I've tried setting up a boiler line that runs off of petroleum gas, but for some reason spent empty barrels can't be removed from the boilers. I've tried setting up filter inserters and they don't seem to be able to 'see' the spent barrels. Standard inserters will remove the full barrels, but ignore the empty ones. Is this a bug, or is this intentional? If I can't remove the spent barrels, it seems like the fuel barrels are only useful for vehicles.

I just had an idea, if it's too hard to code a way to remove the spent barrels, maybe just make a mod setting where barrels can just be 'burned up' and used up instead of leaving spent barrels? I'm fine with paying a cost in steel if it means I get lower pollution and more efficiency.

Or is this a mod conflict? I'm playing with a mod called "Burner Inserter Leech" that causes burner inserters to fuel themselves with burner devices... but then it shouldn't affect regular inserters, right?

This is a (really old) known issue, as stated on mod main page. Wube devs have not fixed it.
Inserters (and loaders) do not interact correctly with spent fuel slots in certain kinds of entities (such as furnaces.)

If you do not mind losing the steel, you may edit my mod file config/config-1.lua. Find the corresponding fuel entry, then remove burnt_result = "empty-barrel" from its item line.
For example, heavy oil would have line 22 changed from:

item = { name = "heavy-oil-barrel", barrel_volume = 50, burnt_result = "empty-barrel"},

to

item = { name = "heavy-oil-barrel", barrel_volume = 50},

However, I would NOT include this in official releases, as I still consider the barrel costs more than the fuel. :-D
You will need to edit your own copy.

Sorry I missed the issue! I saw it talked about furnaces, it makes sense that it applied to steam boilers too. I also agree that losing the steel is expensive, but I'm playing a mod called Next Gen Evolution where you get punished heavily for pollution, so I'm willing to pay extra in steel for not making the biters mad by burning coal. Alternatively, I might just tweak barrels to be cheaper. Thank you so much!!!!

I may look into the prototype definition of boiler, to see whether I can modify it to accept fluid as fuel. If it is doable, then we can just feed the fluid oil directly into it (without need of barrels or any loss of steel).
At the moment, please try the above as a temporary solution.

PS: I also play in Eco-friendly style. I will lower pollution as much as possible, by putting efficiency modules whenever appropriate. Yet not to the extent of having dedicated mods to punish myself. ;-)

Hello again!
I have created a mod Schall Machine Modification, which would especially suit your need.
The "fluid boiler" provided can take fluid fuel directly. So you no longer need any temporary solutions nor to waste any steel or edit any files!
Enjoy!

Hello! I installed the Schall Machine Modification and enjoy it, it's perfect. However, I found someone else made a mod that allows removing spent barrels (or other burnt items). I tested this mod in a sandbox and it seemed to work OK. I am not sure if this is something you could look into for your inserter mod? Here is the link: https://mods.factorio.com/mod/BurntExtractor

Yes, I have known this mod since its birth. :-D
This is done by rather frequent on_tick checking in Lua control script, so running at the expense of UPS.
Maybe it is still affordable for small base and very limited amount of such inserters, but not a feasible solution for larger factories.

In contrast, my fluid boilers do not have such overhead.
Because fluid intake into boiler is directly executed by game engine, hence much faster than such Lua control script.

What about now ?
https://forums.factorio.com/99501
Version 1.1.37
Modding
Inserters can now pull from the main output and burnt result inventory of an entity.

Thanks for the notice. Just did some tests.
Yes, finally fixed by game version 1.1.37, inserters can take the empty barrels off the stone furnaces, steel furnace, boilers, etc.
Loaders are still NOT working though.

So now there are two solutions to choose from:
A/ Update to 1.1.37+, then apply inserters to take the empty barrels.
B/ Use my fluid boiler from Schall Machine Modification, and no need to care for any barrels.

I assume that fake loaders would work ?

B/ Use my fluid boiler from Schall Machine Modification, and no need to care for any barrels.

How does that work with vehicles ?

I assume that fake loaders would work ?

If fake loaders are indeed normal inserters, then they will work like inserters.
If fake loaders are controlled by mod scripts in control stage, then really rely on that.
Better ask author of the mod(s) that creates/controls the fake loaders.

B/ Use my fluid boiler from Schall Machine Modification, and no need to care for any barrels.

How does that work with vehicles ?

The above statement is for boilers.