What types of fuel can I use in my kettle?

If it burns...you can use it!   We use sticks, dry grass, dried peat, paper & cardboard, spirit burner, fistful of sand with petrol poured over it, bean can with petrol in it, your mates shirt, old underpants, etc.!! Sticks work best as they create an inferno in the chimney which rapidly heats the large chimney wall and boils the water very quickly.  
Top Tip if sticks are damp or wet:  You can basically use anything to light a fire and then start feeding long sticks (leave them sticking out the top) into the chimney.  The heat from the smaller fire below will dry the sticks and they will gravity feed down as the bottom most piece burns!  Here in Ireland it can be very difficult to find dry fuel at times so, we use anything we can to start a small fire, then feed longer (usually damp) stick into the chimney using this trick.   Try it out ... it works!