Destructive distillation of coal means heating coal in the absence of air. This process breaks down the complex organic compounds in coal into simpler molecules such as gases, liquids, and solids. These products are then collected and can be used for a variety of purposes such as the production of coal gas, tar, and coke. This process is called destructive distillation because the coal is being broken down or "destroyed" into simpler substances through the process of heating.