Full Length ArticleInsights into the evolution of chemical structures in lignocellulose and non-lignocellulose biowastes during hydrothermal carbonization (HTC)
Introduction
To our knowledge, industrial biowastes are universally considered as byproducts/wastes derived from industrial activities, showing significant characteristics of enormous amount and various species with the development of industrialization process in the past decades all over the world, especially in developing countries [1], [2]. In fact, the effective disposals of industrial biowastes have faced an increasingly serious issue nowadays due to the environmentally unfriendly substances therein. Most of industrial biowastes (sewage sludge, antibiotic residues, etc.) are contaminated with heavy metals, pathogens and a range of hazardous organic components, thereby posing threats to the soil, plants, animals and humans [1], [3], [4]. According to preliminary statistics of world health organization (WHO, data from 2016), 12.6 million people have died from living or working in an unsafe environment caused by unregulated industrial activities, accounting for 23% of all deaths. When accounting for both death and disability, the fraction of the global burden of disease due to the environment is 22%. Consequently, traditional treatments such as landfill, incineration and fertilizer-production are no longer applicable for the sake of human health and environmental protection. However, industrial biowastes can be classified into a type of nontraditional biomass owing to its abundant organic matters introduced during industrial processes, which make the energy utilization become possible [3], [5], [6], [7], [8]. For these reasons, a feasible and economic technology is necessary for solving the pollution and toxic problems associated with industrial biowastes, and concurrently for recovering their potential energy.
Thermochemical and biochemical conversions are recommended as the appropriate treatment methods due to the dual benefits both in environmental safety and resource reuse. Among them, hydrothermal carbonization (HTC) has been developed with the purpose of upgrading biowastes in the presence of water, which can simultaneously avoid the over-consumption of energy in drying process [6], [8], [9]. Based on the previous literatures, it is commonly accepted that hydrolysis and aromatization are two important reactions during HTC process [10], [11]. The former results in the cleavage of linkage bonds in biomacromolecule, and the latter subsequently leads to the formation of coal-like structures [11], [12], [13], [14]. Funke et al. [10] stated that aromatic structure showed a strong stability under hydrothermal environment, which could be similarly considered as a basic building block for the formed HTC-coal. Cross-linking condensation of aromatic rings during HTC process could produce the major constituents similar to natural coal, indicating a good comparability between HTC and natural coalification [6], [13]. He et al. [6], [9], [14] and Zhao et al. [7], [8] also suggested that HTC was an essential thermochemical process focused on simulating natural coalification and converting biowastes into coal-like fuels (hydrochar) under moderate temperature (120–350 °C), holding time (5–60 min) and autogenous pressure (2–16 MPa). Therefore, HTC is served as a key intermediate step optimizing the chemical structures of solid fuels that is subject to the subsequent combustion or pyrolysis [5], [6], [15]. The relationship between hydrothermal conditions and chemical structures of hydrochars is a long-term hot research field. A comprehensive knowledge of mechanisms controlling the evolution of biowaste structures and subsequent structure-reactivity is of crucial for the terminal application of hydrochars.
Chemical structures of hydrochars include but not limit to carbon skeleton, crosslinks of aromatic polymer, surface porosity, functional groups and ultimate components [13], [14], [16], [17]. All of these characteristics can be changed significantly via HTC, thereby influencing the reactivity and fuel properties of hydrochars. For example, He et al. [6], [14] reported that the increase of aromaticity was related to the condensation and polymerization of soluble intermediates from organic matters, which enhanced the coalification of biowastes. Harvey et al. [17] also investigated the primary reactions in HTC, and elucidated the more condensed structures of hydrochar from R-CH2-R to R = C = R by means of two-dimensional perturbation correlation infrared spectroscopy (2D-PCIS). This variation of carbon bonds was further confirmed by the results of Xin et al. [18]. Additionally, Keiluweit et al. [16] revealed the dynamic molecular structures of plant-derived bio-char across a temperature range of 100–700 °C, indicating the progressive transformations of transition, amorphous, composite and turbostratic phase in carbon skeleton structures. More recently, Wei et al. [11] used sewage sludge as feedstock and highlighted that polycyclic aromatic components were the final products of coke, leading to the similar properties to low-rank coal. However, fundamental understandings for the evolution pathways of chemical structures in relation to various biowaste-derived hydrochars are still lacking.
The primary objective of this study is to fill up the knowledge gap on the evolution routes of chemical structures in various biowaste-derived hydrochars during HTC process. Two biowaste types are taken into account for biowastes variability: a lignocellulose one (Herbal tea waste, HTW) and a non-lignocellulose one (Penicillin mycelial waste, PMW). The specific goals of this study are to: 1) identify the difference of biowastes in components and thermal properties; 2) discuss the fundamental characteristics of hydrochars derived at various temperatures; 3) investigate the variation of chemical structures, including aromatic and crystalline structures (characterized via XRD and Raman analysis), carbon skeleton structures (characterized via XPS and 13C NMR analysis) and surface functionalities (characterized via FTIR and 2D-PCIS); 4) establish two comprehensive evolution diagrams for lignocellulose and non-lignocellulose industrial biowastes during hydrothermal process.
Section snippets
Feedstock preparation
Two industrial biowastes used in this study was well characterized by their intrinsic components: lignocellulose (HTW) and non-lignocellulose (PMW). HTW was collected from a traditional Chinese medicine enterprise in Guangdong province, and PMW was sampled from an antibiotic production enterprise in Hebei province. Samples were directly dried at 105 °C for 24 h, and then followed by a series of pretreatments including mechanical-pulverizing, sieving and re-drying. Afterwards, dry samples with
Physicochemical properties of biowastes
According to the component analysis (shown at Table 1) of two biowastes, it was obvious that PMW belonged to non-lignocellulose type due to its considerable amount of protein (almost 50 wt%) and few crude fibers (<0.4 wt%) [1], [27]. Most of carbohydrates in PMW might be attributed to polysaccharides that were artificially added during pharmaceutical process. In comparison, HTW contained more fiber (>20 wt%) and less protein (14.67 wt%) than PMW, implying the properties of HTW was similar to
Conclusions
In this study, two types of biowaste (namely, HTW and PMW) were chosen for the HTC experiments at various temperature regions (120–300 °C, with an interval of 30 °C). HTW was a type of lignocellulose biowaste composed of cellulose, hemicellulose and lignin (14.67% of protein, 80.60% of carbohydrate, and 4.73% of lipid). PMW belonged to non-lignocellulose biowaste as it mainly consisted of proteins (49.47%) and polysaccharides (42.69%). Prior to HTC experiments, PY-GC/MS and TG-DTG analysis were
Acknowledgements
The authors thank the National Natural Science Foundation of China (No. 51661145022), the National Natural Science Foundation of China (No. 51676195), the Special Program for Key Basic Research of the Natural Science Foundation of Guangdong Province (No. 2017B030308002) and the Science and Technology Program of Guangzhou (No. 201803030006) for financial support to this work.
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These authors contributed equally to this work.