What type of combustion is represented by :
burning of coal dust in a coal mine ?

Explanation

**Step-by-Step Solution:**

1. **Understanding Combustion**: Combustion is a chemical process in which a substance reacts with oxygen, releasing energy in the form of heat and light. There are different types of combustion, including complete combustion, incomplete combustion, and spontaneous combustion.

2. **Identifying the Scenario**: The question refers to the burning of coal dust in a coal mine. Coal dust is a fine particulate form of coal that can be easily ignited.

3. **Recognizing the Type of Combustion**: In the case of coal dust in a coal mine, it can ignite without any external flame or spark due to the heat generated from the dust itself or from other reactions. This phenomenon is known as spontaneous combustion.

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