To date smaller diesel engines have managed to meet Eu emissions regulations, and in some cases US Tier II Bin 5 standards, without resorting to Selective Catalytic Reduction (SCR) NOx aftertreatment. This has been accomplished through improved fuel injection and boosting systems, together with the adoption of low-pressure exhaust gas recirculation and Lean NOx traps.
Although current Lean NOx traps are well-proven the rich phases needed to purge them and manage desulfation, where a temperature in excess of 650°C is required in the presence of a reducing gas, has proven problematic. Since the peak temperature of diesel engine exhaust gas is below 400°C, additional heat is required to remove the sulfurs. When the accompanying increase in fuel consumption and the rising cost of precious metals are taken into account, Lean NOx traps are often not viable.
Furthermore the introduction of the world light harmonized test cycle (WLTP) with new elements such as the real driving emissions phase, which could see wide open throttle being used, will make this strategy less robust in many vehicle classes.
Already in 2013 the implementation of Euro VI standards for medium and heavy-duty commercial vehicles prompted truck manufacturers to introduce Selective Catalytic Reduction aftertreatment to all models, often combining this with Exhaust Gas Recirculation (EGR).
With the introduction of Euro 6 emissions legislation and particularly after the introduction of the WLTP, all but the smallest diesel engines will require SCR for NOx abatement: Which according to market research company, Integer, will result in 24 million passenger cars and 6 million light-duty vehicles fitted with SCR technology by 2020.
A solution of 32.5% urea and 67.5% deionized water, known as AdBlue in Europe and diesel exhaust fluid (DEF) in North America, has been used as the source of ammonia in practically all SCR systems for diesel engines; as a result, Integer predicts that cars and vans in Europe will require up to 150 million litres of AdBlue by 2017. Thereafter, with the implementation of Euro 6c legislation and the so-called “real world driving” emissions testing, consumption is likely to accelerate with additional SCR vehicles entering the European market.
- Urea solutions, however, have several disadvantages:
If urea is dosed at insufficient exhaust temperatures, deposits are formed over the catalyst and in the exhaust system. During low temperature operation such as cold start or slow urban driving, a minimum exhaust gas temperature of about 200°C is required to ensure a complete decomposition and hydrolysis of urea to ammonia.
- Creating uniform ammonia distribution is also problematic, typically requiring a static mixer and/or a certain minimum length of piping between the urea injection point and the catalyst inlet, which complicates exhaust packaging.
- AdBlue/DEF has a freezing point of -11°C, necessitating the use of heating in SCR systems in cold climates.
Alternative Amonia storage systems
A number of alternative ammonia storage materials have been identified to address the issues surrounding urea.
In 2013 Mitsubishi Plastics announced that it would be using zeolite AQSOA for SCR catalysts. Being a microporous, aluminosilicate mineral, zeolite (AQua SOrb) was originally developed as a zeolitic water vapor adsorbent used in adsorption heat pumps, adsorption chillers and desiccant air-conditioners.
According to Mitsubishi the AQSOA SCR catalyst delivers superior catalytic performance across a wide temperature range including low temperatures of around 200 °C, and achieves high durability even in a high-temperature environment; which is essential for SCR catalysts.
However, the alternative ammonia storage and delivery concepts that have attracted most attention are those utilizing solid materials, such as ammonium salts or metal ammines, which contain ammonia and release ammonia gas when heated.
In this type of solid SCR system, a cartridge or canister with a solid material (an ammonia precursor or a substance with absorbed ammonia) replaces the urea tank. During vehicle operation, the material is heated to release pressurized ammonia gas, which is metered into the exhaust using a control valve.
In solid SCR systems, gaseous ammonia can be introduced into the exhaust gas at any temperature, including that experienced during slow urban driving or engine idle conditions. If SCR catalysts with good low-temperature activity are used, NOx can be reduced at temperatures significantly below 200°C.
In the solid SCR developed by FEV/ Tenneco, ammonia carbomate is stored in a canister similar to an oil filter. A small amount of diesel fuel is passed through the canister after being heated to 60o C which causes the ammonia carbomate to sublimate directly into gaseous ammonia. The ammonia and diesel are passed through an oil separator to recover the fuel.
Thereafter the ammonia is injected into the exhaust stream to react with the NOx. No diesel is consumed in the process and the solid system is one third the volume of a urea system. This solid system also provides easier handling and packaging and doesn’t require the heated storage tanks that urea needs to keep from freezing in cold weather.
However, it should be noted that solid SCR technologies cannot address low temperature SCR issues that are rooted in the chemistry of ammonia, rather than urea. Such an issue is the formation of ammonium nitrate (NH4NO3) via reactions between NO2 and NH3 at temperatures below 200°C.
Therefore continuous low temperature SCR operation may not always be possible depending on the exact temperature and NO2 concentration. In SCR systems with advanced control algorithms, the quantity of deposits (such as NH4NO3) are determined as a function of the amount of injected reductant and the operating conditions; with reductant dosing being stopped once a maximum allowed deposit mass has been reached.
Hence, while solid SCR systems can help meet NOx emission limits in testing over regulatory test cycles with low temperatures (such as the chassis FTP schedule), they still face limits in reducing NOx in applications characterized by prolonged low exhaust temperature operation.
In stop-and-go urban driving cycles, diesel engines need more time to heat up and urea-based systems will not begin scrubbing NOx until the exhaust gas reaches at least 180º C.
Seeking to improve on this cold temperature performance, Faurecia has developed an Ammonia Storage and Delivery System (ASDS) whereby urea is replaced with pure gaseous ammonia.
According to Robin Willats, chief engineer-innovation for Faurecia Emissions Control Technologies, the dosing is active within two minutes on even the coldest days and is fully functional by 150º C. Even below 150º C, Willats says the ammonia-based SCR system has been proven to convert more than 50% of NOx, compared with 28% for a conventional urea system. “In this instance, we’re emitting 35% to 40% less NOx,” he says.
Furthermore, the conventional systems require a tank, pump, urea lines and a doser or injector to spray the fluid into the exhaust stream; and need to be topped up every 15,000 Km.
Faurecia’s ASDS module replaces the tank with a self-contained ammonia module under the floorpan. An electronic control unit governs its operation, and no doser or injector are required. Instead, a simple pipe connects the ammonia cartridge to the exhaust stream ahead of the SCR catalyst.
The unit actually contains two ammonia cartridges – the large main one and a smaller “starter” cartridge that heats up more quickly so it can treat NOx more rapidly. The cartridges contain strontium chloride salt, which forms a crystal lattice to store the ammonia. Overall, Willats says the module would be about half the size of a urea tank, resulting in a weight saving of up to 10 kg.
Faurecia claim several automakers and commercial-truck manufacturers are studying the ASDS technology, and that the goal is to price it competitively against urea-based SCR systems.
To further complicate matters, as manufacturers comply with ever lower emissions limits, catalysts must achieve much shorter light-off times and spend more time at their optimum operating temperature. However as CO2 and fuel consumption is reduced through improved thermal efficiency exhaust gas temperatures also drop, thereby making aftertreatment more challenging: Active heating is one solution.
Jaguar and IAV developing an electrically-heated oxi-cat SCR system.
Jaguar and development firm IAV are working on an ambitious ultra-clean diesel engine project with the objective of meeting North America’s 2020 fuel economy targets, greenhouse gas emissions targets, and SULEV 30’s NOx and non-methane organic gas emissions limits. The work is being carried out on a Jaguar XF sedan with a 2.2-litre four-cylinder engine.
The chosen technology package comprises a close-coupled electrically-heated oxi-cat from Emitec, a particulate filter and selective catalytic reduction (SCR) system downstream from that, plus an underfloor SCR cat.
In an effort to improve the low temperature performance of SCR’s Emitec introduced the Emicat with a heater element in the brick, which enables rapid light-off, independent of the engine warm-up phase. This also allows the substrate to maintain an ideal temperature even when the engine is under very light loads or after frequent shut-offs because of start-stop system operation.
Because of the short, 260mm, mixing length the first SCR cat uses Delphi’s compact, watercooled dosing module with IAV designed sheetmetal mixing plates.
Initial tests show that the vehicle meets SULEV 30 limits but that catalyst heating strategies increase fuel consumption slightly. Further optimisation could mitigate this, and improvements in sensor technology and control strategies would help to meet on-board diagnostic requirements.
Cost reduction would be essential for series development: IAV’s aftertreatment chief, Dr Lutz Krämer predicts that the cost of the aftertreatment will be about 26% higher.
Although SCR systems appear to have the most support there are those that advocate the use of lean NOx traps as an efficient, low maintenance form of aftertreatment.
Lean NOx Traps as alternatives to SRC
For the time being, Volvo has selected lean NOx traps (LNTs) for its Euro 6 diesels. The technology has been integrated into the firm’s VEA engine family which will be launched in 2014MY applications including the D-segment S60 sedan and XC60 SUV.
Volvo’s VEA programme director, Jan-Erik Larsson, explains: “We will have an LNT on everything. Not having SCR is a convenient factor for the customer, because they don’t need to fill up the additive tank.”
The aftertreatment technology is close-coupled for rapid light-off and was favoured over selective catalytic reduction because it is maintenance-free.
Although SCR systems are capable of higher NOx conversion rates, the higher cost and the need to periodically replenish the urea additive tank were the key drivers in Volvo’s decision.
Nevertheless should Volvo be required to meet growing demand for diesel vehicles in the US, upgrading to SCR might be necessary to meet Tier II BIN 5 emissions legislation. In which case integrating the technology would be relatively straightforward because the system is a modular design. “We can quite easily go into that market with SCR, but we will need to wait to see when the market is ready in the US,” Larsson said.
An alternative to urea as a DEF.
An alternative to urea-based SCR systems was created by Tenneco along with its partners GE and Umicore.
Defined as a Hydrocarbon Lean NOx Catalyst (HC-LNC), either diesel fuel or ethanol is used as the dosing reductant. The technology based on a very futuristic view of the world, where eventually there would be demand for a non-urea NOx abatement technology has found favour in South America.
Brazil has one of the most developed ethanol economies in the world; ethanol fuel can be purchased at virtually any filling station, and when compared to the cost of a DEF dosed SCR the ethanol replacement can save up to 50% in operating costs.
According to Tim Jackson, Tenneco’s head of technology, strategy and business development, a uniformity index of 95% and, at some places in the duty cycle, 100% is being achieved. But Jackson’s aim is to increase efficiency to 98% across the board.
However in order to realize this, the efficiency of the SCR system has to be improved through ensuring a uniform flow of exhaust gas, temperature, and ammonia distribution across the face of the catalyst.
With AdBlue’s limitations regarding cold temperature performance it appears that there’s scope for an alternative carrier for the ammonia or even an entirely new reductant.
But with SRC packaging already being one of the drawbacks, will the industry choose to look at alternative close-coupled hardware architecture to overcome these short comings?