The continuous demand for fossil fuels has directed significant attention to developing new fuel sources to replace nonrenewable fossil fuels. Biomass and waste are suitable resources to produce proper alternative fuels instead of nonrenewable fuels. Upgrading bio-oil produced from biomass and waste pyrolysis is essential to be used as an alternative to nonrenewable fuel. The high oxygen content in the biomass and waste pyrolysis oil creates several undesirable properties in the oil, such as low energy density, instability that leads to polymerization, high viscosity, and corrosion on contact surfaces during storage and transportation. Therefore, various upgrading techniques have been developed for bio-oil upgrading, and several are introduced herein, with a focus on the hydrodeoxygenation (HDO) technique. Different oxygenated compounds were collected in this review, and the main issue caused by the high oxygen contents is discussed. Different groups of catalysts that have been applied in the literature for the HDO are presented. The HDO of various lignin-derived oxygenates and carbohydrate-derived oxygenates from the literature is summarized, and their mechanisms are presented. The catalyst’s deactivation and coke formation are discussed, and the techno-economic analysis of HDO is summarized. A promising technique for the HDO process using the microwave heating technique is proposed. A comparison between microwave heating versus conventional heating shows the benefits of applying the microwave heating technique. Finally, how the microwave can work to enhance the HDO process is presented.

As a consequence of the shortage of traditional resources and escalating environmental constraints, the feedstocks for the production of energy and chemicals are swiftly changing. Biomass has received remarkable attention from both academia and industries as it is the most promising feedstock for these applications. Understanding the conversion mechanisms of such renewable low-value material to value-added products would lead to providing insights to enhance the product yield and/or quality and, in turn, help the new products compete with the traditional ones. In this regard, this paper provides an updated review on the processing of biomass for the production of valuable-products that could replace a part of the fossil fuel-based energy and chemicals. The common technologies that are performed in the conversion processes are demonstrated, and the thermochemical technique is emphasized. Several chemical reactors and their processes for fast pyrolysis applications are presented. The organic chemistry of pyrolysis of a biomass plant and its three constituents (i.e., cellulose, hemicellulose, and lignin) is debated. The effect of the heating mechanism, process parameters, loading of catalyst and other aspects are discussed. Eventually, the economics aspect of fast pyrolysis of biomass is evaluated.