Characterization of empty fruit bunch for microwave-assisted pyrolysis

Abstract

Agricultural waste such as oil palm empty fruit bunch (EFB) is of environmental concern to Malaysia as one of the world's largest oil palm producers. Pyrolysis can be used to treat biomass waste due to its flexibility in producing solid, liquid and gas products. This study attempts to characterize EFB for pyrolysis using microwaves as an alternative heating source. EFB taken from a local oil palm mill was subjected to fuel, chemical and dielectric property analysis. The findings revealed that high moisture and 47% oxygen gave low calorific value of 16 MJ/kg. Notably, high water content is an advantage in microwave heating as water is a good microwave absorber, which results in fast drying. Further, a high volatile content at 70% gave the EFB an advantage of high reactivity. A moderate potassium content of 12.8% could also positively affect microwave absorption. The dielectric properties of EFB were observed to be proportional to the moisture content. Furthermore, the microwave penetration depth was found highest at 20% moisture, i.e. 3.5 cm. However, low values of both dielectric constant and loss of dried EFBs would require the addition of microwave absorbers for pyrolysis reaction. The fuel and chemical characteristics of EFB were found comparable to other biomasses, which indicated a good candidate for microwave pyrolysis treatment.

Dielectric Properties of Oil Palm Trunk Core

“…7 showed dielectric constant was low for low moisture content OPT core because effect of rotation water dipoles diminished when the moisture content was low. These correlation trends are similar with other biomass reported previously such as empty fruit bunch [14] For radial direction, dielectric loss increased rapidly as the moisture content increased (Fig. 8).…”

“…Each OPT cube was polished with sand paper to obtain smooth surface and minimize air bubbles between probe and biomass during conducting measurement because presence of air gap might cause error in measuring by using open-ended coaxial probe. For preparing OPT core biomass with different moisture, the biomass was ovendried at 60 ± 2 o C at different time intervals following method described in a previous study [14].…”

“…Loss tangent is the ratio of dielectric loss to dielectric constant. It determines the attenuation of microwave power in material which resulted in heating [14]. The dipole rotation is depended on several factors such as moisture, frequency and fibre directions [15].…”

“…Each OPT cube was polished with sand paper to obtain smooth surface and minimize air bubbles between probe and biomass during conducting measurement because presence of air gap might cause error in measuring by using open-ended coaxial probe. For preparing OPT core biomass with different moisture, the biomass was ovendried at 60 ± 2 o C at different time intervals following method described in a previous study [14].Dielectric properties of OPT were measured using openended coaxial probe which was attached to an automatic network analyser (model HP 85070-D) as illustrated in Fig 3. It is a non-destructive measuring method as it would not cause any changes on biomass characteristics during measurement. The 3.5 diameter open-ended probe was placed on the biomass according the fibre direction during measurement.…”

“…Therefore, dipole had shorter time to realign itself according the oscillating electric field [13], [28]. Conductive effect of microwave heating also diminished quickly in high frequency [14]. Hence, dielectric constant which indicated the ability of material to store electric energy decreased.…”