- Title
- Molten salt slow pyrolysis for advanced carbon and renewable energy
- Creator
- Jalalabadi, Tahereh
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2022
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- Over the past decades, continuous effort to develop innovative commercialized techniques has been employed in bioenergy generation and biofuel production, including efficient procedure design. As an abundant, low cost and renewable source for green energy supply, biomass from various origins such as agricultural or forest residue, municipality sewage, and different types of wastes have been considered as an alternative to fossil fuels. In recent years individual or combined methods such as pyrolysis, gasification, or oxy-combustion have resulted in a profitable industry in generating renewable energy from biomass with extensive application of its final products. In fact, biomaterials have a bright future as an alternative of fossil fuels for sustainability reasons as well as in addressing the climate crisis. Though thermochemical conversion of biomass is well investigated by researchers, finding an environmentally friendly chemical additive which also reduces demand for energy to accomplish pyrolysis while improving char quality still needs further investigation. Alkali carbonate has excellent synergies with applications in concentrating solar thermal processes due to its high thermal/chemical stability, low vapour pressure, and low viscosity. In considering biomass treatment, its use has benefits such as production of less pollutants during processing, reduced expensive and potential for a simple recycling treatment. In this work, biomass and char treatment with molten carbonate mixtures is investigated under a variety of conditions to modify biomass product morphology and form new advanced carbon materials. In comparison with single salt usage as chemical additives during pyrolysis, application of either binary or ternary molten carbonate mixtures with lower melting points is a promising route to decrease overall energy consumption. In this work focus is on a eutectic mixture of carbonate salts. Specifically, the ternary eutectic with ratio of Li2CO3: 43.5%, Na2CO3: 31.5%, K2CO3: 25% (mole percentage) with melting point ~ 400 °C was chosen to be applied along with biomass precursors (Eucalyptus woody biomass, cellulose, lignin) and graphite as standard carbon. As the main purpose of this project is investigating carbon morphology modification in salt-assisted conversion, slow temperature rate (5°C/min) process (known as slow pyrolysis) was conducted under N2 to the desired temperature under a range of operating conditions. Since pyrolysis conversion and its final product is strongly depending on various operational variables such as the highest heating temperature (HHT), gaseous environment, nature of biomass precursor, heating rate of thermal treatment, working pressure, and mixture ratio (biomass to chemical additives), each chapter is dedicated to deeply study the effect of carbonate on bioconversion during pyrolysis. Various types of solid product’s characterization methods were applied such as analysing porosity in Brunauer–Emmett–Teller (BET), macro scale change in carbon morphology by Scanning Electron Microscope (SEM), nanoscale structural alteration in Transmission Electron Microscopy (TEM), oxygenated bonds in solid Fourier-Transform Infrared Spectroscopy (FTIR), while online changes were racked in a joint setup of devices such as Thermo Gravimetric Analysis (TGA) and Micro Gas Chromatograph (micro-GC). Starting by examining the impact of mixing ratio and the highest heating temperature (HHT) in chapter 3, initial results presented the positive impact of molten carbonate on hindering woody biomass devolatilization and increasing residence time of volatiles. At 600°C it was observed that salt particles melted to form liquid phase coverage on carbon particles and acted to trap volatiles inside particles. This resulted in dominantly a porous carbon structure and also increased char production due to enhanced secondary char-forming reactions. However above 750°C, carbonate vaporization and decomposition in addition to carbonate gasification reactions with carbon were major sources of mass loss and consumed large amounts of starting carbon materials. In chapter 4, lignin and cellulose were added as biomass precursors and more fundamental model compounds. Two working temperatures below (350 °C) and above (600°C) the carbonate melting point were applied to explore various contact phases between carbon and carbonate and overall impact on both reaction rates and char morphology. In contrast with minor changes in solid-solid phase contact (at 350°C) in liquid-solid contact (at 600 °C) surface area improvement of treated samples was observed in comparing with untreated char. This was particularly clear for lignin with a surface area increase of 11 m2g-1 to 209 m2g-1 with salt treatment and for woody biomass from 371 m2g-1 in untreated sample to 516 m2 g-1 once treated. However, pore blockage by liquified molten carbonate in cellulose treatment didn’t improve BET results in comparison with untreated sample. Understanding the impact of the gaseous environment was the principle aim of chapter 5, by employing CO2 and N2 in woody biomass thermochemical conversion with assistance of molten carbonate at 600°C and 900°C. Findings of this chapter expressed similarity between results under CO2 and N2 at 600°C which produced further char, swollen and ruptured porous carbon in treated samples under both gases. Outcomes of 900°C experiments showed reverse Boudouard reaction was consuming all carbon particles in particular under a CO2 atmosphere with an additional observed catalytic effect of carbonate to accelerate this reaction at lower working temperatures (from 735 °C to 640 °C). Results of how pressure influences char making and carbonate gasification reactions on both lignin and woody biomass precursors are presented in chapter 6. Pressurized devolatilization produced highly porous carbon char from biomass at 600°C HHT. This was determined to be due to liquid carbonate coverage on particles with additional external pressure, which further limited volatile freedom to be released. Only surface cracking of lignin particles was observed under the same conditions with minor influence of pressure observed. At elevated temperature of 800°C, carbonate gasification converted biochar into an amorphous form, while lignin was converted to a partially ordered structure at 800°C. These outcomes were intensified by applying 5 bar pressure at 800°C particularly in lignin which resulted in 4 to 6 layered carbonaceous lattices with 0.34 nm distance, similar to standard graphite. Due to these findings, in chapter 7 graphite as a starting material along with carbonate was used to study slow heating (5 °C/min) gasification reaction under N2. These results elaborated turbostratic carbon structure creation, suggested to be due to intercalation of various ions (Li+, Na+, K+, CO32-) between graphite sheets which causes expansion and contraction between lattice layers. By studying variables such as isothermal residence time, pressure effect and mixing ratio for the graphite/carbonate system, it was concluded that higher pressure and higher amount of carbonate could completely turn graphite ordered morphology to disordered carbon structure. The impact of molten carbonate on carbonaceous materials has been found here to be transformative. Effects are variable, depending on types of precursor and working conditions employed, which suggests that this is a promising method to manipulate carbonaceous precursors and shaping desired structure based on its future application.
- Subject
- biomass conversion; pyrolysis; renewable energy; advanced carbon
- Identifier
- http://hdl.handle.net/1959.13/1483965
- Identifier
- uon:51243
- Rights
- Copyright 2022 Tahereh Jalalabadi
- Language
- eng
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