Hey Willie, I answered this one over on TMP but I'll reply here also. they would have been used by the Engineers in the army. Records show that they did have a few of the Biosca Mod. 1918 flamethrowers which I believe the tank is on a cart of sorts. These could possibly been used late in the Rif War and maybe in some of the Peninsular Army's garrisons. Nationalists employed them more than the Republicans, I found no record of the Russians providing any, but the Germans sold at least four Flammenwerfer Mod. 35s to Francos forces in 1936 and trained 30 men through the Condor Legion in their operation in October of the same year. Records show that the Nationalists also acquired several of the Italian Lanciafiamme Mod.36 flamethrowers and these may have been more common. Again these would have been employed by the Engineer sections of the Nationalist or if captured Republican Army, but not by the Militias or Paramilitary forces.
Hope that helps,
Doc