Effects of volatile–char interactions on char during pyrolysis of rice husk at mild temperatures

Effects of volatile–char interactions on char during pyrolysis of rice husk at mild temperatures

Bioresource Technology 219 (2016) 702–709 Contents lists available at ScienceDirect Bioresource Technology journal homepage: www.elsevier.com/locate...

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Bioresource Technology 219 (2016) 702–709

Contents lists available at ScienceDirect

Bioresource Technology journal homepage: www.elsevier.com/locate/biortech

Effects of volatile–char interactions on char during pyrolysis of rice husk at mild temperatures Peng Liu, Yijun Zhao ⇑, Yangzhou Guo, Dongdong Feng, Jiangquan Wu, Pengxiang Wang, Shaozeng Sun Combustion Engineering Research Institute, School of Energy Science and Engineering, Harbin Institute of Technology, 92 West Dazhi Street, Harbin 150001, PR China

h i g h l i g h t s  Volatile–char interactions of in-situ char at mild temperatures were investigated.  Uniformity of the collected char was made sure by the pulse feeding method.  Volatile–char interactions inhibited char reactivity only above 650 °C.  Inhibition intensified with temperature rise, but kept almost unchanged at 700–800 °C.  Volatile–char interactions enhanced volatilization of K even at 600 °C.

a r t i c l e

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Article history: Received 18 July 2016 Received in revised form 10 August 2016 Accepted 11 August 2016 Available online 12 August 2016 Keywords: Volatile–char interactions Mild temperatures Char reactivity Structure Potassium

a b s t r a c t In order to understand the sensitivity of volatile–char interactions to mild temperatures (600–800 °C), insitu rice husk char was prepared from fast pyrolysis (>103 K s1) on a fixed-bed reactor. Retention of K in char, changes in char structure and char reactivity were determined. The results showed that volatile– char interactions did not cause obvious effect on the char yield but showed an inhibitory effect on char reactivity. The inhibition began only above 650 °C and intensified with temperature rise, but kept almost unchanged at 700–800 °C. Char structure and retention of K have a combined effect on char reactivity. The decreased reactivity was caused by additional volatilization of K from char matrix and transformation of relatively smaller aromatic ring systems to large ring systems (>6 benzene rings) above 650 °C. Ó 2016 Elsevier Ltd. All rights reserved.

1. Introduction Biomass has the features of low nitrogen, low sulfur, and carbon neutral, it is considered to be an ideal substitute for fossil fuels (Al-Rahbi et al., 2016; Chen et al., 2016; de Lasa et al., 2011). Rice is a major agricultural crop and annual global rice production is about 700.7 million tonnes (Pinto et al., 2016). Large amounts of rice husk are left as agricultural wastes after harvesting. Gasification is particularly suitable for utilization of low-rank fuels, such as agricultural wastes, by thermo-chemically converting biomass to energy or synthesis gas, because of their high gasification reactivities (Li, 2013; Vakalis et al., 2016). Volatile–char interactions are an important phenomenon during gasification, affecting low-rank fuel gasification significantly (Li, 2013). Volatile–char interactions have been commonly recognized. Majority of the relevant researches were concentrated on ⇑ Corresponding author. E-mail address: [email protected] (Y. Zhao). http://dx.doi.org/10.1016/j.biortech.2016.08.029 0960-8524/Ó 2016 Elsevier Ltd. All rights reserved.

temperatures above 800 °C (Song et al., 2015; Wu et al., 2002; Zhang et al., 2011). Recent years, biomass gasification at mild temperatures has potential applications in sustainable development because it is thermally and economically efficient. Problems caused by slagging by ash in biomass are a challenge in the safe and stable operation of gasifiers. Lowering the gasification temperature is an effective method for preventing agglomeration and ensuring good fluidization (Garcia-Ibanez et al., 2004), more importantly, improving the system efficiency (Xiao et al., 2011). However, the interactions under mild temperatures are limitedly reported. Char structure and retention of inherent catalysts, as two major factors influencing reactivity, are greatly changed by volatile–char interactions. Keown et al. (2007) found that volatile–char interactions changed the biomass char structure drastically during cane trash pyrolysis from 600 to 900 °C in a one-stage fluidized-bed/fixed-bed reactor. Li et al. (2004) reported that volatile–char interactions induced additional volatilization of Na above 700 °C during pyrolysis of Na-form Loy Yang brown coal over the temperature range 530–900 °C in a two-stage

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fluidized-bed/fixed-bed reactor. Quyn et al. (2002) also suggested that volatile-char interactions promoted volatilization of Na during pyrolysis of Loy Yang raw brown coal from 500 to 800 °C in a fluidized-bed/fixed-bed reactor. Choosing experimental methods with reaction conditions corresponding to typical char gasification environments should always be priority for char reactivity investigations. The researchers mentioned above preloaded biomass into the reactor and heated the biomass at a slow rate of about 10 K min1. The formed char then interacted with volatiles. However, in practical gasification processes, the particles were directly carried into the gasifiers by gas flow. They were suddenly exposed to high temperatures and released volatiles rapidly to form chars. Properties of chars prepared at different conditions are different. For example, the volatilization of alkali and alkaline earth metal (AAEM) species for char heated at a slow heating rate (5 K min1) is different from that at a fast heating rate (>103–104 K s1) (Wu et al., 2002). So experiments conducted under slow pyrolysis conditions (10 K min1) are not persuasive to reproduce the practical gasification process. On the other hand, char generated from slow heating (10 K min1) of stacked biomass could not avoid the interactions with volatiles. The majority of volatiles were released below 600 °C and swept out of the reactor during slow pyrolysis (Quyn et al., 2002). But stacking of char particles inevitably facilitates the interactions. This would contribute to the changes in char properties and has obscure effects on the experimental evidence of volatile–char interactions. In order to study the sensitivity of volatile–char interactions to mild temperatures (600–800 °C), the volatile–char interactions of in-situ char were investigated on a fixed-bed quartz reactor operated at a fast heating rate (>103 K s1) during pyrolysis of rice husk at 600–800 °C. A novel pulse feeding method was adopted to instantaneously inject biomass particles into the reactor. This method could enable simultaneous pyrolysis of all particles, the uniformity of the collected char is reliable. More importantly, secondary reactions between volatiles and char were almost totally avoided. In this study, the in-situ char refers to char prepared from fast pyrolysis and no cooling and reheating, similar to char in industrial reactors. So the in-situ char is expected to provide more persuasive evidence to present the sensitivity of volatile–char interactions to mild temperatures, considering its advantages of being close to industrial conditions. 2. Materials and methods 2.1. Biomass The rice was grown in Wuchang, China and crushed to 90–150 lm. The proximate analysis (as received basis) of moisture, volatiles, fixed carbon and ash are 10.50%, 58.53%, 14.63% and 16.34%, respectively. The element content (as received basis) of C, H, N, S and O is 35.89%, 4.23%, 0.19%, 0.13% and 32.72%, respectively. And the content of Na, K, Ca and Mg in the rice husk is 0.13 mg g1, 6.57 mg g1, 0.33 mg g1 and 0.32 mg g1, respectively. 2.2. Experimental system and procedure Fig. 1 shows a schematic diagram of the experimental system. The system was based on the system reported by Guo et al. (2016) and consisted of a quartz reactor, an electrical furnace, a solenoid pulse valve, a feeding tube, a temperature controller, a vibrating feeder, several mass flowmeters, an exhaust trapper and an online process mass spectrometer connected to the outlet of reactor. The reactor was separated into three parts by two

Feeder Quartz reactor

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Valve

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O2

Ar

Rice husk Sample container Furnace

CHCl3:CH3OH=4:1

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Fig. 1. A schematic diagram of quartz fixed-bed reactor used in this study.

porous frits in the middle. The inner diameter of the reactor was 24 mm. The height between the bottom frit and the outlet of the feeding tube was 40 mm. The reactor was heated externally by an electrical furnace. Temperature was monitored precisely by a temperature controller. Biomass was accurately weighed and preloaded into the feeding tube. A solenoid valve was opened for 12 ms to inject biomass into the reactor instantaneously by a pulse compression nitrogen (initial gauge pressure 0.1 Mpa). This feeding method ensured that all the rice husk particles were injected into the reactor simultaneously and pyrolyzed at a rate >103 K s1 simultaneously. So the collected chars were consequently more uniform than those in the previous work by others. The calculation of heating rate is based on the study of Molina and Shaddix (2007). Three types of experiment were performed. In the first type of experiment, 100 ± 0.1 mg of rice husk was preloaded into the feeding tube. The reactor was swept with a flow of high-purity nitrogen gas (>99.999%) to provide an inert atmosphere during heating of it to the desired temperature. After the desired temperature was reached, biomass particles were instantaneously injected into the reactor. The particles would be heated up at a rate >103 K s1 and volatiles would release rapidly after entering the high temperature zone instantaneously. As soon as the volatiles were released, they would be swept out of the reactor immediately by continuous nitrogen flow (3.6 L/min) from the top of the reactor. The formed char particles were dispersed as a thin bed less than 1 mm on the bottom frit. So the secondary reactions between the nascent char and volatiles were almost totally avoided, similar to a wiremesh reactor (Zhang et al., 2015a). The temperature was held for 12 min. Then the reactor was removed out of the electrical furnace and cooled naturally to ambient temperature with continuous nitrogen flow. This is referred to as a ‘‘NTF (no top feeding)” experiment. In the second type of experiment, the feeding procedure and parameters for the nascent char preparation on the bottom frit were exactly same as the ‘‘NTF” experiment. While pyrolysis of rice husk on the bottom frit lasted for 2 min, rice husk entrained in nitrogen gas (3.6 L/min) from a precision feeder, was continuously fed into the reactor from the top at a rate of 140 mg min1 for 5 min. The volatiles evolved from pyrolysis of rice husk would pass through the top frit, with the char particles being retained. The volatiles would severely crack at high temperature zone during

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travelling from the top to the bottom frit. So the char particles on the bottom frit were surrounded by high concentrations of free radicals from thermal cracking of volatiles. The radicals would strongly interact with the char. After feeding, the reactor was held at the target temperature for another 5 min. Then the reactor was removed out and cooled naturally. This is called a ‘‘WTF (with top feeding)” experiment. In both ‘‘NTF” and ‘‘WTF” experiments, chars on the bottom frit were prepared at a fast heating rate and retained at the target temperature for the same time (12 min). So the discrepancies between char properties could only be caused by interactions with volatiles, rather than the reaction time. In the third type, only the top part of the reactor was involved in feeding. When the reactor was heated to the desired temperature, rice husk from the vibrating feeder was fed continuously into the reactor at a rate of 140 mg min1 for 5 min. The biomass particles were heated at a fast rate and released volatiles rapidly, with char being retained on the top frit. After feeding was stopped, the reactor was held at the target temperature for another 5 min. Then the reactor was removed out and cooled naturally. This experiment was conducted to determine the char yield in the ‘‘WTF” experiments. After each experiment, weigh the reactor and calculate the char yield from the difference between the weights before and after the experiment.

2.3. Product characterization 2.3.1. Quantification of alkali metals First, the char sample was digested using a mixture of HNO3 (6 mL), HF (2 mL), and H2O2 (3 mL). All the solutions were electronic purity. An ETHOS-I microwave digestion system (Milestone Corporation, Italy) was used to heat the mixture at 210 °C for 1 h. After digestion, the solution was carefully collected and analyzed using inductively coupled plasma–atomic emission spectroscopy (Perkin–Elmer Optima 5300 DV).

2.3.2. Raman spectroscopy A Renishaw Invia Raman spectrometer with an Ar+ laser (532 nm) was used to obtain Raman spectra of the char samples in the range 800–2000 cm1 at a spectral resolution of 4 cm1. The Raman signals backscattered by the samples were collected using a microscope equipped with a 20 objective. The exposure time was 10 s. The diameter of the laser beam spot projected on the sample was 1 lm, which is much larger than the size of the carbon micro-crystallites in the chars (Liu et al., 2015). Five different locations on each sample were randomly examined, in order to eliminate the effect of char heterogeneity on structure analysis.

2.3.3. Reactivity measurements Another quartz reactor was used in this experiment. The inner diameter and height of the reaction zone were 20 mm and 60 mm, respectively. Biomass char (3 ± 0.1 mg) was loaded into the bottom frit of the reactor using a long sample scoop. The char particles were uniformly distributed on the frit, guaranteeing good heat and mass transfers. The reactor was heated to 600 °C at a rate of 100 °C min1 with a flow of high-purity Ar (>99.999%). When the temperature became constant (±1 °C), the atmosphere was switched from Ar to a mixture of 21% O2 and 79% Ar. The reactivity measurements were started as soon as O2 entered the reactor. A process mass spectrometer was used to monitor CO and CO2 released by char combustion. The CO and CO2 release rates were used to calculate the char reactivity. The detailed procedure has been described in the previous work by Guo et al. (2016).

3. Results and discussion 3.1. Char yield Fig. 2(a) shows the char yields at different temperatures for chars from ‘‘NTF” and ‘‘WTF” experiments. The weight loss increased with increasing pyrolysis temperature. When the temperature increases, chemical bond breakage becomes severity. More fragile fragments consisting of aromatic rings of different sizes are released as volatiles and tar (Li and Nelson, 1996). Volatile–char interactions have caused additional weight loss above 650 °C. During interactions, char particles on the bottom frit (Fig. 1) were surrounded by volatiles released from pyrolysis of rice husk fed into the reactor continuously. The volatiles are mixture, consisting of CO2, H2O, CmHn, etc. The dual effects of volatile–char interactions on char yield are simply expressed as follow:

DW T ¼ W g þ W s

ð1Þ

where DWT stands for changes in char yield caused by volatile–char interactions at a certain temperature, mg. On the one hand, oxidative species (e.g. H2O and CO2) contained in volatiles could react with char particles on the bottom frit to induce the gasification to a certain extent. So this process reduces the final char yield (Wg). On the other hand, some large molecular compounds could form soot on the char surface and increase the char yield (Ws). Judging from results of the experiments (Fig. 2a), the effect of gasification on char yield is dominant compared with the formation of soot on the char surface. With increasing of temperature, the gasification reactivity of char with oxidative species increases, more carbon would be consumed. However, thermal cracking of volatiles in the gas phase becomes severity at higher temperatures. Large molecular compounds are destructed into relatively smaller ones, the sooting propensity of the volatiles is reduced. So the discrepancies between two lines in Fig. 2(a) were more obvious above 700 °C. But from another perspective, the additional weight loss due to volatile–char interactions is almost negligible below 650 °C, only around 2 wt% (db) above 700 °C. Considering the char yield is about 30% at all investigated temperatures, the additional weight loss caused by volatile–char interactions above 700 °C is still slight. So the volatile–char interactions have not caused obvious effect on the final char yield. 3.2. Retention of K in char Due to the highest content (Section 2.1) and highest catalytic activity among main metal catalysts in rice husk (Huang et al., 2009), only the content of K was quantified in this study. Fig. 2 (b) shows that retention of K decreased with increasing pyrolysis temperature; volatile–char interactions led to additional volatilization of K. K in biomass is typically present in two forms, either as organically bound to the char matrix or inorganic salts (Zhang et al., 2015b). In the initial stage of devolatilization, the inorganic K could also be transformed into organic forms, such as (Johansen et al., 2011).

CM—COOH ðsÞ þ KCl ðsÞ ! CM—COOKðsÞ þ HCl ðgÞ

ð2Þ

where CM stands for char matrix. The release of K to gas phase is found to begin at temperature as low as 200–400 °C. During pyrolysis, the organic matrix is partly destroyed, a limited amount of K (<10 wt%) which weakly bonds to char matrix may be released as a liquid tar phase. This process is relatively slow, because it is difficult for K to diffuse from the still intact organic matrix. After initial pyrolysis, the bonding of K to the char matrix (CM–K) could involve oxygen functionalities or intercalation (Jensen et al., 2000). Many

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100

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28 NTF WTF 26

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65

650

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Fig. 2. (a) Char yields on bottom frit and (b) retention of K as a function of pyrolysis temperature for chars prepared from ‘‘NTF” and ‘‘WTF” experiments.

different oxygen-containing functional groups such as lactone, carboxylic, carbonyl, and phenoxide groups may exist on the surface of carbons. Among these functional groups, the thermal stability of phenoxide groups is the best (Boehm, 1994). So K is prone to associate with phenol in the char matrix. On the other hand, K can penetrate between carbon layers by intercalating with the char matrix (Kapteijn et al., 1983). K existing in the form of phenoxide groups and intercalation can survive at temperature as high as 700 °C (Jensen et al., 2000). So the volatilization of K below this temperature was negligible (Fig. 2b). At temperatures higher than 700 °C, sublimation of KCl begins. The melting pointing of KCl is about 770 °C, so KCl is prone to evaporate from char matric due to the increased temperature. However, the decrease in retention of K was not obvious above 700 °C. This phenomenon is largely due to the K-silicate reactions. With the transformation of char matrix into CO2, CO, H2O, and light hydrocarbons, the organic matrix collapses, so K and Si would contact closely. Silicates show great affinity towards K, binding it in an inorganic structure retained in the ash (Johansen et al., 2011). For rice husk, about 90% of ash is silica (Shen et al., 2015). So the K-silicate reactions are more dominant, retaining more K in the ash. In the ‘‘WTF” experiments, volatilization of K was higher than it in the ‘‘NTF” experiments. Additional volatilization of K occurred above 650 °C and was about 20% at 800 °C. This is attributed to reactions with radicals produced by gas-phase thermal cracking of volatiles, and is represented as follow:

CM—K þ H ! CM—H þ K

ð3Þ

where CM stands for the char matrix. Free radicals, especially H, interact with the char by displacing K bonded to the char matrix to gas phase, resulting in additional volatilization of K. More free radicals would be generated by thermal cracking of volatiles resulting in K moving into the gas phase above 700 °C. The additional volatilization of K was observed even at 600 °C in this study. Li et al. (2004) reported that the additional volatilization of Na was negligible in volatile–char interactions at temperatures below 700 °C. In this study, chars on the bottom frit prepared at a fast heating rate (>103 K s1) were more nascent. There was not enough time for K to stabilize in the char matrix and K might exist in a weaker form after the initial pyrolysis. Because of the effects of radicals, K tended to be displaced into the gas phase, even at 600 °C. 3.3. Char structure analysis by Raman spectroscopy Raman spectroscopy is a reliable method for determining the structural features of highly disordered carbonaceous materials

such as biomass char because it is highly sensitive to crystalline, molecular, and amorphous structures (Asadullah et al., 2010). The first-order spectrum of the char shows broad D and G bands, located near 1350 and 1590 cm1, respectively (Tuinstra and Koenig, 1970). However, much useful information about the char structure is obscured by overlap of these two broad bands, therefore it is necessary to deconvolute the spectrum into several bands representing specific char structures. In this study, Peakfit 4.12 software was used to deconvolute the Raman spectrum in the range 800–2000 cm1 into four Gaussian band as shown in Fig. 3(a). The band at 1590 cm1 (G band) is attributed to the stretching vibration (E2g symmetry) of an ideal graphite lattice, and the band at 1340 cm1 (D1 band) represents the defects and heteroatoms in the graphite structure (Sheng, 2007). The band at 1530 cm1 (D2 band) is related to amorphous sp2 carbon bonds, e.g., in aromatics with three to five rings (Jawhari et al., 1995). The band at 1200 cm1 (D3 band) is only observed for highly disordered carbon. It is related to a mixture of sp2 and sp3 sites at crystallite peripheries, which may provide reaction sites (Bar-Ziv et al., 2000). The band area ratios and total area of the deconvoluted Raman spectrum were wildly used to investigate the changes in char structure. The ID1/IG ratio shown in Fig. 3(b) was used to determine the micro-crystallites in the char structure. This ratio is expected to decrease with increasing pyrolysis temperature because the extent of graphitization increases. For the chars in this study, however, the ratio increased significantly with increasing temperature from 600 to 800 °C. Zickler et al. (2006) suggested that ID1/IG shows a positive correlation with temperature from 400 to 900 °C because the crystallites are relatively small. At mild temperatures (600–800 °C), rice husk is in the carbonization stage, far from graphitization. So rice husk chars are highly disordered carbonaceous materials that contain many heteroatoms, cross-linking structures, and small aromatic rings. True graphite structures therefore could not be present in chars. In fact, the G band and D bands do not represent specific structures in highly graphitic materials. For biomass chars, the major component of the G band is probably aromatic ring breathing, and the D band is attributed to large aromatics ring systems (>6 benzene rings) (Keown et al., 2007). The increase in ID1/IG is caused by an increase in the concentrations of large aromatic rings with more than six rings, resulting from enlargement of aromatic rings and dehydrogenation of hydroaromatics (Li et al., 2006a). Fig. 3(c) shows the ID1/ID2 ratio, which is widely used to measure the ratio of large aromatics ring systems (>6 benzene rings) to relatively small ones (3–5 benzene rings) commonly found in amorphous carbon. Transformation of relatively smaller aromatic ring systems to large systems with increasing pyrolysis

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Temperature (°C) Fig. 3. Char structure analysis results by Raman spectra. (a) Curve fitting of a Raman spectra of the char prepared from ‘‘NTF” experiments at 700 °C. (b) ID1/IG and IG/IAll. (c) ID2/ID2 and total area of Raman spectra.

temperature was observed in the ‘‘NTF” and ‘‘WTF” experiments, especially from 600 to 700 °C. At temperatures higher than 600 °C, the loss of substituents and oxygen-containing groups becomes severity (Li and Nelson, 1996), and condensation reactions of aromatic rings are dominant. So the degree of aromatization increases with increasing pyrolysis temperature. The ID1/ID2 ratios for the ‘‘WTF” experiments are higher than those for the ‘‘NTF” experiments above 650 °C. So volatile–char interactions contribute to this transformation for a further step. In the ‘‘WTF” experiments, free radicals, especially H, from thermal cracking of volatiles can penetrate into the char matrix and transform relatively smaller aromatic ring systems to large ring systems (Li, 2013). This transformation is also confirmed by ID1/IG. At temperatures higher than 700 °C, significant amounts of free radicals were generated, which intensified the transformation of aromatic rings, therefore the discrepancies between the two lines were more obvious above 700 °C. IG/IAll in Fig. 3(b) shows that an increase in the number of larger aromatic ring systems (>6 benzene rings) is accompanied by a decrease in the amount of graphite-like structures. But the discrepancies between the IG/IAll ratios for ‘‘NTF” and ‘‘WTF” experiments performed at the same temperature are negligible. So volatile–char interactions only promote condensation of small aromatic rings, and have a negligible effect on the degree of graphitization. As shown in Fig. 3(c), the total areas of the Raman spectra decreased with increasing pyrolysis temperature. The Raman intensity is affected by both the Raman scattering ability and light absorptivity of the sample (Li et al., 2006a). Oxygen atoms, which are electron-rich structures, have a high Raman scattering ability because of resonance effects. They can conjugate with attached aromatic rings and hinder the growth of lamellar aromatic structures in chars and this reduces the sample absorptivity. Aromatization

is another important factor that affects the Raman intensity; it has two opposite effects. During pyrolysis, sp3 structures are transformed into sp2 structures; sp2 structures can conjugate with each other and this increases their Raman scattering ability. However, condensed rings formed by aromatization increase the light absorptivity of the char. The increased light absorptivity is the dominant factor influencing the observed Raman intensity. Rice husk, as a low-rank fuel, has abundant oxygen-containing and substituent groups. The loss of oxygen-containing groups and aromatization of char with increasing pyrolysis temperature contribute to the decrease in the total areas of the Raman spectra. And the effects of pyrolysis reactions on the total areas of the Raman spectra are more obvious in the temperature range from 600 to 700 °C. Volatile–char interactions can decrease the observed Raman intensity at all investigated temperatures, especially at temperatures above 700 °C; this is in agreement with the results reported by Keown et al. (2007). It is possible that during volatile–char interactions small oxygen-containing molecules such as CO2 and H2O produced by thermal cracking of volatiles could induce partial gasification of chars on the bottom frit (Section 3.1). Additional oxygen-containing structures may therefore form on the char and increase the Raman scattering ability. However, the contribution of partial gasification to the observed Raman intensity is small (Li et al., 2006b). This is because the additional oxygencontaining functional groups formed by partial gasification of chars rich in K are not Raman active. The content of K is dominant among the main AAEM species in the rice husk used in this study (Section 2.1), it can participate in radical-based reactions; this enhances the condensation of aromatic rings and decreases the total areas of the Raman spectra. During volatile–char interactions, the chars on the bottom frit were surrounded by high concentrations of free radicals from

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carbon conversion levels. This phenomenon is related to the heterogeneity of char structure. Aromatic ring systems of different sizes were not uniformly distributed in the char matrix. During pyrolysis, large ring systems were more likely to condense than relatively smaller ones. So the structure of rice husk char formed by pyrolysis was also heterogeneous, consisting aromatic ring systems with different degrees of aromatization. In the initial stage of combustion, the relatively smaller aromatic ring systems with low activation energies tend to be consumed at first. With increasing of gas–solid reaction time, the more condensed rings accumulate, accompanied by a significant increase in the activation energies of the reactions (Wu et al., 2005). The reactivity therefore decreases with increasing carbon conversion levels. The decreased reactivity of chars prepared from ‘‘WTF” experiments is also confirmed in Fig. 4, compared with ‘‘NTF” experiments. The inhibition on char reactivity began above 650 °C. Inherent catalysts and char structure are two major factors that influence the char reactivity (Li, 2013). The analysis results

thermal cracking of volatiles. The radicals would penetrate into the char matrix and promote the transformation of relatively small aromatic ring systems to large ring systems (>6 benzene rings), which would increase the light absorptivity of the chars and decrease the total areas of the Raman spectra. As shown in ID1/ID2 ratio (Fig. 3c), the transformation of aromatic ring systems intensified above 700 °C, therefore the negative effects of volatile–char interactions on the Raman intensity are obvious above this temperature. 3.4. Char reactivity The reactivity of chars prepared from ‘‘NTF” and ‘‘WTF” experiments at 600–800 °C is shown in Fig. 4. All the reactivity curves are in the shape of peak. The reactivity of chars increased in the initial carbon conversion levels, showing broad maxima at 20% and 50% carbon conversions for ‘‘NTF” and ‘‘WTF” experiments, respectively. On the other hand, the reactivity decreased in the higher

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Carbon conversion, X Fig. 4. Effects of volatile–char interactions on char reactivity as a function of pyrolysis temperature. (a) Char prepared at 600 °C. (b) Char prepared at 650 °C. (c) Char prepared at 700 °C. (d) Char prepared at 750 °C. (e) Char prepared at 800 °C.

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discussed above reveal that volatile–char interactions can contribute to the transformation of aromatic ring systems and enhance the volatilization of K above 650 °C. The formed condensed rings have higher activation energies, they are more difficult to be consumed. And K makes a catalytic contribution during combustion. Partial bonding of K with carbon atoms can cause electric charge migration and change the distribution of electrons on the surface of the carbon skeleton. As a result, the strength of C–C bond is weakened, but the possibility of formation of C–O increases. The formation of C–O provides more reactive sites to accelerate the gas–solid reactions (Chen et al., 1993). During volatile–char interactions, free radicals enhance the volatilization of K, therefore char from the ‘‘WTF” experiments benefit a weaker catalytic effect compared with ‘‘NTF” experiments. So the volatile–char interactions show an inhibitory effect on char reactivity. And also, the maximum reactivities were reached later in the presence of volatile– char interactions. During combustion, as a result of consumption of carbonaceous matters and oxygen-containing structures, K initially bonds with oxygen in the char matrix will migrate to the char surface, where it can accumulate into K clusters that play a more significant catalytic role in combustion reactions (Quyn et al., 2003). However, the catalytic activity of K clusters is related to the char structure. The active sites that K tends to adsorb on are small aromatic rings or char defects with low activation energies. Volatile–char interactions contribute to volatilization of K from char matrix and promote the formation of less reactive materials, such as condensed aromatic rings, but chars from the ‘‘NTF” experiments have larger amounts of K and relatively smaller rings. In the initial stage of oxidation by O2, K clusters adsorb on these active sites (smaller rings) and accelerate the consumption of carbonaceous matters, so the discrepancies between char reactivity were obvious at lower carbon conversion levels (<30%). However, at higher carbon conversion levels (>70%), the discrepancies shrank and even disappeared. It is understandable that interactions between volatiles and char are more severe on the gas–solid interface, compared with the char matrix. Carbon skeleton and distribution of K on the interface suffer a lot from interacting with volatiles. However, free radicals will gradually die off on the way to deep penetrate into the char matrix, so the properties of char matrix do not suffer much from interactions. They are well preserved, similar to properties for ‘‘NTF” experiments. With increasing of combustion reaction time during reactivity measurement, the suffered aromatic rings on the interface are preferentially consumed. Then the preserved rings are retained, so the discrepancies between char reactivity became smaller. To summarize, the discrepancies between char reactivity in the initial stage of oxidation could be more representative to show the inhibitory effects of volatile–char interactions on char reactivity. So in this study, the discrepancies at 20% carbon conversion were chosen to measure the inhibition degree of reactivity by volatile–char interactions. The inhibition degree (ID) is calculated as follows:

ID ¼

RNTFð20%Þ  RWTFð20%Þ  100% RNTFð20%Þ

ð4Þ

where RNTF(20%) and RWTF(20%) represents the char reactivity at 20% carbon conversion from ‘‘NTF” and ‘‘WTF” experiments, s1, respectively. With increasing of pyrolysis temperatures, the ID is 16.73%, 27.86%, 36.69%, 38.47% and 41.83%, respectively. The inhibition degree of char reactivity follows a similar trend to those of changes in the char structure and retention of K, beginning above 650 °C and intensifying above 700 °C. Two major factors, i.e., the char structure and retention of inherent catalysts, have a combined effect on char reactivity. During volatile–char interaction, more condensed aromatic ring systems with higher activation energies formed and

more K volatilized from char matrix by the displacement of free radicals. So the char reactivity decreases. With the significant generation of free radicals interacting with char on the bottom frit, the inhibitory effects are greater above 700 °C. However, the increase of ID is not obvious from 700 to 800 °C. The effects of volatile–char interaction on char structure (ID1/ID2 in Fig. 3c) and volatilization of K (Fig. 2b) also show the same trend. Under mild conditions, from 600 to 800 °C, the degree of thermal cracking of volatiles is limited. The amounts of free radicals are not enough to cause greater changes in char structure and volatilization of K. So effects of volatile–char interactions on char keep almost unchanged from 700 to 800 °C. 4. Conclusions Volatile–char interactions at mild temperatures (600–800 °C) did not cause obvious effect on the char yield but showed an inhibitory effect on char reactivity. The inhibition began only above 650 °C and intensified with temperature rise, but kept almost unchanged at 700–800 °C. Char structure and retention of K have a combined influence on char reactivity. The decreased reactivity was caused by additional volatilization of K from char matrix and transformation of relatively smaller aromatic ring systems to large ring systems (>6 benzene rings) above 650 °C. The negative effect of volatile–char interactions on gasification reactivities at mild temperatures should be considered properly. Acknowledgements This research was funded by the National Instrumentation Grant Program (2011YQ120039) and the Foundation for Innovative Research Groups of the National Natural Science Foundation of China (Grant No. 51421063). References Al-Rahbi, A.S., Onwudili, J.A., Williams, P.T., 2016. Thermal decomposition and gasification of biomass pyrolysis gases using a hot bed of waste derived pyrolysis char. Bioresour. Technol. 204, 71–79. Asadullah, M., Zhang, S., Min, Z.H., Yimsiri, P., Li, C.Z., 2010. Effects of biomass char structure on its gasification reactivity. Bioresour. Technol. 101, 7935–7943. Bar-Ziv, E., Zaida, A., Salatino, P., Senneca, O., 2000. Diagnostics of carbon gasification by Raman microprobe spectroscopy. Proc. Combust. Inst. 28, 2369–2374. Boehm, H.P., 1994. Some aspects of the surface-chemistry of carbon-blacks and other carbons. Carbon 32, 759–769. Chen, S.G., Yang, R.T., Kapteijn, F., Moulijn, J.A., 1993. A new surface oxygen complex on carbon: toward a unified mechanism for carbon gasification reactions. Ind. Eng. Chem. Res. 32, 2835–2840. Chen, T.J., Zhang, J.Z., Wu, J.H., 2016. Kinetic and energy production analysis of pyrolysis of lignocellulosic biomass using a three-parallel Gaussian reaction model. Bioresour. Technol. 211, 502–508. de Lasa, H., Salaices, E., Mazumder, J., Lucky, R., 2011. Catalytic steam gasification of biomass: catalysts, thermodynamics and kinetics. Chem. Rev. 111, 5404–5433. Garcia-Ibanez, P., Cabanillas, A., Sanchez, J.M., 2004. Gasification of leached orujillo (olive oil waste) in a pilot plant circulating fluidised bed reactor. Preliminary results. Biomass Bioenergy 27, 183–194. Guo, Y.Z., Zhao, Y.J., Meng, S., Feng, D.D., Yan, T.S., Wang, P.X., Sun, S.Z., 2016. Development of a multistage in-situ reaction analyzer based on a micro fluidized bed and its suitability for rapid gas-solid reactions. Energy Fuels 30, 6021–6033. Huang, Y.Q., Yin, X.L., Wu, C.Z., Wang, C.W., Xie, J.J., Zhou, Z.Q., Ma, L.L., Li, H.B., 2009. Effects of metal catalysts on CO2 gasification reactivity of biomass char. Biotechnol. Adv. 27, 568–572. Jawhari, T., Roig, A., Casado, J., 1995. Raman-spectroscopic characterization of some commercially available carbon-black materials. Carbon 33, 1561–1565. Jensen, P.A., Frandsen, F.J., Dam-Johansen, K., Sander, B., 2000. Experimental investigation of the transformation and release to gas phase of potassium and chlorine during straw pyrolysis. Energy Fuels 14, 1280–1285. Johansen, J.M., Jakobsen, J.G., Frandsen, F.J., Glarborg, P., 2011. Release of K, Cl, and S during pyrolysis and combustion of high-chlorine biomass. Energy Fuels 25, 4961–4971. Kapteijn, F., Jurriaans, J., Moulijn, J.A., 1983. Formation of intercalate-like structures by heat treatment of K2CO3-carbon in an inert atmosphere. Fuel 62, 249–251.

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