The Relative Impacts of the use of Calcium and Sodium Dry Sorbent Injection (DSI) Reagents

Transcription

1 2017 World of Coal Ash (WOCA) Conference in Lexington, KY - May 9-11, The Relative Impacts of the use of Calcium and Sodium Dry Sorbent Injection (DSI) Reagents Michael D. Schantz 1 1 Lhoist North America, 3700 Hulen Street, Fort Worth, TX KEYWORDS: DSI, calcium hydroxide, trona, sodium bicarbonate, ESP, resistivity INTRODUCTION Dry Sorbent Injection (DSI) refers to the practice of injecting a dry alkaline mineral into a flue gas stream in an effort to capture acid gases. The use of this technology at utilities and industries continues to expand rapidly as a low capital cost solution for compliance with evolving environmental control requirements. A graphic representation of a typical DSI reagent feed system is illustrated in Figure 1 below. Figure 1 [1]

2 In general, either calcium based or sodium based chemistry is utilized as the alkaline sorbent to capture the acid gases. However the ancillary impacts of the two chemistries can be quite different and these impacts should be considered when selecting a given DSI chemistry solution. In addition to the reagent chemistry, how a given DSI reagent solution is employed will affect the acid gas removal and impact the CCR materials differently. As an example, Figure 2 below is a representation of the various locations used for injection of DSI reagents at a coal fired power plant. Figure 2 [2] This paper discusses how selection of a given DSI reagent chemistry and the employment of this chemistry impacts on the CCR, whether disposed or marketed. THE DRIVERS FOR DRY SORBENT INJECTION (DSI) DSI technology was evaluated as a potential acid gas control technology as early as the 1980 s, however as a general rule, capture efficiencies were not high enough to address the removals required at utility coal-fired boilers under Phase One SO 2 control requirements of the 1990 Clean Air Act Amendments. However, advances in sorbent effectiveness, combined with new regulatory drivers, have DSI playing a major role in U.S. utility and industry compliance strategies.

3 Much of the recent rebirth of DSI technology was driven by the unintended consequence of installing SCR technology on coal-fired utility boilers combusting high sulfur fuel. Unfortunately retrofitted SCR systems designed to capture NO x also convert some percentage of SO 2 to SO 3. This SO 3 has become a major issue for many utilities as the SO 3 reacts with moisture in the flue gas and atmosphere as the gas cools. This has resulted in localized H 2 SO 4 emissions or so called blue plume issues. Over the last ten years, DSI with both calcium and sodium products have proven effective for control of SO 3 emissions and are in use at dozens of facilities today. Additional EPA rule making efforts aimed at coal-fired utilities are expanding the interest in DSI technology. Perhaps most significant is the implementation of the utility MATS rule, which requires control of HCl emissions from coal-fired power plants for the first time. In fact, the U.S. EPA has estimated that 62 GW of coal-fired utilities will utilize DSI technology to comply with the recently finalized utility MATS rule. While the impact of this rule is certainly open for debate, especially given the recent challenge to MATS, the use of DSI technology at coal-fired plants is clearly growing. Also impacting the interest in DSI technology is regulatory uncertainty. In these times of regulator uncertainty there is a clear preference to implement low capital cost solutions as it is difficult to justify major capital expenditures without knowing the anticipated life of a given existing coal-fired generation asset. It is also clear that controlling these acid gases will require utilization of much higher amounts of DSI reagent than that already experienced for basic SO 3 control. It is these higher levels of DSI reagents that have the potential to dramatically impact the properties of the CCR from a given facility. BASIC CHEMISTRY OF DSI REAGENTS The chemistry associated with DSI technology is relatively straight forward and well understood at this point. The two primary chemistries being utilized for utility acid gas control with DSI are calcium based and sodium based. The primary calcium reagent being widely utilized is hydrated lime or calcium hydroxide (Ca(OH) 2 ) and the basic reactions that result in the capture of SO 2 and HCl are outlined below. Ca(OH) 2 + SO 2 +.5O 2 Ca(OH) 2 + 2HCl CaSO 4 + H 2 O CaCl 2 + 2H 2 O Accordingly, the primary reaction products of calcium based DSI are calcium sulfate and calcium chloride. Given the relative volume of sulfur compounds vs. the lower concentration of chlorides in a typical flue gas stream there is generally quite a bit more of the sulfate reaction products than chloride reaction products present. Note that there are other less critical reaction products resulting from interaction with Hydroflouric acid and even CO 2 in the flue gas, but the primary reaction products are those associated with sulfates and chlorides. The sodium based chemistry is a bit more complex in that both trona (sodium sesquicarbonate) and sodium bicarbonate can be used in sodium based DSI

4 applications. However, the reaction products are ultimately the same for either compound. The basic trona reaction for sulfate capture is outlined below [3]. 2(Na 2 CO 3 NaHCO 3 H 2 O) + 3SO 2 3Na 2 SO 3 + 4CO 2 + 5H 2 O Secondary sulfate capture reaction is as follows: 3Na 2 SO O 2 3Na 2 SO 4 The basic chloride capture mechanism for sodium reagents is as follows [3] : Na 2 CO 3 NaHCO 3 H 2 O + 3HCl 3NaCl + 2CO 2 + 4H 2 O In the case of the sodium chemistry the primary reaction products are sodium sulfate and sodium chloride. Just as with calcium, there is generally much more of the sulfate reaction product than the chloride reaction product due to the relative concentration of the pollutants in a coal-fired utility typical fuel gas. IMPACTS OF SODIUM VS. CALCIUM CHEMISTRY (DSI) REAGENTS ON CCR The primary impacts of DSI reagent selection on the CCRs is a function of the resulting reaction byproducts introduced in the fly ash. Assuming the injection location is chosen to take advantage of existing particulate control devices, the reaction byproducts associated with the DSI system, as well as any unreacted reagent, will be comingled with the CCR. This can have implications for the management of the CCRs whether marketed or disposed. Impacts on the Marketability of CCR Materials As outlined above, with sulfur typically being the primary acid gas present in coal-fired generation flue gases, the use of DSI prior to the particulate collection system will result in the sulfate reaction compounds being present in the CCR. The sulfate reaction compounds, to the extent soluble, can lead to sulfate attack in concrete blends produced with the resulting CCRs. As you can see below, sodium sulfate, the reaction compound associated with sodium DSI reagents, is multiple orders of magnitude more soluble than calcium sulfate, the corresponding calcium reaction compound. Reaction Compound Cold H 2 O Solubility* N 2 SO CaSO NaCl 35.7 CaCl *Grams per 100 cc H 2 O Table 1 [4]

5 In addition, when using sodium based DSI reagents, the acid gas capture reaction compounds (N 2 SO 4 and NaCl) and unreacted sodium reagent will contribute to available alkalis. Typically fly ashes to be used in ready mix applications are limited to 1.5% available alkalis [5]. Most ashes already contain some level of available alkalis so the use of a relatively small amount of sodium DSI reagent may preclude marketing the resulting fly ash into ready mix applications. As a result the use of sodium based DSI reagents generally precludes the use of the resulting CCR in any cementious market application. When using calcium based DSI reagents the resulting sulfur compound capture reaction compound is calcium sulfate, which is much less soluble than the corresponding sodium alternative. Accordingly, for a given amount of reaction product, the impact from a sulfate attack potential for the resulting CCR is significantly lower for the calcium DSI solution than the sodium based DSI solution. In short the use of calcium based DSI chemistry is preferred when limiting the amount of soluble sulfate compounds is desired. There is another potential benefit to the use of calcium based DSI reagents in that there is inevitably some unreacted DSI reagent carried over into the CCR material. In the case of the use of calcium based DSI reagents this unreacted material is calcium hydroxide. The additional calcium hydroxide present in the CCR can actually make the pozzolanic CCR more cementitious. This is especially true in instances where the non- DSI reagent fly ash is a Class F material with relatively low amount of available calcium. However, it should be noted that the presence of calcium sulfate in the CCR will have the effect of slowing the cementitious reaction of any material in which it is utilized. Despite this impact, for some fly ashes an increase of calcium reaction compounds, namely CaSO 4 and CaCl 2 (as well as unreacted Ca(OH) 2 ) will not necessarily preclude marketing of the ashes as these are compounds that are present in many fly ashes. Even if the resulting calcium DSI reagent impacted CCR is not an ASTM C-618 material, it may be suitable for non-structure cementitious applications. These CCR materials have also been successfully utilized in soil stabilization applications. An additional factor associated with the choice of alkali DSI reagent selection on the marketability of a given CCR material to be considered is how this reagent will impact the use of activated carbon for mercury control as the carbon from these systems can preclude the use of these materials in ready mix applications. From a positive perspective both sodium and calcium based DSI reagents are highly effective at capturing SO 3, which has been shown to consume activated carbon. Accordingly, the use of these materials can reduce the amount of activated carbon necessary to achieve a given level of mercury control. However under the right conditions, sodium DSI reagents can also catalyze NO x to NO 2 which has been shown to degrade the efficiency of activated carbon. This effect has been noted on a number of field trials. The graph below illustrates this issue and the associated need to increase the amount of activated carbon used for a given level of mercury control as a result of the use of a sodium based DSI reagent (trona in this case). So in some cases when sodium DSI reagents

6 are utilized, the benefits associated with reducing SO 3, namely lower activated carbon requirements, are more than offset by the increased activated carbon demand associated with the increased production of NO 2. Figure 3 [6] Despite the lesser impacts of calcium DSI reagents solutions, at high DSI addition rates, the associated impacts could result in an ash that is problematic for use in ready mix applications or soil stabilization applications so all CCRs should be evaluated on a case-by-case basis. Impacts on CCR Materials Bound for Disposal Recognizing that regardless of DSI reagent chemistry selected for a given coal-fired power plant, some of the resulting CCR materials will not be suitable for marketing applications and will inevitably have to be disposed. Given the relatively new CCR disposal rules requiring management and monitoring of these disposal facilities, understanding these impacts on CCR material bound for disposal is important. The impacts on CCR materials bound for disposal can generally be divided into two categories, physical and chemical impacts. Perhaps the most obvious physical impact will be the increase of the total volume of CCRs when DSI is employed prior to the primary particulate collection system. The relative amount of this increase will be a function of a number of system specific factors, with the two most prominent being the targeted acid gas pollutant and the reagent reactivity towards this pollutant. Methods to minimize the amount of reagent required for a given targeted pollutant and therefore the resulting impacts on the CCR are discussed later.

7 An additional important physical impact is the impact on the geotechnical properties of the resulting material. When considering the impact of DSI reagents on CCR materials it is critical to remember that CCRs are pozzolanic in nature. As a remind the definition of a pozzolan is as follows Pozzolans are a broad class of siliceous or siliceous and aluminous materials which, in themselves, possess little or no cementitious value but which will, in finely divided form and in the presence of water, react chemically with calcium hydroxide at ordinary temperature to form compounds possessing cementitious properties [7]. Given that the primary calcium based DSI sorbent is calcium hydroxide and that there is inevitably residual calcium hydroxide material when employing DSI, the resulting CCR material will be more cementitous in nature than the un-impacted CCR material. This has implications for construction of dry disposal facilities in that these materials generally will allow for steeper slopes and will also likely have reduced permeability. At least as important as these geotechnical property considerations are the chemical property considerations. The two chemical properties that have been shown to differentiate calcium vs. sodium impacted CCR materials are metals leaching potential and total dissolved solids mobility for a given CCR material. The leaching potential for a given metal compound is a function of ph and when the alkaline DSI reagent (whether sodium or calcium) is added to a CCR the ph increases accordingly. In general the associated increase in ph will result in a higher leaching potential for most metal compounds. However the use of calcium DSI reagents brings another mechanism into play, the encapsulation of metals compounds. The fact that calcium DSI reagents trigger the formation of cementitous compounds inherent in the pozzolanic CCR materials can more than offset the increased metal leaching potential related to the increase in ph. This is illustrated in the Figure 4 below where the impact of both sodium and calcium based DSI reagents on this industrial residue material was measured by performing TCLP analysis on both materials. In all cases where metals leached in measureable quantities, the amount of metals leaching from sodium impacted fly ash was significantly higher than the fly ash impacted by calcium DSI reagent usage. Note that while this example is from an industrial application with higher metals concentrations than typically seen in coal-fired utility fly ashes, it illustrates the relative impact of calcium vs. sodium chemistry on the process residue. Since the results from this trial, this facility has since converted from using sodium based DSI reagents to calcium and is experiencing significant cost savings associated with no longer having to pay hazardous waste disposal fees

8 Figure 4 [8] With the new CCR disposal rules requiring the installation and monitoring of groundwater monitoring systems, it is very important to understand the impact of the use of DSI reagents on the solubility of the solid compounds. This can be evaluated by measuring the total dissolved solids (TDS) on DSI reagent impacted materials. These relative impacts have been assessed in a lab study designed to illustrate the differing impacts of sodium vs. calcium DSI reagents. The summary of the results in term of TDS can be below. It is clear that while both the calcium and sodium DSI reagents increase the amount of TDS from the un-impacted fly ash, the impact of the sodium is significantly higher than that of the calcium. The relative TDS leaching potential of CCRs must be considered when selecting a DSI chemistry solution. Table 2 [9]

9 METHODS TO MINIMIZE THE IMPACTS OF DSI REAGENTS ON CCR MATERIALS While important to understand the relative impacts of sodium and calcium chemistry for DSI applications, it is also important to understand that the manner in which a given DSI solution is employed at a given facility can impact the CCRs. Below are some important considerations. Injection Location and Associated Reagent Selection Given the inevitable impacts on CCR materials resulting from the use of DSI reagents there is a desire to evaluate the potential to employ DSI technology after the particulate collection system. However this is not typically possible with sodium as the reagent needs to be injected at high temperatures. However, the use of an optimized calcium DSI reagent post particulate collection system is a possibility worth considering. If the DSI system is being employed for SO 3 control and has an associated wet scrubber system, the scrubber may be able to handle the increased particle loading. However, if the system does not have a scrubber, consideration could be given to adding a small post injection baghouse to capture any used reagent or reaction product. This approach would maintain the marketability of the CCR material. As it is often difficult or impossible to employ a DSI system after the existing particulate collection system, there is a general tread to move the DSI injection location further upstream in the flue gas system to gain operational benefits. This results in the inevitable issue of co-mingling the reaction product with the CCR as well as an increase in the loading to the particulate collection device. In addition to increased particulate loading, in the case of an electrostatic precipitator the resistivity behavior of the resulting DSI reagent impacted CCR must be addressed. There is a general belief that when a plant is using ESP particulate controls sodium reagents must be used to not degrade the performance of the ESP. While the introduction of calcium into the system will increase the resistivity of the CCR materials, this impact will not always result in lesser ESP performance. In fact the impact on performance will be influenced by a number of factors, including the relative amount of unreacted reagent and reaction by-product. As shown below in Figure 5, the relative amount of these materials have a dramatic impact on the laboratory resistivity behavior of the resulting CCR.

10 Figure 5 [10] This has important implications for the use of calcium based DSI materials at facilities employing ESP particulate control systems. In short, if a given acid gas capture target can be achieved with lesser DSI reagent the effects on an ESP may be manageable. The amount of calcium based DSI reagent required can be minimized by ensuring the reagent comes into contact with the desired pollutant in favorable conditions and by improving the reactivity of the reagent itself. While all DSI systems should be designed to insure good distribution of the reagent, perhaps with CFD modeling if justified, there are other ways to insure the best use of calcium reagents as well. As an example, when using DSI technology for HCl control, hydrated lime can be injected at lower temperatures where it s effectiveness in capturing SO 2 is low. The result of this approach is less reagent being consumed by the sulfur compounds in the flue gas and less total DSI reagent being introduced into the system to achieve the targeted HCl capture. Secondly, reagent suppliers continue to develop more reactive reagents not only to improve economics associated with the technology but also to reduce the impacts on the CCR materials. These developments have included improving the physical properties of the reagents to enhance acid gas capture as well as surface treatments of the reagents to reduce the resistivity impacts associated with the calcium reagents. The lesser resistivity impacts associated with the surface treatment reagents is illustrated below.

11 Figure 6 [10] As a result, this perception that the use of calcium based DSI reagents is precluded at units where ESP particulate control is employed is incorrect. A good summary of the data refuting this misperception was reported at the ASTM Symposium on Lime Utilization in 2012 by Lodge Cottrell [11]. The case studies based on laboratory resistivity results concluded that calcium hydroxide injection had little or no influence on ESP performance depending on the ESP design. Furthermore, any potential detrimental effects on different fly ashes could likely be managed by a series of process optimization steps. This thesis was put to the test at full scale trial where an optimized hydrated lime product was used in place of a sodium based DSI reagent for primary SO 2 control on a 200 MW coal-fired unit. This trial concluded that the optimized calcium reagent achieved the very high level SO 2 capture targets with manageable impacts on the ESP system. This result has negated the perceived limitations of calcium based DSI reagents and resulted in a number of utilities converting their DSI systems from sodium chemistry to calcium chemistry to minimize the issues related to CCR impacts [12]. At the trial unit the primary market for the resulting fly ash was soil stabilization for drill pads. When a sodium reagent was being utilized any CCR used for the stabilization application had to have lime added to the system to trigger the pozzolanic behavior in the material. The shift to calcium-based DSI chemistry would significantly lower, and perhaps eliminate, the need for supplemental calcium addition to the CCR stabilization product. The end result of this conversion was a CCR material more suitable for marketing purposes that also created less potential for metals and TDS leaching if disposed.

12 SUMMARY While there is no doubt that DSI technology will continue to be employed for low capital cost acid gas control at more coal-fired units, the development of optimized calciumbased DSI reagents can lessen the potential impact on CCR materials from these units. These optimized reagents have become much more efficient resulting in vastly improved acid gas capture performance with lesser impact on particulate collection systems. As concerns related to the impact on ESP systems was the primary driver for selection of sodium based DSI reagents in many cases, some facilities are converting from sodium to these new optimized calcium based DSI reagent alternatives. The calcium based DSI reagents result in CCR materials that are much more manageable, whether marketed or disposed.

13 REFERENCES [1] Silva, Tony (Babcock and Wilcox); et. al., Institute of Clean Air Companies, Dry Sorbent Injection for Acid Gas Control: Process Chemistry, Waste Disposal and Plant Operational Impacts, ICAC White Paper available at ICAC.org [2] Mastropietro, Bob; Evans, Nick; Nol-Tec Systems: Improved Sorbent Utilization using lanceless Technology causes re-evaluation of Electrostatic Precipitator vs. fabric filter; Power Gen 2016 [3] Allen, Joshua (NatroniX); et. al., Institute of Clean Air Companies, Dry Sorbent Injection for Acid Gas Control: Process Chemistry, Waste Disposal and Plant Operational Impacts, ICAC White Paper available at ICAC.org [4] CRC Handbook of Chemistry and Physics, 93 rd Edition [5] "ASTM C618 12a Standard Specification for Coal Fly Ash and Raw or Calcined Natural Pozzolan for Use in Concrete". ASTM International, Book of Standards, volume [6] Sjostrom, Sharon, ADA Environmental Solutions, Mercury Control from Coal to Stack presentation, Reinhold Environmental NO x control conference, February 18-19, 2013 [7] Wikipedia on-line encyclopedia, on-line definition, April 2, 2017 [8] Hunt, Gerald; Lhoist North America; Ceramic production facility field trial results [9] Schantz, Michael & Sewell, Melissa; Lhoist North America; The Growth of Dry Sorbent Injection (DSI) and the impact on Coal Combustion Residue, WOCA, 2013 [10] Robert Mastropietro & Nick Evans, Nol-Tec Systems, Lodge-Cottrell Division; Improved Sorbent Utiltization using lanceless technology causes re-evluation of electronic precipitator vs. fabric filter; PowerGen 2016 [11] Mastropietro, R. A. Impact of Hydrated Lime Injection on Electrostatic Percipitator Performance. In ASTM Symposium on Lime Utilization; 2012; pp [12] Foo, Rodney, Dickerman, Jim, Hunt, Jerry, Heizenwolf, Johan, Johnson, Landon; ESP Compatible Calcium Sorbent for SO2 Capture at Great River Energy s Stanton Station; Power Plant Pollutant Control and Carbon Management MEGA Symposium August 16-19, 2016