Effect of Calcium Formate on Hydrodeoxygenation of Biomass Model

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Biofuels and Biomass

Effect of calcium formate on hydrodeoxygenation of biomass model compounds Sai Teja Neeli, Rajdeep Shakya, Sushil Adhikari, and Hema Ramsurn Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b04205 • Publication Date (Web): 25 Jan 2019 Downloaded from http://pubs.acs.org on January 27, 2019

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Energy & Fuels

Effect of calcium formate on hydrodeoxygenation of biomass model compounds Sai Teja Neeli,† Rajdeep Shakya,‡ Sushil Adhikari,‡ and Hema Ramsurn∗,† †The Russell School of Chemical Engineering, The University of Tulsa, OK 74104, United States ‡Department of Biosystems Engineering, Auburn University, Auburn, AL 36849, United States E-mail: [email protected] Phone: +1 918 631 2978 Abstract Hydrothermal liquefaction of biomass model compounds (cellulose, xylan and lignin) was carried out between 350-400 ◦ C using calcium formate (Ca(HCOO)2 ) as the in-situ hydrogen source. In this study, high temperatures have been adopted for liquefaction as calcium formate acts a hydrogen donor in that range. Ca(HCOO)2 catalyzes lignin degradation towards liquefaction and consequently, increases the biocrude yields by about 80%. Conversely, cellulose biocrude yields decreased when Ca(HCOO)2 was introduced due to pronounced gasification. It was also observed that Ca(HCOO)2 did not have any effect on xylan biocrude yield which remained almost the same. The maximum biocrude yields of cellulose, xylan and lignin in the presence of Ca(HCOO)2 were 7, 13 and 32 wt% respectively. The excess hydrogen from Ca(HCOO)2 was successful in upgrading the biocrude by removing oxygen which was confirmed by significant decrease in atomic O/C ratio and increase in heating values (by 34.6%). The formate salt slightly upgraded the quality of cellulose and xylan biocrude through increase in H/C ratio which in turn improved the heating values by 15.6% and 8.8% respectively.

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The Gas Chromatography Mass Spectroscopy (GC-MS) chromatograms of biocrudes with and without the hydrogen donor were compared and reaction mechanisms were postulated to better explain the de-oxygenation of biocrude by Ca(HCOO)2 . With excess hydrogen, the biocrude from lignin was rich in alkylated phenols while aromatics were the major components in cellulose biocrude. The formate salt resulted in the formation of alkylated phenols and cylic ketones in xylan biocrude via aldol condensation reactions. Keywords: hydrothermal liquefaction, cellulose, xylan, lignin, calcium formate

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1 Introduction

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Non-renewable exhaustive fossil fuels (crude oil, coal, natural gas) are the main resources of world

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energy supply and are being extracted at an exorbitant rate to meet the present demand. Therefore,

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there is a growing concern for replacing or at least complementing the depleting fossil fuels with al-

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ternative sources of energy. Biomass can be regarded as one of the abundant sources of renewable

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energy for the production of transportation fuels and could be an important part of a more sus-

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tainable future energy system. 1,2 Biomass to biofuel conversion can be achieved through discrete

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thermochemical routes by either gasification to synthetic gas for Fischer-Tropsch fuels production

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or by pyrolysis/liquefaction followed by catalytic upgrading to fuel substitutes. 3 However, gasi-

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fication and pyrolysis conversion methods are problematic, as they require dry feedstocks for the

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synthesis of biofuel. 4,5 To avoid this expensive drying step, hydrothermal liquefaction processes

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have drawn the attention of researchers to convert biomass to value-added products. Hydrothermal

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liquefaction (HTL) utilizes water at 250-550 ◦ C and 5-25 MPa to convert wet biomass to biofuel. 6

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Water is used as a solvent and acts as a catalyst in HTL allowing the biomass to be converted with-

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out the energy-consuming drying step. 7 Among the different available thermochemical processes,

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HTL technology is the most feedstock flexible and results in high conversion efficiencies. 8 Addi-

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tionally, hydrothermal medium helps to overcome heat and mass transfer limitations. 9 Biomass is

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liquefied in sub- and supercritical water because of water0 s unique properties. The physical prop-

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erties such as density, dielectric constant and ionic product of water decrease as the temperature 2


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approaches critical point. 10 Therefore, these properties can be tuned by changing the temperature

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and pressure making water to behave as a non-polar solvent suitable for selective extraction and re-

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action processes involving biomass samples. 11 According to a study, 12 high reaction rates and high

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yields of gaseous products can be obtained in supercritical water. In subcritical region, biomass

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liquefaction is promoted by ionic reactions which are enhanced due to low dielectric constant. In

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the supercritical state (>375 ◦ C, 24 MPa), water acts like a non-polar solvent capable of dissolving

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organic compounds like alkanes, aromatics etc.

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Cellulose, hemicellulose (xylan) and lignin are the major components of biomass whose pro-

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portions vary corresponding to the biomass type. Cellulose is a polysaccharide polymer of cel-

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lobiose (glucose disaccharide) consisting of a linear chain of β-1, 4 glycosidic linkages; xylan (the

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backbone of hemicellulose) is a mixture of several heteropolymers of pentoses, hexoses and acids

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while lignin is a complex organic molecule with high molecular weight compounds. 13 A number

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of studies have been conducted on HTL of biomass model compounds (cellulose, xylan, lignin).

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Cellulose was subjected to HTL under acidic, neutral and alkaline conditions at temperatures of

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275-320 ◦ C with reaction residence times of 0-30 min. 14 The main composition of the resultant

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bio-oil was 5-(Hydroxymethyl) furfural (HMF) under acidic and neutral conditions while under

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alkaline conditions the main components were C2−5 carboxylic acids. Möller et al. 15 also reported

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5-HMF and glucose as major reaction products obtained from the condensation and hydrolysis

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reactions of cellulose in subcritical water. Garrotes0 research group 16 postulated kinetic models by

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means of first order, pseudo-homogeneous reactions for hydrothermal processing of xylan. 94%

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of the initial xylan degraded to xylo-oligomers, with further generation of xylose and furfurals

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when subjected to temperature in the range of 170-216 ◦ C. Yoshida et al. 17 studied lignin depoly-

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merization in supercritical water at 400 ◦ C. They postulated that re-polymerization of monomers

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increased the char content, thereby decreasing the yields of monomeric phenols. This char con-

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tent could be reduced by the addition of phenol which acts as scavenger of the unstable fragments

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produced in the depolymerization of lignin. 18 The decomposition of organosolv lignin has been

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investigated by Saisu et al. 19 in supercritical water with and without phenol at 400 ◦ C. The char

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formation reduced with increase in phenol/lignin ratio as the phenol reacts with active sites of

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decomposed fragments and thereby preventing cross-linking.

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Biocrude obtained from the depolymerization of biomass model compounds is a mixture of dif-

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ferent organic acids, ketones and phenols. 20 The striking difference between biocrude (or bio-oil)

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and commercial crude oil is the high oxygen content, 21 contributing to characteristics such as low

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heating value and high viscosity. Therefore, biocrude must be upgraded through oxygen removal

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before being used as transportation fuel. Hydrodeoxygenation (HDO) is one of the major routes in

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upgrading the biocrude, which consists of hydrogenation and oxygen-removal processes. 22 HDO

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involves treating bio-oils at moderate temperatures of 300-400 ◦ C with high pressure H2 (0.1-

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13.8 MPa) in the presence of heterogeneous catalysts. 23 Previous research utilized metal catalysts

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(CoMo/Al2 O3 , NiMo/ Al2 O3 , Ni/C), metal carbide (Mo2 C/TiO2 ), metal nitride (MoN/C), metal

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phosphide (Ni2 P/SiO2 ) and noble metal catalysts (Pt/Al2 O3 and Pd/C). 24–33 These heterogeneous

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metal catalysts usually need high H2 pressure for HDO reactions and will often deactivate due

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to oxidation by water and coking due to carbonaceous residue deposits. 34 To avoid these issues,

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other studies have focused on the application of in-situ hydrogen donors. For example, a mixture

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of sodium formate and formic acid was proven to be efficient in the depolymerization of oxidized

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aspen lignin. 35 A yield of 91 % was obtained at 110 ◦ C, 24 h reaction time. They postulated that

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lignin converted to low-molecular-mass aromatics by chemoselective aerobic oxidation, followed

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by C-C and C-O cleavage. Onwudili et al . 36 attempted to study the effect of formic acid in the con-

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version of alkali lignin in subcritical water. Such reactions were carried out at 265 ◦ C for 6 h and

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obtained a maximum liquid yield of 33 wt%. Additionally, they observed that guaiacol converted

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to catechol in the presence of formic acid by hydrolysis of O-CH3 ether bonds. Other hydrogen

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donors such as tetralin and isopropanol were employed for the conversion of organosolv lignin to

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short-chain alkyl phenols (SCAP). 37 The highest yield of SCAP was more than 10 wt% obtained

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when lignin was heated with isopropanol in the temperature range of 300-400 ◦ C and reaction

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time of 5-15 min. The hydrogen from tetralin and isopropanol stabilized the primary products

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from lignin decomposition, avoiding unwanted repolymerization reactions. Similar results were

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reported wherein hydrogen from tetralin stabilized the free radicals formed in miscanthus decom-

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position to prevent condensation reactions in order to reduce char formation and result in an oil

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yield of 44 wt%. 38

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While there was considerable research done in the area of hydrogen donor solvents for hy-

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drodeoxygenation reactions, the potential use of calcium salts as hydrogen donors has been ex-

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plored by few researchers. Wheeler and his group 39 stabilized the formic and levulinic acids

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(products from hydrolysis of biomass) by mixing with calcium hydroxide, followed by HDO at

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450 ◦ C. It was observed that the higher heating value of the oil increased from 35 to 40 MJ Kg−1

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with the addition of Ca(HCOO)2 , which proves the presence of highly deoxygenated compounds

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in the oil. Ca(HCOO)2 has also been utilized as a hydrogen donor in the pyrolysis of calcium salts

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of levulinic acid derived from cellulose. 40 The hydrogen produced from the decomposition of cal-

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cium formate reduced the levulinic acid resulting in the production of γ-valerolactone. Our group

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previously worked on hydrothermal deoxy-liquefaction of switchgrass with calcium formate which

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is an environmentally-friendly hydrogen donor and is easier to use in the laboratory, compared to

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formic acid. 8 The use of this inexpensive Ca(HCOO)2 increased the biocrude higher heating value

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from 28 to 34 MJ Kg−1 . Additionally, the biocrude yield nearly doubled by hydro-deoxygenation

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of the depolymerized biomass components. This current research aims at investigating the effect

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of adding calcium formate to the individual biomass model compounds in order to gain a better un-

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derstanding of the mechanistic models and to see how the results differ from those obtained from

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switchgrass (biomass) hydro-deoxygenation. It is expected that the biocrude heating value will

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increase due to deoxy-liquefaction mechanisms. The products obtained after deoxy-liquefaction

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will aid in proposing mechanistic pathways for the deoxy-liquefaction of each model compound.

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The results will also further clarify the difference between deoxy-liquefaction behaviors of model

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compounds and that of the biomass where lignin, cellulose and hemicellulose are intertwined in a

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complex structure.

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2 Experimental section 2.1

Materials and Apparatus

Figure 1: Schematic laboratory setup

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The biomass model compounds and calcium formate used in this study was obtained from Sigma-

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Aldrich. The schematic representation of the reactor setup is shown in Figure 1. The setup consists

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of a Type 316 stainless steel reactor with 1 inch outer diameter and volume of 81 ml. The reactor

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can be operated at a maximum temperature of 425 ◦ C and a maximum pressure of 20,000 psi. The

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temperature inside the reactor was recorded using a 1/16 inch type-K thermocouple (T1) placed

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inside the reactor. The real-time temperature readings from this thermocouple were used for all

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analyses. To record the pressure inside the reactor, two pressure gauges were connected in series

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to the other end of the reactor. P1 recorded the pressure of gas in the range of 0-345 bar while P2

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measured the pressure in the range of 0-41 bar (P2 was closed once the system’s pressure reaches

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above 34 bar). The sole reason for having a second pressure gauge was to accurately read pressures

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below 34 bar. The gas outlet was connected to a desiccator (to absorb moisture) before entering a

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gas chromatograph (GC) for the analysis of residual gases.

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2.2

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2.2.1 Hydrothermal treatment of calcium formate

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Thermogravimetric analysis (TGA) of calcium formate was performed in our previous study 8 in-

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dicated that below 300 ◦ C calcium formate does not act as a hydrogen donor. As such, the HTL

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experiments were carried out in the range of 350-400 ◦ C. 4 g of calcium formate (dry basis) was

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mixed with 40 ml of deionized water and heated to a temperature of 350-400 ◦ C with a reaction

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time of 30 min. The selected reaction time is needed to stabilize the reaction which is confirmed

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by no noticeable pressure fluctuations. Therefore, using less than 30 min time would be detrimen-

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tal to the biocrude yield and anything more would result in the gasification of the products. A

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stainless steel frit (2 µm pore size) was placed at the end of the reactor to avoid the accumulation

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of solid particulates in the exiting stream. The temperature on the furnace was adjusted to ob-

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tain the desired temperature on thermocouple T1. The reaction time was started when the desired

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temperature (T1) was reached inside the reactor. Based on the operating temperature, a pressure

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of 190-450 bar was generated upon heating. On the completion of the reaction, the reactor was

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switched off and air-cooled. Once the pressure reached below 41 bar, an accurate pressure reading

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was obtained from P2 by opening the valve. The residual pressure due to the formed gas was noted

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before performing GC analysis. Aqueous phase and residual solids were collected from the reactor

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after gas analysis. The pH of the water was measured before evaporating it to recover dissolved

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solids, if any.

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2.2.2 Hydrothermal liquefaction of biomass model compounds (cellulose, xylan and lignin)

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Experimental Procedure

with and without calcium formate All the model compounds were subjected to HTL in the temperature range of 350-400 ◦ C. On a dry basis, 2 g of sample (cellulose/xylan/lignin) was mixed with 40 ml of water and fed into the reactor. The same procedure as described in section 2.2.1 was followed. After completion of the reaction, the reactor was depressurized and opened to collect the biocrude and any remaining

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sample. The aqueous phase was procured by collecting the water from the reactor. The reactor walls and the unliquefied sample were washed with acetone and later the acetone was evaporated to obtain biocrude. The biochar/residue left was collected for further analysis. The yield of biocrude was calculated as follows:

Biocrude yield (%) = [weight of biocrude (g)/dry weight of sample (g)] X 100

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For the experiments with formate salt, 2 g (on dry basis) of sample, 40 ml of deionized water

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and 4 g of calcium formate were fed into the reactor. As per our previous study, 8 it was calculated

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that at least 4 g of calcium formate was necessary to theoretically remove all the oxygen from

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the 2 g of sample as CO2 and H2 O. We have therefore used 4 g of calcium formate for all our

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experiments. The same procedure as described earlier in this section was followed. To ensure

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repeatability, all the experiments were run at least three times. The standard deviation for biocrude

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yields was found to be less than 3%. The insoluble calcium carbonate will remain in the residue

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while the unreacted calcium formate would dissolve in the aqueous phase.

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3 Product characterization

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A gas chromatograph (SRI 8610C) equipped with a thermal conductivity detector (TCD) and two

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columns i.e. 6’ molecular sieve 13X and 6” haysep-d maintained at 250 ◦ C with helium and

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nitrogen as the carrier gases, was used to determine the gas composition. Molecular sieve 13X

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column completely separates H2 , O2 , N2 , CH4 and CO before the carrier flow in haysep-d is turned

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on. Haysep-d column separates all the compounds in the range of C1 -C6 . The gas sample was

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injected into the GC online by means of a ten-port injection valve having a sample loop. To ensure

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accuracy, all the readings were taken in triplicate and had a standard deviation of less than 5%. The

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biocrude samples were analyzed using an Agilent 7890 GC/5975 MS equipped with 60 m long

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DB-1701 column. A known amount of biocrude was dissolved in acetone so that the concentration

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was around 50,000 ppm and each sample was injected twice. The column was kept at an initial 8


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temperature of 40 ◦ C for 2 min before ramping up to 250 ◦ C at a rate of 5 ◦ C/min and the column

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was left at 250 ◦ C for a total of 15 min. Ultra-pure helium was used as carrier gas at 1.21 ml/min.

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By comparing the mass spectra with the National Institute of Standards and Technology (NIST)

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mass spectral library, the compounds were identified. Higher heating values (HHV) of the biocrude

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were obtained using an IKA oxygen bomb calorimeter. Elemental composition of the samples was

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determined using a Perkin-Elmer CHNS/O 2400 elemental analyzer. The Total Organic Carbon

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(TOC) analysis was determiend using Shimadzu TOC-LCSN analyzer.

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4 Results and discussion

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4.1

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To understand the behavior of calcium formate in sub- and supercritical water, 4 g (dry basis) of

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calcium formate was hydrothermally treated with 40 ml of deionized water. As seen in the results

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tabulated in Table 1, the residual pressure increases with temperature due to further decomposition

Hydrothermal treatment of calcium formate

of formate salt. Table 1: Hydrothermal decomposition of calcium formate. Temperature (◦ C) 350 375 400

Residual pressure (bar)

Residue (g)

13.78 26.87 37.9

3 2.5 2.5

Gas composition (mmoles) H2

CH4

CO2

10.5 23.5 38.4

10 16.3 13.8

0.7 2.9 8

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In accordance with the TGA results from our previous study, 8 at 350 ◦ C there is partial decom-

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position of calcium formate while at 400 ◦ C most of the salt decomposes. From Table 1, it can

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be observed that the composition of hydrogen and carbon dioxide increases with an increase in

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temperature while the methane composition increases at first but decreases at high temperature. In

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short, hydrogen and methane are the major products at all temperatures. The residue obtained con-

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tains calcium carbonate and unreacted calcium formate. Due to high water solubility (16.6 g in 100

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ml of water), calcium formate dissolves in water at room temperature. The unreacted/unconverted

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calcium formate was recovered by evaporating the water collected after reaction. As the tem-

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perature increased, the amount of residue decreased, indicating an increase in calcium formate

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decomposition. At 400 ◦ C, only 0.06 g of calcium formate was recovered, confirming nearly com-

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plete decomposition of formate to carbonate. The hydrolysis reaction of calcium formate is given

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in reaction 1:

3Ca(HCOO)2 + H2 O → 3CaCO3 + CH4 + 2CO2 + 2H2

(1)

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Theoretically, 4 g of Ca(HCOO)2 should yield 20.49 mmoles of CO2 and H2 each and 10.2

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mmoles of CH4 if the formate salt hydrolyzes completely as in reaction 1. At 350 ◦ C, the yields of

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H2 and CO2 are low since not all formate decomposes. Further, it is postulated that the formed CO2

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and H2 participate in methanation reaction (reaction 2) explaining the high methane yields. Metha-

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nation of carbon-dioxide is a highly exothermic reaction (heat of formation (δH): -164 kJ/mole)

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and is therefore not favored at high temperatures and hence the methane yield decreases at 400

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◦

C and hydrogen yield is high. Both at 375 ◦ C and 400 ◦ C, about 2.5 g of residue was obtained

confirming nearly complete decomposition of the calcium salt.

CO2 + 4H2 ↔ CH4 + 2H2 O

(2)

Ca(HCOO)2 → CaCO3 + CO + H2

(3)

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From Table 1, at high temperatures, it is observed that the yields of CO2 and H2 exceed the ex-

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pected theoretical yields. At low temperatures, reaction 1 (hydrolysis of calcium formate) was

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the main contributor for the hydrogen and carbon dioxide yields. Above 350 ◦ C, at high tempera-

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tures, thermal decomposition of calcium formate (reaction 3) was dominant. The carbon monoxide

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formed in reaction 3 then participates in the water gas shift reaction (reaction 4) resulting in the

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formation of CO2 and H2 . As a result the CO yields were undetectable while the CO2 and H2

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yields increased with increase in temperature.

CO + H2 O → CO2 + H2

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4.2

(4)

Hydrothermal liquefaction of biomass model compounds with and without calcium formate

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4.2.1 Liquefaction product yields

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The effect of temperature and hydrogen donor on the liquefaction of individual biomass con-

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stituents in sub-and supercritical water were studied. Cellulose, xylan and lignin were subjected

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to hydrothermal conditions without calcium formate (hydrogen donor) and the obtained biocrude

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yields are illustrated in Figure 2 (a). It is to be noted that during product recovery, especially during

200

the evaporation of acetone to collect biocrude, some of the components with a low boiling point

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may have evaporated along with acetone.

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From Figure 2 (a), it can be seen that each biomass model compound exhibited different liq-

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uefaction behaviors. Without formate salts, maximum biocrude yields of 12, 9 and 6 wt% were

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obtained at 375 ◦ C, 400 ◦ C, 350 ◦ C for cellulose, xylan and lignin respectively. As the temperature

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increased from 350 to 375 ◦ C, the biocrude yield increased in the case of cellulose liquefaction.

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With a further increase in temperature, gasification rather than liquefaction was favored which ex-

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plains the decrease in biocrude yields. 14 The biocrude yields (8-9 wt%) from xylan liquefaction

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did not change appreciably with varying temperature. Xylan, when compared to cellulose, has a

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branched structure and lower degree of polymerization, making it easily hydrolysable. 41 Most of

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the xylan is expected to decompose below 350 ◦ C and therefore temperatures above 350 ◦ C would

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not have much effect on the product yields. Liquefaction of lignin resulted in low biocrude yields

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when compared to biocrude yields from cellulose and xylan liquefaction, which can be explained

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by lignin’s complex structure. Lignin is reluctant to degradation because of its rigid polyaromatic

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heteropolymers. It was observed that as the temperature was increased from 350 to 400 ◦ C, the

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biocrude yields increased due to an increase in lignin hydrolysis. The heating values of cellulose,

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xylan and lignin biocrudes (without formate) were reported to be around 32 MJ/kg, 34 MJ/kg, 26

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MJ/kg respectively.

Figure 2: (a) Biocrude yields and (b) residue recovered after liquefaction of cellulose, xylan, lignin with and without calcium formate.

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Interestingly, with the addition of calcium formate, the cellulose biocrude yields decreased

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while the xylan biocrude yields did not change appreciably. Both biocrudes had a heating value

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of around 37 MJ/kg, which was an increase of 15.6% and 8.8% respectively. Yin et al. 14 studied

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the hydrothermal liquefaction of cellulose in different conditions. It was reported that the biocrude 12


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yield was lowest in alkaline medium as the biocrude decomposed to gases tthrough acids and alde-

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hydes. As calcium formate in this study provides alkaline medium, similar results were observed.

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On the other hand, the biocrude yields from lignin liquefaction increased significantly (3.5-6.7

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wt% versus 26-31.7 wt%) in the presence of formate salt. The heating value of the biocrude was

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35 MJ/kg which clearly improved when compared to the biocrude from lignin alone which had

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a heating value of 26 MJ/kg (increase of about 34.6%). Similar biocrude yields were reported

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by Mukkamala et al. 42 who studied the effect of formate on pyrolysis of lignin. The calcium

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from calcium formate could promote the lignin degradation reaction towards liquefaction and fa-

230

vor gasification in the case of cellulose and xylan hydrolysis. 43 The role of calcium in catalyzing

231

the degradation of biomass model compounds explains the high biocrude yields of lignin with the

232

addition of calcium formate while there is no apparent improvement in case of biocrude yields

233

from cellulose and xylan. 44 It is clear that the heating values of the biocrude from all the biomass

234

model compounds have improved with the addition of formate and is discussed in detail in Section

235

4.2.4.

236

From Figure 2 (b), the amount of residue/biochar recovered from the liquefaction of cellu-

237

lose, xylan and lignin decreases as the temperature increases from 350 to 400 ◦ C. This decrease

238

in residue with rise in temperature can be related to higher conversion of the biomass model com-

239

pounds to biocrude and residual gases (refer to Section 4.2.2 for more discussion). Moreover, the

240

higher quantity of biochar from lignin when compared to cellulose and xylan was indicative of low

241

lignin conversion. Adding 4 g of calcium formate (hydrogen donor) to 2 g of the model compound

242

had a significant effect on the liquefaction product yields. The residue formed was a mixture of cal-

243

cium carbonate and biochar. It was therefore difficult to deduce the exact biochar yields. However,

244

the residue yields decreased with an increase in temperature representing an increase in conversion

245

of all the model compounds with calcium formate.

13


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246

4.2.2 Residual gas composition

247

The composition of the residual gases obtained from hydrothermal liquefaction of all the biomass

248

constituents with and without formate salt is shown in Figure 3.

Figure 3: Residual gas compositions after hydrothermal decomposition of biomass model compounds with and without calcium formate. 249

The analysis of residual gas shows clearly that CH4 , CO2 and H2 are the major components

250

formed from the liquefaction of cellulose, xylan and lignin without a hydrogen donor. Cellulose

251

undergoes hydrolysis as shown in reaction 5 to form glucose. When temperatures are near the crit-

252

ical point of water, free radical reactions become more important and glucose gasification towards

253

the formation of hydrogen and methane is favored (reactions 6 and 7). 20 Further, the formed CO2

254

can participate in the methanation reaction (reaction 2) to form methane. The hydrogen formation

255

reaction is endothermic (reaction 6) while the methane formation (reaction 7) is exothermic. As the

256

temperature is increased, the equilibrium would shift towards formation of hydrogen while metha-

257

nation reactions would be restrained (or less conversion of carbon-dioxide to methane). Therefore,

258

at high temperatures, more hydrogen and carbon-dioxide yields were observed (Figure 3). n(C6 H10 O5 ) + nH2 O → nC6 H12 O6

(5)

C6 H12 O6 + 6H2 O → 6CO2 + 12H2

(6)

14


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C6 H12 O6 → 3CH4 + 3CO2

(7)

259

Hydrothermal liquefaction of xylan (reactions 8 and 9) produced similar gas yields when compared

260

to the cellulose residual gas composition. 45 As discussed earlier, the hydrogen and carbon-dioxide

261

yields increased with temperature according to Le Chatelier’s principle.

C5 H10 O5 + 5H2 O → 5CO2 + 10H2

(8)

2C5 H10 O5 → 5CH4 + 5CO2

(9)

262

It is well known that lignin in sub- and supercritical water degrades resulting in the formation

263

of phenols such as syringols and guaiacols (reaction 10). 46 These phenolic components further

264

participate in steam reforming reaction (reaction 11) to form CO, CO2 and H2 . The CH4 is formed

265

both by methanation reactions (reaction 2) and by dealkylation of the alkyl groups in lignin. The

266

yields of CO, CO2 and H2 increased with temperature due to an increase in lignin conversion while

267

methane yields (due to an equilibrium shift) dropped at high temperatures.

n(C10 H10 O3 ) + nH2 O → nC10 H10 O4 → phenolics

(10)

phenolics + H2 O → CO + CO2 + H2

(11)

268

The residual gas yields from all the biomass model compounds considerably increased with the

269

addition of formate salt. The analysis of these gas yields unfolded some crucial patterns. While

270

calcium enhances cellulose, xylan and lignin degradation reactions, the excess hydrogen produced

271

from formate is expected to reduce the oxygen content in the biocrude via hydrodeoxygenation

272

reactions (refer to Section 4.2.4 for more details) and thus, resulting in less oxygenated biocrude.

273

The methane yields have increased in case of cellulose and xylan liquefaction due to additional

274

methane formed from the decomposition of formate salt (reaction 1). Conversely, with the addition 15


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275

of calcium formate, the methane yield from lignin liquefaction was low which may be due to

276

equilibrium shift in methanation reaction (reaction 2) resulting in an increase in carbon-dioxide

277

yield.

278

4.2.3 Elemental analysis

279

The recovered biocrude was analyzed for its elemental composition and the results are presented in

280

Table 2. Clearly, the atomic O/C ratios of the biocrudes obtained from hydrothermal liquefaction

281

of lignin, cellulose and xylan ranged from 0.2-0.4 which were significantly lower than the initial

282

values in the respective feedstocks (0.6-0.9) as shown in Table S1 (Supplementary data) confirming

283

the reduction in oxygen content. Table 2: Elemental analysis of biocrude from liquefaction of biomass model compounds with and without calcium formate at 375 ◦ C Biocrude elemental analysis (wt. %)

Feedstock Cellulose Cellulose+formate Xylan Xylan+formate Lignin Lignin+formate

C

H

N

S

O

74.8 75.8 75.1 77.0 58.5 81.5

6.8 7.6 6.8 7.8 6.4 7.3

n.d. n.d. n.d. 0.8 0.2 0.2

0.1 0.2 n.d. n.d. 1.1 0.5

18.3 16.4 18.1 14.4 33.9 10.5

(O/C)*

(H/C)

0.2 0.2 0.2 0.1 0.4 0.1

1.1 1.2 1.1 1.2 1.3 1.1

n.d., not detected; a atomic ratio; * by difference; 284

The addition of calcium formate further decreased O/C ratio in case of biocrude from lignin as

285

evidently seen in Table 2 (from 0.4 to 0.1) along with increase in carbon content (from 58.5% to

286

81.5%). Higher carbon content and lower oxygen content explain the higher heating value of lignin

287

biocrudes i.e. 35 MJ/kg (as discussed in Section 4.2.1) when calcium formate was introduced.

288

However, calcium formate did not appreciably change the elemental composition of cellulose and

289

xylan biocrudes. There was a slight increase in carbon content and less substantial decrease in

290

oxygen content in biocrudes from cellulose and xylan. The small increase in heating value of

291

cellulose and xylan biocrudes with calcium formate is related to O/C and H/C ratios. The increase

292

in H/C ratio accompanied by decrease in O/C ratio improved the heating value of xylan biocrude 16


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293

from formate salt addition better than the cellulose biocrude where in only H/C ratio increased

294

while O/C ratio remained the same.

295

Figure 4 shows the carbon distribution in different product streams calculated using the yields

296

and elemental compositions of biocrude, residue/biochar, TOC content in aqueous biocrude and

297

gaseous product streams. In all the cases, gas phase was analyzed using Gas Chromatography

298

as discussed in Section 4.2.2. The carbon balance closure was between 63% and 72.8%, most

299

probably due to product losses. These losses could be experimental in nature for example during

300

reactor feeding, product recovery and product transfer from reactor to glassware etc. Losses would

301

also arise during different analyses such as volatile loss during drying (light ends such as acetic

302

acid, formic acid and lactic acid 47 in the biocrude), aqueous biocrude adsorbed to the biochar

303

surface and lost due to evaporation etc. It is also to be noted that because of the small reactor and

304

small feed and product amounts, a small loss contributes to a high wt.% loss. This loss range is not

305

out of the ordinary since several studies 48,49 have also reported similar carbon balance losses during

306

separation and processing. It was also mentioned that the filters and funnels used for the separation

307

of the products contribute to additional losses. It is clear that a substantial fraction of carbon content

308

increased in gas fraction when calcium formate was added to cellulose and xylan liquefaction while

309

simultaneously carbon content in biocrude decreased in both the cases. This observation supports

310

the explanation in Section 4.2.1 that calcium formate favored the gasification reactions in case of

311

cellulose and xylan. As a result, there was no significant improvement observed in the biocrude

312

yields. On the other hand, introducing calcium formate decreased the carbon content in solid

313

residue/biochar in lignin liquefaction towards an increase in biocrude carbon content confirming

314

the formate salt favors liquefaction reactions.

17


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

Figure 4: Carbon balance from the hydrothermal liquefaction of cellulose, xylan and lignin with and without calcium formate at 375 ◦ C.

315

4.2.4 GC-MS analysis of biocrude

316

The biocrude samples obtained from the hydrothermal liquefaction of the biomass constituents

317

with and without hydrogen donor were analyzed using GC-MS. A detailed analysis of the chemical

318

composition of biocrude can provide information on the possible reactions in sub- and supercrit-

319

ical water with formate salt. Based on the analysis results, plausible reaction mechanisms were

320

proposed for each model compound.

321

Levulinic acid was the major product detected in the GC-MS chromatogram of the biocrude

322

obtained from cellulose liquefaction in water. As shown in Figure 5, glucose decomposition from

323

cellulose without calcium formate resulted in the formation of levulinic acid with hydroxymethyl

324

furfural as intermediate. 50 However, levulinic acid disappeared with the addition of calcium for-

325

mate. In this study, the major compounds observed in the cellulose biocrude from formate addition

326

were aromatics such as toluene, hexamethyl benzene, 2,3-dihydro 1H-inden-5-ol and 6-methyl-4-

327

indanol. Similar aromatic compounds were reported in other studies in literature. 51,52

18


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Figure 5: Proposed reaction mechanism for production of aromatics from cellulose.

328

Calcium from the hydrogen donor is reacting with levulinic acid to form calcium levulinate as

329

explained in a study by Eaton et al. 53 The calcium salt of levulinic acid can further form α,β-

330

unsaturated ketones evolving water through intramolecular reactions (path A) or intermolecular

331

reactions (path B) as shown in Figure 5. Though the products from both the pathways are unsatu-

332

rated ketones via dehydration reactions, path B leads to formation of cross-linked poly salt, since

333

product from path A might react with calcium levulinate (intermolecular reactions). Aromatic

334

compounds can be formed from the products in path A or path B but it is difficult to predict the

335

exact reaction for the formation of each compound due to several reactions occurring simultane-

336

ously. Around temperatures of 350 ◦ C, it was reported that the dehydration of ketonic oxygen and

337

successive condensation of the methyl ketones to form aromatics was a significant step in the de-

338

oxygenation reaction pathway. 53 Temperatures above 350 ◦ C resulted in the formation of aromatic

339

compounds (toluene, benzene etc.) mainly due to decarboxylation reactions as observed in our

340

study.

341

Figure 6 depicts the possible reaction pathways for lignin decomposition in hydrothermal me-

342

dia. An extensive study of phenolic linkages in lignin was attempted by Mulder et al. 54 This study

343

showed that lignin mainly comprises of β-O-4 phenolic type compounds such as arylglycerol-β19


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

344

aryl ether. In another study by Ehara et al., 55 this β-O-4 phenolic bond was observed to readily

345

dissociate in supercritical water to form guaiacol. We observed that catechol was the major compo-

346

nent in the biocrude from lignin alone without calcium formate in sub- and supercritical conditions.

347

Sasaki et al. 56 conducted experiments to explain the decomposition of guaiacol in sub-critical wa-

348

ter. Clearly, catechol is formed from the homolysis of the weak C-O bond in the guaiacol unit when

349

subjected to sub-critical conditions. Kanetake et al. 57 studied the kinetics of guaiacol decomposi-

350

tion in near-critical and supercritical water conditions which showed that catechol formation was

351

favored when the temperature was increased. It is interesting to see that the quality of biocrude

352

from hydrothermal liquefaction of lignin is significantly different when compared to the biocrude

353

obtained from pyrolysis. Biswas et al. 58 reported that the biocrude obtained from pyrolysis of

354

dealkaline lignin was rich in guaiacols but very low in catechols. This difference in biocrude qual-

355

ity is mainly due to the difference in conditions adapted for the conversion. In contrast, with the

356

addition of calcium formate at 350 ◦ C, significant quantities of guaiacol and alkylated guaiacols

357

were observed. Clearly, calcium from calcium formate promotes the degradation of lignin towards

358

liquefaction by breaking the crucial β-O-4 bonds in complex lignin.

Figure 6: Proposed reaction mechanism for production of alkylated phenols from lignin.

359

Previous literature studies 59,60 showed that lignin degradation is promoted through the cleavage 20


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360

of β-O-4 ether bond in the presence of base pretreatment compounds resulting in the formation of

361

guaiacols and its derivatives. With further increase in temperature, guaiacols and alkylated gua-

362

iacols disappeared resulting in the formation alkyl substituted phenols such as 4-methyl catechol,

363

catechol and p-cresol which contributed to high area% in the GC-MS chromatogram. As men-

364

tioned earlier, high temperatures promote the cleavage of C-O bond in guaiacol resulting in the

365

formation of catechol. On the other hand, it is suggested that hydrodeoxygenation of creosol/4-

366

methyl guaiacol results in the formation of 4-methyl catechol and p-cresol. At first, the weak C-O

367

bond in creosol breaks down to form 4-methyl catechol. Further, the excess hydrogen from calcium

368

formate decomposition at high temperature (refer Section 4.1) would help in cleavage of aromatic

369

C-O bond of the hydroxyl group in 4-methyl catechol to form p-cresol (Figure 6). The presence

370

of alkylated phenols is consistent with the products obtained from the pyrolysis of lignin/calcium

371

formate mixtures. 42

372

The first step in xylan liquefaction as shown in Figure 7 yields xylose. The GC-MS analysis of

373

biocrude from hydrothermal liquefaction of xylan alone in sub- and supercritical water confirmed

374

the presence of oxygenated aromatic hydrocarbons like 2H-1-benzopyranone, 3,5,7-trihydroxy-

375

and 1H-inden-1-one, 2,3-dihydro. Lu et al. 61 also reported the presence of these compounds in the

376

hexane-soluble fraction of biocrude from hydrothermal liquefaction of xylose. Many of the previ-

377

ous literature studies 62,63 have shown that furfural was the major product obtained from the xylose

378

decomposition in high temperature water (180-220 ◦ C) through dehydration reactions. However,

379

different results were reported in a study by Sasaki et al. 64 detailing the kinetics of xylose degra-

380

dation in sub- and supercritical water. It was observed that retro-aldol condensation reactions are

381

more favored over dehydration reactions (which produce furfural) resulting in the formation of

382

glycoldehyde and glyceraldehyde. This glyceraldehyde can either undergo keto-enol tautomerism

383

to dihydroxyacetone or convert to pyruvaldehyde via dehydration. As aldehydes are highly un-

384

stable, they might further participate in polymerization reactions to form the observed aromatic

385

hydrocarbons.

21


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

Figure 7: Proposed reaction mechanism for production of cyclic ketones and phenolic compounds from xylan.

386

Interestingly, with the addition of calcium formate, the formed biocrude was rich in alkylated

387

phenols and cyclic ketones. Oefner et al. 65 proposed the formation of C5 -chain intermediate (as

388

shown in Figure 7) when xylose is degraded in alkaline medium. In the current study, calcium

389

formate provides the alkaline medium which results in the formation of a similar intermediate.

390

Further, the fission of this chain intermediate would result in C1 -C4 fragments which undergo a

391

cyclization via base (calcium in the salt is basic)-catalyzed aldol condensation to form phenols like

392

p-cresol, p-xylenol and also, other cyclic ketones such as 2,3-dimethyl -2-cyclopenten-1-one and

393

1-(4-ethynylphenyl) Ethanone. 66 Some of the other by products formed from xylose degradation

394

in water includes organic acids along with the aldehydes and ketones which are water soluble. The

395

presence of these compounds was reported in the aqueous phase in various studies. 8,67 It is possible

396

that the excess hydrogen (from calcium formate) could have prevented undesirable polymerization

397

reactions of these water soluble products to form oxygenated aromatics/char. These assumptions

398

were supported by the increase in TOC values in the aqueous phase by 57% in the presence of

399

formate salt as shown in Table S2 in the supplementary information.

22


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400

5 Conclusion

401

The effect of calcium formate (hydrogen donor) on the HTL of biomass model compounds (cel-

402

lulose, xylan and lignin) was studied. The formate salt improved the biocrude yields and quality

403

(rich in alkylated phenols) appreciably during lignin liquefaction through hydrodeoxygenation re-

404

actions. Calcium from the formate salt enhanced the gasification of cellulose in hydrothermal

405

media and the excess hydrogen improved the quality of biocrude (rich in aromatics) restricting

406

the formation of PAHs. GC-MS results of biocrude from xylan liquefaction with calcium formate

407

showed the presence of phenolic compounds and cyclic ketone. The presence of these compounds

408

is particularly interesting because majority of research studies in literature have shown furfural as

409

the major product from xylose degradation in water. In this study, the presence of calcium formate

410

creates an alkaline environment catalyzing decomposition of xylan to phenolics and ketones via

411

aldol condensation reactions in sub- and supercritical water.

412

Acknowledgement

413

This work was supported by internal grants from the Russell School of Chemical Engineering at

414

The University of Tulsa and from Phillips 66 Fellowship.

415

Supporting Information Available

416

The supporting information is available free of charge.

417

Elemental analysis of biomass model compounds.

418

Results from hydrothermal liquefaction of biomass model compounds with and without calcium

419

formate.

23


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Effect of Calcium Formate on Hydrodeoxygenation of Biomass Model

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