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Optical methods and results of dew point and deposition rate measurements in salt/ash‐containing combustion gases—B 2 O 3 ( I ) deposition rates by interference methods and comparisons with theory
Author(s) -
Seshadri K.,
Rosner D. E.
Publication year - 1984
Publication title -
aiche journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.958
H-Index - 167
eISSN - 1547-5905
pISSN - 0001-1541
DOI - 10.1002/aic.690300204
Subject(s) - deposition (geology) , dew , combustion , chemistry , dew point , chemical vapor deposition , combustor , analytical chemistry (journal) , mineralogy , condensation , thermodynamics , environmental chemistry , organic chemistry , paleontology , physics , sediment , biology
This study is focused on deposition rate processes leading to inefficiency and “hot corrosion” in fossil‐fuel‐fired furnaces and engines. The inorganic compounds which deposit on heat exchanger surfaces and blades are formed in combustion product gases when the fuel and/or ingested air contains inorganic impurities. An improved understanding of the coupled thermodynamic, kinetic, and transport processes governing the deposition rate of inorganic oxides and salts from hot gases containing these compounds (or their precursors) can suggest more efficient test strategies and control measures. Accordingly, an optical interference method for accurately measuring the growth rate of deposits well before the onset of run‐off under laboratory burner conditions has been developed. To demonstrate the technique and provide data suitable for theoretical model development, a deliberately simple chemical system and target geometry are used. BCl 3 ( g ) is introduced into a premixed C 3 H 8 ‐air flat flame at atmospheric pressure. The growth rate of B 2 O 3 ( I ) on an electrically heated platinum ribbon is then measured interferometrically over a range of fuel/air ratios and seed levels. However, the very existence of B 2 O 3 ( I ) deposition at the present seed levels and surface temperatures (about 1,200–1,300 K) clearly demonstrates the importance of kinetic restrictions on B 2 O 3 ( I ) gasification reactions. Optically measured film growth rates are obtained at film thicknesses small enough to neglect condensate run‐off, hence they yield vapor deposition rates directly. These deposition rates are found to be in good agreement with the predictions of a recently developed multicomponent mass‐transfer boundary layer ( BL ) theory, with a constrained equilibrium ((HBO 2 ) 3 precluded) boundary condition. Remarkably, at a constant value of the BCl 3 flow rate, the Pt ribbon temperature above which there is no B 2 O 3 condensate (i.e., the so‐called dew point) is observed to depend on the fuel/air ratio. Whereas previous equilibrium‐based deposition models cannot embrace such phenomena, a semi‐quantitative argument, based on the nonequilibrium chemistry of B 2 O 3 precursor formation and (HBO 2 ) 3 ‐formation barriers, explains these potentially significant trends. These encouraging results suggest a more general applicability for the optical methods and chemically frozen ( CF ) BL theory described herein, and demonstrate the important role of heterogeneous and homogeneous kinetic barriers in determining dew points and deposition rates in combustion systems.