On-board Reforming Tests

The engine conditions in this section were chosen to help light-off the catalyst, where therein the fuel can be utilised to promote the reformer temperature until the desired reforming reactions can take place for H2 production. Choosing a high load engine condition can be seen as a worst case scenario (i.e. high NOX and soot emissions) where the HC-SCR catalyst deactivation is most prominent. The chosen engine, on-board reformer and aftertreatment reactor conditions are given in Table 10. The HC:NOX ratio was calculated to be in the range of 2.6-3.0, which from the work of DiMaggio et al. (2009) was believed to minimise fuel penalty without affecting the NOX reduction efficiency.

Table 10: Summary of Test Conditions under an Engine Load of 5.3 bar IMEP

Engine Conditions

The engine-out exhaust emissions are presented in Table 11 alongside the product gas from the on-board fuel reformer. The measured emissions from the reformer show an increase in CO2 emissions from complete oxidation (Reaction 41) of the fuel causing an exothermic reaction and raising the local catalyst temperature. The CO emission is approximately half of the corresponding H2 content indicating that H2 is mainly formed during the SR endothermic reactions (Reaction 36).

Table 11: Average Engine-out Emissions and On-board Reformer Product Gas - Engine Load of 5.3 bar IMEP

Engine-out REGR Product Gas

NO ppm 901.61 1.82

NO2 ppm 39.58 98.18

NO_X ppm 941.20 100.00

NH3 ppm 1.95 161.48

Propylene ppm 1.27 199.58

Ethylene ppm 4.00 387.73

Methane ppm 4.25 2999.43

Ethane ppm 0.00 421.56

Total HC ppm 951.45 9254.94

CO ppm 127.09 44041.79

H2O %Vol. 6.68 5.54

CO₂ %Vol. 6.20 9.31

O2 %Vol. 11.91 0.081

The SR reaction absorbs part of the heat generated by the oxidation reaction thus limiting the maximum temperature in the reformer, as shown in Table 10. It is also suggested that the WGS reaction (Reaction 39) has also taken place as there is reduced H2O in the product gas. Another secondary reaction of H with NO seems to have also taken place in the

Chapter 5: Enhancing the Performance of an Ag-Al₂O₃ Lean NO_X Catalyst using On-Board Diesel Exhaust Gas Fuel Reforming

reformer producing NH3 or N2 and H2O. This can be indicated by the reduction in NOX

emissions and the presence of NH3 in the reformer product gas (Tsolakis et al., 2004).

Since pyrolysis is assisted by catalysis, small olefins such as ethylene and propylene were also formed on the surface of the reforming catalyst. In reforming, it is believed that ethylene is a coke precursor. A high ethylene concentration generally means a severe carbon deposition is taking place on the catalyst (Rostrup-Nielsen et al., 1998, Yoon et al., 2008). At higher temperatures (≥ 650°C), the higher hydrocarbons may react by thermal cracking (i.e.

pyrolysis or steam reforming) producing olefins which can easily form coke (Equation 12) as reported by Rostrup-Nielsen et al. (1993). As a result, this can have implications on the overall long-term performance of the reforming catalyst as the active surface sites will become blocked by various types of carbon deposits.

Equation 12: _ → =? → >=Y → ;

In the study of Shimizu et al. (2000), it was concluded that short chain HC species are not active over the Ag-Al2O3 catalyst under normal diesel exhaust temperatures unlike the longer-chained hydrocarbons. Studies have shown that the longer-chained hydrocarbons tend to have a lower activation (i.e. partial oxidation) temperature and therefore enabling the SCR reaction to proceed within a wider activity window under lower exhaust gas temperatures (Eränen et al., 2004, Shimizu et al., 2000, Shimizu et al., 2001, Lindfors et al., 2004).

Using the chosen reformer and engine conditions stated earlier, the HC-SCR catalyst was subjected to various exhaust/reformate ratios. Reducing the overall reformate flow rate had a significant impact on the NOX reduction performance. Figure 31 shows how the catalyst loses approximately 30% of its NOX reduction activity as the reformate flow rate is successively halved. However, the catalyst is able to regain its NOX performance of ~90%

when the exhaust/reformate ratio was brought back to 6, indicating either the amount of H2 or HC were insufficient to promote the HC-SCR reaction. Therefore by controlling the reformate flow with respect to the exhaust gas flow rate over the catalyst; a significant NOX reduction can be achieved.

Figure 31: The Effect of Reformate Addition on the Catalyst Performance

Further, by altering the GHSV while keeping an exhaust/reformate ratio of 6 as shown in Figure 32, it was possible to maintain a high catalyst activity. Although lowering the GHSV resulted in an increased residence time, there was no further improvement in the catalyst performance. This suggests that the catalyst configuration used here may have been the ideal compromise to gain maximum NOX conversion (i.e. the result of a greater active site length).

0 10 20 30 40 50 60 70 80 90 100

NOXConversion (%)

Test Duration

13:1 27:1 6:1

40 min 30 min 30 min 15 min

Baseline Condition

6:1

Exhaust : Reformate (v/v)

Chapter 5: Enhancing the Performance of an Ag-Al₂O₃ Lean NO_X Catalyst using On-Board Diesel Exhaust Gas Fuel Reforming

Figure 32: The Effect of GHSV on the Catalyst NOX Conversion Performance

Considering an exhaust/reformate ratio of 6, Figure 33 shows that a 90% NOX reduction can be achieved and maintained in the presence of an upstream DPF. Once the fuel to the reformer is stopped, the catalyst activity significantly falls as a result of the reduced HC:NOX

ratio and the absence of H2. This highlights the substantial benefit that can be gained from the available HC species which can act as reductants as well as the H2 produced from the on-board fuel reformer. As the reformer product itself contains a plentiful supply of HC species which are sufficient to provide a suitable HC:NOX ratio, additional reducing agents are not required (e.g. external fuel dosing system apart from the fuel supplied to the reformer). This therefore minimises the overall fuel penalty of the system. To achieve an efficient HC-SCR catalyst, oxidation sites that can partially oxidise the reductant are required as observed in the study of Shimizu et al. (2007). The lower amount of oxidation sites (i.e. metallic silver

compared to higher loaded silver catalysts which can result in complete oxidation of the HC as observed in the work of

as well as some of the non exhaust gas downstream the

will be required after the SCR unit. For example

catalyst to oxidise any remaining HC and CO with the aid of any unused H

known, the oxidation reactions over the DOC can be enhanced in the presence of H promoting the catalyst light

(2009).

Figure 33: Lean NOX Catalyst Performance with On

compared to higher loaded silver catalysts which can result in complete oxidation of the HC Kannisto et al. (2009). Figure 34 and Figure

some of the non-active species, with reference to Table 11, that exhaust gas downstream the HC-SCR catalyst. In order to prevent HC-slip

will be required after the SCR unit. For example, a DOC can be utilised after the catalyst to oxidise any remaining HC and CO with the aid of any unused H

the oxidation reactions over the DOC can be enhanced in the presence of H promoting the catalyst light-off temperature as shown in the study

Catalyst Performance with On-board Integrated Fuel Reformer

compared to higher loaded silver catalysts which can result in complete oxidation of the HC Figure 35 show the total HC , that are present in the slip, a clean-up catalyst a DOC can be utilised after the HC-SCR catalyst to oxidise any remaining HC and CO with the aid of any unused H2. As already well the oxidation reactions over the DOC can be enhanced in the presence of H2 by by Katare and Laing

Reformer

Chapter 5: Enhancing the Performance of an Ag-Al₂O₃ Lean NO_X Catalyst using On-Board Diesel Exhaust Gas Fuel Reforming

Figure 34: Hydrocarbon Concentration Downstream the Lean NOX Catalyst 0

0 300 600 900 1200 1500 1800 2100 2400 2700

HC (ppm)

0 300 600 900 1200 1500 1800 2100 2400 2700

The reformer product gas also consists of NH3, which can be beneficial over the HC-SCR catalyst. However from the work of Eränen et al. (2004), it was reported that NH3 is also formed during the HC-SCR reaction via N2 containing species such as isocyanate and isocyanide. The formation of such species is promoted by a high HC:NOX ratio, the presence of H2 and/or increased temperature which has been extensively reported (Burch et al., 2004, Wichterlová et al., 2005, Sazama et al., 2005, Meunier et al., 2000, Breen et al., 2007). Figure 36 shows the combination of unused NH3 from the reformer product as well as that produced over the HC-SCR catalyst. A clean-up catalyst will need to be considered to account for the NH3 slip or, as shown in the work of DiMaggio et al. (2009), the remaining NOX species can be reduced by implementing an NH3-SCR downstream the HC-SCR.

Figure 36: NH3 Slip Downstream Lean NOX Catalyst

From our ongoing investigation with NH3 and H2 over the Ag-Al2O3 catalyst, the quantity of NH3 present in the reformate, after considering the dilution with the exhaust gas

0 20 40 60 80 100 120 140

0 300 600 900 1200 1500 1800 2100 2400 2700

NH3(ppm)

Time (Seconds)

NH3 Linear (NH3)

Chapter 5: Enhancing the Performance of an Ag-Al₂O₃ Lean NO_X Catalyst using On-Board Diesel Exhaust Gas Fuel Reforming

upstream the HC-SCR catalyst, is not enough to have solely promoted the high NOX

conversion as observed in Figure 33. It is suggested that the NH3 measured in Figure 36 is a combination of the unused reformate product and a small concentration produced over the HC-SCR catalyst under the high H2 presence. As a result, there are particular HC species produced from the reformer that are highly active as a reductant and will need to be further investigated through the speciation of the reformate product. It is also believed that under the present conditions, the HC reaction is more dominant than the utilisation of NH3 over the Ag-Al2O3 catalyst.

It is also important to observe the influence temperature plays on the HC-SCR catalyst performance. Figure 37 shows a controlled temperature ramp with the reformer active and the results show how the peak NOX conversion is substantially affected within a narrow temperature window. The low activity at higher catalyst temperatures can be the result of combustion of the reductant (e.g. HC) as there is an increase in CO2 emissions in the product gas (i.e. downstream HC-SCR catalyst). However, the amount of CO2 already present in the feed makes it difficult to distinguish the amount of formed CO2 from this combustion. At high engine loads (i.e. higher exhaust temperatures) the oxidation reaction becomes predominant and further HC is required. Thus this loss in catalyst activity can be alleviated by tuning the HC:NOX ratio as a function of the exhaust temperature. It is also important to note that the result in Figure 37 shows the effect of temperature under a constant engine exhaust composition where the O2 availability does not change. In real engine operation conditions, a high exhaust temperature (i.e. corresponding to a high engine load) would result in a reduction in the O2 availability.

Figure 37: Lean NOX Catalyst Performance over a Wide Diesel Exhaust Gas Temperature Window

To observe the effect of a stop-start sequence, the reformer product gas was redirected away from the HC-SCR catalyst (while keeping the reformer itself active). This scenario demonstrates how the HC-SCR catalyst will behave if the reformate is suddenly stopped and then reintroduced. As shown in Figure 38, this specific test has a significant impact on the catalyst performance and the catalyst cannot regain its efficiency even though the same quantity of reformate is re-introduced each time. Soot which is effectively trapped within the upstream DPF cannot be the cause for the catalyst deterioration. Thus it is suggested that either the excessive CO or the longer chain HC species are having an impact.

0 10 20 30 40 50 60 70 80 90 100

200 250 300 350 400 450

NOx Conversion (%)

Catalyst Inlet Temperature (°C)

HC:NOXRatio = 2.6 - 3.0 GHSV = 30,000 h^-1 H2Availability = 0.8 - 1.0%Vol.

Chapter 5: Enhancing the Performance of an Ag-Al₂O₃ Lean NO_X Catalyst using On-Board Diesel Exhaust Gas Fuel Reforming

Figure 38: The Effect of a Start-Stop Sequence on the Activity of a Lean NOX Catalyst

In order to demonstrate the effectiveness of the upstream DPF, a similar stop-start sequence was carried out with only the HC-SCR catalysts present in the mini-reactor. As illustrated in Figure 39, the catalyst activity is again in the region of 90%. However, after stopping the reformate flow for a short period of time and following a similar stop-start sequence as before, the HC-SCR catalyst performance is significantly affected. In this case the presence of soot is also participating in the catalyst deactivation through the mechanism of coking. It is suggested that H2 needs to be continuously available when the HC-SCR catalyst is subjected to the exhaust gas to promote a clean catalyst. Figure 31 as illustrated previously shows such an example. Although the quantity of reformate was significantly reduced, it was possible to return to a high NOX conversion activity as a result of having H2 present in exhaust gas mixture.

0 10 20 30 40 50 60 70 80 90 100

0 1000 2000 3000 4000 5000

NOx Conversion (%)

Time (Seconds)

Figure 39: The Effect of a Start-Stop Sequence on the Activity of a Lean NOX Catalyst without a DPF