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Wet-limestone scrubber enhancement chemistry improves heat rate for coal-fired plants

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In early 2019, Power Engineering published a report on a chemistry that can significantly enhance limestone reactivity and sulfur dioxide (SO2) removal in wet flue gas desulfurization (WFGD) systems. [1] The article outlined a full-scale application at a coal-fired plant in the eastern U.S. where the chemistry improved scrubbing efficiency.  This in turn allowed operators to reduce the number of slurry recirculating (recycle) pumps, providing operational and maintenance cost savings.

In addition, with fewer recycle pumps in operation, the backpressure on the unit was lowered, reducing booster and ID fan loading.  The ability to cut back parasitic electrical load improve plant heat rate, which could allow some plants greater access to the grid, per the Affordable Clean Energy (ACE) rule. [2] This may allow some plants to stay open, saving local jobs.  Subsequent use of the patented chemical at other plants has achieved similar results and more. 

This article outlines data from the application of this chemistry at City Water, Light & Power (CWLP) in Springfield, Illinois.  Although coal-fired power production in the U.S. has diminished, a core group of plants remain.  Now more than ever, the remaining plants are challenged to find creative ways to reduce operating costs while improving plant performance. 

A Quick Review

Figure 1.  Spray tower, wet-limestone FGD process flow diagram. 

To briefly review, the primary mechanical and chemical processes in wet-limestone scrubbers are as follows:

  • The incoming flue gas is sprayed with a slurry containing finely-ground limestone particles.  The limestone for the CWLP scrubbers contains greater than 95% calcium carbonate (CaCO3) content.
  • SO2 in the flue gas first transfers to the liquid phase.  The SO2 forms an acidic solution of H2SO3.
  • The primary compound in limestone, calcium carbonate (CaCO3), reacts with the acidified liquid to generate an initial reaction product of calcium sulfite hemi-hydrate (CaSO3•½H2O).  The chemistry is more complex than indicated here, but this chemical transformation is the result.
  • In most wet-limestone scrubbers, oxidation air is injected into the reaction tank at the bottom of the absorber tower to convert the hemi-hydrate to gypsum (CaSO4•2H2O).  Gypsum is much easier to dewater and handle than the hemi-hydrate, and serves as a construction material, most notably wallboard.  CWLP has a contract to produce wall board-quality gypsum.
  • The scrubber byproduct is typically dewatered by hydrocyclones, followed by either vacuum drum or belt filtration.

Figure 2.  Scrubber byproduct dewatering on a vacuum drum filter.  Photo courtesy of City Water, Light & Power.

The efficiency and completeness of the reactions depend on kinetics and several important chemical and mechanical factors, most notably:

  • The amount of SO2 in the flue gas
  • Unit load and corresponding gas flow rate
  • The liquid-to-gas ratio in the scrubber
  • The residence time of the flue gas
  • The residence time of the slurry in the reaction tank
  • The efficiency of gas-liquid contact in the scrubber.
    • Plugged, damaged, or broken spray nozzles and poor spray patterns can significantly lower gas-liquid interaction and cause recycle pump mechanical failure.
  • Limestone reactivity, which is a function of:
    • Grind size (90% passing through a 325-mesh screen is a common standard)
    • Percentage of CaCO3 in the limestone.  Typically, limestones with >95% CaCO3 are considered high purity.  As the calcium carbonate content decreases, so does the reactivity.  
  • Liquid-phase alkalinity.  This is primarily a function of the limestone but is supplemented by the chemistry outlined below.
  • Oxidation air flow rates

At many plants, significant benefits are possible if SO2 absorption and limestone reactivity can be enhanced.  One advantage is rotating equipment load reduction (and corresponding operating and maintenance costs) thanks to the ability to shut down one or more recycle pumps during normal operation.  This possibility has been demonstrated in full-scale applications, including the plant in reference 1. 

A second potential benefit concerns limestone selection.  Some plants do not have ready access to 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 used for years is dibasic acid (DBA) addition to scrubber process streams, but, as outlined below, new technology has significantly improved 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 circulate through the process to continue assisting with SO2-absorption chemistry.  However, the availability, cost, and even efficiency of DBA make it a less-than-ideal chemistry. 

The patented ChemTreat product, FGD1105 exhibits much better buffering capacity as outlined in the charts below.

Figures 3a and 3b.  A comparison of buffering capacities of scrubber enhancement chemicals.  Data provided by ChemTreat.

FGD1105 has a significantly higher buffering capacity than DBA when titrated with both sulfuric and hydrochloric acids, and a much higher capacity than the other major alternatives, formic and lactic acids.  Buffering capacity is a critical property of these products.  CWLP personnel have tested all three additives shown in chart 3b and observed and documented the special properties of FGD1105 and its superior performance improvement.

At plants with tighter SO2 emission requirements than the original scrubber design can deliver, enhancement additives may provide an additional efficiency boost.  For example, at CWLP, 95% SO2 removal is the maximum achievable under original design conditions.  An additive is necessary to obtain 97–98% removal.  Building on this concept, another very significant benefit at some plants is that enhanced chemical reactivity may allow use of higher-sulfur, less-expensive coal than lower-sulfur coal whose material cost might be the same but comes from much farther away, increasing transportation costs.  Lower fuel costs dramatically increase the opportunities for plants to be dispatched, thereby improving plant viability and continued local employment at the facility. 

The CWLP Experience

CWLP personnel began evaluating FGD1105 as a potential alternative to DBA on Unit 4 in the fall of 2019. Unit 4 is a 230-MW opposed wall-fired steam generator with locally-mined coal as the fuel. The unit has a complete air quality control system (AQCS), consisting of a selective catalytic reduction (SCR) system for nitrogen oxides (NOx) reduction, a pulse-jet fabric filter (baghouse) for particulate removal, and a WFGD system with a 620,000-gallon absorber.  Scrubber additives such as DBA have been directly injected into the absorber to enhance SO2 removal efficiency and obtain flue gas SO2 concentrations below 0.20lb/MBtu, approximately equivalent to 65 ppmv (parts-per-million by volume).  The additives are batch fed based on unit load and SO2 concentrations as measured by the continuous emissions monitoring (CEM) system.  

One of the significant advantages of FGD1105 that interested CWLP personnel is the product does not need to be heated like DBA, which must be maintained within a 135–150°F range.  Additionally, FGD1105 has a freeze point of -11.2°F and requires no heating for freeze protection, reducing material handling needs.

The evaluation utilized 250-gallon totes of FGD1105 plumbed into the existing DBA feed pump suction after the DBA system had been isolated. As with the previous DBA feed protocol, batch feed was the chosen method for the test.  The evaluation was conducted for 16 days between Sept. 4 and Sept. 19, 2019.  Plant personnel measured and recorded a wide variety of FGD performance data, including unit load (MW), SO2 emissions concentration, and volume of product added. They then compared this data to a 16-day run of DBA with similar operating conditions. Below is a summary of the comparison data.  

FGD1105 vs. DBA: CWLP Trial

(Multiplier to match MW load = 1.08)

FGD1105 performed very well.  An obvious advantage was the reduction in chemical feed (268 fewer gallons) of FGD1105 vs DBA over the same time period, a 21% decrease.  As the charts below indicate, the frequency of FGD1105 feed vs DBA was also reduced.

Another almost immediate observation was increased reaction kinetics of the FGD1105. Prior to the evaluation, even when data suggested the DBA had been nearly consumed, SO2 concentrations remained close to the permitted limit. After initial FGD1105 injection, the SO2 concentration very quickly dropped by approximately 25 ppm and continued to decline. The improved reaction kinetics offer the ability to meet compliance goals during times of load swings, mechanical failures and other upsets, should the SO2 concentration dramatically spike.

An additional observation from this evaluation was the reduction in the stoichiometric ratio of limestone to SO2 and improvement in limestone reactivity with the addition of the FGD1105.

The calculations for limestone utilization (LU) come from thermogravimetric analyzer (TGA) [3] lab tests performed on the byproduct cake.  This data allows calculations for limestone stoichiometric ratio (LSR), which determines the percentage amounts of gypsum (CaSO4∙2H2O), residual calcium carbonate (CaCO3), and residual hemi-hydrate (CaSO3•1/2H2O) that may have escaped the forced-air oxidation process.

These results confirm other evaluations, which have shown the ability of FGD1105 to increase the reactivity of a 90% grade limestone to an equivalent 95% grade material.  This benefit can be significant because of the difference in costs for various limestone grades and may also provide remarkably high performance and efficiency if delivered limestone quality is variable.

Overall, CWLP personnel are very pleased with the evaluation and are currently arranging further full-scale evaluations on other FGD systems at the facility.

ACE Rule Implications

Several power facilities have successfully improved FGD performance and lowered the plant heat rate using this scrubber additive.  Competition among plants for grid access and the need to improve unit availability are influencing efficiency improvement programs.  In some cases, plants are competing within their own fleets to operate.  The ACE regulations reward plants that improve heat rate.

The ACE rule also allows for individual states to implement individual greenhouse gas (GHG) reduction plans.  Ultimately, each state plan rewards electric generating units (EGUs) that operate more efficiently.

Conclusion

FGD1105 is an enhancement product that can improve scrubber efficiency dramatically.  It offers an exciting alternative for those plants seeking to improve scrubber performance and lower parasitic power load.  In some cases, it may offer the opportunity to switch to a lower-cost limestone (although careful testing would be necessary), or perhaps even a coal supply with a higher sulfur content but lower cost than the current fuel.

About the authors: Brad Buecker is Senior Technical Publicist with ChemTreat.  He has four decades of experience in or affiliated with the power industry, much of it in steam generation chemistry, water treatment, air quality control, and results engineering positions with City Water, Light & Power (Springfield, Illinois) and Kansas City Power & Light Company’s La Cygne, Kansas station.  Buecker has a B.S. in chemistry from Iowa State University with additional course work in fluid mechanics, energy and materials balances, and advanced inorganic chemistry.  He is a member of the ACS, AIChE, AIST, ASME, NACE, the Electric Utility Chemistry Workshop planning committee, the International Water Conference Advisory Committee, and the Power-Gen planning committee.  He may be reached at [email protected].

Branden Powell is the Supervisor of Environmental Process at City Water, Light and Power (CWLP) in Springfield, Ill. Branden graduated with a degree in chemistry from the University of Illinois at Springfield in 2001. Following graduation, Branden began his career as an organic chemist for TMI Analytical and then moved to Prairie Analytical Systems, both were IEPA contract laboratories. At Prairie, Branden worked his way up to senior organic coordinator and lab supervisor. In 2015 Branden became the plant laboratory supervisor at CWLP. The following year he was promoted to his current position where he still oversees the plant laboratory and plant chemistry for the electric division, and also oversees the facility’s FGD operations, FGD wastewater operations, and plant outfall wastewater operations. Branden is a committee member and frequent contributor and presenter for the Electric Utility Chemistry Workshop for the University of Illinois held annually in Champaign Illinois.  He may be reached at [email protected]

Dave Karlovich is a Strategic Account Manager with ChemTreat and a US Navy veteran with over 30 years of experience in water treatment applications supporting both the nuclear and fossil-fired power industries.  His recent focus has been on chemistry to improve both the process side and wastewater treatment of wet flue gas desulfurization systems at plants throughout the country.  He is co-inventor of ChemTreat’s patented FGD1105 technology to improve SO2 removal and limestone reactivity in wet scrubbers.