Conventional and Microwave Pyrolysis of Empty Fruit Bunch and Rice Husk Pellets

In recent years, microwave pyrolysis has been the focus of intense research due to the claim that it produced better quality products at a lower power input compared to the electrical furnace pyrolysis system. This study aimed to investigate the influence of both pyrolysis methods on yield and product composition obtained from Malaysian biomass, i.e.: empty fruit bunch and rice husk pellets. They represent lignocellulosic biomass procured as by-products of the milling process.
In the first part of the thesis, an initial characterisation of biomass was conducted to determine the chemical composition. It was found that the biomass in this study has moisture and volatiles content at around 5.4 wt.% and 70 wt.%, respectively which makes them ideal for the pyrolysis process. 200g of biomass was loaded into a 15.8kW fixed bed pyrolysis reactor once the reactor had reached the set temperature. Typically, 40g of biomass was pyrolysed in a specially designed 1000W multi mode microwave oven, where microwaves were fed into the oven cavity through a bottom feed waveguide.
It was found that microwave pyrolysis gave a higher bio oil and char yield than conventional pyrolysis at a similar reaction temperature. Up to 8.40% increase in bio oil yield was observed when rice husk pellets were pyrolysed under microwave radiation at 800ºC. GC-MS analysis revealed a greater content of mono-aromatics compounds obtained from microwave pyrolysis oils with negligible Polycyclic Aromatic Hydrocarbons (PAH) than conventional pyrolysis oils. Similarly, greater cracking of heavier hydrocarbons at high temperature resulted in up to 44% increase in phenol formation from microwave pyrolysis oils. A maximum surface area of 410m2/g was also recorded during microwave pyrolysis of rice husk pellets at 500ºC, where this value reduces with an increase in pyrolysis temperature. Moreover, microwave pyrolysis resulted in up to 29% increase in syngas (H2+CO) evolution and about 42% lower greenhouse gases (CH4+CO2) than conventional pyrolysis. These differences can be attributed to internal heat generation during microwave processing in contrast to conduction from the surface inwards during conventional heating. Energy yield analysis suggested that microwave pyrolysis can be optimised for the production of high quality char and bio oil. Meanwhile, conventional pyrolysis can be optimised to enhance syngas production.
The second part of this thesis looks into the effect of waveguide position and biomass bed height on the electric field and its corresponding temperature distribution. Numerical modelling has shown that higher temperature rise can be generated in a larger load due to greater microwave power deposited. Moreover, an increase in relative permittivity was observed as biomass was converted into char during pyrolysis. This showed that microwave pyrolysis of biomass can be a self-sustaining process, without any addition of microwave absorber.
It was concluded that viable industrial application of microwave pyrolysis is very promising