# Profiles

SO3 issues for coal-fired plant

‘Numerous design and operating parameters influence the amount of SO3 formed in a plant’ ‘503 is difficultto measure fora numberof reasons’ ‘If503 is present in the flue gas, sulphuric acid mist can form’

During the combustion ofcoal in a furnace most ofthe sulphur in the fuel is oxidised to SO2 buta small and variable percentage is furtheroxidised to

## SO3. There are numerous design and operating parameters which influence the amount ofSO3 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 SO3formed will rapidly reactwith watervapourto form H2S04 vapour

1.0

— which can combine with additional

watermolecules to form H2S04 aerosol. Ifthetemperature is sufficiently low, the aerosol can condense on surfaces. The deposited acid forms a sticky film

on airheaterand cold duct surfaces. Coal-fired plants are less affected by this problem as the alkalinity in the ash neutralises the acid. Sulphurtrioxide formation can, however, be a serious problem foroil-fired plant. There are several analytical techniques fordetermining SO3 concentrations in flue gas. Historic methods include the BCURA probe. The Severn Science analyserwas developed forcontinuously measuring SO3 in flue gases. Though this instrumenthas been used extensively, plant operators report thatthe instrument requires the presence ofa very skilled operator and several measurement may be needed to obtain reproducible results. The more modern methods fordetermining SO3 are based

on absorption orcondensation. In the EPA Method 8 method, a sample ofthe flue gas is extracted and passed through

a series ofimpingers to ensure the condensation ofH2S04. 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 ofSO2 may be a thousand times thatofSO3. The 

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Extent ofSO3 neutralisationfollowing alkali injectionControlled Condensation Method has

been designed to measure the vapour phase concentration ofSO3 and H2S04 in flue gas streams. It is reported that this method provides very accurate and reproducible results undercontrolled conditions though biases can arise and there have been extensive laboratory and field trials to determine the extent ofthese biases. In spite ofthe availability ofavariety oftechniques, it is very difficultto measure SO3 due to the low concentrations, its propensity to adsorb on surfaces and the presence of much higherconcentrations ofSO2. Given the problems SO3 can cause both inside and outside a powerplant, several control technologies have been developed eitherto prevent its formation orto 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 ofSO3. Additives operate by either reducing SO3 formation and thus lowering the potential forcorrosion orby coating the exposed surfaces so thatany 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 forachieving proposed PM25 standards orfor removing acid mist. Wet ESPs operate in a similarway as dry ESPs exceptthat the cleaning ofthe collecting electrodes is performed by washing the collecting surface with liquid ratherthan 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 precipitatorperformance since the first commercial use of precipitators early in the 20th century. It involves the use ofan additive to the ESP inlet to increase the surface electrical conductivity ofthe dust. Several conditioning agents such as waterand ammonia have been used but the most common conditioning agent is sulphurtrioxide. SO3 conditioning has been used in several hundred coal-fired plantworldwide and is particularly useful when firing low-S coals. Selective catalytic reduction (SCR) and selective non-catalytic (SNCR) reduction are both post-combustion technologies forreducing NOx in boilers which rely on the injection of ammonia, urea ora similar compound to reactwith 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. Ifthe flue gas contains SO2, the vanadium containing catalyst in the SCR unit can oxidise some ofthe SO2 to SO3. The existing SO3 level in flue gas can be doubled by the installation ofa SCR unit. This is a particularconcern forplant firing high S coal. The visibility ofplumes orplume opacity is ofconcern in some power plant. Visible plumes from modern coal-fired plant tend to be white, blue

orbrown. The amount oflight scattered by particulate matter is determined by the particle numberconcentration, the particle size distribution, the refractive index and the angle between the sun, the plume and the observerand this depends on the concentration ofspecies 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 ofProfiles is based on a detailed study undertaken by lEA Coal Research, the full report of which is available separately. This particular issue ofProfiles is based on the report:
SO3 issues for coal-fired plant Rohan Fernando CCCI72, ISBN 92-9029-387-X 54 pp. September2003, £255*/~85tI~42.50t

* non-membercountries t membercountries t educational establishmentswithin member countries lEA Clean Coal Centre is a collaborative project ofmember countries ofthe International EnergyAgency (WA) to provide information about and analysis of coal technology, supply and use. The service is governed by representatives ofnine countries (Austria, Canada, Italy, Japan, the Netherlands, Poland, Sweden, the United Kingdom and the USA) and the European Commission.



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