A review of the current knowledge and challenges of hydrothermal carbonization for biomass conversion
Introduction
Hydrothermal carbonization (HTC) is a thermochemical process for the pre-treatment of high moisture content biomass to make it viable in several applications. HTC is performed in a temperature range of 180–350 °C during which the biomass is submerged in water and heated under pressure (2–6 MPa) for 5–240 min [1]. The main product of HTC is a solid named hydrochar. It also produces liquid (aqueous soluble) and gas (mainly CO2) by-products [2].
HTC is a promising method for utilizing the potential of biomass for cleaner production. More research on the chemistry of the HTC, its kinetics and heat transfer, the effect of operational parameters and catalysts, energy and heat recovery, combinations with other technologies, and technical and economic aspects are required. The key component of a potential industrial HTC plant is the reactor. Most of the reactors used in the literature are the batch type, however, for an industrial plant, a continuous reactor that can work at a high temperature and pressure is required.
This study is aimed at providing a better understanding of HTC and discussing the recent research progress, gaps and the directions for improvement of this process. To the authors' best knowledge, there is no other review paper in which the suggestions regarding the main challenges of HTC proposed here have been presented. The paper's structure is outlined in Fig. 1.
This review starts with an explanation of biomass structure and thermochemical conversion methods of biomass, followed by a summary of what previous investigations of HTC have covered so far. This section, which can be found in other related review papers as well, includes chemistry, product's specifications, the effect of process parameters and the applications. The next section of the review deals with four challenges encountered in commercializing HTC namely kinetic model, heat transfer model, continuous reactor, and large-scale studies. The strength and focus of the paper are mainly in this section where suggestions will be proposed for each challenge.
Section snippets
Biomass and its structure
Biomass is organic material from plants and animals that has stored sunlight in the form of chemical energy. Biomass can be categorized as non-lignocellulosic and cellulosic. Non-lignocellulosic biomass is usually sewage sludge and animal manure and mostly contains fatty acids, protein and small amounts of hemicellulose, cellulose, and lignin [3]. On the other hand, the main components of lignocellulosic biomass are hemicellulose, cellulose, and lignin. It also contains some amounts of water
Brief chemistry
The aqueous medium used for the HTC process is kept under its critical point. Ionic or polar reactions take place to form the liquid phase at lower temperatures while free radical reactions occur to form the gas state at higher temperatures. Also, molecular reactions mostly occur when the condition is closer to the critical region of the water [14].
Many reactions take place during the process and the detailed nature of the reaction pathways is not fully understood yet. However, as hydrolysis
Kinetic model
Kinetic modeling refers to the mathematical description of changes in properties of the system of interest, as a result of changes in operating parameters such as time and temperature [119]. The order, interactions, and severities of the reactions in HTC are complex and different from one feed to another which makes modeling the degradation kinetics of HTC difficult.
The first step in modeling the kinetics is collecting data (hydrochar yields regarding the temperature and time) by performing HTC
Conclusion
Converting high moisture content biomass to valuable products is more justified with HTC than any other thermochemical process. This paper aimed to provide a review of the current knowledge and challenges for finding the barriers and suggest how to tackle them. It was found that the barriers are mainly due to the lack of knowledge about the chemistry and heat and mass transfer mechanisms during the process and lack of reliable data from continuous HTC reactors. Experimental setups together with
Acknowledgments
The authors would like to acknowledge the research grants from Natural Sciences and Engineering Research Council of Canada (NSERC, Grant no. 400495), and Ministry of the Environment and Climate Change (MOECC) for Best in Science program (Project #053191).
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