IOWA DEPARTMENT OF NATURAL RESOURCES

Environmental Services Division

Air Quality Bureau

Modeling Group

MEMORANDUM

DATE: 12-18-2007

TO: WENDY RAINS

FROM: AMY SCOTT

RE: EMERGENCY EVENT GENERATORS

CC: LORI HANSON, CHRIS ROLING, ANNE PREZIOSI, JIM MCGRAW

ANALYSIS RESULTS

Passed / Acceptable .

Changes required that affect the permits (see below for details)

Does not pass / Not acceptable

ANALYSIS SUMMARY

Modeling has been conducted in support of a rule requested by MidAmerican and Alliant utilities to 

allow alternate electricity generation in a disaster situation.

The model was set up with one 2-MW portable generator as a representative maximum sized 

emission unit operating at 100% load. Emission rates and stack parameters for the representative 

emission unit were provided by the utilities. Table 1 lists the parameters used to evaluate the emission 

unit. 

Down-wash was not included in the model because:

 the emission units are reported to be housed in non-solid structures (portable generator units 

are on wheels),

 portability of the emission units to any location in the state does not allow for the development 

of a representative “facility plan” for purposes of identifying the presence or absence of 

adjacent buildings with the potential to impact the plume. 

No property line credit was given and the terrain was assumed to be flat. The analysis was completed 

for all ten Iowa meteorological datasets. As a conservative measure, the representative emission unit

was modeled as operating 24 hours/day, 8760 hours/year, which exceeds the proposed 250 hour

operational limit per site.

Provided there are no downwash impacts on the stack, the results from the AMS/EPA Regulatory

Model (AERMOD, dated 07026), indicate that the PM10, SO2, NOX and CO emissions from this facility 

will cause predicted concentrations that are less than the applicable PM10, SO2, NOX and CO NAAQS.

The modeling results are listed in Table 2. It should be noted that I have not checked the correctness 

of the parameters used in the modeling, including the emission rates. 
Table 1. Modeled Emission Rates and Stack Parameter – Point Source

Emission Point PM10 SO2* NOX CO Stack 

Height

Stack Gas 

Exit 

Temperature

Stack Tip 

Diameter

Stack Gas 

Flow Rate

(lb/hr) (lb/hr) (lb/hr) (lb/hr) (ft) (F) (in) (acfm)

EP1 

Diesel Engine 

Generator2 MW

1.04 1.03 36.21 1.09 13.80 975 18.0 17,300

* SO2 emission rate based on No. 2 Diesel Fuel with a maximum sulfur content of 0.05%.

Table 2. Worst Case Modeling Results

Pollutant Averaging 

Period

Year and 

location in 

which 

event 

occurred

Predicted 

Concentration*

Background 

Concentration

Total 

Concentration

NAAQS

(g/m3) (g/m3) (g/m3) (g/m3)

PM10 24-hour

2001 

(Sioux 

Falls)

12.04 45 57.04 150

Annual

2002 

(Sioux 

Falls)

1.12 22 23.12 50

SO2 3-hour 2000 

(Omaha)

24.67 20 44.67 1,300

24-hour

2001 

(Sioux 

Falls)

11.93 20 31.93 365

Annual

2002 

(Sioux 

Falls)

1.11 20 21.11 80

NOX Annual

2002 

(Sioux 

Falls)

39.06 11 50.06 100

CO 1-hour 2003 

(Waterloo)

28.65 0 28.65 40,000

8-hour

2004 

(Sioux 

Falls)

21.96 0 21.96 10,000

* The short-term concentrations are the highest-second-highest predicted values. The annual concentrations

are the highest predicted values.