There are three types of fireplace grates. The first two are steel bar, which burns wood, and cast iron, which burns both wood and coal. The third type is a grate heater, which generates return heat into the home.
With steel bar grates, coal will not work. Wood is more versatile as it can burn on steel or cast iron grates.
You cannot burn coal with a steel bar grate because of the space between the bars. Wood is versatile as it can burn on steel or cast iron grates.
However, a cast iron grate offers greater durability and is longer-lasting.