You can safely burn wood in a coal-burning stove. Coal-burning stoves are designed to handle the considerably higher temperatures involved in coal burning, and thus will not be damaged by burning wood. Care should be taken, however, to ensure that the increased smoke output and smell of burning wood does not affect the air in your home.
A Word of Caution
While burning wood in a coal stove is not dangerous, the opposite cannot be said. Coal should never be burnt in a wood stove, as the high temperature at which coal burns can cause serious damage to your stove. With prolonged use of coal in a stove not rated for the temperature, you can expect to see structural failure within the body of the stove, leaks in the smoke flue, and even metal bursting and house fire. Coal should never be burnt in a wood-burning stove.
Burn a Test Log
The smell of burning wood is substantially different from that of coal. Depending on the type of wood, there may be significantly more smoke than with coal as well, so a test log should be burnt before committing to a full load in your coal stove. Take a small piece of hardwood, such as maple or oak, and get a fire started with it in your stove. Evaluate the smell and odour in your home, and use carbon monoxide detectors to make sure the air in your home is safe to breathe. There should be no carbon monoxide in the air. If you feel comfortable with the test log, add more wood and re-evaluate.
Chop Wood Smaller
One issue you may find with a coal-burning stove is that there is much less space inside the stove for adding fuel. Coal comes in smaller chunks for burning, so cramming a log into your stove may prove difficult. Chop precut firewood in half for easier fitting. Logs should be no larger than the stove opening.
Keep the Chimney Clean
Burning wood can deposit more residue in your chimney. Do not switch back and forth between wood and coal. Instead, burn all of the wood you intend to use in your stove for the season at once, and then have your chimney cleaned before switching back to coal. The increased temperature output of coal can set fire to residue deposited from burnt wood in your chimney, causing potential damage to your chimney and home.