- Title
- Alkali ash reactions and deposit formation in pulverised coal fired boilers
- Creator
- Wibberley, Louis James
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 1980
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- During the combustion of pulverised coal (p.f.} the inorganics in the coal are released as fly ash and vapours. Some of this material may adhere to boiler tube surfaces and form deposits which reduce heat transfer and cause other operational problems. Previous research relating to the behaviour of coal mineral matter during combustion and subsequent deposition suggests that deposit initiation and growth involves fine fly ash, alkalis (especially sodium) and sulphur. Speculation has suggested the importance of low melting point alkali salts ("sticky tube mechanism") and low viscosity supercooled alkali silicates ("sticky particle mechanism") on deposit formation. The present study details a theoretical and experimental investigation of the behaviour of alkalis during p.f. combustion to clarify the thermodynamic and kinetic lirni tat ions of the formation of sticky fly ash particles. Thermodynamic calculations were used initially to predict the behaviour of sodium during char combustion and in the furnace gases, at equilibrium. At flame temperatures (>1500 K) the main gaseous species are NaCl and NaOH depending on the Na : Cl ratio of the furnace gases. Elemental sodium appears above 1700 K and is the main sodium specie at the lower oxygen concentrations calculated for the interior of the burning char. The calculations suggest that a major proportion of the volatile sodium in the coal may react with silica or silicate fly ash to form sodium silicates above 1300 - 1400 K. This proportion increases with the Na : Cl ratio of the furnace gases. Below 1300 - 1400 K condensed Na₂SO₄ is stable and at equilibrium will account for the total sodium in the system, for excess sulphur. Gaseous Na₂SO₄ was found to be a minor constituent for all p.f. furnace conditions. The kinetic limitations of these predicted trends were then studied experimentally using short-duration high-temperature reactions involving free-falling silica particles (35, 68 μm) and a synthetic furnace gas containing sodium, chlorine, sulphur and water vapour. These experiments simulated the chemical and thermal conditions experienced by silicate fly ash particles in p.f. furnace gases, remote from the burning char. During the experiments sodium silicate was formed at the surface of the silica, the thickness and composition of the surface layer being estimated by the amount of water soluble sodium and silicon extracted from the reaction residues at 393 K. The thickness of the silicate layer ranged from 0.020 - 0.31 μm and its average composition from 9 - 36 wt.%. Na₂O. A physiochemical model describing the formation of this layer suggests that the rate limiting steps are; below 1300 K solid state diffusion in the silicate product layer, 1300 - 1600 K chemical reaction at the surface. Sulphur was found to reduce the amount of sodium silicate formed below 1300 - 1400 K due to Na₂SO₄ formation, as suggested by the thermodynamic calculations. Above this temperature the reaction was greatly decreased by the presence of chlorine due to the formation of NaCl. The non-isothermal nature of the experiments and other factors enable only semi-quantitative results to be presented for the kinetics ofsilicate formation. The range of rates were 14 - 112 μg (Na₂O) .m⁻² .s¹ for experiments involving NaCl (average for 1000 - 1500 K) and 78 - 234 μg(Na₂O).m⁻².s⁻¹ for experiments involving Na₂CO₃ (average for 1000 - 1600 K. The thickness of silicate formed was therefore shown to depend on the amount of sulphur, chlorine and sodium in the gases, the surface area of the silica and its residence time in the temperature range 1300 - 1600 K. However, on the basis of these experimentally determined rates, calculations suggest that in large p.f. boilers silicate formation is controlled by thermodynamic, rather than kinetic, constraints. Theoretical calculations showed that only a thin (0.01 - 1 μm) low viscosity surface layer of silicate is required to both prevent a sticky ash particle from rebounding elastically from a tube surface on impaction and to withstand abrasion by other impinging fly ash. Calculations showed that this low viscosity material may be established by the supercooling of alkali silicates which may be formed by alkali-ash reactions (or condensing alkalis). Subsequent experiments supported this theoretical speculation and showed that sticky particles will collect on a cool (870 K) surface free from condensed material (i.e. a non-sticky surface). Overall, the results suggested a fouling index based on the surface area of the silicate fly ash, the volatile alkalis and chlorine in the coal. The results of the study are of further practical significance as the presence of the low resistivity silicate surface layer may affect the precipitatability of the fly ash. In addition, the formation of alkali silicates above 1300 K will decrease the formation of alkali sulphates at lower temperatures, causing less sulphur to be retained in the boiler as sulphate deposits.
- Subject
- pulverised coal; alkali ash reactions; heat transfer; alkali behaviour
- Identifier
- http://hdl.handle.net/1959.13/1296648
- Identifier
- uon:19287
- Rights
- Copyright 1980 Louis James Wibberley
- Language
- eng
- Full Text
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