Graywater hardness issue
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In coal water slurry gasification, the hardness of the ash water depends primarily on the quality of the raw coal, but it is also related to the amount of wastewater discharged. The calcium and magnesium ions in the discharged wastewater account for only about 1/10 of those present in the raw coal; the majority of these ions are removed through the ash disposal system. Determining how to remove more calcium and magnesium ions via the ash is related to the gasification process itself. Some say that a higher furnace temperature results in lower ash water hardness, while others claim the opposite. In my opinion, the calcium and magnesium in the raw coal exist in salt form, and in high-temperature conditions they decompose into calcium oxide and magnesium oxide, which are soluble in water, thereby increasing the hardness of the ash water. If the furnace temperature is low, the calcium and magnesium salts precipitate directly in the water and are removed along with the ash. I would appreciate it if any expert out there could shed some light on how the vaporization reaction affects the hardness of gray water. Thank you.
Regardless of the furnace temperature, the composition of the ash changes. In my opinion, in addition to the ash content of the coal used as raw material, the pH value of the water quality is also a very important factor; systems operate more easily under alkaline conditions, which facilitates scaling·
In my opinion, the main components of the slag resulting from the gasification reaction are metal oxides (such as silicon dioxide, iron oxide, magnesium oxide, calcium oxide, titanium oxide, etc.). Coal contains nitrogen, and during the gasification reaction a small amount of ammonia nitrogen is generated and dissolves in water; meanwhile, CO2 dissolves in water to form carbonate and bicarbonate ions. Additionally, the condensate from the conversion process involved in gasification contains large amounts of ammonia nitrogen, which provides a high concentration of hydroxide ions and thus causes an increase in the pH of the water system. The metal oxides resulting from the gasification reaction, together with hydroxide and carbonate ions, constitute the main components of the fouling that forms. Furthermore, the calcium and magnesium ions in the water system mainly come from the raw coal; therefore, the concentration of these ions in the water is directly related to the quality of the raw coal. The ash resulting from the gasification reaction has a specific acid-base ratio, and the higher this ratio is, the lower the hardness of the coal. Meanwhile, coals with a higher acid-base ratio tend to have a higher ash fusion point as well. Therefore, in my opinion, the hardness of water has no direct relation to the vaporization reaction temperature; rather, it depends on the type of coal used.