First operation of chemical-looping combustion with gaseous fuels was demonstrated in 2003,[15] and later with solid fuels in 2006.[16] Total operational experience in 34 pilots of 0.3 to 3 MW is more than 9000 h.[17][18][19] Oxygen carrier materials used in operation include monometallic oxides of nickel, copper, manganese and iron, as well as various combined oxides including manganese oxides.combined with calcium, iron and silica. Also natural ores have been in use, especially for solid fuels, including iron ores, manganese ores and ilmenite.