Need to remove solder from copper toilet drain

I am remodeling a basement bathroom and in the process need to replace the closet flange for the toilet.

This toilet has a 3 inch copper drain below grade. There is some left over solder inside the copper drain that I need to remove before I can attach the new closet flange. I'm just not sure how to go about doing that.

I had considered using a torch to heat it up assuming it would then fall away and I would be left with a clean copper drain pipe. I am concerned though about using a torch near possible sewer gas. I cannot smell any sewer gas but understand it can be quite explosive.

Is there a safe way to heat up and/or otherwise remove the leftover solder from the copper drain pipe? Is it safe to use my Benzomatic Propane torch? Anything I should do first?

Thanks in advance for any advice.

qupte; understand it can be quite explosive.

I have never been able to get it to burn, forget about "explosion".