Impact of Torrefaction on the Fuel Properties of Lignite, Coconut Shells, Cassava Peels and Their Blends

Abstract

The effect of torrefaction temperature and residence time on the fuel properties of lignite, biomass (coconut shells and cassava peels) and their blends was investigated. The samples were subjected to three torrecfaction temperatures (200, 260 and 300oC) and at two residence times (10 and 20 minutes) using programmable muffle furnace. Blends of torrefied lignite and biomass were prepared in two different ratios (80:20 and 70:30). The energy content, proximate and ultimate analyses of the samples were determined using ASTM methods. Scanning electron microscope (SEM) was used to evaluate the pore size, fiber content, topography and the morphology of the samples. The potential emissions of SO2, CO2 and NOx from the torrefied samples were evaluated using emission estimation model for fossil fuel electric power generation. The proximate analysis showed that the ash (8.0 %) and moisture (30.0 %) contents of lignite were higher than that of the biomass. The coconut shells and cassava peels had higher volatile matter of 72.9 % and 68.1 % respectively and much lower fixed carbon. The data showed that release of volatile matter decreased at severe torrefaction condition. The content of fixed carbon and energy increased with the severity of the torrefaction condition except for cassava peels which decreased at 300 oC. One–way analysis of variance on the results of the proximate analysis showed that there was significant difference (P<0.05) between the volatile matter, fixed carbon, energy and ash content of lignite, coconut shells and cassava peels, but no significant difference between the moisture and solid yield. For the blends, volatile matter was found to be higher than that of lignite alone. Increase of biomass ratio in the blends decreased the carbon, nitrogen, oxygen and sulfur content of the samples. Lignite/coconut shells (70:30) had better fuel properties compared to (80:20) and lignite/cassava peels (at both ratios). Results of the ultimate analysis showed that after torrefaction there were large reduction in oxygen and hydrogen content. However, 15 % carbon, 26 %nitrogen and 72 % sulfur was reduced from cassava peels while lignite recorded an increase of 40 % carbon,56 % nitrogen and 48 % sulfur after torrefaction. The SEM image showed that torrefied lignite had a uniform and denser structure compared to the raw. The torrefied coconut shells showed adestroyed and less fibrous structure than the raw while the torrefied cassava peels showed a smootsurface. The fiber length oflignite, coconut shells and cassava peels decreased after torrefaction.Results of theemission potential showed that emissions of SO2, CO2 and NOX from lignite and coconut shells increased after torrefaction, while cassava peels decreased. It was also found that blending biomass and lignite reduced emissions of SO2, CO2 and NOX from lignite. Torrefactionimproved the fuel properties of lignite and biomass such as heating value grindability, hydrophobicity, and uniformity. Blending the two fuels (lignite/biomass) provided a way to compensate the negative effects of each other. Therefore, producers of power and heat should explore the use of torrefied lignite, coconut shells, cassava peels and their blends as suitable fuels.