## 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 a nd 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 SO 2 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 SO 2 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 absorp tion 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 SLURRYThe 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
SO 2 absorption. Conversely, increasing outlet gas temperature shortens the rapid reaction phase and reduces total absorption. Hence, for a given SO 2 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 th e 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
SO 2 absorption and thereby decrease lime consumption.

While gas temperature is readily measured and controlled in dry scrubbers, SO 2 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 a fforded by a more reactive reagent, the slope of the ab sorption 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. TIMESO2ABSORBED DIMINISHED
REACTION RATE LIQUID SLURRY PHASE DRY SOLID PHASE

A B C

TIMESO2ABSORBED `

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 SO 2 AbsorptionFactors 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
documented 4 . 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 scrubbe r 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 establis h “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 b aseline 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 Li me 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 c onfiguration 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 norma l 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’ mill 6 . 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 wate r 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 Te st 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 Reacti vity Results – Baseline vs. Modified Slaking ProcessesFor 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.