OCR Text |
Show Phase I - Char combustion measurements Just after ignition, the char particles burned with a characteristically bright red-white glow. This was often accompanied by a slight peak in the temperature profile. As combustion proceeded, the surface glow slowly diminished and a layer of gray-white ash began to accumulate on the outside of the particles. This process usually continued at a relatively steady rate for a while, with the imbedded thermocouple indicating a nearly constant internal temperature." On the surface at least, particle size did not change much during combustion. The quartz wool tended to keep the ash layer intact and the general particle shape was preserved. Some decrepitation did occur in the latter stages, making particle size difficult to evaluate. The initial stages of extinction were signaled by a steady drop in particle temperature. This continued until the particle approached its final steady-state temperature, usually at or just slightly below the type K thermocouple temperature. The rate of particle weight loss also declined to zero in this period. The final approach weight was assumed to indicate the sample ash content. Two types of measurements were used to quantify the combustion rate. These were the initial rate of particle mass loss, measured in grams per square centimeter of external area per second, and particle burnout time, measured in seconds. The initial rate data are probably most applicable to fluidized bed conditions, since they were evaluated when the effect of the ash layer was minimal. In addition to being affected by the ash layer, the burnout time measurements tend to be more imprecise. This is due to the difficulty in exactly defining the endpoint of combustion. Phase I - Mass transfer vs chemical reaction In order to determine the relative importance of external mass transfer and chemical reaction, it is necessary to know the reaction stoichiometry; i.e., whether the effective surface product is CO or ^Optical pyrometer measurements agreed with the thermocouple to within 6 K (10°F) near the beginning and end of the char combustion. Differences were greater at intermediate stages due to the masking effect of the ash layer. 25 |