Biochar torrefaction from oil palm empty fruit bunches for biomass pellet production

The important in using biomass as an alternative source of energy is growing rapidly. Malaysia has enormous biomass resources, especially from the oil palm industry. In this study, unused oil palm empty fruit bunch (OPEFB) discarded as a by-product from palm oil mills was utilised to produce biochar via torrefaction. This study aims to characterise the properties of torrefied biochar from OPEFB and optimise the torrefied biochar using response surface methodology (RSM) via the Box-Behnken model for enhancing the properties of optimised torrefied biochar from OPEFB by pelletising (densification) using a natural binder (clay). The OPEFB was torrefied based on three independent variables: strand size (250, 500, and 750 μm), holding temperature (200, 250, and 300 °C), and residence time (30, 60, and 90 min). The torrefied biochar was optimised regarding six dependent variables: mass yield, moisture content, volatile matter, ash content, fixed carbon, and calorific value. The optimisation using RSM determined that the most optimal value for torrefied OPEFB biochar was 750 μm (strand size), 274 °C (holding temperature), and 90 min (residence time). It was determined that strand size was not a significant factor to produce good condition for torrefied biochar. A darker colour of OPEFB was observed as higher holding temperature and longer residence time were applied, which might be due to higher carbon content. Moreover, the analyses of the morphology and bonding behaviour of the torrefied biochar were influenced by the degradation of hemicellulose, cellulose, and lignin. Torrefied biochar started to rupture as longer torrefaction residence time was applied due to a longer period of thermochemical reaction. The Fourier transform infrared spectra proved that weak bonds had been diminished at this condition. For thermal behaviour, the degradation started from the removal of moisture content, followed by the removal of hemicellulose, cellulose, and lignin, resulting in the decomposition of torrefied biochar. For elemental properties, the carbon content in the torrefied OPEFB biochar increased but the amount of oxygen decreased as the holding temperature and residence time increased. This is due to the decomposition of hemicellulose in this region. Meanwhile, for phase identification, the crystallinity index declined as the holding temperature and residence time increased from 200 to 300 °C and from 30 to 90 min, respectively. This is because cellulose was completely amorphous as the high temperature and longer time of torrefaction were applied. For the evaluation of the effect of densification on the torrefied OPEFB biochar as pellets, the analysis of morphology and bonding properties demonstrated strong bonds within the internal structure of the produced pellet samples, and the properties could be influenced by clay until a certain breaking point. This study also discovered that energy content (calorific value) increased slightly due to the indirect effect of densification, which occurred for the torrefied pelletised biochar at the lowest binder ratio compared to the non-pelletised torrefied material. The densification with the addition of a low percentage of clay helps in achieving complete burning of the pellets. In addition, the use of a low percentage of clay as a binder improved the durability of the pelletised biochar, where clay is not chemically reactive in influencing the energy content of the pellets.