Direct Injection into the Exhaust Stream of Gaseous Ammonia: Design and Efficiency of Injection and Mixing Hardware (2015)

<div class="section abstract"><div class="htmlview paragraph">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. CO<sub>2</sub>) is achieved via high combustion efficiency that result in higher engine out NOx emissions and lower exhaust gas temperatures [<span class="xref">1</span>, <span class="xref">2</span>]. Conversely, NOx reduction technologies such as SCR require temperatures above 200°C for dosing the reductant (DEF) [<span class="xref">3</span>, <span class="xref">4</span>, <span class="xref">5</span>] as well as for high conversion efficiencies [<span class="xref">1</span>, <span class="xref">2</span>, <span class="xref">6</span>, <span class="xref">7</span>, <span class="xref">8</span>, <span class="xref">9</span>]. 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 NH<sub>3</sub> without deposit formation [<span class="xref">8</span>, <span class="xref">10</span>, <span class="xref">11</span>, <span class="xref">12</span>, <span class="xref">13</span>, <span class="xref">14</span>, <span class="xref">15</span>]. One alternative to dosing DEF, an aqueous solution of 32.5% wt. urea, is to inject gaseous NH<sub>3</sub>. Gaseous NH<sub>3</sub> does not require evaporation of water nor the decomposition/hydrolysis of urea. As a result, gaseous NH<sub>3</sub> 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 NH<sub>3</sub> is introduced into the exhaust as a gas, unlike dosing DEF, injection hardware is less complex and mixers are more compact.</div><div class="htmlview paragraph">The current publication will address the key requirements for the introduction of NH<sub>3</sub> 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) [<span class="xref">16</span>, <span class="xref">17</span>] 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 NH<sub>3</sub> also allows for greater NOx conversion efficiency without deposit formation [<span class="xref">3</span>, <span class="xref">5</span>, <span class="xref">14</span>].</div></div>