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Show sorts of behaviour seen are illustrated by observations on the Series 3-XBM2-F three-burner trials, Table 7. For passably stable operation, the steady-state exhaust gas temperature needs to be over 900 °C, and the average refractory temperature over 830 °C. 7.4.2. Effects of turndown and 02 Stability is, of course, affected by firing rate, and for any given furnace load (other parameters being held constant), there is a minimum firing rate, or maximum "turndown", that marks the lower stability limit, below which combustion failures or flameouts periodically occur. At high Te and Tr, this minimum is low and the available turndown is high. A s Te and Tr drop, the turndown diminishes until instability occurs even at full fire. The exhaust gas O2 level is also an important factor. At very high furnace temperatures, as demonstrated by the Series 1-XBM2-T trials, combustion is stable even with moderate levels of negative excess air, thus under fuel-rich conditions. A s temperatures drop, increasing levels of oxygen are needed to ensure stability under turndown. At the lowest temperatures at which passably stable operation is attainable at full fire, an exhaust-gas O2 level of at least 3 % is required. The determination of turndown limits requires very careful experimentation and observation, and much patience, and is extremely time-consuming. W e therefore had to limit such studies in the present work and our data are not adequate to m a p the turndown limit as a function of Ta, Te, Tr and X0 . However, enough information to provide practical guidelines was obtained. Other data and observations besides those in Table 7, including the single-burner trials, are available in Becker and Sobiesiak (1996). 7.4.3. The upper stability limit of the firing rate No upper stability limit on the firing rate, or "blowout limit", was observed, nor is one to be expected within the practical range of operation of the C G R I burner (see discussion in § 8).The "full-fire" capacity of the burners in our work was actually determined by the available combustion air supply pressure, ca. 8700 Pa. Operating with higher-pressure blowers, C G R I was able to run a burner like the X B M 1 at over 400 k W combustion heat release, compared to our level of a little over 300 k W . Still higher pressures would allow yet higher firing rates but, of course, a point is reached where the associated costs make the further possibilities uninteresting. 7.4.4. Flame detection and the question of the pilot flame In the Series 1-XBM2-T single-burner trials, furnace temperature levels were very high. Under these circumstances, combustion at the higher firing rates was stable with or without the pilot flame. O n turndown, combustion with the pilot flame on was stable down to the lowest attainable firing rates (1/5 to 1/4 of full fire). Without the pilot, flameout typically occurred at levels of 40 to 70 % of full fire. In the Series 3-XBM2-F three-burner trials and the Series 1- X B M 2 - F single-burner trials, the highest temperature levels were well below those in the Series 1-XBM2-T, and stability with the pilot off was erratic at all firing rates, so these trials were done with the pilot on. There are several problems with detection of the C G R I burner flames that were not solved by the present research and will require special investigations to reach an understanding. The main ones are: 13 |