August 19, 2008

TO:  Docket  # EPA-HQ-OAR-2004-008

FROM:  Cheryl Caffrey

RE:  Correspondence with John Reisel of the Center for Alternative Fuels
Mechanical Engineering Department at the University of Wisconsin,
Milwaukee Wisconsin on the paper “Catalyst Deterioration over the
Lifetime of Small Utility Engines”.

________________________________________________________________________

In October 2007, EPA received a copy of the technical paper entitled
“Catalyst Deterioration over the Lifetime of Small Utility Engines”.
 I reviewed the paper that was printed in the October 2007 Journal of
the Air & Waste Management Assocation (ISSN: 1047-3289 J. Air & Waste
Manage. Assoc. 57:1223-1233).  One of the conclusions of the technical
paper is: “The HC emissions did increase over the lifetime of an
engine; they increased routinely as the engine acquired hours. The
efficiency of the catalyst at HC oxidation also decreased as the engines
aged. As a result, for the engines at 187.5 hr, the post-catalyst HC
emissions were higher than the pre-catalyst HC emissions at 0 hr.”
(page 1233-based on Table 1 of page 1227) 

The pre-catalyst HC results presented in Table 1 of page 1227 show that
the engine out emissions have increased noteably.   When calculating the
difference in emissions, pre and post catalyst at zero hour and aged,
one sees that the amount (in g/kWhr) of HC pollutants converted was
slightly increased for Engines 1 and 2 compared to baseline.  

Given the size of the catalyst used on these engines and the space
velocity of the exhaust through the catalyst, the possibility exists
that the catalyst was converting its maximum amount of HC pollutants at
zero hour and was not designed to increase conversion as the engine out
emissions increased, hence “decreased efficiency”.

I discussed this idea with Mr. Reisel, one of the authors of the
article. Mr. Reisel responded by saying the following (summarized):  I
see the difference in how we are interpreting the conversion efficiency
of the catalyst.  We were considering the percent removed, and so when
we concluded that the efficiency was decreasing, as shown in Figure 3. 
But, I see your point with the raw data in Table 1, and that more HC+NOx
is being removed in two of the 3 engines at 187.5 hours.  On the one
hand, that is good because it means that less pollution is getting out,
but we would also argue that it isn't very good because the engines
themselves are producing much more pollution.  But, your view gives me a
different perspective on the data that I should probably look into
further.

