Biodiesel and SCR systems

A look at biodiesel's performance in SCR systems
By Ron Kotrba | December 30, 2009
Like much of the country, we got dumped on here in Northwest Minnesota with more than 25 inches of snow over the Christmas weekend. It's a sort of eerie feeling to be reminded of just how powerful the forces of nature are. High winds can make snow drifts more dense than steel, or so it seems. Man's equipment and machines, as impressive as they are, don't hold a candle to the sheer power and force of Nature.

It's another short work week with New Year's just around the corner. Without much in the way of industry developments since last week, I thought I'd share some good news about biodiesel and its performance in selective catalytic reduction (SCR) systems, the basis of my February feature article. In 2010, SCR systems will be a widespread, commercial technology integrated into the exhaust systems of new on-road diesels. In my feature I discuss how the technology itself is not "foolproof," meaning there are still some uncertainties regarding long-term performance in the face of potential problems, such as urea deposits, ammonia slip, hydrocarbon loading and maybe even unintentional dioxin formation. These are concerns without even considering how biodiesel interacts with the technology-for the concern over biodiesel is minimal in these systems.

In fact, Aaron Williams, NREL engineer, says there are no short-term concerns about using biodiesel in SCR systems. And the only potential long-term goal is trace metals in the fuel accumulating over time-and remember these devices are warrantied up to 435,000 miles for heavy duty vehicles. But despite the fact that the ASTM limit for calcium and magnesium is 5ppm, and the limit for sodium and potassium is 5ppm, most fuel samples tested are well below that, many are even at such low levels they cannot be accurately measured because current tests detect down to 1ppm only.

Furthermore, in many system designs the SCR is downstream of a diesel particulate filter and a diesel oxidation catalyst, so unless those metals are in a vapor state, the SCR system won't even see them.

A possible benefit from using biodiesel is that it could reduce the possibility of hydrocarbon loading on the SCR catalyst. Again, in systems where the SCR system is downstream, there's little concern about the SCR cat and hydrocarbon loading. Even so, hydrocarbons from biodiesel are of a different nature and have shown not to affix themselves to cat surfaces the same way petroleum hydrocarbons do. Check out the feature article in the February issue. Until next week, have a fun and safe New Year's.