Introduction

Anaerobic digestion (AD) is a highly effective technique for the disposal of agricultural residues, organic solid waste, food waste, and sludge (Wang et al. 2021; González-Arias et al. 2020; Xu et al. 2018a) that can process waste while simultaneously generating green fuels. The processed AD slurry and residue are suitable for use as organic manure, fertilizer mixing, and soil improvement (Meng et al. 2018; Möller and Müller 2012; Xu et al. 2021). However, contaminations such as pathogens, humic acids, and organic matters are frequently found in the anaerobic digestion residues (ADR) (Du et al. 2021; KeChrist et al. 2017), which not only complicate ADR utilization but also pose significant risks to the environmental and human health. Additionally, only around 15 ~ 40% of the energy in the feedstock is converted to biogas after the AD process (Opatokun et al. 2015). To enhance the energy efficiency and sustainability, there is pressing need to improve the resource utilization of ADR.

Pyrolysis can convert the ADR into high-value bio-oil, combustible gases, and biochar through the thermal decomposition and cracking reactions. Pyrolysis also achieves the objectives of ADR disposal in a harmless manner and high-efficient cascade usage. Li et al. (2014) examined the energy recovery perspective of ADR formed by anaerobic digestion of manure and straw. They discovered that anaerobic digestion produced 1.5 ~ 2.6 MJ/kg energy, while pyrolysis produced up to 6.1 MJ/kg of energy. The energy recovery efficiency was improved up to 85% by combining digestate with pyrolysis. In a laboratory-scale auger reactor, Liang et al. (2015) investigated the pyrolysis of potato peel and its fermentation residue. The results indicated that the bio-oil and biochar yields via potato peel pyrolysis were 22.7% and 30.5%, respectively, whereas the bio-oil and biochar yields by the pyrolysis of its fermentation residue were 25.6% and 32.2%, respectively. Neumann et al. (2015) pyrolyzed ADR in a continuous pyrolysis reactor operating at a rate of 2 kg/h, suggesting that the heating value of the bio-oil produced at 750 °C was as high as 33.9 MJ/kg, and the oil had a very low moisture content of 1.7 wt.%. It was estimated that more than 91% of the energy contained in the feedstock was converted into usable products. Additionally, the biochar produced by ADR pyrolysis also showed a significant potential for use. Inyang et al. (2010) conducted the pyrolysis experiments of digestate residue and fresh bagasse samples separately and found that the biochar produced from ADR had a larger surface area, higher ion exchange capacity, and better hydrophobicity than the biochar derived from fresh bagasse. Liu et al. (2020) investigated the properties of pyrolysis products from ADR which was generated from food wastes, concluding that the biochar had an excellent porous structure with a lager specific surface areas, which was beneficial for contaminants removal owing to its high adsorption and catalytic reaction site performance. There was limited work performed on the pyrolysis of ADR, with detailed analysis on the pyrolysis distribution and characterization of pyrolysis bio-oil, gases, and char. Besides, previous studies showed that bench scale reactors with electric heating presented a slow pyrolysis rate for processing ADR. When the heat was transferred into the sample particle by diffusion and convection, the heat gradient caused a prolonged pyrolysis time and the serious secondary reactions of volatiles, implying unclear pyrolysis behavior and products characteristics of ADR.

Both heating rate and pyrolysis temperature are critical parameters that affect the volatilization reaction in the ADR pyrolysis process. Fast pyrolysis rapidly breaks the lignocellulosic structure and suppresses the crosslink of radicals, which may effectively decompose feedstocks with minimized secondary reactions, and the process has garnered considerable scientific interest. The conventional analytical fast pyrolysis may be conducting using Py-GCMS (pyrolysis–gas chromatography/mass spectrometry), thermogravimetric analyzer, or Curie-point pyrolyzer, which can only be loaded with a milligram-level mass sample. As a result, liquid and gaseous products could not be collected and further analyzed. In the recent years, fast pyrolysis through infrared heating has attracted research attentions due to its rapid heating rate (> 10 °C/s) and capacity to load materials at gram level. The infrared heating techniques may enable the sample molecules resonation and rapidly heat transfer, thereby substantially improving the heat transfer process between the heat source and the feedstock through heat conduction. It is advantageous for rapid pyrolysis to reduce the residence time of volatiles. Previous studies on pyrolysis of coal (Xu et al. 2019), biomass (Zhu et al. 2019), and oil shale (Siramard et al. 2017) via rapid infrared heating have been reported. Xu et al. (2019) reported that the tar yield climbed from 9.78 to 13.38% as the infrared heating rate was raised from 6 to 677 °C/min, and the highest rate of tar production was 134% of the G-K test oil yield while heating at 667 °C/min and pyrolyzing at 700 °C. The cedar pyrolysis behaviors using infrared heating were conducted, and the result showed that a faster heating rate favored the generation of acids and furans (Zhu et al. 2019). When oil shale pyrolysis was carried out utilizing rapid infrared heating at 0.1 to 0.5 °C/s, the oil yield rose from 9.5 to 11.1% (Siramard et al. 2017). It is worth noting that the ADR pyrolysis behaviors in both fast pyrolysis and slow pyrolysis aspects have not been reported in preceding literature.

The objective of this work is to investigate the pyrolysis behaviors of ADR in a fixed-bed reactor with rapid infrared heating, which was compared to the results from that with electrically heating. The effects of temperature on behaviors of ADR pyrolysis were investigated, and the composition of gaseous products and bio-oil was determined using gas chromatography (GC) and gas chromatography-mass spectrometry (GC–MS). Furthermore, X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and Raman spectrum were employed to characterize the biochar. From analyses of tar components and char characteristics, this work suggested the feasibility of the rapid infrared heating pyrolysis in obtaining a higher tar yield than the electrical heating, as well as an in-depth knowledge of the fast pyrolysis behaviors of ADR.

Material and methods

Materials

Anaerobic digestion residues from Da Xing District of Beijing, China, was used in this study. ADR is comprised of agricultural waste, manure from animals and poultry, and sludge. After 12 h of drying at 105 °C, the dried ADR samples was crushed and sieved to a size below 80 mesh. Proximate analysis of ADR (wt.%, on dry basis) was 4.48 wt.% moisture, 41.88 wt.% ash, 46.29 wt.% volatile matter, and 7.35 wt.% fixed carbon. Elemental analysis of ADR (wt.%, on dry ash-free basis) was 27.33 wt.% C, 3.06 wt.% H, 3.40 wt.% N, 0.14 wt.% S, and 66.06 wt.% O (by difference).

Methods

TG analysis

Prior to bench scale pyrolysis experiments, ADR sample was processed in a thermogravimetric analysis (TGA) to characterize the pyrolysis process in detail. The thermogravimetric experiments were conducted using NETZSCH Simultaneous Thermal Analyzer STA449F3, with a protective gas flow rate of 100 mL/min. Each run utilized a sample mass of 10 mg, and the experiments were performed at heating rates of 10, 20, 30, and 40° C/min from ambient temperature to 900 °C, respectively.

Apparatus and methods

The schematic diagram of the apparatus is shown in Fig. 1. The pyrolysis tests was performed using an innovative infrared heating system and a conventional electric heating system (Hu et al. 2021). Three grams (± 0.001 g) of the anaerobic digestion residues was being put into a quartz tube reactor with an inner diameter of 24 mm and a length of 370 mm in each run. With a flow rate of 100 mL/min, 99.999% pure nitrogen served as the carrier gas. The reactor was heated to the pyrolysis temperature (400, 500, 600, and 700 °C) after the air in the reactor had been removed and held for 30 min to guarantee complete conversion of the ADR samples. A quartz collector submerged in − 25 °C ethylene glycol was utilized to condense the pyrolysis vapor. After passing through three acetone washing beakers, the non-condensable gas was measured using a wet gas flowmeter. Pyrolysis experiments were also done under comparable processing conditions utilizing an electrically heated conventional furnace.

Fig. 1
figure 1

Schematic diagram of the experimental apparatus. 1 High pressure cylinder, 2 control cabinet, 2–1 temperature controller, 2–2 pressure gage, 3 quartz tube, 4 U-shaped tube, 5 acetone washing bottle, 6 saturated sodium bicarbonate bottle, 7 dry bottle, 8 wet flowmeter, 9 gas chromatograph

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Products analysis

By comparing the weights before and after the U-shaped tube, the weight of pyrolysis liquid products was determined. A Karl-Fischer micro-moisture tester was utilized to determine the water content in liquid products (KSL 701, Zibo Kulun). Prior to analysis, the electrolyte was calibrated with filtered water to reach electrolytic equilibrium. We utilized a total of 20 μL of injection and tested the same sample five times. A micro-GC analyzer (INFICON 3000, Switzerland) was used to determine the contents of pyrolysis gas, including H2, CH4, CO, CO2, and C2-C3 (C2H4, C2H6, C3H6, and C3H8) (Hu et al. 2022). Gas chromatography–mass spectrometry was used to measure the composition of the bio-oil (GC–MS Shimadzu, GCMS-QP2010 Plus). The temperature of the GC oven was first set at 50 °C for 5 min and further rose at 5 °C/min to 260 °C, finally keeping at this final temperature for 5 min. The carrier gas was high-purity helium at 1 mL/min. The split mode was used, with a 10:1 split ratio. The intake temperature was 250 °C, and the chromatographic column was Rtx-WAX (30 mm*0.25 mm *0.25 m), with 35–500 m/z mass spectrometric collecting range. The spectrum data were compared to the NIST mass spectral library to determine the bio-oil compositions. Fourier transform infrared spectroscopy was adopted to examine the distribution and change of functional groups on the surface of biochar (FTIR). Firstly, the biochar sample and KBr were crushed to a mass ratio of 1:100 (mass ratio) in an agate mortar to prepare transparent wafers. The spectra were analyzed between 400 and 4000 cm−1 with a resolution of 4 cm−1 and 16 scans. An X-ray diffractometer (XRD, type: D/Max2500pc, Japan) was utilized with filtered Cu-K radiation to identify the phase transformation and crystal structure of biochar. The 2θ scanning ranges of 5 to 55° with a step size of 0.0200° were used to gather data. To investigate the crystalline and amorphous carbon in biochar, Raman measurements were carried out using a Confocal Raman Microspectroscope (LabRAM HR Evolution, France) equipped with a 514.4 nm Ar+ laser beam with a spectral resolution of 1 cm−1. A laser with a power of less than 0.4 mW (at the samples) was employed as an excitation source. Raman spectra were recorded in the 500–2000 cm−1 range.

Results and discussion

TG analysis

Figure 2 illustrates the TG and DTG curves of the ADR samples. The TG curve indicated that the residual mass of the ADR samples was more than 50%, and this matched well with the high ash content of ADR as determined by elemental analysis. ADR pyrolysis process consisted of three steps: dehydration, pyrolysis, and carbonization. The DTG curves revealed that as the heating rate increased, the maximal weight loss rate of ADR increased as well. The maximum weight loss rate increased from − 2.01 to − 10.35%/min with rising temperature from 10 to 40 °C/min, and its maximum weight loss of the 10 °C/min, 20 °C/min, 30 °C/min, and 40 °C/min were 276 °C, 281 °C, 286 °C, and 290 °C, respectively, which is due to the differences in the heat and mass transfer of sample bed (Mallick et al. 2018; Shahbeig and Nosrati 2020). The first stage occurred mainly between 50 and 150 °C and resulted in the removal of moisture and crustal water from ARD, as well as the breakdown of microorganisms such as bacteria and pathogens. The second weight loss peak of ADR occurred from 160 to 600 °C, with the weight loss from 36.10 to 38.65% at different heating rates. In this stage, two peaks appeared at 290 °C and 320 °C in DTG curves accordingly. Bach and Chen (2017) reported that the decomposition of lipid and protein occurs at 150 ~ 550 °C and 210 ~ 310 °C respectively. ADR is made up of agricultural waste, manure from animals and poultry, and sludge; its weight loss peaks at 290 °C and 320 °C are ascribed in part to protein breakdown. The decomposition of cellulose and hemicellulose lignin occurs mostly at 160 ~ 600 °C (Hu et al. 2016; Sophonrat et al. 2018). Carbonation and decomposition of mineral components such as ash and salts of carboxylic acids may result in the third weight loss peak around 600 ~ 800 °C, indicating a sluggish devolatilized rate (Tsemane et al. 2019). In DTG curves, there was a noticeable thermal lag, and it was hypothesized that thermal hysteresis may negatively affect the products of pyrolysis. Due to the fact that the pyrolysis behavior and product distribution of ADR cannot be investigated using the tiny mg-level sample fed into the TG apparatus, it is critical to perform further study using a bench-scale reactor with g-level.

Fig. 2
figure 2

TG (a) and DTG (b) curves of ADR

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Pyrolysis products distribution

Figure 3 shows the effects of temperature on pyrolysis products distribution using infrared heating (IH) and electric heating (EH) techniques. For both IH and EH, the increase in the temperature increased the bio-oil yield first, reaching to a peak at 500–600 °C, and subsequently reduced yield. In comparison to the results obtained with EH, IH presented a higher overall bio-oil yield but a lower water yield. Notably, the infrared heating (IH) and electric heating (EH) techniques had the largest amount of bio-oil yield at 600 °C and 500 °C, respectively. The bio-oil produced by IH increased from 8.35 to 12.56 wt.% with rising temperature from 400 to 600 °C, but further raising the temperature to 700 °C reduced the bio-oil yield to 11.22 wt.%. The bio-oil produced by EH rose from 9.23 wt.% at 400 °C to a maximum value of 12.12 wt.% at 500 °C, before dropping to 7.80 wt.% as the temperature increased to 700 °C. Both infrared heating and electric heating increased pyrolysis gas yields but decreased biochar yields. As the temperature increased from 400 to 700 °C, the biochar yield dropped from 75.95 to 60.59 wt.% and 69.88 to 59.26 wt.% for IH and EH, respectively. On the other hand, the yield of pyrolysis gas yield rose from 12.46 to 24.57 wt.% and 17.18 to 27.41 wt.%, respectively. The bridge bond began to cleave as the temperature increased during ADR pyrolysis, leading in the generation of free radical groups and large quantities of non-condensable gases. As much heat may be transferred into the inside of ADR particles, accelerating the cleavage of macromolecular organics’ alkyl side chains (Gai et al. 2014), which not only generated large condensable gas and small molecule gases but also enhanced the aromatization of biochar. Thermal cleavage of covalent bonds in ADR produced volatiles in a single step that primarily depends on the temperature, while the volatile reactions involve multiple steps and depend on many factors especially the gas phase temperature. Different heating techniques, such as EH and IH, had substantial effects on primary pyrolysis products and volatile repolymerization, resulting in a variation in products distribution (He et al. 2014; Katheklakis et al. 1990).

Fig. 3
figure 3

Effects of temperature on pyrolysis products distribution using infrared heating (IH) and electric heating (EH) techniques

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Figure 4 shows the temperature profiles in reactors. The experimental results indicated that the temperature profile of electrically heated reactor was T1 > T2 > T3, while the infrared heated reactor followed the trend of T1 > T3 > T2. He et al. (2014) discovered that when electric heating was adopted in fixed-bed reactors, a considerable temperature gradient occurred, and consequently the gas phase temperature was much higher than temperature of the materials being processed. When the primary volatiles were exposed to a high-temperature environment, secondary reactions occurred and were exacerbated. This not only deteriorated the quality of oil, but also substantially decreasing the bio-oil production. Infrared heating enabled the material particle to resonate and quickly reach the desired temperature, thus accelerating the heat transfer process. The volatiles produced by IH experienced a lower T2 temperature and a lower rate of secondary reaction of primary volatiles, resulting in an increase in the bio-oil yield (Hu et al. 2017). Due to the temperature differential in EH, the primary volatiles reached a higher T2 temperature and underwent severe secondary reactions, reducing the yield of bio-oil and raising the gas yield (Xu et al. 2019; Zhu et al. 2019).

Fig. 4
figure 4

Profiles of temperature in reactors. T1 refers to the heating temperature; T2 refers to the ambient temperature; T3 refers to the materials temperature

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Bio-oil characterization

Figure 5 illustrates the relative content of main compounds in both IH and EH bio-oil. As can be seen, bio-oil mainly comprised of nitrogen-containing compounds, ketones, alcohols, acids, phenols, alkanes, etc. Infrared heating (IH) presented a greater overall yield of phenolic compounds than EH. Raising temperature increased the phenols contents of bio-oil in IH and EH initially, reaching maximum values of 8.63% and 2.95% at 500 °C and 600 °C, respectively, and subsequently reduced yield. Nitrogen-containing compounds in the IH bio-oil increased with rising temperature, whereas ketones and acids compound reduced. The nitrogen compounds yield in IH and EH bio-oils rose 68.14 to 89.43% and 72.69 to 84.29% with rising temperature from 400 to 700 °C, respectively, whereas the contents of ketones and acids of bio-oil in IH decreased from 17.04 to 4.56% and 5.14 to 1.42%, respectively. Nitrogen compounds accounted for a remarkable proportion in total, which may be ascribed to the high nutritional content such as protein, peptides, and nucleic acids in ADR (Parnaudeau and Dignac 2007). As shown in the thermogravimetric study in the “TG analysis” section, a substantial weight loss occurred at 290 °C and 320 °C of the DTG curves, owing to the breakdown of cellulose, hemicellulose, and lignin, as well as nitrogen-containing compounds in the ADR. Ketones, phenolic acids, alcohols, and other oxygen-containing chemicals were discovered in bio-oil, which may be derived from the undigested lignin, cellulose, and hemicellulose in anaerobic digestion, agreeing well that DTG curve exhibits peaks from 290 to 320 °C. In contrast to the EH, the bio-oil components in the IH changed significantly with increasing temperature. The fast heating by IH increased heat and mass transfer into the particles, allowing the feedstock to reach the desired temperature rapidly, thus intensifying the breakdown of macromolecular organic molecules such as nitrogen and oxygen side chains. EH provided a large temperature gradient between the heat source and the feedstock, which prolonged the reaction time of volatiles, resulting in severe secondary reactions in primary pyrolysis products. Besides, the higher the setting temperature, the larger the temperature gradient in EH reactor, resulting in virtually full volatiles release before the reactor reaches the target temperature. As a consequence of those, the pyrolysis products varied slight with the temperature in EH. The top ten compounds in bio-oil at 500 °C in both reactors are shown in Fig. 6, accounting for about 90% of the total. Piperidine, 1,2-dimethyl-, 4-piperidinone, 2,2,6,6-tetramethyl-, 2-pentanone, and 4-amino-4-methyl- are the top three compounds in bio-oils produced by IH and EH, accounting for more than 50% of the total area. This list is incomplete due to the omission of a few unidentified compounds. The primary difference is that although phenol is the fourth most common compounds in IH bio-oil, it is the ninth most abundant compounds in EH bio-oil, while acetic acid is the fourth most abundant compounds in EH bio-oil. This is because infrared heating promotes pyrolysis of lignin, while EH accelerates cellulose and hemicellulose pyrolysis. In general, the nitrogen-compounds in ADR bio-oil preclude its use as a transportation fuel; it may be used to manufacture plasticizers, rubber softeners, lubricant additives, and insecticide intermediates (Cao et al. 2010), and the extraction of chemical raw materials from ADR bio-oil requires additional research.

Fig. 5
figure 5

Relative content of main compounds in both IH and EH bio-oil. a IH; b EH

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Fig. 6
figure 6

Top ten compounds in bio-oil a IH; b EH

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Gas composition

Figure 7 illustrates the variation of gas compositions, volumes, and high heating value (HHV). The HHV of gases in both EH and IH increased as the temperature rose; raising temperature from 400 to 700 °C increased the gas HHV in EH and IH from 6.82 MJ/Nm3 and 8.43 MJ/Nm3, respectively, to 8.54 MJ/Nm3and 9.67 MJ/Nm3. Infrared heating has a higher HHV value than electric heating. In most cases, the CO and H2 contents of IH were much higher than those of EH. As the temperature increased, the CO2 content of EH and IH decreased. The CO2 content of EH and IH decreased from 62.14 to 49.91 vol.% and from 55.46 to 41.17 vol.%, respectively, when the temperature was increased from 400 to 700 °C. The decrease in CO and CO2 contents indicated that side chains of macromolecules in biochar were greatly reduced, which was consistent with the results in the “Biochar characterization” section that the surface structure of biochar became much ordered. The H2 content in both EH and IH exhibited the opposite trend of CO2 with temperature variation. IH and EH had the lowest H2 yields at 500 °C, with 18.11 vol.% and 15.89 vol.%, respectively. When the temperature was increased to 700 °C, the H2 yields of IH and EH rose to 33.42 vol.% and 26.51 vol.%, respectively. The CH4 and C2-C3 Contents varied slight in both EI and IH.

Fig. 7
figure 7

Composition of gaseous products and its high heating values in the two heating methods; a IH; b EH; c gas volume; d gas HHV

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Biochar characterization

Raman spectroscopy

Figure 8 shows the Raman spectra of the biochar produced by IH and EH. Raman spectra may be used to determine the locations of the D (defects/disordered) and G (graphite) bands, as well as the intensity ratio of the G and D bands (AG/AD) (Asadullah et al. 2010a; Zaida et al. 2007). The Raman signal at 500 ~ 2000 cm−1 was mostly attributable to the polyaromatic ring structure of carbon material (Guo et al. 2008). All char samples exhibited two broad bands at about 1350 cm−1 and 1580 cm−1. The oxygen-containing structure had a high capacity for Raman scattering, and oxygen-containing structures may increase the Raman intensity through a resonance effect with the linked aromatics (Wu et al. 2014). The D band and G band intensities dropped as the temperature increased, which may be ascribed to the gradual removal of aliphatic and oxygen-containing functional groups during the ADR pyrolysis process, as well as the increased aromatic structure in biochar (Asadullah et al. 2010b).

Fig. 8
figure 8

Raman spectra of the biochar produced by the two heating methods. a IH; b EH

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Figure 9 exhibits the Raman spectra of the IH and EH biochars, as well as its AG/AD values. The G band position of two kinds of biochar shifted slightly to the higher frequencies with rising temperature from 400 to 600 °C but shifted to the low frequency at 700 °C, owing to the disappearance of tetrahedral bonds and the formation of trigonal bonds in temperature range from 400 to 600 °C. However, when temperature increased, the D band moved to lower frequencies, indicating that as the temperature increased, the disorder of bond angle decreased and the biochar surface structure became much ordered (Jiu et al. 1999; Yamauchi and Kurimoto 2003). The area ratios of G band and D band (AG/AD) may be utilized to measure the biochar aromatization. The AG/AD at 400 °C is significantly higher than at other temperatures, which may be explained by the fact that biochar heated at 400 °C had much oxygen-containing functional groups, resulting in a stronger Raman spectra (Asadullah et al. 2010b). The ratio of AG/AD in IH biochar increased gradually from 0.17 to 0.20 with rising temperature from 500 to 700 °C. This suggests that the increase in the pyrolysis temperature enhanced the degree of aromatization in biochar structure, resulting in a much organized carbon structure. In comparison to IH biochar, the AG/AD ratio of EH biochar dropped progressively between 400 and 600 °C but substantially rose at 700 °C, owing to the fact that IH and EH used different heating techniques. Clearly, the AG/AD of EH biochar was higher than that of IH at all temperatures, which is due to the slow heating rate and the longer reaction time of ADR in EH. The volatiles were slowly released under the slow heating condition, which is beneficial for maintaining the carbon structure in excellent order. The ADR was quickly heated, and the pyrolysis volatiles released from materials may deteriorate the biochar order. On the other hand, the rapid heating mode of IH significantly reduced the probability of secondary reaction of primary volatiles in this process, whereas the slow heating mode of EH intensified the secondary reaction of the primary volatiles to generate aromatic coke, consequently contributing to a higher AG/AD value than 500 °C and 600 °C.

Fig. 9
figure 9

Band position of G band and D band and ratio of band peak areas AG/AD. a Band position; b ratio of band peak areas AG/AD

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FTIR

Figure 10 illustrates the analysis of FTIR spectra of the biochars produced by EH and IH. The bands intensity in IH biochar was more sensitive with varying temperature than that of EH. With increasing temperature, the amide stretching bands (-CO–NH-) at 1600 ~ 1550 cm−1 got weaker until it disappeared entirely at 700 °C. This is due to the complexation of amide functional groups and metals in ADR. Besides, some nitrogen was volatilized and migrated into the liquid phase, where it was incorporated into aromatic compounds through polymerization and aromatization (Xu et al. 2018b). The intensity of the bands at approximately 1430 cm−1 (for aliphatic chains, including CH3 and CH2 groups (Liu et al. 2016)) decreased gradually with rising temperature for all biochar samples, which was attributed to dehydration, demethylation, and elimination of carbonyl and aliphatic groups during aromatic units formation in the chars. In this process, the structure of biochar can become more orderly, which is consistent with Raman spectroscopy results. The intensity between 700 and 900 cm−1 was attributed to the emergence of aromatic and heteroaromatic compounds (Lu et al. 2013; Xiao and Yang 2013).These bands showed little change as the temperature increased from 400 to 700 °C, suggesting that the aromatic structure existed prior to the biochar being treated at these temperatures. The aromatic structure of the biochar was critical for carbon sequestration, and if the biochar was strongly aromatic, it had the potential to sequester a large amount of carbon. The bands at about 980 ~ 1028 cm−1 correspond to the asymmetric stretching vibration of Si–O-Si in quartz, the stretching vibration of Si–O in aluminosilicates (Zhao et al. 2017), the C-O stretching vibrations of aromatic ring C-O band, and aliphatic C–O–C band, as well as other inorganic substances formed at high temperatures. Both organic and inorganic halogen compounds exhibit stretching vibrations below 600 cm−1 (Hossain et al. 2011).

Fig. 10
figure 10

FTIR spectra of char produced by the two heating methods. a IH; b EH

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Conclusion

In this work, the effects of temperature on pyrolysis behaviors of anaerobic digestion residues (ADR) were investigated in the fixed-bed reactor with rapid infrared heating and electric heating. The pyrolysis process of ADR consisted mostly of three pyrolysis stages. In both IH and EH, the increase in the temperature increased the bio-oil yield initially, reaching to a peak at 500–600 °C, and thereafter reduced yield. It also elevated the gas yield but decreased the char yield. In contrast to EH, IH presented a higher overall bio-oil yield but a lower water yield. The infrared heating (IH) and electric heating (EH) techniques produced the highest amount of bio-oil yield at 600 °C and 500 °C, respectively, which were 12.56 wt.% and 12.12 wt.%, respectively. IH exhibited a higher CO content and gas HHV but a lower CO2 content in comparison to the results obtained by EH. The nitrogen compounds in bio-oil accounted for about 80% of the whole ADR bio-oil. Bio-oil in IH had more phenolic compounds than bio-oil in EH, and the phenol content of bio-oil in both IH and EH increased first and then dropped as the pyrolysis temperature rose, reaching the maximum values of 8.63% and 2.95% at 500 °C and 600 °C, respectively. FTIR and Raman results revealed that the structure of ADR biochar becomes more orderly with rising temperature.