What causes the explosion accident of the furnace of a gas-fired steam boiler

If the following three conditions occur during the burning and flameout process of a gas-fired boiler, the furnace is likely to blow up.

1. The residual gas content in the furnace reaches the ignition limit at the start-up phase of the furnace.
2. The fuel content in the furnace reaches the ignition limit after several times failure of ignition.
3. The fuel is sprayed out because of the halfway extinction of the flame. And, the temperature of the furnace can not meet the spontaneous combustion condition of the sprayed fuel. However, the content of the sprayed fuel reaches the ignition limit.