API RP 536:2006 pdf download
API RP 536:2006 pdf download.Post-Combustion Nox Control forFired Equipment in General Refinery Services.
The ideal temperature range for this catalyst to effect optimum NO reduction is 357°C — 580°C (675°F — 1075°F). It is supplied on ceramic structures in composite honeycomb configurations. Commercial applications of Zeolite catalysts are rare.
4.3 PROCESS CONSIDERATIONS
4.3.1 Effect of Flue Gas Components on SNCR
The dominant factors for NO reduction utilizing either of these processes are the flue gas temperature and temperature profile, rather than the fuel type or its products of combustion.
The SNCR process is affected by the concentrations of 04, H,O, and CO in the flue gas. High CO concentrations are reported to shift the temperature window at the low end, so that NO removal is effective at relatively lower temperatures, i.e., 800°C (1470°F). The SNCR process may retard the oxidation of CO in the flue gas, resulting in slightly higher CO emissions.
The presence of HCI or HF in the flue gas in excess of 5(X) ppmvd may also retard the effectiveness of the SNCR process. The other flue gas components, such as CO,, N,, etc., appear to have no effect on the NO reduction process.
4.3.2 Effect of Flue Gas Temperature, Catalyst Poisons, and Catalyst Masking Agents on SCR
There arc three common reasons that cause deactivation of catalyst: excess flue gas temperature, catalyst poison. and masking of the catalyst.
a. Depending on the catalyst substrate material, the catalyst may be quickly damaged due to thermal stresses at temperatures in excess of the design temperature. High heat up and cool down rates for the catalyst could negatively impact catalyst life. Please refer to the Operations section of this document for commonly used heat up rates.
b. Catalyst poisoning occurs when a component of the flue gas, such as Sodium or Potassium, gets adsorbed on the active surfaces of the catalyst and renders it inactive. Some other known catalyst poisons are arsenic, chrome, and mercury. Arsenic compounds and heavy metal compounds. such as zinc dithiophosphate, tend to accumulate on the periphery of the catalyst. They tend to decompose with time, producing free heavy metals, which then react with the catalytic compounds to produce less active material. Appendix E discusses catalyst poisons.
c. Masking is caused by the accumulation of foreign substances on both the carrier and the catalytic component. Phosphorous components forming a glaze, dust, soot, and oil mist can all block the pores. Catalyst activity can often be regained by removing the material masking the catalyst. Ammonium sulfate and/or hisulfitte salts could be masking the catalyst as well. Soothiowers can sometimes he used to remove some masking agents. Please refer to Item 5 of the Appendix D section of this document for additional soothlower information Other design features to remove masking components could be included upstream of catalyst.
4.3.3 NO Reduction from Initial NO Values
4.3.3.1 SNCR Technology
Whether urea or ammonia based, SNCR is most cost effective in achieving moderate NO reduction in the 4O% — 75% range when the initial N0 values are 100 ppmvd or greater. In general, the NO reduction efficiency decreases as the initial NO value decreases. High NO reductions become more difficult to achieve when the initial N0 value is below 100 ppmvd.
The NO reduction efficiency of both SNCR processes depends on the following factors:
a. Flue gas temperature in reaction zone.
b. Uniformity of flue gas temperature in the reaction zone.
c. Normal flue gas temperature variation with load.
d. Residence time.
e. Distribution and mixing of ammonialurea into the flue gases.
f. Initial N0 concentration.
g. Ammonia/urea injection rate.
h. Physical configuration, which atTects location and design of injection nozzles.
The following are general considerations for selecting a SNCR process for a specific N0 reduction requirement:
a. N0 reduction efficiency required.
h. Allowable NH3 slip to meet requirements.
