Ind. Eng. Chem. Res. 2008, 47, 4883–4890
4883
Reducing the Cost of CO2 Capture from Flue Gases Using Pressure Swing Adsorption Minh T. Ho,†,§ Guy W. Allinson,‡,§ and Dianne E. Wiley*,†,§ School of Chemical Sciences and Engineering, The UniVersity of New South Wales, Australia; School of Petroleum Engineering, The UniVersity of New South Wales, Australia; and The Australian CooperatiVe Research Centre for Greenhouse Gas Technologies (CO2CRC), Canberra, ACT 2601, Australia
Pressure swing adsorption (PSA) processes have been used extensively for gas separation, especially in the separation of hydrogen from CO2, and in air purification. The objective of this paper is to examine the economic feasibility of pressure swing adsorption (PSA) for recovering CO2 from postcombustion power plant flue gas. The analysis considers both high-pressure feed and vacuum desorption using commercial adsorbent 13X, which has a working capacity of 2.2 mol/kg and CO2/N2 selectivity of 54. The results show that using vacuum desorption reduces the capture cost from US$57 to US$51 per ton of CO2 avoided and is comparable in cost to CO2 capture using conventional MEA absorption of US$49 per ton of CO2 avoided. In this paper, a sensitivity analysis is also presented showing the effect on the capture cost with changes in process cycle; feed pressure and evacuation pressure; improvements the adsorbent characteristics; and selectivity and working capacity. The results show that a hypothetical adsorbent with a working capacity of 4.3 mol/kg and a CO2/N2 selectivity of 150 can reduce the capture cost to US$30 per ton of CO2 avoided. Introduction The option of recovering or capturing CO2 for geological storage as a greenhouse mitigation option has become wellpublicized. CO2 can be recovered from flue gases emitted by power plants, steel mills, and cement kilns using gas absorption technology employing carbonates or alkanolamines solvents. However, one of the disadvantages of conventional MEA absorption processes is that the regeneration of the solvent is highly energy intensive.1–4 In addition, the process is plagued with corrosion problems and is expensive. Improved technologies for CO2 capture are necessary to achieve low energy penalties. Pressure swing adsorption (PSA) is one of the alternative technologies that could be applicable for removal of CO2 from flue gas streams. Historically, the recovery of CO2 using pressure swing adsorption (PSA) systems has been in industries such as natural gas processing and hydrogen production. In these industries, the feed gas is available at a high pressure and low temperature, and thus, CO2 can be recovered by using a readily available driving force such as the pressure difference between the high feed pressure for adsorption and a lower pressure for desorption. The driving force is generally referred to as the pressure ratio, the ratio of the desorption pressure to the adsorption pressure. Most adsorbents commonly used in PSA processes have a very strong affinity for CO2 and require a very large pressure difference between the adsorption and desorption stages to enable complete desorption of the CO2. In the 1980s, vacuum swing adsorption (VSA) processes were being explored in place of the traditional PSA systems in order to increase the efficiency of regenerating the adsorbent bed.5–7 In VSA systems, the feed gas is delivered to the adsorber at atmospheric (1 bar) or slightly * To whom correspondence should be addressed. Tel.: +61 2 9385 5541.Fax: +61 2 9385 5456. E-mail:
[email protected]. † School of Chemical Sciences and Engineering, The University of New South Wales. § The Australian Cooperative Research Centre for Greenhouse Gas Technologies (CO2CRC). ‡ School of Petroleum Engineering, The University of New South Wales.
higher than atmospheric pressure. However, the strongly adsorbed gas is regenerated at a much lower pressure, typically 0.01-0.1 bar.8–11 By utilizing a vacuum desorption process, a sufficient pressure ratio is achieved without the need to compress low-pressure feed gases. The inherent advantage of PSA systems is that they are simple to operate. Unlike absorption or cryogenic systems, PSA requires only a few vessels capable of withstanding pressure changes. Like absorption, PSA systems have a regenerative adsorbent that can be reused. The main weakness of PSA compared to other technologies such as absorption is that there has been limited technical experience in recovery of CO2 from industrial streams such as postcombustion flue gas. One of the initial studies by the International Energy Agency Greenhouse Gas R&D Programme (IEA GHG) in 1992 investigated adsorption technology for CO2 capture from power plant flue gases using a PSA system.2 The study found that, using the Gemini process, the feed gas must be compressed to at least 6.5 bar (655 kPa) and cooled to 40 °C for effective removal. This process, which has been designed to recover and purify methane at high pressure, resulted in very high compression costs. On the basis of these results, the report concluded that “gas-solid adsorption, in its present state of commercial development, does not appear to be a suitable technology for the bulk capture of carbon dioxide from flue gas produced by fossil fuel power generating systems”. However, at the time of the analysis, there was little interest in CO2 capture, and it was difficult to perform a thorough analysis. Since then, the increase in interest in Carbon Capture and Storage (CCS) technologies has resulted in many researchers investigating CO2 recovery from flue gases using PSA systems.8,9,12–16 These studies using vacuum desorption conditions show that high and/or moderate rates of CO2 recovery and purity can be achieved without the need for excessive compression of the feed gas stream by utilizing vacuum desorption conditions. To date, all of these studies have only been at the bench or pilot scale and no economic evaluations have been undertaken.
10.1021/ie070831e CCC: $40.75 Published 2008 by the American Chemical Society Published on Web 06/17/2008
4884 Ind. Eng. Chem. Res., Vol. 47, No. 14, 2008
Figure 1. Simplified process flow diagram for the PSA/VSA CO2 capture process. Table 1. Processing Conditions and Composition of a Typical Supercritical Bituminous Power Plant Flue Gas parameter power plant size (net) temperature pressure volumetric flowrate
value 500 MW 110 °C 0.95 bar 670 m3/s
Mole fraction of component gases in the flue gas stream CO2 N2 O2 H2O SOx NOx initial CO2 emission
0.13 0.75 0.05 0.07 220 ppm 99%) CO2 product stream. CO2 capture using chemical absorption is modeled using shortcut methods of suitable accuracy that have been developed through of the extensive experience of engineers with absorption processes in natural gas processing and chemical industries.24–26 The details of the absorption plant have been outlined in our earlier paper.20 Economic Asssumptions. The real cost of CO2 capture is estimated as n
Cost of CO2 avoided )
Ki + Oi
∑ (1 + d)
i )1 n
∑
i )1
i
(CO2 avoided)i
(7)
(1 + d)i
where Ki and Oi are the real capital and operating costs (US$ million) in ith year, d is the discount rate (% pa), and CO2 avoided is the annual amount of CO2 avoided in million tons. Capital and operating costs are estimated for CO2 recovery (capture), precapture treatment, and CO2 compression (postcapture). The total capital cost includes all process equipment shown in Figure 1, plus a general facilities cost. The operating costs includes fixed general maintenance costs comprising labor, nonincome government taxes that may be payable, and general insurance cost. The variable operating costs include costs for the flue gas desulfurization, cooling water, and adsorbent replacement. The analysis assumes a discount rate of 7% over a 25 year project life with 2 year construction period. Additionally, it is assumed that the power plant has an operating capacity of 85%, and the baseline cost of electricity of the reference power plant is 34 $/MWh. The cost for adsorbent zeolite 13X is assumed to be US$5/kg. All costs reported are for the year 2006. Full details of the economic methodology and correlations are discussed in our earlier papers.20,27,28 Results and Discussion Baseline Economic Results for Physical Adsorption. The capture cost using PSA with standard commercial zeolite 13X adsorbent is US$57/ton of CO2 avoided. The CO2 purity from this process is low, at 50% of the total equipment cost. The large number of compressors in the system are needed, first, to compress the feed gas to a high pressure of 6 bar and, second, to compress the low-purity CO2 product for transport. The low percentage of CO2 in the product stream means that large volumes of other gases such as nitrogen and oxygen are present. Therefore, the mixed-gas flowrate is higher and a significantly larger postcapture compressor with higher energy consumption is needed. This, in turn, increases the total capital expenditure and the energy penalty. In comparison, when VSA is used, the cost for capture is US$51/ton of CO2 avoided (Table 3). This cost is comparable
Table 3. Economic Results for CO2 Capture Using Physical Adsorption this study IEA GHG2
this study MEA solvent absorption
VSA PSA
PSA
cost year 2006 2006 CO2 recovery rate (%) 85 85 CO2 purity in recovered 48 48 stream (%) total absorber volume (m3) 630 380 number of adsorber trains 34 2 energy penalty (%) 30 35 capture cost (US$/ton of 51 57 CO2 avoided) additional total capital investment 1300 1450 for capture (US$/kW)
1992 95
2006 90 >99
40 64
34 49
1500
1150
to CO2 capture using MEA solvent absorption and is a reduction of 15% compared to the PSA system. The cost reduction is the result primarily of the lower energy penalty (30% compared to 35%) because there is no need to compress the feed gas. However, the concentration of the CO2 obtained from the desorbed stream in the VSA process is similar to that from the PSA process, that is,
