Ash is the solid material remaining after a substance has undergone combustion. It represents the non-combustible components of the original material, distinct from gases or liquids produced during burning. Ash is primarily inorganic, meaning it is composed of mineral matter. This mineral matter does not vaporize or break down even at high temperatures.
How Ash Forms
Combustion is a chemical process involving the rapid reaction between a substance and an oxidant, usually oxygen, producing heat and light. During this process, organic components within the material, which are typically carbon-based, undergo oxidation. These organic compounds are transformed into gases, primarily carbon dioxide and water vapor, and released into the atmosphere.
The intense heat of combustion causes water to evaporate and organic molecules to break apart. What remains are the inorganic elements and compounds that cannot be burned or converted into gas. These non-combustible residues, which were part of the original material’s structure, coalesce to form ash.
Chemical Composition of Ash
Ash is composed of inorganic minerals, including metal oxides and various salts formed from elements present in the original material. Common elements found in ash include calcium, potassium, magnesium, phosphorus, silicon, iron, and aluminum. While the majority of ash is inorganic, it can also contain trace amounts of unburnt carbon, particularly if the combustion was incomplete. These elements often take the form of oxides, such as calcium oxide (CaO) and potassium oxide (K₂O), or carbonates like calcium carbonate (CaCO₃).
Variations in Ash
The chemical composition of ash varies significantly depending on the material burned. Different source materials contain distinct proportions of inorganic elements, leading to unique ash profiles.
Wood ash, for instance, is rich in calcium (7% to 33%), potassium (3% to 10%), magnesium (1% to 2%), and phosphorus (0.3% to 1.4%). It also contains trace elements like manganese, iron, zinc, and copper, with hardwoods generally yielding more ash and nutrients than softwoods. Bone ash is predominantly composed of calcium phosphate, specifically calcium hydroxyapatite, alongside some calcium oxide and small amounts of magnesium phosphate.
Coal ash, a byproduct of coal combustion, contains major components such as silicon, aluminum, and iron (as oxides). It also includes varying quantities of calcium, magnesium, potassium, and sulfur. Coal ash can also contain trace elements like arsenic, cadmium, lead, and mercury, concentrated from the original coal during burning.