Which of the following conditions can lead to boiler scale formation?

The correct choice pertains to high dissolved solids concentration, which plays a significant role in the formation of boiler scale. When water is heated in a boiler, any impurities and dissolved solids can precipitate out and form deposits on the heat transfer surfaces. This is because the solubility of certain minerals decreases with increasing temperature. As water converts to steam, the concentration of these dissolved solids increases, which can lead to sedimentation and scale buildup.

Boiler scale typically consists of carbonate and sulfate compounds, which are the result of the evaporation of water that leaves behind these impurities. Over time, scale can reduce the efficiency of the boiler, increase fuel consumption, and potentially lead to overheating and damage to boiler tubes.

In contrast, low feedwater temperature may initially seem relevant, but it does not inherently lead to scale formation. The formation of scale is primarily influenced by the concentration of dissolved solids. Excessive oxygen in the water is more related to corrosion than to scale formation, as it promotes oxidation and the breakdown of metal surfaces. Too much chemical treatment might impact water chemistry, but if managed correctly, it is generally intended to prevent scale rather than cause it. Thus, high dissolved solids concentration stands out as the primary condition that directly leads to boiler scale formation.