Thermochemical conversion covers different processes such as direct combustion, gasification, thermochemical liquefaction, and pyrolysis. In pyrolysis, heating rate affects reaction rate which in turn affects product composition. Traditional heating methods require expensive heating mech­anisms to achieve rapid temperature rise with poor process control. Micro­wave assisted pyrolysis (MAP) has following advantages: fine grinding of biomass is not necessary; microwave heating is mature and scalable technology which is suitable for distributed biomass conversion. Due to insufficient understanding of the mechanism of pyrolysis and the lack of effective control of the pyrolysis process, pyrolytic bio-oils are complex mixture with low calorific value, high acidity, high oxygen volume, and poor stability. However, bio-oil from pyrolysis of microalgae appears to have higher quality than those from cellulosics [52].

In recent years, the role of catalyst and minerals in biomass pyrolysis was investigated. Lu et al. [53] reported that ZnCl2 could catalyze fast pyrol­ysis of corn cob with the main products of furfural and acetic acid. Du et al. [54] used 1-butyl-3-methylimidazolium chloride and 1-butyl-3-methylimid — azolium boron tetrafluoride as the catalysts in MAP of straw and sawdust.