The controlling chemistry of surface deposition from sodium and potassium seeded flames free of sulfur or chlorine impurities

Abstract

Sodium and potassium salt deposition have been studied in a series of propane and hydrogen flames free of sulfur or halogen impurities. With the collection probe in the 400 to 800 K range, samples of pure carbonate are observed and more importantly the rates of, for example, sodium carbonate deposition measured in terms of alkali metal are identical to those previously reported for sodium sulfate formation and also those observed for dominant NaCl deposition. Moreover, the behavior of Na 2CO 3 deposition mirrors exactly that of Na 2SO 4 in this temperature range. It shows a corresponding first order dependence on flame total sodium concentration, a zero order dependence on flame carbon, an insensitivity to fuel type, equivalence ratio, flame temperature, flow rate, probe material, or the nature of the sodium speciation in the flame, be it atomic or the hydroxide, or the state of the flame equilibration. A constant rate of deposition between 330 and 800 K conveys formation kinetics with a zero activation energy and that the surface accommodates atomic sodium equally well, be it below or above its dew point temperature and also at a seemingly approximately equal rate to that of flame NaOH. The fact that Na 2CO 3 cannot exist in the gaseous state in a flame finally proves irrefutably that these alkali deposition processes producing sulfate, carbonate or halide salts are heterogeneous in nature. The high collection efficiencies of the surface for alkalis have been confirmed by a further independent new calibration method for flame total alkali content. Also deposition rates are seen to be extremely similar in C 3H 8 /O 2 flames heavily diluted with either He, Ne, or Ar and also in a very fuel rich H 2 or D 2 flame. As with sulfate deposition, the rate of deposition is predominantly controlled by the actual flux of alkali in the flame gases that are intercepted by the collection probe. Moreover, there is an insensitivity to probe geometry and the nature of the flame flowfield, be it laminar or turbulent. The theoretical understanding of the complex boundary layer penetration and deposition mechanism is still inadequate in explaining these observations. The most intriguing results and differences from sulfate deposition have been observed on probes at lower temperatures (330–370 K). Although the formation of NaHCO 3, and more so KHCO 3, was expected to compete with that of their carbonates, in the case of sodium under fuel lean conditions only a small competing contribution of NaNO 3 formation was noted. This was very marginal for fuel rich conditions. However, with potassium the effects were enhanced and KNO 3 competes significantly with K 2CO 3 under fuel lean conditions. However, in fuel rich flames an unexpected dominant formation of potassium oxalate, K 2C 2O 4, was observed, along with some K 2CO 3 and a small amount of KHCO 3. Thermodynamic expectations in this lower temperature regime tend to suggest nitrate>bicarbonate>carbonate>oxalate. This is our first clearly observed non-equilibrium deposition behavior where the flame begins to display a pivotal role in controlling the surface molecular distribution. It also raises the possibility that low temperature surfaces in flames may be a new route for synthesizing certain thermodynamically metastable materials.