Effects of Metal Additives (Fe,Zn,and Sn) on the Co-Pyrolysis of Rice Husk and Cow Manure

Abstract

Co-pyrolysis characteristics of rice husk and cow manure using SnCl₂, ZnCl₂ and Fe₃O₄ as an additive were studied in a fixed tubular furnace. The effects of additive loading amount (0–20 wt.%) on the pyrolysis processes were investigated. The thermogravimetric-derivative thermogravimetric results showed that the weight loss rate was increased with the increase of additive content. There was no obvious shoulder peak appeared after the addition of Fe₃O₄, while the peak of weight loss was moved to the low-temperature region with the ZnCl₂ addition. Gas yield was increased with the increase of Fe₃O₄ content while it was decreased after the addition of SnCl₂ and ZnCl₂. The highest gas yield (0.39 Nm³/kg) was obtained at 20% Fe₃O₄. At the same time, the additives could obviously improve the quality of syngas which promoted H₂ content and reduced CO content. The H₂ content was increased by 7.9%, 8.88%, and 4.44%, respectively, for SnCl₂, ZnCl₂, and Fe₃O₄. The results of GC/MS showed that the additives promoted hydrogen bond breaking and the cracking of macromolecules resulting in an increase of small polycyclic aromatic hydrocarbons production. Meanwhile, the additives made a great influence on the oxygen-containing functional groups in the biochar, which increased the content of O–H and O–C–O functional groups.

Introduction

With the development of economic, the consumption of the energy is more and more large. The traditional fossil fuels cannot satisfy the energy requirement which forces human to develop new energy sources. As a renewable alternative for traditional fossil fuels, biomass utilization had been attracted much attention. Compared with fossil energy, biomass energy can reduce air pollution, cut down CO₂ emissions, and effectively alleviate the greenhouse effect (Brown et al., 2021).

Biomass can be converted into a series of high-value products such as combustible gas, bio-oil, biochar (Rasha et al., 2022), biogas residue, and biogas slurry through anaerobic digestion (Qunpeng et al., 2020; Qunpeng et al., 2016), pyrolysis (Koranit et al., 2021; Shi et al., 2019), and so on. Pyrolysis is considered as one of the most promising technologies for converting biomass into various fuels (Evan and Garcia-Perez, 2019; Henkel et al., 2016). Three kinds of products, including syngas, biochar, and bio-oil can be obtained during the pyrolysis of biomass. However, the characteristic of the products significantly depends on the biomass types and reaction conditions (Jian et al., 2020; Ningbo et al., 2020; Yafei et al., 2020).

To promote the quality of products, co-pyrolysis with other feedstocks was considered as a promise method (Nadia et al., 2017; Shuang et al., 2019; Varsha et al., 2021). Sharypov et al. (2002) found that the addition of polymer on the pyrolysis of wood biomass could increase the syngas yield and promote the quality of syngas. Jiang et al. (2022) investigated the co-pyrolysis characteristics of polypropylene and chlorex and found that the synergistic effect of co-pyrolysis could reduce the activation energy, promote the formation of aliphatic hydrocarbons, and inhibit the formation of aromatization and nitrogen compounds. Wang et al. (2016) investigated the co-pyrolysis of rice husk (RH) and sludge through thermogravimetric analysis (TGA). The results showed that the addition of RH reduced the production of H₂ and CH₄, while increased CO₂ content (Teng et al., 2020).

Meanwhile, the additives, especially containing metallic cations, can play an important role on the pyrolysis process and product characteristics. Many studies have been conducted to evaluate the effects of additives on the biomass pyrolysis (Güray et al., 2013; Makiko et al., 2010). Chen et al. (2003) selected a variety of transition metal oxides for biomass pyrolysis and found that Cr₂O₃ at 850°C was the most effective with a hydrogen content of 56%. Demirbaş (2001) investigated the effect of ZnCl₂, Na₂CO₃, and K₂CO₃ on the catalytic pyrolysis of olive shell, cotton cocoon shell, and tea residue. The results showed that H₂ content of K₂CO₃ in the catalytic pyrolysis of cotton cocoon shell was 4.2% higher than that of Na₂CO₃ at 975 K. However, for olive shell, Na₂CO₃ had a better catalytic effect, and the H₂ content was 5.1% higher than that of K₂CO₃ (Demirbaş, 2001).

Different additives made different effects on pyrolysis products. Meanwhile, the same additive also made great affect on the pyrolysis of products among the different raw materials. Wang et al. investigated the effect of CaO additive on co-pyrolysis behavior of bituminous coal and cow dung. The results showed that CaO promoted the synergistic effect between coal and cow dung during their co-pyrolysis at the range of 350–450°C and CaO also increased the content of high-valued tar products such as benzenes and naphthalenes (Jiaofei et al., 2020). While Chen et al. (2017) investigated the effect of CaO on the product composition of polyethylene, paper pulp, and bamboo and found that CaO could promote H₂ yield and reduce tar yield.

Studies showed that the metals such as Fe, Zn, and Sn had a good catalytic effect on the conversion of biomass into desired products (Feiqiang et al., 2018b; Sangsanga et al., 2014; Zhijie et al., 2021), while the effects of additives, including SnCl₂, ZnCl₂, and Fe₃O₄ on the co-pyrolysis characteristics of RH and cow manure (CM) had not yet been reported. Therefore, in this article, the pyrolysis experiments of co-pyrolysis of RH and CM with three additives were conducted in a fixed bed reactor. Moreover, the influences of additives on the pyrolysis products, including syngas, biochar, and tar components were comprehensively evaluated. This work would be benefit for understanding the function mechanism of additives on co-pyrolysis of RH and CM and provide guidance for the scale-up pyrolysis.

Experiment

Raw material

RH obtained from a grain processing plant in Hubei Province was washed with distilled water and dried in a drying oven at 105°C for 12 h. CM obtained from a breeding farm in Hubei Province was also dried in a drying oven at 105°C for 12 h. RH and CM were physically mixed at a mass ratio of 1:1. The additive amount of SnCl₂, ZnCl₂, or Fe₃O₄ were set as 0%, 3%, 5%, 10%, 15%, and 20% calculated by the mass ratio.

Experiment procedure

The experiments were carried out in a fixed tubular furnace. N₂ was used as the protect gas. The pyrolytic temperature was set at 800°C and the retention time was kept for 20 min. The syngas was collected by the air bag. Tar was collected and dissolved in CH₂Cl₂. Biochar was also collected and weighed.

Methods

X-ray fluorescence was carried out by ZSX Primus II X-ray fluorescence spectrometer (Rigaku, Japan). The component of syngas, including H₂, CO, CO₂, and CH₄ as well as the heat value, was analyzed by the Gasboard Analyzer (TY-6330P). TGA for the samples was determined by thermogravimetric analyzer (TMA-4000; Perkin Elmer, USA). Nitrogen (99.99%) was served as the carrier gas at a flow rate of 80 μL/min, and the heating rate was 10°C/min from ambient temperature to 900°C. The X-ray diffraction (XRD) was analyzed by ShimadzuXRD-600 with scanning speed ≥6°. The surface functional groups of the biochar were measured using an infrared spectrometer (VERTEX 80V; Bruck, Germany). The tar composition was determined by 7890A/5975C gas chromatography-mass spectrometry (Agilent, USA).

Results and Discussion

Effects of additives on weight loss during co-pyrolysis

Figure 1a and d showed the weight loss of the samples with SnCl₂. At 5% SnCl₂, there were two obvious weight loss peaks at 266°C and 330°C, except the weight loss peak caused by free water, and the shoulder peak was appeared at near 266°C. While with the increase of SnCl₂ amount, the weight loss peak was moved to the low temperature region. Meanwhile, it could be seen that the weight loss rate in 10% SnCl₂ was higher than that of 15% SnCl₂ before 526°C, indicating that 15% SnCl₂ had a significant effect on promoting biomass pyrolysis. The final yields of biochar were 33.49%, 43.96%, and 42.21%, respectively, for 5%, 10%, and 15%. Figure 1b and e showed the weight loss of the samples with ZnCl₂. The thermogravimetric (TG) curves in 10% and 15% ZnCl₂ were similar.

FIG. 1.

TG and DTG of curves with SnCl₂ (a and b), ZnCl₂ (c and d), and Fe₃O₄ (e and f). DTG, derivative thermogravimetric; TG, thermogravimetric.

The weight loss was mainly taken place in 170°C–350°C. Derivative thermogravimetric (DTG) curves showed that the corresponding temperatures of weight loss peaks were 332°C, 319°C, and 322°C for 5%, 10%, and 15%, respectively. Similar results were found in the ZnCl₂ compared with SnCl₂. The weight loss peak was moved to the low-temperature region, and the shoulder peak was weakened with the increase of additive amount. Some researches' results indicated that the addition of ZnCl₂ could promote the decomposition of cellulose, hemicellulose, and lignin (Molina-Sabio and Rodríguez-Reinoso, 2004). The yields of biochar were very close which were 34.92%, 34.32%, and 33.81% for 5%, 10%, and 15%, respectively. The weight loss of the samples with Fe₃O₄ could be seen in Fig. 1c and f. The trends of TG and DTG curves in all groups were similar.

The corresponding temperatures of weight loss peaks were about 330°C. There was no shoulder peak appeared which was different from the cases in SnCl₂ and ZnCl₂. The peak at about 300°C was related to the decomposition of hemicelluloses which indicated that the pyrolysis path of hemicellulose was changed (Haiping et al., 2007).

Effects of additives on pyrolysis

The effects of additives on the products of pyrolysis were displayed in Fig. 2. It could be seen that the biochar yield was decreased with the increase of additives. The additives promoted the crack of organic chemical bonds resulting in a decrease of biochar yield. The lowest biochar yield was appeared at 15% SnCl₂. It indicated that the addition of additives could promote the biomass pyrolysis and the decomposition of biomass was sufficiently finished in this case. When the content of SnCl₂ was increased to 20%, the biochar yield was increased caused by the additives.

FIG. 2.

The effect of temperature on the pyrolysis products: (a) Biochar yield; (b) Gas yield.

SnCl₂ could accelerate the primary decarbonization step and reduce solid product yield (Fei et al., 2016), while the effect of Fe₃O₄ on solid product yield was relatively small. The syngas yield was increased with the content of Fe₃O₄, but it was decreased with the content of SnCl₂ and ZnCl₂. The highest syngas yield (0.39 Nm³/kg) was obtained at 20% Fe₃O_4.

Influence of additives on the gas composition in co-pyrolysis

The variation of gas composition affected by the additives was shown in Fig. 3. With the increase of additives amount (Fig. 3a), the content of H₂ and CO₂ were increased, while the CO, CH₄, and C_nH_m were decreased. With the increase of SnCl₂ amount, H₂ content was increased from 4.78% to 14.81%, which was increased by 209.83%. CO content was decreased from 15.78% to 8.11%. The content of CH₄ and C_nH_m were slightly decreased. It could be explained by that the addition of SnCl₂ could reduce the rate of C–O bond cracking to CH₄ and increase the high C–C bond breaking rate for hydrogen production (Huber et al., 2003).

FIG. 3.

Effects of additives on gas composition: (a) SnCl₂; (b) ZnCl₂; (c) Fe₃O₄; (d) comparison of gas components.

The addition of ZnCl₂ could increase the contents of H₂, CO₂, and CH₄, while reduce the contents of CO and C_nH_m (Fig. 3b). This was because ZnCl₂ could reduce the activation energy required for the release of H₂, CO₂, and CH₄ and increase the activation energy for CO production (Feiqiang et al., 2018a). Zn²⁺ could interact with oxygen atoms to form a volatile-Zn²⁺ complex. C–O bond in the side chain and methoxy group was activated and cracked producing CO₂ and CH₄ (Wang et al., 2016). With the increase of ZnCl₂ amount, H₂ content was increased from 4.78% to 15.82% and CO content was decreased from 15.78% to 8.74%.

There were few changes in the content of CO₂, while the content of CH₄ kept decreasing. It indicated that ZnCl₂ enhanced the CO₂–CH₄ reforming reaction (CO₂+CH₄↔2H₂+2CO), which resulting the increase of H₂ and CO. Meanwhile, the decrease of C_nH_m content also indicated that the ZnCl₂ could promote the crack of macromolecular resulting in an increase of H₂ content.

With the addition of Fe₃O₄ (Fig. 3c), H₂ content was increased from 4.78% to 10.39% and CO content was decreased from 15.78% to 12.81%. The mechanism of Fe₃O₄ on the pyrolysis of biomass was different from the above two additives. During the reaction, Fe₃O₄ could be reduced to Fe₂O₃, FeO, and Fe by pyrolytic coke, while Fe₂O₃, FeO, and Fe could also be oxidized to produce Fe₃O₄ by CO and H₂, thus forming a redox cycle reaction. Hence, it had high catalytic activity for tar cracking (Chao et al., 2017).

The comparison of gas components with 10% SnCl₂, ZnCl₂, and Fe₃O₄ was displayed in Fig. 3d. All the three additives could increase H₂ content and reduce CO content. Only Fe₃O₄ promoted the generation of CH₄ and C_nH_m, which increased by 0.52% and 1.27% compared with no addition, respectively. ZnCl₂ has the most significant effect on the H₂ content, which was increased 0.98% and 4.44% more than SnCl₂ and Fe₃O₄, respectively. SnCl₂ could obviously inhibit CO production, which was decreased from 15.78% to 10.61%.

Influence of additives on tar components

The components of tar with three additives were shown in Fig. 4. Strong peaks were appeared at 2.51 and 4.16 min for all the groups, corresponding to benzene and toluene, respectively. The peaks were mainly concentrated in 2–4, 6–8, and 11–15 min. The ion peak appeared after 10 min was enhanced after the additive which indicated that the content of the substance appeared after 10 min was increased.

FIG. 4.

GC/MS of Tar: (a) Blank; (b) SnCl₂; (c) ZnCl₂; (d) Fe₃O₄. GC/MS, gas chromatography-mass spectrometry.

The tar was mainly composed of monocyclic aromatic hydrocarbons (MAHs, mainly benzene and its phenolic derivatives), aliphatic hydrocarbons and oxygen-containing heterocyclic rings (mainly oxindene) (Fig. 4a). Naphthalene was the main polycyclic aromatic hydrocarbons (PAHs) which was accounting for 2.65%. Compared without additive, the intensity of peak at 2.15 and 2.92 min represented for chlorinated hydrocarbons was strengthened after the addition of SnCl₂ (Fig. 4b). The content of phenol at 10.98 min was increased from 3.60% to 4.94%, while the content of o-cresol and p-cresol were decreased, which promoted the formation of monomer phenol. The monomer phenolic compounds and oligomers were mainly derived from the lignin, suggesting that SnCl₂ promoted the pyrolysis of lignin (Rolf and Dietrich, 2009).

Compared with blank group, the addition of SnCl₂ promoted the production of furfural and furfuryl alcohol, which may be caused by the cyclization of the double bond formed by the dehydration of glucose chain (Song et al., 2021). Toluene, m-xylene and p-xylene in MAHs were decreased, while chlorinated hydrocarbons was increased, which indicated that the conversion of dehydrogenation of aliphatic hydrocarbons into aromatic hydrocarbons was inhibited by SnCl₂. The content of oxygen-containing aliphatic hydrocarbons was decreased from 5.44% to 0.39%, in which ketones were decreased from 3.25% to 0.14%. SnCl₂ promoted the cracking of oxygen-containing components resulting in an increase of CO₂ content.

Compared with blank group, an obvious peak occurred at 5.97 min, was repeated for furfural with 13.88%. Furans (furfural) mainly came from the dehydration of carbohydrates in hemicellulose (Chenxi et al., 2017; Qiang et al., 2009), indicating that ZnCl₂ could promote the pyrolysis of hemicellulose (Fig. 4c). ZnCl₂ could inhibit the formation of monomer phenol, resulting that the content of phenol decreasing from 6.13% to 4.78% (Jun et al., 2017). ZnCl₂ had the most obvious effect on H₂ content due to (1) ZnCl₂ greatly promoted the rearrangement reaction of aromatic rings, forming stable carbon structure and releasing H₂; (2) ZnCl₂ also promoted demethylation and part of H₂ came from decomposition of CH₄. The addition of ZnCl₂ could inhibit the formation of tar, in which the toluene content was reduced from 20.60% to 14.03% (Ma et al., 2015).

The components of tar with Fe₃O₄ are shown in Fig. 4d. Benzene and toluene contents in MAHs were decreased, in which benzene was decreased from 33.36% to 30.12% and toluene was decreased from 20.60% to 16.75%. The contents of m-xylene and p-xylene were also decreased correspondingly. CO₂ mainly came from the crack of methyl and methylene. The rise of CO₂ content in gas products indicated that Fe₃O₄ could promote the fracture of methyl. Phenolic substances were increased from 6.13% to 9.52%, which mainly came from the crack of ether bonds in lignin (Linlin et al., 2019).

All the components of tar from pyrolysis were classified according to carbon framework: MAHs, PAHs, aliphatic hydrocarbons, oxygen-containing aliphatic hydrocarbons, and heterocyclic compounds (Table 1). Compared with blank group, the three additives all increased the content of PAHs, and reduced the content of MAHs, aliphatic hydrocarbons, and oxygen-containing aliphatic hydrocarbons. The additives promoted the conversion of heavy aromatic hydrocarbons to light aromatic hydrocarbons, and increased the content of dicyclic and tricyclic hydrocarbons leading to an increase of PAHs. Fe₃O₄ had little effect on monocyclic aromatic hydrocarbons, while SnCl₂ could significantly reduce the content of aliphatic hydrocarbons and oxygen-containing aliphatic hydrocarbons. ZnCl₂ increased the content of furfural resulting in the increase of heterocyclic compounds.

Table 1.

Classification of Tar by Carbon Skeleton

Compound	Relative amount (%)
Compound	Blank	SnCl₂	ZnCl₂	Fe₃O₄
MAH	76.89	62.50	60.73	75.49
PAHs	8.62	13.32	11.41	11.86
Aliphatic hydrocarbon	4.19	0.24	1.86	0.93
Oxygenated aliphatic hydrocarbons	5.57	0.72	3.11	3.16
Heterocyclic compounds	2.96	1.00	18.59	3.53

MAH, monocyclic aromatic hydrocarbons; PAH, polycyclic aromatic hydrocarbon.

Influence of additives on pyrolytic coke characteristics

Fourier infrared spectroscopy was used to analyze the surface functional groups of biochar. As could be seen from Fig. 5, biochar contained functional groups with complex structures such as carboxylic acid, ether, alcohol and substituted benzene groups. The band around 3410 cm⁻¹ was vibration of –OH group (Yue et al., 2011), 2925 cm⁻¹ represented for the vibration of C–H bond, 1620 cm⁻¹ and 1080 cm⁻¹ were the vibration of C = O bond, and C–O out-of-plane bending vibration, respectively. OH, C–O, and C = O were enhanced under the action of additives, indicating that the metal properties in additives had obvious effect on oxygen-containing functional groups. Meanwhile, the wider absorption band caused higher concentration of functional groups.

FIG. 5.

FT-IR diagram of biochar under the action of three additives. FT-IR, Fourier infrared spectroscopy.

The XRD diagram of biochar with three additives at 800°C could be seen in Fig. 6. The results showed that the valence state was changed after the addition of SnCl₂, in which Sn and SnO₂ could be found in biochar, which indicated that Sn²⁺ was reduced to Sn and oxidized to SnO₂ due to disproportionation reaction. SnCl₂ could react with H₂ and CO to reduce to Sn, and combine with the oxygen released from the biomass to oxidize to SnO₂. The fixation of oxygen atom had positive effect on improving the quality of syngas. After the addition of ZnCl₂, a new metal crystal structure was formed. Zn²⁺ reacted with oxygen atoms to form ZnO, which reduced the migration of oxygen atoms into the gas phase.

FIG. 6.

XRD diagram of biochar under the action of three additives: (a) SnCl₂; (b) ZnCl₂; (c) Fe₃O₄. XRD, X-ray diffraction.

However, Fe₃O₄ did not change significantly. Iron atoms had a high affinity for oxygen, leading to a high coverage rate of oxygen species on the metal active site, resulting in the reduced Fe₂O₃ and FeO oxidizing into Fe₃O₄ (Ashok and Kawi, 2014).

Conclusion

The effects of SnCl₂, ZnCl₂, and Fe₃O₄ on the co-pyrolysis of RH and CM were investigated. The results showed that the additives could obviously improve gas quality which increased H₂ content and reduced CO content. At the same time, the additives could also improve the distribution of liquid products, which promoted the cracking of heavy PAHs to light PAHs, and reduced aliphatic hydrocarbons and oxygen-containing aliphatic hydrocarbons. Fe₃O₄ promoted the crack of methyl groups and increased the content of CO₂. ZnCl₂ promoted the pyrolysis of hemicellulose, increased furfural compounds, and inhibited the formation of monomer phenols. SnCl₂ promoted the pyrolysis of lignin, which promoted the increase of monomer phenols. However, the toxicity and environmental impacts about the addition of those metals need to be further studied.

Footnotes

Authors' Contributions

W.Q.: Writing—original draft/review and editing and data curation. Y.L.: Data curation and experiment. J.L.: Software. G.F.: Data curation. G.S.: Conceptualization. Q.C.: Writing—review and editing.

Author Disclosure Statement

No competing financial interests exist.

Funding Information

The authors acknowledge the financial support provided by Hubei Jiebang science and technology project (2021BEC026), Hubei Important Project of Technological Innovation (2022BCA081) and the Central Committee Guides Local Science and Technology Development Special Project of Hubei Province (2019ZYYD059).

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