Synergetic effects of biochar structure and AAEM species on reactivity of H2O-activated biochar from cyclone air gasification

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Synergetic effects of biochar structure and AAEM species on reactivity of H2O-activated biochar from cyclone air gasification Dongdong Feng, Yijun Zhao*, Yu Zhang, Zhibo Zhang, Linyao Zhang, Jianmin Gao, Shaozeng Sun School of Energy Science and Engineering, Harbin Institute of Technology, Harbin 150001, China

article info

abstract

Article history:

In order to investigate the synergetic effects of biochar structure and AAEM species on the

Received 16 January 2017

reactivity of biochar, the H2O activation of the cyclone air-gasified rice husk biochar was

Received in revised form

studied in the one-stage fluidized bed/fixed bed reactor. The details in the transformation

19 May 2017

of biochar structures and the properties of AAEM species were analyzed by FTIR, Raman

Accepted 21 May 2017

and ICPeAES, respectively. The specific reactivity of biochar was determined in a ther-

Available online 12 June 2017

mogravimetric analyzer. The results indicated that those most marked synergetic effects of biochar structure and AAEM species on the H2O-activated reactivity of biochar were

Keywords:

observed in the temperature range from 750
C to 850
C. During the H2O-activation re-

H2O

action, the biochar reactivity would be improved due to the increase of surface oxygen-

Activation

containing functional groups (i.e., ReOH and eCOO) and small aromatic ring structures

Biochar structure

in biochar catalyzed by the AAEM species. The improvement in the reactivity of air-gasified

AAEM species

biochar by the H2O-activation is mainly in the initial carbon conversion (<15%) at 800
C.

Reactivity

© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Introduction Biomass energy is the most abundant renewable resource on earth, which is the world's fourth largest energy and plays an important role in the whole energy system [1]. In addition, biomass is the only carbon-containing renewable sources in the nature with the advantage of CO2 neutral emission [2]. The thermochemical conversion (i.e., torrefaction, liquefaction, pyrolysis and gasification) of biomass energy has a number of realizable social, political, and economic benefits. Among those thermochemical technologies, gasification is a promising way to convert biomass energy into synthesis gas used

for electrical power generation (fuel cells, gas turbine or engine), or as feedstock for the synthesis of liquid fuels and various chemicals [3e7]. Various gasifying agents, such as air, O2, H2O and CO2, are used during the gasification process depending on the desired gas composition [8]. In agreement with the previous studies [9,10], biomass gasification in the cyclone air gasifier can make a great contribution to produce a large number of useful products accompanied by the low-cost inputs. However, the low levels of carbon-conversion and a low calorific value of synthesis gas would be usually obtained during the biomass air gasification. Many technical attempts have been made to improve the efficiency of biomass air gasification and the quality of synthesis gas. Among them,

* Corresponding author. 412B, Jieneng Building, Harbin Institute of Technology, 92, West Dazhi Street, Harbin 150001, China. Fax: þ86 451 8641 2528. E-mail address: [email protected] (Y. Zhao). http://dx.doi.org/10.1016/j.ijhydene.2017.05.153 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

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adding steam into the gasifying agents (H2O-air gasification) is well known to obtain a high thermal efficiency [11e13]. In general, biomass gasification can be conceptually divided into 2 consecutive steps [14], namely the pyrolysis of solid fuel and gasification of biochar. Most current research with H2O-air gasification focuses on the pyrolysis biochar, which is containing a high carbon content [15,16]. However, due to the more rapid interaction of biochareair/O2 than that of biochareH2O, the reaction of H2O-activation actually occurs between H2O and partially air-gasified/oxidized biochar, rather than the pyrolysis biochar. In addition, most of the studies on biomass gasification in cyclone gasifiers are focused on the study of gas-phase products, while the follow-up treatment of solid-phase air-gasified/oxidized biochar sample is rarely investigated. For the H2O-air gasification of biomass, the reactivity of biochar is critical, which is mainly determined by two main factors: (i) the biochar structure, including the carbon skeleton, side chains and substitution groups [17,18], and (ii) the catalytic effects of inherent inorganic species, especially for the alkali and alkaline earth metallic (AAEM) species, such as Na, K, Mg and Ca [6,19e21]. According to Long and Zolin [22,23], the presence of AAEM species could enhance the heterogeneous biochar gasification, as well as the homogeneous hydrocarbon reforming reactions. Zhang et al. [24] studied the added potassium catalysts in H2O gasification of tobacco stalk sample and found the hydrogen yield and carbon conversion increased with the K-loadings increasing. Chen and Yang [25] found that the presence of electropositive atoms like potassium (K) can increase the electron density of carbon atoms, making them more negatively charged. According to Guo et al. [26], both the thermal decomposition of char and the char gasification reactions were related to the changes of char structure during gasification. The O-containing groups in biochar played a significant role in syngas reforming, and the disordered structure of biochar determined the catalytic activity of AAEM species in the process of gasification [27]. In addition, Huang et al. [28] investigated the effect of metal irons on char structures and indicated that a loose flake structure could be observed on the char surfaces in the presence of AAEM species. From the mentioned above, it can be seen that to understand the relationship between biochar reactivity and the AAEM species requires the detail characteristics of biochar structure. Moreover, the changes of the biochar structure (and therefore the biochar reactivity) are often influenced by the properties of AAEM species in the biochar. Thus a good understanding of synergetic effects of biochar structure and AAEM species on biochar reactivity, especially for the H2O-activated biochar from cyclone air gasification, is highly significant for developing H2O-air gasification technology of biomass. The objectives are to provide a theoretical and experimental analysis of the reactivity of H2O-activated biochar to guide process optimization and identify H2O as an activating agent in H2O-air gasification technology. It plays a significant role to further understand the H2O-activation behavior of airgasified/oxidized biochar, the key inherent AAEM species affecting biochar reactivity, and the most probable H2O-activation reaction route with the changes of biochar structure, all

of which are essential to biomass H2O-air gasification process design and development.

Experimental Biochar from cyclone gasification Rice husk, obtained from the northeast of China, was used in the cyclone air gasification system, as shown in Fig. 1. The detail of the cyclone furnace system can be seen in our previous investigation [10]. Air-staged cyclone gasification of rice husk was carried out under the following optimized conditions: a feeding amount of 36.6 kg/h, a total air volume of 34.3 m3/h, an air-equivalence ratio of 0.29, and the ratio of primary to secondary to bottom air of 3:2:5. The proximate and ultimate analyses of the raw rice husk and air-gasified biochar are listed in Table 1.

H2O-activation of biochar The H2O-activation of air-gasified biochar was carried out in an one-stage quartz reactor, which is described in detail elsewhere [29]. The apparatus, a schematic of which is shown in Fig. 2, was operated with a final temperature in the range 700e900
C and a holding time of 10 min. The H2O-activation reaction of biochar is similar to the gasesolid flow in the actual cyclone gasifier. Approximately 1 g of air-gasified biochar was added into the reactor. Ar gas was pumped into the reactor for 10 min to purge any air before the reactor was heated. When the temperature stabilized, the atmosphere was switched to the 15 vol.% H2O atmosphere. The H2O atmosphere was generated by feeding deionized water directly into the reactor with a HPLC pump (Alltech 426). The carrier Ar gas was varied (1.98e1.65 L/min) to maintain a constant residence time at different temperatures. The blank experiment was carried out similarly just without the injection of H2O. The weight loss rate of biochar was determined by direct measurement of the reactor weights before and after each experiment.

Analysis of H2O-activation biochar FTIR analysis Surface functional groups of biochar were analyzed in a Nicolet 5700 Fourier Transform Infrared Spectroscopy (FTIR) (Thermo Fisher Scientific, Waltham, MA). The biochar and KBr were dried at 105
C for 12 h and then ground together in a 1:120 ratio. All spectra were obtained at a resolution of 4 cm1 in the range 400e4000 cm1. 32 scans were collected for each spectrum.

Raman analysis Raman spectra were recorded using an inVia confocal micro Raman spectrometer (Renishaw, New Mills, UK) in an excitation laser of 633 nm. The biochar sample was mixed and ground with spectroscopic-grade KBr (0.25 wt.% biochar). Spectra were recorded for the range 800e1800 cm1 at five different locations in each biochar sample with the averaged values then taken.

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Fig. 1 e Biomass cyclone air-gasification system.

Table 1 e Proximate and ultimate analyses of raw rice husk and air-gasified biochar. Samples

Proximate analysis (%)

Rice husk Cyclone air gasification biochar

Ultimate analysis (%)

Mar

Aar

Var

FCar

Car

Har

Oar(diff.)

Nar

St,ar

10.50 0

16.34 64.19

58.53 12.21

14.63 23.60

35.89 28.66

4.23 1.71

32.72 4.88

0.19 0.45

0.13 0.11

Note: diff. ¼ by difference; ar ¼ as received basis.

Fig. 2 e Schematic diagram of one-stage fluidized bed/fixed bed reactor.

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Metallic species analysis In the microwave system (Ethos 1, Milestone, Sorisole, Italy) at 200
C for 1 h, the biochar (0.1 g) was digested in a 1:3:8 (v/v/v) mixture of 40% HF, 30% H2O2 and 65% HNO3. The metallic species were quantified by the inductively coupled plasmaeatomic emission spectroscopy (ICPeAES) for 3 times.

Reactivity analysis The specific reactivity of biochar was determined in air at 370
C with a thermogravimetric analyzer (TGA: MettlerToledo, Switzerland). Biochar of approximately 4 mg was loaded into platinum crucible and heated to 105
C under a pure N2 atmosphere to remove any residual moisture. The temperature was then increased to 370
C at 50
C/min. After 2 min, the atmosphere was switched into air starting the biochar reactivity test. Once the mass was stable, the sample was heated at 50
C/min to 600
C where it was held for 30 min to ensure complete combustion, with just ash remaining. The reactivity (R) was then calculated from DTG data (dW/dt) and equation (1). 1 dW R¼ * W dt

(1)

where W represents the sample mass at time t. The carbon conversion (C) was calculated using equation (2): C% ¼

W0  Wt *100% W0 ð1  Ad Þ

(2)

where W0 is the mass of the biochar at the beginning of the reaction at 370
C, Wt is the biochar mass at t, and Ad is the ash content. The specific reactivity was then calculated as the product of R and C.

Results and discussion Weight loss rate of biochar As shown in Fig. 3, the weight loss rates of biochar during the H2O-activation increased with the temperature increasing, from 14.1% at 700
C to 29.6% at 900
C. The result was mainly

caused by the H2O-activation (increasing from 4.3% to 15.2% across the range and especially between 750 and 850
C), rather than the secondary pyrolysis (which increased from 9.9% to 14.3%). The weight loss during the secondary pyrolysis stage occurred, because the short residence time of rice husk in the cyclone air-gasification system was insufficient to remove all of its volatile components. This meant that the biochar retained relatively high reactivity and released material during the secondary pyrolysis stage before being exposed to the steam. Air-gasified biochar is known to contain many branched chains, functional groups, small aromatic ring structures [30]. Although the degree of polymerization of aromatic rings in biochar was likely increased by the secondary pyrolysis process, during H2O-activation, the remaining small aromatic ring structures likely experienced ring-opening and bond-breaking in the presence of the H/O/OH free radicals, which led to the additional weight loss of biochar observed. In addition, the concentration of AAEM species (i.e., Na, K, Mg and Ca) may be increased by the consumption of biochar, to further promote the biochar reactivity with H2O. According to Lv et al. [31], the presence of inherent AAEM species has a significant effect on the physical and chemical structure of biochar and facilitates the even dispersion of AAEM species so that to promote biochar gasification (R1eR3) and the wateregas-shift reaction (R13), which is in accordance with above results in the catalytic effects of AAEM species on yields of the syngas [6]. Further increasing of the temperature from 850
C to 900
C slightly increased the weight loss but this was considered marginal and occurred because during the secondary pyrolysis reaction, larger, more stable aromatic ring structures that were more difficult to depolymerize were formed. The potential mechanisms of H2O-activation reaction of air-gasified biochar could be expressed by the heterogeneous and homogeneous reactions shown as below reactions (R1eR13) [32e35], as shown in Table 2. Catalytic reactions cleave the CeC bonds of the carbohydrate backbone to yield a combination of carbon monoxide and hydrogen (R2eR4) [36]. Biochar may react with CO2 to produce the extra CO and CH4 without additional H2O and oxygenated compounds (R7 and R8). If H2O is present, CO2 can be generated via the wateregasshift reaction to produce more H2 (R13).

Analysis of biochar structure FTIR analysis

Fig. 3 e Biochar weight loss rate during H2O-activation.

The results of the FTIR analysis for the biochar samples are shown in Fig. 4, with the spectrum normalized to the maximum absorbance. This allowed us to compensate for different mass loadings and perform a semi quantitative analysis. Painter et al. [37,38] assigned the band at 1600 cm1 to an aromatic ring stretching in their investigation. Similar to Wei et al. [39], they reported that the bands at 1614 and 1633 cm1 were due to the C]C stretching of aromatic rings and the pheC]C stretching. The peak obtained at 3357 cm1 shows the eOH stretching band which is common in carbonaceous materials. As shown in Fig. 4, during the H2O-activation of air-gasified biochar, the increasing of reaction temperature mainly impacted the OH-stretching vibration of alcohol-phenolic ROH at 3645e3230 cm1, the conjugated aromatic ring stretching of C]C at 1650e1470 cm1 and the

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Table 2 e Potential mechanisms of H2O-activation reaction of air-gasified biochar. Potential mechanisms

No. CxHyOz / H2 þ CO þ CO2 þ CH4 þ C2 þ Tar þ Char CnHm þ nCO2 / 2nCO þ (m/2)H2 þ Q CnHm þ nH2O / nCO þ (n þ m/2)H2 þ Q CnHm þ 2nH2O / nCO2 þ (m/2 þ 2n)H2 þ Q CH4 þ H2O 4 CO þ 3H2 CO2 þ 4H2 4 CH4 þ 2H2O C þ 2H2 4 CH4 C þ 1/2O2 / CO, DH298 K ¼ 110.7 kJ/mol C þ O2 / CO2, DH298 K ¼ 405.8 kJ/mol C þ CO2 / 2CO, DH298 K ¼ þ172.1 kJ/mol C þ H2O / CO þ H2, DH298 K ¼ þ131.3 kJ/mol C þ 2H2O / CO2 þ 2H2, DH298 K ¼ þ89.7 kJ/mol CO þ H2O 4 CO2 þ H2, DH298 K ¼ 41.2 kJ/mol

Secondary pyrolysis reaction Dry reforming reaction Steam reforming reaction Methanation reaction

Oxidation reaction Boudouard reaction Water gas reaction Wateregas-shifting reaction

Normalized absorbance (a.u.)

symmetric stretching vibration of carboxylate eCOO at 1420e1290 cm1 in different extents. The change in the conjugated aromatic ring stretching of C]C was mainly caused by the high temperature polymerization of biochar during the secondary pyrolysis process, while the C]C bonds at the more moderate of 700e800
C were less impacted. The absorbance caused by addition polymerization and the growth of aromatic rings were relative weak. Meanwhile, absorbance bands associated with cleavage of C]C bonds during steam activation are similar to those observed for polymerization reactions that form the C]C bonds in biochar, meaning little changes were observed in this range. At higher temperatures (800e900
C), smaller aromatic rings on biochar surface may have detached and/or aggregated to form larger, more stable aromatic ring systems that include at least six fused rings. The degree of self-polymerization significantly increased. During the H2O-activation, fewer aromatic C]C bonds were consumed, so the change in the C]C absorbance was relatively increased with the temperature increasing. According to Li [14], the rate-limiting step of biochar thermal cracking is the breakage of chemical bonds to generate radicals in order for the ring condensation to be initiated and continued. The increase in sustained aromaticity (increasing of aromatic C] C), to some extent will limit the reactivity of biochar. However, the H2O-activation of biochar will produce a large number of active free radicals, especially O/H/OH radicals, being able to

1.0

o

700 C H2O char o

0.8

750 C H2O char o

800 C H2O char

0.6

o

850 C H2O char o

900 C H2O char

0.4 0.2

-OH

Aromatic C=C

0.0 4000

3500

3000 2500 2000 1500 -1 Wavenumber (cm )

(R1) (R2) (R3) (R4) (R5) (R6) (R7) (R8) (R9) (R10) (R11) (R12) (R13)

promote the formation of more oxygen-containing functional groups on biochar surface. In addition, the O-containing functional groups during H2O-activation were closely associated with the aromatic structure and thus tended to loosen the aromatic structure. It facilitates the improvement of biochar reactivity. According to Li et al. [40], with the presence of AAEM species, the O-containing functional groups were not closely associated with the main aromatic structure throughout the biochar. The catalytic H2O-activation reactions were localized on the sites associated with the AAEM catalysts, not only limited on the specific sites (especially sp3rich or sp2esp3 mixture) distributed throughout the biochar. As shown in Fig. 4, the changes of oxygenated functional groups (eOH and eCOO) on the biochar surface were mainly observed during H2O-activation of air-gasified biochar. For eOH bonds, a significant increase in absorbance was observed in the range of 700e850
C though little change occurred when the temperature was increased further. This is explained by considering that the biochar aromatic ring structures were still developing at the moderate temperatures with many small aromatic ring structures present. Thus, the high temperature in the steam-enriched atmosphere caused ringopening to take place forming an active branched chain structure which then reacted with the active free radicals to create branched ROH bonds on the biochar surface. At 850e900
C, the higher degree of polymerization of the biochar decreased the likelihood of forming such branched ROH structures. For the COO bonds, little change was observed at 700e800
C, but from 800 to 900
C the aromatic ring structures were directly opened by H/O/OH free radicals to produce more COO bonds. According to Chen et al. [25], the sites on either zigzag face or armchair face of large aromatic layers in biochar sample are believed to be active for H2O-activation reactions, which are affected by the AAEM species.

Raman analysis -COO 1000

Fig. 4 e FTIR analysis of H2O-activated biochar at 700e900
C.

500

The total area of Raman peaks between 800 and 1800 cm1 (as shown in Fig. 5) is often used to investigate the biochar aromatic structure. Although the light-absorption by the condensation of aromatic rings can decrease the Raman intensity, this is usually outweighed by the electron-rich structures such as O-containing functional groups which tend to have high Raman scattering abilities and enhance the

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3000

6 6

2.4x10

2.03

6

2.2x10

6

2.0x10

1.76

6

1.8x10

6

1.54

2500

2.12

Intensity (a.u.)

Total Raman peak area (a.u.)

2.6x10

1.60

1.6x10

Fitting curve Testing Curve

2000

G

1500

VR

GR

D

VL

1000

SL

6

1.4x10

500

6

1.2x10

0 1800

6

700

750

800 850 o Temperature ( C)

900

Fig. 5 e Total Raman peak area for H2O-activated biochar.

observed Raman intensity [41,42]. Overall, the peak area increased with temperature, particularly at 750e850
C. For the lower temperature (<750
C), the aromatic rings of biochar were less easily destroyed with steam only able to react with unsaturated surface carbons to produce a limited amount of eOH groups, resulting in a marginal peak area increase from 1.54  106 at 700
C to 1.60  106 at 750
C. In addition, the relatively low temperatures could even have led to the condensation of aromatic rings, increasing aromaticity and mitigating increases in the Raman peak area. In agreement with the weight loss findings above, at 750e850
C, where the peak area increased from 1.6  106 to 2.03  106, H2O had larger effect on the biochar structure as H/O/OH free radicals facilitated the breaking of CC and CO bonds and the depolymerization of small aromatic ring structures. This process formed a series of intermediate products and active sites on the biochar surface that the radicals could combine with to form oxygenated functional groups (see for example the significant increase in eOH and eCOO groups observed during the FTIR analysis). Further increasing of the temperature from 850 to 900
C slightly increased the Raman peak area mainly because of an increase in the degree of polymerization of biochar aromatic ring structure. Although some extra H2O-activation was observed, the change was less marked than for previous temperature increases. The Raman spectra of H2O-activated biochar samples were measured by the Raman analysis for 3 times during each experiment. The experimental results proved to be reliable with measurement deviation within 5%. This allowed a single set of results in the range 800e1800 cm1 to be curve-fitted with Peakfit software, using ten Gaussian bands that represent typical structures for biochar samples [43,44], as shown in Fig. 6. Here, the D band represents defect structures that are composed of large aromatic ring systems of at least six fused rings, the Gr band represents structures with three to five fused aromatic rings, and the Vl and Vr bands represent amorphous carbon and small fragments, respectively, which provide links between the aromatic ring systems. As shown in Fig. 7, it shows the ratio between the area of the D band and that of the combined Gr þ Vl þ Vr bands, which can be considered to show the ratio between the large aromatic ring systems and those found in amorphous systems [41]. The increases in the ID/I(GrþVlþVr)

SR

1600

1400

1200

R

1000

-1

800

Raman shift (cm ) Fig. 6 e Curve fitting of Raman spectrum for H2O-activated biochar.

ratio between 700 and 750
C indicates an increase in the concentration of aromatic rings with six or more fused benzene rings. This results from the dehydrogenation of hydroaromatic rings and the growth of aromatic ring systems. The impact of H2O-activation on the biochar reactivity was limited here. Between 750 and 850
C, the ratio increased slightly before decreasing because of the increasing impact of H2Oactivation. Here, although the growth of aromatic rings continued when the CeC/CeO bonds in the small aromatic ring structures were broken, the combination of unsaturated C atoms with H/O/OH radicals became more prominent decreasing the likelihood of polymerization. Small aromatic ring structures were completely opened forming oxygenated and branched chain structures on the biochar surface. The preferential removal of smaller aromatic ring systems and the persistence of cross-linking structures in the presence of AAEM species mean that the large aromatic ring systems were increasingly concentrated with little flexibility, affecting the dispersion of AAEM species [40], which could be a good embodiment of the synergetic effects of biochar structure and AAEM species on reactivity of H2O-activated biochar. Further increasing temperature from 850 to 900
C, the ratio decreased slowly as the secondary pyrolysis reaction created more small ring structures that cannot be transformed into the larger

2.0

ID/I(Gr+Gl+Vr) 1.6

Area ratio

1.0x10

S

GL

1.2 0.8 0.4 0.0

700

750

800

o

850

Temperature ( C)

900

Fig. 7 e Ratio of ID/I(GlþGrþVr) for H2O-activated biochar produced at 700e900
C.

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aromatic systems and fewer active structures in the biochar that can be further H2O-activated. -1

Table 3 shows the analysis of metallic species in the biochar samples, in which the AAEM species, especially K and Ca, occupy a large part. In the previous studies [45], during the H2O-gasification/activation of carbon materials, the AAEM species only increase the concentration of the active complexes on carbon rather than changing the reaction pathway. An oxygen transfer and intermediate hybrid mechanism is applied for understanding of the rate and selectivity of the gasification catalyzed by AAEM species [46,47]. During the H2O-activation of biochar, the concentration and dispersion of AAEMs in biochar vary constantly because of the consumption of carbonaceous matrix as well as the release and agglomeration of AAEM species, accompanied by the gradual evolution of the biochar structures. This in turn affects their catalytic abilities. H2O-activation impacted the content of metallic species in biochar through its impact on overall weight loss. At lower temperatures (700e750
C), the limited transformation of the small aromatic ring structures and the limited migration of AAEM species from the interior of the carbon matrix slightly increased the concentration of AAEM species. According to Kopyscinski et al. [48], the evaporation of AAEM species is negligible at 700
C. The metals concentration increased between 700 and 800
C, mainly because of the increase in K (from 7.30 to 10.33 mg/g) and Ca (from 1.24 to 1.71 mg/g), but also because of the increase in other species such as Na, Mg and Al. This comparison made the impact of H2O-activation most obvious as it enhanced the depolymerization of small aromatic ring structures and biochar weight loss, concentrating the remaining AAEM species [29]. Although H2O-activation also caused AAEM species to migrate to the biochar surface, the increase in concentration suggests that precipitation of AAEM species did not occur significantly. Between 800 and 900
C, the aromaticity of biochar significantly increased reflecting larger more stable ring structures. However, the migration of AAEM species coupled with the increased temperature did appear to lead to the additional precipitation of AEEM species as shown by the decrease in their concentration.

Specific reactivity of biochar Fig. 8 shows the specific reactivity of biochar samples against the degree of carbon conversion measured by the TGA. The

Table 3 e Metallic species in H2O-activated biochar samples.

700 750 800 850 900




C C
C
C
C


Specific reactivity (s )

Analysis of AAEM species

H2O-activation biochar

0.020

Main metallic species (mg/g) Na

K

Mg

Ca

Al

Fe

0.11 0.12 0.16 0.16 0.14

7.19 7.30 10.33 8.77 8.16

0.53 0.55 0.74 0.56 0.51

1.15 1.24 1.71 1.43 1.17

0.06 0.09 0.11 0.11 0.09

0.10 0.21 0.21 0.19 0.04

0.016

Effect of H2O-activation

o

700 C H2O-activated o

750 C H2O-activated

0.012

o

800 C H2O-activated 0.008

o

850 C H2O-activated o

900 C H2O-activated

0.004 0.000

0

20

40

60

Carbon conversion (%)

80

100

Fig. 8 e Specific reactivity of H2O-activated biochar produced at 700e900
C.

changes of biochar structures and the properties of AAEM species in biochar would contribute to the reactivity trend (increase first and then decrease) for each H2O-activated biochar sample. The main reason for the initial increase in the biochar reactivity is due to the accumulation of AAEM species on the biochar surface with the removal of carbonaceous matter. The accumulation of AAEM species on the biochar surface also facilitates the formation of AAEMcontaining clusters that have higher catalytic activities than the finely dispersed AAEM-containing species based on the gasification of carbon [25,49]. As conversion proceeded toward completion decreases in reactivity were attributed to changes in the biochar structure as highly condensed/ graphitized aromatic ring systems, of lower reactivity, formed an increasingly larger portion of the residual material. Due to the biochar heterogeneity, the increased AAEM catalyst concentration was not enough to maintain the high biochar reactivity at high conversion levels [50]. Reactivity generally decreased with the H2O-activation temperature increasing, except for the results with carbon conversion (<15%) at 800
C. The decrease in biochar reactivity at 700e750
C was relatively small because the reaction had little impact on the biochar structure leading to a weaker H2O-activation and less migration of AAEM species to the surface. The biochar would become more ordered during the H2O-activation as the H radical was able to penetrate deep into the biochar to initiate/enhance the ring condensation reactions [40,51], in consistent with the results of Raman analysis. In addition, the particle size of the AAEM species enlarged and the dispersion of the AAEM species decreased due to the consumption of the carbon support and the agglomeration of the AAEM species, which would reduce its activity for its combustion in air. However, for the H2O-activation at 800
C with a carbon conversion below 15%, the biochar reactivity was significantly higher than that at 750
C and even slightly higher than that at 700
C, mainly due to the synergetic effects of the highest AAEM content and welldeveloped biochar structures. The synergetic effect can be realized during the migration of AAEM species and evolution of biochar structure in the H2O-activation reaction of biochar. The AAEM species in the biochar improved O-containing functional groups at the beginning of gasification in the

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biochar. These O-containing functional groups would be consumed quickly in subsequent combustion which would attribute into the increase in biochar reactivity in the low carbon conversion (<15%). As the carbon conversion increases, the surface oxygen-containing functional groups are gradually consumed leading to the decrease in reactivity, as particularly observed for the 800
C sample. Increasing the temperature caused the small aromatic ring structures to gradually be consumed during the H2O-activation of biochar, decreasing the subsequent specific reaction rate. Between 800 and 900
C the decrease in reactivity can be attributed to increased formation of a less reactive biochar structures. The content of AAEM species remaining in the biochar was significantly lower than that at 800
C. The secondary pyrolysis reaction was most likely to impact the reactivity since this had a larger impact on the degree of polymerization of the aromatic ring systems which is the main factor in determining reactivity.

Conclusions (1) The reactivity of H2O-activated biochar from cyclone air gasification is mainly determined by the synergetic effects of biochar structure (i.e., aromatic structure and surface oxygen-containing groups) and the properties of AAEM species. Those most marked synergetic effects on the H2O-activated reactivity (weight loss) of biochar were observed in the temperature range from 750
C to 850
C. (2) During the H2O-activation reaction, the biochar reactivity would be improved due to the increase of surface oxygen-containing functional groups (i.e., ReOH and COO) and small aromatic ring structures in biochar, catalyzed by the AAEM species, overweighing the negative effect of formation of additional aromatic C] C. (3) Through the synergetic effect of biochar structure and AAEM species, the improvement in the reactivity of airgasified biochar by the H2O-activation is mainly in the initial carbon conversion (<15%) at 800
C.

Acknowledgments The Collaborative Innovation Center of Clean Coal Power Plant with Poly-generation, National Key R&D Program of China (2016YFE0102500), the National Natural Science Foundation Innovation Research Group Heat Transfer and Flow Control (51421063) and the CSC Scholar (201606120136 and 201606120135) are gratefully acknowledged.

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