The composition of calcium oxide and sulfur content in fuel have thermal chemical reacted to generate solid calcium sulfate, which can meet the requirements of environmental protection of desulfurization. Theoretically, 1ml calcium is required for 1 mol sulfur content to react and to generate stable calcium sulfate. but in the actual applied limestone desulfurization reaction process , due to the factors of reaction rate, mass exchange rate and reverse reaction, 1mol calcium in limestone can't react 1 mol sulfur content in fuel completely. So, the broad-brush of calcium mole applied actually shall be appropriately increased, in order to improve the desulfurization efficiency. at this moment, the ratio of calcium moles added with the sulfur content S to be removed in coal is called mole ratio of calcium and sulfur, which is indicated with Ca/S. The higher ratio means the lower utilization rate of the limestone in combustion equipment.