Characterization of bio-oil and bio-char produced by low-temperature microwave-assisted pyrolysis of olive pruning residue using various absorbers

Olive pruning residue is largely formed during cultivation, and is usually disposed through open-air combustion directly in the field, but this habit is a possible source of pollution. The pyrolytic conversion of olive pruning residue has been run in a new and very appealing way using microwave as a heating source and different microwave absorbers in a multimode batch reactor. In this way, olive residue is converted into interesting bio-chemical products with a short pyrolysis time, ranging from 15 to 36 min, and with a peak temperature ranging from 450 K to 705 K according to the different microwave absorber. Thus, a very efficient and selective system was realized, which was able to address the process towards the formation of a large amount of bio-char (up to 61.2%) or a high formation of bio-oil (56.2%) and gas (41.7%) with a very low formation of bio-char (2.1%). However, when carbon and iron were used as microwave absorbers, it was possible to obtain an intermediate amount of bio-char (26-30%) and bio-oil (40 wt%). Bio-oils were collected as dark-brown liquids with low viscosity and density. A bio-oil with a low water concentration was obtained using carbon or iron as the microwave absorber. The bio-oils formed in all experiments contained a very large amount of acetic acid, even when NaOH was the microwave absorber. Furthermore, a large amount of aromatics were present in the bio-oil obtained using carbon as the microwave absorber.