Data comparing fuel-grade ethanol to relatively heavy hydrocarbon reductants. 

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... data is available from a separate, but related effort using the same HC-SCR system. The major practical difference is the SV and NO x levels were not held at 50,000/h and 200-240 ppm values used for the main body of data. Results for a low sulfur number 2 diesel fuel, a low sulfur kerosene, an iso- paraffin mixture and fuel grade ethanol are compared in Fig. 7. The compounds other than ethanol are rather ineffective as reductants. A single test using heptane at 100,000/h SV and 350°C exhaust temperature (not shown) gave only a few percent NO x conversion. Including the cyclohexane results discussed earlier, the non-oxygenated reductants tested in this study all gave relatively poor results. These potential reductants were alkanes or contained a large amount of alkanes compounds. Other types of non-oxygenates may give different results. Some descriptions and explanations are offered addressing the hierarchy in performance among reductants tested. Aldehyde formation - There is experimental evidence that ethanol, and 1-propanol undergo oxidation to form acetaldehyde and propionaldehyde, respectively. 1,4 It is likely that 1-butanol also forms a corresponding aldehyde. The aldehydes, which are thought to be good reductants, break down further as part of the reduction process. 1,4,5 It is proposed that 2-propanol forms acetone 4 which then breaks down further. We note that 2- propanol was quite superior as a reductant compared to acetone, especially at low temperatures, so this explanation may not be fully satisfactory. In forming either an aldehyde or ketone, the alcohol donates two H atoms, which presumably enhance in the overall reduction process. Tert-butanol would not be expected to form an aldehyde or a ketone and proved to be relatively unreactive for the tested system. Reactivity – It is obvious that reductants that react or break down easily are likely to create “usable” reactive species, particularly at low temperatures. This might explain ethyl acetate and acetone looking like reasonable reductants at ~ 400°C, but not at low temperature, where they remain relatively stable. There was some expectation that the cyclohexanol could have some reactivity and behave somewhat like hexanol or the 2-propanol. Instead, cyclohexanol was completely unreactive with the tested system, likely due to the high stability of the six carbon ring structure. Reactivity indications - Evidence of oxidation of reductants can be inferred from the measured CO 2 , CO and HC levels and the temperature difference between the catalyst inlet and the catalyst core. The net reactions occurring appear to be quite exothermic. Unfortunately the CO 2 measurement is dominated by the engine-out values (~5-10%) and the increase derived from the reductants is about 0-2500 ppm in the range of interest. Furthermore the flame ionization detector for HC measurement used in this work gives useful information, but has a response that varies widely for many of the species likely present, and the actual slip species are not well characterized. It is not possible to compare and interpret the CO 2 and HC readings with confidence. However, a rise in CO and CO 2 is expected for the compounds that decompose and oxidize along with a relatively low HC reading, and the opposing trends are expected for compounds that are unreactive. Analysis of the CO 2 “rise” data for C/N values of 9-12, gave somewhat crude and scattered results, but a few trends were seen. The poorest performing compounds, cyclohexane, cyclohexanol and tert-butanol, showed virtually no detectable CO and CO 2 formation except at the highest temperature condition where it is estimated 15-30% of the injected carbon ended up as CO and CO 2 . These compounds also gave consistent and high HC readings (accounting for ~68-87% of the injected carbon, depending on the reductant) for the lower temperature conditions (conditions 1-4 in Table 3) with a modest drop in HC value for the highest temperature condition (condition 5 in Table 3). All other reductants gave much higher values for CO + CO 2 production, with increasing values for increasing temperature, and the opposing trend for the HC emissions. Ethylene glycol stood out as having the highest propensity to react to form CO + CO 2 at all temperatures (~ 80 % at the lowest temperature, and rising to ~ 100% at the highest temperature), followed by 1,3-propandiol and ethyl acetate. Ethylene glycol also displayed the highest degree of exothermic activity. Polar compounds, water solubility - It is proposed that a distinct advantage is possessed by the more polar oxygenates, which can compete successfully with water for adsorption sites. 2,3 The environment of interest has abundant water which doubtlessly affects the catalytic process. This property favors the light alcohols and light asymmetric oxygenates. Note that the non-polar diols tested do have very high water-solubility, and may be less disadvantaged compared to low water- solubility compounds. Hexanol and octanol notably have lower water solubility than the lighter alcohols. The non-oxygenated compounds have very low solubility. Molecular mobility - The ability of the compound to diffuse to make intimate contact with the catalyst surface and then be mobile on the surface, could affect the SCR process. This mobility property could be related to the molecular weight, boiling point (listed in Table 2) and other properties of the compound. No attempt to quantify this property is offered. Indirect evidence of a physical interference process, probably involving carbonizing (coking) of the reductant on the catalyst surface, was seen with octanol and compounds of higher molecular weight. The observation was that as injection quantity was increased, NO x conversion began to decrease and would also slowly decrease with time at a given spray rate. The tested HC-SCR system performed well with ethanol, 1- propanol, 2-propanol and 1-butanol as reductants over the range of conditions explored. These light alcohols gave greater than 80% NO x reduction over a broad temperature range for C/N of 9 or greater and 50,000/h SV. A desirable shift toward effective NO x reduction at 260-300°C, was seen for 1-propanol, and 1-butanol. Relatively good performance in the 260-300°C temperature range was also found for 1-hexanol and 1-octanol, but with reduced performance at higher temperatures compared to the lighter alcohols. The tested system gave > 50% NO x reduction at 260-270°C for number of primary alcohols (1- propanol, 1-butanol, 1-hexanol, 1-octanol) for a C/N ratio of 9 or below. 1,3-propanediol is seen to be less effective as a reductant compared to the light alcohols, although it performs as well or better than ethanol at 250-300°C. Ethylene glycol performed relatively well 275°C, but is a relatively poor reductant at higher temperatures. Ethyl acetate, and acetone were both are seen to be good reductants at 400°C and above but not at lower temperatures. Potential reductant candidates that performed quite poorly include tert-butanol, cyclohexanol, cyclohexane, n-heptane, diesel fuel, kerosene and an iso-paraffin mixture. Some overall patterns were observed from the testing of the 17 reductants with this particular SCR system. The results can be associated with certain chemical and physical properties of the reductants tested. Highly polar, water soluble compounds are thought to have a significant advantage, because they compete successfully with water for catalyst surface sites. Low molecular weight may be advantageous, allowing high diffusion rates and good surface mobility. High chemical reactivity in the appropriate temperature range is also desired. This may explain the superior performance of the light alcohols which have the previously mentioned attributes. The primary alcohols appear to readily form aldehydes while donating two protons per molecule in the process. In an analogous fashion, 2- propanol likely forms a corresponding ketone with the same desirable proton donation. These concepts can be applied to the other reductants. The heavier primary alcohols tested, 1-hexanol and 1-octanol, did not perform as well as the lighter alcohols, probably due to being incrementally less polar and mobile. The diols tested were symmetric and non-polar, but appeared to be reactive. Testing a 3 or 4 carbon polar diol could shed more light on these contentions. Ethylene glycol stood out as being exceptionally reactive toward oxidation but was relatively poor at selective reduction of NO x . This may apply to 1,3-propandiol but to a much lesser extent. For the non-alcohol reductants we see that the oxygenates, ethyl acetate and acetone, are low molecular weight and polar, but are not reactive at lower temperatures. The non- oxygenated compounds are not water soluble, and probably have some difficulty competing for active surface sites. The relatively long-chain hydrocarbons showed more reactivity than n-heptane or cyclohexane, a general trend also seen in the literature. More could be learned by examining the HC and nitrogen containing slip species and speices found at different positions within the catalyst through in-catalysts sampling. A follow-on effort of this type for selected reductants could be considered. Another question to investigate is the feasibility of the successful reductants to be fuel-borne or fuel derived. Ethanol/diesel mixtures have been examined due to abundant and relatively inexpensive domestic ethanol production. Such fuel has several drawbacks including flammability/safety issues. More could be done to look into what other alcohols are feasible as either fuel-borne removable reductants, or that could be produced on-board from diesel fuel or a fuel-borne ...