Synthetic Natural Gas (SNG), also known as biomethane or substitute natural gas, is a fuel source designed to be interchangeable with conventional natural gas. It primarily consists of methane (CH₄) and is produced from various non-fossil or fossil feedstocks. The production process typically involves two main stages: creating a synthesis gas (syngas) and then converting that syngas into methane.
The First Step: Producing Synthesis Gas (Syngas)
The journey to creating SNG begins with the production of synthesis gas, or syngas. This is a crucial intermediate step that prepares the raw materials for conversion into methane.
According to the provided reference, syngas is produced through a thermochemical process called gasification. This process converts various carbonaceous materials into a gas mixture primarily consisting of hydrogen (H₂) and carbon monoxide (CO).
- Feedstocks: The raw materials for gasification can be quite diverse. They include:
- Biomass (like wood or agricultural waste)
- Municipal wastes
- Coal
- Petroleum residues
- Tires
- The Process: Gasification takes place inside a specialized vessel called a gasifier. The carbonaceous feedstock is heated to high temperatures in the presence of a controlled amount of oxidant.
- Oxidants Used: Common oxidants include oxygen, air, or carbon dioxide (CO₂).
- Output: The result of this high-temperature conversion process within the gasifier is synthesis gas [7]. This syngas contains the necessary building blocks (H₂ and CO) for making methane.
Converting Syngas to SNG: Methanation
Once syngas is produced, the next major step is to convert its components (primarily H₂ and CO, and sometimes CO₂) into methane (CH₄). This process is called methanation.
Methanation involves reacting the hydrogen and carbon monoxide (or carbon dioxide) in the syngas over a catalyst, usually nickel. The primary reactions are:
- Carbon Monoxide Methanation: CO + 3H₂ → CH₄ + H₂O
- Carbon Dioxide Methanation: CO₂ + 4H₂ → CH₄ + 2H₂O
This catalytic reaction combines the hydrogen and carbon atoms from the syngas to form methane and water. The water is then removed, leaving behind a gas that is largely methane, similar in composition to natural gas.
SNG Production Flow
Here's a simplified flow of the SNG production process:
- Feedstock Preparation: Drying and sizing the carbonaceous material.
- Gasification: Converting the feedstock into syngas in a gasifier using heat and a controlled oxidant [7].
- Syngas Cleaning: Removing impurities from the syngas (like tar, sulfur compounds, and ash).
- Gas Conditioning (Optional): Adjusting the H₂ to CO ratio in the syngas if needed for optimal methanation.
- Methanation: Catalytically converting the cleaned syngas into methane.
- SNG Purification: Removing water, CO₂, and other trace gases to achieve pipeline quality SNG.
| Step | Primary Process | Key Inputs | Key Outputs |
|---|---|---|---|
| Syngas Production | Gasification [7] | Carbonaceous materials, Oxidant [7] | Syngas |
| SNG Conversion | Methanation | Syngas | SNG (Methane) |
| SNG Purification | Separation/Processing | Crude SNG | Pipeline SNG |
By following these steps, various carbon-rich materials can be transformed into a renewable or non-renewable fuel source that can be used interchangeably with conventional natural gas in existing infrastructure.
