Browse Publications Technical Papers 2015-01-1021

Direct Injection into the Exhaust Stream of Gaseous Ammonia: Design and Efficiency of Injection and Mixing Hardware 2015-01-1021

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].


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