Louis Wibberley

Alkali-ash reactions and deposit formation in pulverized-coal-fired boilers: the thermodynamic aspects involving silica, sodium, sulphur and chlorine

Thermodynamic calculations predict the behaviour of alkalis in the furnace gases of pulverized-coal-fired boilers. Results imply that a large proportion of the volatile sodium in the coal reacts with silica or silicate fly ash and forms a low viscosity surface layer of sodium silicate on the ash which, depending on its thickness, may enhance retention of the ash particles impinging on boiler tubes. The thickness of the sodium silicate layer depends mainly on the ratio of alkalis (non-chloride) to ash surface area in the furnace gases and the kinetics of the silicate-forming reaction in the temperature range 1300–1850K. The calculations show that sodium sulphate is stable only below 1300–1400 K in pulverized-coal-fired furnace atmosphere, and, therefore, sodium-ash reactions at higher temperatures will greatly reduce sulphur retention in the boiler as sulphate deposits.

Alkali-ash reactions and deposit formation in pulverized-coal-fired boilers: experimental aspects of sodium silicate formation and the formation of deposits

Abstract The experiments investigate the formation of sodium silicates in reactions between silica particles and a synthetic furnace gas containing sodium, chlorine, sulphur and water vapour. A thin layer of silicate forms on the silica, having a thickness ranging from 0.02 μm to 0.31 μm, and an average composition from 9 to 36 wt% Na 2 O. Sulphur in the furnace gases reduces the silicate formation at temperatures below 1300–1400 K, and chlorine reduces its formation at higher temperatures. The thin, low viscosity layer of sodium silicate allows particles to collect on a cool metal surface (870 K) free from condensed material. Calculations suggest that in large pulverized-coal-fired boilers the formation of silicates by alkali-ash reactions can be predicted by thermodynamic considerations.

An Investigation of Factors Affecting the Physical Characteristics of Flyash Formed in a Laboratory Scale Combustor

Abstract An Australian bituminous coal was burnt in a laboratory drop-tube combustor at 1400, 1500 and 1600°C to determine the effect of p.f. properties and combustion temperature on the character and particle size of the flyash. The experiments showed that the mass mean particle size of the flyash was approximately proportional to that of the p.f., and almost independent of combustion temperature. In contrast, the proportion of fine ash (< 10 μ) was independent of p.f. size but increased markedly with increased combustion temperature. The upper size of the ash was determined by unburnt char for the 175 μm and 240 μm p.f. and by the mineral matter for the 29 μm p.f. Cenosphere formation increased with combustion temperature and dominated the ash formed at 1600°C. The work emphasises the need for more detailed p.f. analysis during laboratory and pilot-scale combustion studies into flyash formation and related phenomena such as fouling, filtration and precipitability. Additional research is required to qua...

Indicators of ignition for clouds of pulverized coal

Experiments are reported giving visual observations of ignition - the emission of light or flashing - and the changes of gas composition - O/sub 2/, CO/sub 2/, and NO - in a laboratory drop-tube furnace fed continuously with pulverized coal and air in near-stoichiometric proportions. As the furnace is slowly heated at 10/sup 0/Cmin an early consumption of oxygen is measured (at gas temperatures of 200-350/sup 0/C); the initial flashing is observed with no associated change in the extent of combustion (400-600/sup 0/C), followed by rapid combustion and NO generation as the char ignites (650-800/sup 0/C). Flashing is shown to be an extremely rare event with only a single flash observed for several thousand coal particles fed. Results are reported for the effect of coal type, particle size, and equivalence ratio.

Nitrogen oxide formation from Australian coals

Abstract The evolution of fuel nitrogen during devolatilization and the formation of NO x during combustion were studied for two Australian coals in crucible, thermobalance, and rapid heating (drop-tube furnace) experiments. The evolution of coal nitrogen during devolatilization was dependent on both temperature and mode of heating. Under near stoichiometric combustion, 20–30% of coal nitrogen was converted to NO x , Conversion increased markedly with increased fuel-lean conditions. The NO x formed from volatiles was proportional to the fraction of coal nitrogen evolved as HCN and NH 3 . The combustion of char at various temperatures and stoichiometries showed that the conversion of char nitrogen to NO x depended primarily on char burnout. The contribution of char nitrogen to NO x formation was greater than that of volatile nitrogen under fuel-rich conditions.

Fly Ash Characteristics and Radiative Heat Transfer in Pulverized-Coal-Fired Furnaces

The properties of a fly ash cloud which determine its radiative influence in furnaces are its dust burden, projected surface area and its mean absorption and scattering efficiency. The first property can be estimated by stoichiometry, the second by laboratory sizing of the ash and the radiative efficiencies should be related to its chemical character. Previous measurements of the absorption and scattering coefficients of ash in several power stations are interpreted in terms of these properties. The resulting estimates of the absorption index of the fly ashes are an order of magnitude higher than the dominant oxides in the ash. Ash is shown to be significant as an emitter and scatterer of radiation, with the present uncertainty in its optical properties leading to unacceptable errors in the calculation of radiative heat transfer.