Elsevier

Fuel

Volume 233, 1 December 2018, Pages 257-268
Fuel

Full Length Article
Impact of hydrothermal carbonization conditions on the formation of hydrochars and secondary chars from the organic fraction of municipal solid waste

https://doi.org/10.1016/j.fuel.2018.06.060Get rights and content

Highlights

  • Time, temperature primary factors impacting HTC as-received organic fraction MSW.

  • As carbonization severity increases, carbon, fixed carbon and HHV increase.

  • Thermal stability hydrochars decreased by presence of amorphous secondary char.

  • Secondary char HHV higher than primary char HHV.

  • Organic acids and furfurals in secondary char extracts peak at 220–240 °C.

Abstract

Hydrothermal carbonization of the organic fraction of municipal solid waste (OFMSW) could mitigate landfill issues while providing a sustainable solid fuel source. This paper demonstrates the impact of processing conditions on the formation and composition of hydrochars and secondary char of OFMSW. Harsher conditions (higher temperatures, longer residence times) decrease generally the solid yield while increasing the higher heating value (HHV), fixed carbon, and elemental carbon. Energy yields upwards of 80% can be obtained at both intermediate and high temperatures (220 and 260–280 °C), but the thermal stability and reactivity of the intermediate hydrochars suggest the formation of a reactive secondary char that condenses on the surface of the primary hydrochar. This secondary char is extractable with organic solvents and is comprised predominantly of organic acids, furfurals and phenols, which peak at 220 and 240 °C and decrease at higher carbonization conditions. The HHVs of secondary char are significantly higher than those of primary char.

Introduction

Global production of municipal solid waste is approximately 1300 million tons per year [1]; by 2025 annual production will reach 2200 million tons [2]. In Italy, municipal solid waste production is about 29 million tons annually [3]. A considerable amount of the organic fraction (OF), which accounts for 30–40% [4] of the total waste, is incinerated or landfilled, low-cost but polluting processes [1]. The remainder undergoes biological treatments such as composting or anaerobic digestion, which are considered more environmentally friendly technologies, but are often not economically viable because of long holding times (20–30 days). In addition, composting has a high energy consumption and CO2 footprint, with a relatively low product sale price [5]. Anaerobic digestion suffers from complexity of reactor start-up, toxic and inhibiting compounds in the OF, and process instability due to feedstock heterogeneity [1].

To address these issues, technologies such as hydrothermal carbonization (HTC) are attracting considerable attention to treat the organic fraction of municipal solid waste (OFMSW). During HTC, wet biomass is reacted in subcritical water up to 300 °C [6], over a few minutes to several hours [7]. HTC converts organic wastes into a carbon-rich solid fuel known as hydrochar, with a high energy density and heating value, high carbon content, homogeneity and grindability [8], [9]. One of the main advantages of HTC is that the heterogeneous wet biomass can be processed without preliminary pre-treatment such as the separating and drying required for pyrolysis and other thermochemical techniques [10]. For these reasons, HTC is applied to various wet residues, including: grape marc [11], off-specification compost [12], olive wastes [7], food wastes [13], digestate [14], sewage sludge [15], [16], [17] and banana stalk [18]. Our group recently demonstrated the feasibility of this technology for large-scale development through a comprehensive economic and process analysis [19]. However, despite its potential, there is no systematic study on the HTC of wet, as-received OFMSW that investigates the influence of process variables on resulting hydrochar formation. Reza et al. [6] carried out HTC tests on OFMSW pulp mixed with paper, pre-treated by steam autoclaving sterilization. Berge et al. [20] demonstrated the feasibility of HTC for mixed MSW, including paper, food, plastics, glass and metals. Lin et al. [21] tested hydrochar from MSW as solid fuel. Ingelia S.L. [22], a small enterprise commercializing HTC plants, lists data related to the energy properties and composition of hydrochar from OFMSW acquired at one operating condition.

Hydrochar forms via two pathways: (1) solid-solid conversion, in which the hydrochar maintains the original structural elements and morphology of the parent biomass; (2) aqueous phase degradation of biomass followed by polymerization of organic molecules into a solid phase [7], [23], [24]. Throughout the literature “primary char” or “char” is often used to describe the hydrochar formed following pathway (1) and “secondary char” or “coke” to refer to the amorphous solid formed following pathway (2)1. This secondary char is thought to result from sequential hydrolysis, dehydration, and isomerization during HTC that produces furfurals, as well as from cleavage reactions yielding intermediate organic acids. These dissolved intermediates can lead to precipitation of the furfurals as a secondary organic phase, which polymerize as microspheres [10], [24], [25], [26], [27], [28]. The spheres can be further carbonized by dehydration reactions, resulting in an amorphous solid that is soluble in organic solvents such as acetone and methanol [29].

It is thought that secondary char formation is promoted at high carbonization temperatures, solid loadings, and residence times. It is characterized by spherical-like structures that deposit on the carbonaceous primary char. The high carbon content and high heating value of secondary char is of interest for its potential use as a biofuel [7], [29]. As reported by Sevilla et al. [30] and Funke et al. [14], the morphology and structure make secondary char suitable for advanced carbonaceous material applications, including lithium ion batteries [31]. To date, most studies focus on the application of secondary char obtained from model compounds such as glucose and fructose. However, no one has yet systematically investigated how HTC reaction conditions affect the formation and characteristics of primary versus secondary char obtained from HTC of a heterogeneous organic residual feedstock. The present paper addresses the gaps identified in the literature by studying the influence of temperature, time and solid load on the mass yields and energy properties of the hydrochar produced. It probes the nature of primary versus secondary char formation resulting from the HTC of OFMSW in terms of composition, heating value and thermal stability. The results of this work suggest the potential for using secondary char as a source of biorefinery platform chemicals (phenols, furfurals, organic acids).

Section snippets

Feedstock

Approximately 30 kg of OFMSW was provided by AMNU, a municipal waste management company in Trento, Italy in November 2016. After elimination of some residual packaging and inert material, the biomass was shredded and homogenized using a knife mill. The average moisture content, evaluated by drying overnight in a ventilated oven at 105 °C, was 78% ± 0.4 wt%. To preserve the biomass, milled samples of ∼16 g each were stored individually in sealed plastic bags in a freezer at −34 °C. OFMSW samples

Results and discussion

Transforming OFMSW into a renewable fuel would mitigate environmental issues associated with landfilling, while providing a renewable energy source. The elemental composition of the raw OFMSW feedstock used is reported in Table 1. Elemental analysis is in accord with average values reported for OFMSW across 18 cities in 12 countries [1]. Sulphur content was lower than 0.2 wt%. The fiber analysis performed on raw biomass shows a high amount of extractives (58.8 ± 0.9%) resulting from protein,

Conclusions

The present work systematically investigates the potential to use hydrothermal carbonization (HTC) to upgrade the organic fraction of municipal solid waste (OFMSW) to energy or by-products from “as-received” feedstocks. Despite the perceived heterogeneity of such feedstocks, clear trends emerge in terms of the impact of carbonization conditions on overall and “secondary” char formation. As the severity of carbonization increases – that is, at higher temperatures and longer residence times – the

Acknowledgements

J. Goldfarb acknowledges support of the U.S.-Italy Fulbright Commission and the Boston University Initiative on Cities. L. Gao acknowledges support of the China Scholarship Council.

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