A systematic review of organic compounds in pyrolysis oil (bio-oil)

<br>Adding value to products is crucial to economic growth, resource efficiency, and most importantly, contributing to a circular economy and sustainability. Hence, the present study systematically reviews the value-added products derived from pyrolysis. Over 50 pyrolysis technologies were classified into eight main categories in order to provide a sound background. A total of 110 relevant studies have been selected from five databases (Web of Science, Scopus, ScienceDirect, Base, and PubMed). In addition to a bibliographic analysis, the purpose of this review was primarily to investigate and analyze the most frequently discussed value-added products, biomasses/feedstocks, and pyrolysis technologies in selected literature. Based on the proposed classification, more than 100 reports related to biomass and feedstock have been identified and categorized. Wastes and by-products from the industrial sector and lignocellulosic biomass were the most important categories. Afterward, more than 20 different pyrolysis technologies were identified by investigating used technologies. According to the results, pyrolysis as a general term, fast pyrolysis, catalytic pyrolysis, co-pyrolysis, microwave-assisted pyrolysis, and slow pyrolysis were the most frequently occurring. Most importantly, value-added products were investigated in the last step. Following the extraction of more than 1700 products, the number of unique chemicals was reduced to approximately 1000 after the removal of duplicates. Afterwards, a keyword analysis was conducted. Methyl, ester, acid, phenol, benzene, and ethyl showed the highest incidences. Furthermore, the analysis of chemicals revealed that the frequency of occurrence indicates that acetic acid, aromatic compounds, biochar, syngas, and phenolic compounds took the lead in the selected literature. In this study, pyrolysis technology is highlighted as an important tool for transitioning towards a circular and sustainable bioeconomy. Further studies may be required to examine and compare the economic (market) and environmental values of such products.