Analysis of a Novel Method for Low-Temperature Ammonia Production Using DEF for Mobile Selective Catalytic Reduction Systems

摘要

The worldwide introduction of new emission standards and new, more encompassing, legislating cycles have led to a need to increase both a selective catalytic reduction (SCR) system’s capacity and conversion efficiency. To this end, it is important for an SCR system to operate to the extremes of its temperature range which in many systems is currently limited by the temperature at which diesel exhaust fluid (DEF) can easily decompose without the formation of deposits. This paper analyses a new system for low-temperature ammonia provision to the SCR reaction. Ammonia Creation and Conversion Technology (ACCT) uses pressure controlled thermal decomposition of DEF followed by re-formation to form a fluid with greater volatility and the same ammonia density as DEF conforming to ISO 22241. A dosing strategy can then be employed where any combination of DEF or ACCT solution can be used to provide ammonia as a reductant over the whole activity temperature range of a catalyst. High-speed shadowgraphy data identifies both fluids’ decomposition rates at several temperatures demonstrating ammonia production from 50°C with rapid decomposition and full water vaporisation from 100°C. This study has also equipped an optically accessible hot flow, diesel exhaust simulation rig with a prototype ACCT device. The optical components allow rapid visual verification of deposit growth for bench-marking urea-based system. At a variety of exhaust temperature and mass flow conditions, the study identified a minimum deposit limited working temperature for DEF of approximately 200°C whereas ACCT solution was shown not to form any deposits and readily generate ammonia as low as 50°C. Further to this, gaseous species quantification using FTIR techniques has shown ammonia release in an 800 mm flow path for ACCT solution to be in excess of 80%. The study has demonstrated the effectiveness of ACCT at extending low temperature operating limits of DEF based SCR systems thereby increasing the total possible NO_x conversion.