Amended Silicates™ sorbents use silicate materials as substrate particles on which chemicals with a strong affinity for heavy metals (e.g., mercury and mercury compounds) are impregnated. Because of their physical structure, the silicates present extended surface area on each particle combined with an easily-generated particle size of a few microns. This configuration promotes maximum exposure of the chemical amendment to the mercury vapor present in the coal-fired flue gas stream.

The base silicate materials typically cost $0.04 to $0.08 per pound, so they represent a very cost-effective sorbent material. Because of this substantial silicate content, when used for removal of mercury from coal-fired utility flue gases, Amended Silicates™ sorbents allow the continued sale of fly ash as a pozzolan material (concrete additive). Tests completed by Boral Materials Technologies have indicated that there is no negative effect on fly ash use in concrete due to the addition of Amended Silicates™ sorbents.

Benefits of Amended Silicates™ Sorbent

Cost-competitive with other sorbent materials (e.g., activated carbon)
Reliable operation using demonstrated injection system equipment
Does not affect the ability of the fly ash to be sold as a concrete additive.
High mercury capture capacity — several times that of activated carbon. Amended Silicates™ sorbents provided 70%-96% mercury capture at injection rates of 1.6-9 lb/MMACF in pilot testing at an operating power plant.
Leachate mercury levels were below the method limit of detection when samples of sorbent mixed with fly ash were collected from the pilot baghouse hopper and subjected to TCLP tests.
Process design and analysis for a commercial-scale manufacturing plant indicate that Amended Silicates™ sorbents will provide a cost-competitive alternative to other sorbent materials for mercury control.

Background

For the past 10 years, ADA Technologies has conducted an extensive program to develop new technologies for the removal of trace levels of mercury from gas, liquid, and solid waste streams. Projects totaling over $8 million have been funded by DOE, EPA, and by DOE's national laboratories.

The 1990 amendments to the Clean Air Act mandated that the EPA study the impact of mercury in the environment and prepare a report to Congress regarding effects and recommendations for mitigation. The report, issued in 1997, cited coal-fired power plants as the largest source of uncontrolled emissions of mercury into the environment in the United States. The assessment of toxic emissions from coal-fired boilers, with an emphasis on mercury, has been underway since 1993 in studies funded by the DOE, EPA, and EPRI. In anticipation of pending regulation of mercury emissions from coal-fired boilers, DOE has been sponsoring a broad program of research into control technologies. The program has included in-house research at the National Energy Technology Laboratory as well as technology development efforts with a number of commercial organizations. To date, no universal control technology has been identified as being the most cost-effective and efficient for mercury removal. Nonetheless, because of the potential health effects of mercury contamination to lakes and rivers, on December 14, 2000, EPA announced a rulemaking that mandated the control of mercury emissions from coal-burning power plants in the United States by 2007.

ADA Technologies was awarded contracts by EPA and DOE to develop a new mercury sorbent for the capture of mercury from coal-fired flue gases. The development of Amended Silicates™ sorbents has followed a careful plan to address several key issues, including the process and conditions for preparing the sorbent material, and evaluating its ability to capture mercury from simulated and actual flue-gas streams.

Several top-performing sorbent formulations have been tested. These tests have been run to gather quantitative data on the ability of Amended Silicates™ sorbents to remove mercury from flue gas at an operating power plant. Results have shown very effective and efficient capture of mercury with the Amended Silicates™ sorbent. The results are summarized in the paper entitled, Evaluation of Amended Silicates™ Sorbents for Mercury Control(259KB PDF), which was presented at the 2003 Combined Power Plant Air Pollutant Control MegaSymposium in Washington, DC.

Several Amended Silicates™ formulations have also been tested at elevated temperatures and pressures as a possible mercury control technology for next-generation coal gasifiers. Laboratory experiments have shown one variant to effectively capture mercury at a temperature of 770°F and 200 psig, with sorbent mercury capacity in excess of 3% by weight. This is a strong indication that Amended Silicates™ technology could offer significant cost advantages in future coal gasification systems. In this application, the Amended Silicates™ sorbent can be disposable or used in a pressure-swing mode where a significant fraction of the captured mercury is liberated when the pressure is released at operating temperature.

Technical Details

See the technical paper entitled Evaluation of Amended Silicates™ Sorbents for Mercury Control(259KB PDF) for more information.