| OCR Text |
Show fuel-to-steam of 5:3, which is consistent with the above computer predictions. This lowering in flame temperatures has a slight effect on lower combustion efficiency by increasing particle burnout time, due to slightly lower particle temperature and longer particle heat-up periods. However, according to Figure 5-5, and the PETCOM coke particle size distribution, a lower particle temperature of 100 to 200°F cannot solely account for the significant reduction in combustion efficiency of steam atomization shown in our experiments. Actually, Libby and Blake (Ref. 25) have shown that at temperatures greater than 1800°K (2780°F), water vapor can enhance the carbon loss rates of particles. At lower temperatures, carbon weight loss rates are unaffected other than by water vapor decreasing the partial pressure of oxygen. Thus, at high temperatures, the following endothermic reactions can become important: C + H20 •» CO + H2 (AH29g = + 131.3 KJ/mol) (5-6) C + 2H20 •* C02 + 2H2 (AH298 = + 90.1 KJ/mol) (5-7) C + H20 •» ^C02 + hCE4 (AH29g = +7.7 KJ/mol) (5-8) If steam concentrations and combustion temperatures are high, all these reactions would tend to cool the flame and particles. With the hot reaction zone, provided initially by oil combustion, these reactions can feasibily take place, albeit for a very short period of time, causing initially higher consumption of coke particles. Other chemical species in the particle combustion environment (NO, CO, C02, S02) are expected to reduce burn rates by actually competing with oxygen for active reaction sites on the coke surface. These non-oxygen reaction rates are typically lower than oxygen at typical combustion conditions and thus these gases decrease coke particle burning rates. 19-31 |
