What is the chemistry behind the reaction of KNO₃, K₂CO₃ and Sulfur?

I'm preparing a chemistry show where I end the show with yellow powder bang.


The powder is a mixture of:


$$\ce{K2CO3 + KNO3 + S} $$
My question is, what's the reaction between these chemicals?




Here's just a guess, but I do have some confidence that I'm right:  

$\ce{KNO3 + S -> K2SO4 + K2SO3 + K2S + K2O + SO2 + N2}$  
  

$\ce{K2CO3 ->[\Delta] K2O + CO2}$  

The first, unbalanced equation is the reaction between potassium nitrate and sulfur, producing quite some gas as product, and a variety of potassium compounds left behind, with the composition depending on your formulation of the powder. Of course there might be some sulfur trioxide produced, but the major oxide of sulfur should be the dioxide. In the second equation is the thermal decomposition of potassium carbonate releasing carbon dioxide. This is the part that I'm not very sure of, but it may be to release more gas to give your reaction more puff. However, IIRC the decomposition requires a very high temperature. In contrast, bicarbonate has a much lower decomposition temperature and also release more gas(carbon dioxide and water), but the products would likely be absorbed by potassium oxide to form hydroxide and carbonate, so bicarbonate might not help much.


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Most likely,
a mixture of potassium polysulfides is formed first. Possibly, a further more complex reaction takes place between the nitrate ion, carbonate ion and sulfide that forms small amounts of tetrasulfur tetranitride, disulfur dinitride and sulfites, evolving nitrogen and sulfur dioxide. The remaining solids are clearly (just look at their possible structure) more reactive than simple polysulfides and ought to be responsible for the final high speed decomposition of the mixture.
On a side note, the less costly way to determine if sulfur nitrides are actually present would be to pass the sample under a Raman spectrometer.


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