Refuel

for i = 1, 16 do -- loop through the slots
  turtle.select(i) -- change to the slot
  if turtle.refuel(0) then -- if it's valid fuel
    local halfStack = math.ceil(turtle.getItemCount(i)/2) -- work out half of the amount of fuel in the slot
    turtle.refuel(halfStack) -- consume half the stack as fuel
  end
end