A comparative study for the organic byproducts from hydrothermal carbonizations of sugarcane bagasse and its bio-refined components cellulose and lignin

Sugarcane bagasse was refined into cellulose, hemicellulose, and lignin using an ethanol-based organosolv technique. The hydrothermal carbonization (HTC) reactions were applied for bagasse and its two components cellulose and lignin. Based on GC-MS analysis, 32 (13+19) organic byproducts were derived from cellulose and lignin, more than the 22 byproducts from bagasse. Particularly, more valuable catechol products were obtained from lignin with 56.8% share in the total GC-MS integral area, much higher than the 2.263% share in the GC-MS integral areas of bagasse. The organic byproducts from lignin make up more than half of the total mass of lignin, indicating that lignin is a chemical treasure storage. In general, bio-refinery and HTC are two effective techniques for the valorization of bagasse and other biomass materials from agriculture and forest industry. HTC could convert the inferior biomass to superior biofuel with higher energy quantity of combustion, at the same time many valuable organic byproducts are produced. Bio-refinery could promote the HTC reaction of biomass more effective. With the help of bio-refinery and HTC, bagasse and other biomass materials are not only the sustainable energy resource, but also the renewable and environment friendly chemical materials, the best alternatives for petroleum, coal and natural gas.


Introduction
Broadly speaking, biomass is all biologically-produced matters, including all plant and microorganism in lands and oceans. Each year large amount of biomass is produced from photosynthesis of plant, containing much more carbon than that consumed from fossil fuel [1]. More specially, biomass matters are the agricultural residues, including straw of wheat, rice, corn and other crops; and forest byproducts, such as saw dust, tree branch, bark, roots, and wood chips. Biomass is not only the largest renewable energy resource [2][3][4], but also the substainable PLOS  resource for chemical industry [5,6] and material industry [7,8]. To deal with the global environmental and ecological problems, caused by over consuming of fossil fuel, including coal, petroleum, and natural gas, which lead to the global warming, frequently happened extreme weather and ecological disaster [9,10], biomass, particularly the waste from agriculture and forest industry, is the only way to replace coal and petroleum as both the energy fuel and as raw materials for chemical and material industries. Biomass is complex organic matters that consist of a number of main organic macromolecules and special minor constituents, depending on different plant species. In order to utilize the biomass materials effectively, economically, and environment friendly, bio-refinery is an important, but difficult technique [11][12][13]. Usually the main organic macromolecules in biomass are cellulose, hemicellulose, and lignin. The goal of the bio-refinery is to separate these components to pure macromolecular compounds effectively without pollution.
Sugarcane is produced in China, Brazil, Cuba, India, and many other countries in large scale. Bagasse is the residue of sugarcane after all sugar juice is squeezed and washed up by clean water for several times, which is the best raw material for development of bio-refinery technique. A bio-refinery scheme of sugarcane bagasse is illustrated in Fig 1. In the above scheme the hemicellulose is used to prepare xylan, a very valuable medicine for hypertensive and diabetic patients [14], also a nutrition and healthy food product. The cellulose is a widely-used material for pulp, glucose, ethanol [12, 15,16], and other products [17,18]. When ethanol is added in gasoline in 10 to 20% ratio, it can reduce the nitrogen Fig 1. Illustration of bio-refinery of sugarcane bagasse. In above scheme the hemicellulose is used to prepare xylan, a very valuable medicine for hypertensive and diabetic patients. The cellulose is a widely-used material for pulp, glucose, and ethanol. The lignin is rich in phenolic compounds, the raw materials for many chemical products. oxides and 2.5 pm particles, discharged from automobiles, remarkably, which is one of the main sources for the severe air pollution and dust fog in north China [19][20][21]. The lignin is rich in phenolic compounds [22,23], the raw materials for many chemical products, such as phenolic plastics, carbon fiber, and binding agent for wood industry [24,25]. In this study a comparative study is conducted for the organic byproducts derived from the hydrothermal carbonizations (HTC) [26,27] of bagasse and its two refined components, cellulose and lignin, to explore what different organic products could be produced before and after biorefinery of bagasse. The conclusions from this study would be helpful for the effective utilization of bagasse as the raw materials of chemical and material industry.

Method and materials
The sugarcane bagasse used in this study was provided by a cane mill of Nanning sugar industry CO., LTD (http://www.nnsugar.com/) in Guangxi, China. The main components of sugarcane bagasse were comprehensively analyzed, and the general results were as follows: cellulose 45-55%, hemicellulose 20-25%, lignin 18-24%, ash 1-4%, and waxes <1, which is consistent with the reference [28].

Bio-refinery of bagasse
In this study the ethanol organosolv technique was used for the refinery of bagasse. Firstly clean and dried bagasse was grinded to 40 mesh (0.45mm) powder, then the bagasse powder was soaked into 55 wt% ethanol-water solution in pH = 3.0~4.0 condition, followed by heating to 180˚C for 90 minutes in a special designed boiler. The dissolved lignin in ethanol-water solution was recovered by solvent vaporization. The undissolved cellulose and hemicellulose were removed from solution to a reactor, where in acidic condition the hemicellulos was hydrolyzed to xylose, and separated with cellulose. The chemical structures of cellulose and lignin are shown in Fig 2. Cellulose is a carbohydrate polymer consisting of glucose monomer through 1-4 glycoside bonds, while lignin is cross linked phenolic polymer, consisting of three phenol monomers, coumaryl alcohol, coniferyl alcohol, and syringyl alcohol.

Hydrothermal carbonization reactions
Usually hydrothermal carbonizations (HTC) of biomass are used to convert the biomass materials to higher calorific biofuel [29][30][31][32]. In HTC reaction elevated temperatures (180-220˚C) is applied to biomass in a suspension with water under saturated pressure for several hours. With this conversion process, a lignite-like, easy to handle biofuel with higher calorific and well-defined properties can be created [6,33]. At the same time many useful organic compounds are produced as byproducts [26,34,35]. In this study we focus on the different organic products obtained from the HTC reactions of the three biomass materials, bagasse and its two refined components, cellulose and lignin. The HTC experimental runs of the three biomass materials were performed in a batch Teflon-lined autoclave boiler with 100 ml volume. The HTC reactor is a stainless steel cylinder 11.5 cm high, 5.5 cm inside diameter, and 150 ml capacity. A 2.5-kW muffle furnace is used for heating the reactor. The reactor is fitted with temperature and pressure sensors, as well as pressure relieve valve, as shown in Fig 3. series, equipped with a HP-5MS (30 m×0.25 mm×0.25 um) column [36,37]. Before analysis, the samples were filtered by a 0.45 μm filter to remove particles. Each time a 2 μL sample was injected into the GC-MS system at initial column temperature 45˚C, holding for 1 min, ramped to 150˚C with 4˚C/min; holding for 3 min, ramped up to 280˚C with 9˚C/min; holding for another 5 min. The injector was kept at 250˚C in spit mode (10:1) with the carrier gas helium. The ion source was maintained at 230˚C. Mass spectrometric measurements were performed using electron impact ionization (EI) at an ionizing voltage 70 eV, and a scanning range of m/z 50-550. Peak identification was accomplished by comparing mass spectra to the

Results
The HTC reactions of three biomass materials are performed at three temperatures (200, 250, and 300˚C). Each time 10 grams dried material and 50 grams deionized water were added in a 100 ml Teflon autoclave reactor. After HTC reactions the organic products in the water solutions were analyzed using GC-MS instrument. The physical conditions of HTC reactions and the yields of solid carbon and organic byproducts are listed in Table 1. The yield of organic products is estimated by subtracting the carbon yield from the total mass of biomass material.

Organic products from HTC reactions of bagasse
The GC-MS spectra of organic byproducts from HTC reactions of bagasse at three temperatures (200, 250, and 300˚C) are shown in Fig 4. Based on the GC-MS analysis, a total of 11, 22 and 4 organic compounds were identified when the HTC reactions were maintained at 200, 250 and 300˚C, respectively. With the temperature increasing from 200˚C to 300˚C, the number of compounds increases firstly, and then decreased. When the temperature was elevated to 300˚C, the compounds mainly were observed after 30 min residence time. The detailed compounds are listed in Table 2. The compounds were classified as phenols, aromatic hydrocarbons, and some other oxygen-containing compounds, such as alcohols, ketones, acids and esters.

Organic products from HTC reactions of cellulose
The GC-MS spectra of cellulose's HTC products at three temperatures (200, 250, and 300˚C) are shown in Fig 5. By GC-MS analysis, a total of 13, 5 and 3 compounds were identified when the temperature was maintained at 200, 250 and 300˚C, respectively. With the increase of temperature, the number of detected compounds decreased. When the temperature was 250 and 300˚C, the compounds mainly were found after 25 min residence time. The detailed information of compounds is listed in Table 3. The compounds were classified as phenols, aromatic hydrocarbons, alkanes, and some oxygen-containing compounds, such as alcohols, ketones, acids and esters.

Organic products from HTC reactions of lignin
The GC-MS spectra of hydrothermal products of lignin at three HTC temperatures (200, 250, and 300˚C) are summarized in Fig 6. Based on GC-MS analysis, total of 19, 14 and 3 compounds were identified when the HTC temperature maintained at 200, 250 and 300˚C, respectively. As shown in Fig 5, when the temperature increased to 300˚C, the compounds only were observed after 30 min residence time. This phenomenon was similar with that of bagasse and cellulose. The detailed compounds are listed in Table 4.
The composition of the chemicals became simple as the temperature increased to 300˚C. The only main compound was dibutyl phthalate, whose shares in all organic products increased to 65.242%, much higher than that at 200 and 250˚C. The contents of 2,2'-methylenebis[6-(1,1-dimethylethyl)-4-methyl-phenol and adipic acid dioctyl ester were also higher than that at 200 and 250˚C.

Discussion
Based on the experimental results it was observed that many valuable organic products could be produced from the HTC reactions of bio-refined components of sugarcane bagasse. Temperature of HTC reactions has significant impact on the chemical composition of the organic byproducts. Many types of organic compounds were produced at the temperature range of 200 to 250˚C for the HTC reactions of bagasse with its component (cellulose and lignin). As the temperature increased to 300˚C, the various types of organic products derived from the HTC reactions, decreased gradually. This is as a result of higher molecular polymeric compound which couldn't be detected via GC-MS, thus increase in temperature consequently lead to less identified organic species. Also at 300˚C, the hydrothermal organic products of bagasse, cellulose and lignin shared a common character: dibutyl phthalate was the dominant component, meaning that at higher temperature, esterification was the dominant reaction, which  Study of bagasse and components allows the active fragments of acids to react with phenols or alcohols and form ester compounds. The most abundant compounds, derived at 250˚C from the HTC reaction of bagasse were phenolic-related componds, which was also observed at the reaction of lignin at 200˚C, 50˚C lower than the bagasse. It has been reported that the phenolic compounds mainly forms from the breakage of β-aryl and benzoylether bonds in lignin and further decomposes to phenolic products, such as benzenediols [40]. In bagasse, the lignin present was chemically connected with cellulose by chemical bonds and it requires higher energy to make these bonds to rupture, so the reaction temperature was higher than that of lignin. Unlike bagasse and lignin, the most abundant compounds, derived from cellulose, were ketones, followed by the esters, and finally the phenolic compounds were the lowest components.
In the HTC reactions of bagasse and its two refined components (cellulose and lignin), more organic byproducts (13+19 compounds) were produced from cellulose and lignin than that from bagasse (22 compounds). Particularly, high valuable catechol products, around US$ 2000~3000/ton [41], were obtained from the HTC reaction of lignin at 250˚C with 56.8% production shares in the total GC-MS integral areas. In contrast, the GC-MS integral area of Study of bagasse and components catechol products from the bagasse HTC reaction was only 2.263%. In HTC reaction the organic byproducts from lignin make up more than half of the total mass of lignin, indicating that lignin is a chemical treasure storage [42]. Finally in general, bio-refinery and HTC are two effective techniques for the valorization of bagasse and other biomass materials from agricultural that are environmental pollutants in many cases [32,[43][44][45]. HTC could convert the inferior biomass to superior biofuel with higher energy quantity of combustion, at the same time many valuable organic byproducts will be produced. Bio-refinery makes the HTC reaction of biomass more effective. With the help of bio-refinery and HTC, bagasse and other biomass materials are not only the sustainable energy resource, but also the renewable and environment friendly chemical materials, the best alternatives for petroleum, coal and natural gas.