Effect of flux addition on gasification

The cosolvent primarily serves to lower the ash fusion point of the raw coal, enabling coals with high ash fusion points to ensure smooth slag discharge without the need for extremely high operating temperatures. Coal with a high ash fusion point necessarily has an imbalance in the acid-base ratio of its ash; this ratio is either too high or too low. When the acid-base ratio of the ash is too high, limestone, which is commonly used, is added as a solubilizing agent. Limestone increases the proportion of calcium oxide in the ash, thereby reducing the acid-base ratio and achieving the goal of lowering the ash fusion point. If the acid-base ratio of the ash is too low, a solubilizer also needs to be added; however, in this case it is not limestone that is used, but rather substances that give the ash acidity, such as silicon oxide (things like sand). By adding a cosolvent, coal with a high ash melting point and that is difficult to react completely becomes easier to react with and reaches a molten state. Once the coal is in this molten state, almost all of the carbon in it can be reacted away. Therefore, it is easy to understand why adding a cosolvent can increase the gas production per unit volume. However, the idea that it can improve the cold gas efficiency seems a bit hard to comprehend. Cold gas efficiency is defined as the calorific value of the gas divided by the calorific value of the raw coal; since the calorific value of the raw coal remains constant, to increase cold gas efficiency one needs to increase the calorific value of the gas. The best way to do this is to increase the content of methane, which has a high calorific value. But for water-coal slurry, it is impossible to increase the methane content. Thus, there is only one option left: to increase the content of useful gases. An increase in the content of useful gases is equivalent to an increase in the calorific value of the gas, which in turn means an increase in cold gas efficiency. So, with this in mind, it becomes easy to understand: coal with a high ash fusion point is more difficult to gasify without the use of solvents; as a result, its gas production rate is low and the content of useful gases is also low. Since a large amount of heat is used to melt the ash and turn water into steam, less heat is available for the gasification reaction, especially for the steam decomposition reaction that produces hydrogen gas. When a cosolvent is added, the ash is more easily melted, which reduces the amount of heat required for its melting. As a result, more heat is available for the reaction, thereby increasing the concentration of useful gas components in the gas. I’m not sure if my explanation works; the reasoning given for improving the efficiency of cold gas always seems a bit forced.

Personally, I believe that the addition of flux lowers the ash melting point; in other words, less heat is required to melt the slag. Since the source of this heat is the heat released by coal combustion (without considering the heat generated by the combustion of volatiles), less coal is needed for combustion. As a result, more coal per unit volume can be used in the gasification reaction rather than being wasted through combustion. That way, it can be explained

We define cold gas efficiency as the carbon in the syngas/carbon participating in the gasification reaction. Personally, I think it’s the carbon-to-carbon ratio of CO in the gas compared to the carbon that goes into the furnace

Your definition seems to be another way of calculating the carbon conversion rate