Profiles

SO3 issues for coal-fired plant

‘Numerous design and

operating parameters

influence the amount of

SO3 formed in a plant’

‘503 is difficult to

measure for a number of

reasons’

‘If 503 is present in the

flue gas, sulphuric acid

mist can form’

During the combustion of coal in a

furnace most of the sulphur in the fuel

is oxidised to SO2 but a small and

variable percentage is further oxidised

to SO3. There are numerous design and

operating parameters which influence

the amount of SO3 formation in a

power plant. Among these are fuel

sulphur content, excess air level, ash

content and composition, convective

pass surface area, gas and tube surface

temperature distributions and coal

fineness. 1~’pical SO3 concentrations

from coal-fired plant are less than

5 ppm whereas for oil-fired plant, SO3

levels are typically 15—50 ppm.

Any SO3 formed will rapidly react with

water vapour to form H2S04 vapour

1.0 —

which can combine with additional

water molecules to form H2S04 aerosol.

If the temperature is sufficiently low,

the aerosol can condense on surfaces.

The deposited acid forms a sticky film

on air heater and cold duct surfaces.

Coal-fired plants are less affected by

this problem as the alkalinity in the ash

neutralises the acid. Sulphur trioxide

formation can, however, be a serious

problem for oil-fired plant.

There are several analytical

techniques for determining SO3

concentrations in flue gas. Historic

methods include the BCURA probe.

The Severn Science analyser was

developed for continuously measuring

SO3 in flue gases. Though this

instrument has been used extensively,

plant operators report that the

instrument requires the presence of a

very skilled operator and several

measurement may be needed to obtain

reproducible results. The more modern

methods for determining SO3 are based

on absorption or condensation. In the

EPA Method 8 method, a sample of the

flue gas is extracted and passed through

a series of impingers to ensure the

condensation of H2S04. This method

had the limitation that a small amount

of SO2 is also absorbed and is oxidised

to sulphate ions. This is a particular

problem in coal-fired boilers where the

concentration of SO2 may be a

thousand times that of SO3. The

November 2003

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500 MW boiler

15 seconds residence time

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0 10 20 30

Initial SO3, ppmv

Extent of SO3 neutralisationfollowing alkali injection
Controlled Condensation Method has

been designed to measure the vapour

phase concentration of SO3 and H2S04

in flue gas streams. It is reported that

this method provides very accurate and

reproducible results under controlled

conditions though biases can arise and

there have been extensive laboratory

and field trials to determine the extent

of these biases. In spite of the

availability of a variety of techniques, it

is very difficult to measure SO3 due to

the low concentrations, its propensity to

adsorb on surfaces and the presence of

much higher concentrations of SO2.

Given the problems SO3 can cause

both inside and outside a power plant,

several control technologies have been

developed either to prevent its

formation or to eliminate it from the

flue gas once formed. Alkaline additives

have been injected either into the

furnace or into the flue gas at the backend to minimise the effects of SO3.

Additives operate by either reducing

SO3 formation and thus lowering the

potential for corrosion or by coating the

exposed surfaces so that any condensed

acid reacts with the additive and not

with the ductwork. Industrial facilities

such as blast furnaces and sulphuric

acid plants have used wet ESPs for

many years but their installation in

power plant is more recent. Power plant

have tended to utilise dry ESPs but dry

ESPs are not ideal for achieving

proposed PM25 standards or for

removing acid mist. Wet ESPs operate

in a similar way as dry ESPs except that

the cleaning of the collecting electrodes

is performed by washing the collecting

surface with liquid rather than

mechanical rapping. The high current

densities possible in wet ESPs enable

them to remove sub-micron particles,

droplets and acid mists as small as

0.01 urn with a collection efficiency

>99.9%.

Flue gas conditioning has been used

to improved precipitator performance

since the first commercial use of

precipitators early in the 20th century. It

involves the use of an additive to the

ESP inlet to increase the surface

electrical conductivity of the dust.

Several conditioning agents such as

water and ammonia have been used but

the most common conditioning agent is

sulphur trioxide. SO3 conditioning has

been used in several hundred coal-fired

plant worldwide and is particularly

useful when firing low-S coals.

Selective catalytic reduction (SCR)

and selective non-catalytic (SNCR)

reduction are both post-combustion

technologies for reducing NOx in

boilers which rely on the injection of

ammonia, urea or a similar compound

to react with NOx in flue gas and

reduce it to molecular nitrogen. In SCR

systems, which are much more common

than SNCR, gaseous ammonia is

injected into the flue gas and passed

over a catalyst. If the flue gas contains

SO2, the vanadium containing catalyst

in the SCR unit can oxidise some of the

SO2 to SO3. The existing SO3 level in

flue gas can be doubled by the

installation of a SCR unit. This is a

particular concern for plant firing high

S coal.

The visibility of plumes or plume

opacity is of concern in some power

plant. Visible plumes from modern

coal-fired plant tend to be white, blue

or brown. The amount of light scattered

by particulate matter is determined by

the particle number concentration, the

particle size distribution, the refractive

index and the angle between the sun,

the plume and the observer and this

depends on the concentration of species

such as SO3 and NO2. In plant

containing wet FGD processes the flue

gas is saturated with moisture and, if

SO3 is present in the flue gas, sulphuric

acid mist can form instantly.

Each issue of Profiles is based on a

detailed study undertaken by lEA

Coal Research, the full report of

which is available separately. This

particular issue of Profiles is based on

the report:

SO3 issues for coal-fired plant

Rohan Fernando

CCCI72, ISBN 92-9029-387-X 54 pp.

September 2003, £255*/~85tI~42.50t

* non-member countries

t member countries

t educational establishments within member

countries

lEA Clean Coal Centre is a

collaborative project of member

countries of the International

Energy Agency (WA) to provide

information about and analysis of

coal technology, supply and use.

The service is governed by

representatives of nine countries

(Austria, Canada, Italy, Japan, the

Netherlands, Poland, Sweden, the

United Kingdom and the USA)

and the European Commission.

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