4.Since coal contains fewer hydrogen atoms than oil,then have to add some extra hydrogen to the gas.

1. 你的说法很对.
2. since所加的原因状语从句一般放在主句之首,表众所周知的原因,有时翻译成"由于/既然"更准确.
3. 此外since还可以做adv/prep/conj,较多出现完成时态.请看例句:
I've lived here since.自那以后我就一直住在这儿.
I've lived here since 1998. 自1998年以来我就一直住在这儿.
It is/ has been8 years since I moved here.自从我搬到这儿来已经8年了.
8 years passed since I had worked there.自从我不在那儿工作以来已经8年过去了.