Wellhead gas, especially mine gas, has a complex and variable composition, usually containing multiple gases, which can be divided into the following categories:
1. Flammable gas
Methane (biogas): It is the most common gas with suffocating and explosive properties, and is also the main combustible component of mine gas or biogas. It usually accounts for 40%~70% of the total composition of biogas. In coal mine production, toxic and harmful gases mainly composed of methane are generally referred to as gas.
Carbon monoxide: a colorless, odorless, and tasteless gas that is slightly soluble in water and can burn. When the volume concentration reaches 13% to 75%, it becomes explosive when exposed to a source of fire.
Hydrogen: It is flammable, and when it reaches a certain concentration in the air, it can easily cause explosion or combustion when exposed to open flames or high temperatures.
2. Toxic gas
Hydrogen sulfide: a colorless, slightly sweet, and slightly foul smelling gas that is easily soluble in water and has explosive properties when the concentration reaches 4.3% to 46%. It can cause blood hypoxia and poisoning in the human body, and has a strong irritant effect on the mucous membranes of the eyes and respiratory tract.
Sulfur dioxide: a colorless gas with a strong sulfur odor and sour taste. When the concentration of sulfur dioxide in the air reaches 0.0005%, a pungent odor can be smelled. It is easily soluble in water and has the highest density among harmful gases underground, often accumulating at the bottom of underground tunnels. Sulfur dioxide in the air reacts with water to form sulfuric acid, which has a stimulating effect on the eyes. Miners call it "blind gas".
3. Other gases
Carbon dioxide: It accounts for 20% to 40% of biogas, and although non-toxic, its large presence may cause suffocation.
Nitrogen: It is one of the main components of air, but in mines, the nitrogen content may also change due to changes in air composition.
4. Special components and sources of mine gas
The methane content of mine gas depends on the amount of coalbed methane emitted. The coalbed methane reserves in old coal seams with deep metamorphism are generally abundant, resulting in a large outflow and high methane content in the produced mine gas. For example, the composition of mine gas in Yangquan smokeless coal mine in Shanxi Province, China is generally about 41% methane, 47% nitrogen, and 12% oxygen.
5. Safety precautions
The composition of mine gas is complex and constantly changes with the progress of mining activities.
When the concentration of toxic gases in the air during underground operations greatly exceeds the national standard and seriously threatens the health of workers, an effective explosion-proof monitoring instrument, such as a toxic gas detector, is needed to detect the concentration of relevant toxic gases in mines and other fields in real time to ensure the safety of workers.
For the victims of harmful gas poisoning, they should be immediately carried to a fresh air tunnel or ground, and emergency measures should be taken according to the poisoning situation.
In summary, the composition of wellhead gas is complex and varied, and corresponding safety measures need to be taken to ensure personnel and production safety.
The content of wellhead gas varies depending on various factors such as mine type, mining depth, coal seam characteristics, geological conditions, and ventilation conditions. The following is a rough range of content and typical content of some common gases:
1. Flammable gas
Methane (CH₄):
It is the main component of mine gas or biogas, with a content usually between 50% and 75% (for biogas), but it may vary in mine gas due to the amount of coalbed methane emitted.
In certain specific mines, methane content may reach as high as 40% to 70% (relative to the total volume of mine gas).
Carbon monoxide (CO):
The content is relatively low, but it is toxic.
In mine air, the content of carbon monoxide is usually very low, but when accidents such as coal seam spontaneous combustion, underground fires, or gas explosions occur, its content will sharply increase.
Hydrogen (H₂):
The content is relatively low, but it may occur under certain specific conditions (such as underground electrolysis of water, coal hydrogenation reaction, etc.).
2. Toxic gas
Hydrogen sulfide (H₂S):
The content varies depending on the mine, but is usually very low.
In some sulfide deposits or coal seams, the content of hydrogen sulfide may be high, even reaching dangerous levels.
Sulfur dioxide (SO₂):
The content is usually low in mines, but it may occur under certain specific conditions (such as underground fires, smoke from coal seam spontaneous combustion, etc.).
3. Other gases
Carbon dioxide (CO₂):
It occupies a certain proportion in the mine air, usually less than 1%.
Under certain specific conditions (such as high carbon dioxide content in coalbed methane, poor underground ventilation, etc.), the carbon dioxide content may increase.
Nitrogen (N₂):
It is one of the main components of air and usually dominates in mines (except for methane).
Its content varies depending on the ventilation conditions of the mine and the amount of coalbed methane emitted.
4. Precautions
Due to the complex and variable composition of mine gas, which constantly changes with the progress of mining activities, it is difficult to provide a specific range of content.
In the actual mining process, the composition and content of mine gas should be regularly monitored to ensure personnel safety and production safety.
When abnormal or excessive gas composition is found in the mine, corresponding measures should be taken immediately for treatment and prevention.
In summary, the gas content at the wellhead is a dynamic process that is influenced by multiple factors. Therefore, close attention should be paid to changes in gas composition and content during the mining process to ensure safe production.
