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Emission Control Technology Spray Dryer Absorber

Spray Dryer Absorbers facilitate the removal of acidic pollutants, heavy metals, and dust from flue- and off-gases at fossil-fuelled power plants, waste incinerators and industrial installations.

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The SDA process

The Spray Drying Absorption process – a semi-dry flue gas desulphurization and cleaning process – facilitates a reaction efficiently transforming gaseous pollutants such as e.g. SO2, SO3, HCl, Hg, and dioxins into a slaked lime Ca(OH)2 absorbent to form a stable and dry powdery product that is easy to store and transport.

Benefits

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Spray Drying Absorption - the unique use of the spray drying technology for acid-gas absorption - was originally invented by GEA in the 1970s. Ever since, it has been subject to continuous further development and optimized to meet changing conditions and requirements. Hence, the process stands as an efficient, versatile and thoroughly tested technology. Every Spray Drying Absorption process is tailored to meet the client’s requirements as well as applicable environmental legislation. The absorbers boast well-proven system features such as e.g. peak-control and activated-carbon injection which have been designed for the purposes of ensuring low mercury and dioxin emission.

Benefits
• High acid-gas removal efficiencies
• Low capital, operating and maintenance costs
• Low power and water consumption – operates on low-quality water
• High plant availability

The Spray Drying Absorption process

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The Spray Drying Absorption process is a semi-dry flue gas desulphurization process. The process uses slaked lime Ca(OH)2 as absorbent and results in a stable and dry end product, mainly consisting of fly ash and various calcium compounds.

Hot, untreated flue gas is introduced into the Spray Dryer Absorber via a flue gas disperser and subsequently comes into contact with a highly reactive absorbent that will be atomized by a Rotary Atomizer. An efficient contact between flue gas and absorbent allows for rapid mass transfer of acidic components from the flue gas into the alkaline absorbent. The absorbent neutralizes the absorbed acid (SO2 + Ca(OH)2 -> CaSO3/CaSO4 + H2O). While this reaction takes place, the water is evaporated, thus forming a dry powder. A fraction of the dry powder will be deposited at the bottom of the absorber chamber and discharged from here, whereas the main part is carried to the downstream dust collector while the cooled flue gas leaves the chamber. The flue gas – now clean – passes from the dust collector to the stack without re-heating.

 
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The Ca(OH)2 – either purchased as slaked lime or (more frequently and economically) prepared on site from burned lime, CaO – is pumped to the Rotary Atomizer from a buffer tank. After separation, the powder is either transported to an end-product storage facility, or recycled in the process for improved utilization of excess absorbent. The end product from the process is a stable and dry powder. This powder is used all over the world, mainly in road construction, as building materials, and for other purposes in the construction industry.

The Spray Drying Absorption technology features excellent performance for absorption – not only of primary pollutants such as SO2 and HCl. For, due to finely atomized absorbent sprayed into the flue-gas stream and the subsequent dust removal, pollutants such as SO3, HF, etc. will practically be completely removed. This facilitates the use of carbon steel as construction material throughout the flue-gas path. And there are two further benefits: In the first place, the process allows for the use of low-quality process water, such as e.g. waste water or even seawater; and, secondly, as the process generates no waste water, there will be no subsequent waste-water treatment or processing.

More than 200 references
Worldwide, more than 200 Spray Drying Absorption plants are installed at power stations, steel plants, waste incinerator plants, and at plants burning hazardous waste. They all share one common trait: They are operated in accordance with or above required performance stipulations as laid down by local authorities. Even today, the very first plants, installed in the 1980’s, are still operating satisfactorily and successfully.

• Total plants built: > 200
• Total number of absorbers: > 350
• Total number of atomizers: > 450
• Process installed at close to 25,000 MWe and 4,300 MWt power-plant capacity
• Process installed at more than 160 incineration lines, worldwide
• Process installed at more than 10,000 m2 sinter band, worldwide

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The Spray Dryer Absorber – the Rotary Atomizer, the Gas Disperser and the Absorber Chamber – is the heart of the Spray Drying Absorption process.

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Ever since the first Spray Dryer Absorbers were designed, both the process and its key components have been subjected to further development – and we will continue to do so.

Rotary Atomizer

SDA key components atomizer wheel
Atomizer wheel

The Rotary Atomizer is the key element in the Spray Dryer Absorber. This piece of equipment atomizes the absorbent liquid into flue gas, basically by feeding the absorbent liquid to the atomizer wheel that rotates at around 10,000 rpm. Slinging the liquid outwards, the fast rotating wheel breaks the liquid into an atomized mist of exceptionally small droplets (less than 50 microns) with immense surface areas.

Using special stainless-steel types, abrasion-resistant wheels and wheel inserts, the Rotary Atomizers applied for the Spray Drying Absorption processes – types F-100, F-360, F-800 and F-1000 – have been modified to withstand the frequently rough environments and abrasive absorbents.

The structure of the Rotary Atomizer comprises an upper and a lower part, separated by a central supporting plate. The upper part houses the gearbox and lubrication system together with the upper oil sump. The Atomizer is powered from a vertical flange motor, located above the gear box. A flexible coupling transmits the power from the motor to the input shaft of the gearbox. The atomizer’s lower part, which is exposed to the hot media inside the Absorber Chamber, mainly consists of the flexible spindle, spindle bearings, feed and flush piping, the liquid distributor, and the Atomizer wheel. Due to the products’ resistance to abrasion, both atomizer parts and atomizer wheel parts are manufactured of ceramic materials. As these parts are exposed to much wear and tear from the absorbent feed, they are replaceable.

The Gas Dispersers

Gas dispersers serve to optimize the distribution of flue gas inside the absorber chamber, facilitating the best contact between the flue gas and the atomized droplets of absorbent feed. The standard gas disperser, type DGA, is a roof gas disperser with adjustable guide vanes. It is used for flue gas with only small contents of abrasive or sticky fly ash. For flue gases containing higher concentrations of abrasive fly ash, the erosion resistant gasp disperser, type DGR, is applied. This gas disperser is used in many Spray Dryer Absorbers applied at municipal waste incineration plants.

Compound gas dispersers are used for flue gas volumes of 400,000 Nm3/h and upwards. Here, the flue gas is split into two streams, where about 60% will enter through the roof gas disperser, while the remaining flue gas enters through a central gas disperser. This setup is often used at power and sinter plants. Most Gas Dispersers are made of mild steel.

 

The Spray Dryer Absorber chamber

Generally made of mild steel, the Spray Dryer Absorber chamber is a cylindrical construction with a coned bottom. The flue gas enters the chamber via the gas disperser(s) and leaves through an outlet duct located at the coned bottom. The Rotary Atomizer is installed centrally in the chamber, encircled by the outlet from the roof gas disperser allowing the gas to mix with the atomized absorbent slurry feed. The central gas disperser is placed directly beneath the atomizer thus facilitating a raise in the atomizer mist for better gas/liquid contact.

The size of the chamber depends on the amount and properties of flue gas; and the shape will vary according to the type of gas disperser. An outlet for larger particles is located at the bottom of the absorber cone.

GEA is constantly developing and optimizing the design and size of both spray-drying absorber chamber and gas disperser. We now provide the capacity for treating more than 2,000,000 Nm3/h in one single absorber chamber; and we are still striving to design the SDA for even higher gas loads.

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With a considerable portion of a power plant’s costs linked to emission control, the right technologies not only cushion environmental impacts, but also further competitive advantages.

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The SDA power process

The Spray Drying Absorption process is a versatile way of cleaning flue gases by the removal of acid gases and particulates generated by fossil-fuel burning, mainly coal. Because of the presence of large volumes of flue gasses, power plant installations frequently have more than one Spray Dryer Absorber module. A compound-gas disperser ensures an optimum gas flow, even at very large volumes. The considerable plant size and gas volumes normally favor recycling of the end product, allowing operation at low outlet temperatures, no more than 10 - 15° C above adiabatic saturation temperature.

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The end product from Spray Drying Absorption consists of the reaction products, excess absorbent and fly ash. The SO2/HCl ratio of the inlet gas is high, thus allowing for operation close to the adiabatic saturation temperature and minimizing the content of lime in the end product. A Spray Drying Absorption system can achieve very high desulphurization rates, practically only limited by the accepted content of excess lime in the end product.

When it comes to gas amount and composition, the normal changes in operation of the upstream flue gas generator are generally so smooth that control of outlet temperatures and emissions are achieved by simply mixing the feed streams of absorbent and recycle slurry (or water) in the above head tank. To a large extent, the absorbent used for power plant applications is purchased and stored as CaO and then slaked on site.

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The hot untreated flue gas is introduced into the absorber module via gas dispersers for optimum gas flow distribution; and contact with the absorbent is facilitated by the rotary-atomizer assisted spray. The efficient contact between gas and atomized absorbent slurry allows for the rapid mass transfer of acidic components occurring during the process from gaseous state into a liquid state.

The alkaline absorbent neutralizes the absorbed acid, and the desired reaction product is generated by simultaneous water evaporation. A fraction of the dried end product will be deposited at the bottom of the absorber chamber from which it will be discharged, whereas the main part is carried to the downstream dust collector together with the cooled flue gas, and then removed from the gas. The now clean gas passes from the dust collector to the stack without re-heating.

Following the chemical reaction of acidic-component absorption and the final drying, the powdery end product is separated from the gaseous state and transported to an end-product storage facility or re-used in the SDA process for improved utilization of excess absorbent. Whether or not to design the SDA plant as a single-pass system or to incorporate a recycle system will depend on the inlet gas quality and emission requirements.

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Spray Drying Absorption is the ideal solution for removing dust and acid-gas exhaust from sinter plants.

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The SDA Sinter process

The Spray Drying Absorption process is a versatile means to clean off- and flue gases by the removal of acid gases and particulates. Spray Drying Absorption installations for sinter plants combine features from power- and waste incineration plants. Flue gas volumes are normally high; and, hence, necessitate the installation of large absorber modules. But, frequently, acid contents will be relatively low, thus allowing for the use of single pass systems.

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Variations in gas quality may occur relatively fast, and therefore the control system must be designed to act accordingly. To a large extent, absorbent used for sinter plant applications will be purchased and stored as CaO for subsequent on-site slaking. The Rotary Atomizers applied in sinter plants are generally F-360, F-800 and F-1000, equipped with stainless steel wheels and central parts.

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The hot, untreated flue gas is introduced into the absorber module via gas dispersers for optimum gas flow distribution; and contact with the absorbent is obtained via Rotary Atomizer spraying. The efficient contact between gas and atomized absorbent slurry allows for rapid mass transfer of acidic components from a gaseous stage into a liquid state.

 
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The alkaline absorbent neutralizes the absorbed acid, and the product from this reaction is formed while water is evaporated simultaneously. A fraction of the dried end product is deposited at the bottom of the absorber chamber from which it will be discharged, whereas the main part is carried to the downstream dust collector with the cooled flue gas and removed from the gas. The clean gas passes from the dust collector to the stack without re-heating.

Following the absorption of acidic components, the chemical reaction, and the final drying, the powdery end product is separated from the gaseous stage and transported to an end-product storage facility or re-used in the Spray Drying Absorption process for improved utilization of excess absorbent. Whether or not to design the plant as a single pass system or to incorporate a recycle system will depend on inlet gas quality and emission requirements.

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Spray drying Absorption is a means to meet the strict emission regulations applying to waste-incineration plants.

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The SDA waste process

Waste incinerator applications are generally characterized by relatively low gas amounts; and, hence, the absorber is constructed with only a roof gas disperser. The smaller plant size and the composition of acid content typically favor single-pass designs. The Rotary Atomizers employed in waste incinerator plants, SDAs, are normally type F-100, equipped with wheels in Hastelloy.

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The high HCl content in the gas gives rise to a considerably higher inlet gas temperature than seen in power plant applications. Consequently, the end products will show different behaviours. Further, the moisture and O2 content of the waste gas is higher. The drying properties of the absorber, high chloride content in the end product, single pass mode, etc., call for a design with higher retention time in the absorber chamber, and therefore the plants are operated at high outlet temperatures.

A peak control system can be incorporated in the design for the purposes of improving the Spray Dryer Absorber system's ability to handle extreme variations in inlet conditions. With this feature, a dry absorbent, powdery Ca(OH)2, is blown directly into the flue-gas stream. An adsorbent system based on the injection of pulverized activated carbon may also be employed to efficiently limit the emission of especially mercury and dioxins. The absorbent employed for waste-incinerator applications can be CaO and Ca(OH)2, depending on local supplies and costs.

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The hot, untreated flue gas is introduced into the absorber module via the roof gas disperser for optimum gas flow distribution; and contact with the absorbent is obtained via Rotary Atomizer spraying. The efficient contact between the gas and the atomized absorbent slurry allows for rapid mass transfer of acidic components from a gaseous stage into a liquid stage.

The alkaline absorbent neutralizes the absorbed acid, and the desired reaction product is achieved. Simultaneous with this reaction, the water is evaporated, thus forming a dry powder. A fraction of the dry powder will be deposited at the bottom of the absorber chamber and discharged from here, whereas the main part is carried to the downstream dust collector together with the cooled flue gas, and removed from the gas. The clean gas passes from the dust collector to the stack without re-heating.

Subsequent to the chemical reaction, setting off the absorption of acidic components, and the final drying, the powdery end product is separated from the gaseous state and transported to an end-product storage facility. For efficient removal of dioxins and mercury from the gas, pulverized activated carbon is also injected into the gas stream The peak control system, which is based on the injection of hydrated lime into the gas stream, can also be incorporated in designs targeted at rapid control of substantial variations in the inlet gas quality. Most waste incinerator Spray Dryer Absorber plants are designed as single-pass systems.

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