Upgrading bio-oil produced from the catalytic pyrolysis of sugarcane (Saccharum officinarum L) straw using calcined dolomite

Sustainable Chemistry and Pharmacy 6 (2017) 114–123

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Upgrading bio-oil produced from the catalytic pyrolysis of sugarcane (Saccharum officinarum L) straw using calcined dolomite

MARK

⁎

Witchakorn Charusiria, , Tharapong Vitidsantb,c a b c

Faculty of Environmental Culture and Ecotourism, Srinakharinwirot University, 114 Soi Sukhumwit 23, Wattana, Bangkok 10110, Thailand Department of Chemical Technology, Faculty of Science, Chulalongkorn University, 254 Phayathai Rd., Patumwan, Bangkok 10330, Thailand Centre of Fuel and Energy from Biomass, Faculty of Science, Chulalongkorn University, Kaengkhoi-Banna Rd., Kaengkhoi, Saraburi 18110, Thailand

A R T I C L E I N F O

A B S T R A C T

Keywords: Catalytic pyrolysis Upgrading bio-oil Sugarcane straw Dolomite

In this study, the catalytic pyrolysis of sugarcane straw (SCS) into bio-oil and chemicals using calcined dolomite was applied for upgraded bio-oil production. Experiments were performed in a custom-built SS316 tube reactor, and the effects of the pyrolysis parameters, including the different dolomite calcination conditions, temperature (400-600 °C), biomass feed rate (0.3–1.2 kg h−1), sweeping gas flow rate (80–200 cm3 min−1) and average size distribution (250–1000 µm), were systematically investigated. The results showed that the SCS catalytic pyrolysis process obtained liquid yields of 36.15 wt%, gas yields of 52.09 wt% and solid yields of 11.76 wt% when using a pyrolysis temperature, biomass feed rate, nitrogen sweep gas, and average biomass size of 450 °C, 0.6 kg h−1, 80 cm3 min−1 and 500 µm, respectively, with 10 wt% calcined dolomite. The calcined dolomite influenced the bio-oil components from the carbonylation and the cracking of volatile vapor and resulted in an upgraded bio-oil with a lower oxygen content, higher gross calorific value and decreased acid corrosion.

1. Introduction Over the last few decades, the depletion of the petroleum reserves and the increase in energy demands due to economic growth and development have caused an energy price crisis. The continuous increase of carbon dioxide emissions and increased awareness of global warming have drawn more attention to bioenergy alternatives to fossil fuels. A lignocellulosic residue from agricultural harvesting and industrial production is considered an alternative energy source and can be a potential reserve of energy with a minimal environmental impact. Biomass has been extensively evaluated to mitigate environmental pollution and does not add to the emission of green-house gases into the atmosphere compared to petroleum energy. Biomass is an established part of the natural carbon cycle. Furthermore, biomass has very low amounts of sulfur and nitrogen compared with other commercial energy sources. Biomass conversion into bioenergy and value feedstocks for chemical processes via thermochemical processes, such as directed combustion, gasification, liquefaction and pyrolysis, has attracted interest as a sustainable energy source (Sutton et al., 2001; Yang et al., 2014). The pyrolysis of lignocellulosic material degrades the material via thermal decomposition, which results in oxygen disappearing as a volatile vapor and condenses the material into a bio-oil. A solid residue and small gaseous products are also produced depending on the thermal

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decomposition process conditions (Angin, 2013; Aysu and Kucuk, 2014). Typically, the pyrolysis temperature and heating rate are produced via a heat transfer from a furnace to the lignocellulosic particles and via heat transfer within the particles. At high pyrolysis temperatures, the yield of bio-oil and aqueous products ranges from 30 to 60 wt %. The bio-oil is composed of very complex compounds, mainly phenol and benzene derivatives and oxygenated compounds, and it requires upgrading before it can be used as a potential fuel in diesel engines (Demiral and Ayan, 2011; Bertero et al., 2014; Morali et al., 2016). In addition, the yield and composition also depend on the biomass characteristics and pyrolysis conditions (temperature, heating rate, and residence time) (Wang et al., 2009; Lee et al., 2010; Duman et al., 2011; Jeong et al., 2015; Henkel et al., 2016). Sugarcane (Saccharum officinarum L.) is an agricultural product in Thailand produced on approximately 8.60 million hectares per year, yielding more than 2.0 × 109 t per year. Assuming a crop to residue ratio of 0.302 for sugarcane straw (SCS), the potential energy from sugarcane residue is 96 million tons of oil equivalent (MTOE) (Pattiya, 2011). Typically, SCS is not collected for utilization after harvesting. The leaves and tops of the sugarcane are burned in the cultivated area to prepare the area for the next season, which is not an efficient use for energy production. The bulky characteristics of this straw have prevented its collection to convert it into energy, and its collection would

Corresponding author. E-mail address: [email protected] (W. Charusiri).

http://dx.doi.org/10.1016/j.scp.2017.10.005 Received 11 August 2017; Received in revised form 19 October 2017; Accepted 20 October 2017 2352-5541/ © 2017 Elsevier B.V. All rights reserved.

Sustainable Chemistry and Pharmacy 6 (2017) 114–123

W. Charusiri, T. Vitidsant

effective catalysts, such as zeolites (Wang et al., 2009; Babich et al., 2011; Choi et al., 2015; Conesa and Domene, 2015; Anand et al., 2016; Wang et al., 2016). The catalysts favorably promote the thermal decomposition and formation of smaller hydrocarbon compounds, and these catalysts also favor hydrogen transfer over the active site of the metal to create suitable structural hydrocarbons, including aromatic hydrocarbons. Therefore, catalytic pyrolysis of SCS collected after harvesting was conducted in a pyrolytic tube reactor using a screw feeder that fed the raw material into the tube pyrolytic reactor over optimized calcined dolomite. This research aims to 1) investigate the catalytic pyrolysis of SCS using calcined dolomite as a catalyst and to 2) characterize the bio-oil obtained from the pyrolysis with and without dolomite to obtain the maximized pyrolysis oil yields. These pyrolysis oil characterizations were performed using chromatographic and spectroscopic techniques, and the physical and chemical properties were also determined according to the American Standard Testing and Material (ASTM) method. The results of this study may provide some insight into the parameters that influence the composition of bio-oil, which provides potential add-on value to alternative energy development.

also slow the harvest and transportation of sugarcane into the sugar mill. The conversion of SCS (leaves and top), which is a lignocellulosic biomass, into energy does not affect the food chain. Additionally, the potential for fuel production and utilization of an agro-residue provide an alternative energy source and aid pollution abatements. However, there are obstacles to the collection of SCS, including light. Increasing the density of SCS or using pyrolysis to transform SCS into a bio-oil for use in the transport industry are interesting as potential replacements for conventional petroleum energy. Furthermore, bio-oil production is advantageous when SCS is located near factories that required a fuel feedstock, and the produced liquid fuels can be rapidly stored and easily transported. Recent studies have shown that the pyrolysis of lignocellulosic biomass can produce bio-oil, which contains complex oxygenate hydrocarbon compounds, with a higher added value than that of products produce from other thermochemical processes, including a small oxygen composition and a higher heating value (Lee et al., 2010; Moraes et al., 2012; Luo et al., 2013; Yang et al., 2014; Lui and Feng, 2016; Oudenhoven et al., 2016). However, the most significant properties of bio-oil that unfavorably affect the pyrolysis quality and make it incompatible with conventional energy are the high oxygen content, high viscosity and high acid value due to the corrosion effect and chemical instability. Several alternative pyrolysis upgrading techniques have been attempted to produce bio-oil from a raw biomass in a single stage, such as co-feeding with an alkaline catalyst or the use of catalytic bed, to improve the bio-oil quality (Bulushev and Ross, 2011; Abu Bakar and Titiloye, 2013; Zhou et al., 2013). Catalytic pyrolysis is a promising technique for improving bio-oil, and the bio-oil produced was used as a fuel without a corrosion effect in the combustion chamber. Typically, several catalysts have been used in the pyrolysis process, e.g., both acid and base catalysts. HZSM-5 zeolite has been widely studied for lignocellulosic biomass pyrolysis and found to intensely change the component of the bio-oils by both reducing the amounts of oxygenated compounds from bio-oil via deoxygenation reactions through dehydration, decarbonylation and decarboxylation (Iliopoulou et al., 2012; Fermoso et al., 2017; Gurevich Messina et al., 2017). In addition, zeolite has some features such as shape-selective due to its small/medium pore size, and strong acid site for its catalyst activity towards cracking and simultaneously increasing the aromatization which obtained high proportion of alkyl-substituted benzene and naphtalenes, but these catalytic reactions resulted in a lower yield of bio-oil accordance with overcracking towards hydrocarbons, vapor volatile gases and coke formation (Nokkosmaki et al., 2000; Iliopoulou et al., 2012; Abu Bakar and Titiloye, 2013; Wang et al., 2014; Anand et al., 2016; Fermoso et al., 2017). Furthermore, ZnO, MgO and other catalysts were investigated to improve the qualities of bio-oil (Sutton et al., 2001; Choi et al., 2015; Elkasabi et al., 2015). A basic metal oxide catalyst such as MgO has been used and produced a lower oxygen content and alkyl-substituted short hydrocarbon chains. CaO could act as a reagent, an absorbent and catalyst depending on the pyrolysis conditions, The use of CaO promoted decarbonylation reactions during biomass pyrolysis, leading to the enhanced formation of CO. CaO can act as an absorbent, a reactant, and a catalyst depending on the pyrolysis conditions employed. The ketone content in the liquid bio-oil increased, and the acid content decreased due to CaO acting as a reagent while the furan and hydrocarbon contents increased, and the ester and anhydrosugar contents decrease and increased the amounts of phenol, cyclopentanone, hydrocarbon and cyclic hydrocarbon compounds due to the role of catalyst (Lin et al., 2010; ThangalazhyGopakumar et al., 2012; Chen et al., 2017). Most investigations into bio-oil production from lignocellulosic biomass have focused on the process conditions, such as the temperature, the mass and heat transfer due to the heat rate efficiency, and the sweep gas flow rate due to the residence time. The properties of bio-oil can be improved by performing catalytic pyrolysis in the presence of

2. Method 2.1. Feedstock analysis The biomass feedstock used in this experiment was SCS (including top and leaves) that was collected after harvesting in the Kanchanaburi province in Thailand and was dried in the ambient air for 7 days to decrease the moisture content. Then, the biomass feedstock was ground using a rotary mill and sieved by a Retsch AS200 sieved shaker according to ASTM E11 into 4 fractions with average sizes of 250 µm, 500 µm, 750 µm and above 750–1000 µm. The moisture was evaporated from the raw feedstock before its use in experiments using an electrical oven at 105 °C for 8 h. The SCS sample was first subjected to a proximate analyses, which were performed according to the ASTM E1756, ASTM E872 and ASTM E1755 methods for moisture, volatile matter and ash content, respectively. An LECO CHN-2000 analyzer (LECO, USA) was used to determine the quantities of carbon, hydrogen and nitrogen in the raw material and liquid product. The oxygen levels in the samples were calculated by the difference method. The thermal decomposition behavior of the SCS raw materials during the pyrolysis process was investigated by thermogravimetric analysis (TGA), which was performed using a Netzch 409 Simultaneous STA (NETZSCH-Gerätebau GmbH, Germany). The samples (4 g) were heated from ambient temperature to 800 °C at heating rate of 30 °C min−1 with a nitrogen gas carrier at 100 mL min−1 in this study. The weight loss and temperature profile were correlated with the decomposition and dehydration (TG) and the derivative due to temperature (DTG). Furthermore, a bomb calorimeter connected to an LECO AC-350 instrument (LECO, USA) was used to determine the calorific heating value in accordance with the ASTM D2015 method. 2.2. Characterization of the natural and calcined dolomite Natural dolomite from the Kanchanaburi province was collected to investigate the effect of the calcination conditions on its use as a catalyst. Natural dolomite was sieved and calcined to remove the impurities and improve the catalyst effectiveness. In a muffle furnace, the dolomite was calcined without CO2 at calcination temperatures from 700 to 900 °C for 2–6 h. Once the calcination finished, the calcined dolomite was stored in desiccators to prevent carbonation and humidification. A Netzch 409 Simultaneous STA (NETZSCH-Gerätebau GmbH, Germany) thermogravimetric analyzer was used to examine the thermal decomposition behavior of the natural dolomite by feeding of 1 g of natural dolomite to the analyzer as the temperature increased from room temperature to 800 °C under a nitrogen atmosphere at a 115

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Fig. 1. Schematic diagram of the catalytic pyrolysis system.

heating rate of 30 °C min−1. The temperature was held for 5 min at the desired temperature, and then, the temperature was increased to 900 °C at a rate of 100 °C min−1. The TG and DTG thermogram was plotted. Subsequently, the crystalline phases of the dolomite powders (natural dolomite and dolomite from the different calcination conditions) were examined by a D8 Advance X-ray diffractometer (XRD) (Bruker, Germany). The XRD patterns were obtained over 10°≤2θ≤80° using Cu kα radiation (λ = 1.5406 nm) and were compared with the Joint Committee on Powder Diffraction Standards (JCPDS) files based on the peak positions and intensities. A S8 Tiger X-ray fluorescence (XRF) spectrometer (Bruker, Germany) was used to determine the mineralogical composition. Furthermore, the calcined dolomite was characterized by morphological studies, and the surface area characteristics were imaged using a JSM-6510LV scanning electron microscope (JEOL, Japan) and ASAP 2020 instrument (Micromeritics Instruments, USA) with the Brunauer-Emmett-Teller (BET) technique.

conducted at temperatures between 400 and 550 °C with a constant biomass feed rate of 0.6 kg h−1 and a nitrogen sweep gas flow rate of 120 cm3 min−1 with 5 wt% calcined dolomite (800 °C for 6 h). The third group of tests was conducted by varying the constant biomass feed rate from 0. 3–1.2 kg h−1 with the other parameters remaining constant at the optimum conditions from the second test. The fourth group of tests was performed with varying sweep gas flow rates from 80 to 200 cm3 min−1, and the final group of tests was performed with an average particle size of 250–1000 µm using the optimum parameters from the fourth group test. A water condenser was used to condense the volatile vapor into biooil products. A solid residue (bio-char and catalyst) was separated and collected, and these solid residues were removed after cooling and weighing. All the test product yields were weighed to determine the total weight of the char, catalyst and liquid (both of organic and aqueous phase). Mass balance calculations were performed to estimate the product yields (= solid + liquid + gas = 100). The product yields and conversions were determined using the following equations:

2.3. Experimental procedure

conversion (%) =

The experimental setup is illustrated in Fig. 1. The catalytic pyrolysis was performed in a bench-scale continuous tube reactor composed of a biomass hopper, a screw feeder for the biomass, a catalytic pyrolysis reactor, a solid residue container, a water condenser and a gas hopper before the vent for the non-condensable gases. The catalytic pyrolysis of SCS was performed in an SS316 custom built reactor (1200 mm length and 40 mm O.D.), and the electrical heating system and the temperature controller used to determine the reaction temperature (a K-type thermocouple) were set according to the experimental operation processes. In each test, the pyrolysis system was heated by an electrical heating furnace to a desired temperature at a rate of 20 °C min−1 with nitrogen sweep gas fed into the pyrolytic tube reactor (80–200 cm3 min−1), and the system was held at the temperature for 15 min. After the test reached a steady state, the screw feeder uniformly fed biomass into the reactor (0.3–1.2 kg h−1) based on the appropriate process parameters according to the nominal feed rate that was determined by the motor speed, in accordance with the pyrolysis residence time. The tests were conducted in five series. The first group of tests was conducted with different dolomite calcination conditions (fresh dolomite, calcined at 700 °C for 2 h, 800 °C for 2 h, 800 °C for 4 h, 800 °C for 6 h, and 900 °C for 2 h), temperatures between 400 and 600 °C, a constant SCS feed rate of 0.6 kg h−1, and a nitrogen sweep gas flow rate of 120 cm3 min−1. Each test used approximately 10 wt% dolomite relative to the biomass raw material. The second group of tests was

(Wbiomass, db − Wchar , db) x100 Wbiomass, db

liquidyield (wt . %) =

solidyield (wt . %) =

Wliquid Wbiomass, db

x100

(Wtotalsolid − Wtotalcatalyst ) Wbiomass, db

(1)

(2)

x100

gasyield (wt . %) = 100 − liquidyield (wt . %) − solidyield (wt . %)

(3) (4)

where Wbiomass,db and Wchar,dv are the weights of the SCS raw materials fed into the pyrolysis reactor and the residual solid residual, respectively, on a dry basis. 2.4. Characteristics of the pyrolysis oil The water/aqueous phases present in the bio-oil were separated from the organic phase using an organic solvent (dichloromethane). The organic phase of the pyrolysis oil were determined by weighed and analyzed its composition by an Agilent 7890B/5977A (Agilent, USA) GC-mass spectrometer (MS) equipped with both a flame ionization detector (FID) and a thermal conductivity detector (TCD), a split/ splitless injection unit and an HP-5 MS capillary column (30 m × 0.25 mm × 0.25 µm). For this analysis, the column temperature was maintained at 40 °C for 1 min, increased to 270 °C at a heating rate of 20 °C min−1 and then maintained for 10 min. The identification of the 116

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Table 1 Characterization of the sugarcane straw. Proximate analyses (wt%) Moisturea Volatile matter Fixed carbon Ash

4.01 73.41 18.20 2.38

ASTM E1756 ASTM 872 by calculation ASTM 1755

Ultimate analyses (wt%) C H N O

40.13 7.72 2.68 36.65

Higher heating value (MJ kg-1)

17.83

a

ASTM D2015

as received. Fig. 2. Thermogravimetric analysis of natural dolomite.

peaks was accomplished by matching the mass spectra with the National Institute of Standard and Technology library. The physicochemical properties of the pyrolysis oil obtained from the catalytic pyrolysis were determined according to the ASTM standard methods as follows: density, kinematic viscosity and higher heating value (HHV) according to ASTM D369, ASTM D445 and ASTM D240, respectively. The modification acid number (MAN) was determined using an 840Trinoplus automated titrator (Metrohm, UK) according to ASTM D664, and the ultimate components of the bio-oil from the catalytic pyrolysis of SCS were determined using a CHN-200 analyzer (LECO, USA). The peaks identified in the GC-MS confirmed the identities of the chemical compounds. The ultimate analyses and physico-chemical property determinations revealed that the properties of the pyrolysis oil from SCS make it a suitable candidate for bio-oil production from biomass residues as a renewable energy source.

decomposing into a bio-char. As seen from this result, the complete thermal decomposition of SCS can occur at a temperature below 700 °C. 3.2. Characteristics of natural and calcined dolomite Fig. 2 shows the thermal decomposition behavior spectra of fresh dolomite (weigh loss versus temperature). As seen in Fig. 2, the dolomite decomposed, and the decomposition of natural dolomite can be divided into two stages at temperatures of 420–460 °C and mainly decomposition at the temperature of 650–760 °C, which correspond to the thermal degradation of dolomite reported in the literature (Shajaratun Nur et al., 2014; Algoufi et al., 2017).

3. Results and discussion

MgCa (CO3)2 → MgO + CaCO3 + CO2

(5)

MgO + CaCO3 → MgO + CaO + CO2

(6)

The TG/DTG shows a flat thermogram at 760 °C, which specifies the maximum weight loss temperature during the thermal decomposition. Therefore, the optimal calcination conditions of dolomite were 800 °C for total devolatilization of the dolomite. The XRD analysis revealed the bulk structure of the catalyst, and Fig. 3 shows the X-ray diffraction pattern of natural dolomite, which was compared with that of the dolomite calcined under different conditions. Fig. 3 shows that the crystalline structures of the natural dolomite and dolomite calcined under the lower calcination conditions are the same. The XRD patterns of natural dolomite presented intense

3.1. Characteristics of the raw material Table 1 shows the results of the proximate and ultimate analyses and the HHV of the SCS samples. The SCS had a low moisture content (only 4.01 wt%) and a low ash content (2.38 wt%). The proximate analysis showed that the volatile matter and fixed carbon content of the SCS were not significantly different from the values defined in the literature (Moraes et al., 2012). The ultimate analysis of the SCS revealed that it contained 40.13 wt% carbon, 7.72 wt% hydrogen, 36.65 wt% oxygen and 2.68 wt% nitrogen. The higher oxygen content causes the lower HHV for SCS than that of other lignocellulosic feedstocks (Sutton et al., 2001; Burhenne et al., 2013; Mythili et al., 2013; Yang et al., 2014; Amutio et al., 2015). TGA was performed to obtain a thermogram of the thermal decomposition behavior of SCS based on the devolatilization spectra of SCS weigh loss versus temperature. The thermogravimetric study revealed that the decomposition of SCS can be divided into three periods. The first stage spans from room temperature to 103 °C and included a slight weight loss and a small peak in the rate of the weight loss curve based on the moisture removal and loss of light volatile vapors from the SCS samples. The second stage corresponded to the main decomposition reaction, which occurred from 238 °C to 323 °C, and the rate of weight loss reached a maximum at 343 °C, which was regarded as the degradation of SCS due to the decomposition of hemicellulose, cellulose and lignin. The decomposition of hemicellulose and cellulose also produced a volatile vapor during the thermal decomposition and continued up to 700 °C, but the rate of the reactions were minimized, which may due to the degradation of the lignin that contributes to the formation of tar. In the final stage, a slow weight loss continued, which revealed the complex lignin and inorganic matter were slowly

Fig. 3. XRD patterns of dolomite. Symbols: ■ Ca(OH)2 • CaCO3 ♦ MgCa(CO3)2 □ CaO ▼ MgO.

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Fig. 4. SEM images of dolomite (A), natural dolomite (B), dolomite calcined at 800 °C for 2 h (C), calcination at 800 °C for 6 h, and (D) calcination at 900 °C for 2 h.

peaks at 2θ= 31.8, 40.8, 51.2; 2θ= 18.8, 29.2, 34.6 and 2θ=47.4 which correspond to MgCa(CO3)2 (JCPDS 36-0426), Ca(OH)2 (JCPDS 78-0315) and CaCO3 (JCPDS 47-1743), respectively. The XRD pattern of dolomite calcined at 800 °C for 6 h did not have peaks that corresponded to MgCa(CO3)2. In addition, the diffraction peaks at 2θ = 32.4, 37.8, 54.1, 64.6, 68.0 and 2θ = 43.1, 62.6, 74.8 signified the presence of CaO (JCPDS 37–1497) and MgO (JCPDS 71-1176), respectively. The high intensities of the diffractograms indicated that a high crystallinity CaO–MgO compound was present. Subsequently, an X-Ray Fluorescence (XRF) spectrometer was used to examined the mineralogical composition, and the calcined dolomite mainly consisted of CaO (49.13 wt%), MgO (14.31 wt%) and other substances such as SiO2, Al2O3 and others (approximately 0.27, 0.21 and 36.08 wt%, respectively). The surface area and morphology of the calcined dolomite were significantly different from that of the natural dolomite because the high temperature accelerated the thermal decomposition of MgCa (CO3)2, which modifies the chemical and physical properties affecting the catalytic reaction. The SEM images (Fig. 4) illustrated the differences in the surface morphologies of the natural dolomite and the calcined dolomite. The calcined dolomite had many orderly pores on the surface, which established a pore structure, and the SEM images agree with the surface area measurement with the BET analysis results, which indicated a total pore volume of 0.10 cm3/g and a surface area of SBET = 18.21 m2/g. The surface area and pore volume values were higher than those of natural dolomite and dolomite calcined at lower temperatures, as shown in Table 2.

Table 2 Morphology studies of natural and calcined dolomite. Calcination conditions

Total surface area m2 g-1

Pore volume cm3 g-1

Pore size °A

natural dolomite 700 °C 2h 800 °C 2h 800 °C 4h 800 °C 6h 900 °C 2h

1.62 1.98 17.14 18.24 18.21 15.55

n/a 0.01 0.06 0.09 0.10 0.06

n/a 139.45 187.02 189.45 179.82 161.73

obtained under several calcination conditions with different pyrolysis temperatures using 10 wt% dolomite are shown in Fig. 5. The temperature ranged from 400 to 600 °C, and the dolomite used was prepared under the following calcination conditions: 700 °C for 2 h, 800 °C for 2 h, 800 °C for 4 h, 800 °C for 6 h, and 900 °C for 2 h. This experiment was used to investigate the product distribution of SCS in the tube pyrolytic reactor with a constant biomass feed rate, N2 sweep flowing rate and average particle size of the biomass of 0.6 kg h−1, 120 cm3 min−1 and 500 µm, respectively. Table 3 shows that the temperature has a significant effect on the product distribution in all the tests and affected the bio-oil and gases yield with and without the dolomite catalyst. According to these results, when the temperature increased from 400 °C to 500 °C, the bio-oil yield increased from 27.44 to 36.73 wt% to 30.16–35.05 wt%. The bio-oil yield decreased from approximately 26.62 to 35.36 wt% to 24.53–31.82 wt% with temperature increasing from 550 °C to 600 °C, whereas the gas yields dramatically increased from 25.96 to 33.62 wt% to 41.07–58.86 wt% as the temperature increased from 400 to 600 °C. Moreover, the trends in the gas yields did not significantly change during the tests using calcined dolomite (700 °C for 2 h and 800 °C for 2 h) and no dolomite.

3.3. Pyrolysis yield and product distribution 3.3.1. Effect of different calcined dolomite on the yield The results of the catalytic pyrolysis and the product distribution 118

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Fig. 5. The effect of the dolomite calcination conditions on the yields and conversion.

liquid product due to the secondary degradation reaction over the active sites of the catalyst, which allow the devolatilization reaction from cellulose and hemicellulose decomposing and forming anhydrosugars, some furan derivatives and favor deoxygenation of the primary and secondary pyrolysis bio-oil proceeds via decarboxylation reactions with limited formation of water and no clear trend can be observed. Hence, the gaseous product yield increased and the liquid yield decreased as the temperature increased due to the influence of the thermal degradation and gasification in the primary reaction to produced volatile vapor. Then, the porosity and surface area of the catalyst aid the formation of secondary cracking reactions of the pyrolysis vapor, and the secondary bio-char reaction can produce a small gaseous yield at higher temperatures, contributing to the increase in the gaseous yields. In particular, the CaO and MgO in the calcined dolomite encourage decarbonylation and decarboxylation reactions during the biomass pyrolysis, leading to the enhanced formation of CO and CO2. However, the pyrolysis tests with SCS without a catalyst and the tests using calcined dolomite (700 °C for 2 h and 800 °C for 2 h) did not have significant differences in the gases and liquid yields. These results indicated that the optimum conditions for the catalytic pyrolysis of SCS (temperature of 450 °C, sweep gas flow rate of 160 cm3 min−1 and average size distribution of SCS of 500 µm) with 10 wt% dolomite (calcined at 800 °C for 6 h) resulted in liquid, gas, and solid fractions of 32.21 wt%, 29.51 wt% and 28.28 wt%, respectively.

Table 3 Product distribution and conversion of the catalytic pyrolysis of SCS.

No catalyst

700 °C 2 h

800 °C 2 h

800 °C 4 h

800 °C 6 h

900 °C 2 h

gas liquid char conversion gas liquid char conversion gas liquid char conversion gas liquid char conversion gas liquid char conversion gas liquid char conversion

400 °C

450 °C

500 °C

550 °C

600 °C

25.96 33.19 40.85 59.15 24.72 35.75 39.53 60.47 25.45 36.73 37.82 62.18 29.36 33.83 36.81 63.19 32.22 28.63 39.15 60.85 33.62 27.44 38.94 61.06

26.93 36.15 36.92 63.08 25.98 37.59 36.43 63.57 25.94 34.63 39.43 60.57 34.43 31.76 33.81 66.19 39.51 32.21 28.28 71.72 39.48 32.51 28.01 71.99

32.66 35.05 32.29 67.71 31.56 37.79 30.65 69.35 33.31 36.70 29.99 70.01 41.64 30.87 27.49 72.51 42.84 30.43 26.73 73.27 43.77 30.16 26.07 73.93

36.39 35.36 28.25 71.75 40.02 34.33 25.65 74.35 39.66 34.83 25.51 74.49 48.26 30.80 20.94 79.06 49.23 29.92 20.85 79.15 53.65 26.62 19.73 80.27

41.07 31.82 27.11 72.89 46.97 29.66 23.37 76.63 47.43 29.12 23.45 76.55 54.72 26.29 18.99 81.01 57.19 25.39 17.42 82.58 58.86 24.53 16.61 83.39

3.3.2. Effect of the temperature on the yields Fig. 6(A) shows the effect of the pyrolysis temperature on the SCS product distribution as the temperature varied from 400 °C to 550 °C with the other parameters fixed (SCS feed rate of 0.6 kg h−1, nitrogen flow rate of 160 cm3 min−1, average size distribution of SCS of 500 µm) using 10 wt% calcined dolomite (800 °C for 6 h). As seen in this figure, at a low pyrolysis temperature, the lignocellulosic biomass cannot completely decompose because of the greater primary decomposition reaction of the lignocellulosic biomass into volatile matter while the

However, the catalyst used influenced the production of gases from the primary thermal decomposition, and the catalyst affects the secondary reaction over the metal active sites on the surface area and pores of the catalyst to form smaller, volatile vapor compounds. Typically, using a catalyst increases both the liquid and gas yields due to its influence on the thermal degradation and secondary cracking reaction of the volatile vapor and bio-char devolatilization, but in this study, dolomite increased the yield of gases and decreased that of the

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products also significantly increased as the SCS feed rate increased. These results indicated that the thermal decomposition of the biomass feedstock influences the cellulose and hemicellulose degradation into volatile vapor based on the long residence time in the reactor due to the lower biomass feed rate. Therefore, the primary reaction dominated the gas production, and the secondary tar cracking and catalytic cracking over the surface of CaO promote the higher yield of gases due to the carbonylation and carboxylation reactions. Furthermore, increasing the biomass feed rate did not significantly affect the liquid yield due to insufficient heat transfer to the biomass particles, and the higher feed rates were dominated by the devolatilization reaction of the bio-char. This observation showed that the optimum conditions for the catalytic pyrolysis of SCS obtained liquid yields of 32.21 wt%, gas yields of 39.51 wt% and solid yields of 28.28 wt% when using a pyrolysis temperature, biomass feed rate, nitrogen sweep gas, and average size of biomass of 450 °C, 0.6 kg h−1, 160 cm3 min−1 and 500 µm, respectively, and 10 wt% calcined dolomite. 3.3.4. Effect of the nitrogen sweep gas on the yield The effect of the nitrogen sweeping gas was tested at a constant pyrolysis temperature, SCS feeding rate, and average size of the biomass feedstock of 450 °C, 0.6 kg h−1, and 500 µm, respectively, and using 10 wt% calcined dolomite (800 °C for 6 h). The nitrogen flow rates tested were 80, 120, 160 and 200 cm3 min−1. The product distribution based on the catalytic pyrolysis conditions is also represented in Fig. 6(C). As seen in this figure, the liquid yields sharply decreased from 36.15 wt% to 26.24 wt% as the sweep gas flow rate increased from 80 to 200 cm3 min−1. The yields of the bio-char dramatically increased from 11.76 wt% to 33.19 wt% as the nitrogen gas feed rate increased from 80 to 200 cm3 min−1, but the gas yields decreased from 52.09 wt % to 40.57 wt% as the nitrogen flow rate increased from 80 to 120 cm3 min−1. The reason for these results is that the higher sweep flow rate affects the residence time of the volatile vapor that results from the decomposition of cellulose and hemicellulose, and the time is not sufficient for the secondary reactions of tar cracking and catalytic cracking via CaO. Therefore, the volatile vapors pass through the pyrolysis reactor before the secondary reactions can be completed. The decrease in the pyrolysis oil yield and increase in the gas yield as the nitrogen sweeping gas flow rate increased can be explained by either the short contact time and poor condensation or the fast release of the devolatilization vapors by the pyrolysis system before the effective condensation. The production of char also increased as the sweep flow rate increased to 200 cm3 min−1, which agreed with the nitrogen atmosphere and led to the devolatilization of the lignocellulosic biomass. The optimum conditions for the catalytic pyrolysis of SCS resulted in liquid yields of 36.15 wt%, gas yields of 52.09 wt% and solid yields of 11.76 wt% when the pyrolysis temperature, biomass feed rate, nitrogen sweep gas, and average biomass size were 450 °C, 0.6 kg h−1, 80 cm3 min−1 and 500 µm, respectively, and 10 wt% calcined dolomite was used.

Fig. 6. The effect of the process parameters on the yields.

secondary cracking of the tar reaction and the secondary decomposition of the bio-char also occurred; therefore, the char yield was relatively higher. As the pyrolysis temperature increased, the yields of the liquid and gas products increased because of the thermal degradation and gasification reactions, e.g., the thermal decomposition and secondary reaction. The higher yield of gases was due to the smaller gases yield dramatically increasing with the increasing temperature, which was caused by the influence of the temperature on the tar-cracking reaction and CaO and MgO promoting the CO formation reaction while the volatile vapor was decomposed to smaller gaseous compounds. Moreover, temperatures above 450 °C resulted in minimal changes in the bio-oil production because the thermal decomposition of tar at high temperatures significantly reducing the amounts of oxygenated compounds from bio-oil via deoxygenation reactions through dehydration, decarbonylation and decarboxylation and overcracking towards hydrocarbons, vapor volatile gases resulting in a dramatic decrease in the bio-oil yield from 32.21 wt% to 29.92 wt% as the temperature increased from 450 °C to 550 °C. and slightly increase in the gaseous product from 32.22 wt% to 49.23 wt% as the temperature increased from 400 °C to 550 °C.

3.3.5. Effect of the biomass particle size on the yield A final series of tests was performed to determine the effect of the particle size using the optimum conditions from the nitrogen sweep flowing rate tests (temperature of 450 °C, SCS feed rate of 0.6 kg h−1, nitrogen flow rate of 80 cm3 min−1 and 10 wt% calcined dolomite). As seen from Fig. 6 (D), the yields of the gas sharply increased from 11.38 wt% to 42.82 wt% where the average SCS particle size was increased from 250 to 1000 µm. The results show that, for the smallest biomass particle size, the gas yields were higher than those for the larger particle size because the heat transfer promotes the primary pyrolysis reaction on the surface of the lignocellulosic biomass, which is affected by the particle size and uniformity of the particle. Therefore, the sufficient and effective heat transfer with the smaller particle size led to the primary reaction and the secondary reaction of either tar cracking of the solid char and catalytic cracking of volatile vapor over

3.3.3. Effect of the biomass feed rate on the yield The effect of the biomass feed rate was investigated with the pyrolysis temperature, nitrogen sweep gas, and average size of the biomass feedstock maintained at 450 °C, 160 cm3 min−1 and 500 µm, respectively, with 10 wt% calcined dolomite (800 °C for 6 h). The SCS feed rates tested were 0.3, 0.6, 0.9 and 1.2 kg h−1. The product distributions for the catalytic pyrolysis of SCS are shown in Fig. 6 (B). The liquid yields increased from to 32.22 wt% to 32.21 wt% as the SCS feed rate increased from 0.3 to 0.6 kg h−1 and decreased to 30.43 wt% and 29.92 wt% as the feed rate increased to 0.9 and 1.2 kg h−1, and the gas yields dramatically increased from 32.22 wt% to 49.23 wt% as the SCS feed rate increased from 0.3 to 1.2 kg h−1. In addition, the bio-char 120

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Table 4 Characteristics of bio-oil production from pyrolysis with and without dolomite. Bio-oil sample

no catalyst with catalyst

Ultimate analysis (wt%) C

H

N

O

42.70 54.74

7.40 18.92

0.45 0.19

33.32 26.15

Density (kg m-3)

Kinematic viscosity (mm2 s-1)

HHV (MJ kg-1)

MAN (mgKOH/g)

10.88 10.79

0.79 0.76

22.62 29.18

23.26 11.43

Table 5 GC-MS identification. RT (min)

3.590 3.854 4.093 4.279 4.305 4.357 4.431 4.513 4.526 4.578 4.600 4.825 4.860 5.016 5.289 5.354 5.389 5.831 5.896 6.022 6.269 6.403 6.512 6.564 6.590 6.681 6.859 7.140 7.179 7.318 7.422 7.512 7.695 7.946 7.882 8.107 8.125 8.146 8.298 8.393 8.454 8.675 8.870 9.065 9.360 9.693 10.047 10.059 10.156 10.162

Relative abundance (peak area) No cat.

with cat.

2.25 – 2.48 3.71 – – 2.49 2.03 – – 4.01 – – 2.03 1.11 – 2.67 – 2.55 3.31 2.29 1.98 11.45 2.13 2.08 – 3.39 5.63 5.02 2.82 2.34 1.56 1.11 2.13 3.65 2.29 1.23 – 0.91 2.62 3.68 5.38 2.49 2.94 0.49 0.14 0.19 0.03 0.38 1.01

– 3.18 2.92 – 4.65 3.52 2.21 1.19 3.13 2.38 3.03 1.24 2.21 1.84 1.67 2.19 2.39 2.36 2.36 3.49 – 1.36 8.64 1.51 2.08 3.47 2.63 3.05 4.69 1.13 1.18 – 1.17 0.59 2.46 2.03 0.56 3.09 0.92 2.49 3.53 4.59 – 2.87 – – – – – –

Chemical Formula

Compounds

C7H6O C9H12 C9H12 C7H8O C10H14 C10H14 C7H8O C10H14 C10H14 C10H14 C7H8O2 C10H14 C10H14 C8H10O C8H10O2 C6H6O2 C8H10O2 C7H8O2 C7H8O3 C7H8O2 C9H10O2 C8H10O2 C8H10O3 C10H12O2 C8H10O3 C8H10O2 C8H8O3 C9H10O4 C10H12O2 C6H12O6 C9H10O3 C6H12O2Si C10H12O3 C6H14O2Si C7H4F4O C11H14O3 C5H4O2 C15H16 C9H12N2O2 C11H14O3 C9H10O4 C11H14O3 C10H12O4 C10H12O5 C13H11N C7H16O2Si C7H14O2Si C8H18O2Si C8H18O2Si C11H12O4

Benzaldehyde Benzene, 1,2,4-trimethylBenzene, 1,2,3-trimethylPhenol, 2-methylBenzene, 1-methyl-3-propylBenzene, 1-methyl−3-(1-methylethyl)Phenol, 3-methylBenzene, 2-ethyl−1,4-dimethylBenzene, 1-ethyl−2,4-dimethylBenzene, 1-ethyl−2,3-dimethylPhenol, 2-methoxyBenzene, 1,2,4,5-tetramethylBenzene, 1,2,3,4-tetramethylPhenol, 2,5-dimethylPhenol, 2-methoxy−3-methyl1,2-Benzenediol Phenol, 2-methoxy−4-methyl1,2-Benzenediol, 3-methyl1,2-Benzenediol, 3-methoxy1,2-Benzenediol, 4-methylPhenol, 2-Methoxy−4-vinylPhenol, 2-methoxy−3-methylPhenol, 2,6-dimethoxyPhenol, 2-methoxy−3-(2-propenyl)Phenol, 3,4-dimethoxy1,2-Benzenediol, 4-ethylBenzaldehyde,4-hydroxy−3-methoxyBenzoic acid, 4-hydroxy−3-methoxyPhenol, E−4-propenyl−2-methoxyD-Aldose Ethenone, 1-(3-hydroxy−4-methoxyphenyl)2-Propenoic acid, trimethylsilyl ester 2,5-Dihydroxy−4-isopropyl−2,4,6-cycloheptatriene−1-one Propionic acid, trimethylsilyl ester Benzene, 1,2,4,5-tetrafluoro−3-methoxyPhenol, 2,6-dimethoxy−4-(2-propenyl)2,4-Pentadienoic acid, 5-hydroxyBenzene, 1,1'-propylidenes’N,4,5-Trimethyl−2-nitroaniline Phenol, 2,6-dimethoxy−4-(2-propenyl)Benzaldehyde, 4-hydroxy−3,5-dimethoxyPhenol, 4-(2-propenyl)−2,6-dimethoxyEthenones, 1-(4-hydroxy−3,5-dimethoxyphenyl)3,5-Dimethoxy−4-hydroxyphenylacetic acid 3-Methyl−2-azafluorene Isobutyric acid, trimethylsilyl ester 3-Butenoic acid, trimethylsilyl ester 2-Methyl-, trimethylsilyl ester 3-Methyl−2-butenoic acid, trimethylsilyl ester 3,5-Dimethoxy−4-hydroxycinnamaldehyde

% in the tests using the 1000 µm SCS particle size. These process conditions for the SCS catalytic pyrolysis resulted in liquid yields of 36.15 wt%, gas yields of 52.09 wt% and solid yields of 11.76 wt% with the pyrolysis temperature, biomass feed rate, nitrogen sweep gas, and average size of biomass of 450 °C, 0.6 kg h−1, 80 cm3 min−1 and 500 µm, respectively, with 10 wt% calcined dolomite.

the active sites on CaO and MgO, resulting in large bio-oil and gas yields. Meanwhile, for the larger particle sizes, the temperature inside the particles may have been insufficient to complete the thermal decomposition reaction during the primary pyrolysis stage. This resulted in large amounts of bio-char (42.82 wt%) due to the lower heat transfer in the larger particle size (1000 µm) during the pyrolysis reaction and a significant decline in the liquid bio-oil. The yield decreased to 17.68 wt 121

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feedstock size of approximately 500 µm. The results obtained in this study indicated that the catalytic pyrolysis produced a higher bio-oil yield at lower temperatures and a lower gases yield than that of the pyrolysis without the catalyst due to the thermal decomposition of the lignocellulosic biomass and the secondary reactions, either the decarbonylation reaction or catalytic cracking of volatile vapor to form smaller organic compounds. Furthermore, the calcined dolomite can reduce the oxygen content in bio-oils by removing the carbonyl and carboxylic functional groups, and CaO and MgO are effective for decreasing the amount of carboxylic acid in the bio-oil, which can then be used as a fuel for combustion engines. This is considered an upgrading process for the pyrolysis of biomass.

3.4. Product characterization The ultimate analyses and physico-chemical properties of the bio-oil obtained from the optimum pyrolysis conditions for SCS with and without dolomite (temperature of 450 °C, biomass feed rate of 0.6 kg h−1, sweep gas flow rate of 80 cm3 min−1, average particle size of 500 µm) were determined and compared in Table 4. As seen from the elemental analysis, the bio-oils obtained with and without dolomite were characterized by a lower oxygen content and higher H/C content than that of the SCS materials from the primary thermal decomposition and gasification reactions and the secondary reaction, i.e., either decarbonylation and decarboxylation reactions on CaO and MgO. The smaller gases such as CO and CO2 were also removed from these stages. A comparison of the bio-oil obtained from the catalytic pyrolysis without dolomite showed a lower oxygen content and higher H/C content than that of the pyrolysis system without dolomite. Moreover, the higher heating value increased the effective removal of oxygenated compounds over the active sites of CaO and MgO, including the pore selectivity of these dolomite catalyst. The HHV of the pyrolysis oil in the absence of dolomite was 22.62 MJ kg−1, and the HHV in the catalytic pyrolysis system was 29.18 MJ kg-1. The HHV of the pyrolysis oil increased after the pyrolysis process. A possible reason for this may be the significant influence of the temperature, biomass feed rate and nitrogen sweep gas on the temperature and residence time of the volatile vapor from the thermal decomposition and the secondary reaction over the active sites of the catalysts, which promotes decarbonylation and decarboxylation reactions, and that the reforming volatile vapor condensed into an upgraded bio-oil with a lower oxygen content, which leads to a higher HHV. The empirical formulas of the bio-oil obtained from the pyrolysis and catalytic pyrolysis were CH2.080O0.585 and CH4.147O0.358, respectively. To quantify the compounds using a GC/MS technique, the chemicals were identified at several retention times, and the percentages of the peak area, chemical formulas and molecular weights are listed in Table 5. The organic compounds were analyzed and classified as aldehydes, ketones, acids, phenols, oxygenate compounds and others. These chemical compounds were produced by the thermal decomposition of cellulose, hemicellulose and lignin. The SCS pyrolysis results with the dolomite catalyst show that some acid compounds were eliminated in the presence of calcined dolomite. CaO and MgO affect the secondary reaction of volatile vapor and can effectively promote the decarboxylation and decarbonylation reactions, resulting in the removal of CO and CO2 and transformation of oxygenated compounds into hydrocarbons. The density of the bio-oil obtained from the catalytic pyrolysis was 10.79 kg m−3, and the kinematic viscosity was 0.79 mm2 s−1, which was higher than that of the bio-oil from the pyrolysis without dolomite. The MAN obtained from product of the dolomite catalytic pyrolysis process was 11.43 mg KOH g−1. The possible reason for this result is that the dolomite catalyst, which is an effective basic catalyst, improved the acidity level of the biofuels, which agreed with the literature (Tani et al., 2011). In the decarboxylation step, dolomite is thought to catalyze the decarboxylation from an acidic organic compound to CO2.

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4. Conclusion In this study, the catalytic pyrolysis of SCS (leave and top) was performed in a pyrolytic tube reactor using dolomite as a catalyst. First, the appropriate calcination conditions for dolomite were determined by investigating the catalytic pyrolysis product distribution. The effects of the catalytic pyrolysis conditions on the product distribution and the composition of products were also investigated. The calcination conditions for dolomite of 800 °C for 6 h, which was used for the catalytic pyrolysis of SCS, obtained the maximized liquid yield with the following process conditions: temperature of 450 °C, biomass feed rate of 0.6 kg h−1, nitrogen sweep gas rate of 80 cm3 min−1 and average 122

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