Come on dothe job properly, do it the right way.

It only takes me 1 hr to remove a post and concrete.

http://www.gb-online.co.uk/gardhand.html

The post will be approx 2ft in the ground, so how can you burn it out.
It will be soaking wet, as the post has rotted. What a mess.
Sodium Chlorate weedkiller has a fire depresant in it, so don't try.
Oxy is too dangerous, and I have not seen a torch with a 2ft extension
nozzle.
Do not use Metapost unless its really really necessary. If you do not
get them straight to start off with you have posts that lean over. Hit
a stone as you drive them in, and they twist. So you have twisted
posts.

Even when I drive in 3inch round to 6 inch round agricultural spiked
wooden posts into the ground by 2ft deep. Its impossible to get them
out by rocking them and pulling. The suction in the ground is
tremendous.

2 ways round it.

1 - Literally dig the concrete out, always dig around the concrete, so
its clear of the soil and is not supported. If possible leave the post
in situ, if not rotted away, as you can lever with it. Chisel the
concrete, or split the concrete in half, look for weak spots. As I say
it takes me 1 hr to get a post and concrete out.

2 - Why not set the posts half a fence panel away or there abouts. So
you are digging and putting in posts in bare ground. Then make up a
half panel to fill in the offset. Leave the old concrete in the
ground.

When I come round to putting in new posts/panels in its amazing what
historical relics of fencing you find.