Fire is a complex phenomenon based on simple chemical reactions: combustion. The basic principle of which is the oxidation process of fuel, of which the rate of reaction depends on various aspects, which resulted in an exothermic reactions, producing heat and reactants.
Thus the core essence of fire itself is combustion. Whether or not flame can be observed from the reaction process is another topic for later discussion (smouldering fire essentially is flameless, and transition from smouldering to flaming combustion is a complex nature in general).
However, what does it defines compare to explosion? The basic difference between fire and explosion is within the rate of reaction from the combustion process, which ultimately for an explosion, causes an overpressure. The pressure wave can be coupled with the flame front which causes the reaction to reach high speed, often times exceeding the speed of sound, > Mach 1.
What causes the overpressure from an explosion reaction is the presence of confinement and dispersion of fuel, that if the fuel reaches the lower flammability / explosivity limit, and energy source exceeding its Minimum Ignition Energy is present, explosion might occur.
However, as first stated, fire is a complex phenomenon, and often times a small fire incident could turn into catastrophic event of fire and explosion, given the right condition. A tragic example of this is the Beirut Explosion which started off as fire incidents from hot works, which trigger the fire of a nearby fireworks being stored in the warehouse. This in turns causes fire, which pre-heated the massive storage of Ammonium Nitrate.
Incidents of Dust Explosion often involving multiple explosion episodes. The initial explosion disperses more combustible dust into the air, which trigger secondary dust explosions. Given the right circumstances, secondary fire could be achieved and worsen the damage of the incident.
To conclude, back to basics, we should focus our attention in preventing any sort of fire and explosion incidents by conducting a thorough risk assessment and making sure risk prevention and risk mitigation measures are in place and dependable.
Written by : Fahri Ali Imran