I've embarked into a mountainous land with no bitumous coal or lignite, and can only find magma in the magma sea. Because of these limitations, I'm going to be migrating my fortress 150 z-levels down; to be closer to my forges (avoiding the fun of magma pump stacks).
My main problem is that 150 squares is a long way to go to put refuse into the outdoor refuse stockpile. Is there some way I can persuade my dwarves to automatically get rid of refuse into the magma sea instead?
If you mean a normal garbage dumping zone (i.e. for things designated for dumping), it should be possible to do this by creating one in the normal way - I guess the most sensible way would be to create a channel down from above the magma. You may need to disable any other dumping zones you have to stop your dwarves deciding they'd rather go to the surface.
However according to this wiki entry (spoilers beware), note that "any items dropped into the magma sea (specifically, onto "magma flow" tiles above semi-molten rock) will vanish without a trace".
If you mean for general refuse, I don't think there's a way to dump this automatically. You could keep a refuse stockpile near your magma dump and manually designate things for dumping now and then, but that may be a hassle. Alternatively you could just create a larger indoor refuse pile with appropriate precautions for sealing the resulting miasma inside the stockpile, let the majority of refuse rot away and use any bits that don't in crafting (bones, shells etc).
I believe what you can do is channel a hole above the magma sea, build a trapdoor over it, then designate that trapdoor as a garbage dump. Put it in a small room by itself, with a door. Then build a lever outside the room and link it to the trapdoor. When you designate something for dumping, it will be placed on the trapdoor. When everyone is out of the room, lock the door (to make sure no one is standing on the trapdoor) and tell a dwarf to pull the lever, dumping the garbage dump into the magma sea. Also, the door will prevent miasma spreading into the rest of the fortress, should you choose to dispose of refuse and corpses this way.
Maybe Minecarts could be helpfull? If you know how minecart railgun works You could make minecart refusegun :). You could shoot refuse into magma, outside area, or closed room. You could even kill some goblins using refuse :D
Although you can design your fortress to dump refuse into the magma sea, it's probably less trouble to instead build a Dwarven Atom Smasher. Atom Smashers have the advantage of not requiring a path to the magma sea; they can be built anywhere. To build one requires only a lever attached to a drawbridge (two mechanisms are also required), where the open drawbridge lays flat on one or more tiles where you designate a dump area. When the drawbridge is opened, it smashes all items under it into oblivion.
Advantages of using an Atom Smasher (rather than the magma sea) to get rid of trash:
1) Avoid the 'fun' of creatures that live in the magma sea entering your fortress.
2) No need to create a chute or other way to get refuse down multiple Z levels to the magma sea.
3) Artifact items may not be destroyed in the magma sea; these undestroyed items will lower game FPS since the game still tracks them.
Disadvantage of using an atom smasher:
1) One of your dwarves needs to pull the drawbridge lever now and then to destroy garbage.
2) Dwarves standing too close to the dump point may be obliterated when the drawbridge is dropped.
