevents catalyst overheating 130, 131... 

In addition to their primary fimction of reducing particulate matter, diesel catalysts must convert gas phase CO and HC. Emission limits have come down sharply for U.S. trucks, for example, 2009 standards allow only about 1/10 as much as HC as was permitted in 1994. For CO, 2003/2004 limits for Japanese heavy-duty trucks are about 1/3 of their values in 2000. The much leaner diesel exhaust, compared with gasoline exhaust, helps reduce CO and HC emissions, but diesel exhaust is generally cooler, which suppresses conversion. Low temperature activity is, accordingly, are research priority. Pt, or Pt/Pd, supported on alumina, is standard, but even small changes in thermal stability can confer a competitive advantage. 

Cerium oxide-based technology was then modified for European diesel passenger cars with the addition of a relatively small amount of zeolite as well as additional precious metal to meet European requirements for gaseous CO and HC reductions. The purpose of the zeolite was to adsorb gaseous HC (mostly vm-burned diesel fuel) during cold start conditions and subsequently release them to the catalyst once the latter was sufficiently active to initiate catalytic oxidation on the Pt (138). As the exhaust temperature and catalyst light-off is achieved, the HC desorbing from the zeolite is catalytically oxidized over the Pt. 

Fig. 21.9 Comparison of underbody exhaust gas temperatures (SCR catalyst location) for a 201IMY diesel truck at 9,500 lbs compared to a prototype truck at 6,000 lbs and a gasoline PFI E450 van at 10,000 lbs. Data are for the first two phases (bags) of the FTP-75, and the effectiveness of rapid warm up is shown for the 9,500 lbs truck...

Finally, sulfur has a negative effect on the performance of the catalyst itself. One sees for example in Figure 5.23 that the initiation temperature increases with the sulfur level in the diesel fuel, even between 0.01% and 0.05%. Yet, in the diesel engine, characterized by relatively low exhaust temperatures, the operation of the catalyst is a determining factor. One can thus predict an ultimate diesel fuel desulfurization to levels lower than 0.05%. 

The crankcase of a gasoline or diesel engine is in reality a hydrocarbon oxidation reactor oil is submitted to strong agitation in the presence of air at high temperature (120°C) furthermore, metals such as copper and iron, excellent catalysts for oxidation, are present in the surroundings. 

The reactor system works nicely and two model systems were studied in detail catalytic hydrogenation of citral to citronellal and citronellol on Ni (application in perfumery industty) and ring opening of decalin on supported Ir and Pt catalysts (application in oil refining to get better diesel oil). Both systems represent very complex parallel-consecutive reaction schemes. Various temperatures, catalyst particle sizes and flow rates were thoroughly screened. 

SCR for heavy-duty vehicles reduces NOx emissions by 80%, HC emissions by 90% and PM emissions by 40% in the EU test cycles, using current diesel fuel (<350 ppm sulphur) [27], Fleet tests with SCR technology show excellent NOx reduction performance for more than 500000 km of truck operation. This experience is based on over 6 000 000 km of accumulated commercial fleet operation [82], The combination of SCR with a pre-oxidation catalyst, a hydrolysis catalyst and an oxidation catalyst enables higher NOx reduction under low-load and low-temperature conditions [83],... 

Apart from the hydrolysis step, the SCR-urea process is equivalent to that of stationary sources, and in fact the key idea behind the development of SCR-urea for diesel powered cars was the necessity to have a catalyst (1) active in the presence of 02, (2) active at very high space velocities ( 500.000 per hour based on the washcoat of a monolith) and low reaction temperatures (the temperature of the emissions in the typical diesel cycles used in testing are in the range of 120-200°C for over half of the time of the testing cycle), and (3) resistant to sulphur and phosphorus deactivation. V-Ti02-based catalysts for SCR-NH3 have these characteristics and for this reason their applications have also been developed for mobile sources. 

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