Modelling of Air Pollution Dispersion in the Utilization of Used Oil as a Fuel

. The need for quicklime has led to increased utilization of used oil as a substitute for fuel in its combustion process. Mahaka Lime Factory produces SO 2 and NO X , which are emitted through the chimney and proportional to lime burning. Therefore, it is necessary to monitor the distribution of the emission concentration in the ambient air. One of the methods used to facilitate the monitoring process to ensure it is kept below the quality standard is dispersion modeling. This is an analytical model processed with the help of AERMOD software and the Gaussian equation. The SO 2 concentrations of 0.001 mg/m 3 and 0.001 mg/m 3 , as well as NO X of 0.027 mg/m 3 and 0.044 mg/m 3 obtained from the calculation, were below the applicable quality standard. Furthermore, emission concentrations directly observed were more significant than the Gaussian modeling results due to the presence of other pollutants around the factory. In conclusion, the dispersion modeling using AERMOD software showed that the largest and lowest distribution of emission concentrations are in the cliff area around the emission source and on the ground surface close to the chimney.


Introduction
The need for limestone is increasing along with the rapid development of the industry [1].Limestone is processed into quicklime to be used as a neutralizer for acidic compounds [2].This material is more practical, inexpensive, and safe for industrial purpose [3].In addition, many lime factories in Indonesia utilize waste as a fuel source during the production, such as used oil [4].Fresh oil utilized for engine maintenance will become waste after some period of upplication, as used oil [5].Used oil sourced from industry is relatively cleaner and easier to purify with simple treatments such as filtering and heating [6].This oil type is composed of long-chain organic components of hydrocarbons which allow it to produce valuable liquid products [7].The combustion process to break down the hydrocarbon chains into simpler compounds is called pyrolysis [8].
One of the sources of pollutants in industrial areas is factory's chimneys, and the concentration produced affects the surrounding environment [9].Factory is always associated with the pollution because of the very visible activities in releasing various chemical compounds into the surrounding environment [10].Lime burning produces harmful pollutants such as COX, NOX, SOX, and other heavy metals [11].At high temperatures of 800 to 1,000℃, it produces gas and dust trapped in the filter [12].Mahaka Lime Factory, located within the area of PT Freeport Indonesia (PTFI), produces quicklime, which is used to process mineral ore.This factory also mixes used oil from operational waste and machine maintenance at the ore processing plant with diesel for fuel [13].Furthermore, from the lime burnt in Mahaka, ambient air produced every semester comprising SO2 gas with a value of less than 2 mg/m 3 at the end of 2019 [14].
Pollutant emissions and atmospheric concentrations can be linked using air quality modeling, such as the dispersion model [15].Dispersion as the continuous pollutant flow released from an immovable source, such as a chimney, which is blown by the wind, spreading its concentrations [16].The mathematical model used to predict pollutant concentrations from various sources is the application of various atmospheric conditions [17].Therefore, to determine the spread of pollutant dispersions, a software is needed to map the spread, such as AERMOD [18].
Lime is one of the mixed ingredients used by PTFI in processing mineral concentrates.Besides the processing, it also produces hazardous and toxic (B3) waste in the form of used oil, which is utilized as a substitute for fuel in lime burning.Apart from saving fuel, this process is also used to reduce the B3 waste produced.In Mahaka, the pollutants produced are comparable to the lime burning performed, hence, they need to be monitored periodically to ensure the concentrations remain below the quality standard [19].However, dispersion modeling is needed to facilitate the monitoring process [20].
Based on this explanation, the problem formulation is strategies needed to determine the distribution of emissions from lime burning in the factory [21].This research aims to determine the distribution of SO2 and NOX emissions due to lime burning in Mahaka Lime Factory.Visualization of the emission distribution was carried out with the help of AERMOD software to easily monitor the resulting emissions and ensure it remains below the quality standard.

Apparatus
This research was conducted in Mahaka Lime Factory in Mimika, Papua, Indonesia, using an analytical modeling method with the help of AERMODView version 8.9.0 software.AERMOD is a spatial distribution model recommended by USEPA to predict the spread of air quality from up to 10 different sources [22].The advantage is that it can predict the ground-level concentration (GLC) due to the effect of the planetary boundary layer (PBL) [23].

Model and data
The data used were SO2 and NOX emissions, chimney height, diameter, coordinates, ambient air temperature, and wind speed each semester in 2019.These parameters were the most pollutant in ambient air and affect human respiration [24].

Procedure of Simulation
The data processing stage is shown in Figure 1.

Result and discussion
PTFI is a copper and gold mining industry operating in the 53S zone according to the Universal Transverse Mercator (UTM) of Mimika Regency, Central Papua.One of the factories within the territory is Mahaka Lime, which uses mineral concentrates manufactured by PTFI to produce lime as a mixture.Mahaka, which has one chimney with a height and diameter of 29.6 m and 2.1 m, is located at an altitude of 3,625 m above sea level with a constant surface wind speed of 1.94 m/s [14].
PTFI utilizes limestone in the Grasberg mining area for mineral concentrate processing, and the lime produced is used to neutralize acidic waters in mineral rocks.Mahaka has a maximum production capacity of 400 tons of lime per day [25].In its production activities, lime burning utilizes used oil as one of its fuels and produces gas in the chimney with an average temperature of 130℃.Besides, the factory still utilizes used oil and diesel at a ratio of no more than 80%.
Used oil is a waste obtained from lubrication, mineral, or synthetic oil [26].According to Proper management is essential for its use as fuel to avoid negative impacts on human health and the environment [27].In addition, burning produces gas emissions in the ambient air, which need to be managed by conducting an inventory process, such as identifying sources and types of emissions, as well as calculating ambient air quality [28].Air dispersion modeling estimates the amount of emission reduction from the source to the ground level [29].Emissions from burning activities in mining areas are a global environmental problem.Dispersion modeling, which includes pollutant concentrations, chemical reactions, building effects, meteorology, plume rise, and pollutant deposition, is accomplished by simulating using an application such as AERMOD [30].Used oil comes from workshop and machine maintenance activities in the Concentrating Division.It is also produced by removing a mixture of oil and water in the Oil Water Separator (OWS).Before being utilized again, used oil is first separated from water using OWS at Likupang Yard.This tool works in accordance with Stokes' Law, stating that the floating speed of a particle is based on its specific gravity and size using the gravitational system [31].
The used oil obtained from use and workshop activities is pumped to Mahaka to be utilized as fuel.In lime burning, different ambient emissions are produced with varying concentrations.The resulting emissions are measured and tested against the applicable quality standards based on the Government Regulation No. 22 of 2021 [28].

Parameter
Unit Standard [28]   Test Results [14]   Semester 1 Semester 2 Sulfur Dioxide (SO2) mg/m 3 0,15 The direct observations result in Table 2 showed that emissions from lime burning utilizing used oil do not meet the applicable quality standards.Therefore, the emission distribution is further visualized using mathematical modeling by employing SO2 and NOX for 2 semesters.The exhaust gas speeds were 43.32 m 3 /s and 33.82 m 3 /s in the first and second semesters, respectively.3 shows a comparison of emission concentrations from direct observations calculated using Gaussian.The calculated emission values showed that SO2 and NOX concentrations are below the quality standard, thereby considering safe for discharge into the ambient air.However, emissions of SO2 as well as NOX in semesters 1 and 2 are higher than concentrations calculated using Gaussian.
The comparison between the direct observation, which is more significant than the modeling, supports the research by Sari [32].This is due to the presence of other pollutant sources in the environment around the factory, which increases the emission concentration during direct measurements [33].Meanwhile, the calculation using the Gaussian model is smaller because the emission concentration is considered pure without any other pollutant sources [34].It is also affected by the wind direction as well as the determination of vertical and horizontal distances [35].
The data in Tables 1, 2, and 3 are entered into the AERMOD software for mathematical modeling, in the form of the NOx distribution produced in semesters 1 and 2 of 2019 in the area around Mahaka. Figure 2 is the distribution map obtained after all the data have been successfully run.Figures 2, 3, 4, and 5 represent the differences in SO2 and NOX concentration values, which affect the distribution of these emissions in ambient air.The red dot in the center is Mahaka chimneys, which are considered a stationary emission source.The minimum and maximum values of SO2 and NOX concentration on both maps are in the ground surface and cliff areas around the factory chimneys [36].Wind speed also affects this distribution, where the greater it is, the lower the concentration [37].The wind speed near the chimneys tends to be faster than in the cliff area because it is blocked by the walls [38].Based on the emission distribution map above, the wind tends to blow from east to west, as seen from the red color on the map, most of which are in the western part.The red color indicates the area with the largest concentration of emissions [39].Meanwhile, when the color turns to purple, it means that the area has the lowest emission concentration [40].Wind that tends to be strong is also caused by high levels of humidity and air pressure [41].

Conclusion
Utilization of used oil as a fuel substitute in burning quicklime in Mahaka Lime Factory produces SO2 and NOX emissions.The data showed a distribution map of SO2 and NOX around the factory area using AERMOD software.The map showed that the highest and lowest emission concentrations were in the cliff area and ground level around the chimneys, respectively.The low concentration was caused by the wind speed, which plays a role in the distribution of air dispersion.Unfortunately, this research has limited detailed data on wind direction as well as wind speed per day, thereby making it impossible to visualize wind direction using Windrose.

Table 1 .
Coordinate Points of Mahaka Lime Factory.

Table 2 .
Parameters of Quality Standards and Emissions from Direct Observation at 10% Oxygen.