BAILs mediated Catalytic Thermo Liquefaction (CTL) process to convert municipal solid waste into carbon densified liquid (CTL-Oil)
Graphical abstract
Introduction
The escalated global energy consumption with the fast depletion of fossil fuel reserves leads to a continual escalation in oil and gas prices. Furthermore, a significant increase in the emission of SOx, NOx, and other greenhouse gases (Exxon-Mobil, 2012, Gracida-Alvarez et al., 2016) created environmental dilemma. Thus, an energy harnessing from alternative abundant sources such as lignocellulosic biomass, wood and municipal wastes provides an effective solution to curb the issues like increased global energy demand and environmental disputes (Nashawi et al., 2010, Slade et al., 2011). Presently, the disposal of municipal solid waste (MSW) generated daily and continuously has become a major problem as well as a challenge for stakeholders in waste management. The humongous amount of waste was generated across the globe and it is increasing tremendously and gradually. The population dense country like India generates on an average of 0.5 Kg/day MSW per capita (Kumar and Samadder, 2017, Beyene et al., 2018, Kumar et al., 2015). Thus, the extent of MSW produced was increasing rapidly and exponentially, which reflected in the form of giant garbage mountains at landfills (Chattopadhyay et al., 2009). Hence, the safest and cost-effective management/disposal of MSW has become an urgent environmental need for modern society (Goal, 2008).
The physical composition of generating MSW is equally miscellaneous which depends on living habitats, living standards and sources of generation hence it could not be predicted precisely (Edjabou et al., 2015). Basically, in an India MSW can be categorized into four major fractions such as; (a) organic biodegradable wet waste: majorly comprises of residues of food, plants, agriculture, garden wastes, and paper, etc. (b) dry incinerable waste: majorly composed of paper, wood, cotton pads, clothes, disposable diapers, sanitary napkins, etc. (c) recyclable and non-biodegradable waste: majorly composed of glass, metals, plastics, leather, etc. and (d) Inorganic and recyclable: majorly composed of debris, ceramics, etc. (Kinnaman, 2009).
Of the possible fractions, inert materials, construction materials, glass, ceramics, plastics, and other inorganic fractions were segregated during the MSW pre-processing. On an average MSW produced in India comprises nearly 70% of organic biodegradable fraction (Zhou et al., 2014). In present-day MSW treatment methods, the wet organic biodegradable fraction was most popularly utilized for composting and anaerobic digestion to produce compost and biogas respectively. While, dry incinerable fraction was subjected to incineration for power generation (Colón et al., 2012, Valentino et al., 2018). The composting, incineration, RDF (Refuse Derived Fuel) burning and dumping in landfills have several major disputes such as; long processing time, the requirement of the large footprint area and environmental pollution, etc.
The exploitation of MSW for energy generation has been promoted considerably for efficient and rapid disposal as well as revenue generation. Thus, the thermochemical processing of MSW gaining popularity because of its potential to produce relatively clean energy as compared to other possible options (Wei et al., 2017). The requirement of dry MSW is the major drawback of present-day thermochemical treatment methods thus a huge amount of energy will be utilized for drying of feedstock and creates a substantial gap between the energy generated and overall energy utilized in the process (Diego et al., 2016, Brown, 2011). However, of the possible thermochemical methods, Hydrothermal liquefaction (HTL) was found to be a promising solution for converting wet organic waste into liquid bio-crude oil (Gollakota et al., 2018, Elliott et al., 2015, Lange, 2018). The conversion of wet waste into liquid bio-crude oil seems to be a beneficial option in various aspects such as; easy transportation, storage and low GHG (greenhouse gases) emission (Ramirez et al., 2015). As a result, an extensive amount of research has been conducted and conveyed in past years on the HTL process to obtain high conversion efficiencies.
The HTL of a broad range of biomass feedstocks such as lignocellulosic biomass, manure, algae and biomass was conducted at elevated temperature 250–400 °C and pressure 40–220 bar. Typically, these reaction conditions allow the breakdown of biomass components into a liquefied substrate called “Biocrude Oil” (Tian et al., 2014). The different attempt made in the literature for liquefaction under different reaction conditions were summarized in Table S1. Several catalysts including alkali (Na2CO3, NaOH, KOH, K2CO3), and transition metals (Pd, Pt, Ru, Ni, HZSM-5, and Raney) have been widely explored for HTL to increase the yield of biocrude oil (Gollakota et al., 2018, Duan and Savage, 2011). The use of heterogeneous catalysts was preferred due their ease of recovery and reuse in comparison with homogeneous catalysts. But in the case of liquefaction, it is difficult to recover solid catalysts from unreacted solid substrate/residue or char. Moreover, in the case of complex feedstock like MSW, there were possibilities of deactivation and decomposition of heterogeneous catalysts (Pieta et al., 2018).
Organic solvents such as ethanol, 1-octanol, and phenol in combination with catalysts are also studied for HTL (Lu et al., 2014, Chen et al., 2012). But the use of organic solvents leads to an operational limitations and separation issues which increases the process complexity. Ionic liquids (ILs) were explored as a promising homogeneous catalyst for liquefaction of biomass due to their characteristic properties such as negligible vapour pressure, high thermal stability and great dissolution ability (Parvulescu and Hardacre, 2007). Fu and Mazza, (2011) have reported liquefaction of wheat straw, in presence 1-butyl-3-methyl-imidazolium chloride to obtain 35% yield of bio-crude oil at 320 °C. Xi and Shi, (2010) have reported liquefaction of wood by using 1-butyl-3-allylimidazolium chloride to obtain 57.2% yield of bio crude oil. Lu et al., (2014), have reported 1–(4-Sulfobutyl)-3-methylimidazolium hydrosulfate Ionic Liquid/1-Octanol system for direct liquefaction of biomass. The characteristic polarity, better dispersion, and dissolution of biopolymeric structure in ILs have a substantial effect on the dissolution of biomass. Thus, the ILs for biomass liquefaction can provide a better platform to produce liquefied biocrude oil.
The HTL was an excellent process for converting waste into biocrude oil to recover the energy from waste, but still there are several drawbacks associated with the present HTL processes such as; (a) use of stringent reaction conditions, (b) formation of char and gases, (c) use of reactive gases such as H2 for reaction (d) non-recyclability of catalyst, etc. (Kim et al., 2019). The substantial formation of char and gases due to uncontrolled side reactions resulted in the loss of carbon causing low biocrude oil yield with the formation of char and gases. ILs have been extensively studied for liquefaction and pretreatment of biomass due to their characteristic property of better penetration into the complex cell wall structure of biomass which increase the dissolution (Brandt et al., 2013). Owing this advantage of physicochemical properties and tunable acidity by changing counter ion, BAILs are selected as a catalyst for the liquefaction of organic degradable waste. The present manuscript explored BAIL mediated Catalytic Thermo Liquefaction (CTL) process to convert organic biodegradable MSW into a carbon densified liquid called “CTL-Oil”. A series of imidazole based BAILs with variable counter ions were -synthesized (Fig. 1) and tested for CTL of organic biodegradable MSW. Various MSW samples were soured from the local area and tested for their compositional behavior followed by the CTL to study the influence of variable counter ions. Moreover, the process intensification study was conducted to check the influence of reaction parameters such as temperature, time, catalyst concentration, reusability and recovery of ionic liquid etc. Detailed analysis of CTL-Oil was conducted by using GC–MS (Gas chromatography–mass spectrometry), ATR-FTIR (Attenuated total reflectance-Fourier transform infrared), elemental and calorific value analysis. The plausible reaction pathways and mechanism for BAILs mediated CTL process was proposed.
Section snippets
Materials and experimental method
All the chemicals and chemical reagents were purchased from synthetic grade chemical suppliers and used for the reaction without further purification. Solvents used for the reaction were procured as synthetic grade solvent and used for the reaction after distillation and drying. The chemicals and solvents used for HPLC (High Performance Liquid Chromatography) mobile phase preparation were HPLC grade chemicals and purchased from S. D. Fine. The standards for HPLC analysis were purchased from
Proximate analysis of organic biodegradable waste
In the present study, the organic biodegradable fraction, majorly composed of garden waste, kitchen waste, road swipe, vegetable waste, papers and cardboard was used for the CTL. Physically composition of organic biodegradable MSW is also highly unpredictable, but chemically it contains > 90% of organic biodegradable solids which are available for CTL reaction (Table 1).
Scientifically, organic biodegradable solids present in the MSW were composed of lignin, cellulose, hemicellulose, pectin,
Conclusion
In the reported study, BAIL mediated CTL process was explored to convert organic biodegradable waste into carbon densified liquid (CTL-Oil). The CTL process provides a simple, non-complex and non-stringent alternative pathway for the liquefaction wet organic biodegradable MSW. The remarkable effect of BAILs as a catalyst, variability of counter ion and reaction conditions was studied. To the best of our knowledge this is the first report to explore BAILs and the influence of counter ion for
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The authors are grateful for the financial support from the Department of Biotechnology (DBT), Ministry of Science and Technology India.
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