Introduction to Carbon Capture and Sequestration

The following sections are included:

• Contents
• Preface
• Acknowledgments
• About the Authors

In this chapter we will be counting carbon atoms. How many carbon atoms do we emit each year? How many of those can we capture? The questions are simple but the answers are enormous. How enormous? That is the topic of this chapter.

There is little scientific overlap between carbon capture and sequestration and the science of the atmosphere and climate modeling. Yet, the justification for carbon capture and sequestration relies on our understanding of the role of CO₂ in the atmosphere and our predictions on how the increase in CO₂ levels will affect our future climate.

CO₂ in the atmosphere plays an important role in regulating the temperature of the earth. Many different mechanisms can influence the CO₂ concentration in the atmosphere. These mechanisms are part of what is commonly referred to as the carbon cycle.

There is nothing magical about carbon capture and storage (CCS). The technology to capture and store CO₂ in geological formations is already available commercially. So why isn't carbon capture technology more widespread? Currently, the costs of separating CO₂ — in terms of required energy and capital costs, and the resulting increase in price of electricity required — are significant. If research were able to reduce the required energy and monetary costs of separation in a meaningful way, it would greatly reduce the barrier to CCS.

In the previous chapter we looked at carbon capture from a purely thermodynamic point of view — depicting the process as a box with an inlet for flue gas and an outlet for the exhaust and the capture stream. In this chapter and in Chapters 6 and 7 we will look inside this box and discuss in detail how such a separation is carried out in practice. For historical reasons, we will start by examining liquid absorption.

Only one letter separates Absorption from Adsorption, yet these two methods to capture carbon dioxide differ in important ways. Adsorption is the process by which the surface of a solid holds on to small molecules in solution or in the gas phase. In other words, the molecules reside on the surface of the material. In absorption, the molecules actually enter into the material. One way these differences play out is in the development of strategies for increasing the capacity of the capture material. With absorption, increasing the capacity of the technology usually involves increasing the amount of capture material. In contrast, with adsorbents, increasing capacity involves increasing the exposed surface area of the material.

One can also use membranes to separate gasses. Normally membranes operate using a large pressure difference, but flue gas is at the “end of the pipe,” and thus the exhaust pressure is very close to atmospheric pressure. Compressing flue gas is very expensive, so membrane separation does not look very attractive for carbon capture. We show that one can nevertheless arrive at promising process options by immersing oneself into the physics and chemistry of the membrane, then using that knowledge in clever process design schemes.

Once we have separated the CO₂ from the flue gas, we need to ensure that the CO₂ is permanently stored. In this and the following two chapters, we discuss geological sequestration, which is at present the most promising option to ensure that the captured CO₂ is not released into the atmosphere.

Geological formations contain enormous volumes of pore space, but most of this space is located in pores with diameters of nanometers to tens of micrometers. What happens when CO₂ is injected into such tiny pores?

This chapter discusses continuum scale physical and chemical storage processes relevant to Geologic Carbon Sequestration. We introduce methods to estimate storage capacity and potential environmental impacts of carbon sequestration, and discuss monitoring approaches to avoid and mitigate those impacts.

Using Carbon Capture and Sequestration, we are able to reduce greenhouse emissions, but this technology will not reduce the absolute CO₂ levels in the atmosphere. For a more complete view on the control of climate change with carbon management, we introduce in this chapter techniques that actually reduce atmospheric CO₂ levels. We divide these techniques into two categories: land use and geo-engineering.

The following sections are included:

• List of symbols
  • Greek symbols
  • Sub- and superscripts

The following section is included:

• Cover figure captions

The following sections are included:

• Glossary
• Answers
• Index