Optical methods and results of dew point and deposition rate measurements in salt/ash‐containing combustion gases—B2O3(I) deposition rates by interference methods and comparisons with theory

Abstract

AbstractThis 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. BCl3(g) is introduced into a premixed C3H8‐air flat flame at atmospheric pressure. The growth rate of B2O3(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 B2O3(I) deposition at the present seed levels and surface temperatures (about 1,200–1,300 K) clearly demonstrates the importance of kinetic restrictions on B2O3(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 ((HBO2)3 precluded) boundary condition. Remarkably, at a constant value of the BCl3 flow rate, the Pt ribbon temperature above which there is no B2O3 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 B2O3 precursor formation and (HBO2)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.