Ana Sofia Mascarenhas Techniques to reduce sulphur oxide emissions Energy Management and Policy

Ana Sofia Mascarenhas

Techniques to reduce sulphur oxide emissions

Energy Management and Policy


Sulphur oxides are emitted from most fossil fuel combustion through oxidation of the sulphur contained in the fuel.


I - Wet Scrubber

• Wet scrubbers, especially the limestone-gypsum processes, are the leading FGD technologies (they have about 80 % of the market share and are used in large utility boilers. This is due to the high SO2 removal efficiency achieved, and the low costs involved).

•Wet lime/limestone scrubber•Seawater scrubber

•Magnesium wet scrubber•Ammonia wet scrubber (Walter process)


• Wet lime/limestone scrubber

This type of plant uses lime slurry and produces a sludge of calcium sulphite/sulphate and fly ash.

Wet lime/limestone scrubbers have become popular in the USA but not in other countries, because of the requirement for large areas of land for sludge disposal.

Limestone is commonly used as a reagent because it is present in large amounts in many countries and is three or four times cheaper than other reagents.

Other reagents also used: magnesium-enhanced lime


• Process:Flue gas leaving the particulate control system usually passes through a heat exchanger and enters the FGD absorber in which SO2 is removed by direct contact with an aqueous suspension of finely ground limestone. Fresh limestone slurry is continuously charged into the absorber. Scrubbed flue gas passes through the mist eliminator and is emitted to the atmosphere from a stack or cooling tower. Reaction products are withdrawn from the absorber and are sent for dewatering and further processing.

The wet limestone scrubber is generally divided into two categories according to the type of oxidation: forced oxidation or natural oxidation mode.


Forced Oxidation

By-product: Gypsum (90%) +

Water (10%)

Size (By-product): 0-100 m

Use: Cement, etc....

Denaturing: Easy (hydrocyclone +

filter)

Reliability: >99 %

Countries: Europe and Japan

Natural Oxidation

By-product: calcium sulphate/sulphite

(50-60%) + Water (50-40%)

Size (By-product): 1-5 m

Use: None

Denaturing: Not easy (thickener +

filter)

Reliability: 95-99 %

Countries: USA


Different types of Lime / Limestone wet scrubbers

Type A:


• The prescrubber removes fly ash (HCL and HF)

•The main role for the prescrubber is to ensure a good and constant gypsum quality.

•The flue gas is cooled to approximately 50ºC and saturated with water vapour in the prescrubber. The flue gas enters then the absorber to remove SO2 and finally emitted from the stack.

•The calcium sulphite slurry produced in the absorber is discharged and sent to an oxidation vessel to produce gypsum. The pH is around 4.0 – 4.5.


Type B:


•The absence of a prescrubber can reduce total capital costs and the amount of waste water.

•The purity of gypsum tends to be slightly lower because it includes a little fly ash.

•The materials used in construction of the system require careful selection to allow for the increase in the chloride concentration.

• The SO2 removal efficiency tends to be lower because aluminium fluoride complexes block the dissolution of limestone in the absorber liquid. The adverse effect of fluoride may be reduced by use of Na2SO4 as additive and by increasing the pH.


Type C:


• It’s now the most common method (in situ oxidation). Although the prescrubber is primarily to remove HCl and HF, a low pH prescrubber also removes more mercury as well as fine particulate carrying other trace elements.

•In situ oxidation has many advantages:- Prevents scaling and plugging problems through complete oxidation of the product in the absorber.- Achieves higher SO2 removal.- In situ oxidation promotes the SO2 removal efficiency even at low pH values.- It reduces the formation of S2O. There is also no need to add H2SO4.


Type D:


It is the simplest configuration in wet limestone scrubbers and has now become the leading FGD system.

All chemical reactions are operated in an integrated single absorber. This can reduce the capital cost and power consumption.

In Germany, most recent FGD installations are type (d)


The design of the absorber is crucial in wet FGD systems.

Type 1: Spray tower

The spray tower predominates in the wet FGD systems.


Type 2: Packed tower

The packed tower lengthens the residence time of gas-liquid contact, resulting in higher SO2 removal efficiency.


Type 3: Jet bubbling reactor

This absorber type is a good example of a simplified FGD process. It eliminates the need for recycle pumps, spray nozzles and headers, separate oxidation tanks and thickeners.


Type 4: Double loop tower


Seawater Scrubber

• Seawater scrubbing utilises seawater inherent properties to absorb and neutralise sulphur dioxide in flue gases.

• The flue gas is filtered in a dust collector, normally a fabric filter or a electrostatic precipitator (ESP). The flue gas subsequently enters the SO2 absorber (packet tower) where the flue gases flow countercurrently to the down-coming seawater in a once-through mode of operation. High removals rates of SO2 are thus obtained.

• The acidified absorber effluent is mixed with the rest of the cooling water prior to the next step which is oxidation (SO2 SO4

2-)

• The water will be discharged back into the sea.


• Magnesium scrubbing (reagent: magnesium hydroxide). It replaced sodium scrubbing, because magnesium hydroxide has become less costly than sodium hydroxide. It produces waste sulphate liquor. (Mainly for coal-fire burners)

• Ammonia wet scrubber (Walter process). In the Walter process, SO2 is absorbed by aqueous ammonia, resulting in ammonium sulphate as the fertiliser by-product. This process is seldom used.(oil-fired boilers)


• Spray dry Scrubbers

• Lime slurry is usually used to remove SO2 from the flue gas in this type of FGD.

• Spray dry scrubbers are generally characterised by low capital costs but higher operating costs.

• Spray dry scrubbers are mostly used for relatively small to medium capacity boilers using low to medium sulphur coal (1.5%)

• The residue is normally a mixture of calcium sulphite, calcium sulphate and fly ash which is less attractive commercially.


•Process:

The process mainly consists of the spray dry absorber, particulate control such as ESP or fabric filter and recycling disposal devices for the reaction products.

The lime slurry dispersion is the distinctive feature in the spray dry absorber.

The sorbent for SO2 absorption is typically lime or calcium oxide.

Lime slurry is atomised to a cloud of fine droplets in the spray dry absorber where SO2 is removed from the flue gas.

Water is evaporated by the heat of the flue gas with sufficient residence time for the SO2 and other acid gases such as SO3 and HCl to react simultaneously with hydrated lime to form calcium sulphite/ sulphate and calcium chloride.


•Process (cont..):

The residue is a dry powder, which is collected by either ESP or fabric filter.

As this residue contains some unreacted lime, part of it is generally recycled and mixed with fresh lime slurry to enhance lime utilisation.

The use of a pre-collector which removes most of the fly ash before it enters the absorber, is a common design feature of most the European spray dry scrubber plants.

Sorbent utilisation in spray dry scrubbers is higher than in sorbent injection processes but unreacted lime amounts to about 10 - 40% of make-up lime and is discharged from the system with calcium sulphite/sulphate.


Furnace sorbent injection

Furnace sorbent injection involves the direct injection of the dry sorbent into the gas stream of boiler furnace.

Typical sorbents include: - pulverised limestone (CaCO3)- hydrated lime (Ca(OH)2)- dolomite (CaCO3MgCO3)

In the furnace, the addition of heat results in calcination of the sorbent to produce reactive CaO particles. The surface of these particles react with SO2 in the flue gas to form calcium sulphite (CaSO3) and sulphate (CaSO4). These reaction products are then captured with fly ash by the particulate control device, typically an ESP or fabric filter.


The sorbent is porous, so the SO2 reaching the surface must diffuse through the CaO pores. In the sulphation process, CaSO4 builds up on the CaO surfaces. This means that the SO2 has to diffuse through the CaSO4 to reach unreacted CaO.

About 50 % of SO2 removal efficiency can be achieved at a sorbent molar ratio (Ca/S) of 2 – 4 when limestone is injected into the boiler furnace at near-optimum operation.There are several measures to improve SO2 removal efficiency at low capital cost by adding some devices to the furnace sorbent injection unit. The simplest method is to spray water into the duct before the precipitator. This results in an improvement in SO2 removal efficiency of about 10 %.


Recycling the reaction product is an effective alternative in order to improve efficiencies of both SO2 reduction and limestone utilisation.

Ash handling and disposal are complications in the furnace sorbent injection, most of all due to the sheer quantity of the reaction products to be processed.

Although many research projects are in progress to utilise the reaction product, most utilities equipping furnace sorbent injection must have a specially prepared disposal site in contrast to wet scrubbers which produce the saleable by-product, gypsum.



Duct sorbent injection

Duct sorbent means injection of a calcium- or sodium-based sorbent into the flue gas between the heated air and the existing ESP or fabric filter.

The humidification water serves two purposes. First, it activates the sorbent to enhance SO2 removal, and second, it conditions the particulate matter to maintain efficient ESP performance.

After injection, the sodium bicarbonate decomposes thermally to form sodium carbonate. After the initial sorbent surface of the sodium carbonate has reacted with SO2 to form sodium sulphite or sulphate, the reaction slows due to pore pluggage.


In order for the reaction to continue, the sorbent particle must decompose further. This decomposition evolves H2O and CO2 gases into the surrounding atmosphere, creating a network of void spaces throughout the particle. This process exposes fresh reactive sorbent and allows SO2, once again, to diffuse into the particle interior. This increase in surface area is in the other of 5-20 times the original surface area, depending on the specific sorbent considered.

The characteristics of duct sorbent injection technologies are low capital cost, simplicity of process and adaptability to difficult retrofit situations, but relatively low SO2 removal efficiency. The goal of SO2 removal efficiencies in duct sorbent injection used to be generally a minimum of 50 %. Spent sorbent recycle is especially important in the economics of duct sorbent injection because shorter sorbent residence times tend to lower sorbent utilisation when compared with conventional spray dry scrubbers. Only 15 % to 30 % of Ca(OH)2 in weight usually reacts with SO2 without spent sorbent recycle



Hybrid sorbent injection• Hybrid sorbent injection is a combination of furnace sorbent injection and duct sorbent injection to improve SO2 removal efficiency. A feature of hybrid sorbent injection is to employ limestone as sorbent because it is cheaper than lime, which is used generally in spray dry scrubbers. • Advantages:

-relatively high SO2 removal rate-low capital and operational costs-easy to retrofit-easy operation and maintenance with no slurry handling-reduced installation area due to compact equipment -no waste water treatment.


Circulating fluid bed (CFB) dry scrubber

• The circulating fluid bed (CFB) process is a dry scrubber, separate from either the spray dryer scrubber or sorbent injection.


Regenerable processes

In regenerable processes, the sorbent is reused after thermal or chemical treatment to produce concentrated SO2, which is usually converted to elemental sulphur. These are complex processes requiring high capital( Wellman-Lord process, magnesium oxide process).

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Ana Sofia Mascarenhas Techniques to reduce sulphur oxide emissions Energy Management and Policy