Hydrothermal carbonization of typical components of municipal solid waste for deriving hydrochars and their combustion behavior

Highlights

• The curves of waste paper hydrochars was shifted to upward and high combustion zone.

• HTC-derived hydrochar has a more thermally stable structure than pure waste wood.

• None enrichments on combustion loss rate were observed for waste food hydrochars.

• The first loss peak of waste rubber hydrochars became significantly intense.

• Almost no chemical reactions occurred during HTC of polypropylene and polyethylene.

Abstract

In this work, five typical components were employed as representative pseudo-components to indirectly complete previous established simulation system during hydrothermal carbonization (HTC) of municipal solid waste. The fuel characteristics and combustion behavior of HTC-derived hydrochars were evaluated. Results clearly illustrated that the energy ranks of hydrochars were upgraded after HTC. For paper and wood, superior combustion performances of their hydrochars could achieve under suitable conditions. While for food, none positive enrichments on combustion loss rate were observed for hydrochars due to its high solubilization and decomposition under hot compressed water. It was noteworthy that a new weight loss peak was detected for paper and food, suggesting that new compounds were formed. For rubber, the HTC process made the properties of styrene butadiene rubber more close to natural rubber. Therefore, the first peak of hydrochars became significantly intense. While for plastic, only physical changes of polypropylene and polyethylene were observed.

Introduction

Due to their simplicity to perform (Prawisudha et al., 2012), the elimination of an energy-extensive drying process (Danso-Boateng et al., 2015, Zhao et al., 2014b), high energy yield (Kim et al., 2014, Lin et al., 2015) and fewer pollutant gases (Berge et al., 2011, Parshetti and Balasubramanian, 2014), hydrothermal carbonization (HTC) process has been considered as one of the most effective and promising thermochemical upgrading technology for municipal solid waste (MSW) (Zhao et al., 2014a). The energy density of HTC-derived hydrochars are upgraded compared to raw MSW (Kim et al., 2015, Lu et al., 2011). And its nature properties are more easily for dewatering, grinding, handling, transport and storage (Zhao et al., 2014a). Subsequently, energy can be recovered by combusting or co-combusting MSW hydrochar with coal (Kim et al., 2015, Lu et al., 2011, Muthuraman et al., 2010). Preliminary investigations clearly showed that HTC process exerted significant effects on the thermal behavior (Lin et al., 2017, Lin et al., 2016b). Under suitable HTC conditions, hydrochars could achieve relatively excellent thermal conversion performance (Lin et al., 2016b). Therefore, these studies had proved that HTC can be performed as both environment-friendly MSW management and efficient energy recovery technology. Moreover, some HTC pilot-scale applications had some successful cooperation cases (Chinnathan Areeprasert et al., 2016, Hitzl et al., 2015, Lu et al., 2014), suggesting its huge application prospects.

Unfortunately, because of complicated components in MSW and undetectable of intermediate process, more-detailed and clear reaction mechanisms are largely unknown. Previous studies had tried to model only a single representative or specific fraction of MSW from its basic constituents, such as paper (Weiner et al., 2014), waste wood (Gao et al., 2016, Zheng et al., 2016), waste food (Deb-Choudhury et al., 2014, Li et al., 2013), waste rubber (Zhang et al., 2016), waste plastic (Poerschmann et al., 2015), lignocellulosic component (Hoekman et al., 2011) and protein (Teri et al., 2014). However, the variation of these studies from researchers to researchers, some results were only meaningful and reproducible for specific fractions of MSW. The comprehensive and systematic integrated research about specific formation pathways of MSW hydrochars is still very difficult and scarce. From this perspective, it seems unfeasible to model complicated MSW directly during HTC. As mentioned in the previous paper (Lin et al., 2016a), HTC mechanism of each representative pseudo-component will be investigated separately. Finally, envisaging integrated and systematic reaction mechanisms during the HTC of complicated MSW or mixtures will be speculated based on the HTC mechanisms of each representative pseudo-component. In the previous paper (Lin et al., 2016a), a possible conversion pathway of HTC of waste textile had been proposed. Follow this representative pseudo-component simulation research system, the remaining typical components in MSW (waste paper, waste wood, waste food, waste rubber and waste plastic) will be studied in this paper. This paper firstly focused on the fuel characteristics and combustion behavior of HTC hydrochars since it was the product fraction of primary interest. And parts of product distribution characteristics and infrared functional group analysis of hydrochars would be referred to support some inferences.