This article discusses classical analytical models with respect to their ability to model bio-oil droplet combustion and, especially, the heating period before the droplet microexplosion. Following an analysis of the droplet combustion histories published in several literature sources, a diffusion limit model with a radiative heating term and Stefan boundary condition was identified as the most adequate for the droplets' preheating period. Considering the multiphase nature of the bio-oil, a bubble-shell model was developed. The basic assumption of this model is that the bio-oil volatile matter forms a core with an expanding radius and growing pressure, while the solids and the low-volatile matter form a membrane (shell). Special attention was paid to the assumption that droplet microexplosions are due to the superheating of droplet kernel liquid. In this regard, several approaches to calculate the limiting temperature, and consequently the droplet preexplosion time, were discussed. These approaches are mainly based on the assumption that impurities in the kernel play roles of nuclei in microbubble growth. An idea for how to define the bio-oil superheat limits via intersection of asymptotes of bio-oil surface temperature evolution with temperature was conceived and a method defining the bio-oil superheat limit based on surface tension variation with temperature was developed. An alternative shrinking-core model of bio-oil droplet explosive disruption was also discussed. A scale analysis defined the parameters that must be determined in preliminary tests of bio-oils, enabling an estimation of the significance of the preexplosion controlling parameters and testing of the results with the published data.