Sulfur content in Compressed Natural Gas (CNG) refers to the concentration of sulfur-bearing compounds such as hydrogen sulfide (H₂S), carbonyl sulfide (COS), mercaptans, and other organic sulfur compounds present in the gas.
Although natural gas is primarily methane (CH₄), sulfur compounds may occur naturally in raw gas extracted from underground reservoirs. For automotive, industrial, and power-generation applications, sulfur content is stringently controlled and minimized.
Due to advanced purification processes, CNG is recognized as one of the cleanest and lowest-sulfur fossil fuels available today.
Sources of Sulfur in Natural Gas
Sulfur compounds may originate from:
Geological formations containing sulfur minerals
Biogenic activity within gas reservoirs
Trace sulfur introduced during gas handling
Odorants (mercaptans) intentionally added for leak detection under regulated limits
Raw natural gas may contain sulfur ranging from a few ppm to several thousand ppm, requiring extensive removal before compression and distribution as CNG.
Key Characteristics of Low-Sulfur CNG
Extremely Low Emissions
Minimal sulfur content results in near-zero sulfur oxide (SOₓ) emissions, reduced acid rain formation, and compliance with stringent air-quality regulations.
Engine & Catalyst Protection
Low sulfur prevents corrosion of fuel systems, protects catalytic converters and oxygen sensors, and preserves long-term engine efficiency.
High Fuel Purity & Combustion Efficiency
Sulfur-free CNG ensures stable combustion, reduced deposits, consistent ignition, and improved thermal efficiency.
Global Standards Compliance
Typical sulfur limits:
Regulatory CNG: ≤20 ppm
Premium automotive CNG: ≤5–10 ppm
Hydrogen-ready or future fuels: <2 ppm
Typical Sulfur Specifications in CNG
| Parameter | Typical Value |
|---|---|
| Total Sulfur | ≤5–20 ppm |
| Hydrogen Sulfide (H₂S) | <4 ppm |
| Organic Sulfur | Trace |
| Methane Content | ≥95% |
| Carbon Dioxide (CO₂) | ≤3% |
| Moisture | <1 ppm |
| Odorant Sulfur | Controlled trace amount |
Refining & Processing of Sulfur in CNG
Before compression, raw natural gas undergoes gas sweetening to remove sulfur compounds.
Primary Desulfurization Methods
Amine treatment for H₂S and CO₂ removal
Adsorption beds using activated carbon or zinc oxide
Molecular sieves for residual sulfur and moisture
Catalytic conversion of COS and organic sulfur into removable H₂S
Final Polishing & Quality Control
Gas dehydration, continuous sulfur monitoring, controlled odorant injection, and compression to 200–250 bar for CNG dispensing.
Why Sulfur Removal Is Critical
Inadequate sulfur removal can cause increased engine wear, catalyst failure, formation of corrosive acids, operational downtime, and non-compliance with emission standards.
Environmental & Regulatory Impact
Low-sulfur CNG meets Euro and Bharat Stage (BS-VI) norms, supports clean-energy policies, and significantly improves urban air quality.
Applications Requiring Low-Sulfur CNG
Automotive & Transport
Passenger vehicles, buses, fleet vehicles, heavy-duty CNG trucks
Industrial & Commercial
Boilers, furnaces, commercial kitchens, gas engines, generators
Power Generation
Gas-based power plants, distributed energy systems, CHP units
Comparison with Other Fuels
| Fuel Type | Sulfur Content |
|---|---|
| CNG | <5–20 ppm |
| LPG | 10–50 ppm |
| Petrol | 10–50 ppm |
| Diesel (ULSD) | ~10 ppm |
| Furnace Oil | 1–3% (10,000–30,000 ppm) |
Summary
Sulfur content control is a defining quality parameter of CNG. Through advanced gas-sweetening and desulfurization technologies, sulfur levels are reduced to negligible ppm levels, ensuring cleaner combustion, lower emissions, engine and catalyst protection, and full compliance with modern fuel standards.
Low-sulfur CNG remains a preferred fuel for sustainable transportation, industry, and power generation.