Environ. Sci. Technol. 2007, 41, 7558-7563
Energy and Material Balance of CO2 Capture from Ambient Air FRANK ZEMAN* Columbia University, Department of Earth and Environmental Engineering, 918 Mudd MC 4711, 500 West 120th Street, New York, New York 10027
Current Carbon Capture and Storage (CCS) technologies focus on large, stationary sources that produce approximately 50% of global CO2 emissions. We propose an industrial technology that captures CO2 directly from ambient air to target the remaining emissions. First, a wet scrubbing technique absorbs CO2 into a sodium hydroxide solution. The resultant carbonate is transferred from sodium ions to calcium ions via causticization. The captured CO2 is released from the calcium carbonate through thermal calcination in a modified kiln. The energy consumption is calculated as 350 kJ/mol of CO2 captured. It is dominated by the thermal energy demand of the kiln and the mechanical power required for air movement. The low concentration of CO2 in air requires a throughput of 3 million cubic meters of air per ton of CO2 removed, which could result in significant water losses. Electricity consumption in the process results in CO2 emissions and the use of coal power would significantly reduce to net amount captured. The thermodynamic efficiency of this process is low but comparable to other “end of pipe” capture technologies. As another carbon mitigation technology, air capture could allow for the continued use of liquid hydrocarbon fuels in the transportation sector.
1. Introduction Research into Carbon Capture and Storage (CCS) has recently been summarized in the Intergovernmental Panel on Climate Change (IPCC) Special Report on CCS (1). The report describes various technologies focused on emitters producing at least 0.1 Mt per year of CO2. All totaled, these sources produce 13.6 Gt of CO2 annually while global emissions are estimated at 25.7 Gt of CO2 (2). The nominal 90% capture rate of most CCS technologies suggests that more than 50% of global emissions would remain unabated even if these were fully deployed. The remaining emissions, from dispersed and often mobile sources, are not amenable to “end-of-pipe” CCS technologies and require other mitigation techniques. Such emissions may be avoided entirely by substituting electricity for carbonaceous fuels. Alternatively, carbon dioxide can be captured from the atmosphere to compensate for the emission generated at the point of energy consumption. CCS is divided into two components, capture and storage, connected by a conduit carrying CO2 at high purity levels. The exact CO2 concentration will depend on specific conditions of the storage site. Air Capture extends the concept of carbon capture from large concentrated sources, typically 5-15% CO2, to dispersed and mobile sources. While it provides a very different approach to carbon capture, the * Phone: 212-854-7065;fax: 212-854-7081;e-mail:
[email protected]. 7558
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storage technologies would be the same used in conventional CCS. A detailed review of storage options is found in the IPCC report on CCS (1). Novel to this approach is that capture is separated from emission. This fixes the concentration of the source to atmospheric levels, currently about 380 parts per million of CO2. Since the CO2 content of the air is spatially constant, capture devices could be installed anywhere. It is worth noting that atmospheric CO2 levels may double in this century. The higher source concentration would make capture proportionally easier. Scrubbing ambient air as a means of reducing greenhouse gas emissions was first suggested in 1999 (3). The concept itself, however, is considerably older. The removal of CO2 from ambient air itself was first studied in the 1940s by Spector and Dodge (4) using a packed tower with an alkaline sorbent. Other scientists later focused on the creation of synthetic fuels from atmospheric CO2 and, in particular, on the unit energy consumed by the process. The unit energy provides an estimate of the thermodynamic efficiency of the process. In 1977, Steinberg conducted a paper study for producing methanol using nuclear energy and estimated that a packed tower would require 101.2 kJe/mol CO2 captured (5). Similar studies estimated that at least 88 kJe/mol CO2 is required for the tower and 308 kJe/mol CO2 for regeneration of the sorbent through electrodialysis (6, 7). The energy requirement for the entire process was estimated at a minimum of 400 kJe/ mol CO2. The primary energy required to generate the electricity would be between 800 and 1200 kJ/mol of CO2, depending on power plant efficiency. While the total energy exceeds that available from typical carbonaceous fuels (4800 kJ/mol CO2), it does not render the process unfeasible. The meter for Air Capture is the amount captured compared to that emitted by the process. Purely renewable systems have been considered (8); cost and intermittency are the dominant concerns of these systems.
2. Process Overview Zeman and Lackner (9) have previously outlined a specific method of Air Capture, shown schematically in Figure 1. The previous work developed the scientific basis for this particular air capture process by establishing its thermodynamic feasibility. The process is summarized in the following paragraphs. First, CO2 is absorbed by an alkaline NaOH solution to produce dissolved sodium carbonate. The absorption reaction is a gas liquid reaction, strongly exothermic, and shown in eq 1.
2NaOH(aq) + CO2(g) f Na2CO3(aq) + H2O(l) ∆H° ) -109.4 kJ/mol (1) The carbonate ion is removed from the solution by reaction with calcium hydroxide (Ca(OH)2), which results in the precipitation of calcite (CaCO3). The causticization reaction is a mildly exothermic, aqueous reaction that occurs in an emulsion of calcium hydroxide. It is presented as eq 2.
Na2CO3(aq) + Ca(OH)2(s) f 2NaOH(aq) + CaCO3(s) ∆H° ) -5.3 kJ/mol (2) Causticization is performed ubiquitously in the pulp and paper industry and readily transfers 94% of the carbonate ions from the sodium to the calcium cation (10). Subsequently, the calcium carbonate precipitate is filtered from solution and thermally decomposed to produce gaseous CO2. The calcination reaction is the only endothermic reaction in the process and is shown in eq 3. 10.1021/es070874m CCC: $37.00
2007 American Chemical Society Published on Web 09/26/2007
CaCO3(s) f CaO(s) + CO2(g)
∆H°) + 179.2 kJ/mol
(3)
The thermal decomposition of calcite is performed in a lime kiln fired with oxygen in order to avoid an additional gas separation step. Hydration of the lime (CaO) completes the cycle. Lime hydration is an exothermic reaction that can be performed with water or steam. Using water, it is a liquid/ solid reaction as shown in eq 4.
CaO(s) + H2O(l) f Ca(OH)2(s)
∆H° ) -64.5 kJ/mol
(4)
The cycle of chemical reactions involved in the process is presented in Table 1. As indicated by Figure 1, the process contains two recycle loops. Recycling of sodium hydroxide and calcium hydroxide is necessary as neither is a naturally sourced mineral. NaOH is predominantly produced through the electrolysis of sodium chloride with a small amount produced via causticization (11). Calcium hydroxide is produced from lime, in turn a product of the calcination of limestone (12). Both of these processes currently result in the emission of CO2 to the atmosphere, thus necessitating sorbent recovery by virtue of the second law of thermodynamics. Practically, materials cannot be recycled indefinitely and will require some new feedstock. Apart from makeup streams, the inputs to the process are air and energy. The energy is provided in the form of electricity or fuel. The outputs are compressed liquid CO2 derived from the air and the combustion of the fuel, and air depleted of CO2 and/or oxygen. An important consideration is the overall energy balance of the process. The sum of all the reaction enthalpies listed in Table 1 is zero. The enthalpy changes are given at standard conditions, but in the process outlined, the CO2 enters and leaves at different pressures. As a thermodynamic minimum, the energy cost is -RT ln (P2/P1) where P1 is the partial pressure of the input stream and P2 of the output stream. Given atmospheric CO2 levels of 380 ppm (13) at ambient temperature (300 K) and a 1 bar output stream, we obtain a minimum energy penalty of 19.6 kJ/mol. The actual penalty will be much higher as each individual process step, shown in Figure 2, will have inefficiencies and limits to the amount of recoverable heat.
3. Capturing CO2 from Ambient Air The challenge in removing CO2 from ambient air lies in its low concentration. It requires more gas to be moved through a larger absorber than with conventional flue gas scrubbing. The volume of air passing through the absorber is controlled by the amount of CO2 targeted for removal as the concentration is fixed. As an example, 133 m3 of ambient air must be processed to capture 1 mole of CO2, assuming a 50% capture rate and 380 ppm (0.015 mol/m3) CO2 concentration. The size of the absorber is a function of the rate of absorption, in turn controlled by the area of solution surface exposed to the air flow and the solution alkalinity. The pH of the solution affects the absorption per unit surface area while the structure dictates the surface area per unit volume. As CO2 is absorbed, the solution is converted from NaOH to Na2CO3. This lowers both the CO2 in the gas phase and the OH in the liquid phase, both of which reduce the uptake rate of the absorber. Absorbing more of the CO2 available in the air reduces the total amount of air that must be passed through the absorber, while absorbing less reduces the area of liquid covered surfaces required. The final design will be a balance between these two conflicting demands. The flux of CO2 into a hydroxide solution has been previously studied and modeled. Spector and Dodge (4) showed experimentally that the absorption reaction is limited by the concentration of CO2 in the gas phase. The limitation is likely kinetic as the solution still contains an abundance
FIGURE 1. Schematic overview of air capture including Air Separation Unit (ASU).
FIGURE 2. Overview of air capture process. of hydroxide ions. Hikita et al. (14) demonstrated that, under these conditions, absorption functions as a first-order reaction, i.e., the flux into solution is proportional to concentration of CO2 in the gas phase. According to Astarita (15), the first-order absorption of CO2 into strong hydroxide solutions can be described by eq 5.
JCO2 ) xDLkd[OH-]KHFCO2
(5)
In this equation JCO2 is the CO2 flux into solution per unit area per unit time. The variables are presented in Table 2 and the equation can be solved to yield a flux of 30 µmol/ m2/s. The alkalinity of the NaOH solution is limited by the causticization reaction to 1 mol/L. This is not far from the practical maximum of 2 mol/L, established by Tepe and Dodge (16), beyond which increases in hydroxide concentration did not produce noticeable increases in CO2 absorption. The kinetic constant is an empirically derived function of the ionic strength; the detailed formula is contained in Astarita (15). The CO2 concentration of 0.0112 mol/m3 represents the average in an absorber that captures 50% of the throughput. Figure 3 shows the instantaneous and cumulative average flux into a 1 M Na+ solution across a range of carbonate concentrations. The cumulative average flux was calculated by averaging the instantaneous flux values up to that point. As can be seen from Figure 3, the cumulative average flux into a 1 molar NaOH solution can be held above 20 µmol/ m2/s even if one approaches complete conversion of OH- to carbonate ions. This value is above the 10 µmol/m2/s achieved by Spector and Dodge but still lower than the highest measured flux of 30 µmol/m2/s (15). To estimate the energy consumption of such a device we will use the previous estimate (7) of 88 kJe/mol CO2 for a packed tower. As an initial estimate, it may be considered a reasonable value that does not consider technological innovations, such as structured packing. However, a design that utilizes natural drafts would greatly reduce the energy consumption by avoiding the fans needed for moving the air. VOL. 41, NO. 21, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Reaction Cycle for Na/Ca Air Capture reaction
∆H° (kJ/mol)
2 NaOH(aq) + CO2(g) f Na2CO3(aq) + H2O(l) Na2CO3(aq) + Ca(OH)2(s) f 2 NaOH(aq) + CaCO3(s) CaCO3(s) f CaO(s) + CO2(g) CaO(s) + H2O(l) f Ca(OH)2(s)
-109.4 -5.3 +179.2 -64.5
TABLE 2. Variables Used for CO2 Flux into NaOH Solution variable
DL kd [OH] KH pCO2
measure
value
diffusivity of CO2 in NaOH kinetic constant hydroxide ion concentration Henry’s constant
1.78 × 10-9
m2/s
17
6745 1
L/mol s mol/L
15 18
0.79
(mol/L)water/ (mol/L)air mol/m3
19
concentration of CO2
0.0112
units
ref
13
4. Regeneration of Sorbent At the end of the capture phase, the CO2 is dissolved into solution in carbonate form and requires further processing to produce a concentrated gas stream. The carbonate ions are removed from solution by causticization using calcium hydroxide, which simultaneously regenerates the absorbing solution. The low solubility of calcium carbonate results in the formation of precipitates. The use of calcium thermodynamically limits the alkalinity to 1 mol/L as higher values would precipitate calcium hydroxide rather than the desired calcium carbonate (18). At this [OH-] concentration, the theoretical conversion from calcium hydroxide to carbonate has been calculated as 96% (9). Achieving a nearly complete conversion of Ca(OH)2 to CaCO3 is important as any Ca(OH)2 entering the kiln will be converted to CaO and consume energy. The precise amount of energy lost will depend on the efficiency of the process equipment as any Ca(OH)2 converted to CaO in the kiln will be subsequently re-hydrated with some energy recovery. The resultant precipitate is removed from solution and dried prior to calcination. The filter cake can be dewatered by mechanical and thermal methods. It is important to dewater the precipitate outside of the rotary kiln as the evaporation of water requires a large energy input and increases fuel consumption. A separate drying process also facilitates recovery of the enthalpy of vaporization and makes steam available for the hydration reaction. Mechanical dewatering through vacuum or pressure filtration of the wet precipitate can remove a significant fraction of the water retained at a low energy penalty. Some thermal drying will be required to remove all water from the precipitate. The efficiency of mechanical dewatering depends on the physical characteristics of the precipitate. The irreducible moisture content of a filter cake is a function of the average particle size and consequently pore space. Research into calcite precipitation (18) found that particles generated at room temperature ranged in size from 1 to 5 µm. Assuming an average particle size of 3 µm and using data in the literature (20, 21), we can estimate the moisture content of the filter cake to be 28%. It is worth noting that the ideal moisture content is not zero. To generate steam for hydrating the lime exiting the kiln, reaction 4, the ideal moisture content is 18% (18/100). A drying process piloted in the pulp and paper industry is well suited to Air Capture (22). The system uses steam as the drying fluid, which avoids a distinct drying fluid and allows for steam hydration and energy recovery as shown in 7560
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name
type
absorption causticization calcination hydration
gas/liquid aqueous gas/solid liquid/solid
Figure 4. The steam loop is heated via a heat exchanger driven by the exothermic hydration reaction. Taking steam as the input, the hydration reaction now releases 105.2 kJ of heat per mol of lime at temperatures up to 400 °C (23), beyond which the reaction is reversed. This amount of thermal energy is capable of evaporating 2.6 mol of water, a moisture content of 46%. This suggests that a steam dryer efficiency of 60% results in no energy penalty for drying the filter cake. The predicted moisture content of 28% (1.5 mole H2O/mol CaO) generates excess steam allowing for some heat recovery. A successful pilot plant is not the same as an industrial scale facility but the technology can be considered viable. Once dry, the calcite precipitate is thermally decomposed in a kiln to regenerate the gaseous CO2. In order to maintain gas purity, this reaction will take place in a CO2 environment. The products are a CO2-rich gas and solid lime. Under these conditions, the reaction temperature is 900 °C (24) and as the highest process temperature, its energy penalty is unavoidable. Oates has summarized the efficiencies of various lime kilns, and the flash calciner, suitable for micron-sized particles, is rated at 70% efficient (12). The best kilns, however, reach efficiencies in excess of 90% (25). Here the efficiency is defined as the enthalpy of reaction 3 over the actual energy consumption of the process. The solid product of the kiln is CaO, which is reacted with steam from the drying step to complete the cycle. We propose calcination in CO2 environment; however, conventional kilns produce gas streams rich in nitrogen not CO2. To avoid a second capture system, the kiln is fired with oxygen using a CO2 sweep gas. This is a variation of oxygen combustion for power generation and is expected to improve kiln efficiency (26) by avoiding heat losses associated with the nitrogen. The use of oxygen for enhanced combustion has been shown to improve kiln efficiency by 10% (27). The most economical source of oxygen is cryogenic distillation, which requires 25 kJe/mol of O2 of electrical energy input (28). The amount of oxygen used will depend on the efficiency of the kiln and the type of fuel. The overall energy balance can now be estimated using the numbers presented above. The energy required to circulate the hydroxide solution is related solely to the pumping height. It can be shown to be less than 1 kJe/mol of CO2 (29) for heights less than 50 m. We assume a kiln efficiency of 70% as a conservative value representing current technology for the expected particle size. The kiln requires 256 kJ/mol heat and if the fuel is coal, containing ∼400 kJ/ mol (29), then the oxygen penalty is 16 kJe/mol CO2. Energy is also required to compress the CO2 to 80 bar prior to storage. This is an established technology and requires 12 kJe/mol of CO2 (30). The amount of CO2 stored will exceed the amount captured from air because of the CO2 generated during combustion. In the case of coal, 1.56 moles are stored for every mole captured. A summary of the energy requirements is presented in Table 3 and shows that the calcination reaction consumes the most energy followed by the air blowers. Applying the heat released from the hydration reaction to the drying, the total energy requirement per mole CO2 captured is 225 kJ thermal and 121 kJe electrical. As such, the primary energy required ranges from 470 to 570 kJ/mol of CO2, about half that reported by Bandi et al. (7). Again, natural draft systems
FIGURE 3. Evolution of CO2 flux of 1 M NaOH at 380 ppm CO2.
FIGURE 4. Steam dryer including (1) steam loop, (2) condensing heat exchanger, (3) hydration heat exchanger, and (4) bleed valve.
TABLE 3. Summary of Energy Requirements for Air Capture item
energy (kJ/mol)
percent total
type
19.9% 3.6% 14.3% 57.9% 4.3% 100%
electrical electrical thermal thermal electrical
energy required air movement cryogenic oxygen drying precipitate calcination CO2 compression total
88 16 63 256 19 442 energy available
heat of hydration
105
23.8%
thermal
could reduce primary energy consumption to 350 kJ/mol CO2 by eliminating the blowers. The electrical conversion efficiencies used were 35% and 48% (1).
5. Results and Discussion 5.1 Other Air Capture Systems. Lackner et al. (3) first proposed the removal of CO2 from ambient air for the purposes of carbon capture and storage in 1999. Other researchers have performed energy and material analyses on air capture systems based on absorption with sodium and regeneration using calcium solids. We have compared
our results to this previous work with the aim of highlighting the most important areas for future work. The comparison with work by Keith et al. (31) and Baciocchi et al. (32) is presented in Table 4. The first item is Air Contacting, which refers to the energy consumed by the pumps and blowers in the absorption tower. The system proposed by Baciocchi et al. is different in that they assume 500 ppm CO2 concentrations. The higher ambient concentration results in a lower throughput for a similar capture rate. For a fixed geometry, the throughput is proportional to the velocity and the power required is proportional to the velocity cubed. To make a direct comparison, the energy listed by Baciocchi et al. would have to be scaled to the atmospheric conditions used by Bandi et al. This factor (500/355)3 can be applied to the 30 kJ/mol listed to produce a value of 84 kJ/mol CO2 captured. The result is similar to our estimate and significantly higher than that of Keith et al. The two values provide a range of possibilities with ours being more conservative than that of Keith et al. One important consideration is that the benefits of a passive draft system, considered later, decrease with the energy consumed during capture. If the energy penalty is as low as Keith et al. suggest, then there is little benefit to a passive system. Keith et al. assume that the sodium carbonate solution must be heated to improve the kinetics of causticization. This requires an additional 96 kJ/mol of CO2 that does not show up in the analysis of Zeman and Baciocchi et al. During separate experimentation, we determined that improving causticization kinetics was not necessary as this reaction is not the rate-limiting step (33). The evaporation of water to dry the filter cake consumes as much heat as the calcination reaction in the analysis of both Keith et al. and Baciocchi et al. The reason is the assumed moisture content of 50% and that the evaporation is done inside the kiln. The magnitude of this heat input is the reason we adopted a separate drying process using more innovative technologies, as shown in Figure 4. Table 4 clearly shows that effective drying and lowmoisture content filter cake is an important aspect of Air Capture. The energy required for calcination is similar in all cases and contains some uncertainty. Oxygen calcination is an unproven technology and also an important research focus. As mentioned earlier, conventional kilns use air as the oxygen source. A “CO2 purification” procedure is necessary VOL. 41, NO. 21, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 4. Comparison of Energy Requirements (in kJ/mol) for Air Capture item energy required air contacting causticization evaporation calcination (eff.) CO2 purificationa CO2 compression total energy available heat of hydration a
Zeman
Keith et al.
Baciocchi et al.
88 0 63 256 (70%) 16 19 442
12 96 180 204 (88%) 168 19 679
30 0 202 239 (75%) 27 18 516
105
0
to elevate the CO2 concentrations in the process. Keith et al. chose to add an amine scrubbing process, effectively a second gas-separation process. Baciocchi et al. and our analysis used high-purity oxygen to avoid the amine process with a much lower energy penalty. The reason is related to the origin of the CO2 in the kiln. During calcination, CO2 is released from the calcium carbonate and produced from the oxidation of the fuel. For an oxygen system, electrical energy is required to produce sufficient oxygen to combust the fuel. In contrast, amine systems must capture all of the CO2, both calcination and oxidation. Considering a kiln running at 90% efficiency and fired by natural gas, we can see that amine scrubbing is concerned with five times the CO2 of an oxygen system. Essentially, in an oxygen system, the calcination CO2 is along for the ride and is not a penalty. The energy penalty for oxygen production in Baciocchi et al. is higher than that in this paper because of the extra heat required for drying. The total energy required for Air Capture is similar among the three analyses, especially if the causticization and amine penalties are removed from Keith et al. It is important to note that the reason is coincidental offsets, which highlights the need for future work on the concept. It also suggests the most important unknowns are the moisture content of the filter cake and feasibility of oxygen calcination. While neither is conceptually difficult, experimental results will determine which assumption was correct. Reviewing Table 4 we can see that calcination, oxygen production, and CO2 compression cannot be avoided. These three processes require a minimum of 230 kJ/mol of CO2 captured, assuming a 90% efficient, coal fired kiln. The other penalties must be minimized to optimize the process. The heat released by steam hydration is not trivial and could ensure that no additional heat is required for drying. 5.2 Relative Mass Flows. This paper outlines the mass flows associated with capturing CO2 from the air. The complete system will also be required to move air through the device and provide a wetted surface area. Assuming a 50% capture rate, we are required to move 3,000,000 m3 of air over 315,000 m2 of surface for each ton of CO2 captured. The total volume of sorbent required is dependent on its capacity to hold CO2 and the film thickness, as the efficient use of surface area requires that it is constantly wetted. Chemically, we require 45 m3 of 1 mol/L NaOH solution to hold 1 ton of CO2. Physically, we require 190 m3 of solution to wet the surface, using a minimum film thickness of 0.6 mm (34). Distributing 1 ton of CO2 evenly throughout the larger volume results in a 0.75 mol/L NaOH solution, which maintains high flux values (eq 5). The mass balance for such an Air Capture system is available by including the solids handling component. Recall, from Table 1, that the mass of lime and limestone relate to the CO2 captured through their molar mass. The relative mass of the CO2, lime, limestone, sorbent solution, and air for this 9
CO2
CaO
Ca(OH)2
1 CaCO3
1.3 sorbent solution
1.7 ambient air
2.3
190
3,000
TABLE 6. Moles of CO2 Released from Electricity Generation Per Mole Captured air movement
0
O2 production (Zeman, Baciocchi et al.), MEA capture (Keith et al.)
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TABLE 5. Mass Ratio for Components of Air Capture Reference System
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 41, NO. 21, 2007
total
electricity source
oxygen and compression
forced draft
50% passive
forced draft
50% passive
natural gas coal
0.08 0.17
0.20 0.42
0.10 0.21
0.28 0.59
0.18 0.38
system is presented in Table 5. The comparison shows that air movement and sorbent recirculation dominate the mass balance. One mass flow not considered is evaporative losses. Dry air can hold 17 g of water vapor per m3 at 20 °C (35). The total water loss associated with capturing 1 ton of CO2 based on a 3,000,000 m3 throughput could be 50 tons. This is indeed significant. The water loss assumes 0% relative humidity, and any background humidity would reduce it proportionally. By comparison, the equivalent maximum water loss at 10 °C would be 26 tons and further reduced to 13 tons if the relative humidity is 50%. This suggests that placement of any Air Capture device would include average annual temperature, relative humidity, and rainfall in the location criteria. Increasing the alkalinity of the absorbing solution can further reduce the water losses although this would complicate the process by introducing a dilution/concentration step between the capture device and causticizer (36). 5.3 Emissions from Electricity Generation. The net amount of CO2 sent to storage by such a facility, disregarding leakage, will depend on the source of the electricity. Renewable energy, such as wind, could be used and the negligible emissions would be balanced against higher cost and intermittency. The emissions from fossil fuel electricity plants can be estimated by using the representative values contained in Table TS.3 of the IPCC Special Report (1). The listed value for a natural gas combined cycle plant is 0.367 kg CO2/kWh, which can be converted to 2.3 mmol/kJe. Similarly, a conventional pulverized coal plant produces 0.762 kg CO2/ kWh or 4.8 mmol CO2/kJe. The emissions for any combination can be obtained by multiplying the fraction of generation capacity by the representative emissions factor. The electricity consumption of the process can be divided into process needs and air movement. The first, CO2 compression and oxygen production, cannot be avoided. The electricity consumption associated with the air blowers can be reduced or eliminated by using passive air movement or newer, more efficient tower designs. Table 6 shows the CO2 emissions due to electricity generation for each mole of CO2 captured. 5.4 Thermodynamic Efficiency. The feasibility of air capture can be quantified using the concept of thermodynamic efficiency. This refers to the ratio of the thermodynamic minimum energy requirement to the actual amount of energy used in the process. We will compare air capture to established capture technologies, not as a direct comparison, rather how well each achieves its objective. MEA flue gas scrubbing is considered the industry standard and consumes roughly 172 kJ/mol CO2 of thermal energy and 9 kJe/mol of electrical energy (37). Advanced amine sorbents, such as KS1, require only 132 kJ/mol CO2 of thermal energy for
TABLE 7. Thermodynamic Efficiencies of Air Capture vs Amine Scrubbing MEA
KS-1
energy (kJ/mol)
air capture
NG
coal
NG
coal
thermodynamics actual efficiency
19.5 328 6.0%
8.4 181 4.6%
5.3 181 2.9%
8.4 141 5.9%
5.3 141 3.8%
regeneration (38). The initial conditions assumed are flue gas at 5% and 15% CO2, from natural gas (NG) and coal, and 65 °C flue gas. The resultant thermodynamic minimum energy (RTln(P2/P1)) of all three technologies is presented in Table 7. It shows air capture comparable to the use of KS-1 on a natural gas plant. This does not include the energy for compression to 80 bar, which applies to all.
Acknowledgments The author acknowledges Dr. Klaus Lackner for his insights, and assistance in editing the work.
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Received for review April 13, 2007. Revised manuscript received August 19, 2007. Accepted August 20, 2007. ES070874M
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