0.2 Properties of Natural Gas, Fuel Gas Classification & Combustion Basics

0.2 Properties of Natural Gas, Fuel Gas Classification & Combustion Basics

Quick Summary: Understanding the chemical composition, physical properties, and combustion behaviour of Natural Gas is fundamental to diagnosing gas appliances, setting burner pressures, and ensuring ventilation safety. Natural Gas is a 2nd Family fuel gas composed mainly of Methane ($CH_4$), possessing a Relative Density of ~0.6 (making it lighter than air) and a flammability range between 5% and 15% by volume in air.

Fuel Gas Families & Classification (BS EN 437)

In the United Kingdom and Europe, fuel gases are classified into three distinct families based on their origin, Wobbe Index, and combustion characteristics as defined in BS EN 437:

1. 1st Family Gases (Manufactured / Town Gas): Produced historically by coal gasification. High Hydrogen content (~50% $H_2$), Relative Density of ~0.4 to 0.5, gross Calorific Value around 18–20 MJ/m³. Highly toxic due to inherent Carbon Monoxide content. Obsolete in UK public distribution networks.
2. 2nd Family Gases (Natural Gas): Extracted from geological reserves under the North Sea or imported via pipelines and Liquefied Natural Gas (LNG) terminals. Consists primarily of Methane ($CH_4$, ~95%), with minor traces of Ethane, Propane, Nitrogen, and Carbon Dioxide. Gross Calorific Value ranges from 37.5 to 40.0 MJ/m³ (10.4–11.1 kWh/m³).
3. 3rd Family Gases (Liquefied Petroleum Gas - LPG): Hydrocarbon gases stored liquid under pressure. Comprises Commercial Propane ($C_3H_8$) and Commercial Butane ($C_4H_{10}$). They have high Calorific Values (Propane ~95 MJ/m³, Butane ~120 MJ/m³) and are significantly heavier than air.

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Physical Properties of Natural Gas ($CH_4$)

Natural Gas possesses several critical physical attributes that directly dictate safety procedures and installation design:

• Relative Density (Specific Gravity): Natural gas has a relative density of approximately 0.6 relative to dry air (Air = 1.0). Because it is lighter than air, escaping natural gas rises vertically and accumulates at high levels or ceiling voids. In well-ventilated rooms, it dissipates rapidly upward. (In contrast, LPG with SG 1.5–2.0 sinks to low levels, basements, and drainage gullies).
• Odorisation: Natural gas in its raw state is completely colourless and odourless. To ensure rapid leak detection, gas transporters add an odorant compound (typically Mercaptan or Dimethyl Sulphide). This odorant is formulated so that gas can be readily detected by smell at a concentration of 1% gas in air—which represents just 20% of the Lower Explosive Limit (LEL).
• Toxicity: Unlike old 1st Family Town Gas, clean Natural Gas is non-toxic. However, it acts as an asphyxiant in high concentrations by displacing atmospheric oxygen.
• Flammability Limits: Natural gas will only ignite or explode when mixed with air in specific proportions:
  • Lower Explosive Limit (LEL): 5% gas in air. Below 5%, the mixture is too "lean" (insufficient fuel) to burn.
  • Upper Explosive Limit (UEL): 15% gas in air. Above 15%, the mixture is too "rich" (insufficient oxygen) to ignite.
  • Stoichiometric Mixture: Approx. 9.5% to 10% gas in air, where perfect chemical combustion occurs.

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Combustion Chemistry: Complete vs. Incomplete

Combustion is a rapid exothermic chemical reaction between fuel gas and oxygen. Domestic appliances use atmospheric air, which consists of approximately 21% Oxygen ($O_2$) and 79% Nitrogen ($N_2$).

Complete Combustion

Complete combustion occurs when there is an adequate supply of oxygen, thorough mixing of fuel and air, and sufficient flame temperature without flame chilling.

$\text{Methane } (CH_4) + 2 \text{ Oxygen } (2O_2) + [N_2] \rightarrow \text{Carbon Dioxide } (CO_2) + 2 \text{ Water Vapour } (2H_2O) + [N_2] + \text{Heat}$

Key facts for complete combustion:

• 1 m³ of Methane requires 2 m³ of pure Oxygen.
• Since air is only 21% Oxygen, 1 m³ of Methane requires ~9.6 to 10 m³ of atmospheric air.
• Products of complete combustion are non-toxic: Carbon Dioxide ($CO_2$), Water Vapour ($H_2O$), inert Nitrogen ($N_2$), and heat.

Incomplete Combustion

Incomplete combustion occurs when there is insufficient primary or secondary oxygen, poor air/gas mixing, damaged burner ports, or when the flame impinging directly onto a cold surface (flame chilling).

$CH_4 + O_2 (\text{restricted}) \rightarrow CO + C (\text{soot}) + CO_2 + H_2O + \text{Heat}$

Products of incomplete combustion include:

• Carbon Monoxide ($CO$): A lethal, colourless, odourless, tasteless gas.
• Carbon / Soot ($C$): Black deposits on heat exchangers, burner ports, and flue terminals.
• Aldehydes & Hydrocarbons: Pungent smelling intermediate breakdown products.

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Carbon Monoxide ($CO$) Toxicity & Flue Gas Analyzer Ratios

Carbon Monoxide ($CO$) is extremely dangerous because it binds to red blood cell haemoglobin with an affinity 200 to 300 times greater than oxygen, forming Carboxyhaemoglobin ($COHb$). This prevents oxygen transportation to critical organs, causing tissue hypoxia, brain damage, and death.

Symptoms of $CO$ Poisoning

Early symptoms mimic flu without fever: headaches, dizziness, nausea, fatigue, confusion, and breathlessness. Exposure to 800 ppm of $CO$ causes severe headache and dizziness within 45 minutes and collapse within 2 hours.

Combustion Performance Benchmarks

Registered operatives use electronic Flue Gas Analysers (FGAs) conforming to BS EN 50379 to measure the ratio of $CO$ to $CO_2$ in appliance flues:

$\text{Combustion Ratio} = \frac{CO \text{ (ppm)}}{CO_2 \text{ (\%) } \times 10,000}$

• Normal Clean Combustion Ratio: Typically below 0.0040.
• Action Required / Unsafe Threshold: A ratio exceeding 0.0080 indicates severe incomplete combustion requiring immediate appliance shutdown and investigation under the Gas Industry Unsafe Situations Procedure.

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Flame Types & Burner Characteristics

1. Aerated (Bunsen-Type) Burners: Primary air is drawn into the burner body via a venturi prior to ignition. Produces a distinct blue inner cone and a lighter blue outer mantle. High flame temperature, clean combustion, no soot formation.
2. Non-Aerated (Neat Gas / Diffusion) Burners: No primary air is mixed before the burner head; combustion air is drawn entirely from surrounding secondary air. Operates with a softer flame. If secondary air is restricted, the flame turns yellow/luminous, generating heavy soot ($C$) and high $CO$.

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Worked Calculation Scenario: Combustion Air Volume

Scenario: A gas operative is assessing an open-flued natural gas boiler with a heat input rating requiring a gas consumption rate of 2.4 m³/h of Natural Gas. Calculate the minimum volume of atmospheric air required each hour to achieve complete combustion.

• Formula / Ratio: 1 m³ of Natural Gas requires 9.6 m³ to 10 m³ of atmospheric air.
• Calculation: $\text{Air Required} = 2.4 \text{ m³/h} \times 9.6 \text{ m³ air/m³ gas} = 23.04 \text{ m³/h of air}$ $\text{Air Required (Upper limit)} = 2.4 \text{ m³/h} \times 10.0 \text{ m³ air/m³ gas} = 24.0 \text{ m³/h of air}$
• Conclusion: The installation room and ventilation openings must supply between 23.04 and 24.0 m³/h of fresh atmospheric air continuously during boiler operation to prevent incomplete combustion and $CO$ creation.