Grate firing is like cooking on a barbecue, but instead of burgers and hot dogs, it's burning things for electricity or energy. It's called "grate firing" because it involves a big "grate" or a metal grid that holds the stuff you want to burn.
When you want to make energy or electricity, you put the things you want to burn on the grate. They can be things like wood, coal, or waste from things like crops or factories.
Once the things are on the grate, they get heated up. This heating makes them start to burn, and the fire creates a lot of energy in the form of heat. That heat turns water into steam, and the steam can be used to turn big turbines or other machines that make electricity.
Sometimes it's not safe to breathe in the smoke that comes out when you burn things, so grate firing has special tools called "scrubbers" that clean the smoke before sending it out into the air.
And that, little one, is how grate firing works!