Slides IFRF Presentation 29
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Energy Management and Policy
Techniques to reduce sulphur oxide
emissions
Ana Sofia Mascarenhas
Techniques to reduce sulphur oxide emissions
Sulphur oxides are emitted from most fossil fuel combustion
through oxidation of the sulphur contained in the fuel.
Techniques to reduce sulphur oxide emissions
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)
Techniques to reduce sulphur oxide emissions
• 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
Techniques to reduce sulphur oxide emissions
• 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 Reaction products are withdrawn from the absorber and are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.
Techniques to reduce sulphur oxide emissions
Forced Oxidation
By-product: Gypsum (90%) +
Water (10%)
Natural Oxidation
By-product: calcium sulphate/sulphite
(50-60%) + Water (50-40%)
Size (By-product): 0-100 m
Size (By-product): 1-5 m
Use: Cement, etc....
Use: None
Denaturing: Easy (hydrocyclone +
filter)
Denaturing: Not easy (thickener +
filter)
Reliability: >99 %
Reliability: 95-99 %
Countries: Europe and Japan
Countries: USA
Techniques to reduce sulphur oxide emissions
Different types of Lime / Limestone wet scrubbers
Type A:
Techniques to reduce sulphur oxide emissions
• 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.
Techniques to reduce sulphur oxide emissions
Type B:
Techniques to reduce sulphur oxide emissions
•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.
Techniques to reduce sulphur oxide emissions
Type C:
Techniques to reduce sulphur oxide emissions
• 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.
Techniques to reduce sulphur oxide emissions
Type D:
Techniques to reduce sulphur oxide emissions
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)
Techniques to reduce sulphur oxide emissions
The design of the absorber is crucial in wet FGD systems.
Type 1: Spray tower
The spray tower
predominates in the
wet FGD systems.
Techniques to reduce sulphur oxide emissions
Type 2: Packed tower
The
packed
tower
lengthens the residence
time of gas-liquid contact,
resulting in higher SO2
removal efficiency.
Techniques to reduce sulphur oxide emissions
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.
Techniques to reduce sulphur oxide emissions
Type 4: Double loop tower
Techniques to reduce sulphur oxide emissions
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 SO42-)
• The water will be discharged back into the sea.
Techniques to reduce sulphur oxide emissions
• 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)
Techniques to reduce sulphur oxide emissions
• 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.
Techniques to reduce sulphur oxide emissions
•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.
Techniques to reduce sulphur oxide emissions
•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.
Techniques to reduce sulphur oxide emissions
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.
Techniques to reduce sulphur oxide emissions
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 %.
Techniques to reduce sulphur oxide emissions
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.
Techniques to reduce sulphur oxide emissions
Techniques to reduce sulphur oxide emissions
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.
Techniques to reduce sulphur oxide emissions
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
Techniques to reduce sulphur oxide emissions
Techniques to reduce sulphur oxide emissions
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.
Techniques to reduce sulphur oxide emissions
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.
Techniques to reduce sulphur oxide emissions
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).