Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture
Reexamination Certificate
2002-02-14
2004-10-26
Langel, Wayne A. (Department: 1754)
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
C423S491000
Reexamination Certificate
active
06808692
ABSTRACT:
FIELD
The present invention relates to a method of reducing the mercury emissions for coal-fired power plants.
BACKGROUND
The United States Environmental Protection Agency (EPA) in its Utility Air Toxins Report to Congress-February 1998 concluded that “mercury from coal-fired power plants is a serious concern. Power plants account for about 52 tons of annual manmade mercury emissions in the country.” The report stated that EPA has been unable to identify any currently feasible, commercially available technology for reducing these emissions. It recommends “further evaluation of potential control strategies” (see www.epa.gov/region2/health/mercury.htm).
The United States Department of Energy (DOE) indicated that it “wants to develop a wider array of mercury control options for power plants that can reliably reduce emissions by 50 to 70% by 2005 and 90% by 2010.” (See www.netl.doe.gov/publications/press/2001/tl_mercurysel2.html)
A variety of previous attempts have been made to capture mercury.
Ide et al describe the conversion of mercury into mercuric chloride via addition of acidic chlorine containing material, especially hydrogen chloride, into mercury-containing gaseous emissions (see Ide, Akiro et al. 1988. “Process for Cleaning Mercury-Containing Gaseous Emissions”. U.S. Pat. No. 4,729,882). This mercury removal technique requires massive hydrogen chloride injection into mercury containing flue gas, at HCl levels of 500 to 1500 ppm, which would result in excessive corrosion of coal fired power plant steel components. Furthermore, combination of this technique with subsequent hydrogen chloride/mercuric chloride containing flue gas in a wet flue gas desulphurization system (FGD) containing alkali for purposes of sulphur dioxide adsorption from flue gas as a sulphite, bisulphate, bisulphate, or sulphate would result in unacceptable consumption of alkali scrubbing chemicals by the adsorbed hydrogen chloride. Therefore, this technique for mercury control is not believed to be commercially viable in coal-fired power plants.
Felsvang et al describe mercury absorption between 110-170° C. in the presence of chlorides, especially hydrogen chloride and alkaline “basic absorbent” introduced as an aqueous aerosol suspension (Felsvang, K et al. 1995. “Method for Improving the Hg-Removing Capability of a Flue Gas Cleaning Process”. U.S. Pat. No. 5,435,980). This technique requires the use of substantial expensive capital equipment foreign to conventional coal fired power plants. It does not utilize existing flue gas desulphurization equipment. Furthermore, alkaline sorbent designed to remove mercuric chloride is partially wasted by hydrogen chloride unreacted with mercury. Finally, its excessive use of hydrogen chloride, especially at 110-170° is certain to cause highly undesirable corrosion of steel containing surfaces in coal-fired power plants to iron chlorides.
Meichen and Pelt al describe the use of precious metals, especially gold, to catalytically convert elemental mercury Hg° to mercuric chloride HgCl
2
in flue gas (Meischen, S. and J. Van Pelt. 2000. “Method to Control Mercury Emissions from Exhaust Gases”. U.S. Pat. No. 6,136,281). This process is limited to oxidation of mercury to mercuric chloride in flue gas below 300° C. due to undesirable loss of gold as volatile gold trichloride. This technique requires the use of substantial expensive capital equipment foreign to conventional coal fired power plants plus the use of very expensive precious metal catalysts.
Biswas and Wu describe the irradiation of solid particles with light, especially ultraviolet light, to induce catalytic photo-oxidation of mercury to forms capable of being adsorbed by solid particles (Biswas, P. and C. Wu. 2001. “Process for the Enhanced Capture of Heavy Metal Emissions”. U.S. Pat. No. 6,248,217). Unfortunately, this technique requires the use of substantial expensive capital equipment foreign to conventional coal fired power plants plus the use of consumable metal catalysts.
Amrhein describes the use of existing wet flue gas desulphurization systems to capture oxidized mercury in a manner which inhibits undesirable partial conversion of adsorbed oxidized mercury back to volatile elemental mercury (Amrhein, Gerald T. 2001. “Mercury Removal in Utility Wet Scrubber Using a Chelating Agent”. U.S. Pat. No. 6,328,939). The disadvantage of this method is that it requires effective but expensive chelating agents with potential problems due to chelation of scrubber metal components with chelates.
Previous publications have indicated that alkaline fly ash containing solids have an affinity for mercury capture. For instance, Galbreath and Zygarlicke have shown that subbituminous coal ash can capture a portion of mercury in a flue gas (Galbreath, Kevin C. and Christopher Zygarlicke. 2000. “Mercury Transformations in Coal Combustion Flue Gas”. Fuel Processing Technology. 65-66, pages 289-310, @ page 304). They also indicated that hydrogen chloride spiking of flue gas inhibited mercury capture by alkaline solids, such as calcium hydroxide or alkaline fly ash by neutralizing the alkaline sites able to complex mercury, especially oxidized mercury. U.S. Pat. No. 6,250,235 issued to Oehr and Yao describes the addition of a fossil fuel and additive in a combustion zone to achieve the following results alone or in combination: accelerated combustion, desulphurization, nitrogen oxides emission reduction, pozzolanic or cementitious products production or combustor anti-fouling (Oehr, Klaus H. and Felix Z. Yao. 2001. “Method and Product for Improved Fossil Fuel Combustion”. U.S. Pat. No. 6,250,235). This is achieved by fusing alkali, such as calcium oxide, to coal ash and sulphur, while the coal is burning, via a flux. Full scale testing of this patented method, using bituminous coal in a 100 megawatt power plant, resulted in 45% reduction of mercury emissions as compared to testing without the use of above fuel additive. X-ray analysis of the resulting fly ash indicated that the normally acidic bituminous fly ash had been converted to an alkaline form containing alkaline cementitious crystals not unlike those found in subbituminous or lignite based fly ash as well as containing small amounts of alkaline calcium oxide and calcium hydroxide.
It is also well known that elemental mercury (Hg°) can be absorbed by activated carbon. Adsorption improves as the temperature of the carbon is reduced or if the carbon is impregnated with halogen species, such as iodine or chlorides and/or sulphides. Unfortunately the use of activated carbon requires extremely high carbon to mercury ratios e.g. 3000-100,000 to 1 carbon to mercury weight ratios. Injection of activated carbon into the cool zones of coal combustors ahead of the ESPs, FFs or BHs, results in unacceptable contamination of coal ash with carbon for purposes of ash recycling into cement/concrete applications.
Finally, previous publications have described the capture of mercury by scrubbing solutions containing oxidants. For instance Mendelsohn describes contacting elemental mercury containing flue gas with oxidizing solutions of halogens to effect mercury capture as a mercuric halide (Mendelsohn, M. H. 1999. “Method for the Removal of Elemental Mercury from a Gas Stream”. U.S. Pat. No. 5,900,042). This technique is not believed to be commercially viable for any or all of the following reasons:
Mercury capture is inadequate. A maximum of 71.1% and 69.6% mercury removal was demonstrated for bromine and chlorine containing solutions respectively.
Halogen reagents are wasted due to the undesirable consumption of halogen oxidant by sulphur dioxide in the flue gas.
Mercury capture does not utilize existing capital equipment including ESPs, FFs, BHs or FGDs. Expensive additional capital equipment is required. Addition of “bubblers” or liquid scrubbers into a coal-fired power plant would substantially increase pressure drops through the system thereby increasing equipment sizing requirements (e.g. air blowers). This would increase both capital and operating costs for the operatio
Langel Wayne A.
Vermette Clifford W.
Vermette & Co.
LandOfFree
Enhanced mercury control in coal-fired power plants does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Enhanced mercury control in coal-fired power plants, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Enhanced mercury control in coal-fired power plants will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3303456