scope:

This recommended practice covers the mechanical description, operation, maintenance, and test procedures of post-combustion NO_x control equipment for fired equipment in general refinery service. It does not cover reduced NOx formation through combustion design techniques, such as flue gas recirculation (FGR) and staged combustion.

This document covers two of the methods of post combustion NO_x reduction:

a. Selective Non-Catalytic Reduction (SNCR).

b. Selective Catalytic Reduction (SCR).

SNCR is a process where the addition of ammonia or urea into the flue gas stream causes the oxides of nitrogen to convert to nitrogen and water vapor. The basis for the selection and limitations of the SNCR systems are described in 4.1.

SCR is a process where the addition of ammonia into the flue gas stream in the presence of a suitable catalyst causes the oxides of nitrogen to convert to nitrogen and water vapor. The basis for the selection of the various catalyst types are described in 4.2.

Table 1 indicates the typical operating performance and limitations of both types of NO_x reduction systems. The reduction efficiency of SNCR is limited because of the flue gas temperature range and difficulty in achieving proper mixing of the reactants, but is often suitable for retrofitting existing equipment for low or moderate NO_x reduction. SCR systems operate at high reduction efficiency at a lower temperature window than an SNCR system and are usually selected for lowest NO_x emission.