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An in-depth analysis of diesel particulate filters (dpfs) and their regeneration systems, specifically continuous regeneration traps (crts). It covers the principles of crt, the role of no2 and noble metals in soot oxidation, passive regeneration using fuel additives, and the impact of fuel sulfur on crt performance. The document also discusses the development of partial diesel particulate filters and their advantages over wall flow filters.
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The principle of CRT is based on the fact that is a much superior oxidizing agent for soot than the molecular oxygen. oxidizes the dry carbon soot trapped in the filter below 300º C by the following reactions:
Although the trap substrate can be coated with a catalyst material to reduce soot oxidation temperatures to as low as 200º C, but installation of an oxidation catalyst upstream of particulate filter where NO is preferentially converted to which then oxidizes the soot has been found more effective. The catalysts used are noble metals. The oxidation catalyst is a flow through ceramic monolith using Pt-Pd catalyst impregnated on Al 2 O 3 washcoat. The schematic of a CRT is shown in Fig. 6.18. is produced by oxidation of NO upstream of DPF. The soot trapped in the downstream DPF is continuously oxidized on the filter substrate by thus keeping the particulate filter essentially clean and the exhaust backpressure remains nearly unchanged.
Fuel sulphur is an important factor that affects conversion of NO to NO 2 on the oxidation catalyst and hence the efficiency of CRT. Fuel sulphur lower than 30-ppm has been found necessary to maintain the functioning of CRT at an acceptable level. In Europe, where low sulphur fuel is available several thousand vehicles with CRT are in operation. To achieve the best performance of CRT, the following conditions should be met:
Sulphur-free fuel (sulphur <30ppm) is necessary to prevent catalyst poisoning For best performance temperature should be in the range 250 – 450º C. The / soot ratio should be adequately high otherwise available will be too low to oxidize soot.
Although wall flow diesel particulate filters have very high particulate trapping efficiency, but their regeneration over the entire life span extending up to 496,000 kms for heavy duty vehicles is a challenging problem. In Europe, many diesel vehicles meeting Euro 4 standards are fitted with ‘wall flow’ particulate filters as original equipment. Metal supported flow- through diesel filters employing CRT operational principle have also been developed recently These filters have been developed to provide 50 to 70 percent reduction in PM emissions and therefore, are called as ‘Partial Particulate Filters’. Like CRT, upstream in the first section an oxidation catalyst is installed where NO is oxidized to NO 2. In the second section, which consists of flow-through type filter element collection of soot and its combustion processes occur. .A schematic cut-away section and working principle of the filter is shown on Fig. 6.19. The metal PM filter consists of flat and corrugated foils in a flow-through monolithic configuration. The corrugated foils are stamped to produce blades like structure to direct the flow towards the flat foil. The flat foil is made of porous sintered metal fleece (wool) packed in the form of a sheet and compressed between metal foils. Part of the exhaust gas is directed by the blades in the corrugated foil towards the porous metal wool that traps the particulate matter. The soot trapped by the metal wool is oxidized by NO (^2) generated on the catalyst in the upstream first section. The design of this diesel particulate filter has open channels and it does not get clogged due to excessive accumulation of soot as happens in the ‘wall flow’ filters on failure of regeneration. All the exhaust is able to flow through the open channels if the metal fleece is choked. However, in such a situation removal of PM from exhaust does not take place. Typical cell density of these filters is 200 cpsi. With use of these particulate filters reduction in PM emissions ranging from 30 to over 70 % have been obtained.
Summary of advancements in diesel emission control technology.
As seen from the Fig 6.20, the engine technologies have been able to reduce emissions to Euro IV emission levels. The technologies employed progressively included;
Turbocharging with inter-cooling Variable geometry turbocharging Improved fuels esp. low sulphur in addition to low final boiling point and closer control on density, viscosity etc. High injection pressures Optimization of combustion bowl geometry and air motion EGR
Aftertreatment Technology for Euro V and beyond:
The second stage is the exhaust after treatment. In some engine models even to meet Euro IV standards diesel oxidation catalysts and SCR de-NOx catalysts were employed. However, for Euro V and later standards some form of exhaust treatment is almost essential. Most engines would employ:
Diesel particulate filters: DPF or CRT de -NOx catalysts: SCR or other types
(6.1) A diesel engine is fitted with mechanical injection pump with 60MPa peak injection pressure. Another engine of the same size developing the same power is employing common rail injection system with 160 MPa injection pressure. Both the engines operate at the same rated speed. Discus the likely differences between the two engines with respect to (i) injection duration (ii) nozzle hole size (iii) atomization and droplet size (iv) injection timing (v) fuel evaporation , mixing (vi) ignition delay and premixed combustion (vii) over all combustion rates (vi) PM and NOx emissions. (6.2) Calculate stoichiometric NH 3 / NO (^) x ratio for reduction of NO (^) x in SCR catalysts if the entire NO (^) x is only NO, and consists of 5 and 10% NO 2 by volume. If 20 % more NH 3 than stoichiometric requirements is supplied calculate ammonia slip in ppm if the NO (^) x concentration in the exhaust gas before conversion was 2000 ppm. (6.3) A diesel particulate filter (DPF) fitted to a 12 litre DI diesel engine is to be regenerated. The engine has volumetric efficiency of 88%, is operating at 67 % excess air and 2000 rpm. The ambient air conditions are 101 kPa and 300 K. For burning the soot collected on the DPF, the exhaust gas temperature is to be raised to 540º C. The exhaust gas temperature entering the DPF is 350º C. Determine the power of an electric heater to raise the exhaust gas temperature to the required level if the entire exhaust gas is to be heated. The specific heat of the gases in the relevant temperature range is N 2 = 30.27, CO 2 = 46.56, O 2 = 31.96, 36.44 kJ/kmol. K. (6.4) Given the LHV of soot = 33.8 MJ/kg , if in a DPF of 1 litre volume 10 g of soot is burned estimate the temperature reached in the DPF. The combustion of soot begins at 540º C. The mass of the DPF is 400 g and its specific heat is 0.9 kJ/kg.K. (6.5) Refer Fig 6.20. Discuss how various technologies have helped in reduction of engine out emissions from the diesel engines. (6.6) A diesel engine has BSFC = 240 g/kWh. In the engine cylinder, lubricating oil enters through piston rings and valve guides which amounts to 0.2 % by mass of the fuel consumption. Of the engine oil in the cylinder, 80% is burned and rest is exhausted as SOF of particulate emissions. Estimate the specific PM emissions solely contributed by the engine oil. How do these compare with the Euro IV PM emission limits for heavy duty engines?
