Briquettes?
The answer generally is it's OK; not harmful anyway (unless of course they are unburned and still contain lighter fluid). But they are alkyline and will raise the pHs so you could do a soil test. If you don't want to, try sprinkling them (no more than 20 lbs per 1000 sq ft) around non acidic plants.

Adding Coal Ashes to Soil - Complete Garden Guide

I found this site that you may find useful, on the front page there is a referance to Briquettes.

I assume ( bord na mona, brand name in ireland) briquettes, or perhaps from a local bog?. a garden centre could be a valuable source of information.

Thanks Joypulv, feel your info is right. Will try a small amount to begin with.