Thus, CO 2 can be captured from dilute gas streams, including air at ~400 ppm CO 2, by contact with basic liquids and solids. These can be introduced either as components of a liquid (usually aqueous) solution, or as functional groups on the surface of a high surface area solid material. Among the simplest of these are hydroxides and amines. Lower concentrations require use of chemicalġ Approximate cost of CO 2 transportation via pipeline is $2.24/tonne CO 2 per 100 km of dedicated pipeline ( DOE, 2015a).īases that react with CO 2, a Lewis acid. Physical solvents are used at high concentrations in natural gas processing and chemical production. The choice of removal technology is governed by the concentration and pressure of the gas stream. The direct air capture approaches described in this chapter are technically feasible, but because CO 2 in air is ~300 times more dilute than in flue gas from a coal-fired power plant, the separation process for the same end CO 2 purity will likely be more expensive than capture from fossil fuel power plants.ĬO 2 removal from gas streams is an important component of many industrial processes. A discussion of the thermodynamic limitations appears in Appendix D. Dilute streams are more difficult to separate and require more energy than more concentrated mixtures. However, thermodynamics sets a lower bound on the energy required to separate a mixture of gases. Furthermore, direct air capture systems have the flexibility to produce CO 2 for the commodity market at a desired purity. In addition to negative emissions potential, direct air capture systems benefit from their inherent flexibility of placement, which can reduce the need for pipelines 1 from the capture site to the sequestration reservoir. Currently, the only reasonable approach to store captured CO 2 is geologic sequestration, which is covered in Chapter 7.ĭirect air capture has received significant attention in the public media because it provides a means to reverse CO 2 emissions, appears to be a relatively “easy fix” to climate change, and is a relatively new and high-tech NET. To be considered a NET, direct air capture systems must sequester the captured CO 2 on a timescale that positively impacts climate change. Among these NETs is the direct removal of carbon dioxide (CO 2) from the atmosphere, commonly referred to as direct air capture. The results of recent integrated assessment modeling ( Fuss et al., 2013) have made clear the need to include negative emissions technologies (NETs) as one component in a portfolio of solutions (e.g., mitigation, energy efficiency, renewables, fuel-switching) to prevent greater than 2☌ global warming by 2100. CHAPTER FIVE Direct Air Capture INTRODUCTION
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