Greetings,

I tried this mod and found two issues :
First, napalm shell is incredibly weak. It barely deals 50 damage to nests and sometimes kills blue biters/spitters. It is far from the destructive power displayed by the gif on your mod page.
I am only using Schall Artillery and its dependecies so the problem doesn't come from other mods.
IIRC I saw you on another post saying that your ammo spawns vanilla projectiles/patches so the damage values might be vanilla values, yet they are incredibly low or fire patches/effects arent triggered correctly, they struggle killing a single blue bitter.

Second, if I use this mod alongside Angels', poison shells have to be crafted in Angels' chemical plants, which is fine, however napalm shells must be produced in vanilla chemical plants, which are not buildable.

Thanks in advance

Thanks for your comment. Maybe first I point out the facts, then my impression and thoughts?

Facts:
1/ Napalm bomb and napalm artillery shell are releasing vanilla "fire-flame", which are the same entity made by vanilla handheld flamethrower and flamethrower turret.
2/ Bombs get damage bonus from stronger explosives tech. Artillery shells do not get any damage bonus at all.
3/ I try to fit vanilla gameplay as much as possible.

Impression:
I also found the napalm artillery shell quite weak. That's why I let the napalm artillery shells generates lots of "fire-flame", and really lots of them. That is the only way I can do to tune up the damage. Since there is no artillery shell damage tech, and I don't want to buff the vanilla "fire-flame". (Yet, players love the vanilla handheld flamethrower and flamethrower turret, and we are just spraying the same "fire-flame"...)
Still it sounds weak compared to other shells, so I set the recipes to be cheaper too (the major ingredients light & heavy oil are basically infinite). They are also unlocked at tech level 1, earlier than atomic shell.

Thoughts:
Maybe I can add a single AoE fire damage to both napalm bomb and artillery shell? (Maybe range 3-6, damage 300-600 fire and 300-600 explosion? Larger range then lower damage, and vice versa.) What do you think?
The artillery shell will always be weaker than its bomb counterpart, because of tech damage bonus. So the damage settings should be compared to the same type, like: vanilla/cluster artillery shell vs napalm artillery shell, or explosive rocket pack vs napalm bomb.
If you have other suggestions, you can also state here.

Angel's Mod:
I never use that or any other overhaul mods. So it seems to have the whole "chemistry" recipe category unused... Maybe you and others can directly provide solutions to me. Like what the original vanilla chemical recipes are turned into? What recipe category I should use? And which of the Angel's mod are directly changing the chemistry part to the game?

Thanks for that detailed answer :D

I think effectively it would fit the napalm shell to deal damage on a wide consistent area yet with lower damage than classic shell. Yet having a one-shot AoE damage would not represent what incendiary weapons are usually used for : area denial.
The current state of the napalm shells falls down only because of the low damage dealt and perhaps a not long enough duration. Its aim would not to destroy whole nests (even if it would be useful) but prevent any spawned creature to move, being instantly burnt. It would be a nice pre-assaul tool however it would be too similiar to poison shell.
The poison shell, for its part, actually does a nice job in cleaning worms after bombing nests with vanilla shells or before an assault. It also quickly kills any creature spawning in the meantime. The mobile nature of poison clouds allow to reach a greater surface yet makes it more imprevisible and above all it doesn't hurt spawners so it's a balanced counterweight.

Here comes the question of the role of the napalm shell.
We have vanilla shell with high concentrated damage, able to be mass-produced
We have cluster shell dealing decent damage on several concentrated zones, a bit more expensive
We have poison shell, large area low damage over time
We have nuclear shell, top tier ammo, high damage, large area, expensive.
Napalm shell should play in the area of large area DoT with the ability to destroy spawners, deal damage for a medium duration, shorter than poison which could be used to temporarily sterilize nests by killing any spawning creature.
Nevertheless I'm wondering where is the difference between the fire patches of the napalm shell and the ones from handheld flamethrower. The latter are just burning enemies like paper.
Don't the napalm fire patches benefit from incendiary damage bonuses ? On the gif of your mod page, I can see flame explosions that utterly destroy nests, what happened since ?

About Angels': it's Petrochem that turns a lot of chemical recipes into custom ones.
For instance, crude oil is replaced with multiphase oil which you seperate to obtain crude oil, raw gas and sulfuric waste water.
Plastic is made from liquid plastic made from (on tier 1) from naphtha (heavy oil) + catalyst or from propene made from methanol made from methane separated from liquid gas made from raw gas made from natural gas
Solid fuel needs coke alongside fluid.
It does deeply changes a lot of things and they all have different categories and subgroups, like

For solid plastic :
category = "crafting-with-fluid",
subgroup = "petrochem-solids",

For solid fuel :
category = "liquifying",
subgroup = "petrochem-fuel",

For crude oil separation :
category = "oil-processing",
subgroup = "petrochem-carbon-oil-feed",

So I don't know what I could tell...

Having some exploration with the fire and flames. There are more findings and solutions that I am still considering:

Note: I will use abbreviations for the frequent nouns in the following. I found that I have to type them a lot...

I guess I figured why the same "fire-flame" from handheld flamethrowers (HFT) and flamethrower turret (FTT) are stronger the from napalm artillery shell (NPAS) now. Because technology "Refined flammables" have buffed the flamethrower ammo and the FTT directly. My napalm rocket (NPR) get alternate buff from technology "Stronger explosives" instead. But the NPAS cannot get neither of them.

So my potential solutions are:
1/ More "fire-flame" generated per ammo NPR and NPAS.
2/ Additional instant fire damage in ammo NPR and NPAS.
3/ A cloned and buffed version of vanilla "fire-flame" for NPR and NPAS.
4/ Put NPR and NPAS under effects of tech "Refined flammables".
5/ Artillery shell damage tech.
6/ Mix of above. (See below)

Not all the them are feasible or I like. Let me explain a bit.
1/ Could be the easiest solution (out of 2) to buff damage. Although more "fire-flame" entities generated could add more workload to game updates thus possibly reducing UPS. Entities having high resistances (3/60% in spawner) may not get really significant damage from this.
2/ Another easiest solution to buff damage. Although it does not fit your wish for using it as area denial. (BTW, you have a very good view on different types of shells. I have similar things in mind but never it down clearly like yours.)
3/ Somewhat a middle of nowhere solution. This solution alone does not really solve anything, as it is a one-time solution. At earlier game, NPR could be overpowered because of the stronger flame. (NPR and NPAS will and should share the same flame. My Napalm MRV missile in my newer mod is reusing this anyway...) But at very late game (going crazy with infinite tech), NPR and NPAS still do not get any tech buff, so will still be weak.
4/ Theoretically the best, but practically may not be feasible at all. Because tech effects (under current game API) is applied to either an ammo category or a turret id. Ammo category is a group of ammo types that can be put into the same gun. For example, "rocket" is the ammo category for all the vanilla explosive rocket, atomic bomb, and my napalm bomb, where rocket launcher can use them all; "flamethrower" is the one for vanilla flamethrower ammo, where HFT and vanilla tank can use. But it seems not possible to set NPR and NPAS under new ammo categories to be included under tech effects, while can still be used in "rocket"-wielding weapons and artillery turrets/wagons.
5/ Somewhat similar to solution 4, although looks a bit better. Having a new tech to buff all artillery shell (including NPAS). But the downside is the vanilla artillery shells and my poison & atomic artillery shells are buffed as well... I think Wube devs intentionally left this out, because this can be too OP for PvP games, and there is no way to counter/prevent artillery shells. Introducing this tech can easily break such balance. So I will reserve this to the last, only if all else failed.
6/ As no single solution is a good fix, so maybe a mix of some of these will be more reasonable. I propose a mix of 1 and 2, which would be a bit conservative, but also mild enough that will not break other things.

About Angel's Petrochem, I have absolutely no idea ever.
I do not want to spend hours/days to study an entirely different chemical systems that I probably will have totally no experience or time to play with.
Maybe you can actually request on Angel's Petrochem mod side, asking what recipes of poison capsule, poison bomb, and poison artillery shell should be changed to (both category and ingeredients). Even better, you can just ask for the compatibility support on their mod request board.
My ammo and recipes are defined in data.lua stage. So they can totally change the recipes in data-updates.lua and data-final-fixes.lua stages. (Also possible in data.lua stage if having optional dependency.)

Just released 0.17.6 / 0.18.3 which has a general buff to napalm artillery shell. I have applied a mix of solution 1 and 2. A customized version of stronger fire flame is used instead of the vanilla one. It has roughly 4 times DPS to entities without fire resistances.

However, it may not satisfy your wish to burn spawners to death, due to a few causes.
A/ Spawners have very strong fire resistance of 3/60%.
B/ There is no vanilla tech for artillery shell damage bonus.
C/ You actually requested for long duration but weak sustained damage, and didn't like the one-shot but strong damage solution.

The vanilla fire does 13/60 per tick. (Not sure if the game actually does it per tick, or say a bigger punch of say 13/6 ~= 2.2 damage every 10 ticks or so...) So even my rough quadruple damage deals little damage because of causes A & B.
But using the same modded fire flame, the incendiary autocannon shells, incendiary bomb and napalm bomb (from Schall Tank Platon) and napalm MRV missile (from Schall Missile Command) have actually become a little bit OP for my taste...
So it means it is nearly impossible to balance napalm artillery shell with others, because of the above causes.

So I am buffing the incendiary and napalm ammo for 4 times DPS for the moment. But maybe some people will say it's too OP, so it is subjective to further changes. Let's see...