OCR Text |
Show 3 operating parameters that directly affects NO emissions is the operating O2 level, typically measured at the economizer exit by on~ or more O2 analyzers. A lower average operating O2 level typically implies lower NOx emissions, all other factors being equal. For a typical coal-fired utility boiler built before the advent of the Federal New Source Performance Standards (NSPS) for NOx emissions, the firing practice that results in the best boiler efficiency also tends to provide lower NOx emissions (Le., operation at the lowest practical O2 level without significant combustible losses, furnace deposits, or degradation in thermal performance). Ideally, combustion conditions in a boiler could be considered to be a well mixed process where the pulverized coal is burned at an airlfuel ratio represented by the total air and fuel flow to the boiler. Unfortunately, boiler combustion conditions are far from ideal and they can be very non-uniform in the lower furnace burner zone. Although many engineers tend to have a macro view of the combustion process occurring at the overall air/fuel ratio, a closer examination (and measurement) of the actual process indicates that coal combustion in the burner zone is occurring in a multitude of local combustion zones, each with a slightly different air/fuel ratio dictated by the nearest burner(s) and local furnace mixing patterns. This stratification is often quite evident, even at the economizer exit, where streamlines of combustion products carried over from the burner zone indicate how the combustion process occurred. Quite often a grid of sample probes installed in the economizer exit duct will show a large gradient in O2, NOx, CO and ash carbon content indicating non-uniform combustion. This variation in combustion uniformity may also be evident on the plant O2 instrumentation in the form of a large difference or "02 split" between the individual O2 analyzers. Non-uniform combustion makes efficient low O2, low NOx firing difficult for two reasons. 1) Local air-rich zones result in combustion that occurs at above average O2 levels. Since NOx emissions are directly linked to O2 level, unnecessarily high O2 results in above average NOx levels. 2) Local fuel-rich zones can result in a) incomplete combustion, CO or elevated ash carbon content, and b) a tendency toward increased ash deposition with coals with a marginal slagging index. Although there is a natural tendency to assume that combustion uniformity is not important because air-rich and fuel-rich zones will eventually mix to the overall average, one must consider how the minimum operating O2 level is established. In general, the overall O2 level is not lowered below the pOint at which significant CO emissions, increased ash carbon content, or ash deposition first become evident at some location in the furnace. Therefore, if one burner region or furnace corner is operating at a much lower O2 level than the others, it will dictate the overall O2 at which the unit is operated, even if the majority of the burners are capable of operating at much lower levels. As a result, the average boiler NOx emissions cannot be reduced below the limitations imposed by the local low O2 region. |