Characteristics of biomass devolatilization and in-situ char

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Biofuels and Biomass

Characteristics of biomass devolatilization and in-situ char gasification tested by the non-isothermal method Xi Zeng, Kaito Kahara, Yasuaki Ueki, Ryo Yoshiie, Guangwen Xu, and Ichiro Naruse Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.9b00672 • Publication Date (Web): 03 Sep 2019 Downloaded from pubs.acs.org on September 3, 2019

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Energy & Fuels

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Characteristics of biomass devolatilization and in-situ char

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gasification tested by the non-isothermal method

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Xi Zeng1, 2*, Kaito Kahara3, Yasuaki Ueki1, Ryo Yoshiie3, Guangwen Xu2, Ichiro Naruse1

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1 Institute of Materials and Systems for Sustainability (IMaSS), Nagoya University, Nagoya

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464-8601, Japan

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2 State Key Laboratory of Multi-Phase Complex Systems, Institute of Process Engineering,

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Chinese Academy of Sciences, Beijing 100190, People’s Republic of China

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3 Department of Mechanical Systems Engineering, Nagoya University, Nagoya 464-8601,

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Japan

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*Authors to whom correspondence should be addressed. Phone: +81-52-789-2710; Fax:

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+81-52-789-5123; E-mail: [email protected] (Xi Zeng).

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ACS Paragon Plus Environment

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ABSTRACT

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This work examined the reaction behavior and kinetics of in-situ char gasification and

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the corresponding biomass devolatilization by TGA based on the non-isothermal method.

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Four kinds of devolatilization atmosphere (N2, N2+CO2 (10%), N2+CO2 (50%), N2+steam

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(10%)), three kinds of gasification atmosphere (N2+CO2 (10%), N2+CO2 (50%), N2+steam

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(10%)) and different heating rates (5, 10, 20, 30, 40 K/min) were adopted. With the increase

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of heating rate in each atmosphere for biomass devolatilization or char gasification, the

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typical reaction temperatures, including TBD-max, TCG-i, and TCG-max, increased obviously due to

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the delayed effect. At a given heating rate, the adopted devolatilization atmospheres displayed

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very weak influence on the volatiles release behavior and kinetics, even for the CO2 content

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of 50 %, but had obvious influence on the char property. The volume reaction model and

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shrinking core reaction model were suitable to describe the behaviors of biomass

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devolatilization and char gasification, respectively. Compared to biomass devolatilization, the

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reaction atmosphere had a remarkable effect on char gasification, especially in the curve

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shape of reaction rate and kinetic parameters. For the in-situ char gasification, the calculated

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activation energy (E) in the steam atmosphere was much lower than that in the CO2

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atmosphere. While for the char gasification in the steam atmosphere, the E of ex-situ char was

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higher than that of in-situ char, indicating the necessity of adopting the in-situ char for the

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char gasification research.

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Keywords: Biomass; Devolatilization; Gasification; In-situ char; TGA;

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1. INTRODUCTION

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As a renewable and sustainable resource, biomass is a direct and promising alternative to

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traditional fossil fuels because of low cost, CO2 neutrality, abundance, and uniform

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distribution throughout the world [1-3]. Among the diverse technologies for biomass

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utilization, gasification represents one of the most attractive and competitive options by

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converting the carbonaceous solid fuel into the combustible gaseous fuel cleanly and

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high-efficiently [4, 5]. Generally, the typical process in an industrial gasifier always involves

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complex physical change, homogeneous and heterogeneous chemical reactions, and the

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corresponding heat and mass transfer, such as drying, devolatilization, gasification,

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combustion, and tar cracking [6]. Among them, char gasification is much related to the

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process of biomass devolatilization by changing the char property. As the primary

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rate-limiting step, char gasification dominates the whole biomass gasification process [7, 8].

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So an improved and comprehensive understanding of biomass devolatilization and char

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gasification becomes essential and necessary for the design, operation, and scale-up of a

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practical gasifier.

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According to the difference in the char preparation approach for the following char

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gasification experiment, the numerous researches in literature can be divided into in-situ char

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gasification and ex-situ char gasification. For the former, biomass devolatilization and char

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gasification were conducted simultaneously or subsequently in the same reactor and

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atmosphere. During this process, the char was always in the hot state without the operation of

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tar collection. While for the latter, they were performed individually in the different reactors

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with the same or the different atmospheres, or the same reactor but with the different 3


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atmospheres. Generally, two approaches were adopted in the research of ex-situ char

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gasification: (1) preparing char in an apparatus under the inert atmosphere (N2, Ar, etc.) and

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then gasifying the pre-prepared cooled char by another reactor (TGA, drop tube reactor, etc.)

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in the reactive atmosphere (O2, air, CO2, steam, etc.) [9-12]; (2) preparing char firstly in an

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inert atmosphere and then directly switching it to the reactive atmosphere in the same

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apparatus at high-temperature [13-16]. In this study, the char samples from the two

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approaches were marked as the ex-situ char 1 and the ex-situ char 2, respectively. As we

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know, the processes of cooling, annealing, and the secondary heating inevitably influence the

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property of char, such as porous structure and the distribution of active site, further leading to

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the loss of char activity [17]. Moreover, in a commercial gasifier, especially for the fluidized

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bed or entrained flow gasifier, the continuous processes of drying, pyrolysis, and gasification

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occur nearly under the same operating condition. So, compared to the ex-situ char, the in-situ

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char can simulate most closely the fluidized bed gasifier and the entrained flow gasifier.

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These make it very necessary to measure the reaction behavior and kinetics of the in-situ char.

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Due to the limitation in measuring principle and instrument structure, the kinetic

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information of in-situ char, especially for the gasification reaction between char and steam,

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was still scarce [18]. Peng et al. [19] compared the gasification reactivity of in-situ and ex-situ

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coal char in the temperature range of 1273-1673 K. The measured TGA curve was divided

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into coal pyrolysis and char gasification by the special mathematical treatment. Compared to

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the ex-situ char, the reaction rate of in-situ char was much higher, reaching up to six times,

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and the curve shapes of reaction rate versus conversion were also different. To optimize the

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design of a kind of air-staged boiler, Chen et al. [20] measured the reaction characteristics of 4


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in/ex-situ chars on a down-firing furnace and further calculated the gasification kinetics. The

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mass balance can be obtained by measuring the gaseous products on a Fourier transform

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infrared spectroscopy (FTIR) gas analyzer and micro gas chromatography (GC). The tested

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reaction rate of in-situ char was about 4.6 times of that ex-situ char. Wang et al. [21]

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examined the gasification characteristics of in-situ char by decoupling pyrolysis and

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gasification in a micro fluidized bed reactor that connected a fast process mass spectrometer.

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Compared to the adopted ex-situ char sample, the in-situ char had the highest reaction rate

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and the lowest activation energy (E). All of these researches strongly verified the necessity

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and availability of using the in-situ char during the gasification reaction analysis. However, in

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the researches mentioned above, due to adopting the complicated mathematical derivation and

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the limitation of low sampling frequency in the gas analyzers, it always brought in serious

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uncertainty during the definition of the critical point between pyrolysis and gasification.

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Meanwhile, for these isothermal gasification researches, it rarely considered the

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corresponding behavior and kinetics of fuel devolatilization. Moreover, nearly all the existing

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researches of in-situ char gasification were conducted for coal char but not for biomass char.

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Although there were numerous reports on the reaction kinetics of biomass

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devolatilization in the inert atmosphere, the related researches in active atmosphere were not

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only scarce, but also filled with obviously different, even contradictory conclusions. Ferreira

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et al. [22] examined the pyrolysis kinetics of sugarcane straw in different atmospheres (N2,

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N2+O2 (3%), N2+O2 (20%)) from 693 K to 993 K. The results showed that, in the atmosphere

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of N2+O2 (3%), it had the lowest activation energy (E), indicating oxidative pyrolysis as a low

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energy cost and environmentally friend process for converting biomass to the valuable 5


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products. However, Chandrasekaran et al. [23] took an opposite conclusion by pyrolyzing

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switchgrass pellet in different atmospheres. The value of E in the oxidative atmosphere (556

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kJ/mol) was much higher than that in the inert atmosphere (314 kJ/mol). A similar result was

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also obtained by Singh et al. [24]. However, an unexpected conclusion from Bach et al. [25]

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demonstrated that, compared to the inert atmosphere, the oxidative atmosphere (CO2,

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CO2+H2O, CO2+H2O+O2) did not have a noticeable influence on the reaction kinetics from

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473 K to 573 K. Perhaps, these differences were mostly related to the adopted experimental

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approach, heating rate, temperature range, pyrolysis apparatus, reaction atmosphere,

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mathematical analysis method, as well as the feedstock. So, the confusing results and even

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contradictory conclusions not only indicate the research value of devolatilization behavior and

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kinetics in different atmospheres, but also demonstrate the research necessity in the char

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gasification and its corresponding devolatilization/pyrolysis together instead of separately

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[26]. Moreover, to obtain a clear and deep understanding in biomass devolatilization and char

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gasification, a reasonable comparison should be conducted under the same experimental

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method, including biomass sample, reaction condition, data analysis approach, and so on [27].

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Isothermal and non-isothermal methods are conventionally used in the analysis of

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reaction behavior and kinetics. Generally, the isothermal method is performed at a fixed

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reaction temperature, indicating the overall reaction characteristics. While the non-isothermal

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method is conducted by loading sample at room temperature and then heating the sample

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according to a given temperature-programmed procedure. Compared to the isothermal method,

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the non-isothermal method owns many advantages, such as less experiment work, shorter

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operating period, and more information about reaction behavior [28, 29]. 6


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In this research, the experiments of biomass devolatilization and the following char

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gasification in different atmospheres were conducted on a thermogravimetric analyzer (TGA)

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according to the non-isothermal approach. The Coats and Redfern method will be adopted for

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the kinetic estimation. The objectives of this study were threefold. Firstly, the effect of inert,

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CO2-containing, and steam-containing atmospheres on the devolatilization behavior and

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reaction kinetics was examined, including TG/DTG curves, conversion degree, reaction rate,

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char structure, and kinetics of volatiles release. Secondly, the gasification characteristics and

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kinetics of the in-situ chars from the different devolatilization atmospheres were tested and

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compared. Thirdly, the difference between in-situ char and ex-situ char, not only in char

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structure and morphology, but also in gasification characteristics and kinetics, were analyzed

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systematically. By this study, it will provide an approach to test the gasification behavior of

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in-situ char, offset the lack of reaction kinetics not only in in-situ char gasification, but also in

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the corresponding biomass devolatilization, and thus deepen the understanding in the whole

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biomass gasification process.

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2. EXPERIMENTAL

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2.1 Material

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In this study, a kind of Japanese black pine was adopted as the experimental feedstock.

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According to the proximate (air dry basis) and ultimate analysis (dry and ash-free basis), the

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mass contents of moisture, ash, volatile matter, and fixed carbon were 6.96 %, 1.84 %, 79.16

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%, and 12.04 %, respectively, and the contents of C, H, S, O, and N were 50.42 %, 6.77%,

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0.00 %, 42.62 % (by difference), and 0.19 %, respectively. The higher heating value of the

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black pine was about 17.81 MJ/kg. These analyses showed that the experimental sample was 7


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rich in volatile matter, C, and O. Prior to experiment, the black pine pellets were ground,

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sieved, and then dried in an air oven at 380 K for 12 h to obtain the experimental samples

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with a particle size in the range of 0.55 mm-0.83 mm.

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2.2 Apparatus and procedure

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As shown in Fig. 1(a), a thermogravimetric analyzer (NETZSCH, TG-DTA

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2000SE/H/VAP) with the sensitivity of sample mass of 0.001 mg was adopted, consisting of

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an electric heating furnace, a mass measuring unit, a set of gas supplying system, a steam

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generator, a temperature controlling and data logging system. Two streams of carrier gas (N2)

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were used in this apparatus, one as the sweeping gas to restrain the secondary reaction of

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volatile matter and the interaction between the released volatile matter and char, the other as

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the carrier gas to entrain the adopted gasification agent (steam/CO2). Steam was generated in

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an electric oven operated at 473 K by vaporizing purified water via a peristaltic pump. To

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avoid the steam condensation, the temperature of the pipeline from the steam generator to the

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electric heating furnace was maintained at 423 K by a ribbon heater.

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To examine the effect of atmosphere’s kind and content on biomass devolatilization and

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the subsequent char gasification, four kinds of atmosphere were employed, including N2,

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N2+steam (10 %), N2+CO2 (10%), and N2+CO2 (50%). Prior to each experiment, about 10 mg

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biomass sample was loaded into the ceramic crucible at room temperature. Below 380 K, the

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biomass sample was heated in N2 with a heating rate of 20 K/min and residence time of 10

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min. Then, the heating rate was changed to the value of 5/10/20/30/40 K/min, and maintained

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throughout the following heating period. For the in-situ char experiment, at 473 K, the

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gasification agent (steam/CO2) was introduced into the heating furnace, and the reaction 8


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temperature rose continuously to 1273 K for biomass devolatilization and the following char

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gasification. The ending temperature of the biomass devolatilization stage and the initial

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temperature of the char gasification stage can be considered as the same value in each

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experiment. Taking the in-situ char gasification at the heating rate of 30 K/min in the

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atmosphere of N2+CO2 as an example, the critical point can be determined by the TG/DTG

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curves, as shown in Fig. 1 (b).

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Prior to the experiments of ex-situ char gasification (biomass devolatilization in N2, char

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gasification in N2+steam (10%)), the corresponding in-situ char gasification experiments

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(both biomass devolatilization and char gasification in N2+steam (10%)) at different heating

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rates were conducted in advance. According to the TG-DTG curve, the initial temperature

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(TCG-i) of in-situ char gasification can be determined. For the ex-situ char gasification with the

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same heating rate, the biomass sample was firstly devolatilized in N2, and then at the

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temperature of TCG-i, a certain flowrate of steam was introduced into the TGA to form the

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atmosphere of N2+steam (10%), and then begin the ex-situ char gasification quickly.

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Hereafter, to distinguish clearly, the gasification atmospheres of N2+steam (10%) for the

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in-situ char produced in N2+steam (10%) and for the ex-situ char produced in N2, were

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marked as N2+steam-1 and N2+steam-2, respectively. Figure 1 (c) shows the typical TG/DTG

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curves of the ex-situ char gasification at the heating rate of 30 K/min. For each condition, the

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experiment was repeated three times, and the average standard deviation of mass loss was

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below 2 %.

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2.3 Analysis and characterization

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To examine the difference in char property, four kinds of biomass char were prepared in 9


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a horizontal tubular furnace, whose operational conditions were consistent with the

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corresponding devolatilization stages with a heating rate of 10 K /min. An automatic

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volumetric sorption analyzer was used to test the pore structure and specific surface area of

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char sample by the N2 adsorption method at 77 K. Barrett-Joyner-Halenda (BJH) method and

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t-plot method were chosen to analyze the structure of meso pore and micro pore, respectively.

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A Fourier transform infrared spectroscopy (BRUKER TENSOR 27, Germany) was employed

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to analyze the distribution of functional groups on the char surface. A scanning electron

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microscopy/energy dispersive spectrometry (SEM/EDS, JSM-7001 F, X-Max 50) was

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adopted to test the surface morphology and element composition of the char samples.

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For the stages of biomass devolatilization (BD) and char gasification (CG), the realized

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conversion (XBD, XCG) and reaction rate (RBD, RCG) can be defined by the following equations

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from (1) to (4),

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X BD 

m0  BD  mi  BD m0  BD  m f  BD

(1)

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X CG 

m0CG  mi CG m0CG  m f CG

(2)

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RBD  

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RCG  

1 m0  BD 1 m0CG



dmi  BD dX BD  dt dt

(3)



dmi CG dX CG  dt dt

(4)

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where m0, mi, and mf are the sample masses at the beginning, given, and final time in the stage

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of biomass devolatilization or char gasification; t is the reaction time in each stage.

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Generally, the typical differential equation of biomass devolatilization and char gasification can be expressed by the equation (5), 10


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Energy & Fuels

E  dX n n RT R  kf  X   k 1  X   Ae 1  X  dt

(5)

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where k is the reaction rate constant expressed by the Arrhenius equation; T, A, and E are the

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absolute reaction temperature, pre-exponential factor, and activation energy, respectively; n is

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the reaction order; R is the universal gas constant.

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For the non-isothermal gasification reaction of char, the common approaches in literature

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include the combination and single heating rate method. Compared to the former, the single

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heating rate method can estimate the reaction characteristics at a given heating rate and obtain

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the corresponding kinetic parameters. A representative single heating rate method was

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proposed by Coats and Redfern [30, 31], as followed by the equation (6) and (7),

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1  1  X 1 n 1   AR  E ln   2   ln   T    E  RT  1  n 

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  ln 1  X    AR  E ln    ln   2 T  E     RT

229 230 231

(for n≠1)

(6)

(for n=1)

(7)

where β is the heating rate adopted in the non-isothermal experiment. To simplify, four notations, namely, Y, Z, m, and c, were employed in the equation (6) and (7), which can be given in the equations from (8) to (11).

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1  1  X 1 n 1  Y=ln   2 T   1  n 

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  ln 1  X   Y  ln   T2  

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m

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 AR  c  ln   E 

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Finally, the equation (6) and (7) can be expressed by the following form.

and

Z

and

Z

1 T

1 T

(for n≠1)

(8)

(for n=1)

(9)

E R

(10) (11)

11


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Y=mZ+c

(12)

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For a given heating rate, by plotting Y versus Z, a good linear relationship will be

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displayed, and the values of E and A can be calculated by the corresponding slope (m) and

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intercept (c) of the fitting straight line, respectively.

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3. RESULTS AND DISCUSSION

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3.1 TG and DTG analysis

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Figure 2 shows the characteristics of biomass devolatilization and the subsequent

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in/ex-situ char gasification under different atmospheres and heating rates. For each

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experiment, the TG/DTG curves can be divided into the biomass drying stage below 473 K,

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the biomass devolatilization stage from 473 K to about 973 K, and the char gasification stage

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from about 973 K to 1273 K. To see more clearly, table 1 summarized the typical

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temperatures from each TG/DTG curve, including the temperatures with the maximum mass

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loss in the stages of biomass devolatilization and char gasification (TBD-max, TCG-max), and the

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initial temperature in the char gasification stage (TCG-i). Here, it is important to point out that

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the ending temperature of biomass devolatilization and the initial temperature of char

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gasification was considered as the same value for a given experiment, which was much higher

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than the TBD-max.

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For a specific reaction atmosphere adopted, with the increase of heating rate from 5

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K/min to 40 K/min, the curves of TG and TDG tended rightward shift, leading to the increase

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of typical reaction temperature. Taking the atmosphere of N2+steam (10%) as an example,

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compared to the values of TBD-max, TCG-i, and TCG-max at the heating rate of 5 K/min, the

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corresponding temperatures at the heating rate of 40 K/min increased from 624 K to 659 K, 12


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900 K to 1010 K, and 1152 K to 1263 K, respectively. Generally, this phenomenon was

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defined as the delayed effect of heating rate, which was much related to the limitations of heat

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and mass transfer between the temperatures of the heating furnace and experimental sample

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[32]. At the lower heating rate, the thermal energy can effectively transfer into the biomass

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sample instantly and heat it sufficiently. However, with the increase of heating rate, the

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heating time became shorter, leading to the inefficient heat transfer between the electrical

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furnace and sample surface [33, 34]. Moreover, the poor property of thermal conductivity in

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biomass material also caused strong temperature gradients between char particles [35].

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For the adopted four kinds of atmosphere in the biomass devolatilization stage, namely,

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both BD and CG in N2+steam (10 %), both BD and CG in N2+CO2 (10 %), BD in N2 and CG

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in N2+steam (10 %), and both BD and CG in N2+CO2 (50 %), by varying the heating rate

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from 5 K/min to 40 K/min, the corresponding TBD-max changed in the range of 624-659 K,

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620-654 K, 626-657 K, and 625-659 K, respectively, having the maximum increment of 35 K.

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While in the char gasification stage, the corresponding TCG-max varied in the range of

273

1152-1263 K, 1141-1243 K, 1147-1251 K, and 1139-1233 K, respectively, having the

274

maximum increment of 111 K. So, compared to the biomass devolatilization stage, the

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delayed influence on char gasification stage was much more distinct. Perhaps, this was

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closely related to the strong endothermic reaction and large energy requirement of char

277

gasification [36]. The results also showed that, for the single process of biomass

278

devolatilization or char gasification, the effect of reaction atmosphere on the TBD-max or TCG-max

279

was relatively weak. Perhaps the limited difference can be attributed to the sample

280

heterogeneity. Interestingly, for the initial temperature of char gasification (TCG-i), the impact 13


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

281

of reaction atmosphere seemed stronger. For example, at the heating rate of 5 K/min, for the

282

chars from the four kinds of atmosphere adopted, the values of TCG-i changed in the range of

283

900-933 K, indicating the different char activity for the gasification reaction.

284

3.2 Reaction behavior analysis

285

Figure 3 illustrated the variation of conversion (X) with reaction temperature for biomass

286

devolatilization and char gasification under different heating rates and reaction atmospheres.

287

In each atmosphere, regardless of biomass devolatilization and char gasification, with the

288

increase of heating rate, the higher reaction temperature was required for reaching a given

289

conversion. For example, in the atmosphere of N2+CO2 (10%), for the heating rate of 5 K/min

290

and 40 K/min, as shown in Fig.3 (a-1) and (b-1), the reaction temperatures reaching the

291

conversion of 50 % were 606 K and 642 K in the biomass devolatilization, respectively; while

292

they were 1106 K and 1199 K in the char gasification, respectively. In other atmospheres, the

293

corresponding reaction temperatures can be seen in Fig. S1 from the supporting document. To

294

see more clearly, Fig.3 (a-2) and (b-2) compared the atmosphere effect on biomass

295

devolatilization and char gasification at the same heating rate of 30 K/min. For the stage of

296

biomass devolatilization, the four curves of X versus T were nearly overlapped. Perhaps the

297

little difference was much related to the sample heterogeneity and the thermos-physical

298

properties of the adopted gas components, demonstrating the weak influence of the reaction

299

atmosphere on the volatiles release during the biomass devolatilization, even for the CO2

300

content of 50 %. So, the experiments with a higher content of CO2/steam were not conducted

301

in this study. However, at the stage of char gasification, for reaching the same conversion

302

below 50 %, the required gasification temperature followed the order of N2+steam-1 ＜ 14


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N2+CO2 (50%) ＜ N2+steam-2 ＜ N2+CO2 (10%). Although the in/ex-situ chars underwent

304

very similar devolatilization behavior in the release of volatiles, their gasification behaviors

305

were much different, even in the same gasification atmosphere of N2+steam (10%).

306

Taking the heating rate of 20 K/min and 40 K/min as two examples, Figure 4 further

307

compares the reaction rate in the biomass devolatilization and char gasification. For the stage

308

of biomass devolatilization, regardless of 20 K/min and 40 K/min, there was not a visible

309

difference in items of curve shape and the value of reaction rate. The maximum reaction rate

310

appeared in the conversion range of 60 % - 80 %, in which the reaction rate in the

311

atmospheres of N2 and N2+CO2 (10%) were just a little higher than that in the atmospheres of

312

N2+steam (10 %) and N2+CO2 (50%), with the similar values of 0.257 min-1, 0.258 min-1,

313

0.249 min-1, and 0.242 min-1, respectively. However, for the stage of char gasification, both of

314

the curve shape and value of reaction rate were much different. In the atmosphere of N2+CO2,

315

with the increase of CO2 content, the reaction rate had a sharp rise in the conversion range of

316

40% - 90%. Compared to the atmospheres of N2+CO2 (10%, 50%), the maximum gasification

317

rate was much higher in the steam-containing atmosphere, and the corresponding conversion

318

lay in the range of 80 % - 90 % instead of 60 % - 80 %. For the N2+steam atmosphere, the

319

in-situ char showed higher gasification rate than the ex-situ char in the range of 10 % - 80 %.

320

For example, for the heating rate of 20 K/min, the reaction rates of in/ex-suit chars at the

321

conversion of 50 % were about 0.135 min-1 and 0.125 min-1 respectively, with a ratio of 1.08;

322

while at the conversion of 90 %, they were about 0.25 min-1 and 0.22 min-1 respectively, with

323

a ratio of 1.14. For the heating rate of 40 K/min, the ratios at the conversions of 50 % and 90

324

% , were about 1.15 and 1.20, respectively. 15


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

325

Table 2 displayed the results of in-situ and ex-situ char gasification in literature. For

326

most of the coal char adopted, one can see that the differences in the reaction rate between

327

in-situ char and ex-situ char were very distinct, displaying a ratio above 1.4. Compared to the

328

results of coal char in literature, the difference in the gasification rate of in/ex-situ chars from

329

biomass sample in this study was relatively lower, with a maximum ratio below 1.20. A

330

possible explanation was the high specific surface area of biomass char than that of coal char

331

[38]. For example, in literature [21], the total specific surface areas of in situ coal char and

332

ex-situ coal char were 40.48 m2/g and 34.63 m2/g, respectively. While in this study, both of

333

them were about 400 m2/g. The details will be discussed in the following section of 3.3.

334

3.3 Property of char from the devolatilization stage

335

Although the reaction atmosphere did not have a visible effect on the release of volatiles

336

from the devolatilization stage, perhaps it was much different in the property of the produced

337

char. Figure 5 further showed the SEM images of biomass char from the different

338

devolatilization stage. All the char samples presented the typical round tubular structure. By

339

analyzing the four kinds of char sample, one can found that the porous structure of char from

340

the atmosphere of N2+CO2 (10%) was the least developed, while that from the atmosphere of

341

N2+CO2 (50 %) and N2+steam (10%) had much border and deeper cavities, especially for the

342

atmosphere with the CO2 content of 50 %.

343

Table 3 displayed the element compositions of the corresponding char sample in figure 5

344

tested by EDS. Compared to the CO2-containing atmosphere, the char samples produced in

345

the atmosphere of N2 and N2+steam (10%) had higher content of O but lower content of C and

346

K. This indicated the promoting effect of CO2 atmosphere on the decomposition of 16


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Energy & Fuels

347

oxygen-containing functional groups and its suppressing effect on the release of K. Moreover,

348

in the inert atmosphere of N2, the element content of Mg, Na, and Ca was higher than that in

349

the oxidative atmospheres. As we know, for char gasification reaction, the alkali metal oxides,

350

especially in K2O, always display good catalytic activity.

351

Table 4 listed the porous property of char samples from the devolatilization stages.

352

Compared to the inert atmosphere, there were different change tendency in the tested active

353

atmospheres. For example, in the atmosphere of N2 + CO2 (10%), both the specific surface

354

area of micro pore and meso pore decreased obviously. However, by increasing the CO2

355

content to 50%, one can see that the specific surface area of micro pore increased sharply,

356

while the change of meso pore was similar. In the atmosphere of N2+steam (10%), the

357

specific surface area of micro pore increased distinctly, while that of meso pore decreased.

358

For the atmospheres of N2 and N2+CO2 (50%), the char samples had the similar value of the

359

specific surface area in meso pore, while for the atmosphere of N2+CO2 (10%) and N2+steam

360

(10 %), they were also similar in meso pore. Although both CO2 (50%) and steam can

361

promote the increase of the total specific surface area, for the former, the main increase was

362

attributed to the new generation of micro pore; while for the latter, perhaps it mainly came

363

from the balance of micro pore generation and meso pore extinction due to the merging of

364

adjacent pore structure. The different pore structure in char samples will inevitably lead to the

365

difference in their gasification reactivity.

366

Figure 6 illustrates the distributions of functional group on the surface of the char

367

samples from the devolatilization stages. Generally, the main functional groups in char

368

included: (1) C-H bond in aromatics between 660 and 1011 cm-1, (2) -OH bond in-plane 17


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

369

deformation between 1011 and 1300 cm-1, (3) -CHx bond out of aromatic plane deformation

370

between 1300 and 1465 cm-1, (4) C=C bond in aromatics between 1500 and 1600 cm-1, (5)

371

aliphatic C-H between 2850 and 2920 cm-1, (6) -OH bond of stretching vibrations between

372

3300 and 3650 cm-1, and (7) C=O bond stretching vibration between 1600 and 1700 cm-1 [39,

373

40]. Compared to the inert atmosphere (N2), the char from CO2 atmosphere had lower

374

adsorption intensity of functional group of (1), (2) and (6), while it was similar in the steam

375

atmosphere. For the functional group (7), it was only present in the char sample produced in

376

the atmosphere of N2+steam. While for the functional group (4), it had the highest intensity in

377

the char generated in N2, indicating the strong aromatic structure.

378

All the analyses mentioned above further demonstrated that although reaction

379

atmospheres had weak effect on the release behavior of volatile matter during the

380

devolatilization stage, it indeed influenced the char property. This will further affect the

381

gasification behavior of not only the in-situ char, but also the ex-situ char.

382

3.4 Reaction order and kinetics

383

According to the equation (6) and (7), for the suitable reaction order n, by plotting Y

384

versus Z, it will display a good linear relationship. By taking the heating rate of 10 K/min in

385

the N2+steam atmosphere as an example, figure 7 illustrated all the curves of Y versus Z at the

386

given reaction order for biomass devolatilization and char gasification. By linear fitting of all

387

the data obtained at the different reaction rates, the correlation coefficients (R2) of these

388

curves were calculated, as shown in Table S1 from the supporting document. It was clear that,

389

for the biomass devolatilization and char gasification, the experimental results presented a

390

good linear relationship with high R2 of 0.9948 and 0.9917 at the reaction order of 1 and 0.67, 18


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Energy & Fuels

391

respectively. Actually, for the other heating rates and atmospheres adopted, similar results can

392

also be obtained, indicating the reliability of the reaction order for 1 and 0.67. The results also

393

verified that the volume reaction model (VRM) and shrinking core reaction model (SCRM)

394

were suitable to describe the behaviors of biomass devolatilization and char gasification,

395

respectively, as shown in the equation (13) and (14) [41],

396 397 398

dX  kv  1  X  dt 2 dX  ks  1  X  3 dt

(13) (14)

where kv and ks are the volumetric and surface reaction rate constant.

399

Figure 8 correlates Y versus Z according to the Coats-Redfern method for biomass

400

devolatilization and char gasification. The corresponding data were displayed in Fig. 3 and

401

Fig. 4. The fitting curves for different heating rates showed good linearity, with all the

402

correlation coefficient above 0.98, further justifying the feasibility of reaction models adopted.

403

Moreover, under each experimental atmosphere, with the increase of heating rate, the curves

404

nearly paralleled with the similar slopes, not only indicating the weak influence of heating

405

rate on reaction kinetics, but also further validating the feasibility of reaction models.

406

According to the slope and intercept of the linear fitting curve in Fig.8 (a, 1-4), the

407

values of E and A for each heating rate in the stage of biomass devolatilization can be

408

calculated, as shown in Table 5. One can see that, for each experimental atmosphere and

409

heating rate, the values of E were much similar, with a fluctuation in the range of ± 4

410

kJ/mol around the average value, showing good reliability. For the same atmosphere of

411

N2+CO2 but with different CO2 content (10 %, 50 %), it found that the values of E were

412

consistent with each other. For the four kinds of atmosphere, the values of E in N2 and 19


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

413

N2+steam atmosphere were a little higher than those in N2+CO2 (10 %, 50%), but were not

414

very obvious, just having a variation from 82.3 kJ/mol to 85.6 kJ/mol. This phenomenon was

415

agreed with the results in the literature [42, 43]. So, it was reasonable to consider that the

416

adopted inert and reactive atmosphere did not have an influence on the reaction kinetics of

417

volatile matter release during the biomass devolatilization, which was just a thermal-driving

418

process. Although the existing researches have demonstrated that atmosphere can influence

419

the pyrolysis behavior, product distribution, and gas components in a bench reactor, such as,

420

fluidized bed, fixed bed, and rotary kiln, from the viewpoint of reaction kinetics, this

421

influence was very weak because of short residence time in TGA and the inhibition of

422

interaction among pyrolytic products.

423

According to the approach as mentioned above, the gasification kinetics of in/ex-situ

424

char in different atmospheres and heating rates can be obtained, as shown in Table 6. At a

425

given atmosphere, the values of E for char gasification had a much more significant

426

fluctuation than that in biomass devolatilization, especially for the N2+steam-1, varying from

427

121.8 kJ/mol to 157.7 kJ/mol. Compared to the ex-situ char produced in N2 atmosphere,

428

in-situ char had a much smaller average value of E and higher average value of A. Meanwhile,

429

for the four kinds of reaction atmosphere adopted, the average values of E for the char

430

gasification were ranked as N2+CO2 (50 %) ＞ N2+CO2 (10 %) ＞ N2+steam-2 (10 %) ＞

431

N2+steam-1 (10 %). Generally, activation energy is defined as the minimum energy barrier to

432

overcome for the initiation of a chemical reaction. A smaller value of E always means higher

433

reactivity of char, lower gasification temperature, and shorter reaction time. Furtherly,

434

compared to N2+steam-2 (10%), the better gasification behavior of in-situ char from 20


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Energy & Fuels

435

N2+steam-1 (10%) was much related to the more developed micro structure, higher content of

436

K, more abundant oxygen-containing group, and poorer aromatic structure.

437

Table 7 further summarized the experimental condition and reaction kinetics in the

438

literature on biomass devolatilization and char gasification adopting a similar feedstock.

439

From these results, one can see that the values of E for biomass devolatilization, char

440

gasification with CO2 and steam were in the ranges of 69.0 kJ/mol-94.0 kJ/mol, 151.0 kJ/mol

441

-199.0 kJ/mol, and 136.0 kJ/mol-139.0 kJ/mol, respectively. In this study, the corresponding

442

experimental results showed good comparability with those in literature, validating the

443

reliability of the experimental method and results.

444

4. CONCLUSIONS

445

The non-isothermal reaction characteristics and kinetics of biomass devolatilization and

446

in-situ char gasification were investigated by TGA under different atmospheres and heating

447

rates. The volume reaction model and shrinking core reaction model were used to describe the

448

reaction behavior of biomass devolatilization and char gasification, respectively, and the

449

approach of Coats and Redfern was adopted to calculate the reaction kinetics. For biomass

450

devolatilization, although the adopted atmospheres, regardless of inert (N2) and reactive gas

451

(N2+CO2 (10%), N2+CO2 (50%) and N2+steam (10%)), did not have obvious influence on

452

TG-DTG curve, reaction rate versus conversion, and the activation energy of volatiles release,

453

even at the CO2 content of 50 %, it had obvious effect on the produced char porous structure,

454

distribution of functional group, and content of alkali metal and alkaline earth metal. The

455

values of E for volatiles release were in the range of 82.3 kJ/mol - 85.6 kJ/mol. Compared to

456

the biomass devolatilization, the delayed effect of the heating rate in char gasification was 21


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

457

much more obvious, and the reaction atmospheres also displayed a stronger effect on the

458

gasification behavior and kinetic parameters. The corresponding maximum reaction rate in the

459

steam-containing atmosphere lay in the conversion range of 80 % - 90 % instead of 60 % - 80

460

% like in the CO2-containing atmosphere. The average values of E in N2+CO2 (50 %),

461

N2+CO2 (10 %), N2+steam-1 (10 %) and N2+steam-2 (10 %) were about 182.7 kJ/mol, 177.2

462

kJ/mol, 144.9 kJ/mol, and 157.5 kJ/mol, respectively. The value of in-situ char gasification in

463

steam was much lower than that in the CO2 atmosphere. Compared to the ex-situ char

464

gasification, the in-situ char gasification in the N2+steam (10%) had higher reaction rate and

465

the lower average value of E. Moreover, it also finds that these differences between in/ex-situ

466

biomass chars were much smaller than that from coal char in literature.

467

ACKNOWLEDGEMENT

468

The authors gratefully acknowledge the financial support provided by the JSPS (Japan

469

Society for the Promotion of Science) International Fellowships for Research in Japan.

470

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(30) El-Sayed, S. A.; Mostafa, M. E. Pyrolysis characteristics and kinetic parameters

552

determination of biomass fuel powders by differential thermal gravimetric analysis

553

(TGA/DTG). Energy Convers. Manage. 2014, 85, 165-172.

554

(31) Bach, Q. V.; Trinh, T. N.; Tran, K. Q.; Thi, N. B. D. Pyrolysis characteristics and

555

kinetics of biomass torrefied in various atmospheres. Energy Convers. Manage. 2017, 141,

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72-78.

557

(32) Montiano, M. G.; Díaz-Faes, E.; Barriocanal, C. Kinetics of co-pyrolysis of sawdust,

558

coal and tar. Bioresour. Technol. 2016, 205, 222-239.

559

(33) Gao, W. H.; Chen, K. F.; Zeng, J. S.; Xu, J.; Wang, B. Thermal pyrolysis characteristics

560

of macroalgae Cladophora glomerata. Bioresour. Technol. 2017, 243, 212-217.

561

(34) Damartzis, T.; Vamvuka, D.; Sfakiotakis, S.; Zabaniotou, A. Thermal degradation

562

studies and kinetic modeling of cardoon (Cynara cardunculus) pyrolysis using

563

thermogravimetric analysis (TGA). Bioresour. Technol. 2011, 102, 6230-6238.

564

(35) Ma, Z. Q.; Chen, D. Y.; Gu, J.; Bao, B. F.; Zhang, Q. S. Determination of pyrolysis

565

characteristics and kinetics of palm kernel shell using TGA-FTIR and model-free integral 26


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methods. Energ. Convers. Manage. 2015, 89, 251-259.

567

(36) Wang, F.; Zeng, X.; Wang, Y. G.; Su, H.; Yu, J.; Xu, G. W. Non-isothermal coal char

568

gasification with CO2 in a micro fluidized bed reaction analyzer and a thermogravimetric

569

analyzer. Fuel 2016, 164, 403-409.

570

(37) Guo, Y. Z.; Zhao, Y. J.; Gao, D. Y.; Liu, P.; Meng, S.; Sun, S. Z. Kinetics of steam

571

gasification of in-situ chars in a micro fluidized bed. Int. J. Hydrogen Energy 2016, 41,

572

15187-15198.

573

(38) Zhou, Q. Q.; Zarei A.; Girolamo, A. D.; Yan, Y. X.; Zhang, L.A. Catalytic performance

574

of scrap tyre char for the upgrading of eucalyptus pyrolysis derived bio-oil via cracking and

575

deoxygenation. J. Anal. Appl. Pyrol. 2019, 139, 167-176.

576

(39) Jian, J.; Lu, Z. M.; Yao, S. C.; Li, Y. Q.; Liu, Z. F.; Lang, B.; Chen, Z. B. Effects of

577

thermal conditions on char yield and char reactivity of woody biomass in stepwise pyrolysis.

578

J. Anal. Appl. Pyrol. 2019, 138, 211-217.

579

(40) Xu, K.; Hu, S.; Su, S.; Xu, C. F.; Sun, L. S.; Shuai, C.; Jiang, L.; Xiang, J. Study on char

580

surface active sites and their relationship to gasification reactivity. Energy Fuels 2013, 27,

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118-115.

582

(41) White, J. E.; Catallo, W. J.; Legendre, B. L. M. Biomass pyrolysis kinetics: A

583

comparative critical review with relevant agricultural residue case studies. J. Anal. Appl.

584

Pyrol. 2011, 91, 1-33.

585

(42) Wang, Z. H.; Ma, X. Q.; Yao, Z. L.; Yu, Q. S.; Wang, Z.; Lin, Y. S. Study of the

586

pyrolysis of municipal sludge in N2/CO2 atmosphere. Appl. Therm. Eng. 2018, 128, 662-671.

587

(43) Senneca, O.; Cerciello, F.; Heuer, S.; Ammendola, P. Slow pyrolysis of walnut shells in 27


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588

nitrogen and carbon dioxide. Fuel 2018, 225, 419-425.

589

(44) Aqsha, A.; Mahinpey, N.; Mani, T.; Salak, F.; Murugan, P. Study of sawdust pyrolysis

590

and its devolatilisation kinetics. Can. J. Chem. Eng. 2011, 89, 1141-1147.

591

(45) Babu, B. V. Biomass pyrolysis: a state-of the-art review. Biofuel Bioprod. Bior. 2008, 2,

592

393-414.

593

(46) Samolada, M. C.; Vasalos, I. A. A kinetic approach to the flash pyrolysis of biomass in a

594

fluidized bed reactor. Fuel 1991, 70, 883-887.

595

(47) Nunn, T. R.; Howard, J. B.; Longwell, J. P.; Peters, W. A. Product composition and

596

kinetics in the rapid pyrolysis of sweet gum hardwood. Ind. Eng. Chem. Process Des. Dev.

597

1985, 24, 836-844.

598

(48) Reschmeier, R.; Karl, J. Experimental study of wood char gasification kinetics in

599

fluidized beds. Biomass Bioenerg. 2016, 85, 288-299.

600

(49) Fermoso, J.; Stevanov, C.; Moghtaderi, B.; Arias, B.; Pevida, C.; Plaza, M. G.; Rubiera,

601

F.; Pis, J. J. High-pressure gasification reactivity of biomass chars produced at different

602

temperatures. J. Anal. Appl. Pyrol. 2009, 85, 287-293.

603

(50) Sircar, I.; Sane, A.; Wang, W. C.; Gore, J. P. Experimental and modeling study of

604

pinewood char gasification with CO2. Fuel 2014, 119, 38-46.

605

(51) Matsumoto, K.; Takeno, K.; Ichinose, T.; Ogi, T.; Akanishi, M. Gasification reaction

606

kinetics on biomass char obtained as a by-product of gasification in an entrained-flow gasifier

607

with steam and oxygen at 900-1000 °C. Fuel 2009, 88, 519-527.

608

(52) Zhang, Y.; Ashizawaa, S. M.; Miura, K. Proposal of a semi-empirical kinetic model to

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reconcile with gasification reactivity profiles of biomass chars. Fuel 2008, 87, 475-481. 28


Page 28 of 43

Page 29 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

610

Table 1 Typical temperatures in BD and CG at different atmospheres and heating rates Heating rate (K/min) Temperature [K] 5

10

20

30

40

case 1

624

634

649

654

659

case 2

620

635

646

651

654

case 3

626

634

647

652

657

case 4

625

636

649

649

659

case 1

900

937

966

973

1010

case 2

931

961

982

997

1017

case 3

918

943

971

984

1015

case 4

933

966

987

1001

1019

case 1

1152

1182

1215

1225

1263

case 2

1141

1167

1202

1220

1243

case 3

1147

1178

1210

1227

1251

case 4

1139

1162

1192

1210

1233

TBD-max

TCG-i

TCG-max

611 612

※

case 1: both BD and CG in N2+steam (10 %); case 2: both BD and CG in N2+CO2 (10 %); case 3: BD in N2, CG in N2+steam (10 %); case 4: both BD and CG in N2+CO2 (50 %);

613 614 615 616 617 618 619 620 29


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

621

Page 30 of 43

Table 2 Comparison of gasification reaction rate between in/ex-situ chars in literature Number

Feedstock

Analyzer

Atmosphere

Temperature

Bituminous coal

Rin/ex situ

Reference

3.44a

Subbituminous 1

TGA

Steam

1273 K

3.85a

[19]

coal Lignite

5.78 a

Bituminous

Down

coal

furnace

firing

2

CO2

1627 K

Yima Bituminous coal

4.54a

[20]

1.4b or 2.7 a

Fluidized bed

3

CO2

1073 K

reactor

[21] 2.00b or

Xilinhaote lignite 3.0 a Shenhua 2.0 b bituminous coal

Fluidized bed

Zhundong

reactor

4

Steam

1073 K

[37] 1.8 b

bituminous coal This 5

Black pine

TGA

Steam

1250 K

1.2b study

622 623

※

a

ex-situ 1: the pre-prepared cool char was used in gasification experiment; was directly used for gasification without cooling and annealing.

624 625 626 627 628 629 30


b

ex-situ 2: char

Page 31 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

630

Table 3 EDS analysis of char sample from the different devolatilization stages Char 1

Char 2

Char 3

Char 4

C

87.06

92.39

95.23

89.97

O

8.98

2.86

2.02

8.69

Na

0.15

0.1

0

0.04

Mg

0.9

0

0

0.04

Si

0.75

0.71

0.78

0.69

S

0.88

1.74

0.53

0

K

0

1.34

1.05

0.18

Ca

1.28

0.86

0.39

0.39

631 632 633 634 635 636 637 638 639 640 641 642 643 644 31


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

645

Page 32 of 43

Table 4 Pore property of char samples from the different devolatilization stage Specific surface area

Pore volume Da*

Char

Atotal

Amicro

Ameso

Vtotal

Vmicro

Vmeso

[m2/s]

[m2/s]

[m2/s]

[mL/g]

[mL/g]

[mL/g]

Char 1

402.4

319.3

83.1

0.29

0.16

0.13

3.824

Char 2

346.2

286.7

59.5

0.25

0.14

0.11

3.828

Char 3

500.0

414.6

85.4

0.33

0.19

0.14

3.822

Char 4

420.8

357.8

63.0

0.27

0.16

0.11

3.821

[nm]

646

Da*: average pore diameter;

647 648 649 650 651 652 653 654 655 656 657 658 659 32


Page 33 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

660

Table 5 Kinetics of biomass devolatilization in different atmospheres Biomass devolatilization in different atmospheres

β

N2+steam

N2+CO2

(10%)

(10%)

N2

N2+CO2 (50 %)

[K/min] E

A

E

A

E

A

E

A

[kJ/mol]

[1/s]

[kJ/mol]

[1/s]

[kJ/mol]

[1/s]

[kJ/mol]

[1/s]

5

82.4

7.4E+7

83.8

9.8E+7

87.1

2.1E+8

86.0

1.5E+8

10

85.6

2.1E+8

84.7

1.8E+8

85.8

2.2E+8

79.2

5.0E+7

20

86.2

3.2E+8

81.8

1.8E+8

85.3

2.8E+8

84.7

2.3E+8

30

83.5

2.5E+8

80.6

1.3E+8

84.5

2.9E+8

79.2

9.3E+7

40

85.2

4.0E+8

81.4

1.8E+8

85.1

3.8E+8

82.6

2.2E+8

84.6

2.5E+8

82.5

1.5E+8

85.6

2.8E+8

82.3

1.5E+8

Averag e 661 662 663 664 665 666 667 668 669 670 671 672 33


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 43

673 674

Table 6 Kinetics of char gasification in different atmospheres Char gasification in different atmospheres

β

N2+steam-1

N2+CO2

N2+steam-2

N2+CO2

(10%)

(10%)

(10%)

(50 %)

[K/min] E

A

E

A

E

A

E

A

[kJ/mol]

[1/s]

[kJ/mol]

[1/s]

[kJ/mol]

[1/s]

[kJ/mol]

[1/s]

5

152.8

8.0E+8

177.8

8.5E+8

159.6

7.3 +E7

171.3

1.1E+8

10

153.9

2.0E+9

184.0

2.0E+9

156.7

3.4 E+7

178.4

1.2E+8

20

157.7

6.8E+8

173.6

6.8E+8

155.3

5.9 E+7

183.6

2.4E+8

30

138.5

8.9E+8

174.5

8.9E+8

161.3

2.2 E+8

195.2

3.4E+9

40

121.8

1.1E+9

176.1

1.7E+9

154.8

1.2 E+8

185.2

2.3E+9

Average

144.9

1.1E+9

177.2

1.2E+9

157.5

7.7 E+7

182.7

1.2E+9

675 676 677 678 679 680 681 682 683 684 685 34


Page 35 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

686 687

Table 7 Kinetics of biomass devolatilization and char gasification in literature

Item

Feedstock

Analyzer

Sawdust

TGA

Pine

TGA

BD

CG

T

E

A

[K]

[kJ/mol]

[1/s]

473-673

79.5

1.1E +8

[44]

798-1133

83.0

7.0E+4

[45]

Gas

Reference

N2 Fire wood

Fluid bed

673-773

94.0

1.9E+4

[46]

Hardwood

Screen heater

600-1400

69.0

4.5E+4

[47]

Spruce/ pine char

Fluidized 1073-1123

151.0

7.0E+3

[48]

(80%/20%)

bed

Slash pine char

TGA

1273-1673

170.5-198.5

8.8E+7

[49]

Pinewood char

Fixed bed

1000-1170

173.0-199.0

CO2

(1.6-12.1) [50] E+3

CG

Spruce/ pine char

Fluidized

(80%/20%)

bed

Japanese

cedar

Drop

char

furnace

Beech char

TGA

tube

H2O

1073 -1123

138.0

3.7E+3

[48]

1173-1473

136.0

9.9E+4

[51]

1073-1223

139.0

2.6E+4

[52]

688 689 690 691 692 693 694 35


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(a)

695

696

697 698

Fig. 1 Schematic of the adopted TG analyzer and typical experiment for biomass devolatilization and

699

in/ex-situ char gasification

700 701 702 703 36


Page 36 of 43

Page 37 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

704

705 706

Fig. 2 TG/DTG curves of biomass devolatilization and in/ex-situ char gasification in different

707

atmospheres and heating rates

708 709 710 711 712 713 714 37


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

715

716 717

Fig. 3 Relationship of X-T for BD and CG in different atmospheres and heating rates

718 719 720 721 722 723 724 725 726 727 38


Page 38 of 43

Page 39 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

728

729 730

Fig. 4 Relationship of R-X for BD and CG in different atmospheres and heating rates

731 732 733 734 735 736 737 738 739 740 39


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 40 of 43

741 742

(a)

(b)

(c)

(d)

743 744 745

Fig. 5 SEM photograph of biomass char prepared in diﬀerent devolatilization stages

746

(a) Char 1 produced in N2 with the final temperature of 937 K and heating rate of 10K/min; (b) Char 2

747

produced in N2+CO2 (10%) with the final temperature of 961 K and heating rate of 10 K/min; (c) Char

748

3 produced in N2+CO2 (50%) with the final temperature of 943 K and heating rate of 10 K/min; (d)

749

Char 4 produced in N2+steam (10%) with the final temperature of 966 K and heating rate of 10 K/min;

750 751 752 753 754 755 40


Page 41 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

756 757

Fig.6 FTIR analysis of char sample from the devolatilization stages

758 759 760 761 762 763 764 765 766 767 768 769 770 41


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

771

772 773

Fig. 7 Curves of Y versus Z at different values of n for BD and CG

774 775 776 777 778 779 780 781 782 783 784 42


Page 42 of 43

Page 43 of 43 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

785

786

787

788 789

Fig. 8 Correlation of Y versus Z for BD and CG under different atmospheres and heating rates

790 791 792 43


Characteristics of biomass devolatilization and in-situ char

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