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Article

Chemical Looping Gasification of Wood Waste Using NiO-Modified Hematite as an Oxygen Carrier

by
Jinlong Xie
1,
Kang Zhu
1,2,
Zhen Zhang
1,2,
Xinfei Chen
2,
Yan Lin
2,
Jianjun Hu
3,
Ya Xiong
4,
Yongqi Zhang
5,
Zhen Huang
2,* and
Hongyu Huang
2
1
School of Mechanical and Electrical Engineering, Guangzhou University, No. 230 Waihuan Xi Road, Guangzhou 510006, China
2
Key Laboratory of Renewable Energy, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences (CAS), No. 2 Nengyuan Road, Wushan, Tianhe District, Guangzhou 510640, China
3
Key Laboratory of New Materials and Facilities for Rural Renewable Energy Ministry of Agriculture and Rural Affairs, College of Mechanical & Electrical Engineering, Henan Agricultural University, No. 63 Agricultural Road, Zhengzhou 450002, China
4
School of Environmental Science and Engineering, Sun Yat-sen University, No. 135 Xingang Xi Road, Guangzhou 510275, China
5
State Key Laboratory of Coal Conversion, Institute of Coal Chemistry Chinese Academy of Sciences, Taiyuan 030001, China
*
Author to whom correspondence should be addressed.
Energies 2023, 16(4), 1847; https://doi.org/10.3390/en16041847
Submission received: 15 January 2023 / Revised: 9 February 2023 / Accepted: 11 February 2023 / Published: 13 February 2023
(This article belongs to the Special Issue Chemical Looping for Syngas Production)
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Abstract

:
Chemical looping gasification (CLG) technology is an effective approach to converting wood waste into high-quality syngas. In the present work, the reactivity of natural hematite is enhanced by doping with nickel oxide (NiO), and the effects of various operating parameters upon the CLG of wood waste are investigated using the NiO-modified hematite as an oxygen carrier. The NiO-modified hematite gives a significantly increased carbon conversion of 79.74%, and a valid gas yield of 0.69 m3/kg, compared to 68.13% and 0.59 m3/kg, respectively, for the pristine (natural) hematite, and 54.62% and 0.55 m3/kg, respectively, for the Al2O3, thereby indicating that the modification with NiO improves reactivity of natural hematite towards the CLG of wood waste. In addition, a suitable mass ratio of oxygen carrier to wood waste (O/W) is shown to be beneficial for the production of high-quality syngas, with a maximum valid gas yield of 0.69 m3/kg at an O/W ratio of 1. Further, an increase in reaction temperature is shown to promote the conversion of wood waste, giving a maximum conversion of 86.14% at reaction temperature of 900 °C. In addition, the introduction of an appropriate amount of steam improves both the conversion of wood waste and the quality of the syngas, although excessive steam leads to decreases in the reaction temperature and gas residence time. Therefore, the optimum S/B (mass ratio of steam to biomass) is determined to be 0.4, giving a carbon conversion and valid gas yield of 86.63% and 0.94 m3/kg, respectively. Moreover, the reactivity of the NiO-modified hematite is well-maintained during 20 cycles, with a carbon conversion and valid gas yield of around 79% and 0.69 m3/kg, respectively. Additionally, the XRD and SEM-EDS analyses indicate no measurable change in the crystal phase of the re-oxidized oxygen carrier.

1. Introduction

With the rapid growth of the economy and population, China has become the largest producer and consumer of wood-based panels in the world. In 2020, the total output of wood-based panels was 311.01 million m3, and the total consumption was 296.16 million m3. Meanwhile, a large amount of wood waste is produced during the processing and production of wood-based panels [1]. Generally, the treatment of wood waste mainly involves recycling and thermochemical conversion [2]. However, the performance of the recycled wood-based panels is poor, and the service life is also significantly shortened. Additionally, more nitrogen-containing effluents are produced during the reprocessing of the waste wood-based panel, thereby further increasing the difficulty of recycling [3]. Wood waste contains a large amount of organic matter, and the volatile content is also high (~70%). Thus, wood waste is viewed as an excellent potential solid fuel. In this context, the thermochemical conversion of wood waste can recover energy or generate high-quality products (including gas, liquid, and solid products).
Among the various thermochemical conversion technologies, gasification is widely used for converting solid fuels to syngas, which has many applications. Additionally, gasification technology has the advantages of low pollutants and high energy recovery [4,5,6].
However, typical gasification media such as air, oxygen, or steam each have specific weaknesses [7,8,9,10]. For example, when air is a gasification medium, syngas has a low heating value due to a high concentration of nitrogen [11]. Meanwhile, pure oxygen can generate high-quality syngas, but this approach is unsuitable for industrial production due to the high cost of producing pure oxygen [12]. Moreover, the introduction of steam can improve the quality of syngas by increasing the H2 concentration. However, the steam gasification of solid waste is an endothermic process that requires a high energy input [13]. In addition, the gasification of solid fuel produces an undesirable by-product (tar), that has many side-effects on the downstream use of syngas, including pipeline blockage, corrosion, catalyst deactivation, and lower gasification efficiency [14,15,16].
Due to the limitations of conventional gasification technology, more advantageous chemical looping gasification (CLG) technology has been proposed. As shown in Figure 1, the CLG process includes two interconnected reactors, namely, a fuel reactor (FR) and an air reactor (AR) [17,18]. In the FR, the oxygen carrier (OC) is reduced, and the solid fuel is partially oxidized into syngas. In the AR, the reduced OC is oxidized to its initial state by air. The use of an OC instead of molecular oxygen has many benefits during the gasification process. For instance, it avoids the dilution of the syngas by nitrogen, and inhibits the generation of nitrogen-containing contaminants [19,20,21]. In addition, the use of an OC reduces the cost of syngas production because pure oxygen is not required [22]. Furthermore, the OC acts as a carrier and provider of heat, so that an external heat source is not needed during the gasification of solid fuels. Moreover, as a metal oxide, the OC can serve as a good catalyst for the cracking of tar and char, thereby reducing the yield tar and the deposition of carbon [23,24].
In view of the above considerations, OC clearly plays a crucial role in the CLG process by providing lattice oxygen and transferring heat energy for the gasification reaction [25,26,27]. The ideal OC should have the following characteristics: (i) a high oxygen-carrying capacity along with a good oxygen-release capacity; (ii) a high redox performance and catalytic activity; (iii) good mechanical strength; (iv) strong thermal stability, without being prone to sintering and agglomeration; and (v) environmental friendliness, with cheap and easy access to raw materials.
Hence, the iron (Fe)-based OCs are currently the most studied in CLG because of their good mechanical properties, economy, stability at high temperatures, and environmental friendliness. Moreover, the Fe-based OCs have been shown to provide good cycling stability during long-term operation in the CLG of biomass [28]. OCs based on hematite as a natural Fe source have shown good advantages for industrial application due to their low cost, good mechanical strength, and ready availability [29,30,31]. However, the application of Fe-based OCs is limited due to their low reactivity and low oxygen transport capacities during CLG. Doping foreign metals is an effective method. Jia L et al. reported that doping multiple metals could reduce the pyrolysis reaction barrier of the modified biomass. On the modified surface of the sample, the doped metals formed aggregated oxides, and the resulting synergistic effect enhanced the oxidative activity of the biochar carriers and the threshold effect of Ce oxide [32]. Meanwhile, although the nickel (Ni)-based OCs have higher reactivities and better catalytic effects upon tar cracking than do the Fe-based OCs, they are easily deactivated due to the carbon deposition and sintering phenomena [33,34]. Hence, to take advantage of both metals, the doping of hematite with Ni or other foreign ions (e.g., Cu or Mn) has been considered to be a good solution for improving the reactivity [35]. Previous studies have also reported that the reactivity of the OC can be further improved by modification with nickel oxide (NiO) [36,37,38]. For instance, the incorporation of NiO within ilmenite has significantly improved the reactivity of the ilmenite-based OC [39]. Furthermore, the modification of the OC with NiO has been shown to enhance the reactivity towards copper slag and promote the conversion of sludge, especially that of sludge char [40]. Hence, the use of NiO-modified hematite as an OC for the efficient conversion of wood waste may be a promising approach [41,42].
Many researchers have studied the biomass chemical looping gasification process, but few works are relevant to wood waste. Hu Z F et al. selected Fe2O3 as an oxygen carrier for the CLG reaction with Chlorella vulgaris. The study found that Fe2O3 had a catalytic effect on the CLG process and could promote the pyrolysis and subsequent gasification reaction of Chlorella vulgaris [43]. Zou J et al. found that the performance of CeO2/Fe2O3 (Ce-Fe molar ratio of 3:7) composite oxygen carriers was better than pure CeO2 or Fe2O3 monometallic oxygen carriers in hydrogen production by cellulose CLG experiments, and the presence of CeO2 not only improved the oxidation capacity of oxygen carriers but also facilitated the oxidation of deposited carbon on the surface of oxygen carriers, while the introduction of Fe enhanced the cleavage of pyrolytic volatiles and promoted the generation of CO and CH4 [44]. Sun Z et al. compared different oxygen carriers in pine wood CLG experiments on gasification, and the results showed that the application of the composite oxygen carrier Ca2Fe2O5 increased the total gas yield and carbon conversion by 5.0% and 5.9%, respectively, over the single-metal oxygen carrier Fe2O3 for the gasification reaction [45].
The innovation of this study is to mix hematite with a small amount of nickel oxide (NiO) to make a bimetallic oxide (NiO-modified hematite) oxygen carrier, and use chemical looping gasification technology to treat wood waste to further realize the stepwise utilization of energy. In this study, the reactivity of NiO-modified hematite is studied using a fixed-bed reactor. The effects of various operating parameters, such as the mass ratio of OC to wood waste (O/W), the reaction temperature, the steam concentration, and the number of cycles, upon the gasification performance are discussed. In addition, the phase structures, morphological features, and elemental distributions of the fresh, reduced, and re-oxidized NiO-modified hematite samples are investigated by X-ray diffraction (XRD) and scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDS).

2. Experimental

2.1. Materials

The wood waste samples were collected from a furniture factory in Guangzhou. Before the experiment, the wood waste was crushed and sieved to obtain powder samples (<150 μm) and dried at 105 °C for 48 h. The ultimate and proximate analyses of the wood waste sample (dry basis) are listed in Table 1. It can be seen that the wood waste has a high quantity of volatiles (73.67%), and the results of ultimate analysis also indicate that the wood waste has a high C content (45.89%). In addition, the calorific value of wood waste is 17,427 kJ/kg.
The hematite was also pulverized into powder (<150 μm) and used as the fresh OC. To remove any impurities and obtain a higher Fe2O3 content, along with a more stable crystal phase structure, the hematite was calcined in a muffle furnace at 900 °C for 3 h prior to the experiment. The hematite powder and was then mixed with NiO powder (Shanghai Macklin Biochemical Technology Company, Analytical reagent) to improve its reactivity. The preparation process for the NiO-modified hematite was as follows: (i) the NiO and hematite were mixed in a mass ratio of 1:9; (ii) the mixture was then heated to 950 °C at a heating rate of 10 °C/min in a muffle furnace, after which the target temperature was maintained for 3 h; (iii) the as-calcined samples were then ground into powder (<150 μm), sealed, and stored for subsequent use. The X-ray fluorescence (XRF) analyses of the pristine hematite and the NiO-modified hematite are presented in Table 2.

2.2. Characterization

The crystalline phases of the fresh, reduced, and re-oxidized OC samples were identified via X-ray diffractometry (XRD; X’Pert Pro MPD) under Cu Kα radiation (λ = 0.1504 nm) at an applied voltage of 40 kV and a current of 40 mA. The OCs were scanned in the range of 5–80° at a rate of 2°/min in steps of 0.02°. The surface structures of the fresh, reduced, and re-oxidized OC samples were examined via scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS; Instrument model, Manufacturer).

2.3. Experimental Set-Up and Procedure

The experimental set-up is shown schematically in Figure 2. Thus, the wood waste gasification experiments were performed in a batch fixed-bed reactor (the length is 800 mm and the inner diameter is 24 mm) connected to a material controller, a mass flow controller, a steam generator, two gas cylinders, a temperature controller, a tail gas treatment system, and a gas chromatograph. The thermocouple is attached to the temperature controller, located in the lower half of the fixed bed, and monitors the temperature of the area near the quartz partition.
The experimental procedure was as follows. First, a small amount of quartz wool was placed on the spacer of the quartz tube, a certain amount of NiO-modified hematite was placed on the quartz wool, and 0.5 g of wood waste was place in the material controller. Then, the fixed bed reactor was heated to the target temperature at a heating rate of 30 °C/min under an argon (Ar) atmosphere. After holding for 3 min, the material controller was turned on and the wood waste was dropped into the reaction zone, where a series of redox reactions occurred in contact with the OC. During the collection process, the gas products were first passed through isopropyl alcohol solution to remove excess tar, and then through a color-changing silica desiccant to remove excess water. After undergoing the cooling, purifying, and drying processes, all the gas products were collected in sampling bags for the gas chromatograph (SHIMADZUA Gas Chromatograph, GC-2014) analysis. After collecting the gas products, the OC was allowed to cool to room temperature under an Ar atmosphere. To regenerate the OC, oxygen was introduced at a flow rate is 100 mL/min for 30 min.
As summarized in Table 3, the reactivities of the NiO-modified hematite OCs during wood waste gasification were investigated by varying the reaction temperature, the mass of the OC, and the amount of steam in the gasification agent. For all experiments, the mass of wood waste was set at 0.5 g, the carrier gas (Ar) and oxidation agent (O2) flow rates were set at 100 mL/min, and the reaction temperature was set at 850 °C. For the wood waste gasification experiments, the mass ratio of OC to wood waste (O/W) is set at 1:1. For the cyclic test, the reduction time was 45 min, the purge time was 30 min, the oxidation time was 30 min, the number of cycles was 20, and the total running time was approximately 48 h. For comparison, Al2O3 was used as an oxygen carrier. In addition, the effect of varying the mass ratio of steam to biomass (S/B) was investigated.

2.4. Data Processing

The total volume of the outlet gas Vout was calculated from the argon balance in accordance with Equation (1):
V o u t = V i n 1 − Σ i y i
where Vin is the volume Vin of inlet gas, and yi is the concentration of each gas product (CO2, H2, CO, CH4, and CnHm).
The carbon conversion rate (ηC) is the ratio of the total molar amount of carbon in the outlet gas to the molar amount of carbon in the wood waste (WC%), and is given by Equation (2):
η C = 12 × V o u t ×   y C O 2 + y C O + y C H 4 + 2 × y C 2 H 4 + 2 × y C 2 H 6 + 3 × y C 3 H 6 22.4 × m × W C %
where m is the mass of wood waste.
The relative concentrations (Ri) of the gas products are given by Equation (3):
R i = y i ∑ y i × 100 %
The volume of combustible gas produced per gram of wood waste, i.e., the yield of valid gas (Vg), was calculated using Equation (4): C 2
V g = V o u t ×   y C O + y H 2 + y C H 4 + y C 2 H 4 + y C 2 H 6 + y C 3 H 6 m

3. Results and Discussion

3.1. The Effect of Various OCs

The XRD patterns of the pristine hematite and the NiO-modified hematite are presented in Figure 3. Here, the main crystal phases of the pristine hematite are seen to be Fe2O3 and Fe3O4, along with a small amount of SiO2. In addition to the Fe2O3 phase, however, the NiO-modified hematite exhibits NiO crystal phases, along with a newly-formed NiFe2O4 crystal phase. The latter is attributed to the reaction between Fe2O3 and NiO at high temperature. These results demonstrate that the crystal phase of the pristine hematite is changed by the NiO-modification, because the order of oxidizability of three species is NiO > NiFe2O4 > Fe2O3 [46]. Huang Z et al. used different oxygen carriers (OCs) for in situ catalytic cracking of biomass tars; NiFe2O4 with a homogeneous Fe/Ni dispersibility shows the best reactivity for toluene cracking with a toluene conversion and H2 yield [47]. The reactivity of the NiO-modified hematite is be higher than that of the pristine hematite during chemical looping gasification of wood waste.
The gas yield results for the individual gases of the wood waste gasification experiments using Al2O3, pristine hematite, and NiO-modified hematite are shown in Figure 4. The experiment was performed at a temperature of 850 °C and a mass ratio of OC to wood waste (O/W) of 1.
Here, the yields of CO, CO2, H2, and CH4 are clearly lowest when the Al2O3 is used, because it hardly provides any lattice oxygen for the gasification reaction. Accordingly, the carbon conversion and valid gas yield are the lowest, at 54.62% and 0.55 m3/kg, respectively. By contrast, the pristine hematite is able to provide lattice oxygen for the gasification reaction, thus providing an improved carbon conversion and valid gas yield of 68.13% and 0.59 m3/kg, respectively. However, the NiO-modified hematite exhibits higher yields of CO, CO2, and H2 because its higher reactivity is beneficial for promoting the conversion of wood waste and the cracking of tar [40]. Consequently, the highest carbon conversion and valid gas yield of 79.74% and 0.69 m3/kg, respectively, are obtained. In addition, the calorific values of the gas products using different oxygen carriers (Al2O3, pristine hematite and NiO-modified hematite) were 13.1 MJ/Nm3, 12.4 MJ/Nm3 and 11.9 MJ/Nm3, respectively. The latter two oxygen carriers, because of the presence of active lattice oxygen, have a reduced proportion of syngas in the gas product. Wei G Q et al. investigated the enhancement of hematite oxygen carriers (OCs) by exogenous active metals during CLG of lignite using TG and fixed-bed reactors with various analytical methods. The experimental results showed that Fe2O3 in hematite reacted with different exogenous metals to form mixed oxides such as CaFe2O5, NiFe2O4, CeFeO3, and CoFe2O4, which increased the oxygen vacancy concentration, enhanced the lattice oxygen transfer, and improved the conversion efficiency of lignite [48]. These results clearly demonstrate that the reactivity of hematite is improved by doping with NiO, and that the NiO-modified hematite can be used as a good OC for the CLG of wood waste.

3.2. The Effect of Varying O/W

As detailed in the Experimental section, the mass ratio of OC to wood waste (O/W) was varied by varying the OC (i.e., the NiO modified hematite) mass for a constant mass of wood waste, giving O/W ratios of 0 to 10. The resulting gas concentrations, carbon conversions, and valid gas yields are shown in Figure 5. In this experiment, the reaction temperature was set to 850 °C.
Here, the concentrations of CO and H2 each exhibit an increasing trend as the O/W increases from 0 to 1 (Figure 5a). This is because the addition of the active OC provides lattice oxygen to promote the conversion of wood waste, along with the catalytic cracking of tar and partial oxidation of the char. However, with the further increase in O/W, the excess lattice oxygen further oxidizes the combustible gases to CO2 and H2O, thereby resulting in a gradual decrease in the concentrations of CH4, CO, and H2. In addition, the iron species are viewed as good catalysts for tar catalytic cracking, and their catalyst activity is determined by availability of reducible iron on the surface of particles [49]. Hence, the concentration of CO2 increases with the increase in O/W because more lattice oxygen is provided for the conversion of wood waste and further oxidation of the carbon-containing gas products. As a result, the carbon conversion also increases with the increase in O/W (black line, left-hand axis Figure 5b), reaching a maximum of 88.41% at an O/W of 10. Meanwhile, the valid gas yield (blue line, right-hand axis, Figure 5b) is seen to decrease gradually as the O/W increases above 1. The is because the increasing amount of lattice oxygen facilitates the depletion of combustible gases, thereby leading to the conversion of CO and H2 to CO2 and H2O, the phase transition of steam occurs from gas to liquid in the condensation process, and the dried gas yield starts reducing [50], as noted above. These results demonstrate that a higher content of the NiO-modified hematite can effectively promote the conversion of wood waste and improve the carbon conversion. However, too low an O/W value is not conducive to the conversion of wood waste due to the shortage of an oxygen source, while too high an O/W value inhibits the wood waste gasification because more syngas is easily oxidized by the relatively abundant oxygen source. Previous experimental results also show that a suitable O/W value can obtain better gasification efficiency. Niu X et al. used Linz-Donawitz converter slag (LD slag) as an oxygen carrier in biomass chemical looping gasification, and the experimental results showed that LD slag exhibited a higher gas yield at the supply oxygen coefficient (O/B) ratio of 1 at 900 °C [51]. Yan B B et al. studied Chlorella CLG based on a NiFe2O4 oxygen carrier (OC). The optimum parameters of Chlorella CLG were the temperature of 800 ℃ and the ratio of OC to biomass (O/B) of 1 [52]. Hence, the O/W value is fixed at the optimum value of 1 in the following investigations, giving a maximum carbon conversion of 79.74% and a maximum valid gas yield of 0.69 m3/kg.

3.3. The Effects of Varying the Reaction Temperature

The reactions occurring during the gasification of wood waste are given by Equations (5)–(7):
Wood Waste → Gas + Char + Tar
Char/Tar → Gas
CO + 3Fe2O3 = CO2 + 2Fe3O4
The effects of various reaction temperatures upon the gasification of wood waste are indicated in Figure 6. In this part of the experiment, the value of O/W was set to 1 and NiO-modified hematite was used as an oxygen carrier. Here, the yields of CO2, CO, H2, and CH4 are seen to increase with the increase in reaction temperature (Figure 6a). This is because the pyrolysis of wood waste (Equation (5)) and the gasification of tar and char (Equation (6)) are both endothermic, and are therefore favored by a high temperature.
Meanwhile, in Figure 6b, the concentration of CO is seen to increase, while that of CO2 decreases, with the increase in reaction temperature. This is because the reaction of CO with OC in Equation (7) is exothermic, so that the increase in reaction temperature favors the reverse reaction, thereby promoting the production of CO and consumption of CO2. By contrast, the concentrations of CH4 and H2 are seen to be relatively stable. At the same time, because the increase in reaction temperature promotes the oxidation of char and tar (Equation (6)), the carbon conversion is seen to increase from 62.15% to 86.77% as the reaction temperature increases from 750 to 900 °C (Figure 6b, green line, right-hand axis). In addition, Huang Z et al. used hematite as an oxygen carrier to treat sewage sludge with a reaction temperature range of 800–950 °C. The experimental results showed that a temperature of 900 °C could achieve better carbon conversion and gas yield [53]. The formation of low-valent iron species not only easily causes the sintering and agglomeration of oxygen carrier particles, but is also unfavorable for the fluidization of particles in a fluidized bed reactor, used in industrial equipment [54]. Therefore, in the following experiments, the reaction temperature was fixed at 850 °C.

3.4. The Effects of Varying the S/B

The presence of steam in the reactor can lead to the reactions shown in Equations (8)–(10):
CO + H2O → CO2 + H2
CH4 + H2O → CO2 + H2
Tar + H2O → CO + CO2 + H2
The effects of varying the mass ratio of steam to biomass (S/B) upon the gas concentrations, the carbon conversion, and the valid gas yield are shown in Figure 7. In this experiment, the reaction temperature was set to 850 °C, NiO-modified hematite as the oxygen carrier, and the O/W value was 1. Here, the concentration of H2 is seen to increase from 19.30% to 32.23%, while that of CO2 increases from 15.21% to 17.66%, as the steam content is increased from 0 to 0.4. Meanwhile, the concentration of CO decreases from 37.43% to 31.07%, and that of CH4 decreases from 9.15% to 6.72%. This is because the addition of steam promotes both the water–gas shift reaction (Equation (8)) and the CH4 reforming reaction (Equation (9)), thus leading to the production of CO2 and H2 and the consumption of CO and CH4.
At the same time, the carbon conversion (black line, left-hand axis, Figure 7b) is seen to increase from 79.74% to 86.61%, while the valid gas yield (blue line, right-hand axis, Figure 7b) increases from 0.69 to 0.94 m3/kg, as the S/B is increased from 0% to 40%. This is because the addition of steam can provide an additional source of oxygen and promote the conversion of tar into gaseous products (Equation (10)) [55]. However, the generation of excess steam requires the absorption of a large amount of heat, thereby decreasing the reaction temperature. Moreover, the introduction of steam increases the gas flow rate and decreases the gas residence time. These factors are not conducive to the contact reaction between the intermediate gaseous products and the OC. Consequently, when the S/B is further increased to 0.5, the carbon conversion and valid gas yield are decreased to 84.82% and 0.89 m3/kg, respectively. The S/B value influences the quality of the syngas. Mu L et al. created a biomass chemical looping gasification model using Aspen Plus software, and the exergy efficiency of the chemical looping gasification process of pine sawdust, rice stalk, and corn stalk were compared. The results showed that the optimal operating conditions were gasification temperature of 850 °C and S/B of 0.4, which could maintain the thermal equilibrium, and the system could achieve high energy efficiency and exergy efficiency [56]. Condori O et al. used ilmenite as an oxygen carrier and pine wood as fuel to produce high-quality syngas. The S/B was selected in the range of 0.05–0.9, and high syngas yields were obtained at S/B values of 0.6 [57]. Experimental results of this study clearly demonstrate that the optimum steam content is 40%, giving a maximum carbon conversion of 86.61% and a maximum valid gas yield of 0.94 m3/kg.

3.5. The Effects of the Number of Cycles

The cycling performance of the NiO-modified hematite is shown in Figure 8. Here, no significant change is observed in the concentration of each gas during the 20 cycles (Figure 8a). However, the carbon conversion (black line, left-hand axis, Figure 8b) is seen to increase slightly as the number of cycles increases. This may be because the accumulation of ash content prolongs the residence time, thereby generating more gas products. Meanwhile, the effective gas yield (blue line, right-hand axis, Figure 8b) is seen to decrease slightly in the 20th cycle. This is because the increase in the gas residence time allows more CO and H2 to be oxidized to CO2 and H2O. Nevertheless, the carbon conversion and valid gas yield do not change significantly during the 20 cycles, remaining at ~79% and ~0.69 m3/kg, respectively. These results clearly demonstrate the good cycling performance of the NiO-modified hematite during the CLG of wood waste. Similar results have been found in previous studies. Huang Z et al. used hematite oxygenate carriers to treat sludge sewage, and the carbon conversion and gas yield were maintained at a constant value during 12 cycles of the experiment [53].

3.6. Characterization of the OC at Various Reaction Stages

The changes in the crystalline phases, morphological features, and elemental distributions of the fresh OC, the reduced OC, and the re-oxidized OC are revealed by the XRD spectra in Figure 9 and the SEM-EDS analyses in Figure 10. Thus, the fresh OC (red spectrum, Figure 9) is seen to consist mainly of the crystal phases NiFe2O4, Fe2O3, and Fe3O4. Meanwhile, due to the release of lattice oxygen, the NiFe2O4 and Fe2O3 phases are replaced by Fe-Ni alloy and FeO in the reduced OC (black spectrum, Figure 9). However, the re-oxidized OC exhibits the same crystal phases as the fresh OC, thus confirming the regeneration of the NiO-modified hematite. Similar results have been found in previous studies. Huang Z et al. used hematite oxygenate carriers to treat sewage sludge, and the carbon conversion and gas yield were maintained at a constant value during 12 cycles of the experiment [53].
Meanwhile, the SEM image of the fresh OC (left-hand panel, Figure 10a) reveals the presence of numerous particles attached to the surface, and the sample exhibits a good porosity. Further, the elemental distribution (right-hand panel, Figure 10a) consists mainly of Fe, O, and Ni, with much higher intensity peaks for Fe and O than for the other elements. By comparison, the surface of the reduced OC (left-hand panel, Figure 10b) is smoother, and the intensity of the O peak in the EDS (right-hand panel) is significantly decreased due to the release of lattice oxygen. Finally, the surface image of the re-oxidized OC (left-hand panel, Figure 10c) reveals that the particles are slightly sintered compared to those of the fresh OC, but the porous structure is maintained. Further, the O peak in the EDS (right-hand panel, Figure 10c) is almost identical to that observed in the fresh OC, thereby indicating that the lattice oxygen of the OC recovered well. However, the intensity of the peak of element Ni was reduced. The reason may be that the EDS characterization means can only detect the distribution of elements on the surface of the sample, the oxidized OC may have some sintering phenomenon, and the Ni elements are covered by impurities, so the intensity of Ni elements is lower than that of fresh OC.

4. Conclusions

In this work, nickel oxide (NiO)-modified hematite was used as an oxygen carrier (OC) in the chemical looping gasification (CLG) of wood waste, and the effects of various parameters were discussed. In addition, the fresh, reduced, and re-oxidized OCs were characterized in order to clarify the cycling performance of the modified OC. Compared with the pristine (unmodified) hematite, the reactivity of the NiO-modified hematite was significantly improved. In detail, the carbon conversion and valid gas yield were increased from 68.13% to 79.74% and from 0.59 to 0.69 m3/kg, respectively. In addition, a suitable increase in the mass ratio of OC to wood waste (O/W) was shown to improve the carbon conversion, whereas too high an O/W value was not conducive to the gasification of wood waste. Consequently, a maximum valid gas yield of 0.69 m3/kg was obtained at an O/W of 1. An increase in reaction temperature was also shown to promote the conversion of wood waste, giving a maximum conversion of 86.14% at 900 °C. Further, the introduction of steam was shown to be beneficial to the conversion of wood waste and the improvement of the syngas quality. Specifically, a maximum carbon conversion of 86.05% and a maximum valid gas yield of 0.94 m3/kg were obtained at a S/B of 0.4. During 20 repeated CLG cycles, the carbon conversion and valid gas yield were maintained at around 79% and 0.69 m3/kg, respectively, and no significant change in the crystal phase of NiO-modified hematite was observed. Taken together, these results demonstrate that the use of CLG with the NiO-modified hematite is a promising approach to the environmentally-friendly handling of wood waste.

Author Contributions

Conceptualization, Y.X.; Methodology, X.C. and J.H.; Validation, Z.Z.; Formal analysis, J.X. and Y.L.; Investigation, X.C.; Resources, Y.X., Y.Z. and H.H.; Data curation, K.Z. and Z.Z.; Writing—original draft, K.Z.; Writing—review & editing, K.Z.; Supervision, J.X., Y.L. and Z.H.; Project administration, Y.L., J.H., Y.Z., Z.H. and H.H.; Funding acquisition, Z.H. All authors have read and agreed to the published version of the manuscript.

Funding

The authors gratefully acknowledge the financial support from the National Key Research and Development Program of China (2021YFC1910404), National Natural Science Foundation of China (52076209, 52006224), the Foundation and Applied Foundation Research of Guangdong Province (2022B1515020045, 2019B1515120022), the Joint Fund of the Yulin University and the Dalian National Laboratory for Clean Energy (2021021), and the Foundation of State Key Laboratory of Coal Conversion (Grant No. J21-22-101).

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. A schematic diagram of the chemical looping gasification process.
Figure 1. A schematic diagram of the chemical looping gasification process.
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Figure 2. A schematic diagram of the experimental set-up.
Figure 2. A schematic diagram of the experimental set-up.
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Figure 3. The XRD patterns of the pristine hematite and the NiO-modified hematite.
Figure 3. The XRD patterns of the pristine hematite and the NiO-modified hematite.
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Figure 4. The effects of various OCs on wood waste gasification.
Figure 4. The effects of various OCs on wood waste gasification.
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Figure 5. The effects of the O/W ratio upon (a) the gas concentrations and (b) the carbon conversion (black) and valid gas yield (blue) during wood waste gasification using the Ni-O modified hematite.
Figure 5. The effects of the O/W ratio upon (a) the gas concentrations and (b) the carbon conversion (black) and valid gas yield (blue) during wood waste gasification using the Ni-O modified hematite.
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Figure 6. The effects of reaction temperature upon (a) the gas yield and (b) the gas concentrations and carbon conversion during wood waste gasification.
Figure 6. The effects of reaction temperature upon (a) the gas yield and (b) the gas concentrations and carbon conversion during wood waste gasification.
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Figure 7. The effect of S/B upon (a) the gas concentrations, and (b) the carbon conversion (black) and valid gas yield (blue) during wood waste gasification.
Figure 7. The effect of S/B upon (a) the gas concentrations, and (b) the carbon conversion (black) and valid gas yield (blue) during wood waste gasification.
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Figure 8. The effect of repeated GLC cycles upon (a) the gas concentrations and (b) the carbon conversion (black line, left-hand axis) and valid gas yield (blue line, right-hand axis) during wood waste gasification.
Figure 8. The effect of repeated GLC cycles upon (a) the gas concentrations and (b) the carbon conversion (black line, left-hand axis) and valid gas yield (blue line, right-hand axis) during wood waste gasification.
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Figure 9. The XRD spectra of the fresh OC (red), the reduced OC (black), and the re-oxidized OC.
Figure 9. The XRD spectra of the fresh OC (red), the reduced OC (black), and the re-oxidized OC.
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Figure 10. The SEM-EDS analysis of (a) the fresh OC, (b) the reduced OC, and (c) the re-oxidized OC.
Figure 10. The SEM-EDS analysis of (a) the fresh OC, (b) the reduced OC, and (c) the re-oxidized OC.
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Table
Table 1. The ultimate analysis and proximate analyses of the wood waste (wt.%).
Table 1. The ultimate analysis and proximate analyses of the wood waste (wt.%).
Ultimate AnalysisProximate AnalysisQ/(kJ/kg)
CHNSO *MoistureVolatilesFixed CarbonAsh
45.895.073.720.1845.148.0273.6716.611.7017,472
* by difference.
Table
Table 2. The XRF analyses of the pristine hematite and the NiO-modified hematite (wt.%).
Table 2. The XRF analyses of the pristine hematite and the NiO-modified hematite (wt.%).
Content (%)FeO *NiSiAlKCaOthers
Pristine hematite62.4432.3803.040.130.020.041.90
NiO-modified hematite50.8529.0615.562.540.200.240.140.31
* by difference.
Table
Table 3. The experimental conditions.
Table 3. The experimental conditions.
ItemOperating parameter
Oxygen carrierNiO-modified hematite or pristine hematite
Reaction temperature750 °C, 800 °C, 850 °C, or 900 °C
Oxygen carrier mass0.25 g, 0.5 g, 1.5 g, 2.5 g, 3.5 g, or 5.0 g
FuelWood waste
Fuel mass0.5 g
Inert gasAr
Inert gas flow rate100 mL/min
Gasification agentH2O
Gasification agent mass (steam)0.03 g, 0.07 g, 0.1 g, 0.2 g, 0.25 g
Oxidation agentO2
Oxidation agent flow rate100 mL/min
Reduction time45 min
Purge time30 min
Oxidation time30 min
Number of cycles20 cycles (48 h)
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Xie, J.; Zhu, K.; Zhang, Z.; Chen, X.; Lin, Y.; Hu, J.; Xiong, Y.; Zhang, Y.; Huang, Z.; Huang, H. Chemical Looping Gasification of Wood Waste Using NiO-Modified Hematite as an Oxygen Carrier. Energies 2023, 16, 1847. https://doi.org/10.3390/en16041847

AMA Style

Xie J, Zhu K, Zhang Z, Chen X, Lin Y, Hu J, Xiong Y, Zhang Y, Huang Z, Huang H. Chemical Looping Gasification of Wood Waste Using NiO-Modified Hematite as an Oxygen Carrier. Energies. 2023; 16(4):1847. https://doi.org/10.3390/en16041847

Chicago/Turabian Style

Xie, Jinlong, Kang Zhu, Zhen Zhang, Xinfei Chen, Yan Lin, Jianjun Hu, Ya Xiong, Yongqi Zhang, Zhen Huang, and Hongyu Huang. 2023. "Chemical Looping Gasification of Wood Waste Using NiO-Modified Hematite as an Oxygen Carrier" Energies 16, no. 4: 1847. https://doi.org/10.3390/en16041847

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