U.S. patent application number 12/557895 was filed with the patent office on 2010-01-28 for system for storing ammonia in and releasing ammonia from a stroage material and method for storing and releasing ammonia.
This patent application is currently assigned to Amminex A/S. Invention is credited to Tue Johannessen, Henning Schmidt.
Application Number | 20100021780 12/557895 |
Document ID | / |
Family ID | 38434816 |
Filed Date | 2010-01-28 |
United States Patent
Application |
20100021780 |
Kind Code |
A1 |
Johannessen; Tue ; et
al. |
January 28, 2010 |
System for Storing Ammonia In and Releasing Ammonia from a Stroage
Material and Method for Storing and Releasing Ammonia
Abstract
A system for storing ammonia in and releasing ammonia from a
storage material capable of binding and releasing ammonia
reversibly by adsorption or absorption for a process with a gradual
ammonia demand that can vary over the time. The system has a
container capable of housing the ammonia-containing storage
material; a heating source arranged to supply heat for the
desorption of ammonia from the solid storage medium; and a
controller arranged to control the heating source to release
ammonia. The heating source is arranged inside the container and
surrounded by ammonia storage material. A controllable dosing valve
is arranged to dose released ammonia according to the ammonia
demand. The controller comprises a feed-forward control arranged to
control the heat supplied by the heating source, based on the
ammonia demand.
Inventors: |
Johannessen; Tue; (Glostrup,
DK) ; Schmidt; Henning; (Allerod, DK) |
Correspondence
Address: |
FROST BROWN TODD, LLC
2200 PNC CENTER, 201 E. FIFTH STREET
CINCINNATI
OH
45202
US
|
Assignee: |
Amminex A/S
Soborg
DK
|
Family ID: |
38434816 |
Appl. No.: |
12/557895 |
Filed: |
September 11, 2009 |
Current U.S.
Class: |
429/421 ; 206/.7;
222/146.2; 222/3; 422/173; 422/177; 60/295 |
Current CPC
Class: |
F01N 2610/146 20130101;
Y02E 60/32 20130101; F17C 13/00 20130101; Y02E 60/50 20130101; B01D
53/9495 20130101; F01N 2610/1406 20130101; F01N 2900/12 20130101;
F17C 2221/01 20130101; H01M 8/222 20130101; F01N 2900/1811
20130101; F01N 2900/0411 20130101; F01N 2900/1808 20130101; C01C
1/006 20130101; B01D 53/8631 20130101; H01M 8/0606 20130101; B01D
2251/2062 20130101; F01N 2610/14 20130101; F17C 2227/0302 20130101;
F01N 2560/026 20130101; F01N 2610/10 20130101; F01N 3/2066
20130101; F01N 2610/06 20130101; B01D 53/9431 20130101; F01N 11/00
20130101; F01N 2900/08 20130101; Y02A 50/20 20180101; F01N 3/208
20130101; F01N 2610/02 20130101; F17C 11/00 20130101; Y02T 10/12
20130101; F17C 2270/0581 20130101; B01D 53/90 20130101 |
Class at
Publication: |
429/19 ; 206/7;
222/3; 222/146.2; 422/173; 60/295; 422/177 |
International
Class: |
H01M 8/18 20060101
H01M008/18; F17C 11/00 20060101 F17C011/00; F17C 13/00 20060101
F17C013/00; B01D 53/94 20060101 B01D053/94; F01N 3/28 20060101
F01N003/28 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 30, 2007 |
EP |
07 006 706.1 |
Mar 26, 2008 |
EP |
PCT/EP2008/002386 |
Claims
1. A system for storing ammonia in and releasing ammonia from a
storage material capable of binding and releasing ammonia
reversibly by adsorption or absorption for a process with a gradual
ammonia demand that can vary over the time, comprising: a container
capable of housing the ammonia-containing storage material; a
heating source arranged to supply heat for the desorption of
ammonia from the solid storage medium; a controller arranged to
control the heating source to release ammonia; a controllable
dosing valve arranged to dose released ammonia according to the
ammonia demand; wherein the controller comprises a feed-forward
control arranged to control the heat supplied by the heating
source, based on the ammonia demand.
2. The system of claim 1, wherein the desorption of ammonia from
the storage material is endothermic, and the feed-forward control
is arranged to control the heat supplied by the heating source such
that it compensates the energy required for the endothermic
desorption of the demanded ammonia from the storage material.
3. The system of claim 1, wherein the controller is arranged to
determine a heat loss of the container to the surroundings, and the
feed-forward control is arranged to control the heat supplied by
the heating source such that it compensates the heat loss to the
surroundings.
4. The system of claim 3, wherein the heat loss to the surroundings
is estimated on the basis of a measurement of at least one of the
temperature inside the container, the temperature of an inner side
of a container wall, the temperature of an outer side of a
container wall, and the temperature of the surroundings.
5. (canceled)
6. The system of claim 1, wherein the controller comprises an
over-laid feedback control arranged, based on a pressure
measurement in the container, to reduce or terminate the supply of
heat by the heating source when the pressure is above an upper
pressure threshold and to increase or start the supply of heat by
the heating source when the pressure is below a lower pressure
threshold.
7. The system of claim 1, arranged to remove NOx from an
oxygen-containing exhaust gas of a combustion engine or combustion
process, further comprising: a feed line arranged to feed released
gaseous ammonia from the container into the exhaust gas, a catalyst
for reducing NOx by reaction with the ammonia, and wherein the
controller is further arranged to obtain the ammonia demand based
on at least one of (i) a measurement of the NOx, and (ii)
information from an engine controller or combustion process
controller.
8. The system of claim 7, wherein the controllable dosing valve is
arranged to dose released ammonia into the exhaust gas.
9. The system of claim 7, further comprising an NOx sensor, on
which the NOx measurement is based.
10. The system of claim 7, wherein the information from the engine
controller is indicative of the engine's operational state, and the
feed-forward control is arranged to estimate the ammonia demand
based on the operational state information.
11. (canceled)
12. (canceled)
13. (canceled)
14. The system of claim 1, arranged to use the desorbed ammonia, or
a derivative of it, as fuel in a fuel cell, further comprising: (a)
a catalytic ammonia cracking reactor to produce hydrogen, a fuel
cell capable of operating on gaseous hydrogen; or (b) a fuel cell
capable on running directly on released ammonia.
15. The system of claim 1, wherein the ammonia storage material is
capable of binding and releasing ammonia reversibly by absorption
and is a chemical complex in the form of an ionic salt of the
general formula: M.sub.a(NH.sub.3).sub.nX.sub.z1 wherein M is one
or more cations selected from alkali metals, alkaline earth metals,
and transition metals or combinations thereof, 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.
16. (canceled)
17. The system of claim 15, wherein the ionic salt is either
chloride or sulphate salts of Mg, Ca, Sr or mixtures thereof.
18. The system of claim 1, wherein the ammonia storage material is
in the form of shaped units of ammonia storage material.
19. The system of claim 18, wherein the ammonia storage material is
compacted to a dense block, rod, cylinder ring or edged unit with a
density above 70% of the theoretical maximum skeleton density of
the saturated solid material.
20. (canceled)
21. (canceled)
22. (canceled)
23. (canceled)
24. (canceled)
25. (canceled)
26. (canceled)
27. (canceled)
28. (canceled)
29. (canceled)
30. (canceled)
31. The system of claim 1, wherein the controllable dosing valve is
also feed-forward controlled to dose released ammonia according to
the ammonia demand.
32. The system of claim 31, wherein an ammonia-demand signal by the
controller is used to feed-forward control both the controllable
dosing valve and the heating source.
33. The system of claim 1, wherein control of the controllable
dosing valve comprises a mass flow meter that is arranged to
measure the mass flow of ammonia dosed by the dosing valve.
34. The system of claim 33, arranged to compare the ammonia demand
and the measured mass flow, and control the controllable dosing
valve such that the measured mass flow matches the ammonia
demand.
35. A method of releasing ammonia stored by storage material housed
in a container and capable of binding and releasing ammonia
reversibly by adsorption or absorption for a process with a gradual
ammonia demand that can vary over the time, comprising: determining
how much heat is to be supplied to the ammonia storage material for
the desorption of ammonia by means of a control comprising a
feed-forward control, based on the ammonia demand; supplying the
heat by a heating source; dosing released ammonia by means of
controllable a dosing valve according to the ammonia demand.
36. The system of claim 1, wherein the heating source is arranged
inside the container and is surrounded by ammonia storage
material.
37. The method of claim 35, wherein the heating source is arranged
inside the container and is surrounded by ammonia storage material.
Description
FIELD OF THE INVENTION
[0001] This invention relates to ammonia storage, and in particular
to a system and method for storing ammonia in and releasing ammonia
from a storage material capable of binding and releasing ammonia
reversibly by adsorption or absorption.
BACKGROUND OF THE INVENTION
[0002] Metal ammine salts which are ammonia absorbing materials can
be used as solid storage media for ammonia (see, e.g. WO
2006/012903), which in turn, for example, may be used as the
reductant in selective catalytic reduction to reduce NO.sub.x
emissions, see e.g. WO 1999/01205.
[0003] Usually, ammonia is released by thermal desorption, e.g.
from metal ammine salts, by external heating of a storage
container, see e.g. WO 1999/01205. The heating elements may also be
placed inside the storage container, see e.g. U.S. Pat. No.
5,161,389 and WO 2006/012903.
[0004] WO 1999/01205 discloses the use of ammonia as the reductant
in selective catalytic reduction to reduce NO.sub.x emissions of
automotive vehicles. The ammonia is released from an either
adsorptive or absorptive solid storage medium, among others
Sr(NH.sub.3).sub.8Cl.sub.2 or Ca(NH.sub.3).sub.8Cl.sub.2 in
granular form, in a storage container and temporarily stored as a
gas in a buffer volume. The amount of ammonia to be supplied to a
reaction volume in the vehicle's exhaust system is dosed under the
control of an electronic engine controller according to the current
operating state of the engine (WO 1999/01205, p. 9, last para.).
The amount of ammonia to be desorbed from the storage medium is
controlled by a feedback control in which the pressure in the
storage container is measured by a pressure sensor, and if the
pressure reaches a pressure threshold, the supply of heat is
interrupted (WO 1999/01205, para. bridging p. 8 and 9).
[0005] U.S. Pat. No. 5,441,716 describes a process for rapid
absorption cycles (less than 30 minutes) using ammoniated metal
halide salts for refrigerating purposes. A suitable reactor is
described having one or more heat transfer tubes inside that are
embedded in the storage material. Heat transfer plates are provided
to increase the heat transfer from the heat transfer tube(s) into
the surrounding storage material. The thermal diffusion path
lengths and mass diffusion path lengths are less than 15 mm and 1.5
mm respectively. A similar reactor is described in U.S. Pat. No.
5,328,671.
SUMMARY OF THE INVENTION
[0006] A first aspect of the invention is directed to a system for
storing ammonia in and releasing ammonia from a storage material
capable of binding and releasing ammonia reversibly by adsorption
or absorption for a process with a gradual ammonia demand that can
vary over the time. The system comprises: a container capable of
housing the ammonia-containing storage material; a heating source
arranged inside the container and surrounded by ammonia storage
material, the heating source being arranged to supply heat for the
desorption of ammonia from the solid storage medium; a controllable
dosing valve arranged to dose released ammonia according to the
ammonia demand; and a controller comprising a feed-forward control
arranged to control the heat supplied by the heating source, based
on the ammonia demand.
[0007] According to another aspect, a method is provided of
releasing ammonia stored by storage material housed in a container
and capable of binding and releasing ammonia reversibly by
adsorption or absorption for a process with a gradual ammonia
demand that can vary over the time. The method comprises:
determining how much heat is to be supplied to the ammonia storage
material for the desorption of ammonia by means of a control
comprising a feed-forward control, based on the ammonia demand;
supplying the heat by a heating source arranged inside the
container and surrounded by the ammonia storage material; dosing
released ammonia by means of a controllable dosing valve according
to the ammonia demand.
[0008] Other features are inherent in the methods and products
disclosed or will become apparent to those skilled in the art from
the following detailed description of embodiments and its
accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] Embodiments of the invention will now be described, by way
of example, and with reference to the accompanying drawings, in
which:
[0010] FIG. 1 shows an embodiment of an ammonia storage and release
system with a storage container in which the ammonia storage
material is heated internally and the heating source is embedded in
the storage material, including a drawing of a suitable shape of an
ammonia-containing storage material unit to be packed in the
container;
[0011] FIG. 2 shows different embodiments in cross-section views of
storage containers in which the heating source is equipped with
heat conducting elements wherein FIG. 2 a corresponds to the
storage container of FIG. 1;
[0012] FIG. 3 shows an embodiment in which the heat conducting
elements, in the form of fins, are circular plates arranged along
an axis of the heating source, including a drawing of a suitable
shape of an ammonia-containing storage material unit to be packed
in the container;
[0013] FIG. 4 shows an embodiment in which the heat conducting fins
are made of porous metal plates to enable desorbed ammonia to flow
in the fins towards the exit of the storage container without
passing through entire blocks of storage material;
[0014] FIG. 5 shows an embodiment similar to FIG. 1, but with a
heating source supplied with a hot fluid as a heating medium;
[0015] FIG. 6 illustrates the notion of a maximum heat diffusion
path length, based on cross-sectional view of the storage container
of FIG. 1;
[0016] FIG. 7 illustrates by experimental data the reduced delay
effects (fast response time, enhanced controllability of the
pressure of desorbed ammonia) of an internal embedded heating
source compared with an external heating source;
[0017] FIGS. 8 and 9 illustrate by experimental data the dosing
ability of an internal embedded heating source compared with an
external heating source;
[0018] FIG. 10 illustrates the feed-forward control of the heat
supply, including real-time estimation of the power demand of the
heating source, based on ammonia demand and observation of
temperature;
[0019] FIG. 11 illustrates the feed forward-control of the heat
supply with an overlaid feedback control, based on measured
pressure in the storage unit;
[0020] FIGS. 12 and 13 illustrate other embodiments in which the
released ammonia is not used to reduce NOx, but serves as a fuel
for fuel cells.
DESCRIPTION OF EMBODIMENTS
[0021] The embodiments pertain to systems and methods for storing
ammonia in and releasing ammonia from a storage material capable of
binding and releasing ammonia reversibly by adsorption or
absorption for a process with a gradual ammonia demand that can
vary over the time. As described in applicant's co-pending
application WO 2006/012903, metal ammine salts can be used as solid
storage media for ammonia. Thus, the metal-ammine salt constitutes
a solid storage medium for ammonia, which represent a safe and
practical option for storage and transportation of ammonia. Ammonia
is released by thermal desorption from the storage material.
[0022] "Gradual ammonia demand" means that the stored ammonia is
not demanded all at once, but in a distributed way over an extended
period of time (for example, over some hours) with a varying rate,
or even intermittently. The ammonia-containing storage material is
held in a storage container from which, in some embodiments, the
released ammonia is dosed through a controllable dosing valve in
the desired proportion. Between the container and the valve, there
is, in some embodiments, a buffer volume.
[0023] For mobile units, it is particularly useful to hold the
storage material (e.g. metal ammine complex) in a container that
can be easily separated from the mobile unit and replaced by a new
metal ammine container or recharged with ammonia in-situ. In one
embodiment of replacement of containers, the metal ammine
containers are recycled and recharged with ammonia in a separate
recharging unit or recharging facility.
[0024] In some embodiments, the desorbed ammonia is to be used as
the reductant in a selective catalytic reduction to reduce NOx
emissions, e.g. from automotive vehicles, boilers and furnaces.
Thus, the system is arranged to remove NOx from an
oxygen-containing exhaust gas of a combustion engine or combustion
process. For example, in some of the embodiments, a feed line
(which may include the buffer volume) is provided which is arranged
to feed released gaseous ammonia from the container directly into
the exhaust gas in the desired proportion, e.g. dosed by the
controllable dosing valve. In a reaction volume in the exhaust
system, a catalyst is provided for reducing NOx by reaction with
the ammonia.
[0025] In some embodiments, the combustion engine is a mobile or
immobile combustion engine unit fuelled by diesel, petrol, natural
gas, coal, hydrogen or other fossil or synthetic fuel. The NO.sub.x
to be removed may be produced by an automobile, truck, train, ship
or other motorized machine or vehicle, or by a power plant for
generating electricity.
[0026] The ammonia demand is substantially that amount of ammonia
that is able to remove all the NOx in the exhaust gas; however, if
it is not tolerable that any ammonia escapes to the atmosphere, a
smaller proportion may be dosed into the exhaust gas to ensure that
substantially all the ammonia is decomposed. In some embodiments,
the ammonia demand is determined based on a measurement of the NOx
in the exhaust gas, e.g. measured by a NOx sensor. In other
embodiments, information from an engine controller or combustion
process controller about the operating state is used to estimate
the NOx expected in the present operating state. For example, the
operating state may be defined by the current engine velocity,
current load, current drive pedal position, etc.; knowing these
parameters enables the engine controller (or combustion process
controller) to determine in real-time the expected NOx in the
exhaust gas. The engine controller is, for example, equipped with a
mapping (e.g. in the form of a look-up table) of the entire engine
operating area to the corresponding expected NOx emission. Such a
real-time predicted NOx signal can be used as an input to the
feed-forward controller to determine the ammonia demand. In some
embodiments NOx measurement and NOx prediction, based on the engine
controller, are combined in order to get a faster, but nevertheless
precise, demand indication; for example, the NOx values predicted
by the mapping (e.g. look-up table) can be compared with the actual
(measured) NOx, and the mapping can be continuously corrected
should there be a discrepancy.
[0027] In other embodiments, the desorbed ammonia is to be used,
directly or indirectly, as a fuel, e.g. for a power generating
unit. For example, in some of these embodiments, the desorbed
ammonia is used to produce hydrogen in a catalytic ammonia-cracking
reactor, and the hydrogen is used as fuel in a fuel cell capable of
operating on gaseous hydrogen. In other embodiments, a fuel for a
fuel cell capable of operating on ammonia is directly operated with
the desorbed ammonia. The gaseous ammonia is dosed into the
ammonia-cracking reactor or directly into the fuel cell, e.g. by
the controllable dosing valve.
[0028] In those embodiments, the ammonia demand is substantially
that amount of ammonia that has to be provided to the reactor, or
the fuel cell, so that the fuel cell is able to produce the power
required.
[0029] The heat used in the thermal desorption of ammonia is
provided by a heating source. In some embodiments, the heating
source is arranged inside the container such that it is surrounded
by, i.e. embedded in the ammonia storage material. Unlike a heating
source arrangement outside the container or inside the container,
but at the container wall, substantially all of the heat supplied
has to enter the storage material. Thus, although a fraction of the
heat is nevertheless lost to the environment, this fraction is
smaller than the fraction that would be lost when the heating
element were not embedded in the storage material.
[0030] The heat supply by the heating source is controlled by a
controller. The amount of ammonia to be supplied, e.g. to a
reaction volume in a vehicle's exhaust system, is, in some
embodiments, dosed by a controlled valve based on the current
ammonia demand, e.g. according to the current operating state of
the engine. Since unloading of ammonia generally varies, there will
be pressure variations in the storage container (if there is a
buffer, the pressure variations will also be in the buffer). For
example, according to WO 1999/01205 the amount of ammonia to be
desorbed from the storage medium is controlled indirectly based on
the pressure variations caused by unloading ammonia from the
container, by a feed-back control in which the pressure in the
storage container is measured by a pressure sensor, and if the
pressure reaches a pressure threshold, the supply of heat is
interrupted. By contrast, in some of the embodiments of the present
invention the controller comprises a feed-forward control arranged
to control the heat supplied by the heating source, based on the
ammonia demand. This, for example, is the current demand or an
estimated future demand, or a combination of current and future
demand. Since the feed-forward control does not only react if the
pressure is already too small or too high, the delay with which the
effective desorption rate is adapted to the rate with which ammonia
is unloaded--which is generally a varying rate--is shortened.
[0031] The heating controller uses information regarding ammonia
delivery demand and an estimated (model-based) heat loss from the
container to ensure that the heating source at all times provides
an amount of energy that does not allow the unit to cool down below
a suitable operation temperature of the dynamic desorption process.
Reaching too low operating temperature would result in a desorption
pressure below the minimum pressure needed to dose ammonia into
e.g. an exhaust line with a pressure slightly above atmospheric
pressure.
[0032] Thus, the feed-forward control is not based on a measurement
of how much ammonia has actually been released; rather the amount
of heat required to release the demanded amount of ammonia is
estimated, e.g. by a model calculation or by experimental data that
links the amount of heat supplied to the resulting ammonia release.
Since the accuracy of such an estimate may be limited, and since
the effect of heating (or terminating the heating) may only appear
after a certain delay, in some embodiments a feedback control is
laid over the feed-forward control, as will be explained in more
detail below.
[0033] In some of the embodiments, the feed-forward control cannot
only simply switch on and off the heating source. Rather, the
feed-forward control is able to adjust the heating source so that
it also can supply intermediate amounts of heat flow between
completely on and off; for example, it is able to adjust the heat
flow to the continuous intermediate values in the range between on
and off. In some embodiments, the heating source itself can be
operated at different powers, e.g. by continuously regulating the
heating current (in an electrically powered heating source) or the
flow of hot liquid (in an hot fluid heating source). In other
embodiments in which the heating source can only be operated at
maximum power, by fast switching the heating source (e.g. switching
the electric supply) with a duty cycle corresponding to the
intermediate value required, an effective amount of heat
corresponding to the intermediate value required is supplied, due
to the thermal inertia of the heating system.
[0034] The desorption rate is a function of the temperature and the
pressure in the storage container. To achieve, or maintain, a
certain desorption rate one might therefore think of measuring the
temperature, and start, or increase, the supply of heat if the
temperature is too low, and stop, or decrease, the supply of heat
if the temperature is too low. However, such a temperature based
feedback control would have similar delays as the pressure-based
feedback control described in WO 1999/01205.
[0035] Generally, the desorption of ammonia from the storage
material is endothermic. Thus, desorbing ammonia has a cooling
effect. In some embodiments, the feed-forward control is arranged
to control the heat supplied by the heating source such that it
compensates the energy required for the endothermic desorption of
the demanded ammonia from the storage material. As explained above,
this is not (primarily) based on a measurement of the temperature
and a feedback control based on the measured temperature, but on a
calculation (i.e. an estimation) of the endothermic desorption
energy required for the desorption of the demanded amount. Since
the desorption energy is proportional to the amount to be desorbed,
the required heat energy is, in some embodiments, obtained by
multiplying the ammonia demand by the proportionality factor.
[0036] Even though the heating source is embedded in the storage
material, so that substantially all the heat is absorbed by the
storage material, a certain fraction of the heat will be lost to
the surroundings through the walls of the storage container. In
some embodiments, this heat loss is taken into account in the
feed-forward control. In these embodiments, the controller is
arranged to determine the heat loss of the container to the
surroundings, and the feed-forward control controls the heat
supplied by the heating source such that it compensates the heat
loss to the surroundings. For example, a simple method of
estimating the heat loss is based on a model description of the
(preferably insulated) storage container in terms of its external
surface area (e.g. in m.sup.2 that the heat has to get out of) and
a heat transfer coefficient (W/m.sup.2K) that is combined with a
temperature gradient from inside the insulation to the outside. In
some of the embodiments, the temperature gradient is taken as the
difference between to actual measurements of the internal and
external temperatures, or, for example, as the difference of an
internal temperature measurement and an average temperature value
of the surroundings.
[0037] In some embodiments, the feed-forward control is such that
the heat supplied by the heating element corresponds to the sum of
the desorption energy required to desorb the demanded amount of
ammonia and the heat loss to the surroundings.
[0038] In some of the embodiments in which the heat loss to the
surroundings is taken into account in the feed-forward control, the
heat loss is estimated on the basis of a temperature measurement.
In principle, to calculate the heat loss, the temperature inside
the storage container (or at the inner side of the container wall)
and the temperature of the surroundings (or at the outer side of
the container wall) should be known. Thus, in some embodiments,
both the temperature inside the storage container (or at the inner
side of the container wall) and the temperature of the surroundings
(or at the outer side of the container wall) are measured and used
in the heat loss calculation. In other embodiments, only one
temperature measurement is made, and for the other temperature a
(constant) average temperature is assumed, and used in the
calculation (the measured temperature may be the inner temperature,
and the average temperature the outer temperature, or vice versa).
In still other embodiments, no temperature measurement is made, and
for both the outer and the inner temperatures average values are
used.
[0039] As mentioned above, in some embodiments a feedback control
is laid over the feed-forward control of the heat supply. The
overlaid feedback control is based on a pressure measurement in the
container. It reduces or terminates the supply of heat by the
heating source when the pressure is above an upper pressure
threshold, and increases or starts the supply of heat by the
heating source when the pressure is below a lower pressure
threshold. In some embodiments, at overpressure the heat supply is
completely switched off, and at underpressure the maximum heat rate
available is supplied. There are generally two reasons why an
overlaid feedback control may be useful: [0040] (i) As explained,
the feed-forward control is based on an estimate of the heat
required to desorb the demanded amount of ammonia; since the
accuracy of such an estimate may be limited, and errors in the
estimation may accumulate over time, the overlaid feedback control
provides a sort of error-correction functionality; and [0041] (ii)
since the effect of heating (or terminating heating) may only
appear after a certain delay, and the demand can suddenly
significantly increase or decrease, it may happen that the pressure
in the storage container exceptionally undershoots or overshoots an
upper or lower pressure limit.
[0042] The overlaid feedback control corrects an accumulated error
of the feed-forward control and constitutes a sort of "emergency
intervention" in the case where the pressure becomes too high or
low.
[0043] The system and process presented here can be used with all
storage materials capable of reversibly releasing ammonia by
thermal desorption. These materials may be ammonia adsorbing or
absorbing materials. Examples of adsorbing materials are carbon
modified with an acid, and zeolites. Examples of absorbing
materials are metal ammine salts.
[0044] Useful metal ammine salts have the general formula
M(NH.sub.3).sub.nX.sub.z, where M is one or more metal ions capable
of binding ammonia, such as Li, Mg, Ca, Sr, V, Cr, Mn, Fe, Co, Ni,
Cu, Zn, etc., n is the coordination number usually 2-12, and X is
one or more anions, depending on the valence of M, where
representative examples of X are F, Cl, Br, I, SO.sub.4, MoO.sub.4,
PO.sub.4 etc.
[0045] During release of ammonia, the original metal ammine salt
M(NH.sub.3).sub.nX.sub.z is gradually transformed into
M(NH.sub.3).sub.mX.sub.z with m<n. When all the desired ammonia
has been released, the resulting M(NH.sub.3).sub.mX.sub.z can
usually be converted back into M(NH.sub.3).sub.nX.sub.z by an
absorption treatment with an ammonia-containing gas stream due to
reversibility of the absorption/desorption process. 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.
[0046] Typical ammonia contents of the metal ammine complexes are
in the range of 20-60 wt %, preferably above 30 wt %. As an
example, a typical and inexpensive compound, such as
Mg(NH.sub.3).sub.6Cl.sub.2, contains 51.7 wt % ammonia. Using a
compaction method such as the one disclosed in applicant's
copending application WO 2006/081824 it is possible to obtain an
ammonia density within a few percent of liquid ammonia (8-9 bar
pressure vessel).
[0047] The use of applicant's technology disclosed in WO
2006/081824 enables storage of ammonia at significantly higher
densities (both on a volume and a weight basis) than both aqueous
ammonia and aqueous urea solutions. Aqueous urea is an example of a
chemical ammonia carrier that may provide ammonia for removal of
NOx by using a catalyst for NOx reduction and generated ammonia as
the reductant.
[0048] It is an advantage to deliver ammonia directly in the form
of a gas, both for the simplicity of the flow control system and
for an efficient mixing of reducing agent, ammonia, with the
exhaust gas. The direct use of ammonia also eliminates potential
difficulties related to blocking of the dosing system which are
caused by precipitation or impurities e.g. in a liquid-based urea
system. In addition, an aqueous urea solution cannot be dosed at a
low engine load since the temperature of the exhaust line would be
too low for complete conversion of urea to ammonia (and
CO.sub.2).
[0049] The arrangement of the heating embedded in the storage
material is in a functional relationship with the feed-forward
control of the heat supply because it enables the estimation of the
heat to be supplied as a function of the ammonia demand to be made
with better precision. Although this relationship is not mandatory,
it is advantageous for the feed-forward control.
[0050] To improve the heat transfer from the embedded heating
source to the storage material, in some embodiments heat conducting
elements are provided that are in thermal connection with the
heating source and ammonia storage material to increase the
internal heat exchanging area between the heating source and
ammonia-containing storage material.
[0051] For example, the heat conducting elements are fins thermally
connected to the heating source and surrounded by
ammonia-containing storage material.
[0052] For example, the heat-conducting elements are made of porous
or dense aluminium, titanium, stainless steel or similar ammonia
resistant metals or alloys. An example of a suitable metal for the
heat conducting elements is aluminium, which is able to tolerate
ammonia and salt--unlike e.g. brass. Furthermore, aluminium has a
low mass density and excellent thermal conductivity and is thus
preferred for efficiently conducting the thermal energy from the
heating element or heating source to the surrounding storage
material kept in the container.
[0053] In some of the embodiments, the heating source has an oblong
shape. For example, the fins are arranged parallel to the heating
source's longitudinal direction. However, in other embodiments,
they are arranged perpendicularly to the heating source's
longitudinal direction.
[0054] In the latter embodiments, if the desorbed ammonia is to be
drawn off at one (or both) of the longitudinal ends of the storage
container, the fins could, in principle, be an obstacle to the gas
flow (if there is, for example, no other way for the gas to flow
around the fins).
[0055] In some embodiments, some, or all, of the heat-conducting
elements are constructed of a porous metal structure that passes
released ammonia from the surface of the storage material being in
contact to the fin. This is not only reasonable in the case of fins
that could otherwise be an obstacle to the gas flow, but it may
also be useful, e.g. with longitudinal fins, to present "channels"
within the storage material for the desorbed gas to facilitate the
gas leaving the storage material.
[0056] The heat conducting metal may be made of porous metal
plates. Porous metal sheets/plates/bodies, for example, made of
partially sintered metal grains will be efficient for heat
conduction from the heating element (heating source) to the storage
material as well as giving increased gas transport channels from
the storage material to the container exit by allowing. ammonia to
flow through the porosity of the heating fins. The porosity of the
heating fins should be limited so that the heat conductivity of the
porous metal is at least 10% of the conductivity of the compact
metal.
[0057] In some of the embodiments, the maximum heat diffusion path
length (the distance from a highly thermally conductive surface to
the point of the storage material farthest away from any highly
conductive surface) is above 15 mm. In some embodiments the mean
mass diffusion path length (the arithmetic mean over all ammonia
molecules of the shortest distance from every ammonia molecule to a
gas permeable surface bordering the ammonia storage material) is
also above 15 mm.
[0058] In some embodiments, the heating element is arranged to be
powered with electrical current to produce heat. The heating source
may comprise a heat exchanger (or a plurality of heat exchangers)
extending into the container. The heat can be obtained from a hot
fluid or gas passing through the heat exchanger. In some
embodiments, the hot fluid or gas is, or is heated by, a hot
product gas or fluid from a chemical reaction or a combustion
process. In some embodiments the heating source is provided in the
form of one or more heating tubes. The container may have a
longitudinal extension, and the heating tube or tubes extend(s) in
the direction of the container's longitudinal extension.
[0059] Thus, this invention relates to the use of storage materials
capable of binding ammonia by ad- or absorption for the storage and
generation of ammonia. For example, solid metal ammine complexes
for storage of ammonia and for release of ammonia from the material
using controlled internal delivery of desorption heat directly
inside a storage container can be used. The release of ammonia may
be further facilitated by internal gas channels in the heat
exchanging material by using porous metal structures. Upon release,
ammonia may be used as the reducing agent in selective catalytic
reduction (SCR) of NO.sub.x in exhaust gases from combustion
processes.
[0060] Other applications using ammonia in mobile or portable units
or in special chemical synthesis routes where storage of liquid
ammonia is too hazardous are also considered embodiments of the
present invention. This also includes fuel cell systems where
ammonia may be considered an efficient hydrogen carrier as well as
other processes consuming ammonia including chemical synthesis
routes involving ammonia where storage of ammonia as liquid ammonia
is not allowed for safety reasons
[0061] The thermal time response of the heated storage medium is,
in most instances, too slow to fit the ammonia demand in real time
(i.e. nearly instantaneously). To fit the ammonia demand in real
time, the controllable dosing valve is provided; it determines the
amount of ammonia actually dosed to the outside (e.g. into the
exhaust gas).
[0062] In some embodiments, the controllable dosing valve is
controlled to dose released ammonia according to the ammonia
demand. For example, the feed-forward ammonia-demand signal
produced by the controller is used to control both the controllable
dosing valve and the heating source. Although the response of the
thermal ammonia release is relatively fast due to the feed-forward
control of the heating source (compared with feed-back control
schemes), it is still relatively slow compared with the nearly
instantaneous dosing response by the dosing valve. As a
consequence, the amount of ammonia released and the amount dosed
may differ from each other at certain instances of time. However,
if the same demand signal is used for both the thermal ammonia
release and released ammonia dosing, these amounts will become
equal, due to an averaging effect, on a time scale comparable to
the time constant of the thermal-ammonia-release-by-heating
mechanism (assuming that the calibration of both processes is
correct).
[0063] As the ammonia amount released instantly not always equals
the amount dosed, the pressure in the container housing the
ammonia-containing storage material may vary. In order to ensure
that the demanded ammonia amount is precisely dosed also in view of
varying pressures, in some embodiments the demand signal does not
directly adjust the controllable dosing valve, but only indirectly,
thereby also relying on a mass flow meter that measure the actual
mass flow of ammonia dosed by the dosing valve. A controller (which
may be the controller mentioned above, or a dedicated mass flow
controller) compares the ammonia demand and the measured actual
mass flow and, based on this comparison, controls the controllable
dosing valve such that the measured mass flow matches the ammonia
demand prescribed by the feed-forward ammonia demand signal.
[0064] In some of the embodiments with an overlaid feedback control
of the heating source to avoid over- and underpressures, the
feedback control signal is only used to control the heating source,
but it is not used in control of the dosing valve. Thus, in such
embodiments, the control valve is always controlled only on the
basis of the feed-forward demand signal, without overlaid feedback
signal, while the feed-forward demand signal also used for the
control of the heating source may be overlaid with the feedback
signal. This ensures that the actually dosed ammonia amount always
matches with the demand as close as possible, while under- and
overpressures due to the above-mentioned greater time constant of
the thermal-ammonia-release-by-heating mechanism, and possibly due
to calibration mismatch of thermal desorption and dosing, are
avoided.
[0065] The embodiments do not only pertain to system (i.e. a
product), but also to a method of releasing ammonia stored by
storage material housed in a container and capable of binding and
releasing ammonia reversibly by adsorption or absorption for a
process with a gradual ammonia demand that can vary over the time.
The method comprises: determining how much heat is to be supplied
to the ammonia storage material for the desorption of ammonia by
means of a control comprising a feed-forward control, based on the
ammonia demand; supplying the heat by a heating source arranged
inside the container and surrounded by the ammonia storage
material; dosing released ammonia by means of a controllable dosing
valve according to the ammonia demand.
[0066] In some embodiments of the method, the desorption of ammonia
from the storage material is endothermic, and the feed-forward
control controls the heat supplied by the heating source such that
it compensates the energy required for the endothermic desorption
of the demanded ammonia from the storage material.
[0067] In some embodiments of the method, the control determines a
heat loss of the container to the surroundings, and the
feed-forward control controls the heat supplied by the heating
source such that it compensates the heat loss to the surroundings.
In some of these embodiments, the heat loss to the surroundings is
estimated on the basis of a measurement of at least one of the
temperature inside the container, the temperature of an inner side
of a container wall, the temperature of an outer side of a
container wall, and the temperature of the surroundings.
[0068] In some embodiments of the method, the feed-forward control
controls the heat supplied by the heating source such that it
compensates both the energy required for the endothermic desorption
of the demanded ammonia from the storage material, and the heat
loss to the surroundings.
[0069] In some embodiments of the method, the control comprises an
overlaid feedback control that, based on a pressure measurement in
the container, reduces or terminates the supply of heat by the
heating source when the pressure is above an upper pressure
threshold and increases or starts the supply of heat by the heating
source when the pressure is below a lower pressure threshold.
[0070] In some embodiments of the method, NOx is removed from an
oxygen-containing exhaust gas of a combustion engine or combustion
process by feeding released gaseous ammonia from the container into
the exhaust gas, and reducing NOx reaction with the ammonia using a
catalyst, wherein the control obtains the ammonia demand based on
at least one of (i) a measurement or estimate of the NOx, and (ii)
information from an engine controller or combustion process
controller. In some of these embodiments, the information from the
engine controller indicates the engine's operational state, and the
feed-forward control estimates the ammonia demand based on the
operational state information.
[0071] In some embodiments of the method, the desorbed ammonia is
used as a fuel for a power generating unit.
[0072] In some embodiments of the method, (a) the desorbed ammonia
is used to produce hydrogen in a catalytic ammonia cracking
reactor, and the hydrogen is used as fuel in a fuel cell capable of
operating on gaseous hydrogen; or (b) a fuel cell capable of
operating on ammonia is operated directly with the desorbed
ammonia.
[0073] Turning now to FIG. 1, the storage container 1 is heated by
a heating element 2 representing a heating source placed inside the
container 1. In order to dissipate the heat from the heating
element 2 there are fins 3 representing heat-conducting elements
attached to the heating element 2. In the example shown, the
heating element is powered by electric current. The fins 3 are
arranged in planes defined by the longitudinal direction of the
container 1 (i.e. along its cylinder axis) and the container's
radial direction. They are suitably made of aluminium or other
light materials with high heat conductivity and resistance to the
environment in the container 1. The ammonia storage material is
made in the shape of blocks 9 to fill out the void in the container
1 (or, in other embodiments, may be put into the unit as powder).
The storage material 9 is shown both separately and inside the
container 1--both places indicated by item 9. When ammonia is
released from the solid by thermal desorption, it passes through a
tube with an on/off valve 4 to a buffer volume 5. A pressure sensor
10 measures the ammonia desorption pressure and a dosing valve 6
doses ammonia into an exhaust line 7 according to the demand given
by a controller 12. The controller 12, for example, communicates
with an engine control unit (ECU; not shown here). An NOx sensor 15
is provided in the exhaust line 7 that delivers an NOx signal to
the controller 12 which, in turns, calculates the ammonia demand to
remove the NOx. In other embodiments, the controller 12 gets a
predicted ammonia-demand signal from the ECU.
[0074] The storage container 1 is insulated by a thermal insulation
8; it also has means for measuring the temperature 11 on the
outside of the container 1 but underneath the insulation material
8. The controller 12 uses the signal from the temperature
measurement 11 to estimate/predict the heat loss through the
insulation material. Since most of the temperature gradient appears
at the insulation material 8, this temperature measurement
approximately corresponds to the higher temperature level to be
used in the heat loss estimation made by the controller 12. The
lower temperature level to be used in the heat loss estimation is,
in some embodiments, measured by a second sensor, e.g. at the outer
surface of the insulation material 8; in other embodiments a
constant average outside temperature is simply assumed. In some
embodiments the heating element itself is constructed with a
built-in thermocouple. This may serve both as a security for
avoiding overheating of the heating element and the temperature
measurement may also be used as a parameter in the prediction of
the temperature gradient.
[0075] This heat loss estimation, combined with the demand to
release ammonia, controls the heating input to the heating element
2, in a feed-forward manner. Of course, it is actually not
necessary to calculate the amount of heat needed to compensate the
desorption energy in a two-step procedure, in which first the
ammonia demand is calculated, and then the heat required to
compensate the desorption energy for this demand is calculated.
Thus, in some embodiments, the (measured or predicted) NOx is
directly mapped to a number indicating the heat required to
compensate the desorption energy for the amount of ammonia to be
released to remove the measured or predicted NOx, by a suitable
mapping table or formula. This heat may then directly be combined
with the estimated heat loss to obtain the amount of heat to be
produced by the heating element 2.
[0076] Based on the result of determination, the controller
controls the electric energy delivered to the heat element 2 such
that the required amount of heat is produced by the heat element 2.
For example, it is able to vary the voltage and/or current in a
continuous manner, according to the need. In other embodiments, the
supply to the heating element 2 is permanently switched on and off,
with a duty cycle corresponding to the amount of heat required.
[0077] The NOx-containing exhaust gas is produced by a combustion
engine or burner, eg. an internal combustion engine 13, and emitted
into the exhaust line 7. The NOx sensor 15 is arranged downstream
in the exhaust line 7. Further downstream the ammonia, dosed by the
dosing valve 6, is discharged into the exhaust line 7. Still
further downstream is an exhaust chamber housing an NOx reduction
catalyst 14 capable of removing NOx by reaction with ammonia. The
ammonia is dosed such by the dosing valve 6 that the dosed amount
is just sufficient to remove the current (measured or predicted)
NOx in the exhaust gas, without any significant amount of ammonia
being emitted to the atmosphere.
[0078] To this end, in some embodiments the ammonia demand signal
(based on calculation or prediction by the controller 12, as
described above) is also used to control the controllable dosing
valve such that it doses the released ammonia according to the
ammonia demand.
[0079] FIG. 2 shows different alternatives (a to c) of applying the
concept of internal embedded heating of the storage material 9 with
large material length scales (the latter will be explained in
connection with FIG. 6): [0080] a) The heating element 2 in the
form of a rod is placed in the middle axis of the container 1, here
a cylindrically shaped container. Four storage blocks 9 are placed
in the container 1. The heating fins 3 conduct the heat from the
heating element 2 to the storage blocks 9. The container 1 is
thermally insulated by means of the insulation 8. [0081] b) The
heating element 2 is placed in the middle axis of the container 1
which is here a rectangular shaped container. Again, four storage
blocks 9 are placed in the container 1, and the heating fins 3
conduct the heat from the heating element 2 to the storage blocks
9. The container 1 is insulated, at 8. [0082] c) Two heating
elements 2 are placed inside a rectangular shaped container 1.
Eight storage blocks 9 are now placed in the container 1. Again,
the heating fins 3 conduct the heat from the heating element 2 to
the storage blocks 9, and the unit is insulated at 8. Using two
internal heating zones may be an advantage for fast start-up with
reduced power demand.
[0083] FIG. 3 shows another embodiment in which the internal
embedded heating is arranged in a cylindrical container 1 with a
heating element on the cylinder axis 2 and heating fins 3 of a
disc-like shape and arranged perpendicularly to the container's
cylinder axis. In this configuration, blocks 9 are, for example, of
cylindrical shape with a central hole in order to fit on the
heating rod 2.
[0084] In some embodiments, the heating rod 2 has separate internal
heating zones, or "sections", and each heating disc (or fin) 3 may
dissipate energy to one section (or two neighboured sections) while
another zone is not heated. This can be an advantage, e.g. when
lower power consumption is desired during start-up, as the system
has an ability to direct more energy locally to reach a desired
desorption pressure without heating the entire storage mass.
[0085] FIG. 4 shows a particular configuration in which heating rod
2 is provided with porous metal sheets acting as heating fins 3,
attached to the heating rod 2 along the longitudinal direction,
similar to FIGS. 1 and 2. The ammonia released from the storage
blocks 9 can then flow in the container's longitudinal direction
through the porous metal sheets 3 which may provide faster ammonia
release. The heating rod 2 may dissipate the heat through
conduction in the porous metal sheet 3. The porosity of the e.g.
sintered metal sheet is below 90% as otherwise the heat
conductivity could be too low. In other embodiments,
perpendicularly arranged fins, as in FIG. 3, are made of porous
metal
[0086] FIG. 5 shows an embodiment similar to FIG. 1, but with a
heat exchanger as the heat element 2. A hot fluid acting as a
heating medium is flowed through a central bore in the heating
element 2. The heating medium conveys some of its heat to the
surrounding heat element 2, due to heat conduction. The heating
medium is, for example, heated by waste heat produced by an engine
(or a burner or chemical reaction chamber etc.) 16. A continuously
regulable valve 17 arranged in the heating-medium circuit is
controlled by the controller 12 to adjust (i.e. vary) the flow of
the heating medium in such a manner that the heating medium conveys
the amount of heat required to the heat element 2.
[0087] FIG. 6 illustrates what is meant by the maximum heat
diffusion path length, based on cross-sectional view of the storage
container of FIG. 1. In the example shown, the container 1 is a
cylinder having a circular cross section with an inner diameter of
10 cm (100 mm). The storage material placed in the range of
distances in which the distance to the central heat element 2, i.e.
is greater than 15 mm is shown as a white area at 18 (at the lower
left quarter in FIG. 6). Taking also the heat conducting elements 3
into account, the storage material placed in the range of distances
in which the distance to nearest hot surface (central heating
element 2 or heat conducting element 3) is greater than 15 mm is
shown as a white area at 19 (at the lower right quarter in FIG. 6).
The latter distance is the "heat diffusion path length". By
contrast, in the shaded area of the lower right quarter in FIG. 6,
the heat diffusion path length is smaller than 15 mm. The maximum
of the heat diffusion path length that appears somewhere in the
container 2 is called the "maximum heat diffusion path length"
herein. In the example of FIG. 6, the "maximum heat diffusion path
length" is greater than 15 mm because there is some storage
material (namely the storage material at 19) for which the heat
diffusion path length is greater than 15 mm.
[0088] Heat diffusion path lengths translate into heat diffusion
times. Thus, the smaller is the maximum heat diffusion path length,
the shorter is the delay between the supply of heat and the
corresponding release of ammonia. Consequently, in a purely
feedback-controlled system, one would tend to adopt a design with a
small maximum heat diffusion path length, significantly smaller
than 15 mm. Since a fast response of the ammonia release is not a
disadvantage in a feed-forward controlled system, either, in some
of the embodiments of the present invention a maximum heat
diffusion path length below 15 mm is chosen.
[0089] However, it has been recognised that based on a feed-forward
control of the heat supply one can better cope with such delays.
Thus, in some embodiments, the maximum heat diffusion path length
is greater than 15 mm, e.g. up to 100 mm or beyond. Such a system
has a less complicated internal structure of fins and is thus be
more interesting from an industrial applicability
point-of-view.
[0090] FIG. 7 shows delays due to heat conduction in two different
experiments that are only based on feedback control of the heat
supply. In both experiments ammonia dosing according to three
consecutive driving cycles with intermediate parking was carried
out. Two different types of federally approved driving cycles are
used: FTP-75 and US-06 (the latter includes more high-speed
driving). The driving cycles simulate certain driving conditions.
They are characterized by a definition of the vehicle speed as a
function of time. When this speed curve is differentiated, one can
get a dynamic curve showing where a car would produce much NOx
(during acceleration) and thus need larger dosing rates of
ammonia.
[0091] The experiment starts from a cold unit (room temperature) at
t=0. Then a given amount of power is applied to the heating element
to reach an ammonia pressure of approximately 2 bars in the buffer.
The testing cycle consist of one FTP-75, a "parking period" of 1
hour, a US06 driving cycle, 2 hours of "parking" and finally again
one FTP-75 cycle.
[0092] One setup consists of: [0093] EXTERNAL HEATING: 2 kg storage
material (Mg(NH.sub.3).sub.6Cl.sub.2) in a container, external
heating element (800 W maximum) wrapped around the container,
insulation material around the heated container, buffer, pressure
sensor, dosing valve (mass flow controller) and a feed-back control
using the pressure as feed-back measurement, i.e. heating is
applied when the pressure is below the set-point and less (or no)
heat is applied when the pressure is above the set-point. The
pressure set-point of the feedback control above which the heating
source is switched off is 2 bars. [0094] INTERNAL HEATING: 2 kg
storage material in a container, as above, but with internal
embedded heating element (500 W maximum), insulation around the
container, buffer, pressure sensor, and dosing valve (mass flow
controller). The pressure set-point of the feedback control above
which the heating source is switched off is 2 bars. [0095] Both
systems are equally insulated by 3 cm Rockwool.
[0096] Looking at the solid curve (EXTERNAL HEATING), this shows
the pressure a function of time during the entire experiment when
the experiment is carried out using an EXTERNAL heating element (as
one traditionally would do). It is easy to see that it takes more
than 10 minutes to reach a suitable buffer pressure and when it is
finally reached, the thermal inertia of the system causes a
dramatic over-shoot of up to 4 bars (also during parking). The next
driving cycle starts with a high pressure because the over-shoot
from the first cycle is not "reduced". But during the next cycle,
the feedback control is unable to avoid a large under-shoot in
pressure (well below the set-point) and a too low pressure makes it
impossible to dose the right amount of ammonia. The last cycle is
seen to have large oscillations in pressure and also the first
period of approximate 10 minutes where the pressure is far too
low.
[0097] The curve using INTERNAL HEATING is dashed and it can be
seen that even using lower power (as the internal heating rod has a
lower maximum power level) one can reach the desired pressure after
cold start much faster. The starting period is only 100-120 seconds
as opposed to more than 10 minutes using EXTERNAL HEATING. Also, in
the remaining period of the experiment, it can be seen that the
pressure is quite stable around the 2 bar set-point. Thus, is it
demonstrated that rapid start and a more stable system is obtained
using the current invention. The total power demand during these
three cycles is less, as shown below (measured in watt hours) for
the entire 3-cycle experiment:
[0098] INTERNAL: 203 W-h
[0099] EXTERNAL: 379 W-h
[0100] Consequently, using the INTERNAL HEATING, the system
performs better while at the same time reducing the power demand by
(379-203)1379%=46%. Embodiments described herein (with feed-forward
control), for example, use an internal embedded heating of this
type.
[0101] FIG. 8 shows the accumulated ammonia dosed during the second
driving cycle of FIG. 5 (from 94-104 minutes). This is a US-06
driving cycle and the assumed ammonia demand amounts to an integral
need of approximately 7 liters of ammonia gas to be dosed. The
figure shows the difference in dosing ability using the INTERNAL
vs. EXTERNAL heating.
[0102] Any increase in the vertical distance between the two curves
means that the system using EXTERNAL heating cannot follow the
demand of ammonia defined by the assumed driving cycle. Especially
in the last 4 minutes of the cycle, almost no increase in the
accumulated ammonia dosing curve is seen. The EXTERNAL heating
delivers less than five out of the seven liters needed. The
INTERNAL heating follows the driving cycle. Once again with regard
to FIG. 5, the US06 cycle (taking place approximately around t=100
minutes) for EXTERNAL is seen to have a low pressure in the last
part of the cycle. Thus corresponds to the severe lack of dosing in
FIG. 6.
[0103] FIG. 9 shows the dosing curve for the final FTP-75 cycle in
the experiment (t=226 to t=257 minutes). Here, the EXTERNAL heating
system only manages to dose approximately four out of six liters of
needed ammonia gas. Here it can be seen that it is mainly in the
first part of the cycle that is difficult to follow. This is also
seen in FIG. 5 where the pressure curve for EXTERNAL heating is
very low in the first 5-10 minutes. INTERNAL heating is able to
deliver the desired amount of ammonia.
[0104] FIG. 10 illustrates the feed-forward control strategy that
further enables the control of a system for large capacity,
including (but not limited to) systems with long heat diffusion
path length scales of the storage material, e.g. above 15 mm. When
the length scales are above 15 mm, the time-delay for heat transfer
is substantial, even with internal embedded heating. An aim of a
control strategy is to avoid reaching a state of "sub-cooling"
created by a large release rates of ammonia, which cools the
material since the desorption is endothermic. This cooling effect
is created locally--where ammonia is desorbed--but the new supply
of heat must reach that desorption "front" in the material and this
is potentially far away from the heating source. And when a high
ammonia release rate takes place when the pressure is above the
set-point, in a conventional feedback control, then that will not
cause the conventional feedback system to increase the energy input
until it is "too late". Therefore, the feed-forward control
algorithm shown in FIG. 10 is advantageous.
[0105] Basically, a storage unit needs heat for two things: a) to
maintain the temperature of the system without ammonia being
desorbed (compensating for heat loss) and b) to supply the
necessary amount of heat for ammonia desorption to avoid cooling of
the material.
[0106] Thus the elements of the control strategy are: [0107] a)
calculate the heat power needed to compensate for the energy demand
for endothermic ammonia desorption. This is done in real time using
an ammonia dosing demand signal received from the engine controller
(or derived from an expected-NOx signal from the engine controller)
or derived from an NOx measurement by an NOx sensor; once the
demand (for example expressed as a rate n, in mol/s) is known, the
corresponding desorption power P.sub.Desorption can be calculated
by:
[0107] P.sub.Desorption[W]=n[mol
NH.sub.3/s].times..DELTA.H.sub.NH3,desorp [J/mol NH.sub.3]; [0108]
b) calculate the heat power necessary for compensating for the heat
loss through the insulation material. This is done in real time
using suitable input such as temperature gradient, the heat
transfer coefficient of the insulation layer and the surface area
of insulated system; for example, temperature measurements provide
an internal temperature T.sub.Cartridge wall [K] and an external
temperature T.sub.Outside [K]; the external surface area of the
storage container is known to be A [m.sup.2]; and the heat transfer
coefficient of the container's insulation is known to be h
[W/K/m.sup.2]; then the power required to compensate the heat loss
can be calculated by
[0108] P.sub.Heat loss comp=A.times.h.times.(T.sub.Cartridge
wall-T.sub.Outside)[W]; [0109] c) add a) and b) in real time to
predict the total power demand P.sub.total at:
[0109] P.sub.total=P.sub.Heat loss comp+P.sub.Desorption; [0110] d)
control the heat element so that it supplies the total power demand
P.sub.total.
[0111] In more practical terms: if one accelerates the car
dramatically, the control system immediately adds more heat to the
storage unit even if the pressure is actually slightly above where
the set-point would be in a conventional feedback control. This
avoids a short period of ammonia deficiency that would show up in a
conventional feedback system.
[0112] If the surface area, A, and the heat transfer coefficient,
h, of the container are not known a priori, the controller may also
comprise an algorithm that estimates the heat-loss parameters
during e.g. a period of 10 minutes of system operation. The
coupling of the knowledge of amount of heat input and the amount of
released ammonia over a specific period of time (e.g. 10 minutes)
will enable the controller to estimate the value of A.times.h (if
the temperature gradient is known). It will not be able to estimate
the value of two parameters, A and h, independently but the
description of the heat loss as a function of temperature gradient
will generally be sufficient if the value of A.times.h is
known.
[0113] FIG. 11 shows an overlaid feedback control to provide an
additional safety feature of the control system. The pressure scale
shown indicates different pressure levels in the pressure control
strategy.
[0114] The basis pressure is the atmospheric pressure of the
surroundings. The pressure in the exhaust line is slightly higher,
e.g. P.sub.Exhaust line=1.2 bar. Dosing of ammonia is not possible
unless the dosing valve gets a certain supply pressure from the
buffer of P.sub.Minimum (as an example say 1.5 bars). The normal
set-point is P.sub.NH3, setpoint (e.g. 1.8 bars). The control
strategy presented in FIG. 8 might only be active in the pressure
range between P.sub.NH3, setpoint and P.sub.Heat-off (e.g. 2.2
bars). Above a certain pressure (P.sub.Heat-off), the heat is
turned off at any rate as a safety feature. P.sub.Heat-off is
higher than the set-point in a conventional feedback control would
be, since it is a safety feature, but the "normal" control is
performed by the feed-forward part. At P.sub.Satefy max an optional
pressure relief valve will open to avoid any pressure above a
mechanical design level.
[0115] When the pressure is below P.sub.NH3, set-point, then
maximum heating should be applied (unless the car is not able to
deliver that much power in the current state of engine load).
P.sub.NH3, set-point is lower than the set-point in a conventional
feedback control would be, since also this is a safety feature, but
the "normal" control is performed by the feed-forward part.
[0116] FIGS. 12 and 13 illustrate other embodiments in which the
released ammonia is not used to reduce NOx, but serves as a fuel
for fuel cells. In the embodiment of FIG. 12, ammonia stored in a
container (1) in storage material (9) is released by a heater (2)
based on a feed-forward control of the heat supplied, as explained
in illustrated in three previous figures. The released ammonia is
supplied to a catalytic cracker (20); the produced hydrogen is fed
to a fuel cell (21a) capable of converting hydrogen to electricity.
In the embodiment of FIG. 13 the released ammonia is directly
supplied to a fuel cell (21b) capable of directly converting
ammonia to electricity.
[0117] The feed-forward control strategy of FIG. 10, with an
optional combination with the pressure level strategy of FIG. 11,
constitutes a control strategy that can handle the long time-delays
of operating combined with a safe ammonia storage system using
endothermic ammonia desorption from storage units with large
material length scales above 15 mm. The strategy of FIGS. 10 and 11
is well-suited for the concept of internal heating as the
heat-compensating term is easier to compute. One reason is that
while the temperature of the internal heating element will
typically fluctuate quite substantially, the temperature of the
container wall will almost be constant over longer periods of
time--and therefore the temperature gradient to the surroundings
does not change rapidly. If an external heating was applied, the
temperature gradient to the surroundings would change very
dynamically because the temperature of the container wall would
increase and decrease with every initiation and ending of a heating
period.
* * * * *