3 C + S + 2 KNO3 → K2S + N2 + 3 CO2
This is an oxidation-reduction (redox) reaction:
3 C0 - 12 e- → 3 CIV (oxidation)
S0 + 2 e- → S-II (reduction)
2 NV + 10 e- → 2 N0 (reduction)
C is a reducing agent, S is an oxidizing agent, KNO3 is an oxidizing agent.