Elsevier

Energy

Volume 113, 15 October 2016, Pages 957-965
Energy

Gasification characteristics of hydrochar and pyrochar derived from sewage sludge

https://doi.org/10.1016/j.energy.2016.07.129Get rights and content

Highlights

  • Low temperature pyrolysis (LTP) of sewage sludge to produce the pyrochar.

  • Hydrothermal carbonization (HTC) of sewage sludge to produce the hydrochar.

  • The hydrochar is more hydrophobic than that of the pyrochar.

  • The hydrochar enhanced hydrogen production from sewage sludge by gasification.

Abstract

Two types of the biochars, pyrochar and hydrochar derived from low temperature pyrolysis (LTP) and hydrothermal carbonization (HTC) of sewage sludge were prepared and characterized. Their gasification properties were further experimentally evaluated. The results showed that the hydrochar was more hydrophobic than the pyrochar. The hydrochar was rich in nitrogen-containing functional groups and increased nickel, iron, alkali and alkaline earth metallic species compared to the raw sludge and pyrochar. It enhanced the interactions between the carbon surface and hydrogen bonding as well as gasification reactivity of the hydrochar, thus resulting in a higher hydrogen concentration and yield than the pyrochar under identical conditions. Additionally, the hydrochar had a more porous structure on the surface, facilitating the pores better accessible for condensable hydrocarbon molecules and thus improved the gas production and gasification efficiency. Although the energy recovery efficiency of LTP-gasification method was higher than that of HTC-gasification approach, the total energy consumption during the HTC pretreatment combined with subsequent gasification was lower than the energy evolved in the corresponding product gas in most gasification conditions. This study demonstrates that the integration of HTC pretreatment and subsequent gasification has promising potential for hydrogen-rich syngas production from sewage sludge.

Introduction

The depletion of fossil-fuel reserves and increasing environmental pollution caused by the large-scale application of fossil fuel make hydrogen an attractive alternative energy carrier and for the production of chemical products (e.g., methanol, ammonia) [1]. Currently, hydrogen is mostly generated from steam reforming of natural gas for industrial application. To achieve the goal of sustainable hydrogen production, the dependence away from fossil-fuel to renewable alternatives, such as biomass resources, is a step in the right direction [2]. As a byproduct from municipal or industrial wastewater treatment processes, sewage sludge is a kind of abundant waste biomass, which contains large amounts of organic components. Until recently, sewage sludge is usually disposed by landfill or incineration, which suffers from secondary pollution and low energy recovery rate [3].

Green Chemistry aspires to reduce consumption of nonrenewable resources and at the same time produce high-quality products in an environmental-friendly manner from renewable resources. Gasification of sewage sludge may be one of the promising sustainable approaches for hydrogen-rich syngas production. However, raw sludge is unsuitable for direct gasification due to the high water content and thus pretreatment is necessary to improve its quality prior to further gasification process [4]. Applying proper thermochemical pretreatment can also improve the energy efficiency and diversify the energy utilization pathways.

As one of the typical thermochemical pretreatment, low temperature pyrolysis (LTP) [5], [6] converts biomass into pyrochar under an inert atmosphere with low heating rate. The pyrochar has improved physical and chemical characteristics, and it has been extensively used for soil amendment, wastewater pollution remediation, carbon sequestration and bioenergy production [7]. An alternative to LTP is hydrothermal carbonization (HTC) [8], [9], [10], which converts biomass into hydrochar in aqueous phase under facile temperatures and self-generated pressure. HTC offers several potential advantages such as high conversion efficiency and the ability to use diverse feedstock without drying pretreatment, which is especially suitable for high water content of biomass like sewage sludge.

The physical properties and chemical functionalities on the surface of the pyrochar and hydrochar are significantly different from each other, both of which thus have respective advantages in various industry applications. Escala et al. [11] reported that conducting HTC and drying the hydrochar have energetic advantages compared with drying the sewage sludge for thermal disposal treatment. In addition, the HTC-pretreated sewage sludge is reported to have improved combustion properties compared to the pyrochar and raw sludge [6], [9], [12], [13]. In terms of gasification of the biochar, Álvarez-Murillo et al. [5] studied the steam gasification characteristics of the hydrochar derived from olive stone as a representative of lignocellulosic biomass. It was observed that the hydrochar modified the gas profiles during gasification, improving H2 and CO production as well as the heating value. Erlach et al. [14] also concluded that pretreating the lingocellulosic biomass with HTC produced a hydrochar that was better suited for entrained flow gasification than raw biomass. These studies successfully verified the improved gasification behavior of lignocellulosic biomass after the hydrothermal pretreatment.

Different from lignocellulosic biomass (mainly composed of biopolymers cellulose, hemicellulose and lignin), the main composition of sewage sludge is protein and lipid. Therefore, different gasification behavior of hydrochar derived from sewage sludge is expected to that of the hydrochar from lignocellulosic biomass. Our previous study [15] investigated the gasification behavior of the hydrochar derived from sewage sludge by hydrothermal carbonization. It was observed that compared to the raw material, the hydrothermally treated sewage sludge had improved gasification characteristics in terms of hydrogen-rich syngas production. However, the lack of understanding on the effect of different pretreatments on the gasification behavior of the same sewage sludge under identical conditions is making the investigation on thermochemical treatment of sewage sludge difficult to go to a more profound level. Therefore, the primary goal of this study is to investigate the gasification properties of pyrochar and hydrochar derived from sewage sludge by LTP and HTC, respectively. Previous literature [16], [17], [18] have shown that steam as the gasifying medium has been proved effective for enhancing hydrogen yield compared to air gasification or air-steam gasification. In this work, the effects of operating conditions, including reaction temperature and the mass ratio of steam to biomass on gasification characteristics of the pyrochar and hydrochar were experimentally evaluated in terms of gas composition, heating value, gasification efficiency and energy recovery efficiency.

Section snippets

Biochars preparation

Sewage sludges were collected from a municipal sewage treatment plant in Beijing, China. The collected raw sludge still contains high content of water. Prior to the LTP reaction, the raw sludge was centrifuged, oven dried at 105 °C for 12 h and then ground into powders. However, no dewatering and drying pretreatment was conducted for the HTC treatment.

Reaction temperature and retention time are two significant factors affecting the properties of the biochars. A recent study [19] investigated

Chemical composition

The properties of the pyrochar and hydrochar depend to a large extent on the chemical compositions, which are shown in Table 1. Proximate analysis illustrated that compared to raw sludge, the amounts of volatile matter (VM) of the pyrochar and hydrochar both decreased while the contents of fixed carbon (FC) and ash both increased. It is mainly attributed to the devolatilization of VM and polymerization during the pyrolysis/hydrothermal process. For the pyrochar and hydrochar, the loss of VM was

Conclusions

In this study, two types of the biochars, pyrochar and hydrochar derived from sewage sludge have been prepared for hydrogen-rich gas production via steam gasification. The pyrochar was rich in aliphatic Csingle bondH functional groups while more aromatic Csingle bondH functional groups were presented in the hydrochar. Compared to the LTP-gasification method, the HTC pretreatment combined with subsequent gasification had a lower energy recovery efficiency of the whole process. However, the steam gasification of the

Acknowledgements

The authors gratefully acknowledge the financial support from the “100 Talents” Program of the Chinese Academy of Sciences to Zhengang Liu, Beijing Natural Science Foundation (Project No.8164064), Opening Project of Key Laboratory for Solid Waste Management and Environment Safety in Tsinghua University (Project No.SWMES 2015-13), and Key Laboratory of Solid Waste Treatment and Resource Recycle in Southwest University of Science and Technology (Project No. 15zxgk01).

Nomenclature

LTP
Low temperature pyrolysis
HTC
Hydrothermal carbonization
TCD
Thermal conductivity detector
HHV
Higher heating value (MJ/kg)
LHV
Lower heating value (MJ/kg)
VM
Volatile matter
FC
Fixed carbon
S/B
Mass ratio of steam to biomass (−)
WGS
Water-gas shift reaction
AAEM
Alkaline earth metallic species
CI
Catalytic index
ERE
Energy recovery efficiency (−)

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