ARTICLE pubs.acs.org/EF
Influence of Lime Addition to Ilmenite in Chemical-Looping Combustion (CLC) with Solid Fuels Guillaume Teyssie,† Henrik Leion,*,‡ Georg L. Schwebel,§ Anders Lyngfelt,|| and Tobias Mattisson|| †
Departement Science et Genie des Materiaux, Institut National des Sciences Appliquees (INSA), F-69100 Lyon, France Department of Environmental Inorganic Chemistry, Chalmers University of Technology, S-412 96 G€oteborg, Sweden § Institut f€ur Energietechnik, Universit€at Siegen, Paul-Bonatz-Strasse 9-11, D-57068 Siegen, Germany Department of Energy and Environment, Chalmers University of Technology, S-412 96 G€oteborg, Sweden
)
‡
ABSTRACT: The influence of calcined and sulfated limestone addition in an oxygen carrier bed of ilmenite has been investigated for chemical-looping combustion (CLC) with solid fuel. The experiments have been performed in a laboratory-batch fluidized-bed reactor where the sample was exposed to alternative oxidizing and reducing conditions at atmospheric pressure and 970 °C. In each reducing period, 0.2 g of petroleum coke was introduced to a bed of 20 g of ilmenite and 5 g of limestone particles (180250 μm). The limestone was calcined and sulfated to different levels (0, 25, or 40%) and mixed with the ilmenite particles (125180 μm). During the reducing period, the mixture was fluidized with a flow of 600 mLn/min of 50% steam in nitrogen, and during the oxidation period, the mixture was fluidized with a flow of 600 mLn/min of 5% O2 in nitrogen. Mixing CaO with ilmenite increased the conversion of the gas as well as the conversion rate of the char. A part of the sulfur released from the fuel reacted with the lime to form CaS/CaSO4. Concerning the mixture of CaSO4/ilmenite, a further improvement was seen on the char conversion because of the release of SO2 from the sulfated limestone particle. However, this beneficial effect disappeared after 10 cycles. Thus, the tests did not show any lasting positive effect from using a mixture of CaSO4/ilmenite compared to CaO/ilmenite. The reason for the improved gas conversion with the addition of CaO to ilmenite seems to be the catalytic effect of CaO on the watergas shift reaction, converting CO to H2, with the latter being much more reactive toward ilmenite.
1. INTRODUCTION Today, the increase in emission of anthropogenic greenhouse gases (GHGs) to the atmosphere has significant and decisive consequences on the global energy balance of the earth. This gives a rise to an increase in the earth surface temperature, thereby causing climate change.1 The CO2 concentration in the atmosphere has increased from a pre-industrial value of 280 to 387 ppm in 2010. In 2004, 80% of the worldwide energy supply was dependent upon fossil fuel.2 Significant reductions in the emission of GHGs, especially CO2, are suggested to limit the increase of the average global temperature to +2 °C compared to the preindustrial level.3 These reductions could be achieved by a portfolio of measures, i.e., renewable energy, increased energy efficiency, nuclear power, etc. However, because the use of fossil fuel energy is not likely to be stopped in the next few decades, technical solutions that allow for a continued use of fossil fuels without substantial CO2 emissions would be useful. Carbon capture and sequestration (CCS) technologies can allow for CO2-free fossil fuel electricity production and may provide a transition between the present society highly dependent upon fossil fuels and the future society less dependent upon fossil fuels.3 A promising combustion technology without an energy-consuming gas-separation process for CO2 is chemical-looping combustion.4 1.1. Chemical-Looping Combustion (CLC). CLC is a combustion technology where CO2 is separated from the rest of the flue gases as part of the process, without an energy-consuming gas-separation process. The ideas of CLC emerged from a CO2 production patent by Lewis and Gilliland in 1954.5 An equivalent concept was proposed for power production in 1983 by Richter r 2011 American Chemical Society
and Knoche.6 Later, Ishida et al. suggested that CLC could be used as a technology to separate CO2 from the rest of the flue gases in a power plant.4 CLC is accomplished by performing the combustion in two interconnected reactors, one with a continuous inlet of air and the other with a continuous feed of fuel. Any form of fuel is feasible, liquid, solid, or gas. The technology requires an oxygen carrier, generally in the form of metal oxide particles, with the purpose of transferring oxygen from the combustion air to the fuel. Because conversion of the fuel and oxidation with air in CLC takes place in separate reactors, direct contact between fuel and air is avoided and a high-purity CO2 stream is obtained from the fuel reactor after condensation of water vapor. The outlet stream from the air reactor will contain nitrogen with some oxygen, which can safely be emitted to the atmosphere. The oxygen carriers are circulated between the two reactors, constantly providing new oxygen for the fuel conversion in the fuel reactor. In the air reactor, the reduced metal oxide is oxidized by air according to reaction 1, and in the fuel reactor, it is reduced, according to reaction 2. O2 þ 2Mex Oy1 S 2Mex Oy
ð1Þ
Cn H2m þ ð2n þ mÞMex Oy S nCO2 þ mH2 O þ ð2n þ mÞMex Oy1
ð2Þ
Received: April 20, 2011 Revised: July 7, 2011 Published: July 07, 2011 3843
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Reaction 1 is strongly exothermic, while reaction 2 could be either exothermic or endothermic depending upon the fuel used and the characteristics of the oxygen carrier. However, the sum of the energy released in these two reactions is always the same as the heat released in conventional combustion. Hence, no extra energy is needed for the CO2 separation. In addition to this, CLC minimizes the formation of NOx because the oxidation of the fuel occurs in a nitrogen-free environment and the temperature in the air reactor is generally too low to generate any thermal NOx. The only potential source of nitrogen oxides is from nitrogen in the fuel, and any nitrogen oxides thus formed can be treated with conventional techniques before or after the CO2 compression. Apart from nitrogen, the fuel can also contain other compounds, such as sulfur, which are released during the fuel conversion process. Because of these gases, the concentration of CO2 after the water condensation will not reach 100%. However, the cleaning of these gases from a CLC fuel reactor could be easier than the cleaning from a conventional boiler because the gas flow out of the fuel reactor is much smaller and the concentration is much higher. The sulfuric compounds are released during the fuel conversion as H2S or SO27 and can have a positive effect on fuel gasification in CLC.8 The further fate of these species in a CLC fuel reactor may depend upon the oxygen carrier used. They may be partially oxidized to SO2 by the oxidant, such as H2O and CO2, or by the metal oxide in parallel to the fuel conversion.9 There is also a risk that they react with the oxygen carrier to solid sulfides and follow the particle stream to the air reactor, such as when a Ni-based oxygen carrier is used.10 1.2. CLC in Solid Fuel Application. Different ways of performing CLC with solid fuels have been proposed. One is when the coal is used in conventional gasification to form syngas products, i.e., CO and H2. This gas can then be converted in CLC.11 However, to avoid dilution of the syngas, the gasification process needs to be performed under an O2 atmosphere and air separation and gasification units are needed, which consequently will increase the cost and complexity of the process. Another option is to insert the solid fuel directly into the CLC fuel reactor, where the oxygen carrier becomes reduced by the fuel.12 This permits a one-step fuel oxidation, avoiding the extra costs and the need of an air separation unit. The volatiles released from the fuel may react according to reaction 2, but oxidation of the remaining char is more complex because the rate of a solidsolid reaction between the oxygen carrier particle and the coal would be negligible.12 The char therefore needs to be gasified in the fuel reactor, and then, the syngas produced may react with the metal oxide particles. Thus, the rate of the char gasification will be the limiting step of char conversion.12 When the fuel is directly introduced to the CLC fuel reactor, there is no need for an external gasifier. Also, gasification in a CLC fuel reactor is faster than conventional gasification because the oxygen carriers effectively consume hydrogen, which is a known inhibitor of gasification.13,14 When the fuel is introduced to the fuel reactor, it will first release its volatiles. These volatiles will react directly with the oxygen carrier. Second, the char fraction of the fuel will be gasified to H2 and CO by steam or CO2 according to reactions 3 and 4. To simplify, the char is assumed here to be carbon. C þ H2 O w CO þ H2
ð3Þ
C þ CO2 w 2CO
ð4Þ
Oxygen can also be transferred within the gas phase by the COshift reaction. CO þ H2 O S CO2 þ H2
ð5Þ
Finally, the syngas products, CO and H2, can react with the oxygen carrier according to Mex Oy þ H2 w Mex Oy1 þ H2 O
ð6Þ
Mex Oy þ CO w Mex Oy1 þ CO2
ð7Þ
1.3. Oxygen Carrier. Oxygen carrier particles are considered to be the key component in the CLC technique. Several essential properties are required for a suitable oxygen carrier9,15 in a circulating system of interconnected fluidized beds, for example, high ability to convert the fuel to H2O and CO2, high reactivity with the fuel in the fuel reactor and with oxygen in the air reactor, low fragmentation and attrition of particles, low tendency to agglomerate, reasonable cost, and low environmental impact. CLC with solid fuel adds some new demands upon oxygen carrier properties.16 The fuel ash may deactivate and decrease the reactivity of the oxygen carrier. The reactivity toward CH4 is less important, as long as the fuel does not contain a high fraction of methane-containing volatiles, but the reactivity toward syngas products, i.e., CO and H2, should be high.16 Also, in CLC with solid fuel, the particle lifetime could be affected by the presence of ash as well as losses in ash separation. Therefore, low-cost materials that have a sufficient reactivity toward H2 and CO can be presumed to be good oxygen carriers in CLC with solid fuel. Ilmenite is such an oxygen carrier that previously has been investigated with both gaseous and solid fuels.1720 Using the system CaSO4/CaS as an oxygen carrier involves the oxidation of CaS in the air reactor
CaS þ 2O2 w CaSO4
ð8Þ
A possible side reaction is CaS þ 3=2O2 w CaO þ SO2
ð9Þ
Generally, under the highly oxidizing conditions prevailing in the air reactor, reaction 8 is thermodynamically favored and the loss of sulfur from the system because of reaction 9 should be small. The situation in the fuel reactor is more complicated. When CaSO4 enters a fuel reactor and is exposed to combustible gases, e.g., CO/H2, two reactions are possible. CaSO4 þ 3CO=3H2 w CaS þ 3CO2 =3H2 O
ð10Þ
CaSO4 þ CO=H2 w CaO þ SO2 þ CO2 =H2 O
ð11Þ
Both reactions involve the oxidation of CO/H2, but reaction 11 is detrimental because the loss of sulfur essentially means the loss of the oxygen carrier. The phase diagram (Figure 1) shows the stability regions of CaS and CaSO4 as a function of the partial pressures of SO2 and oxygen. The corresponding CO/CO2 ratio for 850 °C is also shown on the upper x axis. The phase diagram shows that, in between the regions where CaS and CaSO4 are stable, there is a region where CaO is stable. This CaO region rapidly increases in size as the temperature is increased. Generally, studies of the CaSO4/CaS system under conditions shifting between oxidizing and reducing show a release of SO2 in connection with the shift from oxidizing to reducing and vice versa.7,2123 If fuel gas is in contact with an excess of CaSO4, the 3844
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Figure 2. Fluidized-bed reactor made of quartz. Figure 1. Phase diagram showing the stability regions of CaS, CaSO4, and CaO.
reducing gas will be oxidized until it meets the thermodynamic limit of the reaction. Reaction 10 can theoretically convert CO to CO2 up to 98.3% at 1000 °C and H2 to 99%. At this point, the SO2 concentration is also at a maximum. The equilibrium for this maximum SO2 concentration is given by the equilibrium of 3=4CaSO4 þ 1=4CaS S CaO þ SO2
ð12Þ
For this reaction, the equilibrium concentration of SO2 is 10% at 1000 °C and 4% at 950 °C. Thus, it would generally be expected that CaO is formed and gaseous sulfur is released in the fuel reactor. However, many solid fuels have high sulfur contents, corresponding to rather high concentrations in a fuel reactor, and the conditions also vary considerably locally in the fuel reactor. Therefore, it may be difficult to assess the behavior in real conditions, especially in the presence of an additional oxygen carrier, as investigated in this work. Because limestone is used for both SO2 removal and in the form of CaSO4/CaS as an oxygen carrier,24 the purpose of the present work is to investigate the influence of CaSO4/limestone addition to ilmenite on solid fuel conversion and SO2 capture in CLC. Samples of 5 g of limestone sulfated at different percentages, i.e., 0, 25, or 40%, have been added to a 20 g bed of ilmenite. For each specific mixture of sulfated limestone and ilmenite, alternative oxidizing and reducing conditions in a circulating fluidized bed have been carried out. A sufficient number of cycles has been conducted to make sure that a constant reactivity of fuel and ilmenite has been reached.
2. EXPERIMENTAL SECTION 2.1. Laboratory Setup. The experiments were performed in a system that was built with specific features, making it suitable for CLC with solid fuels. All experiments were conducted with a gas flow of 600 mLn/min (normalized to 1 bar and 25 °C). The temperature was 970 °C, and when steam was used, the inlet concentration of H2O was 50%. It was possible to establish whether the bed was fluidized or not by high-frequency measurements of the pressure drop over the bed.16 The system used for these CLC experiments has been used in previous studies.12,13,16 All of the experiments were conducted with a fluidized-bed reactor of quartz presented in Figure 2. To achieve good mixing of solid in the bed,
the reactor is conically shaped just above the distributor porous plate. The reactor has a total length of 870 mm. For experiments with CO and H2 as fuel, a straight reactor of the same length and with a constant inner diameter of 22 mm was used. A sample of oxygen carrier particles is placed on the porous plate and then initially heated to the reaction temperature in air. The bed is then alternatingly exposed to an oxidation flow of O2 in N2 and to the fuel/ steam mixture, thus simulating the cyclic conditions of a CLC system with alternating oxidizing and reducing conditions. Nitrogen gas was introduced during an inert period of 180 s after each oxidizing and reducing period to purge the system from the gases of the previous phase. During the reducing period, the fluidizing gas was normally a mixture of steam and nitrogen, which was introduced from the bottom of the reactor. At the same time as the fluidizing gas of the reducing cycle entered the bottom of the reactor, solid fuel was inserted through the valve in the top of the reactor, falling down into the fluidized bed. Also, a small flow of nitrogen was added at the top of the reactor, partly to help sweep the fuel down into the reactor and partly to serve as additional carrier gas downstream in the system. The flow of sweep gas was equal to the flow of added steam. Hence, because the steam was condensed before the analyzer, the concentration presented below is very close to the actual concentrations in the reactor with steam substituted for nitrogen. For experiments with CO and H2 as fuel, all of the gases were introduced from the bottom of the reactor. Downstream of the reactor was an electric cooler where the steam was condensed. The gas from the cooler led to a gas analyzer (Rosemount NGA-2000), where the concentrations of CO2, CO, CH4, O2, H2, and SO2 are measured in addition to the gas flow. However, H2 and SO2 could not be measured at the same time. The steam was generated by a steam generator (Cellkraft, Precision Evaporator E-1000). The temperature was measured 5 mm under and 10 mm above the porous plate. It was performed with Pentronic CrAL/NiAl thermocouples enclosed in a quartz shell. The temperature measurement had an accuracy of (5 °C. All temperatures in this paper refer to the upper measurement in the bed of particles. 2.2. Preparation of Oxygen Carriers. In this work, the oxygen carrier used was pure ilmenite, a mixture of activated ilmenite and calcined limestone, or a mixture of activated ilmenite and sulfated limestone. A sample of 20 g of pure ilmenite (125180 μm) was used. In the experiments with limestone, 5 g of calcined or sulfated limestone (180250 μm) was added to the 20 g of ilmenite used in the previous experiments. Ilmenite is an irontitanium mineral, FeTiO3, which frequently has been used as an oxygen carrier in CLC.1720 When used as an oxygen carrier, ilmenite is the most reduced form and the oxidized form is a 3845
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Table 1. Analyses of Petroleum Coke proximate Hi
M
A
ultimate combustibles
VM
C
H
N
S
O
(MJ/kg, as received) (wt %, as received) (wt %, as received) (wt %, as received) (wt %, daf) (wt %, daf) (wt %, daf) (wt %, daf) (wt %, daf) (wt %, daf) 30.9
8.0
0.5
91.5
mixture of pseudobrookite (Fe2TiO5) and rutile (TiO2).18 Ilmenite is the most abundant of all titanium minerals and mined in large quantities. Calcined limestone is obtained by thermal decomposition of limestone, a natural mineral consisting of calcium carbonate (CaCO3), whereby carbon dioxide is released to yield CaO. The ilmenite and calcined limestone were sieved before the experiments to obtain a suitable particle size for fluidization. Because the density of ilmenite particles (4580 kg/m3 for fresh ilmenite and 4250 kg/m3 for activated ilmenite17) is higher than the density of limestone (17602160 kg/m3), it was decided to use different particle size distributions for the two materials. The ilmenite particle size was chosen to be between 125 and 180 μm, and for the limestone particles, the size range of 180250 μm was chosen. To ensure that the given limestone particles were fully calcined before the solid fuel experiment, the limestone was heat-treated well above the CaCO3/CaO phase-transition temperature in a flow of nitrogen until all of the CO2 release in the outlet gas disappeared. The calcination reaction is described by reaction 13. CaCO3 w CaO þ CO2
ð13Þ 7
Sulfation experiments were inspired from Mattisson et al. Calcined limestone particles were heated in fluidized-bed conditions with an inert gas flow of nitrogen, i.e., 900 mLn/min, from room temperature to the reaction temperature of 850 °C. When these conditions were obtained, the calcined limestone particles were exposed to a reactant gas mixture, made of 8000 ppm SO2 and 4% O2. Because the outgoing flow and concentration of SO2 were measured, the time needed for a certain degree of sulfation could be calculated. New samples were then prepared but with a fixed exposer time to SO2 that yield the desired level of sulfation based on the degree of sulfation, i.e., the ratio of moles of CaSO4 to the total moles of Ca desired to form sulfate according to reaction 14. CaO þ SO2 þ 1=2O2 w CaSO4
to obtain particles in the size range of 125180 μm. Proximate and ultimate analyses of the selected fuel are presented in Table 1. Note the high sulfur content of 6.6% in this petroleum coke. 2.4. Data Evaluation. The main parameters used to quantify and qualify the experiments are described in the section below. Every 2 s, the analyzer measures and records the value of CO2, CO, CH4, and SO2 or H2 fractions in the outgoing gas after the water has been removed. Unfortunately SO2 and H2 could not be measured at the same time. Therefore, for the cycles where H2 was measured, additional cycles were performed at the same conditions but with measurement of SO2. From the integration of the flow of carbon-containing gases from the outlet, the total amount of carbon released in each cycle can be calculated. The rate of conversion12 was calculated from 1 mt mtot t
ð15Þ
Here, 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 until time t.
88.8
3.1
1.0
6.6
0.5
Figure 3. Concentration profile during the reduction of 20 g of ilmenite with 0.2 g of petroleum coke. The inlet concentration of H2O is 50%. The temperature is 970 °C. The rate of conversion used in this work is the average rate of carbon converted during the time period needed to reach 95% fuel conversion.12 Because the flow of steam from the steam generator fluctuates, the steam content, calculated from the amount of water condensed in the cooler, is presented for each cycle to facilitate a comparison to similar experiments. To characterize the amount of gas converted to CO2, the gas yield (γCO2) is used, determined as the ratio between the CO2 fraction in the outlet gas and the total carbon content in the outgoing gas, and hence, a γCO2 of 100% represents total conversion of the reactive carbon-containing gases. γCO2 ¼
ð14Þ
2.3. Solid Fuels. The fuel used in this work was crushed and sieved
rave ¼
10.9
XCO2 XCO2 þ XCO þ XCH4 100 ð%Þ
ð16Þ
In the same manner, fCO is the fraction of CO in the outlet gas. fCO ¼
XCO XCO2 þ XCO þ XCH4 100 ð%Þ
ð17Þ
fSO2 is the ratio of SO2 and carbon in the outlet gas stream. fSO2 ¼
XCO2 þ XCO
XSO2 þ XCH4 100 ð%Þ
ð18Þ
Finally, fH2 is the ratio of H2 and carbon in the outlet gas stream. fH2 ¼
XH2 XCO2 þ XCO þ XCH4 100 ð%Þ
ð19Þ
3. RESULTS FROM LABORATORY EXPERIMENTS 3.1. Petroleum Coke Conversion with Pure Ilmenite as an Oxygen Carrier. A sample of 20 g of pure ilmenite (125180
μm) is used as an oxygen carrier in a bed fluidized with 50% of 3846
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Table 3. Experiments with 0.2 g of Coke, 20 g of Pure Ilmenite, and 5 g of Calcinated Limestone steam
conversion
content
rate “r”
(%)
(min1)
1
54.9
8.4
11.3
6.1
92.5
0.7
0.140
2
51.6
7.7
11.8
7.9
90.6
0.9
0.139
3
45.4
7.8
12.1
7.7
90
0.8
0.149
4
44
7.9
12.1
7.9
89.6
0.7
0.151
5
44
7.6
12.5
7.5
90
0.8
0.147
6
47.5
7.3
13
8.1
89
0.7
7
49.8
8.1
11.7
9.2
89.2
1.08
0.161
8 9
46.5 46.2
7.5 7.7
12.6 12.3
7.6 7.7
91.1 90.9
1.37 1.17
0.153 0.164
10
47.2
7.7
12.3
7.3
91.3
1.11
0.168
11
48
7.5
12.6
6.4
92
0.98
0.166
12
47.2
7.5
12.7
6
92.5
0.99
0.171
average
47.7
7.7
12.2
7.4
90.7
1.12
0.156
cycle
Figure 4. Concentration profile during the oxidation of 20 g of reduced ilmenite. The inlet concentration of O2 is 5%. The temperature is 970 °C.
Table 2. Experiments with 0.2 g of Coke and 20 g of Pure Ilmenite total steam content
rate “r”
(%)
(min1)
1
48.5
6.8
2
49.8
6.6
3 4
46.7 48.3
5 6 average
cycle
carbon
conversion t95%
fCO γCO2 fSO2
(min) (%)
f H2
content
(%)
(%) (%)
(g)
14.0
14.8 83.7
1.1 1.90
0.165
14.4
14.3 84.1
1.2 1.85
0.166
6.6 6.6
14.5 14.4
14.9 83.8 15.2 83.4
1.1 1.87 1.89
0.170 0.164
53.9
7.2
13.2
16.1 82.3
0.9
0.147
54.6
7.0
13.6
15.2 83.2
0.8
0.144
50.3
6.8
14.0
15.1 83.4
1.0 1.88
0.159
steam in nitrogen at a temperature of 970 °C. A mass of 0.2 g of petroleum coke is fed to the reactor in each cycle. The experiment with pure ilmenite as an oxygen carrier will be used as a reference in this study of the influence of limestone addition on fuel conversion. Six cycles were made to calculate an average. Figures 3 and 4 show the outlet gas concentration after water condensation in the reduction and oxidation periods as function of time, respectively. During the reduction period, the initial peaks are mainly due to the release of volatiles from the petroleum coke and the reaction of these with the ilmenite. After this, the gasification of the fuel char takes places. The char in the bed reacts with the injected steam to form syngas, i.e., CO and H2, which reacts further with the ilmenite to form CO2 and H2O. In this case, the maximum concentration of CO2 reaches 3.1% and the maximum concentration of CO reaches 1.8%. After approximately 20 min, all of the added fuel has been converted. CO out of the system can partly be related to the mixing of solids in the reactor.12 When the char particles and ilmenite are well-mixed in the reactor, the gasification of char in the upper part of the bed will release syngas, i.e., CO and H2, which will not have sufficient time to react with the oxygen carriers and, therefore, leaves the reactor unconverted. It can be assumed that H2S9 released from the fuel will be partially oxidized to SO2 by the oxide metal and oxidant compounds, such as H2O and CO2. In this case, the maximum
total t95% γCO γCO2 γSO2 (min) (%) (%)
(%)
0.8
γ H2
carbon
(%)
content
0.152
peak of SO2 reaches 0.7% and the ratio of SO2/carbon (eq 18) arrives at 1.1%. The sulfur/carbon molar ratio of the fuel is around 3%. Thus, a major part of the released sulfur is in the reduced state, e.g., H2S. Figure 4 shows the oxidation period, which follows directly after the reduction period presented in Figure 3. The inlet concentration of O2 was 5%, but the final concentration detected by the analyzer is 3.5%. This is due to the injection of nitrogen, i.e., the sweeping gas, in the coal feeding valve. During the first few minutes of the oxidation, all of the O2 injected to the reactor reacts with the bed of reduced ilmenite particles. Consequently, the outlet gas only contains inert nitrogen. After 2 min, the O2 concentration starts to rise rapidly. No CO2 is produced while O2 reacts with the reduced oxygen carriers in the bed, which suggests that no char from the previous reduction remained in the bed. However, a CO2 peak appears when the O2 concentration starts to rise. This is likely due to unconverted char, which was stuck on the upper parts or in the filter of the reactor. The concentration profiles of the oxidation periods are very similar for all cycles, independent of the previous reduction period. From these concentration profiles of the reducing phases, the previous criteria described in eqs 1619 are calculated and will be used as a reference for the following experiments. Table 2 presents these results for the conversion of petroleum coke with pure ilmenite as an oxygen carrier. 3.2. Petroleum Coke Conversion with a Mixture of Ilmenite and Calcined Limestone. In the experiments with limestone, 5 g of calcined limestone (180250 μm) was added to 20 g of ilmenite used in the previous experiments. A slightly bigger particle size was chosen for the limestone particle to compensate for the difference in density between limestone and ilmenite. The target was to obtain good mixing of the bed. Otherwise, the same experimental condition as in the previous experiments has been applied, and the results are presented in Table 3. Figure 5 shows that, by adding limestone, there is a significant decrease of the CO fraction (eq 17) from 15 to 7.4%, which is also seen as an increase in CO2 (Figure 6). The initial peaks that correspond to the devolatilization of the petroleum coke are similar to the peaks seen in the reference experiments with pure ilmenite and can be explained by the same reason as described in 3847
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Figure 5. Comparison of γCO between a bed made of 20 g of ilmenite and the same bed of ilmenite where 5 g of calcined limestone has been added.
Figure 7. Comparison of γH2 between a bed made of 20 g of ilmenite and the same bed of ilmenite where 5 g of calcined limestone has been added.
Figure 6. Comparison of γCO2 for a bed made of 20 g of ilmenite and the same bed of ilmenite where 5 g of calcinated limestone has been added.
Figure 8. Comparison of γSO2 between a bed made of 20 g of ilmenite and the same bed of ilmenite where 5 g of calcined limestone has been added.
the previous paragraph. However, there is a dramatic change in the CO fraction in the period after the release of volatiles, where the remaining char reacts with the added steam to form syngas. In Figure 7, it is also seen that adding limestone also lowers the concentration of H2 in the outlet gas, i.e., a 40% decrease. Figure 8 shows a moderate reduction of the SO2 concentration in the outlet gas. Over six constant cycles, an average diminution of 26% in the SO2 concentration was calculated from the data presented in Table 3. This observation was confirmed by an X-ray analysis, which shows the presence of CaSO4/CaS in the limestone particles collected after the experiment. Figure 9 and Table 4 show an increase in the conversion rate of around 12.5% compared to the reference experiment without limestone addition and, consequently, also show a decrease in the fuel conversion time. The stability over 12 cycles confirms the validity of the result. This can be explained with the decrease of H2 inhibition,13 because the H2 concentration is lower in the outlet gas when limestone is added. 3.3. Petroleum Coke Conversion with a Mixture of Ilmenite and Sulfated Limestone. Two different sets of experiments using a mixture of 20 g of ilmenite (125180 μm) and 5 g of sulfated limestone (180250 μm) have been performed. The difference between the two sets was the sulfation degree of 25 and 40%. The same amount of petroleum coke was added as in the previously presented experiments. In Figure 10, the cycles with the maximum SO2 concentration are shown for four different experiments as a function of time. It is obvious that the introduction of sulfated limestone particles in
Figure 9. Comparison of the average fuel conversion rate between a bed made of 20 g of ilmenite and the same bed of ilmenite where 5 g of calcined limestone has been added.
the initial ilmenite bed is correlated to a significant release of SO2 as the char is converted. The SO2 concentration in the outlet gas is up to 20 times higher than with experiments without any calcium sulfate particles in the bed. As an example, there is an increase in γSO2 from 0.75% with calcined limestone particles in the bed to approximately 16% for the lime with 40% sulfation. However, for both of the experiments with sulfated limestone, this strong release of SO2 was only seen in the initial cycles, which 3848
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can be seen in Figure 11 and Tables 4 and 5. After these initial cycles, the release of SO2 falls, and after 1015 cycles, it reaches a more or less constant level. For the first 4 or 5 cycles, the release of SO2 increases over the cycles, until it reaches a maximum point; i.e., fSO2 = 13% for the lime with 25% sulfation, and fSO2 = 16% for the lime with 40% sulfation. The SO2 concentration in the outlet gas decreases with each cycle until it reaches a level independent of the cycle number. Before this constant level is reached, the SO2 concentration is always higher for the Table 4. Experiments with 0.2 g of Coke, 20 g of Pure Ilmenite, and 5 g of Sulfated Limestone (25%)a steam
total t95%
γCO γCO2 γSO2 carbon
content
conversion
(%)
rate “r”
1 2
47.3 47.4
9 9.4
10.5 10.1
5.6 9.5
93.3 12 88 11
0.120 0.172
3
48
9.5
10
8.1
89.8 12
0.154
4
45
8.9
10.7
8
89.8 13
0.149
5
47.4
9
10.6
8.2
89.7 11
0.152
6
47.4
8.6
11
7.2
90.7
7.2
0.150
7
44.7
9
10.6
6
92.2
5.4
0.148
8
50.4
8.8
10.8
5.8
92.3
4.8
0.153
9 10
50.4 52
8.8 7.2
10.8 13.1
5.8 8.3
92.3 89.3
4.8 2.5
0.146 0.150
11
51.6
7.9
12.1
6.3
91.7
1.8
0.152
12
46.5
7.6
12.5
6
92.1
1.2
0.141
13
48.4
7.2
13.3
6.6
91.5
0.9
0.146
average 15
47.0
9.2
10.4
7.9
90.1 12
0.15
average
48.8
7.6
12.6
6.3
91.8
1.3
0.15
48.2
8.5
11.2
7.0
91.0
6.8
0.1
cycle
(min) (%)
(%)
(%) content
1113 average total a
Averages for initial, final, and all cycles are also shown.
lime with 40% sulfation compared to 25% sulfation. However, the final stable state is reached after more or less the same number of cycles in both cases. It can be seen that this constant release of SO2 is close to the release obtained from the pure ilmenite as well as the release with a mixture of calcined limestone and ilmenite. SO2 is only released during the reduction period when the char is introduced. No trace of SO2 has been observed in the inert and oxidation phase, and there was no trace of SO2 in the short steam/nitrogen mixture of the reduction phase before the fuel was added. Figure 12 shows the average fraction of emitted CO for all cycles with sulfated limestone. In both cases, there is a decrease in fCO from 7.9% with calcined limestone particles to approximately 67% for sulfated limestone. Figure 13 presents γCO2 for the same cycles as in Figure 12. Figure 14 shows the evolution of the conversion rate “r” for the two sets of experiments with 25 and 40% sulfated limestone. In both cases, the significant increase in the SO2 fraction seen in Figure 11 has a beneficial effect on fuel conversion. This has also been observed in previous work.12 Nevertheless, when the large release of SO2 in Figure 11 is compared to the relative increase of the conversion rate in Figure 14, it can be concluded that this advantageous effect is quite moderate. Also, the beneficial improvement drops after a few cycles. After these, the conversion rate reaches a stabile state between the state obtained with pure ilmenite and the state from the mixture of ilmenite and calcined limestone. 3.4. CO and H2 Conversion with Ilmenite and Calcined Limestone. To further investigate the influence of the conversion of CO and H2, a few experiments were made with only these gases as fuel. Here, the bed consisted of 6 g of fresh oxygen carrier mixed with 9 g of inert quartz sand, both in the particle size range of 125180 μm. To investigate the influence of limestone, another set of cycles was performed with the same setting but with 1.5 g of sand substituted with limestone. The fuel was syngas
Figure 10. Comparison of the cycles with maximum SO2 concentrations obtained in the experiment using ilmenite, ilmenite and calcined limestone, ilmenite and sulfated limestone (25%), and ilmenite and sulfated limestone (40%). A total of 0.2 g of petroleum coke has been introduced. The inlet concentration of H2O is 50%. The temperature is 970 °C. 3849
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Figure 11. Evolution of fSO2 for a bed made of 20 g of ilmenite and 5 g of sulfated limestone (25%) and a bed of ilmenite with 5 g of sulfated limestone (40%).
Figure 12. Evolution of fCO for a bed with 20 g of ilmenite and 5 g of sulfated limestone (25%) and a bed of ilmenite with 5 g of sulfated limestone (40%).
Table 5. Experiments with 0.2 g of Coke, 20 g of Pure Ilmenite, and 5 g of Sulfated Limestone (40%) steam cycle
total γCO
γCO2
γSO2 carbon
rate “r”
(min) (%)
(%)
(%) content
content
conversion
(%)
t95%
1
47
8.5
11.17 6.74
91.5
12
0.142
2
53
8.6
11.07 5.6
92.7
13.6
0.143
3
46.2
8.3
11.4
5.74
92.6
15.6
0.154
4
44
8.8
10.8
5.1
93.21
16.1
0.151
5
52
8.24
11.5
7.07
91.28
16
0.147
6
45.2
8.79
10.8
6.2
92.05
9.45 0.149
7
42
9.04
10.5
5.8
92.6
9.32 0.152
8
51
8.96
10.6
6.2
92.1
7.72 0.146
9
52.5
8.05
11.8
5.97
92.4
6.27 0.144
10
48
9.04
10.5
6.36
91.84
3.61 0.148
11
46.2
7.91
12
6.22
91.78
2.11 0.153
12
53.7
7.48
12.7
6.9
91.1
1.12 0.152
13
50
7.36
12.9
6.38
91.7
0.87 0.152
14
45
7
13.6
6.15
92.4
0.59 0.151
15
50.2
6.5
14.6
6.18
92.31
0.58 0.149
16
47.4
7.1
13.3
6.1
92.4
0.74 0.149
17
52.1
7.3
12.1
5.64
92.9
0.9
18
51.4
7.14
13.3
5.97
92.5
0.65 0.148
average 18
47.6
8.7
11.0
6.1
92.3
12.5
0.15
average 1317
49.4
7.1
13.3
6.1
92.4
0.7
0.15
average total
48.7
8.0
11.9
6.1
92.2
6.5
0.1
Figure 13. Evolution of γCO2 for a bed with 20 g of ilmenite and 5 g of sulfated limestone (25%) and a bed of ilmenite with 5 g of sulfated limestone (40%).
0.151
consisting of a 50:50 fraction of CO/H2 or a gas of either 50% CO or 50% H2 in N2. These experiments were repeated for a few cycles to verify that the reactivity was stable over the cycles. The fraction of CO and H2 converted to CO2 and H2O for the different cases is presented in Table 6.25 When a single fuel is used in N2, there is no significant influence on the conversion with or without limestone in the bed, as seen from Table 6. Hence, the limestone did not seem to have any effect on the gas conversion rate. Comparing the gas conversion in a mixture of CO and H2 to a single fuel gas clearly shows that the conversion of CO increases in the presence of H2, whereas the conversion of H2 decreases. In the case of H2 conversion, this change is independent of whether limestone is present or not. However, for CO, the presence of limestone gives a higher fuel conversion. The
Figure 14. Evolution of the conversion rate “r” between a bed made of 20 g of ilmenite and 5 g of sulfated limestone (25%) and a bed of ilmenite with 5 g of sulfated limestone (40%).
overall conversion of CO together with H2, (70 + 92)/2, and in absence of lime is equal to the conversion of CO and H2 when reacted individually, (66 + 96)/2. There is some transfer of CO to H2, but this may well occur above the bed and not affect overall conversion. However, in the presence of lime, the overall conversion of CO together with H2, (78 + 92)/2, is notably higher than the conversion of CO and H2 when reacted individually in the presence of lime, (65 + 98)/2. Because this increase only happens when CO and H2 are reacted together and in the presence of limestone, this clearly indicates that lime improves the overall conversion through catalyzing the watergas shift reaction. 3850
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Table 6. Fuel Conversion for CO and H2 in Ilmenite with and without Limestone CO conversion no limestone
H2 conversion limestone in ilmenite
no limestone
limestone in ilmenite
50% CO in N2
50% CO in H2
50% CO in N2
50% CO in H2
50% H2 in N2
50% H2 in CO
50% H2 in N2
50% H2 in CO
0.66
0.70
0.65
0.78
0.96
0.92
0.98
0.92
4. DISCUSSION The experiments with petroleum coke clearly show an effect of the added lime. What is somewhat less clear is the explanation to this effect. The following possibilities are available: (i) The added lime particles have a physical effect, i.e., changes the fluidization conditions in a way that gives improved contact between the oxygen carrier and fuel particles. Such an effect, although possible, would be highly unlikely to explain, for instance, a halving of the CO fraction. (ii) The lime particles catalyze the watergas shift reaction, H2O + CO T H2 + CO2. This explanation is very clearly supported by the experiments with CO and H2 shown in Table 6. The decrease in CO conversion is somewhat smaller in the gas tests, but that would also be expected because the steam concentration is much lower in these tests. Catalysis of the watergas shift reaction could explain the lower CO concentration, but a higher H2 concentration would be expected, which is also seen in the gas tests but not in the tests with solid fuel, where the opposite is seen. However, the conversion of H2 is much faster than the conversion of CO, and the measured concentrations of H2 are quite small; therefore, perhaps this apparent contradiction should not be given too much attention. (iii) CaSO4/CaS works as an additional oxygen carrier, i.e., between oxidizing and reducing periods. Thus, CaS is formed under the reducing period, which is oxidized to CaSO4 in the oxidizing period, which then oxidizes CO and H2 in the subsequent reducing period, forming new CaS. If this was the dominating effect, it is hard to explain why the unsulfated lime gives a dramatic reduction of CO already in the first cycle. Moreover, no improvement in CO conversion is seen with an increasing number of cycles. (iv) CaSO4/CaS works as an internal oxygen carrier in the bed. This mechanism would transfer oxygen with lime particles moving inside the bed from more oxidizing conditions, e.g., close to ilmenite particles, where concentrations of H2 and CO are low, to reducing conditions in the vicinity of char particles, where H2, CO, and H2S are high. The reactions would be as follows: Reducing conditions: CaO + H2S f CaS + H2O. Reducing conditions: CaSO4 + CO/H2 f CaO + SO2 + CO2/H2O. Oxidizing conditions: CaS + H2O f CaO + H2S. Oxidizing conditions: CaS + 4H2O f CaSO4 + 4H2. In this mechanism, CaSO4/CaS would not contribute to any net oxygen but would locally oxidize CO and H2 and produce a corresponding amount of H2 at another location. Thus, this oxygen demand could be moved from the upper part of the bed to the lower part of the bed and, in this way, provide better contact and accordingly more efficient conversion. Moreover, because mainly H2 would be formed when CaS is oxidized, it would also mean an exchange of CO for H2, and H2 is known to be much more reactive with ilmenite.26 Also, assuming that the oxygen release/uptake by CaSO4/CaS is quicker compared to ilmenite, it would give lower H2 concentrations close to the fuel
particles, which should increase the gasification rate.13 Thus, this mechanism would be able to explain both lower CO and H2 concentrations and more rapid fuel conversion. However, a problem with this explanation is that it assumes faster conversion of char because of the formation of more SO2, whereas the measurements show a decrease in SO2 rather than an increase. Moreover, the very large release of SO2 in the initial cycles with sulfated lime only gave a moderate increase of the char conversion. (v) Another mechanism that should be mentioned is the possible catalytic effect of calcium, because it is known to catalyze char gasification.27 However, it appears that the catalyst should be well-dispersed in the char to be effective,28 and it is doubtful if the calcium available in the form of separate particles would be able to have a similar positive effect. (vi) A lower CO concentration could also explain increased char conversion through lower CO inhibition. In conclusion, it appears that the best available explanation is that lime particles catalyze the watergas shift reaction. With respect to the more rapid char conversion, this could be explained by lower CO inhibition. Another possibility is internal transfer of oxygen in the bed, involving CaS and CaSO4, i.e., mechanism iv. In the tests, lime was added to the bed, thus increasing the bed mass. This was made on purpose to see the effect of CaO addition clearly. From a technical point of view, it is more interesting whether substitution of ilmenite would improve conversion. Otherwise stated, we would have achieved a similar improvement by mixing 25% of lime into ilmenite and comparing the same bed masses. Assuming a first-order reaction, a much larger increase in bed mass should be needed to attain a halving of the CO or H2 concentration. Thus, experiments with 20 g of ilmenite and 0.2 g of petrolium coke as fuel12 gave the same fCO and time for fuel conversion as experiments performed with 40 g of ilmenite and the same condition.16 Because the improvement seen in this paper was accomplished already with a 25% increase of bed mass, it is very likely that also exchanging ilmenite with lime would give significantly improved conversion. Another question is the applicability of these tests to largescale conditions. These tests indicated that CaSO4 is decomposed and no real effect of CaSO4 as an oxygen carrier between the reducing and oxidizing periods could be shown. It should be noted that, for practical reasons, these tests are performed in high excess of steam, which dilutes the gaseous sulfur compounds. A fuel reactor is operated at conditions that should be expected to give high equilibrium concentrations of sulfur gas compounds (see the phase diagram in Figure 1). Thus, the maximum equilibrium concentration of gaseous sulfur compounds is found at H2/H2O and CO/CO2 ratios of around 0.01 and 0.02. This maximum concentration of gaseous sulfur compounds is 4% at 950 °C and 10% at 1000 °C. In these tests, the maximum concentration would be less than 0.1%, if the cycles with the release of SO2 from sulfated lime are excluded. In real operation, the sulfur concentration could be 1 order of magnitude larger. 3851
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Energy & Fuels Higher concentrations of sulfur compounds would be helpful to avoid the formation of CaO and loss of sulfur. Thus, the results in this work should not be taken as any evidence that the formation of CaO and loss of sulfur would make CaS/CaSO4 unsuitable as an oxygen carrier. Consequently, the contribution by reaction mechanisms iii and iv, although likely small in the present tests, could be much more important under conditions when the gases produced by the fuel are much less diluted. It should also be noted that, with CaSO4/CaS as an oxygen carrier, the gas conversion would be maximized to 9899% according to H2/H2O and CO/CO2 ratios for the CaSO4/CaS equilibrium, also seen in the phase diagram (Figure 1). Previous testing of sulfation and reductive decomposition of sulfate under conditions switching between oxidizing and reducing clearly demonstrates the complexity of these reactions.7,2123 The tests generally show strong peaks in SO2 when switching from both oxidizing to reducing and reducing to oxidizing. The size of these peaks is strongly dependent upon the temperature, lengths of oxidizing/reducing periods, reducing compound, and limestone.
’ CONCLUSION The main conclusions of this work are as follows: (1) Unconverted gasification products, i.e., H2 and CO, decreased with the addition of calcined limestone particles in the bed. In particular, the fraction of unconverted CO was halved. (2) Tests with H2 and CO clearly indicated that the improved conversion can be explained by CaO catalyzing the watergas shift reaction. Thus, CO is converted to H2, which has a much higher reactivity with ilmenite. (3) The conversion rate of the char increased moderately with the addition of calcined limestone particles in the bed. (4) Sulfated limestone addition showed a further increase in the char conversion rate, as compared to calcined limestone, during the first 10 cycles. In these cycles, large amounts of SO2 were released. After this, the fuel conversion dropped to a level slightly below the level attained with calcined limestone particles introduced in the bed of ilmenite. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT This work was funded by Vattenfall AB and Chalmers University of Technology via the Energy Area of Advance. ’ REFERENCES (1) Rosenzweig, C.; Karoly, D.; Vicarelli, M.; Neofotis, P.; Wu, Q.; Casassa, G.; Menzel, A.; Root, T. L.; Estrella, N.; Seguin, B.; Tryjanowski, P.; Liu, C.; Rawlins, S.; Imeson, A. Attributing physical and biological impacts to anthropogenic climate change. Nature 2008, 453 (15), 353–357. (2) Intergovernmental Panel on Climate Change (IPPC). Carbon Dioxide Capture and Storage; IPPC: Geneva, Switzerland, 2005; www. ipcc.ch. (3) Radgen, P.; Cremer, C.; Warkentin, S.; Gerling, P.; May, F.; Knopf, S. Assessment of Technologies for CO2 Capture and Storage; Nature Conservation and Nuclear Safety, Federal Ministry of the Environment, Fraunhofer-Institut f€ur Systemtechnik und Innovationsforschung: Karlsruhe, Germany, 2006.
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