To support the required economies of scale for coal-to-liquids (CTL) process to be economical and competitive with crude oil, all stages of the operations, from coal mining to the Fischer-Tropsch process and product work up must be run with great efficiency. Due to the complexity of the Lurgi gasifers used, the quality of the coal was paramount. The initial annual output from the Sigma underground mine in Sasolburg was 2 million tons. Annual coal production from this mine peaked in 1991 at 7.4 million tons. Today, most of the gasifers in Sasolburg have been replaced with autothermal reformers that feed natural gas piped from Mozambique. Natural gas generates about 40–60% less carbon dioxide for the same energy produced as coal, thus is significantly more environmentally friendly. Gas-to-liquids technology converts natural gas, predominately methane to liquid fuels.