Acetic Acid and Sodium Hydroxide-Aided Hydrothermal Carbonization

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Acetic acid and sodium hydroxide-aided hydrothermal carbonization (HTC) of woody biomass for enhanced pelletization and fuel properties Tengfei Wang, Yunbo Zhai, Yun Zhu, Chuan Peng, Bibo Xu, Tao Wang, Caiting Li, and Guangming Zeng Energy Fuels, Just Accepted Manuscript • Publication Date (Web): 27 Sep 2017 Downloaded from http://pubs.acs.org on September 27, 2017

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Energy & Fuels

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Acetic

acid

and

sodium

2

carbonization

3

pelletization and fuel properties

4

Tengfei Wanga,b, Yunbo Zhaia,b *, Yun Zhuc,d*, Chuan Penga,b, Bibo Xua,b, Tao Wanga,b,

5

Caiting Lia,b, Guangming Zenga,b

6

a

7

410082, P. R. China

8

b

9

Ministry of Education, Changsha 410082, P. R. China

(HTC)

of

hydroxide-aided woody

biomass

hydrothermal for

enhanced

，

College of Environmental Science and Engineering, Hunan University, Changsha

10

c

11

d

Key Laboratory of Environmental Biology and Pollution Control (Hunan University),

Office of Scientific R& D, Hunan University, Changsha 410082, P. R. China Shenzhen Institutes of Hunan University, Shenzhen518000, P. R. China

12 13 14 15 16 17 18 19 20 21

*

22

E-mail Address: [email protected](Y.B. ZHAI), [email protected](Y. ZHU)

Corresponding Author. Tel.+86 731 8882 2829, Fax. +86 731 8882 2829.

ACS Paragon Plus Environment

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Abstract: In this study, hydrothermal carbonization (HTC) at 200 °C and 250 °C was

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combined with acetic acid and NaOH (changing feedwater pH from 2-12) to treat

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woody biomass for enhancing its pelletization and fuel properties. The treated

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biomass (hydrochar) was characterized by composition analysis and FTIR, after

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which the energy consumption of pelletization; tensile strength; fuel properties,

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including proximate, elemental analysis, and mass density; and combustion

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characteristics of corresponding pellets were analyzed. The results showed that both

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the acetic acid and NaOH-aided HTC processes decreased the energy consumption of

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hydrochar compression. Weakly basic or acidic feedwater decreased the tensile

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strength of hydrochar pellets, but it increased at pH 2 and 12. The changes in

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treatment pH from 4 to 9 had little effect on the O/C and H/C content of the hydrochar

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pellets, while the addition of acetic acid enhanced oxygen and hydrogen loss more

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than NaOH. The heating values of hydrochar pellets produced at pH 2 from 200 °C

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and 250 °C were 20.7 and 22.8 MJ/kg, respectively. Hydrochar pellets produced from

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HTC at 250 °C and pH 2 had higher mass density and lower equilibrium water

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content than those produced under other conditions. The combustion process was

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promoted with reduced ignition temperature, maximum weight loss rate, and burnout

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temperature. These findings demonstrate that acetic acid-aided HTC of woody

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biomass was more suitable than NaOH for producing hydrochar pellets for use as a

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biofuel.

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Keywords: Hydrothermal carbonization; pH; Pelletization; Tensile strength; Fuel

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properties.


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Introduction The growth of fuel consumption has lagged behind the production rate, which

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has increased focus on the application of biomass as a renewable energy1. It is

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anticipated that dependence on biomass from plants will rise in the coming decades,

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particularly for large-scale production, as such biomass is reproducible as a second

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generation biofuel, and lignocellulosic biomass is the most suitable supplement to

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fossil fuel due to its ability to reduce greenhouse gas emissions (GHGs)2. Many

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conversion technologies have been successfully applied to lignocellulosic biomass.

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However, pelletization is an effective process for the densification of raw biomass

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into pellets with increased bulk density and good mechanical properties, which

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reduces handling, transportation, and storage costs3. However, issues with respect to

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high moisture content, heterogeneity, and low energy density need to be overcome for

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the direct utilization of raw biomass in pellet production4-5.

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Attention is increasingly focusing on the solid product (hydrochar) obtained from

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hydrothermal carbonization (HTC), which is conducted at relatively low temperatures

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(180−250 °C) to convert raw biomass to lignite-like materials6-7. Both oxygen and

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hydrogen contained in the feedstock are significantly reduced by dehydration and

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decarboxylation, producing a solid with highly condensed energy, exhibiting more

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uniform properties and increased friability4. Compared with pellets produced from

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raw biomass, hydrochar pellets have excellent fuel quality due to the high fixed

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carbon content, improved heating values, and mass densities, and these properties

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depend on hydrothermal conditions4, 8. However, besides hydrothermal parameters


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such as temperature, residence time, and feedstock, the quality of feedwater is also

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critical for the hydrothermal reaction9. Notably, acids or alkalis are often added during

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HTC to accelerate the hydrolysis/degradation process by changing ion products10-11.

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The decrease in the pH of the process water is due to the production of organic acids,

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such as acetic, formic, and lactic acids12, and the addition of acetic acid in HTC

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improves the energy content of hydrochar as it catalyzes the decomposition of

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polymers and hydrochar formation13-14. However, introducing NaOH could neutralize

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the generated organic acids, and the hydrothermal process can be influenced by the

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exchange of sodium ions10. A previous study demonstrated that the addition of NaOH

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in the HTC process would change the lignocellulose composition by effectively

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decomposing lignin, whilst the treated biomass had low sodium ion content10. Thus,

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the treated biomass may have different properties depending on whether the

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hydrothermal process was conducted under acidic or alkaline conditions, ultimately

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affecting the densification process.

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The mechanical properties of pellets depend on the properties of the raw

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materials, including constituents, moisture content, particle size, and densification

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conditions, such as temperature and compression force15-16. The binding forces

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between particles are generally categorized into two bonding mechanisms: (1) solid

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bridge-type bonding between particles, formed by the diffusion of molecules from one

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particle to another at contact points, and (2) attraction forces without a solid bridge

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between adjacent particles17-18. Interlocking bonds in raw biomass pellets are

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primarily created through solid bridges between particles by natural binders, including


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lignin and protein after high compression and temperature processing17. It has been

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reported that lignin in lignocellulose exhibits glass transition behavior at temperatures

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around 140 °C8, 19. However, after the HTC process, biomass components affect

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pelletization and the binding mechanism within the pellets. It has been demonstrated

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that the liquid bridge from high molecular organic compounds and enhanced

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attraction forces derived from the surface’s functional groups, contact area, and

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extractives in hydrochar pellets from HTC at 250 °C are critical for mechanical

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strength4. Until now, studies have primarily focused on the fuel characteristics of

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hydrochar produced from HTC with acidic or alkaline assistance, or single HTC of

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biomass for producing fuel pellets10, 20. The pelletization process using biomass

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treated by the acidic and alkaline-aided HTC process has not yet been studied, and

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there is an urgent need to investigate the characteristics of pellets from biomass

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treated for solid fuel preparation.

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In this study, woody biomass treated by acetic acid and sodium hydroxide-aided

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HTC processes at 200 °C and 250 °C and different pH levels (2 to 12) was used to

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prepare fuel pellets via a pelletization process. The composition and surface

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functional groups of the treated biomass were analyzed, followed by investigation of

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energy consumption in the pelletization process. In addition, the mechanical strength;

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fuel properties, including proximate and elemental analysis, and mass density; and

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combustion characteristics of fuel pellets were evaluated to assess their potential for

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biofuel production.

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Materials and methods


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Materials. Hydrochar was obtained by conducting HTC of wood sawdust

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(collected from a local furniture factory and washed with deionized water) in a 500

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mL 316 stainless steel reactor. Acetic acid or NaOH (analytically pure) were used to

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adjust the feedwater pH within a range of 2 to 12. Approximately 10 g of dried

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feedstock loaded with 200 mL of feedwater at a specific pH was dispersed in the

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autoclave, which was heated using a 3.0 kW electric furnace at an approximate rate of

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4 °C/min to 200 °C or 250 °C (pressures of approximately 2.4 or 4.2 MPa,

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respectively) for 30 min, while stirring at 100 r/min. The autoclave was cooled at

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room temperature, and the hydrochar was separated from the mixture and washed

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with deionized water. Following this, the hydrochar was dried at 105 °C for 24 h in an

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oven and ground into a powder by a rotary mill for 2 min, and particle size analysis

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was then conducted (LS-pop(6), China).

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Pelletization of hydrochar. The pelletization process was conducted using a single

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pellet press, which consists of a cylinder-piston unit with heating tape around the

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cylinder, controlled by a thermocouple and temperature controller. The details of the

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apparatus and operation process can be found in previous research16, 21. Prior to

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pelletization, approximately 10% (w/w) of additional water was introduced to the

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sample, based on previous data, to enhance the strength and act as a lubricant during

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the compression process15, 21. Approximately 0.8 g of the sample was compacted at a

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maximum force of 4 kN held for 30 s. The die temperature was maintained at 140 °C

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as lignin easily undergoes glass transition at this temperature, creating a softened

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binder when compressed22-23. The forces and displacement curves were recorded


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online to calculate energy consumption during the compression and extrusion

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processes, respectively. After the extrusion process, the mass, length, and diameter of

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the sample were measured, and the sample was then stored in plastic bottles at 4 °C

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for one week. Length expansion was determined by the following equation:

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Length expansion = (l − l )/l

(1)

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where li represents the initial length of the pellet, and ls represents the length of the

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pellet after it had been stored.

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The pellet consisting of hydrochar produced under certain conditions was

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denoted as T-x-HP, where T represents the temperature, x represents the pH of

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feedwater, and HP indicates that they were hydrochar pellets. The raw biomass pellets

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were denoted as RP. Each experiment was conducted in triplicate.

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Characterization of the hydrochar and pellets. Mass yield was calculated based

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on dry weight, according to the following equation:

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Mass yield(%) = m /m

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where mh is the hydrochar mass and mr is the mass of raw biomass. The lignin

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contents of raw biomass and hydrochar were measured using the 72% sulfuric acid

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method, and the extractives, holocellulose, and hemicelluloses were determined

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following the methods of Carrier et al. and Yang et al.10, 24. Hemicellulose content was

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calculated from the difference between holocellulose and cellulose. Ash was not

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counted as a component. FT-IR analysis of the samples was conducted by an

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FTIR-8400S Spectrometer (USA). The mechanical strength of the pellet was

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measured by tensile strength (Ts), which was tested by the application of a

(2)


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compressive force to a pellet placed horizontally between two anvils. The maximum

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compressive force was recorded when the pellet broke, and Ts was calculated by the

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following equation4, 25:

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Ts = π !

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where F is maximum compressive force, and d and l are pellet diameter and length,

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respectively.

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(3)

Scanning electron microscopy (FEI QuANTA 200，Czech Republic) was used to

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study the pellet surface, and a Zeiss light microscope (Axio Lab.A1, Germany) was

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employed to examine the sectional scratch surface at 100× magnification. Proximate

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analysis of the pellets was conducted according to the Chinese Standard Practice for

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Solid Biofuels (GB/T28731-2012). Elemental analysis of the raw biomass and

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hydrochar pellets was conducted on an Elementar Vario EL cube (Germany). The

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combustion behavior of the pellets was assessed by a thermogravimetric analyzer

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(STA 409, NETZSCH, Germany), with a temperature interval from 30 °C to 800 °C,

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at a heating rate of 15 °C/min with an air (N2 = 80%, O2 = 20%) flow rate of 50

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mL/min. Prior to the experiments, the pellet was manually broken into small pieces

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with a razor blade, and approximately 15 mg of the sample was used for each

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experiment. The equilibrium moisture content (EMC) of the pellets was tested using a

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humidity chamber set at 25 °C and 70 % relative humidity. Prior to this process, the

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samples were dried in an oven set at 105 °C for 24 h and the mass of the pellet was

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measured when it reached constant value.

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Results and discussion


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Yield and composition analysis of hydrochar. Table 1 illustrates the hydrochar

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yields and corresponding components after HTC treatment with varying pH (2-12) at

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200 °C and 250 °C. As anticipated, the mass yield of hydrochar production varied

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more severely with differing HTC temperatures than it did with pH. It varied from

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-56.2% to 69.3% at 200 °C, while it was significantly smaller at 250 °C, ranging from

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39.9% to 45.4%. This result was highly related to the decomposition/hydrolysis of the

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hydrochar’s components. All hydrochar exhibited extreme degradation of

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hemicellulose, but the hydrochar produced at 250 °C had a lower mass yield than that

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produced at 200 °C due to high cellulose degradation. This was due to the dramatic

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changing in water characteristics by a decrease of the dielectric constant, producing

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highly ionized constants when the temperature in the subcritical region improved.

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Degradation of cellulose occurred at temperatures above 200 °C due to disintegration

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by hydrolytic action and further reactions26-27. Furthermore, the pH of 2 at both

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200 °C and 250 °C produced the lowest hydrochar yields of 56.2% and 39.9%,

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respectively. Similar results can be observed in Reza’s study, in which feedwater at

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pH 2 produced the lowest mass yield of hydrochar at both 200 °C and 260 °C, as the

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HTC process was strengthened by acidic catalysis20. The components analysis showed

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that acidic feedwater was more favorable for the degradation of hemicellulose than

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basic feedwater14. In addition, both acidic and basic feedwater resulted in decreased

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cellulose content in hydrochar, excluding the sample produced at 250 °C with pH 2,

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which showed a maximum content of 19.0 %, resulting from the degradation of lignin

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and hemicelluloses. When the conditions became more basic or acidic at 250 °C, high


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degradation of lignin was observed, but this was not notable at 200 °C. Overall,

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varying pH resulted in different solid yields and hemicellulose, cellulose, and lignin

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content as it changed the properties of subcritical water20. Moreover, the extractives

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content of all the hydrochar were generally lower in value than that of raw biomass,

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and did not significantly change with varying feedwater pH at 200 °C, however,

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highly acidic and basic feedwater (pH 2, 3, 11, and 12) at 250 °C produced hydrochar

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with increased extractives content.

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FTIR results of raw biomass and hydrochar. To understand chemical changes in

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the treated biomass during HTC, FTIR analysis was conducted for the hydrochar

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samples as well as the raw biomass. As shown in Figure 1, the IR spectra of raw

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biomass was very similar to that of the biomass treated at 200 °C. The composition

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analysis indicated that the chemical changes in hydrochar from HTC with the addition

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of acetic acid and sodium hydroxide were limited; thus, FTIR was not noticeable. The

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band at 1737 cm-1, indicating the C=O stretching vibrations in the hemicellulose, was

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only identified in the raw biomass, demonstrating the degradation of hemicellulose by

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hydrothermal degradation even at 200 °C28. Peaks between 2820 cm-1 and 2990 cm-1

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were associated with the stretching vibration of aliphatic C-H, indicating the presence

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of aliphatic structures in all hydrochar samples27. The OH stretching vibration at

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approximately 3445 cm-1 was attributed to the hydroxyl and carboxyl groups, which

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did not significantly change when the hydrothermal pH was varied at 200 °C;

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however, they were noticeably weakened at 250 °C, particularly at pH 2. This could

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be because the low pH facilitated the hydrothermal dehydration reaction13. However,


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the peak at 1698 cm-1, corresponding to the carboxyl, carbonyl, or ester groups,

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occurred for the samples prepared at 250 °C, indicating the presence of

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oxygen-containing functional groups on the hydrochar’s surface 27. However, this was

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not noticeable for pH 2, which may be due to enhanced hydrothermal dehydration or

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decarboxylation causing oxygen loss20. The absorptions at 1508 and 1460 cm-1 were

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ascribed to C=C stretching in aromatic lignin groups, and were sharpened in all

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treated biomass samples, but were weaker for both highly acidic and basic

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environments, which confirmed the presence of lignin in the samples. However, a new

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band at 1600 cm-1, corresponding to the presence of C=C, and enhanced bands in the

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region of 897-750 cm-1, attributed to aromatic C-H out-of-plane bending vibrations,

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were observed in 250 °C-derived hydrochar, suggesting that the hydrochar has an

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aromatic skeleton from aromatization27. The bands at 1210 cm-1 and 1031 cm-1 were

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ascribed to C-O in cellulose 28. The small peaks (at 1150, 1110, and 1064 cm-1) in this

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region were sharp after treatment at 200 °C, but became rounded at 250 °C,

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confirming that cellulose was mostly degraded at 250 °C, consistent with the

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composition analysis.

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Pelletization and energy consumption. As shown in Figure 2, typical

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displacement of the samples in the compression process demonstrated that both acid

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and alkali-treated biomass treated had decreased compression displacement until they

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reached the maximum compressive force. Therefore, compared with pH 7, when the

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hydrothermal pH was increased or decreased, the treated material consumed less

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compression energy (calculated from the integral area). The addition of an acid or


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alkali enhanced the hydrothermal hydrochar production process with better

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grindability, and particle packing occurred easily, causing low energy consumption4.

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Thus, hydrochar produced from both basic or acidic environments consumed less

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energy for rearranging particles and disrupting the larger ones, and the interspace

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between particles could be reduced easily when impacting the samples before elastic

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compression29-30. The energy consumption of a single pellet during compression and

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extrusion is presented in Figure 2. The energy consumption was significantly

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different for varied hydrothermal temperatures and pH. The compression energy

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consumption of 200-x-HP was significantly lower than that of 250-x-HP. This was

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because the high temperature enhanced the carbonization of hydrochar by

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decomposing hemicelluloses or soluble substances (such as protein and starch) in the

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wood materials, resulting in an increase in hydrochar grindability, which improved the

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energy consumption31-33. The compression energy consumption of 250-x-HP samples

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exhibited tendency similar to that of 200-x-HP with varying hydrothermal pH. Thus,

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hydrochar produced from HTC with both acidic and basic feedwater decreased energy

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consumption during the compression process. However, the extrusion process was

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highly related to the friction between the pellets and the internal surface of the shield.

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The energy consumption of the extrusion process for hydrochar pellets produced at

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200 °C exhibited a value similar to RP, which consumed 0.11 J/g, but it decreased

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integrally for 250-x-HP when pH increased. However, 250-2-HP consumed

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approximately 0.66 J/g. Basic conditions may provide an environment for

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saponification; the HTC process would include a reaction with grease, producing


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alcohol or higher aliphatic acid as an extractive of hydrochar, generating lower

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friction forces and decreasing energy consumption during extrusion.

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Mechanical strength of the pellets. The mechanical strength of the pellets was

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determined by measuring the tensile strength (Ts). The Ts and average maximum

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breaking forces are shown in Figure 3 and Table 2 respectively. The surfaces of a

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raw biomass pellet and a hydrochar pellet (HTC at pH 2, 7 and 12) are illustrated in

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the images from SEM and Zeiss light microscopy (Figure S1 and Figure S2) to

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determine inter-particle adhesion and mechanical properties.

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The Ts of RP was approximately 0.17 MPa, which was lower than that of all

274

hydrochar pellets. As shown in Figure S1a, melted material integrating with other

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particles by the formation of solid bridges was present on the surface of RP. This

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acted as a natural binder in the raw lignocellulose through melting, creating a glassy

277

coating that resulted in the mechanical interlocking of particles17. Moreover, the

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lateral micrograph showed that RP had a coarser surface, with powders adhering to

279

the surface, and the length expansion reached 5.26% after 7 days (Table 1). This

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indicated that RP easily developed resilience. However, the surfaces of all the

281

hydrochar pellets were smoother than those of RP, indicating reduced intervals

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between the inter-hydrochar particles and the increased in points of contact area18.

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The Ts of 200-x-HP ranged from 1.23 to 2.56 MPa, which was approximately 1 MPa

284

higher than that of 250-x-HP. A similar pattern was identified by Kambo et al., who

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demonstrated that hydrochar had lower compressive strength when it was produced at

286

higher temperatures of between 225 and 260 °C 34. The Ts values in this study were


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lower than those of hydrochar pellets (2.97-7.50 MPa) obtained from woody biomass

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and agro-residues in the study by Liu et al., which may be due to the low compressive

289

forces employed4, 35. The decreased Ts of hydrochar pellets developed at a lower

290

temperature may be due to solid bridge formation from lignin and cellulose contained

291

in 200-x-HP, and the large crystalline cellulose fibers made the pellets more stable

292

than 250-x-HP30. As shown in Figures S1 and S2, the surface of 200-x-HP was

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coarser than that of 250-x-HP, and cross-linking striations can be observed,

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suggesting that 200-x-HP integrated with lignin and cellulose, forming solid bridged

295

and interlocking bonds. There were small flaws present on the surface of 250-x-HP,

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while 200-x-HP had intense scratch lines on the lateral surface, indicating that the

297

solid bridge captured more particles, forming strong attraction forces between

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adjacent particles. Although 250-x-HP possessed higher lignin content than 200-x-HP,

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lignin was primarily formed from phenolic or polyaromatic char derived from liquid

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and non-dissolved lignin conversions (pseudo-lignin) after HTC at a high

301

temperature36. The hydrogen bonds at lignin and cellulose surface areas, and the

302

covalent bonds between the cellulose fibers, were broken at 250 °C, so 250-x-HP

303

contained more brittle lignin than 200-x-HP, which relied more on attraction forces

304

between particles37.

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Figure 3 shows that the Ts patterns of 200-x-HP and 250-x-HP were similar with

306

changing hydrothermal pH. The Ts of pellets from biomass treated with a

307

hydrothermal pH of 7-9 did not significantly change, with relatively high Ts values

308

for 200-8-HP and 250-8-HP (2.56 MPa and 1.34 MPa, respectively). However, the Ts


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of hydrochar pellets subsequently decreased when the hydrothermal pH further

310

increased or decreased, but excellent Ts values were achieved at highly acidic and

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basic hydrothermal environments (pH 2,3,11, and 12). Changes in hydrothermal pH

312

enhanced the severity of the reaction and led to an increase in particle plasticity,

313

consistent with the decreasing energy consumption for compression and particle sizes

314

(Table 1), which slightly decreased when the feedwater pH became more acidic or

315

basic. From the SEM images, more fragments can be observed on pellets derived from

316

highly acidic and basic feedwater (2 and 12) than those produced at pH 7. However,

317

mechanical strength is highly related to the energy consumption of the pelletization

318

process15. However, the pellets derived under highly acidic or basic conditions

319

consumed low amounts of energy with excellent tensile strength. For example, the Ts

320

values of 250-2-HP and 200-12-HP were 1.72 MPa and 1.05 MPa, respectively. The

321

high density (in Table 2) obtained at pH 2 or 3 suggested an increase in the contact

322

area between particles, and highly brittle properties coupled with low particles sizes

323

implied the presence of enhanced attraction forces resulting in high tensile strength

324

for the pellets. On this basis, the high lignin and cellulose content in 200-2-HP and

325

250-2-HP could be another cause of high tensile strength. However, the pellet

326

produced under highly basic feedwater (pH 12) also had high tensile strength. It has

327

been reported that the cellulose structure in a highly basic environment becomes

328

denser and more thermodynamically stable than native cellulose, which may be the

329

reason for the increased tensile strength of this pellet38. The modifications in the

330

crystalline state-cellulose, combined with the high solubility of lignin at pH 12, which


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

331

resulted in the redistribution and condensation of lignin, may also increase the

332

mechanical performance of the pellets. Moreover, the contents of extractives for HP

333

produced under highly acidic and basic feedwater (for example, 13.89% and 19.29%

334

for 250-2-HP and 250-12-HP, respectively) were relatively high and critical to the

335

tensile strength, because large organic compounds bridged adjacent particles4.

336

Proximate and elemental analyses, and fuel properties of the pellets. Proximate

337

analyses of the pellets are presented in Table 2. The RP contained high amounts of

338

volatile matter (VM) and low amounts of fixed carbon (FC), whereas all the

339

hydrochar pellets contained more FC than RP, with 250-x-HP having higher FC

340

content than 200-x-HP. Hydrochar pellets derived from both highly basic and acidic

341

feedwater had increased FC at 200 °C, but the FC of hydrochar pellets produced at

342

250 °C only reached a relatively high value at pH 2 and 3. The HHVs appeared to

343

depend more on hydrothermal temperatures than pH, excluding the pellets produced

344

at pH 2-4, which had high HHVs. It can be deduced that the addition of acetic acid

345

increased the HHVs. However, the ash content in 200-x-HP was lower than that of the

346

raw biomass pellets for the dissolution of the inorganic fraction in feedwater, but the

347

ash content of 250-x-HP was higher.

348

The results of elemental analysis of the raw biomass and hydrochar pellets are

349

shown in Table 2. Elemental carbon increased under an acidic environment at both

350

200 °C and 250 °C, which was closely related to HHVs. Although the hydrochar

351

pellets derived at pH 2 had the highest carbon content of all the samples,

352

hydrothermal temperature affected carbon content to a larger extent. However, while


Page 16 of 35

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Energy & Fuels

353

hydrogen content appeared to be independent of the hydrothermal pH, it decreased

354

slightly at higher temperatures. In contrast, nitrogen was lower in all the hydrochar

355

samples compared to the raw biomass pellet, but became more concentrated in the

356

hydrochar at higher temperatures and in more acidic environments. The highest

357

oxygen content was obtained at pH 12 for both temperatures, while pH from 4 to 10

358

had no significant effect.

359

The van Krevelen diagram in Figure 4 illustrates the degree of hydrothermal

360

carbonization of the biomass under various pH conditions. The H/C and O/C values of

361

raw biomass and hydrochar pellets produced under various hydrothermal conditions

362

were plotted on the van Krevelen diagram. As expected, the raw biomass pellet had

363

the highest O/C and H/C content, located in the upper right corner of Figure 4,

364

whereas all the hydrochar pellets moved to the origin. The hydrochar pellets from

365

biomass treated at pH 2 had lower O/C and H/C content than those treated at other

366

conditions. The O/C of the samples treated at pH 8-12 were slightly higher than those

367

treated at pH 2-7, which changed in accordance with the sample’s oxygen content.

368

This suggests that the addition of acetic acid enhanced dehydration and

369

decarboxylation reactions. Therefore, increased fuel properties with high energy

370

recovery could be achieved when biomass is treated under an acidic hydrothermal

371

environment. Similar results have been presented in previous literature14, 20.

372

The mass density of pellets is closely related to the quality of solid fuel, with

373

high density fuel decreasing transportation costs, energy density, and mechanical

374

properties 4, 29. The mass densities of the samples are presented in Figure 5. All the


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

375

HP had higher mass density, from 1017 to 1213 kg/m3, than RP (865 kg/m3), and the

376

HP produced from high-pH feedwater at both 200 °C and 250 °C had slightly higher

377

mass densities. This may be due to the addition of NaOH, which resulted in

378

modifications to the crystalline structure of cellulose as well as the redistribution and

379

condensation of lignin, and this possessed higher density than the amorphous

380

cellulose and original lignin region38-40. However, hydrochar pellets produced at an

381

initial pH of 4-7 had a decreased mass density, whilst 200-2-HP and 250-2-HP had the

382

highest mass density. This was due to the highly carbonized hydrochar having an

383

aromatic structure, demonstrating high energy density and brittle characteristics,

384

making crushing and compressing to a lower volume more convenient41. The energy

385

densities of pellets are also presented in Figure 5. As expected, energy density varied

386

in a manner similar to mass density with variations in hydrothermal pH. Both

387

200-2-HP and 250-2-HP had higher energy densities (22.60 and 27.65 MJ/m3,

388

respectively) than other pellets. The EMC of the pellets is a vital characteristic of

389

solid fuel, because high moisture may cause the pellets to biodegrade, increase the

390

transportation cost, and decrease the calorific value. The EMC values of the pellets

391

are presented in Figure 5. EMC in the hydrochar pellets decreased more than it did in

392

RP (10.6%), and further decreased at 250 °C due to the loss of hydrophilic functional

393

groups42. Kambo et al. obtained a similar result, demonstrating that the EMC of

394

hydrochar from miscanthus feedstock decreased from 4.3% to 2.3% at temperatures

395

from 190-260 °C34. However, a slight decrease in EMC was observed for pellets

396

derived from both acidic and basic environments, excluding 200-12-HP and


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Energy & Fuels

397

250-12-HP (6.2% and 4.4%, respectively), which may be due to the composition of

398

the pellets and corresponding absorbance properties43.

399

Combustion characteristics. The combustion characteristics of RP and hydrochar

400

pellets produced in the presence of acetic acid or sodium hydroxide in the HTC

401

process were evaluated through thermogravimetric analysis. Representative samples

402

including 200-2-HP, 200-7-HP, 200-12-HP, 250-2-HP, 250-7-HP, and 250-12-HP,

403

exhibited high mechanical strength, and RP was introduced for comparison. The TG

404

and derivative DTG profiles corresponding to the samples’ combustion processes are

405

illustrated in Figure 6. The parameters analyzed from the curve are summarized in

406

Table 3, and the ignition temperature was analyzed following the method reported by

407

Peng et al.44.

408

As shown in Figure 6, two notable peaks can be observed in the DTG curves for

409

RP and 200-HP, whereas single continuous temperature ranges of 231-549 °C,

410

255-565 °C, and 257-580 °C were observed for 250-2-HP, 250-7-HP, and 250-12-HP,

411

respectively. These differing combustion performances were caused by differences in

412

the compositions of the samples, resulting in varied combustion stages for

413

devolatilization, and char combustion4, 45. Higher ignition temperatures were observed

414

for all hydrochar pellets compared to RP, which were due to the reduction of VM

415

caused by HTC. However, 200-2-HP, 200-12-HP, 250-2-HP, and 250-12-HP had

416

lower ignition temperatures than those of 200-7-HP and 250-7-HP, which differed

417

from the general conclusion that solid fuels with high VM content had low ignition

418

temperatures, resulting in pellets that burned more easily29, 44. Moreover, the


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

419

maximum weight loss rate of all hydrochar pellets was lower than that of RP, and the

420

weight loss of HP produced at 250 °C was half of that of HP produced at 200 °C. The

421

pellets produced in acetic acid or sodium hydroxide-aided HTC, at pH 2 and 12, had

422

lower maximum weight loss rates than those produced at pH 7, suggesting that the

423

reactivity of their combustion process was lower4, 45. In addition, the Tm of hydrochar

424

pellets increased with feedwater pH for both sets of samples, and it was

425

approximately 100 °C higher for pellets produced at 250 °C than those produced at

426

200 °C, which was close to the Tm of RP. Coincidentally, the burnout temperature of

427

HP also increased with increasing hydrothermal temperatures from 200 °C to 250 °C

428

and pH from 2 to 12. For example, burnout temperature increased from 549 °C for

429

250-2-HP to 565 °C for 250-7-HP, and then reached 581 °C for 250-12-HP. In general,

430

hydrochar pellets produced under both acidic and basic feedwater (particularly at pH

431

2) had acceptable combustion characteristics, with reduced ignition temperature,

432

wider temperature ranges, and decreased maximum weight loss rate, indicating that

433

combustion under moderate conditions gives rise to pellets with higher thermal

434

efficiency4.

435

Conclusions

436

Acetic acid and sodium hydroxide-aided HTC was an effective method for

437

treating woody biomass for fuel pelletization, and decreased the energy consumption

438

of compression. However, the Ts of hydrochar pellets decreased initially when the

439

feedwater pH changed, but subsequently increased for pellets produced with highly

440

acidic and basic feedwater (pH 2 and 12) due to variation of particle sizes and


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Energy & Fuels

441

components. Moreover, changes in pH from 4 to 9 had no significant influence on

442

H/C and O/C, and proximate characteristics, but hydrochar pellets produced at pH 2

443

had more suitable fuel properties, including high HHVs, FC, and mass density. Low

444

Ti and Rm combustion characteristics were achieved for hydrochar pellets produced at

445

pH 2. For practical applications, pelletized hydrochar produced through HTC with pH

446

2 at 250 °C can be an alternative solid biofuel.

447

Supporting Information

448

SEM image and Ziess micro graph of the pellets.

449

AUTHOR INFORMATION

450

Corresponding Authors

451

E-mail: [email protected], Tel.+86 731 8882 2829, Fax. +86 731 8882 2829.

452

E-mail: [email protected], Tel.+86 731 8882 2829, Fax. +86 731 8882 2829.

453

ORCID iD

454

0000-0001-8164-0613

455

Notes

456

The authors declare no competing financial interest.

457

ACKNOWLEDGMENTS

458

This research was financially supported by the project of National Natural Science


Energy & Fuels

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459

Foundation of China (Nos. 51679083), the Interdisciplinary Research Funds for

460

Hunan University (2015JCA03), the scientific and technological project of Changsha

461

City (KQ1602029), Supported by Petro China Innovation Foundation

462

(2016D-5007-0703) and the project of Shenzhen Science and technology Funds

463

（JCYJ20160530193913646）

464

REFERENCES:

465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 490 491 492 493 494 495 496

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Biomass. Energy & Fuels 2011, 25 (4), 1802-1810. (13) Lu, X.; Flora, J. R. V.; Berge, N. D., Influence of process water quality on hydrothermal carbonization of cellulose. Bioresource Technology 2014, 154, 229-239. (14) Lynam, J. G.; Coronella, C. J.; Yan, W.; Reza, M. T.; Vasquez, V. R., Acetic acid and lithium chloride effects on hydrothermal carbonization of lignocellulosic biomass. Bioresource Technology 2011, 102 (10), 6192-6199. (15) Kaliyan, N.; Vance Morey, R., Factors affecting strength and durability of densified biomass products. Biomass and Bioenergy 2009, 33 (3), 337-359. (16) Li, H.; Jiang, L.-B.; Li, C.-Z.; Liang, J.; Yuan, X.-Z.; Xiao, Z.-H.; Xiao, Z.-H.; Wang, H., Co-pelletization of sewage sludge and biomass: The energy input and properties of pellets. Fuel Processing Technology 2015, 132, 55-61. (17) Kaliyan, N.; Morey, R. V., Natural binders and solid bridge type binding mechanisms in briquettes and pellets made from corn stover and switchgrass. Bioresource Technology 2010, 101 (3), 1082-1090. (18) Kong, L.; Tian, S.; He, C.; Du, C.; Tu, Y.; Xiong, Y., Effect of waste wrapping paper fiber as a “solid bridge” on physical characteristics of biomass pellets made from wood sawdust. Applied Energy 2012, 98, 33-39. (19) Grāvitis, J.; Ābolinš, J.; Tupčiauskas, R.; Vēveris, A., Lignin from steam‐exploded wood as binder in wood composites. Journal of Environmental Engineering and Landscape Management 2010, 18 (2), 75-84. (20) Reza, M. T.; Rottler, E.; Herklotz, L.; Wirth, B., Hydrothermal carbonization (HTC) of wheat straw: Influence of feedwater pH prepared by acetic acid and potassium hydroxide. Bioresource Technology 2015, 182, 336-344. (21) Jiang, L.; Liang, J.; Yuan, X.; Li, H.; Li, C.; Xiao, Z.; Huang, H.; Wang, H.; Zeng, G., Co-pelletization of sewage sludge and biomass: The density and hardness of pellet. Bioresource Technology 2014, 166, 435-443. (22) Reza, M. T.; Yang, X.; Coronella, C. J.; Lin, H.; Hathwaik, U.; Shintani, D.; Neupane, B. P.; Miller, G. C., Hydrothermal Carbonization (HTC) and Pelletization of Two Arid Land Plants Bagasse for Energy Densification. ACS Sustainable Chemistry & Engineering 2016, 4 (3), 1106-1114. (23) M. Toufiq Reza, J. G. L., Victor R. Vasquez, and Charles J. Coronella, Pelletization of Biochar from Hydrothermally Carbonized Wood. Environmental Progress & Sustainable Energy 2012, 31 (2), 9. (24) Carrier, M.; Loppinet-Serani, A.; Denux, D.; Lasnier, J.-M.; Ham-Pichavant, F.; Cansell, F.; Aymonier, C., Thermogravimetric analysis as a new method to determine the lignocellulosic composition of biomass. Biomass and Bioenergy 2011, 35 (1), 298-307. (25) Shaw, M. D.; Karunakaran, C.; Tabil, L. G., Physicochemical characteristics of densified untreated and steam exploded poplar wood and wheat straw grinds. Biosystems Engineering 2009, 103 (2), 198-207. (26) Savage, P. E., Organic Chemical Reactions in Supercritical Water. Chemical Reviews 1999, 99 (2), 603-622. (27) Sevilla, M.; Fuertes, A. B., The production of carbon materials by hydrothermal carbonization of cellulose. Carbon 2009, 47 (9), 2281-2289. (28) Liu, F.; Guo, M., Comparison of the characteristics of hydrothermal carbons derived from holocellulose and crude biomass. Journal of Materials Science 2014, 50 (4), 1624-1631.


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(29) Hu, Q.; Shao, J.; Yang, H.; Yao, D.; Wang, X.; Chen, H., Effects of binders on the properties of bio-char pellets. Applied Energy 2015, 157, 508-516. (30) Lam, P. S.; Sokhansanj, S.; Bi, X.; Lim, C. J.; Melin, S., Energy Input and Quality of Pellets Made from Steam-Exploded Douglas Fir (Pseudotsuga menziesii). Energy & Fuels 2011, 25 (4), 1521-1528. (31) Li, H.; Liu, X.; Legros, R.; Bi, X. T.; Jim Lim, C.; Sokhansanj, S., Pelletization of torrefied sawdust and properties of torrefied pellets. Applied Energy 2012, 93, 680-685. (32) Bach Q, T. K.-Q., Skreiberg Ø., Wet Torrefaction of Norwegian Biomass Fuels. 20th European Biomass Conference and Exhibition,18-22 June 2012, Milan, Italy2012 2012, 1755-1763. (33) Tremel, A.; Stemann, J.; Herrmann, M.; Erlach, B.; Spliethoff, H., Entrained flow gasification of biocoal from hydrothermal carbonization. Fuel 2012, 102, 396-403. (34) Kambo, H. S.; Dutta, A., Strength, storage, and combustion characteristics of densified lignocellulosic biomass produced via torrefaction and hydrothermal carbonization. Applied Energy 2014, 135, 182-191. (35) Liu, Z.; Guo, Y.; Balasubramanian, R.; Hoekman, S. K., Mechanical stability and combustion characteristics of hydrochar/lignite blend pellets. Fuel 2016, 164, 59-65. (36) Kang, S.; Li, X.; Fan, J.; Chang, J., Characterization of Hydrochars Produced by Hydrothermal Carbonization of Lignin, Cellulose,d-Xylose, and Wood Meal. Industrial & Engineering Chemistry Research 2012, 51 (26), 9023-9031. (37) Back, E. L., The bonding mechanism in hardboard manufacture. Holzforschung 1987, 41 (4), 12. (38) Pettersen, R. C., vol. 207., The chemical composition of wood (chapter 2). In: Rowell, R.M. (Ed.), The chemistry of solid wood, Advances in Chemistry Series,. American Chemical Society, Washington, DC 1984., 984. (39) Hendriks, A. T. W. M.; Zeeman, G., Pretreatments to enhance the digestibility of lignocellulosic biomass. Bioresource Technology 2009, 100 (1), 10-18. (40) Gregg, D., Saddler, J.N., A techno-economic assessment of the pretreatment and fractionation steps of a biomass-to-ethanol process. . Appl. Biochem.Biotechnol. 1996, 17. (41) Liu, Z.; Quek, A.; Kent Hoekman, S.; Balasubramanian, R., Production of solid biochar fuel from waste biomass by hydrothermal carbonization. Fuel 2013, 103, 943-949. (42) Reza, M. T.; Uddin, M. H.; Lynam, J. G.; Coronella, C. J., Engineered pellets from dry torrefied and HTC biochar blends. Biomass and Bioenergy 2014, 63, 229-238. (43) Hoekman, S. K.; Broch, A.; Robbins, C.; Zielinska, B.; Felix, L., Hydrothermal carbonization (HTC) of selected woody and herbaceous biomass feedstocks. Biomass Conversion and Biorefinery 2012, 3 (2), 113-126. (44) Peng, C.; Zhai, Y.; Zhu, Y.; Xu, B.; Wang, T.; Li, C.; Zeng, G., Production of char from sewage sludge employing hydrothermal carbonization: Char properties, combustion behavior and thermal characteristics. Fuel 2016, 176, 110-118. (45) He, C.; Giannis, A.; Wang, J.-Y., Conversion of sewage sludge to clean solid fuel using hydrothermal carbonization: Hydrochar fuel characteristics and combustion behavior. Applied Energy 2013, 111, 257-266.

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Energy & Fuels

584

Table 1 Mass yield, percentage of components in the raw biomass and hydrochar

585

after treatment with HTC at varying pH values, final liquid pH, and mean particle

586

size.

587

Figure 1 FT-IR analysis of raw biomass and hydrochar produced at (a) 200 °C and (b)

588

250 °C with hydrothermal pH values of 2-12, respectively.

589

Figure 2 Typical compression displacement of (a) raw biomass and (b) hydrochar

590

pellets produced at 200 °C and 250 °C; (c) energy consumption for raw biomass and

591

(d) hydrochar pellets produced at 200 °C and 250 °C.

592

Figure 3 Tensile strength of RP and HP produced by HTC at varying pH levels.

593

Figure 4 Van Krevelen diagram of RP and hydrochar pellets produced by HTC at

594

varying pH values.

595

Figure 5 Mass and energy density, and EMC of raw biomass and hydrochar pellets

596

produced by HTC at varying pH levels: (a) 200 °C, (b) 250 °C.

597

Table 2 Maximum compressive force, proximate and elemental analyses, and basic

598

fuel properties of pellets

599

Figure 6 TG and DTG curves of (a) RP; (b) 200-2-HP, 200-7-HP, 200-12-HP; (c)

600

250-2-HP, 250-7-HP, and 250-12-HP at an air flow rate of 50 mL/min and a heating

601

rate of 15 °C/min.

602

Table 3 Combustion characteristics of 200-2-HP, 200-7-HP, 200-12-HP, 250-2-HP,

603

250-7-HP, and 250-12-HP. Rm: the maximum weight loss rate; Tm: corresponding

604

temperature of the maximum weight loss rate; Ti: ignition temperature; Tb: burnout

605

temperature.


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

606 Table 1 Samples Mass yield (%) Raw 200-2 200-3 200-4 200-5 200-6 200-7 200-8 200-9 200-10 200-11 200-12 250-2 250-3 250-4 250-5 250-6 250-7 250-8 250-9 250-10 250-11 250-12 607

100.0 56.2 ± 0.6 59.8 ± 0.8 69.3 ± 1.3 66.8 ± 1.6 67.4 ± 2.7 66.8 ± 2.8 68.8 ± 1.8 66.2 ± 2.2 64.2 ± 1.8 68.7 ± 2.4 66.1 ± 1.5 39.9 ± 1.3 42.8 ± 1.7 41.0 ± 2.6 41.1 ± 1.3 41.1 ± 0.8 43.8 ± 1.7 45.4 ± 2.6 44.0 ± 3.2 42.4 ± 1.0 40.9 ± 1.3 40.8 ± 2.8 a

Page 26 of 35

Hemicellulose (%)

Lignin (%)

Extractives (%)

Cellulosea (%)

11.1±1.3 3.4 ± 0.2 3.3 ± 0.2 3.1 ± 0.2 3.1 ± 0.1 2.9 ± 0.3 3.2 ± 0.1 3.1 ± 0.3 3.4 ± 0.2 3.8 ± 0.3 3.7 ± 0.1 3.9 ± 0.3 2.6 ± 0.3 3.1 ± 0.1 3.0 ± 0.2 3.0 ± 0.3 3.3 ± 0.1 3.3 ± 0.1 3.4 ± 0.2 3.5 ± 0.1 3.2 ± 0.2 3.3 ± 0.3 3.4 ± 0.1

34.3 ± 1.7 47.5 ± 1.9 42.1± 2.2 41.6± 1.6 42.2 ± 2.1 42.9 ± 2.0 41.6 ± 1.2 44.8 ± 2.5 43.3 ± 2.4 40.1 ± 3.5 43.1 ± 2.5 42.5 ± 3.4 64.5 ± 2.5 67.9 ± 2.2 66.6 ± 3.2 72.6 ± 1.8 75.1 ± 2.9 73.8 ± 2.5 73.0 ± 1.1 69.4 ± 2.6 67.6 ± 1.9 68.0 ± 1.5 61.1 ± 2.3

16.6 ± 0.8 10.1± 0.6 9.4 ± 1.1 11.6 ± 2.3 9.8 ± 0.8 13.1± 0.2 12.8 ± 0.6 6.4 ± 1.3 7.6 ± 2.3 10.0 ± 1.4 9.1 ± 1.9 11.4 ± 0.7 13.8 ± 1.4 15.5 ± 0.7 15.3 ± 0.6 10.2 ± 1.2 6.5 ± 0.6 6.8 ± 0.4 6.3 ± 0.4 10.5 ± 1.9 14.2 ± 1.9 13.4 ± 2.3 18.3 ± 1.7

37.9 ± 1.2 38.7 ± 0.8 44.7 ± 0.5 43.1 ± 1.0 43.6 ± 1.2 40.9 ± 1.3 42.1 ± 1.5 44.5 ± 1.6 44.8 ± 0.9 43.8 ± 1.3 42.9 ± 0.9 41.9 ± 1.2 19.0 ± 0.7 12.2 ± 0.8 14.1 ± 0.4 13.8 ± 0.5 14.7 ± 0.7 16.1 ± 0.5 16.6 ± 1.1 15.9 ± 0.9 14.4 ± 0.7 14.8 ± 0.6 15.8 ± 0.3

Calculated by deference.

608 609 610 611 612 613 614 615


Final pH

3.8 ± 0.1 3.9 ± 0.0 3.9 ± 0.1 3.9 ± 0.1 3.9 ± 0.0 3.9 ± 0.1 4.0 ± 0.1 4.1 ± 0.1 4.0 ± 0.1 4.1 ± 0.1 4.3 ± 0.2 4.4 ± 0.1 4.2 ± 0.1 4.1 ± 0.1 4.2 ± 0.1 4.3 ± 0.1 4.4 ± 0.2 4.3 ± 0.1 4.5 ± 0.1 4.5 ± 0.1 4.6 ± 0.2 4.7 ± 0.1

Particle mean size (µm) 164 26.6 37.3 55.8 59.8 69.3 63.7 65.0 53.5 55.1 45.0 36.9 18.8 19.1 21.7 20.0 21.0 27.7 23.3 23.4 20.4 19.5 19.2

Page 27 of 35

Figure 1

(a)

C=O C-H

aromatic C=C

(b)

C=C in lignin

O-H

3500

C-H out of plane

C=C in lignin

Raw 200-2 200-3 200-4 200-5 200-6 200-7 200-8 200-9 200-10 200-11 200-12

4000

aromatic C=C

C-O

3000

2500

2000

1500

1000

500

0

Transmittance (%)

616

Transmittance (%)

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

4000

bending vibrations

C=O

C-H

Raw 250-2 250-3 250-4 250-5 250-6 250-7 250-8 250-9 250-10 250-11 250-12

O-H

3500

3000

2500

-1

Wavenumbers (cm )

617 618 619 620 621 622 623 624 625 626 627 628 629 630 631

632


2000

1500

1000 -1

Wavenumbers (cm )

500

0

Energy & Fuels

633

Figure 2

4.5

4.5

Treated biomass

(a)

4.0

2.5 2.0 1.5 1.0

3.0 2.5 2.0 1.5 1.0

Decrease

0.5

0.5

0.0

0.0 0

5

10

250-2-HP 250-3-HP 250-4-HP 250-5-HP 250-6-HP 250-7-HP 250-8-HP 250-9-HP 250-10-HP 250-11-HP 250-12-HP

3.5

Pelletization Force (KN)

Pelletization Force (KN)

3.0

15

20

25

30

35

40

45

Treated biomass

(b)

4.0

RP 200-2-HP 200-3-HP 200-4-HP 200-5-HP 200-6-HP 200-7-HP 200-8-HP 200-9-HP 200-10-HP 200-11-HP 200-12-HP

3.5

Decrease

0

5

10

15

Rod displacement (mm)

20

25

30

35

40

45

50

Rod displacement (mm)

634 0.4

0.3

15

0.2

10

0.1

5

0.0 2

3

4

5

6

7

8

9

10

11

RP and HP from various hydrothermal pH at 200°C

12

Compression process

(d)

Extrusion process 1.5

40

1.0

30

20

0.5

10 2

3

4

5

6

7

8

9

10

HP from various hydrothermal pH at 250°C

635

636 637 638 639 640 641 642

643

644


11

12

Energy consumption of extrusion (J/g)

50

Energy consumption of extrusion (J/g)

20

Raw 1

2.0

Compression process Extrusion process

(c)

Energy consumption of compression (J/g)

25

Energy consumption of compression (J/g)

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

Page 28 of 35

Page 29 of 35

645

Figure 3

3.5

RP 200-x-HP 250-x-HP

3.0

Tensile strength (MPa)

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

2.5

2.0

1.5

1.0

0.5

0.0

Raw 1 2

646

3

4

5

6

7

8

9

10

11

12

RP and HP from HTC at various hydrothermal pH

647 648

649

650

651

652

653

654

655

656

657


Energy & Fuels

658

Figure 4

2.0

latio ethy Dem

1.8 1.6 1.4

Atmotic H/C

on ati dr hy e D

n

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

Page 30 of 35

RP

1.2

Dec arbo xyla tion

1.0 0.8 0.6

Sub-bituminous

Lignite

0.4

Bituminous 250-2-HP

0.2

Anthracite

200-2-HP Raw biomass pellet 200°C (pH 2-7) 200°C (pH 8-12) 250°C (pH 2-7) 250°C (pH 8-12)

0.0 0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

Atmotic O/C

659 660 661

662

663

664

665

666

667

668

669

670


Page 31 of 35

Figure 5

(a)

Energy density

Mass density

EMC

1400 24

Energy density

Mass density

(b) 1400

12

EMC 30

11

1300

7

28 1300

22

6

10

1200

900

16

800

14

700

12

3

3

1200

8 7 6

Energy density (MJ/m )

18

1000

EMC (%)

3

1100

9

Mass density(kg/m )

3

Energy density (MJ/m )

26 20

24 1100

22

20

1000

5

18 900

5

4

3

4 16

600

Raw 1 2

10 3

4

5

6

7

8

9

10

11

RP and HP from various hydrothermal pH at 200°C

12

3

800

2 2

3

4

5

6

7

8

9

10

HP from various hydrothermal pH at 250°C

672 673 674 675 676 677 678 679 680 681 682 683 684 685 686 687 688 689 690 691 692 693 694 695 696 697 698 699 700 701 702 703


11

12

EMC (%)

671

Mass density(kg/m )

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

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

704

Page 32 of 35

Table 2

Samples

Maximum compressi ve force (N)

Length expansion (%)

VM (%)

FC (%)

Ash (%)

C (%)

H (%)

Oa (%)

N (%)

S (%)

HHVsb (MJ/kg)

RP 200-2-HP 200-3-HP 200-4-HP 200-5-HP 200-6-HP 200-7-HP 200-8-HP 200-9-HP 200-10-HP 200-11-HP 200-12-HP 250-2-HP 250-3-HP 250-4-HP 250-5-HP 250-6-HP 250-7-HP 250-8-HP 250-9-HP 250-10-HP 250-11-HP 250-12-HP

77 ± 6 953 ± 27 743 ± 31 537 ± 23 573 ± 19 863 ± 33 1079 ± 24 1125 ± 26 1121 ± 15 585 ± 11 743 ± 10 817 ± 32 754 ± 16 507 ± 8 278 ± 17 354 ± 12 398 ± 32 403 ± 24 588 ± 26 521 ± 15 345 ± 28 342 ± 20 463 ± 14

5.26 ± 0.25 1.94 ± 0.12 2.23 ± 0.21 2.14 ± 0.16 2.56 ± 0.11 2.65 ± 0.23 2.72 ± 0.13 2.67 ± 0.22 2.35 ± 0.30 2.36 ± 0.28 2.21 ± 0.10 2.09 ± 0.16 1.78 ± 0.14 1.85 ± 0.09 1.91 ± 0.17 1.93 ± 0.15 1.94 ± 0.12 2.02 ± 0.22 1.90 ± 0.33 1.92 ± 0.14 1.96 ± 0.11 1.90 ± 0.14 1.89 ± 0.25

83.1 65.9 69.3 72.3 73.2 72.2 72.8 72.9 75.3 69.0 71.5 69.7 46.1 46.4 48.0 49.8 50.4 49.1 50.2 50.6 49.9 48.4 48.2

9.5 26.6 25.0 21.8 21.8 21.4 21.0 21.8 19.5 24.2 22.6 23.5 46.4 45.9 43.7 42.6 41.8 43.0 41.4 41.8 42.9 43.0 43.1

7.4 7.5 5.8 5.9 5.0 6.4 6.2 5.3 5.2 6.8 5.9 6.8 7.5 7.7 8.3 7.6 7.9 7.9 8.4 7.6 7.2 8.6 8.7

46.6 55.8 54.0 51.3 52.2 51.8 50.7 50.9 51.4 51.6 51.4 51.1 63.1 62.5 61.0 61.2 60.1 59.9 59.0 60.2 59.7 59.7 59.4

6.5 5.7 6.5 6.5 5.9 6.2 6.8 6.5 6.3 6.2 6.3 6.1 4.4 4.6 5.4 5.3 5.4 5.1 5.2 5.2 4.8 4.8 4.6

37.6 29.6 32.3 35.0 35.4 34.4 35.2 36.2 35.9 34.3 35.2 35.0 23.2 23.4 23.6 24.3 24.8 25.1 25.7 25.5 26.5 25.3 26.0

1.7 1.1 1.2 1.1 1.3 1.0 0.9 0.9 1.0 0.9 0.9 0.8 1.6 1.6 1.5 1.4 1.6 1.7 1.5 1.3 1.5 1.4 1.1

0.2 0.3 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.3 0.2 0.2 0.2 0.2 0.2 0.2 0.3 0.2 0.2 0.3 0.2 0.2

17.1 20.7 20.5 19.1 18.6 19.1 19.3 18.8 18.8 19.0 18.9 18.6 22.8 22.7 23.1 22.9 22.6 22.1 21.8 22.3 21.4 21.6 21.2

705 706 707 708 709 710 711 712 713 714 715


Page 33 of 35

Figure 6

-1.4

(a)

90

RP-TG RP-DTG

80

-1.0

70

Weight (%)

-1.2

60

-0.8

50

-0.6

40 30

-0.4

20

Mass loss (wt.%)/°C

100

-0.2

10 0 100

200

300

400

500

600

700

0.0 800

Temperature (°C)

717

100

-1.4

200-2-HP-TG 200-7-HP-TG 200-12-HP-TG 200-2-HP-DTG 200-2-HP-DTG 200-12-HP-DTG

(b)

90 80

Weight (%)

70 60

-1.2 -1.0 -0.8

50 -0.6

40 30

-0.4


716

20 -0.2

10 0

100

200

300

400

500

600

700

0.0 800

Temperature (°C)

718 100

-0.6

(c)

-0.5

80

250-2-HP-TG 250-7-HP-TG 250-12-HP-TG 250-2-HP-DTG 250-7-HP-DTG 250-12-HP-DTG

70 60 50

-0.4

-0.3

40 -0.2

30 20

-0.1

10 0

719

0

100

200

300

400

500

600

700

0.0 800

Temperature (°C)

720 721 722 723 724 725 726



90

Weight (%)

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

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

727

Page 34 of 35

Table 3

Sample

Ti

Tm

Tb

Rm (%/°C)

RP

295

321

507

1.28

200-2-HP

297

310

530

1.03

200-7-HP

309

327

540

1.22

200-12-HP

307

330

557

1.08

250-2-HP

313

426

549

0.41

250-7-HP

333

458

565

0.50

250-12-HP

331

477

580

0.45

728 729 730 731 732 733 734 735 736 737 738 739 740


Page 35 of 35

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

741 742

Acetic acid and sodium hydroxide-aidedhydrothermal carbonization (HTC) of

743

woodybiomassfor enhanced pelletizationand fuel properties

744

TengfeiWanga,b, YunboZhaia,b *, Yun Zhuc,d*，ChuanPenga,b, BiboXua,b，Tao Wanga,b,

745

CaitingLia,b, GuangmingZenga,b

746 747 748 749 750 751

For Table of Contents Use Only

，

752 753 754 755 756 757 758


Acetic Acid and Sodium Hydroxide-Aided Hydrothermal Carbonization

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