Performance optimization of blended biomass pellets from corn stover and eucalyptus sawdust using linear low-density polyethylene as a binder

Abstract

Two significant biomass waste streams that are frequently disposed or underutilized are corn stovers and eucalyptus sawdust. Little research has been done on the impact of linear low-density polyethylene (LLDPE) binder on the qualities of blended pellets. Therefore, the main objective of this research was to undertake performance optimization of blended biomass pellets from corn stover and eucalyptus sawdust using linear low-density polyethylene as a binder. The specific objectives were: to perform proximate and ultimate analysis of corn stover and eucalyptus sawdust; to design and fabricate a single pellet press heated mould (SPP) and a pellet durability tester; to fabricate and torrefy blended pellets from corn stover and eucalyptus sawdust using LLDPE as a binder; to carry out characterization of the physical, mechanical and thermochemical properties of blended torrefied pellets and to optimize the pelleting process variables in terms of corn stover to eucalyptus sawdust ratio, the ratio of LLDPE binder and the torrefaction temperature. The Standard ASTM methods such as ASTM E871, E872, D1102 and D5373-02 were used to perform characterization of corn stover and eucalyptus sawdust. SPP heated mould was fabricated using stainless steel and incorporating heating mechanism and temperature controls. Pellet durability tester chamber was fabricated according to ISO 17831-1 standard. The fabricated SPP was used to fabricate the blended pellets and a modified tube furnace was used to torrefy the blended pellets. Taguchi multi-response optimization using grey relational analysis (GRA), through response graphs, ANOVA, contour and response surface plots were used to determine the impact of corn stover: eucalyptus sawdust ratio, LLDPE fraction, and torrefaction temperature variables, as well as their interaction effects, on the pellet properties. Proximate analysis of both feedstocks showed acceptable volatile matter, ash content and fixed carbon while moisture content was slightly lower for pellet production. While the ultimate analysis of both feedstocks indicated high carbon and hydrogen contents suitable for fuel applications. Additionally, the SPP fabricated was able to densify loosely packed ground feedstock to solid pellets, while the durability tester was able to perform tumbling at constant speed of 50 revolutions per minute. Also, the torrefaction of the blended pellets resulted in brownish to black pellets with maintained structural integrity. The optimal variables for blended pellet fabrication were; a 5:5 ratio of corn stover to eucalyptus sawdust, a 2% LLDPE ratio and a torrefaction temperature of 210°C. Consequently, the validation of blended pellet qualities resulted in particle density of 1074.75Kg/m 3 , bulk density of 633.2Kg/m 3 , durability index of 99.07%, Hardness of 1046.972N, Mass yield of 64.45%, higher heating value of 29.894MJ/Kg and Carbon dioxide emissions of 3.55% by volume. In addition, combustion emissions from the optimized blended pellet were: 44.3g/min for CO 2 , 0.40g/min for CO and 7.55mg/min for particulate matter (PM2.5) by gravimetric method. LLDPE significantly influenced most of the properties evaluated. In conclusion, the suitability of corn stover and eucalyptus sawdust for pellet fuel production was guaranteed as demonstrated by their characterization results. From preliminary tests, the designs and fabrications made functioned as required and therefore can be used for production and evaluation of pellets. Furthermore, the validated optimal properties of the blended pellet were within the acceptable European guidelines EN 14961-6 international standard and other published literature for pellets with superior qualities for domestic as well as industrial applications as solid fuels. Furthermore, LLDPE proved to be a significant additive in the pellet blends as a binder. It is highly recommended that the optimized pellet produced be used in improved pellet cookstoves and industrial boilers since they have high energy output and enhanced mechanical properties and physical properties for ease of transportation to their destination of application.