Direct Injection into the Exhaust Stream of Gaseous Ammonia: Design and Efficiency of Injection and Mixing Hardware

Current legislative trends regarding diesel emissions are striving to achieve two seemingly competing goals: simultaneously lowering NOx and greenhouse gas (GHG) emissions. These two goals are considered at odds since lower GHG emissions (e.g. CO₂) is achieved via high combustion efficiency that result in higher engine out NOx emissions and lower exhaust gas temperatures [1, 2]. Conversely, NOx reduction technologies such as SCR require temperatures above 200°C for dosing the reductant (DEF) [3, 4, 5] as well as for high conversion efficiencies [1, 2, 6, 7, 8, 9]. Dosing DEF requires injection pressures around 5 bar to ensure proper penetration into the exhaust stream as well as generate the appropriate spray pattern and droplet sizes. Dosing DEF generally requires long mixing and/or high turbulence (high restriction) areas so that the aqueous urea solution can be converted into gaseous NH₃ without deposit formation [8, 10, 11, 12, 13, 14, 15]. One alternative to dosing DEF, an aqueous solution of 32.5% wt. urea, is to inject gaseous NH₃. Gaseous NH₃ does not require evaporation of water nor the decomposition/hydrolysis of urea. As a result, gaseous NH₃ can be introduced into the exhaust at temperatures below 200°C without the propensity to form deposits as seen with dosing DEF at similarly low temperatures. Since NH₃ is introduced into the exhaust as a gas, unlike dosing DEF, injection hardware is less complex and mixers are more compact.

The current publication will address the key requirements for the introduction of NH₃ into the exhaust with a low cost dosing tube and a compact mixer. Various injection tubes have been evaluated for gas distribution. The tubes were designed with a variety of shapes as well as with multiple holes. Downstream of the injection tube a mixing element was added to the exhaust stream. Performance was validated by measuring gas composition as well as NOx conversion across a downstream SCR substrate or SCR on filter (SCRF) [16, 17] unit. Restriction across the mixing unit was another key parameter that was investigated. The results show that a compact, low cost injection and mixing system can be implemented into a SCR based NOx reduction system. The use of gaseous NH₃ also allows for greater NOx conversion efficiency without deposit formation [3, 5, 14].