acetylene
炔 is a specialized chemical kanji for acetylene, a flammable hydrocarbon gas used in welding and lighting. Its meaning is narrow and technical, with no broader conceptual center.
炔 combines 火 (fire) with 夬, which likely contributes the sound. It was created as a modern chemical character for acetylene, a flammable gas, so the fire radical is semantically fitting.
The fire radical 火 on the left hints at something flammable. The right side 夬 looks like a gas cylinder with a valve. Together they suggest acetylene, a gas that burns with a hot flame.
For ケイ, imagine a gas can labeled 'K' for acetylene: K -> ケイ, and the fire radical reminds you it's flammable.