Boosting Agroforestry Waste Valorization: Red Mud Oxygen Carriers with Tailored Oxygen Release for Enhanced Chemical Looping Gasification

Abstract

In this study, red mud oxygen carriers were obtained by varying the preparation temperature and characterized using XRD, SEM, BET, and H₂-TPR. The results showed that the oxygen carrier prepared at 1000 °C exhibited high reactivity due to clear grain boundaries, uniform size, high porosity, and smooth grain morphology. Additionally, the release of oxygen was accelerated, as indicated by the H₂-TPR results. The water hyacinth, an aquatic plant of agroforestry waste, was selected as the research object, and the chemical looping gasification (CLG) reaction performance with prepared red mud carriers was investigated. The experiment results showed that the total gas yield (Y_g) of the carriers prepared at 1000 °C reached a maximum of 1.02 Nm^3/kg, had a high low-level heating value (LHV) of 12.06 MJ/Nm³, cold gas efficiency (CGE) of 91.49%, and carbon conversion rate (η_c) of 82.65%. This indicated that the red mud carriers synthesized at 1000 °C have a faster oxygen release rate, more concentrated oxygen release, and stronger reaction activity.

1. Introduction

China, as a large agricultural production country, is one of the countries with the largest output of agricultural waste in the world. The comprehensive utilization of agricultural waste is a major issue encountered by not only China but also countries around the world [1]. The development of efficient treatment technologies for converting agricultural waste from low-grade energy into high-grade clean energy is highly significant for meeting the national demand for carbon neutrality, solving the waste of agricultural waste resources, and reducing environmental pollution [2,3].

The gasification technology can convert organic solid wastes into gaseous products (H₂, CO, and CH₄) and reduce the emission of toxic substances. Among them, chemical looping gasification (CLG), as a new and potential gasification technology, decouples conventional gasification into two interconnected reactors: the fuel reactor (FR) and the air reactor (AR). This architecture leverages metal oxide oxygen carriers (OCs) to mediate oxygen transfer from air to fuel, achieving the energy cascade use of the chemical energy of the fuel, effectively improving the gasification quality, and reducing energy consumption [4,5,6,7]. It can be applied to the disposal of organic solid waste and achieve the deep utilization of fuel [8]. CLG has a series of advantages, such as saving cost because the metal oxide carrier can transfer the oxygen from the air to the fuel, improving the calorific value of syngas as the fuel is not in contact with air directly, which avoids the dilution of syngas by nitrogen, possessing the functions of oxygen carrier and heat carrier simultaneously because the heat brought out from the air reactor can compensate the energy for the fuel reactor [9]. Therefore, the choice of oxygen carrier is important to enhance the efficiency of CLG.

In recent years, red mud oxygen carriers, with their potential advantages and prospects, have attracted great attention. Red mud is a type of polluting waste residue discharged from bauxite mines in the process of refining alumina, and the annual global production of red mud exceeds 120 million tons [10]. The traditional landfill method not only occupies land and is prone to air, soil, and lake pollution [11] but also results in a waste of resources, as red mud is rich in metal oxides. Red mud has a sufficient amount of Fe₂O₃ (typically >40 weight percent (wt.%)) to ensure adequate oxygen transfer capacity and suitable heat-carrying properties. The synergistic effect of iron-based and alkali–metal components will significantly increase the activity of biomass gasification reactions. The components of red mud, such as Al₂O₃, TiO₂, MgO, SiO₂, etc., can significantly improve the structure and reaction stability of the solid oxygen transfer material after high-temperature calcination, preventing the loss of oxygen carrier activity and extending its lifecycle. Meanwhile, Al₂O₃ exhibited a positive effect on improving the properties of iron-based oxygen carriers and significantly enhanced the oxygen transfer capacity [12]. Mendiara et al. [13] verified the feasibility of using red mud oxygen carriers for chemical looping combustion in an intermittent fluidized bed and reported that the reaction activity increased with the increasing number of uses of red mud oxygen carriers. In addition, the reaction characteristics of red mud oxygen carriers were further investigated in a 500 Watt thermal (Wth) circulating fluidized bed and found that compared with ilmenite oxygen carriers, the red mud oxygen carriers exhibited good cyclic stability, no sintering, more pores, and higher oxygen transfer rates with under the same experimental conditions [14]. Chen et al. [11] compared the reaction characteristics of ilmenite oxygen carriers, red mud oxygen carriers, and artificially synthesized iron-based oxygen carriers in an intermittent fluidized bed reactor and found that the interaction between active components in red mud and inert carriers results in high reactivity. Bao et al. [15] evaluated the influence of inert components and alkali metals in red mud on its reaction characteristics and studied the multi-cycle characteristics of red mud oxygen carriers. The research results indicate that Al₂O₃ and TiO₂ in the red mud can enhance the mechanical strength of oxygen carriers, and Na can catalyze coal coke gasification reactions, leading to the progress of chemical chain combustion reactions.

Compared with the chemical looping combustion process, the red mud oxygen carriers have obvious two-stage action characteristics in the chemical looping gasification process [16]. In the initial stage of the reaction, the red mud oxygen carrier mainly shows strong oxidative properties, and the combustion reaction is strong, which is favorable for fuel conversion. As the reaction proceeds, the reduced red mud carriers show a catalytic effect on the water vapor conversion reaction, which is conducive to the increase in H₂ production after the reaction. These characteristics of red mud carriers provide theoretical support for simplifying the process and reaction conditions of chemical looping gasification.

Previous work has demonstrated the feasibility of red mud as an oxygen carrier for chemical looping gasification/combustion of coal/coal char (

Table 1. Comparison of the present work with previous chemical looping reactions of bauxite as oxygen carrier.

Oxygen Carrier	Oxygen Carrier Synthesis Methods	Fuel	Reaction Conditions	Gasification Effect	Reference
Bauxite waste	Calcined at 1200 °C for 18 h	Bituminous coal	N2/H2O/CO2, 900 °C	Combustion efficiencies > 0.95 at 900 and 980 °C	[13]
Red mud	Calcined at 1150 °C for 6 h in air	Char from Kentucky Coal (USA)	50 vol.% H2O/N2	/	[11]
Alumina	Calcined at 1200 °C for 18 h	Colombian coal	TFR = 870–930 °C, 100 vol.% H2O	ηcomb,FR = 93.1%	[14]
Red mud	Inert supports (Al2O3,SiO2,TiO2 and CaO) and alkali sodium modification	Coal char	50 vol.% H2O/N2, 950 °C	30th cycle activity	[15]
Red mud	Calcined at 800 °C, 900 °C, 1000 °C, 1100 °C for 8 h	Water hyacinth	0.2 mL/min steam, 950 °C	Gas yield = 1.02 Nm3/kg, LHV = 12.06 MJ/Nm3, CGE = 91.49%, ηc = 82.65%.	This work

). The main objective of this study was to systematically investigate the CLG reaction for the water hyacinth on the red mud oxygen carriers prepared at different temperatures. Based on the CLG experimental results and the characterization analysis of oxygen carriers, the optimum preparation conditions of the red mud oxygen carrier were selected, hence achieving the objectives of waste-to-waste, efficient utilization of waste, and a significant reduction in operating costs.

2. Materials and Methods

2.1. Materials

Water hyacinth, as organic solid agricultural and forestry waste, was obtained from Suzhou, Jiangsu Province. The proximate and ultimate analysis results of water hyacinth are summarized in

Table 2. Proximate and elemental analysis of water hyacinth.

Proximate Analysis (wt.%)				Elemental Analysis (wt.%)
Moisture	Volatiles	Ash	Fixed Carbon	C	H	N	S
11.31	60.43	10.95	17.31	34.71	5.03	1.00	0.32

. The Water hyacinth is dried in a 120 °C oven for 24 h, crushed, and screened into 80–120 mesh sizes for use. Here are some of the reactions that can occur in the calcination of red mud:

2FeOOH → Fe₂O₃ + H₂O

3Fe₂O₃ → 2Fe₃O₄ + O²⁻

The oxygen-carrying red mud was obtained from Shandong Province, and the analysis of the X-ray fluorescence spectrometer (XRF) is shown in

Table 3. XRF analysis results of oxygen-carrying red mud.

Composition	Fe2O3	Al2O3	SiO2	Na2O	TiO2	CaO	MgO	SO3	CuO	Others
Mass fraction (wt.%)	50.06	21.41	11.19	6.77	5.43	1.07	0.13	0.44	0.02	3.47

. Generally, 600 g of fresh red mud with a particle size below 150 μm was screened and poured into a beaker, and then 800 mL of deionized water was added to the beaker under stirring (200 r/min) for 60 min. The homogeneous red mud suspension was dried at 105 °C for 10 h, and calcined at constant temperature (800 °C, 900 °C, 1000 °C, 1100 °C) in the muffle furnace for 8 h, followed by crushing and screening into a 20~80 mesh size. The processed red mud oxygen carriers prepared at different temperatures were characterized using XRD, SEM, BET, and H₂-TPR.

2.2. Experiment Procedures

The experiment of chemical looping gasification was conducted using a self-designed and constructed fixed-bed gasification device (Figure 1). Typically, 1 g of water hyacinth and red mud oxygen carrier (prepared at different temperatures) with a mass ratio of 1:1 was filled into the reactor. The water vapor flow rate was 0.2 mL/min, and the reaction temperature was 950 °C.

2.3. Data Processing

Gas production, total gas production (Y_g), and single-gas component content of each component in gas products were calculated according to the law of nitrogen mass balance. The specific calculation methods are shown in Equations (1)–(3).

$$Y_i={\int }_0^t{\displaystyle \frac{V_{N_2}\times X_i\times 273}{m_0\times \left(1-\sum X_i\right)\times 288}}{d}_t$$

(1)

$$Y_g=\sum Y_i (\mathrm{i}=\mathrm{C}\mathrm{O},{\mathrm{H}}_2,{\mathrm{C}\mathrm{O}}_2,\mathrm{C}{\mathrm{H}}_4)$$

(2)

$$C_i={\displaystyle \frac{Y_i}{Y_g}}\times 100\%$$

(3)

where Y_i is the yield of each single-gas component in the gas product per unit mass of feedstock, Nm³/kg. $V_{N_2}$ is the N₂ flow rate at the inlet at room temperature, m³/s. X_i is the volume fraction of each single-gas component measured online by the gas analyzer, %. m₀ is the total mass of feedstock added in the test, kg. Y_g is the total gas yield per unit mass of feedstock, Nm³/kg. C_i is the content of each single-gas grouping in the gas product, %.

Based on the known yield of each single-gas grouping, it is also possible to calculate the composition of the gas product, e.g., the proportion of H₂ and CO. In order to reflect the gasification reaction characteristics of the material, the carbon conversion rate and cold gas efficiency index are often used. The specific calculation method is shown in Equations (4)–(6):

$${\mathrm{H}}_2/\mathrm{C}\mathrm{O}={\displaystyle \frac{{\mathrm{Y}}_{{\mathrm{H}}_2}}{{\mathrm{Y}}_{\mathrm{C}\mathrm{O}}}}$$

(4)

$${\eta }_c={\displaystyle \frac{12\times {\mathrm{m}}_0\times \left({\mathrm{Y}}_{\mathrm{C}\mathrm{O}}+{\mathrm{Y}}_{{\mathrm{C}\mathrm{O}}_2}+{\mathrm{Y}}_{{\mathrm{C}\mathrm{H}}_4}\right)}{22.4\times f_c}}$$

(5)

$$CGE=(LHV\times Y_g)/q_0$$

(6)

where H₂/CO is the ratio of H₂ and CO in the composition of the gas product, %. η_c is the carbon conversion rate of the material gasification reaction, %. f_c is the content of elemental carbon in the added material, %. CGE is the cold gas efficiency of the material gasification reaction, %. q₀ is the low-level heating value of the added material itself, MJ/kg.

3. Results

3.1. Characterization of Red Mud Oxygen Carriers

XRD patterns of the red mud carriers prepared at different temperatures are displayed in Figure 2. The peaks in Figure 2 were indexed to the diffraction of Fe₂O₃, NaAlSiO_4, and TiO₂, reflecting that Na exists in the form of NaAlSiO₄ [17]. The diffraction patterns of the four oxygen carriers are very similar, indicating that the four oxygen carriers have the same crystal structure. Compared with 1000 °C, the characteristic peak intensities of Fe₂O₃, NaAlSiO₄, and TiO₂ in the oxygen carrier prepared at 1100 °C have decreased, indicating that the crystal phases in the oxygen carrier exhibit an increase in grain size and a decrease in crystallinity due to agglomeration and sintering. Additionally, the diffraction signals of the oxygen carriers prepared at 1000 °C are much stronger, indicating a finer grain size and higher oxygen-carrying capacity.

The SEM of red mud oxygen carriers prepared at different synthesis temperatures is displayed in Figure 3. As can be seen from Figure 3, the morphology and structure of the oxygen carrier were highly affected by the calcination temperatures, thus affecting its performance. There were no obvious crystal shapes and grain boundaries for the red mud oxygen carriers prepared by calcining at 800 °C and 900 °C, as shown in Figure 3a(1)/a(2),b(1)/b(2). Moreover, the low calcination temperature resulted in the low mechanical strength of the carriers, which could not adapt to the wear and tear of the carriers during the reaction process [18]. Clear grain boundaries, uniform size, and high porosity, with relatively dense and smooth grain surface, were exhibited for the red mud oxygen carrier prepared by calcination at 1000 °C in Figure 3c(1)/c(2), which is conducive to the reaction between the oxygen carrier and biomass and beneficial for maintaining the cyclic stability of the oxygen carrier. However, as described in Figure 3d(1)/d(2), the grain size became large for the red mud oxygen carrier prepared at 1100 °C because of the agglomeration phenomenon at higher temperatures, which is consistent with the XRD and BET characterization results.

The XPS spectra of red mud oxygen carriers in the Fe 2p regions are compared in Figure 4. As can be seen in Figure 4, the Fe 2p spectra were divided into two peaks at 709.1 eV and 710.7 eV, which were assigned to Fe(II) and Fe(III) species. The ratio of Fe(II)/Fe(III) were 1.1:1, 0.7:1, 1:1, and 1.3:1 for the material obtained at 800 °C, 900 °C, 1000 °C, and 1000 °C, respectively, which indicated that the change in preparation temperature has an effect on the valence state of iron and that iron oxide undergoes a partial redox reaction. The reversible transition between Fe³⁺ (oxidized state) and Fe²⁺/Fe₃O₄ (reduced state) during redox cycles ensures dynamic replenishment of active oxygen species, minimizing irreversible oxygen depletion. The ratio of Fe³⁺ to Fe²⁺ on the surface of fresh red mud oxygen carriers can reflect the magnitude of lattice oxygen concentration to a certain extent. Among them, the concentration of Fe³⁺ is higher at 900 °C and 1000 °C, which has a higher potential energy difference for Fe species in the direction of low oxidation state evolution toward FeO, Fe₃O₄ during the reduction process, and its lattice oxygen release ability is stronger [19]. High calcination temperatures (1000 °C) induce sinter-resistant Fe–O configurations. This stabilizes the oxygen vacancies while maintaining the crystallinity, thus enabling continuous diffusion of oxygen ions through the Fe–O–Fe network during repeated reduction/oxidation processes. Activated surface oxygen is used for tar cracking while retaining bulk lattice oxygen for CO/H₂ production. It is important for the cracking of undesirable byproducts, such as tar, and the quality improvement in syngas.

As shown in Figure 5, different reduction properties were exhibited for the red mud carriers synthesized at different temperatures. The reduction peak was mainly converted from Fe₂O₃ to Fe₃O₄. The reduction peak of the oxygen carrier synthesized at 1000 °C was advanced to 627 °C with a small peak width and a large peak value, which facilitated the release of oxygen. The lower reduction temperature (627 °C) reduces thermal stress during oxygen release/replenishment cycles. It is inferred that different calcination temperatures affect the valence of the metals in the red mud, especially the concentration of iron ions in different valence states, as analyzed in the XPS section. Consequently, the reduction reaction between hydrogen and oxygen carriers synthesized at 1000 °C was accelerated, which improved the reactivity of the oxygen carriers during high-temperature reactions [20].

3.2. Effect of Different Preparation Temperatures on the Production and Composition

The distribution of gas product yield and composition of the CLG test under the red mud oxygen carrier synthesized at different synthesis temperatures are displayed in

Table 4. The distribution of gas product yield and composition of CLG test under the red mud oxygen carrier synthesized at different synthesis temperatures.

Temperature (°C)	H2 (%)	CO (%)	CO2 (%)	CH4 (%)	Yg (Nm3/kg)
800	44.44	26.94	19.27	9.36	0.87
900	46.37	26.65	16.02	10.96	1.01
1000	47.40	27.18	15.62	9.80	1.02
1100	45.90	27.03	17.09	9.98	0.97

and Figure 6. The total gas yield (Y_g) increased with the increase in the red mud oxygen carrier preparation temperature from 800 to 1000 °C and reached a maximum of 1.018 Nm³/kg, but when the oxygen carrier preparation temperature increased from 1000 °C to 1100 °C, the Y_g decreased to 0.974 Nm³/kg. Similarly, for the H₂ and CO, the volume fraction increased and reached a maximum of 47.395% and 27.182% for the oxygen carriers prepared at 1000 °C and decreased to 45.900% and 27.031% for the carriers prepared at 1100 °C, respectively. Additionally, the volume fraction of CO₂ kept decreasing with the increase in preparation temperature and reached a minimum of 15.622% for the oxygen carrier prepared at 1000 °C. Meanwhile, a relatively high volume fraction of CH₄ (9.8%) was obtained for oxygen carriers prepared at 1000 °C.

3.3. Effect of Different Preparation Temperatures on CLG Characteristics

The influence of the synthesis temperature of red mud oxygen carrier on the characteristics of the CLG reaction is presented in Figure 7 and Figure 8, and the relevant parameters are listed in

Table 5. CLG characteristics of oxygen carriers at different preparation temperatures.

Temperature (°C)	LHV (MJ/Nm3)	CGE (%)	ηc (%)	Effective Gas Composition (%)	H2/CO
800	11.55	74.57	74.29	80.73	1.65
900	12.30	92.24	83.30	83.98	1.74
1000	12.06	91.49	82.65	84.38	1.74
1100	11.95	86.72	81.34	82.91	1.70

.

The effective gas composition was increased with the enhancement of oxygen carrier preparation temperature from 800 to 1000 °C, and reached a maximum of 84.38% at the prepared temperature of 1000 °C, then decreased to 82.91% at the prepared temperature of 1100 °C. H₂/CO has the same pattern, with a maximum of 1.74 at 1000 °C, and decreases to 1.70 at 1100 °C. Meanwhile, the oxygen carriers prepared at 1000 °C exhibit relatively high LHV (12.06 MJ/Nm³), CGE (91.49%), and ${\eta }_c$ (82.65%) (Table 5). It is reasonable that the oxygen carrier synthesized at 1000 °C has a faster oxygen release rate, higher oxygen release concentration, and enhanced reaction activity, according to the characterization results mentioned earlier. Moreover, when the reactivity of the oxygen carrier is moderate, the reaction of water hyacinth and red mud oxygen carriers is promoted, thereby improving the CLG characteristics.

3.4. Analysis of Red Mud Carriers Before and After the Reaction

SEM images of the red mud carriers synthesized at 1000 °C before and after the CLG reaction are present in Figure 9. As shown in Figure 9, the structure of the red mud carriers shows nearly no significant changes before and after the reaction, but the arrangement of materials before the reaction is more compact, with smaller pore structures and pore sizes. After the gasification reaction, the oxygen atoms escaped from the carriers, leading to an increase in pore size and specific surface area (

Table 6. Specific surface area and porosity analysis of red mud oxygen carriers synthesized at 1000 °C before and after reaction.

Oxygen Carrier	SBET (m2/g)	Pore Volume (cm3/g)	Pore Size (nm)
Before reaction	2.451	0.006	10.529
After reaction	2.668	0.007	12.485

).

As can be seen from the XRD patterns before and after the chemical looping gasification reaction of the terracotta carriers prepared at 1000 °C in Figure 10, the main peak line in the reduced state curve pattern of the terracotta carriers after the reaction is the peak line of Fe₃O₄ crystals, and the peak lines of FeO and Fe are not found. Therefore, during the fixed-bed gasification reaction, the oxygen carriers were reduced only to the extent of Fe₃O₄, without continuing to be reduced to FeO or Fe.

4. Conclusions

In summary, four types of red mud oxygen carriers were successfully prepared and characterized by a series of technologies, and their performance in chemical–looping gasification (CLG) reaction with water hyacinth was systematically studied. Characterization results indicated that the red mud synthesized under 1000 °C presented high reactivity due to clear grain boundaries, uniform size, high porosity, and smooth grain surface. Also, the release of oxygen was accelerated according to the H₂-TPR results. CLG experiment results exhibited that the red mud synthesized under 1000 °C has high reactivity with a maximum total gas yield of 1.02 Nm³/kg, effective gas composition (CGE = 91.49%, η_c = 82.65%), and low-level heating value (LHV) of 12.06 MJ/Nm³, which is consistent with H₂-TPR analysis. After the gasification reaction, the oxygen atoms escaped from the carriers, leading to an increase in pore size and specific surface area according to the SEM and BET results of the red mud carriers prepared at 1000 °C before and after the CLG and the oxygen carriers were reduced only to the degree of Fe₃O₄ without continuing to be reduced to FeO or Fe.