Multi-Point Ground Level Flare Modeling Discussion

September 12, 2014 Page 1

Multi-Point Ground Level Flare Modeling Discussion

The flare being installed for this project is a multipoint ground flare that functions as a steam assisted flare 

when receiving vents from normal operations and functions as a pressure-assisted flare during periods of upset, 

maintenance, startup, and shutdown (high-pressure, high flow periods). Pressure assisted flares are also known 

as sonic flares because the exit velocity during periods of high-pressure feeds is at sonic velocities. 

This flare is constructed differently than a normal elevated flare in that it consists of many rows of independent 

flare tips, and all flare tips are approximately eight feet above ground level. The flare is designed to operate in 

stages, with multiple burners (flare tips) in each row, and many rows comprising the flare system. 

Stage 1 is the low pressure stage in which the flare acts as a steam-assisted flare. Stages 2 and beyond are 

activated for high-pressure/high exit velocity flows. The flare system is surrounded by a panel type fence to 

protect nearby workers from the radiant heat from the flare system. At various times, Dow will have emissions 

from the multi-point ground level flare for the following MSS activities: perform plant start-up and shutdown, 

process equipment startup and shutdown, off-spec flaring, non-routine clearing and commissioning of process 

equipment and piping, fuel purging, and flaring to maintain pressure of the net-gas system. 

During MSS activities, the exit velocity from the flare is in a range between 600 feet per second (or 183 meters 

per second) and sonic velocity (sonic velocity ranges from 700 to 1,400 feet per second for common 

hydrocarbon gases that will be flared). There is no available dispersion modeling guidance for multi-point 

ground flares at this time. However, during a meeting with TCEQ on June 11, 2012, TCEQ suggested to 

model this multi-point ground flare as a point source at the center of the ground flare with the actual exit 

velocity for startup, shutdown, and maintenance activities. Therefore, the modeling analysis was performed 

using the expected lower-end velocity of 183 m/s (600 fps) and a higher representative velocity of 300 m/s 

(984.25 fps). 

Per existing modeling guidance for flares, the default exit temperature of 1273 Kelvin was used. There will be 

a fence constructed around the multi-point ground flare with the height of 51 feet above grade. The flare tip 

height combined with the expected flame length makes the effective flare height 45 feet above grade level. As 

the radiation fence is higher than the flame height, it is guaranteed that the emission would not be horizontally 

dispersed until it reaches to the radiation fence height. Therefore, the analysis used the effective height as the 

stack height (45 ft). Dow calculated an effective flare diameter for startup and shutdown activities separately 

according to TCEQ guidance. The modeled emission rates and stack parameters are shown in Table 1. Using 

the generic unit impact from Table 1, the predicted concentration of expected air pollutants from the flare

during startup and shutdown activities were evaluated in Table 2 and 3 for the lower-end exit velocity (183 m/s) 

and in Table 4 and 5 for the higher exit velocity (300 m/s). The predicted impact using lower-end exit velocity 

is slightly greater than the predicted impact using the higher exit velocity.

The analysis used the American Meteorological Society/Environmental Protection Agency Regulatory 

(AERMOD) model (Versions 14134). AERMOD was chosen because it is classified by EPA as 

Preferred/Recommended model and is approved by the TCEQ modeling staff. AERMOD is a steady-state 

plume dispersion model for assessment of pollutant concentrations from a variety of sources. The modeled 

parameters follow the EPA-recommended “regulatory default option”, which includes the use of stack-tip 

downwash and calms and missing data processing routines. The analysis used TCEQ pre-processed 

meteorological data suitable for Brazoria County. AERSURFACE version 13016 was run to estimate which 

land use category best describes the area around the Dow complex where the flare is located. AERSURFACE 

predicted a composite surface roughness of 0.374 meters for a circular area within a 1-kilometer radius 

centered on the main plant site. Based on these results, the meteorological data set that utilized the Medium 

surface roughness length values for Brazoria County was used. 
Multi-Point Ground Level Flare Modeling Discussion

September 12, 2014 Page 2

Table 1. Stack Parameters and Generic Unit Impact

Model 

ID Source Description

Height 

(ft)

Temp 

(K)

Exit 

Velocity 

(m/s)

Effective 

Diameter 

(m)

Unit 

Emission 

Rate (lb/hr)

1-hour Unit 

Impact 

(μg/m3per 1 

lb/hr)

Annual Unit 

Impact 

(μg/m3per 1 

lb/hr)

GF300SU Ground Flare FX-9552 (MSS Startup) 45.00 1273.15 300 10.33 1 0.00311 0.00009

GF183SU Ground Flare FX-9552 (MSS Startup) 45.00 1273.15 183 10.33 1 0.00381 0.00012

GF300SD Ground Flare FX-9552 (MSS Shutdown) 45.00 1273.15 300 14.47 1 0.00211 0.00007

GF183SD Ground Flare FX-9552 (MSS Shutdown) 45.00 1273.15 183 14.47 1 0.00285 0.00009
Multi-Point Ground Level Flare Modeling Discussion

September 12, 2014 Page 3

Table 2. Predicted Maximum Off-Property Concentration from Multi-Point Ground Flare During Start-Up Using 183 m/s of Exit Velocity

1-Hour Analysis Annual Analysis

Compound CAS

Hourly 

Emission 

Rate 

(lb/hr)

Off-site 1-

Hour Unit 

Impact 

(μg/m3

per 1 

lb/hr)

Modeled 

off-site 1-

Hr conc.

(μg/m3)

TCEQ 1-

Hr ESL 

(μg/m3)

Percent 

ESL

Annual 

Emission 

Rate 

(tpy)

Off-site 

Annual 

Unit 

Impact 

(μg/m3

per 1 

lb/hr)

Modeled 

off-site 

Annual 

conc.

(μg/m3)

TCEQ 

Annual 

ESL 

(μg/m3)

Percen

t ESL

Ethylene 74-85-1 2.95 0.00381 0.01 1400 0.00% 0.11 0.00012 2.91E-06 34 0.00%

Butane 106-97-8 75.00 0.00381 0.29 66000 0.00% 0.70 0.00012 1.92E-05 7200 0.00%

Table 3. Predicted Maximum Off-Property Concentration from Multi-Point Ground Flare During Shutdown Using 183 m/s of Exit Velocity

1-Hour Analysis Annual Analysis

Compound CAS

Hourly 

Emission 

Rate (lb/hr)

Off-site 1-

Hour Unit 

Impact 

(μg/m3per 

1 lb/hr)

Modeled 

off-site 1-

Hr conc.

(μg/m3)

TCEQ 1-

Hr ESL 

(μg/m3)

% of

ESL

Annual 

Emissio

n Rate 

(tpy)

Off-site 

Annual 

Unit 

Impact 

(μg/m3

per 1 

lb/hr)

Modeled 

off-site 

Annual 

conc.

(μg/m3)

TCEQ 

Annual 

ESL 

(μg/m3)

% of

ESL

Ethylene 74-85-1 90.08 0.00285 0.26 1400 0.02% 0.19 0.00009 3.92E-06 34 0.00%

Butene 106-98-9 2.49 0.00285 0.01 820 0.00% 0.03 0.00009 5.70E-07 1600 0.00%

Butane 106-97-8 85.49 0.00285 0.24 66000 0.00% 0.95 0.00009 1.96E-05 7200 0.00%

C5's 109-66-0 1.88 0.00285 0.01 4100 0.00% 0.02 0.00009 4.32E-07 7100 0.00%

Benzene 71-43-2 5.86 0.00285 0.02 170 0.01% 0.07 0.00009 1.34E-06 4.5 0.00%

DMDS 

(dimethyl 

disulfide) -- 20.26 0.00285 0.06 20 0.29% 0.23 0.00009 4.64E-06 2 0.00%

Methanol 67-56-1 1.48 0.00285 0.00 3900 0.00% 0.02 0.00009 3.39E-07 2100 0.00%

Note: Emissions of propylene and propane were not evaluated since they are simple asphyxiants.
Multi-Point Ground Level Flare Modeling Discussion

September 12, 2014 Page 4

Table 4. Predicted Maximum Off-Property Concentration from Multi-Point Ground Flare During Start-Up Using 300 m/s of Exit Velocity

1-Hour Analysis Annual Analysis

Compound CAS

Hourly 

Emission 

Rate 

(lb/hr)

Off-site 

1-Hour 

Unit 

Impact 

(ug/m3 

per 1 

lb/hr)

Modeled offsite 1-Hr 

concentration 

(μg/m3)

TCEQ 1-

Hr ESL 

(μg/m3)

Percent 

ESL

Annual 

Emission 

Rate 

(tpy)

Off-site 

Annual 

Unit 

Impact 

(ug/m3 

per 1 

lb/hr)

Modeled offsite Annual 

concentration 

(μg/m3)

TCEQ 

Annual 

ESL 

(μg/m3)

Percent 

ESL

Ethylene 74-85-1 2.95 0.00311 0.01 1400 0.00% 0.11 0.00009 2.18E-06 34 0.00%

Butane 106-97-8 75.00 0.00311 0.29 66000 0.00% 0.70 0.00009 1.44E-05 7200 0.00%

Table 5. Predicted Maximum Off-Property Concentration from Multi-Point Ground Flare During Shutdown Using 300 m/s of Exit Velocity

1-Hour Analysis Annual Analysis

Compound CAS

Hourly 

Emission 

Rate 

(lb/hr)

Off-site 1-

Hour Unit 

Impact 

(μg/m3per 

1 lb/hr)

Modeled offsite 1-Hr 

concentratio

n (μg/m3)

TCEQ 1-

Hr ESL 

(μg/m3)

% of

ESL

Annual 

Emissio

n Rate 

(tpy)

Off-site 

Annual 

Unit 

Impact 

(ug/m3 

per 1 

lb/hr)

Modeled offsite Annual 

concentratio

n (μg/m3)

TCEQ 

Annual 

ESL 

(μg/m3)

% of

ESL

Ethylene 74-85-1 90.08 0.00211 0.26 1400 0.02% 0.19 0.00007 3.05E-06 34 0.00%

Butene 106-98-9 2.49 0.00211 0.01 820 0.00% 0.03 0.00007 4.43E-07 1600 0.00%

Butane 106-97-8 85.49 0.00211 0.24 66000 0.00% 0.95 0.00007 1.52E-05 7200 0.00%

C5's 109-66-0 1.88 0.00211 0.01 4100 0.00% 0.02 0.00007 3.36E-07 7100 0.00%

Benzene 71-43-2 5.86 0.00211 0.02 170 0.01% 0.07 0.00007 1.04E-06 4.5 0.00%

DMDS 

(dimethyl 

disulfide) -- 20.26 0.00211 0.06 20 0.29% 0.23 0.00007 3.61E-06 2 0.00%

Methanol 67-56-1 1.48 0.00211 0.00 3900 0.00% 0.02 0.00007 2.64E-07 2100 0.00%

Note: Emissions of propylene and propane were not evaluated since they are simple asphyxiants.