I'm just a new guy on here, but I have had this very same thing happen to me and have figured out how to replicate it, stop it and control it. I am burning Superior nut, and based on some other threads, I understand it to be especially gassy. Basically, I can cause this "propane torch" anytime after loading the boiler with fresh coal, simply by restricting the volume of under fire air too much too soon. This is after all of the new coal appears to burning with blue flame - no orange glow yet. Stack temp is up good for me at about 325 or 350. If I mostly close the under air fire at this point, I will get blue flames in my flue and within a couple of minutes - maybe less, the stack will go to 700 if I let it. The cure? Simply crack the loading door to allow some over-fire air. This puts this "torch" out almost immediately. Sometimes I have to open it a little more than a crack to get it to go out, but a crack is all it needs to keep from happening while the coal is burning off these volatiles. I know this topic has been discussed other places in this forum, and a lot of time there is talk about how over-air fire is not good for coal - which I totally agree with once the volatiles have burned off. However, after loading new coal, is one situation where over-fire air is useful, at least for the type of coal and the setup I have. I have been experimenting with only allowing a tiny bit of over-fire air, still allowing a flicker of blue flame in the flue, without the crazy torch, to see if it allows me to shut the boiler up sooner after I put new coal in. I'm not sure if it speeds things up or not, but the point is I can control this "torch" by increasing or decreasing the amount of over-fire air after loading new coal.
I just want to share my method of dealing with this for the other new people, because the first time it happened to me, I was terrified because I didn't know for sure what caused it, or what to do about it. After getting up the guts to experiment, it was easy to see how to control the problem. I hope this helps someone else.