The consistent demand for energy with simultaneous concern for environment and climatic changes along with unsustainable fossil fuel resources has attracted the human mankind to produce renewable and clean energy. Renewable energy derived from solar, hydro, tidal, wind, and geothermal fuel sources which is alternative conventional fossil fuels have enormous potential to sustain an eco-friendly approach is required to consolidate bio-economy. Among these energies, bio-fuel produced from biomass with high carbon content at low cost has an imperative role among renewable energies.The demand for bio-fuels globally has necessitated the development of technologies based on energy cropswithout affecting food security and environment. Biomassderived from agricultural residues, energy crops, aquatic plants etc., are an ideal carbon resource for the production offirst generation bio-fuelslike bio-ethanol and bio-diesel (A.R.K Gollakota et al., 2018),chemicals and advanced bio-fuels (bio-hydrogen, bio-gas, bio-oil, bio-char, syngas etc.) Biomass is derived from organic material such as trees, plants, agricultural residues and urban waste comprising stalks, straws, leaves, seeds, nut shells, roots etc. It is subjected to various conversions, bio/ thermochemical conversions techniques to produce energy and biofuels.The agri waste includes straws of rice, wheat, sorgum, bajraetc, with sorghum being an important cereal crop grown globally that can be grown in multiple environmentshavingdrought tolerance. It is in the form of a grain with sweet, forage and low-lignin content present in abundance at the researchers place, near telanganga, india. (Olivera S. et al., 2020)Sorghum (Sorghum bicolour L) is an important staple cereal crop with high phototsyntetic efficiency, which is mostly grown under rain fed conditions in marginal lands and can be effectively used as a feedstock for the production of biofuels or chemicals (Yang Yue et al., 2018). Numerous researchers (Zheting, Bi et al., VarshaNiroula., Thabo, z et al.,) have worked on conversion of sorgumbaggase into various products. (Olivera S.et al., 2020), have presented overview on conversionmethodologies of sorghum biomass into liquid fuels (ethanol, diesel and oil), gaseous fules (hydrogen, bio-gas and syngas) and solid fuels (bio-char). They have narrated in detail, the developments of various pretreatment conversion process and fermentation processes. They have also analysed the complete lifecycle analyses which indicate the challenges, opportunities that augment the proficiency, benefits and tasks in various steps in problems and solutions. The biomass is converted into bio-oils using thermochemical process likecombustion, gasification, pyrolysis and direct liquefaction, where the biomass is pressureized to high pressures. HTL (Christensen et al., 2014) is an emerging technology in conversion of biomass to bio-oils and is particularly useful for wet feedstocks like organic residues, sludges, and is a direct liquefaction process involving high pressures (5‒24 MPa), moderate temperatures(250–375°C), presence of water just below its critical point, absence of oxygen with or without catalyst to produce bio-crude as the major product and small amount of byproducts like solid, gas and aqueous phases were resultedin the presence of water. The yields of bio-oils derived from HTL depends on the source of biomass, operating conditions, initial solvents, catalysts etc.The applicability of wet biomass in HTL direcly without thermal heating is an advatangeous outcome compared to other processes and in reduced cost analysis. (Yang et al., 2014) and without chemicals (Leichang Cao et al., 2016; Christensen et al., 2014; Gan and Yuan, 2014) and less corrositivity (Cybulska et al., 2010). A brief and short review on HTL is narrated by (Gollakota A.R.K et al., 2018). An HTL process on lignocellulosic biomass at 200–350°C, 5–30 MPa was done by and have achived excellent results on bio-oil yield in comparision with (Cybulska et al., 2010). Lignin a by product of biomaas was subjected to HTL to obtain oligomers by (Yuan et al., 2007) using phenol and sodium hydroxide as solvent and at a temperature 260°C for residence time of 60 min. (Liu et al., 2008) used pinewoodwith various solvents obtained the optimum bio-oil yield of 26.5 wt% with a 20minutes retention time.(Shuna, C et al., 2010) derived bio-oil from pine saw dust using HTL with various co-solvents such as water, ethanol and methanol, comparisionof these 3 the most efficient solvent is water. (Shuana et al., 2020) used white pine sawdust to achieve the maximum yield of 65 wt% at 300°C for 15min using two different solvents like water and alcohol. A similar study on saw dust was explored by (Flabianus H et al., 2017) at temperature (180–260°C), reaction time (0–2h) and to yiled an aqueous product yield of 57.2 wt%. The effect of co-solvent on rice straw with varied biomass to solvent ratios and catalysts were explored by (Yerrayya et al., 2020) to obtain a boioil yield of 12.3 wt% at 300◦C with 18MPa pressure in 60min and also identified that the yield increased by 36.8wt% using methanol. (Demirbas et al., 2008) have obtained similar observations using sawdust with different solvents like Na2CO3 and NaOH in the range of temperature 440 to 560K. Similarly, above mentioned researcher used hazelnut shell as biomas with the temperature (250–330°C) and solvent, finally resulting the bio-oil yield decrease with high temperatures. (Yang et al., 2014) conducted HTL on solid biomass at 200–350°C temperature, 5–30 MPa pressure to generate biocrude without oxygen be used as a transportation fuel, which compared is less corrosive than that reported by (Cybulska et al., 2010). Similarly, the works of (Yuan et al., 2007) using water, sodium hydroxide and phenol for degrdatation of high molecular weigth lignin into oligomers at 260°C for 60 min have resulted in successful results. The concept of micro reactor technology for HTL was carried by (Zhe Zhu et al., 2018) barley straw with potassiumcarbonate into the production of bio-crude. The process variables such as temperature, time, catalyst and biomass to water ratios were further optimized with central composite design (CCD) to achieve the optimum condition at 304.8°C, 15.5 min with the bio-oil yield of 38.72 wt%.The effect of cosolvents HTL of rice straw process was studied by (Leichang Cao et al., 2016) in a high-pressure batch reactor. The results on analysys were found that 5 wt% of Na2CO3 at 260°C for 1 h, achieved a bio-oil yield of 50.31 wt%. The effect of CuZnAl catalystcatalyst on the yield of monomeric phenols like 2,6-dimethoxy-phenols and 4-ethyl-phenolbased on bio crude derived from rice straw was studied by (Zhou, C et al., 2016) at yield 26.8wt% at temperature 300°C and reaction time 30 min. (Syamsul Hadi et al., 2014) obtained bio-fuel from rice husk using ethanol as solvent in a HTL process at 325°C and reaction time of 30 minutes. The resulting biooilwith an optimum yield 36.3 wt% was further analysed for physical properties, which indiacted in the presence of lighter fuel oils with a calorific value (12.1–20.2 MJ/kg), viscosity (1.11–1.6 cSt), and flash point (14–29°C). The bio-oil yield of 65 wt % was obtained at 300°C with alcohol for 15min. Conversion of biomass to bio-oil above 300°C temperature repolymerization occurs.The present researchers, (Monika Kashimalla et al., 2021) in their current research have explored using biomass as sorghum bagasse with the effect of process parameters such as temperature (220 to 280°C), time (15 to 60 min)with Na2Co3 catalyst were systematically studied and analysed CHNS, FTIR and GC-MS.To optimize the process CCD was used with the process parameters. The maximum yield of 45.23 wt% was obtained at 260°C for 60 min.
In this work, the researchers initiated this process as there is very little research fouind in the application of HTL for sorgum bagasse. (Zheting Bi et al., 2017)produced bio-oil from sweet sorghum bagasse using HTL process with various catalysts like potassium carbonate (K2CO3), potassium hydroxide (KOH), formic acid (CH3COOH), Ni/SiAAl, Ni2P and zeolites at 300 and 350°C. They observed that the three catalysts K2CO3, KOH, and Ni/SiAAl have resulted in higher yields of bio oil (61.8%, 42.3% and 45.0% ) with low yields of biochar at 300°C, with K2CO3 leadingto higher heating value (HHV) of 33.1 MJ/kg. Similarly, (VarshaNiroula 2014) have used sweet sorghum bagasse with and without catalyst to prodcuce biooil using HTL under varying tmeperaures, catalyst concentrations and reaction times. The process without catalyst has resulted in a bio oil of 7.517%, which has been significantly improved to 45.28% with K2CO3catalsyt. The experimental results were fitted to a cubic model, which on prediction has achivedthe yield of 57% with potassium carbonate (K2CO3) at 320oC for 60 min. The resulting bio-oil was analysed by CHNS analysis with high carbon and low sulohur content.Table 1 indicates the studies conducted by various researchers on application of HTL process to soirghum biomass.
In the present work, HTL for production of bio-oil with sorghum biomass is conducted and the results are analysed to achived the desired yields.
Table 1
Literature survey on sorghum
| S.No | Feedstock | Operating conditions | Solvent/co-solvent | Catalyst | Bio-oil yield % | References |
| 1 | Sorghum bagasse | 300–350ºC | water | K2CO3, KOH, formic acid, Ni/Si-Al, Ni2P and zeolite | 45% @ K2CO3@ 300ºC@60min | Zheting Bi, Ji Zhang etc |
| 2 | Sweet sorghum bagasse | 300ºC | water | K2CO3 | 45.28%@ K2CO3@300ºC @60 min | VarshaNiroula |
| 3 | Sweet sorghum bagasse | 260–320°C | NaOH | ...... | 53.2 wt%@320 ºC | Thabo,z. Sehume, christien. Strydom etc. |
1.1. Process mechanism of HTL
HTL (Leichang Cao et al., 2016) process consists of conversion of biomass into bio-oil which is a multi mixtureof carbohydrates, lignin, proteins, and lipids is depolymerized into crude oil involving several stages like depolymerisation, decomposition and recombination at molecular level thorughdecomposition, depolymerisation and is particularly advantageous for wet biomass which is highly heterogenous and complex. In HTL process, the biomass is decayed and de-polymerized into smaller moleculesthat are highly reactive, to form bio-oil, solid residue and gaseous products. The variousprocessinputs governing process like pressure, temperature, residence time, repolymerization, condensationetc needs to be monited and controlled. Biomas undergoes depolymerisation where in the biomass is sequentially dissolved, with the underlying hemicellulose and cellulose biopolymers undergo a thermal stability change contribtubing to the formation of bio oil. At high pressure and temepraure, the long C-H-O bonds wil be reduced to shorter chains, with energy changes in presence of water. Subsequent to depolymerization, decomposition takes place comprising dehydration, decarboxylation and deamination. Reduction of water, carbondioxide and amino acid is likely to occur in dehydration, decarboxylation and deamination. The macromolcues present in biomass are hydrolizedto oligomers and monomers. Glucoe monomers are generated from cellulose due to the presence of water at high temepraure and pressure. It is understood that the presence of fructose in constrast to glucose, degrades rapidlyintop valuble by products due to various reactions like isomerisation, hydrolysis, dehydration, reverse-aldol defragmentation, rearrangement and recombination reactions etc. The products like polar organic chemicals, furfurals, glycoaldehydes, phenols and organic acids are observed to have highly solubility in water.