Solid Reductant Storage for SCR Systems

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Abstract: SCR systems that store a solid ammonia precursor and deliver gaseous ammonia reductant to the SCR catalyst allow for improved low temperature performance compared with urea-SCR systems. Two main groups of materials that can store ammonia and release it on heating include metal chloride ammines and ammonium salts. Solid SCR systems under development utilize strontium ammine chloride or ammonium carbamate for ammonia storage.

Introduction

Selective catalytic reduction (SCR) has been introduced to control NOx emissions from a range of diesel engines, including light- and heavy-duty highway engines and nonroad diesel engines. Water-based, 32.5% solution of urea—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. Urea solutions, however, show a number of disadvantages, especially during low temperature operation such as cold start of 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. If urea is dosed at insufficient exhaust temperatures, deposits are formed over the catalyst and in the exhaust system. Creating uniform ammonia distributions 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. An additional disadvantage of AdBlue/DEF is its freezing point of -11°C, necessitating the use of heating in SCR systems in cold climates.

A number of alternative ammonia storage materials have been identified to address the issues with urea [2248]. The alternative ammonia storage and delivery concepts that attract most attention are those that utilize solid materials—such as ammonium salts or metal ammines—which contain ammonia and release ammonia gas when heated. In this type of a solid SCR system, a cartridge or canister with a solid material—an ammonia precursor or a substance with absorbed ammonia—is carried on the vehicle. During vehicle operation, the material is heated to release pressurized ammonia gas, which is metered into the exhaust gas using a control valve. Once the cartridge becomes depleted, it has to be removed from the vehicle and re-charged with ammonia using specialized equipment.

In solid SCR systems, gaseous ammonia can be introduced into the exhaust gas at any temperature, including 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. The performance improvement due to the use of gaseous ammonia as the reductant is illustrated in Figure 1 [3533].

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. While examples of SCR operation with continuous ammonia dosing during urban driving were reported in the literature [2256], 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 amount of deposits (such as NH₄NO₃) is calculated as a function of the amount of injected reductant and the operating conditions, and reductant dosing is 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 may face limits in reducing NOx in applications characterized by prolonged low exhaust temperature operation.

Commercial Viability & Infrastructure. The scope of commercial applicability of solid SCR systems will depend on several factors—including their cost competitiveness with urea-SCR systems. At least two solid SCR systems have been under development: (1) a metal ammine chloride system by Amminex and (2) an ammonium carbamate system by FEV/Tenneco.

A limiting factor in the adoption of solid SCR is the lack of infrastructure for the replacement and re-charging with ammonia of used cartridges. Compared to urea-SCR, the logistics are more complex with solid SCR, as both empty and re-charged cartridges need to be handled. A number of types and sizes of cartridges may be also envisioned in solid SCR systems, depending on the type and size of vehicle. Therefore, solid SCR seems to be most suitable—at least in the initial period—for vehicles operated within a limited range. Urban delivery trucks or buses, for example, could be supplied with cartridges within a geographically limited distribution network.

If the commercial reach of solid SCR widens, infrastructure will have to be developed to supply and recharge/recycle ammonia storage cartridges. In the case of metal ammine-based technologies—where the cartridges are charged using pure, anhydrous ammonia—the necessary infrastructure could be built based on the existing ammonia distribution network [2256]. The cartridges could be recharged with ammonia in recharge stations located near ammonia distribution centers. Several industrial gas suppliers operate extensive networks of ammonia ‘hubs’ and each of these ‘hubs’ may potentially serve as the basis for a recharge station.

In an expansion scenario, solid ammonia cartridges can be envisioned to be handled in a similar manner to propane bottles for end-user applications, such as barbecues. End-users would not have to consider the fate of a used solid ammonia storage cartridge—it would be handled by professional organizations that would control the logistics, recharge and quality assurance.

It may be noted that solid ammonia storage also shows certain advantages compared to the urea infrastructure: no multiple quality assurance steps are required in the reductant supply chain. With a well-defined solid cartridge that has been quality assured from production or recharge, the content of the unit will not be exposed to the surroundings and will not degrade under any—hot or freezing—climatic conditions.