**1. Introduction**

Crude oil and coal with high energy density and stable properties to store and transport are preferred fuels for industrial and home applications [1]. The recent reduction in affordable fossil fuels and environmental concerns increased greenhouse gas (GHG) emissions from these fuels that have motivated the public and private sectors to develop renewable energy sources. In recent years, the governments of Canada and the USA adopted regulations to phase out coal-fired power plants by 2030 and investigate other resources to reduce GHG emission. Biomass feedstocks are considered as the renewable resource with a near-zero CO2 input-output balance [2–4]. Biomass is the only renewable energy source which may be used in all solid, liquid and gas phases [5]. But compared to fossil fuels, biomass is lower in energy (17–19 MJ/kg) and bulk density (60–100 kg/m3 ) [6];

heterogeneous in physical, chemical and thermal properties; high in moisture [6], mineral [7] and oxygen contents [8]; highly hygroscopic [9] and difficult to handle [6]. Converting biomass to secondary liquid or gaseous fuels through thermal conversion processes is a way of increasing the energy density and transportability. Mechanical, chemical and thermal pretreatments are able to modify the biomass properties in order to produce a more homogeneous fuel and minimize the post-treatment fuel upgrading.

The current article reviews the feedstock properties that are important for the fast pyrolysis process and the potential pretreatments to modify the biomass properties are explained. Specifically, drying, grinding, washing and torrefaction processes and their influence on the final bio-oil product are thoroughly explained.
