Hmm ... tricky, in as much because I wouldn't think potassium or sodium nitrate would have much of an odor when added to something that burns (that don't themselves burn.) You can try using the bare minimum of oxidizer, or adding more fuel, or maybe change the consistency of what you're making -- more loose to allow more air in. Or drying herbaceous material more so there's less moisture to inhibit burning, so you don't need oxidizer. Off hand, I can't think of a cheaper, cleaner, safer oxidizer.