Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture – Nitrogen or nitrogenous component
Reexamination Certificate
2000-09-14
2002-04-23
Griffin, Steven P. (Department: 1754)
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
Nitrogen or nitrogenous component
C423S24000R, C423S210000, C423S099000, C423S481000, C423S491000, C075S710000, C075S711000
Reexamination Certificate
active
06375909
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to removal of both mercury (Hg) and nitrogen oxides (NO
x
) from exhaust gas generating from combustion of carbonaceous materials and apparatus for effecting such removal.
BACKGROUND OF THE INVENTION
Among the 189 substances listed as hazardous air pollutants in the Clean Air Act Amendments (CAAA) of 1990, mercury is a metal species of great concern due to its extreme toxicity and the risk that it can cause to humans and animals if released to the environment. In several countries, legislation is being prepared to limit the emission of mercury to the atmosphere. While most of the trace metals can be efficiently removed in today's air pollution control system, mercury is present mainly in its vapor phase and is difficult and expensive to remove.
In the US, the EPA maximum achievable control technology (MACT) will limit mercury emissions to 40-110 &mgr;g/dscm at 7%O
2
for hazardous waste incinerator. For municipal sewage sludge combustor, federal regulation (40CFR Part 61, Subpart E) limits mercury emissions to 3,200 grams per 24 hours.
Nitrogen oxides (NO
x
) are also an environmental problem because they can initiate reactions resulting in the production of ozone and acid rain. These pollutants can harm forests and lakes, damage buildings and cause health problems. Guidelines for controlling NO
x
emissions are provided in the 1990 CAAA under the “Nitrogen Oxides Emission Reduction Program” and “Ozone Non-Attainment Program”. For municipal sewage sludge combustors, NO
x
is not regulated at the Federal level yet. However, as a result of the 1990 CAAA, Federal regulation on NO
x
is anticipated. Consequently, NO
x
emission from municipal sewage sludge combustors can be limited to the MACT standard, which is no more than the average emissions achieved by the best performing 12% of all operating incinerators. Most state authorities already regulate NO
x
emissions for their municipal sewage sludge combustors and have very stringent limits.
Of the different technologies available for reducing NO
x
and Hg, most require additional equipment and the use of expensive and/or hazardous chemicals. Therefore, it would be advantageous to develop a method for removing these compounds utilizing only the standard pollution control equipment and cost efficient non-hazardous chemicals, thereby meeting strict emission limits economically.
A number of different methods have been proposed to reduce mercury and/or NO
x
emissions from combustor exhaust gas. However, the majority of these processes are more sophisticated due to either the extension of additional equipment or the hazardous nature of the additives. Very few of those methods propose simultaneous reduction of both mercury and nitrogen oxides.
Mercury Removal:
The chemical form of Hg in the gas to be treated is of considerable interest. Ionic mercury is removed with control processes that employ various aqueous scrubbing techniques. Elemental mercury, however, is essentially unaffected by wet scrubbers and requires some type of sorbent or carbon injection process.
Mercury typically can be removed from the combustor exhaust gas in two ways, (1) adsorption via sorbent injection into the exhaust gas or via flow through fixed sorbent bed at low temperature upstream of a particulate matter collector, and (2) wet scrubbing with conversion of the elemental mercury into a more soluble species that can be easily absorbed in a scrubber.
WO 9,517,240 describes a method for improving mercury removal capability of a flue gas purification system by introducing sulfur vapors into the flue gas stream where admixed flue gases and sulfur vapors contact solid particulate (calcium hydroxide) materials in the flue gas. Calcium hydroxide adsorbs mercury and sulfur vapors and catalyzes reactions forming solid products comprising mercury. The solid products comprising mercury are separated, thereby forming a purified flue gas stream. The solid particulate materials are formed in situ by reaction in a spray dryer between an aqueous dispersion of calcium hydroxide and the acidic materials in the flue gas at a temperature between 70 and 170° C.
U.S. Pat. No. 4,889,698 discloses a process in which powdery activated carbon is injected immediately before, during or after an alkali reagent (limestone or sodium carbonate) spray dryer for simultaneous removal of acid gases and trace contaminants such as mercury. The process requires cooling the flue gas by spray drying in the presence of large amounts of alkali sorbent material together with the activated carbon to enhance overall mercury removal.
U.S. Pat. No. 5,695,726 discloses a process in which toxic mercury vapor is removed from combustion gas by contact with dry alkaline material and dry activated carbon in a reaction chamber followed by solids separation. The adsorptive capacity of activated carbon decreases with increasing gas temperature. US '726 emphasizes that a minimum level of HCl is necessary and a low temperature of the reaction chamber of from about 175° C. to about 235° C. are important for achieving high removal of the mercury. HCl is needed in the gas phase to react with elemental mercury or mercury oxide to convert them to chlorides. US '726 also teaches that in the combustion of the wastes that are chlorine-deficient, an HCl-generating material such as scrap polyvinyl chloride plastic can be added to the chlorine-deficient waste prior to incineration. However, it is well known that adding chlorine to the waste stream and at the same time lowering the flue gas to the temperature range of 200° C.-350° C. are the two most favorable conditions for the synthesis or reformation of dioxins and furans.
U.S. Pat. No. 5,900,042 describes a process to remove elemental mercury from a gas stream by reacting the gas stream with an oxidizing solution to convert the elemental mercury to soluble mercury compounds. The gas stream is then passed through a wet scrubber to remove the mercuric compounds and oxidized constituents. The oxidizing solutions are solutions of aqueous iodine, aqueous bromine, aqueous chlorine, aqueous chloric acid and alkali metal chlorate and others.
U.S. Pat. No. 5,607,496 discloses a removal process, in which the elemental mercury of the combustion gas is first catalytically oxidized to form a mercury compound, and then the mercury compound is either adsorbed on adsorbent particles such as alumina or removed from the gas stream by scrubbing. The catalysts include mostly oxides of existing heavy metals in the combustion gas such as manganese, vanadium, lead, chromium, iron, cobalt, nickel and selenium.
UK Patent No. 1,336,084 discloses a process in which mercury vapour in the flue gas is removed by scrubbing the flue gas with a solution of alkaline earth metal hypochlorite containing an alkali metal chloride or alkaline earth metal chloride in excess of the chemical equivalent of the alkaline earth metal hypochlorite at a pH in the range of 8 to 12.
Nitrogen Oxides Removal:
Nitrogen oxides can be removed from combustor exhaust gas by selective catalytic reduction (SCR), selective non catalytic reduction (SNCR), and wet flue gas denitrification.
U.S. Pat. No. 4,220,632 discloses a process in which ammonia is used to reduce nitrogen oxides in combustion exhaust gas in the presence of a catalyst by SCR. High performance can be achieved with this technique, but it requires injection of ammonia into the exhaust gas prior to entering the SCR reactor. Sometimes it is necessary to first pass through a wet removal process to eliminate dust and poisonous chemicals that hinder the SCR process, then reheat the gas for the SCR. This method requires space due to the extent of the treatment equipment and generates a potential hazardous spent catalyst. Therefore, both capital and operating costs are high.
U.S. Pat. No. 3,900,554 describes a process called selective non-catalytic reduction (SNCR) in which ammonia is used to reduce nitrogen oxide from combustion effluents. Application of the technique is limited, because excessive unreacted ammo
Dangtran Ky
Holst Troy C.
Griffin Steven P.
Infilco Degremont Inc.
Johnson Edward M.
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