Coal Fired

Circulating fluidized bed scrubber vs spray dryer absorber

Issue 8 and Volume 23.

JEA’s Northside Generating Station includes two Amec Foster Wheeler CFB boilers, each producing 831,000 acfm of flue gas.

Credit: Amec Foster Wheeler

Many utilities are under pressure to add flue gas desulphurization to their coal-fired units in response to more stringent air emissions regulations. Matthew Fischer and Greg Darling sort out the difference between state-of-the-art circulating fluidized bed scrubbers and the latest advanced spray dryer absorber designs.

The converging US Environmental Protection Agency (EPA) rules for reducing mercury, metals, acid gases, organic compounds, SO2, NOx and particulates have ratcheted up the pressure on coal-fired generators to quickly reduce a variety of pollutants. The EPA estimates that its Cross-State Air Pollution Rule (CSAPR) alone requires more than 3000 units at more than 1000 plants located in 28 states to reduce emissions that cross state lines and contribute to ground-level ozone and fine particle pollution.

CSPAR Phase 1 compliance takes effect this year, while Mercury and Air Toxics Standards (MATS) and Regional Haze (RH) reduction are ongoing programmes.

The debate over what limits will be imposed has now shifted to how individual units will comply with the prescribed deadlines. There are as many technical approaches to meeting new emission limits as there are differences in plant designs. Adding to the complexity of any solution is the uncertainty of future rules that will require further reductions of an expanding range of pollutants.

In the past, SO2 capture on a large scale was the province of wet flue gas desulphurization (WFGD) technology. It has the advantage of a relatively low operating cost and uses readily available limestone as the reagent, which can be recycled into a number of useful products to offset operating costs.

However, WFGD scrubbers do have disadvantages, such as large capital and high maintenance costs. By design, many WFGD systems require periodic discharge of the scrubber liquor to maintain solids and/or chlorides. This effluent requires additional treatment which adds capital and operating costs. Also the uncertainty of future regulations, specifically the Steam Electric Power Generating Effluent Limitation Guidelines (ELG), may require additional discharge treatment.

WFGD is also limited in its ability to capture mercury and SO3. Some plants have reported increased mercury removal as a desirable but expensive co-benefit when a selective catalytic reduction (SCR) system for NOx removal was installed upstream of the WFGD scrubber.

Other plants have also added injection of one or more proprietary reagents into the furnace, such as dry sorbent injection (DSI), as a means to increase the mercury removal co-benefit. Stacking technologies is not a cost-effective long-term strategy to reduce pollutants – it’s unnecessarily expensive and reduces the overall reliability of the entire unit. A more holistic solution is preferred.

Technology comparison

Interest in dry or semi-dry FGD scrubbers is increasing due to their ability to capture mercury, acid gases, dioxins and furans, in addition to SO2 and particulates. These multi-pollutant technologies also have added benefits: no liquid discharge and significantly reduced water consumption, which is increasingly important to power plants that are under pressure to reduce water consumption.

Two multi-pollutant technologies dominate the utility sector. The fundamental difference between the two technologies is the manner in which the reagent is mixed with the incoming flue gas to remove the desired pollutants.

Figure 1.

The SDA uses hydrated lime to treat flue gas. The heat of the flue gas evaporates the droplets, which cools the flue gas. Cooled flue gas with the dried products is directed to a fabric filter

Source: Amec Foster Wheeler


Figure 2.

The optimized two-fluid nozzle design ensures balanced atomizing air distribution in order to produce a consistent droplet size, and reduced compressed air consumption by a quarter. Also, tungsten carbide inserts have significantly reduced nozzle wear

Source: Amec Foster Wheeler

The first technology is the spray dryer absorber (SDA), which sprays atomized lime slurry droplets into the flue gas. Acid gases are absorbed by the atomized slurry droplets while simultaneously evaporating into a solid particulate. The flue gas and solid particulate are then directed to a fabric filter where the solid materials are collected from the flue gas. Amec Foster Wheeler has installed 60 SDA units representing over 4500 MW of plant capacity.

The second is the circulating fluidized bed scrubber (CFBS), which circulates boiler ash and lime between a scrubber and a fabric filter. Amec Foster Wheeler has installed 78 CFB scrubber units representing over 7000 MW of capacity in the power and industrial industries.

SDA technology operates using absorption as the predominant collection mechanism. In general, the acid gas dissolves into the alkaline slurry droplets and then reacts with the alkaline material to form a filterable solid. Intimate contact between the alkaline sorbent (hydrated lime) and flue gases makes the gas removal process effective.

The key to efficient performance is the means used to atomize the lime slurry into droplets within the gas stream. The SDA offered by Amec Foster Wheeler utilizes a two-fluid nozzle to atomize the lime slurry.

The fine spray provides increased contact area in order for gas absorption to occur compared to the CFBS (it’s easier to mix a gas with a liquid than with a solid). Acid gases are then absorbed onto the atomized droplets. Evaporation of the slurry water in the droplets occurs simultaneously with acid gas absorption. The cooled flue gas carries the dried reaction product downstream to the fabric filter. This dried reaction product can be recycled to optimize lime use (see Figure 1).

Industry experience with earlier SDAs was that they were expensive to operate and maintain regardless of the atomization mechanism used. Amec Foster Wheeler has redesigned its two-fluid nozzle to improve the distribution and mixing of atomizing air with lime slurry, which improves mixing efficiency and decreases operating and maintenance costs (see Figure 2).

The optimized nozzle design delivers even atomizing air distribution to produce a consistent droplet size while providing longer nozzle life. In 14 field applications, the optimized nozzle has demonstrated low cleaning frequency (one to three weeks of continuous operation), reduced cost of operation (20-25 per cent less compressed air consumption), and longer life with its new tungsten carbide inserts. In addition, no special tools are required for routine maintenance.

The SDA design also provides additional operating flexibility for the entire plant. For example, any two-fluid nozzle can be removed for maintenance without decreasing boiler load. Emissions performance is maintained even when multiple two-fluid nozzles are taken out of service. The SDA is also capable of high unit turndown, down to 25 per cent of rated flue gas flow without recirculation of the flue gases while maintaining emission requirements. The design of the unit also provides for fast load response enabling unit cycling or load following. An added advantage is low absorber pressure drop that keeps the parasitic fan power loss to a minimum.

Figure 3.

The principal operating steps are recycling a solids/hydrated lime and water mixture in the flue gas, and then capturing solids in a fabric filter. Other reactive absorbents like activated carbon can be added to target specific pollutants

Source: FWEC


Figure 4.

In turbulent mixing, flue gas enters vertically upward into the scrubber and through a set of venturis that accelerate the gas flow. Wall turbulators increase flue gas and reagent mixing efficiency. Multiple venturis allow a single scrubber to be scaled up to 600 MW in unit capacity

Source: FWEC

Boiler flue gas enters the CFBS (with or without ash) at the bottom of the up-flow vessel, flowing upward through a venturi section that accelerates the gas flow rate, causing turbulent flow. The turbulator wall surface of the vessel causes highly turbulent mixing of the flue gas, solids, and water for four to six seconds to achieve a high capture efficiency of the vapour phase acid gases and metals contained within the flue gas. The gas and solids mixture then leaves the top of the scrubber and the fabric filter removes the solid material (see Figures 3 and 4).

Recycled solids/hydrated lime and water mix with the turbulent flowing gas moving vertically through the vessel, which provides gas cooling, reactivation of recycled ash, and capture of pollutants. The CFBS process achieves a very high solids-to-gas ratio, which dramatically improves the ability of vapour phase pollutants to find adsorption sites on the colliding solid particles. The water plays the important role of cooling the gas to enhance the adsorption of the vapor phase pollutants onto the solid particles.

The gas and solids mixture exits at the top of the scrubber and enters the fabric filter, where solids entrained in the flue gas are captured and recycled back to the scrubber to capture additional pollutants. A portion of the recycled solids is removed from the fabric filter in order to maintain the right quantity of material in the circulating loop.

Performance characteristic



Fuel sulphur content

< 2.5%

< 3.5%

SO2 removal %

95 – 97%

95 – 98+ %

Capacity per vessel

40,000 – 1,000,000 acfm

75,000 – 1,800,000 acfm

Turndown capability, % of MCR flue gas flow

25% without FGR

50% without FGR

25% with FGR


Calcium hydroxide slurry

Dry calcium hydroxide

Sorbent treatment


Dry hydrator

Sorbent utilization

(molar Ca/S ratio)

1.4 – 1.5

(without recycle)

1.3 – 1.4

1.15 – 1.25 (with recycle)

Control flexibility



Water quality



Capital cost

Slightly lower

Slightly higher

Footprint, includes fabric filter

Larger diameter vessel, smaller fabric filter, equal overall footprint

Smaller diameter vessel, larger fabric filter (lower A/C ratio),

equal overall footprint

Table 1. Key technical characteristics of SDA and CFBS. Source: Amec Foster Wheeler

Notes: MCR = maximum continuous rating; FGR = flue gas recirculation; acfm = actual cubic feet per minute

The effectiveness of the sorbent is largely a function of residence time. A CFBS can keep solids in the system from 20 to 30 minutes. This is a sufficient period of time for the sorbent to react with the acid gases. Two independent control systems maintain the dry flue gas at optimum temperature and at adequate removal efficiency by controlling the amount of water added and the amount of fresh sorbent added separately. As a result, unlike the SDA scrubber, pollutant capture is not limited by inlet flue gas temperature.

Technical comparison

Table 1 summarizes the important technical differences between the SDA and CFBS options. Table 2 summarizes the performance differences. In general, the CFBS is slightly better at SO2 control, with up to 98+ per cent capture with high amounts of sulphur in the fuel. Plant turndown capability is equivalent, when the CFBS is equipped with flue gas recirculation.

In general, the CFBS offers slightly greater SO2 removal flexibility when compared to SDA. The amount of fresh lime injection is not limited by flue gas temperature, thus allowing greater SO2 scrubbing performance over a wider range of fuel sulphur content.

SDA systems are temperature-limited because fresh lime is introduced as slurry (lime and water). In addition, due to water being introduced independently and purely for temperature control, the CFBS can utilize lower quality water, as it is not used for pebble lime hydration.

The CFBS has the ability to effectively treat more flue gas volume than an SDA. The multiple venturis present allow a single CFBS vessel to be scaled up to almost twice that of the SDA vessel option.

Turndown capability is built into the SDA design, where a CFBS requires a flue gas recirculation system in order to achieve equivalent turndown. An SDA utilizing the two-fluid nozzle design can maintain required emission levels down to approximately 25 per cent of MCR.

In a CFBS at lower loads, additional recirculated flue gas is required to maintain bed velocities in order to maintain required emission levels. If turndown during non-peak power demands is a consideration, the additional parasitic load is an operating cost consideration for the CFBS.




SO2 removal, %

95 – 97

95 – 98+

SO3 removal, %



HCl/HF removal, %



Total PM removal efficiency, %



Mercury removal efficiency, % (with or without PAC)



Pressure drop, inches H2O



Auxiliary power consumption



Total power consumption (including ID fan)



Availability, %



Water consumption






Table 2. Key performance characteristics of SDA and CFBS. Source: Amec Foster Wheeler

Notes: ID = induced draft; PAC = powdered activated carbon; PM = particulate matter

The CFBS provides greater sorbent utilization compared to a once-through SDA system, as the reagent recycle is incorporated into the design. However, due to the difference in hydration efficiency, a SDA equipped with recycle offers greater overall sorbent utilization compared to a CFBS. In an SDA the recycled solids are slurried within a tank, providing essentially 100 per cent hydration.

In a CFBS water spray nozzles wet the dry recirculated solids as they pass through the vessel. This hydration process is less efficient due to the quantity of recycled solids and the lack of sufficient wetting time.

All the other performance characteristics are relatively equivalent, including net auxiliary power. The pressure drop in the SDA (25 cm H2O) is much less than the equivalently-sized CFBS (40 cm H2O), which is proportional to ID fan power consumed. However, the auxiliary power used by the SDA, principally for compressed (atomizing) air, exceeds that required by the CFBS.

The net result is that the total auxiliary power used by either option is approximately equivalent. However, depending on the unit capacity, pressure drop may have a greater operating cost impact compared to the additional auxiliary power of an SDA.

Both technologies are simple, reliable and robust. When maintenance of the CFBS is required, the accumulated solids can easily be removed through the bottom of the scrubber. Also, the water nozzles are low maintenance and can be replaced with the unit in operation. SDA two-fluid nozzles may also be removed and maintained during plant operation without loss of unit capacity. No special tools are required for two-fluid nozzle maintenance.

No one size fits all

In the past, dry scrubbing technology was typically chosen over WFGD technology for its much lower capital cost and water usage, provided that the boiler size was not too large and the fuel sulphur content was not too high.

Today, CFBS technology has broken through these limitations with single unit designs up to 600 MW backed by operating experience on coal-fired units of over 500 MW, and on fuels with sulphur levels above 4 per cent by weight. SDAs have also been deployed on equal-sized units, but with two absorbers and more limited fuel sulphur ranges. The utility retrofit market has leaned more toward the CFBS technology of late due to the higher SO2 removal performance. However, SDAs are gaining popularity due to a new generation of SDA nozzles which has significantly reduced cleaning frequency, which was a major criticism by early adopters. In addition, the SDA offers greater turndown capability without flue gas recirculation equipment. With extended nozzle life and reduced compressed air consumption, the performance gap between the SDA and CFBS has narrowed. Specific site and environmental permit requirements will be the determining factor.

Matthew Fischer is Product Leader, Dry FGD Systems, and Greg Darling is Product Leader, CFBS Systems, for Amec Foster Wheeler North America Corporation Global Power Group – Environmental Systems