The impact of limestone on water-coal slurry gasification
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Dear seniors, I have a few questions: 1. In the production process, apart from appropriately lowering the ash melting point and increasing the amount of ash in the system, are there any other effects of the amount of limestone added on gasification? 2. How does limestone change the ash fusion point? Does it increase the proportion of alkaline substances to lower the ash fusion point, or does it neutralize the acidic effect of additives?
There is another question: will the addition of limestone have a desulfurization effect?
The ash fusion point of coal is primarily determined by the content of acidic and basic compounds in the coal; the higher the amount of acidic compounds, the higher the ash fusion point, while the higher the amount of basic compounds, the lower the ash fusion point.
If too much limestone is added, calcium silicate with a high ash fusion point may form, which instead hinders the reduction of the ash fusion point. The ash fusion point of coal does not decrease monotonically as the amount of basic compounds increases. The ash fusion point of coal exhibits an inverse normal distribution curve in relation to acidic and basic compounds; that is, it first decreases, reaches a minimum value, and then increases again. Therefore, when adding fluxes, the choice should be based on the actual reason for the high ash melting point; it is not always limestone. But in most cases, there are more acidic compounds; as people say, adding limestone will do. However, attention must be paid to the amount of limestone added; if too much is added, it will instead cause the melting point of the ash to gradually increase after reaching its minimum value.
Adding limestone to water-coal slurry can improve the viscosity-temperature properties of the ash, as calcium oxide acts as an oxidant in the ash, disrupting the formation of silicon polymers and thereby reducing the viscosity of the liquid ash. However, when the amount of limestone added exceeds 30%, the range within which the slag can flow smoothly decreases; the viscosity of the slag increases as the amount of limestone added rises. This is because, when a large amount of limestone is added, more calcium silicate with a high melting point (melting point of 2130°C) is formed in the slag, thereby raising its melting point. Therefore, the amount of limestone added should not be excessive either; it should be determined through experiments based on different types of coal.
Another thing is to add as little of it as possible; that substance isn’t a panacea. Adding too much will cause scaling in the wastewater treatment system, and the rate of scaling **increases**! As a result, the increased maintenance costs have grown exponentially ； If too much is added, the ash melting point actually increases!