Just an FYI, Factorio changed how pollution works in an older version, so it's a fixed amount per minute rather than a calculation. Your mod hasn't updated that, so as of now, all your drills have 0 pollution.

To fix this, you need to change the "protodrill" energysource.emissions = x / 1.5 to energysource.emissions_per_minute = X. Also, you'll need to make several changes to "function(make_drill)" and "data:extend". For ease, here's those sections already fixed:

local function make_drill(name, health, emissions, mining_speed, mining_power, watts, search_radius, slots)
local drill = util.table.deepcopy(protodrill)
drill.name = "skan-" .. name .. "-mining-drill"
drill.icon = "__skan-advanced-mining__/graphics/mining-drills/" .. name .. "-mining-drill.png"
drill.minable.result = "skan-" .. name .. "-mining-drill"
drill.max_health = health
drill.mining_speed = mining_speed
drill.mining_power = mining_power
drill.energy_usage = watts
drill.energy_source.emissions_per_minute = emissions
drill.resource_searching_radius = search_radius
drill.module_specification.module_slots = slots
return drill
end

data:extend({
make_drill("advanced", 600, 8, 1.0, 4.0, "180kW", 2.49, 3),
make_drill("elite", 900, 6, 2.0, 5.0, "240kW", 3.49, 4),
make_drill("ultimate", 1200, 4, 3.0, 6.0, "300kW", 3.49, 4)
})