ARTICLE pubs.acs.org/IECR
Impact of Flare Destruction Efficiency and Products of Incomplete Combustion on Ozone Formation in Houston, Texas Fahad M. Al-Fadhli, Yosuke Kimura, Elena C. McDonald-Buller, and David T. Allen* Center for Energy and Environmental Resources, The University of Texas at Austin, 10100 Burnet Road, Building 133, M.S. R7100, Austin, Texas 78759, United States
bS Supporting Information ABSTRACT: The impact of flare destruction removal efficiency (DRE) and Products of Incomplete Combustion (PICs) on ozone formation was examined using a regional photochemical model. Emission scenarios for five industrial flares were considered. For each flare, four DRE values (95, 90, 75, and 50%) were assumed, along with a base case that assumed 98 or 99% DRE. For each DRE level, a scenario assuming that no PICs and a scenario assuming a level of PICs consistent with full scale flare tests was evaluated. Simulation results indicated that low DREs can increase ambient ozone concentrations by more than 15 ppb under some conditions, but under other conditions, may raise ozone concentrations by 1 ppb or less. Emission rates of unburned flare gases and the chemical reactivity of the unburned hydrocarbons explain much of the variability in ozone formation. The air quality simulations also showed that unburned flare gases can have a larger impact on ozone formation than PICs.
’ INTRODUCTION Flares are designed to combust waste organic gases at very high efficiency. Most flares are designed to have destruction removal efficiencies (DREs), defined as the percentage of waste gas fed to the flare that is destroyed by complete or partial combustion, of 98% or 99%. Flares are also designed to operate over a very large range of flow rates. Emergency flares need to be able to handle the large volumes of gases that may need to be disposed of very rapidly during a process upset or during a process start-up or shut-down. Some of these emergency flares are also used to destroy much lower flows of gases that occur during routine operation. Webster et al. have reported on variability in the flow to a small sample of petrochemical flares;1 Pavlovic, et al.2 (this special issue) have reported on flow variability for a much larger number of flares. Both of these studies have confirmed that flow to flares varies over large ranges, and have presented quantitative characterizations of variability in flare flows. Several studies have examined the impact of flow variability in industrial flare emissions on ambient ozone concentrations, particularly focusing on the Houston Galveston area.1,3 6 However, all of these studies have assumed that the destruction efficiency in flares remains constant at 98 99%. At high flows, and under conditions above a threshold exit velocity, measurements have indicated that flares operate at high DREs.7,8 However, a number of field observations indicate that DRE can fall below the targeted 98 99% values under certain conditions. For example, combustion efficiencies below 85% were measured for two flares (sweet and sour gas flares) in Alberta, under conditions of relatively low flow and liquid carry-over.9 During a field measurement campaign in Houston, the DRE of two flares, estimated using Solar Occultation Flux techniques, were low at low flow rates.10 Recent measurements of full scale flares under controlled flaring conditions, reported by the University of Texas11 and in this special issue12,13 have indicated that for some r 2011 American Chemical Society
types of flares, low flows and high steam or air assist rates lead to destruction removal efficiencies substantially below 98 99%. Low destruction efficiencies were observed even under some conditions when standard emission estimation algorithms would have predicted 98 99% DRE. Recently, field measurement tests were conducted to measure the combustion efficiency of two industrial steam-assisted flares at petroleum refineries in Texas City, Texas and Detroit, Michigan using passive Fourier transform infrared spectroscopy. The tests showed results that were qualitatively and quantitatively similar to those of the University of Texas studies. Increasing the amount of steam assist at low flow can reduce the combustion efficiency dramatically below 98%.14,15 Overall, these studies indicate that low destruction efficiencies are possible in industrial flaring, even when the flares are operated at conditions that may be expected to lead to high DREs. These lower destruction efficiencies at low flows will influence the air quality impacts of flare operation. Computational studies have shown similar results. Casti~ neira and Edgar studied flare DRE using computational fluid dynamics simulations. The simulations indicated that high steam/feed gas and air/gas ratios cause inefficient combustion (decreasing the DRE). Waste gases with lower heating values (LHVs) below 200 Btu/scf were predicted to cause a dramatic decrease in flare efficiency.16 Computational studies of the impact of wind speed indicated that cross winds shortened the flame length, decreasing flame efficiency, and that increasing the exit velocity of high momentum flames decreased the flare combustion efficiency under crosswind conditions.17,18 Special Issue: Industrial Flares Received: July 1, 2011 Accepted: September 23, 2011 Revised: September 22, 2011 Published: September 23, 2011 12663
dx.doi.org/10.1021/ie201400z | Ind. Eng. Chem. Res. 2012, 51, 12663–12673
Industrial & Engineering Chemistry Research
ARTICLE
Table 1. Twenty-Four Refinery Flares with the Highest VOC Emissions, As Reported through a Month-Long 2006 Inventory flow rate
emissions, assuming
(tons/32 day) 98 or 99% DREa (tons/32 day)
process type natural gas
2639.12
52.78
fuel fired equipment
1511.16
30.22
process gas
1515.42
15.15
process gas fluid catalytic cracking unit
716.18 638.60
14.32 12.77
natural gas
462.22
9.24
natural gas
440.83
8.82
natural gas
333.52
6.67
process gas
321.03
6.42
unclassified
311.80
6.24
unclassified
305.56
6.11
process gas natural gas
302.46 297.22
6.05 5.94
unclassified
240.75
4.82
unclassified
207.46
4.15
unclassified
199.54
3.99
unclassified
147.31
2.95
unclassified
87.61
1.75
unclassified
70.84
1.42
fuel fired equipment fuel fired equipment
69.16 48.87
1.38 0.98
process gas
29.98
0.6
fluid catalytic cracking unit
14.60
0.29
2.18
0.04
process gas
Figure 1. Monitored hourly flow rate time series for Refinery Flare 1.
Figure 2. Monitored hourly flow rate time series for Refinery Flare 2.
a DRE of 98% or 99% can be assumed if a flare satisfies all criteria of 40 Code of Federal Regulation (CFR) § 60.18. If the flared gas is alkane (butane +) and hydrogen, a DRE of 98% is assumed, and if the flared gas is propylene, propane, or ethylene, a DRE of 99% is assumed.20
Table 2. Petroleum Refinery Flares Selected for Photochemical Modeling Analyses flare identifier
flow rate
emission assuming 98 or 99% DREa
(tons/32 day) (tons/32 day)
location
average heat content
(lat, lon)
(Btu/Ib)
refinery flare 1
1511.
30.22
29.717, 95.130
19300
refinery flare 2
1515.
15.15
29.723, 95.209
19400
refinery flare 3
716.
14.32
29.371, 94.927
19600
DRE of 98% or 99% can be assumed if a flare satisfies all criteria of 40 Code of Federal Regulation (CFR) § 60.18. If the flared gas is alkane (butane +) and hydrogen, a DRE of 98% is assumed, and if the flared gas is propylene, propane, or ethylene, a DRE of 99% is assumed.20 a
Previous analyses of the air quality impacts of flare emissions have assumed constant destruction efficiencies; these studies have also generally assumed that unburned hydrocarbons exiting the flare have the same composition as the waste gas. Recent measurements have characterized products of incomplete combustion (PICs) in flares, and these PICs include both highly reactive gases (e.g., formaldehyde and acetaldehyde) and less reactive gases (e.g., CO).11,19 These PICs may also influence the air quality impacts of flare emissions.
Figure 3. Monitored hourly flow rate time series for Refinery Flare 3.
This study will use results from recent studies11,19 of the DRE and PICs in full scale flares to extend previous analyses of the air quality impacts of flare emissions. The impact on air quality of flare DREs that are less than 98 99% and the impact on air quality, specifically ozone formation, of products of incomplete composition (PICs), will be examined.
’ METHODOLOGY Industrial Flares. The first step in the analysis was to select specific flares for analysis. Flares from petroleum refineries and from olefin manufacturing operations were chosen, as described below. Petroleum Refinery Flares. Detailed data on the flow rates to petroleum refinery flares have been examined by Pavlovic.2,6 These flares were classified into categories based on the sources of the waste gas fed to the flare, the composition of the waste gas streams, and the variability in the flow rates. Table 1 lists the 24 12664
dx.doi.org/10.1021/ie201400z |Ind. Eng. Chem. Res. 2012, 51, 12663–12673
Industrial & Engineering Chemistry Research
ARTICLE
flares in the Houston Galveston area, reported with refinery source codes, with the highest reported flow rates. The flow data are based on information from a month-long study period (August 15 September 15, 2006) during which hourly mass flows fed to the flares were reported. The flares are grouped into five categories: natural gas flares, process gas flares, fuel fired equipment flares, fluid catalytic cracking unit flares, and unclassified flares. This work will examine the potential air quality impacts of low destruction efficiencies and PICs formation on three of these flares. These flares were selected based on their relatively high average flow rates and the photochemical reactivity of the vent gases. The three flares chosen are described in Table 2. Figures 1, 2, and 3 show the monitored hourly flow rate time series for each of the flares selected for detailed modeling, during the monthlong data collection period starting August 15 and ending September 15, 2006 (768 h). Olefin Flares. Flares used in olefin (ethylene and propylene) manufacturing processes were chosen for analysis because they have relatively high flow rates and because they can emit chemical species that have high photochemical reactivity. Table 3 lists the 17 flares reported with olefin manufacturing source codes that had the highest flow rates during the August 15 through September 15, 2006 reporting period.6 Two of these flares were
chosen for detailed analysis (Table 4), based on their relatively high flow rates and their physical location within a region in which photochemical models with detailed spatial resolution were available. Figures 4 and 5 show the monitored hourly flow rate time series for each of the flares selected for detailed photochemical modeling. The chemical composition of the inlet flow was assumed constant for each flare, and was based on the monthly average composition. Also, the composition of the VOCs reported as unclassified in the flare composition data (ranging from 0 to 54.6% mass fraction) was assumed to have a composition identical to the average composition of identified species. Table 5 shows chemical compositions for each flare. The flared gases contain a mixture of olefins and alkanes, and therefore, the emissions from these flares are expected to have a broad range of chemical reactivity. This variability in the composition of flared gases also indicated that the flares service very different chemical and petroleum processing operations. Further, the labeling of these flares as refinery or olefin manufacturing should be regarded a broad source identifier. Operations serviced by these flares include integrated refining and chemical manufacturing operations, and the identifications used in this work are based on the source code identifiers used in reporting the emissions. Emissions Scenarios. VOC Emissions. Flare DRE and the extent of PICs formation could be influenced by many factors,
Table 3. Seventeen Olefin Manufacturing Flares with the Highest VOC Emissions, As Reported through a Month-Long 2006 Inventory flow rate emissions at 98 99% DREa (tons/32 day) (tons/32 day)
process type ethylene: general
1666.88
ethylene: general
499.13
33.34 9.98
propylene: general
287.95
5.76 4.13
not classified
206.52
ethylene: general
156.38
3.13
ethylene: flue gas vent ethylene: general
119.20 114.44
2.38 2.29
ethylene: general
111.15
2.22
ethylene: general
85.89
1.72
propylene: fugitive emissions
58.58
1.17
not classified
55.88
1.12
not classified
35.01
0.7
propylene: fugitive emissions
27.55
0.55
not classified ethylene: general
24.19 22.57
0.48 0.45
not classified
15.62
0.31
2.09
0.04
ethylene: general
Figure 4. Monitored hourly flow rate time series for Olefin Flare 1.
DRE of 98% or 99% can be assumed if a flare satisfies all criteria of 40 Code of Federal Regulation (CFR) § 60.18. If the flared gas is alkane (butane +) and hydrogen, a DRE of 98% is assumed, and if the flared gas is propylene, propane, or ethylene, a DRE of 99% is assumed.20 a
Figure 5. Monitored hourly flow rate time series for Olefin Flare 2.
Table 4. Olefin Manufacturing Flares Selected for Photochemical Modeling Analyses flare identifier
flow rate (tons/32 day)
emission assuming 98 or 99% DREa (tons/32 day)
olefin flare 1
499.
9.98
29.752,
95.009
19200
olefin flare 2
288.
5.76
29.858,
94.911
19500
location (lat, lon)
average heat content (Btu/Ib flow rate)
DRE of 98% or 99% can be assumed if a flare satisfies all criteria of 40 Code of Federal Regulation (CFR) § 60.18. If the flared gas is alkane (butane +) and hydrogen, a DRE of 98% is assumed, and if the flared gas is propylene, propane, or ethylene, a DRE of 99% is assumed.20 a
12665
dx.doi.org/10.1021/ie201400z |Ind. Eng. Chem. Res. 2012, 51, 12663–12673
Industrial & Engineering Chemistry Research
ARTICLE
Table 5. Composition of Flared Gases
Table 6. Photochemical Modeling Scenarios Performed for Each Flare
mass percentages in waste gases
chemical species
refinery flare
refinery
refinery
olefin
olefin
1
flare 2
flare 3
flare 1
flare 2
unburned scenario
DRE%
hydrocarbons
ratio of PIC
hydrocarbon to PIC
1,3-butadiene
7.81%
0.00%
0.00%
9.75%
acetylene
0.03%
0.00%
0.00%
0.01%
0.00%
A
95
yes
no
1:0
benzene
0.00%
0.00%
0.00%
6.25%
0.00%
n-butane
4.91%
10.14%
7.14%
4.72% 21.69%
B C
95 90
yes yes
yes no
1:1 1:0
butene
0.00%
1.31%
1.45%
0.00%
0.00%
D
90
yes
yes
1:1
1-butene
2.47%
0.00%
0.00%
0.00%
0.00%
E
75
yes
no
1:0
2-methyl-1butene
0.00%
0.00%
0.00%
2.63%
0.00%
F
75
yes
yes
4:1
G
50
yes
no
1:0
3-methyl-1-
0.00%
H
50
yes
yes
9:1
0.00%
0.00%
0.92%
0.00%
flare emissions
0.00%
butene cis-2-butene
1.17%
0.42%
0.00%
1.13%
0.04%
ethyl benzene
0.00%
0.00%
0.00%
0.18%
0.00%
ethylene
7.17%
0.81%
0.00%
3.54%
0.45%
heptane
0.00%
0.00%
0.00%
1.76%
0.00%
hexane indene
0.00% 0.00%
0.00% 0.00%
7.47% 0.00%
5.44% 0.04%
0.00% 0.00%
isobutane
3.39%
9.81%
9.35%
1.43% 36.26%
iso-butene
2.48%
0.00%
1.45%
0.00%
0.00%
isobutylene
0.00%
0.00%
0.00%
5.12%
0.22%
iso-pentane
0.00%
0.00%
7.26%
15.29%
0.00%
ISOPRENE
0.00%
0.00%
0.00%
5.45%
0.00%
nonane
0.00%
0.00%
0.00%
0.50%
0.00%
octane (E)-1,3-
0.00% 0.00%
0.00% 0.00%
0.00% 0.00%
1.22% 1.48%
0.00% 0.00% 0.00%
pentadiene 1,4-pentadiene
0.00%
0.00%
0.00%
1.61%
pentane
0.00%
0.00%
7.26%
4.15% 16.25%
n-pentane
0.00%
0.00%
0.00%
9.74%
0.00%
1-pentene
0.00%
0.00%
0.00%
3.62%
0.00%
propane
1.27%
20.26%
44.01%
13.14% 0.00%
2.26% 0.00%
3.65% 0.00%
4.46% 0.11%
9.05% 0.00%
toluene
0.00%
0.00%
0.00%
1.18%
0.00%
trans-2-butene
1.60%
0.56%
0.00%
1.14%
0.06%
trans-2-pentene
0.00%
0.00%
0.00%
1.52%
0.00%
vinyl toluene
0.00%
0.00%
0.00%
0.06%
0.00%
54.56%
54.44%
10.96%
3.35%
0.00%
propylene styrene
VOC-unclassified
2.21% 15.99%
such as high cross wind speed, over steaming, over aerating, and low heating value of the waste gases. This analysis will examine the impacts on air quality for four assumed levels of DRE (95%, 90%, 75%, and 50%) for each of the five flares chosen for photochemical modeling analyses. These DRE values will be applied to the inlet flow rate data for the five flares to determine the flare emissions. For each flare and each DRE level, two scenarios for PIC emissions will be examined. The first case assumes that VOC emissions are just unburned hydrocarbons (no PICs formation) while the second case assumes that the emissions are a combination of unburned hydrocarbons and PICs. The ratio of unburned hydrocarbon to PICs was based on results reported in the University of Texas flare studies,11 and depended on DRE. Tables S12 and S13 in Supporting Information
summarize the measurement results of the PICs reported by the University of Texas. In this work, it will be assumed that, in cases where the DRE is 95 or 90%, the ratio of unburned hydrocarbon to PIC is 1:1. For 75 and 50% DREs, the ratios of unburned hydrocarbons to PICs are assumed to be 4:1 and 9:1, respectively. In all cases, the PICs are assumed to be 93% (carbon basis) carbon monoxide (CO), 5% formaldehyde, and 2% acetaldehyde. This is a simplification of the full results of PIC composition reported by Allen and Torres,11 but captures the main features of the PIC composition analysis. Much of the mass of PICs is CO, but the remainder includes highly photochemically reactive species such as aldehydes. Taking into account all of the assumed flare emission scenarios, for each of the 5 flares chosen for analysis, 8 scenarios (in addition to the base case of 98 or 99% DRE) were examined. Table 6 summarizes the 8 scenarios examined for each flare. These levels of DRE (95%, 90%, 75%, and 50%) are within the range of measurements reported in the University of Texas flare tests. At world-scale facilities, refinery and olefin emergency flares can accommodate on the order of 500 tons/h of flow. The flow rates of the five flares selected for this analysis do not exceed 25 tons/h and, for most of the time, the reported flows do not exceed 4.5 tons/h (
