An experimental and modelling study on torrefaction and gasification of oil palm biomass

Abstract

Palm oil is one of the most consumed vegetable oils in the world to date. Malaysia being the second largest exporter of palm oil generates large amount of agricultural waste from the oil palm sector. Oil palm biomass is organic and easily renewable feedstock, and has high energy content. With the constant supply of oil palm biomass from processing mills, its utilization will add economic value to the agricultural activities. Despite its advantages, certain inherent characteristics of oil palm biomass such as high moisture content, low density, and rapid iodegradation, which make its direct application in energy sector challenging. Hence torrefaction is proposed in this project as the potential pre-treatment step for oil palm biomass for gasification. This experimental and modelling-based study investigates the torrefaction of oil palm biomass followed by the application of the torrefied chars in gasification. The study focused on empty fruit bunches (EFB), mesocarp fibers (MF) and palm shells (PS). Comprehensive analysis was undertaken including the liquid and gaseous products from oil palm biomass torrefaction. The results showed that torrefaction improved both the physical and energetic characteristics of the biomass. Close examination through micrographs coupled with the thermogravimetric data showed that the fibrous EFB and MF underwent earlier and higher decomposition than the rigid PS. The presence of furan and phenolic compounds in the liquid fraction indicates that lignin and cellulose decomposition is initiated during torrefaction of oil palm biomass. Characterization results indicate that the reaction for oil palm biomass torrefaction mechanism is closely associated to the organic matrix of the biomass. A comparison between model-free isoconversional approach and 2-step consecutive model for torrefaction process indicates that the oil palm biomass can be better predicted with the latter model. The oil palm chars generated were examined in a laboratory scale fixed bed gasification reactor under CO2 and steam environment where the compositional changes in the gaseous product during gasification were examined on-line using Micro Gas Chromatography (Micro-GC). The H2/CO molar ratio for CO2 gasification of torrefied oil palm biomass was found to be below 1, hence more suitable for chemical synthesis. In steam gasification of the torrefied chars, this ratio is in the range of 2-5 indicating potential for liquid fuels synthesis. Kinetic analysis was performed on the gasification of torrefied and untorrefied oil palm biomass using thermogravimetic analyser (TGA). The thermal decomposition of the biomass was examined with five models including Shrinking Core Model (SCM), Volume Reaction Model (VRM), Modified Volume Reaction Model (MVRM), Random Pore Model (RPM) and Modified Random Pore Model (MRPM). The results from the gasification study indicate that the Modified Random Pore Model gave the best prediction without excessive mathematical complexities. The activation energy for the gasification of torrefied PS was in the range of 15.29 kJ/mol to 35.25 kJ/mol while untorrefied PS was 80.07 kJ/mol. This reduced activation energy for gasification can be related to the enhanced morphology and pore structure after torrefaction. Further to this, mass and energy balance was undertaken to examine the energy requirement of the torrefaction and gasification process. Results showed that torrefaction enhanced the energy content of syngas from all three oil palm biomasses. The torrefaction of 10 metric tons oil palm biomass requires about 2217-9645 MJ in the torrefier and this may be self-sustaining with the recovery of heat energy from the combustion of volatiles. Thermodynamic equilibrium model was used for the prediction of the gasification process. The model assumes the process to be adiabatic, and thus heat transfer is negligible. Among the three oil palm biomasses studied, PS showed the highest potential as biofuel source as it has the highest energy content after torrefaction and lowest energy requirement for pre-treatment. The results from this project serve to fill the knowledge gap and ultimately provide a clearer picture for the application of torrefaction of oil palm biomass.