U.S. patent application number 10/757331 was filed with the patent office on 2004-10-14 for coal gasification with alkali additives to reduce emissions of mercury to the atmosphere.
Invention is credited to Ashworth, Robert A..
Application Number | 20040202594 10/757331 |
Document ID | / |
Family ID | 33134936 |
Filed Date | 2004-10-14 |
United States Patent
Application |
20040202594 |
Kind Code |
A1 |
Ashworth, Robert A. |
October 14, 2004 |
Coal gasification with alkali additives to reduce emissions of
mercury to the atmosphere
Abstract
Method for removing mercury emissions from the burning of coal
or other carbonaceous fuels, such as in a power plant or from coal
gasification. Alkali additives are introduced in the coal
gasification and staged coal combustion processes to capture the
mercury in an alkaline molten slag. The combustor is operated at a
stoichiometric air or oxygen to fuel ratio of about 0.40 to 0.80
and a temperature range of about 2200.degree.-3000.degree. F.
During the staged combustion process the molten slag containing
combinations of alkali and mercury is removed and disposed of to
minimize or prevent mercury from escaping in the flue gas.
Inventors: |
Ashworth, Robert A.;
(Wooster, OH) |
Correspondence
Address: |
BUCHANAN INGERSOLL, P.C.
ONE OXFORD CENTRE, 301 GRANT STREET
20TH FLOOR
PITTSBURGH
PA
15219
US
|
Family ID: |
33134936 |
Appl. No.: |
10/757331 |
Filed: |
January 14, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60441005 |
Jan 17, 2003 |
|
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Current U.S.
Class: |
423/99 |
Current CPC
Class: |
B01D 53/64 20130101;
Y02P 10/20 20151101; C22B 7/02 20130101; Y02P 10/212 20151101; C22B
43/00 20130101 |
Class at
Publication: |
423/099 |
International
Class: |
C22B 043/00 |
Claims
What is claimed is:
1. A method for the removal of mercury of carbonaceous fuel
comprising a) introducing any carbonaceous fuel; coal, coke,
bio-mass or combinations thereof containing mercury into a first
stage partial oxidation (gasifier) unit operating at a
stoichiometric air or oxygen air to fuel ratio of 0.40 to 0 0.80,
to provide a reducing operating condition for high levels of
mercury capture in an alkaline molten fuel ash slag under reducing
conditions with carbon, carbon monoxide and hydrogen as the
reducing agents for a partial oxidation (gasifier) temperature
range of 2200.degree. F. to 3000.degree. F.; b) introducing an
alkali or any alkali or combinations thereof from the class
consisting of lime, limestone, dolomite, calcium chloride,
nacholite, and trona, with the said fuel or via a separate stream
into the first stage oxidation unit, the alkali acting as a flux to
reduce molten carbonaceous fuel ash viscosity and to react with the
mercury species being liberated from said fuel; c) fuel gas and
molten slag being separated in a first stage cyclonic device
following the fuel gas-slag mix section and said molten slag
containing combinations of alkalis and mercury compounds being
removed to a water quench system and disposed of.
2. An apparatus for removing mercury during combustion of a
carbonaceous fuel as shown in FIG. 1, and operated according to the
parameters shown in FIG. 2.
Description
[0001] This Application claims priority from Provisional Patent
Serial No. 60/441,005 filed Jan. 17, 2003.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] This invention relates to a method for the reduction of
mercury emissions from coal gasification processes. More
particularly, it refers to an improved method for removal of
mercury through the use of alkali additives in coal gasification
and staged coal combustion processes.
[0004] 2. Description of the Prior Art
[0005] The 1990 Clean Air Act Amendments identified 189 substances
that were designated as hazardous air pollutants (air toxins).
These substances are chemicals, including heavy metals and organic
compounds in both solid and gaseous forms, known to pose a risk to
human health. One metallic element, mercury, is getting much
attention due to its quantity and toxicity.
[0006] Mercury emissions to the air and releases to water occur
naturally and by human activities. According to a fairly recent
emissions inventory (1994-1995), in the United States the major
emitters of mercury to the atmosphere were electric utilities,
municipal waste combustors, commercial and industrial boilers,
medical waste incinerators, and chlor-alkali plants. Until the
middle of the decade, municipal waste combustors, hazardous waste
combustors, and medical waste incinerators were the leading
emitting source category. The United States Environmental
Protection Agency (hereinafter "EPA") now regulates these
industries, and the EPA estimates that emissions from municipal
waste combustors and medical waste incinerators declined by 90%
from 1990 to 2000. This currently makes coal-fired utilities the
leading man-made source of air-borne mercury emissions in the U.S.
Of the estimated 5,000 tons of global mercury emissions emitted to
the atmosphere in 1994-95, U.S. coal-fired power plants contributed
about 51 tons, or 1%. This rate of mercury emissions represented
33% of the 158 tons of mercury released in the U.S. for the same
time period.
[0007] There are several methods for removing elemental mercury and
its compounds from combustion/incineration flue gas. Elemental
mercury removal is somewhat difficult because mercury remains in
the vapor phase at very low temperatures ( boiling point at
674.degree. F.) and does not condense on ash particles in the flue
gas stream so that it may be removed with electrostatic
precipitators. However, removal of mercury from combustion flue gas
(U.S. Pat. No. 4,889,698 and U.S. Pat. No. 5,672,323)) using
activated carbon adsorption is known in the prior art. There are
also other methods of removal; they include the use of oxidizing
agents that convert elemental mercury to its soluble compound forms
(U.S. Pat. No.5,900,042) so that it may be scrubbed from the flue
gas. Another method, U.S. Pat. No. 6,214,304, uses alkali sulfides
to convert elemental mercury to mercury sulfide that is removed by
particulate control devices. Another method uses alkali injection
into the boiler furnace (U.S. Pat. No. 6,372,187); it has been
shown to be somewhat effective in reducing mercury emissions.
However, these methods, if highly effective (90% removal) like
carbon adsorption are very expensive techniques (as high as
$100,000/lb of removal). The oxidizing method (U.S. Pat. No.
5,900,042) and the alkali furnace injection method (U.S. Pat. No.
6,372,187) although less expensive, only remove 50 to 55% of the
mercury.
[0008] It would therefore be advantageous to have an improved
mercury capture technique that will reduce coal mercury emissions
to the atmosphere and do so at a relatively low cost. The method of
the present invention is inexpensive and is as effective if not
more effective than the carbon adsorption method.
SUMMARY OF THE INVENTION
[0009] I have discovered a process employing a staged combustor to
remove mercury in an alkaline molten slag. High levels of mercury
capture were found to be an inherent feature of a staged combustor
(see U.S. Pat. Nos. 4,395,975, 4,423,702 and 5,458,659) developed
for the reduction of sulfur and nitrogen oxides to the atmosphere.
Alkali compounds, such as limestone, lime, hydrated lime, dolomite,
trona, nacholite or combinations thereof are added with the coal
being fired in the first stage of the combustor, or are added
separately into the first stage of combustion operating at 2400 to
2700.degree. F. The first stage of combustion, in effect, is a coal
gasifier operating at an air to fuel stoichiometric ratio of around
0.60. Sulfur and high levels of mercury capture are achieved
through capture in the alkaline molten slag produced from the
partial oxidation of any carbonaceous fuel, including coal, by
incorporating a combustor design that yields a reducing condition
in the alkaline molten slag sulfur capture zone. Nitrogen oxide
emissions are also reduced by firing the coal in a
substoichiometric air condition in the first stage that reduces
NO.sub.x production from the oxidation of fuel bound nitrogen.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] A more complete understanding of the invention can be
obtained by considering the following detailed description in
conjunction with the accompanying drawings, wherein;
[0011] FIG. 1 shows a schematic of a staged combustion system
applied to a coal-fired boiler; and
[0012] FIG. 2 shows the thermo chemical equilibrium for calcium and
magnesium oxide reactions with carbon to form elemental calcium and
magnesium.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0013] It is believed that the reaction mechanism for mercury
capture in a molten slag bath gasifier involves the formation and
capture of amalgams in complex mineral composites. Mercury will
form amalgams with many alkali metals, alkaline earth metals, zinc,
cadmium (Ca), arsenic, antimony, gold, silver and copper. Other
metals like molybdenum, manganese, cobalt and particularly iron are
nearly insoluble in mercury. It is believed that the high melting
point alkaline earth metals Ca (melting point of 2192.degree. F.)
and Magnesium (Mg) (melting point of 2030.degree. F.) that are
combined with their oxide forms CaO (melting point of 4658.degree.
F.) and MgO (melting point of 5072.degree. F.) are the alkaline
earth metals that are forming amalgams with mercury. Under reducing
conditions with carbon as the reducing agent for a gasifier
temperature range of 2400 to 2700.degree. F., both elemental
calcium and magnesium can form; see the thermo chemical equilibrium
coefficients for these reactions in FIG. 2. Although the
equilibrium coefficients are low, still there would be orders of
magnitude more concentration of elemental calcium and magnesium to
react with all of the mercury in the coal. Since the coal is fired
into the alkaline molten slag bath with enough force to swirl the
slag, there should be plenty of carbon formed to create some
quantity of elemental calcium and magnesium. Carbon monoxide will
also react with the oxides of calcium and magnesium to form
elemental calcium and magnesium but the reactions are not quite as
favored as the reactions with carbon.
[0014] To achieve high mercury capture, the combustor is designed
to provide for 1) intimate mixing of the carbonaceous fuel and its
reactants with the reduced alkaline molten slag, and 2) intimate
fuel/air mixing, done in such a way as to eliminate the formation
of localized pockets of unreacted oxygen. By keeping the molten
slag in a hot reducing condition (2200 to 2700.degree. F.), carbon
and carbon monoxide react with certain metals to convert a portion
of those metals to their elemental form that will then combine with
mercury to form an amalgam; for example:
CaO.sub.solid+C.sub.solid.fwdarw.Ca.sub.solid+CO.sub.gas
CaO.sub.solid+CO.sub.gas.fwdarw.Ca.sub.solid+CO.sub.2 gas
[0015] Mercury (Hg) is easily converted from its oxide and sulfide
(Cinnabar) forms to elemental mercury:
HgO.sub.solid+C.sub.solid.fwdarw.Hg.sub.vapor+CO.sub.gas
HgO.sub.solid+CO.sub.gas.fwdarw.Hg.sub.vapor+CO.sub.2 gas
HgS.sub.solid+H.sub.2 gas.fwdarw.Hg.sub.vapor+H.sub.2S.sub.gas
[0016] For example, the elemental calcium then will react with
elemental mercury to produce an amalgam that is tied up in a
complex mineral composite.
Ca.sub.solid+Hg.sub.vapor.fwdarw.Ca.sub.oHg.sub.o solid amalgam
[0017] The conclusion that amalgam formation is probably the cause
of the nearly quantitative capture of mercury in the alkaline
molten slag comes from the work done by Sir Humphrey Davy. In the
early 1800's, Davy attempted to decompose a mixture of lime and
mercuric oxide by an electric current and an amalgam of calcium was
obtained. The separation of the mercury from the calcium was then
so difficult that Davy was not sure if he had obtained pure
metallic calcium. Electrolysis of lime and calcium chloride in
contact with mercury gave the same results.
[0018] Laboratory analysis for a three-stage combustor
demonstration, wherein the first stage was operating at an air to
fuel stoichiometric ratio that ranged from 0.58 of 0.77, firing an
Illinois #5 coal with 3.39 wt % sulfur and with limestone being
added at a Ca/S ratio of 0.85, showed the following results, see
Table 1.
1TABLE 1 Mercury Capture Rate, Hg, Capture, Material: lb/hr ppmw
Hg, lb/hr % Input: Coal 1669.7 0.089 0.00014860 Limestone (as
CaO/MgO) 96.5 0.030 0.00000289 Total 1766.2 0.00015149 Output: Slag
38.9 2.60 0.00010110 66.7 Fly Ash 156.5 0.26 0.00004069 26.9 Total
195.4 0.00014179 93.6
[0019] Although a stack test was not completed for mercury
emissions from the staged combustion system, from the weight rates
and analyses of the different streams, mercury capture in primarily
the first stage (gasifier) molten slag exceeded 90%. Even more
impressive is that when leaching procedure tests were completed on
the first stage (gasifier) slag and the fly ash removed from the
flue gas baghouse, there was no leaching of mercury. Both samples
of leachate yielded 0.0000 mg/l of mercury.
[0020] Mercury analyses were also completed on the ash from a
coal-fired chain grate stoker at the same facility, firing the same
Illinois #5 coal. The mercury in the fly ash was 0.079 ppmw and the
mercury in the grate bottom ash was 0.01 ppmw. This shows that
mercury capture using a stoker is very low compared to the staged
combustion system. This also indicates that for mercury capture to
occur, a reducing condition must exist and limestone or some other
alkali must be added. Data taken from a slagging cyclone boiler
operation, firing Illinois coal wherein alkalis were not added that
was operating under an overall oxidizing condition showed that
about 8% of the mercury was captured in the bottom slag.
[0021] A typical example of the process of the present invention,
preferably using the CAIRE.TM. staged combustor (U.S. Pat. Nos.
4,423,702 and 5,458,659), is shown schematically in FIG. 1. Certain
variations from this schematic could be made with such variations
still being within the context of this invention. It will be
understood by those skilled in the art that certain variations from
this schematic could be made with such variations still being
within the context of the present invention. In the embodiment
shown in FIG. 1, a first stage combustor 10 is located in front of
the entries 12 into the furnace 13. Openings 5 into each of the
combustors receive a conventional fuel such as pulverized coal 2,
and an alkaline product such as lime or limestone 3 with the
carrier primary air 1 and the preheated air or oxygen 4.
Alternatively, a coal water slurry pump could be used to convey
pulverized coal to the combustor. Controlled partial oxidation of
the coal takes place in the combustor by regulation of the
preheated (400.degree. to 700.degree. F.) secondary air or oxygen
flow 4. The air (oxygen) to fuel stochiometric ratio (SR) in first
stage combustor 10 is maintained at about 0.40 to 0.70 (SR.sub.1)
through control of the preheated air or oxygen flow 4, and most for
air preferably at about 0.60. With the first stage combustor 10,
the products of partial combustion in the form of a fuel gas and
the molten slag from the ash portion of the coal plus the inorganic
alkali compounds are separated in the first stage partial oxidation
chamber 10, and a molten slag eutectic 7 containing alkali
compounds and coal ash exit through the bottom opening 8 of the
first stage combustor 10. The molten slag is quenched in a water
quench sluice system 9 and the ash is sluiced to a collection tank
from where it is pumped to a settling pond, or otherwise disposed
of according to conventional known methods.
[0022] The staged combustor 10 has a partial oxidation zone where
mixing at a temperature of about 2200.degree. to 3000.degree. F.
provides intimate contact between the coal and air or oxygen.
Through the use of a staged combustor 10 that has incorporated
molten slag removal, a high percentage (75-90%) of the molten slag
produced during partial oxidation of the coal is removed from the
gas prior to entry into the furnace 14, and prior to further
partial oxidation at entry 12.
[0023] Although certain embodiments of the invention have been
described in detail, it will be appreciated by those skilled in the
art that various modifications and alterations would be developed
in light of the overall teaching of the disclosure. Accordingly,
the particular embodiments and arrangements disclosed herein are
intended to be illustrative only and not limiting as to the scope
of the invention which should be awarded the full breadth of the
following claims and in any and all equivalents thereof.
* * * * *