1.       Introduction

One of the most important industrial wastes in refinery plants are gases which are sent to flares. Flares are high chimneys in which gas burns and there are two types of them, open and closed and they also come in three pressures, low, intermediate and high. Gas flares have 3 benefits in the refinery industry: 1- absorbing shock from the system 2- repairing the refinery 3- burning harmful gases.

The main purpose of burning gas in gas units is to protect the tanks and pipes against the increasing pressure caused by unexpected movements and is the main cause of energy waste in refineries. The highest amount of pollutants is dispersed into the environment through this system. Among the most important gases created as a result of the input gas of flares burning are SO_x and NO_x. These two pollutants each have harmful effects on human health and on society. Both these pollutants are potentially harmful to human health and can cause problems such as olfactory dysfunction, nasal discomfort, dyspnea and respiratory problems, nervous system problems, pulmonary edema and even death. They can also be harmful to plants. This study aimed to display the equivalent financial cost of damages caused by the pollutants which result from flares.

In this study the Risk poll model was used to estimate the amount of financial damage caused. This model is a consistent program for analyzing human health and the environmental effects and costs related to the production of greenhouse gases in the atmosphere from constant sources. The World Bank reported that in 2008 approximately 150 billion meters cubed of natural gas is burnt across the world which annually add 400 million tons of greenhouse gas to the world. The leading countries in burning gas in descending order are: Nigeria, Russia, Iran, Algeria, Mexico, Venezuela, Indonesia and USA and based on presented data Iran ranks third in the world in extracting and burning renewable gas and first in the middle east [1]. The largest flaring sites in the world are in Russia and the Caspian Sea (approximately 60 billion cubic meters), Middle East and North Africa (45 billion cubic meters), African desert (35 billion cubic meters) and Latin America (12 billion cubic meters). In recent years the amount of gas burnt in various countries has oscillated and some of them have increased and some decreased. Researches show that across the world burning gas by humans only creates 1 percent carbon dioxide and 4 percent methane [2]. Some organizations and the national oil industry associations have measured the amount of flared gas on a global scale [3, 4]. Some organizations introduced the OPG device to measure the amount of flared gas [5].

Indonesia’s oil industry in 1950, burnt 95% of the gas in flares, then by using certain strategies they lowered the flared gas percentage by 28% in 1987 [6]. Some researchers recycled the gas sent to flares in Shahid Hashemi refinery which resulted in Sm³/h of acidic gases being recycled when a Sulphur unit was turned off and 7000 sm³/h of the gas sent to flares under normal conditions had suitable economic potential [7].

2.       Methods and Materials 

2.1    Study Area

Assaluyeh (figure 1) is south of Dehno village, east of Nakhl-e-Taghi and west of Nayband national park.

The comparison of the amount of low pressure designs in phase 1 to phase 10 refineries is shown in (Table 1).

The comparison of the amount of low pressure designs in phase 1 to phase 10 refineries is shown in (figure 2).

2.2    Risk Poll Model

Risk poll (Jan 2004 version 1.05) is a suitable software to analyze the various effects and damages of dispersion of pollutants such as SO₂, NO₂, CO and PM10 on health and environment. Also the effects of heavy metals which exist in the atmosphere such as Pb, As, Ni, Mg, Cr, Cd. In this program the effect of pollutants will be analyzed on the following sources: human health, farming products and construction materials. Also these effects will be studied in two group of physical effects and environmental damage. The emphasis of the Risk poll software is to obtain an accurate and logical estimate of the effects with consideration of simplifications. Therefore, this software requires little information to operate and it can even be said that Risk poll requires the least data to operate compared to similar software’s and models. For example, to analyze a certain health risk the user needs the pollutant emission rate, location information and population density. With more accurate data the accuracy of the output of the software increases. This software has to be calibrated to be used in different countries. The calibration coefficient is the value of human life in a country compared to the European Union and the method of calculation has been brought below. It has to be considered that the aim of this software is to calculate the total effect not a section of the area being studied. The study area of Risk poll at maximum is a circle with a radius of 500km from the polluting source.

2.3    Calibration Coefficient

The calibration coefficient is calculated using this formula:

Value of each unit in Europe x PPPGNP  = value of each unit in the  = country being studied.

PPPGNP: power per capita equivalent to gross national product in dollar terms.

According to the most recent data from Iran’s central bank in 2011 PPPGNP was 13053 dollars. Whereas this value in Europe is 31607 dollars.

3.       Results and Discussion

In this study the emission rates of SO_x and NO_x gases were acquired based on the collected data from the chosen flares in phases 1 to 10 in South Pars field. Then the risk of damage to human health was analyzed using the Risk poll software. In this section the method which was used is discussed. Based on the collected data from South Pars field including the amount of input gas and efficiency the reactions for complete and incomplete burning of gases are obtained. With consideration of the input flow rate and output gases the balanced equations are achieved.

By knowing the amount of gas entering the flares and an efficiency of 90%, the burning reaction for each gas can be written based on the intended gas and this can be repeated for complete or incomplete burning reactions. Also by considering the entering gases, the rate of emission for gases in each phase can be acquired by using the amount designed gases and the real rate of emission. The percentage of output gases with the mentioned efficiency is obtained in terms of mol/hour based on this data.

The reactions for complete burning of output gases from flares have been stated below:

The reactions for incomplete burning of output gases from flares:

The amount of carbon dioxide is equal to:

The amount of H2O produced in incomplete reactions:    

The amount of H2O produced in complete reactions:                                                 

The amount of CO2 produced in complete reactions:   

The amount of O₂ produced in complete reactions:

The amount of N₂ entering because of the input O₂ from air:

Considering the calculations and values above, the final percentage of the output gas composition is shown in (table 2).

The rate of transformation for N₂ and H₂S mols into NO_x and SO_x is equal to:

The rate of emission is shown in (Table 3) which is the rate of the actual amount to the designed amount in each phase.

The flared gases (low pressure) in south pars field in Table 3 for the designed amount and actual amount have been reported in terms of kilogram/hours and in this table the height of each flare has been documented for each phase. The difference between the designed amount and actual amount in terms of kilogram/hours can be seen in the table. Considering the analysis of (Table 4), the total designed amount is less that the actual amount.

According to (Table 5) the acquired rate in each phase is based on the actual amount and designed amount. The values presented in the table below have been collected from south pars field phases and also the calculated rate in this table is the rate of operational coefficients in 2011 in relation to designed coefficients for the gases mentioned in Table 5. The gases in this table can be compared to the calculated rate in terms of mol/hour, as a result of which the highest rate belongs to N₂ and the lowest rate belongs to H₂S.

The amount of output gas for each phase has been reported as ton/year for phases 1-10 in Table 6, and this amount has been reported based on the input gases of each phase. For example, the highest output of NO_x can be seen in phases 4 and 5 and least is in phases 2 and 3 and by analyzing the table below these rates can be stated for each gas in each phase. The output gases have been reported in terms of ton/year and the variation in the amount of output of these gases depends and their input.

The results of (Tables 7 And 8) show the damage caused in each phase by nitrate and sulfate.

According to the analysis carried out on Table 7 And Table 8 a pie chart was drawn for sulfates which is presented below which shows that the most damage inflicted is related to death and after that its bronchitis in adults.  The first three destructive effects in Table 9 are: death, chronic bronchitis and daily fatigue which cause more damage compared to other effects. However, it should have considered that the destructive effects of sulfates on the workers’ and locals’ health is negligible compared to nitrates’ effects.

As shown in the table above the destructive effects caused by the emission of NO_x by the flare being studied are significant and costly. The majority of these effects involve death, chronic bronchitis and daily fatigue. The overall cost of these damages add up to 951000 dollars annually, 60% of which is due to death as a result of NO_x emission. The amount of damage caused by each gas is shown in (Table 9).

(Figure 3) has been used to better show the comparison between the damages caused by the two pollutants under study and as it is shown the damages caused by SO_x are negligible compared to those caused by NO_x.

Finally, the cost of damages inflicted in terms of million dollars per year for nitrate and sulfate gases in each phase are presented in (Table 10). Based on the statistical analysis of the data in this table, the cost of damages caused by nitrate gases in each phase was noticeably higher than sulfate gases. The total cost of damages caused by nitrate and sulfate gases is shown at the bottom which was 85.70 million dollars per year for nitrate and 2.71 million dollars per year for sulfate.

4.       Conclusion

Flares of gas refinery plants are one of the major producers of gases such as CO₂, NO_x, SO_x, CO and etc. Therefore, proper management of the input gas of flares is very important. The dispersion of these gases in the surrounding environment has detrimental effects on the environment and human health. In this study the damage caused by the negative effects of SO_x and NO_x emission from phase 1 to 10 flares in Assaluyeh on the health of workers and locals was analyzed. The area studied was a circle with a radius of 50km. Based on the data collected from the refinery in regards to the composition of the input of the flares, annually 666916 tons of NO_x gas and 1.7tons of SO_x gas are dispersed into the environment. As shown above the effects of NO_x are much more significant than the effects of SO_x. Based on the Risk poll model this emission rate causes a damage of 958000 dollars to human health. The majority of these damages are death, chronic bronchitis and daily fatigue. Considering that most flares in this region are active (some permanently and some temporarily) a strategy has to be devised to reduce the rate of flaring or reduce their negative and damaging effects in this region.

Figure1: Assaluyeh region in Iran.

Figure 3: Comparison of damages caused by NO_x and SO_x.

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