Improving Limestone Scrubber Efficiency - True Cost Investigation

Sessions: Cycling Load, Dual Fuel Firing, and Understanding Operating and Maintenance Savings for Air Quality Control Systems will be presented by Suzette Puski, Business Development Manager, Babcock Power Environment. This company has a partner and its first wet FGD order in India.


Exhibitors: AECOM has  a license from Linde to use ozone to capture NOx in FGD scrubbers. Boldrocchi supplies DSI systems. Burns & McDonnell participated in the India SOx-NOx conference. CECO supplies FGD dampers. Duechting supplies FGD pumps. Emerson provides slurry level control and automation. Hamon provides FGD systems. Haynes provides alloys for scrubber vessels and components. Howden provides FGD exhaust fans and oxidation blowers.  Southern Environmental supplies dry scrubber systems. Teledeyne Monitor Labs supplies FGD CEMS as does Thermo Fisher.  Wood has acquired the Wheelabrator APC and wet FGD systems. WSP is a worldwide consultant on power plant systems. We will be continually updating the following  directory of attendees, exhibitors and speakers who are interested in discussing dry and wet FGD.

Issues to be discussed:


·         Limestone forced oxidation with gypsum vs. lime natural oxidation and chemical fixation

·         Oxidation blower options

·         FGD recycle Pump True Costs (see separate analysis)

·         Slurry level control for calcium sulfite

·         FGD chemistry and  emission control

·         Vacuum filter belts and filter options

U.S. and Indian FGD operators both are interested in higher SO2 removal efficiency. In the case of U.S. operators faced with operating higher cost old plants the potential to cut pump connected horsepower is appealing.  For Indian operators there is the potential to reduce capital as well as operating costs. Three options involve scrubber design. scrubber chemicals, and optimization systems.

Relative to scrubber design there are the following choices

  •  A spray tower with liquid flows as high as 100 gpm/1000 cfm and pressure loss of 3-6” w.g  (MET, Doosan, and many others)
  • A modified spray tower (MHPS)
  • A tray or rod deck scrubber with  flows less than 50 gpm/1000 cfm and pressure loss of 6-8” “ w.g. (B&W)
  • A sump scrubber with no liquid flow and pressure loss of 9-11” w.g

Where there is enough fan capacity a tray or rod deck can be added in a spray tower to increase efficiency.

Another alternative is to use dibasic acid or another chemical to enhance scrubber efficiency.  Some plants in Europe add lime during periods when higher efficiency is needed.

India is moving forward with approximately 100,000 MW of FGD systems.  This will require 15-20  million of tons of limestone per year.  SHI-FW has conducted a study which shows that because of the poor limestone a CFB scrubber is the best choice for many plants

This analysis is based on a 300 MW plant with 0.6% sulfur. Limestone cost is 7 rs Crore /yr ($989,000) based on 5 tons per hour and a cost of limestone at  1750 rs./ton ($ 24.50/ton) vs EPA cost estimates based on $30/ton. The tons/MW = 134.7.


Scrubber additives can improve the economics when using a poor quality limestone. One answer is that if treatment chemicals  can be justified for a 0.6% sulfur coal, they will be even more attractive for higher sulfur coals.

Here is the CEA specification for Indian Limestone.





Brad Buecker of Chemtreat has reviewed this analysis and concludes  “With regard to the analysis, the CaO range equates to a calcium carbonate (CaCO3) range of 82 to 91 percent.  A quick thought suggests that this might be an excellent application for ChemTreat’s DBA replacement product.  A general rule-of-thumb that I always followed is that limestones with 94% or greater CaCO3 concentration are usually quite reactive (if ground properly).  My gut feeling says that the stones outlined in the analysis could be significantly enhanced in reactivity using our FGD1105 product.”


Brad wrote an article in Power Engineering earlier this month where he provided further details: “Many plants do not have access to such high-purity limestones. The stone may contain a significant concentration of dolomite (MgCO3∙CaCO3) or inert materials that inhibit reactivity. Thus, supplemental methods are needed to boost the reactions.  A common method that has been used for years is addition of dibasic acid (DBA) to scrubber process streams, but new technology is improving upon this chemistry.  Dibasic acid is a generic name for a blend of relatively short-chain dicarboxylic acids (two COOH functional groups), which add hydrogen ions (H+) to help in the dissociation of limestone, and then circulates through the process to continue assisting with SO2-absorption chemistry.  However, the availability, cost, and even efficiency of DBA have placed limits on the chemical’s effectiveness.  An alternative is available that is much more promising for wet FGD reactivity enhancement.”


Personnel at the Longview Power Plant in Maidsville, West Virginia had faced constant problems with handling and feeding DBA to the wet-limestone scrubber of their 770 MW supercritical steam generator.  For example, the product must be kept warm to prevent solidification.  Exacerbating the issues were DBA delivery constraints, both from a distance perspective and that the tank volume had to be lowered to less than 10% before introduction of a fresh load.  Combined, these factors affected scrubbing efficiency and SO2 removal, the latter of which is a critical aspect of scrubber operation.  Violation of discharge permits can result in load restrictions or forced shutdowns.

Accordingly, the plant staff began full-scale testing and then subsequent application of an alternative, specially-formulated organic acid blend with the product name of FGD1105 (patent pending).  Almost immediately upon chemical addition, SO2 emissions dropped by approximately 35% to 40%, such that even at full load one of the scrubber’s five recycle pumps could be, and was, removed from service.  Stack SO2 emissions only slightly increased from 120 lb/hr to 200 lb/hr following the pump reduction. This action alone reduced auxiliary power consumption by 3 MW, at a projected annual benefit of approximately $700,000.  As a test, the plant staff removed a second recycle pump from service and found that SO2 removal was still more efficient than in the period prior to the FGD1105 addition.  According to Chad Hufnagel, Longview’s Plant Manager, the ability to operate with 3 recycle pumps instead of 4 or 5 has provided additionally flexibility for recycle pump maintenance strategy, as well as offering additional net revenue opportunity with improved unit efficiency.


Therefore the cost saving is more than $700,000/yr less the cost of the additive. There are other questions to pursue such as changes to wastewater treatment costs. This analysis is based on a spray tower.  If a tray tower or rod deck scrubber is used then the saving would have to be in fan rather than pump horsepower. Another question to be answered is the parasitic cost of power for the average Indian plant vs that in the U.S.

The McIlvaine Coal Fired Power Plant Decisions has a number of relevant papers. They include  INVISTA DBA Dibasic Acid - The McIlvaine Company


There are a number of suppliers of DBA around the world. They include

  • Cathay Industrial Biotech
  •  Palmary Chemical
  •  Henan Junheng Industrial Group Biotechnolog
  •  Evonik
  • Invista
  • Zibo Guangtong Chemical


FGD Scrubber Monitoring and Control:  The use of the right chemicals is only part of the true cost reduction. Monitoring and control of parameters is equally important. The CEMS system should be used for process monitoring. In the case above the system could operate with 3, 4, or 5 pumps depending on the reactivity of the limestone. However varying sulfur levels in the incoming coal provide another variable.  If SO2 is measured prior to the scrubber as well as at the stack the right balance between efficiency and energy consumption can be determined.


The measurement of liquids is also important. Brad Buecker addressed this subject in an older Power Engineering article.  He pointed out that it is important to constantly measure the alkalinity in the scrubber module or modules, as too much alkalinity will waste reagent while lean alkalinity will impair SO2 removal. The technique universally employed in wet scrubbers is pH monitoring. These measurements must be continuous, with control of reagent feed rates based upon the readings. For the lab staff, grab-sample pH analyses are very important to make sure that the in-line probes/monitors are accurate.


The slurry circulating pumps can only handle so much mass before electrical requirements are exceeded. Like pH, scrubbers are equipped with continuous density monitors, typically utilizing radioactive detectors. Again, the lab staff needs to monitor density on a grab sample basis to ensure the accuracy of the continuous instruments.


Control of solids chemistry offers interesting challenges and is extremely critical to operation. Experience has shown that operation in either a completely oxidized state (no calcium sulfite-sulfate hemihydrate in the scrubbing slurry) or a completely un-oxidized state (no gypsum in the slurry) minimizes scaling in the scrubber. Scale buildups can be extremely problematic, as deposit formation on scrubber internals and subsequent gas flow restrictions may cause unit de-rates and even forced outages if gas flow is severely restricted.

The technique that has proven itself very well for scrubber solids analysis is thermogravimetry. A thermogravimetric analyzer (TGA) is a quantitative not a qualitative instrument, so the operator needs to have a good idea of the primary constituents in the sample before analysis. If the sample compounds decompose at distinct and separate temperatures, it becomes easy to calculate the concentration of the original materials. Wet-limestone scrubber byproducts lend themselves well to this technique.


The plant can vary the number of pumps in operation. An  optimization system enables the plant to use the minimum number of pumps required to keep the outlet SO2 lower than limits. This also reduces the amount of limestone slurry required, but the major contributor to cost savings is cutting energy use by removing one or more recirculation pumps from service based on the input SO2 value. One optimization system available was described by  Toshihiko Fujii of Yokogawa Electric Corp. in a recent Power Magazine article.


Fig 1_Flue Gas Desulfurization flow diagram


FGD control is normally implemented by the distributed control system through the regulation of limestone slurry flow. The limestone slurry flow is controlled based on the pH value as measured by an analyzer installed in the absorber. Typically, the pH value must be controlled to ensure proper performance of the desulfurization process in the absorber, therefore the limestone slurry flow is controlled to maintain the proper pH value. For this control scheme, other indices such as the FGD inlet and outlet SO2 are not used, and all recirculation pumps are operated regardless of the inlet SO2 value, with slurry flow regulated by the control valve.


The optimization system typically consists of three functions: enhanced regulatory control, model-based prediction, and process value prediction. The system uses these three functions to continuously determine the minimum required number of recirculation pumps in operation, and to calculate the setpoint for the limestone slurry flow PID (proportional-integral-derivative) control loop.


A 700-MW coal-fired power plant in Japan implemented the optimization system. This plant runs about 300 days per year at baseload and is not in operation for the other 65 days of the year. The energy savings realized by running only the required number of recirculation pumps was 12.4% of the unit’s total house load, equating to about $900,000 in energy savings per year based on market conditions in Japan. Another benefit was reduced pump run times, which resulted in lower pump maintenance costs and extended pump life. A third benefit was less limestone usage.


Background Information in FGD &DeNOx Knowledge Network (Free To Power Plant Operators)

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