The thermal conversion of biomass and green synthesis of graphene materials from biochar

Abstract

This work aims to investigate the physicochemical characterisation of biomass, thermal conversion of biomass, including pyrolysis and combustion, and green synthesis of graphene materials from biochar.
A potential alternative method for lignocellulose quantification based on macroscopic imaging and standard Red-Green-Blue (sRGB) colour analysis was proposed. The lignocellulose contents of samples were analysed via a modified version of the Dubois assay. The sRGB values of biomass were determined using an automated image analysis coding specifically developed for this study involving RAL (Reichs-Ausschuß für Lieferbedingungen und Gütesicherung) colour calibration, Roberts edge detection and standard RGB algorithms. A linear correlation was found between sRGB values and lignocellulosic content, indicating the influence of lignin content on R, G and B values, ceteris peribus (especially moisture). Biomass samples with higher lignin content are commonly associated with lower sRGB values. It was shown that the lignocellulosic composition of biomass, particularly lignin, could be predicted based on sRGB values and vice versa. This work forms the basis of future and further studies in establishing a straightforward, low-cost and high-throughput ‘face recognition’ equivalent for identifying and screening biomass, particularly whilst attempting to reduce the utilisation of harsh chemicals.
The relationship between the thermal behaviour of biomass during combustion and lignocellulosic composition was investigated. As with previous study, the lignocellulosic composition was determined via standard biological assays, and it was observed that cellulose is the most abundant organic component in most samples. Combustion kinetics and thermal behaviour were examined using thermogravimetric analysis (TGA) whilst an Ash Fusion Furnace equipped with a camera was used to monitor ignition temperatures in real-time. The kinetic studies were repeated at heating rates of 5, 10 and 17.5 °C/min. Linear correlations were found between the lignocellulosic composition of biomass and its combustion kinetics (both first and second stage), and the ignition point of biomass. Biomass with higher lignin content has higher ignition point and activation energy. This is associated with the chemical characteristics and biological arrangement of lignocellulose in plant cells. It was also established that the pre-exponential factor of combustion is inversely proportional to the burnout temperature interval. This trend was apparent and recorded for all heating rates tested, suggesting an alternative means to evaluate and predict the combustion reaction rate and efficiency of biomass in power plants via simple characterisation of biomass. This work also potentially provides a quick and straightforward indicator for selecting highly reactive and efficient biomass for combustion processes, which is linked directly to biomass composition.The effects of pyrolysis parameters (heating rate, temperature and holding time) on the structural evolution of biochar during rearrangement of the solid phase were investigated. It was found that pyrolysis temperature and heating rate significantly influences the properties of the resultant biochar. Low temperatures would lead to inadequately graphitised biochars with a preserved amorphous, aliphatic and complex cellular structure, whilst high temperatures result in the loss of functional groups and defects, coupled with excessive melting and coalescence of structure. A low temperature, low heating rate and low residence time process would be ideal to maximise the biochar yield during pyrolysis. This work forms the foundation of future and further studies in the synthesis of graphene materials from biochar.
The potential of producing graphene oxide (GO) from biomass via green processing (using comparatively less harsh solvents) and the impact of graphitisation temperature on GO quality was investigated. Our findings show that it is possible to convert biomass into highly pyrolytic biochars akin to graphite, and shear exfoliation was employed to produce few-layer GOs after that. However, the pyrolysis temperature is of prime importance in ensuring that the properties of biochar are suited for effective exfoliation. Raman, X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), atomic force microscopy (AFM), and Brunauer-Emmett-Teller (BET) results have shown 1200 °C to be the optimum graphitisation temperature for miscanthus, where the resultant biochar is highly aromatic with sufficient functional groups to increase interlayer spacing, thus weakening the van der Waals forces and facilitating exfoliation to form 6 layer GO with the specific surface area of 545.3 m2/g.
An effective, sustainable and scalable ultrasound-assisted mechano-chemical cracking method was developed to convert biomass into GO. A typical energy crop, miscanthus, was used as a carbon precursor and pyrolysed at 1200 °C before subjecting to edge-carboxylation via ball-milling in a CO2-induced environment. The resultant functionalised biochar was ultrasonically exfoliated in deionised (DI) water and N-Methyl-2-pyrrolidone (NMP), respectively, in order to investigate the potential of green solvents. The intermediate and end-products were characterised via XRD, Raman, HRTEM and AFM analyses. Results show that the proposed synthesis route can produce good-quality and uniform GOs (8-10% monolayer), with up to 96% of GOs having three layers or lesser when NMP is used. Whilst water is a less effective polar solvent for exfoliation (48% of GO sheets having three layers or lesser), this study establishes the potential of transforming biomass (or solid wastes) into value-added functional materials in a sustainable, effective, and efficient way. In addition, graphene quantum dots (GQDs) were produced within the wrapped GO with dimensions of about 6 nm, hence suggesting the ability to produce different graphene materials in a single step.

Date of Award	13 Nov 2021
Original language	English
Awarding Institution	University of Nottingham
Supervisor	Cheng Heng Pang (Supervisor), Edward Lester (Supervisor) & Tao Wu (Supervisor)