|Biomass feedstock property||Remarks|
• Smaller feedstock particle size generally improves the microwave-assisted pyrolysis process by increasing the heating rate as well as the maximal temperature .|
• The relationship between feedstock particle size and the bio-oil yield is inconsistent. Generally, too small particles may lead to too high external surface area whereas too large particles may result in incomplete pyrolysis and each of these would result in a low bio-oil yield for the microwave-assisted pyrolysis of biomass [111, 112].
• The biomass particle sizes of 2–4 mm are better for microwave-assisted pyrolysis, and these particle sizes are larger than the optimal particle sizes (0.85–2 mm) for the conventional electrical heating pyrolysis [79, 110].
• The microwave absorbability of a biomass feedstock is described by its tangent loss (tan δ). Water has a very good microwave absorbability, the moisture content in a biomass renders the feedstock a high temperature rising rate during the drying process . With increasing temperature the moisture content evaporates, the biomass feedstock becomes less microwave absorptive, and the temperature rising rate then slows down. The tangent loss value (microwave absorbability) of a biomass feedstock would be significantly varied by reaction temperatures [114, 115].|
• Due to the decreases in the tangent loss value, the microwave absorbability of the biomass feedstock becomes lower and lower. This makes the pyrolysis temperature can only achieve a very low value (e.g., less than 200 °C) even if the microwave power is increased. Tangent loss materials (microwave absorbers) which can absorb microwave energy much more efficiently are used to enhance heating [116, 117].
• Ash content is another main inorganic matter in a biomass besides the abovementioned moisture content. The ash components in biomass feedstocks vary significantly from feedstock to feedstock, e.g., rice straw and wood sawdust have high contents of SiO2 (52.66–69.52%) and K2O (10.30–40.13%) while sewage sludges have low contents of SiO2 (1.62–26.40%) and K2O (0.05–1.62%) [118,119,120,121].|
• Generally, some ash components are good microwave absorbers and some are not. e.g., Fe2O3 and TiO2) have higher tan δ values (0.001–0.05) whereas some ash components (e.g., MgO and SiO2) have lower tan δ values (0.0002–0.0005) .
• The ash components themselves cannot be changed to form bio-oil components, the high ash content would therefore reduce the bio-oil yields. Bio-oil yields were still very low. Studied on microwave-assisted heating of three kinds of oil shales, with high ash content showed that low bio-oil yields (0–0.24 wt.%) was obtained; which were mainly caused by the high ash contents (60.5–70.9%) .
• The major organic components in biomass feedstocks are carbohydrates, proteins, and lipids.|
Lignocellulosic biomass is mainly composed of cellulose (23–60%), hemicellulose (25–44%), and lignin (12–49%) . Generally, cellulose and hemicellulose result in more bio-oil than lignin .
With conventional electrical heating, the decomposition temperature ranges are cellulose (315–390 °C), hemicellulose (250–350 °C), and lignin (200–550 °C) . However, with microwave-assisted pyrolysis, the lignocellulose components decomposed at lower temperature (around 100–150 °C lower) [115, 125]. Generally, the lignocellulosic biomass components under microwave-assisted pyrolysis would produce more bio-oil than electrical heating pyrolysis [124, 126].
• Algae biomass is rich in protein (9–30%), lipids (30–62%), and some carbohydrates (2–18%). Algae biomass is usually not a good microwave absorber, even if it has high lipid content. Among carbohydrate, protein, and lipid, the good microwave absorber is lipid . Microwave absorbers are generally required to obtain high temperatures for microwave-assisted pyrolysis of an algal biomass .
• Between microwave-assisted pyrolysis and conventional heating pyrolysis, the bio-oil yield from the microwave-assisted pyrolysis is lower [129, 130]. However, it should be noted that generally, algal biomass produce bio-oil that has much better quality by having higher H/C and H/O ratios and thereby showing higher HHVs (higher heating values) .
• Plastics and rubbers are basically polymers with high hydrocarbons. They are also widely used in microwave-assisted pyrolysis for bio-oil production. Due to the low microwave absorbability of the plastic components (e.g., polypropylene, polystyrene, polyethylene), the plastic cannot achieve a high temperature (lower than 180 °C) if no microwave absorber is added but can increase significantly when microwave absorber are added [132, 133].
• Due to the high organic contents of plastics, the microwave-assisted pyrolysis of plastics would result in high bio-oil yields [132, 133].
• Compared with the plastics, rubbers are better microwave absorbers. The microwave-assisted heating of rubber can achieve a high temperature of 500 °C .
• The high organic contents in rubbers give high bio-oil yields for microwave-assisted pyrolysis of rubbers [132, 134].