Boosting SO2 removal efficiency

A Dual Flow Tray at its 24-month inspection

Credit: Amec Foster Wheeler

New SO2 emission regulations in the US and EU require some utility and power producers to retrofit new flue gas desulfurization units to existing plants. Michael T Hoydick and Hans Jansson discuss cost-effective solutions to achieve these new emission standards

Acid gas removal efficiency (mainly SO2) in a power plant’s limestone-based wet flue gas desulphurization (WFGD) system absorber is governed by two processes: the absorption of SO2 via gas/liquid contact and the rate at which the scrubbing liquor neutralizes the liquid phase acids collected. Improving either process generally enhances SO2 capture.

The rate of SO2 absorption into the absorber liquor is controlled by the mass transfer coefficient, the surface area available for mass transfer, and the difference between the SO2 partial pressure in the flue gas and the vapour pressure of SO2 at the gas/liquid interface.

WFGD system designers can generally influence only the contact surface area and the dissolved alkalinity in the absorber slurry, which, in turn, determines interface vapour pressure. The surface area for mass transfer is determined by the selected liquid-to-gas ratio (L/G) in conjunction with the spray nozzle droplet size distribution.

Open Spray Tower

DFT Tower

73% within 5% of average velocity

99% within 5% of average velocity

96% within 10% of average velocity

100% within 10% of average velocity

Improving SO2 removal performance for existing open tower designs is generally limited to increasing L/G ratio or creating smaller droplet sizes via higher pressure drop nozzles, either of which increase auxiliary pump power. Additionally, smaller spray droplet sizes are only marginally effective due to significant droplet coalescence within the spray zone of the tower.

Flue gas/slurry contact can be significantly enhanced with the use of internal contacting devices. In the past, packing material has been used but has proven unreliable in limestone WFGD systems and is not favoured by the US utility industry. Further development has produced the dual flow tray (DFT) technology that has found favour in US utility applications for over 30 years for new and retrofit applications. In general, the DFT consists of one or more levels of perforated plates that span the entire absorber cross-section. The DFT’s SO2 removal efficiency is improved due to its increased and more effective gas-to-liquid contact area compared to a typical open tower design that relies only on spray droplet surface area.

DFTs improve WFGD performance by improving flue gas distribution at the beginning of the gas-to-liquid contact zone, which takes full advantage of the L/G provided by the slurry sprays. Flue gas distribution in a DFT absorber is markedly better than in open spray tower WFGDs designed with side flue gas entry, where momentum pushes the flue gas to the far wall, thus delaying optimal flue gas/absorber liquor contact. For open spray tower designs, optimal flue gas distribution doesn’t occur until the gas is well into the absorption zone.

DFTs also provide very effective gas-to-liquid contact. Flue gas flowing upward is intimately mixed with the falling absorber slurry. The flue gas velocity travelling through the tray holes causes liquid resistance, thus forming a froth layer on the tray. The froth layer, typically 150 mm–300 mm deep, provides additional mass transfer surface area and contact time in the absorption zone. Each tray level provides an additional one to two seconds of contact time in the absorption zone. Full scale testing of absorber towers with and without DFTs confirm comparable performance for DFT absorbers at L/G ratios 15-30 per cent below open tower designs.

A computational fluid dynamic flow model shows the improved gas flow distribution of an existing Amec Foster Wheeler DFT installation over a comparable side entry wet FGD design. The flue gas distribution is illustrated at 1.5 m above the inlet duct

Credit: Amec Foster Wheeler

Absorber slurry liquid phase chemistry also plays a substantial role in the overall performance of the wet FGD unit. The absorber slurry needs sufficient liquid phase alkalinity to quickly neutralize the absorbed acid to maintain the driving force necessary for SO2 capture.

In limestone-based systems, the alkalinity is produced from dissolved calcium carbonate. The operating pH is a general indicator of the alkalinity of the absorber liquor. The higher the pH, the more dissolved alkalinity is present.


Tower Design


Open Spray


Absorber diameter, m



Recycle tank retention, min.



Recycle tank height, m



Number of recycle pumps

(operating +spare)



Recycle pump flow, m3/hr



Number of trays



Overall tower height, m



Overall liquid recirculation rate, m3/h



Absorber auxiliary power, kW



Pressure drop, kPa



Table 1. Performance comparison between similar open tray and DFT towers in a wet FGD installed on a 500WM coal-fired unit. The fuel sulfur is 1.2% and the systems are designed for 98% SO2 removal. Source: Amec Foster Wheeler

As the absorber slurry falls through the absorber tower, the pH of the solution falls as the acid is absorbed. For an absorber with a reaction tank pH of 5.7, the slurry pH falls to ~3.5-4.5 on the DFT. Since limestone dissolution rate is proportional to the pH, the lower pH on the DFT significantly increases limestone dissolution rates and provides additional dissolved alkalinity needed for further acid neutralization.

Newbuilds and retrofits

A comparison between a typical open spray tower design and an equivalent DFT design for a theoretical 500 MW unit illustrates the performance and equipment size differences between absorber types (see Table 1, p31). Note that the DFT tower is smaller in size than a comparable open tower because of the lower L/G of the DFT absorber, as is the overall liquid recirculation rate. Since limestone dissolution and gypsum crystallization require a minimum retention time in the recycle tank, a lower L/G also allows for a smaller recycle tank. Because a DFT tower requires a lower L/G, it is often possible for a DFT tower to be designed with one less operating spray level and recycle pump.

The DFT can be used as a staging platform during construction

Credit: Amec Foster Wheeler

In this comparison, two operating spray levels are required for the DFT design while three operating spray levels are needed for the open tower design. Note that one less spray level reduces the overall absorber height by over one metre, which may reduce absorber shell thickness and foundation requirements, and therefore overall installation costs. The reduced absorber height will also reduce piping and electrical installation costs. Finally, the DFT can be used as a maintenance platform during construction, and later as an inspection platform for the upper absorber sections.

Performance upgrades

There are several techniques available to improve the performance of an existing wet FGD system. The easiest and most cost-effective is to operate the system with a higher pH.

The typical limestone-based system operates at pH levels between 5.0 and 5.7. A higher operating pH will improve SO2 removal efficiency up to a limit. Slower sulfite to sulfate oxidation rates and high limestone stoichiometry produce unacceptable gypsum quality when pH levels exceed 6.0. Poor oxidation may also produce gypsum scaling, which is not acceptable for long-term operation.

Physical equipment changes are usually the upgrade path. Adding wall rings, improving flue gas or liquid spray distribution, smaller spray droplet spray nozzles, double spray nozzles, more L/G, or the addition of one or more DFTs, alone or in concert, are typical open tower upgrade options.

Wall rings will marginally improve the efficiency of a properly designed wet FGD system. Higher-pressure spray or double spray cone nozzles will produce smaller spray droplets that should help efficiency, in theory.However, droplet coalescence limits the performance improvement.

The remaining option for significantly improving the performance of an existing open spray tower is adding L/G, in conjunction with spray header modification. Unfortunately, increasing L/G in an existing absorber is normally a challenge.

Most sites do not have adequate floor space for additional recycle pumps and not enough tower height for additional spray banks. Modifications to existing pumps are possible, however, recycle pump efficiency will likely be compromised and recycle pipe flow velocities could exceed design limits. Recycle tank retention times must also be considered when adding additional L/G. These solutions, although possible, generally require outages of several months and have high construction costs.

Normally the best physical upgrade option is the addition of one or more DFTs below the bottom spray bank. Many open towers have adequate space between the lowest spray bank and the inlet ductwork to allow installation of a new DFT level. Approximately 3.0-3.5 metres of vertical height is generally required. An added benefit of the DFT is that lower pressure drop nozzles can be used (spray nozzle droplet size is less critical for a DFT) to artificially increase L/G without modification to the existing recycle pump and recycle piping systems.

The Elmer Smith Station in Kentucky, US upgraded an existing open spray chamber absorber with one DFT, thereby increases the wet FGD’s SO2 removal efficiency from 93 per cent to 98 per cent

Credit: Amec Foster Wheeler

Case study

Amec Foster Wheeler’s predictive models indicate that a DFT can improve mass transfer by as much as 50 per cent (1.5 times) from the current design of open tray spray towers. In many instances, the addition of one or more DFTs can achieve desired performance objectives without other modifications.

For even higher levels of performance, a DFT addition in conjunction with spray nozzle modification and pH adjustment is an option. The liquid holdup and low pH on the DFT will allow higher operating pH levels without affecting limestone stoichiometry or gypsum quality.

A DFT retrofit of an existing open spray tower was recently completed at the Elmer Smith Station in the US state of Kentucky, owned by Owensboro Municipal Utilities (OMU). Amec Foster Wheeler supplied two open spray chamber absorbers that began operation in 1995. In 2008, the existing absorber towers were operating at 93 per cent SO2 removal efficiency at an operating pH level of 5.7 when OMU decided to upgrade its system to reach 98 per cent efficiency.

The five-point efficiency increase represented an increase in absorber mass transfer from 2.7 NTU to 3.9 NTU, a 42 per cent increase. Amec Foster Wheeler’s analysis found that adding one DFT level would increased the overall absorber NTU by around 50 per cent, without any additional modifications to the existing recycle pump or spray header system.

Kevin Frizzel, Director of Power Production, notes: “The addition of a Dual Flow Tray level on our two scrubbers was a very cost-effective method for OMU to maintain our commitment to high environmental standards.”

Operational testing of the completed DFT upgrade in 2009 confirmed the expected performance increase was achieved without changes to the operating pH or limestone stoichiometry.

Michael T Hoydick is senior technology manager, FGD Systems, Amec Foster Wheeler, USA. Hans Jansson is Director, Marketing and Business Development, Amec Foster Wheeler, Europe.

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