Effects of Temperature and Mg-Based Additives on Properties of

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Biofuels and Biomass

Effects of temperature and Mg-based additives on properties of cotton stalk torrefaction products Kuo Zeng, Qing Yang, Qinfeng Che, Yang Zhang, Xianhua Wang, Haiping Yang, Xiao He, Sebastian Wright, and Hanping Chen Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b02286 • Publication Date (Web): 16 Aug 2018 Downloaded from http://pubs.acs.org on August 19, 2018

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Energy & Fuels

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Effects of temperature and Mg-based additives on properties

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of cotton stalk torrefaction products

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Kuo Zenga,b, Qing Yanga,b*, Qinfeng Chea, Yang Zhanga, b, Xianhua Wanga, Haiping Yanga, Xiao Heb, Sebastian Wrightc, Hanping Chena,b

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a

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Technology, 1037 Luoyu Road, Wuhan, Hubei 430074, PR China

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b

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Technology, Wuhan, Hubei 430074, PR China

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c

State Key Laboratory of Coal Combustion, Huazhong University of Science and

Department of New Energy Science and Engineering, University of Science and

Honour school of physics and philosophy, University of Oxford, OX1 4AJ, United

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Kingdom.

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Abstract:

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In a new development, Mg-based additives were introduced to the process of

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torrefaction of cotton stalk to enhance the deoxygenation effect. The properties of

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torrefaction products obtained using temperatures in the range 200-350°C and three

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different types of Mg-based additives (MgO, MgO-K2CO3 and MgO-KNO3-NaNO3)

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with varied mass ratios (0.5, 1 and 2) were characterized. The yield of solid product

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significantly declined from 92.02% to 36.95% when the torrefaction temperature rose

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from 200 to 350°C, while gas and liquid yields increased to 24.90% and 38.15%,

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respectively. MgO-K2CO3 was the most effective additive because it not only promoted

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deoxygenation in the solid product (oxygen content decreased by about 43%) but also

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reduced the loss of hydrogen. CO2 was the main gas component and production was

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promoted as the Mg-based additive mass ratio rose from 0.5 to 2. The phenol and

1 ACS Paragon Plus Environment

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ketone content in the liquid product significantly increased, while the acid content

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decreased. Biomass torrefaction with Mg-based additives not only produced a low

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oxygen content solid product but also improved the properties of the by-products (gas

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and liquid) so that they could be recovered and utilized as fuel or chemicals.

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Keywords: Cotton stalk; Additives; Torrefaction parameters; Product properties

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1. Introduction

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Fossil fuels accounted for 85% of the global energy consumption in 2011 [1].

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Fossil fuels exhaustion and the serious environmental problems caused by their use

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drive people to develop renewable energy resources. Biomass is considered a

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sustainable, green fuel and is distributed over much of the world [2]. In fact, biomass is

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the fourth-most important source of energy worldwide and currently provides

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approximately 14% of global energy [3]. In 2050, around 1/4-1/3 of global energy

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consumption will be supplied by bioenergy [4].

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Biomass can be converted to biofuels by thermal-chemical technologies

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(combustion, pyrolysis or gasification) [5]. Raw biomass has drawbacks (high water

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content and low energy density) which make it very hard to store and expensive to

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transport [6]. To overcome these drawbacks, torrefaction may be used to improve

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biomass properties [7]. Torrefaction is conducted in a no-oxygen atmosphere at

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temperatures ranging from 200-350°C [8, 9]. Biomass with a lower ratio of O/C and

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moisture content and with enhanced energy density is obtained [10-12]. During

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torrefaction, oxygen is eliminated from biomass, forming CO2, CO, H2O and small


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amounts of volatile organics [10, 13]. The oxygen content in beech wood reduced from

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43.1 wt.% to 33.8 wt.% when it was torrefied at 280°C [14]. The oxygen content in

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palm decreased from around 49 wt.% to 41 wt.% at a torrefaction temperature of 300°C

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[15]. The oxygen content of torrefied biomass is usually more than 30%, which restricts

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its further use as fuel or pyrolysis feedstock [16, 17]. Increasing the torrefaction

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temperature beyond 300°C could enhance biomass deoxygenation [18-20]. However,

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this would also result in remaining less mass and greater energy consumption [21]. We

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considered what we could do in order to increase the deoxygenation effect at reasonable

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temperatures. We noted the use of Mg-based additives for CO2 capture from plant flue

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gas. MgO reacts with CO2, generating carbonate, when the temperature is 300°C [22].

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The absorption of CO2 is evidently promoted with the introduction of MgO-K2CO3 at

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about 375°C [23]. We thought that this enhanced CO2 absorption might lead to

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decarbonylation instead of deoxygenation, which would allow us to achieve a greater

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deoxygenation effect during low temperature biomass pyrolysis [19]. Alkali and

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alkaline earth metals (AAEMs) can catalyze the ring-opening reactions of primary

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pyrolysis vapors, which increases the acid and water content of bio-oil [24]. In order to

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enhance the properties of the bio-oil, extensive pretreatments (water or acid washing)

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prior to pyrolysis may be conducted to remove the inherent AAEMs to reduce their

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negative catalytic effects on pyrolysis [25]. Also, alkaline earth metals (Mg, Ca) favor

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depolymerisation over dehydration reactions and so could be used as deoxygenation

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catalysts during pyrolysis [26]. The effects of Mg-based additives on biomass


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torrefaction are different from those on biomass pyrolysis to a certain extent, because

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torrefaction temperatures are lower than those used in conventional pyrolysis. However,

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the effects of Mg-based additives on torrefaction behavior of biomass have not yet been

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investigated. The oxygen migration characteristics during biomass torrefaction with

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Mg-based additives are still unknown.

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The biomass torrefaction process and the properties of the products are highly

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dependent on the torrefaction temperature [7]. Studies on the effects of temperature on

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conventional torrefaction were used to guide our study of torrefaction with Mg-based

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additives. We will therefore summarize the effects of temperature on conventional

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biomass torrefaction. Biomass mainly consists of hemicellulose, cellulose and lignin.

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Therefore, its torrefaction is highly dependent on the temperature dependence of the

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evolution of the preceding constituents [27]. Hemicellulose, cellulose and lignin

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decompose at 200-315, 315-400 and 160-900°C, respectively [28]. At temperatures

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between 200-235°C, partial decomposition of hemicellulose by devolatilization and

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carbonization causes minimal mass loss [29]. In the temperature range of 235-275°C,

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hemicellulose and some cellulose decomposes into volatiles [30]. When the temperature

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rises to 275-300°C, almost no hemicellulose is left and cellulose is greatly decomposed

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[31]. At the same time, alcohols, aldehydes, carboxylic acids, ethers and gases (CO2,

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CO and CH4) are released [32]. Little lignin is consumed during the torrefaction process.

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No synergistic effects are exhibited by torrefaction of the three main biomass

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components together [30]. The transformation of oxygen into gas and liquid products


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increases from 19.76%, 5.85% and 16.28 to 71.11%, 33.27% and 44.89% for

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hemicellulose, cellulose and lignin, respectively, when torrefaction temperature is

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increased from 210°C to 300°C [33]

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Biomass torrefaction product properties depend on the thermal degradation of the

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main biomass components with temperature. For light torrefaction at 240°C, over 60%

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of biomass mass is preserved and significant depletion of hemicellulose occurs [27].

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The torrefied biomass (atomic O/C≥0.26) approaches an energy yield of over 88% of

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that of high-volatile coal when the torrefaction temperature is increased from 240 to

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270°C [12]. Chen et al. studied wood torrefaction at temperatures ranging from 220 to

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280°C and found that 50% of the mass was lost but the calorific value of the product

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was increased by 40% at the highest temperature (280°C) [34]. Corn stover was

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torrefied at 200-300°C and the changes (55% mass left with O/C ratio of 0.6 and 19%

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higher energy density) were larger at higher temperatures [16]. 300°C was suggested as

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the optimal temperature in the range of 250-350oC for transforming bamboo into solid

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fuel [21]. The liquid product was relatively rich in acids and alcohols. At the highest

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temperature (300°C), torrefaction improved the fuel quality of olive tree pruning and led

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to a very large decline in crystallinity due to decreasing O/C and H/C ratios [35].

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Nocquet et al. reported that the ratio between CO2 and CO from beech wood

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torrefaction decreased from 6.5 to 2.5 when the temperature increased from 220 to

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300°C [36]. Li et al. studied the torrefaction of bamboo at 220-340°C and found that the

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mass yield and energy for solid product were reduced as torrefaction temperature


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increased while the higher heating value (HHV) increased [37]. Pohlman found that

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torrefaction at 450°C led to an increase of 62% in HHV and a reduction of 78% in

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volatiles of the torrefied biomass [20]. In general, liquid and gas yields from

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torrefaction increased with rising torrefaction temperature, while solid yield decreased

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[38]. There was a sharp increase in aromatization in the solid product and in oxygen

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deletion because of dehydradion and deoxygenation reactions [8, 20].

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Biomass torrefaction products comprise a solid product (char), a permanent gas

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mixture (CO2, CO, H2 and CH4) and a condensed liquid [34]. The effect of torrefaction

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with Mg-based additives on subsequent pyrolysis of torrefied biomass (solid product)

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was investigated in our previous study [39]. However, the impact of Mg-based additives

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on the process of biomass torrefaction has never been investigated. There is also a lack

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of information about the properties of the products, including the liquid and gas

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products, obtained from torrefaction with Mg-based additives. From the perspective of

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practical operation, gas and liquid products should also be recovered and utilized as fuel

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or chemicals. In order to make better use of the products obtained from torrefaction,

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their properties should be investigated in advance. The formation of solid, gas and

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liquid products were investigated in this study to give us a deeper understanding of

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biomass torrefaction with Mg-based additives. The influence of torrefaction

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temperatures and of Mg-based additive types and their mass ratios on the properties of

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the torrefaction products were addressed. We found that the quality of torrefaction

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products can be improved by using particular temperatures and specific types and mass


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ratios of Mg-based additives.

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2. Experimental

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2.1 Experimental sample

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2.1.1 Cotton stalk

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Cotton stalk was collected from Wuhan city, Hubei province, China, and dried in

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an oven before storage in sampling bag. Prior to the experiments, it was cut into small

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particles (125-180µm) and dried at 105°C for 24h. The ultimate and proximate analyses

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of the cotton stalk were performed by an elemental analyzer (EL-2 CHN) and an

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industrial analyzer (SDTGA-2000), respectively. HHV was measured by a Parr6300

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bomb calorimeter. The ultimate and proximate analyses and the HHV of cotton stalk are

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presented in Table 1.

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Table 1

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Ultimate, proximate and HHV of cotton stalk Analysis

Value

Ultimate analysis (wt.%, dry ash basis) C

49.22±0.42

H

7.16±0.11

O

42.22±0.63

N

1.40±0.03

Proximate analysis (wt.%, dry basis)

Volatile

78.62±0.94

Fixed carbon

16.67±0.57

Ash

4.71±0.31

HHV (MJ/kg)

17.86±0.62


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2.1.2 Mg-based additives

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In order to prepare three kinds of Mg-based additives, A1, A2 and A3, four

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substances (all analytical grade, manufactured by Sinopharm Chemical Reagent Co.,

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Ltd.) composed of light MgO, K2CO3, KNO3 and NaNO3 were selected and

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mechanically mixed in certain proportions. The specific preparation method was as

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follows:

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A1: MgO was calcined at 650°C for 3 h.

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A2: MgO-K2CO3 was prepared through the mechanical mixing of calcined MgO

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and K2CO3 at a 3:1 mass ratio. A3: MgO, KNO3 and NaNO3 were mixed mechanically with a mass ratio of

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3:0.5:0.5 to prepare a uniform mixture (MgO-KNO3 -NaNO3).

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2.1.3 Torrefaction feedstock

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The torrefaction feedstocks (S1, S2, S3, S4, S5 and S6) were uniform mixtures of

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cotton stalk with Mg-based additives (A1, A2 and A3) at various mass ratios, given in

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Table 2. High ratios of additives were used in this study to ensure high contact

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efficiency between additives and biomass or intermediate products. The enhanced heat

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and mass transfer in this fixed bed reactor simulated the conditions in upscale systems

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such as fluidized bed reactors. There is therefore no need to use high mass ratios of

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additives in the future.

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Table 2

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The ingredients of the torrefaction feedstocks Sample

Ingredient

Mass ratio of additive to biomass

S1

Cotton stalk

0：1

S2

A1 and Cotton stalk

2：1

S3

A2 and Cotton stalk

2：1

S4

A2 and Cotton stalk

1：1

S5

A2 and Cotton stalk

0.5：1

S6

A3 and Cotton stalk

2：1

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2.2 Experimental procedure

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2.2.1 Experimental setup

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Torrefaction was conducted in a horizontal tube furnace system. The experimental

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setup is shown in Fig. S1. The main components are an electrical furnace, a mass flow

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meter, a quartz tube reactor, a temperature controller, a condensation unit and a gas

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collection unit.

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2.2.2 Experimental procedure

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10g (±5%) of torrefaction feedstock was placed in a quartz boat which was then

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put in a tube furnace. This was then heated to the desired temperature at 50°C/min and

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kept at that temperature for 50 min. Nitrogen (120 mL/min) was used to maintain the

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inert atmosphere and sweep volatiles out of the reactor. The volatiles were cooled and

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condensable species were gathered in the condensation unit. The non-condensable gas

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was filtered and then kept in a sampling bag. The solid residues were left to cool down


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to room temperature and were then regarded as a mixture of bio-char and Mg-based

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additives. Each run was conducted more than 3 times to ensure an RSD of less than 5%.

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2.2.3 Characterization of products

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The composition of the gas in sampling bag was determined by micro gas

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chromatography (Micro-GC, Agilent 3000). The solid residues included Mg-based

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additives, which needed to be removed before further analysis of the torrefied cotton

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stalk (solid product) was conducted. The specific method we used, which involved the

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use of hydrochloric acid solution and deionized water, has already been discussed in our

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previous study [39]. The ultimate, proximate and HHV analyses of torrefied cotton stalk

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were performed using the same instruments as those for cotton stalk. The evolution of

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organic functional groups for torrefied cotton stalk was determined using a VER- TEX

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70 spectrophotometer in the frequency range of 4000-400 cm-1 [37]. The liquid yield

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was determined from the serpentine tube mass difference. The components in the liquid

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product were determined by gas chromatography–mass spectroscopy (GC–MS; 7890A

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GC- HP5975 MS). The mass yields of the products and the energy yield of the torrefied

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cotton stalk were defined as follows:

Ymass ,i =

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Yenergy ,i =

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m pr ,i mrs

×100% (1)

Ymass ,i × HHV pr ,i HHVrs

×100% (2)

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where m and HHV are mass and higher heating value, respectively. Subscripts “pr” and

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“rs” refer to the torrefaction product and the raw sample, respectively. “i” ranges over


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solid, gas and liquid.

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3. Results and discussion

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3.1 Final product distribution

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Fig. 1 demonstrates the influence of torrefaction temperature on the product yields.

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For S1 torrefaction (cotton stalk), the solid yield significantly decreased from 92.02% to

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36.95% when torrefaction temperature increased to 350°C as shown in Fig 1a, the gas

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yield simultaneously increased from 3.79% to 24.90% as shown in Fig 1b, and the

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liquid yield significantly increased from 4.19% to 38.15% as shown in Fig 1c. These

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results are consistent with several previous works on the torrefaction of cotton stalk [10,

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17]. A previous study showed that the solid yield of cotton stalk drastically decreased

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from 90% to 45% when the torrefaction temperature increased from 200 to 290°C,

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while the liquid product yield increased from 4% to 29% due to the enhancement of the

209

devolatilization process [10]. The decrease in solid yield was not obvious at 200°C. The

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reason for the slight weight loss might have been the evaporation of bound water in

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cotton stalk. The solid yield rapidly decreased as the temperature increased from 230 to

212

290°C primarily because of intensified decomposition of hemicellulose and cellulose

213

[7]. The results were similar to the torrefaction of wood block [34]. Hemicellulose was

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almost depleted at 220-315°C and cellulose was also largely consumed, while the

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consumption of lignin, the most thermally stable compound, was very low [40].

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Therefore, there was less of a decrease in solid yield above 320°C. The increase in gas


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yield was much more pronounced between 200 and 230°C than that between 230 and

218

350°C (Fig. 1b). This is explained by greater devolatilization of hemicellulose at 230°C,

219

due to its higher reactivity [30]. However, the increase in liquid yield was most

220

significant between 230 and 260°C as shown in Fig 1c. The main cause of this was

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increased cellulose degradation [30]. Liquid yield was higher than gas yield at

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torrefaction temperatures above 230°C due to the rapid decomposition of other polymer

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fractions for heavier compounds formation in liquid product [16].

224

With the addition of Mg-based additives (S2-S6), torrefied product yields still

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followed the same trends, with increases in torrefaction temperature causing decreases

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in solid product yield and increases in the yields of liquid and gas products. However,

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the size of the change in torrefied product yields varied with the additive types and their

228

mass ratios. As shown in Fig. 1a, the differences in the yields of solid products between

229

different additives were very small from 260-290°C. When the temperature was

230

increased to 320°C, the highest solid yield of 43.44% was obtained from S2, while the

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lowest solid yield of 37.97% was obtained from S6. The solid yield from S6 with the

232

MgO-KNO3-NaNO3 additive was about 30.05%, significantly lower than the 36.95%

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yield obtained from S1 (raw cotton stalk) at the higher temperature of 350°C. This was

234

mainly due to the fact that char aromatization with cleavage of C-H and C-O bonds was

235

promoted by the increased amount of Mg-based additives present in S6, which further

236

reduced solid yield [41]. The increase in the gas yield from S3 (with the MgO-K2CO3

237

additive), from 7.45% to 33.07%, observed when the temperature rose from 200°C to


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350°C was greater than in the case of raw cotton stalk (3.79% to 24.90%) as depicted in

239

Fig 1b. The liquid yield from S3 simultaneously increased more slowly (from 5.80% to

240

27.91%) than in the case of raw cotton stalk (from 4.19% to 38.15%) as shown in Fig 1c.

241

This is explained by the fact that the MgO-K2CO3 increased the occurrence of catalytic

242

reactions which led to the formation of gas products from the liquid products. Also, Fig.

243

1b and Fig. 1c show that the effect of Mg-based additives on gas and liquid product

244

yields was more significant at temperatures higher than 260°C. Compared with raw

245

cotton stalk torrefaction, the introduction of Mg-based additives led to a relatively

246

significant increase in gas yield and a reduction in the liquid yield, except in the case of

247

A1 (MgO). This indicates that the K2CO3 in the additive A2 and the KNO3-NaNO3 in

248

the additive A3 were relatively more active constituents than MgO. During torrefaction,

249

volatiles were mainly formed through the decomposition of hemicellulose and cellulose

250

[29]. More volatiles were converted into gas products with the introduction of Mg-based

251

additives [42, 43]. Moreover, the thermal degradation of intermediate volatiles was

252

enhanced when the mass ratio of the Mg-based additive was increased. This was

253

because increasing the mass ratio of the Mg-based additive led to better contact between

254

it and the intermediate volatiles, which was conducive to the cracking of the

255

intermediate volatiles [43]. Hence, the effects of S4 and S6 were the most significant.

256

The cracking reactions of primary vapors released from cellulose and hemicellulose

257

were catalyzed by K2CO3 in S4 and KNO3-NaNO3 in S6 [44], which caused a

258

fluctuation in product yields.


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259 260

Fig. 1. Effects of temperature on the product yields of cotton stalk torrefaction with

261

Mg-based additives: (a) Solid yield, (b) Gas yield, and (c) Liquid yield.

262

3.2 Solid product characterization

263

3.2.1 Composition variation

264

The variation of solid product composition with torrefaction temperature and

265

Mg-based additives is depicted in Fig. 2. The carbon content of the solid product

266

increased with torrefaction temperature as depicted in Fig. 2a. However, the hydrogen

267

and oxygen content decreased as torrefaction temperature increased as shown in Fig. 2b

268

and Fig. 2c. These trends were in accordance with the results for torrefied cotton stalk

269

obtained at different temperatures [17]. This was mainly because volatiles rich in H and

270

O (such as H2O and CO2) were released [12, 16, 19]. It was evident that additive A2

271

(contained in S3, S4 and S5) led to the greatest improvements in C content and


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reductions in O content. In particular, when the torrefaction temperature was 290°C, the

273

carbon content was increased by 35.44%, 38.57 % and 38.95% for S3, S4 and S5

274

respectively compared with raw cotton stalk (S1, Fig. 2a). Simultaneously, the oxygen

275

content was decreased by 38.34%, 42.25% and 42.80%, respectively (Fig. 2b) and the

276

hydrogen content was decreased by 28.22%, 25.56% and 25.65%, respectively (Fig. 2c).

277

This was attributed to enhanced CO2 absorption caused by the MgO-K2CO3 mixture

278

[23]. CO2 absorption during biomass torrefaction led to decarbonylation instead of

279

deoxygenation, and so a greater deoxygenation effect was achieved. The effect of

280

additive A1 (contained in S2) was weaker. This was because the MgO had a lower

281

theoretical CO2 adsorption value than the MgO and K2CO3 mixture [22]. Additive A3

282

(contained in S6) was the least effective. When the torrefaction temperature was 290°C,

283

the carbon content in torrefied S6 increased by 23.03% less than in the case of S1

284

torrefaction (26.3%). The hydrogen and oxygen content of torrefied S6 decreased by

285

42.45% and 25.7%, respectively. Comparing the oxygen content of torrefied S3, S4 and

286

S5 shows that the deoxygenation effect was not simply enhanced by increasing the

287

Mg-based additive mass ratio from 0.5 to 2. A stronger deoxygenation effect was

288

achieved with the addition of a low proportion of Mg-based additive at temperatures

289

between 260-320°C. In addition, the differences between the deoxygenation effects of

290

different additives were largest in the temperature range of 260-320°C. However,

291

Mg-based additives slightly inhibited the deoxygenation effect at the lower end of the

292

torrefaction temperature range (200-230°C) due to reductions in the rates of dehydration


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

293

reactions. Furthermore, the variation of each element was steep from 230-290°C and

294

flattened out above 320°C. This meant that increasing the torrefaction temperature was

295

not a good way of enhancing the deoxygenation effect.

296 297

Fig. 2. Effects of temperature on change in composition of solid product of cotton stalk

298

torrefaction with Mg-based additives: (a) C, (b) O, and (c) H.

299

3.2.2 van Krevelen diagram and HHV

300

The variation in the composition of the solid product is also illustrated in a van

301

Krevelen diagram (Fig. 3a). The atomic ratios of H/C and O/C for cotton stalk were

302

1.50 and 0.65, respectively. The ratios of H/C and O/C were reduced as the torrefaction

303

temperature increased, in close agreement with previous studies [8, 12, 19]. They

304

dropped fastest from 200-260°C, and little reduction occurred above 320°C. The

305

addition of Mg-based additives led to a greater loss of hydrogen at temperatures below


Page 16 of 33

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Energy & Fuels

306

260°C. Hydrogen was primarily released as H2O [7], indicating that the Mg-based

307

additives promoted the dehydration reaction. Above 260°C, additive A2 (contained in

308

S3, S4, S5) retained more hydrogen while causing efficient deoxygenation. At 290°C,

309

H/C ratios were 0.80, 0.81 and 0.81 for torrefied S3, S4 and S5, which were higher than

310

those of torrefied S1, S2 and S6 with O/C of 0.27. Therefore, from the view of

311

improving both the deoxygenation effect and hydrogen retention, additive A2 performed

312

best. Results were also better with a lower Mg-based additive mass ratio (0.5).

313

These changes in the composition of the solid product improved its higher heating

314

value (HHV) because more oxygen than carbon was lost in volatiles [16]. As shown in

315

Fig. 3b, increasing the torrefaction temperature and using additives were effective ways

316

to increase solid product HHV. Torrefaction of the raw cotton stalk without an

317

Mg-based additive at 320°C increased the HHV by a factor of 1.37 to 27.23MJ/kg.

318

Similar findings have been reported for raw wood torrefaction [34]. By contrast, the

319

addition of Mg-based additives (except for A3, contained in S6) further increased the

320

HHV of the torrefaction solid product for temperatures between 260-320°C. The

321

temperature played the most important role in determining the HHV of the torrefied

322

solid product, while the impact of the Mg-based additives was much less significant.


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

323 324

Fig. 3. Effect of temperature on solid product from cotton stalk torrefaction with

325

Mg-based additives: (a) van Krevelen diagram and (b) HHV change.

326

Fig. 4 shows the energy yield of the solid product from cotton stalk torrefaction at

327

different temperatures with the addition of different Mg-based additives. The energy

328

yield decreased at higher temperatures because solid yield decreased. This result was

329

consistent with previous work on torrefaction of waste biomass [19]. Above 260oC, the

330

energy yields from S3, S4 and S5 could be further improved with the addition of

331

additive A2. For example, the energy yield from sample S5 was 68.14% higher than that

332

from sample S1 (59.89%). It should also be noted that torrefaction with a lower mass

333

ratio of additive to cotton stalk led to an increase in energy yield. Comparing Fig 1a

334

with Fig 4 shows that the energy yields were significantly higher than the mass yields

335

for torrefaction. Therefore, torrefaction is an effective way to increase feedstock energy

336

density, which reduces transportation cost and improves the economy of the overall

337

system. From the perspective of practical operation, the gas and liquid products from

338

torrefaction should be collected and utilized as energy, fuel or chemicals due to their

339

lower energy yields of solid product [34].


Page 18 of 33

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

Energy & Fuels

340

Fig. 4. Effect of temperature on solid product energy yields of cotton stalk

341

torrefaction with Mg-based additives.

342

343

3.2.3 Structural evolution

344

Fig. S2 shows FTIR spectra of torrefied solid products with classifications of

345

characteristic peaks according to literature data [17, 35, 37]. The broad absorption peak

346

(3400-3200 cm-1) was ascribed to intra- and intermolecular O-H bonds from crystalline

347

cellulose [35]. As shown in Fig. S2a, for raw cotton stalk torrefaction, the intensity of

348

this peak declined considerably as torrefaction temperature increased, indicating

349

hydrogen bonds and the release of water through dehydroxylation and condensation

350

reactions of hemicellulose and cellulose [8]. The C=O peak (1765-1715 cm-1) was

351

assigned to the acetyl group in hemicellulose and lignin [37]. It decreased in intensity as

352

the torrefaction temperature increased, resulting in decarbonylation, decarboxylation,


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

353

and C=O bond fracture of acids and aldehydes [35]. The intensity of the 1260 cm-1 (C-O)

354

signal declined sharply as the torrefaction temperature increased, indicating lignin

355

demethoxylation and β-O-4 cleavage [45]. There were smaller changes in these peak

356

intensities between 200-350°C when Mg-based additives (A1, A2 and A3) were added

357

as shown in Fig. S2b, Fig. S2c and Fig. S2d. In addition, the oxygen-containing organic

358

groups in the solid product tended to be simplified when the torrefaction temperature

359

was increased and Mg-based additives were introduced, resulting in the solid products

360

having a lower oxygen content. The improved chemical and structural properties of

361

solid product imply that it can be utilized as an ideal feedstock for fast pyrolysis in

362

terms of enhancing bio-oil yield and quality.

363

3.3 Gas product composition

364

Fig. S3 shows the composition of the gas product from cotton stalk torrefaction at

365

different temperatures. The results closely agree with previous studies on biomass

366

torrefaction [36, 46, 47]. CO2 and CO were the main gas components, while H2 and CH4

367

were trace components. CO2 and CO yields noticeably increased from 1.04 to 1.88

368

mmol/g and from 0.26 to 1.38 mmol/g respectively when the torrefaction temperature

369

rose from 200 to 350°C, and H2 yield also increased slightly from 0 to 0.24 mmol/g.

370

The ratio between CO2 and CO decreased from 4 to 1.36 between 200°C and 350°C,

371

which is consistent with previous studies [48]. The release of gas products was related

372

to the thermal degradation characteristics of the three major components of biomass in

373

the torrefaction temperature range [40]. An increase in splitting of the C-C and C-O


Page 20 of 33

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Energy & Fuels

374

bonds in hemicellulose and cellulose as the torrefaction temperature increased

375

contributed to the increase in CO2 release [48]. CO is mainly generated from the

376

decarbonylation of hemicellulose between 210 and 320°C, which increases as the

377

torrefaction temperature rises [30, 40]. A small amount of CH4 was observed above

378

290°C and might have been produced by the demethylation of lignin [49].

379

The gas products of cotton stalk torrefaction continued to follow the same trends

380

with increasing temperature upon introduction of the Mg-based additives (S2-S6).

381

However, CO2 production was significantly increased by the introduction of Mg-based

382

additives at temperatures higher than 260°C. A2 was the most effective additive.

383

Increasing the mass ratio of the Mg-based additive from 0.5 to 2 also promoted CO2

384

formation. For the torrefaction of S3 (with additive A2), the CO2 yield rose from 1.20 to

385

2.80 mmol/g when the torrefaction temperature was increased to 350°C as shown in Fig.

386

5a. The increased production of CO2 might be due to the introduction of alkali metals,

387

which promoted the formation of carboxyl groups from cellulose during torrefaction [41,

388

43]. Furthermore, carboxyl group cracking was the major means of CO2 formation [48].

389

Although some CO2 was absorbed by the Mg-based additives, the remainder yield still

390

increased due to the presence of alkali metals. As shown in Fig. 5b, the effect of the

391

additive on the amount of CO produced was not as significant as in the case of CO2. The

392

yield rose from 0.07 to just 1.20 mmol/g when the torrefaction temperature was

393

increased to 350°C. The main reason for this was that CO mainly originates from

394

hemicellulose [50], which is not significantly affected by Mg-based additives [41, 51].


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

395

The H2 yield was increased from 0.10 to 0.40 mmol/g as shown in Fig 5c. H2 was

396

mainly produced by the dehydrogenation of benzene rings which are a primary product

397

of lignin pyrolysis [40]. Potassium contained in A2 facilitated dehydrogenation,

398

favoring H2 formation. Gas products with enhanced heating values can be utilized to

399

provide more heat for torrefaction or pyrolysis processes.

400 401

Fig. 5. Effect of temperature on the composition of gas product from cotton stalk

402

torrefaction with Mg-based additives: (a) CO2 molar yield, (b) CO molar yield, and (c)

403

H2 molar yield.

404

3.4 Liquid product characterization

405

The liquid product of torrefaction consisted of two parts: the aqueous and the

406

organic phases. The water content of the liquid product decreased from 87% to 61%

407

when the temperature increased from 200 to 350°C due to increased production of


Page 22 of 33

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Energy & Fuels

408

organic compounds. The organic phase contained many aromatics, which could be

409

utilized as high-value chemicals rather than as fuels [30]. The color of the liquid product

410

gradually changed from light yellow to dark brown as the torrefaction temperature

411

increased. This was mainly due to higher yields of heavier components [34]. We

412

performed an analysis of the liquid product using GC-MS, which can determine the

413

main components in the liquid [48]. Fig S4 depicts the total area from the organic

414

compounds in the liquid product of torrefaction. As a whole, the organic compound total

415

peak area rose as the torrefaction temperature increased, indicating higher liquid yields.

416

The introduction of Mg-based additives made the total peak area decrease further. These

417

results are consistent with the liquid yield variations discussed in 3.1.

418

Fig S5 depicts the peak areas% of each component of the liquid product of

419

torrefaction. Acids, phenols and ketones were relatively abundant in the liquid product.

420

As shown in Fig. S5a, without an Mg-based additive the relative content of phenols,

421

aldehydes, ketones and alcohols increased as the torrefaction temperature rose. There

422

was an increase in the relative content of phenols, indicating more lignin decomposition

423

[29]. The introduction of Mg-based additives clearly affected the relative content of the

424

liquid product components as shown in Fig S5b-f. The relative content of acids

425

decreased, while those of ketones and phenols increased.

426

Four main components (acids, ketones, phenols and aldehydes) are compared in

427

Fig 6 in order to determine the influence of different Mg-based additives on liquid

428

product distribution. The Mg-based additives, especially those with A2 (contained in S3,


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

429

S4 and S5), caused a further decline of acid relative content. At 260°C, the relative

430

content of acids decreased to 15% or less. Acids were mainly produced by the thermal

431

degradation of hemicellulose [50]. As the temperature increased, hemicellulose started

432

to undergo dehydroxylation, decarboxylation and other thermal cracking reactions,

433

which led to the formation of a series of acids [30]. Although the absolute content of

434

acids increased as the torrefaction temperature rose [36], the total liquid product yield

435

increased faster than the increase in absolute content of acids because of the increased

436

thermal cracking of biomass. This might have led to the decrease in the relative content

437

of acids in the liquid which was observed. However, this decrease could also have been

438

caused by decomposition of the acidic compounds into compounds of other kinds with

439

the Mg-based additive acting as a catalyst [43]. The relative content of ketones (Figure

440

6b) and phenols (Figure 6c) significantly increased when an Mg-based additive was

441

introduced, and additive A2 (contained in S3, S4, S5) had the greatest effect.

442

Oxygenates of bio-oil were easily transformed in Mg-based additive basic sites through

443

ketonization and aldol reaction [52]. This shows that torrefaction can produce liquid

444

precursors for the refinery industry. Also, no monotonic relationship between change in

445

the main functional group and the mass ratio of the Mg-based additive was observed.

446

The increase in the content of aldehydes when using additive A2 (contained in S3, S4,

447

S5) was less pronounced, whereas additive A1 (contained in S2) and additive A3

448

(contained in S6) increased aldehyde content for temperatures between 260-290°C.


Page 24 of 33

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Energy & Fuels

449 450

Fig. 6. Effect of temperature on the main functional group distribution of the liquid

451

product from cotton stalk torrefaction with Mg-based additives: (a) Acids, (b) Ketones,

452

(c) Phenols, and (d) Aldehydes.

453

4. Conclusion

454

The yield of solid product significantly decreased from 92.02% to 36.95% when

455

the torrefaction temperature rose to 350°C, while gas and liquid yields increased to

456

24.90% and 38.15% respectively. MgO-K2CO3, the most effective additive, not only

457

decreased oxygen content by 42.8% in the solid product but also reduced the loss of

458

hydrogen. Characterization of the solid product obtained from biomass torrefaction with

459

Mg-based additives suggested that it could be utilized as an ideal feedstock for pyrolysis.

460

Increasing the mass ratio of the Mg-based additive from 0.5 to 2 promoted main gas

461

component CO2 production. It also effectively improved phenols and ketones in liquid


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

Page 26 of 33

462

product and reduced the acid content. Therefore, by-products (gas and liquid) with

463

improved properties could also be recovered and utilized as fuel or chemicals.

464 465 466 467 468 469 470

Acknowledgement

471

This work was supported by the National Natural Science Foundation of China

472

(51576087, 51706083), the Fundamental Research Funds for the Central Universities

473

(2016YXZD007),

474

(2016CFB132).

and

the

Natural

Science

Foundation

475 476


of

Hubei

Province

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Energy & Fuels

477

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Effects of Temperature and Mg-Based Additives on Properties of

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