56
3.6.6 Deposit build up on Spray
The AdBlue urea solution has a tendency to crystallize when exposed to the air. This produced a white deposit build up around the spray and tubes. Some of this white deposit hardened if exposed
to high temperature in the exhaust stream in the range of 250
O
to 400
O
C. Under some conditions melamine formation occurred inside the spray injector opening. Some examples of these white
deposits are shown in figure 3.6.6a and 3.6.6b. When this happened, cleaning the spray by soaking with warm water may not be suitable and an ultrasonic cleaning unit was needed.
3.6.6a Deposit on injector sleeve 3.6.6b Deposit around injector
3.6.6c Ultrasonic cleaning
-half immerse
3.6.6d Ultrasonic cleaning full submerge
Figure 3.6.6 White deposit build up and ultrasonic cleaning
The use of the ultrasonic cleaning unit is subject to special attention in order to protect the electrical contact point of the spray unit. The spray unit was disassembled from the main unit and the outer
cover sleeve and the removable part were submerged in the ultrasonic cleaning unit as shown in figure 3.6.6d. The cleaning normally took approximately two minutes. If necessary, the procedure
was repeated. For the main unit with electrical parts, only the mechanical part of the spray was submerged in the ultrasonic cleaning as shown in figure 3.6.6c and the electrical contact point
57 remained above the water level at all times. Once the cleaning was completed the spray unit was
dried completely using compressed air. Further inspection was needed to ensure none of the electrical parts were exposed to water or any debris from the crystallized AdBlue solution.
Sometimes certain parts of the spray injector cleaning could be done manually using tweezers. This procedure depends on the hardness of the deposit formed. An example in this case is shown in
figure 3.6.6e
Figure 3.6.6e Manual cleaning of injector sleeve with tweezers.
3.6.7 Cleaned Spray inspection
Final visual inspection was needed after the cleaning procedures were completed. The areas to be inspected were the main injector sleeve, the injector opening, the supply inlet and outlet and also
the complete assembly as shown in figure 3.6.7. The cleaned sleeve is shown in figure 3.6.7a while figure 3.6.7b shows the main injector opening. Clean inlet and outlet supply lines are shown in figure
3.6.7c. The overall inspection of the spray injector required looking for any debris around the main assembly of the spray as shown in figure 3.6.7d.
58 3.6.7a Cleaned sleeve
3.6.7b Main injector opening
3.6.7c Cleaned inlet supply 3.6.7d Completely assembled clean injector.
Figure 3.6.7 Final visual inspection of fully cleaned injector
59
3.7 NH
3
Gas Experimental Setup
As a comparison with the AdBlue Urea Spray experiment, NH
3
gas at 4 and 5 concentration, the balance being N
2
, was used. The gas experiment was conducted in order to isolate NH
3
species from urea decomposition processes. In the urea spray experiment, it was expected that the urea droplets
would be converted to NH
3
gas. The phase changed and the time taken for it to decompose in the exhaust system before reacting with the SCR catalyst is difficult to predict. Using NH
3
gas should provide information as to SCR performance when 100 of the urea droplet had transformed to gas
phase. When compared to the urea spray experiments it should also provide insight into droplet behaviour.
3.7.1 NH
3
Gas Supply and Nozzle Location.
Initially the test was done utilizing gas bottles containing 4 NH
3
, the balance being N
2
gas, however only approximately 4 to 6 hours of testing was possible. To reduce costs and extend the testing time,
a 5 NH
3
gas was later introduced. The flow rate was lowered about 20 from the 4 gas in order to get similar concentration in ppm. Gas was injected into the exhaust stream at the first
instrumentation module in the same location as the EXSA 1500 sampling point. A nozzle with a J-shape was fabricated of internal diameter 4 mm and 6 mm outside diameter. Since
the pipe diameter of the instrumentation module was 120 mm, the nozzle was designed such that is measured 60 mm from the wall; the centre of the pipe. The nozzle was also pointed to the direction
of the flow. Before connecting the nozzle with the NH
3
gas supply, the nozzle was carefully inserted in the instrumentation module and turned to face the mixing chamber. As the NH
3
gas reached the mixing chamber, it was expected that it would mixed uniformly with the exhaust gases. Then it
would continue flowing through the long diffusing cone, as an approximate one dimensional flow, eventually reaching the SCR catalysts for reduction with NOx. The gas injector geometry is shown in
figure 3.7.1b
60 Figure 3.7.1b NH
3
Gas Injection Nozzle.
3.7.2 Gas flow meter and pressure gauge.