Integration of hydrothermal liquefaction and supercritical water gasification for improvement of energy recovery from algal biomass

Accepted Manuscript Integration of hydrothermal liquefaction and supercritical water gasification for improvement of energy recovery from algal biomass Pei-Gao Duan, Shi-Kun Yang, Yu-Ping Xu, Feng Wang, Dan Zhao, Yu-Jing Weng, Xian-Lei Shi PII:

S0360-5442(18)30864-8

DOI:

10.1016/j.energy.2018.05.044

Reference:

EGY 12877

To appear in:

Energy

Received Date: 21 February 2018 Revised Date:

4 May 2018

Accepted Date: 6 May 2018

Please cite this article as: Duan P-G, Yang S-K, Xu Y-P, Wang F, Zhao D, Weng Y-J, Shi X-L, Integration of hydrothermal liquefaction and supercritical water gasification for improvement of energy recovery from algal biomass, Energy (2018), doi: 10.1016/j.energy.2018.05.044. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT Integration of hydrothermal liquefaction and supercritical water gasification for improvement of energy recovery from algal biomass Pei-Gao Duan, Shi-Kun Yang, Yu-Ping Xu*, Feng Wang, Dan Zhao, Yu-Jing Weng, Xian-Lei Shi* College of Chemistry and Chemical Engineering, Department of Energy and Chemical Engineering, Henan Polytechnic University, No. 2001, Century Avenue, Jiaozuo, Henan 454003, P.R. China

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Abstract Herein, we report on a combined process that incorporates hydrothermal liquefaction (HTL) and supercritical water gasification (SCWG) to improve energy recovered from algal biomass. Eight

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algal biomasses, including four microalgae and four macroalgae with a large difference in biochemical compositions, were screened for this dual process. The algal biomass feedstocks

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significantly affected the carbon and energy distribution in the product fractions (crude bio-oil, solid, gas, and water-soluble products). 62.50-71.34% energy of microalgae and 6.03-41.06% energy of macroalgae could be recovered as crude bio-oil. 11.86-21.55% carbon of the microalgae and 8.01-15.82% carbon of the macroalgae was distributed in the HTL process water in form of water

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soluble products after the HTL process. 14.3-33.7% energy of microalgae and 30.18-36.34% energy of macroalgae was retained in the HTL process water. SCWG could convert the organics in the HTL process water into fuel gases consisting mainly of H2 and CH4. 54-91% carbon of the HTL process

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water was transformed into the fuel gases, which correspond 5.53-18.30% energy of the algal biomass. Thus, this work shows that the integration of HTL and SCWG could improve energy

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recovery from algal biomass relative to the HTL process alone.

Keywords: Algae; integration; hydrothermal liquefaction; supercritical water gasification; energy recovery *Corresponding author. Tel.: +86(0391) 3986820; fax: +86(0391) 3987811; Email address: [email protected] (X-Y. Xu);[email protected](X-L Shi)

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1. Introduction

Algae are emerging as one of the most promising long-term, sustainable sources of biomass and oils for fuel, food, feedstock, and other co-products because of their higher photosynthetic efficiency, faster growth rate, and higher area-specific yield than terrestrial biomass [1,2]. Two

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major types of algae are macroalgae (seaweeds), which occupy the littoral zone and include green algae, brown algae, and red algae, and microalgae, which are found in both benthic and littoral habitats and throughout the ocean as phytoplankton. Compared with microalgae, macroalgae are

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multicellular and eukaryotic marine algae. They possess plant-like characteristics, making them easier to harvest than microalgae. Algae contain various amounts of protein, starch-like

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carbohydrates, and lipids. The biochemical composition of algae is not an intrinsically constant factor; it varies over a wide range, depending on the species and on cultivation conditions. Macroalgae and microalgae have a common feature in that they have high moisture after harvesting. Therefore, hydrothermal processing of algal biomass feedstocks is attractive from an energy

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consumption perspective because it obviates the need for dewatering, which is required for other conversion methods such as pyrolysis and gasification [3]. Other researchers, employing either a batch reactor or a continuous-flow reactor, have

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examined either microalgae or macroalgae alone [4-11] or microalgae and macroalgae together [12] as a feedstock for hydrothermal liquefaction (HTL). Four product fractions-crude bio-oil, gas, solid

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residue, and water-soluble products (WSPs)-are produced from the HTL of algae. Among these, crude bio-oil is considered a potential alternative to fossil fuels and their derivatives. Most of these previous studies on the HTL of algae have concentrated on optimizing the reaction parameters to maximize yield and improve the quality of the crude bio-oil [4-11]. The use of water as the processing medium in the HTL of algae results in a large water handling requirement. Therefore, recycling and reuse of the HTL process water has become a major consideration in the design of the HTL process for algal biomass in view of environmental and economic factors. Previous studies on the analysis and utilization of algal HTL process water are rare. Yang et al. [13] reported that 2

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approximately 40 wt.% of carbon and 60 wt.% of hydrogen are retained in the HTL process water of the microalga Microcystis viridis. For HTL of the microalga Auxenochlorella pyrenoidosa, Yu et al. [14] suggested that 35-40% of carbon in the feedstock were remined as WSPs when the reaction temperature was higher than 220 °C and the retention time was longer than 10 min. Jazrawi et al.

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[15] found that a substantial fraction of the feedstock carbon-as much as 60%-was in the HTL process water. Cherad et al. [16] claimed that the WSP fraction ranged between 30% and 50% of the product composition and could be as high as 68%, as demonstrated in the HTL of Spirulina. All

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these previous studies indicate that a large proportion of carbon remained in the HTL process water, causing a large quantity of energy loss, which clearly affects the economic feasibility of the HTL

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process. Furthermore, if these HTL process water are discharged directly into the ambient environment, they will cause environmental pollution. Therefore, recovery or reuse of the HTL process water is essential for economical processing by HTL.

To date, many researchers have studied the potential recycling of nutrients from HTL process water for algae cultivation. The recycling of nutrients is beneficial to carbon utilization efficiency

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and can improve biomass yields [17-19]. Other researchers have suggested that the recycling of HTL process water could greatly increase bio-oil yields. However, an increase in bio-oil yield is

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usually accompanied by a substantial increase in the oxygen content of the bio-oil [20,21]. In addition, the levels of potentially toxic compounds in HTL process water have been shown to be

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dramatically reduced following the cultivation of algae. Therefore, treatment of the HTL process water by anaerobic digestion has received limited study due to the high nitrogen content of HTL process water [22]. Another option being developed is supercritical water gasification (SCWG). Via this process step, the organics in the HTL process water can be converted to hydrogen and methane, which can be easily separated from the water [9, 15,16, 23-25]. These previous studies suggest that SCWG is an effective approach to HTL process water cleanup and fuel gas production. An almost complete conversion of 99.2% of the carbon left in the HTL process water has been observed, allowing the water to be considered for recycling of nutrients to the algae growth ponds [9]. This 3

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almost complete gasification is one of the advantages of the SCWG. The produced hydrogen can also be used in subsequent hydrotreating of the crude bio-oil produced from the HTL of algal biomass. Finally, minerals obtained from the HTL process water after the SCWG process can be used as nutrients for growing plants or other biomass sources [26]. In terms of current technology,

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SCWG usually leads to energy penalty which will cut down economic feasibility of energy return from the HTL process water. However, at least but not limited, this technique provides a new concept to improve energy recovery from algal biomass and reduce or even eliminate potential

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pollutant sources from the HTL process water to ambient environment. Moreover, the environmental benefits of this SCWG process are far greater than its economic benefits.

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Thus far, only the separate SCWG of process water produced from the HTL of microalgae or macroalgae have been examined [9, 15,16, 23-25]. These previous studies have indicated that the biochemical compositions of various microalgae and macroalgae species generally exhibit pronounced seasonal variation. In addition, such species can be strongly affected by temperature, geographical location, water salinity, and aqueous nutrient content. Hence, samples from the same

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species grown in different climates can differ substantially. Therefore, a side-by-side comparative evaluation of SCWG of the process water derived from the HTL of different algal species under

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identical conditions has not been reported. Given the ongoing research centered on energy recovery from algal HTL process water, for SCWG of HTL process water, further information is still needed

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to understand the relationship between the biochemical compositions of algae and the properties of HTL process water, as well as the yield and composition of the resulting fuel gas. Therefore, in the present study, a two-step process combined HTL of algal biomass and SCWG of the HTL process water was designed to convert algal biomass into bio-oils suitable for the generation of bio-based liquid fuels and into HTL process water for producing fuel gases. This combined process not only can increase total carbon conversion efficiency and energy recovergy for utilization of algal biomass energy but also can eliminate or reduce environmental impact of the HTL water. First, we provide a comparative assessment of the effect of biochemical composition on 4

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product distributions, crude bio-oil quality, and process water properties from the HTL of eight different algal species grown under identical conditions. Subsequently, all of the generated HTL process water was then hydrothermally gasified under identical conditions to compare the compositions and yields of the resulting gaseous products. Finally, energy recoveries from HTL and

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SCWG process was evaluated and compared.

2 Experimental section 2.1 Materials

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Eight aquatic biomasses were used, including four microalgae (Nannochloropsis oceanica: NO, Auxenochlorella pyrenoidosa: AuP, Arthrospira platensis: ArP, and Schizochytrium limacinum: SL)

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and four macroalgae (Ulva prolifera: UP, Saccharina japonica (Areschoug): SJ, Zostera marina: ZM, and Gracilaria eucheumoides harvey: GEH). Each aquatic biomass was dried in an oven at 110 °C for 12 h and pulverized into particles (≤100 mesh) prior to their analysis and use. The proximate and ultimate analyses of each aquatic biomass is listed in Table 1.

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Freshly deionized water, prepared in lab, was used throughout the experiments. High-pressure stainless-steel autoclaves were used to perform the HTL and SCWG reactions. These reactors consisted of 500 mL and 127 mL autoclaves surrounded by an electrical heater. The power of these

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two autoclaves was 2.5 and 1.2 kW, respectively. The 500 mL autoclave was equipped with a mixer and mixer controller, a safety relief valve, and a pressure gauge. No mixer was installed for the 127

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mL autoclave which was heated by a custom-built molten-salt bath that consists of KNO3 and NaNO3 at a mass ratio of 5:4. Schematics of autoclave reactor for HTL of algal biomass and apparatus and batch stainless-steel reactor for SCWG of HTL process water were shown in Fig. 1. These two reaction systems were loaded with water and conditioned at 400 °C for 1 h to remove any lubricants/oils that remained from the manufacture of the reactor parts. The reaction systems were then cleaned with acetone and air dried prior to use. 2.2 Experimental 2.2.1 HTL 5

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The HTL experiments were performed in the 0.5 L stainless-steel autoclave. In a typical run, the autoclave was filled with 60 g of algal biomass feedstock and 200 mL of deionized water and was then tightly sealed. The air inside the autoclave was displaced by purging the loaded autoclave headspace with helium and charging to 0.01 MPa gauge, which served as an internal standard for

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the quantification of gas yields. After the autoclave was sealed, the reaction was initiated by switching on an electrical heater in the autoclave. The speed of the mixer was set at 400 rpm. The autoclave reached the setpoint temperature of 350 °C after approximately 50 min, and it remained

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within 350 ± 2 °C for 60 min. The pressure inside the autoclave was maintained at approximately 15-18 MPa during the reaction, depending on the species of algal biomass used. The power was

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then turned off, and the autoclave was allowed to cool to room temperature for approximately 90 min. At this time, the pressure in the reactor was between 1-2 MPa depending on species of the algal biomass. After the autoclave had cooled, the outlet valve was opened to collect the gaseous products for analysis using a 0.5 L gas aluminum-plastic composite film bag. After the collection of gaseous products, the autoclave was reduced to atmospheric pressure and then opened. The reaction

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mixture consisted of a tar-like material floating on the surface of the aqueous phase. Most of the aqueous phase was poured into a 500 mL beaker and was used as the feedstock for subsequent

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SCWG. We added dichloromethane to dissolve the crude bio-oil and separate it from the mixture. Please note that the industrial process does not require solvent extraction process as the crude

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bio-oil and water can be separated directly by gravity. The dichloromethane extract was then filtered, and the solvent was vaporized by using a rotary evaporator at 35 °C under vacuum; the remaining material was the crude bio-oil. After filtration, the filter paper with solid residue was dried in an oven at 105 °C for 12 h and then weighed. The bio-oil and solid yields were calculated as their mass divided by the mass of algal biomass feedstock loaded into the reactor. 2.2.2 SCWG SCWG experiments were performed in the aforementioned 127 mL stainless-steel autoclave. In a typical run, 30 mL of aqueous phase from Section 2.2.1 was loaded into the reactor. This loading 6

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resulted in a water density of 0.236 g/cm3 at supercritical conditions. Prior to the reaction, air inside the reactor was displaced by purging its headspace with helium for approximately 15 min. The purged reactor was then pressurized with helium to 0.01 MPa. This added helium served as a standard in the quantification of gas yields. After pressurization, the reactor valve was closed and

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the reactor assembly was disconnected from the helium. With the reactor thusly pressurized, the reaction was initiated by switching on the autoclave’s electrical heater. After approximately 60 min, the reactor reached 600 °C and the reaction time was set to zero. The autoclave was maintained at

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600 ± 5 °C using a temperature controller. The pressure inside the reactor, which was strongly affected by the aqueous phase type, varied between 30 and 50 MPa. The more formation of the

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gaseous products, the larger the pressure in the reactor. After a reaction time of 120 min had elapsed, the power switch was turned off and the reaction was quenched by the autoclave being temporarily placed in an ice-water bath. The autoclave was then cooled to room temperature over approximately 60 min. Finally, the reactor was left under ambient conditions for at least 4 h to allow the liquid-gas system to reach equilibrium. At this time, the pressure inside the reactor was

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between 0.4 and 1.4 MPa, depending on the type of the loaded aqueous phase. After the gas fraction was detected by gas chromatography, the reactor was opened and the aqueous phase was collected

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for subsequent analysis.

2.2.3 Freeze-drying of the HTL process water

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Freeze-drying instead of direct-drying was used to determine the total molecular composition of the organics in the HTL process water. A LGJ-10 freeze dryer (Beijing Songyuanhuaxing Technology Development Co., Ltd., China) was used to perform freeze-drying of the HTL process water. The temperature limit that the freeze dryer can reach is -80 °C and its vacuum can reach 10 Pa. The normal working temperature is 10-30 °C. While precooling the freeze dryer, the temperature of the cold hydrazine reached -40 °C. Then, 5 mL of HTL process water was placed on an evaporating dish, and this dish was loaded into the carrier plate. The precooled frame was then placed on the cold hydrazine, and the freeze dryer was tightly sealed. The temperature of the freeze 7

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dryer was reduced to -60 °C and held at this temperature for 12 h. Then, the freeze dryer was opened, and the evaporating dish was removed and placed on the drying frame. The vacuum pump was turned on and the vacuum degree of the freeze dryer reached 5 MPa. The vacuum was then turned off after 10 h. In this way, the water portion of the HTL process water was removed. Finally,

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WSPs in the remaining liquid were collected and weighed. The yield of WSP was calculated as its total mass in the HTL process water divided by the mass of algal biomass feedstock loaded into the reactor.

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2.3 Analysis

The gaseous products were analyzed using a GC-7900 gas chromatograph (Shanghai

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Techcomp Scientific Instrument Co., Ltd., Shanghai, China) equipped with a thermal conductivity detector (TCD). The gas component was separated using a 15 ft × 1/8 in i.d. stainless-steel column packed with 60 × 80 mesh Carboxen 1000 (Supelco). Argon (15 mL/min) served as the carrier gas for the analysis. The temperature of the column was maintained at 70 °C for 120 min. The reactor gas-collection bag was connected to the GC gas sampling valve, and the gases in the reactor flowed

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into the sample loop as the reactor valve was opened slowly (and slightly) to allow a predetermined amount of sample to exit. The gas sample was then sent to the column via a switching valve. After

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the switching valve was closed, the reactor valve was also closed. To ensure that the GC sample was representative of the gas mixture, we conducted a subsequent analysis. Thus, two consecutive

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analyses of the gas mixture were carried out for each run, and the values were presented as the average values of these two runs. Gas standards were purchased from Changzhou Jinghua Industrial Gases Co., Ltd. (Changhong Rd, Wujin, Changzhou, Jiangsu, China) and were analyzed to generate a calibration curve for each component, which in turn was used to calculate the mole fraction of each component in the reactor samples. The amount of helium added to the reactor was then used as an internal standard to determine the molar amount of each constituent. One-dimensional chromatographic processes are widely used in the analysis of bio-oil products. Although such methods often provide informative analytical results, the complexity of the bio-oil 8

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exceeds the capacity of any single separation system. As a consequence, considerable research has been dedicated in recent years to combining independent techniques with the aim of strengthening the resolving power. Comprehensive two-dimensional gas chromatography (GC×GC) employs two orthogonal mechanisms to separate the constituents of the sample within a single analysis. The

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technique is based on the application of two GC columns coated with different stationary phases, such as apolar and polar phases, and connected in series through a special interface (modulator). The interface cut small (several seconds) portions of the first-dimension eluate by cryofocusing and

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re-injected it into the second column. Each first-dimension peak was modulated several times, which enabled the preservation of the first-dimension separation. The second column was very short

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and narrow; consequently, each modulated portion was flash separated before the next modulation started. Using this instrumental approach, compounds co-eluting from the first column underwent additional separation in the second column. Therefore, the separation potential was greatly enhanced compared with that of one-dimensional GC.

In addition to chromatographic separation, the sensitivity and limits of detection were also

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improved because of the focusing of the peak in the modulator and the separation of analytes from the chemical background. High-speed time-of-flight mass spectrometry (TOF-MS), with maximum

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acquisition rates of 500 spectra s−1, provided sufficient data density to address the requirements of GC×GC separations. In addition, TOF-MS offers other advantages such as full mass spectra

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acquisition at trace-level sensitivity and mass spectral continuity, which enables deconvolution of the spectra for co-eluted peaks. The molecular composition of the bio-oil and organic matter in the WSPs were analyzed by comprehensive two-dimensional gas chromatography-time-of-flight mass spectrometry (GC×GC-TOF-MS) with a LECO Pegasus-IV CTDMGC/TOF-MS (LECO Corp., USA) system equipped with 2 GC columns: a non-polar Rxi-5Sil MS (30 m × 0.25 mm ID × 0.25 µm film thickness) and a polar Rxi-17 (1 m × 0.1 mm ID × 0.1 µm film thickness). The samples were prepared by redissolution in ethanol at a concentration of 10 (wt./vol)%. Because some inorganic salts are insoluble in the ethanol, only the supernatant was collected and analyzed. The 9

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sample injection volume was 1 µL at split ratio of 30:1. The inlet temperature was 300 °C. The first-dimension column was initially held at 50 °C for 4 min. The temperature was ramped to 300 °C at 5 °C/min and held isothermally for 10 min. The second-dimension column was initially held at 50 °C for 4 min. The temperature was ramped to 300 °C at 4 °C/min and held isothermally

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for 10 min. The modulation period, the hot-pulse duration, and the cool time between stages were set at 4.0, 1.4, and 0.6 s, respectively. The transfer line to the TOFMS detector source was operated at 280 °C. The ion source temperature was 250 °C with a filament bias voltage of 70 eV. The data

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acquisition rate was 100 spectra/s for the mass range of 35 to 500 amu. The detector voltage was 1500 V. Identification of compounds was realized using Chroma TOF v4.51.6.0 by comparing the

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acquired spectra with the NIST11 database. Helium flowing at 2 mL/min served as the carrier gas. The inorganic composition of the algal biomass feedstocks was measured via X-ray fluorescence (XRF) using a Bruker S8 TIGER XRF spectrometer, and the results are also provided in Table 1. The ammonia nitrogen in the HTL process water was detected by a TR-208 ammonia nitrogen tester (Shenzhen Tonggao Technology Co., Ltd., Shenzhen, China). The HTL process water was analyzed

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using an ICS-2000 ion chromatograph (Dionex, USA) to identify and quantify the main anions and cations present. Total organic carbon (TOC) analyses of the HTL process water were performed on

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a TOC analyzer (TOC-V CPH, Shimadzu, Japan).

3. Results and discussion

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3.1. Effect of algal feedstocks on the yield of product fractions and properties of the crude bio-oils The yields of WSP and other product fractions such as solid, gas and WSP obtained from HTL of four microalgae (NO, AuP, ArP, and SL) and four macroalgae (UP, SJ, ZM, and GEH) at 350 °C for 1 h are shown in Fig. 2. In all runs, the total weight of gaseous products was calculated by gas chromatography, as described in Section 2. “Lost” represents the total mass loss of all product fractions during their handling procedure. Fig. 2 shows that the algal species strongly affected the yield of each product fraction because of their different content of lipid, protein, and carbohydrate. Crude bio-oil was the dominant fraction (ranging from 35.20 to 52.68 wt.%) for the microalgae, 10

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whereas WSP was the dominant fraction (ranging from 23.23 to 52.33 wt.%) for the macroalgae. Essentially, the higher the lipid content of the feedstock, the higher the resulting crude bio-oil yield. Of course, the compounds derived from the decomposition of carbohydrates and protein, which depended on their solubility in water, also contributed to the production of crude bio-oil. Higher

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WSP yields of 25.25 and 27.70 wt.% were also observed for the microalgae AuP and ArP because of their substantial protein content. Therefore, as we expect that a certain amount of energy was distributed among the HTL process water for the HTL of algae. However, we note that a higher

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yield of WSP would not imply higher energy loss in HTL process water because inorganic salts were the major fraction of the WSP for the macroalgal species. Previous research has suggested that

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carbohydrate decomposition would result in the formation of WSPs [14]. Therefore, a clear trend of increasing WSP yield with increasing carbohydrate content was also observed. In addition, the results of this study also indicate that a higher protein content would also result in a higher WSP yield. Although SJ contains a lower carbohydrate content, the highest WSP yield was observed for this macroalga, which we ascribed to water-soluble inorganic salts in SJ remaining in the process

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water after the HTL process. Fig. 2 also indicates that solid residue yields, except for SL and GEH, are all lower than the ash contents of their initial feedstocks, again indicating that some inorganic

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salts in the feedstocks remained in the process water after the HTL process, which contributed the yield of WSPs. The solid residue yields produced from SL and GEH are higher than the ash

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contents of their initial feedstocks, implying that some organic matter was not completely converted during the HTL process. A certain amount of gaseous products was also formed during HTL of the microalgae and macroalgae and mainly consisted of H2, CO, CH4, and CO2 together with a small amount of C2-C5 hydrocarbons. Fig. 3(a) and 3(b) show the effect of algal biomass feedstocks on the mole percentage and yield of gaseous products. Clearly, CO2 is the absolutely dominant fraction of the gaseous products, which ranged from 87 to 97% depending on the algal biomass feedstocks. CH4 and H2 are the second most abundant fractions, which account for 1.3-4.1% and 1.5-6.5% in the gaseous products, respectively. It is worth noting that about 6.5% CH4 was observed in the 11

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gaseous products produced from the HTL of GEH. The mole fraction of CO produced from the macroalgae except UP is much higher than from the microalgae. The yield of CO2 ranged 2.13-4.47 mmol/g for the microalgae and 4.82-8.03 mmol/g for the macroalgae, respectively. The yield of CH4 produced from the HTL of GEH is about 0.60 mmol/g. Table 1 shows that either the

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microalgae or the macroalgae commonly contained a certain amount of ash, which would affect the HTL process and, subsequently, the product distribution as well as the WSP yield. Of course, this catalytic role was likely a consequence of the different reactivities of the individual lipids, proteins,

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and carbohydrates. Moreover, solvent extraction also affected the yield of WSP [15]: if the HTL process water was extracted with an organic solvent, lower WSP yield was observed because some

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water-soluble compounds would also be distributed in the organic solvent.

All of the crude bio-oils produced either from the microalgae or macroalgae were dark-brown, viscous, and had a smoky odor. GC×GC-TOF-MS was used to separate and tentatively identify many of the molecular components in the crude bio-oils produced from the HTL of different algae. More than 1500 peaks appeared in the GC×GC-TOF-MS chromatogram, which is ten times larger

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than that detected by one-dimensional GC-MS, indicating that GC×GC-TOF-MS is superior to the traditional GC-MS in determining the molecular composition of the bio-oil. Fig. S1 compares the

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GC×GC-TOF-MS chromatograms of crude bio-oils produced from the HTL of the eight different algae feedstocks. The identified components in the crude bio-oil mainly include long chain

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hydrocarbons, aromatics, saturated and unsaturated hydro-carbons, alcohols, esters, ketones, amides, S-, O-, N-, and N,O-containing compounds, consistent with the results of previous research [12]. Interestingly, a certain number of S-containing compounds were also detected in the crude bio-oils; these compounds usually cannot be detected by one-dimensional GC-MS. Table 2 provides a corresponding summary of the relative amounts of the different compound classes in the crude bio-oils. Clearly, the relative amounts of the different compound classes in the crude bio-oils show numerous differences from each other, indicating that the biochemical components of the algal biomass feedstocks strongly affected the molecular composition of the crude bio-oil, as did the 12

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molecular composition of the HTL process water. The dominant fractions of the crude bio-oils produced from the microalgae mainly consisted of amides and N-containing compounds because of the high protein content of the microalgae. The amides originated from the reaction of fatty acids with ammonia [27]. By contrast, the crude bio-oils produced from macroalgae mainly consisted of

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ketones and N-containing compounds because of the high carbohydrate content of the macroalgae. Crude bio-oil produced from the SL contains 20.22% fatty acids because SL has the highest lipid content. The hydrocarbons (saturated and unsaturated) were produced from the deoxygenation or

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decarboxylation of fatty acids [28]. Therefore, we inferred that the crude bio-oil is a very complex mixture containing various compounds derived from proteins, lipids, and carbohydrates, similar to

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the composition of the organics in the HTL process water. These crude bio-oils are potential feedstocks for the production of bio-based fuels. However, it should be noted that crude bio-oils with high content of N-containing compounds will cause downstream process problems. 3.2. Effect of algal biomass feedstocks on carbon distribution

Because a certain amount of carbon is distributed in the process water after the HTL

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processing of algae, understanding how the algal biomass feedstocks affect the carbon distribution in each product fraction would be helpful. Fig. 4 shows the carbon distribution in the different

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product fractions, as calculated using the yields and elemental compositions of the crude bio-oil, solid residue (See Table S1), and gaseous product fractions, along with the carbon content of the

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HTL process water (by TOC analysis). In all runs, the gas phase was calculated by gas chromatography, as described in Section 2. Loss represents the carbon loss during the handling process of each product fraction, which is common for the HTL of all of the algae examined. Another possible reason for carbon loss is that the total carbon in the feedstock was detected by elemental analysis; however, only the total organic carbon was detected in the HTL process water by the TOC analyzer. Fig. 4 shows that more than half of the organic carbon was distributed in the crude bio-oil from the microalgae. By contrast, only one-third of the organic carbon was distributed in the bio-oil from 13

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the macroalgae, except for GEH which is only 4.54%. Approximately 21-49% of the organic carbon was distributed in the solid residue of the macroalgae due to the difficult conversion of organics in the HTL process water. By contrast, approximately 2-15% of the carbon was distributed in the solid residue of the microalgae. Almost half of the organic carbon was distributed in the solid residue of

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the GEH, indicating that the carbohydrates in the GEH were difficult to convert. The carbon distribution in the HTL process water is greatly affected by the species of algae, which ranges 11.86 to 21.55% for the microalgae and 8.01 to 15.82% for macroalgae, respectively. The highest organic

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carbon distribution of 21.55% was observed for ArP followed by AuP, NO, and SL. The higher the carbon distribution in the HTL process water, the higher the energy loss. However, Jazrawi et al.

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[15] observed that approximately 35% of organic carbon was distributed in HTL process water when they hydrothermally treated AuP at 350 °C, which might be due to the use of a much lower feedstock concentration (5 wt.% vs. 30 wt.% in the present study). Although much higher WSP yields were observed for macroalgae, the HTL process water produced from the macroalgae contained less organic carbon, indicating that WSPs produced from the macroalgae mainly consist

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of water-soluble inorganic salts. For the macroalgae, the highest organic carbon distribution of 15.82% was observed for ZM followed by SJ, UP, and GEH. Only 8.01% of the organic carbon was

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distributed in the HTL process water for GEH because the carbohydrates in the GEH were difficult to convert, as previously mentioned. The carbon distribution in the HTL process water from

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macroalgae is double or five times higher than that in the HTL process water from microalgae as the yield of CO2 derived from macroalgae is almost double or five times than that from microalgae, 4.62-12.34% for the microalgae and 23.42-27.14% for the macroalgae. 3.2 Characterization of the HTL process water As previously mentioned, the HTL process water mainly consisted of soluble inorganic salts and organic matter derived from the algal biomass. Therefore, better understanding its composition is needed for effective utilization of this HTL process water. The HTL process water was characterized by TOC analysis, ion-exchange chromatography, pH, and GC×GC-TOF-MS. The 14

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TOC in the HTL process water is shown in Table 3, together with the pH and the concentrations of ammonium, chloride, nitrate, phosphate, and sulfate. Ion-exchange chromatography of the HTL process water indicated the presence of the anions CH3COO-, Cl-, NO 3− , PO 34− , and SO 24 − and the cations K+, Na+, Mg2+, Ca2+, Fe3+, and NH +4 . X-ray fluorescence (XRF) analysis of the algal

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biomass feedstocks suggested that the algae contained K, Na, Ca, Mg, Fe, Si, I, S, P, Cu, Ni, Mn, Zn, Sr, As, Cr, Mo, Br, and Ti. These elements would remain in the process water after HTL of the algae. Among the aforementioned ions, chloride and acetate are present in the highest

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concentrations in the HTL process water. Table 1 shows that the macroalgal UP and SJ exhibit the highest ash contents and that the ash mainly consisted of chlorides; thus, the highest Cl-

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concentration was observed for the process water produced from HTL of the UP and SJ. Acetates originated from acetic acid, which was in turn derived from the decomposition of carbohydrates in the algae. Acetate levels in the HTL process water showed large differences depending on the algal species. Essentially, the HTL process water produced from macroalgae contained higher

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concentrations of acetate than those produced from microalgae. Of course, the elements in the algal biomass can be strongly affected by the temperature, geographic location [29], water salinity, and aqueous nutrient content [30]. Hence, the elemental compositions of samples of the same species

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grown in different climates can differ dramatically. The TOC analysis indicates that a certain proportion of components in the crude bio-oils were

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water soluble. A higher content of polar compounds in the crude bio-oil results in a higher TOC of the HTL process water. The choice of algal biomass influences the amount of carbon dissolved in the HTL process water. The TOC obtained from the microalgae were all higher than those obtained from the macroalgae. Thus, the higher the protein of the feedstock, the higher the TOC of the HTL process water. The TOC in the HTL process water for microalgae ranges from 19.70 to 30.04 g/L, and that for macroalgae ranges from 7.32 to 14.43 g/L. However, Fig. 2 shows that the WSP yields produced from the macroalgae are all higher than those obtained from the microalgae, indicating that the WSPs produced from the macroalgae mainly consisted of soluble inorganic salts. Ross et al. 15

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[31] reported TOCs for ArP and AuP of 0.374 and 0.720 g/L, respectively, after hydrothermal treatment of the AuP and ArP at 350 °C for 60 min with added Na2CO3. Such a low TOC was due to their utilization of dichloromethane as the extraction solvent; therefore, solvent extraction also strongly affects the amount of carbon dissolved in the HTL process water. This solvent extraction

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process pollutes the water because some organic solvent remains in the HTL process water as a consequence of its solubility. A lower TOC was observed when no solvent was used for crude bio-oil extraction [16]. In this case, the pH of the aqueous phase was typically ~8 because of the

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presence of ammonia, which was derived from the decomposition of protein in the algal biomass. Thus, the higher the protein content of the feedstock, the higher the pH of the HTL process water.

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More detailed chemical analyses of these dissolved organics in the HTL process water are needed to understand our observations and will be the focus of future research. Unfortunately, information is scant on process waters after the HTL of algae. We used GC×GC-TOF-MS to separate and tentatively identify many of the molecular components in the HTL process water. The HTL process water was first freeze-dried to remove water from the WSPs. The obtained WSPs were

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then redissolved in ethanol and only the supernatant was analyzed because some of the inorganic salts are insoluble in water. The components of the WSPs included alcohols, fatty acids, fatty acid

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amides, esters, N-containing compounds (not including the amides), O-containing compounds (not including the acids and amides), and N,O-containing compounds (containing O and N at the same

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time), consistent with the molecular compositions of the bio-oils. During the HTL of algae, carbohydrates were rapidly hydrolyzed to monosaccharides with glucose as one of the main products. The glucose was readily converted to fructose, an isomer of glucose. Finally, the glucose and fructose were converted into furfurals, short chain carboxylic acids, aldehydes, ketones, and ketenes [32]. The proteins mainly consist of linear polymers of amino acids. The peptide C-N bonds link the amino acids together between their carboxyl and amine groups, which can be hydrolyzed to amino acids. The amino acids rapidly undergo decarboxylation and deamination and consequently produce hydrocarbons, amines, aldehydes, and carboxylic acids [33]. Table 4 provides the tentative 16

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molecular identities and peak area percentages for each individual compound in the WSPs. Only those compounds for which the area percentage in the GC×GC-TOF-MS chromatogram exceeded 0.5% for the WSPs are listed. The molecular composition of the WSPs produced from the microalgae is more complex than that produced from the macroalgae. Acids, amides, and ketones

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were the most abundant components in the WSPs, and these were higher than 80% due to the very high carbohydrate and protein contents of the selected algae. They mainly consisted of short-chain fatty acids and amides. The acid content of the WSPs produced from the microalgae (except SL)

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were all less than those produced from the macroalgae. Table 4 shows that acetic acid and acetamide are the two most abundant fatty acids and amides in the WSPs, respectively. For the

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macroalgae, the acetic acid contents are all greater than 50% and can reach 83.02% for GEH. However, only a small amount of acetic acid was observed in the crude bio-oil, indicating a large proportion of acetic acid was distributed in the HTL process water. A possible pathway for the formation of amides in the WSPs is the reaction of fatty acids with ammonia [27]. 2-Pyrrolidinone and its derivatives, which were derived from reactions among the intermediates produced from

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carbohydrates and protein, were also observed in the WSPs [32]. Approximately 16% of amines was observed for the WSPs produced from the ArP because of its high protein content; these amines

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were derived from the hydrolysis of amino acids. Only a small amount of hydrocarbons was detected in the HTL process water because their solubility in water is low.

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3.3 SCWG of the HTL process water

Because the HTL process water contained a certain amount of organics, conversion of the aqueous byproducts can improve energy efficiency of algal biomass conversion in HTL. Therefore, the process waters generated from the HTL of different algae were gasified at 600 °C for 60 min. The carbon gasification efficiency (CGE) was calculated from the following equation: CGE =

m total carbon in gaseous products m total organic carbon in aqueous phase

× 100%

where mtotal carbon in gaseous products and mtotal organic carbon in aqueous phase represent the total masses of carbon in the gaseous products and organic carbon in the HTL process water, respectively. 17

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Fig. 5 shows a comparison of the CGE and yields (mmol/g(aq)) of the various gaseous product fractions for SCWG of the HTL process water produced from different algae. The reactions were performed at 600 °C for 60 min at a water density of 0.236 g/cm3. The CGE varies from 54.18 for UP to 93.52 for SL depending on the algal species. The higher the content of N- or O-containing

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compounds in the WSPs, the lower the CGE. Moreover, the larger the total acid and amide contents of the HTL process water, the higher the CGE. Therefore, lower CGEs were observed for the ArP, UP, and the ZM. Furthermore, if the total contents of acids and amides are similar to one another,

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then a higher acids content leads to a higher CGE. The highest CGE of 93.52% was observed for the process water produced from HTL of SL. These results indicate that more than half of the TOC

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in the HTL process water can be converted to gaseous products during the non-catalytic SCWG process. Catalysts are possibly needed for those HTL process waters with high TOC contents after treatment by the SCWG process if a complete conversion of organics is expected. Fig. S2 shows a comparison of the color of the HTL process waters before and after the SCWG treatment process.

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Clearly, all the process waters produced from the HTL of algae are brown, whereas the HTL process water produced from SJ and ZM exhibit a deeper brown color. By contrast, all the treated HTL process waters subjected to SCWG exhibit a light-yellow color, indicating that some organic

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matter was removed during the SCWG process. Table 5 shows a comparison of the TOCs for the HTL process water before and after the SCWG process. Clearly, the TOC content of the HTL

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process water after treatment by the SCWG process was substantially reduced. The reduction ratio of TOC is consistent with the carbon gasification efficiency as shown in Fig. 5. The higher the CGE, the lower the TOC of the HTL process water. The SCWG process yielded a nutrient-rich aqueous phase that contained minerals, ammonium, and dissolved carbon dioxide and was thus suitable for growing plants or algae [33]. Fig. 5(a) and 5(b) also show the mole percentage, identities, amounts, and yields of the various gas products from the SCWG of process waters produced from the HTL of eight different algal feedstocks. Note that the gas yield is expressed in millimoles of gas per gram of HTL process water. 18

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Thus, the higher the organic matter content of the HTL process water, the higher the yield of the gaseous products. The inorganic salts in the HTL process water might affect the yield of each gas product fraction. The identified gaseous products mainly include H2, CH4, CO2, CO, and C2-C3 hydrocarbons. Clearly, the HTL process water strongly influenced the mole percentage and yield

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distribution of each product fraction. The mole percentages and yields of these gaseous products are very different from those yields produced from the HTL of algae, indicating that the chemical compositions of the feedstocks strongly affected the yield of each gas product fraction. Three major

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reaction pathways are responsible for the formation of these gases, including steam reforming, water-gas shift, and methanation. Decarboxylation of carboxylic acids can also contribute the

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production of CO2. CO was mainly produced from the steam-reforming reactions and was subsequently consumed, perhaps in the water-gas shift and/or methanation reactions. Fig. 5(a) shows that H2, CH4, and CO2 are the major components of the gaseous products for all of the HTL process waters. The mole percentages of both H2 and CH4 are higher than 20%. By contrast, CO2 was the overwhelming fraction obtained from the HTL of algae (see Fig. 3(a)). Only a small

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percentage of CO and C3H8 were observed in all of the HTL process water. At severe temperature, CO would have been consumed in the water-gas shift and methanation reactions during the SCWG

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process. Fig. 3(b) shows that the total gas yields produced from the SCWG of HTL process water produced from microalgae are all higher than those produced from macroalgae because of the

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higher TOC content of HTL process water from microalgae. For the HTL process water from microalgae, CH4 always shows the highest yield, followed by H2 and CO2. The highest CH4 and H2 yields of 1.13 and 1.08 mmol/g(aq), respectively, were achieved with the NO HTL process water, followed by the ArP, AuP, and the SL HTL process waters. The CH4 yield (15.13 mmol/g) is almost five times higher than the H2 yield (3.08 mmol/g) when the microalga AuP is gasified at 600 °C for 60 min, indicating that the gasification activity of AuP biomass is similar to that of its HTL process water but with a higher gas yield [34]. Cherad et al. [16] reported yields of 1.79 and 3.31 mmol/g of CH4 and H2, respectively, when they gasified the HTL process water of Chlorella at 550 °C min for 19

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30 min. They also found that a decrease in TOC content from 11 to 2 g/L increased the H2 yield seven-fold. In the present study, the TOC of the HTL process water from AuP is approximately 24 g/L; therefore, a lower H2 yield is expected. For the SCWG of process waters produced from the HTL of macroalgae, the CH4 yields produced from UP and GEH are higher than the H2 yields,

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whereas contrary results were observed for SJ and ZM. A small amount of CO was observed for all the HTL process water produced, and it varied from 0.003 to 0.199 mmol/g(aq). The C2H6 yield ranged 0.06 to 0.22 mmol/g(aq) for microalgae and 0.02 to 0.06 mmol/g(aq) for macroalgae,

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respectively. 3.4 Energy balance

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The energy of algal biomass feedstocks would distribute among different product fractions after they were subjected to HTL. The energy recovery from the HTL process water is mainly achieved in terms of gaseous products. Table 6 shows the energy ratios of each product fraction produced from the HTL of different algal biomass to the algal biomass and the energy ratios of gaseous products produced from SCWG of different HTL process water to the algal biomass and the HTL

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process water, respectively. HTL(O), HTL(S), HTL(AP), and HTL(G) represent the energy ratio of the crude bio-oil, solid, aqueous phase, and gaseous products produced from the HTL of algal biomass

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to the algal biomass feedstocks, respectively. SCWG(GA) and SCWG(GAP) represent the energy ratio of gaseous products produced from SCWG of HTL process water to the algal biomass feedstocks

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and aqueous phase, respectively. For the microalgae, 62.50-71.34% energy of the algal biomass was distributed in the crude bio-oil and 14.30-25.64% energy was distributed in the HTL processed water; the energy retained in the HTL processed solid was lower than 4% of the algal biomass with the exception of SL which remained 15.69% energy of the algal biomass. In contrast, for the macroalgae, 6.03-41.06% energy of the algal biomass distributed in the crude bio-oil, which are all lower than those of microalgae due to the lower yield of crude bio-oil from macroalgae; 20.31-51.97% of energy remained in the solids due to the existence of a large quantity of unconverted raw materials in the solids; more than one third of energy (30.18-36.34%) remained in 20

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the aqueous phase. For both the microalgae and macroalgae, all the energy ratios of the gaseous products to the algal biomass are very low, which ranged from 0.62 to 9.77% depending on the species of algal biomass. 54.82-75.12% energy of the microalgal HTL process water and 18.31-49.34% energy of the macroalgal HTL process water were transferred into the gaseous

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products after the HTL process waters were subjected to SCWG process. If these ratios were converted to energy recovery relative to the algal biomass feedstocks, 5.53-18.30% energy lost in the HTL water can be recovered as gaseous biofuel depending on species of the algal biomass

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feedstocks. Therefore, energy recovery from the HTL process water of the macroalgae is more difficult than from the microalgae. Possibly, more severe temperature or catalyst is needed if one

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expects a higher energy recovery from the HTL process water. Finally, we include that the integration of HTL and SCWG could improve energy recovery from algal biomass than HTL process alone.

4. Conclusions

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HTL effectively converted algal biomass into the four product fractions of crude bio-oil, solid residue, gas, and water-soluble products. The type of algal biomass strongly affected the HTL product distribution and properties of the crude bio-oil due to the different biochemical

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compositions of the biomass. The crude bio-oils are suitable feedstocks for the generation of bio-based fuels. The process water generated from the HTL of algae is often rich in organic carbon.

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The type of algal biomass feedstock also significantly affected carbon distribution in the HTL process water, accounting for 8.01-21.55% of the carbon from the examined algae. Acetic acid and acetamide were the most abundant fatty acids and amides in HTL process water. The TOC of HTL process water was substantially reduced when it was subjected to the supercritical water gasification process (SCWG). CO2 is the absolutely dominant fraction of the gaseous products for the HTL of algal biomass, while H2 and CH4 were the dominant fractions of gas produced by SCWG of HTL process water. This generated H2 can be used as a hydrogen source for subsequent crude bio-oil upgrading. Integration of HTL and SCWG could increase 5.53-18.30% energy recovery from algal 21

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biomass than HTL process alone. Therefore, this study shows that the successful integration of HTL and SCWG can not only improve the energy utilization efficiency of algal biomass and reduce the environmental impacts of HTL but can also increase the economic feasibility of the HTL process. Acknowledgments

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We gratefully acknowledge the financial support of the National Science Foundation of China (21776063), the United Fund for NSFC and Henan Province (U1704127), the Scientific and Technological Innovation Team of the University of Henan Province (18IRTSTHN010), and

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Outstanding Youth Foundation for Scientific and Technological Innovation in Henan Province (184100510013).

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25

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GEH

1.24 32.75

2.43 31.23

4.25 1.01 28.81 50.26 20.28 29.46

4.62 5.5 33.35 58.33 17.42 24.25

7.75 1.1 61.69

15.19 0.4 60.16

2.28 0.62 0.04 0.10 0.03 5.83 4.76 1.77

6.30 0.61 0.19 0.61 0.07 6.91 4.16 0.77 0.08

0.04

0.10

0.08 0.09

0.01 0.01 0.05 0.01

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Table 1 Proximate, ultimate analyses (wt. %) and inorganic composition (wt%, biomass AuP ArP SL NO UP SJ 8.81 10.79 2.42 6.59 3.63 2.13 N 53.52 47.49 61.31 51.01 31.04 23.7 C 7.45 6.63 8.65 7.36 4.62 3.43 H 0.67 0.56 0.74 0.67 1.5 0.11 S 22.61 22.48 16.96 21.7 23.55 18.52 O 74.54 66.63 79.66 73.97 48.07 39.38 V 20.3 22.29 10.82 16.18 19.7 11.05 FC 5.16 11.08 9.52 9.86 32.23 49.57 A 55.06 67.44 15.13 41.19 22.69 13.31 Protein 20.7 6.2 41.6 21.3 3.2 2.7 Lipid 19.08 15.28 33.75 27.65 41.88 34.42 CH+others Na 1.03 2.64 3.08 4.26 8.85 5.08 Mg 0.35 1.41 0.45 2.65 0.57 0.29 Al 0.15 0.34 0.14 0.11 1.11 Si 0.79 4.39 1.79 1.79 0.42 3.63 P 0.45 2.49 0.18 0.10 0.73 1.71 Cl 2.44 0.62 0.44 1.07 19.90 4.92 K 0.38 4.64 2.73 2.51 4.52 Ca 3.92 0.87 0.99 1.61 2.31 5.70 Ti 0.00 0.01 0.01 0.02 Cr 0.02 0.01 Mn 0.06 0.01 0.01 0.03 0.34 0.01 Fe 0.08 0.73 0.67 0.28 0.21 3.50 Ni 0.00 0.00 0.06 Cu 0.03 0.02 0.00 0.02 0.01 Zn 0.00 0.01 0.01 0.01 0.02 0.20 Br 0.04 0.39 Sr 0.18 0.01 0.64 Mo 0.01 0.03 V: volatiles; FC: fixed carbon; A: ash; CH: carbohydrate

ACCEPTED MANUSCRIPT Table 2 Composition of the bio-oils produced from the HTL of different algae (% of total peak area by GC-MS) ArP 5.42 1.67 3.40 1.50 1.74 1.59 7.78 19.75 2.10 3.70 31.16 19.92

SL 8.42 0.93 5.49 20.22 2.00 3.15 6.76 35.75 0.93 3.86 7.48 4.90

NO 7.46 3.09 5.40 0.92 2.71 2.37 12.26 26.71 1.43 7.03 17.74 12.69

UP 7.73 2.53 4.37 0.09 2.14 0.92 21.21 4.59 6.17 9.31 25.56 15.29

SJ 4.97 2.97 6.62 1.07 2.64 1.03 23.03 4.45 1.28 11.79 25.25 14.51

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ZM 5.17 2.45 6.68 1.87 2.45 1.35 36.59 3.06 2.37 11.97 16.93 8.71

GEH 7.44 7.67 6.51 1.09 0.98 1.63 17.10 14.29 7.74 15.76 14.52 4.79

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AuP 4.90 1.69 4.33 2.68 1.06 1.53 8.31 30.09 2.15 5.44 23.00 14.76

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Aromatics Sat. hydrocarbons Unsat. hydrocarbons Acids Alcohols Esters Ketones Amides S-comp. O-comp. N-comp. N,O-comp.

ACCEPTED MANUSCRIPT Table 3 Total organic carbon content (TOC), acetate, pH, ammonium, chloride, nitrate, phosphate and sulfate content of the aqueous phase. Cl(mmol/L) 65.90 408.59 844.29 1245.06 3434.45 881.37 628.69

NO3(mmol/L)

1.33

1.38

PO43(mmol/L)

SO42(mmol/L)

8.59 2.01 2.26 0.83 0.62 0.82

25.51 72.98 36.46

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NH4+ (g/L) 22.58 28.10 5.88 15.91 8.53 4.52 2.57 5.93

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CH3COO(mmol/L) 289.15 222.56 411.96 344.48 353.08 486.56 462.24 551.35

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AuP ArP SL NO UP SJ ZM GEH

TOC (g/L) 23.78 30.04 21.95 19.70 7.28 10.03 14.43 7.32

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Algae

17.37 18.76

pH 8.23 8.34 7.93 8.49 7.78 8.04 7.99 8.33

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SC

RI PT

Table 4 Tentative identities and area % of major peaks in total ion chromatograms for WSPs. Name Area (%) Name Area (%) 2-Piperidinone 1.67 AuP Acetic acid 30.54 2,5-Pyrrolidinedione, 1-ethyl0.69 Isobutylamine 0.88 SL Ethanol, 2-nitro-, propionate (ester) 0.50 5-Hexyn-3-ol 0.57 Cyclopropanecarboxaldehyde 1.24 Acetic acid 52.76 Acetamide 19.51 Ethanol, 2-nitro-, propionate 5.13 Butanoic acid 0.52 Ethane, 1,1-diethoxy0.65 Acetamide, N-methyl3.15 Acetamide 8.77 Propanamide 4.36 Butanoic acid 1.28 Propanedioic acid, propyl1.03 Acetamide, N-methyl0.65 Acetamide, N-ethyl0.81 Butanoic acid, 3-methyl0.99 Propanamide, N-methyl0.70 Propanamide 1.79 Butanamide, 3-methyl1.04 Butyrolactone 1.97 Butanenitrile, 3-methyl1.54 Ammonium Chloride 0.60 2-Pyrrolidinone, 1-methyl1.32 2-Pyrrolidinone, 1-methyl2.25 2-Pyrrolidinone 11.58 2(3H)-Furanone, 5-ethyldihydro0.97 2,5-Pyrrolidinedione, 1-methyl1.34 4-Aminobutanoic acid 14.21 1-Ethyl-2-pyrrolidinone 0.54 1-Ethyl-2-pyrrolidinone 0.98 3-Aminopyridine 0.66 3-Pyridinol 0.79 3-Pyridinol 0.68 NO 3-Pyridinol, 2-methyl0.59 1-Propanol 0.49 2(1H)-Pyridinone, 3,6-dimethyl0.53 Acetic acid 38.47 2-Piperidinone 1.95 Ethanol, 2-nitro-, propionate 7.55 Ethane, 1,1-diethoxy1.69 ArP Acetic acid 19.83 Acetamide 2.40 1-Propanol, 2-amino5.03 Butanoic acid 0.92 Isobutylamine 11.42 Acetamide, N-methyl2.12 (R)-(-)-2-Amino-1-propanol 1.89 Butanoic acid, 3-methyl2.69 Cyclobutanecarbonitrile, 3,3-dimethyl8.47 Butanoic acid, 2-methyl0.57 Propanoic acid 0.67 Propanamide 6.06 1-Butanamine, 2-methyl2.03 Propanamide, N-methyl0.89 1-Butanamine, 3-methyl2.63 Butanamide, 3-methyl0.72 Acetamide 8.62 Glycerin 2.10 Acetamide, N-methyl2.84 Ammonium Chloride 3.02 2-Methyl-1,2-propanediamine 0.62 2-Pyrrolidinone, 1-methyl5.73 Butanoic acid, 3-methyl1.45 2-Pyrrolidinone 9.74 Propanamide 1.02 1-Ethyl-2-pyrrolidinone 0.95 Propanenitrile 1.35 2-Piperidinone 1.24 Acetamide, N-ethyl1.26 Isosorbide 3.56 Propanamide, N-methyl1.42 UP Butanamide, 3-methyl0.87 1-Propanol 1.12 Glycerin 0.52 Acetic acid 52.29 2-Pyrrolidinone, 1-methyl2.87 Ethanol, 2-nitro-, propionate 8.13 2-Pyrrolidinone 9.53 Ethane, 1,1-diethoxy0.82 2,5-Pyrrolidinedione, 1-methyl1.20 Acetamide 13.87 1-Ethyl-2-pyrrolidinone 0.95 Acetamide, N-methyl1.24 3-Aminopyridine 0.84 Butanoic acid, 3-methyl1.67

ACCEPTED MANUSCRIPT

M AN U

61.62 0.69 1.99 4.42 3.45 5.79 1.12 1.44 2.68 0.59 2.35 5.18 5.27

EP

AC C

50.06 0.80 2.78 1.62 0.48 2.87 3.25 3.54 0.96 0.73 3.88 1.51 1.38 1.29 0.74 2.65 4.13 1.17 0.53 0.88 1.23

Area (%) 1.15 74.54 5.55 1.37 4.89 0.83 1.26 0.64 0.58 0.51 0.51 2.53 0.92

RI PT

0.65 1.14 1.98 0.57 5.29 3.52 0.49 0.55 1.10

Name 1-Propanol Acetic acid Propanoic acid Ethane, 1,1-diethoxyAcetamide Butanoic acid Butanoic acid, 3-methylPropanenitrile Butyrolactone 2(3H)-Furanone, dihydro-5-methyl2-Pyrrolidinone, 1-methyl2-Pyrrolidinone 3-Pyridinol, 6-methyl-

SC

Area (%)

TE D

Table 4 (continued). Name UP Propanamide Butyrolactone 2-Pyrrolidinone, 1-methyl1-Butanol 2-Pyrrolidinone 3-Pyridinol 4-Acetylbutyric acid 2-Piperidinone 2(1H)-Pyridinone, 5-methylSJ Acetic acid Ethyl Acetate Ethanol, 2-nitro-, propionate (ester) Ethane, 1,1-diethoxyAcetamide Acetic acid, cyanoButanoic acid Butanoic acid, 3-methylButyrolactone 2(3H)-Furanone, dihydro-5-methyl2-Pyrrolidinone, 1-methyl2-Pyrrolidinone 3-Pyridinol ZM Acetic acid Ethyl Acetate Propanoic acid Oxiranemethanol, (S)Propanoic acid, ethyl ester Ethane, 1,1-diethoxyAcetamide 3-Methyl-1,2-diazirine Formamide, N,N-dimethylButanoic acid, 3-methylButyrolactone 2(3H)-Furanone, dihydro-5-methyl2-Pyrrolidinone, 1-methyl2(3H)-Furanone, dihydro-4-methyl(S)-(+)-2',3'-Dideoxyribonolactone 2-Pyrrolidinone 3-Pyridinol 3-Pyridinol, 2-methyl3-Pyridinol, 6-methyl2(1H)-Pyridinone, 1-ethylIsosorbide GEH

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

Table 5 Comparison of TOC(g/L) of the HTL process water before and after the SCWG process Before SCWG After SCWG TOC reduction% AuP 23.78 3.72 84.36 ArP 30.04 8.83 70.61 SL 21.95 1.23 94.40 NO 19.7 1.45 92.64 UP 7.28 3.12 57.14 SJ 10.03 0.78 92.22 ZM 14.43 4.8 66.74 GEH 7.32 0.98 86.61

ACCEPTED MANUSCRIPT Table 6 Energy balance of the HTL and SCWG process HTL(gas) SCWG(gas) relative to HTL(solid) HTL(aqueous phase)

AuP ArP SL NO UP SJ ZM GEH

71.34 62.50 69.38 69.64 41.06 37.35 40.55 6.03

1.78 2.39 15.69 3.88 27.73 22.06 20.31 51.97

a

25.64 33.37 14.30 24.54 30.18 35.94 36.34 32.23

1.25 1.74 0.62 1.94 1.03 4.65 2.80 9.77

algal biomass

aqueous phase

16.31 18.30 10.74 15.40 5.53 17.73 11.60 6.31

63.60 54.82 75.12 62.77 18.32 49.34 31.93 19.57

AC C

EP

TE D

M AN U

SC

a. calculated by difference

SCWG(gas) relative to

RI PT

HTL(oil)

SC

RI PT

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

(a) (b) Fig. 1. Schematic of apparatuses and reactors for (a) HTL of algal biomass; (b) SCWG of HTL process water

ACCEPTED MANUSCRIPT

52.68

50

52.33

Oil Solid

46.62

Gas

44.43

WSP

28.17 24.70 25.33

19.69

19.57

19.38 15.47

9.93

10

9.77

3.84

ArP

14.70

SL

15.61

9.72 9.83

6.94

3.77

2.71

0 AuP

14.86

14.64 8.48

6.83 4.60

23.23

22.17

20.33

27.15 25.97

26.90

NO

UP

7.12

5.95

M AN U

20

RI PT

28.67

27.70 25.25

SC

30

37.81

Lost

35.20

SJ

1.95

ZM

EP

TE D

Fig. 2. Effect of algal biomass feedstocks on the yields of product fractions

AC C

Yield (wt.%)

40

GEH

ACCEPTED MANUSCRIPT CH₄ CO₂

(b) 100

6.0

12

80

3.0

70

2.0 60

1.0 0.0

50 NO

UP

SJ

ZM GEH

0.5 9

0.4 0.3

0.2 0.1 0

AuP ArP SL

NO

SC

AuP ArP SL

Yield(mmol/g)

Percentage

4.0

CH₄ CO₂

CO C₃H₈

0.6

90

5.0

H₂ C₂H₆

UP

SJ

6

Yield(mmol/g)

CO C₃H₈

RI PT

H₂ C₂H₆

Percentage

(a) 7.0

3 0

ZM GEH

AC C

EP

TE D

M AN U

Fig. 3 Effect of algal biomass feedstocks on the (a) mole percentage and (b) yield of gaseous products

ACCEPTED MANUSCRIPT 100

Biocrude

Solid

6.98

7.52

2.58 4.62

10.56

12.34

11.86

Aqueous phase

Gas

1.68

5.13

10.57 8.64

12.16

25.06

23.42 27.14

80 14.90

15.36

8.06

2.00

60

25.99

12.38

21.55 4.02

15.82 15.31

8.01

29.69

RI PT

2.50

22.91

21.40

40 65.58

65.55

64.40

56.09

20

0

ArP

SL

NO

UP

M AN U

AuP

31.03

SC

35.51

SJ

49.30

34.11

4.54

ZM

GEH

EP

TE D

Fig. 4. Effect of algal biomass feedstocks on the carbon distribution in the product fractions

AC C

Carbon/ wt.%

Loss 3.75

ACCEPTED MANUSCRIPT H₂

(a)

CH₄

CO

CO₂

C₂H₆

C₃H₈

38.35

37.35 35.11 33.91

36

36.20

35.95

34.48 31.37

30.11

29.35

28.30

28.17

27.83

27.77 26.46

Percentage

33.81

33.72

32.40

25.90

26.68

25.73

24

21.05 17.30

RI PT

15.26 12.40

11.22 9.76

9.89

9.76

7.24

6.65

6.06

2.69

2.57

0.63

0.27

0 AuP (b)

ArP

SL

NO 0.79 0.77

0.21 0.76

SJ

0.70

GEH

C₂H₆ CH₄

C₃H₈ CO₂

0.8

0.70 0.66

93.52

90

84.15

91.11

90.54

83.57

69.05

0.6

CGE

0.63

70

0.57

0.57

ZM

CO H₂

0.22

0.1990.20

0.65

0.15

61.83

50

54.18

30

0.1

0

AuP

0.010

0.012

0.006

0.006

EP

0.010

ArP

SL

0.34

0.077

0.30 0.27

0.06

0.05

0.4

0.34

0.32

0.33

0.31

TE D

Yield (mmol/g aq)

UP

M AN U

0.2

0.27

0.00

SC

0.46

5.62

0.26 0.15

0.06

0.16 0.13 0.02

0.03 0.000

0.001

0.24

0.2 0.067 0.06

0.12

0.007

Yield(mmol/gaq)

12

0.002

0.003

0.003

0.001

0

NO

UP

SJ

ZM

GEH

AC C

Fig. 5 Effect of HTL process water on the (a) mole percentage and (b) CGE and yield of each gas product fraction

ACCEPTED MANUSCRIPT

Highlights

Integration of HTL and SCWG can improve energy recovery from algal biomass Algal biomass significantly affected carbon and energy distribution in the products 14.3-36.34% energy of algal biomass distributed in the HTL process water 57-94% TOC of the HTL process water after SCWG was substantially reduced

AC C

EP

TE D

M AN U

SC

RI PT

5.53-18.30% energy of the algal biomass was recovered from the HTL process water

1