What are the different methods of capturing and storing CO2 in carbon capture processes?

There are several methods of capturing and storing CO2 in carbon capture processes, including post-combustion capture, pre-combustion capture, and direct-air capture. Post-combustion capture involves capturing CO2 from flue gases after fuel combustion, while pre-combustion capture involves converting fossil fuels into a mixture of hydrogen and CO2 before combustion. Direct-air capture technology captures CO2 directly from the atmosphere. Once captured, the CO2 can be stored in various ways, such as underground storage in depleted oil and gas reservoirs, deep saline aquifers, or through mineralization processes where it is chemically converted into stable carbonate minerals.

Long answer

1. Post-combustion Capture: This method involves capturing CO2 from flue gases after fuel combustion in power plants or industrial facilities. The primary technique used is absorption, where the flue gas comes into contact with an absorbent material that selectively captures the CO2. Common absorbents include amine-based solvents that chemically react with the CO2 to form a liquid solution that can be separated and further processed for storage.

2. Pre-combustion Capture: Pre-combustion capture is applied in integrated gasification combined cycle (IGCC) power plants or facilities with gasification processes like coal-to-liquid plants. In this method, fossil fuels are converted into a mixture of hydrogen (H2) and carbon monoxide (CO), known as synthesis gas or syngas. The syngas is then treated to separate hydrogen for use as fuel while capturing the resulting CO2 for storage. One common approach is called water-gas shift reaction, which converts CO to additional H2 by reacting it with steam.

3. Direct-Air Capture: Unlike post- and pre-combustion capture, direct-air capture (DAC) technology aims to extract CO2 directly from ambient air. Various methods exist, including chemical, physical, and biological capture processes. Chemical absorbents are commonly used to selectively bind with CO2 molecules, and once captured, the CO2 is separated from the absorbent for storage or utilization. DAC is particularly useful in situations where point sources of CO2 emissions are impractical to capture.

Storage:

Following the capture phase, the next step is to store the captured CO2 securely to prevent its release into the atmosphere. Some common storage methods include:

1. Geologic Storage: The most widely implemented approach is geologic storage, which involves injecting captured CO2 deep underground into geologically stable formations. Examples include depleted oil and gas reservoirs, deep saline aquifers (porous rock formations containing high volumes of water), or unmineable coal seams. Geologic storage relies on several mechanisms to securely trap CO2 over long periods, including structural trapping within porous rocks and caprock formations that act as impermeable seals.

2. Mineral Carbonation: This process involves converting CO2 into stable carbonate minerals through chemical reactions with specific rocks or industrial by-products rich in metal oxides such as basaltic rocks or steel slag. This method offers the potential for long-term storage while also creating value by producing building materials from carbonated minerals.

3. Ocean Storage: Although still in the research phase and subject to extensive environmental assessment, ocean storage involves injecting purified CO2 into deep ocean waters away from sensitive ecosystems. It is considered for scenarios where other carbon capture alternatives may not be viable; however, its implications on marine life and ecosystem health require careful evaluation.

Each carbon capture process and storage method has its advantages and challenges depending on the specific situation it’s implemented in. The choice of method often depends on factors such as cost-effectiveness, availability of suitable geological formations or infrastructures, local regulations, and general project objectives