U.S. patent application number 11/078517 was filed with the patent office on 2006-09-14 for production of activated char using hot gas.
Invention is credited to Lawrence E. III Bool, Jurron L.D. Bradley, Chien-Chung Chao, Mark K. Weise.
Application Number | 20060204429 11/078517 |
Document ID | / |
Family ID | 36971155 |
Filed Date | 2006-09-14 |
United States Patent
Application |
20060204429 |
Kind Code |
A1 |
Bool; Lawrence E. III ; et
al. |
September 14, 2006 |
Production of activated char using hot gas
Abstract
A gas mixture preheated to high temperatures using an oxy-fuel,
an oxygen-enriched air-fuel or an air-fuel burner is used to
devolatilize and partially oxidize carbonaceous feedstock, thereby
producing an active residual char that can be used in applications
utilizing activated carbon. Use of hot gas and ground carbonaceous
feedstock allows the equipment to be minimized, thereby allowing
the activated carbon to be produced at or near points of use, for
example the production of activated char at or near utility boilers
for use in the reduction of mercury emissions from flue gas
streams.
Inventors: |
Bool; Lawrence E. III; (East
Aurora, NY) ; Chao; Chien-Chung; (Williamsville,
NY) ; Weise; Mark K.; (Orchard Park, NY) ;
Bradley; Jurron L.D.; (Buffalo, NY) |
Correspondence
Address: |
PRAXAIR, INC.;LAW DEPARTMENT - M1 557
39 OLD RIDGEBURY ROAD
DANBURY
CT
06810-5113
US
|
Family ID: |
36971155 |
Appl. No.: |
11/078517 |
Filed: |
March 14, 2005 |
Current U.S.
Class: |
423/449.1 |
Current CPC
Class: |
C10J 2300/0959 20130101;
C10J 2300/1846 20130101; C10J 2300/0973 20130101; B01J 20/20
20130101; C01B 32/336 20170801; C10J 2300/093 20130101; C10J 3/485
20130101 |
Class at
Publication: |
423/449.1 |
International
Class: |
C01D 3/00 20060101
C01D003/00 |
Claims
1. A method for producing activated char, the method comprising:
generating at least one gas stream containing an oxidant, the gas
stream having a temperature of at least 800.degree. F.; mixing
carbonaceous feedstock with the at least one gas stream such that
the carbonaceous feedstock undergoes devolatilization and partial
oxidation, thereby producing activated char and byproducts.
2. The method of claim 1, wherein the byproducts comprise carbon
monoxide and hydrogen.
3. The method of claim 2, wherein the byproducts are suitable for
use as fuel.
4. The method of claim 3, wherein the byproducts are used as a
reburning fuel.
5. The method of claim 1, wherein the activated char and the
byproducts are produced at a facility that utilizes the activated
char.
6. The method of claim 1, further comprising quenching the powdered
activated char with a quenching media.
7. The method of claim 6, wherein the quenching media comprises a
fog of water droplets.
8. The method of claim 7, wherein the quenching media includes a
dopant for doping the activated char.
9. The method of claim 8, wherein the dopant is KBr.
10. The method of claim 7, wherein the activated char is further
treated with steam to increase the surface area thereof.
11. The method of claim 6, wherein the quenching media comprises
nitrogen.
12. The method of claim 11, wherein the activated char is further
treated with steam to increase the surface area thereof.
13. The method of claim 1, further comprising adding a secondary
reactive gas prior to mixing the carbonaceous feedstock with the at
least one gas stream.
14. The method of claim 13, wherein the secondary reactive gas
stream comprises steam.
15. The method of claim 1, further comprising adding a solid, a
liquid or a gaseous additive subsequent to devolatilization and
partial oxidation.
16. The method of claim 1, wherein the temperature of the at least
one gas stream is at least 2000.degree. F.
17. The method of claim 1, wherein the activated char is a
powder.
18. The method of claim 1, wherein the carbonaceous feedstock is
selected from coals, petroleum coke, biomass materials, nutshells,
nut hulls and mixtures thereof.
19. The method of claim 18, wherein the carbonaceous material
comprises coal.
20. The method of claim 1, wherein the step of generating the at
least one gas stream comprises mixing an oxidant and a fuel.
21. The method of claim 20, further including mixing steam with the
oxidant and the fuel.
22. The method of claim 20, wherein the oxidant and the fuel are
mixed at or near the stochiometric ratio of the oxidant and the
fuel.
23. The method of claim 20, wherein the oxidant and the fuel are
mixed in a ratio of excess oxidant than the stochiometric ratio of
the oxidant and the fuel.
24. A method for producing activated char, the method comprising:
generating a superheated gas stream, the gas stream having a
temperature of at least 800.degree. F.; mixing carbonaceous
feedstock with the superheated gas stream such that the
carbonaceous feedstock undergoes devolatilization and partial
oxidation, thereby producing activated char and byproducts.
25. The method of claim 24, wherein the superheated gas stream
comprises an inert gas and the carbonaceous feedstock undergoes
devolatilization and pyrolysis in substitution of devolatilization
and partial oxidation.
26. The method of claim 25, wherein the inert gas comprises
nitrogen.
27. The method of claim 1, wherein the byproducts comprise carbon
monoxide and hydrogen.
28. The method of claim 24, wherein the byproducts are suitable for
use as fuel.
29. The method of claim 28, wherein the byproducts are used as a
reburning fuel.
30. The method of claim 24, wherein the activated char and the
byproducts are produced at a facility that utilizes the activated
char.
31. The method of claim 24, further comprising quenching the
powdered activated char with a quenching media.
32. The method of claim 31, wherein the quenching media comprises a
fog of water droplets.
33. The method of claim 32, wherein the quenching media includes a
dopant for doping the activated char.
34. The method of claim 33, wherein the dopant is KBr.
35. The method of claim 31, wherein the activated char is further
treated with steam to increase the surface area thereof.
36. The method of claim 31, wherein the quenching media comprises
nitrogen.
37. The method of claim 36, wherein the activated char is further
treated with steam to increase the surface area thereof.
38. The method of claim 24, further comprising adding a secondary
reactive gas prior to mixing the carbonaceous feedstock with the at
least one gas stream.
39. The method of claim 38, wherein the secondary reactive gas
stream comprises steam.
40. The method of claim 24, further comprising adding a solid, a
liquid or a gaseous additive subsequent to devolatilization and
partial oxidation.
41. The method of claim 24, wherein the temperature of the at least
one gas stream is at least 2000.degree. F.
42. The method of claim 24, wherein the activated char is a
powder.
43. The method of claim 24, wherein the carbonaceous feedstock is
selected from coals, petroleum coke, biomass materials, nutshells,
nut hulls and mixtures thereof.
44. The method of claim 43, wherein the carbonaceous material
comprises coal.
Description
TECHNICAL FIELD
[0001] The present invention relates generally to methods and
systems to produce activated char such that production can occur at
or near the end use point.
BACKGROUND OF THE INVENTION
[0002] Activated carbon is a widely used adsorbent in industrial
processes to remove contaminants from gas or liquid streams. For
example, attempts to meet currently pending mercury emissions
limits for fossil fuel fired power plants by injecting powdered
activated carbon (PAC) into the flue gas upstream of a particulate
control device in order to remove contaminants from the flue gas
are being investigated.
[0003] The removal of mercury from flue gas streams from combustion
processes is of significant interest. The toxicity of mercury to
humans has long been known. An example of the devastating effects
of mercury exposure occurred in Minamata, Japan in the 1950's where
organic mercury byproducts of acetaldehyde production were
discharged into the local bay, and were ingested and metabolized by
fish. By consuming fish in the bay, wide spread neurological damage
and birth defects to the local population were reported.
[0004] Coals used for various combustion processes typically
contain about 0.1 ppm mercury. In the United States alone, about 50
tons of mercury are discharged as vapor in stack gas every year.
Through chemical and biological processes, such mercury can become
concentrated by many thousand-fold into fish, thus entering human
food supplies at harmful levels. In December 2000, the
Environmental Protection Agency (EPA) made its regulatory decision
that mercury emissions from coal-fired electric generating plants
need to be controlled.
[0005] One barrier to the use of adsorbents, however, has been the
high cost of both producing and shipping PAC to the end use point.
PAC is typically produced from carbonaceous starting materials such
as coal, wood, biomass materials, nutshells (e.g., walnut shells,
palm nut) or nut hulls (e.g., coconut) that initially do not have
high adsorptive characteristics. The carbonaceous starting
materials are converted into PAC materials exhibiting higher
adsorptive properties by energy and capital intensive processes
that include pyrolyzing the feedstock in a rotary kiln, activating
the carbon with an activation media (i.e. steam), and grinding or
pulverizing the resulting char. The activated carbon material must
then be shipped to its end use point, such as a coal-fired power
plant.
[0006] Attempts to produce PAC near an end use point have been
made. For instance, the use of a combustion process to produce char
for mercury control has been discussed in the patent literature.
U.S. Pat. No. 6,451,094 B1 to Chang, et al. discuss injecting a
feedstock into a hot flue gas and activating the feedstock in
situ.
[0007] U.S. Pat. No. 6,595,147 to Teller et al. relates to adding a
carbonaceous char to the flue gas while it is still within a
resource recovery unit at a temperature high enough to devolatilize
the material to form activated char in situ.
[0008] Attempts have also been made to use carbon found in fly ash
to capture mercury from flue gas in coal-fired processes. U.S. Pat.
No. 5,787,823 to Knowles proposes a method in which
carbon-containing fly ash is captured in a cyclone upstream of a
conventional particulate control device (PCD). The captured
material is then injected into a duct to capture mercury. U.S. Pat.
No. 6,027,551 to Hwang, et al. teach separation of the carbon from
fly ash captured in a conventional PCD and injection of the
carbon-rich portion as a mercury sorbent.
[0009] U.S. Pat. No. 6,521,021 B1 to Pennline et al. teach removing
partially oxidized coal from the combustion zone of a boiler. The
coal is separated out and injected further downstream as a mercury
sorbent.
[0010] Lanier, et al. (U.S. Pat. No. 6,726,888 B2) suggest
controlling the combustion process such that both the NOx emissions
and the characteristics of the native fly ash for mercury removal
are controlled.
[0011] Given both the constraints of normal boiler operation, and
the fact that the activity of the native residual carbon decreases
as it moves through the boiler, separate production processes for
activated carbon can provide a better opportunity to produce
activated carbon having characteristics favorable for an intended
and particular end use. It would therefore be desirable to provide
systems and methods for onsite production of activated carbon
suitable for a wide range of processes, thereby improving the cost
and efficiency of activated carbon production use.
BRIEF SUMMARY OF THE INVENTION
[0012] The present invention provides methods and systems for
production of activated char that are sufficiently flexible and
efficient such that production can occur near or at the end use
site of the activated char. The methods and systems provided herein
can also be used in other arrangements. For example, the present
invention can be implemented for a central facility to produce
activated char as described herein and to serve multiple utilities
or the like. In another implementation, the present invention can
be used by a utility to make activated char for its own plant and
ship excess activated char to other locations.
[0013] Such methods and systems include preheating a gas mixture to
high temperatures using an oxy-fuel, an oxygen-enriched air-fuel or
an air-fuel burner to form a hot gas stream. The hot gas stream is
mixed and reacted with a carbonaceous feedstock (i.e. carbonaceous
raw material) in a manner such that the carbonaceous feedstock is
devolatilized and partially combusted to thereby produce an active
residual char that can be implemented in a variety of applications
that use activated carbon.
[0014] Use of hot gas and ground carbonaceous feedstock allow the
equipment to be minimized, thus allowing the activated carbon to be
produced at or near the point of use, for example to reduce utility
boiler mercury emissions from flue gas.
[0015] The present invention includes a method to produce activated
char at or near the end use point. A hot (preferably about
2000-3000.degree. F.) oxidizing gas stream mixes and reacts with a
ground or pulverized carbonaceous feedstock to create powdered
activated char with adsorbent properties similar to powdered
activated carbon produced with the same carbonaceous feedstock.
Alternatively, an inert gas could be heated and used to pyrolize
the feedstock. It will be appreciated by those skilled in the art
that such adsorptive properties are dependent on the feedstock
utilized. It will also appreciated by those skilled in the art that
while adsorptive properties of an activated char may be sufficient
for some applications, the activated char may need to be altered
for other applications.
[0016] The present invention thus provides several benefits
including, but not limited to, the ability to produce activated
char at or near the end use point, lower cost and more efficient
production methods for activated char relative to large, rotary
kiln methods and an option to use a carbonaceous feedstock for
producing activated char that may be different from the fuel used
in the main combustion process of a given facility. Consequently,
the present invention provides the flexibility to alter activated
char properties for a specific application at or near the end use
point.
[0017] The surface area of the activated char produced in
accordance with the present invention may be less than, and in some
cases significantly less than, the surface area of currently and
commercially available PACs. Given the efficient and flexible
methods provided herein, however, the overall economics may still
favor use of activated char produced in accordance with the present
invention, even in situations where more activated char may be
necessary relative to currently and commercially available
PACs.
[0018] The hot gas stream, which can include steam, oxygen, or
mixtures of gasses, is produced by preheating a gas stream with an
oxy-fuel, an oxygen-enriched air-fuel or an air-fuel burner to
create a hot and highly reactive gas mixture. The high turbulence
from the hot-gas serves to rapidly mix the carbonaceous feedstock
with the hot-gas. In a preferred embodiment of the invention, the
elevated temperature and the oxygen concentration of the hot gas
cause rapid ignition, devolatilization and partial oxidation of the
carbonaceous feedstock.
[0019] Because both elevated temperature and high oxygen
concentrations have been shown to significantly increase the
devolatilization rate of carbonaceous fuels, the use of hot
oxidizing gas reduces the residence time required to produce the
activated carbon material. Consequently, small reactors can be used
to replace the large rotary kilns typically used to produce
activated carbon. In addition, by-products (such as CO and H.sub.2)
produced in accordance with this process can be used either as fuel
for the main boiler or as a reburning fuel.
[0020] The present invention provides methods and systems for
separating activated char production from the main combustion
process, thereby making it possible to produce activated char
having properties (e.g., adsorptive) desirable for a specific
application. For example and while not to be construed as limiting,
the present invention enables the onsite (or near end use)
production of activated char for a pulverized fuel-fired utility
for the removal of mercury in a flue gas stream. Alternatively or
in addition, the present invention enables the production of
activated char to be tailored for use in a fixed bed arrangement
for waste water treatment to remove hydrocarbons and other
contaminants. Another advantage of the present invention is the
ability to use the partial oxidation gas as a useful fuel in the
process, either by recirculating this material to the hot gas
burner or by firing it into a boiler that may, or may not, be part
of the process. For example, the gas products could be sent to the
boiler as a reburning fuel for NOx control.
[0021] As discussed above, the hot oxygen or hot gas burner as used
in accordance with the present invention allows for on-site
activated char production. The combination of high temperatures and
good mixing achieved with this burner allows activated char
production with relatively small, simple process equipment
(especially as compared to the rotary kilns currently used to
produce PAC). Moreover, activation of the activated char of the
present invention occurs as a result of the process as opposed to
rotary kiln methods which typically require separate activation
steps.
[0022] Accordingly, the present invention can significantly reduce
the cost of activated char for the end user by both improving the
production efficiency and versatility as well as minimizing
shipping requirements.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] For a more complete understanding of the present invention
and the advantages thereof, reference should be made to the
following Detailed Description taken in conjunction with the
accompanying drawings in which:
[0024] FIG. 1 illustrates a schematic view of apparatus to produce
activated carbon in accordance with one embodiment of the present
invention;
[0025] FIG. 2 illustrates a schematic view of apparatus to produce
activated carbon in accordance with an alternative embodiment of
the invention; and
[0026] FIG. 3 illustrates a schematic view of apparatus to produce
activated carbon in accordance with yet another alternative
embodiment of the invention.
[0027] Similar reference characters refer to similar parts
throughout the several views of the drawings.
DETAILED DESCRIPTION
[0028] The present invention provides methods and systems for
production of activated char near or at the end use point of the
activated char. Such methods and systems include preheating a gas
mixture to high temperatures using an oxy-fuel, an oxygen-enriched
air-fuel or an air-fuel burner to form a hot gas stream. The hot
gas stream is mixed and reacted with a carbonaceous feedstock (i.e.
carbonaceous raw material) in a manner such that the carbonaceous
feedstock is devolatilized and partially combusted to thereby
produce an active residual char that can be implemented in
applications that use activated carbon.
[0029] Use of hot gas and ground carbonaceous feedstock allow the
equipment to be minimized, thus allowing the activated carbon to be
produced at or near the point of use, for example to reduce utility
boiler mercury emissions from flue gas.
[0030] The present invention includes a method to produce activated
char at or near the end use point. A hot (preferably,
2000-3000.degree. F.) oxidizing gas stream mixes and reacts with a
ground or pulverized carbonaceous feedstock to create powdered
activated char with adsorbent properties similar to activated char
produced with the same carbonaceous feedstock from typical methods
such as rotary kiln processes. It will be appreciated by those
skilled in the art that such adsorptive properties are dependent on
the feedstock utilized. It will also be appreciated by those
skilled in the art that while adsorptive properties of an activated
char may be sufficient for some applications, the activated char
may need to be altered for other applications.
[0031] The present invention thus provides several benefits
including, but not limited to, the ability to produce activated
char at or near the end use point, lower cost and more efficient
production methods for activated char relative to large, rotary
kiln methods and an option to use a carbonaceous feedstock for
producing activated char that may be different from the coal used
in the main combustion process of a given facility. Consequently,
the present invention provides the flexibility to alter activated
char properties for a specific application at or near the end use
point.
[0032] The hot gas stream, which can include steam, oxygen, or
mixtures of gasses, is produced by preheating a gas stream with an
oxy-fuel, an oxygen-enriched air-fuel or an air-fuel burner to
create a hot and highly reactive gas mixture. The gas mixture is
then rapidly mixed with the carbonaceous feedstock. In a preferred
embodiment of the invention, the elevated temperature and the
oxygen concentration of the hot gas cause rapid ignition,
devolatilization and partial oxidation of the carbonaceous
feedstock.
[0033] Because both elevated temperature and high oxygen
concentrations have been shown to significantly increase the
devolatilization rate of carbonaceous fuels, the use of hot
oxidizing gas reduces the residence time required to produce the
activated carbon material. Consequently, small reactors can be used
to replace the large rotary kilns typically used to produce
activated carbon. In addition, by-products (such as CO and H.sub.2)
produced in accordance with this process can be used either as fuel
for the main boiler or as a reburning fuel.
[0034] The present invention thus provides methods and systems for
separating activated char production from the main combustion
process, thereby making it possible to produce activated char
having properties (e.g., adsorptive) desirable for a specific
application. For example and while not to be construed as limiting,
the present invention enables the production of activated char for
a pulverized fuel-fired utility for the removal of mercury in a
flue gas stream. Alternatively or in addition, the present
invention enables the production of activated char to be tailored
for use in a fixed bed arrangement for waste water treatment to
remove hydrocarbons, water and other contaminants. Another
advantage of the present invention is the ability to use the
partial oxidation gas as a useful fuel in the process, either by
recirculating this material to the hot gas burner or by firing it
into a boiler that may, or may not, be part of the process. For
example, the gas products could be sent to the boiler as a
reburning fuel for NO.sub.x control.
[0035] As discussed above, the hot oxygen or hot gas burner as used
in accordance with the present invention allows for on-site
activated char production. The combination of high temperatures and
good mixing achieved with this burner allows activated char
production with very small, relatively simple process equipment
(especially as compared to the rotary kilns currently used to
produce PAC). Moreover, activation of the activated char of the
present invention occurs as a result of the process as opposed to
rotary kiln methods which typically require separate activation
steps.
[0036] Accordingly, the present invention can significantly reduce
the cost of activated carbon for the end user by both improving the
production efficiency and versatility as well as minimizing
shipping requirements.
[0037] As previously discussed, the present invention can be used
to produce activated char suitable for a wide range of industrial
processes. The optimal configuration for production of activated
char for a particular application therefore depends strongly on the
end use and the desired char characteristics. For purposes of
illustration, ratios of hot gas to carbonaceous feedstock,
residence time, temperature of the hot gas and additives to the
process can be determined based on the intended end use of the
activated char and economic factors.
[0038] Referring now to FIG. 1, a schematic view of apparatus to
produce activated carbon in accordance with one embodiment of the
present invention is shown.
[0039] Burner 1 can be used in a variety of modes, with the burner
design being altered as necessary to account for the mode of
operation. In one preferred embodiment, burner 1 is an oxy-fuel
burner and operates on fuel 10 and oxidant source 11 to produce a
very hot flue gas. In this embodiment, oxidant 11 is pure oxygen.
In another embodiment, burner 1 is used in an oxygen-enriched mode
of operation. More specifically, oxidant 11 has an oxygen
concentration less than pure oxygen, but greater than air (e.g, an
oxygen concentration of between about 21 and less than 100%). This
embodiment may not be as preferred because of the presence of
nitrogen in the air. The N.sub.2 in the hot gas can act as a
diluent for the reaction with the carbonaceous feedstock and lowers
the temperature of the hot gas. In yet another embodiment, burner 1
is operated as an air burner. Oxidant 11 in this embodiment is air.
This third embodiment may also be not as preferred as the oxy-fuel
mode because of the presence of N.sub.2 in the air. The presence of
N.sub.2 in the hot gas can act as a diluent for the reaction with
the carbonaceous feedstock and lowers the temperature of the hot
gas.
[0040] Fuel 10 and oxidant 11 are fed to hot oxygen burner 1.
Exemplary fuels for fuel 10 include, but are not limited to,
natural gas (NG), methane, propane, hydrogen, light oil, LPG, fuel
oil and coke oven gas. Fuel 10 can be liquid, but is preferably a
gas.
[0041] In some embodiments, it may be preferred to introduce
reactant gas 12 into burner 1 where it is mixed and reacted with
oxidant 11 and fuel 10. Reactant gas 12, which can be steam, can be
used primarily to obtain a desirable composition for gas stream 7
for proper reaction with carbonaceous feedstock in reaction vessel
3. Reactant gas 12 is also used to modify the properties of the
activated char produced in reaction vessel 3. For example, reactant
gas 12 can be used to enhance the surface area of the resulting
char produced in reaction chamber 3. It will be appreciated that
reactant gas 12 may not always be necessary.
[0042] In particular embodiments where burner 1 is used in the
oxy-fuel or oxygen-enriched air-fuel modes, burner 1 can be
configured as a hot oxygen burner such as those disclosed in U.S.
Pat. No. 5,266,024 to Anderson, the entire contents of which are
incorporated herein by reference. These hot oxygen burners can
produce a high velocity, hot and highly reactive gas mixture known
as "hot oxygen".
[0043] Regardless of whether burner 1 is operated in an oxy-fuel,
oxygen-enriched or air mode of operation, it is necessary to have a
sufficient amount of oxygen in gas stream 7 to burn and partially
oxidize carbonaceous feedstock 13 to thereby generate activated
char having adsorptive properties for its intended end use. The
oxidation potential of gas stream 7 is such that carbonaceous
feedstock 13 will be partially oxidized and will not be completely
consumed in order to generate the desired activated char. The
amount of oxygen in gas stream 7 is accordingly adjusted based on
the amount of desired reaction of feedstock 13. In order to
generate the proper amount of oxygen in gas stream 7, the amount of
oxidant 11 and/or fuel 10 can then be adjusted. It is important
that too much oxygen not be used in order to avoid too much
consumption of feedstock 13, which would result in poor product
yield.
[0044] If excess oxygen relative to fuel 10 is desirable in stream
7 (for example to enhance the reaction with carbonaceous feedstock
13 when no other streams such as steam are being used), the
stochiometric ratio of oxidant 11 fed to burner 1 will be in vast
excess for fuel 10. In other modes of operation, burner 1 will be
operated between near stoichiometric and in vast oxygen excess of
oxidant 11 to fuel 10.
[0045] Hot oxygen burner 1 produces a high temperature gas stream
7, preferably having a temperature equal to or greater than
800.degree. F. and most preferably greater than 2000.degree. F. The
temperature of hot gas stream 7 is sufficiently high to cause the
desired reaction with feedstock 13 (and any carrying material for
feedstock 13). Gas stream 7 will primarily contain products of
combustion (e.g., CO.sub.2 and H.sub.2O), residual oxygen, any
unreacted gas from gas stream 12 and possibly N.sub.2 if air is
used as part of or all of oxidant 11. In some embodiments, gas
stream 7 may contain greater than 70% by volume residual oxygen
with the balance being products of combustion.
[0046] As further illustrated in FIG. 1, ground or pulverized
carbonaceous feedstock 13 is fed to mixing section 2 and mixed with
hot gas mixture 7. Carbonaceous feedstock 13 can be selected from a
variety of carbonaceous raw materials such as a variety of coals,
petroleum coke, biomass materials (e.g., saw dust) or nutshells
(e.g., walnut shells, palm nut) or nut hulls (e.g., coconut).
Carbonaceous feedstock 13 can be conveyed to mixing section 2 by a
variety of methods.
[0047] Carbonaceous feedstock 13 may be conveyed to mixing section
2 by entrainment in a carrier gas such as air or flue gas,
pneumatically supplied, in a slurry such as a water slurry. It will
be appreciated by those skilled in the art that other methods of
conveying feedstock 13 may also be employed, including supplying
the feedstock by itself, without a carrying or conveying material.
Any oxygen in the conveying stream should be accounted for in the
overall ratio of oxygen to feedstock (i.e., oxygen in the conveying
stream combines with the oxygen in stream 7).
[0048] The velocity of gas stream 7 produced from burner 1 and fed
to mixing section 2 is sufficiently fast such that carbonaceous
feedstock 13 together with any conveying material, regardless of
how feedstock 13 is supplied (e.g. entrained in a carrier gas,
pneumatically supplied, as a slurry) to mixer 2, will be entrained
in gas stream 7.
[0049] Those skilled in the art will appreciate that carbonaceous
feedstock 13 can be selected based on a variety of criteria,
including the end use of the activated carbon produced in
accordance with the present invention. For example, it may be
desirable to use a particular pulverized coal for applications
using activated carbon in a dispersed phase capture mode (e.g.
capture of mercury in a flue gas stream). In contrast, it may be
preferred to use a crushed coal in applications where the activated
carbon is to be used in packed beds (e.g., fixed bed arrangement
for waste water treatment to remove hydrocarbons and other
contaminants from gas or liquid streams).
[0050] It will likewise be appreciated by those skilled in the art
that the residence time in reaction vessel 3 (discussed herein)
will be affected by the selection of feedstock 13. For example, the
residence time for pulverized feedstocks for dispersed phase modes
of adsorption may be on the order of seconds as compared to
residence times for crushed feedstocks, which may be on the order
of minutes. Carbonaceous feedstock 13 is thus ground or pulverized
to a desirable size depending on the end use and the equipment
design.
[0051] In some embodiments, it may be desirable to pretreat
carbonaceous feedstock 13 with a dopant. For example, carbonaceous
feedstock 13 can be pretreated with a halide salt (e.g., KBr) such
that the halide salt is dispersed in carbonaceous feedstock 13
prior to being introduced into mixing section 2. This may be
beneficial in applications where the activated carbon will be used
for mercury capture and removal from flue gas streams. The halide
salt (e.g., KBr) can improve the activated char characteristics in
this type of application. Preferred examples of such treatment can
be found in commonly owned U.S. patent application Ser. No. ______
entitled "Catalytic Adsorbents For Mercury Removal From Flue Gas
and Methods of Manufacture Therefor" to Chien-Chung Chao et al.,
filed on even date herewith, the entire contents of which are
hereby incorporated by reference.
[0052] Combined stream 8 thus contains a mixture of feedstock 13
and hot reactant gas 7. Stream 8 is introduced into reactor vessel
3. Reactor vessel 3 may be a refractory lined pipe with water
cooling as needed. Alternatively, water sprays could be used to
control the temperature in the reactor.
[0053] As also shown in FIG. 1, other additive fluids, solids or
gases 14b may be added into reaction vessel 3. Exemplary additive
14b may include, but is not limited to, steam, N.sub.2, water
and/or material(s) that have a specific activity for an intended
use of the activated char. For instance, stream 14b may be used to
adjust the temperature within reactor 3 and/or provide steam for
the reaction within reactor 3. In the alternative or in addition to
additive 14b, additive fluid, gas or solid (e.g., lime) 14a may
also be mixed with stream 8 prior to injection into reaction vessel
3.
[0054] As previously discussed, the materials in reaction vessel 3
undergo devolatilization and partial oxidation, resulting in a
product stream containing partial oxidation gasses (e.g., CO and
H.sub.2) and activated char. The partial oxidation gases and
activated char exit reactor vessel 3 as stream 9.
[0055] Those skilled in the art will appreciate that the residence
time, ratio of residual oxygen in gas stream 7 to carbonaceous
feedstock, and reaction vessel 3 temperature are controlled based
on carbonaceous feedstock 13 and desired characteristics of the
activated char. For example, if the residence time is too long, or
the ratio of hot oxygen to feedstock is too high, too much of the
feedstock will be consumed. This can result in reduced product
(i.e. activated char) yields. If the residence time or reaction
temperatures are too low, the devolatilization and activation may
be incomplete, thereby reducing product (i.e. activated char)
quality.
[0056] Partial oxidation gases and activated char mixture 9 exiting
reaction vessel 3 are quenched with a quenching media 15 to cool
the products. In some embodiments, it may be desirable to include
additives (such as a halide salt (e.g, KBr) for use of the
activated char in removal of mercury from flue gas streams) in
quench media 15 which are mixed with the activated char. Preferred
examples of such treatment can be found in commonly owned U.S.
patent application Ser. No. ______, entitled "Catalytic Adsorbents
For Mercury Removal From Flue Gas and Methods of Manufacture
Therefor" to Chien-Chung Chao et al., filed on even date herewith,
the entire contents of which are hereby incorporated by reference.
The quenching media could be a fog of water droplets containing the
desired additive, or a gas such as nitrogen.
[0057] Cooled mixture 21 may then be separated in a cyclone (or
other particulate collection device) 4. Cyclone 4 may not always be
necessary, for example in direct injection modes of use (see for
example, FIG. 3).
[0058] A portion or all of partial oxidation gas 16 (e.g., carbon
monoxide and hydrogen) exiting cyclone 4 can then be fed to boiler
5 as a reburning fuel 18a, or returned to the combustion zone of
the boiler as fuel 18b. Alternatively or in addition to using
partial oxidation gas 16 as reburning fuel 18a and/or fuel 18b, a
portion or all of partial oxidation gas 16 may be used in burner 1
as burner fuel 19.
[0059] The activated char produced in accordance with the invention
exits cyclone 4 as stream 17 and is processed for its intended end
use. When the activated char is to be used to capture mercury in a
flue gas stream for example, stream 17 is entrained with a carrier
gas (not shown) and injected into the flue gas at a location where
the temperature is within the desired range for mercury capture.
The activated char is then collected along with the fly ash in a
particulate control device (PCD) 6 (e.g., electrostatic
precipitator or filter fabric) similar to conventional PAC
injection for mercury control. As an alternative, the activated
char could be injected downstream of the PCD so that the carbon
content of the flyash does not destroy the ability to sell flyash
as a component for cement. In other embodiments, activated char in
stream 17 is transported to its intended end use (e.g., a fixed bed
for waste water treatment).
[0060] The embodiment illustrated in FIG. 1 can be altered such
that many of the process steps can be combined into the actual
process equipment. For example and while not to be construed as
limiting, a schematic representation of a laboratory-scale system
to produce activated char in accordance with the present invention
is shown in FIG. 2.
[0061] In this embodiment, burner 1 and reactor 3 are combined
within the process equipment. Experience has shown that
carbonaceous feedstock 13 (coal in this example) ignites while
still in mixing section 2 (not shown in FIG. 2), indicating
ignition is extremely fast. Mixing section 2 is attached to reactor
vessel 3, which is a refractory lined pipe. The design of the
reactor 3 can be adjusted to account for proper residence time
within the reaction zone 3. Additive fluids or gasses 14, such as
water or steam, could be mixed anywhere in this embodiment,
including mixing upstream of the hot oxygen nozzle of burner 1 or
into reaction vessel 3 (as shown in FIG. 2).
[0062] As further shown in FIG. 2, stream 9 may contain CO,
H.sub.2, CO.sub.2 and N.sub.2 (for example, about 40% CO, 20%
H.sub.2, 20% CO.sub.2 and 20% N.sub.2 of the gases on a dry basis
in stream 9) in addition to the char produced in the reaction zone
3.
[0063] Nitrogen 15 is used to quench the products which are sent to
cyclone 4. The use of nitrogen 15 as a quench media could be
replaced with cooling tubes such that the composition of stream 9
is not altered. Alternatively, steam could be used as quench media
15. In this case, the concentration of hydrogen and carbon dioxide
entering the cyclone would be altered from that in stream 9.
[0064] Cyclone 4 can be made from stainless steel. Combustible
gasses can be flared using a natural gas-supported flame. Nitrogen
22 is used as an eductor gas to pump gases out of the cyclone. Gas
24 is thus heavily concentrated in nitrogen. Burner 26 shown in
FIG. 2 is used for safety precautions in order to combust gases 24.
As shown, the gases can be run through a natural gas-oxygen
flare.
[0065] As further shown in FIG. 2, activated char 17 is collected
from cyclone 4 and can be further processed and used as discussed
above with reference to FIG. 1.
[0066] FIG. 1 illustrates the use of hot partial oxidation gases 19
as fuel to burner 1 to replace some, if not all, of fuel 10 to heat
oxidant 11. Another configuration of the present invention is shown
in FIG. 3. In this exemplary embodiment, products 9 from reaction
vessel 3 could be injected directly into flue gas 20 for the
removal of mercury from flue gas 20. It will be appreciated that
stream 9 could be used for direct injection modes other than for
the removal of mercury. As also shown in FIG. 3, stream 9 may be
quenched using stream 15 as discussed hereinabove.
[0067] There is no combustion of the partial oxidation gases in the
embodiment illustrated in FIG. 3. Accordingly, the activated char
and partial oxidation gases formed in reactor 3 are injected
together into flue gas 20. In situations where no cyclone is used,
the point of injection of stream 9 (containing activated char and
partial oxidation gases) is likely to be upstream in flue gas 20
relative to a configuration where a cyclone is used. This is due to
the additional time needed to allow the partial oxidation gases to
completely combust. The additional residence time and temperature
of the flue gas 20 allows the carbon monoxide to burn out (i.e.
completely combust). More specifically, the temperature of the flue
gas at the point of injection in a configuration shown in FIG. 3
may be about 2000.degree. F. as opposed to about 600.degree. F. at
the point of injection in FIG. 1.
EXAMPLE 1
[0068] Several samples of activated char were produced using the
experimental set-up illustrated in FIG. 2. Referring to FIG. 2,
natural gas was used for stream 10, oxygen was used for stream 11
and Powder River Basin (PRB) coal was used for stream 13.
[0069] Listed in Table 1 are the stochiometric ratios (SR) used to
make the activated chars and the resulting properties of the chars.
For comparison purposes, the properties of Darco.RTM. FGD, a powder
activated carbon commercially available from Norit America, Inc.,
is also listed in Table 1.
[0070] SR.sub.HOB was calculated by dividing the amount of oxygen
required to completely combust the natural gas fed in stream 10 by
the amount of oxygen fed in stream 11. SR.sub.Reaction Vessel was
calculated by dividing the amount of oxygen to completely combust
the PRB coal fed in stream 13 by the amount of oxygen entering
reaction vessel 3.
[0071] The carbon content of the activated char was determined by
using a muffle furnace to dry a sample of activated char and then
to ignite the dried activated char sample. The carbon content was
then calculated by dividing the difference between the initial mass
and the final mass of the ignited sample by the initial mass of the
dry activated char.
[0072] BET surface area of the activated char was measured using a
Micrometrics.RTM. ASAP 2000 analyzer. It is noted that the BET
surface area of raw PRB coal is 5 m.sup.2/g.
[0073] The yield was calculated by using the carbon content of the
activated char and by performing a material balance on the ash
content of the PRB coal fed in stream 13. The ash content of the
PRB coal on a wet basis is 4.47 %.
[0074] Mercury removals by the activated chars were evaluated using
Electric Power Research Institute's (EPRI's) Pollution Control
System (PoCT) at We Energies' Pleasant Prairie Power Plant, a 605
MW unit that burns PRB coal. PoCT is a residence chamber used to
simulate injection into the first field of a large scale ESP.
During the mercury removal evaluation, a slipstream located
upstream of Pleasant Prairie's cold side ESP was extracted and then
injected into the PoCT. Residence times of about 2 and 4 seconds
were tested and an activated char injection rate of about 6
lb/MMacf was used. Percent mercury removal was calculated by
subtracting the outlet concentration of mercury from the inlet
concentration of mercury and then dividing the sum by the inlet
mercury concentration and multiplying by 100. Experiment number
three was not evaluated for mercury removal. A more detailed
description of the mercury removal experimental set-up can be found
in Sjostrom, et al., "Assessing Sorbents for Mercury Control in
Coal-Combustion Flue Gas", Journal of the Air and Waste Management
Association, Vol. 52, p. 902-911 (August 2002). TABLE-US-00001
TABLE 1 Yield BET surface area Mercury Removal Mercury Removal
Experiment Number SR.sub.HOB SR.sub.ReactionVessel Carbon Content %
lbchar lbcoal ##EQU1## m 2 g ##EQU2## 2 seconds residence time % 4
seconds residence time % 1 6 0.7 58 0.11 237 41 44 2 6 0.3 82 0.25
225 22 24 3 3 0.3 81 0.24 361 ND ND Darco .RTM.FGD NA NA 72 NA 474
48 56
[0075] As shown in Table 1, as SR.sub.HOB was decreased, while
holding SR.sub.Reaction Vessel constant, the surface area
increased. This is believed to be a result of the increased
temperature and steam amount of the stream leaving section 1 of
FIG. 2. Temperature, the amount of oxidant and the exposure time of
the char to the oxidant are important factors in determining the
surface area. Generally, increasing any of the three factors
increases the surface area.
[0076] Also, when SR.sub.Reaction Vessel was increased, yield and
carbon content decreased, surface area increased and mercury
removal increased. It is believed that the yield and carbon content
decreased because more oxygen was supplied to the reaction vessel
and as a result more carbon was consumed. It is also believed that
the surface area increased as a result of the explanation given
above for increased surface due to increased SR.sub.HOB.
[0077] If the invention is used to produce activated char at a
central facility such as a utility or cement kiln or a facility
where the off-gas can be burned, the partial oxidation gases could
be used in a boiler (or the burner) and the cooled activated char
could be stored for use elsewhere. Alternatively or in addition,
the activated char could be further processed (i.e. post-processed)
to achieve specific desired characteristics (e.g., steam treatments
to increase surface area) or doping of the activated char with a
halide salt such as those disclosed in U.S. patent application Ser.
No. ______, entitled "Catalytic Adsorbents For Mercury Removal From
Flue Gas and Methods of Manufacture Therefor" to Chien-Chung Chao
et al., referenced hereinabove.
[0078] In other embodiments of the invention, a hot gas stream
other than hot oxygen can be created and used to activate the
carbonaceous feedstock material. For example, steam could be
dramatically superheated by mixing the steam with the products of a
near stoichiometric oxy-fuel burner. This superheated steam would
then be used to react with, and activate the carbonaceous
feedstock. It will also be appreciated by those skilled in the art
that if it is desirable to solely pyrolyze (i.e., without
combustion), then other gasses such as nitrogen could also be
superheated in a similar fashion to pyrolyze the coal. In this
particular embodiment, no residual oxygen would be present in
stream 7 (see figures above).
[0079] As discussed above, the present invention provides methods
and systems for production of activated char that are sufficiently
flexible and efficient such that production can occur near or at
the end use site of the activated char. It will be appreciated,
however, that production of the activated chars in accordance with
the present invention is not limited to onsite production. The
methods and systems provided herein can also be used in other
arrangements. For example and while not to be construed as
limiting, activated char produced by the present invention could be
produced at a central facility and used to serve multiple utilities
or the like. Another exemplary implementation could include
producing activated char at a utility for that plant and shipping
excess activated char to other locations. It will also be
appreciated that the activated chars produced in accordance with
the present invention can be used to replace PAC production used
for other applications. The present invention thus provides
versatility and flexibility with respect to the site of
production.
[0080] It should be appreciated by those skilled in the art that
the specific embodiments disclosed above may be readily utilized as
a basis for modifying or designing other structures for carrying
out the same purposes of the present invention. It should also be
realized by those skilled in the art that such equivalent
constructions do not depart from the spirit and scope of the
invention as set forth in the appended claims.
* * * * *