Co-hydrothermal carbonization of digested sewage sludge and cow dung biogas residue: Investigation of the reaction characteristics

Highlights

• Digested sewage sludge (DSS) and cow dung biogas residue (CDBR) were recovered energy using co-HTC method.

• The reducing O/C and H/C atomic ratios revealed that the dehydration was main reaction during the co-HTC.

• The interaction of DSS and CDBR can be confirmed for TOC value of mixture was far exceeded that of the raw materials.

• The combustion properties of hydrochar were improved.

Abstract

In order to recovery energy from digested sewage sludge (DSS) and cow dung biogas residue (CDBR), co-hydrothermal carbonization (co-HTC) was employed to obtain the hydrochar for the use as solid fuel. Carbon content and higher heating value of hydrochars were increased, while the O/C and H/C atomic ratios reduced, revealing the dominance of the dehydration reaction during the co-HTC of DSS and CDBR. The interaction between DSS and CDBR or their derivate intermediates took place during the co-HTC for the TOC value exceeded far more than that in the raw materials and the intensity of mixture was lower than pure material. The CDBR can notably promote hydrochar formation during the co-HTC with DSS, resulting in a high fraction of nitrogen content in hydrochar of DSS. The combustion properties of hydrochar were improved notably when CDBR was added, evidencing by the increasing ignition temperature and the comprehensive devolatilization index. The activation energy of the hydrochar from co-HTC increased notably especially the hydrochar with 75% CDBR. This study suggested that co-HTC of the mixed feedstocks was a promising strategy to prepare a high-quality hydrochar for recovery energy.

Introduction

The effective disposal of digested sewage sludge (DSS), generated from wastewater treatment plants, has become a serious issue worldwide for its huge yield, the lack of effective method for processing and the strict environmental regulations of governments [1]. For its poor dewaterability, DSS often contains about 80–85% water after mechanical filtration. In general, DSS has been considered as a resource to generate renewable gas [2], biofuels [3] or solid fuel [4] from the organic matter with the content in the range about 32%–74% in dry matter [5]. How to recovery the energy from the organic matter in DSS has been became hot topic in processing of DSS.

Many technologies have been used to recover organic matter for DSS, like composting [6], pyrolysis [7], hydrothermal [8], fermentation [9], incineration [10] and etc. Each method has its own advantages and limitations. In these methods, hydrothermal carbonization (HTC) has attached much attention for its mild condition. Generally, the reaction temperature is confined in the range of 180–250 °C and the pressure is maintained above saturation pressure to ensure the liquid state of water [11]. It is a fast and efficient thermochemical process to convert wet waste material to obtain solid residue, which is defined hydrochar. The most important advantage of HTC process to wet feedstock is that drying step can be avoided, which saves energy. Hence, this process is best suitable for DSS disposal to obtain hydrochar, which can be used as solid fuel [12], soil ameliorant [13], wastewater pollution adsorbent [14] and etc. However, compared to coal, the higher heating value (HHV) of hydrochar for DSS was still relative low for containing large amount of ash [15], which limited its application as solid fuel. Hence, co-hydrothermal carbonization (co-HTC) of DSS with other waste feedstock has been developed [16,17]. The co-HTC can notably increase HHV and decrease the amount of ash in hydrochar.

There are large amount of waste materials are produced every year worldwide, like animal manure, food waste, forestry and agricultural residues, DSS and etc. Partial of them are decomposed by microorganisms under anaerobic conditions to produce biogas, which is rich in methane. Chinese government is supporting the development of the biogas industry with program of “The National Rural Biogas Development Plan in 13th Five-Year” [18]. However, during the anaerobic digestion to prepare biogas, large amounts of biogas residues (BR), which are difficult to handle and can easily cause secondary pollution, is produced. BR contains lignocellulosic materials, which is a typical waste organic matter [19]. How to dispose BR is also very important to the healthy development of biogas industry using waste material as raw materials. It also can be used to recover energy from the wet lignocellulosic materials using HTC method. Hence, co-HTC maybe is a good method to recover energy from waste organic matter. In addition, some interactions were reported between two types of biomass during co-HTC. Lang et al. found that synergy effect occurred between swine manure and lignocellulosic biomass during co-HTC, in which the fixed carbon content and energy yield of the hydrochar were increased [20]. Zhang et al. found that the interactions between pinewood sawdust and SS during co-HTC resulted in increase of hydrochar yield, organic and carbon retentions [21]. In addition, Zhai et al. found that the energy recovery rate was remarkably increased during the co-HTC of SS with lignocellulosic biomass [22]. Hence, synergy effect possibly occurred during co-HTC of DSS and CDBR.

The main purpose of this work was to recover energy from DSS and CDBR using co-HTC method to obtain hydrochar for the use as solid fuel. During the co-HTC, the characteristics of liquid and solid phase were analyzed using different methods. The objectives of this paper were: (1) to investigate the influence of process condition on co-HTC performance in terms of HHV and carbon content; (2) to determine the physicochemical properties of hydrochar (the composition, surface functional group property and thermal decomposition property); (3) to explicate the synergistic effects during the co-HTC process of DSS and CDBR.