The LEL for H2S in air is stated to be 4.5% v/v. In a CO2 rich atmosphere, the LEL would be higher, since CO2 has a much higher specific heat than air.
Given the autoignition temp for H2S is 232degC, think you can manually calculate if combustion will self sustain at this 5% v/v concentration. Compute the adiabatic flue gas temp for stoichiometric combustion and see if it gets beyond 232degC.