Some F-T factories use coal, biomass or other solid compounds as a starting point. Before these factories can begin the F-T process, they must turn the solids into gases such as CO, H, and alkanes. Changing solid chemical compounds into gas is called gasification. The gas collected from coal gasification often has a CO/H ratio of ~0.7 instead of the optimal ratio of ~2. This ratio can be adjusted from 0.7 to 2.0 using the water-gas shift reaction. Gasification is a dirty and expensive process. Coal-based Fischer–Tropsch factories start out with coal, gasify the coal, and then use the resulting gas as feed stock for the Fischer-Tropsch process. These factories can produce large volumes of CO in this way. One of the main reasons for this approach is the large amounts of energy required for a coal-based gasification process.