Wood ash is the powdery residue remaining after the combustion of wood, such as burning wood in a fireplace, bonfire, or an industrial power plant. It is largely composed of calcium compounds along with other non-combustible trace elements present in the wood. It has been used for many purposes throughout history.
A comprehensive set of analyses of wood ash composition from many tree species has been carried out by Emil Wolff, [1] among others. Several factors have a major impact on the composition: [2]
According to one research on the average the burning of wood results in about 6–10% ashes. [2] The residue ash of 0.43 and 1.82 percent of the original mass of burned wood (assuming dry basis, meaning that H2O is driven off) is produced for certain woods if it is pyrolized until all volatiles disappear and it is burned at 350 °C (662 °F) for 8 hours. [lower-alpha 1] Also the conditions of the combustion affect the composition and amount of the residue ash, thus higher temperature will reduce the ash yield. [4]
Typically, wood ash contains the following major elements: [2] [ clarification needed ] [5]
As the wood burns, it produces different compounds depending on the temperature used. Some studies cite calcium carbonate (CaCO3) as the major constituent, others find no carbonate at all but calcium oxide (CaO) instead. The latter is produced at higher temperatures (see calcination). [3] The equilibrium reaction CaCO3 → CO2 + CaO has its equilibrium shifted leftward at 750 °C (1,380 °F) and high CO2 partial pressure (such as in a wood fire) but shifted rightward at 900 °C (1,650 °F) or when CO2 partial pressure is reduced. [6]
Much of wood ash contains calcium carbonate (CaCO3) as its major component, representing 25% [7] or even 45% of total ash weight. [8] At 600 °C (1,112 °F) CaCO3 and K2CO3 were identified in one case. [lower-alpha 2] Less than 10% is potash, and less than 1% is phosphate. [7]
There are trace elements of iron (Fe), manganese (Mn), zinc (Zn), copper (Cu) and some heavy metals. [7] Their concentrations in ash vary due to combustion temperature. [3] Decomposition of carbonates and the volatilization of potassium (K), sulfur (S), and trace amounts of copper (Cu) and boron (B) may result from increased temperature. [3] The study has found that at raised temperature K, S, B, sodium (Na) and copper (Cu) decreased, whereas Mg, P, Mn, Al, Fe, and Si did not change relative to calcium (Ca). All of these trace elements are, however, present in the form of oxides at higher temperature of combustion. [3] Some elements in wood ash (all fractions given in mass of elements per mass of ash) include: [2] : 304
One study has determined that a slowly burning wood (100–200 °C (212–392 °F) ) emissions typically include 16 alkenes, 5 alkadienes, 5 alkynes and several alkanes and arenes in proportions. [lower-alpha 3] [9] Ethene, acetylene and benzene were a major part at efficient combustion. [9] Proportion of C3-C7 alkenes were found to be higher for smouldering. [9] Benzene and 1,3-butadiene constituted ~10–20% and ~1–2% by mass of total non-methane hydrocarbons. [9]
Wood ash can be used as a fertilizer used to enrich agricultural soil nutrition. In this role, wood ash serves as a source of potassium and calcium carbonate, the latter acting as a liming agent to neutralize acidic soils. [7]
Wood ash can also be used as an amendment for organic hydroponic solutions, generally replacing inorganic compounds containing calcium, potassium, magnesium and phosphorus. [10]
Wood ash is commonly disposed of in landfills, but with rising disposal costs, ecologically friendly alternatives, such as serving as compost for agricultural and forestry applications, are becoming more popular. [11] Because wood ash has a high char content, it can be used as an odor control agent, especially in composting operations. [12]
Wood ash has a very long history of being used in ceramic glazes, particularly in the Chinese, Japanese and Korean traditions, though now used by many craft potters. It acts as a flux, reducing the melting point of the glaze. [13]
For thousands of years, plant or wood ash was leached with water, to yield an impure solution of potassium carbonate. This product could be mixed with oils or fats to produce a soft "soap" or soap like-product, as was done in ancient Sumeria, Europe, and Egypt. [14] However only certain types of plants could produce a soap that actually lathered. [15] Later, medieval European soapmakers treated the wood ash solution with slaked lime, which contains calcium hydroxide, to get a hydroxide-rich solution for soapmaking. [16] However it was not until the invention of the Leblanc process that high quality sodium hydroxide could be mass produced, rendering obsolete the earlier forms of soap using crude wood or plant ash. [17] This was a revolutionary discovery that facilitated the modern soapmaking industry. [18]
The ectomycorrhizal fungi Suillus granulatus and Paxillus involutus can release elements from wood ash. [19]
Wood ash is sometimes used in the process of nixtamalization, where corn is soaked and cooked in an alkali solution to improve nutritional content and decrease risk of mycotoxins. The alkali solution has historically been made from wood ash lye.
Nixtamalization was originally practiced in Mesoamerica, from which it spread northwards through various indigenous tribes of North America. In eastern North America, nixtamalized corn was traditionally eaten in porridges and stews, a dish that Europeans would call hominy. [20] Wood ash is also used as a preservative for some kinds of cheese, such as Morbier and Humboldt Fog. [21] [22]
An early leavened bread was baked as early as 6000 BC by the Sumerians by placing the bread on heated stones and covering it with hot ash. The minerals in the wood ash could have supplemented the nutritional content of the dough as it was baked. [23] In present day, the amount of wood ash content in bread flour, as measured by the Chopin alveograph, [24] is strictly regulated by France. [25]
Calcium is a chemical element; it has symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar to its heavier homologues strontium and barium. It is the fifth most abundant element in Earth's crust, and the third most abundant metal, after iron and aluminium. The most common calcium compound on Earth is calcium carbonate, found in limestone and the fossilised remnants of early sea life; gypsum, anhydrite, fluorite, and apatite are also sources of calcium. The name derives from Latin calx "lime", which was obtained from heating limestone.
A carbonate is a salt of carbonic acid, H2CO3, characterized by the presence of the carbonate ion, a polyatomic ion with the formula CO2−3. The word "carbonate" may also refer to a carbonate ester, an organic compound containing the carbonate groupO=C(−O−)2.
A cement is a binder, a chemical substance used for construction that sets, hardens, and adheres to other materials to bind them together. Cement is seldom used on its own, but rather to bind sand and gravel (aggregate) together. Cement mixed with fine aggregate produces mortar for masonry, or with sand and gravel, produces concrete. Concrete is the most widely used material in existence and is behind only water as the planet's most-consumed resource.
Calcium carbonate is a chemical compound with the chemical formula CaCO3. It is a common substance found in rocks as the minerals calcite and aragonite, most notably in chalk and limestone, eggshells, gastropod shells, shellfish skeletons and pearls. Materials containing much calcium carbonate or resembling it are described as calcareous. Calcium carbonate is the active ingredient in agricultural lime and is produced when calcium ions in hard water react with carbonate ions to form limescale. It has medical use as a calcium supplement or as an antacid, but excessive consumption can be hazardous and cause hypercalcemia and digestive issues.
Calcium oxide, commonly known as quicklime or burnt lime, is a widely used chemical compound. It is a white, caustic, alkaline, crystalline solid at room temperature. The broadly used term lime connotes calcium-containing inorganic compounds, in which carbonates, oxides, and hydroxides of calcium, silicon, magnesium, aluminium, and iron predominate. By contrast, quicklime specifically applies to the single compound calcium oxide. Calcium oxide that survives processing without reacting in building products, such as cement, is called free lime.
Sodium carbonate is the inorganic compound with the formula Na2CO3 and its various hydrates. All forms are white, odourless, water-soluble salts that yield alkaline solutions in water. Historically, it was extracted from the ashes of plants grown in sodium-rich soils, and because the ashes of these sodium-rich plants were noticeably different from ashes of wood, sodium carbonate became known as "soda ash". It is produced in large quantities from sodium chloride and limestone by the Solvay process, as well as by carbonating sodium hydroxide which is made using the Chlor-alkali process.
Calcium hydroxide (traditionally called slaked lime) is an inorganic compound with the chemical formula Ca(OH)2. It is a colorless crystal or white powder and is produced when quicklime (calcium oxide) is mixed with water. It has many names including hydrated lime, caustic lime, builders' lime, slaked lime, cal, and pickling lime. Calcium hydroxide is used in many applications, including food preparation, where it has been identified as E number E526. Limewater, also called milk of lime, is the common name for a saturated solution of calcium hydroxide.
Hard water is water that has a high mineral content. Hard water is formed when water percolates through deposits of limestone, chalk or gypsum, which are largely made up of calcium and magnesium carbonates, bicarbonates and sulfates.
Calcium sulfate (or calcium sulphate) is the inorganic compound with the formula CaSO4 and related hydrates. In the form of γ-anhydrite (the anhydrous form), it is used as a desiccant. One particular hydrate is better known as plaster of Paris, and another occurs naturally as the mineral gypsum. It has many uses in industry. All forms are white solids that are poorly soluble in water. Calcium sulfate causes permanent hardness in water.
The Solvay process or ammonia–soda process is the major industrial process for the production of sodium carbonate (soda ash, Na2CO3). The ammonia–soda process was developed into its modern form by the Belgian chemist Ernest Solvay during the 1860s. The ingredients for this are readily available and inexpensive: salt brine (from inland sources or from the sea) and limestone (from quarries). The worldwide production of soda ash in 2005 was estimated at 42 million tonnes, which is more than six kilograms (13 lb) per year for each person on Earth. Solvay-based chemical plants now produce roughly three-quarters of this supply, with the remaining being mined from natural deposits. This method superseded the Leblanc process.
Calcination is thermal treatment of a solid chemical compound (e.g. mixed carbonate ores) whereby the compound is raised to high temperature without melting under restricted supply of ambient oxygen (i.e. gaseous O2 fraction of air), generally for the purpose of removing impurities or volatile substances and/or to incur thermal decomposition.
Ammonium sulfate (American English and international scientific usage; ammonium sulphate in British English); (NH4)2SO4, is an inorganic salt with a number of commercial uses. The most common use is as a soil fertilizer. It contains 21% nitrogen and 24% sulfur.
Lime is an inorganic material composed primarily of calcium oxides and hydroxides, usually calcium oxide and/or calcium hydroxide. It is also the name for calcium oxide which occurs as a product of coal-seam fires and in altered limestone xenoliths in volcanic ejecta. The International Mineralogical Association recognizes lime as a mineral with the chemical formula of CaO. The word lime originates with its earliest use as building mortar and has the sense of sticking or adhering.
A pyrotechnic composition is a substance or mixture of substances designed to produce an effect by heat, light, sound, gas/smoke or a combination of these, as a result of non-detonative self-sustaining exothermic chemical reactions. Pyrotechnic substances do not rely on oxygen from external sources to sustain the reaction.
Ash glazes are ceramic glazes made from the ash of various kinds of wood or straw. They have historically been important in East Asia, especially Chinese pottery, Korean pottery, and Japanese pottery. Many traditionalist East Asian potteries still use ash glazing, and it has seen a large revival in studio pottery in the West and East. Some potters like to achieve random effects by setting up the kiln so that ash created during firing falls onto the pots; this is called "natural" or "naturally occurring" ash glaze. Otherwise the ash is mixed with water, and often clay, and applied as a paste.
Cement kilns are used for the pyroprocessing stage of manufacture of portland and other types of hydraulic cement, in which calcium carbonate reacts with silica-bearing minerals to form a mixture of calcium silicates. Over a billion tonnes of cement are made per year, and cement kilns are the heart of this production process: their capacity usually defines the capacity of the cement plant. As the main energy-consuming and greenhouse-gas–emitting stage of cement manufacture, improvement of kiln efficiency has been the central concern of cement manufacturing technology. Emissions from cement kilns are a major source of greenhouse gas emissions, accounting for around 2.5% of non-natural carbon emissions worldwide.
Oxy-fuel combustion is the process of burning a fuel using pure oxygen, or a mixture of oxygen and recirculated flue gas, instead of air. Since the nitrogen component of air is not heated, fuel consumption is reduced, and higher flame temperatures are possible. Historically, the primary use of oxy-fuel combustion has been in welding and cutting of metals, especially steel, since oxy-fuel allows for higher flame temperatures than can be achieved with an air-fuel flame. It has also received a lot of attention in recent decades as a potential carbon capture and storage technology.
Calcium looping (CaL), or the regenerative calcium cycle (RCC), is a second-generation carbon capture technology. It is the most developed form of carbonate looping, where a metal (M) is reversibly reacted between its carbonate form (MCO3) and its oxide form (MO) to separate carbon dioxide from other gases coming from either power generation or an industrial plant. In the calcium looping process, the two species are calcium carbonate (CaCO3) and calcium oxide (CaO). The captured carbon dioxide can then be transported to a storage site, used in enhanced oil recovery or used as a chemical feedstock. Calcium oxide is often referred to as the sorbent.
A carbonate fluoride, fluoride carbonate, fluorocarbonate or fluocarbonate is a double salt containing both carbonate and fluoride. The salts are usually insoluble in water, and can have more than one kind of metal cation to make more complex compounds. Rare-earth fluorocarbonates are particularly important as ore minerals for the light rare-earth elements lanthanum, cerium and neodymium. Bastnäsite is the most important source of these elements. Other artificial compounds are under investigation as non-linear optical materials and for transparency in the ultraviolet, with effects over a dozen times greater than Potassium dideuterium phosphate.