I eat a lot of raw hazelnuts (I love them!) I have been trying to use the cracked shells in the garden for years. Here is a list some of my attempts:
Composting them. This wasn't very effective, as they hardly break down at all within a composting cycle for my large bin (9 months)
Vermi-composting them. Not even the worms seem to be interested in them, because they go mouldy very slowly and even when they do, they are still very tough.
Mulching with them. Now, this wasn't much of an failure, but I still have some doubts about the process. My first concern is that they may attract slugs; then, that they might get mixed up in the bulk of the soil lock some of the nitrogen that would otherwise be available to plants.
The best plan for this season is to mulch around soft fruit, whose beds do not get any disturbance apart from mulching in early spring.
Does anyone have any preference, suggestion, experience on what to do with them?