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Co2 Recovery from Flue Gas
Cassman
The fundamental principle of CO₂ recovery is to selectively separate it from a mixed gas stream. This is achieved through a standardized four-step process:
The raw flue gas must first be cleaned to prevent corrosion, blockages, and performance degradation in the downstream capture equipment.
Dust Removal: Electrostatic precipitators or baghouse filters are used to remove particulate matter (dust), which can clog adsorbents or solvents and reduce separation efficiency.
Desulfurization & Denitrification: Wet flue-gas desulfurization (e.g., limestone-gypsum method) and Selective Catalytic Reduction (SCR) are employed to remove sulfur dioxide (SO₂) and nitrogen oxides (NOₓ). These impurities cause equipment corrosion, react to create unwanted byproducts with capture solvents, and reduce the final CO₂ purity.
Dehydration: Coolers and adsorption dryers remove water vapor to prevent ice formation and pipeline blockages during low-temperature capture processes and to avoid the formation of corrosive carbonic acid.
This is the core of the recovery process and the most technologically intensive and costly stage, accounting for 60-70% of the total investment. The pre-treated gas enters a capture unit where CO₂ is selectively separated from N₂ and O₂ using physical or chemical methods.
The captured "raw" CO₂ (typically 85-95% pure) often requires further purification to remove residual impurities like N₂, O₂, and H₂S. The required purity level dictates the technology used.
Food-Grade (≥99.9% Purity): For applications like beverage carbonation or as a food additive, a combination of adsorption towers (using molecular sieves to remove N₂/O₂) and distillation columns (to separate light-component impurities) is required.
Industrial-Grade (95-98% Purity): For uses such as enhanced oil recovery (EOR) or chemical synthesis, purity requirements are less stringent, allowing for a simplified and more cost-effective purification process.
The purified CO₂ gas is pressurized and cooled for efficient storage and transport.
The gas is fed into compressors and pressurized (typically to 2.0-7.38 MPa) while being cooled to -20°C to -30°C.
This process converts the CO₂ into a liquid or supercritical state (CO₂ critical point: 7.38 MPa and 31.1°C), which is then stored in specialized, insulated tanks awaiting utilization or sequestration.
The choice of capture technology depends heavily on the flue gas characteristics, cost considerations, and operational requirements.
| Technology Route | Core Principle | Advantages | Disadvantages | Ideal Applications |
|---|---|---|---|---|
| Chemical Absorption | Utilizes an alkaline solvent (e.g., MEA, DEA) that chemically reacts with CO₂ to form a stable carbamate. The solvent is then heated (120-150°C) to break the bond, releasing the CO₂, and regenerating the solvent. | 1. High Selectivity: Excellent capture efficiency (≥90%) even at low CO₂ concentrations. 2. Mature Technology: Widely proven with numerous industrial applications. | 1. High Regeneration Energy: Solvent heating accounts for over 70% of total energy consumption, leading to high operating costs. 2. Solvent Degradation: Solvents degrade and volatilize, requiring replenishment and potentially causing secondary pollution. 3. Corrosive: Requires expensive, corrosion-resistant materials for equipment. | Scenarios with low CO₂ concentration (10-15%) and stable gas flow, such as coal-fired power plants and waste-to-energy incinerators. |
| Physical Adsorption | Employs solid adsorbents (e.g., molecular sieves, activated carbon, MOFs) that capture CO₂ on their surface at low temperatures/high pressures. The CO₂ is released (desorbed) by raising the temperature or lowering the pressure (TSA/PSA). | 1. Low Regeneration Energy: Energy consumption is 30-50% lower than chemical absorption. 2. Non-Corrosive: Adsorbents are inert, leading to longer equipment life. 3. Environmentally Friendly: No solvent loss or associated pollution. | 1. Lower Efficiency at Low Concentrations: Best suited for flue gas with CO₂ concentrations ≥15%. 2. Limited Capacity: Adsorbents have a finite capacity, requiring frequent regeneration cycles and larger equipment volumes. 3. Susceptible to Impurities: Water vapor and other impurities can deactivate the adsorbent material. | Scenarios with higher CO₂ concentration (15-25%) and low impurity levels, such as cement kilns and steel mill coke oven gas. |
| Membrane Separation | Uses polymer membranes (e.g., polyimide) that are selectively permeable to CO₂. CO₂ molecules pass through the membrane 5-10 times faster than N₂, creating a CO₂-rich stream on one side and a CO₂-depleted stream on the other. | 1. Compact Footprint: No large towers or vessels are required. 2. Low Maintenance: No moving parts, leading to simple operation. 3. Flexible & Scalable: Modular design allows for easy adjustment to varying gas flow rates by adding or removing membrane units. | 1. Low Separation Efficiency: Single-stage separation yields only 80-85% purity, often requiring multiple stages in series, which increases cost. 2. Sensitive to Conditions: Membranes are susceptible to damage from high temperatures and impurities, requiring strict pre-treatment and a typical membrane life of 3-5 years. 3. High Energy Use for Low Concentrations: Energy consumption increases significantly for dilute gas streams. | Small to mid-sized facilities with fluctuating gas flows (e.g., small chemical plants, distributed power stations), or as a pre-concentration step in a hybrid process. |
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