OCR Text |
Show High Heat Release Rates Presented in Table # 1 is the range of space heat release rates (SHRR), in BTU/HR per cubic foot of furnace volume, for package boilers started up with the Coen Company second generation OAF burner between January, 1992 and July, 1992. As shown, the majority of these units contained SHRR's well over 75,000 BTU/HR-FT3. For comparative purposes; SHRR's typically encountered in refinery fired heaters are on the order of 20,000 BTU /HR-FT3. For gas or oil fired utility boilers, SHRR's of under 60,000 BTU/HR-FT3 are the usual norm. A troublesome goal to obtain in a high SHRR package boiler unit is that of flame containment, or more specifically, the avoidance of sidewall and/or rear wall flame ,impingement. In addition to the obvious consequences of heat mal-distribution, flame impingement can quench the combustion process, resulting in unacceptable levels of carbon monoxide, unburnt hydrocarbons, and soot. In addition, there is the problem of superheat temperature control, which may also occur when flame lengths become unacceptably long. Therefore, for a burner specifically designed for package boiler applications, in addition to ensuring that both sidewall and rear wall flame impingement is avoided, the burner's flame shaping ability should be flexible enough to accommodate all envisioned operating restraints. An additional problem which may present itself in a high SHRf1 package boiler unit is that of unacceptable levels of combustion induced rumble and vibration. Although a detailed discussion of this subject is beyond the scope of this paper, it can be noted that burner pulsations resulting from the firing of gaseous fuels is a strong function of the rate of fuel admission and the rate of fuel/air mixing.1 In a high SHRR package boiler unit, where a limited combustion chamber residence time dictates a high level of fuel admission and mixing, the overall air/fuel turbulent diffusion 2 process, as well as local entrainment rates must be reviewed to guard against detonation (vs deflagration) and exceedingly rapid thermal gradients. When NOx reducing mechanisms are incorporated in a high SHRR package boiler, the above named problems are usually augmented. For example; One of the most successful burner controlled mechanisms for thermal NOx reduction (independent of flue gas recirculation) is fuel or air staging.2 However, a detriment of fuel or air staging is the consequential flame lengthening. Therefore, for high SHRR package boiler applications, where flame containment problems are commonly encountered without staged burners, the problems of increased flame length associated with staged burners must be carefully considered. In addition to making implementation of NOx reduction techniques more difficult, a high SHRR will also translate into a higher baseline thermal NOx level: Thermal NOx generation will follow an exponential function of integrated time-temperature flame zones in accordance with the Zeldavich' mechanism. For a given excess air level and rate of flue gas recirculation, adiabatic flame temperature will be a strict function of fuel composition.4 However, actual flame temperatures will deviate from this theoretical value as a result of three main factors: (1) dissociation of combustion products, (2) conduction and convection losses due to non-instantaneous heat generation, and (3) radiation losses. When a given quantity of heat is released in a diminutive volume, heat losses from the flame (factors 2 & 3) will be Significantly reduced. Accordingly, baseline thermal NOx values have been found to be a strong function of SHRR and related parameters. |