With the compliance deadline for the EPA’s Mercury and Air Toxics Standards less than a year off, plant owners need to decide now on the best approach to control mercury emissions. The use of non-carbon sorbents may offer substantial benefits over standard powdered activated carbon injection.
The upcoming implementation of the Environmental Protection Agency’s (EPA’s) Mercury and Air Toxics Standards (MATS) is putting pressure on coal-fired power plants to develop and implement compliance strategies that are effective, economically efficient, and environmentally advantageous. MATS, scheduled to go into effect in April 2015 for most power plants, requires that vapor-phase mercury stack emissions be less than 1.2 pounds per trillion Btu (lb/TBtu) for bituminous- and subbituminous-fired units and 4 lb/TBtu for lignite units.
Plant compliance with these emissions standards must be reported on a 30-day rolling average basis. The EPA has authorized a one-year extension for utilities that apply to state regulators; a second-year extension is available only under extreme circumstances. Compliance options include mercury capture in a wet flue gas desulfurization (FGD) scrubber or the use of powdered sorbents to capture mercury upstream of the particulate control device (PCD), with the best solutions dependent on individual unit configuration and operating conditions.
Wet FGD capture requires high (typically >90%) levels of mercury oxidation, as oxidized mercury has much greater solubility in the scrubber solution. Oxidation occurs naturally with high-chloride-content coals or can be facilitated by the addition of oxidizing agents to the boiler coal feed. An alternate method relies on mercury oxidation as a co-benefit of installation of a selective catalytic reduction (SCR) module for NOx control. Capital and operating costs for these options depend on the technology employed and site conditions.
Wet scrubbers are installed on many plants that burn higher-sulfur eastern bituminous coals. While these systems typically provide high native capture of vapor-phase mercury, most cannot consistently meet the MATS standard. In addition to coal oxidation additives noted above, wet FGD units sometimes employ additives to the scrubber liquor to reduce or eliminate a phenomenon called “re-emission.”
In the circulating FGD liquid, oxidized mercury in solution may be reduced back to its elemental state, with lower solubility. This leads to saturation of the liquid with respect to elemental mercury and release of the elemental mercury back into the gas phase. Strong evidence of this process is measurement of vapor-phase elemental mercury at higher concentration in the stack than at the inlet to the wet FGD. Several scrubber additive products are commercially available to convert the soluble oxidized mercury to another form, which eliminates the potential for re-emission.
Dry sorbent injection captures mercury on fine particulates added to the flue gas after the economizer, typically either upstream or downstream of the air heater. The mercury-containing particles are then removed in the unit’s PCD. Powdered activated carbon (PAC) and non-carbon sorbents, such as Novinda’s Amended Silicates products, are options. PAC is commercially available in standard and brominated forms, with brominated materials used where vapor-phase mercury is mostly elemental. The bromine promotes oxidation on the particle, facilitating capture and removal from the flue gas.
The most efficient and economic option for a specific coal-fired unit depends on many factors and is best determined via a trial where mercury capture is measured during implementation of candidate technologies. Trials require temporary installation of equipment to deliver the mercury control technologies of interest, as well as mercury measurement instruments to document mercury capture. Trials can be as short as a week or continue for a month or more if the host utility is interested in evaluating longer-term performance and balance-of-plant impacts. Utilities often wish to understand the effect of mercury control on the salability of fly ash as a cement replacement or on operation of the host unit PCD.
Prior to the availability of the Amended Silicates reagent technology, coal-fired power plants that were looking for a dry sorbent mercury (Hg) removal solution had little choice but to use carbon-based products. The non-carbon Amended Silicates platform was under development for several years, with the most recent version—Amended Silicates HgX-ESP—introduced in June 2014.
The Amended Silicates family of products employs a non-carbon mercury capture reagent that removes mercury via chemical reaction, providing economic and environmental advantages unavailable with carbon-based products.
Amended Silicates employs a bentonite clay substrate amended with a metal sulfide that acts as the reagent to capture mercury. This occurs via a chemical reaction that forms mercuric sulfide on the reaction sites of particle surfaces in flue gas. Mercuric sulfide (cinnabar in its naturally occurring state) is the most stable form of mercury and the dominant mercury mineral in Earth’s crust.
Amended Silicates directly reacts with elemental and oxidized mercury to form stable, sequestered mercury compounds on the surface of the amended clay particles. This means that oxidizing coal additives or wet scrubber re-emission chemicals are not needed to maintain compliance with MATS, making Amended Silicates a single-product solution. Fly ash samples containing collected Amended Silicates particles have been subjected to the EPA Method 1311 TCLP test, and leachable mercury has been found to be nondetectable, a further indication that the mercury is sequestered as an insoluble solid.
Full-Scale Trial Examples
Novinda is supplying Amended Silicates to multiple coal-fired generation units in the western and midwestern U.S., including four units at the Gillette Energy Center (GEC) complex operated by Black Hills Power in Gillette, Wyo.
In a series of trials conducted in each of the four Powder River Basin (PRB)–fired units at GEC, Novinda demonstrated the superior performance of Amended Silicates. Trials were completed to optimize mercury control, as well as to demonstrate extended operations and assess balance-of-plant impacts. Typical results for one of the units are shown in Figure 1 for a 30-day trial of AS-HgX. The uncontrolled mercury emissions from the host unit were between 5 lb/TBtu and 6 lb/TBtu—well above the MATS standard. Once injection was started, mercury stack emissions quickly dropped below the MATS level; the injection rate was adjusted over the first week of the trial to minimize cost while continuing to meet the standard. The optimized injection rate was determined to be a low 0.26 pounds per million actual cubic feet (lb/MMacf) of flue gas.
|1. Quick results. The cumulative average of stack Hg emissions during injection of Amended Silicates AS-HgX for a Powder River Basin (PRB) coal-fired unit are shown here. Courtesy: Novinda|
In trials completed in other units at GEC, AS-HgX consistently out-performed brominated PAC.
“Black Hills continuously makes responsible investments in technology to help us comply with state and federal environmental policies,” said Mark Lux, vice president and general manager of power delivery at Black Hills Corp. “Amended Silicates has proven to be a consistent performer in achieving our mercury removal objectives, and Novinda’s technology delivers the desired results with less than half the material required by alternative products, minimizing the emissions control costs for our customers.”
Another long-term trial was completed at a proprietary plant burning PRB coal and equipped with an SCR, circulating dry scrubber, and pulse-jet baghouse. AS-HgX was injected at this site for 18 days, with results shown in Figure 2. The plant was able to tune the injection ratio to meet the target mercury emission rate for the entire period at an average injection ratio of 0.11 lb/MMacf. For this graph, mercury is plotted as a cumulative average, the required reporting standard of the EPA MATS rule.
|2. Meeting targets. Test results show the effectiveness of AS-HgX on cumulative stack mercury in a PRB plant equipped with a circulating dry scrubber. Courtesy: Novinda|
Other benefits of Amended Silicates’ bentonite clay substrate include compatibility with continued sale of fly ash as a replacement for Portland cement. (PAC typically contaminates the fly ash, requiring landfill disposal.) Novinda’s Amended Silicates has passed ASTM’s C618 test for use as a cement replacement in every sample evaluated to date. The combined financial impact realized from the sale of fly ash and avoided disposal costs for that same fly ash if contaminated by PAC are often greater than the operating costs for mercury control in a host unit.
AS-HgX also has been subjected to the ASTM E1226-10 explosibility test and found to be not explosible. Therefore, injection systems for Amended Silicates do not require explosion suppression or inerting systems, as are specified for PAC silos in recent procurements.
Some utilities have explored a hybrid approach for compliance with MATS. One option is to add an oxidizing agent to the coal feed coupled with the injection of PAC or brominated PAC to capture the oxidized mercury from the flue gas. A concern has emerged at some plants over evidence of corrosion associated with the use of halogen oxidizing agents such as calcium bromide (CaBr2). Novinda’s Amended Silicates offers a single-product solution without the use of corrosive halogen compounds; AS-HgX serves to both oxidize and capture mercury from the flue gas.
Amended Silicates has a higher bulk density compared to PAC and may require minor modifications to PAC injection systems to efficiently disperse in the flue gas. Novinda recommends lance spacing of 30 inches and discharge locations along the length of the lance of 30 inches as well. To flow reliably in PAC silo systems, minor modifications to the fluidization air pad configuration are recommended. These changes are typically implemented in less than one week at a cost of less than $50,000. The use of higher injection air velocities at the lance discharge is recommended as well, which may require an upgrade to the transport air blower in existing injection systems.
Due to its efficient capture of mercury and one-product solution in certain plant configurations, Amended Silicates generates significant cost savings for many users. Details for several cases are presented here.
Comparative economics for a baseload (90% availability) 110-MW unit burning PRB coal are shown in Table 1. This unit is equipped with SCR, a spray dryer absorber, and a pulse-jet baghouse for particulate control. In addition to AS-HgX, brominated PAC was tested in this unit, both as the sole mercury control sorbent and in combination with calcium chloride (CaCl2) added to the coal as a mercury oxidizer. AS-HgX met the pending MATS emissions standard with a 20% margin at an injection rate of 30 lb/hr (1.3 lb/MMacf) while a higher rate was needed for brominated PAC. The simultaneous addition of CaCl2 to the coal feed allowed for a reduced brominated PAC injection rate, but the combined costs were still higher than AS-HgX. Use of Novinda’s AS-HgX is projected to save the utility 63% to 70% of the mercury abatement costs of the brominated PAC alternatives.
|Table 1. Comparative economics for 110-MW unit burning PRB coal. Source: Novinda|
In another trial, shown in Table 2, MATS compliance was achieved with Amended Silicates injection, and a concrete-friendly PAC was evaluated as well. To meet MATS, the PAC product required concurrent use of a coal additive for mercury oxidation, as well as hydrated lime to reduce flue gas SO3 content, which degrades PAC mercury capture efficiency. In spite of the use of a concrete-friendly PAC, fly ash samples did not meet specifications for sale as a cement replacement. In this trial, Novinda’s single product offered a 33% savings over the multiproduct alternative and preserved the salability of fly ash, estimated to be worth an additional $5,000,000 in annual revenue and avoided disposal costs.
|Table 2. Comparative economics for 760-MW unit burning eastern bituminous coal. Annual costs do not include the effect on revenue from fly ash sales and avoided disposal costs. Source: Novinda|
Novinda’s Amended Silicates was also evaluated in a unit burning a moderate sulfur (~1.7%) eastern bituminous coal. The host unit was fitted with SCR, which contributed to a SO3 concentration of about 20 ppm at the Amended Silicates injection location. This condition was similar to another unit burning eastern bituminous coal where a sulfur-tolerant PAC had been tested with results reported in the literature. To determine injection rates of sulfur-tolerant PAC and hydrated lime, a proportioning factor was calculated as a simple ratio of plant outputs for the Novinda host unit and the literature unit. Results of the economic analysis are presented in Table 3. The Novinda single-product option of AS-HgX injection to meet MATS showed a 45% cost savings compared to the combination of an SO3-tolerant PAC and hydrated lime.
|Table 3. Comparative economics for a 315-MW unit burning eastern bituminous coal. Source: Novinda|
Amended Silicates products provide a number of important environmental benefits when compared to the use of PAC. In light of climate change concerns, Novinda commissioned CH2M HILL to compare the greenhouse gas (GHG) emission impacts for production of Amended Silicates versus PAC.
Life-cycle GHG emissions for Amended Silicates were estimated using process-specific data and the GaBi 4 life-cycle assessment (LCA) software. These results were compared with existing literature values for production and use of PAC. The LCA included mining, manufacturing, use in a power plant, disposal, and transportation of materials between each step in the process. The LCA concluded:
■ Per pound of material produced and used, GHG emissions associated with Amended Silicates are 11% of the average literature values for GHG emissions of PAC.
■ The use of Amended Silicates would deliver a potential reduction in carbon dioxide–equivalent (CO2e) emissions of 174,000 metric tons per year if just 10% of the market substituted Amended Silicates for PAC, based on Novinda estimates of a total mercury sorbent market of 225,000 metric tons per year.
■ The ability to reuse fly ash results in avoided GHG emissions of an estimated 91 lb. of CO2e for each pound of Amended Silicates used, or 1.03 million metric tons per year of avoided CO2e emissions, based on the assumption that only 50% of fly ash is reused and the same Amended Silicates 10% market usage with an average Amended Silicates content of 1% in the fly ash.
The full report is available at http://bit.ly/1p8w918.
A Cost-Effective Solution
The EPA’s MATS regulation for control of mercury emissions requires technology at most coal-fired plants to capture vapor-phase mercury in the emissions control train. Options include the injection of sorbents, the use of additives to oxidize elemental mercury for capture in wet FGDs, and other specialized hardware solutions. Most common will be the use of injected sorbents such as Amended Silicates or PAC, which represent a low–capital expense approach.
Novinda’s Amended Silicates product line offers a viable alternative to PAC for compliance with MATS. For most plant configurations Amended Silicates is cost-effective, generates low balance-of-plant impact, and preserves the value of fly ash for resale. ■
— Jim Butz is vice president product management for Novinda Corp. in Denver.