Flue Gas Desulphuriser Explained
What is flue gas desulphurisation?
Flue gas desulphurisation (FGD) describes a process that removes sulphur dioxide (SO2) from a flue gas (exhaust gas) stream. Sulphur dioxide is released to the atmosphere when fossil fuels are burnt and it is a leading contributor to acid rain. The FGD process has become critical to many industrial plants due to increasingly stringent environmental legislation. Although the FGD process is present in many industries, this article focuses upon FGD equipment associated with the power generation industry, particularly for coal fired power stations.
Coal Power Station Exhaust System with Flue Gas Desulphuriser Highlighted
Note
‘Desulphurisation’ is also spelt ‘desulfurization’, the former is British English whilst the latter is American English.
Why do we need flue gas desulphurisation?
Most fossil fuels (coal, oils etc.) contain some sulphur. When a fossil fuel is burnt, the sulphur it contains is released to atmosphere via the process of combustion. Some coals may contain up to 4% sulphur, which is a significant amount considering that a coal power station may burn in excess of 5,000 tonnes of coal per day.
Sulphur dioxide combines readily with water and consequently combines readily with moisture clouds in the atmosphere. Once a cloud has become sufficiently saturated with moisture, water droplets form and fall to the ground due to gravity; this process is known as precipitation (rain).
Unfortunately, as water absorbs sulphur dioxide it becomes more acidic. Consequently, as clouds of moisture absorb the sulphur dioxide gas in the atmosphere, the pH value of the suspended water molecules (moisture) decreases, and it becomes more acidic. The acidic rain -colloquially referred to as acid rain- then falls to the ground due to gravity.
Forest Damaged by Acid Rain
Acid rain damages crops, infrastructure, vegetation, soil and contributes to ocean acidification. Because sulphur dioxide is a large contributor to the causes of acid rain, environmental laws have been enacted to force SO2 producers to reduce the amount of SO2 they generate. One of the main produces of sulphur dioxide are coal fired power stations, consequently, they are forced to install FGD systems in order to reduce their SO2 emissions and comply with environmental legislation.
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Flue Gas Desulphurisation (FGD)
FGD processes are termed either ‘wet’ or ‘dry’. Dry type FGD systems utilise a reagent in powder form (dry form). Wet type FGD systems utilise an alkaline slurry that is formed after mixing a dry reagent with water. Although two main types of FGD designs are possible, more than 75% of power generation FGD systems are wet.
Wet Flue Gas Desulphuriser Schematic
How Flue Gas Desulphurisation Works
The most economical means of removing SO2 from a flue gas stream is via a chemical reaction with a reagent. A reagent is a substance or compound added to a system to cause a chemical reaction. Suitable reagents should render the SO2 harmless to the environment whilst also producing a by-product that does not damage the environment.
The most common reagents used in FGD systems are lime (calcium oxide) and limestone (CaCO3). Other reagent alternatives exist e.g. ammonia, but limestone is the most widely adopted. The main reason for limestones widespread adoption is that it is plentiful, cheap and easy to access; all these factors depend however upon geographical location.
By-products of the flue gas desulphurisation process are usually calcium sulphite (CaSO3) and/or calcium sulphate (CaSO4). The by-product produced depends upon which reagent and which FGD system design is used. Irrespective of the reagent and design, the by-product is usually calcium based.
The wet ‘throwaway’ FGD design is the most common FGD design employed by fossil fired power stations today. The next section describes how a typical wet limestone absorber tower works.
Wet Flue Gas Scrubber Tower
How does wet flue gas desulphurisation work?
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Limestone is delivered to the plant in crushed, or whole form. Crushed limestone may be delivered directly to a storage silo before being mixed with water in a dedicated mixing unit. Un-crushed limestone will need to pass through a size reduction stage prior to being mixed with water or stored. Size reduction may be achieved using on-site crushers or mills e.g. jaw crushers, gyratory crushers, ball mills, cone crushers etc.).
Pulverised limestone is mixed with water to form an alkaline based slurry. An alkaline slurry is any slurry with a pH exceeding 7.0, but for operational purposes the pH desired for the slurry is usually 8.0 (system design dependent).
Flue gas is discharged from the power station watertube boiler(s), passes through a bag house or electrostatic precipitator (ESP) and is then passed to the desulphuriser. The flue gas temperature is approximately 150 ⁰C (300 ⁰F) or more when it enters the flue gas desulphuriser. Sulphur dioxide gas entrained within the flue gas and is separated via wet scrubbing.
Wet Flue Gas Desulphuriser
Wet scrubbing is achieved by passing the flue gas from the bottom of the scrubber tower to the top. The alkaline slurry travels in the opposing direction (top to bottom); this arrangement is termed ‘counter flow’ due to the opposing flow directions of the two flowing mediums. Note that the counter flow design is also sometimes referred to as contra-flow. Of all the flow designs (counter, cross and parallel flow), the counter flow design is the most efficient for the transferring heat and mixing of flowing mediums.
In order to ensure efficient direct contact between the alkaline slurry and flue gas, a series of spray decks fitted with spray nozzles are used. Spray nozzles discharge the alkaline slurry uniformly within the tower, which ensures the flowing mediums have a high contact surface area with each other. The lower spray decks operate at a pH of approx. 4.0 whilst the higher decks operate at a pH of approx. 6.0 or more. Spray nozzles operate at low pressure, approximately 1 bar (14.5 psi).
The alkaline slurry falls from the spray deck to a perforated tray. The perforated tray forces the flue gas to bubble through the slurry as it passes through the tower, this ensures good direct contact between the slurry and flue gas.
After passing through the holes in the perforated tray, the slurry falls to the base of the tower due to gravity and is collected in the effluent holding tank (EHT) (sometimes called a reaction delay tank). Slurry that is entrained with the flue gas, is separated by a demister at the top of the tower and returned to the EHT.
Demister (green indicates gas, blue indicates slurry)
Water in the slurry readily absorbs the sulphur dioxide gas whilst the alkaline nature of the slurry neutralises the gas’ acidity. The water is termed the absorbent whilst the limestone is termed the reagent. The remaining flue gas is discharged at the top of the tower, but up to 99% of the SO2 may have now been removed (typically 90% to 95% is removed).
Reacting the alkaline slurry with sulphur dioxide produces calcium sulphite (CaSO3), this chemical reaction can be expressed as:
CaCO3 + 1 SO2 → CaSO3 + CO2
Further oxidation of calcium sulphite produces calcium sulphate (CaSO4), this chemical reaction can be expressed as:
CaSO3 + 2H2O + ½O2 → CaSO4 · 2H2O
Compressed air (pressure of approx. 1 bar / 14.5 psi) is injected into the base of the effluent holding tank where it bubbles upwards through the slurry. Due to the injection of compressed air into the slurry, forced oxidation of the calcium sulphite occurs and calcium sulphate is formed. Some slurry held by the EHT is circulated back to the spray deck, but some of the slurry is discharged from the tower for dewatering. Agitators (propellers connected to three phase motors) prevent calcium solidification within the EHT.
The dewatering process separates FGD by-products from the slurry. The slurry contains approximately 10-15% calcium-based solids when discharged from the tower. Machinery items used in the dewatering process often include vacuum filters, hydro-cyclones and clarifiers (thickeners). Once the crystalline calcium-based substance has been extracted, it can be either sold or disposed of.
FGD by-products are often saleable and can be sold to reduce the plant’s overall operating costs. Calcium sulphate is also known as ‘gypsum’ and is used for many commercial products. The most common usage of gypsum is for plasterboard (wallboard) in the construction industry, but it also used in the agricultural industry as fertiliser. If the by-product can not be sold, it is often mixed with fly ash and sent to a landfill site.
Man Holding Plasterboard
Wet Scrubber Tower Construction Materials
It is necessary to select tower construction materials carefully due to the corrosive and abrasive environment within the tower. Construction materials depend upon the components and design of the tower, but stainless steel, fibreglass and rubber lined carbon steel, are common construction materials.
FGD Process Efficiency
The liquid to gas (L/G) flow rates through a wet scrubber tower have a large effect upon its operating efficiency. Typically, a high L/G ratio is desired as this ensures as much SO2 is removed from the flue gas as is economically possible, whilst also preventing solidification of the alkaline slurry within the tower. Solidification of the slurry leads to reduce flow paths within the tower, blocked spray nozzles, and it is difficult to remove (very hard and adhesive).
The pH of the alkaline slurry increases when it reacts with sulphur dioxide, it is therefore necessary to continually supply limestone to the EHT so that the pH of the slurry can be held constant. A reduction in the slurry pH will lead to a resultant reduction in FGD efficiency.
Flue Gas Desulfurization Reducing Acid Rain
Flue gas desulfurization (FGD) is a set of technologies used to remove sulfur dioxide (SO2) from flue gases produced from industrial combustion at petrol refineries, chemical manufacturing industries, mineral ore processing plants, and power stations to name a few.
The removal of sulfur dioxide is critical to establishing a safe and clean environment where toxic emissions are kept to a safe low.
Where Does Sulfur Dioxide Come From?
Fossil fuels such as coal and oil often contain high amounts of sulfur, and when these fuels are burned around 95% or more of the sulfur is converted to sulfur dioxide (SO2) which is emitted as flue gas.
The main source of sulfur dioxide in the air is a result of industrial activity that processes or uses materials that contain sulfur.
Electricity generation from coal, oil or gas
Mineral processing incl. copper extraction
Other industrial activities that involve burning fossil fuels
Transporation, motor vehicles
Why Does Sulfur Dioxide Need To Be Removed?
Sulfur dioxide is an acidic gas that reacts easily with other substances to commonly form harmful compounds such as sulfuric acid, sulfate particles, and sulfurous acid.
When breathed in, it can irritate the nose, throat and airways with a risk of developing more severe problems over prolonged exposure.
Sulfur dioxide in itself is a major air pollutant which impacts all life. It is also a precursor of acid rain which has significant adverse impacts on forests, freshwaters and soils, in turn killing insect and aquatic life-forms, causing paint to peel, corrosion of steel structures such as bridges, and weathering of stone buildings and statues.
Due to the damaging nature of sulfur dioxide, stringent environmental regulations have been placed limiting so2 emissions and urging industries to take more action to remove sulfur dioxide from their flue gases.
How is sulfur dioxide removed?
As sulfur dioxide is an acidic gas it will need to be neutralised before disposal. To do this an alkaline-based sorbent is used to bring the pH level of the sulfur dioxide gas closer to neutral.
Scrubber systems are one of the most efficient ways of reducing sulfur dioxide emissions caused by industrial combustion, and a classic example of an acid-base chemical reaction performed on an industrial scale.
Flue Gas Desulfurization Systems: Scrubbers
Flue gas desulfurization systems (FGD) or scrubbers are devices capable of sulfur removal efficiencies between 50% to 98%. Typically the highest removal is achieved by wet scrubbers and the lowest by dry scrubbers.
They are used in coal-and-oil fired combustion units including utility and industrial boilers, municipal and medical waste incinerators, petroleum refineries, cement and lime kilns, metal smelters, and sulphuric acid plants.
Spray Tower Scrubber
In the wet scrubbing process, flue gas exits the water tube boiler and is drawn into the scrubberspray tower by a fan. The temperature of the flue gas will be in excess of 150-degree Celcius and therefore may be passed through a heat exchanger before entering the spray tower.
The lower third of the spray tower is where the alkaline based slurry is held, this is referred to as the effluent holding tank (EHT). The slurry is most commonly a mixture of ground-up limestone and water.
From the bottom of the tower, a heavy-duty centrifugal pump is used to pull through and discharge the slurry from the spray headers nearer the top of the tower.
Often the spray headers are in different levels called a spray deck where the slurry flows along before coming out of the spray nozzles. The alkaline slurry gets sprayed across the entire tray area where the gas meets the slurry.
As the sulfur dioxide comes into contact with the slurry it is absorbed by the water and the limestone within the slurry neutralises it.
The gas that continues to rise is now at a pH level closer to neutral, the slurry then drops back down into the EHT. Any remaining slurry that continues to rise is prevented to pass by a mist eliminator.
The sulfur dioxide gases are now neutralised and the reaction that has taken place in the slurry has produced calcium sulphite (CaSO3) which in turn can be turned into calcium sulphate (CaSO4) using a process known as forced oxidation.
Oxygen in the form of compressed air is drawn directly into the slurry in the EHT via nozzles from a compressed airline. This bubbles air into the slurry which enables the calcium sulphite to oxidise into calcium sulphate which is also known as gypsum.
This step is advantageous as gypsum is able to be sold as a byproduct helping with cost-effectiveness, and it is also easier to separate from the water than calcium sulphite.
The calcium sulphite will gradually settle out to the base of the tank where they can be extracted via a discharge hole.
Around 15% of the discharged slurry will contain the suspended solids including gypsum - now referred to as sludge.
Once the gypsum is discharged it is sent for dewatering where it can go on to be used as a byproduct for cement and agricultural industries.
The flue gas now exiting the absorber contains very low traces of sulfur dioxide, as little as 2%, significantly reducing the impact that these combustion processes have on the environment.
The Future For Sulfur Dioxide Emissions
With the stringent environmental regulations limiting S02 emissions across the world it makes both economical and environmental sense to make flue gas desulfurization a priority in the industrial sector.
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