----- 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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| Cc: |

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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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| Date: |

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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.