When deployed, all players may use clean herbs on it and it will provide 10 times the Herblore experience given for cleaning herbs, as well as Firemaking experience, while destroying the herbs. Similarly to portables, it lasts for 5 minutes after being deployed, after which it must be redeployed to resume. However, it can be used an unlimited number of times and only works on clean herbs. Also, multiple herb burners can be placed as long as they're 5 spaces apart from each other. Approximately 1000 herbs can be burned per hour. Testing implies 28 herbs are burned in 1 minute 38 seconds or one herb in 3.5 seconds.
There was a bug with the herb burner and clean spirit weed. A left click would show the message that you have no herbs for use. You needed to right-click and choose 'Use' and use the spirit weed on the herb burner. This got fixed on 10 July 2017.