U.S. patent application number 11/658986 was filed with the patent office on 2009-11-12 for solid ammonia storage and delivery material.
Invention is credited to Claus Hviid Christensen, Tue Johannessen, Jens Kehlet, Ulrich Quaade, Rasmus Zink.
Application Number | 20090280047 11/658986 |
Document ID | / |
Family ID | 35531460 |
Filed Date | 2009-11-12 |
United States Patent
Application |
20090280047 |
Kind Code |
A1 |
Christensen; Claus Hviid ;
et al. |
November 12, 2009 |
Solid ammonia storage and delivery material
Abstract
A solid ammonia storage and delivery material A solid ammonia
storage material comprising: an ammonia absorbing salt, wherein the
ammonia absorbing salt is an ionic salt of the general formula:
M.sub.a(NH.sub.3).sub.nX.sub.z, wherein M is one or more cations
selected from alkaline earth metals, and/or one or more transition
metals, such as Mn, Fe, Co, Ni, Cu, and/or Zn, X is one or more
anions, a is the number of cations per salt molecule, z is the
number of anions per salt molecule, and ri is the coordination
number of 2 to 12, wherein M is Mg provides a safe, light-weight
and cheap compact storage for ammonia to be used in the automotive
industry.
Inventors: |
Christensen; Claus Hviid;
(Lynge, DK) ; Johannessen; Tue; (Glostrup, DK)
; Quaade; Ulrich; (Bagsvaerd, DK) ; Kehlet;
Jens; (Naerum, DK) ; Zink; Rasmus; (Vedbaek,
DK) |
Correspondence
Address: |
EDWARDS ANGELL PALMER & DODGE LLP
P.O. BOX 55874
BOSTON
MA
02205
US
|
Family ID: |
35531460 |
Appl. No.: |
11/658986 |
Filed: |
August 3, 2005 |
PCT Filed: |
August 3, 2005 |
PCT NO: |
PCT/DK05/00516 |
371 Date: |
October 17, 2008 |
Current U.S.
Class: |
423/239.1 ;
252/193; 422/174 |
Current CPC
Class: |
F01N 2610/02 20130101;
Y02T 10/12 20130101; C01G 53/12 20130101; C01F 5/00 20130101; F17C
11/00 20130101; C01C 1/006 20130101; C01P 2006/10 20130101; F01N
2610/10 20130101; C01G 49/00 20130101; C01G 9/00 20130101; F01N
2610/06 20130101; Y02T 10/24 20130101; B01D 53/9431 20130101; B01J
20/28011 20130101; B01D 53/565 20130101; B01J 20/3035 20130101;
B01D 53/90 20130101; C01G 3/14 20130101; B01J 20/046 20130101; C01G
51/12 20130101; F01N 3/2066 20130101; C01G 45/00 20130101 |
Class at
Publication: |
423/239.1 ;
252/193; 422/174 |
International
Class: |
B01D 53/56 20060101
B01D053/56; C09K 3/00 20060101 C09K003/00 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 3, 2004 |
PA |
2004-01179 |
Feb 3, 2005 |
PA |
2005-00166 |
Jun 25, 2005 |
PA |
2005-00926 |
Claims
1. A solid ammonia storage and delivery material capable of
absorption/desorption of ammonia and comprising: an ammonia
absorbing salt, wherein the ammonia absorbing salt is an ionic salt
of the general formula: M.sub.a(NH.sub.3).sub.nX.sub.z, wherein M
is one or more cations selected from alkaline earth metals, and/or
one or more transition metals, such as Mn, Fe, Co, Ni, Cu, and/or
Zn, X is one or more anions, a is the number of cations per salt
molecule, z is the number of anions per salt molecule, and n is the
coordination number of 2 to 12, wherein M is Mg, and wherein the
ammonia storage and delivery material has been saturated with
ammonia and compressed to a density of 0.9 to 1.3 g/cm.sup.3 and a
bulk density above 70% of the skeleton density.
2. A solid ammonia storage material as claimed in claim 1 wherein
the anion is selected from fluoride, chloride, bromide, iodide,
nitrate, thiocyanate, sulphate, molybdate, and phosphate ions.
3. A solid ammonia storage material as claimed in claim 2 wherein
the anion is the chloride.
4. A solid ammonia storage material as claimed in claim 3 wherein
the salt is Mg(NH.sub.3).sub.6Cl.sub.2.
5. A solid ammonia storage material as claimed in claim 1, wherein
the solid storage material has a density of 0.9 to 1.3
g/cm.sup.3.
6. A solid ammonia storage material as claimed in claim 1, wherein
the solid storage material has a density of 1.1 to 1.3
g/cm.sup.3.
7. A solid ammonia storage material as claimed in claim 1, wherein
the solid storage material has bulk density above 85% of the
skeleton density.
8. A method of producing a solid ammonia storage material capable
of absorption/desorption of ammonia and comprising: an ammonia
absorbing salt, wherein the ammonia absorbing salt is an ionic salt
of the general formula: M.sub.a(NH.sub.3).sub.nX.sub.z, wherein M
is one or more cations selected from alkaline earth metals, and/or
one or more transition metals, such as Mn, Fe, Co, Ni, Cu, and/or
Zn, X is one or more anions, a is the number of cations per salt
molecule, z is the number of anions per salt molecule, and n is the
coordination number of 2 to 12, said method comprising the steps of
1) providing the solid salt, 2) saturating the salt fully with
ammonia, and 3) compressing the saturated ammonia salt complex to a
bulk density above 70% of the skeleton density.
9. A method as claimed in claim 8, wherein the ammonia salt complex
is compressed to a bulk density above 80% of the skeleton
density.
10. A method as claimed in claim 9, wherein the ammonia salt
complex is compressed to a bulk density above 85% of the skeleton
density.
11. A method for selective catalytic NO.sub.x reduction in waste
gases containing oxygen, using ammonia and a reduction catalyst
wherein gaseous ammonia is provided by heating a solid storage
medium capable of absorption/desorption of ammonia and comprising
one or more ionic ammonia absorbing salts of the general formula:
M.sub.a(NH.sub.3).sub.nX.sub.z, wherein M is one or more cations
selected from alkaline earth metals, and transition metals such as
Mn, Fe, Co, Ni, Cu, and/or Zn, X is one or more anions such as
chloride or sulphate ions, a is the number of cations per salt
molecule, z is the number of anions per salt molecule, and n is the
coordination number of 2 to 12, wherein M is Mg, wherein the
ammonia storage and delivery material has been saturated with
ammonia and compressed to a bulk density above 70% of the skeleton
density and wherein the release rate of ammonia is controlled in
consideration of the content of NO.sub.x in the waste gases.
12. A method for selective catalatic reduction (SCR) of NO.sub.x in
exhaust gases from combustion processes comprising contacting an
exhaust gase with a solid ammonia storage material capable of
absorption/desorption of ammonia and comprising: an ammonia
absorbing salt, wherein the ammonia absorbing salt is an ionic salt
of the general formula: M.sub.a(NH.sub.3).sub.nX.sub.z, wherein M
is one or more cations selected from alkaline earth metals, and
transition metals such as Mn, Fe, Co, Ni, Cu, and/or Zn, X is one
or more anions such as chloride or sulphate ions, a is the number
of cations per salt molecule, z is the number of anions per salt
molecule, and n is the coordination number of 2 to 12, wherein M is
Mg and wherein the ammonia storage and delivery material has been
saturated with ammonia and compressed to a bulk density above 70%
of the skeleton density as a source of ammonia as the reducing
agent in selective catalytic reduction (SCR) of NO.sub.x in exhaust
gases from combustion processes.
13. An ammonia delivery device comprising a container comprising an
ammonia absorbing salt capable of absorption/desorption of ammonia,
wherein the ammonia absorbing salt is an ionic salt of the general
formula: M.sub.a(NH.sub.3).sub.nX.sub.z, wherein M is one or more
cations selected from alkaline earth metals, and/or one or more
transition metals, such as Mn, Fe, Co, Ni, Cu, and/or Zn, X is one
or more anions, a is the number of cations per salt molecule, z is
the number of anions per salt molecule, and n is the coordination
number of 2 to 12, wherein M is Mg and wherein the ammonia storage
and delivery material has been saturated with ammonia and
compressed to a bulk density above 70% of the skeleton density,
said container being provided with one or more closable outlet
opening(s) connected to a pipe and further being provided with
means for heating the container and the ammonia absorbing salt for
release of gaseous ammonia.
14. An ammonia delivery device as claimed in claim 13, wherein the
closure of the closable outlet opening(s) is (are) in the form of
one or more valve(s).
15. An ammonia delivery device as claimed in claim 13, wherein the
heating means is in the form of an electrical resistive heating
device.
16. An ammonia delivery device as claimed in claim 13, wherein said
means for heating is provided by a heat produced by chemical
reactions.
17. An ammonia delivery device as claimed in claim 13, wherein the
release rate of ammonia is controlled by accurate control of the
heating of the container and the ammonia absorbing salt for release
of gaseous ammonia.
18. An ammonia delivery device as claimed in claim 13, where the
release of ammonia is further controlled by reduction valves, flow
controllers or similar.
19. A method for selective catalytic reduction (SCR) of NO.sub.x in
exhaust gases from combustion processes comprising contacting an
exhaust gase with an ammonia delivery device comprising a container
comprising an ammonia absorbing salt capable of
absorption/desorption of ammonia, wherein the ammonia absorbing
salt is an ionic salt of the general formula:
M.sub.a(NH.sub.3).sub.nX.sub.z, wherein M is one or more cations
selected from alkaline earth metals, and/or one or more transition
metals, such as Mn, Fe, Co, Ni, Cu, and/or Zn, X is one or more
anions, a is the number of cations per salt molecule, z is the
number of anions per salt molecule, and n is the coordination
number of 2 to 12, wherein M is Mg and wherein the ammonia storage
and delivery material has been saturated with ammonia and
compressed to a bulk density above 70% of the skeleton density,
said container being provided with one or more closable outlet
opening(s) connected to a pipe and further being provided with
means for heating the container and the ammonia absorbing salt for
release of gaseous ammonia as a source for ammonia in selective
catalytic reduction of NO.sub.x in exhaust gases from combustion
processes.
20. A solid ammonia storage and delivery material capable of
absorption/desorption of ammonia and comprising: an ammonia
absorbing salt, wherein the ammonia absorbing salt is an ionic salt
of the general formula: M.sub.a(NH.sub.3).sub.nX.sub.z, wherein M
is one or more cations selected from alkali metals, alkaline earth
metals, and transition metals such as Li, Na, K, Cs, Mg, Ca, V, Cr,
Mn, Fe, Co, Ni, Cu, or Zn or combinations thereof such as NaAl,
KAl, K.sub.2Zn, CsCu, or K.sub.2Fe, X is one or more anions
selected from fluoride, chloride, bromide, iodide, nitrate,
thiocyanate, sulphate, molybdate, and phosphate ions, a is the
number of cations per salt molecule, z is the number of anions per
salt molecule, and n is the coordination number of 2 to 12, said
storage and delivery material having been saturated with ammonia
and compressed to a bulk density above 70% of the skeleton density
and a density of 0.9 to 1.3 g/cm.sup.3.
21. A solid ammonia storage material as claimed in claim 20,
wherein the solid storage material has a density of 1.1 to 1.3
g/cm.sup.3.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to the storage of ammonia in a
solid form using metal ammine complexes and delivery there from.
The invention also relates to the use of ammonia stored in a solid
form as the reducing agent in selective catalytic reduction (SCR)
of NO.sub.x in exhaust gases from combustion processes, to methods
for producing such complexes and to an ammonia delivery device
comprising such complexes.
[0003] 2. Description of the Related Art
[0004] Current environmental regulations necessitate the use of
catalysts in the exhaust gas from automotive vehicles, boilers and
furnaces for control of NO.sub.x emission leaving the system.
Particularly, vehicles equipped with diesel engines or other lean
burn engines offer the benefit of improved fuel economy, but suffer
from the drawback of increased formation of NO.sub.x being noxious
and which must be eliminated from the exhaust gas. However,
catalytic reduction of NO.sub.x using conventional so-called
three-way exhaust catalysts for automobiles is impossible because
of the high content of oxygen in the exhaust gas. Instead,
selective catalytic reduction (SCR) has proven useful for achieving
the required low levels of NO.sub.x in the exhaust gas both in
stationary and mobile units. In such systems NO.sub.x is
continuously removed from the exhaust gas by injection of a
reducing agent into the exhaust gas prior to entering an SCR
catalyst capable of achieving a high conversion of NO.sub.x. So
far, ammonia has proven to be the most efficient reducing agent,
which is usually introduced into the exhaust gas by controlled
injection of gaseous ammonia, aqueous ammonia or aqueous urea. In
all cases, the amount of reducing agent being dosed has to be very
precisely controlled. Injection of a too large amount of reducing
agent will cause emission of ammonia with the exhaust gas whereas
injection of a too small amount of reducing agent causes a less
than optimum conversion of NO.sub.x.
[0005] In many mobile units powered by combustion engines, the
preferred technical solution is to use an aqueous solution of urea
as the reducing agent since in this way potential hazards or safety
issues relating to the transport and handling of liquid ammonia in
high pressure containers are eliminated. However, there are several
disadvantages related to the use of aqueous urea as the reducing
agent. First of all, the use of urea solutions requires the
carrying of a relatively large volume in order to provide
sufficient amounts of ammonia to allow a vehicle to drive e.g.
about 20,000 kilometres without having to substitute or refill the
source of ammonia. In typical systems, an aqueous solution
comprising about 30 wt % of urea is preferred meaning that about 70
wt % of the content of a container holding the urea solution is
used only to transport water. During the decomposition, one
molecule of urea forms two molecules of NH.sub.3 and one molecule
of CO.sub.2 and thus, ammonia only constitutes roughly 50 wt % of
the weight of the urea molecule and hence, the concentration of
ammonia of the reducing agent is very low. Similar concentrations
of ammonia can be achieved using aqueous solutions of ammonia as
reducing agents. Furthermore, when using solutions, specially
designed spray nozzles combined with a precision liquid pump are
required to ensure that a) the aqueous urea is delivered to the
exhaust system at a desired (and dynamically changing) flow rate
and b) the aqueous urea is efficiently dispersed in the gas phase
before entering the catalyst. Finally, the aqueous solutions might
freeze in cold weather conditions (below minus 11.degree. C.), or
the urea solution may simply form precipitates, which will block
the dosing system, e.g. the nozzle. Altogether, these difficulties
may limit the possibilities of using SCR technology in abatement of
pollution from NO.sub.x, particularly in connection with mobile
units.
[0006] Transporting ammonia as a pressurized liquid is hazardous as
the container may burst or a valve or tube might break in an
accident giving a discharge of poisonous/lethal gaseous ammonia. In
the case of the use of a solid storage medium containing
absorbed/adsorbed ammonia, the safety issues are much less critical
since a small amount of heat is required to release the ammonia
from the storage medium.
[0007] International Patent Publication No. WO 99/01205 discloses a
method and a device for selective catalytic NOx reduction in waste
gases containing oxygen, using ammonia and a reduction catalyst.
According to the method, gaseous ammonia is provided by heating a
solid storage medium comprising one or more salts, especially a
chloride and/or sulphate of one or more cations selected from
alkaline earth metals, and/or one or more transition metals,
preferably Mn, Fe, Co, Ni, Cu, and/or Zn, said storage medium being
introduced into a container. In the preferred embodiments of WO
99/01205 the cation is Ca.sup.2+ or Sr.sup.2+. The inventive method
and device are stated to be particularly suitable for use in
automobiles.
[0008] However, the use of the ammonia storage media known from WO
99/01205 suffers from various draw-backs hampering a wide-spread
use in the automotive industry. The vapour pressure of ammonia
above a solid salt phase is e.g. about 1 bar at room temperature
and atmospheric pressure for calcium octa ammine chloride and
strontium octa ammine chloride complexes rendering the use somewhat
complicated due to the high pressures that must be taken into
account in view of the toxicity of ammonia. Having such a high
partial pressure of ammonia, the handling and transportation of the
saturated storage material is still difficult and also dangerous.
Still further, the use of granulated storage materials--stated as
preferred embodiments in WO 99/01205--requires measures for
preventing the storage material from leaving the container during
use in moving vehicles. Furthermore, a granulated material will
have a considerable void between the granules which will
drastically reduce the bulk density of the storage material by a
factor 1.5-2.
[0009] It has now been found that these drawbacks may be overcome
using a magnesium salt complex according to the present invention
providing a compact, light-weight, cheap and more safe storage for
ammonia having a very low vapour pressure of ammonia below 0.1 bar
at room temperature to be used in the automotive industry.
SUMMARY OF THE INVENTION
[0010] The present invention relates to a solid ammonia storage and
delivery material comprising:
an ammonia absorbing salt, wherein the ammonia absorbing salt is an
ionic salt of the general formula:
M.sub.a(NH.sub.3).sub.nX.sub.z,
wherein M is one or more cations selected from alkaline earth
metals, and/or one or more transition metals, such as Mn, Fe, Co,
Ni, Cu, and/or Zn, X is one or more anions, a is the number of
cations per salt molecule, z is the number of anions per salt
molecule, and n is the coordination number of 2 to 12, wherein M is
Mg.
[0011] In a second aspect the invention relates to a method of
producing a solid ammonia storage and delivery material
comprising:
an ammonia absorbing salt, wherein the ammonia absorbing salt is an
ionic salt of the general formula:
M.sub.a(NH.sub.3).sub.nX.sub.z,
wherein M is one or more cations selected from alkaline earth
metals, and/or one or more transition metals, such as Mn, Fe, Co,
Ni, Cu, and/or Zn, X is one or more anions, a is the number of
cations per salt molecule, z is the number of anions per salt
molecule, and n is the coordination number of 2 to 12, said method
comprising the steps of 1) providing the solid salt, 2) saturating
the salt with ammonia, and 3) compressing the ammonia salt
complex.
[0012] In a third aspect the invention relates to a method for
selective catalytic NO.sub.x reduction in waste gases containing
oxygen, using ammonia and a reduction catalyst wherein gaseous
ammonia is provided by heating a solid storage medium comprising
one or more ionic ammonia absorbing salts of the general
formula:
M.sub.a(NH.sub.3).sub.nX.sub.z,
wherein M is one or more cations selected from alkaline earth
metals, and transition metals such as Mn, Fe, Co, Ni, Cu, and/or
Zn, X is one or more anions such as chloride or sulphate ions, a is
the number of cations per salt molecule, z is the number of anions
per salt molecule, and n is the coordination number of 2 to 12,
wherein M is Mg and wherein the release rate of ammonia is
controlled in consideration of the content of NO.sub.x in the waste
gases.
[0013] In a fourth aspect the invention relates to the use of a
solid ammonia storage and delivery material comprising:
an ammonia absorbing salt, wherein the ammonia absorbing salt is an
ionic salt of the general formula:
M.sub.a(NH.sub.3).sub.nX.sub.z,
wherein M is one or more cations selected from alkaline earth
metals, and transition metals such as Mn, Fe, Co, Ni, Cu, and/or
Zn, X is one or more anions such as chloride or sulphate ions, a is
the number of cations per salt molecule, z is the number of anions
per salt molecule, and n is the coordination number of 2 to 12,
wherein M is Mg as a source of ammonia as the reducing agent in
selective catalytic reduction (SCR) of NO.sub.x in exhaust gases
from combustion processes.
[0014] In a fifth aspect the invention relates to an ammonia
delivery device comprising a container comprising
an ammonia absorbing salt, wherein the ammonia absorbing salt is an
ionic salt of the general formula:
M.sub.a(NH.sub.3).sub.nX.sub.z,
wherein M is one or more cations selected from alkaline earth
metals, and/or one or more transition metals, such as Mn, Fe, Co,
Ni, Cu, and/or Zn, X is one or more anions, a is the number of
cations per salt molecule, z is the number of anions per salt
molecule, and n is the coordination number of 2 to 12, wherein M is
Mg, said container being provided with one or more closable outlet
opening(s) connected to a pipe and further being provided with
means for heating the container and the ammonia absorbing salt for
release of gaseous ammonia.
[0015] In a sixth aspect the invention relates to the use of an
ammonia delivery device comprising a container comprising
an ammonia absorbing salt, wherein the ammonia absorbing salt is an
ionic salt of the general formula:
M.sub.a(NH.sub.3).sub.nX.sub.z,
wherein M is one or more cations selected from alkaline earth
metals, and/or one or more transition metals, such as Mn, Fe, Co,
Ni, Cu, and/or Zn, X is one or more anions, a is the number of
cations per salt molecule, z is the number of anions per salt
molecule, and n is the coordination number of 2 to 12, wherein M is
Mg, said container being provided with one or one or more closable
outlet opening(s) connected to a pipe and further being provided
with means for heating the container and the ammonia absorbing salt
for release of gaseous ammonia as a source for ammonia in selective
catalytic reduction of NO.sub.x in exhaust gases from combustion
processes.
[0016] In a seventh aspect the invention relates to a solid ammonia
storage and delivery material comprising:
an ammonia absorbing salt, wherein the ammonia absorbing salt is an
ionic salt of the general formula:
M.sub.a(NH.sub.3).sub.nX.sub.z,
wherein M is one or more cations selected from alkali metals,
alkaline earth metals, and transition metals such as Li, Na, K, Cs,
Mg, Ca, V, Cr, Mn, Fe, Co, Ni, Cu, or Zn or combinations thereof
such as NaAl, KAI, K.sub.2Zn, CsCu, or K.sub.2Fe, X is one or more
anions selected from fluoride, chloride, bromide, iodide, nitrate,
thiocyanate, sulphate, molybdate, and phosphate ions, a is the
number of cations per salt molecule, z is the number of anions per
salt molecule, and n is the coordination number of 2 to 12, said
storage and delivery material having a density of 0.9 to 1.3
g/cm.sup.3.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] The invention is disclosed more in detail with reference to
the drawings in which
[0018] FIG. 1 shows a photograph of an ammonia delivery device of
the invention,
[0019] FIG. 2 shows schematically a top view of the location of
thermocouples in the device of FIG. 1,
[0020] FIG. 3 shows a diagram of the experimental setup,
[0021] FIG. 4 shows a graphical representation of the desorption of
the ammonia,
[0022] FIG. 5 is a plot of the buffer pressure during desorption of
the ammonia,
[0023] FIG. 6 shows the pressure and flow in a single pressure
oscillation period during desorption,
[0024] FIG. 7 schematically shows a device for compression of an
ammonia delivery material,
[0025] FIG. 8 schematically shows an embodiment of an ammonia
delivery device of the invention,
[0026] FIG. 9 schematically shows another embodiment of an ammonia
delivery device of the invention,
[0027] and FIG. 10 shows an equilibrium phase-diagram for the
MgCl.sub.2--NH.sub.3 system.
DETAILED DESCRIPTION OF THE PRESENT INVENTION
[0028] The present invention is related to the use of metal-ammine
salts as safe and efficient solid storage media for storage and
controlled delivery of ammonia, which in turn is used as the
reduction agent in selective catalytic. reduction to reduce
NO.sub.x emissions, especially from automotive vehicles, boilers
and furnaces.
[0029] The present invention relates to a solid ammonia storage and
delivery material comprising:
an ammonia absorbing salt, wherein the ammonia absorbing salt is an
ionic salt of the general formula:
M.sub.a(NH.sub.3).sub.nX.sub.z,
wherein M is one or more cations selected from alkaline earth
metals, and/or one or more transition metals, such as Mn, Fe, Co,
Ni, Cu, and/or Zn, X is one or more anions, a is the number of
cations per salt molecule, z is the number of anions per salt
molecule, and n is the coordination number of 2 to 12, wherein M is
Mg.
[0030] Suitable anions to be used according to the present
invention may be selected from fluoride, chloride, bromide, iodide,
nitrate, thiocyanate, sulphate, molybdate, and phosphate ions. The
anion is preferably the chloride.
[0031] A preferred solid ammonia delivery material according to the
invention is Mg(NH.sub.3).sub.6Cl.sub.2.
[0032] It has been found that although Mg(NH.sub.3).sub.6Cl.sub.2
has a very low partial pressure of ammonia above a salt phase,
below 0.1 bar at room temperature, it is very suitable for use as a
source of ammonia in SCR technology for abatement of pollution from
NO.sub.x.
[0033] A solid ammonia storage and delivery material
comprising:
an ammonia absorbing salt, wherein the ammonia absorbing salt is an
ionic salt of the general formula:
M.sub.a(NH.sub.3).sub.nX.sub.z,
wherein M is one or more cations selected from alkaline earth
metals, and/or one or more transition metals, such as Mn, Fe, Co,
Ni, Cu, and/or Zn, X is one or more anions selected from chloride
and sulphate ions, a is the number of cations per salt molecule, z
is the number of anions per salt molecule, and n is the
coordination number of 2 to 12, and the use thereof are also
considered aspects of the present invention.
[0034] An ammonia saturated material of the invention may be
prepared by exposing the dry salt to gaseous ammonia in a manner
known per se. The ammonia saturated delivery material as prepared
is powdery and rather "fluffy" and difficult to handle or transport
and may be--during transport or use--transformed into small
particle fragments thereby potentially blocking the dosing system
of a device or give rise to hazardous dust into the environment.
Furthermore, the powder has a low bulk density.
[0035] In a preferred embodiment of the invention the solid
delivery material has a density of 0.9 to 1.3 g/cm.sup.3, more
preferred a density of 1.1 to 1.3 g/cm.sup.3 giving a very
high-density storage and delivery material.
[0036] The compacted material shows a very low release rate of
ammonia at room temperature and atmospheric pressure and may be
handled without special precautionary measures for protection
against the action of ammonia. The compacted material can easily be
handled during transport and during and after the final
application.
[0037] It has surprisingly been found that a powdered ammonia
delivery material of the present invention having a very low vapour
pressure of ammonia at room temperature may be compacted to a very
high density using several different methods for shaping of the
material into a desired form and still be capable of delivery of
ammonia at a sufficient rate to be suitable for use as a source of
ammonia for a SCR reduction of NOx in e.g. automotive vehicles,
boilers and furnaces. Such methods are e.g. pressing, extrusion,
and injection moulding. In the case of pressing, a pressure might
be applied in several different ways in a manner known per se. In
one embodiment, the material is compressed to shapes like dense
blocks or tablets by placing the saturated salt in a
groove/dent/hole/pit in a metal block (e.g. in a cylindrical hole)
and applying pressure to compress the material using a
corresponding piston.
[0038] The saturated material is preferably compressed to a bulk
density above 70%, more preferred above 80% and even more preferred
above 85%, of the skeleton density. In a preferred embodiment the
present invention is related to the compaction and shaping of the
saturated ammonia delivery material.
[0039] When an ammine complex of a salt is compressed to such a
high extent, desorption from such a compacted material would be
expected to be extremely slow, mostly due to diffusion hindrance.
In most dense materials desorption of ammonia would involve solid
state diffusion which is known to be a slow process for virtually
all materials. This has surprisingly been found not to be the case
for the dense materials according to the present invention. It has
been found that when ammonia desorbs, a progressing nano-porous
structure is formed as the "reaction front" proceeds, leaving open
paths for additional ammonia to leave from the central parts of the
body of storage material. This is in contrast to e.g. classical
heterogeneous catalysis where conversion of reactants is only
possible, if reactants are able to diffuse into the catalyst pore
structure and the products are able to diffuse out of the catalyst
pore structure.
[0040] The metal-ammine salts constitute a solid storage medium for
ammonia, which represent a safe, practical and compact option for
storage and transportation of ammonia (a single-crystalline
compound of Mg(NH.sub.3).sub.6Cl.sub.2 has an ammonia density of 38
kmole NH.sub.3/m.sup.3, whereas that of liquid ammonia is only
slightly higher (40 kmole NH.sub.3/m.sup.3)). Ammonia may be
released from the metal ammine salt by heating the salt to
temperatures in the range from 10.degree. C., to the melting point
of the metal salt ammine complex, preferably to a temperature from
100 to 700.degree. C., more preferred to a temperature from 150 to
500.degree. C.
[0041] During release of ammonia the metal-ammine salt of the
formula M.sub.a(NH.sub.3).sub.nX.sub.z wherein M, X, a, n, and z
has the meaning stated above, is gradually transformed into a salt
of the formula M.sub.a(NH.sub.3).sub.mX.sub.z wherein
0.ltoreq.m<n. When the desired amount of ammonia has been
released, the resulting salt of formula
M.sub.a(NH.sub.3).sub.mX.sub.z can usually be converted back into
the salt of the formula M.sub.a(NH.sub.3).sub.nX.sub.z by treatment
with a gas containing ammonia.
[0042] Anhydrous MgCl.sub.2 absorbs up to six moles of NH.sub.3
(Gmelins Handbuch, 1939; Liu, 2004) according to reactions 1 to
3:
MgCl.sub.2(s)+NH.sub.3(g)Mg(NH.sub.3)Cl.sub.2(s) (1)
Mg(NH.sub.3)Cl.sub.2(s)+NH.sub.3(g)Mg(NH.sub.3).sub.2Cl.sub.2(s)
(2)
Mg(NH.sub.3).sub.2Cl.sub.2(s)+4NH.sub.3(g)Mg(NH.sub.3).sub.6Cl.sub.2(s)
(3)
[0043] Typical ammonia contents of the metal ammine complexes are
in the range of 20-60 wt %, and preferred complexes comprise above
30 wt % ammonia, more preferred above 40 wt % ammonia. The
inexpensive compound Mg(NH.sub.3).sub.6Cl.sub.2 contains 51.7 wt %
ammonia.
[0044] The present invention offers ammonia storage at
significantly higher densities (both on a volume and a weight
basis) than both aqueous ammonia and aqueous urea. For several
metal ammine salts it is possible to release all ammonia and then
transform the resulting material back into the original metal
ammine salt in a large number of cycles. Additionally, the ammonia
is directly delivered into the exhaust pipe as a gas, which is an
advantage in itself--both for the simplicity of the flow control
system and for an efficient mixing of reducing agent, ammonia, in
the exhaust gas--but it also eliminates possible difficulties
related to blocking of the dosing system because of precipitation
in the liquid-based system.
[0045] Some metal ammine complexes offer a further advantage in
that the vapour pressure of ammonia above a solid salt phase is
relatively low. It is preferred that the vapour pressure is below
0.1 bar at room temperature, preferably below 0.01 bar.
Specifically for Mg(NH.sub.3).sub.6Cl.sub.2 the vapour pressure is
as low as 0.002 bar at room temperature and atmospheric pressure
which in practice eliminates any noxious effect of the ammonia as
the release of ammonia is as low or lower than the release from
common cleaning materials containing ammonia.
[0046] For Mg(NH.sub.3).sub.6Cl.sub.2 the partial pressure of
ammonia at room temperature is 0.002 bar. Even though a partial
pressure of ammonia of 0.002 bar at ambient temperature in itself
could cause health problems, the compacted material according to
the invention saturated with ammonia releases ammonia at a very
slow rate at ambient temperature and an equilibrium pressure of
0.002 bar will only be obtained after a considerable span of time,
even if the material is placed in a very confined space. However,
when raising the temperature e.g. in the delivery device, a quite
quick desorption of ammonia is observed as discussed above.
[0047] For mobile units, it is particularly useful to use an
ammonia delivery device comprising a container containing the metal
ammine complex as such a container may easily be separated from
mobile unit and replaced by a fresh at suitable intervals. In
preferred embodiments, the metal ammine containers are recycled and
recharged with ammonia in a separate recharging unit.
[0048] Due to the slow release of ammonia in open atmosphere at
ambient temperatures for the compressed materials of the present
invention, the handling of the materials does not require extensive
safety precautions and substitution of exhausted storage and
delivery material with fresh material does not require an
encapsulation of the material facilitating the handling as compared
to the handling of the materials used in the methods of the state
of the art.
[0049] In another aspect the invention relates to a method of
producing a solid ammonia storage and delivery material comprising
an ammonia absorbing salt, wherein the ammonia absorbing salt is an
ionic salt of the general formula:
M.sub.a(NH.sub.3).sub.nX.sub.z,
wherein M is one or more cations selected from alkaline earth
metals, and/or one or more transition metals, such as Mn, Fe, Co,
Ni, Cu, and/or Zn, X is one or more anions, a is the number of
cations per salt molecule, z is the number of anions per salt
molecule, and n is the coordination number of 2 to 12, said method
comprising the steps of 1) providing the solid salt, 2) saturating
the salt with ammonia, and 3) compressing the ammonia salt complex
to a dense, shaped body.
[0050] In a preferred embodiment of the method of the invention,
the ammonia salt complex is compressed to a density of 0.9 to 1.3
g/cm.sup.3, more preferred to a density of 1.1 to 1.3
g/cm.sup.3
[0051] In a further aspect the invention relates to a method of
selective catalytic NO.sub.x reduction in waste gases containing
oxygen, using ammonia and a reduction catalyst wherein gaseous
ammonia is provided by heating a solid storage and delivery medium
comprising one or more ionic ammonia absorbing salts of the general
formula:
M.sub.a(NH.sub.3).sub.nX.sub.z,
wherein M is one or more cations selected from alkaline earth
metals, and transition metals such as Mn, Fe, Co, Ni, Cu, and/or
Zn, X is one or more anions, a is the number of cations per salt
molecule, z is the number of anions per salt molecule, and n is the
coordination number of 2 to 12, wherein M is Mg and wherein the
release rate of ammonia is controlled in consideration of the
content of NO.sub.x in the waste gases.
[0052] In a yet further aspect the invention relates to the use of
a solid ammonia storage and delivery material comprising:
an ammonia absorbing salt, wherein the ammonia absorbing salt is an
ionic salt of the general formula:
M.sub.a(NH.sub.3).sub.nX.sub.z,
wherein M is one or more cations selected from alkaline earth
metals, and transition metals such as Mn, Fe, Co, Ni, Cu, and/or
Zn, X is one or more anions, a is the number of cations per salt
molecule, z is the number of anions per salt molecule, and n is the
coordination number of 2 to 12, wherein M is Mg as a source of
ammonia as the reducing agent in selective catalytic reduction
(SCR) of NO.sub.x in exhaust gases from combustion processes.
[0053] In yet another aspect the invention relates to an ammonia
delivery device comprising a container comprising
an ammonia absorbing salt, wherein the ammonia absorbing salt is an
ionic salt of the general formula:
M.sub.a(NH.sub.3).sub.nX.sub.z,
wherein M is one or more cations selected from alkaline earth
metals, and/or one or more transition metals, such as Mn, Fe, Co,
Ni, Cu, and/or Zn, X is one or more anions, a is the number of
cations per salt molecule, z is the number of anions per salt
molecule, and n is the coordination number of 2 to 12, wherein M is
Mg, said container being provided with one or more closable outlet
opening(s) connected to a pipe and further being ing provided with
means for heating the container and the ammonia absorbing salt for
release of gaseous ammonia.
[0054] In the ammonia delivery device according to the invention,
the closure of the closable outlet opening(s) may be in the form of
one or more valve(s) known per se for use in connection with
ammonia.
[0055] Heating means may be in the form of an electrical resistive
heating device known per se.
[0056] The heating means may alternatively be provided as heat
produced by chemical reactions.
[0057] The salt is normally heated to temperatures in the range
from 10.degree. C. to the melting point of the metal salt ammine
complex, preferably to a temperature from 100 to 700.degree. C.,
more preferred to a temperature from 150 to 500.degree. C.
[0058] In a preferred embodiment of the invention the release rate
of ammonia is controlled by accurate control of the heating of the
container and the ammonia absorbing salt for release of gaseous
ammonia. The release of ammonia is preferably further controlled by
reduction valves, flow controllers or similar equipment or units.
The release of ammonia from a container is preferably controlled by
interaction with an electronic engine control system for delivery
of an optimum amount of ammonia in a specific ratio (e.g.
NH.sub.3:NOx=1:1) of the changing emission of NOx from the
engine.
[0059] In still another aspect the invention relates to the use of
an ammonia delivery device comprising a container comprising
an ammonia absorbing salt, wherein the ammonia absorbing salt is an
ionic salt of the general formula:
M.sub.a(NH.sub.3).sub.nX.sub.z,
wherein M is one or more cations selected from alkaline earth
metals, and/or one or more transition metals, such as Mn, Fe, Co,
Ni, Cu, and/or Zn, X is one or more anions, a is the number of
cations per salt molecule, z is the number of anions per salt
molecule, and n is the coordination number of 2 to 12, wherein M is
Mg, said container being provided with one or more closable outlet
opening(s) connected to a pipe and further being provided with
means for heating the container and the ammonia absorbing salt for
release of gaseous ammonia as a source for ammonia in selective
catalytic reduction of NO.sub.x in exhaust gases from combustion
processes.
[0060] In yet a further aspect, the invention relates to a solid
ammonia storage and delivery material comprising:
an ammonia absorbing salt, wherein the ammonia absorbing salt is an
ionic salt of the general formula:
M.sub.a(NH.sub.3).sub.nX.sub.z,
wherein M is one or more cations selected from alkali metals,
alkaline earth metals, and transition metals such as Li, Na, K, Cs,
Mg, Ca, V, Cr, Mn, Fe, Co, Ni, Cu, or Zn or combinations thereof
such as NaAl, KAl, K.sub.2Zn, CsCu, or K.sub.2Fe, X is one or more
anions selected from fluoride, chloride, bromide, iodide, nitrate,
thiocyanate, sulphate, molybdate, and phosphate ions, a is the
number of cations per salt molecule, z is the number of anions per
salt molecule, and n is the coordination number of 2 to 12, said
storage and delivery material having a density of 0.9 to 1.3
g/cm.sup.3.
[0061] Still further, the invention relates to a method of
producing a solid ammonia storage and delivery material comprising
an ammonia absorbing salt, wherein the ammonia absorbing salt is an
ionic salt of the general formula:
M.sub.a(NH.sub.3).sub.nX.sub.z,
wherein M is one or more cations selected from alkali metals,
alkaline earth metals, and transition metals such as Li, Na, K, Cs,
Mg, Ca, V, Cr, Mn, Fe, Co, Ni, Cu, or Zn or combinations thereof
such as NaAl, KAl, K.sub.2Zn, CsCu, or K.sub.2Fe, X is one or more
anions selected from fluoride, chloride, bromide, iodide, nitrate,
thiocyanate, sulphate, molybdate, and phosphate ions, a is the
number of cations per salt molecule, z is the number of anions per
salt molecule, and n is the coordination number of 2 to 12, said
storage and delivery material having a density of 0.9 to 1.3
g/cm.sup.3, said method comprising the steps of 1) providing the
solid salt, 2) saturating the salt with ammonia, and 3) compressing
the ammonia salt complex to a density of 0.9 to 1.3 g/cm.sup.3.
[0062] The present invention is especially suitable for use in
reduction of emission of NO.sub.x in exhaust gases from stationary
and mobile combustion engines or power plants fuelled by diesel
oil, petrol, natural gas or any other fossil fuels.
[0063] Thus, the present invention is also especially suitable as a
source for providing ammonia in selective catalytic reduction in
exhaust gasses for reduction of emission from stationary and mobile
combustion engines or power plants fuelled by methanol, ethanol,
hydrogen, methane, ethane or any other synthetic fuels.
[0064] Mobile combustion engines for which the invention is
suitable are may e.g. be automobiles, trucks, trains, ships or any
other motorised vehicle.
[0065] The invention is particularly suitable for use in connection
with reduction of NO.sub.x in combustion gases from automobiles and
trucks.
[0066] Stationary power plants for which the invention is suitable
are preferably power plants generating electricity.
[0067] The present invention is also especially suitable for use in
reduction of emission of NO.sub.x in exhaust gases from solid oxide
fuel cells (SOFC). SOFC's are operated at a high temperature, where
there may be a small production of NO.sub.x, which can be removed
by adding ammonia using the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0068] The invention is now explained more in detail with reference
to the drawings showing preferred embodiments of the invention.
Materials and Methods
[0069] MgCl.sub.2 powder: anhydrous, >98% purity, Merck
Schuchardt.
[0070] Glove-bag: Aldrich premium AtmosBag from Aldrich Chemical
Company, Inc., 1001 West Saint Paul Ave., Milwaukee, Wis. 53233,
USA.
[0071] Ammonia Gas: Ammonia Gas 99.9% from Hede Nielsen,
Industriparken 27-31, 2750 Ballerup, Denmark.
Preparation of Ammonia Saturated MgCl.sub.2 Powder.
[0072] The ammonia carrier, Mg(NH.sub.3).sub.6Cl.sub.2, was
prepared by placing MgCl.sub.2 powder for several days in a
glove-bag containing ammonia gas at atmospheric pressure (1 bar)
and at room temperature. The degree of saturation was checked by
temperature programmed desorption (TPD) and verified to be near
100% of the theoretical amount. The absorption/desorption was found
to be fully reversible.
[0073] The rate of absorption is dramatically increased at higher
NH.sub.3 pressures (minutes rather than days) (Touzain and
Moundamga-lniamy, 1994).
[0074] An equilibrium phase-diagram of the MgCl.sub.2--NH.sub.3
system is shown in FIG. 10 also showing the process of saturation
when carried out at 5 bars.
[0075] Starting from point B' (pure MgCl.sub.2) the pressure was
increased to 5 bars at point A corresponding to an isothermal
absorption at T=298.15K and resulting in the formation of fully
saturated Mg(NH.sub.3).sub.6Cl.sub.2. The rate of formation depends
on the equilibrium pressure drop; however absorption is generally
quite fast at ammonia pressures of more than 4 bars (Touzain and
Moundamga-Iniamy, 1994). As mentioned, the desorption reaction only
proceeds at elevated temperatures, which is also indicated on the
path from A to B (isobaric desorption at P=5 bar).
Example 1
[0076] An ammonia delivery device was made from stainless steel in
the form of a cylindrical container, having the dimensions 2R0=H=10
cm, where R0 is the inner radius and H is the height of the
reactor. A photograph of the device is shown in FIG. 1. The device
was provided with five wells for insertion of thermocouples placed
perpendicular to the tangent of the reactor circumference for
determining the radial temperature distribution. Furthermore, a
thermocouple was placed on the outer wall to measure the actual
temperature of the heated steel wall. Ammonia-saturated MgCl.sub.2
powder (258.8 grams) was then placed and compacted slightly by
manual pressure to a bed density of approximately 331 kg/m.sup.3. A
thin sheet of quartz cotton was placed on top of the salt to
prevent any grains from being carried out of the reactor. As
desorption of ammonia from the complex requires elevated
temperature, a heating wire was wrapped around the reactor and
thermal insulation (Rockwool) was placed on top of this. Using a PC
with labview interface, the power to the heating tape surrounding
the storage container was turned on/off when the pressure in the
buffer was below/above the pressure set-point. The desorbed gaseous
ammonia flowed into a small buffer container, that was placed after
the desorption unit and having a volume of approximately 200
millilitres. The pressure inside the buffer was measured using a
digital Kobold SEN-87 pressure meter. The position of the
thermocouples (denoted T1 to T5) are shown in the below table 1 and
illustrated in FIG. 2.
TABLE-US-00001 TABLE 1 Radial position of five thermocouples
Thermocouple Radial distance from inner wall (mm) T1 0 (inner wall)
T2 12 T3 26 T4 45 T5 50 (centre) The total test setup is shown in
FIG. 3.
[0077] A mass-flow controller (Brooks Smart Mass Flow 5850S
calibrated to NH.sub.3) dosed the ammonia from the buffer container
into a "tailpipe" conveying a stream of carrier gas of air (1000
litres/minute) corresponding roughly to the current of exhaust gas
from a small car. In addition to dosing ammonia, the same device
also measured the actual flow through the valve in
millilitres/minute (at T=298.15 K and P=1 bar). According to the
stoichiometry of the SCR reaction, the mixture ratio between
NH.sub.3 and NOx should be approximately 1:1 (e.g. (Koebel and
Kleeman, 2000; Fang and DaCosta, 2003)). The transient NOx
concentration (and by that the transient NH3 concentration) in the
exhaust gas from a car is very complicated to describe a continuous
mathematical function and as an approximation, a sinusoidal
function was used. The amplitude was set to 210 millilitres/minute
and the period was set to 120 seconds. Due to limitations in the
dynamics of the mass-flow controller, however, the resulting outlet
flow was not completely sinusoidal. Heating of the desorption unit
was controlled using the ON-OFF controller programmed along with
the datacollection in Labview. The control parameter was chosen as
the buffer pressure and the set-point pressure was set to 5 bars.
At such high pressures it is fairly safe to assume that there is no
pressure gradient across the porous bed (Lu and Spinner, 1996), and
therefore the reactor pressure should be equal to the buffer
pressure. It is possible to reduce the set-point pressure, which
will reduce the temperature required for desorption. However, the
main reason for the choice of set-point pressure was to maintain a
well-defined flow through the mass-flow controller. With proper
flow-mapping the flow through the valve should in principle be
independent of the back-pressure.
[0078] The result of the experiment appears from FIG. 4 which shows
a graphical representation of recorded temperature at positions T1
and T5 (the melting point for MgCl.sub.2 is 714.degree. C.) during
desorption of ammonia from the ammonia-saturated MgCl.sub.2 powder,
and FIG. 5 shows the recorded pressure in the buffer container
during the experiment. Phases I-IV and an intermediate phase (phase
(II-III)) have been indicated in FIGS. 4 and 5 and are explained
more in detail below.
[0079] During the experiment, a controller increased the
temperature of the unit slowly to sustain a desired pressure of
ammonia in the buffer. In the case of the experiment in FIGS. 4 and
5, the buffer was kept at a pressure close to 5 bars. Other
experiments were done at lower buffer pressures. This resulted in a
lower operating temperature due to the thermodynamics of the
storage material. A higher ammonia pressure gives a higher
desorption temperature.
Phase (I)-Warm-up
[0080] During this phase the pressure was allowed to build up to
the set-point, no ammonia was extracted during this initial warm-up
phase.
Phase (II)--Desorption of First Four Moles of Ammonia
[0081] When reaching the set-point buffer pressure, heating was
turned off and the mass-flow controller began to dose ammonia. This
eventually caused the pressure to drop and heating was turned on
again. Switching between on and off for dosing ammonia caused the
pressure to oscillate around the set-point. The small oscillations
in the measured out-flow (nearly sinusoidal flow) can also be seen
in the large oscillations around the set-point, as it is shown in
FIG. 6.
[0082] The large (and slow) oscillations are also seen in the
temperature, and due to the low effective thermal conductivity of
the porous solid matrix this is most pronounced in the vicinity of
the source of heat (T1). During phase II, the oscillations of the
pressure and temperature continued to increase in amplitude and
decrease in frequency. Combined with the large temperature
gradients observed from FIG. 4 this indicates, that the reaction
proceeds along an inward moving reaction front. Such a moving front
will increase the way of heat-transfer, which will result in
increasing amplitude and decreasing frequency of the
pressure/temperature oscillations.
Phase II-III--Transition Phase
[0083] As the front moves toward the centre of the reactor, less
and less ammonia is available for desorption. Eventually, a new
front builds up near the heat-source, in which the fifth mole of
coordinated ammonia is desorbed. This again reduces the way needed
for heat-transfer and thus reduces the amplitude and increases the
frequency of the oscillations. The apparently lower amplitude of
the oscillations in the transition phase as compared to phase III,
indicates that there still is ammonia bound as
Mg(NH.sub.3).sub.6Cl.sub.2 left for desorption near the centre.
Phase III--Desorption of Fifth Mole of Ammonia
[0084] The sudden increase of the amplitude marks the end of the
transition phase and the beginning of the third phase, in which
only the fifth mole of ammonia desorbed. The decrease of the
pressure overshoot as compared to phase II is most obviously due to
the fact that only one mole of ammonia was released as compared to
the four moles released during phase II (cf. reactions 2 and 3),
therefore reducing the overall desorption rate. However, the
increased loss of heat to the surroundings due to the higher
temperature level at T1 could also help moderating the
temperature/pressure peaks. During this phase it is difficult to
see any increase of amplitude and decrease of frequency of the
pressure oscillation; however the peak temperature does increase
throughout the phase.
Phase IV--Desorption of Sixth Mole of Ammonia
[0085] The amplitude of the pressure overshoot was even smaller
during this phase than for phase II. One mole of ammonia was
desorbed in both phase III and IV, but the increased loss of heat
at T1 during phase IV helped moderating the temperature peaks and
hence, also the overshoot of pressure. Since this experiment was
run for several hours, including a night, a maximum allowable
temperature of 500.degree. C. was set as a safety parameter. From
FIGS. 4 and 5 it can be seen, that this temperature was reached,
which resulted in the loss of buffer pressure. As this occurred
during the night, it was not corrected until the following morning
by setting the maximum allowable temperature to 650.degree. C.
(64.degree. C. less than the melting point temperature of
MgCl.sub.2). The temperature and pressure fluctuated a somewhat
during this phase. This might indicate some sort of build-up of
pressure and subsequent release due to mass-transfer hindrance.
During phase IV, the temperature throughout the reactor became
quite high. However, in an optimized system the set-point for
pressure will certainly be lower than the chosen 5 bars, which will
also reduce the required temperature for all three desorption
reactions to occur.
[0086] Integration of the outlet flow curve in time yields the
total accumulated volume of NH.sub.3 released through the valve.
This value can be converted to number of moles by means of the
ideal gas law. In order for the process to be efficient, this value
should be close to the theoretically predicted amount of NH.sub.3
contained in the salt. This is easily calculated, since the ratio
of MgCl.sub.2 and NH.sub.3 in a 100% saturated salt is 1:6. The
stoichiometric calculations showed that, theoretically, 7.88 moles
of ammonia should be stored within the given mass of
Mg(NH.sub.3).sub.6Cl.sub.2 salt (258.8 grams), while the
integration of the calibrated signal from the mass flow controller
revealed that 7.86 moles of ammonia was released through controller
giving a very high storage efficiency of approximately 99.8%.
Example 2
Compression of Solid Ammonia Storage Medium
[0087] FIG. 7 schematically shows a device compression of the solid
ammonia delivery material for the preparation of cylindrical
tablets (dimensions: 13 mm in diameter; 10 mm high). In this
embodiment, the solid ammonia delivery material was compressed in a
chamber by applying a pressure of 2-4 tons/cm.sup.2 using a piston
compressing the powdered saturated storage material. When the
piston was removed, the delivery material was in the desired shape
of e.g. a tablet, a cylinder or a rod, and had a density above 80%
of the theoretical crystal density.
[0088] FIG. 8 schematically shows an embodiment of an ammonia
delivery device of the invention for desorption of the compressed
delivery material. In this embodiment, one or more tablets of solid
ammonia delivery material 1 are placed in a container 2, which can
be heated by a heating device 3. Desorbed ammonia leaves the
container through a nozzle 4. Heat for the heating device 3 may be
provided by e.g. resistive electric heating or chemical reactions.
Such chemical reactions could be generated e.g. by combustion of a
part of the released ammonia. If the delivery device is used in
connection with SCR of NOx in exhaust gases, waste heat from the
engine producing the gases can also be applied.
[0089] FIG. 9 schematically shows another preferred embodiment
where only a part of the stored solid delivery material 1 is heated
at a time. The solid delivery material is stored in compressed
form, and introduced into a hot chamber 2 one at the time at
intervals corresponding to the requirement for gaseous ammonia. The
hot chamber is heated by a heating device 3 operated after the same
principles as described above 2. Gaseous ammonia leaves the hot
chamber through a nozzle 4, and when all ammonia is desorbed from a
tablet 5 of solid ammonia delivery material, it is discarded into a
separate container 6.
[0090] In a similar type of embodiment, the entire storage material
is separated into a number of compartments each having their own
heating source so that it is possible to have complete desorption
of a given fraction of the material without using any moving parts
to replace saturated/unsaturated salt e.g. on-board the vehicle
during use.
[0091] The bed-density of the delivery material used in Example 1
was quite low (331 kg/m.sup.3 when compacted gently by hand) when
compared to the density of MgCl.sub.2 and
Mg(NH.sub.3).sub.6Cl.sub.2 (1252 kg/m.sup.3, cf. the below table
2).
TABLE-US-00002 TABLE 2 Mass density Molar volume Salt kg/m.sup.3
cm.sup.3/mole Source MgCl.sub.2 2325 40.86 CRC Handbook 2004
Mg(NH.sub.3).sub.6Cl.sub.2 1252 157.4 Gmelins Handbook 1939
[0092] A low density means that the entire storage system would
require more space. This problem was solved by compressing
Mg(NH.sub.3).sub.6Cl.sub.2 into solid rods having a density of 1219
kg/m.sup.3(97% of the solid density). TPD experiments confirmed
that it was possible to desorp all ammonia from this tablet, thus
increasing the potential storage capacity by a factor of 3.7 (on a
molar basis) to approximately 93% of the volumetric ammonia storage
capacity of liquid ammonia. A nearly quantitative desorption of
ammonia from the dense material is possible because the front of
desorption leaves behind a porous layer of anhydrous MgCl.sub.2.
This automatically generates the required pore system needed for
mass-transfer through the structure. This is considered an ideal
combination of a) an initially very compact structure having almost
no void and being easy to handle, b) a high capacity for containing
and delivering ammonia, c) a low external surface area, and d) a
high degree of safety.
Example 3
Comparison of the Use of Ammonia Delivery Devices of the Invention
and Urea-technology
[0093] By calculating the amount of NOx (assumed to be pure NO)
generated per kilometre in a model fuel (taken as pure n-octane,
.rho.=696.8 kg/m3), the corresponding driving distance for a given
amount of ammonia or urea can be found.
[0094] Based on the EURO 3 standards (The European Parliament,
1998) as well as by the values used by some researchers (Hyundai
Motor Co.: Choi et al., 2001), the assumed NOx concentrations and
the fuel economies are: 150 ppm and 10 km/litre for stoichiometric
combustion (typically gasoline) and 300 ppm and 15 km/litre for
lean burn combustion (.lamda.=1.5, typically diesel).
[0095] Taking 1 litre (or 696.8 g) of fuel as a basis of
calculations the generated NOx per. kilometre would be
5.8710.sup.-2 mole/km or 0.18 g/km for stoichiometric combustion,
while for lean burn combustion (.lamda.=1.5), the NOx emission is
1.1510.sup.-2 mole/km or 0.34 g/km.
[0096] In the below table 3 is shown the required mass/volume of
the high-density Mg(NH.sub.3).sub.6Cl.sub.2 needed to drive 20,000
km (excluding the mass/volume of any equipment) compared to that of
the 32.5% urea solution. The comparison is based on the combustion
of pure n-octane and the above assumptions. The fuel is assumed to
be pure n-octane and the exhaust is assumed to have an average
molar fraction of 300 ppm NOx. The engine is assumed to run lean
with a fuel economy of 15 km/litre fuel. The density of a 32.5%
wt/wt urea solution is 1090 kg/m.sup.3 and the density of the
Mg(NH.sub.3).sub.6Cl.sub.2 is assumed to be 1219 kg/m.sup.3.
TABLE-US-00003 TABLE 3 Mass/20,000 km Volume/20,000 km Material kg
litre 1 Urea 21.2 19.4 2 Mg(NH3).sub.6Cl.sub.2 7.5 6.2 Ratio 1:2
2.8 3.1
[0097] The results clearly show the superior ammonia storage
capacity of Mg(NH.sub.3).sub.6Cl.sub.2 over that of the urea
solution. Combined with a high efficiency of approximately 99% this
ammonia storage compound is very appropriate for the purpose. Due
to the nature of the compaction of the materials, the present
invention is also superior to the disclosures of WO 99/01205, where
the preferred embodiment of granulated material of either
Ca(NH.sub.3).sub.8Cl.sub.2 or Sr(NH.sub.3).sub.8Cl.sub.2 will
results in a reduction in volumetric capacity of the theoretical
salt densities by a factor of 1.5-2.
REFERENCES
[0098] Fang, H. L., DaCosta, H. F., 2003. Urea thermolysis and NOx
reduction with and without SCR catalysts. Applied Catalysis B:
Environmental 46, 17-34. [0099] Goetz, V., Marty, A., 1992. A model
for reversible solid-gas reactions submitted to temperature and
pressure constraints: Simulation of the rate of reaction in
solid-gas reactor used as chemical heat pump. Chem. Eng. Sci. 47
(17-18), 4445-4454. [0100] H. Van Vlack, L., 1989. Elements of
Materials Science and Engineering, sixth Edition. Addison-Wesley
Publishing Company Inc. [0101] Hyundai Motor Co.: Choi, S.-m.,
Yoon, Y.-k., Kim, S.-j., Yeo, G.-k., Heesung Engelhard Corp.: Han,
H.-s., 2001. Development of UREA-SCR system for light-duty diesel
passenger car. SAE Technical Paper Series. [0102] Koebel, M.;
Elsener, M., Kleeman, M., 2000. Urea-scr: A promising technique to
reduce NOx emissions from automotive diesel engines. Catalysis
Today 59, 335-345. [0103] Liu, Chun Yi & Aika, K.-i., 2004.
Ammonia absorption on alkaline earth halides as ammonia separation
and storage procedure. Bull. Chem. Soc. Jpn. 77 (1), 123-131.
[0104] Lu, Hui-Bo; Mazet, N., Spinner, B., 1996. Modelling of
gas-solid reaction coupling of heat and mass transfer with chemical
reaction. Chem. Eng. Sci. 51 (15), 3829-3845. [0105] Matsumoto, S.,
1997. Recent advances in automobile exhaust catalyst. Catalysis
Surveys from Japan 1, 111-117. [0106] Olovsson, I., 1965. Packing
principles in the structures of metal ammine salts. Acta Cryst. 18,
889-8.93. [0107] CRC Handbook, 2004. Handbook of chemistry and
physics (web edition). Gmelins Handbuch, 1939. Magnesium teil b:
Die verbindungen des magnesiums (27). [0108] The European
Parliament, 1998. Directive 98/69/ec. [0109] The European
Parliament, 1999. Directive 1999/96/ec. [0110] The European
Parliament, 2003. Directive 2003/17/ec. [0111] Touzain, P.,
Moundamga-lniamy, 1994. Thermochemical heattransformation: [0112]
Study of the ammonia/magnesium chloride-GIC pair in a laboratory
pilot. Mol. Crys. Liq. Cryst. 245, 243-248.
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