Characteristics of a CaSO₄ oxygen carrier for chemical-looping combustion: reaction with polyvinylchloride pyrolysis gases in a two-stage reactor

Abstract

Chemical-looping combustion (CLC), which is a promising technique that includes an inherent separation of CO₂ may reduce the generation of dioxins in municipal solid waste (MSW) disposal because in a CLC system, no free oxygen is available for incineration process. Polyvinylchloride (PVC) and kitchen garbage are the main chlorine substances in MSW. The reaction between PVC pyrolysis gas and a calcium (Ca)-based oxygen carrier was investigated in a two-stage reactor in this study. The thermal decomposition and reduction/oxidation cycle behaviors of the oxygen carrier were investigated by analyzing methane (CH₄) using a thermogravimetric analyzer (TGA). The characteristics of CaSO₄/Fe₂O₃ oxygen carrier were determined by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results showed that the addition of Fe₂O₃ into CaSO₄ can enhance the reaction rate, and also that the CaSO₄/Fe₂O₃ oxygen carrier showed a good heat stability at the temperature of 900 °C. The reduction/oxidation cycles confirmed that the decomposition of the CaSO₄/Fe₂O₃ oxygen carrier is usually accompanied by some side reactions. These side reactions could cause the loss of their regeneration ability. The CaSO₄/Fe₂O₃ oxygen carrier successfully reacted with PVC pyrolysis gas in a two-stage reactor and the complete reaction ratio of m_PVC to m_{oxygen carrier} is 8. The research documented herein provides a useful reference for the utilization of MSWs.

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1. Introduction

Waste-to-energy is recognized as a renewable energy technology and is increasingly playing an important role in municipal solid waste (MSW) management.¹ Of all the MSW treatments, incineration is considered as the most important method in the waste-to-energy process.² However, there are major concerns for incineration of persistent organic pollutants (POPs) such as polychlorodibenzo-p-dioxins (PCDDs) and polychlorodibenzofurans (PCDFs).³ PCDD/Fs are almost always formed in all thermal processes when chlorine, oxygen, hydrogen and carbon are present.^4,5 Chlorine (Cl), which can form acidic pollutants, is the key element in the formation of dioxins.⁶ PVC is the main Cl source in MSW, and its incineration can cause serious ecological and environmental problems because of the emission of toxic dioxins from the incineration of MSW disposal.⁷

Chemical-looping combustion (CLC) is a new concept for the incineration of MSWs. The unique and attractive ability of CLC is that it involves an inherent CO₂ separation.⁸ In CLC, a solid material is used as an oxygen carrier, which transfers oxygen from air to the fuel in the CLC system. The system consists of two reactors: a reducer and an oxidizer. In the reducer, oxygen carrier particles are reduced by the fuel, which is oxidized to CO₂ and H₂O. In the oxidizer, the reduced oxygen carrier particles are re-oxidized by air and they revert back to the reducer for another cycle.⁹ The reaction equations are illustrated in eqn (1) and (2):

(2x + y)Me_mO_n + C_xH_2y → (2x + y)Me_mO_n−1 + xCO₂ + yH₂O (1)

where Me_mO_n represents a metal oxide and Me_mO_n−1 represents a metal or reduced metal oxide.

Because the conversion of air and fuel takes place in separate reactors, a concentrated CO₂ stream is obtained from the reducer after the condensation of the water vapor.¹⁰ The outlet stream from the oxidizer, which contains nitrogen and unreacted oxygen, can be safely emitted to the atmosphere.¹¹ Moreover, because the oxygen in the oxidizer does not directly come into contact with the fuel in the reducer, free oxygen is not available for the combustion process. In addition, the use of CLC in MSW incineration would not only separate CO₂, but also decrease the generation of dioxins.

Oxygen carrier is a key element in the CLC of MSW. The use of oxides of metals, such as Fe, Ni and Cu, in a common fossil fuel CLC system has been proven to be feasible.¹² Lyngfelt¹³ and Mattisson^14,15 studied the oxides of Fe, Ni and Cu. The price of Fe₂O₃ is very low and it is environmentally friendly; however, it has the disadvantage of a low oxygen transfer capacity. The performance of nickel oxide as an oxygen carrier is good, but it is expensive and environmentally unfriendly. The key disadvantages of copper oxide are that it has a low melting point and it is prone to sintering at high temperatures.¹⁶ However, when the metal oxygen carrier used in the CLC of MSW contains more chlorine substances such as PVC, the metal oxide may easily react with HCl produced from the chlorine substances and can be deactivated. Therefore, there is a need to develop an oxygen carrier with high chlorine resistance. Compared to metal oxygen carriers, the reaction between HCl and CaSO₄ is less likely to occur at the same temperature.¹⁷ This suggests that CaSO₄ could be used as an oxygen carrier for the CLC of MSW.

Because of its high oxygen transfer capacity, low cost and environmental friendliness, this Ca-based oxygen carrier has recently attracted more attention.^18–20 Xiao¹⁸ and Zhang¹⁹ discussed the reduction characteristics of CaSO₄ and detailed its kinetic model. Song²⁰ investigated a CaSO₄/Fe₂O₃ oxygen carrier and found that the addition of elemental iron could improve the conversion and decrease the emissions of sulfur. The CaSO₄/Fe₂O₃ oxygen carrier also showed a better recyclability and stability over pure CaSO₄ oxygen carrier. These investigations prove the feasibility of using CaSO₄ as an oxygen carrier; however, few studies focus on the chlorine resistance of the Ca-based oxygen carrier, which is important for the CLC of MSWs.

Herein, CaSO₄/Fe₂O₃ was used as an oxygen carrier, and its heat stability, reactive performance and recyclability were investigated using TGA. XRD and SEM analyses were used to further determine the characteristics of the fresh oxygen carrier and the solid products of reduction/oxidation. The reaction between the pyrolysis gases of PVC and the CaSO₄/Fe₂O₃ oxygen carrier was tested in a two-stage reactor at 900 °C under atmospheric pressure.

2. Experimental section

2.1. Materials

Pure CaSO₄ was obtained from calcium sulfate dihydrate (99.0%, Sinopharm Chemical Reagent Co., Ltd.). Fe₂O₃ was obtained from nitrate nonahydrate (98.5%, Sinopharm Chemical Reagent Co., Ltd.). PVC samples (Sinopec Qilu Co., Ltd, Shandong, China) with a diameter of 150 μm were used in this study.

2.2. Oxygen carrier preparation

The impregnation method was used to prepare the CaSO₄/Fe₂O₃ oxygen carrier with 4.0 wt% of Fe₂O₃. In this CaSO₄/Fe₂O₃ oxygen carrier, CaSO₄ is the active support and Fe₂O₃ is the additive. First, an aqueous solution of iron nitrates was added to CaSO₄ particles with constant stirring. The produced sample was then dried, ground, and finally calcined in a muffle oven at 550 °C for 2 h. When the impregnation was completed, the prepared composite oxygen carrier particles with the size between 60 and 20 mesh were ground and sieved for further use.

2.3. TGA analysis

The TGA experiments were performed using a thermogravimetric analyser (Rubotherm, Germany), operating at a heating rate of 10 °C min⁻¹. A thin layer of the oxygen carrier (around 50 mg) was distributed evenly at the bottom of a crucible. Methane (CH₄) and hydrogen (H₂) were used as the fuel gas in this research. Thermal stability experiments were conducted using ultra-high purity Ar as the inert carrier gas at a heating rate of 10 °C min⁻¹. The multiple reduction/oxidation cycle tests were conducted using the following procedure. In the first step, the reacting temperature was increased from room temperature to 900 °C in an inert atmosphere, and then the condition was switched to a CH₄ or H₂ atmosphere, but with the temperature kept stable. After the reduction process was finished, the gas was switched to O₂. The flow rate was set at 50 mL·min⁻¹ for each period. The multiple reduction/oxidation cycle tests were repeated for five more times. In addition, between the reduction and oxidation periods, an evacuation period of 30 min duration was also allowed to ensure the clean-up of residual gaseous products. As soon as the reaction was finished, the heater was switched-off and the solid products were cooled in an inert atmosphere to room temperature and then collected for further analysis.

2.4. Two-stage reactor

A two-stage reactor was used to investigate the reaction between the pyrolysis gas of PVC and the oxygen carrier. A schematic diagram of the two-stage reactor is shown in Fig. 1. The system consisted of three main sections: carrier gas system, two stage reactors and an anti-suck back device system. Both reactors had a 900 mm length stove with a silica tube (i.d. = 55 mm, length = 1200 mm). The reaction temperature was controlled by a temperature controller and the flow of the carrier gas was controlled by a rotor flow meter. The anti-suck back device was used to prevent air from entering the furnace.

Fig. 1 The schematic of the two-stage pipe reactor.

In this experiment, PVC samples with different weights were placed in a silica boat in 1^st stage reactor, and 200 mg of oxygen carrier particles were placed in a silica boat in the 2^nd stage reactor. Before the reaction, the silica boats were pushed into tubular furnaces. Then, flanges were used to seal and connect these two tubular furnaces, which constitute the two-stage pipe reactor. When the reaction started, the temperature of the 2^nd stage reactor increased to 900 °C in N₂ atmosphere at a constant flow rate of 200 mL min⁻¹ to provide an inert gas environment. Then, the temperature of the 1^st stage reactor was increased to 900 °C at the heating rate of 10 °C min⁻¹. With the increase in temperature, the pyrolysis gas of PVC from the 1^st reactor was conveyed to the 2^nd reactor with N₂ to react with the oxygen carrier. The reduced oxygen carrier particle was then cooled to room temperature in N₂ atmosphere. After testing the reduced oxygen carrier, the atmosphere was switched to air. During this oxidation period, the oxygen carrier reacted with the O₂ in the air.

3. Results and discussion

3.1. TGA analysis of oxygen carriers

Fig. 2(a–c) show the TG curves of pure CaSO₄ and CaSO₄/Fe₂O₃ in inert atmosphere and reduction/oxidation agents. As shown in Fig. 2(a), the final weight of both pure CaSO₄ and CaSO₄/Fe₂O₃ oxygen carriers in CH₄ agent are almost the same. It is obvious that the reaction rate of the CaSO₄/Fe₂O₃ oxygen carrier is considerably higher than that of the pure CaSO₄ oxygen carrier, suggesting that the presence of elemental iron could obviously improve the reaction rate. This phenomenon is ascribed to the following explanations. On the one hand, the equilibrium constant of Fe₂O₃, which may work as an additional oxygen carrier, is considerably higher than that of CaSO₄; a higher equilibrium constant represents a higher reaction rate.²⁰ As Fe₂O₃ has a pronounced catalytic effect on the reductive decomposition of CaSO₄ to CaS,²¹ the activation energy of CaSO₄/Fe₂O₃ is lower than that of pure CaSO₄. On the other hand, the BET results showed that the surface area of the oxygen carrier increased significantly from 3.80 (pure CaSO₄) to 14.0 m² g⁻¹ (CaSO₄/Fe₂O₃ oxygen carrier). Zhang et al.²¹ also reported that fresh CaSO₄/Fe₂O₃ oxygen carrier becomes more porous than pure CaSO₄. Fig. 2(a) also shows the TG curve of CaSO₄/Fe₂O₃ oxygen carrier in H₂ agent. The results show that the reaction times of both H₂ and CH₄ are almost the same (about 30 min). The reaction is given by eqn (3).

CaSO₄ + 4H₂ → CaS + 4H₂O (3)

Fig. 2 TG curves of the oxygen carrier: (a) comparison of the CaSO₄/Fe₂O₃ and CaSO₄ oxygen carriers at 900 °C in CH₄ and H₂ atmospheres; (b) thermal stability of the CaSO₄/Fe₂O₃ oxygen carrier in N₂ atmosphere; (c) the multi-cycle test of the CaSO₄/Fe₂O₃ oxygen carrier in CH₄ and O₂ atmospheres.

The thermal decomposition behavior of the CaSO₄/Fe₂O₃ oxygen carrier in an Ar agent from room temperature to 1200 °C at a heating rate of 10 °C min⁻¹ is shown in Fig. 2(b). The CaSO₄/Fe₂O₃ oxygen carrier is considerably stable when the temperature is increased, until the temperature reaches 1100 °C; the samples then begin to decompose slowly. When the temperature reaches 1200 °C and is maintained for 30 min at 1200 °C, the weight begins to decline from 96.7% to 92.7%. A decomposition reaction takes place at the high temperature, which causes a decline in the quality. The decomposition reaction is illustrated as eqn (4).

When the pressure of sulfur dioxide is lower than the equilibrium pressure, this complicated reaction takes place. When the temperature is below 1200 °C, the reaction rate is very slow.²² To sum up, the thermal stability of the CaSO₄/Fe₂O₃ oxygen carrier is considerably good. When the temperature approaches 1100 °C, only 2.00% of the CaSO₄/Fe₂O₃ oxygen carrier is decomposed, and the CaSO₄/Fe₂O₃ oxygen carrier only decomposes by about 1.00% at a reaction temperature of 900 °C. The thermal stability and the advantage of the CaSO₄/Fe₂O₃ oxygen carrier were thus observed and confirmed in the above sections. This gives useful information regarding the potential performance of the CaSO₄/Fe₂O₃ oxygen carrier in the CLC process with a CH₄ atmosphere at a reaction temperature of 900 °C.

Fig. 2(c) shows the multi-cycle test of the CaSO₄/Fe₂O₃ oxygen carrier at 900 °C. It can be clearly seen that the mass percentage of the solid products reduces from 100% to 54.0% during the reduction period, while the proportion of the active component CaSO₄ in the fresh CaSO₄/Fe₂O₃ oxygen carrier is 95.2%. Through calculation, the maximum mass of decomposition is 50.2%. Therefore, the majority of the CaSO₄/Fe₂O₃ oxygen carrier loses its lattice oxygen to form CaS during the reduction period, as illustrated in eqn (5).

CaSO₄ + CH₄ → CaS + CO₂ + 2H₂O (5)

During the oxidation period, CaS combines with O₂ to form initial CaSO₄, as illustrated in eqn (6).

CaS + 2O₂ → CaSO₄ (6)

The weight of the reduced CaSO₄/Fe₂O₃ oxygen carrier returned 89.2% after the oxidation period. This implies that the following reactions may occur, as illustrated in eqn (7) and (8), because the partial pressure of CO₂ in the reactor is almost zero.²³

CaS + 3CaSO₄ → 4CaO + 4SO₂ (8)

After six cycles, the mass percentage drops to 64.6%, indicating that almost 35.4% of the CaSO₄/Fe₂O₃ oxygen carriers lost their regeneration ability. The capacity of the CaSO₄/Fe₂O₃ oxygen carrier was therefore deemed to be considerably low. The mass ratio of the CaSO₄/Fe₂O₃ oxygen carrier equally lost their regeneration ability for every cycle. The increase in the mass of the CaSO₄/Fe₂O₃ oxygen carrier in the subsequent oxidation period cycle was the same as the decrease in the prior reduction period cycle. It is thus evident that the mass fraction of CaO increases in the solid products with an increase in cycling times, indicating that CaO is piling up because of the formation of CaO in every cycle. The high oxygen ratio of the calcium-based oxygen carrier compared to that of the metal oxide-based oxygen carrier¹⁸ can make up for the disadvantages of circularity.

3.2. The reaction of PVC pyrolysis gas with the oxygen carrier

The TG/DTG curve of PVC at a heating rate of 10 °C min⁻¹ is shown in Fig. 3. The PVC curve showed no weight loss up to 250 °C; thus, the pyrolysis process of PVC could be divided into two stages: 250–350 °C and 350–520 °C. The chemical bonds of the 2nd pyrolysis stage were more difficult to break than the 1st stage. The stability of the chemical bonds of C–Cl is lower than the C–C bonds.²⁴ It is clear that when the PVC is pyrolyzed, the chemical bonds of C–Cl would break first. The stage II weight loss of PVC was thus assumed as the breaking of the C–C bonds. The first reaction involves the formation of HCl and intermediates,²⁵ whereas the second reaction involves the formation of some volatiles (namely, benzene, naphthalene and anthracene), polyene chains of toluene and other aromatics, as well as the final residue.²⁶

Fig. 3 TG/DTG curves of PVC at the heating rate of 10 °C min⁻¹.

In this study, the reaction of PVC pyrolysis gas with the oxygen carrier was investigated in a two-stage reactor. Fig. 4 shows the relationship between the weight of the oxygen carrier and the m_PVC/m_{oxygen carrier} ratio of the CaSO₄/Fe₂O₃ oxygen carrier. When the ratio is 1, the weight is 78.0%, and when the ratio is 8, the weight is 50.0%.The main content of the chemical compounds from the pyrolysis of PVC are the groups containing 2 rings, 3 rings, quasi-4 rings and 4 rings, as detailed in the literature.²⁷ According to eqn (1), the C_xH_2y compounds obtained from the pyrolysis of PVC can react with the oxygen carrier. When the ratio is 8, the weight stops decreasing and remains constant at about 48.0%, which is similar to the results of the TGA.

Fig. 4 Plot of weight loss (%) as a function of the ratio of PVC to the oxygen carrier.

3.3. Characteristics of the CaSO₄/Fe₂O₃ oxygen carrier

In this study, XRD and SEM analyses were used to further determine the characteristics of the fresh oxygen carrier and the solid products obtained from reduction/oxidation. Fig. 5 shows the XRD patterns for the fresh CaSO₄/Fe₂O₃ oxygen carrier and for the reduction/oxidation solid products. As shown in Fig. 5(a), the main component of the fresh oxygen carrier is CaSO₄. When the content of Fe₂O₃ in the fresh oxygen carrier was 4.0 wt%, a small portion of Fe₂O₃ was also detected in the fresh oxygen carrier. Fig. 5(b) shows that the main component of the reduced CaSO₄/Fe₂O₃ oxygen carrier is CaS. The reaction can be illustrated by eqn (4) during the reduction period. After reduction, there is a formation of new phase (Ca₂Fe₂O₅), which indicates that the product CaO reacted with Fe₂O₃. It can be clearly seen that CaS is the dominant reduced product after the reduction period and that the CaSO₄ diffraction peak is absent. However, a CaO diffraction peak was found, which implies that reaction illustrated in eqn (8) may occur, which implies that the CaO product is formed during the reduction period. As shown in Fig. 5(c), it could be confirmed that CaSO₄ is the main product after one cycle, and a low-intensity CaO diffraction peak was found. CaO may be formed from two sources: the reduction period and the oxidation period, according to eqn (6) and (7). Therefore, the mass percentage of the solid products fails to return to 100% after use. By comparing Fig. 5(c) and (d), it can be seen that the intensity of CaO becomes higher, which suggests the occurrence of a much higher degree of side reactions. This result demonstrates the decrease of the regeneration ability of the CaSO₄/Fe₂O₃ oxygen carrier.

Fig. 5 XRD analysis of the oxygen carrier: (a) fresh; (b) reduced by CH₄; (c) after oxidation with O₂; (d) after six cycles of reduction (CH₄)/oxidation (O₂) test; (e) after reduction with the pyrolysis gas of PVC; (f) after being used once in the two-stage pipe reactor.

Fig. 5(e) and (f) show the XRD patterns of the CaSO₄/Fe₂O₃ oxygen carrier after reduction with the pyrolysis gas of PVC and after oxidation with air. It can be confirmed that a CaS diffraction peak is found, which confirms the target product as expected. A new phase formation (Ca₂FeO₃Cl) is also found after the reduction, which indicates that the previously formed Ca₂Fe₂O₅ reacted with the pyrolysis product HCl. Though this reaction indicates a decrease regeneration ability of the CaSO₄/Fe₂O₃ oxygen carrier, it implies the formation of HCl, which can stop the spread of emitted toxic dioxins. From Fig. 2(c), it is obvious that the content of CaO increases with the increase in cycling times, whereas the product CaO could absorb chlorine compounds such as HCl and Cl₂.²⁸ Moreover, according to a study by Gruncharov,²⁹ CaCl₂ could improve the reaction performance of CaSO₄. During the oxidation period, CaS can react with O₂ in the air to form CaSO₄, which indicates that the CaSO₄/Fe₂O₃ oxygen carrier can transfer oxygen as well as inhibit the formation and emissions of dioxins.

CH₄ + 4CaSO₄ → 4CaO + 4SO₂ + CO₂ + 2H₂O (9)

SEM images of the oxygen carrier were obtained to investigate their shape and morphology. Fig. 6 shows a comparison of the SEM images of the pure CaSO₄, fresh CaSO₄/Fe₂O₃ and used CaSO₄/Fe₂O₃ oxygen carrier. These images visually suggest that the pure CaSO₄ have smooth surfaces. The fresh CaSO₄/Fe₂O₃ oxygen carrier comprises of grains, which made their surfaces considerably rough. The fresh CaSO₄/Fe₂O₃ oxygen carrier displayed a very impervious surface with presence of little pores, while the CaSO₄/Fe₂O₃ oxygen carrier after its use displayed a smooth surface with the formation of more pores. The sintering behavior between the different grains can also be observed, which caused the agglomeration of many large particle.²² This made the oxygen carrier more suitable for adsorbing the reducing gas and improved its reaction with the adsorbed gas as compared to pure CaSO₄.²¹ This fact can explain why the reaction rate of the CaSO₄/Fe₂O₃ oxygen carrier is considerably higher than that of the pure CaSO₄ oxygen carrier.

Fig. 6 SEM images of the CaSO₄/Fe₂O₃ oxygen carrier: (a) pure CaSO₄; (b) fresh CaSO₄/Fe₂O₃; (c) used CaSO₄/Fe₂O_3.

4. Conclusions

TGA was used to analyze the thermal decomposition and reduction/oxidation cycle behaviors of the Ca-based oxygen carrier. The reaction of PVC pyrolysis gas with the Ca-based oxygen carrier was investigated in a two-stage reactor. It was found that the addition of Fe₂O₃ could obviously improve the reaction rate of the CaSO₄ oxygen carrier. The fresh CaSO₄/Fe₂O₃ oxygen carrier is more porous than pure CaSO₄ and Fe₂O₃, which has a catalytic effect on the reductive decomposition of CaSO₄ to CaS. The reduction/oxidation test showed that the CaSO₄/Fe₂O₃ oxygen carrier has high oxygen ratios, but the content of CaO increased with time, suggesting the occurrence of side reactions during the cycles. The fresh CaSO₄/Fe₂O₃ oxygen carrier displayed a very impervious surface with the presence of few pores, whereas the CaSO₄/Fe₂O₃ oxygen carrier after its use displayed a smooth surface with the formation of more pores. The CaSO₄/Fe₂O₃ oxygen carrier successfully reacted with the pyrolysis gases of PVC in the two-stage reactor. The formation of a new phase (Ca₂FeO₃Cl) was found after reduction, which indicates that the previously formed Ca₂Fe₂O₅ reacted with the pyrolysis product HCl. The CaSO₄/Fe₂O₃ thus can not only be used as an oxygen carrier in a CLC system, but could also be useful for absorbing chlorine compounds, such as HCl and Cl₂, during the incineration of MSW.

Acknowledgements

Financial support from the National Basic Research Program of China (973 program, Grant no. 2011CB201502), National Natural Science Foundation of China through contract (Grant no. 51406220) and China Postdoctoral Science Foundation funded project (Grant no.2014M551978) are greatly acknowledged.

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