**4. Conclusions**

remarkably decreased with the increase of the microwave power. The peaks at 2920 and 2880 cm−<sup>1</sup> indicated aliphatic methylene groups. The peak intensity of pretreated PKS was smaller than raw PKS at higher microwave power of 450 and 600 W. The carbonyl group (C=O) bonds were

by decomposition of cellulose and hemicellulose. The peak was smaller at higher torrefaction tem-

of C=C stretching. The most concentrated peaks were observed in the range of 1500–1000 cm−<sup>1</sup> and assigned to C=O stretching and O–H deformation at organic compounds containing oxygen

The torrefied PKS at 8 min processing time with microwave power of 300, 450, and 600 W were chosen for thermal decomposition in TGA. The analysis of pretreated sample at 200 W was not chosen because its characteristic was similar with the untreated sample as discussed in the earlier section. The DTG curve of untreated and pretreated PKS is presented in **Figure 9**. The untreated and pretreated PKS showed three noticeable peaks existed in the DTG curve.

perature, which was linked with breakdown of hemicellulose. Peaks at 1550 cm−<sup>1</sup>

(alcohols, phenols, and ethers). Aromatic groups are represented by peak 790 cm−<sup>1</sup>

corresponding to various acids, aldehydes, and ketones, which were formed

present alkenes

for PKS.

observed at 1750 cm−<sup>1</sup>

42 Biofuels - Challenges and opportunities

**3.7. Thermal decomposition of torrefied PKS**

**Figure 9.** DTG curve of the untreated and pretreated PKS.

This study presents the properties of torrefied PKS using thermal pretreatment via microwave irradiation. The torrefied PKS underwent physical and chemical modifications, which include mass reduction, rise in energy content, and change in chemical compositions. The increase in microwave power level showed the significant effect, which decreased the mass and energy yield of torrefied PKS. As the microwave power level increased, the moisture, volatile mater, oxygen content, and O/C ratio decreased. Among the microwave powerlevel variation studies, the carbon content and calorific value were enhanced to 55.94% and 21.20 MJ/kg, respectively, at microwave power of 450 W. The peak intensity of oxygenated functional group was reduced with the increase of the microwave power as presented in FTIR spectra. The TGA analysis has correlated the thermal decomposition with hemicellulose, cellulose, and lignin in torrefied PKS. The research can be concluded that the PKS can be upgraded via MI pretreatment to a value-added feedstock at microwave power of 450 W with processing time of 8 min. Thus, the torrefied PKS has the prospective to be applied in thermochemical conversion such pyrolysis, liquefaction, and gasification or co-conversion with coal.

### **Acknowledgements**

This research project is funded by the Ministry of Higher Education, Malaysia, under Fundamental Research Grant Scheme (FRGS/1/2017/TK10/UITM/02/11). The authors acknowledge Universiti Teknologi MARA and Universiti Malaysia Perlis for providing facilities during the research work.
