Gasification characteristics of hydrochar and pyrochar derived 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 CH functional groups while more aromatic CH 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 (−)
References (43)
- et al.
Estimation of efficiency of using hydrogen and aluminum as environmentally-friendly energy carriers
Int J Hydrogen Energy
(2015) - et al.
Hydrogen production from the catalytic supercritical water gasification of process water generated from hydrothermal liquefaction of microalgae
Fuel
(2016) - et al.
Review of sewage sludge management: standards, regulations and analytical methods
J Clean Prod
(2015) - et al.
Characterization and comparison of biomass produced from various sources: suggestions for selection of pretreatment technologies in biomass-to-energy
Appl Energy
(2012) - et al.
Biomass pyrolysis toward hydrocarbonization. Influence on subsequent steam gasification processes
J Anal Appl Pyrol
(2015) - et al.
Upgrading of waste biomass by hydrothermal carbonization (HTC) and low temperature pyrolysis (LTP): a comparative evaluation
Appl Energy
(2014) - et al.
Conversion of tomato-peel waste into solid fuel by hydrothermal carbonization: influence of the processing variables
Waste Manag
(2016) - et al.
Fate of inorganic material during hydrothermal carbonization of biomass: influence of feedstock on combustion behavior of hydrochar
Fuel
(2016) - et al.
Comparison of liquid and vapor hydrothermal carbonization of corn husk for the use as a solid fuel
Bioresour Technol
(2016) - et al.
A comparative review of biochar and hydrochar in terms of production, physico-chemical properties and applications
Renew Sust Energy Rev
(2015)
Conversion of sewage sludge to clean solid fuel using hydrothermal carbonization: hydrochar fuel characteristics and combustion behavior
Appl Energy
Combined hydrothermal carbonization and gasification of biomass with carbon capture
Energy
Hydrogen-rich gas production by steam gasification of hydrochar derived from sewage sludge
Int J Hydrogen Energy
Steam gasification of plant biomass using molten carbonate salts
Energy
High quality syngas production via steam-oxygen blown bubbling fluidized bed gasifier
Energy
Assessment of steam gasification kinetics of the char from lignocellulosic biomass in a conical spouted bed reactor
Energy
Hydrothermal carbonization of anaerobically digested sludge for solid fuel production and energy recovery
Fuel
Kinetics of faecal biomass hydrothermal carbonisation for hydrochar production
Appl Energy
Experimental study on non-woody biomass gasification in a downdraft gasifier
Int J Hydrogen Energy
High temperature steam gasification of wastewater sludge
Appl Energy
Influence of pyrolysis temperature on physical and chemical properties of biochar made from sewage sludge
J Anal Appl Pyrolsis
Cited by (95)
Hydrothermal liquefaction of municipal sludge and its products applications
2024, Science of the Total EnvironmentInterface engineering of Ce/La-doped hydrochar-supported nickel catalysts for enhanced tar reforming performance at low temperature
2024, Journal of Analytical and Applied PyrolysisCo-hydrothermally carbonized sewage sludge and lignocellulosic biomass: An efficiently renewable solid fuel
2023, Arabian Journal of Chemistry