Characteristic evolution of hydrochar from hydrothermal carbonization of corn stalk

Highlights

• Hydrothermal carbonization of corn stalk was carried out at reaction severity of 5.05–8.29.

• The hydrochar exhibited coal-like O/C and H/C atomic ratios at the reaction severity log R₀ > 7.11.

• Correlations between the hydrochar properties and the reaction severity were modeled by dose-response functions.

• The FTIR functional groups showed different profiles with reaction severity.

Abstract

Reaction severity is a comprehensive parameter which combines temperature and time to describe the severity of a process. In this work, hydrothermal carbonization (HTC) of corn stalk was carried out at reaction severity levels of 5.05–8.29. The effect of reaction severity on the solid yield, the chemical, energetic, and structural properties of hydrochar products were studied in detail. The hydrochar yield varied from 71 to 36%. The higher heating value increased from 20.8 MJ/kg at reaction severity of 5.05 to the maximum value of 29.79 MJ/kg at severity of 8.29. When the reaction severity was greater than 7.11, the hydrochar exhibited coal-like oxygen/carbon and hydrogen/carbon atomic ratios. The evolution of microcrystalline structure for hydrochar was examined by XRD and the results showed that the component degradation led to develop hydrochar structure. The functional group results revealed that the high aromatization performance and carbonization extent of the hydrochar could be achieved with reaction severity increasing. The correlations between solid yield, carbon content, oxygen content, HHV, relative peak intensity ratios of the functional groups for hydrochar and reaction severity were fit with the experimental data. The results indicated that the dose-response models fit these characteristics data well, which would be favorable to understand and design the hydrothermal carbonization process for corn stalk.

Introduction

Biomass is an important renewable resource which can be potentially utilized to cope with the rising problem of energy shortage and environmental pollution. In past decades, various techniques such as combustion, pyrolysis and gasification have been successively applied in biomass treatment and conversion [1], [2], [3]. Hydrothermal carbonization (HTC) is currently one of the most interesting processes for biomass thermochemical conversion. HTC has some key benefits compared to other techniques, such as its simplicity to perform, mild reaction condition, wide availability of raw materials and less greenhouse gas emission. During HTC process, the biomass should be submerged in saturated water to ensure the hydrothermal reaction. The final solid residue of the HTC process, often called hydrochar, is potentially suitable for wide scale applications such as for carbon sequestration, adsorbents, container nurseries, fuels and even as soil additive [4], [5].

In the recently reported studies, various types of feedstock ranging from simple model compounds (d-xylose, cellulose, lignin, etc.) to complex biomass materials (maize silage, wheat, sunflower stem, etc.) [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18] have been treated by HTC for hydrochar production. These HTC studies have illuminated that after different reactions including hydrolysis, dehydration, decarboxylation, polymerization, aromatization, and solid–solid reactions [4], [5] most of the carbon content of feedstock remains in the solid hydrochar and the higher heating value (HHV) of hydrochar can be significantly increased. In addition, the chemical and energetic characteristics of hydrochar are dependent on reaction conditions, that is reaction temperature, residence time and solid load (ratio of feedstock and water) [9], [10], [11], [12], [16], [17], [18], [19], [20], [21], [22], [23]. However, the effect of solid load seems to be less strong as those of temperature and time [9], [10], [23]. During hydrothermal carbonization process, the feedstock is usually heated from about environmental temperature to a given reaction temperature, and held at this temperature for a certain residence time, then cooled down to room temperature. In recent years, staged temperatures in the same HTC process have been also proposed [14], [19]. During these heating, cooling and other temperature variation periods, the components of biomass may undergo significant conversion and carbonization, which may thus augment the difficulty in comparing and analyzing the experimental results. To exactly describe the correlation between hydrochar formation and reaction conditions for a whole HTC process including heating, holding, cooling and other temperature variation stages, it is necessary to employ a comprehensive reaction parameter. Reaction severity, which combines both temperature and reaction time, can be used to examine the combined effect of reaction conditions and is favorable to understand the influence of reaction conditions on formation and evolution of hydrochar. This parameter has been successfully applied in studying xylan, hemicellulose, lignin removal, and solubilization of lignocellulose materials [25]. Correlations between residual of feedstock and reaction severity have been also established and interpreted. However, little information has been provided to describe the dependence of the chemical, energetic, and structural characteristics for hydrochar on the whole process reaction severity with temperature variation. Understanding such relationships is critical in process control and parameter selection.

For the present work, corn stalk was chosen as the precursor for hydrothermal conversation. This precursor was a major agricultural waste and was abundantly available in China and its annual production was over 200 million tons in 2009–2013 [24]. In addition, as a lignocellulosic biomass which contained 35–39.6% cellulose, 16.8–35% hemicelluloses, and 7–18.4% lignin, it was a promising low-cost material for production of biofuels [25]. There were limited reports on hydrothermal carbonization of corn stalk in the past few years [26], [27]. Fuertes et al. [26] addressed the physical and chemical properties of hydrochar from corn stover at a reaction temperature of 250 °C and a 4 h reaction time, and found that the characteristics of hydrochar approached those of the low-grade coal. Xiao et al. [27] also discuss the physical and chemical properties of hydrochar from corn stalk obtained at 250 °C for 4 h, they revealed that hydrochar was lignite-like and contained a large amount of oxygen-containing groups. These studies helped to understand the physical and chemical properties of hydrochar at the given reaction condition. However, the specific aims of this work are to: (1) understand how reaction severity influences the yield, carbon content, HHV, functional groups characteristic of corn stalk hydrochar; (2) try to explore the correlations between reaction severity and characteristics of corn stalk hydrochar and provide valuable information for process scale-up.