Hydrochar derived from green waste by microwave hydrothermal carbonization
Graphical abstract
Introduction
The amount of green waste (GW) greatly increases with afforestation and urbanization. For instance, GW accounts for approximately 50% of the municipal solid waste in Beijing, China [1]. GW is the biodegradable organic fraction of municipal solid waste and generally consists of grass, leaves, tree trimmings, and other similar constituents [2]. Additionally, GW also contains a high content of moisture, oxygen and alkaline earth metals, which is unfavorable for incineration. Furthermore, direct incineration of GW results in large volumes of flu gas emissions that increases air pollution levels. As a consequence, disposal of GW is a major problem affecting the environment and the sustainable development of cities [3,4]. Recently, there has been a rise in the number of studies focusing on composting techniques to recycle GW rather than landfill or incineration [1,[5], [6], [7]]. In composting, aerobic microorganisms transform organics into hygienic and biostable products that can be used as soil amendments, organic fertilizers, or as alternatives to peat in soilless cultures [8]. However, compositing still faces challenges that require circumventing such as lowering odorous gas emissions and improving compost product stability. Furthermore, the composting period is time consuming, especially for GW because of the high proportion of cellulose and hemicellulose [9,10].
In terms of the high cellulose and hemicellulose content in GW, hydrothermal carbonization (HTC) transformation has attracted increasing attention. HTC is regarded as an effective thermochemical conversion technology, which can convert biomass waste (e.g. biomass, corn stalk, municipal waste, and nutshells) into useful products [11]. At present, HTC receives significant attention because of its several advantages. For instance, HTC can be operated at mild temperatures (160–270 °C) [12] when compared with other thermochemical conversion technologies such as gasification and pyrolysis. HTC is also considered as a pathogen removal process [13]. Water is typically the reaction medium during HTC. It can serve as a catalyst or solvent for the conversion of lignocellulosic biomass [14]. Hydrochar is the major product from HTC and has a higher carbon content and calorific value than the feedstock, hence hydrochar is useful as a fuel source [15]. Furthermore, hydrochar can also be employed as a soil amendment [16], carbon-based catalyst [17] or adsorbent [18].
Compared with traditional HTC, microwave HTC (MHTC) [[19], [20], [21]] has additional advantages. Studies have reported that the MHTC process promotes faster processing times as a result of rapid volumetric heating; facilitates higher processing rates because of relatively lower residence times; improves dewaterability and most importantly consumes less energy (about half the energy required for conventional HTC processes) to convert biomass materials to valuable products [20,22,23]. The high calorific value (HHV) of hydrochar obtained by MHTC was significantly higher than that obtained by HTC under the same conditions [24].
This research firstly used MHTC to treat GW to determine the influence of the hydrothermal holding temperature, holding time and liquid-to-solid ratio on the product when subjected to the microwave hydrothermal process and to determine the hydrochar with HHV as a potential fuel which can alleviate the current energy shortage.
Section snippets
Sample preparation
In this work, the GW sample comprises fallen leaves and deadwood as the raw material derived from the green belts in Hangzhou. The GW was pretreated before using. First, the inert fractions such as pebbles and dust in the GW were removed. Thereafter the GW was milled into small particles of less than 1 mm. This study did not consider the influence of moisture content on the MHTC process. Hence, samples were oven-dried at 105 °C for 24 h before using.
MHTC conditions testing
MHTC conditions testing were conducted in a
HHV
HHV was determined to evaluate the potential value of hydrothermal carbon as a solid fuel. Fig. 1 shows how hydrochar HHV is influenced as a function of MHTC parameters. Increasing the MHTC holding temperature from 130 °C to 190 °C results in the hydrochar HHV increasing from 17.91 ± 0.08 MJ kg−1 to 23.01 ± 0.06 MJ kg−1 with a corresponding holding time of 1 h and a liquid-to-solid ratio of 8:1 (Fig. 1a). Compared with the HHV of the raw GW (18.15 ± 0.09 MJ kg−1), the MHTC holding temperature
Conclusion
Using RSM design, a maximal hydrochar HHV (∼23.01 MJ kg−1) was produced at a MHTC holding temperature of 190 °C, a holding time of 1 h and a liquid-to-solid ratio of 8:1. When subjecting the GW-derived MHTC hydrochar to holding temperatures of 130–190 °C, hydrochar HHV and hydrochar yield were positively and negatively correlated to MHTC holding temperature, respectively. SEM observations revealed hydrochar microspheres at an MHTC holding temperature of 130 °C, a holding time of 1 h and a
Acknowledgements
This work was financially supported by the National Natural Science Foundation of China (51778579, 41471408, and 51608480), and Natural Science Foundation of Zhejiang Province of China (LQ16E080001).
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