Complex sentence: A pyrotechnic heat source can be a direct part of a pyrotechnic composition e.g. in chemical oxygen generators a heat source composition with large surplus of oxidizer is used; the heat produced by burning the composition is used for thermal decomposition of the oxidizer.
Split into simpler sentences: A pyrotechnic heat source can be a direct part of a pyrotechnic composition. E.g. in chemical oxygen generators a heat source composition with large surplus of oxidizer is used; the heat produced by burning the composition is used for thermal decomposition of the oxidizer.