Evaluation of ground level concentration of pollutant due to gas flaring by computer simulation: A case study of Niger - Delta area of Nigeria
A. S. ABDULKAREEM
Chemical Engineering Department, Federal University Of Technology, Minna, Nigeria
Abstract
The disposal of associated gases through flaring has been a major problem for the Nigerian oil and gas industries and most of theses gases are flared due to the lack of commercial out lets. The resultant effects of gas flaring are the damaging effect of the environment due to acid rain formation, green house effect, global warming and ozone depletion.
This writes up is aimed at evaluating ground level concentration of CO2, SO2, NO2 and total hydrocarbon (THC), which are product of gas flared in oil producing areas. Volumes of gas flared at different flow station were collected as well as geometrical parameters. The results of simulation of model developed based on the principles of gaseous dispersion by Gaussian showed a good agreement with dispersion pattern.
The results showed that the dispersion pattern of pollutants at ground level depends on the volume of gas flared, wind speed, velocity of discharge and nearness to the source of flaring. The results shows that continuous gas flaring irrespective of the quantity deposited in the immediate environment will in long run lead to change in the physicochemical properties of soil.
Keywords
Gas flaring, Niger – Delta, Modeling, Simulation and environmental pollution
Introduction
The presence of unaccepted levels of foreign gaseous and particulate matters in the atmosphere is referred to as air pollution (Odigure, 1999). The deterioration of the environmental quality, which began when man first collected into villages and utilizes fire, has existed as a serious problem under the ever-increasing impacts of exponentially increasing population and industrial society (Russel, 1976). Environmental contamination of air, water, soil and food has become a threat to continue existence of many plant and animal communities of ecosystem and ultimately threatens the very survival of the human race. Gas being flare into atmosphere is one of such indiscriminate discharge. Petroleum industry operation is characterized by sets of hazards (Onosode, 1996). The nature and magnitude of these hazards are dependent on series of factors and vary significantly from one sector to another. While it is true that Niger – Delta of Nigeria suffers a great deal from immediate impact of gas flaring. It is now increasingly recognized that the contribution of the petroleum industry to the environment degradation goes far beyond the immediate vicinity of the oil producing areas. The Nigeria oilfield in the Niger – Delta area produce about two million barrels of oil and most of this oil comes from reservoirs containing gas, which is produced with the oil (Ikelegbe, 1993). This associated gas, a by-product of the country’s lifeline petroleum exploration activities is separated from the oil at flow station and more than 95% of it is flared. Currently are total sum of two billion standard cubic feet per day (scf/day), which is estimated to be about a quarter of the world gas flares (Onosode, 1996).
The impact of gas flared is of local and global concern. The main components of this flared include carbon (iv) oxide (CO2), methane (CH4), nitrous oxide (NO2), water vapour and sulphur dioxide (SO2). It is estimated that the Nigerian gas flared has released 35 million tons of carbon dioxide and 12 million tons of methane in a year. The low combustion efficiency of Nigerian flare stack (60 – 80%) results in a large portion of the gas emitted being methane and since methane has a higher global warming potential (64 against 1 for CO2) (Sawaragi and Akashi, 1978). The Nigerian oil industry probably contributes more than any other company of these serious global commons environmental problems. As carbon dioxide and methane are the main green house effect and consequently, this phenomenon has been confirmed to raise the average global temperature by about 0.5°C within the last century (Thomas and Allen, 1999 and Penner, 1999). The flaring stations in the Niger – Delta area has been blamed for smoke and flames that damage the air and soil quality in the ecologically fragile delta and contribute to the global green house gases. The gas flaring in Nigeria have for years been criticized by environmental groups as the world’s largest sources of global warming pollutants this acts, which experts attributes to the rise in sea level. In Nigeria however, environmental policies and regulations can be conveniently characterized as minimal and poor, as the Nigerian government puts profits ahead of the environment and the welfare of its citizens. Nigeria, which according to UN economic commission for Africa is categorized as a gas – surplus country, still has limited associated gas sales as 68% of the gas is flared and as a result has been associated with climate change and related warming, deforestation and acid rain with attendant impact on agriculture and other physical infrastructure (Onosode, 1996). Heat and noxious gases may contribute to environmental health problem in the Niger – Delta region. Also, there has been timely warming and an increasing global awareness on the need to protect our environment and improve our economy. Typical gas flare in Nigerian oil field are located at ground level and surrounded by thick vegetation, farmland and villages huts 20 – 30m from the flare. The heat radiation is a function of the flare temperature, gas flow rate and geometrical design of flare stack. There is a great physiological impact on crops planted in the vicinity of the gas flare. The soil mantle of the earth is indispensable for the maintenance of the plant life, affording mechanical support and supplying nutrient and water.
Also, the soil constitutes a major storage location for heat, acting as sink for energy during the day and a source to the surface at night. Soil temperature is one of the most critical factors that influence important physical, chemical and biological processes in soil and plant. Soil temperature affects plant growth first during seed germination. Metabolically regulated plant processes, such as water and nutrient uptake can be diminished below optimum rates at both low and high temperature, resulting in temperature dependent growth and yield pattern. For instance, corn yields were observed to increase almost linearly as a function of soil temperature between 15 – 25°C, above 25°C the yield decreases (Allmaras et- al, 1964)). With respect to the gas flaring, its effect on vegetation, health and microclimate are equally searing. The free disposal of gas through flaring generates tremendous heat, which is felt over an average radius of 0.5 kilometer there by causing thermal pollution (Ikelegbe, 1993). It has been reported that there could be about 100% loss in yield of crops cultivated 200m away from the flares, 45% loss in yield of crops at 600m away and 10% loss in yield for crop planted 1000m away (Oyenkunle, 1999). This work focuses on the evaluation of ground level concentration of CO2, SO2, NO2 and THC due to the flaring of associated gases during exploration of oil in the Niger – Delta area by computer simulation. This will be achieved via realization of the following objectives:
1. Develop a predictive model for the dispersion of pollutant from the flaring point;
2. Find interaction between various parameters that affects pollutant dispersion pattern i.e. volume of gas flared, temperature, wind speed etc;
3. Simulate the developed predictive model using computer program.
Experimental Methodology
The experimental methods carried out in this project aimed at quantifying the volume of gas flared at flow station in the Niger-Delta area for different stations. The experiments were performed by industries situated in the Niger-Delta. The data were collected from their logbook.
Conceptualization of the modeling technique
A pollutant concentration may be considered either as a climatological parameter in the same way as air temperature or humidity or as a complex resultant of “pure” climatological parameters (wind rise, stability wind rose, temperature, e.t.c.) Every population of climatological descriptors is essentially random. Since every function of a random variable is also a random variable, the concentration will be subject to random variation similar to that of a climatological series. The modeling is a resultant of pure climatological parameters. To compute base on climatological parameters an adequate amount of meteorological input about the state of the atmosphere as well as detailed data about emission were collected.
During the development of the mathematical model for the ground level concentration of pollutant using the Gaussian formula, the following assumptions were made:
1. A constant wind speed and turning of wind due to frictional effect is neglected;
2. Eddy diffusivities are constant throughout the space;
3. Flat topography;
4. Complete combustion and flare stack efficiency is 64% (46% of the flare gas are released as hydrocarbon while the remaining 64% undergo combustion);
5. Pollutants are chemically inert.
The resultant equation for pollutant concentration from an elevated continuous source is given by (Odigure and Abdulkareem, 2001):
(1)
where: Ci - pollutant concentration in air at any point (x,y,z) (ug/m3), Q - emission rate from the stack (gm/sec), Us - wind speed at source height (m/sec),σy - horizontal dispersion coefficient (m), σz - vertical dispersion coefficient (m), y - cross wind distance (m), z - vertical distance (m), H - effective stack height (m).
Equation above can be simplified for ground level concentration by setting Z = 0:
(2)
But Gwendolyn and Lewis, 1993 stated that:
(3)
(4)
From equations 3 and 4:
(5)
Substituting equation 5 into equation 2:
(6)
where:
Q = ρV, (7)
, (8)
and hs - physical chimney height, ht - maximum terrain height between the release point and the point for which the calculation is made. In this case a uniform topography is assumed, therefore ht = 0.
Then equation (8) becomes:
(9)
Note that Δh is the increase in stack height or the plume rise that occurs due to the effect of temperature and pressure and Δh can be calculated as function of temperature, pressure and other variables as follows:
(10)
where: Vs - stack discharge velocity (m/s), d - stack diameter (m), U - wind velocity (m/s), Ts - Stack discharge temperature (°K) , Ta - Ambient temperature (°K), P - Atmospheric pressure (mbar), C - weather condition;
(11)
Also:
(12)
(13)
The composition of associated flare gas is shown in table 1 below (Helen, 2003):
Table 1. Percentage by volume composition of flare gas
|
Component |
CH4 |
C2H6 |
C3H8 |
C4H10 |
C5H12 |
Others |
|
% Composition by volume |
47 |
18 |
20 |
5 |
9 |
1 |
The above named compounds present in the flared gas undergo combustion during flaring to release CO2, SO2, NO and total hydrocarbon.
Amount of CO2 released by flaring 1m3 of associated gas can be calculated from stiochiometry of burning. Total amount of CO2 released by burning 1m3 of gas with 64% stack efficiency was calculated to be 1.3312m3. Consequently, the concentration of CO2 released and dispersed at ground level is equal to:
CCO2 = 
(14)
Also, on the basis of 1m3 of flared gas and 64% efficiency of flare stack with composition by volume of sulphur containing compound and nitrogen containing compound of 0.18% and 0.02% respectively. The volume of SO2 and NO2 produced are 0.001152 m3 and 0.000128m3 respectively. Consequently, the concentration of SO2 and NO2 released and dispersed at ground level is equal to:
CNO2
= 
(15)
CSO2
= 
(16)
Also the volume of total hydrocarbon produced by flared 1m3 of associated gas with 64% efficiency of the stack is 0.365m3. The concentration of THC released and dispersed at ground level is equal to:
CTHC
= 
(17)
Results
Experimental results are presented in tables 1 and 2 below.
Table 1. Volume of gas flared (m3/s) at various times for station 1
|
Month |
Volume of gas flared (m3/s) |
Discharge velocity (m3/s) |
Wind speed (m/s) |
Surrounding temperature (°K) |
Stack temperature (°K) |
|
January |
1.174 |
14.0 |
1.5 |
320 |
1000 |
|
February |
4.480 |
14.0 |
2.0 |
325 |
950 |
|
March |
1.121 |
13.5 |
1.5 |
318 |
1100 |
|
April |
1.137 |
12.5 |
1.0 |
319 |
1000 |
|
May |
0.622 |
12.5 |
1.3 |
325 |
900 |
|
June |
0.877 |
12.0 |
2.0 |
320 |
1100 |
|
July |
0.947 |
12.0 |
1.5 |
328 |
1150 |
|
August |
1.126 |
12.5 |
1.8 |
320 |
900 |
|
September |
1.103 |
14.0 |
2.0 |
318 |
1080 |
|
October |
1.071 |
14.0 |
2.5 |
320 |
1000 |
|
November |
1.092 |
12.0 |
3.0 |
320 |
950 |
|
December |
0.977 |
13.0 |
2.8 |
328 |
1100 |
Height of the stack = 6.1m Diameter of the stack = 0.34m
Table 2. Volume of gas flared (m3/s) at various times for station 2
|
Month |
Volume of gas flared (m3/s) |
Discharge Velocity (m3/s) |
Wind speed (m/s) |
Surrounding temperature (°K) |
Stack temperature (°K) |
|
January |
0.831 |
12.5 |
2.5 |
328 |
1100 |
|
February |
0.514 |
14.0 |
3.0 |
318 |
1000 |
|
March |
0.582 |
13.0 |
3.0 |
320 |
900 |
|
April |
0.497 |
14.0 |
1.5 |
320 |
900 |
|
May |
0.485 |
13.0 |
2.0 |
320 |
1100 |
|
June |
0.421 |
12.5 |
3.0 |
318 |
1000 |
|
July |
0.365 |
12.5 |
3.0 |
328 |
1050 |
|
August |
0.411 |
14 |
1.3 |
319 |
1000 |
|
September |
0.372 |
13.5 |
3.0 |
325 |
980 |
|
October |
0.336 |
12.0 |
1.3 |
328 |
900 |
|
November |
0.307 |
12.5 |
2.0 |
318 |
1000 |
|
December |
0.293 |
14.0 |
2.9 |
320 |
900 |
Height of the stack = 7.53m Diameter of the stack = 0.56m
Simulation Results
Simulation of the model means the use of computer codes to show the operation and behaviour of the system. The model equations were simulated using Q-Basic programme. The results obtained are presented in tables 3 and 4.
Table 3. Computed concentrations of pollutant (μg/m3) for station 1 in the month of February
|
Distance (m) |
Polluants concentration (μg/m3) |
|||
|
|
CO2 |
NO2 |
SO2 |
THC |
|
100 |
02.03912E-4 |
3.268312E-8 |
2.0936374E-7 |
2.139874E-5 |
|
150 |
58.94978 |
9.448499E-3 |
8.488887E-2 |
6.186251 |
|
200 |
3443.891 |
0.5519885 |
4.959272 |
361.4054 |
|
250 |
18716.9 |
2.999954 |
26.95271 |
1964.171 |
|
300 |
41449.18 |
6.643494 |
59.68764 |
4349.719 |
|
350 |
61275.14 |
9.821209 |
88.23743 |
6430.276 |
|
400 |
73905.47 |
11.8456 |
106.4254 |
7755.716 |
|
450 |
79811.27 |
12.79219 |
114.9298 |
8375.477 |
|
500 |
80907.05 |
12.96782 |
116.5078 |
8490.469 |
|
550 |
79003.65 |
12.66274 |
113.7668 |
8290.724 |
|
600 |
75421.2 |
12.08855 |
108.608 |
7914.777 |
|
650 |
71021.76 |
11.3834 |
102.2728 |
7453.097 |
|
700 |
66333.98 |
10.63204 |
95.52226 |
6961.156 |
|
750 |
57188.73 |
9.883905 |
88.80071 |
6471.325 |
|
800 |
52986.82 |
9.166238 |
82.35292 |
6001.444 |
|
850 |
52986.82 |
8.492754 |
76.30209 |
5560.492 |
|
900 |
49095.85 |
7.869107 |
70.69901 |
5152.169 |
|
950 |
45521.98 |
7.296285 |
65.55256 |
4777.123 |
Table 4. Computed concentrations of pollutant (μg/m3) for station 2 in the month of February
|
Distance (m) |
Polluants concentration (μg/m3) |
|||
|
|
CO2 |
NO2 |
SO2 |
THC |
|
100 |
1.167116E-9 |
1.87066E-13 |
1.680671E-12 |
1.224784E-10 |
|
150 |
6.808103E-2 |
1.091206E-5 |
9.803804E-5 |
7.144493E-3 |
|
200 |
25.14161 |
4.02971E-3 |
3.620443E-2 |
2.638386 |
|
250 |
319.5423 |
0.0512164 |
0.4601474 |
33.5331 |
|
300 |
1120.959 |
0.1796678 |
1.614203 |
117.6346 |
|
350 |
2185.434 |
0.3502824 |
3.147069 |
229.3417 |
|
400 |
3153.504 |
0.5054451 |
4.541109 |
330.9319 |
|
450 |
3850.266 |
0.6171225 |
5.544461 |
404.0509 |
|
500 |
4260.925 |
0.682943 |
6.135817 |
447.1458 |
|
550 |
4439.391 |
0.7115478 |
6.392812 |
465.8742 |
|
600 |
4452.173 |
0.7135964 |
6.411218 |
467.2155 |
|
650 |
4356.291 |
0.6982285 |
6.273147 |
457.1536 |
|
700 |
4194.348 |
0.6722721 |
6.039945 |
440.1591 |
|
750 |
3995.986 |
0.6404786 |
5.7543 |
419.3429 |
|
800 |
3780.928 |
0.606009 |
5.444613 |
396.7745 |
|
850 |
3561.841 |
0.5708936 |
5.129122 |
373.7832 |
|
900 |
3346.569 |
0.5363897 |
4.819126 |
351.1924 |
|
950 |
3139.744 |
0.5032369 |
4.521294 |
329.4879 |
Discussion of results
The operation at gas plant and flow station in the Niger – Delta area of Nigeria involves flaring of excess gas on twenty-four hourly basis. Combustion of gas flare contributes to the atmospheric content of carbon, nitrogen, sulphur and total hydrocarbon. Most pollutant is emitted into the atmosphere from elevated sources such as chimneystacks and transported through the atmosphere by wind currents from their point of release to downwind receptors (Gwendolyn, 19993). The major meteorological parameters controlling atmospheric dispersion are atmospheric stability and wind speed (Gavriel, 1991). One of the major issues in Nigeria is the flaring of gas that is produced with oil in the Niger – Delta area. The flaring of gas has been an integral part of the operation associated with the exploitation of crude oil and natural resources in Nigeria since inception. Flaring of gas has been as a result that most of Nigerian’s oil facilities were built in 1960s and 1970s (Kuranga, 2002). This implies that they were built to environmental standard of those days. Gas flaring has exposed the people of Niger – Delta area to a lot of hazard.
From experimental results shown in tables 1 to 2, it could be observed that the volume of gas flared varies from station to station and month. This could be attributed to the fact that the production rate and well properties are not constants for all stations. The simulation results of the predictive model are presented in tables 2 to 4. It could be seen from the tables that the most dangerous zone is within the 200 – 600m radius from the flare station (figs. 1-4).
However, effect of gas flared are felt within the radius range of 1000m away from the flaring source depending on the volume of gas flared, wind speed, surrounding temperature, velocity of discharged and height of stack. It could be observed from the results that the concentration of pollutant in the ground level increases as the volume of gas flared increase and vice versa. This implies that the concentration of the pollutant is directly proportional to the volume of gas flare. Simulation results of the predictive model shows that cross wind distance and distance across the x – axis affect the concentration of the pollutants at the ground level.
Figure 1. SO2 concentration for station 2
Figure 2. SO2 concentration for station 1
Figure 3. CO2 concentration for station 1
Figure 4. CO2 concentration for station 2
Note that at the further distance, lesser concentration of the pollutants (figs. 1-4). This could be attributed to the fact the pollutants are engaged in other reaction due to dispersion of pollutants and other components. Results show that the people of Niger - delta area are exposed to danger due to continuous pollution of soil from these sources. Also, the results obtained showed that dispersion pattern conform to the modified principles by Gaussian.
Conclusions
From this research, the following conclusions can be deduced:
1. It was observed that the result of the simulation of model developed based on the modified principles of gas dispersion showed a remarkable agreement with dispersion pattern;
2. The dispersion pattern of pollutant showed that the concentration of pollutant from the flare source depends on the volume of gas flared, height of stack and wind speed;
3. Model equation that represent ground level concentration of pollutants are:
CCO2 = 
CNO2
= 
CSO2
= 
CTHC
=
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