Investigation of element migration characteristics and product properties during biomass pyrolysis: a case study of pine cones rich in nitrogen

Abstract

To further understand the element migration characteristics and product properties during biomass pyrolysis, herein, pine cone (PC) cellulose and PC lignin were prepared, and their pyrolysis behavior was determined using thermogravimetric analysis (TGA). Subsequently, the PC was pyrolyzed in a vertical fixed bed reactor system at 400–700 °C for 60 min. The characteristics of element migration and the physicochemical properties of the pyrolysis products were analyzed and discussed. In the pyrolysis temperature range from 200 °C to 500 °C, there were two distinct weight loss peaks for PC. During the pyrolysis process, the C element was primarily retained in the biochar, while the O element mainly migrated into liquid and gaseous products in the form of compounds such as CO₂, CO, and H₂O. Besides, 28.42–76.01% of the N element in PC migrated into biochar. Of the three-phase products, the gases endow the lowest energy yield, while the energy of the biochar dominates the pyrolysis of the PC. Additionally, the N content and specific surface area for the PC-derived biochar obtained at 400 °C in a N₂ atmosphere were higher than those of the biochar derived from fiberboard.

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1. Introduction

Biomass is a clean and renewable resource, resulting from its wide distribution, abundance, CO₂ neutrality, and low emission of nitrogen and sulfur compounds.^1–3 Unfortunately, the high water and oxygen contents of biomass lead to some drawbacks such as low burning rates and low flame temperatures for biomass combustion, as well as transport difficulties.^4–6 Thus, relevant researchers often employ thermochemistry or biotechnology to convert biomass into solids, gases, and liquids to enhance the quality of biomass for integrated use, of which pyrolysis is a promising thermochemical process for biomass utilization.⁷ As is known, the pyrolysis of biomass involves a transformation of its phase. Meanwhile, the parent substance (i.e., biomass) and the derivatives (i.e., solids, gases, liquids) possess inherited relationships. As a result, the element content of the parent substance and the derivatives is a critical indicator of the degree of inheritance. It follows that it is essential to investigate the migration of elements during biomass pyrolysis.

Previous works have indicated that the release of CO₂ and CO during low-temperature (200–350 °C) pyrolysis (viz. torrefaction) of biomass allows for the elimination of oxygen faster than that caused by pure dehydration.^8–15 Recently, numerous studies have shown that torrefaction can increase the carbon content and calorific value of biomasses, and reduce their oxygen content.^16–19 According to these, we can conclude that torrefaction leads to changes in the content of O and C elements, especially the O element. In other words, torrefaction causes the migration of O and C elements from biomass. Notably, most of the biomass torrefaction research has focused on the solid products of biomass, and the physiochemical changes, fuel properties, and further uses of torrefied biomass have been extensively reported.^20,21 In fact, in addition to the solid products, there are small amounts of gaseous and liquid products.^22,23 However, relatively few systematic studies have been performed on the migration laws of carbon and oxygen elements as the collection of small amounts of gaseous and liquid products is difficult. Biomass pyrolysis (400–800 °C) can significantly increase the yield of gaseous and liquid products.

On the other hand, previous studies have indicated that the doping of carbon-based materials with other heteroatoms (e.g., N, P, and S) can further improve their properties.^24–26 Schnucklake et al.²⁷ synthesized nitrogen-doped porous carbon using phenolic resin as the carbon source and pyrrole-2-carboxaldehyde as the nitrogen source. It was presented that the nitrogen-doped porous carbon shows promising activity for the positive side reaction. Laheäär et al.²⁸ used ammonia gas as the nitrogen source to modify activated carbon and indicated that the nitrogen-doped activated carbon with a nitrogen content of 4.50 wt% and a specific capacitance of 152 Fg⁻¹. Xu et al.²⁹ obtained biochar with 3.16 wt% N content by pyrolyzed the fiberboard containing phenolic resin. It was found that the biochar derived from fiberboard had a better adsorption effect on tetracycline. These previous studies have made significant progress in understanding that doped with N can enhance the active site of carbon. However, exogenous nitrogen (e.g., pyrrole-2-carboxaldehyde, ammonia gas, and phenolic resin) doping is not conducive to the uniform distribution of nitrogen in carbon materials, accompanied by the high cost. Therefore, biomass feedstocks with high nitrogen possess the potential to address the problem of uneven nitrogen doping and improve the active site of biomass-derived carbon. It follows that the nitrogen migration laws during pyrolysis of biomass feedstocks with high nitrogen are worthy of prior investigation.

Interestingly, pine cone (PC) is rich in nitrogen as an agroforestry waste, which is higher than that of other biomass like rice straw and grape stem, as well as is abundant in China.^7,13 However, until now, most of the research on PC has focused on the development and investigation of natural medicinal extracts.³⁰ Consequently, the pyrolysis behavior of PC and the migration law of C, O, and N elements in the pyrolysis process have not been reported yet. In the present work, the physicochemical properties for biochar, liquid, and gaseous products generated at different temperatures were determined via proximate analysis, ultimate analysis, Karl–Fischer titration, and so on. Overall, this work has made further understand of the deoxygenation, decarbonization, and denitrogenation from PC during pyrolysis at different temperatures.

2. Experimental section

2.1. Materials

Pine cone (PC), employed as the feedstock for the pyrolysis experiment, was sourced from the Kuandian, Liaoning Province of China. Before the test, the PC was dried in an oven at 105 °C for 24 h and then ground into a powder with a particle size of 0.130–0.180 mm. The PC cellulose and the PC lignin were prepared regarding the Chinese National Standards GB/T2677.10-1995 and the Chinese National Standards GB/T2677.8-1994, respectively.

2.2. Thermogravimetric experiment

Weigh loss characteristics of samples were carried out in TGA (TGA STA8000, PerkinElmer, Waltham, MA, USA). For each experiment, the sample weight, nitrogen flow rate, heating rate, and final pyrolysis temperature were ∼10 mg, 50 mL min⁻¹, 20 °C min⁻¹, and 700 °C, respectively.

2.3. Pyrolysis experiment

The pyrolysis experiment was employed in a vertical fixed-bed (Blue M, Thermo Scientific, USA) reactor system, as presented in Fig. 1.

Fig. 1 A vertical fixed-bed reactor system.

Approximately 30 g of PC was placed into the reactor, and before the experiment, the reactor was purged with nitrogen to remove air from the reactor, and then the nitrogen was turned off after 10 min. Subsequently, the operating parameters (temperatures) of the vertical fixed-bed reactor system were varied to examine the properties of biochar, liquid, and gaseous products at a residence time of 60 min. During the pyrolysis process, the biochar remained in the reactor. Meanwhile, the liquid and gaseous products were collected in a liquid collecting device and a gas collecting bag, respectively. The pyrolysis experiment was repeated 3 times under the same experimental conditions to obtain enough products for subsequent testing. Besides, the volume concentration of each gas in the gaseous product was determined by a gas analyzer (Gasboard, Ruiyi Automatic Control Co., Ltd, Hubei, China).

2.4. Sample labels

Herein, the pine cone was named PC. Besides, PC-derived biochar (pyrolyzed pine cone) was labeled as PPC-X, where X is the pyrolysis temperature. As an example, PPC-400 stands for the PC-derived biochar obtained by pyrolyzing PC upon 400 °C under a heating rate of 20 °C min⁻¹ with a residence time of 60 min.

2.5. Analysis of pyrolysis products

Proximate analysis, ultimate analysis, and HHV of PC and PPC-X were performed regarding the Chinese National Standards GB/T28731-2012, an element analyzer (Vario macro cube, Elementar, Germany), and an adiabatic oxygen bomb calorimeter (XRY-1A, Changji Geological Instruments, China), respectively. The HHV of gaseous was determined via the gas analyzer.

The water content, pH, ultimate analysis, and composition of liquid were determined via Karl Fischer titration (KF-200, Mitsubishi Corporation, Japan), pH tester (E-201-C, Precision Scientific Instruments Co., Ltd, Shanghai, China), an element analyzer, and gas chromatography-mass spectrometry (Trace DSQ II, Thermo Scientific, USA), respectively.

2.6. Mass yield and energy yield of samples

Mass yield and energy yield were calculated by (1) and (2), respectively.Where R, m₀, and m_i are the mass yield (%) of the sample, the mass (g) of PC, and the mass (g) of the sample, respectively. Other symbols of R_i, HHV₀, and HHV_i represent the energy yield (%) of the sample, the HHV₀ (MJ kg⁻¹) of PC, and the HHV_i (MJ kg⁻¹) of the sample, respectively.

3. Results and discussion

3.1. Thermogravimetric analysis of PC, PC cellulose, and PC lignin

The TGA (a) and DTG (b) curves of PC, PC cellulose, and PC lignin pyrolysis with a heating rate of 20 °C min⁻¹ are given in Fig. 2.

Fig. 2 TGA (a) and DTG (b) curves of PC, PC cellulose, and PC lignin.

A notable difference was found in the pyrolysis behaviors of PC, PC cellulose, and PC lignin. The weight loss of PC happens early in the temperature ranging from 110 °C to 200 °C. Moreover, the DTG curve of PC endows two distinct weight loss peaks in the pyrolysis temperature ranging from 200 °C to 500 °C, at around 330 °C and 450 °C, respectively. Of these, the weight loss peak at about 330 °C was caused by the pyrolysis of lignin or hemicellulose, while the weight loss peak at about 450 °C was primarily ascribed to the existence of non-structural substances (e.g., wax, fat, resin, tannin, sugars, and pigments). This is significantly different from other biomass shown only one peak such as bamboo,³¹ pinewood,³² and wheat straw,³³ while similar to palm kernel had two weight loss peaks.⁸ The weight percentage for the pyrolysis residue of PC being ∼26%, which was consistent with the proximate analysis of PC (Table 1). Compared with the PC, PC cellulose was pyrolyzed in the lower temperature ranging from 200 °C to 380 °C and endows a narrow range of maximum weight loss upon 220–370 °C. Meanwhile, the weight percentage for pyrolysis residue was ∼23%. Unlike the sharper DTG peaks of PC and PC cellulose, PC lignin was slowly pyrolyzed in the wider temperature ranging from 160 °C to 700 °C and generated the highest weight percentage for residue (∼40%), this result was similar to the previous publication.⁸ Thence, the thermal stability of samples was determined: PC lignin > PC > PC cellulose.³⁴

Table 1 Physicochemical properties for PC and biochar

Samples	Proximate analysis (wt%, db)			Ultimate analysis (wt%, db)					HHV (MJ kg^−1)	Energy yield (%)
	Volatile	Fixed carbon	Ash	C	H	N	S	O
PC	73.98 ± 0.95	23.41 ± 0.44	2.61 ± 0.26	43.99 ± 0.49	3.65 ± 0.05	1.67 ± 0.07	0.51 ± 0.02	47.57 ± 0.89	17.86 ± 0.52	100
PPC-400	20.46 ± 0.77	71.69 ± 0.46	5.70 ± 0.32	66.11 ± 0.52	2.21 ± 0.04	2.79 ± 0.06	0.79 ± 0.03	22.40 ± 0.97	28.92 ± 0.59	73.68
PPC-500	16.57 ± 0.81	75.30 ± 0.49	7.99 ± 0.29	70.63 ± 0.48	1.09 ± 0.06	1.72 ± 0.05	0.53 ± 0.01	18.04 ± 0.89	29.68 ± 0.45	54.39
PPC-600	11.99 ± 0.87	79.47 ± 0.51	8.61 ± 0.34	74.07 ± 0.55	0.63 ± 0.03	1.65 ± 0.07	0.20 ± 0.02	14.84 ± 1.01	30.08 ± 0.65	51.01
PPC-700	9.48 ± 0.79	81.15 ± 0.45	8.96 ± 0.27	74.63 ± 0.51	0.53 ± 0.04	1.63 ± 0.06	0.15 ± 0.01	14.10 ± 0.88	30.31 ± 0.63	49.42

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3.2. Pyrolysis of PC

3.2.1 Product distribution. Natural polymeric components inter biomass primarily consist of hemicellulose, cellulose, and lignin.³⁵ Consequently, Fig. 3 indicates the main chemical reaction illustration for the decomposition of PC.

Fig. 3 Illustration of the main chemical reactions for the decomposition of PC.

It is well known that the weight loss of PC during pyrolysis primarily resulted from the evaporation of volatiles. Fig. 4 shows the product distribution of the PC pyrolysis at different temperatures.

Fig. 4 Product distributions of the PC pyrolyzed at different temperatures.

From Fig. 4, it can seem that an increase in pyrolysis temperature, resulting in an outstanding reduction in the yield of biochar, but the yield of liquid presented a gradual increase. Meanwhile, the yield of gas has no significant difference.

The weight loss gradually increased with the pyrolysis temperature from 400 °C to 700 °C, and had a relatively considerable weight loss (70.88%) at 700 °C, since the hemicellulose, cellulose, lignin, and non-structural substances of PC strongly decomposed.³⁶ However, the increase in weight loss gradually slows down as the pyrolysis temperature increases. The sum of the yield of liquid and gas increased as the pyrolysis temperature increases, which was mainly attributed to the thermal cracking and lead to the formation of more volatiles.³⁷ The effect of pyrolysis temperature on PC mass yield was similar to the effect on other biomass (e.g., cotton stalk, eucalyptus, and rice husk).^38–40

3.2.2 Biochar.
3.2.2.1 Physicochemical properties of biochar. Physicochemical properties for biochar are listed in Table 1.

The pyrolysis temperature has a notable effect on the physicochemical properties of biochar. The content of volatile gradually decreased as the pyrolysis temperature increases, such as 20.46% for PPC-400 and only 9.48% for PPC-700. Conversely, as the pyrolysis temperature increases, the ash content increased as a result of the ash remaining in the PC. Meanwhile, the content of fixed carbon equally increased significantly. Similar results were obtained for coffee chaff⁴¹ and Japanese cedar.⁴² This suggests that the pyrolysis temperature resulted in lower volatile content, while the higher ash content was due to enhanced thermal decomposition for the PC. In addition, the nitrogen content of PPC-400 was higher, which was 2.79%, and the nitrogen content was over those of other biochar such as coffee chaff biochar,⁴¹ rick husk biochar,⁴³ and date seeds biochar.⁴⁴ However, the nitrogen content of PPC-400 was similar to that of fiberboard biochar.²⁹ In reviewing the literature, Li et al.⁴⁵ indicated the nitrogen content of bamboo biochar detected by element analyzer and XPS being 0.6% and 0.4%, respectively. Wang et al.⁴⁶ demonstrated the nitrogen content for lignin-derived porous carbon determined by element analyzer and XPS is 0.24% and 0%, respectively. Similarly, Xu et al.²⁹ illuminated the nitrogen content for fiberboard biochar detected by element analyzer and XPS being 3.42% and 2%, respectively. Notably, the nitrogen element content detected by an element analyzer is higher than the nitrogen element content detected by XPS. A plausible explanation for this result can be given as follows: element analyzer and XPS work on different principles. For the elemental analyzer, the element content of samples is determined by the detector after combustion, secondary combustion, reduction, and interference gas removal, while XPS according to Einstein's photoelectron effect to determines the elemental content on the surface of samples. In other words, ultimate analysis demonstrates the elemental composition of samples, while XPS determines elemental content on the surface of samples. As such, when discussing the migration law of the N element, the N element determined by XPS will not be discussed if not mentioned otherwise.

3.2.2.2 H/C, O/C, and N/C element rations. The S content of PC was intensely low and did not change much during the pyrolysis process. As the pyrolysis temperature increases, the formation and release of CH₄ and C₂H₆ lead to a slight decrease in H content. Besides, the content of both C and O in PC exceeds 40%. A sharp decrease in O content, from 45.57% for PC to 14.10% for PPC-700 with increasing the pyrolysis temperature. Notably, the C element was released during pyrolysis, but the C content increased increasingly, from 43.99% for PC to 74.63% for PPC-700, which was because of the weight loss of PC and the removal of a major amount of O element. In addition, the N element enrichment and N element release have effects on the N content of biochar during the pyrolysis process. The N content of PPC-400 was higher than that of PC, PPC-500, PPC-600, and PPC-700; from this, we can infer that the enrichment of N element was higher than the release of N element upon pyrolysis temperature was 400 °C.

Fig. 5 shows variation in H/C, O/C, and N/C element rations for the PC and biochar along with pyrolysis temperature.

Fig. 5 H/C, O/C, and N/C element rations for PC and PPC-X.

With an increase in pyrolysis temperature, the element ratios of N/C increased at first, and then they slowly decreased and tended to be stable. However, the element ratios of H/C and O/C for the biochar gradually decreased as the pyrolysis temperature increases. This indicates that large amounts of H and O elements were removed from the PC, while the C element was enriched during the pyrolysis process. Compared to the ranges of the H/C and O/C element ratios found in the previous publication for lignite, peat, bitumen, and anthracite, the PC contains more O and H elements than all coals.⁴⁷

3.2.3 Liquid. Results of water content, pH, and element analysis are shown in Table 2.

Table 2 Physicochemical properties for liquids obtained by pyrolysis of PC at different temperatures^a

Samples	Ultimate analysis (wt%)				Water content of liquid (wt%)	pH	HHV (MJ kg^−1)	Energy yield (%)
	C	H	N	O
a L-XXX stands for liquid obtained from pyrolysis of PC at XXX temperature.
L-400	8.21 ± 0.32	4.76 ± 0.03	0.62 ± 0.03	86.41 ± 0.62	73.03 ± 1.32	3.25 ± 0.27	10.40	16.80
L-500	7.99 ± 0.41	6.13 ± 0.04	0.64 ± 0.02	85.24 ± 0.53	74.56 ± 1.27	3.71 ± 0.32	7.89	17.60
L-600	7.76 ± 0.35	5.41 ± 0.05	0.81 ± 0.03	86.02 ± 0.57	76.45 ± 1.36	4.08 ± 0.38	8.74	20.60
L-700	7.82 ± 0.31	4.92 ± 0.03	0.92 ± 0.04	86.34 ± 0.62	77.89 ± 1.32	4.42 ± 0.29	10.12	24.54

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Liquid had a C content of 7.76–8.21% and an O content of 85.24–86.41%. Meanwhile, with an increase in pyrolysis temperature, the water content of liquid gradually increased from 73.03% to 77.89%. This result suggests that increased temperature promotes the dehydration reaction. Besides, with an increase in pyrolysis temperature, the pH also increased from 3.25 to 4.42. This was mainly due to the water content of liquid gradually increased.

To compare the composition of the liquids obtained at different temperatures, L-400 and L-700 were subjected to GC/MS tests. As is known, the chromatographic peak area of a compound is considered to be linearly related to its content. Several representative compounds were selected as their comparatively higher peak area%. Table 3 lists the 30 components of L-400 with a relative content above 0.15%. Similarly, Table 4 exhibits the 34 components of L-700 with a relative content above 0.15%.

Table 3 Composition of L-400 and the relative content of the corresponding composition

Time (min)	Relative content (%)	Compound name	Molecular formula
5.774	17.12	Acetic acid	C2H4O2
11.961	9.08	Furfuryl alcohol	C5H6O2
18.256	5.21	Guaiacol	C7H8O2
17.945	4.23	Phenol	C6H6O
12.130	2.78	2-Methyl-2-cyclopenten-1-one	C6H8O
10.411	2.13	2-Cyclopentenone	C5H6O
20.411	1.95	p-Cresol	C7H8O
15.185	1.92	3-Methyl-2-cyclopenten-1-one	C6H8O
8.704	1.89	Pyrrole	C4H5N
8.415	1.86	Cyclopentanone	C5H8O
12.632	1.81	3-tert-Butylcyclobutene	C8H14
8.049	1.65	Propionic acid	C3H6O2
16.734	1.62	2,3-Dimethyl-2-cyclopenten-1-one	C7H10O
19.309	1.59	o-Cresol	C7H8O
21.169	1.57	2-Methoxy-4-methylphenol	C8H10O2
15.338	1.53	2,3-Dimethyl-butane	C6H14
6.522	1.21	Hydroxyacetone	C3H6O2
23.433	1.12	4-Ethyl-2-methoxyphenol	C9H12O2
9.484	1.09	Dimethylmalonic acid	C5H8O4
6.615	1.03	2,4-Pentadienenitrile	C5H5N
21.633	1.01	2,4-Dimethylphenol	C8H10O
15.561	0.89	3,4-Dimethyl-2-cyclopenten-1-one	C7H10O
7.378	0.85	2-Methyl-2-pentanol	C6H14O
14.918	0.80	2,3-Dimethyl-2-cyclopenten-1-one	C7H10O
9.670	0.72	2-Methylcyclopentanone	C6H10O
17.007	0.58	2,3,4-Trimethyl-2-cyclopenten-1-one	C8H12O
22.332	0.41	Maltose	C6H6O3
8.234	0.34	4-Methylphenol	C7H8O
13.758	0.22	Phenol	C6H6O
22.243	0.16	Hydroxycarbamide	CH4O2N2

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Table 4 Composition of L-700 and the relative content of the corresponding composition

Time (min)	Relative content (%)	Compound name	Molecular formula
1.769	11.88	Acetic acid	C2H4O2
12.893	8.92	1-2-Benzenediol	C6H6O2
10.492	4.36	Valeraldehyde	C5H10O
13.604	3.38	4-Methylcatechol	C7H8O2
5.150	2.46	2-Furan methanol	C5H6O2
11.248	1.96	3-Hydroxypyridine	C5H5ON
1.589	1.94	2-Thiourea	CH4N2S
2.229	1.89	3-Hydroxypropionic acid	C3H6O3
2.149	1.68	Acetoxyacetic acid	C4H6O4
8.977	1.38	2-Hydroxy-3-methylcyclopent-2-enone	C6H8O2
6.406	1.34	2-Methyl-2-imidazoline	C4H8N2
10.282	1.20	Guaiacol	C7H8O
8.497	1.14	N-Methylpiperidine	C6H13N
8.056	0.63	Phenol	C6H6O
10.903	0.45	3-Ethyl-2-hydroxy-2-cyclopenten-1-one	C7H10O2
18.486	0.43	Lauric acid	C12H24O2
10.808	0.40	Maltose	C6H6O3
15.750	0.37	4-Ethylresorcinol	C8H10O2
6.516	0.36	4-Hydroxybutyric acid	C4H8O3
7.656	0.36	2,4-Dimethylfuran	C6H8O
1.534	0.35	Hydroxycarbamide	CH4O2N2
12.458	0.35	2-Methoxy-4-(methoxymethyl)phenol	C9H12O3
5.450	0.32	Acetonyl acetate	C5H8O3
9.572	0.31	Gamma-valerolactone	C5H8O2
10.082	0.31	4-Methylphenol	C7H8O
17.980	0.31	4-Hydroxy-3-methoxyphenylacetone	C10H12O3
13.879	0.27	4-Hydroxy-3-methoxystyrene	C9H10O2
13.984	0.27	m-Dihydroxybenzene	C6H6O2
7.846	0.25	Dibuthylamine	C8H19N
19.751	0.24	Homovanillic acid	C9H10O4
1.564	0.18	Isopropylamine	C3H9N
7.001	0.17	3,5-Lutidine	C7H9N
8.822	0.17	1,2,5-Trimethylpyrrole	C7H11N
6.606	0.16	2,6-Dimethylpyrazine	C6H8N2

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Table 3 indicates the top 5 products were acetic acid (17.12%), furfuryl alcohol (9.08%), guaiacol (5.21%), phenol (4.23%), and 2-methyl-2-cyclopenten-1-one (2.78%). Table 4 lists the top 5 products were acetic acid (11.88%), 1-2-benzenediol (8.92%), valeraldehyde (4.36%), 4-methylcatechol (3.38%), and 2-furan methanol (2.46%).

The liquids (i.e., bio-oils) are intensely complex in composition and contains an army of compounds. Although they were not likely to identify each ingredient, the liquids can be divided into several main categories according to the functional groups determined by GC/MS: acids, phenols, ketones, esters, alcohol, N-containing compounds, aldehydes, sugar, and furans. Total content for liquids ranged from 68.37% for L-400 to 50.19% for L-700, resulting from some components present in liquids were not calculated. The main components of L-400 and L-700 are shown in Fig. 6(a) and (b), respectively.

Fig. 6 Main components for L-400 and L-700 are shown in (a) and (b), respectively.

As seen in Fig. 6(a), the relative contents of acids, phenols, ketones, alcohols, N-containing compounds, and sugar were 19.86%, 17.27%, 13.30%, 11.14%, 3.08%, and 0.41%, respectively. However, the relative content of sugar was below 1.00%. Compared with L-400, L-700 (as indicated in Fig. 6(b)) had fewer acids and more N-containing compounds, which is consistent with the results of the PH as well as the ultimate analysis. Notably, the composition of L-700 is more complex than that of L-400, which suggests the high temperature leads to the secondary decomposition of many substances.

3.2.4 Non-condensable gas. Fig. 7 shows the volume concentration of gases obtained from the PC pyrolysis.

Fig. 7 Gases compositions and HHV with pyrolysis temperatures.

The gaseous products were primarily CO₂, CO, CH₄, and H₂, which are derived from the decomposition of cellulose, hemicellulose, and lignin, and the mechanisms of their formation have been reported in the previous work.⁴⁸ Of these, CO₂ endows the highest volume concentration, followed by CO and CH₄, but pyrolysis temperature was 700 °C, the CH₄ volume concentration slightly more than the volume concentration of CO. With an increase in pyrolysis temperature, the CO volume concentration and the C_mH_n volume concentration have no significant difference, while the CH₄ volume concentration and H₂ volume concentration gradually increased. In reviewing the literature, CH₄ is mainly derived from the pyrolysis of methoxy, methyl, and methylene in lignin.⁴⁹ This suggests an increase in pyrolysis temperature, resulting in the lignin intensely decomposition, and produce more CH₄, which coincides with the change of CH₄ volume concentration in this study. HHV for the gases of 400 °C, 500 °C, 600 °C, and 700 °C was 4.93 MJ kg⁻¹, 7.81 MJ kg⁻¹, 11.19 MJ kg⁻¹, and 13.68 MJ kg⁻¹, respectively. These results manifested that with an increase in pyrolysis temperature, the heating values of gaseous products gradually increased.

3.2.5 Distribution of O, C, and N in pyrolysis products. In the pyrolysis of biomass, the biomass was decomposed to produce biochar, liquid, and gaseous products. Besides, during pyrolysis, the O, C, and N elements from the biomass migrate into biochar, liquid, and gaseous products. Fig. 8(a–c) show the distribution of O, C, and N in the pyrolysis products, respectively.

Fig. 8 Distribution of oxygen (a), carbon (b), and nitrogen (c) elements in the pyrolysis products.

When the pyrolysis temperature increased from 400 °C to 700 °C, the proportion of O in biochar decreased from 24.42% to 8.63%. Similarly, the proportion of O in gaseous products decreased from 26.30% to 12.76%. In contrast, the proportion of O in liquid products increased from 52.28% to 78.13%. This means that higher temperatures intensified the deoxygenation reaction. Element O was removed from the PC and migrated into the liquid and gaseous products in the form of CO, H₂O, CO₂, and oxygen-containing organic compounds. Notably, element O in PC was primarily transferred into H₂O, followed by CO₂ and CO. These results suggest that water and gaseous products play a vital role in deoxygenation.

When the pyrolysis temperature increased from 400 °C to 700 °C, the proportion of C in liquid products increased from 5.37% to 7.70%. Similarly, the proportion of C in gaseous products increased from 25.89% to 42.90%. In contrast, the proportion of O in biochar decreased from 68.38% to 49.40%. These results show that the C loss in PC was less than 50.60% over the pyrolysis temperature range. Meanwhile, with an increase in pyrolysis temperature, the proportion of N decreased from 76.01% for PPC-400 to 28.42% for PPC-700. As a result, the pyrolysis process of PC being a deoxygenation process along with C and N elements migration.

3.2.6 Energy yield for pyrolysis products. Energy yield for pyrolysis products is presented in Fig. 9. The total energy yield of pyrolysis products was below 100% resulted from the experimental errors during the collection and analysis of the pyrolysis products.

Fig. 9 Energy yield for pyrolysis products.

With an increase in pyrolysis temperature, the energy yield of biochar gradually decreased from 73.86% for PPC-400 to 49.42% for PPC-700, but the energy yield of liquid increased from 18.60% (400 °C) to 24.54% (700 °C). Among the three-phase products, the gases endow the lowest energy yield because it has the lowest mass yield. Notably, the energy of the biochar dominates the pyrolysis of the PC, which was consistent with the previous study.⁵⁰

3.3. Analysis of the potential for removal of tetracycline

Recently, it has been reported that biochar derived from fiberboard can remove tetracycline due to its high content of nitrogen and a certain specific surface area.²⁹ In this work, the nitrogen content of PPC-400 was similar to that of the biochar derived from fiberboard and its specific surface area was slightly higher than that of the biochar derived from fiberboard.²⁹ Interestingly, in this work, PPC-400 was obtained by pyrolysis of PC at 400 °C under the conditions of insufficient nitrogen atmosphere. Therefore, the biochar (PPC-N-400) prepared by pyrolyzing PC at 400 °C in a nitrogen atmosphere for 60 min has more nitrogen content and its specific surface area further increased. The nitrogen content and specific surface area of PPC-N-400 were higher than that of the biochar derived from fiberboard, as listed in Table 5.

Table 5 Studies concerning the removal of tetracycline employed biochar with high nitrogen content reported in the literature

Samples	Surface area (m^2 g^−1)	Pore volume (cm^3 g^−1)	Average pore size (nm)	Nitrogen content (%)	Tetracycline removal efficiency (%)	Reference
a BCXXX stands for biochar derived from pyrolysis of fiberboard prepared at XXX °C, XXX can be 300, 500, and 800.
BC300^a	32.2	0.022	2.5	3.54	55.45%	29
BC500^a	34.9	0.033	4.3	3.16	63.29%
BC800^a	135.1	0.108	3.3	3.42	68.60%
PPC-400	65.3	0.066	2.0	2.79	Further investigation	This study
PPC-N-400	246.4	0.230	1.9	3.98

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Consequently, we can infer that the PPC-400 and PPC-N-400 have potential in the field for removing tetracycline. Particularly, PPC-N-400 had a significant removal effect, and its removal efficiency may be over 68.60%.²⁹

4. Conclusions

When PC was pyrolyzed, the effect of its pyrolysis temperature on the products was similar to the effect of torrefaction pretreatment on the products. PC endows two distinct weight loss peaks in the pyrolysis temperature ranging from 200 °C to 500 °C. Of these, the weight loss peak at about 450 °C was primarily ascribed to the existence of non-structural substances. During the pyrolysis process, the deoxygenation, decarbonization, and denitrogenation of PC endow a significant effect on the physicochemical properties of biochar, liquid products, and gaseous products. After pyrolysis, oxygen was present in the gaseous state as CO₂ and CO, and also in the liquid products as oxygen-containing organic compounds and H₂O. Unlike oxygen, over 49.40% of carbon in PC was remained in PPC-X, while a small amount of carbon migrated into the liquid and gaseous products. Of note is that the energy of biochar dominates the pyrolysis of the PC. Furthermore, the nitrogen content and specific surface area of the PPC-N-400 were higher than that of the biochar derived from fiberboard. Therefore, we can infer that the PPC-N-400 has a significant effect on the removal of tetracycline.

Author contributions

Jielong Wu: data curation, investigation, and writing-original draft. Liangcai Wang: project administration, writing-original draft, editing, and supervision. Huanhuan Ma: visualization and methodology. Jianbin Zhou: supervision, writing-review, and funding acquisition.

Conflicts of interest

The authors declare no competing financial interest.

Acknowledgements

This work was supported by the National Promotion Project of China (No. 2020133136), the National Key Research and Development Plan of China (2016YFE0201800).

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Footnote

† These authors contribute equally to this work.

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