Complex sentence: Depending on the situation, the amount of chemical exergy added can be very small, or if the system being evaluated involves combustion, the amount of chemical exergy is very large and necessary to find the total exergy of the system.
Split into simpler sentences: Depending on the situation, the amount of chemical exergy added can be very small. If the system being evaluated involves combustion, the amount of chemical exergy is very large and necessary to find the total exergy of the system.