*4.2. Chemical Properties*

Wood combustion produces highly alkaline ash (pH varies from 9 to 12) [62]. Ash yield is decreased approximately to 45% with an increase in burning temperature from 538 ◦C to 1093 ◦C. The metal contents in ash increase with the increase in the burning temperature. With the increase in the burning temperature, elements such as calcium, iron, magnesium, manganese, and phosphorus increase, and elements such as zinc, potassium, and sodium decrease [62].

The alkalinity of WWA depends on the carbonate, bicarbonate, and hydroxide content in it. WWA composition also varies during storage and under different environmental conditions as carbon dioxide and moisture react with WWA to form carbonates, bicarbonates, and hydroxides [62]. An increase in potassium, sodium, and manganese concentration increases linearly with ash concentration. The leaching of these elements increased to result in a decrease in pH. Table 1 shows the chemical elements of different types of ashes.


**Table 1.** Chemical elements of the ashes (ppm) [53].

Zajac et al. [53] divided ash components into three categories based on their concentration: 1. macro-elements: phosphorus, potassium, calcium, and sulfur; 2. micro-elements: manganese, iron, copper, and zinc; 3. toxic elements: chromium, nickel, arsenic, and lead. The quantity and quality of WWA content depend upon the organic, inorganic, and impurity elements present in it. The chemical and physical characteristics of WWA depend upon on the sampling point, the sort of biomass, plant kind, growth process, growth circumstances, plant age, fertilization, the applied dosage of plant protection products, harvesting conditions, and process of burning (preparation of fuel, burning method used, and circumstances) [51,53].

Szakova et al. [64] determined the chemical composition of WWA (wood chips and wood waste) using the XRF technique. Different elements analyzed (in ppm) were P: 5300–10,800; S: 1200–11,100; K: 38,000–58,000; Ca: 78,000–159,000; Cr: 118; Mn: 6200–10,700; Fe: 29,300–34,800; N: 28.9; Cu: 153; Zn: 300–1100; As: 9.8; and Pb: 313. Tarun et al. (2003) revealed the subsequent elements in wood ash: C (5% to 30%), Ca (5% to 30%), carbon (7% to 33%), K (3% to 4%), Mg (1% to 2%), P (0.3% to 1.4%), and Na (0.2% to 0.5%). Elemental arrangement varies for WWA because ashes derived from branches and roots are rich in many elements than those derived from stem wood [51].

The following compound composition limits were also reported: titanium dioxide (0% to 1.5%), sulfur trioxide (0.1% to 15%), silica (4% to 60%), aluminum oxide (5% to 20%), ferric oxide (10% to 90%), magnesium oxide (0.7% to 5%), potassium oxide (0.4% to 14%), calcium oxide (2% to 37%), loss of ignition (0.1% to 33%), moisture content (0.1% to 22%), and available alkalis (0.4% to 20%). Table 2 shows the chemical compounds in waste wood ash that were obtained in past research.

WWA is usually very low in nitrogen because it evaporates during incineration. Trace elements such as boron (B), molybdenum (Mo), copper (Cu), and zinc (Zn) have been observed in WWA, which are called micronutrients [65].


**Table 2.** Chemical compounds of WWA from past studies.
