What is kiln drying and why is it good?

Trees have plenty of water inside while growing, excess moisture is removed from timber during the kiln drying process. There might be around 80% moisture content in the timber originally. It must be dried down well below 20% in order to use for fuel safely. Theoretically it is possible to season or air-dry Logs in a shed but it would take 2-3 years get moisture down below 20% in the core of the log. Kiln drying allows you to undertake this process in 4-5 days using special kiln ovens where temperature is controlled at the optimum level.

What is the difference between Birch and Ash firewood?

These are both Hardwoods. Ash will burn a bit longer and hotter than Birch due to the fact Ash is around 10% denser than Birch

What is the best type of wood to use for fuel?

Hardwood species are best for fuel use because hardwoods are more dense and the denser the wood the longer and hotter it burns. Softwoods like Spruce or Pine tend to spit while burning so hardwoods are by far better and safer.