fossil fuel power stations. Calcination equilibrium Calcination of limestone using charcoal fires to produce quicklime has been practiced since antiquity by cultures all over the world. The temperature at which limestone yields calcium oxide is usually given as 825 °C, but stating an absolute threshold is misleading. Calcium carbonate exists in equilibrium with calcium oxide and carbon dioxide at any temperature. At each temperature there is a partial pressure of carbon dioxide that is in equilibrium with calcium carbonate. At room temperature the equilibrium overwhelmingly favors calcium carbonate, because the equilibrium pressure is only a tiny fraction of the partial pressure in air, which is about 0.035 kPa. At temperatures above 550 °C the equilibrium pressure begins to exceed the pressure in air. So above 550 °C, calcium carbonate begins to outgas into air. However, in a charcoal fired kiln, the concentration of will be much higher than it is in air. Indeed, if all the oxygen in the kiln is consumed in the fire, then the partial pressure of in the kiln can be as high as 20 kPa. The table shows that this partial pressure is not achieved until the temperature is nearly 800 °C. For the outgassing of from calcium carbonate to happen at an economically useful rate, the equilibrium pressure must significantly exceed the ambient pressure of . And for it to happen rapidly, the equilibrium pressure must exceed total atmospheric pressure of 101 kPa, which happens at 898 °C. {| class="wikitable" |+ Equilibrium pressure of over (P) versus temperature (T). |- !P (kPa) |0.055||0.13||0.31||1.80||5.9||9.3||14||24||34||51||72 ||80||91||101||179||901||3961 |- !T (°C) |550||587||605||680||727||748||777||800||830||852||871||881||891||898||937||1082||1241 |} Solubility With varying pressure Calcium carbonate is poorly soluble in pure water (47 mg/L at normal atmospheric partial pressure as shown below). The equilibrium of its solution is given by the equation (with dissolved calcium carbonate on the right): {| width="500" | style="width:50%; height:30px;"| | Ksp = to at 25 °C |} where the solubility product for is given as anywhere from Ksp = to Ksp = at 25 °C, depending upon the data source. What the equation means is that the product of molar concentration of calcium ions (moles of dissolved per liter of solution) with the molar concentration of dissolved cannot exceed the value of Ksp. This seemingly simple solubility equation, however, must be taken along with the more complicated equilibrium of carbon dioxide with water (see carbonic acid). Some of the combines