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Enhanced performance of Chemical Looping Combustion of CO with CaSO4-CaO oxygen carrier Min Zheng, Yanbing Xing, Kongzhai Li, Simei Zhong, Hua Wang, and Baoling Zhao Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b03078 • Publication Date (Web): 14 Mar 2017 Downloaded from http://pubs.acs.org on March 15, 2017
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Enhanced performance of Chemical Looping Combustion of CO with CaSO4-CaO oxygen carrier Zheng Min1,2, Xing Yanbing1,3, Li Kongzhai1,2*, Zhong Simei1,2, Wang Hua1,2*, Zhao Baoling2 1
State Key Laboratory of Complex Nonferrous Metal Resources Clean Utilization, Kunming University of Science and Technology, Kun Ming 650093, China 2
Faculty of Metallurgical and Energy Engineering, Kunming University of Science and Technology, Kunming 650093, China
3
Faculty of Material Science and Engineering, Kunming University of Science and Technology, Kunming 650093, China
Hua Wang and Kongzhai Li contributed to this work equally. *Corresponding authors. Tel.: +86-871-65153405 E-mail address:
[email protected] (Wang Hua )； E-mail address:
[email protected] (Li Kongzhai).
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Abstract CaSO4 is an interesting alternative oxygen carrier for Chemical-Looping Combustion (CLC). Utilization of CaSO4 oxygen carrier suffers deactivation caused by sulfur loss. To prevent sulfur loss, a small amount of CaO particles were mixed with CaSO4 oxygen carrier. The predominant regions of CaSO4, CaS and CaO have been obtained with consideration of SO2 and COS emissions. The predominant region of CaO is increasing with the temperature and CO2 partial pressures. Isothermal as well as non-isothermal kinetics of CaSO4-based oxygen carrier reduction, and reduction/oxidation cycle tests have been carried out. The effects of reaction temperature, CO concentration, and molar ratio of CaO to CaSO4 and cycle number on sulphur emission and CO2 generation efficiency are taken into account. XRD, XRF and gas analyses were performed to investigate the variations of solid phase change, elements composition in solid residual and sulfur release with reaction time. The results show the use of fresh CaO additive not only enhances CO2 yield, but also captures SO2 and COS, which are identified by XRD and XRF analysis. During cycle tests, the use of CaO additive also improves CO2 yield. CO2 yields drop a little after the 1st cycle, but they can maintain at high levels subsequently in cases with the use of CaO additive. SO2 released from the Fuel Reactor (SO2-FR) is dominant over COS released from the Fuel Reactor (COS-FR) and SO2 released from the Air Reactor (SO2-AR). With the use of CaO additive, the releases of COS-FR and SO2-AR decrease, but SO2-FR emission increases a little. The comprehensive roles of CaO additive on CO2 generation and gas sulfide releases are evaluated by the Environmental factor. The CLC with CaSO4-CaO oxygen carrier with CO, ensuring a low environmental impact during the early 1-6 cycles, could achieve a high CO2 generating efficiency. In subsequent cycles, there was no obvious drop in the E-factor of SO2. Keywords: Chemical Looping Combustion; CO2 capture; CaSO4 oxygen carrier; SO2 release; SO2 suppression.
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1. Introduction Chemical-looping combustion (CLC) is a promising alternative to conventional combustion technique within the framework of the CO2 capture options.1-3 The CLC is an indirect combustion with the uses of an oxygen carrier, an air reactor and a fuel reactor. The oxygen carrier transfers oxygen from air in air reactor to fuel in fuel reactor. Thus, the mixture of air and fuel is avoided. Without the dilution of N2 from air, a high concentrated CO2 stream is available. As early as the year 2010, the total operation time (about 3500 h) in different continuous CLC units within the size range of 0.3-120 kWth demonstrated the feasibility of the technology and its further maturity.1 The successful over-1000-hour long-term operation, with 40 wt% free NiO on NiAl2O4 manufactured by spray-drying of commercial raw materials has been carried out at CHALMERS’s 10kWth CLC plant.4 The scaling-up of CLC technology, from 1-3 MWth scale to larger scale, is needed to testify the feasiblity before the pre-commercial units.1,5 Recently, 4000 h of operation with FeTiO3 oxygen carrier for CLC of municipal solid waste in a 75 MWth CFBboiler (industrial relevant scale) has been sucessfully performed, which contributes knowledge in the scale-up of CLC operation.6 More than 700 different materials1 based on Ni4,7,8, Cu9,10,11, Fe6,12, Mn13,14, Co15,16, as well as other mixed oxides (CuO-Fe2O3, Mn2O3-Fe2O3, NiO-CoO el al.) and low cost materials (cheap natural ore (iron natural ore, nature gypsum ore et al.), and by-products of industry), have been compiled. The metal oxides NiO, Fe2O3, CuO, Mn2O3, CoO as well as their derivations raise considerable concerns due to their high reactivities. For further large-scale commercial use, the lowcost materials also raise a growing concern.1 CaSO4 is an optional oxygen carrier for further large-scale commercial use of CLC because of its rich reserves, low price and high oxygen carrier capacity (0.46).17-19 Fig. 1 shows a diagram of the CO-fueled CLC process based on CaSO4 oxygen carrier. The general reaction approach is as below. CaSO4 is reduced by CO to CaS in the fuel reactor: 1 1 θ CaSO 4 + CO → Ca S + CO 2 ∆ H 298.15 = −4 2.76kJ / mol 4 4 3 Environment ACS Paragon Plus
(R1)
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Then CaS is oxidized by air to CaSO4 in the air reactor, and the molecular oxygen of O2 in air is transferred to lattice oxygen in CaSO4. θ CaS + 2O2 →CaSO4 ∆H298.15 = −960.89kJ / mol
(R2)
Without the dilution of N2 from the air, the flue gas of the fuel reactor mainly consists of CO2. Previous investigations20-22 on reaction performances of CH4, synthesis gas and coal with anhydrite ore oxygen carrier. Although the relativity of anhydrite ore with fuels is lower than Ni, Fe, Cu oxygen carriers, the outlet gas is mainly composed of CO2, as well as a very small amount of unconverted CH4, CO and H2. A long-time cycle test, which has been performed on anhydrite ore oxygen carrier with the use of gaseous fuels,20-21 exhibited a stability performance during the 15 cycles. Additionally, the mixed CaSO4-based oxygen carriers CaSO4-Fe2O3 and CaSO4-CuO have been compiled23-24, and the reactivities of the mixed oxygen carriers (OC) with coal have been enhanced remarkably due to beneficial complementary effect of mixed OC as well as heat balance. Besides, the reactivity of the nano-sized CaSO4 oxygen carriers supported by Al2O3 binder was investigated.25 The reaction rates of the nano-sized oxygen carriers are significantly increased, because the fresh samples have higher surface areas. And the nanosized oxygen carriers have excellent thermal stability, which may be promote CaAl2O4 formation.25 The utilization of CaSO4 as oxygen carrier suffers the problem of sulfur emission during reducing/oxidation cycling17-28. In literature20, it has been proposed that the sulfur release should be captured as CaS in fuel reactor or CaSO4 in air reactor with the use of Ca-based sorbent. However, the roles of sorbents on sulfur capture lack verification. Lime and limestone are typical sorbents and extensively used for sulphur capture in both conversional coal-fired combustion systems and coal gasification systems.29-30 However, the sulphur capture in the present work differs from that either in coal gasification systems or in conversional coal combustion systems. For the CaSO4 reduction by CO in fuel reactor, SO2 is the main sulfur-derived gas. However, the desulphurization environment used in a fuel reactor is different from that for coal combustion. The sulphur capture in
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fuel reactor is undertaken under reductive atmosphere, which is composed of CO and CO2 rather than O2 in coal combustion system. Since the introduction of lime and limestone sorbents for sulphur capture in CLC systems, some promising results have been obtained for the systems with Fe2O3-based oxygen carrier, but little work on CaSO4-CaO oxygen carrier has been done. It is interesting the Tierga iron ore with high CaO content is self-desulfurization of coal during a CLC process.31 The influences of limestone and lime additions to ilmenite in CLC with solid fuels are respectively investigated in literature32-33. The addition of limestone to ilmenite gives significant improvements of both gas conversion and char conversion at 950 °C. Both H2S and SO2 released from CaSO4 oxygen carrier and coal fuel in the fuel reactor may react with Ca-base sorbent to form CaS. Limestone is calcined and sulfated to different extent. With the use of the preparing limestone as additive, at 970 oC, the conversion of gas and rate of char conversion are enhanced. Partial sulfur released from the fuel was captured as CaS/CaSO4.32-33 Thus, both limestone and lime sorbents can capture a part of sulphur emissions and enhance the CO2 generation for CLC of coal with ilmenite oxygen carrier. Thermodynamics and kinetics study on the CaSO4 reduction, CaSO4-CaO oxygen carrier reduction by CO as well as multi-cycle with CaSO4-based oxygen carrier were undertaken in this paper. The effects of reaction temperature, CO concentration, and molar ratio of CaO to CaSO4 and cycle number on sulphur emission and CO2 generation efficiency are taken into account.
2. Thermodynamic There is significant literature on CaSO4 reduction, which occurs in CLC system with CaSO4 oxygen carrier 18,26,27 and in traditional coal combustion system with SO2 desulfurization29,30,34. The behaviors of CaSO4 reduction at different CO concentrations are investigated. In the fuel reactor of CLC system, the common reduction of CaSO4 with CO to CaS is via reaction (R1). The side reactions with SO2 release (R3) and (R4) may occur simultaneously.18 θ C aSO 4 + C O → C aO + C O 2 + SO 2 ∆ H 298.15 = 2 1 9.19 kJ / m ol
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3
1 θ CaSO4 + CaS → CaO+SO2 ∆H 298.15 = 261.94kJ / mol 4 4
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(R4)
The SO2 released may further reacts with CO to COS as follows18: θ SO2 +3CO ↔ COS+2CO2 ∆H 298. 15 = −300.59kJ / mol
(R5)
The sulfur released could be adsorbed by Ca-based sorbent to form CaS in the fuel reactor or CaSO4 in the air reactor as below: θ CaO+3CO+SO 2 → CaS+3CO 2 ∆ H 298.15 = − 390.22 kJ / mol
(R6)
θ CaO+SO 2 → CaSO 3 ∆H 298.15 =931.93kJ/mol
(R7)
θ CaO+COS → CaS+CO2 ∆H298.1 5 = − 562.84kJ/mol
(R8)
1 θ CaO+SO 2 + O 2 → CaSO 4 ∆H 298.15 =-503.00kJ/mol 2
(R9)
Fig. 2 shows the standard-state Gibbs free energy changes for CaSO4-CaO reduction under CO atmosphere. With respect to sulfur migration from CaSO4 to gas phase, SO2 emissions via reactions (R3) - (R4) are spontaneous at high reaction temperatures. The sulfur migration of SO2 to COS is spontaneous within the temperature range of 0 - 1000 oC, but it becomes less spontaneous with the rising reaction temperature. SO2 and COS emissions can be absorbed and solidified into CaS species, but the formation of CaSO3 via reaction (R7) is not a spontaneous process. Thus, the product of desulfurization would be CaS. However, the sulfur migration of SO2 to COS, and the absorptions of SO2 and COS by CaO sorbent become less spontaneous with rising reaction temperature. To better understand the complex reaction system, the chemical stability of CaSO4/CaS/CaO under CO atmosphere is studied. In literature18,29,34, the chemical stability of CaSO4/CaS/CaO under CO atmosphere has been obtained without the consideration of COS release. With consideration of SO2 and COS emissions, the phase diagrams of the CaSO4/CaO/CaS equilibrium at 850 and 950 oC are calculated mainly under the guidance of the work by Shen et al18. The predominance region of
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CaSO4, CaS and CaO species is dependent on five factors: temperature (T), CO2, SO2 and COS partial pressures (PCO2, PSO2, PCOS), and reductive potential (PCO/PCO2) defined in literature18. In fuel reactor of CO-fueled chemical looping combustion, the predominance diagram of CaSO4, CaS and CaO species as a function of T, PSO2, PCO/PCO2 and PCOS are indicated in Fig. 3 and Fig. 4. They are calculated from the equilibrium constants of reactions (R1), (R3), (R5) and (R6). The predominance diagrams show that the regions of I, II, and III, divided by the three Lines (1), (2) and (3), are the predominance areas of CaSO4, CaS and CaO species respectively. Lines (1), (2) and (3) are respectively the equilibrium lines for reactions (R1), (R3) and (R6). The boundary lines between any two stability fields denote the coexistence of two phases (CaSO4/CaS, CaSO4/CaO, CaS/CaO). At the intersection point of three solid lines, CaSO4, CaS and CaO species are in coexistence. CaS is stable under high partial pressures of SO2 and COS as shown in Figs. 3 and 4. At a given reaction temperature, there exits critical partial pressures of SO2 and COS (PSO2-θ, PCOS-θ) for CaSO4 conversion to CaS and CaO. The critical partial pressure of SO2 and COS at 950 oC under 101325 pa CO2 partial pressure is 3850.437 and 18.015 pa respectively. When either partial pressure of SO2 and COS in the gas phase is higher than the critical partial pressure, CaS is in stable existence. However, when either partial pressure of SO2 and COS is lower than the critical partial pressure, there is a stability range for CaO species. The predominance region of CaO substance enlarge with temperature and CO2 partial pressures. It is better to remove the CO2 generation from the reactor to the bulk phase to suppress CaO generation. The result of the predominance diagrams implies the complexity of utilizing CaSO4-based oxygen carriers. In a real reaction system, when the values of real reaction temperature (T) and rection atmosphere (PCO/PCO, PSO2, PCOS, PCO2) fall into the region of II, the final product should be CaS substance. As illustrated in Fig 3 and Fig 4, the equilibrium partial pressure of SO2 is much higher than that of COS at the equilibrium triple point of CaSO4-CaS-CaO. With an increase in reaction temperature, the partial pressures of SO2 and COS increase, and the growth of SO2 partial pressure
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is more remarkable. Thus, within the reaction temperature of 850-950 oC and at the equilibrium point of CaSO4/CaS/CaO, SO2 is the main gas sulfide, and COS release is very small.
3. Kinetics 3.1 Non-isothermal kinetics of CaSO4 reduction and CaSO4-CaO reduction 3.1.1 Experimental Section The general non-isothermal chemical reaction kinetics of CaSO4 reduction and CaSO4-CaO reduction were carried out under a non-isothermal condition by a Catlab system. The catlab system comprises a catlab bench-top micro-reactor and a hiden quadrupole analytical mass spectrometer (Qantitative Gas Analysis, QGA). The mass spectrometer (MS) monitors the evolved gases from the micro-reactor. A quartz tube was fixed in the bench-top micro-reactor. The length of the reactor in the electric heater is 200 mm. The constant temperature length and the inner diameter of the reactor is 40 mm and 10 mm, respectively. The micro-reactor can be operated within a wide temperature range of 20-1000 °C. A sample of the CaSO4-based oxygen carrier was put in the quartz reactor and Silica wool was put both under and on the sample. The temperature of the sample in the isothermal area of the tube was measured with an in-bed 10% Pt/Rh thermocouple. The gas flow rate is regulated by mass flow controllers. The evolved gases from the micro-reactor were measured online by the mass spectrometer, which can simultaneously measure eight kinds of gas species. Mass range of mass spectrometer is 200 amu with an ultimate detection limit of 100 ppb and better than 500 ms response time to change in gas composition. Experiments were performed following the procedures. Approximately 30 mg of analytical reagent samples (0.125 - 0.15 mm) were placed in the middle of the isothermal area of the tube, which was placed into catlab micro-reactor. First, the micro-reactor was heated from room temperature to 200 oC at 10 oC/min heating rate in Ar atmosphere with the flow rate of 80 ml/min, and then the temperature is held constant with 20 min dwell time. Second, the sample temperature of the micro-reactor was heated to 950 oC with a 10 oC/min ramped heating rate in the desired
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reductive gas atmosphere at the same flow rate. Afterwards, it was maintained at 950 oC with 2 min dwell time in a Ar atmosphere at constant flow rate.
3.1.2 Reduction of CaSO4 with CO The reductions of CaSO4 with different CO concentrations (0%, 1%, 5%, 10%) were carried out. The reactions were complex processes, with CO2 formation as well as sulfur emission depending on reaction temperatures. Take the reaction under 1% CO concentration, as shown in Fig. 5, for example, at low reaction temperatures, it was a single reaction (R1), just with CO2 production in the flue gas, whereas the side reactions were suppressed at the low temperatures, and no sulphur released was observed. At high reaction temperatures, the CaSO4 reduction process was accompanied by sulfur-derived gas liberations with the molar mass of 64 and 70. Combined with our previous FTIR study18, it can be confirmed that both SO2 and COS species are formed. A small amount of S2 may also be released, but no CS2 was detected. Compared with SO2 species, COS was generated much easier and reached the maximum release rate at lower reaction temperatures. It may be because the SO2 generation reactions ((R3) and (R4)) are endothermic, while the reaction for COS generation (R5) is exothermic. The variations of CO2, SO2 and COS released from CaSO4 with CO concentration and reaction temperature are illustrated in Fig. 6. The CaSO4 decomposition under Ar atmosphere did not occur even at 950 oC, while it took place at about 778 oC under different CO concentrations. The reaction rate of CaSO4 reduction, presented as the increasing peak value of CO2 generating rate and the declining reaction temperature for the peak value, increased remarkably with the rising CO concentration. The higher the CO concentration was, the higher the peak value of CO2 release rate, and the lower reaction temperature for the peak value of CO2 release rate would be. It can be observed that the curves of CO2 releases at different CO concentrations followed certain regularity in the process: it showed an exponential increase at lower reaction temperatures, and then it increased almost linearly at higher reaction temperatures. After reaching the maximum, the CO2 release rate dropped almost along a curve as the reaction progressed, even when different
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CO concentrations were utilized. There was a transition period for CO2 release from exponential increase stage towards linear increase stage. The SO2 generation reactions ((R3), (R4)) are endothermic, while the reaction for COS generation ((R5)) is exothermic. Thus, rising reaction temperature aggravates SO2 generation, but it is adverse to COS formation, as demonstrated in Fig. 5. In addition, the higher the CO concentration was, the less SO2 was released. The COS emission, the product of SO2 reacting with CO via reaction (R5), firstly increased and then decreased with CO concentration.
3.2 Isothermal kinetics of CaSO4-CaO reduction 3.2.1 Experimental Section 3.2.1 Materials and Methods The analytical reagent CaSO4 (Tianjin Fuchen Chemical Factory) and CaCO3 (Tianjin Fengchuang Chemical Technology CO. Ltd.) particles after calcination were used to produce oxygen carrier sample. The calcination of CaSO4 was carried out at 170 °C for 12 h with the use of a muffle oven. The calcination of CaCO3 was carried out in a muffle oven at 900 °C for 40 min. Then both CaSO4 and CaO particles within the size range of 0.125 - 0.147 mm were pressed respectively by a tablet press under the pressure of 5 atm for 10 min. The tablets were pulverized and sieved respectively, and the particle diameters were 0.147 - 0.177 mm. Finally, the CaSO4based oxygen carrier and CaO-based additive mixed mechanically in various proportions, and were ready for experiments. CO substance is extremely toxic and explosive. The lower and upper explosion concentration limits for CO are respectively 12.5% and 80% (% by volume of air). From a security standpoint, the experimental tests in this manuscript were carried out in the CO concentration range of 1-15%, mainly around 5%.
3.2.2 Experimental Setup The isothermal reduction reaction of CaSO4-CaO as well as CaSO4 oxygen carrier is investigated with a tubular reactor, which is very similar to what we did in our previous work22. The
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outlet gas was analyzed quantitatively by an Gas Chromatograph (GC 9790II, Fuli Instrument) and an Infrared Flue Gas Analyser (York MGA5). The length of the reactor in the electric heater is 600 mm. The constant temperature length and the inner diameter of the reactor is 80 mm and 13 mm, respectively. The temperature of the tubular reactor was measured with a K-omega thermocouple. The high-purity reactant gases (N2 and CO) are provided by Shanghai Weichuang Gas Co., Ltd. The flow rates of reactant gases were measured by mass flow controllers (MFC, Beijing Sevenstar Huachuang Electronics Co., Ltd.). A water cooler was used to cool the hot gas stream from the tubular reactor. After having been cooling, the exit gas from the cooler was sampled by syringes for analysis. The exit gas from the cooler was separated into two streams for off-line concentration measurement. One stream was measured by the Gas Chromatograph, which is equipped with a thermal conductivity detector (TCD) to detect CO, CO2 and a flame photometric detector (FPD) to detect COS. The other stream was first diluted, and then analyzed by an infrared flue gas analyzer to detect SO2 and O2. After reduction, the oxygen carriers were analyzed by an X-ray diffractometer (XRD, Rigaku MiniFlex600) and an X-ray Fluorescence spectrometer (XRF, Rigaku ZSX100E XRF Analyzer).
3.2.3 Experimental procedure The oxygen carrier reduction and the following cycle tests were also designed similarly to our previous work22. The details are as follows. A sample of the CaSO4-CaO or CaSO4 was placed in the reactor. Then it was heated to the desired reaction temperature in N2 atmosphere. After the temperature stabilized at the desired temperature, switch the nitrogen to gas reactant. Subsequently, the gas reactant was introduced, and the experiment started. When the reaction finished, switched the gas reactant to nitrogen to blow, and shut down the heater. When the solid residual was cooled to room temperature, it was collected and preserved for analysis. For cycle test, the reduction time for oxygen carrier was designated to 10 min. The oxidation reaction was carried out at 800 oC, with the oxidizing gas concentration 5% O2 in N2. The oxidation
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reaction is intense exothermic. To avoid a fierce increase in granular temperature, a low oxidizing atmosphere would be suitable according to the publication.21 The oxidation duration was designated to 12 min. Table 1 and Table 2 show respectively the operation conditions concerning the oxygen carrier reduction and the specific cycle tests.
3.2.4 Data evaluation According to our previous work22, the terms of CO2 yield
η
CO
and environmental factor Ei
2
were introduced to facilitate the discussion on CO2 generating and other gas emissions. The CO2 yield
η
CO
is calculated as follows22:
2
T
∑ NCO2,out,t
η CO2 =
t =0
where NCO,out,t , NCO2,out,t and NCOS,out,t are the molar amount of interval
∆t
(1)
( NCO,out,t+NCO2,out,t+NCOS,out,t) i
product gas species within the time
in sample collector. Since the molar amount of COS species is very small, the item of
COS amount is ignored while calculating CO2 yield. The cumulative amount of gas product is the integration of the corresponding evolution rate22. The environmental factor Ei is the mass ratio of the product gas species i to CO2 product, as defined in our previous work22. Ei =
where
mi
mi (i=CO,SO2,COS) mCO2
refers to the mass amount of product species i, and
m CO 2
(2)
is the mass amount of CO2
produced.
3.2.5 Results and Discussion 3.2.5.1 Effects of reaction temperature on gas products The effects of reaction temperature on CO2 generating and evolutions of SO2 and COS during the reaction process are shown in Fig. 7. The reactions were carried out with 5% CO concentration and with CaO/CaSO4 molar ratio of 1. CO2 yield first rose remarkably and then dropped a little with rising reaction temperature. The decline in CO2 generation at 950 oC is ascribed to thermodynamic 12 Environment ACS Paragon Plus
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limits on CaSO4 reduction. Within the reaction temperature of 850 - 950 oC, SO2 was the main gas sulfides, and SO2 and COS releases increased with reaction temperature. The releases of SO2 and COS decreased in the presence of CaO additive. It is because the gas sulfides are absorbed by CaO additive via reactions ((R6) and (R8)). The suppression of CaO additive on total gas sulfide (SO2+CO) also dropped at 950 oC compared with that at 900 oC, where the main gas sulfides SO2 dropped slightly and COS decreased remarkably at 950 oC. Thus, the role of CaO additive on less sulfide release was decreasing with rising reaction temperature. The following study was centered on 900 oC with respect to CO2 generation and gas sulfide releases.
3.2.5.2 Effects of CO concentration on gas products Fig. 8 shows gas product evolutions as a function of CO concentration during reactions at 900 o
C and with CaO/CaSO4 molar ratio 1. With the increase in CO concentration, the CaSO4 reduction
rate was accelerated, but the conversion of CO to CO2 dropped. Meanwhile, SO2 evolution dropped dramatically because of the rising reductive potential of the gas phase (PCO/PCO2)18, while COS species, which was the product of the reaction between SO2 and CO, increased first (
