| dc.description.abstract | An increased reliance on energy production and the consumption thereof is the result of an exponentially growing world population. Current global energy resources rely greatly on coal, natural gas and to a small extent on nuclear power. A major drawback in using fossil fuels for energy production is due to its non-renewable nature and thus it is becoming scarcer. With the use of fossil fuels, four major energy concerns arise, i.e. the depletion of fossil fuel reserves, the rise in greenhouse emissions causing global warming, the security in which energy can be produced and provided, and due to scarcer resources, the rising energy costs. All these factors contribute to the need for alternative resources to be used to provide sustainable energy production and to reduce greenhouse emissions.
One such alternative is the utilization of agricultural wastes (being widely available and abundant) alongside thermochemical conversion processes to produce solid (biochar), liquid (bio-oil) and gaseous (bio-gas) fuels. This technology uses biomass to obtain low molecular weight bio-products with various applications, ranging from bio-composites for carbon sequestration, bio-energy, soil remediation products (such as adsorbents) and it can also be used for gasification and co-gasification in coal-fired furnaces.
The aim of this study was to investigate the effect of operating parameters on the production of bio-products from the liquefaction of quinoa lignocellulose. To reach this aim, the objectives of the study were to determine the effect of biomass loading, temperature and heating rate on the product yield as well as its effect on the structural and chemical compositions of these products.
The study was conducted using a grade 316 stainless steel autoclave equipped with a variable speed magnetic stirrer and a removable heating jacket. Each experiment was carried out with a starting pressure of 10 bar, a stirring speed of 720 rpm and water as the only solvent. All experiments were conducted in a nitrogen gas atmosphere and with a residence time of 15 min.
The biochar, bio-oil and bio-gas products were characterised regarding their chemical and structural characteristics. The chemical analysis included proximate analyses, elemental analyses, higher heating values (HHV), Fourier-transform Infrared Spectroscopy (FTIR), total organic carbon (TOC) analysis using the UV-Persulphate oxidation technique and gas chromatography. The structural analysis included Brunauer-Emmet-Teller (BET), Scanning Electron Microscopy (SEM), and UV-spectrophotometry.
It was evident from the study that biomass loading had an influence on the chemistry of the liquefaction process. At a low biomass loading, hydrolysis reactions were promoted and more bio-oil and bio-gas were produced. On the other hand, at a higher biomass loading, the production of biochar through a carboxylation reaction was favoured. Temperature played a crucial role in the liquefaction process, influencing not only the product yields, but structure and composition. At a low temperature, the production of biochar was favoured and the process delivered higher yields of biochar. However, with increasing temperature the fixed carbon content and volatile matter decreased and the ash content increased. The elemental analysis showed similar carbon, hydrogen and oxygen contents, and subsequently similar fuel properties to that of lignite coal, but with a higher H/C molar ratio. The HHV for biochar declined with increasing temperature, and that of bio-oil increased. Through SEM analysis, a clear increase in devolatilization holes could be seen with increasing temperatures, confirming the destruction of biomass. BET analysis indicated that by increasing the temperature, the surface area of the produced biochar would increase. Lastly, the destruction of cellulose, hemicellulose and to a small degree lignin was evident when performing a FTIR analysis.
The heating rate of the liquefaction had a similar effect on biochar yields than temperature. A decrease in biochar yields was observed with increasing heating rates. However, the fixed carbon content increased and the volatile matter and ash content of biochars increased. When increasing the heating rate, a reduced carbon content and higher oxygen content was observed through elemental analysis. The SEM analysis showed that increasing the heating rate had a significant effect on the formation of larger devolatilization holes. The BET surface area obtained at higher heating rates also showed a significantly greater surface area when compared to the surface area at lower heating rates. Lastly, the FTIR analysis indicated that biomass constituents had undergone additional destruction at higher heating rates.
Thus, in this study it was demonstrated that operating parameters had a tremendous effect on the distribution, structural and chemical properties of the bio-products obtained. Producing fuels with a high carbon and HHV content with low volatile matter and oxygen content proved that these bio-products can be used alongside coal for co-gasification and gasification purposes. High surface areas suggest that these biochars can be used as adsorbents and soil amendments with prior activation of the biochars | en_US |
