Page 183 - New Trends In Coal Conversion

146 New Trends in Coal Conversion

boilers, but higher CO emission and O 2 imbalance results are problems derived from
their operation. However, these problems could be minimized by the appropriate selec-
tion of burners (Bell and Buckingham, 2010).
Low excess air. The use of low excess air in the combustion is an economical alter-
native for reducing NO x because it is not necessary to acquire new hardware or to
modify the existing facilities to implement this control technology. Moreover, there
are many authors who think that low excess air should be part of a normal furnace
operation, requiring only process control tuning to obtain the optimal operation point
(Bell and Buckingham, 2010).

6.2.1.3.3 Postcombustion control techniques
Selective noncatalytic reduction (SNCR). SNCR works by injecting and mixing a re-

agent into a flue gas stream that contains NO x at temperatures of around 800e1200 C
to produce a reaction in a gas-phase homogeneous atmosphere that allows for a reduc-
tion of the percentage of oxygen that reacts with NO. The scheme of the SNCR
concept is shown in Fig. 6.2 (Tayyeb et al., 2007). An SNCR facility consists of
two operational units:

Reagent
storage

Air/
reagent Heater
mix
Carrier air
Vaporizer
Stack

Air
precheater
Wind
box

Forced
draft fan
Figure 6.2 Selective noncatalytic reduction.
Adapted from Bell, R.D., Buckingham, F.P., 2010. An Overview of Technologies for Reduction
of Oxides of Nitrogen from Combustion Furnaces. MPR Associates.