Agricultural use of fly ash: Expected benefits and consequences

Coal is one of the largest sources of energy production in the world accounting in 2006 for about 25% of the world’s primary energy supply, that amounts to almost with a worldwide consumption of 3,090 million tons of oil equivalent. Fly ash (FA), is a residue of coal combustion consisted of amorphous ferro-alumino silicate minerals with a matrix very similar to soil. Its elemental composition including both nutrient and toxic elements varies according to types and sources of coal used. It is estimated that the annual amount of the FΑ produced globally is about 750 million tons that requires a proper management including its disposal which is becoming a very serious problem for the countries using coal as a primary source of energy. Thus, appropriate measures for FA management including safe disposal and utilization are necessary for sustainable management of this waste. So far two alternative options of FA disposal are in use, i.e. its utilization in construction materials and application to the land as soil amendment. In the first case, FA is used in the production of cement clinker or is blended with cement. In the second case FA is applied to the land, utilizing its properties as soil amendment and source of nutrient supplementation. The appropriatness of FA as soil improvement is related to its mineralogical, physical and chemical properties of FA which depend on the nature and properties of the coal and the conditions under which it was produced. Fly ash is comprised of very fine particles, with an average diameter <10 μm, which are aggregated into spherical particles of 0.01–100 μm sizes which are hollow spheres (cenospheres) filled with smaller amorphous particles or crystals (pelospheres). These components create interesting properties in the FA such as very large specific surface area ranging from 2,500 to 4,000 cm 2 g -1 , and consequently high sorption capacity which makes FA suitable as sorbent for flue gas cleaning from sulphur components, NOx, gaseous organics and for removal from wastewater of several toxic metal ions, and inorganic anions. The bulk density of FA is low to medium ranging from 1 to 1.8 g cm -3 and its moisture retention capacity ranges from 6.1% at 15 bars to 13.4% at 1/3 bar. Its organic carbon content i.e. the un-burnt coal estimated as loss on ignition, ranges from 0.5 to 12% giving the black or grey appearance of FA. In addition FA contains high amounts of Fe2O3 giving a dark colour. The chemical characteristics of FA depend on geological factors related to the coal deposits and on the operating conditions at the power plants.The main components of FA are silica, alumina and iron oxides with varying amounts of carbon, calcium, magnesium, and sulphur. Fly ashes are distinguished into two classes, i.e. Class F produced from anthracite, bituminous and sub-bituminous coals containing less than 7% CaO, and Class C produced from lignite coal containing more liming material, up to 30%. The pH of FA ranges between 4.5 and 13.25, depending on the sulphur and CaO contents of the parent coal. Fly ashes produced from coals containing high amounts of anthracite (usually contains high amounts of sulphur, S) are acidic while FA including that from lignite (usually lower in S and higher in Ca) are alkaline. The S content of FA usually varies from 0.1 to 1.5% while that of soils ranges from 0.01 to 2%