Characterization and Kinetics for Co-Pyrolysis of Zhundong Lignite

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Characterization and Kinetics for Co-pyrolysis of Zhundong Lignite and Pine Sawdust in a Micro Fluidized Bed Feiqiang Guo, Yuan Liu, Yan Wang, Xiaolei Li, Tiantao Li, and Chenglong Guo Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b01115 • Publication Date (Web): 11 Jul 2017 Downloaded from http://pubs.acs.org on July 15, 2017

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Characterization and Kinetics for Co-pyrolysis of Zhundong Lignite and Pine Sawdust

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in a Micro Fluidized Bed

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Feiqiang Guo*,†, Yuan Liu†, Yan Wang†, Xiaolei Li†, Tiantao Li† and Chenglong Guo†

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†

School of Electrical and Power Engineering, China University of Mining and Technology,

Xuzhou 221116, People’s Republic of China

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* Corresponding author. Tel.: +86 516 83592000. E-mail address: [email protected].

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Abstract

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Co-pyrolysis behaviors of Zhundong lignite and pine sawdust under isothermal conditions were

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investigated with a focus on the gas releasing characteristics using a micro fluidized bed reactor. The kinetic

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parameters were deduced by measuring time dependent composition changes of evolved gases from the

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pyrolysis reactions, employing the universal integral method. The gas releasing intensity and conversion rate of

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pine sawdust was quite different from Zhundong lignite due to the difference in chemical structure and element

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content. The effect of blending ratio on the conversion degree varied significantly for different gas components,

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H2, CO and CH4 conversion became quicker with increasing biomass ratio, while the variation of CO2 was not

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obvious. The generally used mechanism models for gas-solid reactions were examined as ways to interpret the

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experimental data and reaction kinetics was deduced with respect to the formation of individual gas component.

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The obtained activation energies for the generation of H2, CO and CH4 was much lower from the pyrolysis of

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pine sawdust than Zhundong lignite, while CO2 was a little higher. The activation energies of the blending

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samples was not following a linear fashion with the changing of blending ratio, showing the interaction between

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Zhondong lignite and pine sawdust. Further comparing the measured and calculated activation energies,

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interaction was noted between these two fuels for generating the gas components, while the interaction differed

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for different gas component and different blending ratios.

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Key word: Co-pyrolysis; Micro-fluidized bed; Isothermal; Activation energy; Interaction

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1. Introduction

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Ever since the first discovery of huge coal reserves in east Junggar Basin (Zhundong), Xinjiang, China,

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efficient and clean utilization of Zhundong coal has drawn a lot of attention. Zhundong coal reserves are

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extensive enough to meet the coal consumption of China for the next one hundred years [1]. However,

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unacceptably severe slagging and fouling originating from the alkali metal vapors has plagued combustion of

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Zhundong coal in traditional boilers, limiting its utilization as clean fuel [2,3]. Pyrolysis/gasification of

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Zhundong coal is a promising alternative for producing clean gas products at temperatures well below the ash

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fusion temperatures to circumvent ash melting, thereby avoiding slagging and fouling [4].

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For pyrolysis/gasification technology to be an effective strategy, the low hydrogen to carbon ration (H/C) of

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coal presents a key barrier limiting the production of gas products [5]. Supplying hydrogen from other sources is

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necessary to render this approach feasible. Toward this end, co-utilization of Zhundong coal with hydrogen-rich

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organic matter such as biomass provides a strategy for more efficient pyrolysis/gasification of Zhundong coal.

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Compared with other materials, biomass is seen as a carbon-neutral fuel since its carbon is generated from

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atmospheric CO2 by photosynthesis [6], indicating that the use of biomass can also assist in mitigating net CO2

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emissions. Also, the production of biomass is enormous globally and can ensure that adequate supplies are

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available for co-utilization with coal. Thus, co-pyrolysis/gasification of coal and biomass is considered as a

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bridge between energy production based on fossil fuels and sustainable energy production based on renewable

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fuels [7, 8].

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As the initial step for co-thermochemical gasification, co-pyrolysis is a key process that influences the final

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product composition of the process. Comprehensive studies of the co-pyrolysis of biomass and coal, particularly

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for the high-alkali Zhundong coal, is necessary to understand the co-thermochemical process. Previous

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investigations have paid attention to the mechanism of synergistic effects for better conversion of biomass and

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coal blends. Generally, synergystic effects are expected for better conversion of biomass and coal. Some studies

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suggested that synergetic effects between biomass and coal are due to the high H/C in biomass which may act as

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a H2-donor in the co-pyrolysis process [9,10]. Haykiri-Acma and Yaman [11] found that the addition of biomass

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led to some increases in the volatilization rates of coals and the char yields revealed unexpected variations in

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case of low rank coals. Sonobe et al. [12] found that the biomass volatiles react with coal particles during the

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co-pyrolysis process. Li et al. [13] investigated the co-pyrolysis characteristics of biomass and bituminous coal,

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showing that increased biomass ratios led to larger relative deviations between experimental weight fractions and

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calculated ones. However, some other studies reported that a lack of significant synergetic effects occur in the

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coal and biomass blend reactions. Krerkkaiwan et al. [14] believed that the higher reactivity of coal/biomass

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blend could be related to the increased surface area and pore volume of chars from the blend as well as the

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influence of volatile K released from the biomass. Kastanaki et al. [15] also reported no significant interaction

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was detected in the solid phase between the components of the coal–biomass blends. Thus, further study is still

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needed to establish definitively whether co-pyrolysis of coal and biomass occurs in a synergistic manner to

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promote the overall conversion process, especially in the case of the Zhungdong coal.

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Previously co-pyrolysis characteristics and the interaction between biomass and coal was mainly studied

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using a variety of approaches that include fixed bed reactors, drop tube reactors, fluidized bed reactors, and

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specifically thermogravimetric analyzers (TGA) under non-isothermal conditions. Among them, TGA was

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shown to be a useful, simple and representative analyzer, which can continuously record the weight loss of the

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samples. However, the pyrolysis assays in TGA are accomplished via programmed heating (usually below

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100 ℃/min) for preset samples, and thus the decomposition of coal and biomass occurs at different stages during

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the co-pyrolysis process. However, under isothermal conditions, we can speculate that the interaction between

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biomass and coal should be different from TGA approaches when biomass and coal decompose nearly at the

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same time by proper selection of an optimum temperature. Recently, a micro fluidized bed reaction analyzer

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(MFBRA) has been successfully used for kinetics studies of gas-solid reactions, such as biomass pyrolysis [16,

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17], coal gasification [18, 19], NO reduction [20], tar cracking [21, 22], etc. Rapid heating and inhibition of

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external diffusion can be achieved by this micro fluidized reaction analyzer, making the obtained kinetics more

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closely related to the intrinsic chemical kinetics [23]. The use of the micro fluidized reaction analyzer allowed

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the determination of the kinetic parameters by on-line monitoring of the released gas product, which can be

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employed to study the co-pyrolysis of biomass and coal in a novel manner.

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With the aim of clarifying the co-pyrolysis mechanism for biomass amended Zhundong lignite, a new

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laboratory-scale fluidized bed experimental system was developed based on the MFBRA to investigate the gas

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release of Zhundong lignite, biomass and their blends under isothermal conditions. The generation trends for

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four major gas components (H2, CO, CO2 and CH4) were investigated by a continuously-recording mass

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spectrometer, and the kinetic parameters for forming these gas components were determined.

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2. Material and methods

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2.1. Sample preparation

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The biomass in this paper was pine sawdust (PS) from the surrounding areas of Xuzhou, Jiangsu province.

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The Zhundong lignite (ZL) in this study was mined from East Junggar Basin in Xinjiang province, which was

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received without any pretreatment. Both the biomass and coal samples have been crushed preciously and sieved

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to obtained a mean size between 150 to 250 µm. Biomass and coal were mixed in different weight ratios, with

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biomass mass ratios of 100%, 75%, 50%, 25% and 0% (wt.%, dry basis), respectively. Proximate and ultimate

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analyses of samples are summarized in Table 1. It can be seen that the H:C molar ratio for the ZL sample is

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relatively low compared to that of the PS. The higher H:C molar ratio of PS suggests that PS in the mixture may

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act as a hydrogen donor to the co-pyrolysis process, which may lead to the interaction between biomass and coal.

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The ZL sample has high ash content, suggesting that it has a higher content of metal-containing elements

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which may have a catalytic effect on the co-pyrolysis process. Accordingly, the elemental composition of ZL

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were detected by a Sequential X-ray Fluorescence Spectrometer (XRF-1800), and the relative content of these

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elements is listed in Table 2, which shows that ZL was rich in Fe, Ca, K and Mg which have be known to have a

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significant positive performance on biomass and coal pyrolysis.

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2.2. Apparatus and operational conditions

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The schematic diagram of the experiment system is illustrated in Figure 1. A micro fluidized bed reactor

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was employed to study the fast pyrolysis of the samples under isothermal conditions, and a mass spectrometer

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(AMETEK, American) was used for on-line gas detection. The fluidized bed reactor designed here is 20 mm in

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inner diameter and 150 mm in height and consists of two porous plates to separate it into three zones. The zone

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between the two porous plates of 40mm in height is the main reaction zone. The detailed depiction of the

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experimental system has been described in our previous study [16].

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Before each test, three grams of quartz sand (74-125 µm) was put into the middle zone between the two

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porous plates as the fluidized medium. The reactor was heated to a desired temperature and a stream of

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high-purity argon(99.999%) at 300 ml/min was used ensure the good fluidization. Then, 10 mg fuel sample was

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injected into the reactor by compressed gas and the product gases (H2, CO, CH4 and CO2) were measured by the

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mass spectrometer continuously. Each experiment was repeated three times to assure the reliability of the test

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results.

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2.3. Kinetics modeling

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During the experiments, the release characteristics of individual gas component with time under different

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reaction temperatures was measured by the mass spectrometer. As suggested in the literatures about the micro

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fluidized bed reaction analyzer[24, 25], the conversion of the gas component is defined according to the MS data

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measured, as shown in Eqs.(1)-(3) . te

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WRe =

qv × ∫ ϕ R × M R t0

(1)

22.4 t

qv × ∫ ϕ R × M R t0

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WRt =

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Wt x = Re = WR

(2)

22.4 t

∫ ϕ dt ∫ ϕ dt t0 te t0

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i

(3)

i

1 dWRt dx =− e dt WR dt

(4)

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Where, t0, t and te represent the different stages of the reactions at time 0, t and the end, respectively; φi

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represents the concentration of gas specie i, vol. %; and qv denotes the flow rate of the product gas from the

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reactor, L/min; 22.4 is the molar volume of gas at standard condition; x is the conversion degree of product gas.

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The reaction rate is generally expressed by the differential equation f(x) and the integral equation G(x), as shown in Eqs.(5) and (6).

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dx = k (T ) f ( x) dt

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G( x ) = ∫

x

0

(5)

dx = k (T )t f ( x)

(6)

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Where, f(x) is a function, the type of which depends on the reaction mechanism; k(T) is the reaction rate

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constant defined by the Arrhenius equation and it is a constant in isothermal process, which means it can be

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separated from f(x); G(x) is the integral reaction model.

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In order to characterize the reaction behavior, the generally used gas-solid reaction models were employed

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to fit the experimental data of co-pyrolysis from the micro fluidized bed reactor. The points of G(x) versus t at

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different reaction temperatures can be fitted to a straight line. Then, the activation energies and pre-exponential

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factor can be obtained according to Arrhenius Eq. (7):

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ln(k (T )) = −

Ea + ln( A) RT

(7)

Where, Ea is the apparent activation energy, kJ/mol; A denotes to the pre-exponential factor, 1/s; T refers to

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the temperature, K; R represents the gas constant, 8.314 J/(mol·K).

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3. Results and discussion

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3.1 Structure analysis of Zhundong lignite and pine sawdust

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The surface chemical structure of Zhundong lignite and pine sawdust was investigated with FTIR

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(VERTEX 80v). During the test, the sample was first crushed to below 75 um and then blended with KBr

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uniformly with a mass ratio of 1:100. After that, the prepared sample was compressed for FTIR investigation,

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and the major absorption bands obtained was shown in Figure 2. The characteristic bands at 3600-3000cm-1

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indicate the existence of hydrogen-bonded OH and N-H in the two fuels. The adsorption bands at 3420cm-1

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associated with the hydroxyl (O-H), showing that PS has a much higher intensity of hydroxyl groups than ZL,

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most likely due to the polysaccharides in the PS. The appearance of the bands in the range

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3000-2800cm-1originate from the stretching vibration of aliphatic carbon (C-H), and PS has a higher intensity.

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The bands at 1610 and 1032cm-1 for both ZL and PS indicate the presence of aromatic structures C=C and C=O.

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In comparison with ZL, more absorption bands were found in PS at 1510, 1368, 1316 and 1272cm-1,

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representing the existence of different carboxylic groups and ether bonds. Therefore, ZL and PS have some

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similarities in structure, while more aliphatic groups and hydroxyl exist in PS but ZL contains relatively more

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aromatic groups. Particularly, the groups related to hydrogen and oxygen differed significantly for these two

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samples, also indicating the difference in elemental composition and chemical structure. The difference between

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ZL and PS in structure leads to different chemical reactivity, which would affect the release of gas components in

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pyrolysis.

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3.1 Pyrolysis Characteristics of Zhundong lignite and Pine Sawdust

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For both coal and biomass materials, the most significant pyrolysis species are H2, CO2, CO and CH4, and

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releasing of these four gas species was detected continuously under 600-900 ℃ for all the samples. Figure 3

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shows the evolution of four main gas components with the pyrolysis temperature for pure Zhundong lignite and

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pine sawdust. As a result of the rapid pyrolysis in the micro fluidized bed reactor, the gas release was generally

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completed in around 15 seconds under isothermal conditions. Different gas components showed different

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evolution times, and the gas intensity differed significantly at different temperatures.

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CO was the dominant gaseous product for pine sawdust and coal in the temperature range of 600 to 900℃,

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while PS was shown to produce about two times as much CO as ZL. Similarly, the concentration of H2, CO2 and

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CH4 released from biomass was much higher compared with lignite, indicating that more gases are generated

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from biomass pyrolysis under the same conditions. The results illustrated the remarkable difference in the

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pyrolysis characteristics between the two kinds of fuels. The difference in the molecular structure of biomass and

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coal was generally used to explain this. The biomass mostly comprises cellulose, hemicellulose and lignin,

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which are linked together with relatively weak ether bonds (R-O-R) with bond energies of 380-420 kJ/mol [26].

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In comparison, the dense polycyclic aromatic hydrocarbons in coal are generally linked together by aromatic

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ring bonds (C=C), which is much stronger with bond energy of about 1000 kJ/mol [27]. Furthermore, it is

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important to note that the release intensity of H2 and CH4 from pine sawdust pyrolysis was much higher

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compared with coal under the same temperature, indicating that biomass could supply hydrogen for the coal

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decomposition, possibly leading to interaction during the co-pyrolysis [6].

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At 600 ℃, CO2 was the released first during the pyrolysis process, and in succession CO, CH4 and H2 are

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produced for all the samples at lower temperature. With the increased pyrolysis temperature, the gas release

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pattern changed, as illustrated in Figure 3. The release of H2 initiated earlier than CH4 at 900℃, and the

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concentration of H2 increased significantly as well. The intensity of CO, H2 and CH4 increased obviously when

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the temperature increased from 600 to 900℃, while the variation of CO2 was negligible. The results indicate that

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the these gas components were generated via different chemical routes and mechanics. During the pyrolysis of

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biomass, CO generally forms from the cracking of carbonyl (C-O-C) and carboxyl (C=O) groups as well as the

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secondary cracking of volatiles [28,29], all of which are favored at higher temperatures. The formation of H2 is

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generally attributed to radical polycondensation and dehydrogenation reactions and CH4 is generally released

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from -OCH3- and -CH2- [30] structures in coal and biomass, and all these reactions occur likely at high

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temperatures as well. In comparison, the release of CO2 is related to the cracking and reforming of C=O and the

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decarboxylation of -COOH functional groups [31]. The bond energy was relatively smaller for generating CO2,

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and therefore its release was insensitive to pyrolysis temperature.

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On the basis of Eq.(3), the conversion degree of the gas components as a function of time is illustrated in

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Figure 4. For both PS and ZL, the conversion trend of the four gas components showed that, as the pyrolysis

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temperature increased, the time for complete conversion became shorter for all the samples, indicating that the

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generation of the gas species was highly temperature dependent. However, the time for complete evolution of

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different gas species obviously differed, representing the different chemical routes for their formation. This

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higher reaction rate of gas release can be further confirmed from the relationship between reaction rate and

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conversion yield based on Eq. (4), as shown in Figure 5.

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Figure 5 shows that the reaction rate is higher with increasing temperature under the same conversion for all

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the gas components, indicating that the reaction accelerated at higher temperature, which is in good agreement

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with theory. Furthermore, the reaction rate also increased quickly at the beginning of the reactions and reached

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the maximum value at conversion below 0.2 under all experimental conditions. This rapid increase of reaction

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rate represented the rapid temperature rise period after the fuels were injected into the reactor. After the rapid

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increase, the reaction rate gradually slowed as the conversion increased. The results are greatly attributed to the

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evolution of the biomass and coal particles during the pyrolysis process under high temperatures. With

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increasing reaction time, the formation of gas components mainly depends on the condensation of the aromatic

201

nuclei into char, resulting in a decrease in the reaction rate [32].

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3.2 Pyrolysis Characteristics of The Blends

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Under the co-pyrolysis conditions, the conversion rate of the gas species was calculated to provide

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information regarding the effect of blending ratio on the pyrolysis behaviors. Figure 6 represents the conversion

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rate of the four gas components for the thermal decomposition of pine sawdust/ Zhundong lignite under various

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blending ratios at 800℃. Focusing on the conversion ratio of the four species, the conversion rate of H2, CH4 and

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CO increased gradually with the increased biomass ratio, while the conversion rate of CO2 showed minimal

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decrease. For the blended samples, the gradational trend for the conversion rate appeared to be inherited from the

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behavior of the parent materials. Pine sawdust is richer in hydrogen containing groups in comparison with ZL,

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including hydroxyl and aliphatic carbon (Figure 2), which would lead to a more rapid release of H2 and CH4 at

211

higher biomass blend ratios. However, the C=O and -COOH functional groups which associate with the

212

formation of CO2 are more prevalent in ZL, resulting in the decreased conversion rate of CO2 at higher biomass

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blend ratios. For CO, its formation originates from the carbonyl (C-O-C) and carboxyl (C=O) groups and ether

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bonds which also are more prevalent in various forms in PS. From an energy barrier perspective, since

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conversion rate of the gas species depends upon the energy barrier of the corresponding reactions, the variation

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of the conversion rate of the gas species can also be traced back to the activation energy of formation.

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Energy & Fuels

3.3 Analysis of Reaction Kinetics

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Considering the isothermal nature of the pyrolysis reactions in the micro fluidized bed, the kinetic

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parameters can be determined by fitting of G(x) with the reaction time t according to Eq. (6). Concerning the

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thermochemical reactions between gas and solid, there are twenty reaction models that are used commonly for

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heterogeneous solid-state reactions, as listed in Table 3. By exploring the linearity of G(x) with the reaction time

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t, G3 and G17 (three-dimensional diffusion (Jander) and Order based n=3/2), enabled a better description for the

223

isothermal data of the four gas component release in the micro fluidized bed. Figure 7 illustrates the resulting

224

fittings of the two mechanism models for ZL:PS=50:50, and the reaction rate constant k(T) was calculated from

225

the slope of the curve based on Eq.(6). Both of the two models provided a good linear fit with a linear correlation

226

factor R2 mostly above 0.97.

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Table 4 shows the activation energies and pre-exponential factor calculated by the mechanism function

228

models G3 and G17. Obviously, the obtained activation energies by the two mechanistic models are basically the

229

same for all conditions, indicating that the values represent well the isothermal pyrolysis of ZL, PS and their

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blends. Figure 8 shows the comparison of experimental and model data for the co-pyrolysis of ZL and PS at

231

biomass blend ratios of 50%. The other four kinds of samples showed similar results. As observed in Figure 8 (a),

232

the three-dimensional diffusion (Jander) model (G3) gives a better fit than does the Order based n=3/2 model for

233

the gas release during the pyrolysis. The obtained estimated data by model G3 for CO, CO2 and CH4 release can

234

be seen in Figure 8(b),(c) and (d), indicating that the model G3 here is adequate for the prediction of the four

235

main gas components during the pyrolysis of ZL, PS and their blends.

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The activation energies for generating H2, CO, CO2 and CH4 from pure ZL obtained from the G3 model are

237

63.9, 32.4, 18.3 and 30.7 kJ/mol respectively. For pure PS, the activation energies for generating H2, CO, CO2

238

and CH4 are 39.4, 27.1, 23.4 and 18.7 kJ/mol respectively. The values obtained here are in a similar range with

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activation energies of pyrolysis determined via the micro fluidized bed for similar samples throughout the

240

literature. In our previous study [16], the activation energies for generating H2, CO, CO2 and CH4 from herb

241

residue pyrolysis were 18.9, 12.0, 10.5 and 11.3 kJ/mol respectively using direct Arrhenius plot method. Yu et al.

242

[17] used the micro fluidized bed reactor to describe the pyrolysis of beer lees, finding activation energies for

243

generating H2, CO, CO2 and CH4 were 28.2, 12.4, 10.9 and 12.5 kJ/mol respectively, which were in the same

244

order with the results in this study. Likewise, Lv et al. [33] reported activation energies calculated using a

245

fluidized bed for generating CO and the mixture gases from biomass fast pyrolysis were 24.4 and 15.1 kJ/mol.

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In comparison with pure biomass, the activation energies are much higher for generating H2, CO and CH4

247

from ZL pyrolysis, while the activation energies for generating CO2 are a little lower. The results demonstrate

248

that the main non-condensable gases such as H2, CO and CH4 are generated much easier from the pyrolysis of

249

biomass than coal. These observations comply with the fact that biomass fuel contains more volatiles and has

250

higher reactivity towards pyrolysis. Furthermore, it is apparent that the activation energies of the blended

251

samples is significantly affected by the parent materials. Generally speaking, as the blend ratio of biomass

252

increases, the corresponding activation energy for the mixture should decrease as well. However, from Table 4, it

253

can be seen that the activation energies for the blended samples are not varying linearly with the mass fraction of

254

biomass, indicating that interaction between PS and ZL likely occurs during the co-pyrolysis process.

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3.4 Interaction of Zhundong Lignite And Pine Sawdust

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Based on the obtained activation energies, the interaction of ZL and PS can be described by predicting the

257

activation energy of the blended samples. As suggested by Goldfarb et al. [34], if the blended samples

258

experience no synergistic effects in terms of reaction kinetics, it can be expected that the activation energy of a

259

blend would be the additive summation of each individual fuel’s contribution. In this study, the calculated

260

activation energies (EC) of the blended samples were obtained by Eq. (8).

12


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Energy & Fuels

ECi = E Pi ν P + EZi (1 −ν P )

261 262

where

(8)

ECi is the calculated activation energy of gas i in the pyrolysis process of the blends, kJ/mol.

263

E Pi and E Zi represent the measured activation energy of the pure PS and ZL, kJ/mol. νP denotes to the biomass

264

ratio of the blended samples.

265

Figure 9 shows the measured and calculated activation energies for the four gas species released from the

266

co-pyrolysis of ZL and PS, from which the relationship between them can be observed. It is evident that the

267

calculated activation energies are consistent with the measured values in the entire trend, which also represents

268

the influence of the parent materials on the blended samples. However, there is considerable discord between a

269

linearly calculated activation energies and the measured values, indicating that the barrier to generate these gas

270

components differed as a result of blending of PS and ZL. Thus, it can be speculated that the interaction between

271

PS and ZL might occur during the co-pyrolysis process. In order to further show the interactions between

272

biomass and coal, the relative deviation (δ, %) between the calculated values and experimental values could be

273

calculated by Eq. (9)

274

δ = [( Ea i ) measured − ( Ea i ) calculated ] ( Ea i ) measured × 100%

(9)

275

Figure 10 illustrates the relative deviation in calculated activation energies and measured activation energies,

276

showing that the relative deviation varies significantly for different gas components. For H2, obvious deviations

277

are noted for the ZL:PS 50:50 and 25:75 blends. For the ZL:PS 50:50 blend, the value for relative deviation is

278

negative, indicating that the release of H2 is much easier for the blended samples compared with the two pure

279

materials. However, for the ZL:PS 25:75 blend, the measured activation energy is larger than the calculated

280

value (a positive value for relative deviation), indicating that the releasing of H2 becomes more difficult at higher

281

biomass blend ratio.

282

For CO, the relative deviation is negative for all the blended samples and reaches a peak of over 40% at

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

283

biomass weight ratio of 50%, while the value decreases sharply if biomass weight ratio increases to 75%. Thus,

284

it can be speculated that the release of CO becomes easier if blending a certain ratio of PS in ZL is used, while

285

the excessive higher blend ratio of biomass is not conducive to the release of CO. For CO2, a negative relative

286

deviation in activation energies is observed for the three blended samples, while the values are small and cannot

287

be simply regarded as a synergistic effect. The activation energy for generation CH4 shows varying degrees of

288

negative and positive deviations from an additive activation energy scheme. Obvious positive deviation is

289

observed only at a 50:50 blend sample, indicating that the co-pyrolysis at this ratio makes it difficult to release

290

CH4. In a previous study [34], the relative deviation between 10% and 40% was generally seen as a synergistic

291

effect. The relative deviation of the energy was predominantly over 10% for the four gas components released in

292

this study, and thereby a synergistic effect between ZL and PS existed during the formation of the gas species,

293

including both negative and positive effects for different gas species and biomass blend ratios.

294

Compared with results obtained by other researchers regarding the interaction between biomass and

295

bituminous coal [13], lignite [11] and other rank coal samples under non-isothermal conditions, the synergistic

296

effect between ZL and PS are much more obvious in this study. The pyrolysis of biomass and coal almost occurs

297

simultaneously under isothermal conditions using the micro fluidized bed reactor. Benefiting from this advantage,

298

the interaction evaluated by the activation energies using the micro fluidized bed should be closer to the intrinsic

299

chemical change as a result of the co-pyrolysis process.

300

In summary, co-pyrolysis of a mixture of biomass and coal is a complex process which consists of different

301

reactions. The activation energies for generating the non-condensable gases from the blended samples are

302

influenced by the blend ratio. A majority of the activation energies for the gas release from the ZL-PS blends,

303

especially for CO and CO2, are over-predicted, indicating that the blended samples appear to lower the activation

304

barrier to pyrolysis. Furthermore, the inorganic components in Zhundong lignite may have catalytic effects on

14


Page 14 of 32

Page 15 of 32

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Energy & Fuels

305

the pyrolysis of biomass and the coal itself, resulting in the variation of activation energy, an observation that is

306

currently under investigation and will be the subject of a future report.

307

4. Conclusion

308

The isothermal reaction characteristics and kinetics of Zhongdong lignite, pine sawdust and various blends

309

were investigated on the basis of the pyrolysis gas release behavior using a micro fluidized bed reactor. The

310

reaction kinetics for the formation of each pyrolysis gas species were analyzed in terms of the universal integral

311

method based on the gas-solid mechanism model. For the blended samples, the resulting activation energies

312

indicated that the blended samples did not follow the behaviors of their parent materials in an additive manner.

313

Comparing the calculated and measured activation energies, the deviations varied significantly for different gas

314

components and different blend ratios. This suggests that the interaction between biomass and coal during

315

co-pyrolysis can be analyzed based on the formation activation energy of the gas components instead of the

316

overall pyrolysis gas mixture. The blending of Zhundong lignite and pine sawdust can change the gas release

317

behaviors by changing the activation energy barrier.

318

Acknowledgement

319

This work was financially supported by the Fundamental Research Funds for the Central Universities

320

(2015XKMS055). We gratefully acknowledge the valuable cooperation with Prof. Dong Yuping and the

321

members of his laboratory.

322

References

323

(1) Jin, H.; Chen, Y.; Ge, Z.; Liu, S.; Ren, C.; Guo, L. Int. J. Hydrogen Energy 2015, 40,16096-16103.

324

(2) Xu, J.; Yu, D,; Fan, B.; Zeng, X.; Lv, W.; Chen, J. Energy Fuels 2014, 28, 678-684.

325

(3) Song, G.; Song, W.; Qi, X.; Lu, Q. Energy Fuels 2016, 30, 3473-3478.

326

(4) Minchener , A.J. Fuel 2005, 87, 2222-2235.

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327

(5) Weiland, N.T.; Means, N.C.; Morreale, B.D. Fuel 2012, 94, 563-570.

328

(6) Zhang, L.; Xu, S.; Zhao, W.; Liu, S. Fuel 2007, 86, 353-359.

329

(7) Li, K.; Zhang, R.; Bi, J. Int. J. Hydrogen Energy 2010, 35, 2722- 2726.

330

(8) Wu, Z.; Wang, S.; Zhao, J.; Chen, L.; Meng, H. Energy Fuels 2015, 29, 4168-4180.

331

(9) Park, D.; Kim, S.; Lee, S.; Goo, L.J. Bioresour. Technol. 2010, 101, 6151-6156.

332

(10) Chen, C.; Ma, X.; He, Y. Bioresour. Technol. 2012, 117, 264-273.

333

(11) Haykiri-Acma, H.; Yaman, S. Renew. Energy 2010, 35, 288-292.

334

(12) Sonobe, T.; Worasuwannarak, N.; Pipatmanomai, S. Fuel Process. Technol. 2008, 89, 1371-1378.

335

(13) Li, S.; Chen, X.; Liu, A.; Wang, L.; Yu, G. Bioresour. Technol. 2015, 179, 414-420.

336

(14) Krerkkaiwan, S.; Fushimi, C.; Tsutsumi, A.; Kuchonthara, P. Fuel Process. Technol. 2013, 115, 11-18.

337

(15) Kastanaki, E.; Vamvuka, D.; Grammelis, P.; Kakaras, E. Fuel Process. Technol. 2002, 77-78, 159-166.

338

(16) Guo, F.Q.; Dong, Y.P.; Lv, Z.C.; Fan, P.F.; Yang, S.; Dong, L. Energy Convers. Manage. 2015, 93, 367-376.

339

(17) Yu, J.; Yao, C.; Zeng, X.; Geng, S.; Dong, L.; Wang, Y.; Gao, S.; Xu, G. Chem. Eng. J 2011, 168, 839-847.

340

(18) Guo, Y.; Zhao Y.; Gao, D.; Liu, P.; Meng, S.; Sun, S. Int. J. Hydrogen Energy 2106, 41, 15178-15198.

341

(19) Zeng, X.; Wang, F.; Wang, Y.; Li, A.; Yu, Jian.; Xu, G. Energy Fuels 2014, 28, 1838?1845.

342

(20) Song,Y.; Wang,Y.; Yang, Wu.; Yao, C.; Xu, G. Fuel Process. Techno. 2014, 118, 270-277.

343

(21) Guo, F.Q.; Dong, Y.P.; Fan, P.F.; Lv, Z.C.; Yang, S.; Dong, L. Int. J. Hydrogen Energy 2016, 41,

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13380-13389.

345

(22) Guo, F.Q.; Dong, Y.P.; Fan, P.F.; Lv, Z.C.; Yang, S.; Dong, L. J. Anal. Appl. Pyrol. 2016, 118, 155-163.

346

(23) Yu, J.; Zeng, X.; Zhang, J.; Zhong, M.; Zhang, G.; Wang Y.; Xu G. Fuel 2013, 103, 29-36.

347

(24) Wang, F.; Zeng X.; Wang Y.; Yu, J.; Xu G. Fuel Process. Techno 2016,141,2-8.

348

(25) Wang, F.; Zeng, X.; Wang, Y.; Su, H.; Yu, J.; Xu G. Fuel 2016, 164, 403-409.

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349

(26) Blazej, A.; Kosik, M. In: Ellis Horwood (Ed.) 1993, 173-221.

350

(27) Moghataderi, B.; Meesri ,C.; Wall, T.F. Fuel 2004, 83, 745-750.

351

(28) Yang, H.; Yan, R.; Chen, H.; Lee, D.H.; Zheng, C. Fuel 2007, 86, 1781-1788.

352

(29) Greenwood, P.F.; van Heemst, J.D.H.; Guthrie,E.A.; Hatcher, P.G. J. Anal. Appl. Pyrol 2002, 62, 365-373.

353

(30) Liu, Q.; Wang, S.; Zheng, Y.; Luo, Z.; Cen, K. J. Anal. Appl. Pyrol 2008, 82, 170-177.

354

(31) Park, H.J.; Dong, J.I.; Jeon, J.K.; Park, Y.K.; Yoo, K.S.; Kim, S.S.; Kim, J.; Kim, S. Chem Eng J 2008, 143,

355

124-132.

356

(32) Porada, S. Fuel 2004, 83, 1191-1196.

357

(33) Lv, P.M.; Chang, J.; Wang, T.J.; Wu, C.Z.; Tsubaki, N. Energy Fuels 2004, 18, 1865-1869.

358

(34) Goldfarb, J.L.; Ceylan, S. Fuel 2015, 160, 297-308.

359

17


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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

360

Page 18 of 32

Table 1. Ultimate and proximate analysis of samples Sample

ZL

PS

Carbon

49.26

51.05

Hydrogen

3.71

5.78

Oxygen (diff)

44.46

40.88

Nitrogen

1.02

0.75

Sulfur

1.55

1.54

Volatiles

35.15

81.05

Fixed carbon

50.73

17.69

Ash

14.12

1.26

Ultimate analysis (wt%, dry basis)

Proximate analysis (wt%, dry basis)

361

18


Page 19 of 32

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

362

Table 2. Ash analyses of Zhundong lignite Element

SiO2

Al2O3

SO3

Fe2O3

CaO

MgO

K2O

Na2O

TiO2

49.62

20.50

15.40

5.18

4.28

2.01

1.83

0.51

0.48

Content （wt.%） 363

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

364

Page 20 of 32

Table 3. Typical mechanism functions using in gas-solid reactions Symbol

Model

f(x)

G(x)

G1

Diffusional

1-D diffusion

1/2 x

x2

G2

2-D diffusion

[-ln(1-x)]-1

x+(1-x) ln(1-x)

G3

3-D diffusion(Jander)

[1.5(1-x)2/3]/[1-(1-x)1/3]

[1-(1-x)1/3]2

G4

3-D diffusion(G-B)

1.5[1-(1-x)1/3]-1

1-2x/3-(1-x)2/3

G5

3-D diffusion(A-J)

1.5(1-x)2/3[(1+x)1/3-1]-1

[(1+x)1/3-1]2

n=2/3

1.5(1-x) [-ln(1-x)]1/3

[-ln(1-x)]2/3

G7

n=1/2

2(1-x) [-ln(1-x)]1/2

[-ln(1-x)]1/2

G8

n=1/3

3(1-x) [-ln(1-x)]2/3

[-ln(1-x)]1/3

G9

n=1/4

4(1-x) [-ln(1-x)]1/3

[-ln(1-x)]1/4

x (1-x)

ln[x (1-x)]

n=1/2

2x1/2

x1/2

G12

n=1/3

3x2/3

x1/3

G13

n=1/4

4x3/4

x1/4

n=3

(1-x)3

[(1-x)-2-1]/2

G15

n=2

(1-x)2

(1-x)-1-1

G16

n=1

1-x

-ln(1-x)

G17

n=3/2

(1-x)3/2

2(1-x)-1/2-2

Mampel Power law

1

x

G19

Contraction sphere

3(1-x)2/3

1-(1-x)1/3

G20

Contraction cylinder

2(1-x)1/2

1-(1-x)1/2

G6

Nucleation and growth

G10

Autocatalytic reaction

G11

Mampel Power law

G14

G18

Order based

Phase interfacial reaction

20


Page 21 of 32

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Energy & Fuels

365

Table 4. Activation energies of different gas components from pyrolysis of ZL, PS and their blends PS blending ratio

EH2(kJ/mol)

ECO(kJ/mol)

ECO2(kJ/mol)

ECH4(kJ/mol)

(wt.%)

G3

G17

G3

G17

G3

G17

G3

G17

0

63.9

64.2

32.5

33.1

18.3

19.8

30.7

31.0

25

57.0

57.6

27.0

27.2

21.8

21.9

23.9

24.4

50

46.6

48.7

20.7

21.3

19.9

20.4

31.4

35.3

75

58.5

59.1

23.0

26.0

17.1

20.6

26.5

26.8

100

39.5

40.0

27.1

27.2

23.4

25.2

19.8

18.7

366

21


Energy & Fuels

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367

Page 22 of 32

Table 5. Pre-exponential factor of different gas components from pyrolysis of ZL, PS and their blends PS blending ratio

AH2(kJ/mol)

ACO(kJ/mol)

ACO2(kJ/mol)

ACH4(kJ/mol)

(wt.%)

G3

G17

G3

G17

G3

G17

G3

G17

0

6.28

96.63

0.21

3.38

0.04

1.11

0.25

3.83

25

4.93

77.78

0.18

2.68

0.09

1.32

0.13

2.01

50

1.40

25.86

0.07

1.09

0.08

1.17

0.27

6.14

75

4.43

69.55

0.09

3.44

0.06

1.23

0.16

2.45

100

0.94

14.68

0.15

2.27

0.12

2.25

0.11

1.42

368 369

22


Page 23 of 32

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Energy & Fuels

Exhaust

Mass spectrometer

Flow sensor

Gas filter

Needle vavle

Pressure sensor Electric furnace Temperature and pressure sensor

Ar

Electromagnetic valve

Mass flowmeter

Gas valve

Display interface The loaded samples

Quartz tubular reactor Mass flowmeter

370 371

Figure 1. Schematic diagram of the experimental system

372

23


Energy & Fuels

Pine Sawdust

Transimittance (%)

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 24 of 32

C-H C=C C=O

High-alkali lignite

O-H

4000

373 374

3500

3000

2500

2000

1500

1000

500

-1

Wave number (cm )

Figure 2. FTIR spectra of the pine sawdust and Zhundong lignite

375

24


Page 25 of 32

376 -6

-6

1.0x10

1.0x10

CO2

CO2 CH4

-7

8.0x10

CH4

-7

8.0x10

H2

H2

CO

CO -7

-7

6.0x10

Pure ZD o T=600 C

Intensity

Intensity

6.0x10

-7

4.0x10

-7

-7

4.0x10

2.0x10

0.0 10

Pure ZD o T=900 C

-7

2.0x10

0.0

20

30

40

50

10

20

Time (s)

30

40

-6

-6

2.5x10 CO2

CO2

CH4

-6

2.0x10

-6

1.0x10

-7

5.0x10

-6

1.0x10

-7

5.0x10

0.0

0.0

20

30

40

50

10

Time (s)

377

Pure PS o T=900 C

-6

1.5x10

Intensity

-6

H2 CO

CO Pure PS o T=600 C

1.5x10

CH4

-6

2.0x10

H2

10

50

Time (s)

2.5x10

Intensity

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

20

30

40

50

Time (s)

Figure 3 Gas releasing characteristics of (a) ZL at 600 ℃, (b) ZL at 900 ℃, (c) PS at 600 ℃ and (d) PS at 900 ℃

378

25


Energy & Fuels

379 (a)

1.0

1.0

0.8

0.8 o

600 C o 700 C o 800 C o 900 C

600 C o 700 C o 800 C o 900 C

0.6

0.4

Conversion (%)

o

Conversion (%)

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 26 of 32

0.4

0.2

0.2 H2 0.0

0.6

CO

CH4

CO2

0.0

0

100

0

100

0

100

0

100

0

100

0

Time (S)

380

100

0

100

Time (S)

Time (s)

Time (S)

Figure 4 Gas conversion as a function of reaction time for (a) ZL and (b) PS

381

26


CH4

CO2

CO

H2

0

100 Time (S)

Page 27 of 32

(a)

H2

0.10 0.05

-1

dx/dt [s ]

0.00 0.10

o

600 C o 700 C o 800 C o 900 C

CO

0.05 0.00 0.10

CO2

0.05 0.00 0.10

CH4

0.05 0.00 0.0

0.2

0.4

0.6

0.8

1.0

x

382

(b)

0.15

o

600 C o 700 C o 800 C o 900 C

H2

0.10 0.05 0.00 0.10

CO

0.05

-1

dx/dt [s ]

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

0.00

CO2

0.05 0.00

CH4

0.10 0.05 0.00 0.0

383 384

0.2

0.4

0.6

0.8

1.0

x Figure 5 Reaction rate of gas releasing versus conversion of (a) ZL and (b) PS

385

27


Energy & Fuels

386 0.14

H2

0.14 ZL ZL:PS 75:25 ZL:PS 50:50 ZL:PS 25:75 PS

0.12

CO ZL ZL:PS 75:25 ZL:PS 50:50 ZL:PS 25:75 PS

0.12 0.10

0.08

dx/dt [s-1]

-1

dx/dt [s ]

0.10

0.06

0.08 0.06

0.04

0.04

0.02

0.02

0.00

0.00

0.0

0.2

0.4

0.6

0.8

1.0

0.0

0.2

0.4

x

0.12

CO2 ZL ZL:PS 75:25 ZL:PS 50:50 ZL:PS 25:75 PS

0.08

0.8

1.0

CH4 ZL ZL:PS 75:25 ZL:PS 50:50 ZL:PS 25:75 PS

0.10 0.08

-1

-1

dx/dt [s ]

0.06 0.04

0.06 0.04

0.02

0.02

0.00

0.00

0.0

0.2

0.4

0.6

0.8

0.0

1.0

0.2

0.4

0.6

x

x

387

0.6

x

0.10

dx/dt [s ]

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 28 of 32

Figure 6. Variation of reaction rate with biomass blending ratio at T=800℃

388

28


0.8

1.0

Page 29 of 32

389 5

G3-600 C o G3-700 C o G3-800 C o G3-900 C o G17-600 C o G17-700 C o G17-800 C o G17-900 C

4

2


4

3

2

1

1

0

CO o

G(x)

3

G(x)

5

o

H2

0 0

20

40

60

80

0

20

Time (s)

40

60

Time(s)

5 CO2 G3-600 C o G3-700 C o G3-800 C o G3-900 C o G17-600 C o G17-700 C o G17-800 C o G17-900 C

3

2

4

3

2

1

0

o


G(x)

4

CH4

5

o

G(x)

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

1

0

20

0

40

0

20

Time (s)

390

40

Time(s)

Figure 7. Mechanism function model determination for ZL:PS=50:50

391

29


60

Energy & Fuels

392 1.0

1.0

(a) H2

0.8

(b)

0.8 600 C-Mea o 600 C-Cal-G3 o 600 C-Cal-G17 o 700 C-Mea o 700 C-Cal-G3 o 700 C-Cal-G17 o 800 C-Mea o 800 C-Cal-G3 o 800 C-Cal-G17 o 900 C-Mea o 900 C-Cal-G3 o 900 C-Cal-G17

0.6

0.4

0.2

0.0

0

50

100

150

CO

Conversion (-)

Conversion (-)

o

o

600 C-Mea o 600 C-Cal o 700 C-Mea o 700 C-Cal o 800 C-Mea o 800 C-Cal o 900 C-Mea o 900 C-Cal

0.6

0.4

0.2

0.0

200

0

50

1.0

1.0

(c)

0.8

Conversion (-)

CO2 o


0.4

0.2

200

(d)

0

50

100

150

CH4 o

0.6


0.4

0.2

0.0

200

0

50

Time (s)

393

150

0.8

0.6

0.0

100

Time (s)

Time (s)

Conversion (-)

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 30 of 32

100

150

200

Time (s)

Figure 8. Comparison between experimental data and those estimated using the kinetic model for ZL:PS=50:50

394

30


Page 31 of 32

H2 CO CO2

60

CH4

Ea (kJ/mol)

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

40

20

0

395 396

20

40

60

80

100

Biomass weight ratio (wt.%) Figure 9. Measured and calculated Ea values as a function of biomass weight ratio for the four gas components

397

31


Energy & Fuels

20

Relative deviation (%)

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 32 of 32

0

-20 PS 25% PS 50% PS 75%

-40

398 399

H2

CO2

CO

CH4

Figure 10. Relative deviation in activation energies for ZL and PS blends

32


Characterization and Kinetics for Co-Pyrolysis of Zhundong Lignite

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