Tackling particulates

In developing countries like India and China, where high ash, low heating value coal is fired; flue gas conditioning to control particulates can provide significant cost savings compared to installing a much larger precipitator. PEi puts the spotlight on this technology and highlights what`s on offer.

Anupam Sanyal,

International Environmental & Energy Consultants

USA;

Ken Parker,

Environmental and Energy Consultant Associates

UK.

The impact on electrostatic precipitator performance arising from the firing of low sulphur coals, producing fly ash having high electrical resistivity, is well established. Where low NOx burners have been employed, the carryover of carbon also has a pronounced impact on performance and these two effects can lead to an almost ten fold emissions increase on existing plant.

Power generation in the developing world will expand significantly during the next decade. Much of this power will be generated from pulverized coal (PC) fired plant. The indigenous fuel supplies in some countries, eg. India, although plentiful, often have a low heating value, high ash and moisture content. Combustion of these coals, usually low in sulphur, can be difficult, leading to high gas flow rates and temperatures, while the high resistivity fly ash can adversely impact on precipitator performance.

World Bank assisted power plants have to comply with emission levels of 50 mg/Nm3 and this can necessitate attaining a collection efficiency of 99.8+ per cent, depending on the operational parameters. To meet compliance, large precipitators are required. These not only have a large footprint, but add significantly to the overall cost of the boiler installation. For these coals, as cost is of prime concern, it is commercially viable to modify the operational characteristics by means of flue gas conditioning (FGC) agents, so that a smaller and less costly precipitator installation can be supplied. Although it is difficult to accurately ascribe costs in developing countries, using typical US costings, it can be shown that FGC can reduce potential installation costs by up to 40 per cent.

Proven technology

FGC is a well proven technology, where small concentrations of chemical reagents are injected into the flue gas upstream of the precipitator to modify the gas and fly ash characteristics, to enhance precipitator performance. FGC of power plant precipitators dates back to the early 1960s, when there were concerns over increasing widespread atmospheric pollution from particulates and “acid rain”. FGC has been shown to be completely effective to overcome emission increases arising from coals producing a “difficult” fly ash, for example:

a) Plants firing low sulphur coals can produce a fly ash of high resistivity giving rise to reverse ionisation. To minimise this effect, the ash resistivity can be reduced to acceptable levels by the addition of gaseous sulphur trioxide to the gas stream, typically at rates of 5 to 15 ppm v/v. The required quantity being dependent on the ash composition, gas moisture and most important the flue gas temperature. On injection, the sulphur trioxide reacts with the flue gas moisture and the acid mist formed uses the particles as condensation nuclei to give a conductive coating which lowers their electrical resistivity.

b) A further need arises with dusts having poor cohesive properties, eg. those with high carbon contents. In this instance, 5-15 ppm ammonia additions to the flue gas significantly reduce rapping re-entrainment losses. The injected ammonia reacts with the SO2 /SO3 in the flue gases to form ammonium salts. Some of these are fused at the operating temperature, which when collected with the fly ash increases the general cohesivity of the deposit. Gaseous NH3 also improves the electrical operating conditions which also aids performance.

c) An Australian fly ash, exhibiting both high resistivity and poor cohesive properties, is encountered and for this situation, “dual conditioning” where both sulphur trioxide and ammonia are injected to resolve the difficulty. The fly ash is typically characterised by having a silica plus alumina content in excess of 90 per cent. While dual conditioning was developed for this particular Australian ash, in practice there are other coals, eg. some US Western sub-bituminous, which exhibit similar ash characteristics and dual conditioning has been used to overcome both deficiencies.

Although the theory of FGC is readily understood and proven, the equipment for injecting the conditioning agents is quite complex. There are three specialist US companies – Seattle-based, Chemithon Corp., Wahlco Inc., based in California and Wilhelm Environmental, Indianapolis. Indiana. There are three major US companies which offer typically different types of equipment, and together with their overseas licensees, have perhaps the most global FGC experience.

The basic approaches to produce the conditioning agents are similar, in that for SO3, the system converts sulphur dioxide to trioxide by means of a catalyst before dispersion into the ductwork, while for NH3, gaseous ammonia is mixed with air before dispersion. Each supplier offers different forms of equipment, having preferred or patented approaches. These are concerned with improving the overall plant efficiency in terms of feedstock usage and operating power, which tend to be supplier and site specific. To minimise site installation work, all supply equipment is largely skid mounted, prewired and checked prior to delivery. The site work is thus limited to the distribution pipework, electrical hook-up and interfacing with the plant control system.

Sulphur trioxide conditioning

The type of sulphur trioxide equipment supplied by Chemithon Corp. depends on the unit size. For smaller units, or those which operate infrequently, Chemithon offers granular sulphur or liquid sulphur dioxide based feedstock systems. For plants over 100 MW, the equipment is usually of the sulphur burner type. Molten sulphur is stored at a temperature around 140 degrees C and carefully metered, using a mass flow element, through a gear or centrifugal pump into the sulphur burner.

The variable speed gear pump has advantages in having no valves and more accurate flow control. In the refractory lined sulphur burner, molten sulphur mixes with air and ignites to form sulphur dioxide, which then passes through an SO2 cooler (unique to Chemithon) to control gas temperature entering the converter in the optimum 415 to 455 degrees C range. The air is supplied from a variable frequency drive, positive displacement blower, which is more economical to operate and the output can be varied to maintain SO2 strength.

The SO2/air mixture passes through a two stage converter (using typically vanadium pentoxide catalyst), and is converted to sulphur trioxide. The efficiency of conversion depends on the catalyst temperature and sulphur dioxide strength. The exothermic reaction effectively stops when the gas temperature exceeds 600 degrees C, so cooling air is added between stages to lower the gas temperature to about 415 degrees C. By controlling the air flow to give an SO2 strength of around six to eight per cent, and cooling the SO2, the system can achieve conversion efficiencies above 95 per cent.

The heat from the exothermic reactions of sulphur burning and SO2 conversion using this uniquely higher gas strength, means that the external heat supply can be discontinued, which results in significant operational cost savings. The hot SO3 and air mixture exiting the convertor is then piped through a manifold system maintained under pressure, at about 300 mbar compared to around 50 mbar as used by other companies, and at a temperature above the acid dew point to minimize corrosion and pluggage situations.

In Wahlco`s system, like Chemithon, the feedstock is liquid sulphur contained in a steam heated vessel containing a fully submerged metering gear pump which eliminates any risk of leakage. The liquid sulphur then passes into a combined sealed burner/catalyst converter vessel. Wahlco employs a cascade burner, which it claims is superior to a spray burner. Although the converter has only a single stage, efficiencies of up to 95 per cent are claimed with the aid of a new proprietary catalyst, which also has a lower start up temperature saving on installed power.

The variable speed blower is of a multistage cast type, requiring no silencing, and is fitted with a 10 micron washable filter to prevent blinding of the catalyst bed. The heater elements are of a low temperature design for longevity. The heater plenums are designed using CFD techniques to ensure optimum air flow and heat transfer. Because of the exothermic reactions, once the plant is operating the supplemental heat requirements can be significantly reduced. In addition to employing liquid sulphur, Wahlco also offers a granular sulphur burning system

Wilhelm Environmental currently favours the use of granular sulphur for its latest design, although it also has FGC systems based on molten sulphur. The granulated sulphur is metered and pneumatically conveyed through an atomising burner into a preheated furnace where it oxidises to form SO2 . The gaseous SO2 is then conveyed through fully insulated hot gas piping which acts as a “free” heat exchanger between the furnace and convertor vessels. The converter vessels are located adjacent to the injection manifold, thereby avoiding pipework heat loss and ensuring that the SO3 /air mixture is maintained well above acid dew point prior to injection.

The small “close coupled converters” dissipate heat very quickly and maintain high conversion efficiencies, particularly with multistage converters. If necessary, several converters can be installed across large ductwork systems ensuring optimum distribution of the SO3. Similar to the other suppliers, the system incorporates a variable speed blower, so gas strengths can be maintained for any required feed rate. However, in this case to avoid catalyst pluggage the inlet air supply is cleaned through a small baghouse to sub-micron size, thereby eliminating the need to periodically wash or change filters and further, reduce contamination of the converter catalyst.

To eliminate the need of a sulphur feedstock, equipment is now being developed by Wilhelm to use and convert the naturally occurring SO2 in the flue gases, either,by having a variable area catalyst inserted into the ductwork ahead of the air heater, or by having a fixed bed bypass catalyst and varying the gas flow through it.

Although all of these systems refer to the use of elemental sulphur, all three companies also offer systems based on liquid sulphur dioxide.

Ammonia conditioning

As the equipment for ammonia conditioning is far simpler than for sulphur trioxide, all offer similar systems. These comprise a pressurized liquefied ammonia storage vessel, the outlet of which passes to a vapouriser, the gaseous ammonia is then metered and injected with dilution air into the flue gas stream.

Where ammoniacal liquor is used as a feed stock, the liquor is metered from the storage vessel, evaporated, then mixed with hot air, prior to injection into the flue gases. The use of ammoniacal liquor offers an advantage in simplified site storage. The availability and transportation may, however, favour liquefied ammonia in many instances. An alternative feedstock would be to use Urea, but costs may outweigh the advantages of having an easily handleable dry product.

Dual conditioning

Where the situation calls for dual conditioning, each company offers a combined plant based on their preferred design of SO3 and NH3 systems. The equipment is normally supplied on a single skid, since by changing the fan and heater specifications, a single blower can supply both duties. Separate feed stock storage, control and injection systems are required, usually with the NH3 being injected upstream of the SO3 .

Injection rates

While the required quantity of conditioning agent depends on various factors, such as the required performance enhancement factor, gas flow rate, gas temperature etc. Considering a nominal 15 ppm v/v injection rate, the weight of sulphur, sulphur dioxide and ammonia required per million Nm3/hr is: sulphur 22.5 kg/hr, sulphur dioxide 45 kg/hr, both based on 95 per cent conversion efficiency, and ammonia 11.4 kg/hr or 93.9 kg/hr for 25 per cent ammoniacal liquor. The actual injection rate will be site dependent and is controlled, usually by microprocessor devices, using feedback from the boiler load, DCS, CEM, ESP Electrics, etc. Nominal injection rates, however, can be preset based on the coal and ash analyses. The use of microprocessors also enables SO2 conversion and electrical usage to be optimized, using information from the conditioning plant flows, temperatures etc.

To maximize the conditioning effect in an available contact time of less than 1 second, it is important that the reagent gases are introduced into the flue gas having as wide a cover as possible. A typical injection manifold would comprise a number of probes spaced across the duct, each having injection nozzles positioned at the “centres of equal areas”.

Some typical examples of particulate emission reductions achieved by flue gas conditioning on a world wide basis are shown in Table 1.

Conclusions

Using flue gas conditioning to modify the characteristics of a “difficult” fly ash has been shown to be effective, on a global range of different coals, to enhance the performance of electrostatic precipitators. The tabulated data indicates that, dependent on the application, FGC results in a minimum emission reduction of 50 per cent and in certain circumstances, can approach nearly 90 per cent.

Table 1 lists some emission enhancements that have been obtained as a result of flue gas conditioning, installed by the major equipment suppliers, to satisfy emission requirements.

With the expansion of electricity generation equipment in developing countries, eg. India and China, where high ash, low heating value coal is to be fired, flue gas conditioning can provide a significant cost saving over a much larger precipitator installation to meet emission requirements. For example, to attain an efficiency of 99.8 per cent, based on such coal, a conventional precipitator having a specific collection area of some 220m2/m3/s. may be required. If, however, FGC was to be applied with a precipitator of only 120m2/m3/s, then the installation costs would decrease by around 40 per cent .

Flue gas conditioning – as an alternative to a much larger precipitator, and/or as a way of overcoming precipitation difficulties – is commercially viable and should be considered as a positive and economic solution for precipitator performance attainment.

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Figure 1. Power generation in the developing world will expand significantly during the next decade

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Figure 2. World Bank assisted power plants have to comply with specified particulate emission levels

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