Document ID: EPA-HQ-OAR-2008-0708-0526
Agency: epa
Document Type: Supporting & Related Material
Title: 
Posted Date: 2010-08-12T04:00Z

----- Forwarded by Melanie King/RTP/USEPA/US on 08/05/2010 04:41 PM
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  |"Bromberg, Kevin L." <kevin.bromberg@sba.gov>                                                                                             
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  |Melanie King/RTP/USEPA/US@EPA, RobertJ Wayland/RTP/USEPA/US@EPA                                                                           
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  |"Higgins, Cortney" <Cortney_Higgins@omb.eop.gov>, "Kymn, Christine J." 
<Christine_J._Kymn@omb.eop.gov>, Michael Horowitz/DC/USEPA/US@EPA, |
  |Fred Talcott/DC/USEPA/US@EPA                                                                                                              
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  |08/03/2010 05:15 PM                                                                                                                       
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  |FW: NOx Reductions vs. HCHO Reductions - More Technical Information - can 
be shared with EPA                                              |
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I told Peter that I would share this with EPA so we can get its comments in a 
timely manner.  This affects the NOx benefits from 4SRB engines.

Cortney – I’m working on a consolidated document to share with EPA.
This has more details on the NOx issue than that document will have.

Kevin

From: Bromberg, Kevin L.
Sent: Monday, August 02, 2010 1:31 PM
To: 'Higgins, Cortney'; 'Kymn, Christine J.'
Cc: 'Lee, Amanda I.'; 'Nancy.Johnson@hq.doe.gov'; 'Zwicke, Greg - Portland, 
OR'
Subject: NOx Reductions vs. HCHO Reductions - More Technical Information
- can be shared with EPA

If you get NOx reduction with NSCR it is usually relatively high – e.g., 90% 
or more.  The problem is that the NOx performance is very sensitive to the air 
to fuel ratio (AFR) and the stream needs to be “more rich” (i.e., more fuel 
rich or air deficient).  A lesser NOx reduction (e.g., 20 – 60% NOx reduction) 
with NSCR is typically not seen, because the performance curve (versus AFR) is 
very steep – meaning that you go from no NOx reduction to relatively high NOx 
reduction (e.g., 80% or
more) with very small changes to AFR.  Similarly, if set for “richer”
operation with NOx reduction, relatively small changes in AFR can move the AFR 
more lean and significantly impact NOx performance.  Thus, if an operator 
installs NSCR to meet a NOx limit, the set to provide a better assurance of 
emission performance is often accomplished by establishing a “richer” set 
point (which can result in significant ammonia emissions).  In this scenario, 
there is more of an operating window before minor AFR changes can move “too 
lean” and significantly compromise NOx performance.  Those richer set point 
also result in lower efficiency / higher fuel use, more hydrocarbon (HC) 
emissions, and hotter operation – and these conditions affect maintenance 
requirements because the HC and higher temps impact performance and increase 
maintenance frequency (and related costs).

An example of NOx (post NSCR catalyst) versus AFR for a RB with NSCR is 
provided here. Note the “stoichiometric ratio” (SR) is the theoretical amount 
of air relative to the amount required to burn all of the fuel – i.e., at 1.00 
there is just enough air to burn all the fuel.  From the scale on the x-axis, 
a change in the amount of air by 1 or 2% dramatically changes the NOx 
emissions (and ammonia and CO emissions).
This level of AFR control is difficult to maintain on a continuous basis. It’s 
also not reasonable to operate in a region of the “NOx curve” that provides 
intermediate levels of NOx reduction so in general NOx reduction with NSCR 
close to an all or nothing proposition. Also – as a reminder, the definition 
of “rich burn” is an engine with a stoichiometric ratio up to 1.10 (beyond the 
scale of this graph).  As you can see from the graph, the NOx emissions on the 
right side of the graph (from SR of about 0.99 or higher) have high NOx 
indicative of very little or no NOx reduction.  This relates to our discussion 
that NOx reductions cannot be presumed with NSCR – and operators can achieve 
better efficiency, lower fuel use, and lower greenhouse gas emissions by 
running leaner where NSCR will not reduce NOx.

Also for the NOx data – the marginal relative change in the 3 data points to 
the right (from about 2100 to about 2650 ppmv) is due to changes in the inlet 
NOx and not from NSCR reduction – i.e., the inlet NOx versus AFR is also 
dependent on AFR at this operating threshold where you cross from “rich” to 
“lean”, and the NOx reduction across the catalyst is insignificant.  Note that 
the one NOx point at about 850 ppmv is 58% reduction and the three low NOx 
points on the left are about 98% reduction for these tests.

(Embedded image moved to file: pic09770.gif) Since facilities will be 
attempting to meet a HCHO standard in the neighborhood of 3.7 ppm, the 
preferred operation will use a stoichiometric ratio of 0.99 or more, which 
would be on the “fuel lean”
side (right hand side) of the graph.  However, the NOx reductions will only 
occur on the rich side (left hand side) of the graph – in this case 
approximately 0.98 or less.  In addition to good HCHO emissions performance, 
there are additional reasons for being on the lean side of the curve – reduced 
fuel consumption, associated reductions in greenhouse gas emissions (i.e., 
lower CO2 and methane), reduced maintenance, and avoidance of ammonia and CO 
emissions.  The NOx reductions go very quickly from roughly 98% to 0% once you 
move from
richer settings to leaner operation above about 0.98.   Also, the air to
fuel ratio differences are very small and given some instability of the 
settings of these engines, maintaining a precise setpoint is difficult and you 
will need to set an even leaner setting optimize engine operability and 
effieciency – or, conversely, set a richer setting to
ensure NOx reductions.   The above graph is from a rich burn engine
tested by Colorado State University. Unless EPA is also setting a NOx limit 
with the HCHO limit, these desired NOx reductions are unlikely to occur.

EPA may wish to review additional data to see how under what AFR ratios, both 
the HCHO number and the desired NOx reductions can occur (by how much and 
whether these overlap), although I don’t know how this result can be achieved 
under the current standard.

This argues that our NOx reductions will be lower for the 300-500 HP major 
sources and the >500 HP area sources, and reinforces our argument to drop the 
NSCR for sources <750 HP and in rural America.