in coastal communities it would often be dried and burned - however it is a moot point whether this was to deliver the potash but obviously in poorer times and communities you do not waste heat, so vraic may have been burned like peat. In the petro chemical age burning peat for warmth seems nonsensical - burning seaweed, several times more so. First it was discovered to be easier to hunt whales and make oil that way then black gold arrived extracted with even less effort. So now we have an environmental conscience and scientists are continuing to push the boundaries, can seaweed provide fuel?
This has been tried in many ways such as mimicking the extraction of sugars from land grown plants. A major project in Chile was shut down within the last decade because the process appeared to be uneconomic. Since then oil prices have risen and then tumbled making North Sea oil almost uneconomic despite sunk investment.
The future for extracting bio fuel from seaweed looks bleak but at least the scientists are still trying.