Article pubs.acs.org/EF
Application of CaO-Decorated Iron Ore for Inhibiting Chlorobenzene during In Situ Gasification Chemical Looping Combustion of Plastic Waste Jinxing Wang and Haibo Zhao* State Key Laboratory of Coal Combustion, School of Energy and Power Engineering, Huazhong University of Science and Technology, Wuhan 430074, People’s Republic of China ABSTRACT: Plastic waste has been considered as a renewable energy source, because of its high calorific value. Chemical looping combustion (CLC), with the characteristic of inherent CO2 separation, is a promising alternative for translating plastic wastes into energy with potential to drastically suppress the generation of PCDD/Fs, since it provides an O2-free combustion pattern to inhibit the de novo synthesis of polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/Fs). However, due to the existence of much chlorine element in plastic wastes, the generation of chlorobenzene during the CLC processes should be seriously addressed. CaO decoration to oxygen carrier (OC) has been identified as an effective in situ dechlorination route during the CLC processes. In this paper, an inexpensive material, natural iron ore, was considered as the OC. First, the CLC tests of HCl-containing syngas were carried out in a laboratory-scale fluidized bed to compare the difference of the decoration methods (wet impregnation method and ultrasonic impregnation method) and to determine the optimal CaO loading as well as CLC operational parameters. It was found that the ultrasonic impregnation can attain a higher dechlorination efficiency for the type of relatively dense OC material, i.e., natural iron ore. Then, the in situ gasification chemical looping combustion (iG-CLC) experiments of plastic wastes were conducted in the same reactor and the effect of supply oxygen to fuel ratio on combustion efficiency was investigated. Increasing the supply oxygen to fuel ratio can obviously improve combustion efficiency, and the adverse effect of CaO decoration on combustion efficiency can be obviously mitigated when the ratio of supply oxygen to fuel reaches 2.5. Lastly, the stability of CaO-decorated iron ore was evaluated through 10 continuous cyclic redox tests. The organic compounds, including chlorobenzene, were measured via gas chromatography−mass spectrometry (GC-MS). The combustion efficiency was well-maintained during the cyclic redox process and the emission of chlorobenzene was significantly inhibited in the iG-CLC processes. Overall, these results provide sound evidence for controlling chlorobenzene during the utilization processes of plastic waste.
1. INTRODUCTION A fast developing economy is significantly swelling the amount of the solid waste (SW), including plastic waste, in China.1 Hence, the disposal of SW has become a significant problem that must be urgently solved.2 Compared with landfills, incineration has been considered as a preferable approach for waste management, because of its advantages of mass reduction, volume reduction, hygienic control, and energy recovery.3 Generally, plastic waste often contains chlorine sources, which may result in a significant production of chlorobenzene [which is related to the formation of PCDD/Fs (two series of toxic matter)] upon combustion.4 Therefore, it is necessary to control the emission of chlorobenzene during the incineration process of plastic waste. In our previous publications, chemical looping combustion (CLC), which provides an O2-free combustion pattern, has been speculated as a superior technology for managing plastic waste, together with the advantage of suppressing PCDD/Fs formation.5,6 What is more, CLC has been considered as an alternative technique, because of the inherent enrichment of CO2 during the combustion process.7−10 The schematic description of CLC process is shown in Figure 1. The fuel reacts with the oxygen carrier (OC) to form CO2 and H2O in a fuel reactor (FR), as shown in reaction 1: © 2016 American Chemical Society
Figure 1. Schematic description of chemical-looping combustion.
CnH 2m + (2n + m)MexOy ↔ nCO2 + mH 2O + (2n + m)MexOy − 1
(1)
The OC at the reduction state then is circulated back into the air reactor (AR) for regeneration by air, according to reaction 2. Received: May 7, 2016 Revised: June 7, 2016 Published: June 7, 2016 5999
DOI: 10.1021/acs.energyfuels.6b01102 Energy Fuels 2016, 30, 5999−6008
Article
Energy & Fuels
Table 1. Proximate Analysis, Elemental Analysis (on wt %, As Received), and Lower Heating Value (LHV) of the Plastic Waste Proximatea (wt %, as received)
a
Elemental (wt %, d.a.f)
sample
FCad
Aad
Vad
Mad
LHV (MJ/kg, db)
Cad
Clad
Oad
Sad
Nad
Had
plastic waste
0.08
6.1
93.79
0.03
33.87
73.89
4.92
3.39
0.16
0.72
10.82
FCad = fixed carbon adsorbed, Aad = ash adsorbed, Vad = volatile matter adsorbed, and Mad = moisture adsorbed.
Table 2. Ash Components of the Plastic Wastea Ash Components (wt %)
a
sample
SiO2
Fe2O3
CaO
SO3
ZnO
Al2O3
CuO
Mn2O3
other elements
plastic waste
34.92
19.23
19.54
4.48
2.39
15.63
1.89
1.39
0.53
As received.
O2 + 2MexOy − 1 ↔ 2MexOy
Fe2O3/Al2O3 OC.6 The practicability of iG-CLC of plastic waste was validated, and the effects of reaction atmosphere, reaction temperature as well as the ratio of supply oxygen to fuel on the performance of iG-CLC of plastic waste were examined. It was found that the practical supply oxygen to fuel ratio is 2.5, the suitable reaction temperature is 900 °C, and the optimal fluidizing agent is 40 vol % H2O + 60 vol % N2. Different from previous works, a much less-expensive material, natural iron ore, was used as the OC in this paper. Generally, iron ore has a relatively pyknotic surface, compared with synthetic OC, regardless of fresh or used OC particles.29 More importantly, the specific surface area (BET), as well as pore volume of the iron ore (Table 4, presented later in this paper), in this work is far less than those of synthetic Fe-based OC particles in the previous publication.6 Therefore, probably, it is more difficult for the relatively dense material to decorate CaO (as dechlorination adsorbent) on its surface and inside. CaO-decorated iron ore used as OC has not been evaluated during the CLC process of plastic waste. This work was conducted to examine the CLC performance of plastic waste using CaO-decorated iron ore as OC. Besides, the emission of chlorobenzene (which is not only a toxic organic matter but also can be viewed as an indicative intermediate of PCDD/ Fs30,31) during these iG-CLC processes were also measured. This work was organized as follows. First, iron ore and ores decorated by CaO through two methods (wet impregnation and ultrasonic impregnation, respectively) were used as OCs for the dechlorination investigation in the CLC process of HClcontaining syngas. Ten successive redox experiments using two types of CaO-decorated iron ore as OCs were conducted, and these used OCs were characterized by an environmental scanning electron microscopy system, coupled with an energydispersive X-ray spectroscopy (ESEM-EDX) component (FEI, Model Quanta 2000), tested by using an X-ray diffraction (Shimadzu, Model XRD-700), and detected by a surface area and porosity analyzer (Micromeritics, Model ASAP3000). Next, the iG-CLC experiments of plastic waste using iron ore decorated by 5 wt % CaO through ultrasonic impregnation method as OC were executed in a laboratory-scale fluidized bed reactor to determine the optimal supply oxygen to fuel ratio and examine the stability of combustion efficiency. In addition, the organic matters, including chlorobenzene of two exhaust gases, were measured via gas chromatography−mass spectrometry (GC-MS), using an Agilent Model 7890A gas chromatograph and an Agilent Model 5975C mass spectrograph.
(2)
To date, three main patterns for CLC with solid fuels have been widely adopted.7 The first is to transform solid fuels to syngas as the input of FR, namely, syngas-CLC.7,11 Obviously, the complexity of the system is a significant deficiency, because of the addition of a gasifier. The second is the pyrolysis and gasification of solid fuel, as well as subsequent combustion reactions with OC, which can occur simultaneously after solid fuels are directly introduced into the FR via a gasification agent (CO2/H2O), namely, iG-CLC.12,13 The third is to burn the solid fuels with gaseous oxygen released from special OCs in the FR. The last approach is the so-called “chemical looping with oxygen uncoupling” (CLOU).14,15 However, this approach may not be conducive to inhibiting the formation of PCDD/Fs, because of the existence of gaseous oxygen in FR.16 Therefore, the iG-CLC of plastic waste has been considered as an optimal choice for disposing plastic waste. The OCs are critical for the application of the CLC process.17,18 At present, some combustive gases (methane, hydrogen, and syngas from coal gasification) were used as fuel to test the properties of some metal oxides in either a thermogravimetric analyzer or a fluidized bed reactor, and these metal oxides included Ni-, Co-, Mn-, Cu-, and Fe-based oxides, as well as their metal blends.19 Among them, Ni-based OC should be restricted due to its toxicity, although Ni-based OCs generally demonstrate a better reactivity than other metal oxides.20 In thermodynamics, the Co-based, Mn-based and Cubased OCs are the candidates of the OCs which can release gaseous O2 at high temperatures.21 Alternatively, Fe-based OC may be an attractive OC for CLC technique, because of its abundance, low price, moderate reactivity with fuels, and its ability to endure physical and thermal stress during hightemperature fluidization.20,22,23 Notably, iron ore has emerged as a very attractive OC candidate, because of its inexpensive and environmentally friendly characteristics.24,25 Effective dechlorination during the CLC process may be conducive to suppressing the generation of PCDD/Fs through the precursor conversion approach.26−28 In our previous publications, the impacts of adsorbent types (i.e., Na2O, K2O, and CaO), decoration method (physical mixture, coprecipitation, and wet impregnation) as well as loading content for synthetic Fe2O3/Al2O3 OC on dechlorination efficiency in the syngas-CLC have been evaluated using a syngas that was primarily dependent on the pyrolysis result of plastic waste at 900 °C.5 This work confirmed CaO as the optimal adsorbent, wet impregnation as the best decoration method, and 5 wt % as the applicable loading content. Next, the iG-CLC tests of plastic waste were carried out using the optimal CaO-decorated 6000
DOI: 10.1021/acs.energyfuels.6b01102 Energy Fuels 2016, 30, 5999−6008
Article
Energy & Fuels
μm) was used as the precursor and ∼100 mL of solution was produced through dissolution in deionized (DI) water. The fresh iron ore then was soaked in the solution. For the wet impregnation method, the mixture of the precursor and iron ore was successively churned for 12 h at 90 °C and dried at 105 °C. For the ultrasonic impregnation method, the mixture of the precursor and iron ore was heated for 6 h at 90 °C with under established conditions (40 000 Hz and 100 W) in a ultrasonic cleaner (Model SB-100DT, XiAn ChaoJie Biotechnology Limited Company). The dried iron ore impregnated by Ca(NO3)2· 4H2O was first calcined at 900 °C for 1 h to release NOx and then chilled down to ambient temperature; these particles were ultimately resieved to 0.2−0.3 mm. In this study, three mass ratios of CaO to iron ore particles were considered, i.e., 5, 10, and 15 wt %. 2.2. Apparatus and Procedure. The syngas-CLC and iG-CLC tests were studied in a laboratory-scale fluidized bed reactor. An overview of the experimental system is shown in Figure 2. The system
2. EXPERIMENTAL SECTION 2.1. Materials. In this work, a perfusion tube was selected as a representative plastic waste sample. The proximate analysis, elemental analysis, and lower heating value (LHV) of the plastic waste are presented in Table 1, and its ash components are shown in Table 2. A detailed description of the proximate and elemental analyses can be found elsewhere.32 A type of natural iron ore was selected as OC. To improve the crushing strength and eliminate the volatile components of the iron ore, these iron ore particles were successively calcined at 500 °C for 3 h and at 1000 °C for 6 h, using a muffle oven. Eventually, they were ground and samples in the diameter range of 0.2−0.3 mm were selected as the fresh OC particles. The composition analysis of fresh iron ore was performed by X-ray fluorescence (XRF) spectrometry (EDAX EAGLE III), as shown in Table 3. It can be found that the
Table 3. Compositional Characteristics of Iron Ore component
content (wt %)
Fe2O3 SiO2 Al2O3 TiO2 others
81.89 8.42 8.37 0.74 0.58
main component of this iron ore includes 81.89 wt % Fe2O3, 8.42 wt % SiO2 and 8.37 wt % Al2O3. The physical and chemical properties of the fresh iron ore particles are shown in Table 4. The oxygen transport
Table 4. Physical and Chemical Properties of Fresh Iron Ore property
value
particle size attrition index oxygen transport capacity, ROC specific surface area, BET pore volume average pore diameter density XRD main phases crushing strength
0.2−0.3 mm 2.1%/h 2.73 wt % 0.31 m2/g 2.03 × 10−3 cm3/g 0.641 μm 4.92 × 103 kg/m3 Fe2O3, SiO2, Al2O3 2.61 N
Figure 2. Overview of the fluidized-bed reactor.
mainly includes a gas supplying unit, a reactor unit, NaHCO3 aqueous solution, toluene solution, and an online gas analyzer (Wuhan, Model Gasboard-3151). Note that the dechlorination in syngas-CLC was evaluated via detection of the chlorine ion concentration in NaHCO3 aqueous solution by an ion chromatograph (ThermoScientific, Model ICS-90), while the toluene solution was used for absorbing the organic matter generated in iG-CLC. The chlorobenzene in toluene solution was measured by the GC-MS with an Agilent Model 19091S-433 column. The optimized GC temperature-programmed procedure was as follows: splitless injection of 1 μL at 50 °C, original oven temperature of 50 °C for 3 min, then heated at 20 °C min−1 to 300 °C and maintained for 3 min at the final temperature. For the laboratoryscale fluidized bed reactor, the reaction chamber was an erect stainless steel tube (i.d. = 26 mm, length = 892 mm) and it was electrically heated by a furnace. A porous distributor plate is located at 400 mm from the bottom, and a thermocouple is placed at the center of the furnace to monitor the furnace temperature, which is controlled by a temperature controller. It is worth noting that the actual bed temperatures of the fluidized bed were measured by using a portable thermocouple before these experiments. The exhaust gas first entered into a filter to extract particulate matter (ash) and then went through either the toluene solution for absorbing the organic matters (including chlorobenzene) under the ice-bath condition or the NaHCO3 aqueous solution for absorbing the remaining HCl. The vast majority of ash can be separated in the reactor, because of the density variation and then collected in the filter. Next, the offgas was introduced into a condenser to remove the vapor and next to the online gas analyzer for measuring the concentrations of CO2, CO, CH4, H2, and O2; then, these concentration values were dynamically recorded by a computer. The fluidized bed reaction system was also described elsewhere.5,22,24,34 Note that redox experiments of 60 cycles have been conducted to examine the reactivity persistence of analogous OC in our previous publication.5 Therefore, in this study, little attention was given to the long-term stability of CaO-decorated
capacity was defined as the ratio of the mass of available active oxygen to the mass of oxygen carrier, where the mass of available active oxygen was calculated on the premise that Fe2O3 is the sole oxidation state and Fe3O4 is the single reduction state. The specific surface area (BET) and pore volume of iron ore particles were evaluated by using a surface area and porosity analyzer (Micromeritics, Model ASAP3000) to test adsorption/desorption of nitrogen at 77 K. The real density and attrition index of the iron ore particles were examined using an automatic true density analyzer (AccuPyc 1330) and an abrasion tester. To test the attrition index, ∼30 g of iron ore particles was placed within a stainless cylinder (i.d. = 12.0 cm, length =14.5 cm), which has a 1.5 cm baffle, and rotated on a ball-mill roller for 50 min at a constant rate (10 rpm). After resieving and weighing the attrited particles, the attrition index was obtained using the expression m − m2 δ (%) = 1 × 100 m1 where m1 and m2 are the initial and final mass of iron ore, respectively. Besides, the crushing strength and the crystalline phases of the iron ore were measured by a crushing strength apparatus (Shimpo, Model FGJ5) and an XRD system, respectively. Note that the crushing strength is the mean value of 20 measurements. The extra ions were added to the fresh iron ore via wet impregnation5,33 and ultrasonic impregnation methods. For the two decoration methods, Ca(NO3)2·4H2O (>99.9 wt % purity; size of
