Composition and Properties of Oil and Natural Gas

Oil is a combustible substance extracted from underground, usually a flowing or semi-flowing viscous liquid. In terms of color, most oil is black, but it can also be dark black, dark green, dark brown, or even reddish-brown, light yellow, or colorless. In terms of relative density, most oil ranges between 0.9 and 0.98, but some oils have relative densities greater than 1.02 or less than 0.71. The fluidity of oil can vary greatly; some oils have a kinematic viscosity of 1.46 mm²/s at 50°C, while others can be as high as 20,392 mm²/s. Many oils have a strong odor due to the presence of sulfur compounds that have an unpleasant smell.

Section 1: Composition of Oil

The differences in the appearance and properties of oil reflect its varied composition. While the composition of oil is extremely complex, its elemental composition is relatively simple. Oil is primarily composed of five elements: carbon, hydrogen, sulfur, nitrogen, and oxygen. The carbon content ranges from 83% to 87%, hydrogen from 11% to 14%, totaling 96% to 99% together. The combined content of sulfur, nitrogen, and oxygen ranges from 1% to 4%. Additionally, oil contains trace amounts of iron, nickel, copper, vanadium, arsenic, and others.

These elements exist in oil in the form of organic compounds. It has been determined that the organic compounds in oil are divided into two major categories: hydrocarbons composed of carbon and hydrogen, and non-hydrocarbon compounds containing elements like sulfur, nitrogen, and oxygen. The hydrocarbons in oil mainly consist of alkanes, cycloalkanes, and aromatic hydrocarbons. The sulfur, nitrogen, and oxygen elements in oil exist in the form of non-hydrocarbon compounds. Although these elements only account for 1% to 4% of the content, the proportion of non-hydrocarbon compounds is quite high.

I. Hydrocarbon Composition in Oil

1. Alkanes in Oil: Alkanes are one of the main components of oil. As molecular weight increases, alkanes exist in oil in three states: gas, liquid, and solid.

   At room temperature, methane to butane are gases and are the main components of natural gas. At room temperature, C5 to C15 alkanes are liquids, mainly found in gasoline and kerosene, with boiling points increasing with molecular weight. During oil distillation, C5 to C10 alkanes mostly enter the gasoline fraction (below 200°C), while C11 to C15 alkanes enter the kerosene fraction (200–300°C).

   Alkanes above C10 are solids at room temperature and generally exist in oil in a dissolved state. When the temperature drops, they crystallize and precipitate out. Industrially, these solid hydrocarbons are called waxes. The wax content significantly affects the pour point of oil products.

2. Cycloalkanes in Oil: Cycloalkanes are another major component of oil and are the main constituents of lubricating oils. The cycloalkanes present in oil are primarily cyclopentane, cyclohexane, and their derivatives.

   The content of cycloalkanes varies across different oil fractions. Their relative content increases with the boiling point of the fraction but gradually decreases in heavier oil fractions due to an increase in aromatic hydrocarbons. Generally, cycloalkanes in the gasoline fraction are mainly monocyclic; in kerosene and diesel fractions, in addition to monocyclic cycloalkanes (which have longer side chains or more side chains than those in the gasoline fraction), there are also bicyclic and tricyclic cycloalkanes. In high-boiling fractions, monocyclic, bicyclic, tricyclic, and polycyclic cycloalkanes are present.

   Cycloalkanes have a significant impact on the viscosity of oil products. Generally, higher cycloalkane content leads to higher oil viscosity.

3. Aromatic Hydrocarbons in Oil: Aromatic hydrocarbons are also one of the main components of oil. They are present in small amounts in light gasoline (below 120°C) but are more abundant in higher boiling fractions (120–300°C). The gasoline fraction generally contains monocyclic aromatic hydrocarbons; the kerosene, diesel, and lubricating oil fractions contain not only monocyclic aromatic hydrocarbons but also bicyclic and tricyclic ones. Tricyclic and polycyclic aromatic hydrocarbons are mainly found in high-boiling fractions and residual oils.

II. Non-Hydrocarbon Compounds in Oil

In addition to various hydrocarbons, oil also contains a considerable amount of non-hydrocarbon compounds, especially in the heavier oil fractions. The main non-hydrocarbon compounds in oil are sulfur-containing, oxygen-containing, nitrogen-containing compounds, as well as resins and asphaltenes.

1. Sulfur-Containing Compounds: Sulfur is one of the common elements in oil. The sulfur content varies greatly among different oils, ranging from a few thousandths to several percent. Oils with sulfur content greater than 2% are usually referred to as high-sulfur oils, those below 0.5% as low-sulfur oils, and those between 0.5% and 2% as sulfur-containing oils. Most oils in China are low-sulfur oils.

   The distribution of sulfur in oil generally increases with the boiling range of the oil fraction, with most sulfur concentrated in the residual oil. Sulfur in oil exists mostly in the form of organic sulfur compounds, with a very small portion as elemental sulfur. Sulfur compounds can be classified into three categories: acidic sulfur compounds, neutral sulfur compounds, and sulfur compounds with high thermal stability.

   Sulfur compounds affect oil processing and product quality in various ways, including:

   • Severe corrosion of equipment.
   • Production of toxic gases like H₂S and low-molecular-weight mercaptans during processing, causing air pollution harmful to human health.
   • Presence of sulfur compounds in gasoline reduces its stability.
   • Catalytic poisoning during gas and various oil fraction catalytic processing due to sulfur compounds.
2. Oxygen-Containing Compounds: The oxygen-containing compounds in oil are minimal, approximately a few thousandths. Most of the oxygen in oil is concentrated in resins and asphaltenes. Oxygen-containing compounds in oil can be divided into acidic and neutral oxygen-containing compounds.

   Acidic oxygen-containing compounds are highly corrosive and can corrode equipment. Neutral oxygen-containing compounds can further oxidize to eventually form gums, affecting the performance of oil products.

3. Nitrogen-Containing Compounds: The nitrogen content in oil is minimal, generally a few ten-thousandths to a few thousandths. The nitrogen content in oil generally increases with the boiling point of the fraction. Therefore, most nitrogen compounds exist in the form of resins and asphaltenes in residual oil.
4. Resins and Asphaltenes: Resins and asphaltenes constitute a large class of substances among the non-hydrocarbon compounds in oil. They are present in considerable amounts in oil; in China’s major crude oils, resins and asphaltenes account for about 10% to over 40%.

   Resins are viscous liquids or semi-solid substances, ranging in color from yellow to dark brown. The color of oil products is mainly due to the presence of resins.

   Asphaltenes are black, amorphous, brittle solids with a relative density greater than 1.

Section 2: Physical and Chemical Properties of Oil

During transportation and storage, evaporation of oil products can cause many issues. For example, evaporation can cause vapor lock in pipelines; significant evaporation of light components in crude oil increases evaporation losses; oil vapors can easily cause fires and make breathing difficult, even leading to suffocation and death. These issues are closely related to the evaporation properties of oil products. The evaporation properties are usually expressed by vapor pressure and distillation range.

I. Vapor Pressure

Vapor pressure is the pressure exerted by the vapor in equilibrium with its liquid phase at a certain temperature, referred to as the saturated vapor pressure or simply vapor pressure. The magnitude of the vapor pressure indicates the ability of molecules within the liquid to vaporize or evaporate. At the same temperature, liquids with higher vapor pressures vaporize more easily than those with lower vapor pressures.

Crude oil is a mixture composed of various hydrocarbons. Its vapor pressure under certain conditions equals the sum of the saturated partial pressures of its components under those conditions.

II. Distillation Range

For pure substances at a certain external pressure, when heated to a specific temperature, the saturated vapor pressure equals the external pressure. This temperature is called the boiling point. At a constant external pressure, the boiling point is a fixed value.

The vapor pressure of oil is affected not only by temperature and pressure but also changes with the vaporization rate. Under a certain external pressure, the boiling point of oil continuously rises with an increase in vaporization rate. Therefore, the boiling point of crude oil is not a single temperature point but a temperature range, which is called the distillation range.

The distillation range of oil varies depending on the measurement equipment used. Using a simple distillation method (Engler distillation), when 100 ml of test oil is distilled in a specified apparatus at a specified distillation rate, the first substances to vaporize are hydrocarbon molecules with low boiling points. The vapor temperature at which the first drop of condensate appears is called the initial boiling point. Hydrocarbon molecules gradually vaporize in order of increasing boiling points, causing the vapor temperature to gradually rise. The volumes of distillate reach successive points: 10%, 20%, …, up to 90%. The highest vapor temperature reached at the end of distillation is called the dry point or final boiling point. The temperature range from the initial boiling point to the dry point (or final boiling point) is called the distillation range. High boiling point components in oil can decompose at high temperatures. Therefore, when distilling crude oil, the entire oil sample is generally not distilled to dryness. Distillation is stopped when the vapor temperature reaches 300°C, and the corresponding amount of distilled oil is recorded.

III. Density

Density is the mass of a substance per unit volume in a vacuum, with units of g/cm³, g/ml, or kg/l.

According to China’s national standard GB1884-83, the standard density of oil and liquid petroleum products is specified at 20°C, denoted as ρ₍₂₀₎. If the density is measured at other temperatures, it is called apparent density, denoted as ρ₍ₜ₎.

The relative density of oil is the ratio of the oil’s density to that of water at a specified temperature, a dimensionless quantity. Since the density of pure water at 4°C is approximately 1 g/cm³, it is often used as the reference standard. The relative density at t°C compared to water at 4°C is expressed as dₜ⁴. Its numerical value is approximately equal to the apparent density of the oil at t°C. In China, the commonly used relative density is d₂₀⁴.

European and American countries often use specific gravity to express the density of oil products, also known as API gravity at 60°F, denoted as API°. The API° value is inversely related to density—the larger the API° value, the lower the density.

Factors Affecting Oil Density

Oil density is related to the fractional composition of the oil, chemical composition, temperature, and pressure.

1. Relationship Between Oil Density and Fractional Composition and Chemical Composition

   For different fractions from the same crude oil, density increases with higher boiling points. Different hydrocarbons with the same carbon number have different densities; aromatic hydrocarbons have the highest density, followed by cycloalkanes, with normal alkanes having the lowest. The more rings in a molecule, the higher its density. Thus, two fractions with the same distillation range produced from different crude oils can have significant density differences due to varying chemical compositions. For oil fractions with the same distillation range, those rich in aromatic hydrocarbons have higher densities than those rich in alkanes.

2. Effect of Temperature on Density

   Temperature greatly affects oil density. As temperature increases, the volume of oil expands, reducing its density. The density of crude oil at different temperatures can be converted using the “Petroleum Measurement Tables.”

   If the density of crude oil at 20°C is known, the density at other temperatures can be calculated using the following formula:

   ��=�20−�(�−20)ρt​=ρ20​−γ(t−20)

   Where:

   • ��ρt​ = density of the oil at t°C
   • �20ρ20​ = density of the oil at 20°C
   • $\gamma$ = temperature coefficient of oil density
3. Effect of Pressure on Density

   Since liquids are almost incompressible, the effect of pressure on the density of liquid oil products can be neglected under moderate temperatures. Only under extremely high pressures is the impact of external pressure considered. However, it is important to note that when liquid oil products are heated under constant volume, the pressure can increase sharply. If all the valves at the inlet and outlet of a pipeline or container filled with oil are closed, heating the oil can generate extremely high pressures, potentially causing the container to rupture.

4. Effect of Mixing Oils on Density

   When several oil products are mixed, if the volume after mixing is additive, the density of the mixed oil can be calculated using additive methods:

   ��=∑��ρm​=∑ρi​

   When two components with significantly different densities are mixed, the volume is often not additive—for example, mixing crude oil with low-molecular-weight hydrocarbons.

IV. Viscosity

Viscosity is an index used to evaluate the flow properties of crude oil and oil products. The viscosity of oil directly affects the energy consumption in oil pipelines. Especially when the flow rate of oil is low and the liquid is in a laminar flow state, the pressure drop in the pipeline is directly proportional to the viscosity of the oil. Taking necessary viscosity-reducing measures is highly beneficial for the collection and transportation of oil.

When liquid molecules move relative to each other, similar to solids moving relative to each other, frictional resistance is generated. The greater the resistance, the poorer the flow properties of the liquid. This internal friction phenomenon of the liquid is usually expressed by viscosity.

Factors Affecting Oil Viscosity

1. Relationship Between Oil Viscosity and Composition

   Oil viscosity reflects the internal molecular friction within the oil and is therefore closely related to molecular size and structure. Viscosity increases with increased density, higher boiling points, or increased hydrocarbon molecular weight. Among hydrocarbons with similar molecular weights, alkanes have the lowest viscosity, cycloalkanes have the highest, and aromatic hydrocarbons fall in between.

2. Relationship Between Oil Viscosity and Temperature

   Oil viscosity is very closely related to temperature.

   Temperature has a significant impact on oil viscosity. As the temperature of the oil increases, the viscosity decreases rapidly. Therefore, viscosity data without specified temperatures are meaningless. The property that describes how the viscosity of an oil changes with temperature is called its viscosity-temperature characteristic. Oils with good viscosity-temperature characteristics exhibit smaller changes in viscosity with temperature changes.

3. Relationship Between Viscosity and Pressure

   When the pressure is below 4.0 MPa, the effect of pressure on the viscosity of liquid oil products is minimal and can be ignored. When the pressure exceeds 4.0 MPa, viscosity gradually increases with increasing pressure and increases significantly under high pressure.

V. Pour Point and Cloud Point

There are two cases where oil loses its fluidity at low temperatures:

1. Oil with Little or No Wax: As the temperature decreases, the viscosity increases rapidly. When the viscosity increases to a certain level (approximately 3×10⁵ mm²/s), crude oil becomes an amorphous glass-like substance and loses fluidity. This kind of solidification is called viscosity-temperature solidification.
2. Wax-Containing Oil: As the temperature decreases, wax in the oil gradually crystallizes. Initially, tiny crystallization centers appear, and high-melting-point hydrocarbon molecules crystallize on these centers. The crystals gradually grow, causing the originally transparent oil to become cloudy; this temperature is called the cloud point. If the temperature continues to decrease, the wax crystals grow, and the temperature at which crystals just become visibly discernible is called the crystallization point. As the temperature further decreases, a large amount of crystals precipitate and form a network-like crystalline skeleton. The wax crystal skeleton encloses the oil, which is still liquid at this temperature, causing the entire oil to lose fluidity. This phenomenon is called structural solidification. Under specific conditions, the temperature at which the oil just loses fluidity is called the pour point.

   The pour point of an oil is the lowest temperature at which it continues to flow under specified cooling conditions. Since it better reflects the low-temperature performance of oil products than the pour point, it has been stipulated as an international standard method. China has begun to adopt it, gradually replacing the pour point as an oil quality index.

   The pour point and cloud point of oil are related to its chemical composition. The higher the boiling point of the oil, the higher its pour point and cloud point.

VI. Flash Point, Fire Point, and Autoignition Temperature

Oil is a complex mixture with hydrocarbons as the main components. Its combustion is essentially a rapid oxidation process of hydrocarbons. However, at normal temperature and pressure, the oxidation reaction proceeds very slowly, and combustion does not occur. If hydrocarbons evaporate into gases and mix with air, and then encounter an external open flame, the oxidation reaction is greatly accelerated. In a very short time, a large number of hydrocarbon molecules oxidize rapidly, producing high temperatures and pressures, resulting in a flash fire. If there are many hydrocarbon molecules, the oxidation produces a large amount of gas, causing temperature and pressure in the local space to rise suddenly. This can generate shock waves with propagation speeds of 2000–3000 m/s, resulting in strong destructive power and causing explosions. If, after the flash fire, no fresh hydrocarbon vapor and air are supplied, the flame extinguishes immediately. If hydrocarbon vapor and air continue to be supplied after the flash fire, a continuous flame forms, leading to sustained combustion.

From this, it is evident that for hydrocarbons to combust, three conditions must be met: hydrocarbon vapor, oxygen, and an open flame. Research has found that even if these three conditions are met, combustion does not necessarily occur. Only when the concentration of hydrocarbon vapor in the air is within a certain range will combustion occur. If the hydrocarbon vapor concentration is below this range, there is not enough fuel; if it is above this range, there is insufficient oxygen. In both cases, combustion cannot occur. This concentration range is called the explosive limit or explosion range, generally expressed as a percentage by volume of the combustible gas. The highest concentration that can cause combustion is called the upper explosive limit, and the lowest is called the lower explosive limit.

The explosive limit is related to the state of the mixed gas. The explosive range widens with increasing temperature and pressure, increasing the risk of ignition. For example, at a pressure of 1×10⁵ Pa, the explosive range of methane is 5.2%–15.0%; at 10×10⁵ Pa, it is 5.8%–17.0%; at 50×10⁵ Pa, it is 5.8%–29.5%.

With increasing hydrocarbon molecular weight, both the upper and lower explosive limits generally decrease, but the reduction varies. The overall trend is that the explosive range narrows.

Explosive Limits of Mixed Gases

Classification of Oil Flash Points

The flash point is the lowest temperature at which oil vaporizes under specified conditions to form a mixture with air that can produce a momentary flash when exposed to a flame.

The flash point of oil is related to its boiling point; the lower the boiling point, the lower the flash point, and the less safe it is. Atmospheric pressure affects the flash point; the flash point increases with rising pressure. Usually, the flash point refers to the value at atmospheric pressure. The flash point of crude oil is also related to its chemical composition; among oils with the same viscosity, those with higher paraffin content have higher flash points.

The flash point is a safety index for oil products. The hazard level of flammable liquids is classified based on the flash point. Since the flash point of crude oil is very low, it is classified as a Class I flammable material. From a safety perspective, it is safer to pour oil at temperatures about 17°C below its flash point.

The fire point is the lowest temperature at which, under specified conditions, the oil can be ignited by a flame and continue to burn for at least five seconds. The fire point is generally 20–60°C higher than the flash point measured in an open cup.

The autoignition temperature is, as the name suggests, the lowest temperature at which oil will spontaneously ignite without an external flame due to intense oxidation when heated in contact with air.

Flash point, fire point, and autoignition temperature are all related to the combustion and explosion of oil products and are also related to their chemical and fractional compositions. For the same oil, the autoignition temperature is the highest, followed by the fire point, with the flash point being the lowest. For different oils, higher flash points generally correspond to higher fire points, but the autoignition temperature may be lower.

Section 3: Composition of Natural Gas

Natural gas is a gaseous mixture primarily composed of various hydrocarbons. Its main components are methane, ethane, propane, isobutane, normal butane, pentane, trace amounts of heavier hydrocarbons, and small amounts of non-hydrocarbon gases such as nitrogen, hydrogen sulfide, carbon dioxide, and helium.

Natural gas can be divided into three types: gas field gas, oil field gas, and condensate gas field gas.

Gas field gas mainly contains methane, with a content of about 80%–98%. The content of ethane to butane hydrocarbons is generally not high, and it contains little or no hydrocarbons heavier than pentane or non-hydrocarbon gases.

Oil field gas includes dissolved gas and gas cap gas. Its characteristic is a higher content of hydrocarbons from ethane to butane and higher, similar in composition to condensate gas field gas after removing condensate oil.

Natural gas extracted from condensate gas fields contains large amounts of methane and ethane, as well as certain amounts of propane, butane, pentane, and heavier hydrocarbons, including gasoline and kerosene components.

Natural gas can also be classified into two types: dry gas (or lean gas) and wet gas (or rich gas). Generally, natural gas with methane content over 90% is called dry gas. If methane content is below 90%, and the content of alkanes like ethane and propane is over 10%, it is called wet gas.

If classified based on the sulfur content in natural gas, gas containing less than 1 gram of sulfur per cubic meter is called sweet gas, while gas containing more than 1 gram per cubic meter is called sour gas.

I. Methods of Expressing Natural Gas Composition

There are three ways to express the composition of natural gas: mass composition, volume composition, and molar composition. Each composition can be expressed in percentages or decimals.

1. Mass Composition
   • Expressed as a percentage:
     ��=(���)×100gi​=(mmi​​)×100
   • Expressed as a decimal:
     ��=���gi​=mmi​​
2. Volume Composition
   • Expressed as a percentage:
     ��=(���)×100Vi​=(VVi​​)×100
   • Expressed as a decimal:
     ��=���Vi​=VVi​​
3. Molar Composition
   • Expressed as a percentage:
     ��=(���)×100yi​=(nni​​)×100
   • Expressed as a decimal:
     ��=���yi​=nni​​

II. Conversion Between the Three Methods

1. If the mass composition of natural gas is known and needs to be converted to volume or molar composition, the following formula can be used:
   ��=����∑(����)yi​=∑(Mi​gi​​)Mi​gi​​​

   Where ��Mi​ is the molar mass of component $i$.

2. If the volume percentage of natural gas is known, it can be converted to mass percentage using the following method:
   ��=��×��∑(��×��)gi​=∑(Vi​×Mi​)Vi​×Mi​​

Section 4: Physical and Chemical Properties of Natural Gas

1. Pressure of Natural Gas: This refers to the total force from collisions between a large number of molecules in random motion within the natural gas, indicating the magnitude of the natural gas’s energy.
2. Temperature of Natural Gas: Represents the intensity of thermal motion of molecules within the natural gas. The temperature depends on the internal thermal motion state of the natural gas.
3. Critical Temperature of Natural Gas: For every pure gas, there exists a certain temperature. Above this temperature, no matter how much pressure is applied, the gas cannot be converted into a liquid. This temperature is called the critical temperature, denoted as ��Tc​. The critical temperature is the highest temperature at which the gas can exist in a liquid state.
4. Critical Pressure of Natural Gas: The pressure applied when the gas and liquid phases are in equilibrium at the critical temperature. It can also be defined as the saturation pressure corresponding to the critical temperature, denoted as ��Pc​. It is the minimum pressure required to convert the gas from a gaseous to a liquid state at the critical temperature.
5. Dew Point of Natural Gas: Natural gas contains a certain amount of water vapor. When the temperature drops or the pressure rises to a certain value, water vapor in the natural gas begins to condense. At this point, the natural gas is saturated with water vapor (i.e., reaches saturation), and the temperature at this moment is called the dew point of the natural gas.
6. Molecular Mass of Natural Gas: Natural gas is a mixture composed of various gases; it does not have a single molecular formula and cannot have a constant molecular mass calculated from it like a pure gas. However, for convenience in engineering calculations, the mass of natural gas occupying 22.4 m³ at 0°C and 101,325 Pa is considered its molecular mass. In other words, the molecular mass of natural gas numerically equals the mass of 1 mole of natural gas under standard conditions.

   If the molar composition ��yi​ (or volume composition, both expressed as decimals) and molar mass ��Mi​ of component $i$ in natural gas are known, the molecular mass of natural gas can be calculated using:

   �=∑(��×��)M=∑(yi​×Mi​)
7. Density of Natural Gas: The density of natural gas is defined as the mass per unit volume of natural gas, denoted by $\rho$:
   �=��ρ=Vm​

   Where:

   • $m$ = mass of natural gas (kg)
   • $V$ = volume of natural gas (m³)

   Since at 0°C and 101,325 Pa, 1 mole of any gas occupies 22.4 liters (Note: This should be 22.4 liters, not m³. There’s an error in the original text). Therefore, the density of any gas under these standard conditions is:

   �0=�22.4ρ0​=22.4M​

   The density of a gas is related to pressure and temperature, and at low temperatures and high pressures, it’s also related to the gas compressibility factor.

Basic Properties of Common Hydrocarbons in Natural Gas (at 101,325 Pa, 0°C)

8. Relative Density: The relative density of natural gas is the ratio of the density of natural gas to the density of air under the same pressure and temperature conditions, i.e., �gas/�airρgas​/ρair​, a dimensionless quantity. The relative density of natural gas is generally between 0.58 and 0.62. The relative density of associated petroleum gas is between 0.58 and 0.62, but some oil field gases rich in heavy hydrocarbons can have relative densities greater than 1.
9. Specific Heat of Natural Gas: The amount of heat required to raise the temperature of a unit quantity of gas by 1°C is called the specific heat (heat capacity) of the gas, with units of J/kg·K or J/kg·°C. Previously used units were kcal/kg·°C.

   1 kcal/kg·°C = 4.18 J/kg·°C.

10. Calorific Value of Natural Gas: One of the important uses of natural gas is as fuel; the calorific value is one of its economic indicators.

    The amount of heat released when one kilogram or one cubic meter of natural gas is completely combusted is called the calorific value of natural gas, with units of kJ/kg or kJ/m³. Complete combustion refers to the combustion reaction where the most stable oxides or elements are formed after the reaction.

    There are two ways to express the calorific value of natural gas: higher heating value and lower heating value. When natural gas combusts, water vapor is produced. Condensing water vapor into water releases latent heat of vaporization (the latent heat of vaporization of water is 2256.7 kJ/kg). The calorific value that includes the latent heat of vaporization of water vapor is called the higher heating value. In practice, since the temperature of flue gas in chimneys is high and the steam produced during combustion does not condense (the latent heat of vaporization cannot be utilized), the lower heating value is obtained by subtracting the unusable latent heat of vaporization from the higher heating value. The lower heating value is commonly used in engineering.

    The combustion calorific value of natural gas determines its thermal value and is an important quality index. Among several commonly used fuels, natural gas has the highest combustion calorific value:

    • Natural gas: 46,055 kJ/m³
    • Gas field gas: 35,588–41,868 kJ/m³
    • Oil field gas: 35,588–66,989 kJ/m³
    • Coal: 29,308 kJ/kg
11. Explosiveness of Natural Gas: When the content of natural gas in the air reaches a certain proportion, it can form an explosive mixture with air. This gas mixture can explode and combust when it encounters an ignition source.
12. Toxicity and Corrosiveness of Natural Gas: Natural gas extracted from gas wells contains varying amounts of acid gases such as hydrogen sulfide and carbon dioxide, as well as harmful substances like mercaptans, carbon sulfide, and carbon disulfide. Among these, acid gases, especially hydrogen sulfide, are the most harmful. Hydrogen sulfide is highly toxic and poses serious hazards to humans, pipelines, equipment, and instruments.