- Email: [email protected]
Solid fuels in chemical-looping combustion
international journal of greenhouse gas control 2 (2008) 180–193
available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/ijggc
Solid fuels in chemical-looping combustion Henrik Leion a,*, Tobias Mattisson b, Anders Lyngfelt b a b
Department of Environmental Inorganic Chemistry, Chalmers University of Technology, S-412 96 Go¨teborg, Sweden Department of Energy and Environment, Chalmers University of Technology, S-412 96 Go¨teborg, Sweden
article info
abstract
Article history:
The feasibility of using a number of different solid fuels in chemical-looping combustion
Received 23 April 2007
(CLC) has been investigated. A laboratory fluidized bed reactor system for solid fuel,
Received in revised form
simulating a chemical-looping combustion system by exposing the sample to alternating
28 September 2007
reducing and oxidizing conditions, was used. In each reducing phase 0.2 g of fuel in the size
Accepted 6 October 2007
range 180–250 mm was added to the reactor containing 40 g oxygen carrier of size 125–
Published on line 26 November 2007
180 mm. Two different oxygen carriers were tested, a synthetic particle of 60% active material of Fe2O3 and 40% MgAl2O4 and a particle consisting of the natural mineral ilmenite.
Keywords:
Effect of steam content in the fluidizing gas of the reactor was investigated as well as effect
Chemical-looping combustion (CLC)
of temperature. A number of experiments were also made to investigate the rate of
Fluidized bed
conversion of the different fuels in a CLC system. A high dependency on steam content
Oxygen carrier
in the fluidizing gas as well as temperature was shown. The fraction of volatiles in the fuel
Solid fuel
was also found to be important. Furthermore the presence of an oxygen carrier was shown
Ilmenite
to enhance the conversion rate of the intermediate gasification reaction. At 950 8C and with
Iron oxide
50% steam the time needed to achieve 95% conversion of fuel particles with a diameter of
Volatiles
0.125–0.18 mm ranged between 4 and 15 min depending on the fuel, while 80% conversion
Steam
was reached within 2–10 min. In almost all cases the synthetic Fe2O3 particle with 40% MgAl2O4 and the mineral ilmenite showed similar results with the different fuels. # 2007 Elsevier Ltd. All rights reserved.
1.
Introduction
Since Arrhenius first discovered the connection between the amount of CO2 in the atmosphere and the global average temperature (Arrhenius, 1886) the concentration of CO2 in the atmosphere has risen approximately 30% higher than the preindustrial value. The main reason for the increased CO2 levels is combustion of fossil fuels, the world’s dominant energy source. Separation and sequestration of the CO2 formed has been recognized as one very important option to accomplish a reduction in CO2 emissions. Chemical-looping combustion (CLC) is one of the techniques that can be used to capture CO2. The main advantage with CLC is that CO2 is inherently separated from the other flue gas components, thus avoiding costly equipment
and energy consumption for separation of gas (Cho et al., 2004, 2005). The CLC process is composed of two fluidized bed reactors, an air and a fuel reactor, shown in Fig. 1. The fuel is introduced to the fuel reactor where it reacts with an oxygen carrier to CO2 and H2O. The reduced oxygen carrier is transported to the air reactor where it is oxidized back to its original state by air. As a result, CO2 can be inherently separated in this combustion process. The total amount of heat released in the air and the fuel reactor is equal to the heat released from normal combustion thus separating CO2 without any losses in energy. There have been a number of publications where different aspects of the CLC process have been investigated (Adanez et al., 2004, 2005; Ishida and Jin, 1994; Ishida et al., 1996;
* Corresponding author. Tel.: +46 31 7722886; fax: +46 31 7722853. E-mail address: [email protected] (H. Leion). 1750-5836/$ – see front matter # 2007 Elsevier Ltd. All rights reserved. doi:10.1016/S1750-5836(07)00117-X
181
international journal of greenhouse gas control 2 (2008) 180–193
reactions are C þ H2 O ) CO þ H2
(1)
CO þ H2 O ) CO2 þ H2
(2)
C þ CO2 ) 2CO
(3)
The main reactions with the metal oxide are
Fig. 1 – Schematic picture of the CLC process. Two interconnected fluidized bed reactors, one air and one fuel reactor, with circulating oxygen carrying particles.
Lyngfelt et al., 2001; Mattisson et al., 2004). Most of the work around chemical-looping combustion has focused on the use of gaseous fuel and only a few publications on solid fuel (Cao et al., 2006, 2004; Cao and Pan, 2006; Dennis et al., 2006; Leion et al., 2007; Lyon and Cole, 2000; Scott et al., 2006). Since coal is considerably more abundant than natural gas it would be highly advantageous if the CLC process could be adapted for solid fuels. One way of performing this is by first gasifying the coal to a syngas consisting mainly of CO and H2. This gas could then be burned in CLC. However, in order to obtain undiluted syngas the gasification needs to be carried out with O2 and thus an air separation unit would be needed. Another option is to introduce the solid fuel directly into the fuel reactor where the oxygen carrier is reduced by the fuel giving a one step fuel oxidation without any need for air separation. However, the solid–solid reaction between coal and an iron based metal oxide is not very likely to occur at any appreciable rate (Leion et al., 2007). Instead the solid fuel needs to be gasified and then the oxygen carrying particles react with the gas produced, making the gasification the time limiting step (Dennis et al., 2006; Leion et al., 2007; Scott et al., 2006). An important advantage compared to normal gasification is that it will take place in a high concentration of H2O and/or CO2 which is beneficial for the reaction rates. The main gasification
MeX OY þ H2 ) MeX OY1 þ H2 O
(4)
MeX OY þ CO ) MeX OY1 þ CO2
(5)
This work investigates the reaction of two oxygen carriers, one synthetic iron based particle (Fe2O3/MgAl2O4) and one natural mineral, ilmenite, or mainly Fe2TiO5, in a laboratory fluidized bed reactor. A number of different solid fuels were used, Table 1, as well as a gaseous fuel mixture of 50% H2 and 50% CO (syngas). The concentration of steam in the fluidizing gas, as well as the temperature in the fluidized bed, has been varied in order to investigate the effect on the reaction rate. Also the some experiments were made where the oxygen carriers were replaced by sand to further investigate gasification of solid fuel in a CLC system.
2.
Experimental
2.1.
Experimental setup and procedure
The experiments were conducted in a fluidized bed reactor of quartz, Fig. 2, using either Fe2O3/MgAl2O4 or ilmenite as bed material. In order to achieve good solids mixing in the bed, the reactor was conically shaped just above the distributor plate. The reactor had a total length of 870 mm with a porous quartz plate placed 370 mm from the bottom of the reactor. The porous plate and the reactor below the plate had an inner diameter of 10 mm. Above the distributor plate the inner diameter of the reactor increased to reach 30 mm at a height of 20 mm above the porous plate. The diameter was then constant for 250 mm. The reactor diameter was then increased further to 45 mm for a length of 100 mm. This
Table 1 – Solid fuels used in this work Fuel
Petroleum coke
South Africa
China
Indonesia
Taiwan
S. France Raw
Volatiles (%) Moisture (%) Ash (%) HHV (MJ/kg) C (%) H (%) S (%) N (%) O (%)
10.0 8.0 0.5 31.7 81.3 2.9 6.0 0.9 0.5
21.6 8.3 15.9 29.9 62.5 3.5 0.7 1.4 7.7
8.8 1.0 21.0 26.6 70.3 2.8 0.6 1.1 3.1
45.3 8.5 1.4 26.4 66.1 4.7 0.1 0.7 18.5
31.5 2.5 12.3 27.9 70.6 4.5 0.5 1.7 7.9
25.0 1.5 21.8 24.9 63.9 3.6 0.8 0.8 7.7
Sieved 25.0 1.5 21.8 24.9 63.9 3.6 0.8 0.8 7.7
182
international journal of greenhouse gas control 2 (2008) 180–193
Fig. 2 – Fluidized bed reactor of quartz.
disengaging section was constructed in order to avoid that smaller coal and metal oxide particles leave the reactor. A sample of 40 g of oxygen carrier particles of size 0.090– 0.125 mm was placed on the porous plate and was then initially heated in an inert atmosphere to the reaction temperature. When the bed was not fluidized the bed height was approximately 35 and 30 mm for Fe2O3/MgAl2O4 and ilmenite, respectively. The particles were then alternatingly exposed to 5% O2 and the fuel/steam mixture, thus simulating the cyclic conditions of a CLC system burning solid fuel. Nitrogen gas was introduced for 180 s between each reducing and oxidizing period. During the reducing period, the fluidizing gas was a mixture of steam and nitrogen, which was introduced from the bottom of the reactor. At the start of the reducing period, i.e. at the same time as the fluidizing gas of the reducing cycle, mainly consisting of steam and nitrogen, started to come into the bottom of the reactor, the fuel was inserted in the top of the reactor, falling down into the fluidized bed.
The diameter of the solid fuel particles was between 125 and 180 mm, i.e. somewhat larger than the oxygen carrier particles. The experiments were performed in this manner for two to six cycles in each case, where the different cases are defined by the value of temperature, fraction of steam in the fluidizing gas during reduction and the fuel used. The temperature was measured 5 mm below and 10 mm above the porous quartz plate, using 10% Pt/Rh thermocouples enclosed in quartz shells. Since reaction heat affects the temperature in the bed, the temperature below the bed was used to control the reactor temperature, and all reactor temperatures in this work refer to the temperature measured below the bed. The temperature in the bed was generally 20 8C higher. The gas from the reactor was led to an electric cooler, where the water was removed, and then to a gas analyzer (Rosemount NGA-2000) where the concentrations of CO2, CO, CH4, and O2 were measured in addition to the gas flow. A schematic layout of the laboratory setup is presented in Fig. 3. All fuels contained some sulphur and the presence of SO2 may have a significant impact on the gasification rate (Leion et al., 2007; Lyon and Cole, 2000). SO2 also interfered with the CH4 channel. Therefore in Tables 3, 4, 6 and 8 the concentration of CH4 as a fraction of the total amount of carbon containing gases during a cycle is presented both as the measured values and the corrected values under the assumption that all sulphur in the fuel leaves as SO2. As seen, this interference can be neglected for all fuels except for petroleum coke which has comparably high sulphur content. For a few cycles gas from the exiting stream was collected in bags and analysed with a gas chromatograph (Varian MicroGC with Molsieve and Poraplot column). Gases were sampled during 30 s of a reducing period. The average concentrations of CO2, CO, CH4 and H2 were measured with the GC. The measurements from the gas chromatograph were made to confirm the accuracy of the CO2, CO and CH4 measurements from the gas analyzer and to measure the hydrogen concentration. The exothermic nature of the oxidation reaction means that there will be release of heat and therefore a subsequent temperature rise. To limit this temperature increase, a gas mixture with 5% O2 in N2 was used instead of air. Thus, large temperature increases were avoided. In a real system it will of course be possible to cool the air reactor, but this was not the case with this small laboratory setup. All experiments were conducted with a gas flow of 600 mL/min (at 1 bar, 0 8C), both
Fig. 3 – Schematic layout of the laboratory setup.
international journal of greenhouse gas control 2 (2008) 180–193
for the reducing and oxidizing periods. This corresponds to a velocity ratio, u/ umf, in the 10 mm section just above the porous plate, of 53 and 70 for the oxygen carriers, i.e. synthetic Fe2O3/MgAl2O4 particles and the ilmenite mineral, and fuel, respectively. u is the gas velocity and umf is the minimum fluidization velocity. However, the velocity was considerably lower in the wider 45 mm part of the reactor, corresponding to a u/umf of 2.6 and 3 for the oxygen carriers and for the fuel particles, respectively. In the 30 mm part u/umf was 5.8 and 7.2 for the oxygen carriers and a for the fuel particles, respectively. These velocity ratios are calculated for nitrogen but differs only a few percent for the other gases used in this work. However, reference experiments indicate that small fuel particles still left the reactor, also some fuel was lost in the feeding device. These losses are excluded from the result and all calculations are made on the carbon containing gases leaving reactor. From high frequency measurements of the pressure drop it was possible to see whether the bed was fluidized or not (Cho et al., 2006).
2.2.
Material preparation
Two types of oxygen carrier were tested. One was a synthetically produced iron based particle previously used in experiments with gaseous fuel and therefore having known reaction and fluidizing properties (Johansson et al., 2004). These particles contained 60% active material of Fe2O3 and 40% MgAl2O4. They were produced by freeze granulation and then sintered at 1100 8C for 6 h using a heating rate of 10 8C/min. Each batch was sieved to obtain particles in the size range 0.090–0.125 mm. The fresh particles had a BET area of 8.58 m2/g as determined by a Micromeritics Gemini 2362. The actual Fe2O3/MgAl2O4 particles used in this work have previously been used in a 10 kW CLC system at Chalmers (Lyngfelt and Thunman, 2005). The other oxygen carrier was a natural mineral, ilmenite (FeTiO3), was obtained from Titania A/S. Ilmenite is concentrated from ore containing 40% ilmenite, 37% plagioclase, 8.6% ortopyroxene, 6.5% clinopyroxene, 4.2% biotit and some minor other phases. The ilmenite received from Titania is 94.3% pure. The molar ratio of iron and titanium is close to 1:1. XRD measurements on fresh samples show ilmenite and small fractions of hematite. Fresh ilmenite had a BET area of 0.11 m2/ g as determined by a Micromeritics Gemini 2362. To make sure that the ilmenite mineral was in its most oxidized state the ilmenite particles were first heat treated in air for 24 h at 950 8C. Also, for many oxygen carrier materials the reactivity is different during the first cycles and then stabilizes (Mattisson et al., 2004). In order to avoid differences in reactivity because of this, the ilmenite particles were, after the heat treatment, exposed to a flow of 450 N mL/min of either 5% O2 in nitrogen or syngas (50% CO, 50% H2) in a cyclic manner at 950 8C for four cycles, before the testing with solid fuels started. The fuels used are presented in Table 1. All fuels were crushed and sieved to obtain particles in the size range 0.125– 0.180 mm. 0.2 g of fuel was used for every reducing cycle, which is equivalent to a maximum mass reduction of the bed of 1 wt% when the fuel with the highest oxygen demand, petroleum coke, is used in 40 g of bed material. This corresponds to approximately 50% conversion of the synthetic iron particles from Fe2O3 to Fe3O4. In this application it would
183
not be useful to reduce iron oxide to lower forms than Fe3O4, i.e. FeO and Fe, since thermodynamic restrains would prevents full conversion of the fuel. In other words the concentration of oxidizing gases, such as H2O, in the fluidizing gas should be high enough to prevent the further reaction from Fe3O4, to FeO under conditions with high conversions of fuel (Jerndal, 2005). The different fuels used in this work have different composition and therefore will result in somewhat different degrees of reduction of the oxygen carrier. But it has previously been shown that the conversion time for petroleum coke in Fe2O3/MgAl2O4 particles was not affected when the ratio of fuel and oxygen carrier was varied(Leion et al., 2007). The reduction of ilmenite is somewhat more complicated. The oxidation/reduction of ilmenite proceeds through three levels. The reduced level is ilmenite, FeTiO3, corresponding to FeO + TiO2, an intermediate level is Fe3Ti3O10, corresponding to Fe3O4 + 3TiO2, and the most oxidized level is Fe2TiO5 + TiO2, corresponding to Fe2O3 + 2 TiO2 (Zhang and Ostrovski, 2002). The total oxygen transfer capacity from the most oxidized to the most reduced level of ilmenite is 5%, and since the added fuel in this work only corresponds to a mass reduction of 1%, the ilmenite shifts between the most oxidized level is Fe2TiO5 + TiO2 and the intermediate level, Fe3Ti3O10. Thermodynamics calculations indicate that when ilmenite moves between these levels the conversion of CO and H2 is over 99.99%.
3.
Data evaluation
The rate of conversion of the fuel is given as an average rate of the carbon converted in the fuel, and was calculated from
rave ¼
1 mt mtot t
(6)
where t is the time elapsed since the start of the cycle, mtot is the total mass of carbon converted during the entire reducing period and mt is the mass of carbon converted up until the time t. The total amount of carbon is determined from the integration of the outgoing CO and CO2 concentrations for an entire reduction period. The CO fraction i.e. the fraction of carbon released as unconverted CO was defined as
CO ¼
COcum ðCO þ CO2 Þtot
(7)
COcum in the cumulative amount of carbon converted to CO up to a given degree of conversion, whereas (CO + CO2)tot is the total amount of carbon released as CO and CO2 during an entire cycle.
4.
Results
4.1.
Concentration profiles
Fig. 4 shows the outlet gas concentrations after condensation of water as a function of time for a reducing period where 0.2 g
184
international journal of greenhouse gas control 2 (2008) 180–193
Fig. 4 – Concentration profile during the reduction of 40 g Fe2O3/MgAl2O4 with 0.2 g petroleum coke. The inlet concentration of H2O is 50%. The temperature is 950 8C.
Fig. 5 – Concentration profile during the reduction of 40 g ilmenite with 0.2 g petroleum coke. The concentration of H2O is 50%. The temperature is 950 8C.
of petroleum coke was used as fuel and Fe2O3/MgAl2O4 was used as oxygen carrier. Hydrogen concentration obtained from bag samples analyzed with gas chromatography is also shown. CO2, CO and CH4 measurements from the gas chromatograph are not presented but agreed very well with the measurements from the gas analyzer. The fluidization gas contained 50% steam and had a temperature of 950 8C. The initial peak of CH4 is mainly from devolatilization of the petroleum coke. As mentioned earlier, some of the measured methane could actually be due to release of SO2. The initial peaks of CO and CO2 are due to release of volatiles and reaction of these with the iron oxide. After this the remaining char reacts with the added steam to syngas, i.e. CO and H2, which reacts further with the metal oxide to CO2 and H2O. In this case the CO2 concentration reaches a maximum just below 8% and the CO a maximum of roughly 0.5% if the initial peak is not taken into account. The CO2 concentration starts to decrease after reaching a maximum, approximately 5 min into the cycle. After about 20 min of reaction all of the added petroleum coke has reacted. Some CO was found in all experiments, and can be related to the gas–solid mixing. Assuming that the metal oxides and fuel particles are well mixed in the reactor there will be fuel particles all the way up to the top of the bed, and thus there will always be a fraction of the gasification products, i.e. CO and H2, which will not have sufficient contact with the oxygen carrier even if the reactivity of the metal oxide particles is high. It cannot be excluded that there also is some segregation, increasing the fraction of lighter fuel particles in the top of the bed. In Fig. 5 the gas concentrations for a reducing period are shown for the same conditions as in Fig. 4. The difference being that ilmenite is used as oxygen-carrying material instead of Fe2O3/MgAl2O4. The concentrations of all measured species were very similar for the two different oxygen carriers independent of which fuel that was used.
Figs. 6 and 7 shows the concentration profiles for experiments conducted with the South African- and Indonesiancoal, respectively. The reaction is again carried out with 50% steam in nitrogen. In Fig. 6 hydrogen concentration obtained from bag samples analyzed with gas chromatography is also shown. These coals have higher contents of volatiles in comparison to petroleum coke, and therefore a higher fraction of the solid fuel is reacting during the devolatilization stage, resulting in higher initial peaks of CO2, CO and in the case of the Indonesian coal also CH4/SO2. In the case of petroleum coke the reaction of the volatiles ends before the char reaction reaches its maximum conversion rate, giving a dip in the concentrations of CO2 after the release of volatiles. For the other solid fuels it is not possible to separate the char reaction
Fig. 6 – Concentration profile during the reduction of 40 g ilmenite with 0.2 g South African coal. The inlet concentration of H2O is 50%. The temperature is 950 8C.
185
international journal of greenhouse gas control 2 (2008) 180–193
Fig. 7 – Concentration profile during the reduction of 40 g ilmenite with 0.2 g Indonesian coal. The inlet concentration of H2O is 50%. The temperature is 950 8C.
from the reaction of the volatiles, giving a gradual decrease of CO2 and CO from the initial peaks. Fig. 8 shows the oxidation period following immediately after the reducing period shown in Fig. 6. The inlet O2 content was 5%. During the first minutes all incoming oxygen reacts with the reduced metal oxide particles resulting in only inert nitrogen in the outgoing flow. Then the oxygen concentration increases rapidly during the following minutes after which the rise gradually slows down. Since there is no CO2 produced during the first period of oxidation, i.e. the period when all oxygen is consumed, this indicates that there is no carbon left in the bed. To confirm this a few experiments were performed: 0.1 g of petroleum coke was added at the very beginning of the oxidation to a reduced bed. In these cases the added fuel was immediately converted to CO2 showing that if there is char residues in a reduced bed these react with O2 in the very beginning of the oxidation. The CO2 and CO peaks that appear when the O2 concentration increases are most likely due to unburnt carbon which elutriated and stuck to the walls in the top section of the reactor. The oxidation looked very much the same in all cases, independent of fuel, oxygen carrier or other parameters in the previous reduction period. The same batch of 40 g of Fe2O3/MgAl2O4 particles was used for all the different experiments with the coals, with a total of
Fig. 8 – Concentration profile during the oxidation of reduced ilmenite. The inlet concentration of O2 is 5% and the temperature is 950 8C.
30 cycles during approximately 30 h of experiments. The particles were cooled down to room temperature and heated up again several times during this test since the oven had to be turned off at night. A small amount of ash particles was visible in the bed at the end of the tests with Fe2O3/MgAl2O4. Furthermore the same batch of 40 g of ilmenite particles was used in the same way as the synthetic Fe2O3/MgAl2O4 particles. In addition, these particles were exposed to different temperatures and steam concentrations, giving a total of 50 cycles during 45 h with this batch of particles. Just as for the Fe2O3/ MgAl2O4 particles a small amount of ash particles was visible in the bed at the end of these experiments. On one occasion during an experiment at a temperature of 1000 8C the bed stopped fluidizing, but the fluidization started again after a few minutes. Table 2 shows the time needed to reach 95% and 80% conversion for different solid fuels with Fe2O3/MgAl2O4 and ilmenite as oxygen carrier as well as with an inert sand bed. The time it takes to convert the fuel is not dependent on whether synthetic Fe2O3/MgAl2O4 or ilmenite was used as oxygen carrier, thus confirming that the gasification reactions are the rate limiting reactions. However, the time needed for conversion is much longer in the sand bed. Tables 3 and 4 present the measured fraction of unconverted CH4 as a fraction of the total amount of carbon
Table 2 – Time in min for 95% and 80% of conversion for different solid fuels in Fe2O3/MgAl2O4, ilmenite and sand Oxygen carrier
Conversion (%)
Petroleum coke
South Africa
China
Indonesia
Taiwan
S. France Raw
Fe2O3/MgAl2O4 Ilmenite Sand Fe2O3/MgAl2O4 Ilmenite Sand
95 95 95 80 80 80
15.0 14.1 38.0 10.8 10.0 28.7
9.9 10.8 18.8 4.1 5.0 10.7
13.7 13.4 29.9 8.5 8.2 17.6
5.1 4.2 10.3 1.5 1.2 3.2
11.9 9.6 22.9 6.0 4.9 14.4
9.6 10.7 21.3 4.6 4.7 13.1
Sieved 8.5 9.8 16.3 4.0 4.9 8.8
186
international journal of greenhouse gas control 2 (2008) 180–193
Table 3 – Measured fraction of unconverted CH4 during one cycle for different fuels in Fe2O3/MgAl2O4 and the recalculated fractions of unconverted CH4 compensated for the SO2 interference under the assumption that all sulphur in the fuel leaves as SO2 CH4/(CO2 + CO + CH4)
Petroleum coke
South Africa
China
Indonesia
Taiwan
S. France Raw
Meassured Corrected for SO2
0.0429 0.0362
0.0576 0.0566
0.0261 0.0253
0.0697 0.0696
0.0759 0.0753
0.0568 0.0557
Sieved 0.0736 0.0725
Table 4 – Measured fraction of unconverted CH4 during one cycle for different fuels in ilmenite and the recalculated fractions of unconverted CH4 compensated for the SO2 interference under the assumption that all sulphur in the fuel leaves as SO2 CH4/(CO2 + CO + CH4)
Petroleum coke
South Africa
China
Indonesia
Taiwan
S. France Raw
Meassured Corrected for SO2
0.0460 0.0393
0.0720 0.0710
0.0307 0.0299
0.0791 0.0790
0.0844 0.0838
0.0814 0.0804
Sieved 0.0788 0.0777
containing gases during a cycle for different fuels in Fe2O3/ MgAl2O4 and ilmenite, respectively. Also shown in Tables 3 and 4 are the recalculated fractions of unconverted CH4 as a fraction of the total amount of carbon containing gases during a cycle when compensated for the SO2 interference under the assumption that all sulphur in the fuel leaves as SO2. The differences in unconverted CH4 between the different fuels are fairly small, except for the lower fractions for the two fuels with low volatile content petroleum coke and China coal. Fe2O3/MgAl2O4 generally has lower fractions of unconverted CH4 compared to ilmenite. Laboratory tests also show that Fe2O3/MgAl2O4 is more reactive towards CH4 compared to ilmenite. Figs. 9 and 10 show the average rate of reaction as a function of the amount of volatiles for the same set of experiments. The rate of reaction increases with the content of volatiles, the coal from Taiwan being somewhat an
Fig. 10 – Average rate of reaction during the period to reach 80% and 95% conversion for different solid fuels in ilmenite as a function of the amount of volatiles in the different fuels. The fluidizing gas contained 50% H2O and the temperature was 950 8C.
Fig. 9 – Average rate of reaction during the period to reach 80% and 95% conversion for different solid fuels in Fe2O3/ MgAl2O4 as a function of the amount of volatiles in the different fuels. The fluidizing gas contained 50% H2O and the temperature was 950 8C.
exception. Volatiles are, due to the high temperature, rapidly released when the coal particles are introduced in the reactor and can therefore react directly with the metal oxide. The solid char in the fuel takes longer time to convert since it first has to be gasified, for example by steam, before it can react with the metal oxide. In Figs. 11 and 12 the fraction of CO, as calculated in Eq. (7), after 2 min and at full conversion of the fuel is shown as a function of volatiles for different fuels for synthetic Fe2O3/ MgAl2O4 and ilmenite mineral, respectively. Two minutes were chosen because at that time all volatiles had been released for all fuels. Just as for the reaction rates there is no major difference between Fe2O3/MgAl2O4 and ilmenite as
187
international journal of greenhouse gas control 2 (2008) 180–193
Table 5 – Average time needed, in minutes, to reach 95% and 80% conversion of 0.2 g of South African coal for different steam conditions Conversion (%) 95 80
12.5% H2O
25% H2O
21.9 8.8
12.6 6.0
50% H2O 10.8 5.0
Fig. 11 – Fraction of CO after 2 min and at full conversion, Eq. (7), for different fuels in Fe2O3/MgAl2O4 as a function of volatiles in the fuels. The fluidizing gas contained 50% H2O and the temperature was 950 8C.
oxygen carrier even if the fraction of CO seems slightly higher for ilmenite. However it is clear that a high amount of volatiles results in a higher fraction of the fuel released as CO. This may be due to the high fraction of combustible gases released in the top of the reactor for high volatile fuels in the beginning of the cycle. The oxygen carriers do not have sufficient time to convert all these gases before they leave the reactor with the fluidizing gas, giving only partly conversion of high volatile fuels in the beginning of the cycle. As a result, the CO released during the first two minutes accounts for the majority of the total amount of CO released for high volatile fuels. Also the CO
Fig. 13 – Average rate of reaction during the period to reach 80% and 95% conversion for 0.2 g of South African coal in 40 g of ilmenite as a function of the inlet concentration of steam. The temperature was kept constant at 950 8C.
released after 2 min is apparently similar for all of the fuels, around 10%. However, this does not apply to the Indonesian coal which has almost full conversion after 2 min.
4.2.
Fig. 12 – Fraction of CO after 2 min and at full conversion, Eq. (7), for different fuels in ilmenite as a function of volatiles in the fuels. The fluidizing gas contained 50% H2O and the temperature was 950 8C.
Effect of steam content
Table 5 shows the time needed to reach 95% and 80% conversion of 0.2 g of South African coal in 40 g of ilmenite with different steam content, and Fig. 13 shows the average rate of reaction reached at 80% and 95% for the same experiments. These experiments were conducted at 950 8C. There is a clear trend of increasing conversion rate with increasing steam content. With 50% steam it takes roughly 10 min to reach 95% conversion of the fuel, with 12.5% steam the conversion time is more than doubled. It was not possible to go higher than 50% with the present equipment, but the trend of increasing conversion rate is expected to continue for steam contents above 50%. Table 6 presents the measured fraction of unconverted CH4 as a fraction of the total amount of carbon containing gases during a cycle for South African coal in ilmenite for different steam contents in the fluidizing gas. Also shown in Table 6, are the recalculated fractions of unconverted CH4 with compensation for the SO2 interference under the assumption that all sulphur in the fuel leaves as SO2. No significant effect of the steam content on the conversion of CH4 is seen.
188
international journal of greenhouse gas control 2 (2008) 180–193
Table 6 – Measured fraction of unconverted CH4 during one cycle for South African coal in ilmenite for different steam conditions and the recalculated fractions of unconverted CH4 with compensation for the SO2 interference under the assumption that all sulphur in the fuel leaves as SO2 CH4/(CO2 + CO + CH4) Meassured Corrected for SO2
12.5% H2O
25% H2O
0.0675 0.0665
0.0616 0.0606
50% H2O
Table 7 – Average time needed, in minutes, to reach 95% and 80% conversion of 0.2 g of South African coal for different temperature conditions Conversion (%) 95 80
850 8C
950 8C
43.2 25.3
10.8 5.0
1000 8C 5.1 2.4
0.0720 0.0710
In Fig. 14 the fraction of CO, as calculated in Eq. (7), is plotted as a function of total converted carbon for different steam content in the fluidizing gas. There is no visible effect of steam on the CO production in this plot. However, for the first part the slope of the CO curve is somewhat larger. This is when mostly volatiles react with the oxygen carrier which is in agreement with what is seen in Figs. 11 and 12. The oxygen carriers are less efficient at converting the volatiles released, compared to the syngas produced by reaction (2) and (3).
4.3.
Effect of temperature
Table 7 shows the time needed to reach 95% and 80% conversion of 0.2 g of South African coal in 40 g of ilmenite for different temperatures and Fig. 15 shows the average rate of reaction reached at 80% and 95% for the same experiments. It is evident that the reaction rate is strongly temperature dependent. At the higher temperature 1000 8C, some temporary defluidization was noted, but no agglomeration occurred. Table 8 presents the measured fraction of unconverted CH4 as a fraction of the total amount of carbon containing gases during a cycle for South African coal in ilmenite for different temperatures. Also shown in Table 8, are the recalculated fractions of unconverted CH4 as compensate for the SO2 interference under the assumption that all sulphur in the fuel
Fig. 15 – Average rate of reaction to reach 80% and 95% conversion for 0.2 g of South African coal in 40 g of ilmenite as a function of temperature. The fluidizing gas contained 50% H2O.
leaves as SO2. A higher conversion of CH4 can be noted at the highest temperature. In Fig. 16 the fraction of CO, as calculated in Eq. (7), is shown as a function of total converted carbon for different temperatures. It is clear that a higher temperature gives an improved conversion of CO. This is most likely explained by the slower reaction of oxygen-carrying particles at lower temperatures (Mattisson et al., 2006). In Fig. 16, just as in Fig. 14 it is also seen from the slope that a higher fraction of CO is obtained when volatiles are released in the beginning of the cycle.
4.4.
Syngas experiments
In order to investigate the reactivity of the two oxygen carriers with CO and H2, experiments were also made with syngas as
Table 8 – Measured fraction of unconverted CH4 during one cycle for South African coal in ilmenite for different temperatures and the recalculated fractions of unconverted CH4 to compensate for the SO2 interference under the assumption that all sulphur in the fuel leaves as SO2 Fig. 14 – Fraction of CO, Eq. (7), as a function of conversion for 0.2 g of South African coal in 40 g of ilmenite at 950 8C presented for different steam concentrations.
CH4/(CO2 + CO + CH4)
850 8C
950 8C
1000 8C
Meassured Corrected for SO2
0.0686 0.0676
0.0720 0.0710
0.0437 0.0427
international journal of greenhouse gas control 2 (2008) 180–193
Fig. 16 – Fraction of CO, Eq. (7), as a function of conversions for 0.2 g of South African coal in 40 g of ilmenite for different temperatures. The fluidizing gas contained 50% H2O.
189
Fig. 18 – Fraction of CO converted to CO2 as a function mass reduction with previously used ilmenite and Fe2O3/ MgAl2O4 as oxygen-carrying particles. The fuel, also used as fluidizing gas, contained 50% H2 and 50% CO and the temperature was 950 8C.
fuel, i.e. 50% CO and 50% H2. Fresh oxygen carriers and the old ilmenite and Fe2O3/MgAl2O4 particles which were used for the solid fuel experiments were tested. The temperature was 950 8C and the flow of syngas was 450 mLn/min during 100 s. Figs. 17 and 18 shows the reactivity of fresh and used oxygen carriers with syngas, with the fraction of CO converted to CO2 as a function of the mass based degree of conversion of the oxygen carrier. The mass-based degree of conversion is defined as the mass of the oxygen carrier divided with the mass of the oxygen carrier in its most oxidized state. As can be seen, both fresh and used particles display a high reactivity
Fig. 19 – Fraction of CO at full conversion, Eq. (7), for different fuels in sand as a function of volatiles in the fuels. The fluidizing gas contained 50% H2O and the temperature was 950 8C.
with almost complete yield of the syngas to CO2 and H2O. Fe2O3/MgAl2O4 shows no change, or maybe a small increase, in the conversion of CO and ilmenite shows a small increase in reactivity.
4.5. Fig. 17 – Part CO converted to CO2 as a function mass reduction with fresh ilmenite and Fe2O3/MgAl2O4 used as oxygen-carrying particles. The fuel, also used as fluidizing gas, contained 50% H2 and 50% CO and the temperature was 950 8C.
Sand experiments
To investigate gasification of the different fuels the oxygen carrying particles were replaced by quartz sand. The temperature was 950 8C and a flow of 50% steam and 50% nitrogen was used as fluidization gas when 0.2 g of fuel was introduced
190
international journal of greenhouse gas control 2 (2008) 180–193
Fig. 20 – Average time of reaction to reach 95% conversion for different fuels in ilmenite and Fe2O2/MgAl2O4 as a function of the average time of reaction to reach 95% conversion in sand. The fluidizing gas contained 50% H2O and the temperature was 950 8C.
Fig. 22 – SEM images of Fe2O3/MgAl2O4 used in solid fuel experiments.
to the bed. The fuel slowly reacts with the steam forming CO and H2, however only the amount of CO could be measured. Some of this CO reacts further with the steam forming CO2, and more H2 giving a fraction of CO in the range of 60–70% as this is shown in Fig. 19. The fraction of CO does not differ significantly for the different fuels. The time it need to convert 95% of a fuel with an oxygen carrier is about a half of the time to convert the same fuel in sand, Fig. 20. One difference between CLC and gasification is the concentrations of H2 and CO, which are lower in a CLC reactor, as the oxygen carrier reacts with CO and H2. H2 is known to inhibit the gasification reaction (Barrio et al., 2000; Johnstone et al., 1952; Weeda et al., 1993).
Fig. 23 – SEM images of fresh Fe2O3/MgAl2O4.
4.6.
Fig. 21 – SEM images of fresh Fe2O3/MgAl2O4.
Scanning electron microscope (SEM) images
The surface of both fresh and used oxygen carriers were analysed in a scanning electron microscope, SEM. In Figs. 21– 24 the SEM images are shown in two different magnifications for fresh and used Fe2O3/MgAl2O3 particles. The used particles seem to have a somewhat rougher surface. The BET area of the reacted particles, as determined by a Micromeritics Gemini 2362, was only 0.76 m2/g as compared to the BET area of 8.58 for fresh Fe2O3/MgAl2O3 particles. Thus it is quite clear that the interior particle structure of these particles seems to sinter during testing. As this effect has been noticed also during batch experiments with gaseous fuels, it is likely not due to the solid fuel ash (Johansson et al., 2004). Interestingly, the particles maintain a high-reactivity even after the particle restructuring, see Fig. 18.
international journal of greenhouse gas control 2 (2008) 180–193
Fig. 24 – SEM images of Fe2O3/MgAl2O4 used in solid fuel experiments.
191
Fig. 26 – SEM images of ilmenite used in solid fuel experiments.
In Figs. 25–28 the SEM images are shown in to different magnifications for fresh and used ilmenite particles. It is clear that the used particles have much smoother edges and rougher surfaces that the fresh ilmenite particles. Also the BET area of the used ilmenite increased to 0.58 for the used particles as compared to 0.11 for fresh ilmenite. The increase BET area likely means that the particles are gaining porosity during the reduction/oxidation experiments.
5.
Discussion
This work shows that a number of solid fuels can be used directly in chemical-looping combustion. As shown in Figs. 17 and 18, both the synthetic Fe2O3/MgAl2O3 particles
Fig. 27 – SEM images of fresh ilmenite.
Fig. 25 – SEM images of fresh ilmenite.
and the natural ilmenite mineral react fast with syngas. The intermediate gasification rates are dependent on the type of solid fuel that is used, e.g. Fig. 20. Although the presence of an oxygen carrier doubles the conversion rate, the fuel gasification is still the time limiting step. However, a higher reaction temperature or a higher fraction of steam in the fluidizing gas will increase the rate of conversion, Figs. 13 and 15. The slow reaction means that a high solids inventory may be necessary in the fuel reactor of a real system. Leion et al. (2007) have estimated the needed solid inventory in a CLC system, when synthetic Fe2O3/MgAl2O3 particles where used as oxygen carrier and petroleum coke as fuel, to 2000 kg/MW. It is then assumed a temperature difference of 50 8C between the reactors of the CLC system, due to the endothermic reaction in the fuel reactor, a solid flow of 380 kg/MWth, minute of Fe2O3/MgAl2O3 particles and a
192
international journal of greenhouse gas control 2 (2008) 180–193
6.
Conclusions
The results show that the CLC-technique can be used for a number of different solid fuels with different composition. Two different oxygen carriers were used, one natural ilmenite mineral and one synthetic Fe2O3/MgAl2O4 particles:
Fig. 28 – SEM images of ilmenite used in solid fuel experiments.
needed residence time of five minutes to convert the petroleum coke. If a more reactive coal with high content of volatiles is used, such as the South African or Indonesian coal in this work, the residence time will be shorter and the needed solids inventory less. The actual residence time will depend, not only on fuel, but also on reaction temperature, steam content in the fluidizing gas, reactor design and needed CO2 capture efficiency. When solid fuels are burned, ash is produced. In a CLC system this ash has to be separated from the oxygen carrying particles. This separation can be accomplished by feeding small fuel particles to the system. During conversion the fuel particles will produce even smaller ash particles, small enough to be elutriated with the fluidizing gas and separated in a cyclone. Nevertheless it cannot be avoided that some of the oxygen carrying particles are lost with the ash. Especially particles those due to breakage and fragmentation are so small that they have the same fluidizing properties as the ash. The particles may also suffer from reactivity loss due to reaction with ash components, although the current experiments saw no such deactivation. This means that the oxygen-carrying particles used with solid fuel in a CLC-boiler may have to be replaced fairly often. The fact that similar rates of reaction were seen with the natural ilmenite and the synthetically produced iron oxide particles would clearly indicate that the more inexpensive ilmenite would be more feasible to use in a real system, where perhaps the oxygen carrier may need to be replaced more often than in a gas fired system. In these tests, the same sample of ilmenite was used for over 45 h. During this time the particles were heated up to 950 8C and cooled down again to room temperature several times since the oven had to be turned off at night. The particles were also exposed to ash and combustion gases from the different fuels. Still the particles showed good fluidization properties throughout all experiments and the particles did not in any way lose in reactivity.
The natural mineral ilmenite reacts just as well with all investigated fuels as the synthetic Fe2O3/MgAl2O4 particles. The gasification of the fuel about two times faster when conducted in the presence of oxygen-carrying particles. This is believed to be explained by efficient removal of gases like H2 that is known to have an inhibiting effect on gasification. The oxygen carriers, Fe2O3/MgAl2O4 and ilmenite, react fast with intermediate gasification products such as CO and H2. The gasification reaction is slow compared to the reaction of the gasified components with the metal oxide, thus gasification is the time limiting step. The rate of conversion together with oxygen carrier particles was determined for a range of fuels with highly different volatiles content. The conversion rate of South African coal increased significantly with increased fraction of steam in the fluidizing gas as well as temperature. No agglomeration could be seen for the synthetic Fe2O3/ MgAl2O4 particles and only minor agglomeration for the ilmenite mineral was seen. This occurred most likely during a period of defludization at high temperature, 1000 8C. At 950 8C and with 50% steam the time needed to achieve 95% conversion of fuel particles with a diameter of 0.125– 0.18 mm ranged between 4 and 15 min depending on the fuel, while 80% conversion was reached within 2–10 min.
Acknowledgment This work was made in the EU financed research project Enhanced Capture of CO2 (ENCAP), SES6-2004-502666.
references
Adanez, J., de Diego, L.F., Garcia-Labiano, F., Gayan, P., Abad, A., Palacios, J.M., 2004. Selection of oxygen carriers for chemical-looping combustion. Energy Fuels 18 (2), 371–377. Adanez, J., Garcia-Labiano, F., de Diego, L.F., Gayan, P., Abad, A., Celaya, J., 2005. Development of oxygen carriers for chemical-looping combustion. Carbon Dioxide Capture for Storage in Deep Geologic Formations—Results from the CO2 Capture Project, vol. 1. pp. 587–604. Arrhenius, S., 1886. On the influence of carbonic acid in the air upon the temperature of the ground. Philos. Mag. 41, 237–277. Barrio, M., Gøbel, B., Risnes, H., Henriksen, U., Hustad, J.E., Sørensen, L.H., 2000. Steam gasification of wood char and the effect of hydrogen inhibition on the chemical kinetics. Prog. Thermochem. Biomass Conver. 1, 32–46. Cao, Y., Pan, W.-P., 2006. Investigation of chemical looping combustion by solid fuels. 1. Process Analysis. Energy Fuels 20 (5), 1836–1844. Cao, Y., Casenas, B., Pan, W.-P., 2006. Investigation of chemical looping combustion by solid fuels. 2. Redox reaction
international journal of greenhouse gas control 2 (2008) 180–193
kinetics and product characterization with coal, biomass, and solid waste as solid fuels and CuO as an oxygen carrier. Energy Fuels 20 (5), 1845–1854. Cao, Y., Liu, K., Riley, J.T., Pan, W.-P., 2004. Application of a circulating fluidized bed process for the chemical looping combustion of solid fuels. Preprints of Symposia—American Chemical Society. Div. Fuel Chem. 49 (2), 815–816. Cho, P., Mattisson, T., Lyngfelt, A., 2004. Comparison of iron-, nickel-, copper- and manganese-based oxygen carriers for chemical-looping combustion. Fuel 83 (9), 1215–1225. Cho, P., Mattisson, T., Lyngfelt, A., 2005. Carbon formation on nickel and iron oxide-containing oxygen carriers for chemical-looping combustion. Indus. Eng. Chem. Res. 44 (4), 668–676. Cho, P., Mattisson, T., Lyngfelt, A., 2006. Defluidization conditions for a fluidized bed of iron oxide-, nickel oxide-, and manganese oxide-containing oxygen carriers for chemical-looping combustion. Indus. Eng. Chem. Res. 45 (3), 968–977. Dennis, J.S., Scott, S.A., Hayhurst, A.N., 2006. In situ gasification of coal using steam with chemical looping: a technique for isolating CO2 from burning a solid fuel. J. Energy Inst. 79 (3), 187–190. Ishida, M., Jin, H., 1994. A new advanced power-generation system using chemical-looping combustion. Energy 19 (4), 415–422 (Oxford, United Kingdom). Ishida, M., Jin, H., Okamoto, T., 1996. A Fundamental study of a new kind of medium material for chemical-looping combustion. Energy Fuels 10 (4), 958–963. Jerndal, E., 2005. Thermal analysis of chemical-looping combustion. Div. Energy Technol.. Johansson, M., Mattisson, T., Lyngfelt, A., 2004. Investigation of Fe2O3 with MgAl2O4 for chemical-looping combustion. Indus. Eng. Chem. Res. 43 (22), 6978–6987.
193
Johnstone, H., Chen, C., Scott, D., 1952. Kinetics of the steamcarbon reaction in porous graphite tubes. Indus. Eng. Chem. 44, 1564–1569. Leion, H., Mattisson, T., Lyngfelt, A., 2007. The use of petroleum coke as fuel in chemical-looping combustion. Fuel 86 (1213), 1947–1958. Lyngfelt, A., Thunman, H., 2005. Construction and 100 h of operational experience of a 10-kW chemical-looping combustor. Carbon Dioxide Capture for Storage in Deep Geologic Formations—Results from the CO2 Capture Project, vol. 1. pp. 625–645. Lyngfelt, A., Leckner, B., Mattisson, T., 2001. A fluidized-bed combustion process with inherent CO2 separation; application of chemical-looping combustion. Chem. Eng. Sci. 56 (10), 3101–3113. Lyon, R.K., Cole, J.A., 2000. Unmixed combustion: an alternative to fire. Combust. Flame 121, 249–261 (1/2). Mattisson, T., Johansson, M., Lyngfelt, A., 2004. Multicycle reduction. Oxidation of different types of iron oxide particles-application to chemical-looping combustion. Energy Fuels 18 (3), 628–637. Mattisson, T., Johansson, M., Lyngfelt, A., 2006. CO2 capture from coal combustion using chemical-looping combustion—reactivity investigation of Fe Ni and Mn based oxygen carriers using syngas. Clearwater Coal Conf.. Scott, S.A., Dennis, J.S., Hayhurst, A.N., Brown, T., 2006. In situ gasification of a solid fuel and CO2 separation using chemical looping. AICHE J. 52 (9), 3325–3328. Weeda, M., Abcouwer, H., Kapteijn, F., Moulijn, J., 1993. Steam gasification kinetics and burn-off behavioiur for a bituminous coal derived char in the presence of H2. Fuel Process. Technol. 36, 235–242. Zhang, G., Ostrovski, O., 2002. Effect of preoxidation and sintering on properties of ilmenite concentrates. Int. J. Miner. Process. 64, 201–218.
available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/ijggc
Solid fuels in chemical-looping combustion Henrik Leion a,*, Tobias Mattisson b, Anders Lyngfelt b a b
Department of Environmental Inorganic Chemistry, Chalmers University of Technology, S-412 96 Go¨teborg, Sweden Department of Energy and Environment, Chalmers University of Technology, S-412 96 Go¨teborg, Sweden
article info
abstract
Article history:
The feasibility of using a number of different solid fuels in chemical-looping combustion
Received 23 April 2007
(CLC) has been investigated. A laboratory fluidized bed reactor system for solid fuel,
Received in revised form
simulating a chemical-looping combustion system by exposing the sample to alternating
28 September 2007
reducing and oxidizing conditions, was used. In each reducing phase 0.2 g of fuel in the size
Accepted 6 October 2007
range 180–250 mm was added to the reactor containing 40 g oxygen carrier of size 125–
Published on line 26 November 2007
180 mm. Two different oxygen carriers were tested, a synthetic particle of 60% active material of Fe2O3 and 40% MgAl2O4 and a particle consisting of the natural mineral ilmenite.
Keywords:
Effect of steam content in the fluidizing gas of the reactor was investigated as well as effect
Chemical-looping combustion (CLC)
of temperature. A number of experiments were also made to investigate the rate of
Fluidized bed
conversion of the different fuels in a CLC system. A high dependency on steam content
Oxygen carrier
in the fluidizing gas as well as temperature was shown. The fraction of volatiles in the fuel
Solid fuel
was also found to be important. Furthermore the presence of an oxygen carrier was shown
Ilmenite
to enhance the conversion rate of the intermediate gasification reaction. At 950 8C and with
Iron oxide
50% steam the time needed to achieve 95% conversion of fuel particles with a diameter of
Volatiles
0.125–0.18 mm ranged between 4 and 15 min depending on the fuel, while 80% conversion
Steam
was reached within 2–10 min. In almost all cases the synthetic Fe2O3 particle with 40% MgAl2O4 and the mineral ilmenite showed similar results with the different fuels. # 2007 Elsevier Ltd. All rights reserved.
1.
Introduction
Since Arrhenius first discovered the connection between the amount of CO2 in the atmosphere and the global average temperature (Arrhenius, 1886) the concentration of CO2 in the atmosphere has risen approximately 30% higher than the preindustrial value. The main reason for the increased CO2 levels is combustion of fossil fuels, the world’s dominant energy source. Separation and sequestration of the CO2 formed has been recognized as one very important option to accomplish a reduction in CO2 emissions. Chemical-looping combustion (CLC) is one of the techniques that can be used to capture CO2. The main advantage with CLC is that CO2 is inherently separated from the other flue gas components, thus avoiding costly equipment
and energy consumption for separation of gas (Cho et al., 2004, 2005). The CLC process is composed of two fluidized bed reactors, an air and a fuel reactor, shown in Fig. 1. The fuel is introduced to the fuel reactor where it reacts with an oxygen carrier to CO2 and H2O. The reduced oxygen carrier is transported to the air reactor where it is oxidized back to its original state by air. As a result, CO2 can be inherently separated in this combustion process. The total amount of heat released in the air and the fuel reactor is equal to the heat released from normal combustion thus separating CO2 without any losses in energy. There have been a number of publications where different aspects of the CLC process have been investigated (Adanez et al., 2004, 2005; Ishida and Jin, 1994; Ishida et al., 1996;
* Corresponding author. Tel.: +46 31 7722886; fax: +46 31 7722853. E-mail address: [email protected] (H. Leion). 1750-5836/$ – see front matter # 2007 Elsevier Ltd. All rights reserved. doi:10.1016/S1750-5836(07)00117-X
181
international journal of greenhouse gas control 2 (2008) 180–193
reactions are C þ H2 O ) CO þ H2
(1)
CO þ H2 O ) CO2 þ H2
(2)
C þ CO2 ) 2CO
(3)
The main reactions with the metal oxide are
Fig. 1 – Schematic picture of the CLC process. Two interconnected fluidized bed reactors, one air and one fuel reactor, with circulating oxygen carrying particles.
Lyngfelt et al., 2001; Mattisson et al., 2004). Most of the work around chemical-looping combustion has focused on the use of gaseous fuel and only a few publications on solid fuel (Cao et al., 2006, 2004; Cao and Pan, 2006; Dennis et al., 2006; Leion et al., 2007; Lyon and Cole, 2000; Scott et al., 2006). Since coal is considerably more abundant than natural gas it would be highly advantageous if the CLC process could be adapted for solid fuels. One way of performing this is by first gasifying the coal to a syngas consisting mainly of CO and H2. This gas could then be burned in CLC. However, in order to obtain undiluted syngas the gasification needs to be carried out with O2 and thus an air separation unit would be needed. Another option is to introduce the solid fuel directly into the fuel reactor where the oxygen carrier is reduced by the fuel giving a one step fuel oxidation without any need for air separation. However, the solid–solid reaction between coal and an iron based metal oxide is not very likely to occur at any appreciable rate (Leion et al., 2007). Instead the solid fuel needs to be gasified and then the oxygen carrying particles react with the gas produced, making the gasification the time limiting step (Dennis et al., 2006; Leion et al., 2007; Scott et al., 2006). An important advantage compared to normal gasification is that it will take place in a high concentration of H2O and/or CO2 which is beneficial for the reaction rates. The main gasification
MeX OY þ H2 ) MeX OY1 þ H2 O
(4)
MeX OY þ CO ) MeX OY1 þ CO2
(5)
This work investigates the reaction of two oxygen carriers, one synthetic iron based particle (Fe2O3/MgAl2O4) and one natural mineral, ilmenite, or mainly Fe2TiO5, in a laboratory fluidized bed reactor. A number of different solid fuels were used, Table 1, as well as a gaseous fuel mixture of 50% H2 and 50% CO (syngas). The concentration of steam in the fluidizing gas, as well as the temperature in the fluidized bed, has been varied in order to investigate the effect on the reaction rate. Also the some experiments were made where the oxygen carriers were replaced by sand to further investigate gasification of solid fuel in a CLC system.
2.
Experimental
2.1.
Experimental setup and procedure
The experiments were conducted in a fluidized bed reactor of quartz, Fig. 2, using either Fe2O3/MgAl2O4 or ilmenite as bed material. In order to achieve good solids mixing in the bed, the reactor was conically shaped just above the distributor plate. The reactor had a total length of 870 mm with a porous quartz plate placed 370 mm from the bottom of the reactor. The porous plate and the reactor below the plate had an inner diameter of 10 mm. Above the distributor plate the inner diameter of the reactor increased to reach 30 mm at a height of 20 mm above the porous plate. The diameter was then constant for 250 mm. The reactor diameter was then increased further to 45 mm for a length of 100 mm. This
Table 1 – Solid fuels used in this work Fuel
Petroleum coke
South Africa
China
Indonesia
Taiwan
S. France Raw
Volatiles (%) Moisture (%) Ash (%) HHV (MJ/kg) C (%) H (%) S (%) N (%) O (%)
10.0 8.0 0.5 31.7 81.3 2.9 6.0 0.9 0.5
21.6 8.3 15.9 29.9 62.5 3.5 0.7 1.4 7.7
8.8 1.0 21.0 26.6 70.3 2.8 0.6 1.1 3.1
45.3 8.5 1.4 26.4 66.1 4.7 0.1 0.7 18.5
31.5 2.5 12.3 27.9 70.6 4.5 0.5 1.7 7.9
25.0 1.5 21.8 24.9 63.9 3.6 0.8 0.8 7.7
Sieved 25.0 1.5 21.8 24.9 63.9 3.6 0.8 0.8 7.7
182
international journal of greenhouse gas control 2 (2008) 180–193
Fig. 2 – Fluidized bed reactor of quartz.
disengaging section was constructed in order to avoid that smaller coal and metal oxide particles leave the reactor. A sample of 40 g of oxygen carrier particles of size 0.090– 0.125 mm was placed on the porous plate and was then initially heated in an inert atmosphere to the reaction temperature. When the bed was not fluidized the bed height was approximately 35 and 30 mm for Fe2O3/MgAl2O4 and ilmenite, respectively. The particles were then alternatingly exposed to 5% O2 and the fuel/steam mixture, thus simulating the cyclic conditions of a CLC system burning solid fuel. Nitrogen gas was introduced for 180 s between each reducing and oxidizing period. During the reducing period, the fluidizing gas was a mixture of steam and nitrogen, which was introduced from the bottom of the reactor. At the start of the reducing period, i.e. at the same time as the fluidizing gas of the reducing cycle, mainly consisting of steam and nitrogen, started to come into the bottom of the reactor, the fuel was inserted in the top of the reactor, falling down into the fluidized bed.
The diameter of the solid fuel particles was between 125 and 180 mm, i.e. somewhat larger than the oxygen carrier particles. The experiments were performed in this manner for two to six cycles in each case, where the different cases are defined by the value of temperature, fraction of steam in the fluidizing gas during reduction and the fuel used. The temperature was measured 5 mm below and 10 mm above the porous quartz plate, using 10% Pt/Rh thermocouples enclosed in quartz shells. Since reaction heat affects the temperature in the bed, the temperature below the bed was used to control the reactor temperature, and all reactor temperatures in this work refer to the temperature measured below the bed. The temperature in the bed was generally 20 8C higher. The gas from the reactor was led to an electric cooler, where the water was removed, and then to a gas analyzer (Rosemount NGA-2000) where the concentrations of CO2, CO, CH4, and O2 were measured in addition to the gas flow. A schematic layout of the laboratory setup is presented in Fig. 3. All fuels contained some sulphur and the presence of SO2 may have a significant impact on the gasification rate (Leion et al., 2007; Lyon and Cole, 2000). SO2 also interfered with the CH4 channel. Therefore in Tables 3, 4, 6 and 8 the concentration of CH4 as a fraction of the total amount of carbon containing gases during a cycle is presented both as the measured values and the corrected values under the assumption that all sulphur in the fuel leaves as SO2. As seen, this interference can be neglected for all fuels except for petroleum coke which has comparably high sulphur content. For a few cycles gas from the exiting stream was collected in bags and analysed with a gas chromatograph (Varian MicroGC with Molsieve and Poraplot column). Gases were sampled during 30 s of a reducing period. The average concentrations of CO2, CO, CH4 and H2 were measured with the GC. The measurements from the gas chromatograph were made to confirm the accuracy of the CO2, CO and CH4 measurements from the gas analyzer and to measure the hydrogen concentration. The exothermic nature of the oxidation reaction means that there will be release of heat and therefore a subsequent temperature rise. To limit this temperature increase, a gas mixture with 5% O2 in N2 was used instead of air. Thus, large temperature increases were avoided. In a real system it will of course be possible to cool the air reactor, but this was not the case with this small laboratory setup. All experiments were conducted with a gas flow of 600 mL/min (at 1 bar, 0 8C), both
Fig. 3 – Schematic layout of the laboratory setup.
international journal of greenhouse gas control 2 (2008) 180–193
for the reducing and oxidizing periods. This corresponds to a velocity ratio, u/ umf, in the 10 mm section just above the porous plate, of 53 and 70 for the oxygen carriers, i.e. synthetic Fe2O3/MgAl2O4 particles and the ilmenite mineral, and fuel, respectively. u is the gas velocity and umf is the minimum fluidization velocity. However, the velocity was considerably lower in the wider 45 mm part of the reactor, corresponding to a u/umf of 2.6 and 3 for the oxygen carriers and for the fuel particles, respectively. In the 30 mm part u/umf was 5.8 and 7.2 for the oxygen carriers and a for the fuel particles, respectively. These velocity ratios are calculated for nitrogen but differs only a few percent for the other gases used in this work. However, reference experiments indicate that small fuel particles still left the reactor, also some fuel was lost in the feeding device. These losses are excluded from the result and all calculations are made on the carbon containing gases leaving reactor. From high frequency measurements of the pressure drop it was possible to see whether the bed was fluidized or not (Cho et al., 2006).
2.2.
Material preparation
Two types of oxygen carrier were tested. One was a synthetically produced iron based particle previously used in experiments with gaseous fuel and therefore having known reaction and fluidizing properties (Johansson et al., 2004). These particles contained 60% active material of Fe2O3 and 40% MgAl2O4. They were produced by freeze granulation and then sintered at 1100 8C for 6 h using a heating rate of 10 8C/min. Each batch was sieved to obtain particles in the size range 0.090–0.125 mm. The fresh particles had a BET area of 8.58 m2/g as determined by a Micromeritics Gemini 2362. The actual Fe2O3/MgAl2O4 particles used in this work have previously been used in a 10 kW CLC system at Chalmers (Lyngfelt and Thunman, 2005). The other oxygen carrier was a natural mineral, ilmenite (FeTiO3), was obtained from Titania A/S. Ilmenite is concentrated from ore containing 40% ilmenite, 37% plagioclase, 8.6% ortopyroxene, 6.5% clinopyroxene, 4.2% biotit and some minor other phases. The ilmenite received from Titania is 94.3% pure. The molar ratio of iron and titanium is close to 1:1. XRD measurements on fresh samples show ilmenite and small fractions of hematite. Fresh ilmenite had a BET area of 0.11 m2/ g as determined by a Micromeritics Gemini 2362. To make sure that the ilmenite mineral was in its most oxidized state the ilmenite particles were first heat treated in air for 24 h at 950 8C. Also, for many oxygen carrier materials the reactivity is different during the first cycles and then stabilizes (Mattisson et al., 2004). In order to avoid differences in reactivity because of this, the ilmenite particles were, after the heat treatment, exposed to a flow of 450 N mL/min of either 5% O2 in nitrogen or syngas (50% CO, 50% H2) in a cyclic manner at 950 8C for four cycles, before the testing with solid fuels started. The fuels used are presented in Table 1. All fuels were crushed and sieved to obtain particles in the size range 0.125– 0.180 mm. 0.2 g of fuel was used for every reducing cycle, which is equivalent to a maximum mass reduction of the bed of 1 wt% when the fuel with the highest oxygen demand, petroleum coke, is used in 40 g of bed material. This corresponds to approximately 50% conversion of the synthetic iron particles from Fe2O3 to Fe3O4. In this application it would
183
not be useful to reduce iron oxide to lower forms than Fe3O4, i.e. FeO and Fe, since thermodynamic restrains would prevents full conversion of the fuel. In other words the concentration of oxidizing gases, such as H2O, in the fluidizing gas should be high enough to prevent the further reaction from Fe3O4, to FeO under conditions with high conversions of fuel (Jerndal, 2005). The different fuels used in this work have different composition and therefore will result in somewhat different degrees of reduction of the oxygen carrier. But it has previously been shown that the conversion time for petroleum coke in Fe2O3/MgAl2O4 particles was not affected when the ratio of fuel and oxygen carrier was varied(Leion et al., 2007). The reduction of ilmenite is somewhat more complicated. The oxidation/reduction of ilmenite proceeds through three levels. The reduced level is ilmenite, FeTiO3, corresponding to FeO + TiO2, an intermediate level is Fe3Ti3O10, corresponding to Fe3O4 + 3TiO2, and the most oxidized level is Fe2TiO5 + TiO2, corresponding to Fe2O3 + 2 TiO2 (Zhang and Ostrovski, 2002). The total oxygen transfer capacity from the most oxidized to the most reduced level of ilmenite is 5%, and since the added fuel in this work only corresponds to a mass reduction of 1%, the ilmenite shifts between the most oxidized level is Fe2TiO5 + TiO2 and the intermediate level, Fe3Ti3O10. Thermodynamics calculations indicate that when ilmenite moves between these levels the conversion of CO and H2 is over 99.99%.
3.
Data evaluation
The rate of conversion of the fuel is given as an average rate of the carbon converted in the fuel, and was calculated from
rave ¼
1 mt mtot t
(6)
where t is the time elapsed since the start of the cycle, mtot is the total mass of carbon converted during the entire reducing period and mt is the mass of carbon converted up until the time t. The total amount of carbon is determined from the integration of the outgoing CO and CO2 concentrations for an entire reduction period. The CO fraction i.e. the fraction of carbon released as unconverted CO was defined as
CO ¼
COcum ðCO þ CO2 Þtot
(7)
COcum in the cumulative amount of carbon converted to CO up to a given degree of conversion, whereas (CO + CO2)tot is the total amount of carbon released as CO and CO2 during an entire cycle.
4.
Results
4.1.
Concentration profiles
Fig. 4 shows the outlet gas concentrations after condensation of water as a function of time for a reducing period where 0.2 g
184
international journal of greenhouse gas control 2 (2008) 180–193
Fig. 4 – Concentration profile during the reduction of 40 g Fe2O3/MgAl2O4 with 0.2 g petroleum coke. The inlet concentration of H2O is 50%. The temperature is 950 8C.
Fig. 5 – Concentration profile during the reduction of 40 g ilmenite with 0.2 g petroleum coke. The concentration of H2O is 50%. The temperature is 950 8C.
of petroleum coke was used as fuel and Fe2O3/MgAl2O4 was used as oxygen carrier. Hydrogen concentration obtained from bag samples analyzed with gas chromatography is also shown. CO2, CO and CH4 measurements from the gas chromatograph are not presented but agreed very well with the measurements from the gas analyzer. The fluidization gas contained 50% steam and had a temperature of 950 8C. The initial peak of CH4 is mainly from devolatilization of the petroleum coke. As mentioned earlier, some of the measured methane could actually be due to release of SO2. The initial peaks of CO and CO2 are due to release of volatiles and reaction of these with the iron oxide. After this the remaining char reacts with the added steam to syngas, i.e. CO and H2, which reacts further with the metal oxide to CO2 and H2O. In this case the CO2 concentration reaches a maximum just below 8% and the CO a maximum of roughly 0.5% if the initial peak is not taken into account. The CO2 concentration starts to decrease after reaching a maximum, approximately 5 min into the cycle. After about 20 min of reaction all of the added petroleum coke has reacted. Some CO was found in all experiments, and can be related to the gas–solid mixing. Assuming that the metal oxides and fuel particles are well mixed in the reactor there will be fuel particles all the way up to the top of the bed, and thus there will always be a fraction of the gasification products, i.e. CO and H2, which will not have sufficient contact with the oxygen carrier even if the reactivity of the metal oxide particles is high. It cannot be excluded that there also is some segregation, increasing the fraction of lighter fuel particles in the top of the bed. In Fig. 5 the gas concentrations for a reducing period are shown for the same conditions as in Fig. 4. The difference being that ilmenite is used as oxygen-carrying material instead of Fe2O3/MgAl2O4. The concentrations of all measured species were very similar for the two different oxygen carriers independent of which fuel that was used.
Figs. 6 and 7 shows the concentration profiles for experiments conducted with the South African- and Indonesiancoal, respectively. The reaction is again carried out with 50% steam in nitrogen. In Fig. 6 hydrogen concentration obtained from bag samples analyzed with gas chromatography is also shown. These coals have higher contents of volatiles in comparison to petroleum coke, and therefore a higher fraction of the solid fuel is reacting during the devolatilization stage, resulting in higher initial peaks of CO2, CO and in the case of the Indonesian coal also CH4/SO2. In the case of petroleum coke the reaction of the volatiles ends before the char reaction reaches its maximum conversion rate, giving a dip in the concentrations of CO2 after the release of volatiles. For the other solid fuels it is not possible to separate the char reaction
Fig. 6 – Concentration profile during the reduction of 40 g ilmenite with 0.2 g South African coal. The inlet concentration of H2O is 50%. The temperature is 950 8C.
185
international journal of greenhouse gas control 2 (2008) 180–193
Fig. 7 – Concentration profile during the reduction of 40 g ilmenite with 0.2 g Indonesian coal. The inlet concentration of H2O is 50%. The temperature is 950 8C.
from the reaction of the volatiles, giving a gradual decrease of CO2 and CO from the initial peaks. Fig. 8 shows the oxidation period following immediately after the reducing period shown in Fig. 6. The inlet O2 content was 5%. During the first minutes all incoming oxygen reacts with the reduced metal oxide particles resulting in only inert nitrogen in the outgoing flow. Then the oxygen concentration increases rapidly during the following minutes after which the rise gradually slows down. Since there is no CO2 produced during the first period of oxidation, i.e. the period when all oxygen is consumed, this indicates that there is no carbon left in the bed. To confirm this a few experiments were performed: 0.1 g of petroleum coke was added at the very beginning of the oxidation to a reduced bed. In these cases the added fuel was immediately converted to CO2 showing that if there is char residues in a reduced bed these react with O2 in the very beginning of the oxidation. The CO2 and CO peaks that appear when the O2 concentration increases are most likely due to unburnt carbon which elutriated and stuck to the walls in the top section of the reactor. The oxidation looked very much the same in all cases, independent of fuel, oxygen carrier or other parameters in the previous reduction period. The same batch of 40 g of Fe2O3/MgAl2O4 particles was used for all the different experiments with the coals, with a total of
Fig. 8 – Concentration profile during the oxidation of reduced ilmenite. The inlet concentration of O2 is 5% and the temperature is 950 8C.
30 cycles during approximately 30 h of experiments. The particles were cooled down to room temperature and heated up again several times during this test since the oven had to be turned off at night. A small amount of ash particles was visible in the bed at the end of the tests with Fe2O3/MgAl2O4. Furthermore the same batch of 40 g of ilmenite particles was used in the same way as the synthetic Fe2O3/MgAl2O4 particles. In addition, these particles were exposed to different temperatures and steam concentrations, giving a total of 50 cycles during 45 h with this batch of particles. Just as for the Fe2O3/ MgAl2O4 particles a small amount of ash particles was visible in the bed at the end of these experiments. On one occasion during an experiment at a temperature of 1000 8C the bed stopped fluidizing, but the fluidization started again after a few minutes. Table 2 shows the time needed to reach 95% and 80% conversion for different solid fuels with Fe2O3/MgAl2O4 and ilmenite as oxygen carrier as well as with an inert sand bed. The time it takes to convert the fuel is not dependent on whether synthetic Fe2O3/MgAl2O4 or ilmenite was used as oxygen carrier, thus confirming that the gasification reactions are the rate limiting reactions. However, the time needed for conversion is much longer in the sand bed. Tables 3 and 4 present the measured fraction of unconverted CH4 as a fraction of the total amount of carbon
Table 2 – Time in min for 95% and 80% of conversion for different solid fuels in Fe2O3/MgAl2O4, ilmenite and sand Oxygen carrier
Conversion (%)
Petroleum coke
South Africa
China
Indonesia
Taiwan
S. France Raw
Fe2O3/MgAl2O4 Ilmenite Sand Fe2O3/MgAl2O4 Ilmenite Sand
95 95 95 80 80 80
15.0 14.1 38.0 10.8 10.0 28.7
9.9 10.8 18.8 4.1 5.0 10.7
13.7 13.4 29.9 8.5 8.2 17.6
5.1 4.2 10.3 1.5 1.2 3.2
11.9 9.6 22.9 6.0 4.9 14.4
9.6 10.7 21.3 4.6 4.7 13.1
Sieved 8.5 9.8 16.3 4.0 4.9 8.8
186
international journal of greenhouse gas control 2 (2008) 180–193
Table 3 – Measured fraction of unconverted CH4 during one cycle for different fuels in Fe2O3/MgAl2O4 and the recalculated fractions of unconverted CH4 compensated for the SO2 interference under the assumption that all sulphur in the fuel leaves as SO2 CH4/(CO2 + CO + CH4)
Petroleum coke
South Africa
China
Indonesia
Taiwan
S. France Raw
Meassured Corrected for SO2
0.0429 0.0362
0.0576 0.0566
0.0261 0.0253
0.0697 0.0696
0.0759 0.0753
0.0568 0.0557
Sieved 0.0736 0.0725
Table 4 – Measured fraction of unconverted CH4 during one cycle for different fuels in ilmenite and the recalculated fractions of unconverted CH4 compensated for the SO2 interference under the assumption that all sulphur in the fuel leaves as SO2 CH4/(CO2 + CO + CH4)
Petroleum coke
South Africa
China
Indonesia
Taiwan
S. France Raw
Meassured Corrected for SO2
0.0460 0.0393
0.0720 0.0710
0.0307 0.0299
0.0791 0.0790
0.0844 0.0838
0.0814 0.0804
Sieved 0.0788 0.0777
containing gases during a cycle for different fuels in Fe2O3/ MgAl2O4 and ilmenite, respectively. Also shown in Tables 3 and 4 are the recalculated fractions of unconverted CH4 as a fraction of the total amount of carbon containing gases during a cycle when compensated for the SO2 interference under the assumption that all sulphur in the fuel leaves as SO2. The differences in unconverted CH4 between the different fuels are fairly small, except for the lower fractions for the two fuels with low volatile content petroleum coke and China coal. Fe2O3/MgAl2O4 generally has lower fractions of unconverted CH4 compared to ilmenite. Laboratory tests also show that Fe2O3/MgAl2O4 is more reactive towards CH4 compared to ilmenite. Figs. 9 and 10 show the average rate of reaction as a function of the amount of volatiles for the same set of experiments. The rate of reaction increases with the content of volatiles, the coal from Taiwan being somewhat an
Fig. 10 – Average rate of reaction during the period to reach 80% and 95% conversion for different solid fuels in ilmenite as a function of the amount of volatiles in the different fuels. The fluidizing gas contained 50% H2O and the temperature was 950 8C.
Fig. 9 – Average rate of reaction during the period to reach 80% and 95% conversion for different solid fuels in Fe2O3/ MgAl2O4 as a function of the amount of volatiles in the different fuels. The fluidizing gas contained 50% H2O and the temperature was 950 8C.
exception. Volatiles are, due to the high temperature, rapidly released when the coal particles are introduced in the reactor and can therefore react directly with the metal oxide. The solid char in the fuel takes longer time to convert since it first has to be gasified, for example by steam, before it can react with the metal oxide. In Figs. 11 and 12 the fraction of CO, as calculated in Eq. (7), after 2 min and at full conversion of the fuel is shown as a function of volatiles for different fuels for synthetic Fe2O3/ MgAl2O4 and ilmenite mineral, respectively. Two minutes were chosen because at that time all volatiles had been released for all fuels. Just as for the reaction rates there is no major difference between Fe2O3/MgAl2O4 and ilmenite as
187
international journal of greenhouse gas control 2 (2008) 180–193
Table 5 – Average time needed, in minutes, to reach 95% and 80% conversion of 0.2 g of South African coal for different steam conditions Conversion (%) 95 80
12.5% H2O
25% H2O
21.9 8.8
12.6 6.0
50% H2O 10.8 5.0
Fig. 11 – Fraction of CO after 2 min and at full conversion, Eq. (7), for different fuels in Fe2O3/MgAl2O4 as a function of volatiles in the fuels. The fluidizing gas contained 50% H2O and the temperature was 950 8C.
oxygen carrier even if the fraction of CO seems slightly higher for ilmenite. However it is clear that a high amount of volatiles results in a higher fraction of the fuel released as CO. This may be due to the high fraction of combustible gases released in the top of the reactor for high volatile fuels in the beginning of the cycle. The oxygen carriers do not have sufficient time to convert all these gases before they leave the reactor with the fluidizing gas, giving only partly conversion of high volatile fuels in the beginning of the cycle. As a result, the CO released during the first two minutes accounts for the majority of the total amount of CO released for high volatile fuels. Also the CO
Fig. 13 – Average rate of reaction during the period to reach 80% and 95% conversion for 0.2 g of South African coal in 40 g of ilmenite as a function of the inlet concentration of steam. The temperature was kept constant at 950 8C.
released after 2 min is apparently similar for all of the fuels, around 10%. However, this does not apply to the Indonesian coal which has almost full conversion after 2 min.
4.2.
Fig. 12 – Fraction of CO after 2 min and at full conversion, Eq. (7), for different fuels in ilmenite as a function of volatiles in the fuels. The fluidizing gas contained 50% H2O and the temperature was 950 8C.
Effect of steam content
Table 5 shows the time needed to reach 95% and 80% conversion of 0.2 g of South African coal in 40 g of ilmenite with different steam content, and Fig. 13 shows the average rate of reaction reached at 80% and 95% for the same experiments. These experiments were conducted at 950 8C. There is a clear trend of increasing conversion rate with increasing steam content. With 50% steam it takes roughly 10 min to reach 95% conversion of the fuel, with 12.5% steam the conversion time is more than doubled. It was not possible to go higher than 50% with the present equipment, but the trend of increasing conversion rate is expected to continue for steam contents above 50%. Table 6 presents the measured fraction of unconverted CH4 as a fraction of the total amount of carbon containing gases during a cycle for South African coal in ilmenite for different steam contents in the fluidizing gas. Also shown in Table 6, are the recalculated fractions of unconverted CH4 with compensation for the SO2 interference under the assumption that all sulphur in the fuel leaves as SO2. No significant effect of the steam content on the conversion of CH4 is seen.
188
international journal of greenhouse gas control 2 (2008) 180–193
Table 6 – Measured fraction of unconverted CH4 during one cycle for South African coal in ilmenite for different steam conditions and the recalculated fractions of unconverted CH4 with compensation for the SO2 interference under the assumption that all sulphur in the fuel leaves as SO2 CH4/(CO2 + CO + CH4) Meassured Corrected for SO2
12.5% H2O
25% H2O
0.0675 0.0665
0.0616 0.0606
50% H2O
Table 7 – Average time needed, in minutes, to reach 95% and 80% conversion of 0.2 g of South African coal for different temperature conditions Conversion (%) 95 80
850 8C
950 8C
43.2 25.3
10.8 5.0
1000 8C 5.1 2.4
0.0720 0.0710
In Fig. 14 the fraction of CO, as calculated in Eq. (7), is plotted as a function of total converted carbon for different steam content in the fluidizing gas. There is no visible effect of steam on the CO production in this plot. However, for the first part the slope of the CO curve is somewhat larger. This is when mostly volatiles react with the oxygen carrier which is in agreement with what is seen in Figs. 11 and 12. The oxygen carriers are less efficient at converting the volatiles released, compared to the syngas produced by reaction (2) and (3).
4.3.
Effect of temperature
Table 7 shows the time needed to reach 95% and 80% conversion of 0.2 g of South African coal in 40 g of ilmenite for different temperatures and Fig. 15 shows the average rate of reaction reached at 80% and 95% for the same experiments. It is evident that the reaction rate is strongly temperature dependent. At the higher temperature 1000 8C, some temporary defluidization was noted, but no agglomeration occurred. Table 8 presents the measured fraction of unconverted CH4 as a fraction of the total amount of carbon containing gases during a cycle for South African coal in ilmenite for different temperatures. Also shown in Table 8, are the recalculated fractions of unconverted CH4 as compensate for the SO2 interference under the assumption that all sulphur in the fuel
Fig. 15 – Average rate of reaction to reach 80% and 95% conversion for 0.2 g of South African coal in 40 g of ilmenite as a function of temperature. The fluidizing gas contained 50% H2O.
leaves as SO2. A higher conversion of CH4 can be noted at the highest temperature. In Fig. 16 the fraction of CO, as calculated in Eq. (7), is shown as a function of total converted carbon for different temperatures. It is clear that a higher temperature gives an improved conversion of CO. This is most likely explained by the slower reaction of oxygen-carrying particles at lower temperatures (Mattisson et al., 2006). In Fig. 16, just as in Fig. 14 it is also seen from the slope that a higher fraction of CO is obtained when volatiles are released in the beginning of the cycle.
4.4.
Syngas experiments
In order to investigate the reactivity of the two oxygen carriers with CO and H2, experiments were also made with syngas as
Table 8 – Measured fraction of unconverted CH4 during one cycle for South African coal in ilmenite for different temperatures and the recalculated fractions of unconverted CH4 to compensate for the SO2 interference under the assumption that all sulphur in the fuel leaves as SO2 Fig. 14 – Fraction of CO, Eq. (7), as a function of conversion for 0.2 g of South African coal in 40 g of ilmenite at 950 8C presented for different steam concentrations.
CH4/(CO2 + CO + CH4)
850 8C
950 8C
1000 8C
Meassured Corrected for SO2
0.0686 0.0676
0.0720 0.0710
0.0437 0.0427
international journal of greenhouse gas control 2 (2008) 180–193
Fig. 16 – Fraction of CO, Eq. (7), as a function of conversions for 0.2 g of South African coal in 40 g of ilmenite for different temperatures. The fluidizing gas contained 50% H2O.
189
Fig. 18 – Fraction of CO converted to CO2 as a function mass reduction with previously used ilmenite and Fe2O3/ MgAl2O4 as oxygen-carrying particles. The fuel, also used as fluidizing gas, contained 50% H2 and 50% CO and the temperature was 950 8C.
fuel, i.e. 50% CO and 50% H2. Fresh oxygen carriers and the old ilmenite and Fe2O3/MgAl2O4 particles which were used for the solid fuel experiments were tested. The temperature was 950 8C and the flow of syngas was 450 mLn/min during 100 s. Figs. 17 and 18 shows the reactivity of fresh and used oxygen carriers with syngas, with the fraction of CO converted to CO2 as a function of the mass based degree of conversion of the oxygen carrier. The mass-based degree of conversion is defined as the mass of the oxygen carrier divided with the mass of the oxygen carrier in its most oxidized state. As can be seen, both fresh and used particles display a high reactivity
Fig. 19 – Fraction of CO at full conversion, Eq. (7), for different fuels in sand as a function of volatiles in the fuels. The fluidizing gas contained 50% H2O and the temperature was 950 8C.
with almost complete yield of the syngas to CO2 and H2O. Fe2O3/MgAl2O4 shows no change, or maybe a small increase, in the conversion of CO and ilmenite shows a small increase in reactivity.
4.5. Fig. 17 – Part CO converted to CO2 as a function mass reduction with fresh ilmenite and Fe2O3/MgAl2O4 used as oxygen-carrying particles. The fuel, also used as fluidizing gas, contained 50% H2 and 50% CO and the temperature was 950 8C.
Sand experiments
To investigate gasification of the different fuels the oxygen carrying particles were replaced by quartz sand. The temperature was 950 8C and a flow of 50% steam and 50% nitrogen was used as fluidization gas when 0.2 g of fuel was introduced
190
international journal of greenhouse gas control 2 (2008) 180–193
Fig. 20 – Average time of reaction to reach 95% conversion for different fuels in ilmenite and Fe2O2/MgAl2O4 as a function of the average time of reaction to reach 95% conversion in sand. The fluidizing gas contained 50% H2O and the temperature was 950 8C.
Fig. 22 – SEM images of Fe2O3/MgAl2O4 used in solid fuel experiments.
to the bed. The fuel slowly reacts with the steam forming CO and H2, however only the amount of CO could be measured. Some of this CO reacts further with the steam forming CO2, and more H2 giving a fraction of CO in the range of 60–70% as this is shown in Fig. 19. The fraction of CO does not differ significantly for the different fuels. The time it need to convert 95% of a fuel with an oxygen carrier is about a half of the time to convert the same fuel in sand, Fig. 20. One difference between CLC and gasification is the concentrations of H2 and CO, which are lower in a CLC reactor, as the oxygen carrier reacts with CO and H2. H2 is known to inhibit the gasification reaction (Barrio et al., 2000; Johnstone et al., 1952; Weeda et al., 1993).
Fig. 23 – SEM images of fresh Fe2O3/MgAl2O4.
4.6.
Fig. 21 – SEM images of fresh Fe2O3/MgAl2O4.
Scanning electron microscope (SEM) images
The surface of both fresh and used oxygen carriers were analysed in a scanning electron microscope, SEM. In Figs. 21– 24 the SEM images are shown in two different magnifications for fresh and used Fe2O3/MgAl2O3 particles. The used particles seem to have a somewhat rougher surface. The BET area of the reacted particles, as determined by a Micromeritics Gemini 2362, was only 0.76 m2/g as compared to the BET area of 8.58 for fresh Fe2O3/MgAl2O3 particles. Thus it is quite clear that the interior particle structure of these particles seems to sinter during testing. As this effect has been noticed also during batch experiments with gaseous fuels, it is likely not due to the solid fuel ash (Johansson et al., 2004). Interestingly, the particles maintain a high-reactivity even after the particle restructuring, see Fig. 18.
international journal of greenhouse gas control 2 (2008) 180–193
Fig. 24 – SEM images of Fe2O3/MgAl2O4 used in solid fuel experiments.
191
Fig. 26 – SEM images of ilmenite used in solid fuel experiments.
In Figs. 25–28 the SEM images are shown in to different magnifications for fresh and used ilmenite particles. It is clear that the used particles have much smoother edges and rougher surfaces that the fresh ilmenite particles. Also the BET area of the used ilmenite increased to 0.58 for the used particles as compared to 0.11 for fresh ilmenite. The increase BET area likely means that the particles are gaining porosity during the reduction/oxidation experiments.
5.
Discussion
This work shows that a number of solid fuels can be used directly in chemical-looping combustion. As shown in Figs. 17 and 18, both the synthetic Fe2O3/MgAl2O3 particles
Fig. 27 – SEM images of fresh ilmenite.
Fig. 25 – SEM images of fresh ilmenite.
and the natural ilmenite mineral react fast with syngas. The intermediate gasification rates are dependent on the type of solid fuel that is used, e.g. Fig. 20. Although the presence of an oxygen carrier doubles the conversion rate, the fuel gasification is still the time limiting step. However, a higher reaction temperature or a higher fraction of steam in the fluidizing gas will increase the rate of conversion, Figs. 13 and 15. The slow reaction means that a high solids inventory may be necessary in the fuel reactor of a real system. Leion et al. (2007) have estimated the needed solid inventory in a CLC system, when synthetic Fe2O3/MgAl2O3 particles where used as oxygen carrier and petroleum coke as fuel, to 2000 kg/MW. It is then assumed a temperature difference of 50 8C between the reactors of the CLC system, due to the endothermic reaction in the fuel reactor, a solid flow of 380 kg/MWth, minute of Fe2O3/MgAl2O3 particles and a
192
international journal of greenhouse gas control 2 (2008) 180–193
6.
Conclusions
The results show that the CLC-technique can be used for a number of different solid fuels with different composition. Two different oxygen carriers were used, one natural ilmenite mineral and one synthetic Fe2O3/MgAl2O4 particles:
Fig. 28 – SEM images of ilmenite used in solid fuel experiments.
needed residence time of five minutes to convert the petroleum coke. If a more reactive coal with high content of volatiles is used, such as the South African or Indonesian coal in this work, the residence time will be shorter and the needed solids inventory less. The actual residence time will depend, not only on fuel, but also on reaction temperature, steam content in the fluidizing gas, reactor design and needed CO2 capture efficiency. When solid fuels are burned, ash is produced. In a CLC system this ash has to be separated from the oxygen carrying particles. This separation can be accomplished by feeding small fuel particles to the system. During conversion the fuel particles will produce even smaller ash particles, small enough to be elutriated with the fluidizing gas and separated in a cyclone. Nevertheless it cannot be avoided that some of the oxygen carrying particles are lost with the ash. Especially particles those due to breakage and fragmentation are so small that they have the same fluidizing properties as the ash. The particles may also suffer from reactivity loss due to reaction with ash components, although the current experiments saw no such deactivation. This means that the oxygen-carrying particles used with solid fuel in a CLC-boiler may have to be replaced fairly often. The fact that similar rates of reaction were seen with the natural ilmenite and the synthetically produced iron oxide particles would clearly indicate that the more inexpensive ilmenite would be more feasible to use in a real system, where perhaps the oxygen carrier may need to be replaced more often than in a gas fired system. In these tests, the same sample of ilmenite was used for over 45 h. During this time the particles were heated up to 950 8C and cooled down again to room temperature several times since the oven had to be turned off at night. The particles were also exposed to ash and combustion gases from the different fuels. Still the particles showed good fluidization properties throughout all experiments and the particles did not in any way lose in reactivity.
The natural mineral ilmenite reacts just as well with all investigated fuels as the synthetic Fe2O3/MgAl2O4 particles. The gasification of the fuel about two times faster when conducted in the presence of oxygen-carrying particles. This is believed to be explained by efficient removal of gases like H2 that is known to have an inhibiting effect on gasification. The oxygen carriers, Fe2O3/MgAl2O4 and ilmenite, react fast with intermediate gasification products such as CO and H2. The gasification reaction is slow compared to the reaction of the gasified components with the metal oxide, thus gasification is the time limiting step. The rate of conversion together with oxygen carrier particles was determined for a range of fuels with highly different volatiles content. The conversion rate of South African coal increased significantly with increased fraction of steam in the fluidizing gas as well as temperature. No agglomeration could be seen for the synthetic Fe2O3/ MgAl2O4 particles and only minor agglomeration for the ilmenite mineral was seen. This occurred most likely during a period of defludization at high temperature, 1000 8C. At 950 8C and with 50% steam the time needed to achieve 95% conversion of fuel particles with a diameter of 0.125– 0.18 mm ranged between 4 and 15 min depending on the fuel, while 80% conversion was reached within 2–10 min.
Acknowledgment This work was made in the EU financed research project Enhanced Capture of CO2 (ENCAP), SES6-2004-502666.
references
Adanez, J., de Diego, L.F., Garcia-Labiano, F., Gayan, P., Abad, A., Palacios, J.M., 2004. Selection of oxygen carriers for chemical-looping combustion. Energy Fuels 18 (2), 371–377. Adanez, J., Garcia-Labiano, F., de Diego, L.F., Gayan, P., Abad, A., Celaya, J., 2005. Development of oxygen carriers for chemical-looping combustion. Carbon Dioxide Capture for Storage in Deep Geologic Formations—Results from the CO2 Capture Project, vol. 1. pp. 587–604. Arrhenius, S., 1886. On the influence of carbonic acid in the air upon the temperature of the ground. Philos. Mag. 41, 237–277. Barrio, M., Gøbel, B., Risnes, H., Henriksen, U., Hustad, J.E., Sørensen, L.H., 2000. Steam gasification of wood char and the effect of hydrogen inhibition on the chemical kinetics. Prog. Thermochem. Biomass Conver. 1, 32–46. Cao, Y., Pan, W.-P., 2006. Investigation of chemical looping combustion by solid fuels. 1. Process Analysis. Energy Fuels 20 (5), 1836–1844. Cao, Y., Casenas, B., Pan, W.-P., 2006. Investigation of chemical looping combustion by solid fuels. 2. Redox reaction
international journal of greenhouse gas control 2 (2008) 180–193
kinetics and product characterization with coal, biomass, and solid waste as solid fuels and CuO as an oxygen carrier. Energy Fuels 20 (5), 1845–1854. Cao, Y., Liu, K., Riley, J.T., Pan, W.-P., 2004. Application of a circulating fluidized bed process for the chemical looping combustion of solid fuels. Preprints of Symposia—American Chemical Society. Div. Fuel Chem. 49 (2), 815–816. Cho, P., Mattisson, T., Lyngfelt, A., 2004. Comparison of iron-, nickel-, copper- and manganese-based oxygen carriers for chemical-looping combustion. Fuel 83 (9), 1215–1225. Cho, P., Mattisson, T., Lyngfelt, A., 2005. Carbon formation on nickel and iron oxide-containing oxygen carriers for chemical-looping combustion. Indus. Eng. Chem. Res. 44 (4), 668–676. Cho, P., Mattisson, T., Lyngfelt, A., 2006. Defluidization conditions for a fluidized bed of iron oxide-, nickel oxide-, and manganese oxide-containing oxygen carriers for chemical-looping combustion. Indus. Eng. Chem. Res. 45 (3), 968–977. Dennis, J.S., Scott, S.A., Hayhurst, A.N., 2006. In situ gasification of coal using steam with chemical looping: a technique for isolating CO2 from burning a solid fuel. J. Energy Inst. 79 (3), 187–190. Ishida, M., Jin, H., 1994. A new advanced power-generation system using chemical-looping combustion. Energy 19 (4), 415–422 (Oxford, United Kingdom). Ishida, M., Jin, H., Okamoto, T., 1996. A Fundamental study of a new kind of medium material for chemical-looping combustion. Energy Fuels 10 (4), 958–963. Jerndal, E., 2005. Thermal analysis of chemical-looping combustion. Div. Energy Technol.. Johansson, M., Mattisson, T., Lyngfelt, A., 2004. Investigation of Fe2O3 with MgAl2O4 for chemical-looping combustion. Indus. Eng. Chem. Res. 43 (22), 6978–6987.
193
Johnstone, H., Chen, C., Scott, D., 1952. Kinetics of the steamcarbon reaction in porous graphite tubes. Indus. Eng. Chem. 44, 1564–1569. Leion, H., Mattisson, T., Lyngfelt, A., 2007. The use of petroleum coke as fuel in chemical-looping combustion. Fuel 86 (1213), 1947–1958. Lyngfelt, A., Thunman, H., 2005. Construction and 100 h of operational experience of a 10-kW chemical-looping combustor. Carbon Dioxide Capture for Storage in Deep Geologic Formations—Results from the CO2 Capture Project, vol. 1. pp. 625–645. Lyngfelt, A., Leckner, B., Mattisson, T., 2001. A fluidized-bed combustion process with inherent CO2 separation; application of chemical-looping combustion. Chem. Eng. Sci. 56 (10), 3101–3113. Lyon, R.K., Cole, J.A., 2000. Unmixed combustion: an alternative to fire. Combust. Flame 121, 249–261 (1/2). Mattisson, T., Johansson, M., Lyngfelt, A., 2004. Multicycle reduction. Oxidation of different types of iron oxide particles-application to chemical-looping combustion. Energy Fuels 18 (3), 628–637. Mattisson, T., Johansson, M., Lyngfelt, A., 2006. CO2 capture from coal combustion using chemical-looping combustion—reactivity investigation of Fe Ni and Mn based oxygen carriers using syngas. Clearwater Coal Conf.. Scott, S.A., Dennis, J.S., Hayhurst, A.N., Brown, T., 2006. In situ gasification of a solid fuel and CO2 separation using chemical looping. AICHE J. 52 (9), 3325–3328. Weeda, M., Abcouwer, H., Kapteijn, F., Moulijn, J., 1993. Steam gasification kinetics and burn-off behavioiur for a bituminous coal derived char in the presence of H2. Fuel Process. Technol. 36, 235–242. Zhang, G., Ostrovski, O., 2002. Effect of preoxidation and sintering on properties of ilmenite concentrates. Int. J. Miner. Process. 64, 201–218.