Lloyd L. Handbook of Industrial Catalysts

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448 Chapter 11

trioxide or tungsten trioxide. Both oxides are known to form a wide range of

mixed oxides with vanadium pentoxide and can presumable spread active sites

over the catalyst surface.

The reaction is through to proceed via the following mechanism. Ammonia

is adsorbed on one of the V=O groups of an active site and then reacts with ni-

tric oxide:

+ NO + V=O → N

+ H

O + V–OH (11.1)

The V-OH group is then reoxidized by lattice or molecular oxygen leaving the

active site in its original state:

2 V−OH + O → 2 V=O + H

O (11.2)

It has been suggested that the V=O and V–OH groups are in equilibrium on

the catalyst surface and that the hydrogen atoms of the hydroxyl groups are mo-

bile.

It is possible that at low vanadia loading on unpromoted titania, with no

promoter, that part of the vanadium is reduced to low activity V

and there is a

lower proportion of Bronsted acid sites.

The presence of a promoter may avoid

this and provide the active catalyst. Mixed oxides equivalent to the compound

(β-bronze, see Table 4.7), containing no V

also undergo a phase

change to V

with some reduction of V

to V

, in the presence of arse-

nic oxide.

The following substances are known to poison the reaction:

• Arsenic oxide which reduces the vanadium component of the active sites.

• Alkalis which neutralize the acid sites.

• Calcium sulfate or silica deposition on the catalyst surface , which covers

the active sites.

11.1.2.4 Removal of Sulfur Dioxide as Sulfuric Acid

The SNOX process developed by Haldor Topsøe avoids problems associated

with undesirable sulfur dioxide oxidation in the SCR process.

Sulfur dioxide is

removed from the flue gas in a small contact process unit after the nitrogen ox-

ides have been removed by the usual SCR procedure. Nitrogen oxide conversion

is as high as 95% and sulfur oxides are almost completely recovered as com-

mercially useful sulfuric acid. Apart from the environmental benefits, such a

design allows the use of higher sulfur content coals in power generation and

avoids the need for gypsum disposal.

Conventional SCR and sulfuric acid catalysts are used. Nitrogen oxides are

removed with maximum conversion at about 390°C, following dust removal,

with a typical promoted vanadium pentoxide/titania catalyst. Sulfur dioxide is

Environmental Catalysts 449

then converted to sulfuric acid, using a vanadium pentoxide/kieselguhr catalyst,

by the wet gas sulfuric acid process.

Residual ammonia is also removed during

the oxidation.

This process was first employed in 1991 in a 300 MW coal-based power

plant in Denmark. It produced about 5 tonne h-1 of sulfuric acid from a total gas

flow of up to 900,000 m

/hr at 100% load using 0.38 tonnes per hour of ammo-

nia for the SCR reaction.

11.1.3 Gas Turbine Exhausts

Up to one million tonnes of NOX emissions per year were produced by gas tur-

bines fired with natural gas and distillates in 1992/3.

Typical volumes of NOX

in exhaust gases for different operating conditions involving a number of me-

chanical or catalytic processes are shown in Table 11.4. In future, to cope with

the rapid growth of power generation, the cheaper and cleaner gas turbines will

replace coal-fired boilers as a major source of energy.

At the same time, more

stringent regulations will probably limit NOX emissions to less than 5 ppm.

11.1.3.1 Low Temperature Vanadium Pentoxide Catalysts

It has been convenient to use a low temperature selective catalytic reduction

catalyst operating in the range 160°–190°C after the heat recovery steam genera-

tor in a gas turbine. A promoted vanadium pentoxide catalyst supported on tita-

nia extrudates has been used in these duties, and is stable between 160°–360°C,

with no sintering.

Nitrogen oxides are reduced by up to 95% and turbines of up

to 20MW capacity have been retrofitted. If sulfur dioxide is present in the ex-

haust gas then a small conversion to sulfur trioxide may occur depending on the

temperature.

11.1.3.2 Catalytic Combustion Processes

Catalytic combustion processes are now being developed in which the high tem-

perature thermal formation of NOX is avoided by operating at a lower tempera-

ture. This could avoid the current use of SCR processes to remove thermally

generated NOX from the effluent of conventional gas turbines. Reactions that

generate NOX during the typical homogeneous combustion hydrocarbons with

air take place at temperatures as high as 1500°C and are listed in Table 11.3.

The NOX concentration depends on the maximum temperature and contact time.

While water/steam injection or low fuel/air ratios do improve performance they

increase operating costs or affect reliability.

Catalytic combustion can produce a stable surface flame at low fuel/air ratio

and temperatures as low as 1300°C, and this avoids the formation of thermal

NOX. Operating costs are also significantly lower.

Compression up to an oper-

450 Chapter 11

ating level of 12–16 bar can increase the gas temperature to 350°–410°C. This

may not be high enough to initiate catalytic combustion and a gas burner may be

reach an outlet temperature of up to 1300°C in modern turbines so a very stable

catalyst has to be used. Typical space velocities in the small turbine combustion

chamber are up to 300,000 h

-1

which demands a high catalyst activity. A prob-

lem during operation, however, is that both thermal and catalytic combustion

occur at temperatures exceeding 1000°C.

So far, it has not been possible to develop a thermally stable catalyst that

can operate at temperatures of up to 1300°C when burning the fuel/air mixture

in an economic combustor design. This has led to several modified catalytic

procedures. Two-stage burning procedures have been developed. One burns

only part of the fuel in the first catalytic stage. This limits the catalyst tempera-

ture to less than 1000°C and forms less than 3 ppm NOX. Remaining fuel is

added to the second stage and good mixing with the residual air is essential to

achieve a maximum temperature below 1300°C and avoid temperature hot spots

that can then give rise to additional NOX.

In a second process, air is added in two stages. A high fuel ratio in the first

stage limits the temperature rise in the catalyst and produces a carbon monox-

ide/hydrogen mixture with 2–14 ppm NOX. Once again, good mixing of fuel

with the remaining air is needed in the second thermal stage to avoid the for-

mation of more NOX.

with three sections. The first section contains an active palladium oxide catalyst

that can operate up to about 800°C before being reduced to palladium metal

which is less active. The palladium oxide catalyst is regenerated by reoxidation

of the metal as temperature falls. A more stable catalyst in the second section

continues the catalytic combustion. In the third section, combustion is completed

by thermal reaction and the gas temperature increases to 1300°–1400°C. Over-

all, less than 1 ppm NOX is formed. Palladium oxide is supported on a monolith

coated with temperature resistant barium hexaaluminate.

11.1.4 Nitric Acid Plant Exhaust Gas

Nitrogen oxides are removed from the exhaust gas of nitric acid plants by vari-

ous procedures. A typical exhaust gas contains about 4000 ppm NOX

(NO:NO

= 1) with 3% oxygen and the balance nitrogen. Non-selective removal

is possible using a conventional supported palladium catalyst, with added hy-

drogen, carbon monoxide or a hydrocarbon, at a temperature in the range 200°–

400°C. Both oxygen and NOX are removed with a temperature rise depending

on the oxygen content.

Investigations to develop base metal catalysts have led to the use of selec-

tive catalytic reduction processes with supported vanadium pentoxide catalysts

In a more interesting process, all of the fuel/air mixture is added to a combustor

required to trigger the reaction. Catalytic combustion units must be designed to

Environmental Catalysts 451

and ammonia as the reductant. More than 90% NOX conversion can be

achieved, with acceptable levels of ammonia in effluent, by operating at temper-

atures in the range 170°–250°C and a space velocity up to 40,000 h

-1

Vanadium pentoxide catalysts, similar to those used to remove nitrogen ox-

ides from gas turbine effluent, have also been used to treat the effluents from a

nitric acid plant.

The catalyst is based on extruded titania, impregnated with

vanadium pentoxide and any promoters, or cordierite monoliths covered with a

washcoat of the active material.

An active catalyst based on an extruded large-pore mordenite support has

also been used.

11.1.5 Ion-exchanged ZSM-5 Zeolites

Direct decomposition of nitric oxide could be more attractive than selective

catalytic reduction but, although the reaction is thermodynamically favourable

up to 1000°C, this has not so far been achieved. It is possible, however, that cat-

alysts may be developed in the future to make the process attractive at commer-

cial temperatures. Nitrogen oxides decompose in the ten membered rings of the

zeolite structure where metal has been exchanged.

zeolite has a stable activity in the range 400

°-

when using ammonia as reductant, particularly

metal such as magnesium.

The presence of

reduce activity.

Nitric oxide can be reduced by low molecular weight hydrocarbons in the

presence of oxygen. This avoids the use of ammonia and, potentially, makes the

metal exchanged ZSM-5 suitable for NOX removal from some exhaust emis-

sions. Palladium exchanged ZSM-5 was found to be more active than the copper

exchanged catalysts, at significantly lower temperature.

So far, the practical problems of commercial operation and the effects of

poisons limit the efficiency of metal exchanged ZSM-5 catalysts. No large-scale

operations have been reported for the treatment of effluents from either station-

ary or mobile sources.

In general, the use of most zeolites has been limited both by the significant-

ly higher costs of these materials and by problems with poisons and deactiva-

tion. The efficiency and longer life of cheaper alternatives are still more attrac-

tive. As the energy business changes from coal to natural gas as the preferred

source of energy during the next 10-20 years, retrofits and any changes to the

types of catalysts used will be delayed.

Copper exchanged ZSM-5

500°C with conversion up to 60%

when promoted with a second

oxygen in the flue gas does, however,

452 Chapter 11

11.2 MOBILE SOURCES

The rapid increase in the use of automobiles throughout the world has generated

a high proportion of the pollution responsible for low level ozone and smog

formation since the 1950s. The significance of ozone in photochemical smog

was discovered following work by Arie J Haagen Smit on the components of

pineapple fragrance in 1952! He realized that the ozone concentration during his

experiments was higher on foggy days than on clear days.

The paper on ‘The

Chemistry and Physics of Los Angeles Smog’ described the importance of sun-

light in smog formation.

It has been reported that Los Angeles, with mountains

on three sides, has up to 320 days per year with inversion conditions.

The term inversion in this context refers to the atmospheric condition

whereby there is an increase in temperature with altitude, in contrast with the

norm, where there would be a decrease in temperature with altitude. Such an

inversion can lead to pollution and smog being trapped close to the ground. It is

believed that the exhaust emissions from automobiles contribute strongly to the

genesis of this type of inversion.

The scale of the problem led to the introduction of legislation. In 1966 the

California state government specified state limits for hydrocarbon and carbon

monoxide emissions from automobiles. These were followed, in 1970, by the

US Federal Clean Air Act. Since then the emission limits from automobiles have

become increasingly rigorous, and a zero emission vehicle limit (ZEVL) in 1998

requiring that, in certain cases, hydrocarbons, carbon monoxide and nitrogen

oxides be completely removed was imposed in California. This must obviously

refer to electric powered vehicles but does indicate how tight the limits may

become in future.

possible by the use of automobile emission control catalysts.

These were first

used in the US during 1975 and in Europe from 1993. As a result of continuous

improvement to design and manufacture, the catalysts have been able to con-

form with the increasing severity of the regulations.

11.2.1 Automobile Emission Control

Measures to control the level of carbon monoxide and hydrocarbons in automo-

bile exhaust emissions began during the 1960s with modifications to the fuel

management system. At the same time, base metal oxidation catalysts were test-

ed and were found to be easily poisoned by other impurities. From 1975–77

precious metal catalysts containing platinum and palladium,

together with bet-

ter fuel management, were found to be the essential to remove carbon monoxide

and hydrocarbons from the exhaust. These early catalysts became more success-

ful as the levels of lead additives in gasoline were first decreased and then com-

The trends in the automobile emission standards for the USA and Europe since

1966 are given in Table 11.7. Compliance with the standards set has been made

Environmental Catalysts 453

TABLE 11.7. Exhaust Emission Limits for US and European Union.

US Federal (g/mile) HC CO NOX

Pre-control 15 90 6

1968-69 275 ppm 1.5% –

1970 4.1 34 4

1975 1.5 15 3.1

1977 1.5 15 2.0

1981 0.41 (0.39)

3.4 1.0

1994 0.25

3.4 0.4

2003 0.125

1.7 0.2

EU (g/km) HC CO NOX

1993 – 2.72 0.97

1996 0.5 2.2 –

2000 proposed 0.2 2.3 0.15

2005 indicative 0.1 1.0 0.08

Non methane hydrocarbons. Californian regulations have generally been stricter than in the rest of

the US and limits have been introduced on transitional low emission vehicles (TLEV-1994); low

emission vehicles (LEV-1997); ultra-low emission vehicles (ULEV-1997) and zero emission vehi-

cles (ZEV-1998).

Includes hydrocarbons.

pletely removed. There was no NOX regulation during that period but, where

necessary, some NOX reduction was made possible by recycling exhaust gases.

More stringent engine control and efficient catalysts were demanded from

1977 when legislation calling for NOX removal was also introduced. Nitrogen

oxides were difficult to remove in conjunction with the other pollutants, particu-

larly in the presence of oxygen. This led to strict control of the air/fuel ratio

(A/F) in the engine by the use of oxygen sensors in the exhaust and the introduc-

tion of efficient three-way catalysts incorporating rhodium.

In simple terms, the two-way catalyst is solely an oxidation catalyst, which

is used to convert carbon monoxide and residual hydrocarbons to carbon diox-

ide. The three-way catalyst also provides a capability for reduction, which is

required for the removal of nitrogen oxides.

The air/fuel ratio, based on the mass of air and fuel used in an engine, has a

significant effect on the proportion of the undesirable impurities in exhaust gas

as shown in Table 11.8. At the stoichiometric air/fuel ratio of about 14.7 there is

just enough air for complete hydrocarbon combustion. With less air, that is with

rich.

Under rich conditions, the exhaust will contain more reducing impurities such as

carbon monoxide and hydrocarbons. With an excess of air, or a ‘lean’ mixture,

the exhaust will contain more oxidizing impurities such as oxygen and less ni-

trogen oxides and carbon monoxide. Exhaust gas compositions are related to the

lambda (λ) value which is defined as the ratio of the actual and stoichiometric

value of the A/F ratio. This variable ratio does not require knowledge of the

gasoline composition.

excess fuel, there will be incomplete combustion and the mixture is said to be

454 Chapter 11

TABLE 11.8. Variation of Exhaust Gas Composition with A/F Ratio.

Air/Fuel

λ-value

CO vol% HC ppm NOX ppm O

vol%

13.2 0.9 (rich) ~3 ~1000 ~1600 ~0.8

14.7 1.0 (stoichiometric) ~1.3 ~400 ~2000 ~1.0

16.2 1.1 (lean) ~0.2 ~250 ~1500 ~2.2

Notes: Usually about 3 mol H

/1 mol CO.

content in ppm is (1.1837) x (mg of S/liter fuel) / (A/F ratio).

Engine performance is optimum at λ~0.9. λ = air:fuel / 14.7

Many chemical reactions can take place in the engine effluent to remove

undesirable impurities and some of these are shown in Table 11.9. Reactions A-

C can all take place in a catalytic converter although, if possible, reactions C-D

should be avoided.

The basic catalytic systems that have been used to control emissions are:

• From 1977, closed loops with an oxygen sensor have been used to control

the air/fuel ratio and to optimize conversion of carbon monoxide, hydro-

carbons and NOX with a three-way catalyst system. Before the develop-

ment of fuel management systems the lower conversions needed to meet

legislation could be achieved with a variable air/fuel ratio.

• Two catalytic converters may be used with the engine operating in a rich

mode. The first catalyst reduces nitrogen oxides in an oxygen deficient

exhaust. Oxygen can then be added to the exhaust to remove carbon

monoxide and hydrocarbons with an oxidation catalyst in the second con-

tainer.

• If extra oxygen is added before treating a rich exhaust mixture in a cata-

lytic converter only carbon monoxide and hydrocarbons are removed.

• Lean operating engines may operate with an air/fuel ratio as high as 26

(λ ~ 1.8) which produces low nitrogen oxide concentrations in the efflu-

ent. This demands the use of higher activity, oxidation catalysts because

of the lower exhaust gas temperature.

Not all of these applications can meet the current strict emission legislation.

The autocatalyst operates under conditions that are constantly changing.

Cold starts may be required several times a day and even on a motorway, auto-

mobiles are regularly accelerating and braking. This will influence the air/fuel

ratio and engine temperature. These factors all have an effect on catalyst per-

formance. A typical operating cycle is, therefore:

• Following ignition, the temperature increases from ambient to 600°C.

Catalyst becomes active, or lights off, in the temperature range 250°–

300°C.

• Conversion then increases rapidly up to ~600°C and conversion reaches

95–100%. Temperatures reach 900°C under full load.

Environmental Catalysts 455

TABLE 11.9. Catalytic Removal of Exhaust Gas Impurities.

Reactant Reaction

A: Oxygen

+ (m + 0.25n) O

→ m CO

+ 0.5n H

CO + 0.5 O

→ CO

+ 0.5 O

→ H

B: Nitrogen oxides

2 CO + 2 NO → 2 CO

+ N

+ 2(m + 0.25n) NO → (m + 0.25n) N

+ 0.5n H

O + m CO

2 H

+ 2 NO → N

+ 2 H

C: Rich gas

CO + H

↔

+ H

(HTS)

+ 2m H

O ↔ m CO

+ (2m + 0.5n) H

D: Other reactions

2 SO

+ O

↔

2 SO

+ 3 H

↔

S + 2 H

2 NO + O

→ 2 NO

2 NO + 5 H

→ 2 NH

+ 2 H

2 NO + CO → N

O + CO

• Air/fuel ratio is usually stoichiometric at about 14.7.

• Under fuel rich conditions NOX conversion is enhanced while under fuel

lean conditions there is more carbon monoxide/hydrocarbon conversion.

There is normally some oscillation around the stoichiometric air/fuel ratio

that is adjusted by the feedback system to control air addition to the engine.

in the engine are richer and when

decelerating conditions are leaner. Operation at the stoichiometric level provides

sufficient carbon monoxide and hydrocarbons to reduce the NOX concentration.

11.2.2 Automobile Emission Control Catalysts

The choice of catalyst used is critical in removing the three major impurities and

it must be operated under conditions that are effective for the oxidation of car-

bon monoxide and hydrocarbons and the reduction of the nitrogen oxides. This

demanding operation was necessary from about 1980 when more comprehensive

effluent control legislation was introduced in the US. By 1980, the original bead

catalysts were being replaced by the more efficient extruded ceramic honey-

combs or monoliths, which were impregnated with the active catalyst. Honey-

combs have the advantage of a reduction in pressure drop through the catalyst

bed, improved gas distribution, and a reduction in the size and weight of the

equipment used. Metal monoliths were also developed, although these were not

as widely used as ceramics. Platinum group metals are the most efficient and

active catalysts available and are more resistant to poison than base metals.

During acceleration, the conditions with

456 Chapter 11

11.2.2.1 Bead Catalysts

Bead catalysts were based on typical supports such as , or alumina with

surface areas of about 100 m

/g. Alumina was easily impregnated with the plati-

1000–1100°C before the phase change to alumina took place. Alumina can

be stabilized to a limited extent by the addition of other refractory oxides but

will still sinter at high temperature.

A disadvantage of the bead catalysts, when used in small volume reactors,

is the need to select an appropriate size to compromise between an acceptable

pressure drop through the bed and a satisfactory rate of gas mixing. A complex

design of the catalyst container is also required, to provide for a uniform gas

flow and to avoid disintegration of the catalyst by physical shock as the vehicle

passes over uneven ground. They were, however, easily produced and withstood

Platinum group metals are best adsorbed onto the surface of alumina parti-

cles during impregnation, which limits the amount of metal used and minimizes

costs.

Typical bead properties were in the ranges:

• Diameter 0.3–0.4 cm

• Pore volume 0.5–1.0 cm /g

• Bulk density 0.43– 0.67 kg/liter

11.2.2.2 Monolith Catalysts

The production of honeycomb monolith catalysts is relatively more complicated

than the formation of bead catalysts. It requires several stages:

• Extrusion of the cordierite monolith followed by firing at a high tempera-

ture.

• Application of an alumina washcoat to the honeycomb surface again fol-

lowed by firing at an appropriate temperature.

• Impregnation of the platinum group metals, together with other metal ox-

ide promoters, onto the wash coat.

The most common support is a cylinder of cordierite (2MgO.2Al

.5SiO

extruded as a honeycomb, and covered with a washcoat of γ-, θ- and δ- alumina

with a surface area of about 100 m

-1

. The components all have a similar ther-

mal coefficient of expansion, and hence the composite is most able to withstand

the heating and cooling cycles without disintegrating during use. Cordierite also

has a bulk porosity in the range 20–40 %vol, mainly as macropores, so that good

wash coat adhesion is achieved and the overall bulk density is low. Typical cor-

num and palladium and the resulting catalysts were stable at temperatures up to

the significant thermal shock when used in an automobile exhaust. Under steady

conditions, beads were stable for long periods.

Environmental Catalysts 457

TABLE 11.10. Properties of Cordierite Monoliths.

Property Values

Cell density 400/in

(62/cm

)

Wall thickness (mm) 0.15

Channel size (mm) 0.10

Channel length (cm) 5–15

Open area (%) 75

Specific surface area (m

/g) 2.8

Bulk density (kg/liter) 0.41

Maximum operating temperature (°C)

1200–1300

dierite honeycombs contain 400–600 channels per square inch of the face area

and typical properties are given in Table 11.10.

Metallic monoliths are very light and made up of alternating flat and corru-

gated foil plates about 0.05 mm thick in a cylindrical container. They present a

large open area for gas to pass through the channels and have a high thermal

conductivity. Metals are, however, non-porous and special alloys, containing

iron, chromium, aluminum (Fecralite), must be pre-treated to provide a suitable

oxide layer which bonds with the alumina washcoat.

Ceramic or metal monoliths are packed into a stainless steel metal can with

a ceramic or wire mesh mat as packing which acts as protection against mechan-

ical vibration as well as providing insulation.

11.2.2.3 Washcoat Composition

The surface of the honeycomb is covered by the alumina washcoat, which can

be stabilized against sintering at high temperature by the addition of more re-

fractory materials such as barium oxide, calcium oxide, magnesium oxide or

lanthanum oxide. A very thin surface layer of the washcoat, up to about 10-

30μm, is applied although the thickness increases to about 150μm at the corners

of the square channels.

The surface area of the washcoat can be varied within a range of 50–250

/g of coating, depending on the platinum group metal loading and the stability

required. The washcoat is applied as aqueous slurry that is then dried and cal-

cined to provide the appropriate crystalline form and surface area. Some oxides

are added as stabilisers, to prevent phase changes at the higher temperatures

experienced in the exhaust, while others may also be added to improve the per-

formance of the catalyst, as shown below:

• Cerium oxide acts as an oxygen sink which can absorb oxygen during

lean operation to assist in oxidation when it is later released during peri-

ods of rich operation. Ceria will also stabilize the distribution of platinum

during operation and promote the water gas shift reaction to produce hy-