Oklahoma Gas & Electric Muskogee Generating Station Best ...

Oklahoma Gas & Electric
Muskogee Generating Station – BART Determination
May 28, 2008
that can result in reflective ash deposits on the waterwall surfaces. Because most heat
transfer in the furnace is radiant, reflective ash can result in less heat removal from the
furnace and higher exit gas temperatures. If ammonia is injected above the appropriate
temperature window, it can actually lead to additional NOx formation.
SNCR control systems have not been designed or installed on large subbituminous coalfired
boilers, and, as described above, there are several currently unresolved technical
difficulties with applying SNCR to large subbituminous coal-fired boilers (including the
physical size of the boiler, inadequate NH3 mixing, and ash characteristics). Even
assuming that SNCR could be installed on Muskogee Units 4 & 5, NOx control
effectiveness would be marginal, and, depending on boiler exit temperatures, could actually
result in additional NOx formation. Because SNCR has not been designed for, or
demonstrated on, a large subbituminous coal-fired boiler, it was determined that the control
technology is not applicable to Muskogee Units 4 & 5, and SNCR will not be evaluated
further in the BART determination.
3.2.2.2 Selective Catalytic Reduction
Selective Catalytic Reduction (SCR) involves injecting ammonia into boiler flue gas in the
presence of a catalyst to reduce NOx to N2 and water. Anhydrous ammonia injection
systems may be used, or ammonia may be generated on-site from a urea feedstock. The
overall SCR reactions are:
4NH3 + 4NO + O2 → 4N2 + 6H2O
8NH3 + 4NO2 + 2O2 → 6N2 + 12H2O
The performance of an SCR system is influenced by several factors including flue gas
temperature, SCR inlet NOx level, the catalyst surface area, volume and age of the catalyst,
and the amount of ammonia slip that is acceptable.
The optimal temperature range depends on the type of catalyst used, but is typically
between 560 o F and 750 o F to maximize NOx reduction efficiency and minimize ammonium
sulfate formation. This temperature range typically occurs between the economizer and air
heater in a large utility boiler. Below this range, ammonium sulfate is formed resulting in
catalyst deactivation. Above the optimum temperature, the catalyst will sinter and thus
deactivate rapidly. Another factor affecting SCR performance is the condition of the
catalyst material. As the catalyst degrades over time or is damaged, NOx removal
decreases.
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