Process Overview
Gas Cleanup is an important step in the production of sustainable aviation fuel (SAF) production. The purpose of gas cleanup is to remove contaminants that would poison catalysts or interfere with downstream synthesis steps such as Fischer–Tropsch (FT) or alcohol synthesis.
Process Schematic
Depending on the contaminants in the syn-gas, several or all of the steps shown below are utilized for the cleanup.
1. Particulate Removal: In this step ash, char, soot, and entrained solids are removed from raw syngas. Technology options include:
• Cyclones: First-stage bulk solids removal using centrifugal separation.
• Filters: Hot gas filter or ceramic filter is used for high temperature fine particulate removal.
• Electrostatic precipitators (ESP): EPS’s are used for very fine particulates when lower temperatures are acceptable.
2. Tar and Heavy Hydrocarbon Reforming: In this step tars (heavy hydrocarbons that condense and foul equipment are destroyed or converted. Technology options include:
• Thermal cracking: High temperature treatment to break down tars.
• Catalytic tar reforming: Nickel based or dolomite catalysts convert tars into CO and H₂.
• Plasma reforming: High energy plasma to decompose heavy organics (less common, high cost).
3. Acid Gas Removal (H₂S, COS, CO₂): In this step the goal is to remove sulfur species (catalyst poison) and adjust CO₂ levels to protect catalysts and optimize syngas composition. Technology options include:
• Amine scrubbing (MEA, MDEA): Chemical absorption of H₂S and CO₂.
• Physical solvents (Selexol, Rectisol): Applied for syngas at high pressures and low temperatures.
• Zinc oxide beds: This is used as a polishing step to remove residual H₂S to very ppm levels.
4. Nitrogenous Contaminant Removal (NH₃, HCN): In this step, nitrogenous compounds are removed as they are catalyst poisons. Technology options include:
• Water scrubbing: Removes ammonia and some soluble species.
• Catalytic hydrolysis: Converts HCN and NH₃ into more manageable species.
5. Halide Removal (HCl, Cl , F ): In this step halide compounds are removed to protect metal catalysts and prevent corrosion. Technology options include alkali sorbents (for example, sodium bicarbonate) for capturing halides and use of activated alumina for removing trace halides.
6. Metal Removal: In this step goal is to remove vapor phase metals (e.g., Na, K, Zn) that can deposit on catalysts. Technology options include high temperature sorbents and condensation/filtration systems.