Energy Fuels 2010, 24, 3034–3048 Published on Web 05/06/2010
: DOI:10.1021/ef100068m
Gasification and Chemical-Looping Combustion of a Lignite Char in a Fluidized Bed of Iron Oxide T. A. Brown,† J. S. Dennis,*,† S. A. Scott,‡ J. F. Davidson,† and A. N. Hayhurst† †
Department of Chemical Engineering and Biotechnology, University of Cambridge, Pembroke Street, Cambridge CB2 3RA, United Kingdom, and ‡Department of Engineering, University of Cambridge, Trumpington Street, Cambridge CB2 1PZ, United Kingdom Received January 20, 2010. Revised Manuscript Received April 15, 2010
Gasification and chemical-looping combustion experiments with a lignite (Hambach) char are reported, using an electrically heated fluidized bed in a 25 mm diameter tube at about 1073 K. The fluidizing gas was CO2 in nitrogen, with either batch or continuous feed of char into a bed of sand or particles of Fe2O3. The experiments were also modeled using the two-phase theory of fluidization; a well-mixed particulate phase is assumed, and the bubble flow is augmented by gasification products. This was combined with LangmuirHinshelwood (L-H) kinetics of gasification deduced from the experiments on gasification, with CO2, of this char in sand. These L-H kinetics are complex; the rate constants are different for two ranges of partial pressure of CO2: (i) from 0 to 0.05 bar and (ii) from 0.05 to 0.9 bar. The theory gives good predictions of (i) off-gas concentrations of CO and CO2 and (ii) accumulation of carbon in the bed. Combustion also occurred when the char was fed into a bed of Fe2O3 particles fluidized by nitrogen, giving the appearance of a solid-solid reaction, i.e., char oxidized by Fe2O3. The occurrence of a solid-solid reaction is however very unlikely, and it is believed that the reaction actually occurs via gaseous intermediates, CO and CO2, and is triggered by small amounts of oxygen in the char or air entrained with the char. This hypothesis is well-supported by the theoretical model. The results are particularly relevant for the chemical-looping combustion of char in Fe2O3.
fuel enters the fuel reactor, which contains a metal oxide, MeO, and reacts in
1. Introduction The urgent need to apply chemical-looping techniques to burning solid fuels arises from the requirement to capture and sequester CO2 emitted from the expanding use of coal for generating electricity.1-6 The global supply of electricity accounts for ∼38% of total anthropogenic carbon emissions to the atmosphere or ∼2400 megatons/year (carbon basis), a figure projected5 to exceed 4000 megatons/year by 2020. The principal means of controlling emissions of CO2 from burning coal will be to capture it from flue gases and sequester it in suitable geological structures. Such disposal is only feasible if the CO2 is available in almost pure form, largely free of nitrogen and other gases. Chemical-looping combustion (CLC) offers the inherent feature of isolating CO2, avoiding the need for costly separation processes. The basic concept for the looping combustion of gaseous fuels typically involves two interconnected fluidized beds; the
ð2n þ mÞMeO þ Cn H2m f ð2n þ mÞMe þ mH2 O þ nCO2 ðR1Þ so that the exit stream contains largely CO2 and steam, yielding almost pure CO2 after the steam has been condensed. The reduced metal oxide or metal, Me, is transferred to the oxidation reactor, where it is oxidized in air by Me þ 1=2O2 f MeO
The oxidized MeO is recycled to the first reactor to undergo a new cycle of reduction while oxidizing more fuel. The metal oxide thus acts as an oxygen carrier. A full conversion from MeO to Me and vice versa is not necessarily obtained in a real system, neither is it essential, because, depending upon the thermodynamics of the oxygen carrier, complete conversion to the metal could mean incomplete conversion of the fuel gas.7,8 The exit gas from the oxidation reactor is N2 containing unused O2. Taking reactions R1 and R2 together, the fuel has been combusted but resulting CO2 has been separated from N2 in the air, while the total heat evolved is the same as for the direct combustion of the fuel in air. Dependent upon the metal oxide, reaction R1 is often but not always endothermic and reaction R2 is always exothermic. When the two reactors are fluidized beds and
*To whom correspondence should be addresssed. Telephone: þ44-(0)1223-334787. Fax: þ44-(0)1223-334796. E-mail:
[email protected]. (1) Anthony, E. J. Ind. Eng. Chem. Res. 2008, 47 (6), 1747–1754. (2) Song, Q. L.; Xiao, R.; Deng, Z. Y.; Shen, L. H.; Xiao, J.; Zhang, M. Y. Ind. Eng. Chem. Res. 2008, 47 (21), 8148–8159. (3) Scott, S. A.; Dennis, J. S.; Hayhurst, A. N.; Brown, T. AIChE J. 2006, 52 (9), 3325–3328. (4) Leion, H.; Mattisson, T.; Lyngfelt, A. Fuel 2007, 86 (12-13), 1947–1958. (5) Intergovernmental Panel on Climate Change (IPCC). Third Assessment Report. IPCC, Geneva, Switzerland, 2001. (6) International Energy Agency (IEA). World Energy Outlook. IEA, Paris, France, 2006. (7) Lyngfelt, A.; Leckner, B.; Mattisson, T. Chem. Eng. Sci. 2001, 56 (10), 3101–3113. r 2010 American Chemical Society
ðR2Þ
(8) Kronberger, B.; Lyngfelt, A.; Loffler, G.; Hofbauer, H. Ind. Eng. Chem. Res. 2005, 44 (3), 546–556.
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Energy Fuels 2010, 24, 3034–3048
: DOI:10.1021/ef100068m
Brown et al.
where reaction R1 is endothermic, it is possible to use the interchange of the solid oxygen carrier between the two beds to balance the heat on the fuel reactor. The heated flue gas of oxygen-depleted air leaves the oxidation reactor at a high temperature (∼1273 K) and can be used to raise steam or, when the operation is pressurized, to drive a gas turbine topping cycle. There is now an increasing body of literature on using CLC with solid fuels;2-4,9-12 the key difficulty is that the particles of fuel and oxygen carrier cannot be easily separated. Without separation, a solid fuel would enter the oxidation (second) reactor along with the partially reduced oxygen carrier and give CO2 in the off gas, thereby defeating the objective of the technique. A number of strategies have evolved to cope with this problem, but all start from the basic tenet that the solid fuel must first be gasified to synthesis gas (CO and H2) for it to react with the solid oxygen carrier efficiently. The primary gasification reactions are ðR3Þ CðsÞ þ CO2 ðgÞ f 2COðgÞ CðsÞ þ H2 OðgÞ f COðgÞ þ H2 ðgÞ
feature is that the heat load on the gasifying section is now balanced by the sensible heat conveyed by the solids circulating from the oxidation unit. An alternative approach3 is to gasify the fuel in situ using mixtures of steam and CO2 in the presence of a batch of appropriate oxygen carrier in a single, fluidized bed. Such a reactor contains a bed of oxygen-carrier particles, which are cycled between three different stages. The choice of oxygen carrier is restricted, because it requires an oxide, which is exothermic during reduction to balance the endothermic gasification reactions; copper has such oxides. During stage 1, the oxygen carrier is fluidized with steam or CO2 or a mixture thereof and solid fuel is fed continuously into the bed. During this first stage, the fuel is gasified and the resulting syngas is oxidized by the oxygen carrier. At some point, before the oxygen carrier is fully reduced, the feeding of fuel is stopped and the char in the bed is allowed to gasify to completion. This is stage 2. In the final stage, the fluidizing gas is switched to air and the oxygen carrier is reoxidized. The process would consist of several fluidizedbed reactors operating in parallel and out of phase with one another to achieve a continuous supply of energy. This process has the real advantage of avoiding the separation of unreacted carbon from the oxygen carrier required with separate reactors for oxidation and reduction. Furthermore, the high mechanical stresses experienced by the metal oxide particles in a circulating fluidized-bed reactor are reduced, thereby increasing the longevity of the carrier. Whatever the scheme, the buildup of any unreactive char during the second stage is still a factor, which could reduce the efficiency for capturing CO2. For example, in the scheme proposed by Scott et al.,3 if the inventory of carbon is large and unreactive, the second stage would have to be prolonged, otherwise a large amount of CO2 would be generated during the oxidation stage, reducing the efficiency of capture of CO2. This could limit the types of fuel that could be used to those with highly reactive chars, such as low-rank coals and biomass. There is, however, evidence to suggest that the oxygen carrier might increase the rate of gasification. Thus, it is known15 that certain inorganic species in the ash catalyze gasification, with Fe, Ca, and Mg being of prime interest. Leion et al.4 conducted experiments with batches of petroleum coke gasified with steam in the presence of Fe2O3 particles supported on MgAl2O4. They noted that the reaction of the gaseous products of gasification with the oxygen carrier was much faster than the gasification of the fuel, so that gasification is the rate-limiting step. They observed that gasification was faster in the presence of iron oxide than in a bed of inert material, such as silica. In one experiment,4 it took 900 s to achieve 95% conversion of the carbon in a batch of petroleum coke; this compares with 2400 s to gasify the same mass of fuel in the presence of inert quartz sand. Three possible factors might explain the increased rate of gasification in the presence of the oxygen carrier: (i) There is less inhibition of gasification by CO produced, because CO immediately reacts with the adjacent carrier particles. (ii) Because the particles of the oxygen carrier could come into contact with solid carbon, it is possible that the oxygen carrier might have a catalytic effect similar to that of elements, such as Fe, in fuel ash. (iii) There might be a direct reaction between the solid oxygen carrier and the solid carbon, without the involvement of a gaseous
ðR4Þ
In addition to the gasification reactions, the homogeneous water-gas shift reaction is also important. COðgÞ þ H2 OðgÞTCO2 ðgÞ þ H2 ðgÞ
ðR5Þ
In principle, the solid fuel could be gasified separately and the synthesis gases passed to a conventional chemical-looping system, as used for gaseous fuels, described above. The complication, however, is that the gasifying agent would need to be pure O2 or a mixture of O2 and CO2 to ensure that the synthesis gas was mainly CO and H2, without N2. This would require an air separation unit and partly defeat the objective of using chemical looping. Alternatively, gasification could be undertaken in pure CO2 or mixtures of CO2 and steam, but to balance these endothermic gasification reactions, heat would have to be transferred to the gasifier from the oxidation reactor, constituting a problem in heat integration at high temperatures. A range of oxides13 might be used as oxygen carriers, provided a reaction scheme is available to balance the endothermic gasification reactions. For example, Lyngfelt and colleagues4,12 have developed a fuel reactor to separate the unburnt fuel particles from the spent metal oxide before the carrier particles are transferred to the air reactor. The overall plant includes an air reactor, operated as a fast fluidized bed, connected to a riser leading to a cyclone, where elutriated particles are separated. This section is similar to that used previously in the work on natural gas combustion.14 The fuel reactor, however, has been modified so that (i) gasification and oxidation of the products occur and (ii) the solids pass to a downstream section, where the fuel particles can be separated from the carrier particles (ilmenite, FeTiO3, mean size of 150 μm) on the basis of different rates of elutriation of the carrier and the fuel particles, with the latter being of lower density and, most likely, smaller in size at this stage. A key (9) Dennis, J.; Scott, S. Fuel 2010, DOI: 10.1016/j.fuel.2009.08.019. (10) Leion, H.; Mattisson, T.; Lyngfelt, A. Int. J. Greenhouse Gas Control 2008, 2 (2), 180–193. (11) Dennis, J. S.; Scott, S. A.; Hayhurst, A. N. J. Energy Inst. 2006, 79 (3), 187–190. (12) Berguerand, N.; Lyngfelt, A. Fuel 2008, 87 (12), 2713–2726. (13) Cao, Y.; Pan, W. P. Energy Fuels 2006, 20 (5), 1836–1844. (14) Lyngfelt, A.; Kronberger, B.; Adanez, J.; Morin, J.-X.; Hurst, P. Presented at the Seventh International Conference on Greenhouse Gas Control, Vancouver, British Columbia, Canada, 2004.
(15) Laurendeau, N. M. Prog. Energy Combust. Sci. 1978, 4 (4), 221– 270.
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: DOI:10.1021/ef100068m
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Table 1. Ultimate Analysis of Hambach Lignite fuel
Hambach lignite (dried)
Hambach lignite char
C (%) H (%) O (%) (balance) N (%) S (%) ash (%)
66.6 5.18 22.4 0.892 0.305 4.59
84.21 1.07 4.64 1.10 unknown 8.94
intermediate. However, previous workers have postulated that solid-solid reactions between carbon and an oxygen carrier do not occur at an appreciable rate and can be ignored.4 This paper investigates the mechanism by which an oxygen carrier, in this work, Fe2O3, influences the rate of gasification of lignite char by CO2. The study accordingly also concerns the accumulation of carbon chars during chemical looping.
Figure 1. Particle size distribution of the char feed by sieve analysis.
2.1. Continuous Feeding of Solid Fuel to a Fluidized Bed. At the start of an experiment, the empty reactor was filled with either 20 mL of silica sand or 40 g of iron oxide and then heated to 1073 K with the bed fluidized with nitrogen at a rate of 40 mL/s [all flow rates are given at standard temperature and pressure (STP)]. It should be noted that the densities of the iron oxide particles (2060 kg/m3) and the sand (2650 kg/m3) are similar. Thus, the minimum fluidization velocity, Umf, for both materials was roughly equal, 0.05 m/s, at 1073 K. The same velocity through the bed, U, was used irrespective of the bed material, giving U/Umf ∼ 6. Furthermore, 40 g of iron oxide occupy ∼20 mL, giving an initial bed height above the distributor plate, Hmf = 40 mm, similar for both materials. Once the operating temperature had been reached, the fluidizing gas was switched to 44 mL/s of 55 mol % CO2 in N2 and, when the outlet [CO2] was steady, feeding of the char at ∼0.18 g/ min was commenced. At the end of the feeding period, CO2 and fuel were switched off simultaneously and the bed was purged with 40 mL of N2/s. After 60 s of purging, the purge N2 was changed to a gas mixture containing 3 mol % O2 in nitrogen at a flow rate of 40 mL/s to burn off the carbon in the bed and, when the bed material was iron oxide, to regenerate the carrier back to Fe2O3. Once oxidation was complete and all of the remaining carbon in the bed had been combusted, as measured by the absence of CO and CO2 detected in the sampled gases, the next feeding period was commenced and the process was repeated. The mass of carbon accumulated in the bed during the feeding period was determined directly from the total number of moles of CO2 and CO released during both the purge and oxidation stages after the end of the period of feeding char. The experiment was repeated several times, with different time periods of feeding. In this way, the amount of carbon accumulated in the bed could be determined as a function of time during the approach to a steady state. A table of the experimental parameters is provided in Table 2. 2.2. Batch Experiments To Measure the Kinetics of Gasification. The fluidized bed used for studying the kinetics of gasification of the lignite char has been described by Chuang et al.17 Briefly, a quartz reactor (inner diameter of 30 mm) was filled with 20 mL of silica sand (sieved to 300-425 μm) and heated by an electric furnace. Experiments were performed at 10731173 K. The mole fraction of CO2 in the fluidizing gas was varied between 0.25 and 86 mol %, and the background concentration of CO was 0-9 mol % with the balance nitrogen. The total volumetric flow rate was always 50 mL/s (at STP), giving U/Umf ∼ 5-6. The char was divided into four narrower size fractions, viz., 355-600, 600-850, 850-1000, and 1000-1400 μm. In a typical experiment, a batch of char, of known mass (usually 0.01-0.05 g), was added to the bed fluidized with the
2. Experimental Section The particles of iron oxide were prepared using the method of “mechanical mixing” described elsewhere.16 Briefly, this entails spraying powdered hemeatite (Fe2O3) with a fine mist of water. High-purity hemeatite (>99 wt %, sized to
