I found this video from the Discovery Channel. It doesn’t appear that there is anything “bad” in briquettes to preclude them from being used in the garden.
The recipe for briquettes is:
- Wood
- Anthracite coal
- Lime
- Corn starch
Note – a couple of other non-official sites also list a few other ingredients – borax, sodium nitrate, sawdust, mineral char. Site 1, site 2.
Butterfly Gardening 
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