Green coal, also known as biomass, is a renewable source of energy that can be used as a substitute for fossil fuels. However, it requires certain conditions to be met before it can be burnt efficiently.

Heating to ignition point:
The correct answer to the question is option 'D', which states that green coal must be heated to its ignition point in order to be burnt. This means that the temperature of the biomass must be raised to a point where it can spontaneously ignite and sustain combustion.

Why is heating to ignition point necessary?
Heating green coal to its ignition point is necessary because it is a solid fuel that contains moisture and volatile compounds. These compounds need to be evaporated and driven off before the solid fuel can ignite and burn efficiently. Heating the biomass to its ignition point also ensures that the fuel burns completely and produces less smoke and pollutants.

Other factors that affect the burning of green coal:
Apart from heating the fuel to its ignition point, there are other factors that affect the burning of green coal. These include:

- Burning in excess air: Burning green coal in excess air ensures that the fuel burns completely and produces less smoke and pollutants. It also helps to maintain a stable flame and prevents the formation of soot and tar.

- Burning as powder: Burning green coal as a powder increases the surface area of the fuel and allows it to burn more efficiently. It also helps to reduce the amount of unburnt fuel that is produced during combustion.

Conclusion:
In conclusion, heating green coal to its ignition point is necessary in order to burn it efficiently. This ensures that the fuel burns completely and produces less smoke and pollutants. Other factors that affect the burning of green coal include burning in excess air and burning as powder.