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Effects of particle size, pretreatment, and catalysis on microwave pyrolysis of corn stover
Accepted Manuscript Effects of particle size, pretreatment, and catalysis on microwave pyrolysis of corn stover Yu-Fong Huang, Wen-Hui Kuan, Chun-Yuan Chang PII:
S0360-5442(17)31872-8
DOI:
10.1016/j.energy.2017.11.022
Reference:
EGY 11810
To appear in:
Energy
Received Date: 18 April 2017 Revised Date:
26 October 2017
Accepted Date: 5 November 2017
Please cite this article as: Huang Y-F, Kuan W-H, Chang C-Y, Effects of particle size, pretreatment, and catalysis on microwave pyrolysis of corn stover, Energy (2017), doi: 10.1016/j.energy.2017.11.022. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Effects of particle size, pretreatment, and catalysis on microwave
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pyrolysis of corn stover
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Yu-Fong Huang a, Wen-Hui Kuan b,*, Chun-Yuan Chang b
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Chou-Shan Rd., Taipei 106, Taiwan, ROC b
Department of Safety, Health and Environmental Engineering, Ming Chi University of Technology, 84 Gong-Juan Rd., Taishan, New Taipei City 243, Taiwan, ROC
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Graduate Institute of Environmental Engineering, National Taiwan University, 71
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Corresponding author. Tel.: +886 2 2908 9899 ext 4653; fax: +886 2 2908 0346. Email address: [email protected] (W.-H. Kuan).
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Abstract
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The effects of particle size, pretreatment (acid pretreatment and steam explosion), and catalysis of aluminum oxide (Al2O3) on heating efficiency, product distribution, and
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gaseous product composition of corn stover pyrolysis using microwave heating were
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investigated in this study. Both maximum temperature and heating rate increased with
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decreasing particle size of corn stover. The heating efficiency was also improved over
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Al2O3 and by applying both pretreatment methods. Adding 10-mesh Al2O3 increased the
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gaseous yield but decreased the liquid yield. However, this phenomenon did not exist
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when 50-mesh Al2O3 was used. This may be attributable to that small catalyst particles
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are encapsulated by other catalyst and biomass particles to reduce their catalytic activity.
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Applying acid pretreatment substantially decreased the gaseous yield but increased the
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liquid yield, whereas the effect of steam explosion was not significant. Approximately
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half of CO was produced during the first 5 min of experiment, but the yields of H2, CH4,
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and CO2 produced from microwave pyrolysis of corn stover pretreated by acid were
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relatively small or even none during this initial period. The reaction kinetics for
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microwave pyrolysis of corn stover was analyzed by using the first- and second-order
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reaction models.
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Keywords: Microwave pyrolysis; Corn stover; Particle size; Pretreatment; Catalysis; Reaction kinetics
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1. Introduction
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Renewable energy systems, such as solar photovoltaics, wind power, and biofuels, have attracted increasing interest mainly because of concerns about growing demand for
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limited, non-renewable fossil fuels and issues related to climate change (i.e., global
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warming, extreme events and disasters, sea ice melting, sea level rising, ocean
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acidification, etc.) caused by emissions of anthropogenic greenhouse gases [1–7]. The
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world’s total energy consumption has been almost doubled from 4,667 Mtoe (million
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tonnes of oil equivalent) in 1973 to 9,301 Mtoe in 2013 [8]. The Paris Agreement in the
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21st Conference of the Parties (COP 21) to the United Nations Framework Convention
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on Climate Change (UNFCCC) has reset the collective global climate ambition from the
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aspiration of keeping temperature rise below 2 °C (relative to pre-industrial levels), to a
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new aim of limiting it to well below 2 °C with efforts to pursue 1.5 °C, so attention
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must turn to further reducing emissions remaining within energy scenarios consistent
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with 2 °C warming [9]. The Paris Agreement is the first climate deal with universal
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contributions to mitigation action, and its strengthened long-term objectives require
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even stronger actions than previously identified [10]. Therefore, the research and
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development of renewable energy is so important and urgent, to reduce the dependence
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on fossil fuels and thus to mitigate the climate change.
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Biomass energy (bioenergy) is one of the forms of renewable energy. Biomass has
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been recognized as an abundant carbon-neutral renewable resource for the production of
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value-added bioenergy and biomaterials [1–3]. In contrast to other renewables
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supplying heat and power, biomass is the only currently source of solid, liquid, and
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using biochar (solid carbonaceous residues from biomass pyrolysis) for carbon
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sequestration [12] or by bio-energy with carbon capture and storage (BECCS) [13].
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Lignocellulosic biomass is mainly composed of structural constituents (hemicelluloses,
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cellulose, and lignin), and it can be converted into bioenergy by using thermochemical
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and biological processes [14]. Pyrolysis, which is the direct thermal decomposition of
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biomass in the absence of oxygen to obtain solid, liquid, and gaseous products, has been
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widely recognized as the preferred conversion method of various thermochemical
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processes available [11,14–16].
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Biomass pyrolysis is conventionally carried out by means of electric heating furnaces. However, microwave heating can offer a number of advantages over
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conventional heating such as: (i) higher heating rates and efficiencies, (ii) non-contact
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heating, (iii) material-selective heating, (iv) volumetric and uniform heating, (v) energy
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transfer rather than heat transfer, (vi) energy savings at significantly reduced processing
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temperatures (150–300 °C), (vii) less feedstock pretreatment needed, (viii) quick
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start-up and stopping, (ix) greater control and safety, and (x) reduced equipment size
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and waste [17–22]. Therefore, microwave heating has been applied in various fields.
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Biomass pyrolysis heated by microwave irradiation has been proven as an efficient
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technique for the production of fuels and materials [20,22–26]. More work is still
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needed to extend existing understanding of microwave pyrolysis characteristics in order
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to improve the technique and then to transform it into a commercially viable route for
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the production of fuels and materials in an environmentally sustainable manner [24].
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Corn stover, the crop residue left in cornfields, is produced at a ratio of approximately 1 kg per kg of corn grain (on dry basis) [27]. Corn stover is abundant
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since more than 216 Tg can be produced annually in the United States [28]. The annual
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corn stover production was approximately 114 Gg in Taiwan, based on the corn grain
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production data in 2014 [29]. Corn is one of the most important crops in the world, so
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its residues can be regarded as a sustainable bioenergy resource [27,30]. This study was
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aimed at investigating the effects of particle size, catalysis, and pretreatment on the
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gaseous products produced from corn stover by using microwave pyrolysis. A number
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of studies have focused on the effect of either particle size or catalysis on microwave
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pyrolysis of biomass, but there is seldom work about the effect of pretreatment on
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product distribution as well as the explanation of the effect. Besides, the reaction
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kinetics for microwave pyrolysis of corn stover under different conditions was analyzed.
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2. Material and methods
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2.1 Materials
Raw corn stover feedstock was collected in the cornfields and markets of Taiwan.
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Prior to pretreatment and microwave pyrolysis, the corn stover was naturally dried by
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exposure to air in an open area for 14 days, and then it was mechanically shredded and
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manually sieved. By using different sieves, five corn stover samples with different
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particle sizes were obtained: >4.00, 2.00–4.00, 0.425–2.00, 0.250–0.425, and
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0.150–0.250 mm, simply named as 5, 10, 40, 60, and 100 mesh, respectively, in this
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study. The catalyst used in this study was Al2O3 purchased from Kokusan Chemical
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Works (Japan), and it was sieved by 10 and 50 mesh screens.
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2.2 Pretreatment
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stirring into an aqueous solution containing 1 mL deionized water and 10 mL
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phosphoric acid (85 %). Sufficient materials for all experiments were pretreated in
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advance. Phosphoric acid was used in this study because it is one of the common acids
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for the acid pretreatment of lignocellulosic biomass [31,32]. The solution was heated in
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a 323 K water bath for 10 h, and then it was mixed with 50 mL deionized water with
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strong stirring. Afterwards the corn stover was washed with plenty of deionized water.
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The pH value of corn stover was adjusted to neutral by using a NaOH solution. In the
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steam explosion, the dry corn stover was put into an autoclave under the condition of
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160–260 °C and 6–34 bar. The environmental condition was held for 1 min when it was
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reached. The pretreated corn stover was naturally dried by exposure to air in an open
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area for 14 days and shredded to the designated particle size.
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2.3 Experimental apparatus
Microwave pyrolysis of corn stover was carried by using a single-mode microwave
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oven operating at a frequency of 2.45 GHz. The microwave oven was purchased from
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MUEGGE (Germany). The layout of the overall microwave pyrolysis system can be
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found elsewhere [33]. Both reaction tube (40 cm in length and 5 cm in outer diameter)
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A K-type thermocouple sensor was placed outside the bottom of the quartz crucible to
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measure the real-time temperature of corn stover sample. Pure nitrogen gas (99.99%)
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was purged into the system at a flow rate of 50 mL/min to ensure the anoxic condition
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in the reaction system. The actual working microwave power level was determined by
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the incident power level subtracting the reflected power level.
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The corn stover sample or its blend with Al2O3 was added to the quartz crucible and then placed inside the quartz reaction tube. Each pretreated or non-pretreated corn
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stover was weighed to 2 g. In catalytic pyrolysis experiments, 2 g corn stover was
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blended with the catalyst by 5 wt% (i.e., totally 2.1 g). The height of the quartz crucible
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was adjusted to be located in the path of microwave propagation. After sufficient
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nitrogen gas purging, the power supply was turned on and the adjustment knob was
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switched to a microwave power level of 500 W. The power level was selected because
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of insufficient and over heating for biomass pyrolysis at lower and higher power levels,
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respectively. After microwave irradiation for 20 min, the power supply was turned off,
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and the carrier gas purging was stopped. The vapor produced during the experiment
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immediately passed through a condenser tube whose temperature was controlled at 4 °C
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by a thermostat. The condensable and non-condensable parts of the product vapor were
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regarded as liquid and gaseous products, respectively. The gaseous product was
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collected every 5 min in a 5 L Tedlar bag. After self-cooled down to room temperature,
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the solid residues remained in the quartz crucible were removed, weighed, and then
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stored in a desiccator. All of the experiments were performed at least in triplicate to
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obtain average values for the experimental results.
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The proximate analysis of corn stover was carried out according to the standard
test method E1131–08 of the American Society for Testing and Materials (ASTM). The
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moisture content of corn stover was 4.97 wt%. The volatile matter, fixed carbon, and
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ash contents (on dry basis) of corn stover were 90.59, 6.94, and 2.47 wt%, respectively.
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The higher heating value (HHV) of corn stover was measured in a CAL2K ECO
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adiabatic oxygen bomb calorimeter. Each sample (approximately 0.5 g) was dried at 105
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°C prior to the heating value analysis. The average HHV of corn stover was 18.41
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MJ/kg. Gaseous product analysis was carried out by using the Perkin–Elmer Auto
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System XL gas chromatography–thermal conductivity detector (GC–TCD) equipped
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with a Supelco Carboxen 1010 PLOT column. The temperatures of injector, oven, and
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detector were controlled at 120, 100, and 150 °C, respectively. The flow rate of carrier
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gas (He or N2) was set at 10 mL/min (25:1 split).
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3. Results and discussion
3.1 Temperature profiles
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The temperature profiles of microwave pyrolysis of corn stover are illustrated in Fig. 1. As can be seen in, both maximum temperature and heating rate generally
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100 mesh corn stover were 475, 478, 557, 522, and 557 °C, and the average heating
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rates during the first 5 min were 84.6, 75.4, 95.2, 86.4, and 94.6 °C/min, respectively.
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The reaction temperature was continuously recorded to obtain the real-time temperature
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change and the arithmetic average heating rate. The average heating rate during the first
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5 min was considered because the heating ramp in this period was much more
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significant than afterwards, no matter which particle size it was. The effects of
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pretreatment and catalysis on the temperature profiles of microwave pyrolysis of corn
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stover can be observed by Fig. 1b. It can be seen that, without any pretreatment, the
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effect of adding 10-mesh Al2O3 was insignificant. However, after either acid
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pretreatment or steam explosion, the catalytic effect turned to be substantial.
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Furthermore, after acid pretreatment, the maximum temperature of catalytic microwave
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pyrolysis of corn stover was highest.
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To further observe the effects of pretreatment and catalysis, the maximum temperatures and average heating rates during the first 5 min of microwave pyrolysis of
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corn stover under various conditions are shown in Fig. 2. In general, both maximum
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temperature and heating rate increased with decreasing particle size of corn stover, no
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matter the catalyst was added or not. When the particle size was reduced from 5 mesh to
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100 mesh, the maximum temperature and heating rate increased by 67–82 °C and 10–18
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°C/min, respectively. Besides, the addition of catalyst promoted both maximum
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temperature and heating rate in some cases. After acid pretreatment and steam explosion,
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(approximately 106 °C/min), and the heating rates of catalytic microwave pyrolysis
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were also close to each other (approximately 110 °C/min). On the other hand, the
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maximum temperatures of microwave pyrolysis of corn stover were not so different
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(604–610 °C). However, the maximum temperature of catalytic microwave pyrolysis of
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acid pretreated corn stover was 654 °C, much higher than that (628 °C) when steam
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explosion was applied. According to the aforementioned experimental results, it is clear
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that all the three factors (particle size, pretreatment, and catalysis) had substantial
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effects to increase the efficiency of microwave heating. In addition, the effect of
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pretreatment seems to be stronger than that of catalysis. Microwave heating can be
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referred to as dielectric heating [17,19]. The different maximum temperatures and
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heating rates could be attributable to the difference in dielectric property of corn stover
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with and without catalyst or pretreatment, since the composition of the biomass would
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be changed after the processes.
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3.2 Product distribution
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After microwave pyrolysis, corn stover was converted into solid, liquid, and
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gaseous products. The three-phase product distributions under various operational
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conditions are shown in Fig. 3. It should be noted that the quantities of solid and
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gaseous products were directly measured, and the solid amount is without the catalyst.
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However, the percentage of liquid product was taken as the difference between 100 and
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product. In general, gaseous yield increased but both solid and liquid yields decreased
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with decreasing particle size. The addition of 10-mesh Al2O3 increased the gaseous
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yield by approximately 4–9 wt%, but adding 50-mesh Al2O3 decreased the yield by
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approximately 3–7 wt%. The results indicate that the relatively small particle size of
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catalyst may hinder the further (secondary) degradation of the organic vapor released by
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corn stover pyrolysis. Consequently, the three-phase product distributions of microwave
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pyrolysis of corn stover with and without 50-mesh Al2O3 were similar to each other. The
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gaseous yields of microwave pyrolysis of 10–100 mesh corn stover samples were up to
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70–72 wt%, when the 10-mesh Al2O3 was added. The gaseous yield of 60-mesh corn
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stover without adding catalyst was 73 wt%. This could be attributable to the impurity of
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biomass sample or the unknown characteristics of microwave heating. It is a
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coincidence that the product yields of 40-mesh in Fig. 3c are very close to those of
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SE-NC in Fig. 3d. This may imply that the addition of 50-mesh Al2O3 and the
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pretreatment of steam explosion provide similar effect on product distribution. It has
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been reported that the gaseous yield of catalytic microwave pyrolysis was
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approximately 40–60 wt% [34–36], lower than the data in this study. This could be
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attributable to the catalytic effect accomplished by Al2O3 and the heating efficiency
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provided by single-mode microwave oven.
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Figure 3
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The catalytic effect and the difference in the product distributions of biomass pyrolysis over 10- and 50-mesh Al2O3 can be explained by Fig. 4. The product vapor
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on the flow rate of carrier gas and the configuration of reactor and pipelines), when
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there is no catalyst added. By using a condenser, the product vapor is divided into
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condensable and non-condensable parts, namely liquid and gaseous products,
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respectively. When a catalyst is added to the biomass, part of the product vapor can be
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entrapped by the catalyst particles and then decomposes into smaller substances such as
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light hydrocarbons (CH4, C2H4, C2H6, etc.) and permanent gases (H2, CO, and CO2)
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[37–39]. Consequently, catalytic pyrolysis produces more gaseous product than
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conventional pyrolysis. However, if the catalyst particles are too small, they may be
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encapsulated by other catalyst and biomass particles to reduce their catalytic activity. As
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a result, the gaseous yield of biomass pyrolysis over small catalyst particles would be
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lower than expected.
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Figure 4
After acid pretreatment, the liquid yield of microwave pyrolysis of corn stover
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substantially increased by approximately 15 wt% whereas the gaseous yield decreased
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by approximately 14 wt%. The result could be attributable to the depolymerization and
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hydrolysis of hemicellulose and cellulose during the acid pretreatment [40–45], so there
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was less formation of primary products that can be further converted into permanent
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gases. On the other hand, the effect of steam explosion pretreatment was not significant.
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The product distribution of microwave pyrolysis of corn stover pretreated by steam
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explosion was not so different from that without any pretreatment. Therefore, the extent
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of hemicellulose and cellulose solubilization accomplished by steam explosion could be
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microwave pyrolysis of pretreated corn stover was higher than that without adding
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catalyst by approximately 3 wt%, no matter which pretreatment method was used. The
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solid yields, which were nearly the same under different operational conditions, could
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be regarded as an allowable minimum value for the microwave pyrolysis of corn stover
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at a microwave power level of 500 W.
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The quantities of H2, CH4, CO2, and CO produced by microwave pyrolysis of 40-mesh corn stover with and without pretreatment and catalysis are shown in Fig. 5.
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For the total processing time of 20 min, the overall production quantities of H2, CH4,
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CO2, and CO under different conditions were in the ranges of 0.001–0.05, 0.02–0.04,
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0.01–0.05, and 0.91–1.15 L, respectively. It can be seen that of quantity of CO was
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mush bigger than those of other gases. The composition of gaseous product was
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different from those reported in the literature [35,36]. This could be attributable to the
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difference in product selectivity among the catalysts (char-supported metallic catalysts
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[35], HZSM-5 aluminosilicate zeolite [36], and Al2O3 in this study). Approximately half
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of CO was produced during the first 5 min, and then it was stably produced until the end
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of the process. H2, CH4, and CO2 were not regularly generated as CO was. Most of them
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were produced during 5–20 min. In general, adding catalyst increased but applying
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pretreatment decreased the production quantities of the gases. When the corn stover was
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acid pretreated, the quantity of H2 was largely reduced from 0.04–0.05 L to 0.001–0.005
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L. It has been reported that the formation of H2 from agricultural residues by microwave
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hemicellulose or even cellulose of corn stover would be partly removed during the acid
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treatment, resulting in its lower yield of H2. In fact, all the four gases should be
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primarily generated from the hemicellulose and cellulose fractions of corn stover, so it
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is not surprising that their production quantities were reduced after acid pretreatment.
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Furthermore, the productions of H2, CH4, and CO2 were relatively small or even none
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during the first 5 min, where the reaction temperature had not yet reached the maximum
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temperature. The lower temperature means that the more gases were produced from
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components of biomass with higher thermal reactivity (i.e., hemicellulose and cellulose).
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This provides another proof that the acid pretreatment should take the responsibility for
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the reduction on H2, CH4, and CO2 yields.
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3.4 Reaction kinetics
In this study, the reaction kinetics for microwave pyrolysis of corn stover under different conditions was analyzed by using a first-order reaction model:
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dqt = k1 (qm − qt ) dt
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where k1 is the first-order reaction rate constant, qm is the maximum quantity of gaseous
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product from corn stover by microwave pyrolysis, and qt is the quantity of gaseous
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product at any time, t. The condensable gases were not considered here. By plotting
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dqt/dt versus qt, a straight line can be obtained after linear regression. The first-order
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reaction rate constant and the maximum quantity of gaseous product can be determined
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from the slope and intercept of the straight line, respectively. The result of first-order
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reaction kinetic analysis is listed in Table 1.
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Besides, a second-order reaction model was also applied:
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dqt 2 = k 2 (qm − qt ) dt
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where k2 is the second-order reaction rate constant. Integrating Eq. (2) followed by a
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rearrangement results in:
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(2)
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(3)
By plotting t/qt versus t, a straight line can be obtained after linear regression. The
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maximum quantity of gaseous product and the second-order reaction rate constant can
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be determined from the slope and intercept of the straight line, respectively. The result
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of second-order reaction kinetic analysis is listed in Table 2. Furthermore, to testify the
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accuracy of the two reaction kinetics models, the average absolute error (AAE) was
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applied [46,47]:
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AAE =
1 n qtexp − qtcal ∑ q exp × 100% n i =1 t
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where n is the number of data points, and qtexp and qtcal are experimental and calculated
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quantities of gaseous product, respectively.
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Table 2
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range of 0.83–0.91, lower than those of the second-order reaction model (0.95–0.99).
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Under different conditions, k1 and k2 are between 0.10–0.25 and 0.04–0.16, respectively.
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The maximum quantities of gaseous product determined by the two models are not very
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different. The maximum quantities determined by the first-order reaction and the
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second-order reaction models are in the ranges of 1.34–2.21 L and 1.39–2.30 L,
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respectively. Both of the models are helpful to predict the allowable maximum gaseous
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yield of microwave pyrolysis of corn stover. However, the AAE of the second-order
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reaction model is much lower than that of the first-order reaction model. Therefore, the
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reaction kinetics of gaseous production from corn stover by microwave pyrolysis can be
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better fitted by the second-order reaction model. However, it should be noted that the
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first-order reaction model and the second-order reaction model are differential and
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integral, respectively. Therefore, the different kinetic parameters and fitting errors could
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be also attributable to the different methods. Both of the models indicate that there were
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the highest reaction rate constants when the 5-mesh corn stover was microwave
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pyrolyzed with and without 50-mesh Al2O3, but the maximum quantity of gaseous
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product was lowest. These results show that the driving force of reaction may mainly
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come from the magnitude of maximum gaseous productivity. As aforementioned, the
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product distribution of microwave pyrolysis of corn stover was similar to that of
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catalytic microwave pyrolysis using 50-mesh Al2O3. Although adding 10-mesh Al2O3
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substantially reduced the reaction rate constants, the larger catalyst provided higher
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maximum quantity of gaseous product that is the primary driving force of reaction.
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There was the lowest reaction rate constant when the 40-mesh corn stover pretreated by
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steam explosion was microwave pyrolyzed with 10-mesh Al2O3. However, the effect of
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acid pretreatment on the reaction rate constant was insignificant.
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The kinetic parameters (activation energy and pre-exponential factor) of microwave pyrolysis of corn stover can be determined from the reaction rate constants
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as above mentioned and their corresponding reaction temperatures by using the
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Arrhenius equation:
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k = A exp(− Ea / RT )
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where A is the pre-exponential factor, Ea is the activation energy, R is the gas constant,
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and T is the maximum temperature. Taking natural logarithms on both sides of Eq. (5)
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leads to:
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ln k = − Ea / RT + ln A
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(6)
By plotting lnk versus T, a straight line can be obtained after linear regression. The
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second-order reaction rate constants were applied here because of their higher accuracy
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as aforementioned. The result shows that the activation energy and pre-exponential
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factor are approximately 21 kJ/mol and 0.02 1/s, respectively, with R2 value of 0.96.
383
The activation energy of microwave pyrolysis of corn stover is similar to that of
384
microwave pyrolysis of other lignocellulosic biomass, and it is much lower than those
385
of conventional pyrolysis of various biomass feedstocks [48,49]. The pre-exponential
386
factor of microwave pyrolysis of corn stover is lower than that of microwave pyrolysis
387
of rice straw by approximately one order of magnitude [49].
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4. Conclusions
390 391
The heating efficiency and gaseous yield of microwave pyrolysis of corn stover
17
ACCEPTED MANUSCRIPT increased with decreasing particle size of corn stover. Adding catalyst or applying
393
pretreatment also promoted the reactivity of corn stover pyrolysis. According to the
394
literature, the hemicellulose and cellulose fractions of corn stover may be
395
depolymerized and hydrolyzed during acid pretreatment, resulting in less gas
396
(non-condensable part of pyrolysis product vapor) but more liquid (condensable part)
397
produced. The effect of steam explosion was not significant. A secondary reaction
398
occurred after microwave pyrolysis may exist, where the product vapor released by
399
biomass pyrolysis was entrapped by catalyst particles, and then it further decomposed
400
into light hydrocarbons (CH4, C2H4, C2H6, etc.) and permanent gases (H2, CO, and CO2).
401
However, when the catalyst particles were too small, they could be encapsulated by
402
other catalyst and biomass particles to reduce their catalytic activity. The reaction
403
kinetics of microwave pyrolysis of corn stover can be well described by a second-order
404
reaction model. The activation energy of microwave pyrolysis of corn stover is similar
405
to that of microwave pyrolysis of other lignocellulosic biomass, and it is much lower
406
than those of conventional pyrolysis of various biomass feedstocks.
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409 410
Acknowledgments
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The authors gratefully acknowledge the financial support from the Ministry of
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Education (Project: Development of the effective production of bio-fertilizers recycled
412
from agricultural waste) and Ministry of Science and Technology, Taiwan, ROC (MOST
413
102-2221-E-131-001-MY3).
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Appendix A. Supplementary material
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ACCEPTED MANUSCRIPT 416 417 418
Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/
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[45] Dagnino EP, Chamorro ER, Romano SD, Felissia FE, Area MC. Optimization of the acid pretreatment of rice hulls to obtain fermentable sugars for bioethanol
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[47] Sheng C, Azevedo JLT. Estimating the higher heating value of biomass fuels from basic analysis data. Biomass Bioenerg 2005;28:499–507.
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[48] Dong Q, Xiong YQ. Kinetics study on conventional and microwave pyrolysis of moso bamboo. Bioresour Technol 2014;171:127–31.
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Figure captions
537 538
Fig. 1. Temperature profiles of (a) microwave pyrolysis of corn stover with different particle sizes and (b) catalytic microwave pyrolysis of pretreated 40 mesh corn
540
stover (AP: acid pretreatment; SE: steam explosion; NP: no pretreatment; AC:
541
adding catalyst; NC: no catalyst).
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Fig. 2. Maximum temperatures (a) and heating rates (b) of microwave pyrolysis of corn stover under various conditions.
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Fig. 3. Product distributions of microwave pyrolysis of corn stover: (a) no catalyst, (b) adding 10-mesh Al2O3, (c) adding 50-mesh Al2O3, (d) 40-mesh corn stover
548
with and without pretreatment and catalysis.
551 552 553 554 555 556
Fig. 4. Decomposition of product vapor of catalytic pyrolysis in comparison with conventional pyrolysis.
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Fig. 5. Production of (a) H2, (b) CH4, (c) CO2, and (d) CO from corn stover by
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microwave pyrolysis at different time intervals.
24
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Table 1
558
First-order reaction kinetic analysis of gaseous product from corn stover by microwave
559
pyrolysis. Catalysis (Al2O3)
k1 (1/min)
qm (L)
5 mesh
–
–
0.25
1.37
10 mesh
–
–
0.18
1.61
40 mesh
–
–
0.12
2.21
60 mesh
–
–
0.14
100 mesh
–
–
0.14
5 mesh
–
10 mesh
10 mesh
–
40 mesh
AAE* (%) 25.21
0.84
27.18
0.89
27.24
2.21
0.88
27.46
2.02
0.87
27.49
0.14
1.72
0.88
27.39
10 mesh
0.15
1.92
0.86
27.55
–
10 mesh
0.15
1.96
0.90
27.42
60 mesh
–
10 mesh
0.13
2.00
0.88
27.41
100 mesh
–
10 mesh
0.12
2.13
0.88
27.17
5 mesh
–
50 mesh
0.25
1.35
0.89
25.26
10 mesh
–
50 mesh
0.20
1.34
0.87
26.66
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50 mesh
0.16
1.79
0.88
27.45
–
50 mesh
0.16
1.83
0.88
27.48
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60 mesh
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0.86
40 mesh
560
R2
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Pretreat ment
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Particle size of corn stover
100 mesh
–
50 mesh
0.12
2.11
0.85
27.22
40 mesh
SE
–
0.11
2.06
0.91
26.57
40 mesh
SE
10 mesh
0.10
2.17
0.83
26.64
40 mesh
AP
–
0.13
1.74
0.88
27.33
40 mesh
AP
10 mesh
0.15
1.60
0.90
27.43
*AAE: average absolute error.
561
25
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Table 2
563
Second-order reaction kinetic analysis of gaseous product from corn stover by
564
microwave pyrolysis. Catalysis (Al2O3)
k2 (1/min·L)
qm (L)
5 mesh
–
–
0.15
1.42
10 mesh
–
–
0.10
1.67
40 mesh
–
–
0.05
2.30
60 mesh
–
–
0.05
100 mesh
–
–
0.06
5 mesh
–
10 mesh
10 mesh
–
40 mesh
AAE* (%) 4.42
0.98
5.33
0.98
4.55
2.30
0.98
4.71
2.10
0.97
4.93
0.07
1.78
0.98
4.58
10 mesh
0.07
2.00
0.97
5.09
–
10 mesh
0.07
2.03
0.98
3.76
60 mesh
–
10 mesh
0.06
2.08
0.97
4.83
100 mesh
–
10 mesh
0.05
2.22
0.97
4.74
5 mesh
–
50 mesh
0.16
1.39
0.99
3.95
10 mesh
–
50 mesh
0.13
1.39
0.98
4.53
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50 mesh
0.08
1.86
0.98
4.62
–
50 mesh
0.07
1.90
0.98
4.67
AC C
60 mesh
SC
0.99
40 mesh
565
R2
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Pretreat ment
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Particle size of corn stover
100 mesh
–
50 mesh
0.05
2.19
0.96
5.37
40 mesh
SE
–
0.05
2.14
0.98
3.95
40 mesh
SE
10 mesh
0.04
2.26
0.95
5.83
40 mesh
AP
–
0.06
1.81
0.97
4.86
40 mesh
AP
10 mesh
0.08
1.65
0.98
4.41
*AAE: average absolute error.
566
26
569 570 571 572
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Fig. 1. Temperature profiles of (a) microwave pyrolysis of corn stover with different particle sizes and (b) catalytic microwave pyrolysis of pretreated 40 mesh corn
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stover (AP: acid pretreatment; SE: steam explosion; NP: no pretreatment; AC: adding catalyst; NC: no catalyst).
573
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577 578
corn stover under various conditions.
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Fig. 2. Maximum temperatures (a) and heating rates (b) of microwave pyrolysis of
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579 580
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Fig. 3. Product distributions of microwave pyrolysis of corn stover: (a) no catalyst, (b)
581
adding 10-mesh Al2O3, (c) adding 50-mesh Al2O3, (d) 40-mesh corn stover
582
with and without pretreatment and catalysis.
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586
Fig. 4. Decomposition of product vapor of catalytic pyrolysis in comparison with conventional pyrolysis.
587
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589 590 591
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Fig. 5. Production of (a) H2, (b) CH4, (c) CO2, and (d) CO from corn stover by microwave pyrolysis at different time intervals.
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ACCEPTED MANUSCRIPT Highlights:
Gaseous yield increased with decreasing particle size of corn stover.
Catalytic effect was not found when catalyst particles were small.
Liquid yield was increased when acid pretreatment was applied.
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S0360-5442(17)31872-8
DOI:
10.1016/j.energy.2017.11.022
Reference:
EGY 11810
To appear in:
Energy
Received Date: 18 April 2017 Revised Date:
26 October 2017
Accepted Date: 5 November 2017
Please cite this article as: Huang Y-F, Kuan W-H, Chang C-Y, Effects of particle size, pretreatment, and catalysis on microwave pyrolysis of corn stover, Energy (2017), doi: 10.1016/j.energy.2017.11.022. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT 1
Effects of particle size, pretreatment, and catalysis on microwave
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pyrolysis of corn stover
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Yu-Fong Huang a, Wen-Hui Kuan b,*, Chun-Yuan Chang b
4 5 a
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Chou-Shan Rd., Taipei 106, Taiwan, ROC b
Department of Safety, Health and Environmental Engineering, Ming Chi University of Technology, 84 Gong-Juan Rd., Taishan, New Taipei City 243, Taiwan, ROC
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Graduate Institute of Environmental Engineering, National Taiwan University, 71
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*
Corresponding author. Tel.: +886 2 2908 9899 ext 4653; fax: +886 2 2908 0346. Email address: [email protected] (W.-H. Kuan).
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Abstract
12 13
The effects of particle size, pretreatment (acid pretreatment and steam explosion), and catalysis of aluminum oxide (Al2O3) on heating efficiency, product distribution, and
15
gaseous product composition of corn stover pyrolysis using microwave heating were
16
investigated in this study. Both maximum temperature and heating rate increased with
17
decreasing particle size of corn stover. The heating efficiency was also improved over
18
Al2O3 and by applying both pretreatment methods. Adding 10-mesh Al2O3 increased the
19
gaseous yield but decreased the liquid yield. However, this phenomenon did not exist
20
when 50-mesh Al2O3 was used. This may be attributable to that small catalyst particles
21
are encapsulated by other catalyst and biomass particles to reduce their catalytic activity.
22
Applying acid pretreatment substantially decreased the gaseous yield but increased the
23
liquid yield, whereas the effect of steam explosion was not significant. Approximately
24
half of CO was produced during the first 5 min of experiment, but the yields of H2, CH4,
25
and CO2 produced from microwave pyrolysis of corn stover pretreated by acid were
26
relatively small or even none during this initial period. The reaction kinetics for
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microwave pyrolysis of corn stover was analyzed by using the first- and second-order
28
reaction models.
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Keywords: Microwave pyrolysis; Corn stover; Particle size; Pretreatment; Catalysis; Reaction kinetics
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1. Introduction
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Renewable energy systems, such as solar photovoltaics, wind power, and biofuels, have attracted increasing interest mainly because of concerns about growing demand for
38
limited, non-renewable fossil fuels and issues related to climate change (i.e., global
39
warming, extreme events and disasters, sea ice melting, sea level rising, ocean
40
acidification, etc.) caused by emissions of anthropogenic greenhouse gases [1–7]. The
41
world’s total energy consumption has been almost doubled from 4,667 Mtoe (million
42
tonnes of oil equivalent) in 1973 to 9,301 Mtoe in 2013 [8]. The Paris Agreement in the
43
21st Conference of the Parties (COP 21) to the United Nations Framework Convention
44
on Climate Change (UNFCCC) has reset the collective global climate ambition from the
45
aspiration of keeping temperature rise below 2 °C (relative to pre-industrial levels), to a
46
new aim of limiting it to well below 2 °C with efforts to pursue 1.5 °C, so attention
47
must turn to further reducing emissions remaining within energy scenarios consistent
48
with 2 °C warming [9]. The Paris Agreement is the first climate deal with universal
49
contributions to mitigation action, and its strengthened long-term objectives require
50
even stronger actions than previously identified [10]. Therefore, the research and
51
development of renewable energy is so important and urgent, to reduce the dependence
52
on fossil fuels and thus to mitigate the climate change.
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Biomass energy (bioenergy) is one of the forms of renewable energy. Biomass has
55
been recognized as an abundant carbon-neutral renewable resource for the production of
56
value-added bioenergy and biomaterials [1–3]. In contrast to other renewables
57
supplying heat and power, biomass is the only currently source of solid, liquid, and
3
ACCEPTED MANUSCRIPT gaseous fuels [11]. Besides, negative carbon dioxide emissions can be accomplished by
59
using biochar (solid carbonaceous residues from biomass pyrolysis) for carbon
60
sequestration [12] or by bio-energy with carbon capture and storage (BECCS) [13].
61
Lignocellulosic biomass is mainly composed of structural constituents (hemicelluloses,
62
cellulose, and lignin), and it can be converted into bioenergy by using thermochemical
63
and biological processes [14]. Pyrolysis, which is the direct thermal decomposition of
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biomass in the absence of oxygen to obtain solid, liquid, and gaseous products, has been
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widely recognized as the preferred conversion method of various thermochemical
66
processes available [11,14–16].
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Biomass pyrolysis is conventionally carried out by means of electric heating furnaces. However, microwave heating can offer a number of advantages over
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conventional heating such as: (i) higher heating rates and efficiencies, (ii) non-contact
71
heating, (iii) material-selective heating, (iv) volumetric and uniform heating, (v) energy
72
transfer rather than heat transfer, (vi) energy savings at significantly reduced processing
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temperatures (150–300 °C), (vii) less feedstock pretreatment needed, (viii) quick
74
start-up and stopping, (ix) greater control and safety, and (x) reduced equipment size
75
and waste [17–22]. Therefore, microwave heating has been applied in various fields.
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Biomass pyrolysis heated by microwave irradiation has been proven as an efficient
77
technique for the production of fuels and materials [20,22–26]. More work is still
78
needed to extend existing understanding of microwave pyrolysis characteristics in order
79
to improve the technique and then to transform it into a commercially viable route for
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the production of fuels and materials in an environmentally sustainable manner [24].
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Corn stover, the crop residue left in cornfields, is produced at a ratio of approximately 1 kg per kg of corn grain (on dry basis) [27]. Corn stover is abundant
84
since more than 216 Tg can be produced annually in the United States [28]. The annual
85
corn stover production was approximately 114 Gg in Taiwan, based on the corn grain
86
production data in 2014 [29]. Corn is one of the most important crops in the world, so
87
its residues can be regarded as a sustainable bioenergy resource [27,30]. This study was
88
aimed at investigating the effects of particle size, catalysis, and pretreatment on the
89
gaseous products produced from corn stover by using microwave pyrolysis. A number
90
of studies have focused on the effect of either particle size or catalysis on microwave
91
pyrolysis of biomass, but there is seldom work about the effect of pretreatment on
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product distribution as well as the explanation of the effect. Besides, the reaction
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kinetics for microwave pyrolysis of corn stover under different conditions was analyzed.
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2. Material and methods
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2.1 Materials
Raw corn stover feedstock was collected in the cornfields and markets of Taiwan.
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Prior to pretreatment and microwave pyrolysis, the corn stover was naturally dried by
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exposure to air in an open area for 14 days, and then it was mechanically shredded and
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manually sieved. By using different sieves, five corn stover samples with different
103
particle sizes were obtained: >4.00, 2.00–4.00, 0.425–2.00, 0.250–0.425, and
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0.150–0.250 mm, simply named as 5, 10, 40, 60, and 100 mesh, respectively, in this
105
study. The catalyst used in this study was Al2O3 purchased from Kokusan Chemical
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Works (Japan), and it was sieved by 10 and 50 mesh screens.
107 108
2.2 Pretreatment
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112
stirring into an aqueous solution containing 1 mL deionized water and 10 mL
113
phosphoric acid (85 %). Sufficient materials for all experiments were pretreated in
114
advance. Phosphoric acid was used in this study because it is one of the common acids
115
for the acid pretreatment of lignocellulosic biomass [31,32]. The solution was heated in
116
a 323 K water bath for 10 h, and then it was mixed with 50 mL deionized water with
117
strong stirring. Afterwards the corn stover was washed with plenty of deionized water.
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The pH value of corn stover was adjusted to neutral by using a NaOH solution. In the
119
steam explosion, the dry corn stover was put into an autoclave under the condition of
120
160–260 °C and 6–34 bar. The environmental condition was held for 1 min when it was
121
reached. The pretreated corn stover was naturally dried by exposure to air in an open
122
area for 14 days and shredded to the designated particle size.
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2.3 Experimental apparatus
Microwave pyrolysis of corn stover was carried by using a single-mode microwave
127
oven operating at a frequency of 2.45 GHz. The microwave oven was purchased from
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MUEGGE (Germany). The layout of the overall microwave pyrolysis system can be
129
found elsewhere [33]. Both reaction tube (40 cm in length and 5 cm in outer diameter)
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A K-type thermocouple sensor was placed outside the bottom of the quartz crucible to
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measure the real-time temperature of corn stover sample. Pure nitrogen gas (99.99%)
133
was purged into the system at a flow rate of 50 mL/min to ensure the anoxic condition
134
in the reaction system. The actual working microwave power level was determined by
135
the incident power level subtracting the reflected power level.
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The corn stover sample or its blend with Al2O3 was added to the quartz crucible and then placed inside the quartz reaction tube. Each pretreated or non-pretreated corn
141
stover was weighed to 2 g. In catalytic pyrolysis experiments, 2 g corn stover was
142
blended with the catalyst by 5 wt% (i.e., totally 2.1 g). The height of the quartz crucible
143
was adjusted to be located in the path of microwave propagation. After sufficient
144
nitrogen gas purging, the power supply was turned on and the adjustment knob was
145
switched to a microwave power level of 500 W. The power level was selected because
146
of insufficient and over heating for biomass pyrolysis at lower and higher power levels,
147
respectively. After microwave irradiation for 20 min, the power supply was turned off,
148
and the carrier gas purging was stopped. The vapor produced during the experiment
149
immediately passed through a condenser tube whose temperature was controlled at 4 °C
150
by a thermostat. The condensable and non-condensable parts of the product vapor were
151
regarded as liquid and gaseous products, respectively. The gaseous product was
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collected every 5 min in a 5 L Tedlar bag. After self-cooled down to room temperature,
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the solid residues remained in the quartz crucible were removed, weighed, and then
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stored in a desiccator. All of the experiments were performed at least in triplicate to
155
obtain average values for the experimental results.
156 2.5 Analytical methods
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The proximate analysis of corn stover was carried out according to the standard
test method E1131–08 of the American Society for Testing and Materials (ASTM). The
161
moisture content of corn stover was 4.97 wt%. The volatile matter, fixed carbon, and
162
ash contents (on dry basis) of corn stover were 90.59, 6.94, and 2.47 wt%, respectively.
163
The higher heating value (HHV) of corn stover was measured in a CAL2K ECO
164
adiabatic oxygen bomb calorimeter. Each sample (approximately 0.5 g) was dried at 105
165
°C prior to the heating value analysis. The average HHV of corn stover was 18.41
166
MJ/kg. Gaseous product analysis was carried out by using the Perkin–Elmer Auto
167
System XL gas chromatography–thermal conductivity detector (GC–TCD) equipped
168
with a Supelco Carboxen 1010 PLOT column. The temperatures of injector, oven, and
169
detector were controlled at 120, 100, and 150 °C, respectively. The flow rate of carrier
170
gas (He or N2) was set at 10 mL/min (25:1 split).
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3. Results and discussion
3.1 Temperature profiles
175 176 177
The temperature profiles of microwave pyrolysis of corn stover are illustrated in Fig. 1. As can be seen in, both maximum temperature and heating rate generally
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179
100 mesh corn stover were 475, 478, 557, 522, and 557 °C, and the average heating
180
rates during the first 5 min were 84.6, 75.4, 95.2, 86.4, and 94.6 °C/min, respectively.
181
The reaction temperature was continuously recorded to obtain the real-time temperature
182
change and the arithmetic average heating rate. The average heating rate during the first
183
5 min was considered because the heating ramp in this period was much more
184
significant than afterwards, no matter which particle size it was. The effects of
185
pretreatment and catalysis on the temperature profiles of microwave pyrolysis of corn
186
stover can be observed by Fig. 1b. It can be seen that, without any pretreatment, the
187
effect of adding 10-mesh Al2O3 was insignificant. However, after either acid
188
pretreatment or steam explosion, the catalytic effect turned to be substantial.
189
Furthermore, after acid pretreatment, the maximum temperature of catalytic microwave
190
pyrolysis of corn stover was highest.
193 194
Figure 1
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To further observe the effects of pretreatment and catalysis, the maximum temperatures and average heating rates during the first 5 min of microwave pyrolysis of
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corn stover under various conditions are shown in Fig. 2. In general, both maximum
197
temperature and heating rate increased with decreasing particle size of corn stover, no
198
matter the catalyst was added or not. When the particle size was reduced from 5 mesh to
199
100 mesh, the maximum temperature and heating rate increased by 67–82 °C and 10–18
200
°C/min, respectively. Besides, the addition of catalyst promoted both maximum
201
temperature and heating rate in some cases. After acid pretreatment and steam explosion,
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203
(approximately 106 °C/min), and the heating rates of catalytic microwave pyrolysis
204
were also close to each other (approximately 110 °C/min). On the other hand, the
205
maximum temperatures of microwave pyrolysis of corn stover were not so different
206
(604–610 °C). However, the maximum temperature of catalytic microwave pyrolysis of
207
acid pretreated corn stover was 654 °C, much higher than that (628 °C) when steam
208
explosion was applied. According to the aforementioned experimental results, it is clear
209
that all the three factors (particle size, pretreatment, and catalysis) had substantial
210
effects to increase the efficiency of microwave heating. In addition, the effect of
211
pretreatment seems to be stronger than that of catalysis. Microwave heating can be
212
referred to as dielectric heating [17,19]. The different maximum temperatures and
213
heating rates could be attributable to the difference in dielectric property of corn stover
214
with and without catalyst or pretreatment, since the composition of the biomass would
215
be changed after the processes.
216
219 220 221
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Figure 2
3.2 Product distribution
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After microwave pyrolysis, corn stover was converted into solid, liquid, and
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gaseous products. The three-phase product distributions under various operational
223
conditions are shown in Fig. 3. It should be noted that the quantities of solid and
224
gaseous products were directly measured, and the solid amount is without the catalyst.
225
However, the percentage of liquid product was taken as the difference between 100 and
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227
product. In general, gaseous yield increased but both solid and liquid yields decreased
228
with decreasing particle size. The addition of 10-mesh Al2O3 increased the gaseous
229
yield by approximately 4–9 wt%, but adding 50-mesh Al2O3 decreased the yield by
230
approximately 3–7 wt%. The results indicate that the relatively small particle size of
231
catalyst may hinder the further (secondary) degradation of the organic vapor released by
232
corn stover pyrolysis. Consequently, the three-phase product distributions of microwave
233
pyrolysis of corn stover with and without 50-mesh Al2O3 were similar to each other. The
234
gaseous yields of microwave pyrolysis of 10–100 mesh corn stover samples were up to
235
70–72 wt%, when the 10-mesh Al2O3 was added. The gaseous yield of 60-mesh corn
236
stover without adding catalyst was 73 wt%. This could be attributable to the impurity of
237
biomass sample or the unknown characteristics of microwave heating. It is a
238
coincidence that the product yields of 40-mesh in Fig. 3c are very close to those of
239
SE-NC in Fig. 3d. This may imply that the addition of 50-mesh Al2O3 and the
240
pretreatment of steam explosion provide similar effect on product distribution. It has
241
been reported that the gaseous yield of catalytic microwave pyrolysis was
242
approximately 40–60 wt% [34–36], lower than the data in this study. This could be
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attributable to the catalytic effect accomplished by Al2O3 and the heating efficiency
244
provided by single-mode microwave oven.
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Figure 3
247 248 249
The catalytic effect and the difference in the product distributions of biomass pyrolysis over 10- and 50-mesh Al2O3 can be explained by Fig. 4. The product vapor
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ACCEPTED MANUSCRIPT released by biomass pyrolysis leaves the reaction system within a short time (depending
251
on the flow rate of carrier gas and the configuration of reactor and pipelines), when
252
there is no catalyst added. By using a condenser, the product vapor is divided into
253
condensable and non-condensable parts, namely liquid and gaseous products,
254
respectively. When a catalyst is added to the biomass, part of the product vapor can be
255
entrapped by the catalyst particles and then decomposes into smaller substances such as
256
light hydrocarbons (CH4, C2H4, C2H6, etc.) and permanent gases (H2, CO, and CO2)
257
[37–39]. Consequently, catalytic pyrolysis produces more gaseous product than
258
conventional pyrolysis. However, if the catalyst particles are too small, they may be
259
encapsulated by other catalyst and biomass particles to reduce their catalytic activity. As
260
a result, the gaseous yield of biomass pyrolysis over small catalyst particles would be
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lower than expected.
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Figure 4
After acid pretreatment, the liquid yield of microwave pyrolysis of corn stover
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substantially increased by approximately 15 wt% whereas the gaseous yield decreased
267
by approximately 14 wt%. The result could be attributable to the depolymerization and
268
hydrolysis of hemicellulose and cellulose during the acid pretreatment [40–45], so there
269
was less formation of primary products that can be further converted into permanent
270
gases. On the other hand, the effect of steam explosion pretreatment was not significant.
271
The product distribution of microwave pyrolysis of corn stover pretreated by steam
272
explosion was not so different from that without any pretreatment. Therefore, the extent
273
of hemicellulose and cellulose solubilization accomplished by steam explosion could be
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275
microwave pyrolysis of pretreated corn stover was higher than that without adding
276
catalyst by approximately 3 wt%, no matter which pretreatment method was used. The
277
solid yields, which were nearly the same under different operational conditions, could
278
be regarded as an allowable minimum value for the microwave pyrolysis of corn stover
279
at a microwave power level of 500 W.
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The quantities of H2, CH4, CO2, and CO produced by microwave pyrolysis of 40-mesh corn stover with and without pretreatment and catalysis are shown in Fig. 5.
285
For the total processing time of 20 min, the overall production quantities of H2, CH4,
286
CO2, and CO under different conditions were in the ranges of 0.001–0.05, 0.02–0.04,
287
0.01–0.05, and 0.91–1.15 L, respectively. It can be seen that of quantity of CO was
288
mush bigger than those of other gases. The composition of gaseous product was
289
different from those reported in the literature [35,36]. This could be attributable to the
290
difference in product selectivity among the catalysts (char-supported metallic catalysts
291
[35], HZSM-5 aluminosilicate zeolite [36], and Al2O3 in this study). Approximately half
292
of CO was produced during the first 5 min, and then it was stably produced until the end
293
of the process. H2, CH4, and CO2 were not regularly generated as CO was. Most of them
294
were produced during 5–20 min. In general, adding catalyst increased but applying
295
pretreatment decreased the production quantities of the gases. When the corn stover was
296
acid pretreated, the quantity of H2 was largely reduced from 0.04–0.05 L to 0.001–0.005
297
L. It has been reported that the formation of H2 from agricultural residues by microwave
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299
hemicellulose or even cellulose of corn stover would be partly removed during the acid
300
treatment, resulting in its lower yield of H2. In fact, all the four gases should be
301
primarily generated from the hemicellulose and cellulose fractions of corn stover, so it
302
is not surprising that their production quantities were reduced after acid pretreatment.
303
Furthermore, the productions of H2, CH4, and CO2 were relatively small or even none
304
during the first 5 min, where the reaction temperature had not yet reached the maximum
305
temperature. The lower temperature means that the more gases were produced from
306
components of biomass with higher thermal reactivity (i.e., hemicellulose and cellulose).
307
This provides another proof that the acid pretreatment should take the responsibility for
308
the reduction on H2, CH4, and CO2 yields.
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309 Figure 5
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3.4 Reaction kinetics
In this study, the reaction kinetics for microwave pyrolysis of corn stover under different conditions was analyzed by using a first-order reaction model:
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dqt = k1 (qm − qt ) dt
(1)
317
where k1 is the first-order reaction rate constant, qm is the maximum quantity of gaseous
318
product from corn stover by microwave pyrolysis, and qt is the quantity of gaseous
319
product at any time, t. The condensable gases were not considered here. By plotting
320
dqt/dt versus qt, a straight line can be obtained after linear regression. The first-order
321
reaction rate constant and the maximum quantity of gaseous product can be determined
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from the slope and intercept of the straight line, respectively. The result of first-order
323
reaction kinetic analysis is listed in Table 1.
324 Table 1
326 327
Besides, a second-order reaction model was also applied:
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dqt 2 = k 2 (qm − qt ) dt
329
where k2 is the second-order reaction rate constant. Integrating Eq. (2) followed by a
330
rearrangement results in:
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(2)
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(3)
By plotting t/qt versus t, a straight line can be obtained after linear regression. The
333
maximum quantity of gaseous product and the second-order reaction rate constant can
334
be determined from the slope and intercept of the straight line, respectively. The result
335
of second-order reaction kinetic analysis is listed in Table 2. Furthermore, to testify the
336
accuracy of the two reaction kinetics models, the average absolute error (AAE) was
337
applied [46,47]:
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AAE =
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(4)
339
where n is the number of data points, and qtexp and qtcal are experimental and calculated
340
quantities of gaseous product, respectively.
341 342
Table 2
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345
range of 0.83–0.91, lower than those of the second-order reaction model (0.95–0.99).
346
Under different conditions, k1 and k2 are between 0.10–0.25 and 0.04–0.16, respectively.
347
The maximum quantities of gaseous product determined by the two models are not very
348
different. The maximum quantities determined by the first-order reaction and the
349
second-order reaction models are in the ranges of 1.34–2.21 L and 1.39–2.30 L,
350
respectively. Both of the models are helpful to predict the allowable maximum gaseous
351
yield of microwave pyrolysis of corn stover. However, the AAE of the second-order
352
reaction model is much lower than that of the first-order reaction model. Therefore, the
353
reaction kinetics of gaseous production from corn stover by microwave pyrolysis can be
354
better fitted by the second-order reaction model. However, it should be noted that the
355
first-order reaction model and the second-order reaction model are differential and
356
integral, respectively. Therefore, the different kinetic parameters and fitting errors could
357
be also attributable to the different methods. Both of the models indicate that there were
358
the highest reaction rate constants when the 5-mesh corn stover was microwave
359
pyrolyzed with and without 50-mesh Al2O3, but the maximum quantity of gaseous
360
product was lowest. These results show that the driving force of reaction may mainly
361
come from the magnitude of maximum gaseous productivity. As aforementioned, the
362
product distribution of microwave pyrolysis of corn stover was similar to that of
363
catalytic microwave pyrolysis using 50-mesh Al2O3. Although adding 10-mesh Al2O3
364
substantially reduced the reaction rate constants, the larger catalyst provided higher
365
maximum quantity of gaseous product that is the primary driving force of reaction.
366
There was the lowest reaction rate constant when the 40-mesh corn stover pretreated by
367
steam explosion was microwave pyrolyzed with 10-mesh Al2O3. However, the effect of
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acid pretreatment on the reaction rate constant was insignificant.
369 370
The kinetic parameters (activation energy and pre-exponential factor) of microwave pyrolysis of corn stover can be determined from the reaction rate constants
372
as above mentioned and their corresponding reaction temperatures by using the
373
Arrhenius equation:
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374
k = A exp(− Ea / RT )
375
where A is the pre-exponential factor, Ea is the activation energy, R is the gas constant,
376
and T is the maximum temperature. Taking natural logarithms on both sides of Eq. (5)
377
leads to:
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ln k = − Ea / RT + ln A
378
(6)
By plotting lnk versus T, a straight line can be obtained after linear regression. The
380
second-order reaction rate constants were applied here because of their higher accuracy
381
as aforementioned. The result shows that the activation energy and pre-exponential
382
factor are approximately 21 kJ/mol and 0.02 1/s, respectively, with R2 value of 0.96.
383
The activation energy of microwave pyrolysis of corn stover is similar to that of
384
microwave pyrolysis of other lignocellulosic biomass, and it is much lower than those
385
of conventional pyrolysis of various biomass feedstocks [48,49]. The pre-exponential
386
factor of microwave pyrolysis of corn stover is lower than that of microwave pyrolysis
387
of rice straw by approximately one order of magnitude [49].
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4. Conclusions
390 391
The heating efficiency and gaseous yield of microwave pyrolysis of corn stover
17
ACCEPTED MANUSCRIPT increased with decreasing particle size of corn stover. Adding catalyst or applying
393
pretreatment also promoted the reactivity of corn stover pyrolysis. According to the
394
literature, the hemicellulose and cellulose fractions of corn stover may be
395
depolymerized and hydrolyzed during acid pretreatment, resulting in less gas
396
(non-condensable part of pyrolysis product vapor) but more liquid (condensable part)
397
produced. The effect of steam explosion was not significant. A secondary reaction
398
occurred after microwave pyrolysis may exist, where the product vapor released by
399
biomass pyrolysis was entrapped by catalyst particles, and then it further decomposed
400
into light hydrocarbons (CH4, C2H4, C2H6, etc.) and permanent gases (H2, CO, and CO2).
401
However, when the catalyst particles were too small, they could be encapsulated by
402
other catalyst and biomass particles to reduce their catalytic activity. The reaction
403
kinetics of microwave pyrolysis of corn stover can be well described by a second-order
404
reaction model. The activation energy of microwave pyrolysis of corn stover is similar
405
to that of microwave pyrolysis of other lignocellulosic biomass, and it is much lower
406
than those of conventional pyrolysis of various biomass feedstocks.
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Acknowledgments
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The authors gratefully acknowledge the financial support from the Ministry of
411
Education (Project: Development of the effective production of bio-fertilizers recycled
412
from agricultural waste) and Ministry of Science and Technology, Taiwan, ROC (MOST
413
102-2221-E-131-001-MY3).
414 415
Appendix A. Supplementary material
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Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/
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from lignocellulosic biomass by catalytic microwave pyrolysis. Bioresour Technol
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2011;102:7004–7.
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microwave pyrolysis of rice husk with char-supported metallic catalysts. Bioresour
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[37] Li S, Xu S, Liu S, Yang C, Lu Q. Fast pyrolysis of biomass in free-fall reactor for
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promising technologies for pretreatment of lignocellulosic biomass. Bioresour
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Figure captions
537 538
Fig. 1. Temperature profiles of (a) microwave pyrolysis of corn stover with different particle sizes and (b) catalytic microwave pyrolysis of pretreated 40 mesh corn
540
stover (AP: acid pretreatment; SE: steam explosion; NP: no pretreatment; AC:
541
adding catalyst; NC: no catalyst).
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Fig. 2. Maximum temperatures (a) and heating rates (b) of microwave pyrolysis of corn stover under various conditions.
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Fig. 3. Product distributions of microwave pyrolysis of corn stover: (a) no catalyst, (b) adding 10-mesh Al2O3, (c) adding 50-mesh Al2O3, (d) 40-mesh corn stover
548
with and without pretreatment and catalysis.
551 552 553 554 555 556
Fig. 4. Decomposition of product vapor of catalytic pyrolysis in comparison with conventional pyrolysis.
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Fig. 5. Production of (a) H2, (b) CH4, (c) CO2, and (d) CO from corn stover by
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microwave pyrolysis at different time intervals.
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Table 1
558
First-order reaction kinetic analysis of gaseous product from corn stover by microwave
559
pyrolysis. Catalysis (Al2O3)
k1 (1/min)
qm (L)
5 mesh
–
–
0.25
1.37
10 mesh
–
–
0.18
1.61
40 mesh
–
–
0.12
2.21
60 mesh
–
–
0.14
100 mesh
–
–
0.14
5 mesh
–
10 mesh
10 mesh
–
40 mesh
AAE* (%) 25.21
0.84
27.18
0.89
27.24
2.21
0.88
27.46
2.02
0.87
27.49
0.14
1.72
0.88
27.39
10 mesh
0.15
1.92
0.86
27.55
–
10 mesh
0.15
1.96
0.90
27.42
60 mesh
–
10 mesh
0.13
2.00
0.88
27.41
100 mesh
–
10 mesh
0.12
2.13
0.88
27.17
5 mesh
–
50 mesh
0.25
1.35
0.89
25.26
10 mesh
–
50 mesh
0.20
1.34
0.87
26.66
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50 mesh
0.16
1.79
0.88
27.45
–
50 mesh
0.16
1.83
0.88
27.48
AC C
60 mesh
SC
0.86
40 mesh
560
R2
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Pretreat ment
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Particle size of corn stover
100 mesh
–
50 mesh
0.12
2.11
0.85
27.22
40 mesh
SE
–
0.11
2.06
0.91
26.57
40 mesh
SE
10 mesh
0.10
2.17
0.83
26.64
40 mesh
AP
–
0.13
1.74
0.88
27.33
40 mesh
AP
10 mesh
0.15
1.60
0.90
27.43
*AAE: average absolute error.
561
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Table 2
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Second-order reaction kinetic analysis of gaseous product from corn stover by
564
microwave pyrolysis. Catalysis (Al2O3)
k2 (1/min·L)
qm (L)
5 mesh
–
–
0.15
1.42
10 mesh
–
–
0.10
1.67
40 mesh
–
–
0.05
2.30
60 mesh
–
–
0.05
100 mesh
–
–
0.06
5 mesh
–
10 mesh
10 mesh
–
40 mesh
AAE* (%) 4.42
0.98
5.33
0.98
4.55
2.30
0.98
4.71
2.10
0.97
4.93
0.07
1.78
0.98
4.58
10 mesh
0.07
2.00
0.97
5.09
–
10 mesh
0.07
2.03
0.98
3.76
60 mesh
–
10 mesh
0.06
2.08
0.97
4.83
100 mesh
–
10 mesh
0.05
2.22
0.97
4.74
5 mesh
–
50 mesh
0.16
1.39
0.99
3.95
10 mesh
–
50 mesh
0.13
1.39
0.98
4.53
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50 mesh
0.08
1.86
0.98
4.62
–
50 mesh
0.07
1.90
0.98
4.67
AC C
60 mesh
SC
0.99
40 mesh
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R2
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Pretreat ment
TE D
Particle size of corn stover
100 mesh
–
50 mesh
0.05
2.19
0.96
5.37
40 mesh
SE
–
0.05
2.14
0.98
3.95
40 mesh
SE
10 mesh
0.04
2.26
0.95
5.83
40 mesh
AP
–
0.06
1.81
0.97
4.86
40 mesh
AP
10 mesh
0.08
1.65
0.98
4.41
*AAE: average absolute error.
566
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569 570 571 572
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Fig. 1. Temperature profiles of (a) microwave pyrolysis of corn stover with different particle sizes and (b) catalytic microwave pyrolysis of pretreated 40 mesh corn
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stover (AP: acid pretreatment; SE: steam explosion; NP: no pretreatment; AC: adding catalyst; NC: no catalyst).
573
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corn stover under various conditions.
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Fig. 2. Maximum temperatures (a) and heating rates (b) of microwave pyrolysis of
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579 580
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Fig. 3. Product distributions of microwave pyrolysis of corn stover: (a) no catalyst, (b)
581
adding 10-mesh Al2O3, (c) adding 50-mesh Al2O3, (d) 40-mesh corn stover
582
with and without pretreatment and catalysis.
583
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586
Fig. 4. Decomposition of product vapor of catalytic pyrolysis in comparison with conventional pyrolysis.
587
AC C
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589 590 591
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Fig. 5. Production of (a) H2, (b) CH4, (c) CO2, and (d) CO from corn stover by microwave pyrolysis at different time intervals.
592
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ACCEPTED MANUSCRIPT Highlights:
Gaseous yield increased with decreasing particle size of corn stover.
Catalytic effect was not found when catalyst particles were small.
Liquid yield was increased when acid pretreatment was applied.
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