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Show 2. PRESENT CONTROL PHILOSOPHY (cont'd) Lack of Automatic Combustion Control (cont'd) The problem with this philosophy is two fold. The first is that the resultant high firebox draft is compounding the high oxygen problem due to the before mentioned leakage. The reason the stack damper is designed to maintain firebox draft at a minimum is that any higher draft will result in pulling in more unwanted air. The oxygen analyzer in the stack is representing the measurement after air has ingressed into the heater. Therefore, while the operator may think the heater is operating at safe oxygen levels, a majority of this may be due to leakage. In actuality the oxygen level in the firebox (at the burners) may be sub-stoichiometric leading to an fuel rich firebox which is considered unsafe. A CO analyzer is used to combat this problem (a fuel rich combustion zone would result in high CO readings), however, as previously stated many heaters presently don't have these analyzers. The second and utmost problem with this philosophy is that high oxygen levels result in low heater efficiency and high NOx formation. Since fuel prices are currently relatively low, the efficiency issue may not be the first thing on the operator's mind. Once a NOx reduction system is installed, this philosophy will have to change. Once the system is installed and NOx levels exceed the permitted values, plant shutdown (loss of production opportunity) will be mandated. With the whole thrust of this work being NOx reduction, it is easy to understand why automatic combustion control is desirable. 3. NEW SYSTEM DESIGN This section of the paper deals with the philosophy used in revamping an existing heater control system to one that will aid in maintaining NOx reduction day in and day out. One of the most pronounced changes that may occur in the NOx program is the introduction of either a Distributed Control System (DCS) or Programmable Logic Controllers (PLC). As previously mentioned, if the plant has not yet converted over to a DCS or PLC system, this would be the optimum time. Most of the discussion thus far has dealt with the more commonly encountered natural draft heater. It sho~ld be noted that if the viable NOx reduction technology is the installation of LoNOx burners, automatic combustion control would be the major control philosophy. If this is not the case and a Selective Catalytic Reduction Unit (SCR) is to be installed on a natural draft heater, in addition to Automatic Combustion Control (ACC) the following are control and reliability issues: 4 |