Lloyd L. Handbook of Industrial Catalysts

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ENVIRONMENTAL CATALYSTS

The effects of atmospheric pollution from the combustion of fossil fuel have

been recognized for many years. Acid rain from industrial areas of Europe and

North America devastated forests. The pea-souper fogs in Donora, Pennsylva-

nia, London, UK and Belgium caused by emissions from power plants, steel-

works and metal smelters as well as domestic coal fires affected public health

and led to thousands of deaths. All this, together with the photochemical smogs

in Los Angeles, California, brought a growing demand for environmental protec-

tion.

Early measures were almost all restricted to imposing limits on particulate

emission and in the UK, the first Clean Air Act of 1956 resulted in the introduc-

tion of smokeless zones. Since then further legislation has led to the develop-

ment of catalytic processes that also reduce the concentration of nitrogen oxides,

hydrocarbons and carbon monoxide in the various exhaust emissions to speci-

fied levels.

Legislation was passed in the 1970s to limit the release of sulfur and nitro-

gen oxides from power plants in Japan and later in Germany. This led to the

investigation of new catalytic processes. Selective Catalytic Reduction (SCR)

was developed to enable power companies to reduce NOX emissions from coal

and oil based power plants. Since then another procedure has been developed to

remove NOX from the gaseous effluent produced by several other combustion

and chemical processes.

The Clean Air Act, introduced by the US Government during 1970, set spe-

cific targets for the control of emissions from automobiles to be achieved by

1975. Catalytic converters are now fitted to automobiles in many parts of the

world to remove nitrogen oxides, hydrocarbons and carbon monoxide in accord-

L. Lloyd, Handbook of Industrial Catalysts, Fundamental and Applied Catalysis,

DOI 10.1007/978-0-387-49962-8_11, © Springer Science+Business Media, LLC 2011

439

440 Chapter 11

ance with local regulations. An additional environmental benefit from the use of

catalytic converters has been the need to stop the addition of tetraethyl lead to

gasoline. Tetraethyl lead was used originally to increase the octane rating of the

gasoline, but lead is a powerful catalyst poison, and would cause a rapid deacti-

vation of the catalytic converter. This led to a need for major changes to be

made to the composition of the US gasoline pool and new Environmental Pro-

tection Agency (EPA) regulations covering the reformulated gasoline were de-

veloped. The use of low-boiling, high-octane hydrocarbons and toxic aromatic

components which may be released to the atmosphere is now restricted.

A third major group of emissions controlled by environmental legislation

comprises volatile organic compounds (VOCs) from all industrial and refinery

effluents. The US Clean Air Act amendments of 1990 called for the reduction in

the concentration of no fewer than 188 toxic chemical air pollutants before

2000. Many of these chemicals are VOCs and significant reductions have been

achieved by the introduction of a range of new catalytic oxidation processes.

The magnitude of the problems involved in environmental control and the po-

tential demand for catalysts is demonstrated by Table 11.1 that shows the esti-

mated volume of some effluents in the USA during 1995.

Reports show that about 1.27 million tons of major chemicals were released

in the US during 1993, some of which are listed in Table 11.2.

Sulfuric acid

made up about 73,000 tons of this total. During 1999 up to 22 million tons of

sulfur was recovered from crude oil fractions and hydrocarbon gases by the cata-

lytic Claus Process.

Although sulfur from power plant emissions has usually

been removed chemically as gypsum, it can be converted to useful sulfuric acid

by a modified form of the contact process.

TABLE 11.1. US Gaseous Emissions During 1995.

Emission Source %

NOX Transport 49

Combustion 46

Industrial Processes 3

Chemical Processes 1

Miscellaneous 1

Total 21.8 million tons

VOCs Non Chemical Processes 51

Transport 37

Chemical Processes 7

Fuel Combustion 3

Miscellaneous 2

Total 22.9 million tons

SOX (1986) Fuels 85

Transport 7

Industry 8

Total 18.5 million tons

Environmental Catalysts 441

TABLE 11.2. Major Chemicals Released to Atmosphere in 1993 in the United States.

Chemical Percentage of total

Ammonia 12.6

Hydrochloric Acid 8.0

Phosphoric Acid 7.6

Methanol 7.5

Toluene 6.4

Sulfuric Acid 5.7

Acetone 4.6

Xylenes 4.0

Carbon Disulfide 3.3

Methyl Ethyl Ketone 3.0

Chlorine 2.7

Zinc compounds 2.6

Dichloromethane 2.3

1,1,3 Trichloromethane 2.2

Total released 1.27 million tons

The catalytic processes used for environmental protection demand an enor-

mous capital investment but are relatively uncomplicated and more efficient

than physical procedures. Demands imposed by environmental legislation have

created a significant and rapidly expanding sector of the worldwide catalyst

business.

11.1 STATIONARY SOURCES

High temperature combustion of coal and oil to provide heat and generate elec-

tricity resulted in huge volumes of nitrogen oxides (NOX), sulfur dioxide and

carbon oxides being vented to the atmosphere. The most widely used fixed, or

stationary, sources which produce nitrogen oxides are steam generating boilers,

combustion turbines, process heaters, waste incinerators and reciprocating en-

gines. A number of chemical processes such as nitric acid, adipic acid, caprolac-

tam and acrylonitrile also produce effluents containing large volumes of nitro-

gen oxides.

It is now generally necessary to treat the flue or exhaust gases from new sta-

tionary sources to remove NOX to a statutory level by the most convenient and

economic procedure available (

Figure 11.1). This usually requires a reduction

process using added ammonia. Although some power plants have been retrofit-

ted with SCR equipment, many older installations have not been covered by the

legislation. This has been described as being ‘grandfathered’ but, as a typical

power plant has a life of at least 30-40 years, means that considerable NOX

would still release continue for several years.

442 Chapter 11

Figure 11.1. Coal-fired power plant fitted with SCR (Selective Catalytic Reduction) unit

to decrease NOX content of the flue gas. Reproduced with permission from Haldor

Topsøe A/S.

Coal and fuel oil can contain up to 5 wt% sulfur and during combustion

most is converted to gaseous sulfur oxides. This amounts to about 1800 volume

ppm sulfur dioxide in flue gas for 2.5 wt% sulfur in coal. Up to 95% of the sul-

fur in the flue gas can be removed by scrubbing with an aqueous slurry of lime-

stone to produce calcium sulphate, or gypsum. This procedure, however, re-

quired facilities for the disposal of the large amount of gypsum thus produced.

Lower sulfur dioxide emissions can be obtained where possible by using low

sulfur fuel. Alternatively, a mini sulfuric acid unit may now be installed to re-

cover several tonnes of acid per day from a single power plant unit.

Nitrogen oxides are formed at the high boiler temperatures, which can ex-

ceed 1500°C, from either the nitrogen compounds present in the fuel or by the

direct oxidation of elemental nitrogen in air as shown in Table 11.3.

The NOX

content of flue gas in modern power plants is usually in the range 800–1800

ppm as shown in Table 11.4.

Environmental Catalysts 443

TABLE 11.3. Formation of Nitrogen Monoxide.

Mechanism Reaction

Thermal (>1500°C) O + N

→ NO + N

Oxidation of molecular nitrogen

N + O

→ NO + O

N + OH → NO + H

Prompt (low temperature, fuel rich)

CH + N

→ HCN + N

Initiation by hydrocarbon radicals N + HCN converted to NO by reaction with

oxygen and hydrogen atoms in the flame.

This can be controlled to a certain extent by the use of low nitrogen fuels or

by modifying the combustion conditions. Low excess air addition, flue gas re-

circulation, low NOX burners or water/steam injection can provide 50-60% re-

ductions in NOX concentration. The scale of the operation is enormous and a

typical US power plant burning 9500 tons of selected coal per day will have to

remove about 135,000 tons of sulfur dioxide (95% conversion) and 45,000 tons

of NOX per year. During 2000 about 50% of US power was generated from coal

but by 2020 60% is expected to come from natural gas.

11.1.1 Selective Catalytic Reduction

Selective Catalytic Reduction (SCR) of nitrogen oxides with ammonia using

specially developed catalysts was investigated in Japan from 1974 and then in-

troduced commercially from about 1980.

It is now the most important process

for removing nitrogen oxides (NOX) from the effluent gas of power plants and

many other stationary sources. German power plants installed SCR units based

on Japanese licenses from about 1984 following the introduction of legislation

limiting the NOX content of effluent gases in 1983.

TABLE 11.4. NOX Concentration in Flue Gas.

A: Power Plant

Plant Type NOX Concentration (mg/m

)

Coal (Pre 1980) 1200–2800

Coal (Post 1984) 800–1800

Lignite 600– 800

Oil 500– 800

Gas 500–1200

B: Gas Turbines

Fuel

NOX Concentrations (mg/m

)

Normal Wet Injection Low NOX Burner SCR

Natural gas 100–400 25–40 15–25 5–10

Distillate 150–700 40–65 65 15–18

444 Chapter 11

Japanese power plants generally used low sulfur coal with dry fired boilers

and accepted relatively low NOX conversion with the SCR catalysts. This also

limited the catalytic oxidation of sulfur dioxide to sulfur trioxide and avoided

excessive deposition of sulfates in downstream equipment. German power

plants, with higher sulfur coals and slagging boilers, had difficulties resulting

from high sulfur dioxide conversion to sulfur trioxide as well as catalyst poison-

ing from volatile arsenic oxides. These problems were gradually overcome

through catalyst development and process modifications that included NOX

removal following sulfur oxide removal.

In the SCR process, NOX impurities are reduced with added ammonia in

the presence of some residual oxygen from the furnace. The main NOX reduc-

tion reactions are shown in Table 11.5 together with some of the undesirable

oxidation reactions, which can both produce sulfur trioxide and waste some of

the added ammonia. Between 0.6–0.9 moles of ammonia per mole of NOX are

added to limit the ammonia slip to downstream equipment where it would de-

posit as sulfates. NOX conversion is therefore limited to between 60-90%. At

low NOX levels, there is little conversion to nitrous oxide. Nitrous oxide

formation is also inhibited by water. Gas leaving the boiler is usually at a tem-

perature in the range 300–430°C and contains dust. Dust is removed in an elec-

trostatic precipitator with little heat loss before sulfur dioxide is removed as

gypsum by reaction with lime. Alternatively, sulfur dioxide can also be convert-

ed to sulfuric acid.

The effluent gas is then vented to atmosphere. In the first

power plants to be retrofitted with SCR units there were three possible locations

for the catalyst bed:

• At the boiler exit. In this position the temperature was suitable for the

NOX reaction and allowed some conversion of sulfur dioxide to trioxide.

Dust in the gas stream led to catalyst problems.

• Following the electrostatic precipitator. The gas temperature was still

high enough for both NOX removal and some sulfur dioxide oxidation.

Dust problems were, however, avoided.

• At the tail end following flue gas desulfurization. Dust problems and sul-

fur dioxide oxidation were avoided but the gas had to be reheated for the

catalytic NOX reduction to be effective.

TABLE 11.5. Reduction/Oxidation Reactions with SCR Catalysts.

Process Reaction

DENOX

4 NO + 4 NH

+ O

→ 4 N

+ 6 H

6 NO

+ 8 NH

→ 7 N

+ 12 H

OXIDATION

2 SO

+ O

→ 2 SO

4 NH

+ 5 O

→ 4 NO + 6 H

4 NH

+ 3 O

→ 2 N

+ 6 H

Environmental Catalysts 445

Figure 11.2. DeNOX catalyst for use in SCR (Selective Catalyt-

ic Reduction) plants. Reproduced with permission from Haldor

Topsøe A/S.

In the first processes, the choice was to use the catalyst in the most conven-

ient high dust location at the boiler exit where dust blocked the catalyst bed and

eroded the catalyst surface in a relatively short period of time. This led Japanese

operators to prefer the low dust location after the electrostatic precipitator. Ger-

man operators, however, decided to remove NOX after the sulfur dioxide re-

moval stage despite the need for heat exchange to reheat the gas when using

typical catalysts.

From 1980, a variety of catalyst shapes and compositions was developed to

overcome operating problems and provide better catalyst selectivity. Following

the US Clean Air Act Amendments in 1990, it became necessary to consider

SCR flue gas treatment from some coal based plants and to install suitable NOX

removal procedures with new boilers, turbines and furnaces.

11.1.2 Selective Catalytic Reduction Catalysts

The most important reactions taking place over SCR catalysts are shown in Ta-

ble 11.5. In power plants using coal or hydrocarbon fuels there is typically a

larger excess of nitric monoxide compared with nitrogen dioxide (~19:1) than

in, say, nitric acid plant tail gas (~1:1). A number of catalysts have been intro-

duced (Figure

11.2) that are able to operate in the available temperature range

from about 200°–430°C. The development, preparation and structure of the cata-

446 Chapter 11

lysts, the supports used and operating performance are described in several re-

views.

11.1.2.1 Catalyst Composition

SCR catalysts are prepared from vanadium pentoxide promoted with either mo-

lybdenum trioxide or tungsten trioxide and supported on the anatase form of

titanium dioxide. The kinetics of the reaction are diffusion limited, and conse-

quently, a thin layer of catalyst is usually deposited on a suitable high geometric

area framework or honeycomb. When the flue gas has high dust content, at the

boiler outlet, the honeycomb is formed from thin, notched, metal plates. These

can be stacked as layers in modules that provide appropriate channel openings in

the reactor. Alternatively, an extruded ceramic honeycomb with smaller chan-

nels and a higher surface area has been used when the gas has low dust content,

after having passed through the electrostatic precipitator or desulfurization unit.

Small honeycomb blocks are stacked together and loaded as beds in the reactor.

Details of catalyst composition and dimensions, together with operating condi-

tions, are summarized in Table 11.6.

Catalysts based on zeolites have also been used successfully to remove NOX

from the emissions of large and small-scale stationary sources. The zeolite can

be extruded directly as a monolith or applied as a washcoat on preformed cordi-

erite supports. The use of several zeolites—including wide-pore mor-

denite—for this reaction has been patented.

Up to 95% NOX conversion can

be achieved with mordenite, with no promoters, and some zeolites are stable at

temperatures up to 600°C. Long catalyst lives have been achieved in retrofitted

coal based SCR units with low sulfur trioxide formation and good poisons re-

sistance. Spent zeolite catalysts can be disposed of in approved landfill sites

because they have negligible heavy-metal content.

Promoted vanadium pentoxide/titania and zeolite based catalysts have been

used to reduce NOX emissions at low temperature from nitric acid plants, gas

turbines, industrial heaters, incinerators and boilers.

TABLE 11.6. SCR Catalyst and Operating Conditions.

Exit Boiler Exit deduster or low dust fuels

Gas Temperature (°C)

320–430 280–430

Dust content (g/SCF) > 6.5 0.02–6.5

Honeycomb Metal Titania/ceramic

Channel opening (mm) 4–6 3–4

Surface area (m

/g) 280–350 400–800

Honeycomb block size (cm) 65 x 46 x 46 10 x 15 x 15

Catalyst volume (m

per megawatt) ~0.6–1.2

Reactor volume (m

per megawatt) ~1.7–3.4

Catalyst composition V

/MoO

or WO

/TiO

Low dust fuels are hard coal, oil or natural gas.

Environmental Catalysts 447

11.1.2.2 Catalyst Operation

Most catalysts supported on titanium dioxide reach an optimum NOX reduction

temperature that depends on the catalyst composition and the treated gas. Activi-

ty then declines as the secondary reactions compete for the ammonia reductant

and sulfur dioxide oxidation becomes excessive. Typical operation is in the

range 300°–425°C although zeolite catalysts operate from 300°–600°C.

12,13

Catalysts may therefore be designed for use in specific duties. For power

plant, the design must balance the reaction rates of NOX reduction and sulfur

dioxide oxidation in the restricted range of temperature of flue gas leaving the

boiler, or at the dust and sulfur dioxide removal stages. A low activity catalyst

that reaches maximum NOX reduction between, say 380°–400°C, can be more

efficient than a catalyst that is more active between 300°–350°C because, over-

all, it produces less sulfur trioxide at the fixed operating temperature.

Vanadium pentoxide/titania catalysts, promoted with molybdenum trioxide

or tungsten trioxide, have lower sulfur dioxide oxidation activity under power

plant conditions than unpromoted catalysts. However, while catalysts containing

tungsten trioxide are initially more active, any arsenic oxides in flue gas more

easily poison them than similar catalysts containing molybdenum trioxide. Ar-

senic is a typical impurity in coal. Flue gas from slagging boilers contains up to

twenty times more than flue gas from dry ash boilers. Catalysts promoted with

molybdenum trioxide have been more widely used in Europe for this reason.

The vanadium pentoxide content of NOX reduction catalysts is probably less

than 1 wt% which maximizes the rate of nitrogen oxide reduction while limiting

the rate of sulfur dioxide oxidation. The total oxide content including promoter

would be less than 10 wt%.

11.1.2.3 Reaction Mechanism

Selective Catalytic Reduction catalysts are similar to the vanadium pentoxide-

anatase catalysts introduced by BASF and von Hayden in the 1960s for the oxi-

dation of methyl groups in ortho-xylene. They were also coated onto cordierite

supports.

Vanadium pentoxide reacts with surface hydroxyl groups on the tita-

nia to form active surface sites. In the case of oxidation catalysts, the mono-

vanadyl species are active. However, for NOX reduction, at least two vanadyl

groups, linked by an oxygen atom, form the selective site. These sites must be

maximized.

When the vanadium content of the catalyst is low, the monovanadyl surface

species predominates. At a concentration around 5%, the monovanadyl species

begin to polymerize, forming V-O-V bridges by dehydration mechanism, and at

concentrations above about 10%, crystalline vanadium pentoxide is deposited.

Preparation of the necessary active sites for maximum activity and selectivity

has been achieved by promoting the catalyst with an excess of molybdenum