Biomass Pyrolysis Using Microwave Technology

A series of biomass wastes from Malaysia known as Malaysian wood pellets, and rubberwood were employed in the present work. Using these materials as the feedstock, two different heating techniques; external heating by means of conventional slow pyrolysis (SP) and volumetric heating by means of microwave pyrolysis (MP) were carried out. Two distinct temperatures; 500°C and 800°C were used. The main objective was to characterise both the microwave-pyrolysed products and slow pyrolysed products including the influence of temperature so as to compare and contrast in terms of yield, and composition of the char, oil and high-value fuel gas (H2) or syngas (H2+CO). Whilst there is an increasing interest in comparing microwave pyrolysis with conventional pyrolysis, much of the research work done in the past focussed on using domestic microwave ovens with power control features where indirect temperature measurements were carried out at different power and time settings. In the present research, the control feature for both heating techniques is similar, where the user can conveniently set the desired pyrolysis temperature and therefore, this would allow for a more direct and reliable comparison of products obtained from conventional pyrolysis and microwave pyrolysis. The research found that the use of the microwave oven system to conduct pyrolysis boosted the production of oil but reduced the total gas yield. The char proportion also reduced when microwave heating method was applied. This research also revealed that the configuration of the microwave oven with mode stirrer and bottom-fed waveguide that produces a cyclic controlled output power of 1000 W at any set temperature has yielded different results when compared to previous studies and so provides a new understanding for the microwave pyrolysis community. The results demonstrated that the microwave-pyrolysed chars were slightly more porous than slow-pyrolysed chars at 500°C. However, at a higher temperature of 800°C, lower surface area was obtained from microwave pyrolysis which can be attributed to significant damage to the char structure as the consequence of high power supplied into the cavity and high temperature used. SEM microphotographs revealed that microwave pyrolysis at 500°C led to the formation of char with clearly defined pore structure. In the case of gaseous product, both heating approaches were found to produce a comparable level of H2+CO content except those produced by MP at higher temperature (800°C). Regarding bio-oil quality, the microwave-pyrolysed oil was found to present compounds with higher aliphatic content and contain less polycyclic aromatic hydrocarbon (PAH) content, which is an added quality value as PAH is toxic to the environment. As demonstrated in the present work, employing a microwave oven to conduct pyrolysis process leads to a great time saving where the woody samples required only 8-10 minutes and 15-16 minutes to reach 500 and 800ºC respectively. On the other hand, the electric furnace used to conduct conventional pyrolysis process demonstrated a slower performance where the time required to reach 500 and 800ºC were about 49 and 72 minutes respectively. This again emphasizes that microwave oven is powerful to speed up the pyrolysis process due to the nature of rapid heating within the internal body of the sample. Additionally, from the viewpoint of energy consumption, microwave oven used approximately 62% less energy than the electric furnace to conduct pyrolysis process and therefore leads to greater energy saving. In the present work, COMSOL Multiphysics software has successfully demonstrated solutions of the numerical coupled electromagnetic and heat transfer equations. The results extracted from the simulation using specified cavity geometry, dielectric properties and thermal properties were seen to agree reasonably well with the experimental data in terms of the temperature profile and heating behaviour of the biomass. The location of hot spots and cold spots from the simulation also agreed with that observed from the experiment. The simulation work has proved that the inhomogeneity of temperature of the biomass is reflected by the local occurrence of hot spots and cold spots. These are influenced by the standing waves of different electric field concentration formed at different areas inside the cavity, and this phenomenon is very common for biomass treatment in a microwave environment. The effect of different positions of the waveguide is remarkable where the bottom-fed microwave energy oven was shown to have a poor electric field distribution. However, when simulation was done on combining the effect of having the microwave energy fed from the bottom and the presence of the mode stirrer, the electric field was greatly improved with the heating distribution of the biomass resembling that obtained from the side-fed microwaves energy oven (usually refers to a common home microwave oven). The effect of having a mode stirrer rotating inside the microwave oven is also pronounced where the mode stirrer acts to stir the electric field strength within the cavity so that a more uniform heating within the biomass can be achieved. The simulation work also demonstrated that the amount of microwave power absorbed in the biomass materials varies according to the changes in loading height of the biomass, and sample positioning inside a microwave oven also contributes to the electric field distortion and heating behaviour of the biomass. Interestingly from the simulation, for a specified microwave cavity, an optimum bed size of biomass was found at 50mm height where maximum microwaves energy absorption takes place. In this sense, more microwaves energy can be converted into heat thereby ultimately helping the biomass to reach the desired pyrolysis temperature in shorter time. The COMSOL modelling on microwave heating therefore has shown to be simple and practical for use as a framework in predicting temperature profile of the biomass and intensity of the electric field.