Chemical and structural changes of biomass during pyrolysis and the influence on gasification reactivity in coal-biomass blends
In this study, the conversion of biomass for thermochemical energy applications is studied. In the first section, the effect of biomass upgrade, via torrefaction, on the structural and chemical properties of lignocellulosic biomass, was investigated. Three biomass samples; softwood chips (SW), hardwood chips (HW) and sweet sorghum bagasse (SB) were used for this study. SW and HW showed similarities in characteristics in terms of the ultimate and proximate analysis, fibre analysis, X-ray diffraction, solid state 13C nuclear magnetic resonance (NMR) and CO2 adsorption, whilst these were significantly different for SB. The torrefaction conditions, with a weight loss target of 30%, were determined in a thermogravimetric analyser and then torrefaction experiments were performed in a tube furnace, in a N2 atmosphere. The torrefaction times, at 260 °C were 110, 100 and 20 minutes for SW, HW and SB respectively. Torrefaction was accompanied by a decrease in the H/C and O/C ratios and a significant increase in the calorific value and fixed carbon. The hemicellulose content was significantly reduced by torrefaction. There were no significant changes for cellulose and lignin amounts after torrefaction. These changes were accompanied by the aromatization of biomass where the net aliphatic fractions were reduced whilst the aromatic fraction increased by approximately 40%, for all biomass samples investigated. The crystallite lattice was also affected by torrefaction, where significant decreases in the crystallite size (La) which also resulted in the increases in the micropore volume, were observed. There was a significant micropore surface area increase for SB; from 42 m2/g for raw SB increasing to 92 m2/g after torrefaction and insignificant changes were observed for SW and HW after torrefaction. This was as a result of the melting of lignin at torrefaction conditions which were in higher amounts for SW and HW. The second part of this study included the investigation of the char formation process. Chars were prepared from the torrefied material to final temperatures of 300, 400, 600 and 1100 °C and a holding time of 60 minutes. The progressive decrease in O/C and H/C ratios as temperatures were increased, from torrefaction conditions to 1100 °C, was accompanied by other chemical and structural changes and obtained results were comparable for SW and HW than SB. For all biomass samples, the calorific value (CV) increased from torrefaction conditions (22.3, 22.4 and 23.0 MJ/kg for SW, HW and SB respectively), a maximum observed for chars prepared at 600°C (33.1, 33.7 and 30.1 MJ/kg for SW, HW and SB respectively) and slightly decreased for chars prepared at 1100°C (32.3, 32.1 and 26.6 MJ/kg respectively). This was as a result of the reduction of elemental O (resulted in the initial increase) and then the graphitization of the carbon structure at higher temperatures (resulted in the slight decrease beyond 600°C) This observation was confirmed by wide angle X-ray diffraction carbon fraction analysis (WA-XRD-CFA) data. From WA-XRD-CFA, the increase in crystalline diameter (La) was accompanied by decreases in interlayer spacing (d002), crystalline height (Lc) and the average number of aromatic layers per carbon crystallite (Nave) which was a sign that the carbon lattice was stretchered into sheets as pyrolysis temperature increased. The use of attenuated total reflectance Fourier Transforms infrared (ATR-FTIR) spectroscopy was extended by developing a method of evaluating the aromaticity. Results from this new method were comparable to the well documented 13C NMR method. Data from char samples prepared at 1100 °C did not give any peaks for either method as the bonding between elements was almost completely destroyed. The aromaticity increased from approximately 20% at torrefaction conditions rising to approximately 90 % for chars prepared at 600 °C, for all biomass samples. These findings were accompanied by increases in the degree of aromatic ring condensation (R/C)u and a decrease in the CH2/CH3 ratio and the fraction of amorphous carbon (XA). As char formation progressed, with increasing pyrolysis temperature, below 600 °C the aromatization process was as a result of the removal of the aliphatic components from the matrix while above 600 °C, the condensation of aromatic bonds was a significant contributor to the aromatization as char forms. From the generated results, correlations between the characteristics were drawn where there were linear correlations between the aromaticity and H/C, (R/C)u with H/C and a power law could related CH2/CH3 with H/C ratio, for all samples with correlation coefficients > 85%. Chars prepared at 1100 °C were then used to investigate CO2 gasification under isothermal conditions between 850 and 950 °C in a thermogravimetric analyser. Bituminous coal char samples were prepared at 1100 °C and then gasified for comparison with biomass char. SB had the highest gasification reactivities whilst SW and HW had comparable gasification reactivities while coal char showed the lowest gasification rates. All biomass char gasification resembled catalytic gasification, showing gasification reactivity maximums at conversions above X=0.5. As a result, the gasification reactivities were better predicted by the modified random pore model. It was also observed that the different biomass samples exhibited different values of the structural parameter (ψ), and the empirical constants c and p were similar for all samples whilst the p was varied. The gasification characteristics were related to the char characteristics; surface area, La, Lc/d002, H/C and AI2. The addition of biomass char, to coal char, resulted in increased reactivities (Ri and slightly Rs) for HW and SW compared to coal, however, the addition of SB resulted in an improved gasification reactivity throughout the conversion range (increased Ri, Rs and Rf). The differences in effect, between the woody biomass and SB were a result of the mineral content and the possible interaction between the minerals contributed by coal and biomass.