What is Coal Ash? A Combustion File downloaded from the IFRF Online Combustion Handbook ISSN Combustion File No: 38

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1 What is Coal Ash? A Combustion File downloaded from the IFRF Online Combustion Handbook ISSN Combustion File No: 38 Version No: 1 Date: Author(s): Source(s): Referee(s): Sub-Editor Status: Sponsor: Flemming Frandsen See CF Jörg Maier, Peter Roberts Flemming Frandsen Published DTU 1. Background Industrial fuels are introduced in Combustion File (CF) 62 and more specifically coal is introduced in CF177. The present CF introduces coal ash as an introduction to a portfolio of CFs examining coal ash in more detail, with a particular emphasis upon problems created by coal ash in combustion equipment such as pf fired boilers slagging and fouling - and the disposal of fly ash. For a detailed introduction, see references 1 and Ash forming species There is no ash present in coal. However there are ash-forming species present, which during thermal conversion of coal are transformed into ash being present as fly ash, bottom ash or in deposits after the thermal conversion process. Thus, ash is formed from inorganic, non-combustible material, which is either an integrated part of the fuel or introduced to the combustor with the fuel (such as dirt, clay etc.). The content of inorganic constituents in coal are usually given as the oxides of the relevant metal. In a standard bulk chemical ash analysis, the composition is given as [2]: SiO 2 + Al2O3 + Fe2O3 + CaO + MgO + Na2O + K2O + TiO2 + P2O5 + SO3 = 100% where SiO 2 is the weight percentage of Si, expressed as SiO 2 in the ash. The coal ash composition varies over a large range, being strongly dependent on the rank as well as geographic origin of the coal, as seen in Table 1.

2 Oxide Component: Percentage: SiO Al 2 O Fe 2 O CaO MgO Na 2 O K 2 O TiO SO Table 1: Chemical composition ranges for coal ash. 3. Form of inorganic constituents The inorganic constituents in coal can occur in several forms in amorphous phases [1]: As discrete minerals (mineral grains), As organically associated cations, As salts (cations) dissolved in pore water in the coal. Figure 1 shows these forms of occurrence. The mineral grains normally comprise a large part of the inorganics in coal. For the higher rank coals, bituminous coal and anthracite, most of the inorganic species are present as minerals. Minerals present in the coal matrix can be classified as included or excluded minerals. The included minerals are closely associated with the organic coal matrix, where excluded minerals are not. A list of mineral species often found in coal and their chemical formula is in provided in Table 2.

3 Included minerals Organic coal matrix C O - OH Na + Organically associated elements C C - HO O - O OH Ca 2+ Excluded minerals Figure 1: Illustration of modes of occurrence for minerals and other inorganic constituents in coal [1]. Mineral Formula Mineral Formula Albite Na 2 O.Al 2 O 3.6SiO 2 Calcite CaCO 3 Dolomite CaCO 3.MgCO 3 Gypsum CaSO 4.2H 2 O Halite NaCl Hematite Fe 2 O 3 Illite K 2 O.3Al 2 O 3.6SiO 2.2H 2 O Kaolinite Al 2 O 3.2SiO 2.H 2 O Magnetite Fe 3 O 4 Muscovite K 2 O.3Al 2 O 3.6SiO 2.2H 2 O Orthoclase K 2 O.Al 2 O 3.6SiO 2 Pyrite FeS 2 Quartz SiO 2 Rutile TiO 2 Siderite FeCO 3 Sylvite KCl Table 2: Name and chemical formula of some minerals commonly found in coal. The fraction of organically associated (= organically bound = atomically dispersed) inorganic constituents varies with coal rank. When the fuel rank decreases (H:C and O:C ratios increase), the fraction of organically associated inorganic components increase. Approximately 25% of the oxygen is present as carboxylic acid groups, which can act as bonding sites for cations such as Na +, K +, Ca 2+ etc [1-2]. Another association form are the chelates, where the cations form a chelate coordination complex with pairs of adjacent organic oxygen functional groups.

4 4. Conclusions This foregoing competes the answer to the basic question what is ash?. The subject is pursued further with CF39 and CF77, dealing respectively with factors affecting coal ash formation and the analysis of coal ash chemistry. Glossary terms Anthracite The highest rank coal characterised by low volatile matter - always less than 10% - and high carbon content it has a semi-metallic lustre and is capable of burning, relatively easily, without smoke see also Semi-anthracite. Ash - Ash is the non-combustible material that is contained by a liquid or solid fuel, which is left as a residue after the completion of the combustion process. (See also Fly ash). Bituminous Coal - Coal in rank between sub-bituminous coal and semi-anthracite. Volatile matter on a dry ash free basis ranges from between 10-14% to 40% and over. Usually divided into three sub groups: high volatile, medium volatile and low volatile. Chelates - Chemical compounds in the form of a heterocyclic ring, containing a metal ion attached by coordinate bonds to at least two nonmetal ions. Coal - A solid fossil fuel in the form of dark, compact, stratified rock-like mass of decayed plant debris interspersed with smaller amounts of inorganic matter and covered with sedimentary rock Fouling - Deposits of impurities, dirt or foreign matter that clog systems or restrict flow and interfere with heat transfer: e.g. sodium hardening on water tubes in the boiler. Fly ash - Fine ash carried out of a solid fuel fired combustion chamber or furnace, entrained in the gaseous products of combustion or "flue gas" stream. (See also Ash). Slagging - Process when the mineral impurities (ash) melt due to hot temperature and adhere to the walls/heat transfer surfaces of a boiler. Keywords Ash; bottom ash; deposits; Coal ash; mineral grains; organically association; coal rank; slagging; fouling; fuel; fly ash; fly ash; emissions, boilers, power generation, PowerFlam Related Combustion Files 39 What factors affect coal ash formation? 62 What are industrial fuels? 77 How do I analyse coal ash chemistry? 177 What is coal?

5 Sources [1] Benson, S.A., Jones, M.L. and Harb, J.N., 1993, Ash formation and deposition. In Smoot, L.D. (Ed), Fundamentals of Coal Combustion - for Clean and Efficient Use, Coal Science and Technology 20, Elsevier Science Publishers, Amsterdam, ISBN , Chapter 4, pp [2] Bryers, R.W., 1996, Fireside slagging, fouling and high temperature corrosion of heat-transfer surface due to impurities in steam-raising fuels, Prog. Energy Combust. Science, 22 (1), pp Acknowledgements CHEC is co-funded by ELSAM, ENERGY E2, the Danish Technical Research Council, the Danish and Nordic Energy Research Programmes, and the European Union. File Placing [Slagging, Fouling and Corrosion]; [Ash]; [Basics] Access Domain [Open Domain] Parity between this pdf and the present html version of this Combustion File The information contained in this pdf Combustion File edition is derived from html edition of the same number and version, as published in the IFRF Combustion Handbook ( The information published in this pdf edition, is that which was included in the original html edition and has not been updated since. For example there may have been minor corrections in the html version, of errors, which have been drawn to our attention by our readers. What is more important is that with the passage of time and the continuous growth of the handbook, a number of other changes may have been made to the published html version, such as: The related combustion files may have been augmented; The filing system may have been further developed; The Access Domain may have changed. These changes can be made without substantial changes being made to the main text and graphics. If there have been substantial changes made, then a new version of the Combustion File will have been published. Thus to be sure of up-to-date information, go to the Handbook and download the latest html version of the Combustion File. Limits of Liability A full Limits of Liability declaration is shown at the entry of the IFRF ONLINE Combustion Handbook at Through possession of this

6 document, it is assumed that the holder has read and accepted the limits. The essential limitation is that: The International Flame Research Foundation, its Officers, its Member Organisations its Individual Members and its staff accept no legal liability or responsibility whatsoever for the consequences of unqualified use or misuse of the information presented in the IFRF Combustion Handbook or any results derived from the Combustion Files which comprise this Handbook. IFRF