Page 331 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

308 10 Post-combustion Air Emission Control

of Hg, indicates that physical sorption is the main mechanism for Hg capture by
activated carbon.
Meanwhile, the activated carbon is not adsorbing Hg exclusively, as there are
many other species present in the flue gas, such as H 2 O, NO 2 ,SO 2 , and other trace
elements. For example, the presence of NO 2 and SO 2 has a negative effect on
mercury adsorption. In addition, the presence of HCl, if any, supports the trans-
formation of physisorbed elemental Hg to stronger bound chemisorbed Hg. A 90 %
of Hg reduction can be achieved by activated carbon sorption.
Much higher Hg removal can be achieved when activated carbons are impreg-
nated with sulfur or iodide. In sulfur-impregnated activated carbons, Hg is strongly
bound as HgS, sorption capacities of which may be 100–1000 times higher than
those of activated carbon alone. It is especially effective at high temperatures [33].
Iodide-impregnated activated carbons show the improved performance by forming
stable Hg–I complexes [6].

10.8.3 Mercury Captured by Metal Oxides, Silicates, and Fly
Ashes

Alternative Hg sorbents using metal oxides, silicates, fly ashes, etc., have been
developed to replace activated carbons. Metal oxides such as MnO 2 ,Cr 2 O 3 and
MoS 2 showed moderate capacities, making them possible alternatives for activated
carbons [15]. As an example, in a Finish cement plant, the Hg emission was
reduced by approximately 90 % when 10 % of the coal/petcoke fuel was replaced
by car tire scrap. This benefit is due to the metals such as Mn and Cr in the tires that
were introduced to the combustion system [21].
Fly ash may also absorb or adsorb Hg [30]. The performance seems to depend
on many factors such as the carbon content of fly ash, temperature, gas phase
composition, as well as the property of the fossil fuel.

10.9 Proper Layout for Post-combustion Air Pollution
Control Devices

After fossil fuel combustion and in-combustion controls, multiple air pollutants
present in the flue gas must be resolved. The air pollutants of concern depend on the
fuel and local regulations on emission control. NO x is the major concern for natural
gas combustion, while SO 2 may also need attention when oil is used for com-
bustion. For solid fuels, particulate, SO 2 (except for biomass) and NO x emissions
must all be reduced to certain levels before the flue gases being discharged to the
atmosphere for dispersion. Similarly, solid fuel-fired IGCC processes involve hot
gas cleanup to upgrade the fuel gas to meet the gas turbine specifications.