(127a) Cryogenic CO2 Capture for Improved Efficiency at Reduced Cost

Abstract

This document outlines the cryogenic carbon capture (CCC) process, which is projected to be a much more energy efficient and cost effective alternative to solvent-based CO₂ separation processes. CCC involves cryogenically separating CO₂ from flue gas via desublimation and compressing the solid and liquid products. Detailed process analyses and laboratory-scale experiments predict potential energy and cost savings of over 50% relative to amine absorption processes. In addition to the energy and cost savings, CCC recovers most of the flue gas moisture in usable forms, represents a true bolt-on technology requiring minimal upstream modification, removes most other gas-phase pollutants, and eliminates the large quantities of hazardous materials and atypical power plant processes associated with amine absorption. Innovation

The cryogenic carbon capture (CCC) process is a retrofit, post-combustion technology that desublimates CO₂ in the flue gas, separates the resulting solid from the remaining light gases, pressurizes the solid CO₂, melts the CO₂ and warms the light gas, and completes the CO₂ pressurization with the liquid CO₂. The process (Figure 1):

(1)   Dries and cools flue gas,

(2)   Compresses it to 5-7 bar,

(3)   Cools it in a heat recovery heat exchanger to nominally -107 °C,

(4)   May extract condensed contaminants such as mercury, SO₂, NO₂, Hg, and HCl at various stages during cooling and condenses most (about 75%) of the CO₂ during cooling (cooling happens in stages even though it is illustrated in a single step),

(5)   Expands the remaining light gas to further cool it to -120 °C (90 % capture) to -135 °C (99 % capture),

(6)   Separates the remaining solid CO₂ that forms during cooling from the remaining gas,

(7)   Pressurizes the solid CO₂ to 70-80 bar,

(8)   Reheats the CO₂ and the remaining flue gas to near ambient conditions (15 °C) by cooling the incoming gases, and

(9)   Compresses and pressurized the now melted CO₂ stream to final delivery pressure (nominally 150 bar).

There is a small external refrigeration loop in the process that transfers the enthalpy of pure CO₂ melting to cooler temperatures to avoid a heat exchanger temperature cross over. One batchwise CO₂ frosting technology under development in Europe has some features in common with CCC, but differs in large and efficiency-degrading ways from this technology. Otherwise, this process is unique among those under investigation for CCS.

The final products of the process are a liquid CO₂ stream at about 150 bar pressure and a gaseous nitrogen stream at atmospheric pressure, both near room temperature. The CCC process exhibits low energy and total costs compared with the current state of the art with high capture efficiency and CO₂ purity. This process is discussed in more detail in several earlier publications and patent applications [1,2,3,4].

Figure 1.   Simple schematic diagram of the cryogenic carbon capture (CCC) process that uses the flue gas as its own coolant. Performance Advantages, Development Status

The capture efficiency depends on the operating pressures of the compressor and success of heat integration steps. Table 1 summarizes representative results for the first version of the process assuming a best-of-class compressor (92% polytropic efficiency) and a dry, simple flue gas (Simple System column) and for a complex flue gas containing SO_x, NO_x, Hg, HCl, and saturated in moisture using a system with a supplemental refrigerator to address CO₂ melting issues (discussed later). Also, the simulation that produced these results assumes an equilibrium reactor, which converts some pollutants to forms that may not actually occur in a realistic system. Despite these simulation shortcomings, the table provides a reasonable measure of system performance.

Table 1     Figures of merit for the CCC process under simple (only N₂, O₂, and CO₂) and complex (flue gas containing S, N, Hg, and Cl impurities) assumptions. .

CCC is capable of very high capture efficiencies without large increases in energy demand. For example, capture efficiencies of 99.7% are achieved when the coolest temperature is -143 °C, and these correspond to capturing all of the CO₂ produced from coal combustion and a fraction of that in the incoming air, where the nominal concentration of CO₂ in the air is 380 ppm. This can be achieved with a relatively modest increase in the specific energy requirements for the process with CCC. Table 2 summarizes performance comparisons between CCC and amine absorption technologies. As is clear, CCC has major potential advantages.

Table 2     Comparison of several figures of merit between amine absorption and CCC processes, with CCC validation status. All results assume 90% capture and product CO₂ pressurization to 150 bar. Amine absorption data are from DOE estimates using the same software and approach as our estimates for CCC [5-7]. American Air Liquide and GE Global Research analyzed the process independent of SES and of each other and confirmed the cost and energy estimates. They and Dong Energy (Europe) have reviewed the process details and confirmed feasibility and qualitative advantages.

The cost and energy efficiency of this process are its most compelling advantages. Several additional major advantages to this process include

·         Bolt-on retrofit to existing processes with minimal upstream modification

·         Simultaneous pollutant removal eliminates or reduces other pollution control processes

·         Reduces power plant water demand, recovering up to 90% of flue gas water in useful form.

·         Eliminates toxic and hazardous streams and uses equipment and processes common to power generation.

  Bibliography

(1)   Baxter, L. (2008). Carbon Dioxide Capture from Flue Gas. US Patent Office . USA: Brigham Young University.

(2)   Baxter, L. (2008). Simultaneous CO2 Capture and Energy Storage. US Patent Office . USA: Brigham Young University.

(3)   Baxter, L., Baxter, A., & Burt, S. (2009). Cryogenic CO2 Capture as a Cost-Effective CO2 Capture Process. International Pittsburgh Coal Conference. Pittsburgh, PA.

(4)   Baxter, L., Baxter, A., & Burt, S. (2009). Cryogenic CO2 Capture to Control Climate Change Emissions. Coal: World Energy Security - The Clearwater Clean Coal Conference - The 34th International Technical Conference on Clean Coal & Fuel Systems. Clearwater, FL.

(5)   US Department of Energy, N.E.T.L. (2007). Cost and Performance Baseline for Fossil Energy Plants. Volume 1: Bituminous Coal and Natural Gas to Electricity .

(6)   US Department of Energy, N.E.T.L. (2007). Pulverized Coal Oxycombustion Power Plants. Volume 1: Bituminous Coal to Electricity .

(7)   US Department of Energy, N.E.T.L. (2008). Pulverized Coal Oxycombustion Power Plants. Volume 1: Bituminous Coal to Electricity: Revision 2 .