Page 189 - New Trends In Coal Conversion

152 New Trends in Coal Conversion

operational problems derived from the different characteristics of the fuel compared
with the designed fuel constrain its application at large scale (Roy and Sardar, 2015).
There are several techniques based on physical and mechanical processes that are
focused on sulfur removal from fuel before combustion. Coal washing and crushing
are the most popular alternatives. Coal liquefaction, gasification, and pyrolysis are
also reported in the literature (Wang et al., 2005). Most of the techniques provide
elevated operating costs and require a trade-off between DeSO x and precombustion
techniques to select the more feasible option for each plant.

6.2.2.3.2 Cocombustion control techniques
Cocombustion control techniques consist of the combination of a SO 2 sorbent and
solid fuel before combustion occurs. Commercialized technologies are based on
both fluidized bed (FBC) and circulating fluid bed (CFBC) combustors that provide
the contact time required for SO 2 -sorbent reactions. Pressurized circulating fluid bed
combustors (PCFBCs) have been also proposed.
Sorbents such as dolomite, limestone, lime, and calcium hydroxide are crushed and
mixed with coal before their injection into the boiler. Air is injected from the bottom of
the boiler to support and transport the solid mass inside the boiler. Once the combus-
tion occurs, SO 2 reacts with the sorbent to form calcium sulfate, which is disposed in
combination with the ash derived from the coal. CFBC should provide the proper tur-
bulence and contact time that lead to high SO 2 removal efficiency up to 98%
(European Commission, 2006).

6.2.2.3.3 Postcombustion control techniques
6.2.2.3.3.1 Regenerative processes WellmandeLord process. The Wellmande
Lord process uses sodium sulfite (Na 2 SO 3 ) to absorb SO 2 from flue gas, producing
a highly concentrated SO 2 stream. A process flow diagram (PFD) is shown in Fig. 6.5:
Flue gas is treated before being introduced into the absorber. NH 3 is injected at
170 C in a prescrubber to eliminate the SO 3 that is converted to solid ammonium sul-

fate ((NH 4 ) 2 SO 4 ). The solid ammonium sulfate is then collected in combination with
ash. Chlorides and fluorides must also be eliminated to prevent operational issues in
the absorber by adding hydrochloric acid into the bottom absorber (C ordoba, 2015).
The flue gas is then introduced in countercurrent with aqueous N 2 SO 3 and sodium
bisulfite is formed following Reaction 1. Sodium pyrosulfite is then precipitated as
temperature is lowered, as expressed in Reaction 2 (Poullikkas, 2015):

Na 2 SO 3 þ SO 2 þ H 2 O/2NaHSO 3 (R1)

2NaHSO 3 /Na 2 S 2 O 5 Y þ H 2 O (R2)

Heat is required in the regeneration stage to release SO 2 and recover sodium sulfite
ready for being used in the absorber. The concentration of the SO 2 stream reaches
values of over 90% saturation in water. The rich SO 2 gas stream can be used either
by comprising and liquefaction SO 2 or in the production of sulfuric acid, elemental