Research on Oxygen Supply System of Coal Mine Refuge Chamber Based on Personnel Location and Programmable Logic Controller

Abstract

The refuge chamber provides a safe space where people can stay safe when they are trapped by an accident in a coal mine. The refuge chamber includes several systems such as oxygen supply, air purification, cooling, and dehumidifying. The oxygen supply system is the core of the refuge chamber because it is a closed environment. The oxygen supply time is 96 h according to the relevant standards in America and China. If people stay in the refuge chamber for more than 96 h, the oxygen supply may not be enough, and they can be in danger. It is necessary to efficiently use oxygen and extend the oxygen supply time. Based on the experimental platform of the refuge chamber, this paper conducts an in-depth study on the automatic control of the oxygen supply system. The oxygen supply system and the harmful gas removal system can be automatically and efficiently controlled by the programmable logic controller (PLC) according to the personnel number and the oxygen concentration. The experiment has proved that the system is good and reliable.

1. Introduction

Coal mine production conditions are very complex and dynamic [1]. With the increase in mining depth, the risk of each coal mine accident has increased significantly, especially the gas explosion and the fire accident [2,3,4]. When a coal mine gas explosion or fire accident happens, toxic and harmful gases such as carbon dioxide, carbon monoxide, and sulfur dioxide rapidly diffuse into the roadway [5,6]. The accident may also cause a roadway to collapse and block escape routes. It will cause the workers trapped underground to be directly exposed to the disaster environment. The high temperature and high-pressure environment formed by the disaster and the toxic and harmful gases produced will make the workers’ lives unsafe [7]. The people who could not escape from the mine can quickly stay in the coal mine refuge chamber and wait for rescue. It is very important to establish a permanent or temporary underground refuge chamber, which can provide a safe place to temporarily escape and reduce casualties [8,9].

The refuge chamber is a closed space [10]. The environmental conditions in the refuge chamber are artificial and not natural. It needs ventilation, harmful gas removal, and heat dissipation to ensure air quality and comfort [11,12]. At present, many scholars have conducted research on the environmental conditions of refuge chambers. Xiong [13], Jin [14], Yang [15] et al., analyzed air or oxygen as a reliable supply method. Gao [16,17], Zhang [18], Yang [19], Zhang [20] et al., studied the heat-efficient removal of harmful gases. Wang [21], Mejías [22], Hao [23], Wang [24], and Jia [25] investigated the heat and humidity removal method. Zhang [26] et al., designed the external environment monitoring system of the mobile refuge chamber. All these studies safeguard the reliable and comfortable environment in the refuge chamber. Oxygen concentration, as the most important environmental factor, has always been the main limitation affecting the survival time of refugees. Zhang [27] et al., investigated the use of compressed O₂ cylinders and air purification devices to ensure indoor air quality. Zhang [28] et al., experimentally determined a ventilation rate of 0.1 m³/min per person in a 50-person refuge. Li [29] et al., studied the oxygen supply system using potassium superoxide as a raw material. But the refuge chamber only guarantees people can stay for 96 h safely [30]. So, if the utilization efficiency of oxygen can be improved, the service time of oxygen will be prolonged, and the safe stay time in the refuge chamber will also be prolonged. The oxygen concentration shall be maintained at 18.5% to 23% [30]. If the oxygen concentration is below 18.5%, people could have dyspnea. And if the oxygen concentration is up 23%, it will increase the risk of fire and explosions. The oxygen flow meter is set at 0.5 LPM/person, based on the number of people in the refuge chamber, using the oxygen flow meter [31]. But according to a study by Hao Shao [32], when the flow meter of oxygen is set at 0.5 LPM/person, the oxygen concentration could exceed 23% and stay at 34%. When the concentration of O₂ is between 18.5% and 23%, the oxygen supply is between 0.405 L/min per person and 0.43 L/min per person. Therefore, a constant flow rate cannot be preferred, and the oxygen supply to the refuge should be dynamic. The most straightforward way to achieve this is to intervene in the oxygen supply rate through real-time human monitoring. However, the limited energy of trapped workers makes it difficult to monitor the oxygen supply 24/7, and real-time monitoring can increase the burden on the trapped candidate.

In conclusion, scholars have studied the environmental parameters in refuge chambers in detail, but none have proposed a study of the control methods used in the practical application of various oxygen supply methods. Stable and efficient methods of oxygen supply control can greatly improve the efficiency of oxygen utilization. This paper proposes a new method for automatically controlling oxygen concentration and carbon dioxide concentration at appropriate levels based on real-time monitoring of personnel numbers and oxygen concentration, thereby increasing the success rate of rescue through longer-term protection for those affected in the refuge.

2. Analysis of the Current Situation of Oxygen Supply in Refuge Chamber

The oxygen supply in the refuge chamber mainly includes pressurized air, compressed oxygen, and self-rescuers. When people are trapped in a mine disaster, they could stay in the refuge chamber and open the pressurized air first. When pressurized air is polluted or the pressurized air equipment is destroyed, the compressed oxygen should be opened immediately. When the compressed oxygen is exhausted, the self-rescuer can be used.

The pressurized air is supplied by an air compressor and stored in the gasholder. Firstly, the pressurized air is transported by pipe transport and is filtered by an air filter to remove the water, dust, and oil in the air. Secondly, clean, pressurized air enters the refuge chamber through a pressure-reducing valve. The pressurized air can guarantee a stable oxygen concentration and dilute and remove toxic or harmful gases. But the pressurized air pipeline is easily destroyed by explosion accidents and cannot supply adequate air. The schematic diagram is shown in Figure 1.

If the pressurized air fails, the compressed oxygen should be opened so that it can supply continuous and effective oxygen for at least 96 h. The high-pressure oxygen bottle releases oxygen, and the oxygen gas passes through the pressure-reducing valve and globe valve, finally slowing emissions to the refuge chamber evenly. The schematic diagram is shown in Figure 2.

When all the pressurized air and compressed oxygen do not work, the self-rescuer can be used. The self-rescuer can afford oxygen for two to three hours if the people stay quietly in the refuge chamber. So, this is the final method to buy as much time as possible until rescue arrives.

3. The Methods of Compressed Oxygen Supplying

3.1. Manual Oxygen Supply

Currently, the main method for continuously supplying oxygen to the refuge chamber is to release oxygen manually. The operator observes the change in oxygen concentration in the refuge chamber through the oxygen concentration sensor. When the concentration is lower than the minimum oxygen concentration demanded, the oxygen valve will be manually activated to release oxygen. When the oxygen concentration rises to the upper limit demanded, the oxygen valve is closed again and thus repeated. However, this method will cause the oxygen concentration to rise and fall, and it is difficult to maintain stability. In order to ensure that the oxygen concentration is not lower than the minimum value, the oxygen concentration in the refuge chamber is maintained at a high level so that a higher concentration of oxygen is discharged outside. This causes unnecessary waste and shortens the time available for oxygen, which is very unfavorable for the disaster-stricken personnel waiting for rescue.

3.2. Two-Stage Oxygen Supply Method

Oxygen demand in the refuge chamber is closely related to the number of people, but the real-time oxygen demand per person varies. Therefore, this paper proposes a two-stage control method for oxygen supply control. Through the refuge chamber personnel location system, the number of people in the refuge chamber can be monitored in real time. According to the per capita oxygen demand formula for people sitting in the sitting position, the minimum real-time oxygen demand can be calculated. An oxygen flow automatic regulator is installed at the end of the busbars of high-pressure oxygen bottles so that the oxygen flow rate is automatically adjusted to the minimum oxygen demand to ensure a stable and sustained minimum oxygen demand in the chamber. If the person enters the motion state from the static state, the oxygen demand will increase. At this time, the PID algorithm can adjust the fine adjustment valve according to the oxygen concentration monitored by the oxygen sensor to ensure the additional oxygen demand. The two-stage oxygen supply method can ensure the safe and efficient use of oxygen. In addition, the automatic control system can also achieve a reasonable range of carbon dioxide by adjusting the rotational speed of the air blower of the indoor air purifier in the refuge, and indirectly ensuring the stability of the oxygen concentration. The control schematic diagram is shown in Figure 3.

4. Realization of Compressed Oxygen

4.1. Design and Establishment of Refuge Chamble Model

4.1.1. Overview of the Refuge Chambre Model

In order to verify the reliability of automatic oxygen supply based on a real-time predicted amount of oxygen supply, a refuge chamber model was established, which is 8 m long, 4 m wide, and 2.5 m high. The refuge chamber model is divided into a living room and a transition room, as shown in Figure 4.

The transition chamber has an area of 2 m × 4 m. An automatic sprinkler system, an air curtain system, and a compressed oxygen cylinder are installed in the transition chamber. The living room area is 6 m × 4 m. A personnel location system, a gas sensor, a seat, a cooling and dehumidifying system, and the like are installed in the living room. In addition, an evacuation diverticulum monitoring center has been established outside of the refuge chamber, including audio and video surveillance, monitoring of environmental parameters, and a terminal display system.

Personnel location systems include a host in the evacuation diverticulum monitoring center, a sub-station reader in the living room, and a personnel location identification card worn by each person. The host and the sub-station reader are connected by industrial Ethernet. The sub-station reader reads the personnel location identification card information and transmits it to the host. At the same time, the environmental monitoring system can also read the personnel positioning data in real time through the database sharing function and provide a basis for predicting the environmental changes that may occur in the living room.

The environmental monitoring system controls the oxygen supply system, the cooling and dehumidification system, and the air purification system. It uses oxygen demand forecasting and real-time monitoring of oxygen concentration to control oxygen supply, as shown in Figure 5.

4.1.2. Oxygen Supply System

The oxygen supply system of the refuge chamber model includes a pressurized air supply system and a compressed oxygen supply system. It mainly includes the following equipment: an air compressor, gas storage tank, pressure-reducing valve, pressure control cabinet, special connecting parts and piping, filter device, and high-pressure hose. The gas storage tank of the pressurized air supply system and the oxygen bottles of the compressed oxygen supply system are shown in Figure 6. A total of six oxygen bottles are used. The oxygen supply system layout is shown in Figure 7.

4.1.3. Automatic Control of Carbon Dioxide Elimination

The carbon dioxide automatic elimination system uses an air purifier (Figure 8) and a CO₂ adsorbent. The air purifier is used to force ventilation to achieve the absorption of carbon dioxide by the CO₂ adsorbent. The CO₂ adsorbent uses LiOH as the main raw material, and the adsorption efficiency can reach 50% or more. The color of the adsorbent is pink when it is not reacted, and the color is pure white after the reaction. The air purifier can use the continuously variable speed fan to adjust the fan speed through the current control signal, thereby controlling the flow rate of the gas through the medicament and the speed of eliminating the carbon dioxide gas. In the process of use, the fan is generally driven to the maximum to ensure that the indoor carbon dioxide does not exceed the standard. But this increases energy consumption. Because the power of the air purifier comes from the refuge chamber. After the accident, the power supply outside the refuge is likely to be interrupted and can only be powered by the internal battery. At this time, the energy-saving potential of the equipment in the refuge chamber is very high. Oxygen consumption is directly related to the amount of carbon dioxide produced. The air purifier can be indirectly controlled by the oxygen consumption in the refuge chamber to achieve the purpose of air purification and energy saving.

4.2. Real-Time Oxygen Supply Prediction Method

A compressed oxygen supply is different from pressurized air. The amount of oxygen in the oxygen bottle is limited, so it is very important to avoid wasting oxygen so that the oxygen supply time of the oxygen bottles can be extended as much as possible.

Assuming that the amount of oxygen supply is Q, the oxygen supply concentration is O_IN, the refuge chamber oxygen concentration is O_OUT, and the oxygen consumption is U, the discharge amount of the exhaust gas at the outlet end is equal to Q, and the oxygen concentration of the exhaust gas is equal to the oxygen concentration in the chamber. At this point, the equation is satisfied: Q * O_IN = Q * O_OUT + U, after finishing:

$$Q=\frac{U}{Q_{in}-Q_{out}}$$

(1)

Regarding the value of per capita oxygen consumption U, the oxygen consumption of adults in a minute of sitting is about 0.25 L, and the average is generally not more than 0.33 L/min. There is a difference between the populations, and the actual situation should be based on actual conditions. According to the per capita oxygen consumption U of 0.33 L/min, Q is 0.5 L/min, O_IN is 100%, and O_OUT is calculated to be 34%; that is, the refuge chamber oxygen concentration is 34%, far exceeding the upper limit demand of 23%. If O_OUT is 18.5~23% and U is 0.33 L/min according to the requirements, the calculated per capita oxygen supply Q is 0.4~0.43 L/min. That is to say, in the case of the normal operation of each system in the chamber, when the per capita oxygen supply is 0.4~0.43 L/min, the breathing requirement can still be met.

The above is the calculation method for per capita oxygen supply. According to the number of people in refuge chamber X and the average oxygen consumption Y of a single person at rest, the total oxygen consumption of the refuge chamber can be calculated: U = XY. According to Formula (1), the total oxygen supply Q_t of the refuge chamber can be calculated as

$$Q_t=\frac{XY}{Q_{in}-Q_{out}}$$

(2)

At this time, the calculated total oxygen supply is only the minimum oxygen supply capacity of the people in the refuge chamber at rest. Due to individual differences and differences in personnel activities, the real-time oxygen supply of the refuge chamber will be slightly larger than the minimum oxygen supply. It is necessary to use an oxygen sensor to monitor the difference between the real-time oxygen concentration of the refuge chamber and the target oxygen concentration and use the PID algorithm to control the fine-tuning of the valve to supplement the oxygen required for personnel activities.

4.3. Strategy for Automatic Oxygen Supply Based on Real-Time Predicted Oxygen Supply

The refuge chamber automatic control system first monitors whether the pressure of the pressurized air is normal. If the pressure is too low, the pressurized air supply system is faulty. And the compressed oxygen supply system should be turned on. The personnel location system is used to monitor the number of refuge chamber personnel in real time, and the minimum oxygen demand of the refuge chamber is calculated according to the number of personnel. The minimum oxygen demand is a relatively constant amount, at which point the minimum oxygen demand valve is controlled to ensure a stable supply of the lowest oxygen demand in real time. The oxygen sensor is used to monitor the oxygen concentration in real time, calculate the difference between the real-time oxygen concentration and the target oxygen concentration, and use the PID algorithm to adjust the fine adjustment valve to meet the amount of oxygen required for the personnel activity. The system flow chart is shown in Figure 9.

4.4. Experimental Verification of Compressed Oxygen Supply

After the hardware construction and software design of the oxygen supply system are completed, the debugged system can run normally and realize automatic control, but the system needs to conduct real-life experiments to confirm that it can meet the oxygen supply needs of the avoidance personnel.

4.4.1. Experiment Preparation

Before the real experiment begins, it is necessary to first check all parts of the system to ensure that the functions of each part are normal and to ensure the safe development of the experiment.

First of all, it is necessary to conduct an air tightness test to ensure that the refuge chamber is airtight and there is no gas leakage. During the test, close the airtight door and the automatic pressure relief valve, and turn on the compressed air oxygen supply system. When the pressure difference between the chamber outside and the chamber inside is about 500 Pa, Stop the compressed air oxygen supply, wait for 60 min, and observe whether there is any obvious change in the pressure difference. If it is above 150 Pa, the airtightness is good; otherwise, it is necessary to carefully check the sealing condition. Second, check the oxygen supply system. Check whether the hardware equipment and connection of the compressed air oxygen supply and compressed oxygen supply are in good condition, whether the valves are opened and closed normally, and whether the readings of the flow meter are normal. Finally, it is necessary to check whether the auxiliary equipment, such as the spray air purifier, can work normally and whether the effect of the air treatment can meet the requirements. After the above work is completed, it is necessary to cooperate with the monitoring system to detect data reading and remote control functions, whether the upper computer data are consistent with the sensor value in the chamber, and whether the remote equipment starts and stops normally. The experiment can only be carried out after the debugging is confirmed to be correct.

4.4.2. Experimental Process

The initial number of experimenters was 10, the initial concentration of oxygen in the refuge was 20.9%, and the concentration of carbon dioxide was 0.05%. After the experimenters entered the refuge chamber, the pressurized air supply system was automatically turned on, and then the experimenters closed the gas control valve of the pressurized air supply system, and the compressed oxygen supply system was automatically turned on at this time. The refuge chamber oxygen concentration target value was set to 21.0%, and the minimum amount of oxygen was set to 0.38 L/min per person. Experimenters who entered the refuge chamber at 8:00 kept their meditation state for four hours. Lunch and free time were from 12:00–13:00. At 13:00, 5 people left. Sit-in time was from 13:00–17:00, and the experiment ended at 17:00.

The manual oxygen supply method is to supply oxygen at a certain rate. During the experiment, the oxygen supply rate was set at 0.5 L/min for each experimenter. The oxygen supply starts when the space personnel enter the refuge chamber, stops when the oxygen concentration reaches 23%, and starts again when the oxygen concentration drops to 18.5%.

4.5. Analysis of Results

(1)

Experimental results of manual oxygen supply method

The oxygen concentration was recorded after the experimenters entered the refuge chamber. Figure 10 shows the constant of oxygen concentration change. When the oxygen supply rate is constant, it can be seen that the oxygen concentration continues to rise because the oxygen supply rate is greater than the oxygen consumption rate. At 240–300 min, the oxygen ascent rate decreases; this is because the experimenter eats and the oxygen consumption increases. However, due to keeping the internal pressure of the refuge chamber stable and releasing harmful gases, part of the air will be exhausted from the refuge chamber. Higher oxygen concentrations will waste oxygen.

(2)

Experimental results of the two-stage oxygen supply method

Figure 11 shows the trend of oxygen concentration change. As show in Figure 11, at the beginning of the experiment, the minimum oxygen demand control valve kept the oxygen output continuously at a flow rate of 3.8 L/min. The oxygen concentration sensor monitored an initial oxygen concentration of 20.9%, which was slightly lower than the target value. At this point, the PLC issued a command to open a control valve to deliver oxygen to the refuge. The amount of oxygen delivered was calculated by the PLC using a PID algorithm to automatically calculate the ambient oxygen concentration. At 20 min, the concentration increased to 21%, and at 40 min, the concentration increased to 21.1%. The concentration then decreased gradually. It was reduced to 20.9% at 160 min. Then it rose to 21%. At 240 min, it was lunch and spreading time, the amount of people’s activity increased, and the oxygen concentration decreased to 20.9% at 260 min. Then the oxygen concentration gradually rose again. Because only 5 people were left in the refuge chamber after 300 min, the oxygen consumption was reduced and the oxygen concentration was maintained at 21.1 for a longer period of time, after which the concentration was reduced to 21%. It can be seen that after the oxygen is supplied by the compressed oxygen, the oxygen concentration is basically within the set range, which meets the design requirements. The total amount of oxygen released during the experiment was 1360 L, compared to a manual oxygen release of 2400 L over the same period of time, reducing oxygen consumption by 43.3% compared to a manual release. When the PLC is used to automatically control the release of oxygen, the amount of oxygen required for the barrel time is less. That is to say, if the same amount of oxygen is auto-matically controlled by the PLC, it will allow those who are in danger to survive longer and have more time to wait for rescue. In addition, the oxygen concentration in the refuge was maintained at 21%, which is close to the oxygen content in the air. This does not make the oxygen concentration too low, which could cause discomfort, or too high, which could increase the risk of combustion in the refuge.

During the experiment, the air purifier must be turned on; otherwise, the carbon dioxide concentration will rise sharply. The change in CO₂ concentration during the experiment is shown in Figure 12. As can be seen in Figure 12, the CO₂ concentration in the refuge space began to rise rapidly after the experiment began, slowed down after 100 min, and remained at about 0.5% for the time period after 110 min. This is because when the experiment began, people were breathing and producing CO₂, but the CO₂ concentration was low, so the air purification unit was turned on, and as the CO₂ concentration in the air accumulated, the purification unit began to work at 100 min, so the CO₂ concentration slowed down. Between 110 and 240 min, the CO₂ concentration increased significantly because the CO₂ production rate increased during the meal and activity time of the evacuees. After this time, the five experimenters withdrew, and the CO₂ concentration began to decrease slowly.

5. Conclusions

The refuge chamber oxygen supply system mainly includes pressurized air supply and compressed oxygen supply, and the living room is also equipped with a self-rescuer. A pressurized air supply is the first choice. It not only provides oxygen but also eliminates toxic and harmful gases and can rely on external pipelines to continuously supply gas without restrictions. However, when the pipeline is damaged and the pressurized air oxygen supply fails, it is necessary to apply compressed oxygen to supply oxygen. Compressed oxygen is generally stored in an oxygen bottle and is limited. Therefore, when supplying oxygen, it is necessary to ensure demand and extend the oxygen supply time as much as possible. Generally, the oxygen supply is controlled by a manual valve, which tends to cause the oxygen concentration to rise and fall. If the control of the oxygen supply relies solely on PID adjustment, the oxygen concentration also varies, rising and falling easily.

This paper proposes a two-stage oxygen supply method. The minimum oxygen demand of the refuge chamber is selected according to the number of people, that is, the oxygen demand of the person in the sitting state. This value will only change according to the number of people and is relatively stable. Then, according to the difference between the actual oxygen concentration and the target oxygen concentration monitored by the oxygen sensor, the PID algorithm is used to control the supplementation of the extra oxygen required for personnel activities.

This adjustment mode ensures the minimum oxygen demand of the refuge chamber, does not cause the oxygen concentration to be too low, guarantees the extra oxygen demand of people, and does not cause the problem of insufficient oxygen. At the same time, by using this method, the rotational speed of the fan of the air purifier can also be controlled, and the minimum energy requirement of the air purifier can be realized while ensuring that the carbon dioxide concentration is not exceeded. Field experiments show that this method can maintain the oxygen concentration and carbon dioxide at a relatively stable level, improve the oxygen utilization efficiency, reduce the energy consumption of the air purifier, and be of great significance for prolonging the survival time of people in the refuge chamber.