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Experimental and modeling study of potassium catalyzed gasification of woody char pellet with CO2
Energy 171 (2019) 678e688
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Experimental and modeling study of potassium catalyzed gasification of woody char pellet with CO2 Qiang Hu a, b, Haiping Yang a, Zhiqiang Wu b, c, C. Jim Lim b, Xiaotao T. Bi b, *, Hanping Chen a, d, ** a
State Key Laboratory of Coal Combustion, School of Energy and Power Engineering, Huazhong University of Science and Technology, Wuhan, 430074, PR China Clean Energy Research Centre, Department of Chemical and Biological Engineering, University of British Columbia, Vancouver, V6T 1Z3, Canada c School of Chemical Engineering and Technology, Xi'an Jiaotong University, Xi'an, Shaanxi, 710049, PR China d Shenzhen Huazhong University of Science and Technology Research Institute, Shenzhen 523000, PR China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 September 2018 Received in revised form 20 December 2018 Accepted 12 January 2019 Available online 16 January 2019
Understanding the catalytic effect of potassium on CO2 gasification of biochar can help clarify the complex catalytic conversion mechanism of biochar. In this study, effects of temperature and KOH content on conversion performance and reaction kinetics of gasification of biochar pellet were investigated by using a macro-thermogravimetric analysis (MTGA) unit. Results showed that the increase of gasification temperature and KOH content promoted the conversion of biochar pellet and enhanced the gasification rate (dX/dt). With the reaction proceeded, dX/dt increased initially then decreased, and reached the peak values at X of about 0.15e0.40. A faster expansion of pore structure at both higher temperature and higher KOH content would contribute to the shift of peak values to lower X. Phase boundary-controlled model fitted well to the gasification of biochar pellet with added KOH at 750 and 800 C, while the possible melting of K species at 850 C would change the gasification reaction to phase change model. The inferred catalytic role of K in pellet gasification is that it reduced the activation energy and increased the active sites for carbon to react with CO2, and the activation energy was reduced with the increase in K content (0e20 wt. %). © 2019 Elsevier Ltd. All rights reserved.
Keywords: Potassium catalyzed gasification Reaction rate constant Gas-solid reaction Phase boundary surface reaction Phase change model
1. Introduction Developing high-efficiency conversion technologies for biomass is of great importance for solving the problems of climate change, energy crisis, and environmental pollution [1] as biomass is a renewable, clean, and CO2-neutral source of energy [2]. As one of the most promising technologies applicable to biomass conversion, gasification has attracted ample attention for decades due to its flexibility, scalability, and high thermal efficiency [3,4]. Through gasification, various types of biomass can be converted into syngas (mainly hydrogen, carbon monoxide, carbon dioxide, and methane, depending on the gasification conditions). The gasification of biomass with a gasifying medium can be divided into three steps: pyrolysis of biomass, volatile reforming, and gasification of biochar
* Corresponding author. 2360 East Mall, Vancouver, BC, V6T 1Z3, Canada. ** Corresponding author. 1037 Luoyu Road, 430074, Wuhan, PR China. E-mail addresses: [email protected] (X.T. Bi), [email protected] (H. Chen). https://doi.org/10.1016/j.energy.2019.01.050 0360-5442/© 2019 Elsevier Ltd. All rights reserved.
[5,6]. The gasification of biochar step is extremely slow compared with the other two steps because of the different phase of reaction, different chemical activity and reaction, and mass transfer resistance of biochar [7e9]. Consequently, gasification of biochar has great importance in the overall biomass gasification process and needs to be understood. To solve the slow reaction rate problem of char gasification, alkali and alkaline earth metallic (Na, K, Ca, and Mg) species (AAEMs) have been implemented as useful catalysts to promote the gasification conversion of biochar [10,11]. The gasification conditions of temperature, pressure and gasification agent, char structure of surface area, morphological structure, and the content and dispersion of inorganic elements are the main factors influencing the catalytic effect of AAEMs [12,13]. Dupont et al. [10] investigated the steam gasification of biochars derived from wood, short rotation forestry/coppice, agricultural biomass, and microalgae in France. Results showed that inorganic elements influenced the gasification performance of biochar, and the ratio of K/(Si þ P) could determine the gasification rate. Nzihou et al. [12] reviewed the
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catalysts for the gasification of biomass char and pointed out that Li and K were the best catalysts for promoting the gasification rate. Huang et al. [14] investigated the CO2 gasification reactivity of fir char in China using different metals as catalysts and found that the gasification rate improved by metal catalyst addition in the order of K > Na > Ca > Fe > Mg. The Chinese researchers Feng et al. [15] loaded the AAEMs before and after pyrolysis of rice husk at 800 C and then used them for CO2/H2O gasification. It was revealed that AAEMs pre-loading on biochar surface could provide better availability of active sites, resulting in the high specific reactivity of biochar during gasification. It has been concluded that alkali metals (Na, K) are more effective for char catalytic gasification than alkaline metals (Ca, Mg). Additionally, K has been proven to be the most significant catalyst among AAEMs for biochar gasification [12,15,16]. Therefore, the catalytic effect, behavior, and mechanism of K in biochar conversion need to be highlighted for gasifier design and optimization. In recent years, several studies have investigated the catalytic influence of K on biochar gasification. The majority of these studies focused on catalytic performance, char reactivity, and comparisons with other inorganics [4,17e19], and have utilized many kinetic models to fit the gasification process and reaction mechanism. Prestipino et al. [4] examined the steam gasification kinetics of chars from six agro-industrial biomass residues in Italy, and showed that the gasification reactivity was correlated with (K þ Ca)/Si at both different temperatures and steam concentrations. The Swedish researchers Kirtania et al. [17] performed the catalytic study of K and Na salts on pine sawdust biochar gasification using the random pore model (RPM) and modified random pore model (MRPM) and found that biochar had high catalytic activity and low carbon leaching with the addition of K2CO3 (0.5 M). In Finland, Perander et al. [19] compared the catalytic effect of K and Ca on CO2 gasification of Norwegian spruce biochar, and the results demonstrated that the catalytic activity of Ca was higher than that of K in the beginning but then decreased quicker than the catalytic effect of K. However, studies focused on the specific indepth effect of K on biochar CO2 gasification performance, reactivity, and mechanism are very limited. The reaction kinetics and mechanism of catalytic gasification of biochar pellets with K addition still remain incompletely understood, especially for the catalytic gasification process and its reaction rate-determining factors. On the other hand, it has been shown that the weight of sample influences the reaction kinetics in thermal conversion process [20]. For the char powders, the shrinking core model, volumetric model and random pore model, etc. are the kinetics models frequently used to describe their gasification process and reveal the kinetic mechanism [21e23]. Whereas, it is hard for those models to explain the solid-state reaction process and the reaction controlling steps in macro-mass conversion of pellets, due to the different solid diffusion, gas diffusion, and solid-gas interaction behaviors for a mass of solid particles. The gas-solid state reaction analysis method has been used successfully to fit the macro-mass reaction process, such as for iron ore pellet reduction, char pellet gasification, and carbonate decomposition [24e27], which may be applied to identify the role and mechanism of K in char pellet gasification process. In this study, CO2 gasification of biochar pellets with different contents of KOH was carried out at different temperatures in a macro-thermogravimetric analysis (MTGA) reactor. Gasification performance and catalytic mechanism were the key points examined during the experimental and modeling process. The gas-solid state analysis method, based on AvramieErofe'ev generalized equation, was applied to analyze the catalytic CO2 gasification of the K contained biochar pellet [24,25]. Firstly, the classical basis of the method for reaction kinetics analyzing depended on the
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nucleation and growth gas-solid reaction model. The mechanism related indicator (m) was calculated from nucleation and growth gas-solid reaction model. And then the detailed reaction model was chosen based on indicator m and the fitting quality, which was used for further mechanism identification and reaction rate-limit step explanation. Given the description above, biochar obtained from pyrolysis of woody biomass at 550 C was first washed by hydrochloric acid to eliminate the influence from its inherent inorganic elements. Then biochar pellets were made by compressing HCl-treated biochar with different contents of KOH. The K contained biochar pellets were pyrolyzed at 850 C to remove volatile matter and subsequently gasified at 750e850 C in CO2 with the interval of 50 C using a MTGA reactor. Effects of temperature and KOH content on the gasification rate of the pellets were investigated to explain the catalytic gasification performance of K. The gasification kinetics, mechanism, and reaction controlling steps were elucidated based on the gas-solid reaction analysis method. Particular attention was paid to exploring the activation energy variation with the addition of different contents of K, which would then facilitate the understanding of the mechanism of potassium catalyzed gasification of char pellets. The catalysis of K has significate influence on the conversion performance and reaction kinetics of the gasification of biochar pellet. And the influence in different temperatures and K contents is expected to be different. The present research aimed at: 1) investigating the gasification performance of biochar pellets with different contents of KOH added, and exploring the effect of temperature and KOH content on the gasification process and conversion rate of biochar pellet; 2) identifying the conversion kinetics and understanding the reaction rate-limit step and catalytic mechanism during CO2 gasification of K contained biochar pellet. 2. Experimental 2.1. Materials Wood shavings obtained from a furniture factory were crushed and then dried in an oven at 105 C for 24 h. Then, the regular, dried, raw woody biomass was selected for pyrolysis in a fixed-bed tubular reactor for the preparation of woody char samples. When the temperature was raised to 550 C, the quartz arc with the wood shavings was placed into the reaction zone with a flow of nitrogen (99.99%, 0.4 L/min) for a pyrolysis time of 30 min. The detailed pyrolysis process is described in our previous reports [28,29]. The solid residue after pyrolysis was collected as the biochar used in this study. To remove acid-soluble mineral matter from the woody char, washing with hydrochloric acid was performed by mixing 20 g of biochar with 200 ml of 5 M HCl in a 500 ml glass jar. Then, the mixture was stirred at 30 C for 24 h, followed by filtering and washing repeatedly with distilled water until a pH of 7 was reached. The filtered solid char was then dried at 105 C for 24 h. After the drying process, the residual biochar was ground in a blade mill (XY-1000A, Songqing Hardware Company, China) for 3 min. The obtained HCl-washed char, called “HCl-treated char” in this paper, was used for the subsequent pellet formation and gasification to reduce the catalytic effects of any inherent metallic elements in the raw biochar. 2.2. Properties of chars The ultimate analysis of the char was carried out in a CHNS/O elemental analyzer (Vario MICRO cube, Elementar, Germany), and proximate analysis was performed based on the standard of the
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Proximate Analysis of Solid Biofuels (GB/T 28731-2012) using a muffle furnace (KSL-1200X, Kejing Materials Technology Co. Ltd., China). The lower heating value (LHV) was measured using an oxygen bomb calorimeter (Parr 6300, USA). The true density was determined in a true density determinator (AccuPyc 1330, USA). The particle size distribution of the char was analyzed using a laser particle-size analyzer (MS2000, Malvern Instruments Ltd., U.K.). The HCl washing effect was determined by quantifying the alkali and alkaline earth metal species in the chars before and after acid treatment using inductively coupled plasma mass spectrometry (ICP-MS) (ELAN DRC-e, PerkinElmer, USA). The results were the average of two tests. The surface characteristics of the char were examined by scanning electron microscopy (SEM). Before SEM analysis, the samples were dried at 105 C for 24 h. The biochar powder was attached to a metal stub by conductive carbon paste and then sputter coated with gold for 180 s in a sputter coater (SCD 050, Bal-Tec, Liechtenstein). SEM images were recorded using a scanning electron microscope (Quanta 200, FEI Company, Netherlands) operated at 20 kV. The results were the most representative image from more than five images of each sample. Table 1 shows the properties of the raw wood shavings, the 550 C pyrolytic char, and the HCl-treated char. After anoxic thermal cracking, the true density, particle size, and volatile and oxygen contents of the raw samples decreased, whereas the calorific value, fixed carbon content, and carbon content increased. A comparison of the char before and after acid washing showed that the HCltreated char had higher mean particle size, increased LHV, and decreased ash content. The mean particle size of the HCl-treated char increased because fine particles could pass through the filter paper during the filtration process. In addition, several contents of the inorganic ash could dissolve in the hydrochloric acid, which caused the ash content to decrease from 1.67 to 1.08% and the LHV to increase from 22.42 to 30.35 MJ/kg. The ICP-MS testing results of different samples could indicate which types of inorganic elements or especially AAEMs were washed out and exactly how much of these species were removed. Table 2 shows the AAEMs and Fe contents in the raw biomass, char, and HCl-treated char. The inorganic content in the char and the HCl treatment efficiency were calculated as follows:
Inorganic content in char ¼
HCl treatment efficiency ¼
Wchar char yield 100% Wbiomass
(1)
Wchar WHCltreated char 100% Wchar (2)
where wi is the weight of the inorganic species in sample i. As Table 1 Basic properties of wood shaving biomass and char samples.
True density (kg/m3) Mean particle size (mm) LHV (MJ/kg) Proximate analysis, dry, wt.% Ash Volatile Fixed carbon Ultimate analysis, dry, wt.% C H N S O (by difference)
Biomass
Char
HCl-treated char
1555.03 0.32 17.20
1674.30 0.25 22.42
1649.67 0.30 30.35
0.37 86.08 13.55
1.67 20.57 77.76
1.08 22.50 76.42
50.73 6.70 0.01 0.45 41.74
87.56 2.81 0.18 0.21 7.57
83.63 2.74 0.18 0.15 12.22
Table 2 Contents of inorganic species in biomass, char, and HCl washed char from ICP-MS tests (wt. %). Sample
Na
K
Mg
Ca
Fe
Biomass Char HCl-treated char Inorganic content in char HCl treatment efficiency
0.00394 0.01386 0.00404 73.70 70.85
0.03435 0.12874 0.02297 78.52 82.16
0.01671 0.08702 0.06769 109.10 22.21
0.12400 0.61322 0.43273 103.60 29.43
0.02283 0.09321 0.03087 85.53 66.88
shown in Table 2, Na and K were partly removed from the biomass (about 73.70% and 78.52% remained in the char, respectively) by pyrolysis at 550 C. However, Mg and Ca contents had no obvious change from the biomass to the 550 C pyrolytic char. The reason for this result may be the thermal decomposition of carboxylates of AAEM species, and the release of gaseous K, Na while Mg, Ca were difficult to be released at the present pyrolysis temperature [30]. After HCl washing of the pyrolytic char, up to 70% Na, 82% K, 22% Mg and 29% Ca were removed from biochar, respectively. The results agreed with Yip et al. [31]. This means that most of K was washed out and the inherent catalytic effect of K in the HCl-treated char could be negligible. 2.3. Woody char pellets preparation In this study, KOH (ACS reagent, purity > 99%), serving the role of binder and catalyst, was mixed with 0.5 g of the HCl-treated char and deionized water to form a single pellet (diameter of 6.35 mm, height of 10e12 mm) in an MTI 50K press machine (Measurement Technology Inc.). The detailed densification process is described elsewhere [28,32,33]. The mass ratio of KOH to biochar in the pellets varied from 0.5:10 to 2:10 (KOH was 5e20 wt. % of biochar). The obtained biochar pellets with different contents of KOH were named as “n-pellet,” where n indicates the KOH content (wt. % of biochar). 2.4. Gasification procedure A high-pressure macro-thermogravimetric analyzer (MTGA) (Thermax500, Thermo Fisher Scientific Inc., USA) was used for pellet gasification study, as shown elsewhere [34,35], CO2 gasification of pellets was performed in atmospheric pressure at temperatures of 750e850 C. Pellets were loaded into a stainless steel basket (12 mm in diameter and 20 mm in height) which is connected to the arm of a balance at the top via a thin metal wire. The maximum weight allowance of the balance was 10 g, and its sensitivity was 10 mg. A thermocouple was inserted into the reactor to detect the temperature in the reaction zone. The gases were introduced from the bottom of the reactor and passed through the sample during the experiments [26]. In each run, the whole pellet was loaded into the basket which went through three reaction stages: a) pyrolysis stage: the pellet was heated in N2 (200 ml/min, purity 99.99%) from room temperature to 110 C at 10 C/min and held for 30 min to remove water, then continuously heated in N2 (200 ml/min, purity 99.99%) to 850 C at 25 C/min and held for 30 min to achieve complete pyrolysis; b) temperature adjustment stage: the reactor was cooled in N2 to adjust the temperature to the target gasification temperature; c) gasification stage: the reaction gas was switched to N2 (200 ml/ min, purity 99.99%) and CO2 (200 ml/min, purity 99.99%) as soon as temperature reached the target gasification temperature and then held for gasification until the weight became constant. The weight, time and temperature were recorded by TherMax DAQ software. Before each run, the TGA load cell was calibrated using a 1 g
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standard weight. The typical reaction process and temperature profile (Fig. S1 in Supplementary Material) showed that the weight of the pellet decreased initially due to evaporation of water, then decreased further continuously during the pyrolysis (pyrolysis weight loss), and finally decreased again during the CO2 gasification of the residual char (gasification weight loss).
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E k ¼ Aexp RT
(7)
E lnk ¼ þ lnA RT
(8)
The detailed flowchart of kinetic calculation and study methodology is shown in Fig. 1. 2.5. Kinetic model CO2 gasification of a char pellet with potassium as catalyst can be treated as a catalytic Boudouard reaction:
C ðsÞ þ CO2 ðgÞ/2CO ðgÞ
(3)
This gasification process can be analyzed using the gas-solid reaction method, presented by Hancock and Sharp, for comparing the isothermal conversion kinetics [24]. The method is based on the general equation to describe the nucleation and growth gas-solid state reaction [26,27,36]:
X ¼ 1 expð kt m Þ
(4)
lnðlnð1 XÞÞ ¼ lnk þ mlnt
(5)
where k is the reaction constant, and m is the constant associated with geometry of the reaction system. X is the mass conversion at time t (s) and is calculated by
X¼
m0 mt m0 m∞
(6)
where m0 , mt , m∞ are the initial, at time t, and final weights of the sample, and dX/dt shows the gasification conversion rate. Higher gasification rate (dX/dt) means higher conversion rate with a higher reactivity. The constant of m can be recognized from the slope of the curve of ln [-ln (1-X)] against ln(t) based on Equation (5). As shown in Table 3, if m is less than 1, the reaction mechanism is more likely to be a diffusion process; whereas if 1 < m < 2, the phase boundary control seems to be in dominant; in addition, if m2, the reaction process is in favor of a random growth of a nuclei [37]. The reaction mechanism and rate-limiting step are diagnosed depending on these various gas-solid reaction models. The most suitable reaction model then can be used to determine the reaction constant (k) at different gasification temperatures. After obtaining the reaction constant (k) at different gasification temperatures, the kinetic parameters of both activation energy (E) and pre-exponential factors (A) can be calculated based on Arrhenius Equations.
3. Results and discussion 3.1. Pyrolysis behavior of pellets In this study, pellets with different contents of K were first subjected to pyrolysis at 850 C for 30 min and then gasification in CO2 at different temperatures. Thus, possible volatiles in the char after pyrolysis would not influence the subsequent gasification because the gasification temperatures were no higher than 850 C. Table 4 shows the weight loss of pellets during pyrolysis. The weight loss of the HCl-treated char after pyrolysis was 14.82%. Furthermore, with the increase of K content from 5% to 20%, the weight loss of pellets increased from 18.22% to 21.51%. The added K influenced the pyrolysis of char in terms of improved devolatilization, enhanced volatile-char interaction, and changed char structure [38,39]. The K metal or ion could penetrate the carbon matrix to generate micropores and mesopores. Moreover, the residual K in the char mixture would impede N2 or CO2 adsorption, which made it difficult to determine the pore properties of pyrolyzed chars [40]. The changed properties of chars after pyrolysis of different pellets would be expected to influence the following gasification performance. The morphologies of the chars after pyrolysis are presented in Fig. S2 in the Supplementary Material. After pyrolysis, the HCl-char
Fig. 1. Kinetic calculation and study methodology flowchart.
Table 3 Kinetic models for gas-solid reaction [36,37]. Kinetic model
Equation
m
First order reaction Phase boundary-controlled (contracting sphere)
lnð1 XÞ ¼ kt 1 ð1 XÞ1=3 ¼ kt
1 1.07
Phase boundary-controlled (contracting cylinder)
1 ð1 XÞ1=2 ¼ kt
1.11
One-dimensional diffusion
X 2 ¼ kt ð1 XÞlnð1 XÞ þ X ¼ kt
0.62
Two-dimensional diffusion Three-dimensional diffusion (Jander eq.)
Two-dimensional growth of nuclei
ð1 ð1 XÞ1=3 Þ2 ¼ kt 2 2 1 X ð1 XÞ3 ¼ kt 3 ðlnð1 XÞÞ1=2 ¼ kt
Three-dimensional growth of nuclei
ðlnð1 XÞÞ2=3 ¼ kt
Three-dimensional diffusion (G-B eq.)
0.57 0.54 0.57 2 3
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Table 4 Weight loss of char and pellets during pyrolysis at 850 C for 30 min (wt.%). Sample
HCl-treated char
5%-pellet
10%-pellet
15%-pellet
20%-pellet
Weight loss
14.82 ± 0.52
18.22 ± 0.45
19.33 ± 0.06
20.45 ± 0.58
21.51 ± 0.38
maintained the woody structure with little pore formation (Fig. S2a). However, with the addition of KOH, the surface of the chars from pyrolysis of the pellets was covered with some agglomerates, which became larger with the increase of KOH content. This may be due to the salt melting at a pyrolysis temperature of 850 C. With the increase of KOH addition, the melting and condensation were enhanced. In addition, a pit and ravine structure appeared when the KOH content was higher than 10% (Figs. S2c, d, and e). Apparently, the melting, penetrating, and catalytic effects of the potassium salt were responsible for the destroyed woody structure. Bubbles would form in the molten phase during the devolatilization process and volatiles would then escape, creating pores and pits in the surface of the char [17]. When the KOH content was 15% or 20%, the salt condensed at the char surface and covered the pores, as shown in Figs. S2d and S2e. However, these pores in the 15%-pellet char and 20%-pellet char would be exposed and beneficial for the gas-solid reaction when the salt melted and flowed at high temperatures [39]. 3.2. Gasification performance of char pellets 3.2.1. Effect of temperature on CO2 gasification of K contained char pellets The gasification process may include char reaction and volatilization of K species, so the total weight loss during gasification is the conversion of the whole pellets instead of the char/carbon conversion. Fig. 2 shows the mass conversion of HCl-treated char and the 15%-pellet at three different gasification temperatures. At the temperatures studied, the reaction rate of the HCl-treated char was constant, as embodied by the linear increase of the conversion, and higher temperature promoted the conversion rate of the char. This suggests that the HCl-treated char remained homogeneous at constant concentration during the whole gasification process. Increasing temperature also increased the conversion rate of the Kcontaining pellets, as shown in Fig. 2b. An increase in gasification temperature enhanced the reactivity of both char matrix and K species which are helpful for pore development in the char, thus leading to promoted char pellet conversion [41,42]. The time required to reach a conversion of 95% for the 15%-pellet was reduced from about 2.4 to 0.5 h when the gasification temperature
was increased from 750 to 850 C. Fig. 3 presents the dX/dt variation with the mass conversion of the pellets at three different temperatures. It shows that the conversion rate was increased with the increase of temperature. Higher temperature enhanced the heat and mass transfer and increased the reactivity of the molecular carbon with CO2, which facilitated the increase in reaction rate [43]. The peak value of reaction rate of the pellets gasified with CO2 occurred at a mass conversion of about 0.15e0.40, as shown in Fig. 3. The random pore model, which describes the modification of the pore structure of chars throughout the gasification reaction process, can be used to explain the behavior of the gasification reaction rate (dX/dt, s1) versus mass conversion (X) [22,44,45]. Thus, in this study, when the mass conversion was lower than the peak value of dX/dt, the pore structure of the char was developed, gas-solid interface reaction was enhanced, and specific surface area was increased. Those together resulted in the increase of the gasification reaction rate. However, with further conversion of the char, the mass conversion exceeded the peak value of dX/dt, the pores and holes collapsed and disappeared, which decreased the gasification reaction rate. Moreover, higher gasification temperature shifted the peak position to lower mass conversion. For example, the peak position of dX/dt of the 15%-pellet changed from X ¼ 0.40 to about X ¼ 0.25 when gasification temperature increased from 750 to 850 C, as shown in Fig. 3c. The likely reason is that higher temperature enhanced gas diffusion, gas molecule collision with solid surface, and K species penetration, which resulted in faster expansion of the pore structure [46]. In this way, the increase of temperature improved gasification process efficiency in terms of char pellet conversion, reaction rate, and pore expansion. 3.2.2. Effect of added KOH content on CO2 gasification of char pellets Fig. 4 shows the effect of KOH content in the pellets on mass conversion at gasification temperatures of 750, 800, and 850 C. The result shows that the complete gasification conversion time of the HCl-treated char was shortened by about 50% when 5% KOH was added, and further increase of KOH content increased the conversion rate and decreased the complete conversion time further. For example, at the gasification temperature of 800 C, the time needed for 95% conversion of the HCl-treated char was more
Fig. 2. Mass conversion of the gasification of (a) HCl-treated char powder and (b) 15%-pellet at different temperatures.
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Fig. 3. Variation of dX/dt with mass conversion at different gasification temperatures for (a) 5%-pellet, (b) 10%-pellet, (c) 15%-pellet, and (d) 20%-pellet.
than 8 h, but the times for the 5%-pellet, 10%-pellet, 15%-pellet, and 20% pellet were 4.0, 1.5, 0.9, and 0.6 h, respectively. It indicates that the facilitating effect of KOH on gasification was marginalized as KOH content increased from 15% to 20% compared with that as KOH content increased from 0 (HCl-treated char) to 15%. The bonding of K with carbon matrix and the distribution of K on the surface might be the main factors for the catalytic effect of the K species [47]. Moreover, the K catalyzed gasification rate slowed down significantly after about 90% mass conversion. The final 10% of the mass may have been covered with an agglomerated K2CO3 crystalline layer, which impeded the contact of char with CO2 [14,19]. Fig. 5 shows the effect of KOH content on dX/dt as a function of mass conversion at a gasification temperature of 850 C. The value of dX/ dt was enhanced by higher KOH content, and the peak value of dX/ dt, as shown in Fig. 5, was also shifted to lower mass conversion with higher KOH content. For example, the peak value of dX/dt of the 5%-pellet was at the position of about X ¼ 0.40, but the peak value for the 20%-pellet was shifted to about X ¼ 0.20. This result indicates that the increase of KOH content not only enhanced the conversion rate but also facilitated char to reach the maximum reaction rate during biochar pellet gasification process. Under the catalysis of K in biochar pellet gasification, the pores expanded, which enhanced the char conversion. And high KOH promoted the expansion of pores at lower mass conversion during CO2 gasification of char pellet [22]. 3.3. Mechanism analysis of potassium catalyzed char pellet gasification In this study, char pellets formed with different contents of KOH addition were first subjected to pyrolysis at 850 C to remove volatiles, and subsequently to gasification at different temperatures. We assume that: the pellet is isotropic without internal mass or
temperature gradient in the isothermal reaction; the diffusion of external nonreactive nitrogen into the pellet through the boundary layer is negligible; the char and K particles are distributed uniformly in the pellet; and the K components escape completely during the pyrolysis for 30 min and thus would not decrease during the gasification period. Then the catalytic Boudouard reaction process mainly consists of: (1) CO2 diffusion into the char matrix; (2) C reacting with CO2 and the catalysis of the K components to produce CO, and (3) CO diffusion out of the char particles. However, in an actual reaction, K distribution, gas or solid diffusion, phase change, boundary conditions, and particle growth or nuclei could influence the gasification process and performance. As shown in Fig. 4, the catalytic Boudouard reaction (X ¼ fðtÞ) was sigmoidshaped and could be divided into three distinct regions: incubation, acceleration, and decay [36]. With the increase of KOH content, the incubation period was shortened and the acceleration velocity was enhanced. This was especially visible when KOH content increased from 0 to 15%. It indicates that char gasification was launched more quickly under the catalysis of potassium. In addition, based on the gas-solid nucleation and growth model, the incubation period tends to be dominated by nucleation events, whereas growth plays a leading role in acceleration. The decay region is related to the termination of growth upon impingement of different growth regions, or at grain boundaries [48]. In this study, pellets formed with more KOH incurred more growth but a reduced nucleation. The growth may have resulted from phase boundary reaction or random growth of nuclei [37]. As explained previously, the peak values of dX/dt occurred at mass conversion of about 0.15e0.40, so the gasification results were modeled with X restricted to 0.15e0.40 first based on isothermal gas-solid state reaction in order to discern the kinetics of the pore development process. The parameter values of m were calculated and listed in Table 5. The values of m ranged from 0.45 to 0.72 when
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Fig. 5. Variation of dX/dt with mass conversion of pellets containing different contents of KOH at the gasification temperature of 850 C.
Fig. 4. Effect of KOH content on mass conversion of the CO2 gasification of the pellets at (a) 750 C, (b) 800 C, and (c) 850 C.
HCl-treated char was gasified at 750e850 C, which means that the reaction process is closer to diffusion-controlled model. In addition, the HCl-treated char probably exhibited a nucleation process because m is less than 1. With the increase of temperature, the m values of the HCl-treated char increased, indicating slower nucleation rate [36]. When K was added, the average value of m was 1.11, suggesting that the reaction was mainly a phase boundarycontrolled surface gasification reaction and proceeded through the one-dimensional growth (with integer value of 1) with a relatively fast contribution of nucleation (with decimal value of 0.11)
[36]. Thus, for the HCl-treated char, the diffusion-controlled nucleation process was the reaction-limiting step for gasification, which led to the slow development of pore structure. Differently, for the K-added pellets, the rapid development of pores and surface area over 0.15 < X < 0.40 most likely resulted from phase boundarycontrolled surface reaction. Since the values of m varied from 0.45 to 1.25 with an average value of 1.01 when X was in the range of 0.15e0.40 (Table 5), it showed an inconspicuous mechanism based on Table 3. The integrated gasification might have some other mechanism to some limited extent. Thus, different models were applied to fit the whole range of experimental data, with results shown in Figs. 6e9 and Table 6. Considering the overall gasification process, the phase boundary-controlled model was more suitable than the first-order nucleation, growth model or diffusion-controlled model according to a comparison of correlation coefficients (R2) from all gasification conditions. Moreover, it was found that the phase change model could better fit the experimental data at the gasification temperature of 850 C, whereas the phase boundary-controlled model fitted better at 750 and 800 C. From the fitting results of the four models, it can be concluded that the phase boundary-controlled reaction model (m ¼ 1.07) can describe the gasification of the HCl-treated char and the K-added pellets at the temperatures of 750 (Figs. 7a) and 800 C (Fig. 7b), whereas the gasification reaction at 850 C (Fig. 6c) is better captured by the phase-change model. The HCl-treated char gasification showed some diffusion resistance at the beginning (X less than 0.4, Table 5), but the whole reaction process was not controlled by gas-solid diffusion. The reason may be that initially CO2 diffusion into char matrix and diffusion of CO produced by Boudouard reaction out of solid occurred, and pores and holes of the solid state developed along with increased surface area (based on the random pore model results), thus causing the reaction mechanism to change to surface gas-solid boundary control in the following stages. When KOH was added, the surface-controlled reaction of carbon continued for the whole conversion process. At the gasification temperature of 850 C, K salts might have melted, which resulted in a more uniform distribution of K species. The gradual melting of K species with the progress of conversion changed the reaction to the phase change model. The activation energy (E) was calculated using the phase boundary-controlled model results at gasification temperatures of 800 and 750 C and the phase change model results at 850 C based on the Arrhenius equation (8). Plots of lnk versus 1/T are presented
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685
Table 5 Calculated parameter m at different gasification temperatures. 750 C
HCl-treated char 5%-pellet 10%-pellet 15%-pellet 20%-pellet a
800 C
850 C
ta (s)
m
R2
ta (s)
m
R2
ta (s)
m
R2
15830 8739 7185 3219 2296
0.45 1.23 1.25 1.24 1.11
0.948 0.997 0.999 0.998 0.999
12737 7587 2019 1175 952
0.63 0.77 1.14 1.07 1.13
0.977 0.960 0.996 0.995 0.999
4201 1513 806 597 454
0.72 1.09 1.08 1.11 1.16
0.994 0.994 0.999 0.997 0.999
t means the time needed for mass conversion to reach 0.40.
Fig. 6. Experimental and modeling results of char pellets gasification at different temperatures with phase change model (a) 750 C, (b) 800 C and (c) 850 C.
Fig. 7. Experimental and modeling results of char pellets gasification at different temperatures with phase-boundary control model (a) 750 C, (b) 800 C and (c) 850 C.
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Fig. 8. Experimental and modeling results of char pellets gasification at different temperatures with first order model (a) 750 C, (b) 800 C and (c) 850 C.
in Fig. 10a. The slopes were used to estimate the activation energy. As shown in Fig. 10b, the activation energy for the gasification of HCl-treated char was 389 kJ/mol. And as a result of the catalytic effect of K, the activation energy required for char pellet gasification was reduced, from 356 to 273 kJ/mol, with K content increased from 5 to 20 wt. %. The reduced activation energy for higher Kadded pellets would benefit for the molecules activation and gasification reaction. Therefore, when considering the gasification of biochar pellets, higher KOH content promotes the activation of molecules and provides more active sites for char conversion. It is
Fig. 9. Experimental and modeling results of char pellets gasification at different temperatures with diffusion control model (a) 750 C, (b) 800 C and (c) 850 C.
inferred that the catalytic role of K in the char gasification was that it reduced activation energy and increased the active sites for carbon to react with CO2. 4. Conclusions The potassium catalytic effect on the behavior and mechanism of gasification of biochar pellets with CO2 was studied in an MTGA at different temperatures with different amounts of KOH addition.
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687
Table 6 Fitted results of the gasification process based on different models.
750 C
Phase change model (m ¼ 2)
800 C 850 C Phase boundary-controlled model (m ¼ 1.07)
750 C 800 C 850 C
First-order nucleation and growth model (m ¼ 1)
750 C 800 C 850 C 750 C
Diffusion-controlled model (m ¼ 0.54)
800 C 850 C
k R2 k R2 k R2 k R2 k R2 k R2 k R2 k R2 k R2 k R2 k R2 k R2
HCl-treated char
5%-pellet
10%-pellet
15%-pellet
20%-pellet
4.34E-05 0.864 4.96E-05 0.879 1.59E-04 0.962 2.67E-06 0.953 1.33E-05 0.988 4.47E-05 0.959 4.00E-05 0.922 4.57E-05 0.972 1.57E-04 0.924 2.56E-06 0.723 2.89E-06 0.807 1.13E-05 0.737
5.55E-05 0.904 1.01E-04 0.977 4.47E-04 0.992 1.06E-05 0.976 2.68E-05 0.886 1.29E-04 0.976 5.54E-05 0.951 8.98E-05 0.846 4.80E-04 0.939 4.07E-06 0.786 5.45E-06 0.634 4.50E-05 0.781
9.32E-05 0.911 3.18E-04 0.970 7.80E-04 0.996 2.68E-05 0.976 8.92E-05 0.977 2.20E-04 0.990 9.91E-05 0.939 3.16E-04 0.946 8.20E-04 0.971 9.02E-06 0.772 2.36E-05 0.761 7.79E-05 0.837
2.00E-04 0.925 5.38E-04 0.978 1.01E-03 0.993 5.57E-05 0.982 1.53E-04 0.990 2.79E-04 0.991 2.07E-04 0.969 5.71E-04 0.969 1.04E-03 0.983 1.97E-05 0.842 5.50E-05 0.836 1.00E-04 0.873
2.69E-04 0.959 6.48E-04 0.960 1.32E-03 0.991 7.46E-05 0.986 1.83E-04 0.992 3.68E-04 0.990 2.77E-04 0.979 6.71E-04 0.969 1.37E-03 0.983 2.62E-05 0.859 5.74E-05 0.812 1.32E-04 0.877
Fig. 10. (a) Arrhenius plot, (b) activation energy versus KOH content added in char pellet.
The following conclusions are drawn: (1) Washing with HCl effectively removed K from the biochar with an efficiency of 82.16%. The ash content and mean particle size were reduced while the lower heating value was increased by the HCl treatment. After pyrolysis of the Kadded pellets, there were agglomerated deposits covering the char surface, which blocked the pores of the char. (2) The gasification reaction rate and mass conversion were promoted by KOH addition, and the promotion was enhanced at higher added amount of K at higher temperature. The peak value of reaction rate of the pellets gasified with CO2 occurred at mass conversion of about 0.15e0.40, and the increase of gasification temperature or added K content shifted the peak to lower mass conversion. (3) The gasification of the HCl-treated char showed some diffusion resistance at the beginning (X < 0.4) but then changed to a phase boundary-controlled surface reaction in the following stages. The reaction of the K-added pellets with CO2 was a phase boundary-controlled surface reaction at the gasification temperatures of 800 and 750 C but a phase
change model reaction at 850 C because of the melting of K species. With the increase of K-added content from 0 to 20 wt. %, the activation energy of K-added pellets decreased from 389 to 273 kJ/mol. The catalytic role of K in the biochar gasification was that it reduced the activation energy and increased the active sites for carbon to react with CO2.
Acknowledgements The authors express sincere appreciation for the financial support from National Key Research and Development Plan （2017YFB0602701-02), the National Natural Science Foundation of China (51622604) and the Natural Science Foundation of Shenzhen (JCYJ20170307172032901). The authors are also grateful for the assistance in the experimental studies provided by the Analytical and Testing Center in Huazhong University of Science & Technology (http://atc.hust.edu.cn), Wuhan 430074, China. The assistance in device debugging from Dr. Yonghua Li at the University of British Columbia is also gratefully acknowledged.
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Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.energy.2019.01.050.
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Experimental and modeling study of potassium catalyzed gasification of woody char pellet with CO2 Qiang Hu a, b, Haiping Yang a, Zhiqiang Wu b, c, C. Jim Lim b, Xiaotao T. Bi b, *, Hanping Chen a, d, ** a
State Key Laboratory of Coal Combustion, School of Energy and Power Engineering, Huazhong University of Science and Technology, Wuhan, 430074, PR China Clean Energy Research Centre, Department of Chemical and Biological Engineering, University of British Columbia, Vancouver, V6T 1Z3, Canada c School of Chemical Engineering and Technology, Xi'an Jiaotong University, Xi'an, Shaanxi, 710049, PR China d Shenzhen Huazhong University of Science and Technology Research Institute, Shenzhen 523000, PR China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 September 2018 Received in revised form 20 December 2018 Accepted 12 January 2019 Available online 16 January 2019
Understanding the catalytic effect of potassium on CO2 gasification of biochar can help clarify the complex catalytic conversion mechanism of biochar. In this study, effects of temperature and KOH content on conversion performance and reaction kinetics of gasification of biochar pellet were investigated by using a macro-thermogravimetric analysis (MTGA) unit. Results showed that the increase of gasification temperature and KOH content promoted the conversion of biochar pellet and enhanced the gasification rate (dX/dt). With the reaction proceeded, dX/dt increased initially then decreased, and reached the peak values at X of about 0.15e0.40. A faster expansion of pore structure at both higher temperature and higher KOH content would contribute to the shift of peak values to lower X. Phase boundary-controlled model fitted well to the gasification of biochar pellet with added KOH at 750 and 800 C, while the possible melting of K species at 850 C would change the gasification reaction to phase change model. The inferred catalytic role of K in pellet gasification is that it reduced the activation energy and increased the active sites for carbon to react with CO2, and the activation energy was reduced with the increase in K content (0e20 wt. %). © 2019 Elsevier Ltd. All rights reserved.
Keywords: Potassium catalyzed gasification Reaction rate constant Gas-solid reaction Phase boundary surface reaction Phase change model
1. Introduction Developing high-efficiency conversion technologies for biomass is of great importance for solving the problems of climate change, energy crisis, and environmental pollution [1] as biomass is a renewable, clean, and CO2-neutral source of energy [2]. As one of the most promising technologies applicable to biomass conversion, gasification has attracted ample attention for decades due to its flexibility, scalability, and high thermal efficiency [3,4]. Through gasification, various types of biomass can be converted into syngas (mainly hydrogen, carbon monoxide, carbon dioxide, and methane, depending on the gasification conditions). The gasification of biomass with a gasifying medium can be divided into three steps: pyrolysis of biomass, volatile reforming, and gasification of biochar
* Corresponding author. 2360 East Mall, Vancouver, BC, V6T 1Z3, Canada. ** Corresponding author. 1037 Luoyu Road, 430074, Wuhan, PR China. E-mail addresses: [email protected] (X.T. Bi), [email protected] (H. Chen). https://doi.org/10.1016/j.energy.2019.01.050 0360-5442/© 2019 Elsevier Ltd. All rights reserved.
[5,6]. The gasification of biochar step is extremely slow compared with the other two steps because of the different phase of reaction, different chemical activity and reaction, and mass transfer resistance of biochar [7e9]. Consequently, gasification of biochar has great importance in the overall biomass gasification process and needs to be understood. To solve the slow reaction rate problem of char gasification, alkali and alkaline earth metallic (Na, K, Ca, and Mg) species (AAEMs) have been implemented as useful catalysts to promote the gasification conversion of biochar [10,11]. The gasification conditions of temperature, pressure and gasification agent, char structure of surface area, morphological structure, and the content and dispersion of inorganic elements are the main factors influencing the catalytic effect of AAEMs [12,13]. Dupont et al. [10] investigated the steam gasification of biochars derived from wood, short rotation forestry/coppice, agricultural biomass, and microalgae in France. Results showed that inorganic elements influenced the gasification performance of biochar, and the ratio of K/(Si þ P) could determine the gasification rate. Nzihou et al. [12] reviewed the
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catalysts for the gasification of biomass char and pointed out that Li and K were the best catalysts for promoting the gasification rate. Huang et al. [14] investigated the CO2 gasification reactivity of fir char in China using different metals as catalysts and found that the gasification rate improved by metal catalyst addition in the order of K > Na > Ca > Fe > Mg. The Chinese researchers Feng et al. [15] loaded the AAEMs before and after pyrolysis of rice husk at 800 C and then used them for CO2/H2O gasification. It was revealed that AAEMs pre-loading on biochar surface could provide better availability of active sites, resulting in the high specific reactivity of biochar during gasification. It has been concluded that alkali metals (Na, K) are more effective for char catalytic gasification than alkaline metals (Ca, Mg). Additionally, K has been proven to be the most significant catalyst among AAEMs for biochar gasification [12,15,16]. Therefore, the catalytic effect, behavior, and mechanism of K in biochar conversion need to be highlighted for gasifier design and optimization. In recent years, several studies have investigated the catalytic influence of K on biochar gasification. The majority of these studies focused on catalytic performance, char reactivity, and comparisons with other inorganics [4,17e19], and have utilized many kinetic models to fit the gasification process and reaction mechanism. Prestipino et al. [4] examined the steam gasification kinetics of chars from six agro-industrial biomass residues in Italy, and showed that the gasification reactivity was correlated with (K þ Ca)/Si at both different temperatures and steam concentrations. The Swedish researchers Kirtania et al. [17] performed the catalytic study of K and Na salts on pine sawdust biochar gasification using the random pore model (RPM) and modified random pore model (MRPM) and found that biochar had high catalytic activity and low carbon leaching with the addition of K2CO3 (0.5 M). In Finland, Perander et al. [19] compared the catalytic effect of K and Ca on CO2 gasification of Norwegian spruce biochar, and the results demonstrated that the catalytic activity of Ca was higher than that of K in the beginning but then decreased quicker than the catalytic effect of K. However, studies focused on the specific indepth effect of K on biochar CO2 gasification performance, reactivity, and mechanism are very limited. The reaction kinetics and mechanism of catalytic gasification of biochar pellets with K addition still remain incompletely understood, especially for the catalytic gasification process and its reaction rate-determining factors. On the other hand, it has been shown that the weight of sample influences the reaction kinetics in thermal conversion process [20]. For the char powders, the shrinking core model, volumetric model and random pore model, etc. are the kinetics models frequently used to describe their gasification process and reveal the kinetic mechanism [21e23]. Whereas, it is hard for those models to explain the solid-state reaction process and the reaction controlling steps in macro-mass conversion of pellets, due to the different solid diffusion, gas diffusion, and solid-gas interaction behaviors for a mass of solid particles. The gas-solid state reaction analysis method has been used successfully to fit the macro-mass reaction process, such as for iron ore pellet reduction, char pellet gasification, and carbonate decomposition [24e27], which may be applied to identify the role and mechanism of K in char pellet gasification process. In this study, CO2 gasification of biochar pellets with different contents of KOH was carried out at different temperatures in a macro-thermogravimetric analysis (MTGA) reactor. Gasification performance and catalytic mechanism were the key points examined during the experimental and modeling process. The gas-solid state analysis method, based on AvramieErofe'ev generalized equation, was applied to analyze the catalytic CO2 gasification of the K contained biochar pellet [24,25]. Firstly, the classical basis of the method for reaction kinetics analyzing depended on the
679
nucleation and growth gas-solid reaction model. The mechanism related indicator (m) was calculated from nucleation and growth gas-solid reaction model. And then the detailed reaction model was chosen based on indicator m and the fitting quality, which was used for further mechanism identification and reaction rate-limit step explanation. Given the description above, biochar obtained from pyrolysis of woody biomass at 550 C was first washed by hydrochloric acid to eliminate the influence from its inherent inorganic elements. Then biochar pellets were made by compressing HCl-treated biochar with different contents of KOH. The K contained biochar pellets were pyrolyzed at 850 C to remove volatile matter and subsequently gasified at 750e850 C in CO2 with the interval of 50 C using a MTGA reactor. Effects of temperature and KOH content on the gasification rate of the pellets were investigated to explain the catalytic gasification performance of K. The gasification kinetics, mechanism, and reaction controlling steps were elucidated based on the gas-solid reaction analysis method. Particular attention was paid to exploring the activation energy variation with the addition of different contents of K, which would then facilitate the understanding of the mechanism of potassium catalyzed gasification of char pellets. The catalysis of K has significate influence on the conversion performance and reaction kinetics of the gasification of biochar pellet. And the influence in different temperatures and K contents is expected to be different. The present research aimed at: 1) investigating the gasification performance of biochar pellets with different contents of KOH added, and exploring the effect of temperature and KOH content on the gasification process and conversion rate of biochar pellet; 2) identifying the conversion kinetics and understanding the reaction rate-limit step and catalytic mechanism during CO2 gasification of K contained biochar pellet. 2. Experimental 2.1. Materials Wood shavings obtained from a furniture factory were crushed and then dried in an oven at 105 C for 24 h. Then, the regular, dried, raw woody biomass was selected for pyrolysis in a fixed-bed tubular reactor for the preparation of woody char samples. When the temperature was raised to 550 C, the quartz arc with the wood shavings was placed into the reaction zone with a flow of nitrogen (99.99%, 0.4 L/min) for a pyrolysis time of 30 min. The detailed pyrolysis process is described in our previous reports [28,29]. The solid residue after pyrolysis was collected as the biochar used in this study. To remove acid-soluble mineral matter from the woody char, washing with hydrochloric acid was performed by mixing 20 g of biochar with 200 ml of 5 M HCl in a 500 ml glass jar. Then, the mixture was stirred at 30 C for 24 h, followed by filtering and washing repeatedly with distilled water until a pH of 7 was reached. The filtered solid char was then dried at 105 C for 24 h. After the drying process, the residual biochar was ground in a blade mill (XY-1000A, Songqing Hardware Company, China) for 3 min. The obtained HCl-washed char, called “HCl-treated char” in this paper, was used for the subsequent pellet formation and gasification to reduce the catalytic effects of any inherent metallic elements in the raw biochar. 2.2. Properties of chars The ultimate analysis of the char was carried out in a CHNS/O elemental analyzer (Vario MICRO cube, Elementar, Germany), and proximate analysis was performed based on the standard of the
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Proximate Analysis of Solid Biofuels (GB/T 28731-2012) using a muffle furnace (KSL-1200X, Kejing Materials Technology Co. Ltd., China). The lower heating value (LHV) was measured using an oxygen bomb calorimeter (Parr 6300, USA). The true density was determined in a true density determinator (AccuPyc 1330, USA). The particle size distribution of the char was analyzed using a laser particle-size analyzer (MS2000, Malvern Instruments Ltd., U.K.). The HCl washing effect was determined by quantifying the alkali and alkaline earth metal species in the chars before and after acid treatment using inductively coupled plasma mass spectrometry (ICP-MS) (ELAN DRC-e, PerkinElmer, USA). The results were the average of two tests. The surface characteristics of the char were examined by scanning electron microscopy (SEM). Before SEM analysis, the samples were dried at 105 C for 24 h. The biochar powder was attached to a metal stub by conductive carbon paste and then sputter coated with gold for 180 s in a sputter coater (SCD 050, Bal-Tec, Liechtenstein). SEM images were recorded using a scanning electron microscope (Quanta 200, FEI Company, Netherlands) operated at 20 kV. The results were the most representative image from more than five images of each sample. Table 1 shows the properties of the raw wood shavings, the 550 C pyrolytic char, and the HCl-treated char. After anoxic thermal cracking, the true density, particle size, and volatile and oxygen contents of the raw samples decreased, whereas the calorific value, fixed carbon content, and carbon content increased. A comparison of the char before and after acid washing showed that the HCltreated char had higher mean particle size, increased LHV, and decreased ash content. The mean particle size of the HCl-treated char increased because fine particles could pass through the filter paper during the filtration process. In addition, several contents of the inorganic ash could dissolve in the hydrochloric acid, which caused the ash content to decrease from 1.67 to 1.08% and the LHV to increase from 22.42 to 30.35 MJ/kg. The ICP-MS testing results of different samples could indicate which types of inorganic elements or especially AAEMs were washed out and exactly how much of these species were removed. Table 2 shows the AAEMs and Fe contents in the raw biomass, char, and HCl-treated char. The inorganic content in the char and the HCl treatment efficiency were calculated as follows:
Inorganic content in char ¼
HCl treatment efficiency ¼
Wchar char yield 100% Wbiomass
(1)
Wchar WHCltreated char 100% Wchar (2)
where wi is the weight of the inorganic species in sample i. As Table 1 Basic properties of wood shaving biomass and char samples.
True density (kg/m3) Mean particle size (mm) LHV (MJ/kg) Proximate analysis, dry, wt.% Ash Volatile Fixed carbon Ultimate analysis, dry, wt.% C H N S O (by difference)
Biomass
Char
HCl-treated char
1555.03 0.32 17.20
1674.30 0.25 22.42
1649.67 0.30 30.35
0.37 86.08 13.55
1.67 20.57 77.76
1.08 22.50 76.42
50.73 6.70 0.01 0.45 41.74
87.56 2.81 0.18 0.21 7.57
83.63 2.74 0.18 0.15 12.22
Table 2 Contents of inorganic species in biomass, char, and HCl washed char from ICP-MS tests (wt. %). Sample
Na
K
Mg
Ca
Fe
Biomass Char HCl-treated char Inorganic content in char HCl treatment efficiency
0.00394 0.01386 0.00404 73.70 70.85
0.03435 0.12874 0.02297 78.52 82.16
0.01671 0.08702 0.06769 109.10 22.21
0.12400 0.61322 0.43273 103.60 29.43
0.02283 0.09321 0.03087 85.53 66.88
shown in Table 2, Na and K were partly removed from the biomass (about 73.70% and 78.52% remained in the char, respectively) by pyrolysis at 550 C. However, Mg and Ca contents had no obvious change from the biomass to the 550 C pyrolytic char. The reason for this result may be the thermal decomposition of carboxylates of AAEM species, and the release of gaseous K, Na while Mg, Ca were difficult to be released at the present pyrolysis temperature [30]. After HCl washing of the pyrolytic char, up to 70% Na, 82% K, 22% Mg and 29% Ca were removed from biochar, respectively. The results agreed with Yip et al. [31]. This means that most of K was washed out and the inherent catalytic effect of K in the HCl-treated char could be negligible. 2.3. Woody char pellets preparation In this study, KOH (ACS reagent, purity > 99%), serving the role of binder and catalyst, was mixed with 0.5 g of the HCl-treated char and deionized water to form a single pellet (diameter of 6.35 mm, height of 10e12 mm) in an MTI 50K press machine (Measurement Technology Inc.). The detailed densification process is described elsewhere [28,32,33]. The mass ratio of KOH to biochar in the pellets varied from 0.5:10 to 2:10 (KOH was 5e20 wt. % of biochar). The obtained biochar pellets with different contents of KOH were named as “n-pellet,” where n indicates the KOH content (wt. % of biochar). 2.4. Gasification procedure A high-pressure macro-thermogravimetric analyzer (MTGA) (Thermax500, Thermo Fisher Scientific Inc., USA) was used for pellet gasification study, as shown elsewhere [34,35], CO2 gasification of pellets was performed in atmospheric pressure at temperatures of 750e850 C. Pellets were loaded into a stainless steel basket (12 mm in diameter and 20 mm in height) which is connected to the arm of a balance at the top via a thin metal wire. The maximum weight allowance of the balance was 10 g, and its sensitivity was 10 mg. A thermocouple was inserted into the reactor to detect the temperature in the reaction zone. The gases were introduced from the bottom of the reactor and passed through the sample during the experiments [26]. In each run, the whole pellet was loaded into the basket which went through three reaction stages: a) pyrolysis stage: the pellet was heated in N2 (200 ml/min, purity 99.99%) from room temperature to 110 C at 10 C/min and held for 30 min to remove water, then continuously heated in N2 (200 ml/min, purity 99.99%) to 850 C at 25 C/min and held for 30 min to achieve complete pyrolysis; b) temperature adjustment stage: the reactor was cooled in N2 to adjust the temperature to the target gasification temperature; c) gasification stage: the reaction gas was switched to N2 (200 ml/ min, purity 99.99%) and CO2 (200 ml/min, purity 99.99%) as soon as temperature reached the target gasification temperature and then held for gasification until the weight became constant. The weight, time and temperature were recorded by TherMax DAQ software. Before each run, the TGA load cell was calibrated using a 1 g
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standard weight. The typical reaction process and temperature profile (Fig. S1 in Supplementary Material) showed that the weight of the pellet decreased initially due to evaporation of water, then decreased further continuously during the pyrolysis (pyrolysis weight loss), and finally decreased again during the CO2 gasification of the residual char (gasification weight loss).
681
E k ¼ Aexp RT
(7)
E lnk ¼ þ lnA RT
(8)
The detailed flowchart of kinetic calculation and study methodology is shown in Fig. 1. 2.5. Kinetic model CO2 gasification of a char pellet with potassium as catalyst can be treated as a catalytic Boudouard reaction:
C ðsÞ þ CO2 ðgÞ/2CO ðgÞ
(3)
This gasification process can be analyzed using the gas-solid reaction method, presented by Hancock and Sharp, for comparing the isothermal conversion kinetics [24]. The method is based on the general equation to describe the nucleation and growth gas-solid state reaction [26,27,36]:
X ¼ 1 expð kt m Þ
(4)
lnðlnð1 XÞÞ ¼ lnk þ mlnt
(5)
where k is the reaction constant, and m is the constant associated with geometry of the reaction system. X is the mass conversion at time t (s) and is calculated by
X¼
m0 mt m0 m∞
(6)
where m0 , mt , m∞ are the initial, at time t, and final weights of the sample, and dX/dt shows the gasification conversion rate. Higher gasification rate (dX/dt) means higher conversion rate with a higher reactivity. The constant of m can be recognized from the slope of the curve of ln [-ln (1-X)] against ln(t) based on Equation (5). As shown in Table 3, if m is less than 1, the reaction mechanism is more likely to be a diffusion process; whereas if 1 < m < 2, the phase boundary control seems to be in dominant; in addition, if m2, the reaction process is in favor of a random growth of a nuclei [37]. The reaction mechanism and rate-limiting step are diagnosed depending on these various gas-solid reaction models. The most suitable reaction model then can be used to determine the reaction constant (k) at different gasification temperatures. After obtaining the reaction constant (k) at different gasification temperatures, the kinetic parameters of both activation energy (E) and pre-exponential factors (A) can be calculated based on Arrhenius Equations.
3. Results and discussion 3.1. Pyrolysis behavior of pellets In this study, pellets with different contents of K were first subjected to pyrolysis at 850 C for 30 min and then gasification in CO2 at different temperatures. Thus, possible volatiles in the char after pyrolysis would not influence the subsequent gasification because the gasification temperatures were no higher than 850 C. Table 4 shows the weight loss of pellets during pyrolysis. The weight loss of the HCl-treated char after pyrolysis was 14.82%. Furthermore, with the increase of K content from 5% to 20%, the weight loss of pellets increased from 18.22% to 21.51%. The added K influenced the pyrolysis of char in terms of improved devolatilization, enhanced volatile-char interaction, and changed char structure [38,39]. The K metal or ion could penetrate the carbon matrix to generate micropores and mesopores. Moreover, the residual K in the char mixture would impede N2 or CO2 adsorption, which made it difficult to determine the pore properties of pyrolyzed chars [40]. The changed properties of chars after pyrolysis of different pellets would be expected to influence the following gasification performance. The morphologies of the chars after pyrolysis are presented in Fig. S2 in the Supplementary Material. After pyrolysis, the HCl-char
Fig. 1. Kinetic calculation and study methodology flowchart.
Table 3 Kinetic models for gas-solid reaction [36,37]. Kinetic model
Equation
m
First order reaction Phase boundary-controlled (contracting sphere)
lnð1 XÞ ¼ kt 1 ð1 XÞ1=3 ¼ kt
1 1.07
Phase boundary-controlled (contracting cylinder)
1 ð1 XÞ1=2 ¼ kt
1.11
One-dimensional diffusion
X 2 ¼ kt ð1 XÞlnð1 XÞ þ X ¼ kt
0.62
Two-dimensional diffusion Three-dimensional diffusion (Jander eq.)
Two-dimensional growth of nuclei
ð1 ð1 XÞ1=3 Þ2 ¼ kt 2 2 1 X ð1 XÞ3 ¼ kt 3 ðlnð1 XÞÞ1=2 ¼ kt
Three-dimensional growth of nuclei
ðlnð1 XÞÞ2=3 ¼ kt
Three-dimensional diffusion (G-B eq.)
0.57 0.54 0.57 2 3
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Table 4 Weight loss of char and pellets during pyrolysis at 850 C for 30 min (wt.%). Sample
HCl-treated char
5%-pellet
10%-pellet
15%-pellet
20%-pellet
Weight loss
14.82 ± 0.52
18.22 ± 0.45
19.33 ± 0.06
20.45 ± 0.58
21.51 ± 0.38
maintained the woody structure with little pore formation (Fig. S2a). However, with the addition of KOH, the surface of the chars from pyrolysis of the pellets was covered with some agglomerates, which became larger with the increase of KOH content. This may be due to the salt melting at a pyrolysis temperature of 850 C. With the increase of KOH addition, the melting and condensation were enhanced. In addition, a pit and ravine structure appeared when the KOH content was higher than 10% (Figs. S2c, d, and e). Apparently, the melting, penetrating, and catalytic effects of the potassium salt were responsible for the destroyed woody structure. Bubbles would form in the molten phase during the devolatilization process and volatiles would then escape, creating pores and pits in the surface of the char [17]. When the KOH content was 15% or 20%, the salt condensed at the char surface and covered the pores, as shown in Figs. S2d and S2e. However, these pores in the 15%-pellet char and 20%-pellet char would be exposed and beneficial for the gas-solid reaction when the salt melted and flowed at high temperatures [39]. 3.2. Gasification performance of char pellets 3.2.1. Effect of temperature on CO2 gasification of K contained char pellets The gasification process may include char reaction and volatilization of K species, so the total weight loss during gasification is the conversion of the whole pellets instead of the char/carbon conversion. Fig. 2 shows the mass conversion of HCl-treated char and the 15%-pellet at three different gasification temperatures. At the temperatures studied, the reaction rate of the HCl-treated char was constant, as embodied by the linear increase of the conversion, and higher temperature promoted the conversion rate of the char. This suggests that the HCl-treated char remained homogeneous at constant concentration during the whole gasification process. Increasing temperature also increased the conversion rate of the Kcontaining pellets, as shown in Fig. 2b. An increase in gasification temperature enhanced the reactivity of both char matrix and K species which are helpful for pore development in the char, thus leading to promoted char pellet conversion [41,42]. The time required to reach a conversion of 95% for the 15%-pellet was reduced from about 2.4 to 0.5 h when the gasification temperature
was increased from 750 to 850 C. Fig. 3 presents the dX/dt variation with the mass conversion of the pellets at three different temperatures. It shows that the conversion rate was increased with the increase of temperature. Higher temperature enhanced the heat and mass transfer and increased the reactivity of the molecular carbon with CO2, which facilitated the increase in reaction rate [43]. The peak value of reaction rate of the pellets gasified with CO2 occurred at a mass conversion of about 0.15e0.40, as shown in Fig. 3. The random pore model, which describes the modification of the pore structure of chars throughout the gasification reaction process, can be used to explain the behavior of the gasification reaction rate (dX/dt, s1) versus mass conversion (X) [22,44,45]. Thus, in this study, when the mass conversion was lower than the peak value of dX/dt, the pore structure of the char was developed, gas-solid interface reaction was enhanced, and specific surface area was increased. Those together resulted in the increase of the gasification reaction rate. However, with further conversion of the char, the mass conversion exceeded the peak value of dX/dt, the pores and holes collapsed and disappeared, which decreased the gasification reaction rate. Moreover, higher gasification temperature shifted the peak position to lower mass conversion. For example, the peak position of dX/dt of the 15%-pellet changed from X ¼ 0.40 to about X ¼ 0.25 when gasification temperature increased from 750 to 850 C, as shown in Fig. 3c. The likely reason is that higher temperature enhanced gas diffusion, gas molecule collision with solid surface, and K species penetration, which resulted in faster expansion of the pore structure [46]. In this way, the increase of temperature improved gasification process efficiency in terms of char pellet conversion, reaction rate, and pore expansion. 3.2.2. Effect of added KOH content on CO2 gasification of char pellets Fig. 4 shows the effect of KOH content in the pellets on mass conversion at gasification temperatures of 750, 800, and 850 C. The result shows that the complete gasification conversion time of the HCl-treated char was shortened by about 50% when 5% KOH was added, and further increase of KOH content increased the conversion rate and decreased the complete conversion time further. For example, at the gasification temperature of 800 C, the time needed for 95% conversion of the HCl-treated char was more
Fig. 2. Mass conversion of the gasification of (a) HCl-treated char powder and (b) 15%-pellet at different temperatures.
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Fig. 3. Variation of dX/dt with mass conversion at different gasification temperatures for (a) 5%-pellet, (b) 10%-pellet, (c) 15%-pellet, and (d) 20%-pellet.
than 8 h, but the times for the 5%-pellet, 10%-pellet, 15%-pellet, and 20% pellet were 4.0, 1.5, 0.9, and 0.6 h, respectively. It indicates that the facilitating effect of KOH on gasification was marginalized as KOH content increased from 15% to 20% compared with that as KOH content increased from 0 (HCl-treated char) to 15%. The bonding of K with carbon matrix and the distribution of K on the surface might be the main factors for the catalytic effect of the K species [47]. Moreover, the K catalyzed gasification rate slowed down significantly after about 90% mass conversion. The final 10% of the mass may have been covered with an agglomerated K2CO3 crystalline layer, which impeded the contact of char with CO2 [14,19]. Fig. 5 shows the effect of KOH content on dX/dt as a function of mass conversion at a gasification temperature of 850 C. The value of dX/ dt was enhanced by higher KOH content, and the peak value of dX/ dt, as shown in Fig. 5, was also shifted to lower mass conversion with higher KOH content. For example, the peak value of dX/dt of the 5%-pellet was at the position of about X ¼ 0.40, but the peak value for the 20%-pellet was shifted to about X ¼ 0.20. This result indicates that the increase of KOH content not only enhanced the conversion rate but also facilitated char to reach the maximum reaction rate during biochar pellet gasification process. Under the catalysis of K in biochar pellet gasification, the pores expanded, which enhanced the char conversion. And high KOH promoted the expansion of pores at lower mass conversion during CO2 gasification of char pellet [22]. 3.3. Mechanism analysis of potassium catalyzed char pellet gasification In this study, char pellets formed with different contents of KOH addition were first subjected to pyrolysis at 850 C to remove volatiles, and subsequently to gasification at different temperatures. We assume that: the pellet is isotropic without internal mass or
temperature gradient in the isothermal reaction; the diffusion of external nonreactive nitrogen into the pellet through the boundary layer is negligible; the char and K particles are distributed uniformly in the pellet; and the K components escape completely during the pyrolysis for 30 min and thus would not decrease during the gasification period. Then the catalytic Boudouard reaction process mainly consists of: (1) CO2 diffusion into the char matrix; (2) C reacting with CO2 and the catalysis of the K components to produce CO, and (3) CO diffusion out of the char particles. However, in an actual reaction, K distribution, gas or solid diffusion, phase change, boundary conditions, and particle growth or nuclei could influence the gasification process and performance. As shown in Fig. 4, the catalytic Boudouard reaction (X ¼ fðtÞ) was sigmoidshaped and could be divided into three distinct regions: incubation, acceleration, and decay [36]. With the increase of KOH content, the incubation period was shortened and the acceleration velocity was enhanced. This was especially visible when KOH content increased from 0 to 15%. It indicates that char gasification was launched more quickly under the catalysis of potassium. In addition, based on the gas-solid nucleation and growth model, the incubation period tends to be dominated by nucleation events, whereas growth plays a leading role in acceleration. The decay region is related to the termination of growth upon impingement of different growth regions, or at grain boundaries [48]. In this study, pellets formed with more KOH incurred more growth but a reduced nucleation. The growth may have resulted from phase boundary reaction or random growth of nuclei [37]. As explained previously, the peak values of dX/dt occurred at mass conversion of about 0.15e0.40, so the gasification results were modeled with X restricted to 0.15e0.40 first based on isothermal gas-solid state reaction in order to discern the kinetics of the pore development process. The parameter values of m were calculated and listed in Table 5. The values of m ranged from 0.45 to 0.72 when
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Fig. 5. Variation of dX/dt with mass conversion of pellets containing different contents of KOH at the gasification temperature of 850 C.
Fig. 4. Effect of KOH content on mass conversion of the CO2 gasification of the pellets at (a) 750 C, (b) 800 C, and (c) 850 C.
HCl-treated char was gasified at 750e850 C, which means that the reaction process is closer to diffusion-controlled model. In addition, the HCl-treated char probably exhibited a nucleation process because m is less than 1. With the increase of temperature, the m values of the HCl-treated char increased, indicating slower nucleation rate [36]. When K was added, the average value of m was 1.11, suggesting that the reaction was mainly a phase boundarycontrolled surface gasification reaction and proceeded through the one-dimensional growth (with integer value of 1) with a relatively fast contribution of nucleation (with decimal value of 0.11)
[36]. Thus, for the HCl-treated char, the diffusion-controlled nucleation process was the reaction-limiting step for gasification, which led to the slow development of pore structure. Differently, for the K-added pellets, the rapid development of pores and surface area over 0.15 < X < 0.40 most likely resulted from phase boundarycontrolled surface reaction. Since the values of m varied from 0.45 to 1.25 with an average value of 1.01 when X was in the range of 0.15e0.40 (Table 5), it showed an inconspicuous mechanism based on Table 3. The integrated gasification might have some other mechanism to some limited extent. Thus, different models were applied to fit the whole range of experimental data, with results shown in Figs. 6e9 and Table 6. Considering the overall gasification process, the phase boundary-controlled model was more suitable than the first-order nucleation, growth model or diffusion-controlled model according to a comparison of correlation coefficients (R2) from all gasification conditions. Moreover, it was found that the phase change model could better fit the experimental data at the gasification temperature of 850 C, whereas the phase boundary-controlled model fitted better at 750 and 800 C. From the fitting results of the four models, it can be concluded that the phase boundary-controlled reaction model (m ¼ 1.07) can describe the gasification of the HCl-treated char and the K-added pellets at the temperatures of 750 (Figs. 7a) and 800 C (Fig. 7b), whereas the gasification reaction at 850 C (Fig. 6c) is better captured by the phase-change model. The HCl-treated char gasification showed some diffusion resistance at the beginning (X less than 0.4, Table 5), but the whole reaction process was not controlled by gas-solid diffusion. The reason may be that initially CO2 diffusion into char matrix and diffusion of CO produced by Boudouard reaction out of solid occurred, and pores and holes of the solid state developed along with increased surface area (based on the random pore model results), thus causing the reaction mechanism to change to surface gas-solid boundary control in the following stages. When KOH was added, the surface-controlled reaction of carbon continued for the whole conversion process. At the gasification temperature of 850 C, K salts might have melted, which resulted in a more uniform distribution of K species. The gradual melting of K species with the progress of conversion changed the reaction to the phase change model. The activation energy (E) was calculated using the phase boundary-controlled model results at gasification temperatures of 800 and 750 C and the phase change model results at 850 C based on the Arrhenius equation (8). Plots of lnk versus 1/T are presented
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685
Table 5 Calculated parameter m at different gasification temperatures. 750 C
HCl-treated char 5%-pellet 10%-pellet 15%-pellet 20%-pellet a
800 C
850 C
ta (s)
m
R2
ta (s)
m
R2
ta (s)
m
R2
15830 8739 7185 3219 2296
0.45 1.23 1.25 1.24 1.11
0.948 0.997 0.999 0.998 0.999
12737 7587 2019 1175 952
0.63 0.77 1.14 1.07 1.13
0.977 0.960 0.996 0.995 0.999
4201 1513 806 597 454
0.72 1.09 1.08 1.11 1.16
0.994 0.994 0.999 0.997 0.999
t means the time needed for mass conversion to reach 0.40.
Fig. 6. Experimental and modeling results of char pellets gasification at different temperatures with phase change model (a) 750 C, (b) 800 C and (c) 850 C.
Fig. 7. Experimental and modeling results of char pellets gasification at different temperatures with phase-boundary control model (a) 750 C, (b) 800 C and (c) 850 C.
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Fig. 8. Experimental and modeling results of char pellets gasification at different temperatures with first order model (a) 750 C, (b) 800 C and (c) 850 C.
in Fig. 10a. The slopes were used to estimate the activation energy. As shown in Fig. 10b, the activation energy for the gasification of HCl-treated char was 389 kJ/mol. And as a result of the catalytic effect of K, the activation energy required for char pellet gasification was reduced, from 356 to 273 kJ/mol, with K content increased from 5 to 20 wt. %. The reduced activation energy for higher Kadded pellets would benefit for the molecules activation and gasification reaction. Therefore, when considering the gasification of biochar pellets, higher KOH content promotes the activation of molecules and provides more active sites for char conversion. It is
Fig. 9. Experimental and modeling results of char pellets gasification at different temperatures with diffusion control model (a) 750 C, (b) 800 C and (c) 850 C.
inferred that the catalytic role of K in the char gasification was that it reduced activation energy and increased the active sites for carbon to react with CO2. 4. Conclusions The potassium catalytic effect on the behavior and mechanism of gasification of biochar pellets with CO2 was studied in an MTGA at different temperatures with different amounts of KOH addition.
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Table 6 Fitted results of the gasification process based on different models.
750 C
Phase change model (m ¼ 2)
800 C 850 C Phase boundary-controlled model (m ¼ 1.07)
750 C 800 C 850 C
First-order nucleation and growth model (m ¼ 1)
750 C 800 C 850 C 750 C
Diffusion-controlled model (m ¼ 0.54)
800 C 850 C
k R2 k R2 k R2 k R2 k R2 k R2 k R2 k R2 k R2 k R2 k R2 k R2
HCl-treated char
5%-pellet
10%-pellet
15%-pellet
20%-pellet
4.34E-05 0.864 4.96E-05 0.879 1.59E-04 0.962 2.67E-06 0.953 1.33E-05 0.988 4.47E-05 0.959 4.00E-05 0.922 4.57E-05 0.972 1.57E-04 0.924 2.56E-06 0.723 2.89E-06 0.807 1.13E-05 0.737
5.55E-05 0.904 1.01E-04 0.977 4.47E-04 0.992 1.06E-05 0.976 2.68E-05 0.886 1.29E-04 0.976 5.54E-05 0.951 8.98E-05 0.846 4.80E-04 0.939 4.07E-06 0.786 5.45E-06 0.634 4.50E-05 0.781
9.32E-05 0.911 3.18E-04 0.970 7.80E-04 0.996 2.68E-05 0.976 8.92E-05 0.977 2.20E-04 0.990 9.91E-05 0.939 3.16E-04 0.946 8.20E-04 0.971 9.02E-06 0.772 2.36E-05 0.761 7.79E-05 0.837
2.00E-04 0.925 5.38E-04 0.978 1.01E-03 0.993 5.57E-05 0.982 1.53E-04 0.990 2.79E-04 0.991 2.07E-04 0.969 5.71E-04 0.969 1.04E-03 0.983 1.97E-05 0.842 5.50E-05 0.836 1.00E-04 0.873
2.69E-04 0.959 6.48E-04 0.960 1.32E-03 0.991 7.46E-05 0.986 1.83E-04 0.992 3.68E-04 0.990 2.77E-04 0.979 6.71E-04 0.969 1.37E-03 0.983 2.62E-05 0.859 5.74E-05 0.812 1.32E-04 0.877
Fig. 10. (a) Arrhenius plot, (b) activation energy versus KOH content added in char pellet.
The following conclusions are drawn: (1) Washing with HCl effectively removed K from the biochar with an efficiency of 82.16%. The ash content and mean particle size were reduced while the lower heating value was increased by the HCl treatment. After pyrolysis of the Kadded pellets, there were agglomerated deposits covering the char surface, which blocked the pores of the char. (2) The gasification reaction rate and mass conversion were promoted by KOH addition, and the promotion was enhanced at higher added amount of K at higher temperature. The peak value of reaction rate of the pellets gasified with CO2 occurred at mass conversion of about 0.15e0.40, and the increase of gasification temperature or added K content shifted the peak to lower mass conversion. (3) The gasification of the HCl-treated char showed some diffusion resistance at the beginning (X < 0.4) but then changed to a phase boundary-controlled surface reaction in the following stages. The reaction of the K-added pellets with CO2 was a phase boundary-controlled surface reaction at the gasification temperatures of 800 and 750 C but a phase
change model reaction at 850 C because of the melting of K species. With the increase of K-added content from 0 to 20 wt. %, the activation energy of K-added pellets decreased from 389 to 273 kJ/mol. The catalytic role of K in the biochar gasification was that it reduced the activation energy and increased the active sites for carbon to react with CO2.
Acknowledgements The authors express sincere appreciation for the financial support from National Key Research and Development Plan （2017YFB0602701-02), the National Natural Science Foundation of China (51622604) and the Natural Science Foundation of Shenzhen (JCYJ20170307172032901). The authors are also grateful for the assistance in the experimental studies provided by the Analytical and Testing Center in Huazhong University of Science & Technology (http://atc.hust.edu.cn), Wuhan 430074, China. The assistance in device debugging from Dr. Yonghua Li at the University of British Columbia is also gratefully acknowledged.
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Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.energy.2019.01.050.
[26]
[27]
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