U.S. patent application number 10/738346 was filed with the patent office on 2004-09-23 for method for reducing the formation of nitrogen oxides in steam generation.
This patent application is currently assigned to Fina Technology, Inc.. Invention is credited to Butler, James R..
Application Number | 20040185398 10/738346 |
Document ID | / |
Family ID | 32682254 |
Filed Date | 2004-09-23 |
United States Patent
Application |
20040185398 |
Kind Code |
A1 |
Butler, James R. |
September 23, 2004 |
Method for reducing the formation of nitrogen oxides in steam
generation
Abstract
Disclosed herein is a method for generating steam, comprising
oxidizing a fuel to generate heat via a flameless reaction; and
using the heat generated via the reaction to convert water to
steam. In an embodiment, the amount of NO.sub.x present is flue gas
from the reaction is less than about 10 PPMv. In an embodiment, the
reaction temperature is less than about 2600.degree. F.
(1430.degree. C.). In an embodiment, the method further comprises
controlling the reaction temperature to minimize the formation of
NO.sub.x. In an embodiment, controlling the reaction temperature
further comprises sensing one or more process variables and
adjusting a process controller in response to the sensed process
variable. Also disclosed herein is a steam generator comprising a
reaction zone wherein fuel is oxidized to generate heat via a
flameless reaction and a heating zone wherein water is converted to
steam via heat from the reaction.
Inventors: |
Butler, James R.;
(Friendswood, TX) |
Correspondence
Address: |
FINA TECHNOLOGY INC
PO BOX 674412
HOUSTON
TX
77267-4412
US
|
Assignee: |
Fina Technology, Inc.
|
Family ID: |
32682254 |
Appl. No.: |
10/738346 |
Filed: |
December 17, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60435503 |
Dec 20, 2002 |
|
|
|
Current U.S.
Class: |
431/2 ;
431/268 |
Current CPC
Class: |
F23C 99/00 20130101;
F22B 7/00 20130101; Y02E 20/34 20130101; F22B 31/00 20130101; F23C
13/00 20130101; F23C 2900/99001 20130101 |
Class at
Publication: |
431/002 ;
431/268 |
International
Class: |
F23Q 011/00 |
Claims
I claim:
1. A method for generating or heating steam or heating other
process streams, comprising: oxidizing a fuel at a reaction
temperature to generate heat via a flameless reaction; and using
the heat generated via the flameless reaction to convert water to
steam or heat other process streams, wherein the reaction
temperature is controlled to minimize the formation of
NO.sub.x.
2. The method of claim 1 wherein the reaction temperature is
controlled by adjusting the sizing or porosity of a fuel
distribution system.
3. The method of claim 1 wherein the reaction temperature is
controlled by adjusting one or more process variables selected from
the group consisting of reactant flow rates, reactant pressures,
reactant concentrations, reactant ratios, and the use or non use of
an oxidation catalyst.
4. The method of claim 3 wherein the one or more process variables
affecting the combustion temperature are adjusted by a
computer.
5. The method of claim 4 wherein the computer is a controller.
6. The method of claim 5 wherein the controller is a feedback loop
between a sensor and a process controller.
7. The method of claim 6 wherein the process controller is a flow
controller or a pressure controller.
8. The method of claim 6 wherein the sensor is a flue gas NOx
sensor.
9. The method of claim 8 wherein the temperature is controlled by
adjusting the reactant pressure, the reactant is fuel, and which in
turn adjusts the amount of fuel being fed to the flameless
reaction.
10. The method of claim 1 wherein the amount of NO.sub.x present is
in a flue gas from the flameless reaction, and is present at less
than about 10 PPMv.
11. The method of claim 1 wherein the amount of NO.sub.x present is
less than about 5 PPMv.
12. The method of claim 1 wherein the amount of NO.sub.x present is
less than about 3 PPMv.
13. The method of claim 1 wherein the reaction temperature is less
than about 2600.degree. F. (1430.degree. C.)
14. The method of claim 1 wherein the reaction temperature is less
than about 1600.degree. F. (871.degree. C.).
15. The method of claim 1 wherein the reaction temperature is
greater than the auto-ignition temperature of the fuel and the
difference between the reaction temperature and the auto-ignition
temperature of the fuel is less than or equal to about 100.degree.
F. (38.degree. C.).
16. The method of claim 15 wherein the difference between the
reaction temperature and the auto-ignition temperature of the fuel
is less than or equal to about 75.degree. F. (24.degree. C.).
17. The method of claim 16 wherein the difference between the
reaction temperature and the auto-ignition temperature of the fuel
is less than or equal to about 50.degree. F. (10.degree. C.).
18. The method of claim 17 wherein the difference between the
reaction temperature and the auto-ignition temperature of the fuel
is less than or equal to about 25.degree. F. (-4.degree. C.).
19. The method of claim 1 further comprising adding steam into the
fuel to reduce coking during the reaction.
20. The method of claim 1 further comprising using the heater in a
hydrocarbon cracking process, a distillation process, a reforming
process, to heat a process stream, or combinations thereof.
21. A process heater comprising: a reaction zone wherein a fuel is
oxidized to generate heat via a flameless reaction; a heating zone
wherein a process stream temperature is increased using heat from
the flameless reaction; and a device for adjusting one or more
process variables selected from the group consisting of reactant
flow rates, reactant pressures, reactant concentrations, reactant
ratios, and the use or non use of an oxidation catalyst.
22. The process heater of claim 21 wherein the device for adjusting
one or more process variables is a computer.
23. The process heater of claim 22 wherein the computer is a
controller.
24. The process heater of claim 23 wherein the controller is a
feedback loop between a sensor and a process controller.
25. The process heater of claim 24 wherein the process controller
is a flow controller or a pressure controller.
26. The process heater of claim 24 wherein the sensor is a flue gas
NO.sub.x sensor.
27. A process heater, comprising: a reaction zone wherein fuel is
oxidized to generate heat via a flameless reaction; a heating zone
wherein a process stream temperature is increase using heat from
the flameless reaction; and a fuel distribution system wherein the
sizing or porosity of the fuel distribution system is selected to
control the reaction temperature of the process heater.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This Application claims priority from the U.S. Provisional
Patent Application having Serial No. 60/435,503, filed Dec. 20,
2002.
FIELD OF THE INVENTION
[0002] The present application relates to a method for steam
generation.
BACKGROUND OF THE INVENTION
[0003] Steam is useful in a variety of industrial applications such
as petroleum or chemical plants. Traditionally, stream is produced
using a boiler, wherein a fuel is combusted to supply the heat
needed to convert water to steam. Environmental regulations require
reduced emissions of nitrogen oxides (NO.sub.x), such as nitric
oxide (NO) and nitrogen dioxide (NO.sub.2), from combustion
processes and equipment such as steam boilers. Thus, a need exists
for improved combustion processes and equipment that reduces the
amount of NO.sub.x emissions in the flue gas, especially to
ultra-low levels below about 10 parts per million by volume
(PPMv).
SUMMARY OF THE INVENTION
[0004] Disclosed herein is a method for generating steam,
comprising oxidizing a fuel to generate heat via a flameless
reaction; and using the heat generated via the reaction to convert
water to steam. In an embodiment, the amount of NO.sub.x present is
flue gas from the reaction is less than about 10 PPMv. In an
embodiment, the reaction temperature is less than about
2600.degree. F. (1430.degree. C.). In an embodiment, the method
further comprises controlling the reaction temperature to minimize
the formation of NO.sub.x. In an embodiment, controlling the
reaction temperature further comprises sensing one or more process
variables and adjusting a process controller in response to the
sensed process variable.
[0005] Also disclosed herein is a steam generator comprising a
reaction zone wherein fuel is oxidized to generate heat via a
flameless reaction and a heating zone wherein water is converted to
steam via heat from the reaction In an embodiment, the steam
generator further comprises a means for controlling the reaction
temperature to minimize the formation of NO.sub.x.
BRIEF DESCRIPTION OF THE DRAWINGS
[0006] FIG. 1 is a cross-section diagram of a flameless distributed
combustion heater.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0007] Description of Heater
[0008] Referring to FIG. 1, flameless distributed combustion (FDC)
heater 10 is in contact with a process fluid 20. Heater 10 includes
a fuel distribution system for distributing and metering a fuel
within the heater for a flameless oxidation reaction, for example
one or more porous tubes 15, pipes, or other structurally defined
flow passageways, channels, and the like. Porous tubes 15 include
openings or passages (i.e., pores) for the metering of fuel, and
the number, size, and arrangement of the pores may vary to achieve
desired fuel metering. Heater 10 further comprises one or more
reaction zones 5 in communication with the fuel distribution system
(e.g., porous tubes) and configured for receipt of and flameless
oxidation of the fuel therein. In an embodiment, an oxidation
catalyst may be disposed within the reaction zone for catalyzing
the flameless oxidation reaction. Heater 10 further includes one or
more heating zones 22 wherein a process fluid is heated via heat
generated from the oxidation reaction. The heating zones 22 may be
integral with the heater body (i.e., the process fluid passes
through the heater within walls 12), or located adjacent to the
heater in embodiments where the heater is placed directly in a
process stream or vessel within walls 12.
[0009] In the embodiment shown in FIG. 1, the reaction zone and
fuel distribution system are arranged in a shell and tube
configuration, respectively, wherein a plurality of porous tubes 15
are disposed within a reaction zone defined by outer shell wall 13
and inner shell wall 14. Alternatively, the reaction zone and fuel
distribution system may be arranged in a tube within a tube (e.g.,
concentric, offset, etc.) configuration, respectively, wherein a
plurality of porous inner tubes 15 are disposed within a reaction
zone defined by outer tube walls 13 and 14. The porous tubes may
support one or more oxidation catalysts on their outer surface. In
an embodiment, the heating zone is integrated into the heater as
shown by walls 12 with the process fluid 20 flowing through the
body of the heater as represented by arrows 17. Dashed arrow 17
represents that the heater may be configured to allow process fluid
20 to flow in interior portions of a shell and/or tube
configuration. Furthermore, the flow of process fluid 20 may be
concurrent, countercurrent, or crosscurrent with the flow of
components (e.g., reactants such as fuel and air and reaction
products such as flue gas) to and from the oxidation reaction. In
such an embodiment, the walls 12 include an outer shell enclosing
the inner shell or concentric tubes comprising the fuel
distribution system and reaction zone. In an alternative
embodiment, walls 12 define a process stream or tank, wherein the
heater is disposed therein.
[0010] The heater may include a preheat zone, wherein one or more
reactants such as air is preheated prior to entering the reaction
zone. For example, flue gas produced from the reaction may be used
to preheat the air in a preheat zone. In an embodiment, the preheat
zone is integrated within a shell and tube heater, for example by
placing the preheat zone upstream of the reaction zone and using
flue gas produced via the reaction to preheat one or more reactants
as they flow from the preheat zone to the reaction zone. The heater
may include one or more sensors, for example a NO.sub.x and/or
temperature sensor in flue gas outlet 30, and such sensors may be
coupled to one or more process controllers to control operation of
the heater. The fuel distribution system, reaction zone, and
heating zones may have alternative structural configurations, for
example a plate type heater wherein a plurality of shaped and/or
porous plates form the fuel distribution system and one or more
reaction zones. Examples of various FDC heater structural
configurations are shown shell and/or tube configurations in U.S.
Pat. Nos. 5,255,742; 5,297,626; 5,392,854; and 5,404,952; and for
plate-type configurations in U.S. Pat. No. 6,274,101, each of which
is incorporated herein by reference in its entirety.
[0011] Description of heater operation
[0012] Flameless oxidation of the fuel in heater 10 generates heat,
which is transferred to and heats the process fluid. More
specifically, fuel is fed into the fuel distribution system by one
or more inlets 23 and subsequently travels through the fuel
distribution system as shown by arrows 27. An oxidizer such as air
is preheated in a preheater (not shown) to greater than the
auto-ignition temperature of the fuel and fed to the combustion
chambers via inlet 24. A fuel's auto-ignition temperature (AIT) is
the temperature at which the fuel self-ignites in the presence of
the oxidizer (e.g., air) without an external source of ignition,
such as a spark or flame. During startup of the heater, the
oxidizer is preheated via an external heat source since heat from
the oxidation reaction is not yet available to preheat the
reactants and drive the reaction. Within the reaction zone, the
fuel mixes with the preheated air by passing or diffusing through
the walls of the fuel distribution system (e.g., porous tubes 15),
and the fuel undergoes flameless oxidation upon contact with the
oxidizer, that is the direct oxidation without a flame or
flamefront being generated. Upon initiation of the oxidation
reaction (i.e., light-off of the heater), the reactants (e.g., air)
may be heated to greater than the auto-ignition temperature using
heat produced via the reaction, thus creating an autothermal
reaction that may be self sustained.
[0013] Typically, the pressure in the fuel distribution system is
greater than the pressure in the reaction zone, thereby creating a
pressure differential that drives the diffusion of fuel into the
reaction zone as shown by arrows 26. The pressure in the fuel
distribution system may be increased or decreased, for example
using a process controller, to regulate the amount of fuel fed to
the reaction zone, which in turn controls the amount of heat
generated by the heater. When present, the oxidation catalyst
disposed within the reaction zone catalyzes the flameless oxidation
of the fuel. The oxidation of the fuel heats the surfaces of the
heater that are in contact with the process fluid (e.g., the
reaction chamber walls such as shell walls 13 and/or 14), and heat
is exchanged between the surfaces and the process fluid according
to known heat transfer means and technology. Flue gas comprising
reaction products (e.g., CO, CO.sub.2, H.sub.2O) and unreacted fuel
and oxidizer circulate through the heater as shown by arrows 28 and
exits via outlet 30.
[0014] The oxidizer may be oxygen, air, oxygen-enriched air, oxygen
mixed with an inert gas (i.e., diluent), and the like. Suitable
oxidization catalysts are known in the art, for example metal
catalysts such as platinum or palladium. The fuel may be hydrogen,
one or more hydrocarbons, or combinations thereof Typically, the
fuel contains a minimal amount of nitrogen chemically bound in the
fuel, thereby further minimizing the amount of nitrogen available
to form NO.sub.x. The fuel may be gas and/or vaporizable liquid,
with the fuel distribution system being configured (e.g., tube
porosity, that is the size and number of pores in the tubes, which
may be controlled by the manufacturing process and/or materials
selected) to allow diffusion of the fuel into the reaction zone
based upon the particular fuel to be used in the heater. In an
embodiment, the fuel is gaseous hydrocarbons comprising from about
1 to 4 carbon atoms. In an embodiment, the fuel further comprises
hydrogen. In another embodiment, the fuel consists essentially of
methane.
[0015] Description of Heater Operation to Minimize NO.sub.x
[0016] Thermal NO.sub.x and fuel NO.sub.x account for the majority
of NO.sub.x formed during the combustion of fossil fuels. Thermal
NO.sub.x is formed by the oxidation of molecular nitrogen in the
combustion air. Formation of thermal NO.sub.x is temperature
dependent, with greater amount of thermal NO.sub.x being formed at
higher temperatures, especially temperatures greater than about
2600-2800.degree. F. (1430-1540.degree. C.) wherein NO.sub.x
formation may begin to increase exponentially. Fuel NO.sub.x is
formed by the oxidation of nitrogen chemically bound within the
fuel. Formation of fuel NO.sub.x is oxygen concentration dependent
(in relationship of a perfect stoichiometric ratio), with NO.sub.x
formation the highest at fuel-to-air combustion ratios producing
about 5-7% O.sub.2 in the flue gas (25-45% excess air). Lower
excess air levels starve the fuel NO.sub.x reaction for oxygen, and
higher excess air levels drive down the flame temperature, slowing
the rate of the fuel NO.sub.x reaction.
[0017] In an embodiment, the heater is operated in a manner to
minimize the formation of NO.sub.x during combustion of the fuel,
for example to achieve what is referred to in industry as ultra low
NO.sub.x formation. For example, the temperature within the
reaction zone of the heater (i.e., reaction temperature) may be
controlled to minimize the formation of NO.sub.x. In an embodiment,
the flue gas comprises less than about 10 PPMv of NO.sub.x,
alternatively less than about 5 PPMv of NO.sub.x, and alternatively
less than about 3 PPMv of NO.sub.x. In an embodiment, the reaction
temperature is controlled to remain less than about 2600.degree. F.
(1430.degree. C.), which is about the temperature of a burner
flamefront as well as about the temperature at which NO.sub.x
begins to form at an exponential rate. In another embodiment, the
reaction temperature is controlled to remain substantially less
than 2600.degree. F. (1430.degree. C.). In another embodiment, the
reaction temperature is controlled to remain less than about
1600.degree. F. (871.degree. C.). At a reaction temperature of less
than about 1600.degree. F. (871.degree. C.), many fuels having an
AIT of less than 1600.degree. F. (871.degree. C.) in air are
available. Furthermore, at less than about 1600.degree. F.
(871.degree. C.), conventional materials such as grade 304 steel
may be used to construct the heater components (e.g., reaction
zone, fuel distribution system, etc.) rather than more expensive
materials having a higher heat tolerance. In another embodiment,
the reaction temperature is controlled to remain less than about
150.degree. F. (816.degree. C.).
[0018] In an embodiment, the target temperature or temperature
range for a heated process stream is provided. Other process
variables such as pressure, phase, and flow rate may also be
provided for the heated process stream. For example, the heated
process stream may be steam having a desired temperature and
pressure (e.g., superheated steam). For an available fuel and
oxidizer type and concentration, the AIT of the fuel is determined.
The reaction is lit-off by heating the oxidizer to greater than the
AIT of the fuel, and subsequently introducing fuel to the heated
oxidizer. In order for the oxidation reaction to continue, the
reaction temperature is controlled such that it remains about equal
to or greater than the AIT of the fuel, otherwise the oxidation
reaction would terminate. In order to provide a buffer for
temperature fluctuations, the reaction temperature may be
controlled to remain at about a set point (i.e., tolerance) greater
than the fuel AIT, for example about 25, 50, 75, 100.degree. F.
(-4, 10, 24, 38.degree. C.) or greater above the fuel AIT. In an
embodiment, the reaction temperature is controlled such that the
difference between the reaction temperature and the AIT of the fuel
is minimized within a given tolerance, e.g., about 25, 50, 75,
100.degree. F. or greater (-4, 10, 24, 38.degree. C.). In an
embodiment, the reaction temperature is controlled such that it
remains about equal to or greater than the AIT of the fuel and less
than about 1600.degree. F. (871.degree. C.), alternatively less
than about 1500.degree. F. (816.degree. C.).
[0019] The reaction temperature may be controlled by equipment
configuration such as heater sizing, porosity (size and number of
pores) of the fuel distribution system, etc.; adjusting one or more
process variables such as reactant (e.g., fuel and/or oxidizer)
flow rates, pressures, concentrations, ratios, etc.; and/or by use
of one or more oxidation catalysts. One or more of the process
variables affecting the combustion temperature may be computer
controlled, for example via a feedback loop between a sensor and a
process controller such as a flow controller, pressure controller,
etc. In an embodiment, a temperature controller is coupled to a
flue gas NO.sub.x sensor, allowing computerized feedback control of
one or more process variables to control the temperature. In an
embodiment, the reaction temperature is controlled by adjusting the
fuel pressure, which in turn adjusts the amount of fuel being fed
to the reaction.
[0020] The type and amount of oxidation catalyst may be selected
and/or adjusted to assist in the control of the reaction
temperature. In an embodiment, the presence of an oxidation
catalyst lowers the reaction temperature in comparison to similar
reaction conditions with no oxidation catalyst present. The amount
of oxidizer present in the reaction zone (i.e., the molar ratio of
oxygen to fuel) may be selected and/or adjusted to assist in the
control of the reaction temperature. In an embodiment, an increase
in the molar ratio of oxygen to fuel lowers the AIT of the fuel in
comparison to similar reaction conditions with a lower molar ratio
of oxygen to fuel, thereby allowing the reaction temperature to be
likewise reduced. Such an embodiment might be temporarily used
during start up of the heater, with a subsequent shift to lower
oxygen to fuel ratios. For example, oxygen enriched are may be used
to start the heater at a lower temperature, with a gradual switch
to air as temperatures increase.
[0021] Typically, a temperature gradient exists with higher
temperatures at or near the outer surface of the fuel distribution
system (referred to as the skin temperature, which typically is
about equal to the reaction temperature) and decreasing
temperatures at increasing distances from the outer surface. In an
embodiment, the skin temperature is controlled to minimize coke
formation on the outer surface of the fuel distribution system, for
example by controlling the skin temperature to about less than the
coking temperature of the fuel. Formation of coke may be further
minimized by adding steam to the fuel prior to introduction into
the reaction zone. In an embodiment, about 0.1 to about 0.2 weight
percent steam is added to the fuel prior to the fuel being
introduced into the reaction zone. In an embodiment, the oxidation
catalyst is present at or near the outer surface of the fuel
distribution system, thereby lowering the skin temperature thereof
required to maintain the oxidation reaction. In an embodiment, the
flow of reactants through the combustion chamber is selected to
assist with heat transfer to the process fluid, for example by
maintaining turbulent rather than laminar flow across the outer
surface of the fuel distribution system.
[0022] The heater 10 may be used to heat a process fluid, for
example a feed, intermediate, or product stream within a
manufacturing facility such as a chemical plant or petroleum
refinery. For example, the process heater 10 may be disposed within
a process flow line or process vessel such as a tank as defined by
walls 12, or alternatively the process fluid may be passed through
the heater as defined by walls 12. In an embodiment, the heater is
used as a process heater in a hydrotreater. In an embodiment, the
heater is used as a reboiler in a distillation column. In an
embodiment, the heather is used to heat a process stream in a
reformer, for example between catalyst beds. The process fluid may
be a solid, semi-solid, liquid, or gas, and the heater is
configured for heat exchange with the physical state of the process
fluid according to known heat exchange technology. In an
embodiment, the process fluid does not chemically react upon being
heated, and thus the heating zone does not function as a reaction
zone. In an embodiment, the process fluid chemically reacts upon
being heated, and thus the heating zone also functions as a
reaction zone. In an embodiment, the process fluid is crude oil
being distilled in a petroleum refinery, for example preheating
crude oil for distillation in a crude tower.
[0023] In a steam boiler embodiment, the process fluid is water,
which is converted to steam by contact with heater 10. The water
may optionally include additives such as anti-scale additives. In
an embodiment, steam is produced at a temperature greater than
about 400.degree. F. (204.degree. C.), alternatively greater than
about 500.degree. F. (260.degree. C.). In an embodiment, a portion
of the steam generated is recycled and combined with the fuel prior
to oxidation of the fuel to reduce coking of the fuel. In an
embodiment, the steam boiler is employed within a petroleum
refinery and the steam is used in a hydrocarbon cracking process,
to power a steam turbine, to heat a process stream, or combinations
thereof. In an embodiment, the steam is used to facilitate heat
transfer at another location, for example via a steam jacket or
increasing the temperature of a heating fluid such as a heating oil
or antifreeze. In an embodiment, light off gas comprising, or
alternatively consisting essentially of, hydrocarbons having less
than about 4 carbon atoms from one or more petroleum refining
processes is used as a fuel to the heater.
[0024] While the preferred embodiments and examples of the
invention have been shown and described, modifications thereof can
be made by one skilled in the art without departing from the spirit
and teachings of the invention. Heater design criteria (including
sizing, selection of construction materials, and fabrication),
pendant processing equipment, and the like for any given
implementation of the invention will be readily ascertainable to
one of skill in the art based upon the disclosure herein. The
embodiments and examples described herein are provided for
illustration and are not intended to be limiting. Many variations
and modifications of the invention disclosed herein are possible
and are within the scope of the invention. Accordingly, the scope
of protection is not limited by the description set out above, but
is only limited by the claims which follow, that scope including
all equivalents of the subject matter of the claims.
* * * * *