Study of CO Gas Dilution in Forcing Ventilation System at Ramp Down KKRB 4 Utama, Pongkor, West Java

CO is a hazardous gas formed from the reaction of explosives that lack oxygen balance. These explosives have the potential to generate toxic gases, including CO. It is crucial to minimize CO concentration at the work site by ensuring proper air circulation. This study aims to examine the impact of varying ducting distances on CO gas dilution, using the gas diffusion coefficient value as a basis. The research method employed is observational, involving the assessment of CO gas dilution in the tested tunnel at varying distances from the working front to the sensor. The research was conducted on the Ramp Down KKRB 4 Utama. Result of the research is the most ideal carbon monoxide reduction time at RD KKRB 4 Utama is shown at sensor placement distance 1 using the blow system, where A Sensor takes 2.21 hours and B Sensor takes 1.06 hours to reach a concentration level below the TLV (Threshold Limit Value) 50 ppm.


Introduction
The location of mines poses a high risk of danger, especially underground mines.With highly limited working conditions and space, the potential hazards are so significant [1].These conditions can lead to work accidents, which can occur due to unsafe actions and unsafe conditions [2].Among the various factors that contribute to an unsafe location in underground mines, toxic and hazardous gases can disrupt activities at the workplace, making it necessary to plan for proper air ventilation to ensure that work can be carried out safely and comfortably [3].
In underground mining, ventilation plays a crucial role in supporting every task [4].Ventilation in underground mines functions to supply fresh air into the mine, which is needed for the availability of clean air for workers in the mine and for all processes that require oxygen inside the mine [5].Additionally, ventilation functions to remove heat that accumulates in underground mines due to the activities carried out by workers and the equipment used [6].
CO gas is caused by incomplete combustion resulting from blasting activities and the operation of heavy machinery in underground mines [7].The nature of CO gas is colorless and odorless.CO gas is highly dangerous, so its presence needs to be carefully monitored, especially during blasting at the working front.At the research site in PT Antam UBPE Pongkor, carbon monoxide (CO) gas resulting from blasting often becomes an obstacle.Referring to the Director General of Minerals and Coal's Decision No.185K/37.04/DJB/2019[8], the volume of CO gas should not exceed 0.005% or 50 ppm and not more than 0.04% or 400 ppm for an exposure time of 15 minutes.The timing of the reduction of CO gas is important to consider during blasting in the development tunnel because if CO gas does not 2 dissipate when workers re-enter for work, it can cause poisoning [9].To meet the needs of air and heat flow and to dilute toxic gases in underground mines, careful calculations are required for the placement of ventilation ducts, fan speed, and good air circulation conditions, ensuring that fresh air and heat can circulate properly, thereby creating a safe and comfortable working environment [10].The purpose of this research is to study how quickly CO gas dissipates at the observation distance used.The ventilation system used at the research site for Ramp Down KKRB 4 Utama (RD KKRB 4 Utama) is forcing system as given in Figure 1, where this system provides a blast of fresh air to the working front [5].

Method
The research was conducted at PT Antam UBPE Pongkor using the observation method.Observations were made by testing several different observation distances, resulting in data on the quality and quantity of air.The data on air quality resulted in the concentration of CO gas in parts per million (ppm).
The measurement of tunnel dimensions was carried out using the Leica Disto D2, which is used to measure the length, width, and height of the tunnel in meters.The measurement of tunnel dimensions was adjusted according to the actual conditions and the progress of the tunnel.Figure 2 is Leica Disto D2, the device utilized for measuring the dimensions of tunnels.

Figure 2. Measurement of tunnel dimensions
The measurement of CO gas concentration is carried out in order to obtain the curve results of the gas dilution reduction process.The process of collecting data on CO gas concentration uses robotic 3 hardware with Arduino coding software.The hardware used will be placed on the tunnel wall, with its two sensors positioned at an initial distance of 50 meters from the working front for the first sensor and 25 meters from the first sensor for the second sensor with two observation distance.Figure 3 depicts the robotic equipment employed to measure CO gas within the tunnel, while Figure 4 illustrates the schematic showcasing the positioning of the sensor during the research within the tunnel.Table 1 displays the observed distances utilized in the research.Ansys Fluent is used to identify the direction of airflow occurring in the tunnel.The data used consists of tunnel dimensions.The creation of tunnel dimensions is followed by the creation of a mesh in the simulation tunnel.The creation of this mesh is useful to ensure that the created geometry is wellstructured and orderly.The smaller the mesh, the more accurate the results obtained.
Equations are used to determine the conditions of the airflow.Viscosity (μ), density (ρ), and the diameter of the airway are three parameters that affect the flow conditions (d).The Reynolds number is the relationship between μ, ρ, and d, which has the same dimensions as velocity and can be studied using the following equation: The direction of laminar or turbulent flow is determined by the Reynolds number.If the value of Re is less than 2,000, the flow is identified as laminar flow.If the value of Re is greater than 4,000, the flow is identified as turbulent flow.If the value of Re falls within the range of 2,000 and 4,000, the airflow is referred to as transitional flow.
The diffusion coefficient describes the dispersion of chemicals released in a closed flow in laminar flow.The diffusion coefficient is a function of molecular diffusion in the radial direction and the velocity profile of the closed channel in the axial direction.The diffusion coefficient is known as the virtual diffusion coefficient, and it is determined by observing the concentration distribution at the observation location.It is proportional to turbulent flow.The equation for the diffusion coefficient in laminar flow can be seen in this equation written by Taylor (1974): The following equation can be calculated using Taylor's test principle (1954), where the concentration follows the Gaussian distribution theory with the assumption of one dimension:

Result and discussion
The research was conducted at the Kubang Kicau location at the working front of the access tunnel, namely Ramp Down KKRB 4 Utama (RD KKRB 4 Utama) at PT Antam UBPE Pongkor.The CO gas under investigation is the CO gas resulting from blasting, which is measured at the end of each shift.Data collection at each working front was conducted three times to ensure the accuracy of the results.The type of airflow in the tunnel at the research site can be determined by the Reynolds number.The Reynolds number is obtained from calculations of air density, air flow velocity, and the diameter of the cross-section at the research site.The air flow velocity used is the average of the measurements taken at 16 observation points at the working front.All conditions have known Reynolds number values, and the results indicate that there is no airflow present at RD KKRB 4 Utama.
The testing diffusion coefficient is obtained using the Taylor equation (1954), as in equation 2. The required data includes the diameter of the tunnel airflow, the average air flow velocity, and the friction factor obtained from the Reynolds number value using the Colebrook equation, due to the rough tunnel surface conditions in the field.Table 3 represents the values of the diffusion coefficient during data collection in the field for RD KKRB 4 Utama, which includes the initial testing diffusion coefficient value when carbon monoxide (CO) gas resulting from the blast is at the 0-second mark.The value of the testing diffusion coefficient is very low because the CO gas has not yet been fully diluted.
The testing diffusion coefficient is obtained using the Taylor equation (1954), as in equation 2. The required data includes the diameter of the tunnel airflow, the average air flow velocity, and the friction factor obtained from the Reynolds number value using the Colebrook equation, due to the rough tunnel surface conditions in the field.Table 4 represents the values of the diffusion coefficient during data collection in the field for RD KKRB 4 Utama, which includes the initial testing diffusion coefficient value when carbon monoxide (CO) gas resulting from the blast is at 0 seconds.The value of the testing diffusion coefficient is very low because the CO gas has not yet been fully diluted.and the actual placement of the fan in the field.The maximum smoke clearing process can be determined when there is good air circulation and the decrease in CO gas occurs rapidly.
Figure 5 shows the graph of carbon monoxide (CO) gas concentration over time at RD KKRB 4 Utama at observation distance 1.At this observation distance, it can be seen that the dilution process for the two sensors differs significantly.This is because, considering the position of the sensors, A Sensor is closer to the working front and positioned in the middle of the blow duct's length, resulting in a longer CO gas dilution process compared to B Sensor.The dilution process at A Sensor takes 2.21 hours, while B Sensor requires 1.06 hours to reach a concentration level below the TLV (Threshold Limit Value) 50 ppm.The significant time difference is also attributed to the presence of an exhaust duct at the end of the RD KKRB 4 Utama tunnel with a fan power of 37 kW.This causes the CO concentration to dissipate faster at B Sensor compared to A Sensor, which is positioned farther from the exhaust duct.
Referring to the Director General of Minerals and Coal's Decision No.185K/37.04/DJB/2019[8], which stipulates a 400 ppm concentration limit for a 15-minute CO gas exposure, the conditions at RD KKRB 4 Utama already meet this requirement because the 15-minute CO gas exposure concentration at RD KKRB 4 Utama is 383.43 ppm.
The value of the graphic method diffusion coefficient is obtained from the carbon monoxide (CO) gas dispersion graph according to the actual field conditions, considering the position of the vent duct and the air velocity in the blow duct.The value of the graphic method diffusion coefficient is empirically derived based on the Taylor diffusion coefficient (1954) using equation 4 with a curve matching method (trial and error in Microsoft Excel Solver).
At observation distance 2, it can be observed that the dilution of the two gases at A Sensor and B Sensor is not significantly different.Referring to the Director General of Minerals and Coal's Decree No. 185K/37.04/DJB/2019[8], which stipulates a 400 ppm concentration limit for a 15-minute CO gas exposure, the conditions at RD KKRB 4 Utama at observation distance 2 already meet this requirement, as the 15-minute CO gas exposure concentration at RD KKRB 4 Utama is 337.54 ppm.
The dilution process at A Sensor takes 2.59 hours, while B Sensor requires 2.34 hours to reach a concentration level below the TLV 50 ppm.The CO gas dissipation time in observation placement 2 does not yet meet the company's requirements, which is 2 hours.Sensor one requires a longer time for CO gas dilution because A Sensor is closer to the working front, which is the source of CO gas due to blasting activities.
Another factor that can be observed is the presence of holes in the flexible blow duct.These holes in the flexible blow duct are intentionally created by the workers at the working front due to the hot working conditions, necessitating a flow of fresh air under the flexible blow duct through the holes.During 7 blasting, these holes enlarge, reducing the airflow at the end of the flexible blow duct and limiting the optimal expulsion of CO gas at the working front.Figure 6 shows the holes in the flexible blow duct at observation distance 2.  The results of the experiment using the matching curve method are the method graphic diffusion coefficient values.Table 4 represents the diffusion coefficient values from the decrease in carbon monoxide (CO) gas concentration.After analyzing the data from the RD KKRB 4 Utama field on carbon monoxide (CO) gas resulting from blasting, the data processing results in the table show that at observation distance 1, the graphic method diffusion coefficient obtained is the highest, with the distance from the working front to the blow duct being 40.64 m and the air velocity at the end of the blow duct being 1.72 m/s.The CO gas dissipation time is relatively fast compared to the dissipation of CO gas under other conditions at RD KKRB 4 Utama, which is 1.07 hours.CO gas requires 1.06 hours at B Sensor, with a peak value at B Sensor of 616.1 ppm. Figure 8 shows a bar chart comparing the diffusion coefficients of each sensor testing under various different conditions at RD KKRB 4. The results of the testing diffusion coefficient calculation using the Taylor equation (1954) differ from the graphic method diffusion coefficient values obtained using the matching curve method (trial and error in Microsoft Excel).The testing diffusion coefficient is obtained when CO gas is initially blasted, thus not undergoing complete turbulence, whereas the graphic method diffusion coefficient is obtained when perfect turbulence occurs in the airflow, and the CO gas undergoes dilution from the peak concentration until complete dissolution.The diffusion coefficient is greater than the testing diffusion coefficient because when the airflow experiences perfect turbulence, CO gas will be diluted rapidly.In the RD KKRB 4 Utama simulation, the ratio of the graphic method coefficient to the testing coefficient ranges from 138 to 314 times.Table 4 represents a comparison of the graphic method diffusion coefficient (Dx) with the analytical diffusion coefficient (DA).
After conducting simulations using the physical tunnel model, Ansys Fluent software was used to determine the airflow turbulence.Observation distance 1 showed a high diffusion coefficient value during the simulation when air recirculation did not occur.Carbon monoxide (CO) gas is diluted rapidly because the placement of the blow duct is appropriate.This condition will be optimal if it follows the company's regulations, which state that if the end of the blow duct is more than 30 meters away, the blow duct will be advanced.The clean air blown by the blow duct goes directly to the working front and undergoes turbulence to dilute the CO gas.Subsequently, the contaminated air containing CO gas is sucked out by the exhaust duct located outside the tunnel without experiencing air recirculation.Figure 9 shows airflow at RD KKRB 4 Utama observation distance 1 (a) and 2 (b).

Conclusion
The most ideal carbon monoxide reduction time at RD KKRB 4 Utama is shown at sensor placement distance 1 using the blow system, where A Sensor takes 2.21 hours and B Sensor takes 1.06 hours to reach a concentration level below the TLV 50 ppm.Referring to the Director General of Minerals and Coal's Decree No. 185K/37.04/DJB/2019,which stipulates a 400 ppm concentration limit for a 15minute CO gas exposure, the conditions at RD KKRB 4 Utama at placement distance 1 already meet this requirement, as the 15-minute CO gas exposure concentration at RD KKRB 4 Utama is 383.43 ppm.Adjustments to the blow duct and improvements to the flexible duct are necessary to optimize CO gas smoke clearing at the research site.This can be seen from the coefficients of each condition.The larger the diffusion coefficient value, the easier it is for CO gas to be diluted.

Figure 4 .
Figure 4.The schematic of the placement of robotic hardware in the tunnel Number v: air velocity (m/s) μ: dynamic viscosity of airflow (Ns/m 2 ) ρ: air density (kg/m 3 ) d: airway diameter (m) flow diffusion coefficient (m 2 /s) u': Friction velocity (m/s) r: Diameter of the orifice (m) τ: Shear stress (Pa) u: Mean velocity of air flow (m/s) ρ: Fluid mass density (kg/m 3 ) f: Friction factor

Figure 5 .
Figure 5. Graph of carbon monoxide (CO) concentration over time at RD KKRB 4 Utama at observation distance 1

Figure 6 .Figure 7
Figure 6.Holes in the flexible duct at RD KKRB 4 Utama at observation distance 2 Figure 7 represents the analysis of CO gas concentration data testing under Condition 1 at RD KKRB 4 Utama, with the CO gas concentration graphic method results at B Sensor using equation 4, resulting in the empirically derived diffusion coefficient.

Figure 7 .
Figure 7. Graph of carbon monoxide (CO) concentration over time at RD KKRB 4 Utama at observation distance 1

Figure 8 .
Figure 8. Graph of carbon monoxide (CO) concentration over time at RD KKRB 4 Utama at observation distance 1

Table 2 .
Table 2 explain the primary data of vent duct at RD KKRB 4 Utama.Primary data of vent duct at RD KKRB 4 Utama

Table 3 .
The values of the Taylor (1954) testing diffusion coefficient in each tunnel under various conditions The research was conducted at 2 different observation distances based on the progress of the tunnel 6

Table 4 .
Graphic method diffusion coefficient values from the decrease in carbon monoxide (CO) gas concentration at various observation distances