Hydrothermal treatment of rice straw for carbohydrate production

This study focused on the effect of hydrothermal (HT) treatment at 180 – 210 °C for holding 0 - 15 min on the solubilization of rice straw and the changes of HT residue. The optimum 12 treatment conditions for the highest solubilization and solid reduction of rice straw was 210 °C for holding 0 min. Under this condition, the extraction yield and total organic carbon (TOC) concentration of the HT liquid part were the highest, about 44% and 7850 mg/L, respectively. 15 The dry residue showed that the HT conditions above 200 °C for holding a short time were more efficient, which was confirmed by FT-IR and the changes of surface morphology under microscope. The reactor headspace could be an important factor because HT treatment with 18 a lower headspace (HTp210-0(15)) yielded more soluble carbohydrate under the test conditions. Also, energy input calculated based on the 1 ton removed hemicellulose (extraction yield) in the headspace experiments proved this finding.


INTRODUCTION
Increasing energy usage and the rapid depletion of fossil fuels require renewable energy development to reduce pollutions generated by fossil fuels. According to the Energy Information Administration of the United States (EIA), worldwide energy demand will 30 increase by 40% by 2040, reaching about 800 quadrillion British thermal unit, with the rising countries accounting for the majority of the demand increases [1]. Due to its use as a fuel and other value-added chemical production, biomass or biofuel has attracted increasing 33 attention among the numerous renewable energy sources such as geothermal, hydro, wind, and solar [2].
The fact that lignocellulosic biomass makes up a large portion of plant matter makes it the 36 most plentiful renewable resource on earth. It is a desirable feedstock for making chemicals and fuels since it is accessible and affordable. The three main components of lignocellulosic biomass are cellulose (40-50%), lignin (15-20%), and hemicellulose (25-35%). Rice 39 straw's recalcitrant nature is one of the challenges for its biochemical conversion to bioethanol and methane. To convert biomass to biofuel, cellulose and hemicellulose molecules must be broken down into monomers or simple sugars. Rice straw fermentation 42 is difficult in practice, which creates a significant barrier for lignocellulosic biomass in the bioconversion process [3]. In a sugar platform bio refinery, pretreatment is an important step in increasing biomass digestibility. Several criteria define the goal of any pretreatment 45 procedure: (1) maximizing the final yield such as ethanol and other valuable products; (2) high amount lignin removal; and (3) reducing the formation of degradation products that can inhibit the action of produce biofuels [3,4]. There are four main types of treatments, i.e., 48 physical, chemical, physicochemical, and biological methods in the hydrolysis of lignocellulosic biomass, and some of these treatment methods are very effective.
Many chemical, thermal, and biological pretreatment procedures have been extensively 51 researched to increase lignocellulosic biomass susceptibility to later enzymatic hydrolysis [5]. The use of water as the primary reaction medium with no other chemical additives makes hydrothermal pretreatment one of the most effective pretreatment techniques in terms of 54 both practical and environmental considerations [6].
Hydrothermal (HT) processing of lignocellulosic materials has been studied under a variety of operational conditions in the past. HT treatment operating temperatures are typically 57 between 100 and 230 °C, though higher temperatures can be used. The efficiencies vary depending on the applied temperature and time, which are co-related factors. Generally, 180 -210 °C with a short holding time (1-15 minutes) can achieve the best sugar refinery 60 [7,8]. Some researchers suggested HT treatment with some chemicals such as acid and A c c e p t e d m a n u s c r i p t alkali addition. According to Imman et al. [9], the carbohydrate yield from HT treatment with acid and alkali were about 30 -40% higher than that without chemical addition. However, 63 adding chemicals is not regarded as environmentally friendly. The liquid to solid ratio (LSR) of solid concentrations can range from 2 to 100 (w/w), with the most typical values being around 10 [5]. The interaction between HT temperature and holding time has a significant 66 impact on the selection of both the liquid and solid phases. It is widely assumed that HT treatment at a higher temperature for a short time will result in slightly better pentoses yield and less inhibitor formation [10,11]. HT treatment at 200 -210 °C for a short period is 69 effective: When corn stover was hydrothermally treated at 210 °C for 0 and 10 minutes, more than 90% of the xylan was solubilized [12,13]. One of the most critical aspects in the process economics of commercializing lignocellulosic biomass conversion is energy 72 consumption. That's why energy balance analysis is very important. According to He et al. [13], HT pretreatment gained energy about 2741 MJ/t-rice straw when the process was performed at 150 °C for 20 minutes, which was 300 MJ/t-rice straw more compared to the 75 methane production from no pretreatment group. In addition, the energy recovery from the HT and microwave pretreatment was 43 -53% and 57 -79%, respectively [14].
Many researchers investigated the HT treatment on various types of lignocellulosic 78 materials, but there is little information about HT reactor head space's influence on sugar recovery. This research aimed to determine the suitable HT treatment conditions for rice straw to achieve digestible sugars which can be used for maximal ethanol production.

Materials:
In this investigation, rice straw was collected from a farm area in Tsukuba 84 (Ibaraki-ken, Japan) and cut up into small pieces and then be air-dried. The air-dried rice straw particles were milled for the experiment, with particle sizes ranging from 0.27 to 0.56 mm. Before use, the milled straw was kept in a plastic container in the dark at ambient 87 temperature. The original rice straw used in this study contains 92.56% total solids, 50.3% total carbohydrates, 27.8% lignin and 10.44% ash.

Apparatus and procedure:
In a 200 mL stainless-steel reactor, HT treatment was 90 performed. Rice straw was treated at 4 temperature levels in the range of 180 -210 °C for 0 min, 5 min, 10 min, and 15 min, respectively. The temperature in the HT reactor was increased at 12 °C/min on average, and the pressure was around 1 bar. In addition, when it 93 reached the holding time, the heater was powered off, and a table fan was used to cool it.
The average cooling rate was 2 °C/min. Nine different HT treatment conditions were performed in this study, which were labelled as HT180-10, HT180-15, HT190-10, HT200-5, A c c e p t e d m a n u s c r i p t HT200-10, HT210-0, HTP210-0(5), HTP210-0(9), HTP210-0(15). The first six experimental tests were to find out the suitable HT condition, and the last three experiments were to check whether the reactor pressure had any influence on the sugar yield. The installed pressure 99 meter was used to read the reactor pressure, which was around 1 -2 MPa depending on HT conditions. The treated rice straw was centrifuged after HT treatment, and the solid HT residue was rinsed with distilled water. The pH value, total organic carbon (TOC), volatile 102 fatty acids (VFAs), and total carbohydrate of the isolated supernatant were all measured.
After being washed with deionized water, the solid residue from HT was dried at 105 ºC for 24 hours and used to calculate the total yield based on the weight difference [15]. For future 105 usage, the pretreated dry biomass was packaged in plastic bags and stored in the dark.

Analytical methods:
The National renewable energy laboratory (NREL) method was used to determine total solid (TS), volatile solid (VS), and calculate yield [15]. The concentration 108 of total soluble carbohydrates was measured using the phenol sulfuric acid technique with glucose as reference [16]. A pH meter was used to determine the pH value. Individual VFA species in the liquid from rice straw during HT treatment was determined using gas 111 chromatography with a flame ionized detector (GC-8A, Shimadzu Corporation, Japan).
VFAs were calculated as the sum of acetic, propionic, iso-butyric, n-butyric, iso-valeric, and n-valeric acids. A TOC analyzer was used to determine the total organic carbon (TOC) of 114 the HT liquid component (TOC-V CSN, Shimadzu, Japan).
The modified method was used to determine the amounts of lignin, cellulose, and hemicellulose in HT treated dry biomass [15,17]. In brief, 0.3 g of solid residue was mixed 117 with 3 mL of 72% w/w H2SO4 on a laboratory shaker for 4.5 hours at ambient temperature (25 °C). The solution was then diluted to 4% and hydrolyzed overnight to convert cellulose to glucose. The liquid and solid components were then separated using vacuum filtration, 120 and the solid part was dried at 105 °C for lignin analysis. Acid-soluble lignin and total carbohydrate were determined in the separated liquid.
All the trials were done three times and the average results were presented. The structural 123 morphology of HT treated biomass and the raw rice straw were observed by optical microscopy. The structural modifications during the HT treatment were also investigated using an FT-IR spectrophotometer.

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The energy consumption was estimated according to Eq. 1 [13]. The rice straw disposal capacity was expected to be 1 ton in this study.
°C); γrs is the specific heat capacity of rice straw (1.67 kJ/kg °C); THT (°C) is the HT treatment temperature (180-210 °C in this study); Tat is the temperature of the environment (25 °C in this study).

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The out wall of the HT reactor would be supplied with thermal insulation material if it were implemented in practice; however, heat loss through the reactor wall during the HT process was ignored in this study. The pH values varied from 3.31 (HT200-5) to 4.31 (HT180-10). From Fig.1, the pH value 156 was decreased from 4.31 to 3.31 (HT200-5), then slightly increased to 3.55 at HT210, probably due to a higher temperature especially > 200°C can break down some organic acids [18]. Generally, when compared to the total extraction yield and pH value, a reverse 159 tendency was noticed: the increased extraction yield was accompanied by a decreased pH value, probably owing to the production of organic acids from the dissolved hemicellulose. This means that the reactor headspace or pressure may influence the extraction yield and 168 the liquid pH value.

Dissolved carbohydrate and TOC from rice straw by HT treatment:
The total dissolved carbohydrate was determined using the phenol-sulfuric acid method. This method can 171 reflect all types of sugars such as xylose, glucose and others. Fig. 2   suggesting that a higher temperature is beneficial for VFAs decomposition.
In addition, a lower VFAs was detected in the liquid from HTp210-0(5) and HTp210-0(9) in 210 comparison to HTp210-0(15). There were 3 noticeable unknown peaks from the gas chromatography results that need further confirmation by additional VFA standards. These unknown VFA products were observed to increase when HT treatment was conducted at The effect of HT treatment on the solubilization of rice straw was studied. The HT treatment yielded various amounts of carbohydrate and other products from rice straw. HT210 was 219 found to have a considerable impact on rice straw solubilization, boosting dissolved carbohydrate production with lower pH while also increasing VFA production. This observation suggests that this HT temperature is more suitable for hemicellulose 222 decomposition from rice straw. The reactor headspace experiment found that a smaller   HTp-210-0(9) and HTp-210-0(15), respectively. The lowest energy was consumed under HTp210-0(15) condition due to its higher extraction yield than the other two HT210

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conditions. This result also suggests that the HT reactor headspace is critically essential for the enhanced breakdown of rice straw when energy consumption is taken into consideration.
However, a more detailed energy balance analysis is necessary when the final products 294 such as ethanol, methane and other useful products are considered, which might be different when different final products being concerned.

CONCLUSIONS
In this study, we investigated the effects of HT treatment on rice straw solubilization and residue changes. In terms of achieving optimal results, the HT treatment conducted at 210 300 °C for 0 minutes yielded the best outcome, with a soluble carbohydrate yield of 44% and a total organic carbon (TOC) content of 17.1%. The temperature of HT treatment was found to exert a significant influence on the production of volatile fatty acids (VFAs), with acetic A c c e p t e d m a n u s c r i p t acid being the predominant species in this condition. Moreover, this study showed that HT treatment demonstrated higher efficiency at temperatures above 200 °C and short holding times, which was supported by evidence from FT-IR spectra and morphological changes.

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Furthermore, we observed that reducing the headspace in the reactor resulted in a more efficient recovery of carbohydrates from rice straw with lowest energy usage. https://doi.org/10.1007/s12010-008-8420-z