The energy stored in coal is directly form the sun which is solar energy. This energy are preserved in carbon atoms and they originate from the life processes of a plant.
Plants carry out photosynthesis and produce their energy by this process.
Green plants have special adapted structures by which they trap solar energy from the sun and use it to produce their own food.
The food is used by plant for nutrition and life activities.
In essence the solar energy is converted to chemical energy in plants.
To form coal, when plants are rapidly buried their organic matter is preserved from decay by combination with oxygen.
More burial preserves the complex molecules that have stored during photosynthesis.
