Glow plugs, circulating oil heaters, circulating coolant heaters

Adding a small amount of kerosene in your fuel

Can anyone add more light on this??

Can we use air heaters for the same?

well, you have to monitor some critical points: ensure that all your fluids are ready for this start : liquid phase, not too high viscosity. The fuel should be vaporized in your combustion chamber in order to enable the combustion start. You can either increase the volatility of the fuel by adding high volatiliyt fractions (see my comment on kerozene above) or heat the fuel in order to decrease its viscosity (heater on the fuel line, circulation of the fuel in the circuit before injection...). Another way could be to heat the intake air in order to enhance this volatilization.

Keeping the engine warm is the best method for cold weather starting. Any method that keeps the engine warm would work. Before engine heaters came along, people would actually build small fires under equipment to warm the engine up before starting.

Glow plugs, coolant line heater, or engine oil pan heater are different means to start a diesel engine at -15 deg C.  Air heaters may not work as the temperature required for combustion inside the combustion chamber is not sufficient enough to sustain self combustion because of the cool engine block and the relatively cold fuel.