Environ. Sci. Technol. 2010, 44, 6901–6904
New CO2 Capture Process for Hydrogen Production Combining Ca and Cu Chemical Loops J . C . A B A N A D E S , * ,† R . M U R I L L O , ‡ J. R. FERNANDEZ,† G. GRASA,‡ AND I . M A R T ´I N E Z ‡ Spanish Research Council, INCAR-CSIC, C/Francisco Pintado Fe, 26, 33011 Oviedo, Spain, and Spanish Research Council, ICB-CSIC, Miguel Luesma Casta´n 4, 50015 Zaragoza, Spain
Received May 20, 2010. Revised manuscript received July 29, 2010. Accepted July 30, 2010.
This paper presents a new solids looping process for capturing CO2 while generating hydrogen and/or electricity from natural gas. The process is based on the sorption enhanced reforming of CH4, employing CaO as a high temperature CO2 sorbent, combined with a second chemical loop of CuO/Cu. The exothermic reduction of CuO with CH4 is used to obtain the heat necessary for the decomposition of the CaCO3 formed in the reforming step. The main part of the process is completed by the oxidation of Cu to CuO, which is carried out with air diluted with a product gas recycle of this reactor at sufficiently low temperatures and high pressures to avoid the decomposition of a substantial fraction of CaCO3.
1. Introduction The capture and storage of the CO2 produced in large scale power plants and industrial processes may be essential for an effective mitigation of climate change in the next few decades. There is a great interest worldwide in developing new CO2 capture technologies aimed at reducing the energy penalty and the cost of existing capture equipment (1). Particularly important are two families of gas separation processes which use reversible, high-temperature gas-solid reactions to transport CO2 or O2 between different reactors or steps. The following reactions are considered in this work
CaO(s) + CO2 / CaCO3(s) ∆H298 ) -178.8 kJ/mol (1) Cu(s) + 1/2O2 / CuO(s) ∆H298 ) -156.1 kJ/mol (2) This paper also focuses on the application of the first reaction to the steam reforming reaction of natural gas in the presence of CaO (for sorption enhanced reforming reviews see refs 2 and 3). CH4 + CaO(s) + 2H2O / CaCO3(s) + 4H2 ∆H298 ) - 13.7 kJ/mol
(3)
The benefit that can be derived from the presence of CaO in reaction 3, or its equivalents with other carbonaceous fuels and hydrocarbons, has been frequently referred to in the state-of-the-art literature (3–10). The enthalpy of the * Corresponding author e-mail:
[email protected]. † INCAR-CSIC. ‡ ICB-CSIC. 10.1021/es101707t
2010 American Chemical Society
Published on Web 08/12/2010
overall reaction is almost neutral, and the equilibrium of the intermediate step, the water gas shift reaction, is shifted to the right due to the absorption of CO2 by CaO. However, a significant problem still to be solved for CO2 capture systems is that for the entire process to work, the subsequent calcination of CaCO3 to produce CO2 and CaO must be performed in CO2-rich atmospheres. In these conditions, equilibrium demands very high calcination temperatures (about 900 °C in pure CO2 at atmospheric pressure), and the heat transfer to this part of the system is in practice highly problematic in all the processes proposed (11). Here is where the second chemical loop, the reversible redox reaction 2, comes into play. Lyon (12) was the first to propose a reforming method with CaO as CO2 sorbent, using a Fe/FeO chemical loop to solve the problem of the endothermic regeneration of the CaCO3 generated during the reforming step. In this way, the exothermic oxidation reaction from Fe to FeO with air is able to generate the heat required to decompose the CaCO3. According to a review by Dupont et al. (13), this method has been extended to other systems with CaO and other redox systems. Such methods take advantage of the efficiency with which the heat is transferred from the metal particles oxidized with air to the CaCO3 particles in the same bed. Moreover, a certain experimental validation of the operation of these fixed beds has already been achieved (14), showing that all the reactions take place on narrow reaction fronts which makes it possible to design each cyclic operation in a single bed of solid materials. This is accomplished by alternating the gaseous reactants that are fed in, adjusting the temperature and pressure conditions in the fixed beds, and employing different time cycles (13, 14). However, it must be stressed that these processes only partially solve the problem of CO2 emissions from the overall system. This is because the CO2 generated in the decomposition of CaCO3 leaves the metal oxidation reactor highly diluted by N2, and, as a result, it is emitted to the atmosphere. This paper proposes a solution to the critical problem of CaCO3 calcination raised above. We outline here a new process for H2 production and/or power generation from natural gas, with inherent CO2 capture combining the known benefits in terms of energy efficiencies of sorption enhanced reforming processes and chemical looping processes. The objective of this conceptual paper is to demonstrate the theoretical viability of the new process by incorporating state of the art knowledge on the thermodynamics and kinetics of the main gas-solid reactions involved. Some design and operational challenges are also outlined for future R&D work on this process.
2. Fundamentals of the Proposed Process The most important step of the CO2 capture process discussed in this paper is the regeneration of CaCO3 using heat from the reduction of CuO with a fuel gas (assumed to be natural gas in this work). Both of these reactions are made to take place simultaneously by feeding the natural gas into a fixed bed containing adequate proportions of CaCO3 and CuO. The general scheme of Figure 1, includes the following main steps: a) The production of a hydrogen-rich stream by the sorption enhanced reforming of CH4 with steam and the simultaneous carbonation of CaO with the CO2 resulting from the reforming reaction. This reaction will require a suitable reforming catalyst (e.g., noble transition metals or Ni) as Cu may not provide sufficient activity. VOL. 44, NO. 17, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. General scheme of the Cu-Ca three step chemical loop.
FIGURE 2. A possible integration of the Cu-Ca chemical loop for power generation employing natural gas as energy source. b) The oxidation of Cu to CuO with air, in conditions such that no major decomposition of CaCO3 takes place (this will be favored by high pressure operation, low temperatures, and the partial recycling of the N2 product gas as a way to lower O2 partial pressures). c) The calcination of CaCO3 and the reduction of CuO with a fuel gas (syngas with a high CO content yields the highest reduction reaction enthalpies) in order to obtain most or all of the heat necessary for calcination. Compared to other methods in which CaCO3 is calcined under rich CO2 atmospheres, the advantage of the simultaneous reduction and calcination step lies in the high efficiency with which heat is transferred between reactions that take place in the same reactor. This would allow the use of moderate operation temperatures and so lead to the saving of energy and special materials and the avoidance of complex heat exchange steps at very high temperatures. The possible process schemes based in Figure 1 will depend on the characteristics of the fuel used and on the end product (15). We report here on a particular case of sorption enhanced reforming of natural gas for the production of H2 and power generation (Figure 2). In order to illustrate the potential of high efficiency systems based on these processes, Table 1 summarizes the operating conditions and stream compositions when equilibrium composition are achieved for the gas product streams in all the critical steps outlined in Figure 2. HYSYS software has been employed in 6902
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order to calculate the adiabatic temperatures and equilibrium compositions resulting at the exit of the reaction fronts depicted in Figure 2. As can be seen from Figure 2, the reforming reactor A is fed with a stream (11) of natural gas and steam and reacts in a narrow reaction front in the fixed bed of solids initially containing a certain fraction of CaO and Cu to allow the sorption enhanced reforming reaction (eq 3) to take place. The solid CaCO3 generated in reactor A has a high concentration of CaCO3 and Cu and is left behind the reaction front as this front advances toward the end of the reactor A. Since the overall reaction is slightly exothermic, the feed gas should be at slightly lower temperature (see Table 1) to close the heat balance in the reaction front. This heat balance has been solved for a case example in Table 1 for a set of temperatures indicated in the table. It must be noted that these inlet temperatures to the reaction front may change as the reaction front advances toward the exit of the bed. Therefore, they need to be considered as a representative temperature in the overall operation of reactor A. When most of the CaO has reacted to form CaCO3, the fixed bed is switched to a mode of operation characteristic of reactor B, where the target is the oxidation of Cu to CuO by means of air (stream 27). Since this is the main output of energy in the system, the oxidation reaction in reactor B should be made to take place at the highest possible temperature. At the same time, the decomposition of CaCO3
TABLE 1. Temperature and Pressure Conditions of Mass Streams in Figure 2 reforming reactor
oxidation reactor
air compressor
gas turbine
regenerator reactor
gas streams
11
12
52
21
25
22
41
27
42
23
24
31
32
temperature (K) pressure (kPa) molar flow (mol/s) composition (% v) H2O O2 CO2 N2 CH4 H2 CO
821 1961 1.39
973 1961 1.63
973 1961 1.42
515 1961 5.09
618 1961 3.06
1121 1961 4.70
293 981 5.25
515 1961 2.04
515 1961 3.21
1848 1961 5.86
1022 981 5.86
933 981 0.77
1123 981 1.39
80.00 20.00 -
39.10 0.18 2.45 58.10 0.17
39.10 0.18 2.45 58.10 0.17
8.41 1.21 90.38 -
2.10 97.90 -
2.10 97.90 -
21.00 79.00 -
21.00 79.00 -
21.00 79.00 -
24.72 3.28 1.23 70.73 0.04
24.72 3.28 1.23 70.73 0.04
72.17 27.83 -
70.63 29.37 -
in this oxidation reactor must be kept to a minimum, since the CO2 released here is emitted to the atmosphere in the exhaust gases of the turbine (stream 24). An acceptable tradeoff can be arranged by operating the oxidation reactor at 20-30 atm and around 850 °C (very fast oxidation rates as reported in refs 16–18) and by mixing the air with the recycled stream 25, which is very rich in N2, before the resulting stream 21 enters the reactor. The avoidance of hot spots in the oxidation bed has to be achieved mainly by controlling the temperature of the recycle gas 25, by altering the oxygen concentration of the stream 21, and by selecting appropriate cycle times (14). Other possible solutions under research for the application of fixed beds to chemical looping combustion (19) could also be considered. It may also be possible to include between reaction steps A and B an intermediate heat exchange step between the solids and a nonreactive gas in order to lower the temperature of the solids and heat the gas. Although this may tend to complicate the final process scheme, there is substantial experience in different industrial processes (20) where “fixed bed regenerative heat exchangers” are used as compact heat exchangers in which heat is alternately stored and removed from a solid heat storage matrix. These heat exchangers may need to be used in this process to extract heat from stream 25 (heat exchanger not included in the figure for the sake of simplicity) before this is mixed to stream 21. In any case, the resulting stream from reactor B (23) will then be expanded and drive the gas turbine. In order to maximize the electricity efficiency of the gas turbine, it may be of interest to increase the mass flow and temperature of this gas stream 23 by burning the gas generated in reactor A (primarily H2 and, therefore, with a low carbon content, represented by stream 52) using a second air stream obtained from compressor D. Another option would be to burn a small part of the natural gas with air under pressure. However, this is not further considered in this process because the carbon contained in the extra natural gas fed in would be emitted to the atmosphere as very diluted CO2 in the turbine exhaust gases (stream 24), reducing the system’s overall CO2 capture efficiency. A third and critical step mentioned above consists of regenerating the CaCO3 formed in reactor A by means of the reduction of the CuO formed in reactor B. In the example of Figure 2 and Table 1, reactor C is fed with stream 31, consisting of natural gas and steam in order to reduce the partial pressure of CO2 in the reaction atmosphere of reactor C, so as to accelerate the decomposition reaction of CaCO3. As indicated in Figure 2, the resulting solids from reactor B (primarily CaCO3 and CuO) are progressively converted in reactor C into a mixture of CaO and Cu with which a new cycle may be started in reactor A. The operating temperature of reactor C must be high, preferably greater than 800 °C in order to favor the rapid decomposition of CaCO3, and preferably lower than 900 °C, in order to reduce the problems of CaO deactivation and/or unwanted reactions of CuO (16–18). The
working pressure of reactor C must be atmospheric or lower than atmospheric pressure in order to favor the decomposition of the carbonate. A carbon balance on the results obtained in the example of Table 1 indicates an overall CO2 capture efficiency of 0.84 (CO2 coming out from in step C divided by total carbon fedin as CH4) when 87% of the hydrogen obtained from reactor A is burnt to increase the inlet gas temperature of turbine E. The energy efficiency of the system to hydrogen is 0.58 LHV of hydrogen coming out from reactor A divided by LHV of the total CH4 fed to reactors A and C. The remaining energy will be released in gas streams at sufficient high temperatures for effective integration of a steam cycle coupled to this system. Since the process proposed in this work is at a very early stage of development, it is out of the scope of this paper to propose a detailed and complete heat integration scheme to allow full energy efficiency estimation. However, we note that, as it is the case in similar chemical looping processes, the absence of process steps with a substantial energy penalty (other than the purification and compression of CO2, common to all CO2 capture systems) is an indication that high energy efficiencies should be expected for the novel proposed system. The ideal proportions of the main solids using for the calculations of Table 1 have been determined taking into account the enthalpies of reaction under reaction conditions (21) that favor neutrality for the overall reaction in the calcination-reduction step. A Cu/Ca molar ratio of 3.1 is obtained in the case of a pure CH4 feed to the reactor C. Given the high reduction enthalpy of CuO with CO (-126.9 kJ/mol at 298 K) the theoretical Cu/Ca ratio would decrease to 1.3, but this would require a source of CO readily available for the calcination step. Several authors working in the field of chemical looping combustion (16–18) have reported fast reaction rates for reduction and oxidation reactions of CuO and Cu with similar materials and conditions to those required by this system. The presence of an inert support to accompany CaO and Cu, or of a third solid with catalytic properties for the reforming reactions, must be kept to the minimum allowed by the sorbent preparation method, in order to minimize the demand for additional heat in the calcination steps. If the real process conditions do not permit this, it might be necessary to supply external heat to the CaCO3 calcination step so that all the heat required by the CaCO3 regeneration reactor does not have to come from the exothermic CuO reduction reaction. The operating temperatures in Table 1 have been chosen, taking into account the information available on carbonation and sorption enhanced reforming kinetics, the oxidation of Cu to CuO, and the calcination and reduction of CuO to Cu. However, further investigation is needed to derive specific rate expressions for the specific materials that may be required for the process to work in practice. What is more, experimental and modeling work VOL. 44, NO. 17, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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has to be undertaken in the near future in order to validate the performance of the main reactors, estimate expected temperature profiles in the different reaction steps, and establish cycle times for all reaction and possible heat exchange steps. However, by drawing on established knowledge about sorption enhanced reforming in fixed beds, copper oxidation at low partial pressures of oxygen, and the existing information on calcination of CaCO3 and reduction of CuO to Cu, it should be possible to design operating windows and strategies to exploit the inherent benefits of this new process.
Acknowledgments The authors acknowledge the grant provided by the Spanish Ministry of Industry and Commerce and the project with reference ENE2009-11353 from the Spanish Science and Innovation Ministry for the financial support.
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