by-products (waste prevention). The catalyst is
not consumed during the reaction it supports;
therefore its lifetime should, in theory, be
unchanged after the reaction. In practice, however, the stability might be reduced by chemical
interaction of reactants with the active sites of the
catalyst called poisoning. In addition, performance is limited by physical blocking of the
active sites by species deposited on the catalyst
and its pores called fouling. Both processes, as
well as thermal sintering due to high temperatures,
inhibit the catalytic reaction rate and deactivate
the catalyst in the long term [80].
Catalytic technology is widely present in
industrial applications such as oil refining, energy,
transport, chemicals, polymers, and textiles but
also in pollution control and monitoring. Catalysts
can be roughly classified into metals (Ag, Pt, Pd),
oxides (MnO 2 , NiMoO 4 ), and acids (H 3 PO 4 ,
H 2 SO 4 , zeolites). Each of the classes has its
advantages and can be used in different applications [80].
Selective Catalytic Reduction
Selective catalytic reduction (SCR) is a chemical
reduction reaction via a reducing agent supported
by a catalyst. SCR is used mainly in fossil fuel
combustion units and nitric acid plants for reduction of nitrogen oxides. The SCR process reduces
NO x chemically into molecular nitrogen and
water vapor with the help of nitrogen-based
reagent (usually ammonia or urea) and catalyst
(noble metals, molecular sieves, metal oxide,
etc.). The SCR catalysts are divided into four
groups: (1) V 2 O 5 -WO 3 /TiO 2 type, (2) three-way
catalyst (TWC) type, (3) molecular sieve type,
and (4) AC-supported type [89]. The TiO 2 -WO 3 /
V 2 O 5 catalyst is the most widely used commercial
SCR catalyst due to its high De-NO x efficiency
and stability.
Honeycomb catalysts, plate-type catalysts, and
corrugated catalysts are the three most widely
used SCR catalyst types [90]. The catalyst type
and composition have an influence on the efficiency of NO x reduction, lifetime, and its stability.
Operation works by injection of the reagent into
the hot waste gas stream. The mixture then enters
the reaction chamber which contains catalyst bed
and diffuses through it ensuring selective reaction
of the reagent and NO x under specific temperature
(EPA).
SCR is very efficient and can operate at significantly lower temperatures than non-catalytic systems, but their operation and installation cost
increases considerably [91]. Almost all coal-fired
power plants have installed deNO x systems due to
the strict limitation of NO x emission rule. And
95% of them are using SCR deNO x catalyst.
However, the activity of commercial SCR catalysts would decrease because of the complex
chemical composition in the flue gas of power
plants. Thus, the lifetime of SCR catalysts is normally around 3 to 4 years. The regeneration and
recycling of poisoned or wasted catalysts have
become a severe problem.
Hydrodesulfurization
Hydrodesulfurization (HDS) is a catalytic process
widely used to remove sulfur from natural gas and
refined petroleum products, such as gasoline, diesel fuel, etc. Its purpose is to reduce SO 2 emissions from vehicles, power plants, and industrial
processes. The product of this process, hydrogen
sulfide (H 2 S), can be readily converted into elemental sulfur or sulfuric acid in petroleum refineries. In fact, the majority of world sulfur
production comes from its recovery from natural
gas and petroleum processing [92]. The catalysts
used in HDS are usually based on aluminum
oxides mixed with metals like Co, Mo, Ni,
etc. [93] and supported on materials with high
surface area, therefore increasing the amount of
catalytically active sites [80].
Catalytic removal of VOCs was briefly
described in a chapter on thermal oxidation. Catalytic incinerators are one of the most efficient
methods for the destruction of VOCs from the
gas effluent [9]. The catalyst can decrease the
required oxidation temperature significantly,
therefore allowing VOC conversion to H 2 O and
CO 2 to occur with much less external heat and
hence at a much lower cost. Among various types
of catalysts, noble metals are highly effective in
catalytic oxidation. However, their high cost and
poor stability, especially in the presence of chlorine, set back their development. Metal oxides of
Industrial Emissions Control Technologies: Introduction
497
not consumed during the reaction it supports;
therefore its lifetime should, in theory, be
unchanged after the reaction. In practice, however, the stability might be reduced by chemical
interaction of reactants with the active sites of the
catalyst called poisoning. In addition, performance is limited by physical blocking of the
active sites by species deposited on the catalyst
and its pores called fouling. Both processes, as
well as thermal sintering due to high temperatures,
inhibit the catalytic reaction rate and deactivate
the catalyst in the long term [80].
Catalytic technology is widely present in
industrial applications such as oil refining, energy,
transport, chemicals, polymers, and textiles but
also in pollution control and monitoring. Catalysts
can be roughly classified into metals (Ag, Pt, Pd),
oxides (MnO 2 , NiMoO 4 ), and acids (H 3 PO 4 ,
H 2 SO 4 , zeolites). Each of the classes has its
advantages and can be used in different applications [80].
Selective Catalytic Reduction
Selective catalytic reduction (SCR) is a chemical
reduction reaction via a reducing agent supported
by a catalyst. SCR is used mainly in fossil fuel
combustion units and nitric acid plants for reduction of nitrogen oxides. The SCR process reduces
NO x chemically into molecular nitrogen and
water vapor with the help of nitrogen-based
reagent (usually ammonia or urea) and catalyst
(noble metals, molecular sieves, metal oxide,
etc.). The SCR catalysts are divided into four
groups: (1) V 2 O 5 -WO 3 /TiO 2 type, (2) three-way
catalyst (TWC) type, (3) molecular sieve type,
and (4) AC-supported type [89]. The TiO 2 -WO 3 /
V 2 O 5 catalyst is the most widely used commercial
SCR catalyst due to its high De-NO x efficiency
and stability.
Honeycomb catalysts, plate-type catalysts, and
corrugated catalysts are the three most widely
used SCR catalyst types [90]. The catalyst type
and composition have an influence on the efficiency of NO x reduction, lifetime, and its stability.
Operation works by injection of the reagent into
the hot waste gas stream. The mixture then enters
the reaction chamber which contains catalyst bed
and diffuses through it ensuring selective reaction
of the reagent and NO x under specific temperature
(EPA).
SCR is very efficient and can operate at significantly lower temperatures than non-catalytic systems, but their operation and installation cost
increases considerably [91]. Almost all coal-fired
power plants have installed deNO x systems due to
the strict limitation of NO x emission rule. And
95% of them are using SCR deNO x catalyst.
However, the activity of commercial SCR catalysts would decrease because of the complex
chemical composition in the flue gas of power
plants. Thus, the lifetime of SCR catalysts is normally around 3 to 4 years. The regeneration and
recycling of poisoned or wasted catalysts have
become a severe problem.
Hydrodesulfurization
Hydrodesulfurization (HDS) is a catalytic process
widely used to remove sulfur from natural gas and
refined petroleum products, such as gasoline, diesel fuel, etc. Its purpose is to reduce SO 2 emissions from vehicles, power plants, and industrial
processes. The product of this process, hydrogen
sulfide (H 2 S), can be readily converted into elemental sulfur or sulfuric acid in petroleum refineries. In fact, the majority of world sulfur
production comes from its recovery from natural
gas and petroleum processing [92]. The catalysts
used in HDS are usually based on aluminum
oxides mixed with metals like Co, Mo, Ni,
etc. [93] and supported on materials with high
surface area, therefore increasing the amount of
catalytically active sites [80].
Catalytic removal of VOCs was briefly
described in a chapter on thermal oxidation. Catalytic incinerators are one of the most efficient
methods for the destruction of VOCs from the
gas effluent [9]. The catalyst can decrease the
required oxidation temperature significantly,
therefore allowing VOC conversion to H 2 O and
CO 2 to occur with much less external heat and
hence at a much lower cost. Among various types
of catalysts, noble metals are highly effective in
catalytic oxidation. However, their high cost and
poor stability, especially in the presence of chlorine, set back their development. Metal oxides of
Industrial Emissions Control Technologies: Introduction
497