How to Train Your Dragon did explain that each dragon species has a limited to its shots and each dragon would be "out" of fire for a time period with this in mind and different dragons employed different methods of projection (the Jock Guys was a flaming liquid like an oil, the girlfriend's dragon had a more firework style flame, the twins used a dragon with a separate explosive heavy gas and a spark like flame, both in-effective when used on their own. The Gronkl would swallow rocks and then puke them out with a coating of a flaming substance for an added weight to their shots and were limited by the availability of boulders to use as a projectile. Toothless used a plasma-like burst that complemented its speedy flight and wasn't all that well described. The exact shot limit of the Night Fury as a whole was not documented as their attacks relied on hard fast moving hits).
This has a basis in the real world, namely venomous snakes, who's venom is delivered via hollowed fangs. Snakes have a fixed supply of venom and adult snakes can regulate the amount delivered in each bite, allowing them to conserve the supply to an effective killing dose. Baby snakes don't have the muscular control down and thus tend to inflict a single venomous bite that dumps all of their supply in one shot. This will make them deadlier if you are the first bite victim, but the survival is much better if you are the second and they haven't replenished their venom. Snake Venom is produced in venom sacks located in the upper skull and giving a distinctly wider skull then constrictor snakes, not unlike a "fire sack". Most dragons in Train seem to have two separate mechanisms, a fuel sack, and a heat ignition source, the fuel sack would create a substance with a low combustion threshold, typically a liquid or gaseous secretion. Again, venomous snakes support this in that they are both modified saliva, and different species produce venoms that do different things to kill. They are either nerotoxins (itself with three such methods of disrupting the nerves from sending and reciveing signals from the brain... which includes the "Pump heart", "breath in", "breath out", and "move it you're about to be swallowed whole" signals), Cytotoxins which target other non-nerological vital system (including killing cells indiscriminantly, killing blood cells to stop oxygen distrobution, and killing the heart, stopping blood flow) or a combination of both Neortoxins and Cytotoxins for "nuke it from orbit" levels of assurance that it's dead. This is supported by the description of the two headed dragon's method of flame, which has the ignition source and fuel source in separate heads, allowing for the combustible gas to linger until the ignition source lit the gas (a one headed dragon will likely have control over the ignition source so that it doesn't accidentally light itself on fire on the inside, which also supports the idea that dragons are not internally fire proof, a plot point in the first movie. A dragon sending fire into another dragon's throat could ignite the fuel secretion sack, which may be lethal to the dragons (as it proves to the huge dragon in the climax) but could be survivable (as the tiny dragon merely puffed up and scurried away). It should be noted that the small dragon was not in the act of releasing his fire and probably had a very small fuel sack that would have combusted without lethal injury which, given they tend to scavenge from larger dragons, it could mean that this behavior of killing dragons is used by dragons when fighting one another and would be an adaptation that allowed the tiny dragons to survive a capture, while the giant dragon was in the process of launching a fire attack when the killing blow was delivered.
Again snakes are similarly designed as a venomous snake is immune to self envenoming by biting itself, suggesting it's body adapts to handle it's own venom but not the venom of another snake, which could prove fatal.
It also supports some of the non-fire dragons, as DVD extras suggest there are multiple dragons with multiple heads that comprise of a "mystery class" dragon and they favor deceptive hunting practices to using fire projection as the gas head could possibly create a fatal situation where it's fuel sack is ignited while it is deploying it's gas. In film, I do recall it the gas head closing it's mouth prior to the spark head igniting the gas. Other dragons do not use fire at all as ocean dwelling dragons favor either skalding water, suggesting a modified fuel sac that can draw in water and expel a boiling jet. It might be the ignition mechanism is super heated as cold sea water of a North-Atlantic-esque ocean would be difficult to super heat, but such a heat source would provide body heat in the cold waters. Another aquatic dragon uses concussive sounds (and has a wider jaw than most dragons) suggesting that this dragon could have a fuel that explodes quickly and in cold conditions but is loud enough to break materials while in a gaseous atmosphere and is widely lethal underwater (a concussive force in water is more lethal because living things are made of mostly water.... the force doesn't change it's momentum when it goes through a body... unlike in air, where the body is denser and thus a more effective barrier closer to a comparable blast). This would suggest that certain dragons adapted these methods to better perform in niche environments where they could have an edge over land dragons, who specifically are shown to be defanged when doused with water. Again, similar behaviors can be found in nature. There are frogs that use water to shoot flies out of the air, and the pistol shrimp uses underwater sound to stun it's prey. This sound is so loud that it competes with the Blue Whale for loudest sound produced by a living organism, and the mechanics of the sound generation produce a quick burst of heat measured at 4,700 degrees Celcius, which is about 800 degrees cooler than the surface of the sun. The sheer speed of the whole "click" event prevents it from vaporizing itself because the entire click of a pistol shrimp lasts only a millisecond. The Sound is lethal to fish in a 4 cm radius from the claw that generates the blast.
For your dragon, it could have a fuel source that is flamable on air with oxygenated air. So it doesn't burn it's lungs, the the sack opening is covered by a mucus membrane that is not permeable to oxygen. When the dragon shoots an explosive sphere, this mucas membrane will envelop the fuel, creating a barrier that prevents combustion as it leaves the throat, the final product exiting the mouth would behave like a water balloon, and will burst on impact, causing the fuel to ignite. Since it is compacted, the fuel explodes on impact, splashing the immediate area with flaming residue. The ripped mucas sack could stick , trapping the now burning oily fuel to skin, which would estinguiesh the fuel after oxygen is consumed, but could leak as the body moves, causing slow drip of unconsumed fuel to reignite on contact with oxygen. The sudden light and sound energy released in an explosive ignition could potentially blind and deaffen those out of the "splash zone". As an added fun, since the fire sack is the only part of the dragon that is not fireproof, this dragon would be able to switch from the explosive attack to a wider flame attack by using it's teeth to puncture the mucas mebrane as it leaves it's jaws, (the lack of a single penetrative point would mean this has a less deafening sound that the dragon could tolerate. As a final bit of fun, like a water balloon, the distortion of the sack as it is rotating in the air could cause it to break before impact, creating a wider splash zone as the oil spreads overhead, rather then spashed onto a surface. It could also provide a nice "fireball" effect if the glob starts to break while flying, as small amounts of fuel ignite but lack sufficient oxygen to be explosively lethal.
I'm not sure which specific chemicals would cause this effect if any, but the physics are based on water balloons... but the liquid is much more volatile than water. It's even easier to source if your dragon's element is "acid" because acid secretion is quite common in wildlife... Your own digestive tract contains some of the strongest acids known to science. And human gastric acid is especially potent as humans are omnivores and thus will ingest a greater range of things that need to be converted into energy to fuel the body... and with that are susceptible to a host of diseases that typically don't survive the stomach's acid bath (I so want an Osmosis Jones-esque premise of human body being a cell city... but the white Blood cells lowering a germ into the stomach ala Bond villains, telling the hapless virus "No Mr. Streptococcus, I don't expect you to talk. I expect you to die"). The stomach lining that contains it isn't safe for long and is constantly replaced and the stomach also produces sodium bicarbonate, a base, to regulate the acid strength, with the chemical waste product being dissolved Carbon dioxide that is expelled through the lungs.
For everything else, you should probably list the specific element that is expelled.