There is some debate about whether coal and logs can be burned together in a multi-fuel stove. Coal lasts long and produces a high heat output. Wood produces long flames and gives off a pleasant aroma. Picture the two fuels together and a roaring fire from a Christmas movie comes to mind.
Although a coal-wood combination sounds like a great idea, it is important to understand whether mixing fuels in a stove system is possible and even safe.
Wood and coal burn differently, with coal giving off sulphur dioxide and logs relatively high in water. Coal requires an open grate for oxygen to enter from below for combustion to occur effectively. Wood burns by mixing with oxygen above the log.
If you burn wood above a bed of coal, the top air intake will need to be opened. This results in the coal burning poorly because air enters above the fuel instead of through it. If you burn coal above the logs, ash from the burning wood will block the grate and leave the coal bathed in carbon dioxide. The only exception being kindling a fire with wood.
Also if the two fuels are burned together, the sulphur and water combine to create a acidic deposit. This solution can stick to and corrode your stove system and flue lining.
To sum up, multi-fuel stoves are designed to burn coal/smokeless coal and wood but not at the same time. Wood burners should only be used to burn wood and never coal. If you are still unsure, follow the manufacturer’s instructions for your appliance about choosing the right fuel and mixing fuels.