Explain the concept of carbon capture and storage (CCS) and its potential application in reducing greenhouse gas emissions from refineries.

Carbon Capture and Storage (CCS) is a technology designed to reduce greenhouse gas emissions, particularly carbon dioxide (CO2), from industrial sources by capturing CO2 emissions, transporting it, and storing it permanently underground, preventing it from entering the atmosphere. Refineries are significant sources of CO2 emissions due to the combustion of fuels for heat and power and from certain chemical processes. Therefore, CCS has the potential to significantly reduce a refinery's carbon footprint. The CCS process typically involves three main stages: capture, transport, and storage. CO2 capture involves separating CO2 from other gases in the flue gas stream. Several capture technologies are available, including pre-combustion capture, post-combustion capture, and oxy-fuel combustion. Post-combustion capture, which involves removing CO2 from the flue gas after combustion, is the most commonly used method in refineries. It typically uses chemical solvents to absorb the CO2. CO2 transport involves transporting the captured CO2 to a suitable storage site. This is typically done via pipelines, but it can also be done by trucks or ships. CO2 storage involves injecting the captured CO2 into deep underground geological formations, such as depleted oil and gas reservoirs or deep saline aquifers. The CO2 is permanently stored in these formations, preventing it from escaping into the atmosphere. The suitability of a geological formation for CO2 storage depends on several factors, including its porosity, permeability, and sealing capacity. Porosity refers to the amount of empty space in the rock, permeability refers to the ability of fluids to flow through the rock, and sealing capacity refers to the ability of the overlying rock layers to prevent the CO2 from escaping. In a refinery setting, CCS could be applied to capture CO2 emissions from various sources, such as the combustion of fuels in furnaces and boilers, the production of hydrogen in steam methane reformers, and the flue gas from the FCC unit. The captured CO2 could then be transported via pipeline to a suitable geological storage site. The implementation of CCS in a refinery is a complex and costly undertaking. It requires significant capital investments in capture equipment, pipelines, and storage facilities. It also requires careful planning and permitting to ensure the safety and environmental integrity of the storage site. However, CCS offers a significant opportunity to reduce greenhouse gas emissions from refineries and to contribute to a more sustainable energy future. For example, a refinery could capture CO2 from its steam methane reformer, which is used to produce hydrogen for hydrotreating units. This hydrogen is vital for producing low-sulfur fuels. The captured CO2 could then be injected into a nearby depleted oil reservoir, both storing the CO2 and potentially enhancing oil recovery. Therefore, CCS has the potential to play a significant role in reducing greenhouse gas emissions from refineries, helping to mitigate climate change.