Carbon Capture and Storage (CCS) has become top of mind in oil and gas, energy policy, and sustainability conversations worldwide. But few, apart from the geologists and engineers who work directly in CCS, understand what it is.
This article will be the second in our series on “What is CCS“ and will serve as an introduction into the surface science of CCS, so that it can be understood by everyone.
CCS is a broad term that represents several technologies which capture CO2 emissions from facilities or directly from the atmosphere. The process is designed to help prevent the accumulation of greenhouse gases in the atmosphere to reduce global warming. Once capturing CO2 is accomplished it is re-used as a gas in manufacturing processes or is stored via enhancedoil recovery.
There is growing interest in the application of carbon capture and storage technologies to help reduce greenhouse gas emissions in Canada and around the world.
Many large carbon dioxide emitters are studying, and applying, various CO2 capture technologies – especially pre-combustion, post-combustion and oxy-fuel combustion. These include:
Below is a brief introduction into these processes.
Pre-Combustion Carbon Capture
In pre-combustion, CO2 is removed from fossil fuels before the combustion of these fuels is completed. For example, coal is heated under pressure to create asynthesis gas – a process known as gasification. This gas contains hydrogen, carbon monoxide, CO2, and several other lesser components.
Pre-combustion carbon capture removes CO2 from this gasification or reforming process. A synthetic gas is then produced to manufacture hydrogen or generate power in an integrated gasification combined cycle (IGCC) power plant.
Using the water gas shift reaction (carbon monoxide + water vapor), the synthetic gas is converted to hydrogen and CO2. This CO2 can be captured prior to combustion in a gas turbine, or the fuel gas stream for a steam methane reformer. As pre-combustion carbon capture occurs at relatively high pressures it can result in an efficient route to produce a higher purity CO2 stream. Thereby providing a pure burning fuel which does not release CO2.
Typical technologies used to recover the CO2 include:
Solvent absorption (typically amine based)
Adsorbents (physical and chemical)
Cryogenic (freezing) separation
Membrane assisted separation
The technology selection will depend on several factors including:
Capacity
Fuel impurities
Capital and operating costs
Ease of operation and maintenance
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Pre-combustion carbon capture integrates easily into a steam methane reformer for hydrogen production. It is typically more efficient than post-combustion carbon capture in IGCC power plants.
Because of the higher levels of hydrocarbons present before combustion, pre-combustion capture is often more efficient, but more expensive. This is due to the additional processes required for capture. It is not currently cost effective to retrofit old facilities with this technology at this time, so its capabilities are not as well-tested.
Post-combustion carbon capture
In post-combustion carbon capture, CO2 is captured from flue gases from power plants, process heaters, or industrial processes that involve burning residual coke (a byproduct of the refining process).
Since air is used in the burning of fuels, the concentration of CO2 in the flue gas is low. Efficient and economical separation of this CO2 from the large quantity of nitrogen in the flue gas can be challenging. The flue gas is at essentially atmospheric pressure and requires significant compression to recover the CO2 for transportation and storage.
Post-combustion carbon capture is an option for conventional power plants
Again, typical approaches to capturing this CO2 involve:
Solvent absorption
Adsorbents
Membranes
Post-combustion carbon capture could be attractive for existing power plants. However, the scale of the system required to capture the CO2 in large power plants presents significant energy-related and economic challenges.
Post-combustion carbon capture reduces the thermal efficiency of a power plant, requiring more fuel to achieve the same power generation. It is also unlikely to be economical for the typical process heaters used in refineries and chemical plants due to their small size.
The greatest challenge of successful and efficient post combustion capture is separating the carbon from the flue gas. Each type of fuel combustion contains different elements and amounts of CO2. Capture methods must be purpose built for each type of energy source.
Oxy-fuel combustion carbon capture
Oxy-fuel combustion is the burning of a fuel using pure oxygen, or a mixture of oxygen and recirculated flue gas, instead of air.
The advantage of this approach is that it avoids the heating of large amounts of inert nitrogen in the air. Instead, a concentrated CO2 stream can be produced directly. Other advantages of oxy-fuel combustion include a higher adiabatic flame temperature which can increase fuel combustion efficiency and the absence of NOx (nitrogen oxides) formation. A drawback for this approach is that it requires a large quantity of oxygen.
Air separation units to produce oxygen are costly and require large amounts of energy to operate. Additionally, significant modifications to an existing heater or boiler and flue gas ducting are required. Currently, there is a lot of interest in oxy-fuel combustion as a means of reducing carbon emissions.
Vista Projects completed a very interesting study in recent years regarding oxy-fuel combustion on a 50 MMBTU/h steam-assisted gravity drainage once through heat recovery steam generator.
Understanding and executing CCS surface projects
In its limited history CCS has been championed as a one-size-fits-all solution to increased emissions caused by burning fossil fuels. It has also been widely accepted that CCS will play a significant role in the race to net-zero emissions. However, due to the recent boom in CCS throughout North America, CCS has been receiving increased notoriety as a method of emissions reduction. In fact, many CCS projects throughout North America have been shown to be emission contributors instead of emission reducers.
On a closing note, it’s important to mention that CCS technology should not be approached as a mere public relations exercise. Organizations need to embrace their role in reducing emissions. It’s important to take the time to research and understand what it means to invest in complex CCS technologies to be successful.
Vista Projects is an integrated engineering services firm able to assist with your pipeline projects. With offices in Calgary, Alberta, Houston, Texas and Muscat, Oman, we help clients with customized system integration and engineering consulting across all core disciplines.
