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innovative environmental solutions, inc .

MEMORANDUM

Date: July 27, 2011

To: Lisa Beal, INGAA

From: Jim McCarthy and Jeff Panek, IES Inc.

Re: Concentration versus distance profiles from reciprocating internal combustion engines –

Dispersion modeling results for two example engines from previous INGAA modeling.

Emission impacts from reciprocating internal combustion engines (RICE) have been discussed in 

conjunction with ongoing negotiations regarding the RICE NESHAP, 40 CFR 63, Subpart ZZZZ. In a 

recent conference call, it was noted that RICE dispersion modeling could be used to show concentration

degradation versus distance plots for example RICE. In the call, it was noted that previously completed 

modeling may be available to demonstrate how modeled ground-level impacts decrease with distance 

from the stack. The concentration versus distance plots could thus support approaches being discussed 

for establishing a distance criterion for RICE located in sparsely populated areas. 

IES completed dispersion modeling for INGAA in 2002 and early 2003 during development of the 

original RICE NESHAP. That rule was proposed in December 2002 and promulgated in June 2004. 

Although the rule was a technology-based standard, modeling was conducted to understand the range of 

modeled impacts from example engines to support comments on the proposed rule and discussion 

associated with the Industrial Combustion Coordinated Rulemaking process. Modeling was completed 

for two example reciprocating engines, and the effect of stack height on modeled impacts was assessed. 

Dispersion modeling was completed using ISC3, the preferred EPA model at that time. Assumptions 

associated with the two examples are:

Example engine #1 – Natural gas-fired lean burn engine

 800 hp capacity (modeling based on full load operation)

 30% efficiency (i.e., 8,470 Btu/hp-hr heat rate based on lower heating value)

 8% oxygen in the exhaust (i.e., commensurate with 4-stroke lean burn engine)

 700 oF exhaust temperature

 15 inch stack diameter

 18 ft stack height (“baseline” – a range of stack heights were investigated)

 Exhaust velocity based on the above parameters of 51.1 ft/sec

Example engine #2 – Natural gas-fired rich burn engine

 1,000 hp capacity (modeling based on full load operation)

 27% efficiency (i.e., 9,400 Btu/hp-hr heat rate based on lower heating value)

 0.5% oxygen in the exhaust (i.e., commensurate with 4-stroke rich burn engine)

 1,000 oF exhaust temperature

 12 inch stack diameter

 12 ft stack height (“baseline” – a range of stack heights were investigated)

 Associated exhaust velocity based on the above parameters of 91.5 ft/sec
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The concentration versus distance plots for the two example engines are presented in two figures below. 

The concentration plotted is the maximum modeled 1-hour average concentration associated with RICE 

emissions. These graphs were developed in the 2002 – 2003 timeframe and were not created for this 

memo (e.g., not updated using AERMOD, the currently preferred model). The range of stack heights 

shown were modeled to assess the effect of stack height, and the taller heights modeled are not 

indicative of actual installations or stacks that could actually be installed. Flat terrain was assumed for 

the modeling runs.

Modeling was conducted using a “unitized” emission rate of one gram per second. Pollutant-specific 

impacts can easily be scaled based on proportionality to the unitized emission rate. Thus, the modeled 

impact (shown on the y-axis) is not based on any particular exhaust pollutant. This is a standard 

approach for dispersion modeling when pollutant in-plume reactions are not assumed – i.e., the exhaust

species does not change from the stack to the point of impact – because it eliminates the need for 

multiple, pollutant specific model runs. In contrast, this approach would not be appropriate for 

modeling NOx impacts when NO to NO2 conversion reactions are assumed as the plume disperses 

downwind. When applying a unitized emission rates for modeling, impacts for an array of pollutants 

can be assessed by scaling the modeled impact based on the actual emission rate for the pollutant(s) of 

interest, as compared to the unitized rate. Actual RICE hazardous air pollutant emission rates are much 

lower than 1 g/s. Thus, the scale on the y-axis is not shown in the figures below because it is not 

relevant – i.e., the plots are intended to demonstrate how emission impacts degrade with distance rather 

than assess the impact for comparison to a threshold level. 

The first figure shows concentration versus distance plots for the example lean burn engine. The second 

figure presents the rich burn engine results. The RICE NESHAP discussions have considered different 

distances. When interpreting downwind impacts with distance on the x-axis in the figures below, 402 

meters is equivalent to a quarter mile (i.e., the impact at 400 meters is indicative of a 1⁄4 mile radius).

As expected, both figures show that increasing the stack height can significantly reduce the maximum 

modeled impact by minimizing or eliminating the effects of structure-related downwash. More 

importantly, for the recent RICE NESHAP discussions, the distance plots show that higher modeled 

impacts (associated with shorter stacks) decrease rapidly with distance and the results converge over a 

relatively short distance. 

For the example lean burn engine, maximum model predicted impacts from all stack heights 

investigated occurred at a distance of less than 50 meters (equivalent to 164 feet, 55 yards and 1/32nd of 

a mile). At a distance of 100 meters (equivalent to 328 feet, 109 yards and 1/16th of a mile), higher 

impacts (from shorter stacks) are greatly diminished and at a distance of 150 meters (492 feet, 164 

yards, just under 1/10th of a mile) the impacts are minimal and indistinguishable for the broad range of 

stack heights modeled. 

For the example rich burn engine, the maximum impacts from all stack heights investigated are

predicted to occur at a distance less than 130 meters (equivalent to 427 feet, 142 yards and 1/12th of a 

mile) primarily attributable to building “downwash”. At 200 meters (656 feet, 2190 yards and 1/8th of a 

mile), the higher impacts from shorter stacks are greatly diminished and modeled impacts for various 

stack heights converge. At 400 meters (1312 feet, 437 yards, 1⁄4 mile) the impacts are minimal and 

indistinguishable for the broad range of stack heights modeled.

The figures demonstrate the dispersion and rapid decrease in concentration with distance from “typical” 

engines, and support the use of a distance criterion on the order of 1⁄4 mile or less. 
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Lean Burn RICE: Concentration vs Distance for different stack heights 

("unitized" emission rate used for modeling)

0 50 100 150 200 250

Distance (m)

N

ormalized C

oncentration (mg/m3 per g/s)

15 foot

20 foot

25 foot

35 foot

50 foot

55 foot

70 foot

80 foot

115 foot

150 foot

180 foot

Rich Burn RICE: Concentration vs Distance for different stack heights 

("unitized" emission rate used for modeling)

0 50 100 150 200 250 300 350 400 450

Distance (m)

N

ormalized C

oncentration (mg/m3 per g/s)

10 foot

15 foot

22 foot

35 foot

60 foot

80 foot