Explosion is essentially a fast cascading combustion reaction. Combustion requires three things: Fuel, oxygen, and heat. In an explosion or any continuous combustion (candle, campfire), the heat requirement is satisfied by the reaction itself and only an initial ignition source is required. The difference between a continuous combustion and an explosion is basically the rate of reaction. Continuous reactions have their reaction rates limited in some way. These are the factors I can think of affecting a potential explosion:
- Oxygen-fuel mixture: In a campfire, combustion is limited by the supply of oxygen, which cannot penetrate into the fuel source (wood). In a dust cloud, oxygen will be mixed with the fuel. There is some optimum ratio of fuel to oil outside of which your explosion will happen less completely or not at all.
- Homogeneity: If the fuel-oxygen mixture is "lumpy", then any explosion could stop at localized areas of high or low fuel-oxygen ratio.
- Fuel quality: Things like dampness or contamination by incombustible material will raise the activation energy of ignition, making an explosion weaker and less likely to occur.
- Temperature: Likewise, fuel/air temperature will also affect activation energy of ignition. If something is already near combustion temperature, it's easier to ignite. Hot fuels also tend to be dryer.
- Fuel energy: How much energy is released when your fuel burns? If it's a lot, it's more likely to explode.
You asked specifically about the ignition requirement. If everything else is optimized (for volatility, or explosiveness), an extremely tiny spark would be able to ignite the whole mess. The spark in your car's engine is only a couple millimeters long. But as with most things in life, bigger is better. Having a larger, hotter ignition source increases the chance of ignition.
That's all I can think of. With that many factors, the exact requirements for explosiveness are way too complex to go into here but I hope this gives you a better idea of how such a system works.