Optimizing Scrubber Performance

Introduction 

Coal fired power generating facilities in 

the U.S. currently spend over $200 

million per year on lime for flue gas 

treatment[1]. This expenditure is expected 

to increase by more than $100 million per 

year by 2020, as more stringent emission 

regulations take effect and 31,000 

megawatts of new coal-fired generating 

capacity are added[2]

. 

At facilities equipped with dry scrubbers, 

recent research and practical experience 

shows that lime consumption can be 

substantially reduced by optimization of 

the reagent preparation process. In 

addition to reducing lime consumption, 

other financial and operational benefits 

may include reduced corrosion in 

downstream components, greater safety 

and improved dependability. 

Sunflower Electric’s Holcomb Station is typical of many 

coal fired facilities using dry scrubber technology for flue 

gas treatment. This paper describes a general model 

applicable to dry scrubber processes and successful 

application of that model and related principles to reduce 

lime consumption at Holcomb Station. 

Dry Scrubber Operation 

A typical dry scrubber type flue gas treatment (FGT) 

process is shown in Fig. 1. Boiler flue gas enters the 

spray drier absorber (SDA) at point A where reagent 

slurry is sprayed into the gas stream. At coal-fired 

facilities, gas temperature at this point is approximately 

equal to air preheater temperature, typically ranging from 

250F to 350F. The reagent slurry typically consists of a 

mixture of solid hydrated lime (calcium hydroxide) 

suspended in water which may also include recycled 

flyash solids. Evaporation of the liquid water begins 

immediately at point A and continues until the drying 

process is complete at point B where gas temperature is 

typically 150F – 190F. Gas temperature at this point is 

controlled by varying the water (or recycle ash slurry) 

flowrate to the SDA while SO2 removal is controlled by 

varying reagent flowrate. 

 

The Gas Temperature Effect and Scrubber 

Process Model 

Common practical experience shows that gas 

temperature at the SDA outlet (point B) significantly 

affects SO2 removal and lime consumption. This effect 

is substantial; measurements at operating facilities show 

that a five degree reduction in SDA outlet temperature 

can reduce lime consumption by 10-30%. 

The effect of SDA outlet temperature on performance 

can be modeled by assuming a relationship between SO2

absorption rate, drying time and temperature. Referring 

to Figures 1 and 2 together, reagent enters the gas 

stream at point A as a slurry of solid calcium hydroxide 

particles suspended in liquid water. During the 

evaporation phase (between points A and B), the 

presence of liquid water enables the solid lime to 

dissolve and react rapidly with the gaseous sulfur 

dioxide. This rapid absorption process continues until 

the liquid water is fully evaporated at point B. After 

evaporation is complete, the remaining dry solids 

continue absorbing acid gas between points B and C, 

however the reaction rate in this dry solid phase is 

substantially reduced. 

LIME

SLURRY

BOILER

FLUE

GAS

CLEAN

FLUE GAS

REACTOR STACK

FLYASH

PROCESS

WATER

SPRAY

DRYER

ABSORBER BAGHOUSE

STACK TEMP

SO2 A

 B

C

SO2

Stewart Nicholson 

Primex Process Specialists, Inc. 

Corey Linville 

Sunflower Electric Power Corp.. 

FIGURE 1. Typical Dry Scrubber FGT Process 

PROCESS WATER 

OR RECYCLE ASH 

SLURRY 
The model illustrated in Fig. 2 

shows that decreasing SDA outlet 

temperature will prolong the rapid 

reaction phase (shifting point B to 

the right), thereby increasing total 

SO2 absorption. Conversely, 

increasing outlet gas temperature 

shortens the rapid reaction phase 

and reduces total absorption. 

Hence, for a given SO2 removal 

target, this model predicts that lime 

consumption will be affected 

significantly by SDA outlet 

temperature. 

The Lime Slurry Reactivity 

Effect 

In addition to gas temperature 

effects, recent experience at 

facilities such as Holcomb Station shows that lime 

consumption can be significantly affected by changes to 

the lime slaking process. This effect is believed to be 

related to reagent properties (such as hydrated lime solids 

surface area) that affect the dissolution and absorption 

rates. The same principles discussed above predict this 

effect; as shown in Figure 3, accelerating the absorption 

rate (at a fixed SDA outlet temperature) should increase 

SO2 absorption and thereby decrease lime consumption. 

While gas temperature is readily 

measured and controlled in dry 

scrubbers, SO2 absorption rate and 

the factors affecting it are not 

practically measurable in operating 

systems. However, a method for 

measuring lime slurry reactivity 

(defined herein as the rate at which a 

particular lime slurry absorbs sulfuric 

acid in the liquid phase) has been 

developed[3]. Reactivity 

measurements using this method 

show a significant correlation to 

scrubber performance. This 

correlation suggests that reactivity is 

valid as a key indicator of absorption 

rate and that the absorption rate 

model presented in Fig. 3 is also 

valid. 

Figure 3 suggests another important possibility. With 

the accelerated absorption afforded by a more reactive 

reagent, the slope of the absorption curve is flatter at 

the evaporation point (C); therefore, absorption should 

be less sensitive to gas temperature changes. While this 

effect is yet to be verified experimentally, it suggests 

that increasing the absorption rate may not only reduce 

lime consumption but may also enable higher operating 

temperatures, thereby reducing corrosion and solids 

buildup. 

TIME 

SO2 ABSORBED

DIMINISHED 

REACTION RATE

LIQUID SLURRY PHASE DRY SOLID PHASE

A

B C

TIME 

SO2 ABSORBED

`

NORMAL 

REACTION RATE

ACCELERATED

REACTION RATE

REMOVAL 

EFFICIENCY 

GAINED

LIQUID SLURRY PHASE DRY SOLID PHASE

A

B C

FIGURE 3. Effect of Increasing Absorption Rate 

FIGURE 2. Effect of Evaporation on SO2 Absorption 
Factors Affecting Lime Slurry Reactivity 

At most dry scrubber facilities, reagent is produced by 

onsite slaking of quicklime (calcium oxide) to produce a 

slurry of solid hydrated lime (calcium hydroxide) 

particles. The factors affecting the lime slaking process 

and their effect on lime slurry properties have been well 

documented4

. These factors include: 

 Reactant Chemical and Physical Properties 

 Slaking Water Temperature 

 Water to Quicklime Ratio 

 Slaker Residence Time 

 Slaking Process Stability 

Recent research indicates that these same factors affect 

lime slurry reactivity and that they can be optimized to 

improve reactivity considerably. 

Holcomb Station – A Case Study 

Summary

Located near Garden City, Kansas, 

Holcomb Station is a 360 megawatt 

coal-fired power generating station 

owned by Sunflower Electric Power 

(SEP) Corporation. The station became 

operational in 1983 and is equipped 

with a dry scrubber type FGT system 

consisting of three SDA modules and 

fabric filter baghouse, similar to the 

configuration shown above in Fig. 1. 

Reagent is prepared by slaking 

quicklime to produce hydrated lime 

slurry as shown in Fig. 4. The lime 

slurry is then mixed with recycled 

(baghouse) flyash slurry and fed to the 

SDA modules through rotary atomizers. 

An evaluation of scrubber performance 

at Holcomb Station in March 2003 

revealed low slurry reactivity. The 

evaluation report concluded that 

optimization of the reagent preparation processes could 

substantially improve scrubber performance. Actions to 

improve slurry reactivity were identified and implemented 

during a series of reagent preparation and scrubber 

performance tests in August and October 2003. 

Test results yielded two significant conclusions. First, that 

lime slurry reactivity could be approximately doubled by 

changes to reagent handling and preparation processes; 

and second, that lime consumption at full load was 22% 

lower with the more reactive slurry. 

Test Methodology

Scrubber performance was evaluated by measuring 

specific lime consumption (the mass ratio of quicklime 

consumed to SO2 removed). The test plan included two 

basic steps to quantify the effect of lime slurry reactivity 

on specific lime consumption at Holcomb Station. The 

first step was to establish “baseline” performance with 

the existing reagent preparation processes. The second 

step was to modify the reagent preparation process and, 

to the extent practical, hold all other variables constant 

(as compared to the baseline tests) while measuring 

specific lime consumption. Lime slurry reactivity was 

measured during each of the test steps. 

Quicklime used during the baseline test was received, 

conveyed as usual into one of the two lime day bins. 

This material handling process resulted in significant 

pulverization of the quicklime prior to slaking. 

Quicklime used during the modified process test was 

conveyed pneumatically by self unloading bulk delivery 

trucks directly into the other lime day bin, thus 

bypassing the storage silo and two-step pneumatic 

conveying process. In addition, during the modified 

process test, slurry was prepared with slaking process 

conditions chosen to maximize lime slurry reactivity. 

Comparing the modified slaking process to the baseline 

process, the principal changes were: 

 Use of pebble quicklime (modified test) vs. 

pulverized quicklime (baseline test). 

BALL MILL 

LIME SLAKER

TO 

ATOMIZER

FEED TANK LIME DAY 

BIN

LIME SLURRY 

SUMP

GRIT 

SCREEN

WEIGHBELT FEEDER

PROCESS 

WATER

LIME SLURRY 

TANK

WATER

HEATER

BALL MILL 

LIME SLAKER

TO 

ATOMIZER

FEED TANK LIME DAY 

BIN

LIME SLURRY 

SUMP

GRIT 

SCREEN

WEIGHBELT FEEDER

PROCESS 

WATER

LIME SLURRY 

TANK

WATER

HEATER

FIGURE 4. Holcomb Station Lime Slurry Preparation System 
 Slaking water temperature was increased and 

controlled to enable higher slaking temperatures 

during the modified test. 

 Slaking water flowrate and quicklime feedrate were 

controlled to stabilize slaking temperature at the 

maximum safe temperature during the modified test. 

Lime slurries produced during the baseline and modified 

slaking processes were stored in separate holding tanks. 

These slurries were then fed to the scrubber during two 

separate 24 hour periods, during which SDA outlet 

temperature was set to 175F and all other operating 

parameters recorded. 

Observations and Results 

Scrubber operating parameters and performance indicators 

are presented in Table 1 below. Lime slurry reactivity 

results are shown in Fig. 5. As expected, the multiple 

storage and conveying steps caused substantial 

pulverization of quicklime used during the baseline test, 

with approximately 65% passing #40 mesh measured at 

the slaker inlet. Quicklime for the modified test was 

mostly pebbled material with only 6% passing #40 mesh. 

However as slaking progressed the quantity of fines 

increased to 52% passing #40 mesh. 

 

The existing electric circulation heater used for slaking 

water had been refurbished to enable optimum slaking 

process control. The piping configuration is such that 

slaking water is supplied to both ball mills through this 

heater, hence the ‘B’ mill water was heated whenever the 

‘A’ mill was operating[5]. Reliable slaking water 

temperature indication was not available, however 

measurements taken during the flushing cycles indicated 

that the heater was yielding a temperature increase of 

approximately 15F with normal slaking water flow for 

one operating mill. During slaker operation slaking 

temperatures were 145-162F in the ‘B’ (baseline) mill and 

195-200F in the ‘A’ mill. Temperature in the ‘A’ mill 

was not reliably indicated with the plant instrumentation 

due to the cooling effect of the trommel screen spray 

water. This temperature was measured manually for the 

duration of the modified slaking process and controlled by 

manual adjustment of lime feedrate and slaking water 

flowrate. 

Lime slurry produced during modified slaking in mill ‘A’ 

appeared to be substantially more viscous than slurry 

produced in the ‘B’ mill6

. Additional dilution water was 

therefore required to maintain sufficient agitation in the 

‘A’ mill sump. This additional water resulted in a 

reduction in solids concentration (after dilution) to 

approximately 15% in the modified slurry as compared to 

21% in the baseline slurry. While lime slurry flowrate to 

the scrubber was proportionately higher, the solids 

difference is not believed to have affected scrubber 

performance significantly. 

Other factors that may influence specific lime 

consumption were comparable for the duration of the 

test periods as shown in Table 1 below. Inlet SO2 (and 

therefore SO2 removed) was higher during the modified 

slaking test than the baseline test, however previous 

data at Holcomb suggests this difference should not 

significantly affect specific lime consumption. 

Conclusions and Recommendations 

Analysis of scrubber performance data from the two 

tests yielded the following principal conclusions: 

 

 Lime consumption over 24 hours averaged 14% 

lower with the modified slaking as compared to 

baseline consumption. 

 Lime consumption over the 16 hour full load (365 

mW) period averaged 22% lower with the 

modified slaking as compared to baseline 

performance. 

Laboratory analysis of lime slurry samples as shown in 

Figure 5 indicates that lime slurry produced during the 

modified slaking process was approximately twice as 

reactive (acid absorbed vs. time) than the baseline 

slurry. 

Following analysis of test results and conclusions, 

Primex offered the following recommendations 

1. Stabilize and Optimize Quicklime Properties 

a. Modify or bypass the existing quicklime 

unloading, storage and conveying system to 

minimize quicklime particle size reduction. 

b. Review quicklime supplier options, change 

supplier and/or supply specifications as 

required to maximize “as delivered” quicklime 

quality and value. 

c. Test quicklime properties (temperature rise, 

available CaO and particle size) on an ongoing 

basis to ensure compliance with supply 

specifications. 

2. Reconfigure Lime Slaking Process Equipment, 

Instrumentation and Controls to Maximize Lime 

Slurry Reactivity 

a. Relocate dilution water flow transmitter or 

trommel screen spray water takeoff branch to 

enable accurate dilution water flow indication. 
b. Relocate thermowell for accurate indication of 

slaking temperature 

c. Add slaking water inlet temperature 

instrumentation to indicate inlet water 

temperature locally and remotely. 

d. Modify 120 kW heater controls as required to 

maintain water temperature within specified 

limits and to prevent excessive or uncontrolled 

heat addition. 

e. Reconfigure slaking system automatic controls to 

enable safe and stable slaker operation at 195-

200F operating temperature. 

f. Reconfigure slaking system automatic controls 

to enable continuous (demand paced) slaker 

operation. 

3. Education and Training for Operations and 

Maintenance Personnel 

a. Initial training for all scrubber operations and 

maintenance personnel in dry scrubber theory 

and practice, lime slaking concepts, reagent 

key performance indicators, measurement and 

performance optimization techniques. 

b. Recurrent (annual) training as required for 

existing personnel, new hires and transfers. 

Process Variable Baseline Modified Slaking Baseline Modified Slaking

 Coal Flow Rate (tons/ hr) 189 188 201 201

 Inlet SO2 (lbs/ mmbtu) 0.74 0.84 0.73 0.87

 Outlet SO2 (lbs/ mmbtu) 0.17 0.18 0.19 0.20

 SO2 Removed(lbs/ hr) 1853 2154 1908 2376

 SDA Inlet Temp Avg. 290 292 295 297

 SDA Outlet Temp Avg. 175 176 175 175

 Lime Slurry Density(% Solids) 20.7 15.5 20.5 15.2

 Lime Slurry Flow Rate(gpm) 30 41 33 45

 Quicklime Consumed (lbs/ hr) 2679 2686 2930 2860

 Reactor Slurry Flow Rate Total(gpm) 291 298 320 331

 Additive Tank Dilution Water Flow Rate(gpm) 183 180 201 203

 Ash Flow Rate(100 lbs/ hr) 928 947 1018 1057

 SO2 Removal Rate % 77% 78% 74% 78%

 Specific Lime Consumption(quicklime/ SO2) 1.46 1.26 1.57 1.20

24 hr Period Variable Load 16 hrs Steady State Full Load

Table 1. Holcomb Station Scrubber Test Results – 24 Hr. Average Values

0%

20%

40%

60%

80%

100%

120%

00:00 00:10 00:20 00:30

TI ME ( m in:sec)

% Acid Absorbed

(From pH 1.8 to pH 11.0)

Baseline 10/ 7/ 03, 02:00

Modified 10/ 8/ 03, 20:00

Optimized Ball Mill Slaker

PrimeX P R O C E S S S P E C I A L I S T S 

Figure 5. Lime Slurry Reactivity Results – Baseline vs. Modified Slaking Processes
For More Information 

For more information, contact 

Corey Linville 

Sunflower Electric Power Corporation 

2440 Holcomb Lane 

PO Box 430 Holcomb, KS 67851 

Tel 620 277 4517 

email: linville@sunflower.net

www.sunflower.net

Stewart Nicholson 

Primex Process Specialists, Inc. 

PO Box 571, Doylestown, PA 18901. 

Tel (215) 340 3648 

Email: stewart@primexprocess.com

www.primexprocess.com 

 

NOTES AND REFERENCES 

[1] USGS Mineral Survey – 2003 Lime Use Statistics 

[2] Department of Energy, Energy Information 

Adminstration (EIA) – available at 

www.eia.doe.gov. 

[3] Using test methods based on ASTM C400 Standard 

Test Methods for Quicklime and Hydrated Lime for 

Neutralization of Waste Acid, ASTM Philadelphia, 

PA (1995)

[4] Miller, T.C. A Study of the Reaction Between 

Calcium Oxide and Water, National Lime 

Association, Washington D.C. (1960)

[5] Slurry reactivity results indicate this inadvertent 

heating of the ‘B’ mill slaking water did not 

significantly affect baseline scrubber performance. 

[6] Slurry viscosity changes are usually attributable to 

inconsistent quicklime properties. These changes are 

unlikely to persist, however dilution water controls 

can and should be configured to compensate for 

viscosity effects.