Photometric investigations of the behaviour of chromium additives in premixed H 2 + O 2 + N 2 flames

Abstract

The chemistry of a flame gas + metal additive system which may contain particles of involatile metal oxide has been studied photometrically, and the results interpreted in detail for the first time. The principal experiments discussed concern H 2 + O 2 + N 2 flames into which traces (10 -6 to 10 -8 mole fraction of the total flame gases) of either gaseous chromium carbonyl or aqueous chromium salt sprays are introduced. In the temperature-range examined (1800 to 2700 K), for both types of additive the gaseous phase results are interpreted in terms of the presence in comparable proportion of four species, Cr, CrO, CrO 2 and HCrO 3 which interact in the balanced reactions Cr + OH ⇌ CrO + H, CrO + OH ⇌ CrO 2 + H, CrO 2 + H 2 O ⇌ HCrO 3 + H, or their equivalents. The species HCrO 3 , which has not been reported before, was found by a Second-Third law procedure to have an atomization energy of 1940 ± 40 kJ mol -1 . The equilibrium constant of Cr + 3H 2 O ⇌ HCrO 3 + 2H 2 + H at 2300 K is 0.050 ± 0.008. In addition to the gas phase constituents (which alone cause the behaviour of chromium in flames to be the most complex of that of any metal studied to date) further features arise from the usual presence of solid material. In the case of salt spray additives, solid is detected even under conditions of thermodynamic instability although in such circumstances it is only a minor constituent. In the burned gases, there is no evidence for significant change in particle size or for significant interaction between solid and gaseous chromium-containing constituents in the available time scale—up to 10 ms. These ‘inert’ particles do, however, participate in the spin destruction processes Cr 2 O 3 (s) + H + H (or OH) → Cr 2 O 3 (s) + H 2 (or H 2 O) in which the rate-determining step is the rate at which the radicals strike the particle surface. The ‘hot’ particles thereby produced behave as approximately grey body emitters with temperatures which may be up to 500 K above that of the ambient flame gases. Significant catalysis of radical recombination is caused by amounts of additive as low as 1/10 6 of the total burned gases. The preferred explanation of this striking effect involves a homogeneous route in which oxygenated chromium species participate.