By Lindsay Morris, Associate Editor
Mercury-specific control technologies include Activated Carbon Injection (ACI), halogen addition, and various co-benefit methods of control, such as particulate matter (PM) controls, dry sorbent injection (DSI), and dry and wet scrubbers. A few of the many available solutions on the market are mentioned here.
Mitsubishi Heavy Industries (MHI) has released several technologies for controlling mercury. One of them, the Mercury Removal System integrated with SCR and FGD, uses the injection of ammonium chloride (NH4Cl) into the ductwork, upstream of the SCR catalyst and provides NOx control and improved oxidation of elemental mercury. MHI’s downstream wet FGD technology, known as the Double-Contact-Flow-Scrubber (DCFS), removes oxidized mercury. Re-emission of the mercury from the DCFS is minimized by control of the absorbent Oxidation-Reduction Potential (ORP) by adjusting the amount of oxidation air supplied to the DCFS.
During the creation of MHI’s technology, engineers discovered that the level of halogen in coal sometimes varied and was not sufficient to result in a high level of mercury oxidation. They also found that chloride-containing salts to promote mercury oxidation could increase the risk of boiler corrosion and slugging. Therefore, to mitigate this possibility, MHI patented the injection of chloride compounds, such as HCl or NH4Cl, into an SCR process to boost the conversion while limiting the impact to the balance of the flue gas path, namely the boiler.
MHI created the ORP to address the issue of re-emission. This phenomenon occurs when mercury that has been previously captured by an FGD process is chemically reduced in the absorbent and escapes the process. ORP functions by manipulating the oxidation air flow to the FGD to maintain an appropriate level of ORP while minimizing the re-emission of captured Hg.
Initially, MHI tested its mercury control system during a 5 MW scale demonstration at Gulf Power Company’s Mercury Research Center. At the end of the demonstration projects, results revealed that HCl, which was injected before the SCR, was effective at achieving increased mercury oxidation. Additionally, mercury oxidation across the FGD scrubber was controlled by 5 percent or less by increasing ORP, and total mercury removal greater than 90 percent was achieved.
Following the successful demonstration project, Southern Company commissioned MHI to complete a full-scale demonstration testing for the NH4Cl injection and ORP control technologies at Alabama Power’s Plant Miller 720 MW Unit 1 in April 2011. The coal-fired unit, which burns Powder River Basin coal, experienced over a 95 percent oxidized mercury ratio at NH4Cl injection of more than 50 ppm. In addition, no mercury re-emission was seen during NH4Cl injection in the appropriate ORP conditions.
Nalco’s approach has been to minimize mercury compliance costs by maximizing mercury removal across existing air quality control devices, said John Meier, product manager with Nalco’s air protection technology group. All of Nalco’s MerControl Technologies are liquid based applications reducing required capital for implementation and reducing reliability concerns associated with solids conveying.
It is well known that mercury exists primarily in the flue gas as elemental mercury (Hg0) and oxidized mercury (Hg2+), with oxidized mercury more readily captured by carbon based sorbent and acid gas scrubbing equipment (wet FGD and spray dryer absorber (SDA)). By increasing the fraction of oxidized mercury in the flue gas, our clients can maximize the potential mercury removal across existing air quality control devices. Nalco’s MerControl 7895 technology augments the oxidation of mercury released during the combustion of coal, can reduce the amount of activated carbon required, thus preserving fly ash quality, and can remove over 90 percent of mercury when used in conjunction with an SDA or fabric filter.
“There is no one-size fits all solution for controlling mercury,” Meier said. “It is important to maximize the potential of the existing equipment, with each one having different strengths and attributes.”
With that in mind, Nalco saw an opportunity to reduce mercury compliance costs for units equipped with SDAs. Nalco will be introducing MerControl 6012 to the industry over the coming months, designed specifically for mercury control on semi dry acid gas scrubbers. MerControl 6012 is a patented, inorganic, non-halogen mercury sorbent for use in SDAs or circulating dry scrubbers (CDSs). MerControl 6012 can eliminate the need for supplemental oxidants for a simplified and more effective control program across all coal types. The MerControl 6012 program has been successfully demonstrated at full scale demonstrations and has shown to be more cost-effective compared to alternative technologies, Meier said.
Nalco’s technology is a patented chemistry that was developed to prevent the mercury re-emission process, which is of large concern in the Mercury and Air Toxics Standard (MATS). MerControl 8034 can typically completely eliminate mercury re-emissions. MerControl 8034 has been proven effective in commercial demonstrations in a wide variety of scrubber configurations/chemistries, and in some cases, is the only technology required for mercury emissions compliance, Meier said.
Shaw Environmental & Infrastructure
Shaw Environmental & Infrastructure has been working on a solution called Enhanced Mercury Oxidation (EMO) halogen solution that has undergone 14 full-scale trials at coal-fired utilities over the last 30 months, including units at TVA, Ameren, San Miguel Power Cooperative, GenOn, City Power & Light of Springfield and Arizona Electric Power. Tests have been conducted on all three primary types of coal: bituminous, Powder River Basin and lignite.
|Shaw Environmental & Infrastructure’s Enhanced Mercury Oxidation (EMO) halogen solution has undergone 14 full-scale trials at coal-fired utilities over the last 30 months. Photo courtesy Shaw.|
The system involves a halogen solution being injected in a semi-diluted form directly into the duct at the economizer outlet. The halogen solution is mixed with water by a mixing and pumping system near the chemical storage tank, which is typically at ground elevation. The diluted solution and air are then pumped separately up to the duct, where individual valves control the amount of air and the amount of chemical solution that enters each individual lance. The solution is then atomized by atomizing spray nozzles as it leaves the lance. As it exits, a very fine mist is evaporated into the gas stream.
Full-scale trials have recorded a 90 percent removal of mercury from coal-fired boiler stack gases, said Terry Marsh, vice president, integrated emissions solutions for Shaw Environmental & Infrastructure. Capital costs for the solution are in a range of $1.5 to 2.5 million, and chemical costs can range from $300,000 to $1.5 million. One primary benefit of this technology is that the user can optimize their hg oxidation and capture medium. In addition to mercury capture, the EMO has also been effective at increasing SCR performance by 7 to 12 percent and demonstrates noticeable improvements in particulate capture.
W. L. Gore & Associates
Instead of using methods like a wet scrubber or ACI to remove mercury, W.L. Gore & Associates has created a fixed bed system. The Gore Mercury Control System uses low pressure drop sorbent modules placed in the flue gas stream to allow for the continuous absorption of mercury for a long period of time without deactivation, said Jeff Kolde, product specialist.
|W. L. Gore’s Mercury Control System modules, seen here, contain a Sorbent Polymer Composite (SPC) material that captures both elemental and oxidized mercury and does not require regeneration. Photo courtesy W. L. Gore & Associates.|
The modules contain a Sorbent Polymer Composite (SPC) material that captures both elemental and oxidized mercury and does not require regeneration. It is robust to changes in coal type or process adjustments that can impact mercury speciation or flue gas composition. In addition, the modules provide additional SOx reduction as a co-benefit.
Gore’s method offers a low operating cost and requires minimal maintenance. “It can be scaled to meet each particular plant’s reduction needs,” Kolde said, “including the provision for changing the number of modules to meet future compliance needs.”
The Gore Mercury Control System is a self-contained sorbent system, which means it results in no contamination of fly ash and has no impact to particulate collection devices, Kolde said. The modules will continually capture mercury for typically several outage cycles before needing to be replaced.
|Southern Company commissioned Mitsubishi Heavy Industries to complete a full-scale demonstration testing for the NH4Cl injection and ORP control technologies at Alabama Power’s Plant Miller 720 MW Unit 1 in April 2011. Photo courtesy Mitsubishi Heavy Industries.|
In 2010, Gore, along with the Electric Power Research Institute and URS, completed a pilot-scale demonstration of the Mercury Control System at Southern Company’s Plant Yates in Newnan, Ga. The results were a 90 percent reduction of mercury and a 50 to 70 percent reduction of SO2 over the six-month test period. Currently, several other long-term demonstrations are taking place, including one at Southern Company’s Plant Gorgas near Parrish, Ala. that will be completed sometime in 2013.
In July, URS announced that it is collaborating with Gore on the development of the technology and has become an authorized integrator of the Gore Mercury Control System for power applications.