LEcture 3.pptx Coal Gasification Process
- 2. Coal Gasification Technology • Coal gasification technology efficiently converts coal into: Power, chemicals, fertilizers, and fuels. • Clean technology that decomposes coal into components using steam and oxygen at high pressure and temperature. • Produces synthesis gas (syngas): a mixture of carbon monoxide (CO) and hydrogen (H₂). • Utilized for methanol production as an alternative energy source.
- 3. Major Applications of Coal Gasification • Power generation • Fertilizer production • Methanol synthesis • Hydrogen production • Hydrocarbon production Coal gasification is the only direct route to produce hydrogen from coal.
- 4. Process Overview • Reaction: • Stages of the process: 1. Pre-treatment of Coal 2. Gasification 3. Gas Cleaning 4. Gas Beneficiation
- 5. Reactions in Coal Gasification
- 7. Selection of Suitable Coal for Gasification • Critical properties to choose appropriate coal at the initial stage based on : • Sulfur content • Fixed carbon • Oxygen levels • Ash and volatile content Sub-bituminous coal: Lowest sulfur content compared to lignite and bituminous coal.
- 8. Gas Cleaning & Acid Gas Removal • Removal of acid gases (H₂S, CO₂) is crucial for syngas preparation. • Low sulfur content in coal minimizes unwanted products like H₂S and CO2. Methanol Production • Syngas ideally has a 1:2 CO to H₂ ratio.
- 9. Coal Preparation Pre-treatment step: • Coal is dried and crushed/pulverized for fluid or entrained bed gasifiers. • Moisture removal by drying is essential. • Caking coals may require partial oxidation for easier gasifier operation. • For fixed bed gasifiers, briquetting of fine coal particles is necessary.
- 10. Gasification • Reactor Charging: Pre-treated coal is fed into the gasification reactor. • Reactions: Coal reacts with oxygen (or air) and steam. • Operates at 800–1900°C and up to 10 MPa pressure. • Gas Content: When coal is burned with less than a stoichiometric quantity of air, with or without steam, the product is a low-heat- content gas, which after purification can be used as fuel gas. • Using oxygen instead of air yields a medium-heat-content gas. • Shift Conversion: Some of CO must be reacted with steam by shift conversion to get additional hydrogen • Ash Removal: Ashes are removed as molten slag or in dry form.
- 11. Purification of Product Gas Gas Composition: Contains CO, CO₂, H₂, CH₄, organic vapors, and H₂S. If air is used, nitrogen is also present. Impurities: Particulates like coal dust, ash, and tar aerosols. Purification Steps: • Gas Quenching: Removes tar and oil. • Acid Gas Removal: Uses solvent treatment technology to absorb acid gases (e.g., H₂S, COS, CO₂). • Methanol can be used as a solvent for absorption.
- 12. Different Gasification Techniques 1. Fixed Bed Gasifier 2. Moving Bed Gasifier 3. Fluid Bed Gasifier 4. Entrained-Flow Gasifiers
- 13. Moving Bed Gasifier • Coal bed moves downwards counter-currently with air. • Lowest oxygen consumption. • Operates at low temperatures, leading to: • Slower reaction rates. • Increased maintenance costs.
- 14. Fluid Bed Gasifier • Provides good mixing and has low overall cost. • Carbon conversion rate is lower due to: • Some carbon loss with ash. • Suitable for low-rank coals like lignite as opposed to sub- bituminous.
- 15. Entrained-Flow Gasifier • Fine coal particles react with steam and oxygen co- currently. • Operates at high temperatures, leading to: • High carbon conversion (~99%). • Destruction of tar and oil, yielding pure syngas. • High oxygen demand and sensitivity to high ash content.