Closed control loop with ammonia slip and NOx sensor feedback for use in selective catalytic reduction system

Abstract

A selective catalytic reduction system control system (10) and method of its use include an ammonia (“NH.sub.3”) slip sensor (13) located within an interior space (27) of an exhaust stack (15) of a selective catalytic reactor (31), toward an inlet end (25) of the stack (15); a housing (17) located within the interior space of the exhaust stack; the housing including face panels 19; a nitrogen oxides (“NOx”) sensor (11) contained within an interior space (29) defined by the face panels of the housing, at least two of the face panels (19.sub.I, 19.sub.O) containing an oxidation catalyst; and a dosing controller (59) in communication with the NH.sub.3 and NOx sensors, the dosing controller including a microprocessor with dosing logic embedded thereon. The housing with oxidation catalyst acts as a linear box, isolating the NOx sensor from NH.sub.3 slip, linearizing the NOx sensor signal.

Claims

1. A selective catalytic reduction system control system (10) comprising: an ammonia (“NH.sub.3”) slip sensor (13) located within an interior space (27) of an exhaust stack (15) of a selective catalytic reactor (31); a nitrogen oxides (“NOx”) sensor (11) contained within the interior space (27), the NOx sensor isolated from NH.sub.3 slip within the exhaust stack and located downstream of the NH.sub.3 slip sensor; and a dosing controller (59) in communication with the NH.sub.3 slip sensor and NO.sub.x sensor, the dosing controller including non-transitory machine-readable storage medium containing dosing logic executable by the dosing controller; wherein the dosing logic corrects sensor readings sent to it by the NOx sensor by sensor readings sent to it by the NH.sub.3 slip sensor; wherein the sensor readings of the NH.sub.3 slip sensor are real values of NH.sub.3 slip; and wherein the dosing logic adjusts an amount of liquid reduction agent being injected into an exhaust stream based upon the corrected NOx readings.

2. A method of controlling a dosing system (53) of a selective catalytic reduction system (30) configured for use with a diesel engine (60), the dosing system including a dosing controller (59) in communication with an-ammonia (“NH.sub.3”) slip sensor (13) located within an interior space (27) of an exhaust stack (15) of a selective catalytic reactor (31) and a nitrogen oxides (“NOx”) sensor (11), the NOx sensor isolated from NH.sub.3 slip within the exhaust stack and located downstream of the NH.sub.3 slip sensor, the dosing controller including a non-transitory machine-readable storage medium executable by the dosing controller, the method comprising: obtaining sensor readings from the NH.sub.3 slip sensor indicating a concentration of ammonia in an exhaust stream of the diesel engine; obtaining sensor readings from the NOx sensor indicating a concentration of nitrogen oxides in the exhaust stream; correcting the sensor readings of NOx by using the sensor readings of NH.sub.3 by calculating a NOx real value and a NH.sub.3 slip real value, wherein NOx sensor reading=NOx real value+NH.sub.3 slip real value, the NH.sub.3 slip real value being the NH.sub.3 sensor reading, and wherein NOx real value=NOx sensor reading−NH.sub.3 sensor reading; and adjusting an amount of liquid reduction agent being injected into exhaust stream of the diesel engine in response to the NOx real value.

3. Non-transitory machine readable storage medium containing dosing logic executable by a dosing controller (59) for controlling a dosing system (53) of a selective catalytic reduction system (30) configured for use with a diesel engine (60), the dosing logic comprising: obtaining sensor readings indicating a concentration of ammonia in an exhaust stream of the diesel engine by way of an ammonia (“NH.sub.3”) slip sensor (13) located within an interior space (27) of an exhaust stack (15) of a selective catalytic reactor (31); obtaining sensor readings indicating a concentration of nitrogen oxides in the exhaust stream by way of a nitrogen oxides (“NOx”) sensor (11), the NOx sensor isolated from NH.sub.3 slip within the exhaust stack and located downstream of the NH.sub.3 slip sensor; correcting the sensor readings of NOx by using the sensor readings of NH.sub.3 by calculating a NOx real value and a NH.sub.3 slip real value, wherein NOx sensor reading=NOx real value+NH.sub.3 slip real value, the NH.sub.3 slip real value being the NH.sub.3 sensor reading, and wherein NOx real value=NOx sensor reading−NH.sub.3 sensor reading; and adjusting an amount of liquid reduction agent being injected into exhaust stream of the diesel engine in response to the NOx real value.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic of an embodiment of a selective catalytic reduction (“SCR”) closed loop control system of this disclosure. The system includes dual feedback sensors, one for nitrogen oxides (“NOx”) and another for ammonia (“NH.sub.3”) slip

(2) FIG. 2 shows an embodiment of a stack of an SCR reactor including a housing (“box”) with oxidation catalysts located inside the stack and housing a NOx sensor, thereby isolating the sensor from any NH.sub.3 slip flowing through the stack. Essentially the housing with the oxidation catalysts is acting as a linear amplifier, a linear response, a linear regression, a linear equation, or their equivalents, making the NOx sensor's signal linear and behaving as a straight line or slope when graphed. The housing, therefore, may be referred to as a “linear box.” The sensors are in communication with a controller. The mixing blades located downstream of an SCR catalyst bed remix all molecules just upstream of a NOx sensor and a NH.sub.3 slip sensor. This lowers variation in sensor readings and improves accuracy and precision of both sensors' readings.

(3) FIG. 3 is an isometric view of SCR reactor of FIG. 2. The NOx sensor is located inside the box. The NH.sub.3 sensor is located adjacent to the linear box and within the stack.

(4) FIG. 4 is an isometric view of the box of FIG. 2. Oxidation catalyst panels are located at the top and bottom, thereby isolating the NOx sensor from NH.sub.3 slip.

(5) FIG. 5 is an isometric view of the box of FIG. 4. The linear box includes a NOx sensor port arranged so the sensor lies in between the top and bottom oxidation catalyst panels. Exhaust flow may be reversed in some applications as may the orientation of the box to accommodate a horizontal exhaust flow.

(6) FIG. 6 is a schematic of an embodiment of a dual sensor system of this disclosure. The sensors are located in the exhaust stack. The housing with the oxidation catalysts linearizes the NOx sensor signal.

DEFINITIONS

(7) For the purpose of this disclosure, a “linear box” is a housing that acts as a sensor tunnel or slipstream assembly, permitting an exhaust flow through it, and containing a NOx sensor and including oxidation catalysts that isolate the sensor from NH.sub.3 slip flowing in the exhaust stack containing the housing. The housing with the oxidation catalyst acts as a linear amplifier, a linear response, a linear regression, a linear equation, or their equivalents, making the NOx sensor's signal linear and behaving as a straight line or slope when graphed.

DETAILED DESCRIPTION

(8) Referring to the drawings, embodiments of a selective catalytic reduction (“SCR”) closed loop control system 10 of this disclosure includes dual feedback sensors 11, 13, one for nitrogen oxide (“NOx”) and another for ammonia (“NH.sub.3”) slip. The sensors 11, 13 are located at the stack 15, downstream of a selective catalytic reduction (“SCR”) catalyst bed, with the NOx and NH.sub.3 sensors 11, 13 providing information to control the injection of a liquid-reductant agent to the SCR system. The NOx sensor 13 is housed within a housing 17 that isolates it from the total exhaust flow through the stack but permits a portion of that total exhaust flow to flow through the housing 17. The housing 17, which is not a completely closed structure, may be thought of as a sensor tunnel or slipstream assembly. In some embodiments, the liquid-reductant agent may be DEF. The ammonia needed for NOx reduction comes from the decomposition of the urea solution or from aqueous ammonia that is injected and evaporated within an exhaust duct, pipe, or reactor. In embodiments, both sensors 11, 13 can be located toward in inlet end 25 of an exhaust stack 15 of an SCR reactor 31. The NH.sub.3 slip sensor 13 reading provides an accurate and precise bias correction value in real time to the NOx sensor 11 reading.

(9) In embodiments, the control loop 10 includes an NH.sub.3 slip sensor 13 located within an interior space 27 of an exhaust stack 15 of an SCR reactor 31 and a housing 17 containing the NOx sensor 11. Both sensors 11, 13 should be located at the same stream location. The stack 15 is wide enough to allow for the location of two ports 14, 27, for example, athwart (traverse) from the exhaust flow direction. In a square stack 15 the sensors 11, 13 may be located on the same wall 45 of the stack 15 but do not have to be located on the same wall. In some embodiments, the NH.sub.3 sensor 13 may be upstream or downstream of the NOx sensor 22, depending of the application. This is to adapt to the addition of specific catalyst bed (e.g., oxidation, formaldehyde, other) for custom emission control requirements. The housing 17 may be located upstream or downstream and on a same or different side of the stack 15 as the NH.sub.3 slip sensor 13, adjacent to the NH.sub.3 slip sensor 13. The housing 17 may include one or more oxidation catalyst panels 19 that surround and isolate the NOx sensor 11 that is contained within an interior space 29 of the housing 17 defined by the face panels 19. Other walls 35 of the box may be solid walls. A flange 33 may be used to mount the box 17 to a sidewall 45 of the stack 15. The sensor 11 may be inserted into the housing 17 through a port 27. The housing 17 may be any shape preferable, for example, a square- or rectangular-shaped box. In other embodiments, the housing 17 is sized and shaped appropriate to a stack 15 having a circular-shaped cross section.

(10) The housing 17 with the oxidation catalysts linearizes the NOx sensor signal. Essentially, the housing 17 with the oxidation catalysts acts as a linear amplifier, a linear response, a linear regression, a linear equation, or their equivalents, making the NOx sensor's signal linear, behaving as a straight line or slope when graphed. The housing 17 when configured in this way, therefore, may be referred to as a “linear box.” With the linear box, the NOx sensor 11 signal (the input) is always truly proportional to the real NOx value measured in the exhaust stack 15 and because the signal slope is a straight line, the NOx measurement is simple to perform and accurate. Without this linear box 17, the NOx sensor 11 is sending a signal that is not proportional to real NOx value. The non-linearized signal is truly aberrant (at fault and therefore not capable of consistent bias correction) and indicates a NOx value far from the real NOx value in the stack. The NH.sub.3 sensor 13 may also be in communication with signal processing means 73 or may be sent directly to the dosing controller 59.

(11) The NOx and NH.sub.3 sensors 11, 13 may be automotive-style sensors similar in size to an automotive vehicle's oxygen sensor and connectable to computer processing means by way of a controller area network (“CAN”) bus. For purposes of this disclosure, an automotive-style sensor is a sensor including a CAN bus connection. The sensors 11, 13 should be installed so that they sample directly from the exhaust stack without the need for sampling probes or sampling lines, thereby avoiding issues like sample line plugging and NOx value averaging. In some embodiments, the sensors 11, 13 are mounted toward the same side of the stack 15. In other embodiments, the sensors 11, 13 may be mounted on different sides of the stack 15. The sensors 11, 13 may be oriented in a sideway direction relative to the stack, that is, mounted transverse to a direction of exhaust flow through the stack 15 For example, the sensors 11, 13 may be oriented parallel to a lateral axis 12 of the stack 15, the exhaust flow being perpendicular to the longitudinal axis 16 of the exhaust stack 15. the linear box 17 being oriented so exhaust flows through the panels 19.sub.I, 19.sub.O.

(12) The exhaust flow may be flowing upward or downward or, for that manner, in any other direction, and a smaller portion of this total exhaust flow is flowing through the linear box 17. By way of a non-limiting example, the linear box 17 may be sized to handle or accommodate no more than 10%, no more than 5%, and no more than 1% of the total exhaust flow through the stack 15. By way of a non-limiting example, if the total exhaust flow is 14,000 actual cubic feet per minute (“ACFM”) (about 396 cubic meters per minute), then the exhaust flow going through the linear box 17 may be range from 140 to 1,400 ACFM. Another non-limiting example would be for a larger exhaust flow of 50,000 actual cubic feet per minute (“ACFM”) (about 1416 cubic meters per minute), where the exhaust flow through the linear box 17 could be in the range from 500 to 5,000 ACFM.

(13) Because the exhaust gas stream flowing through the stack 15 may still contain NH.sub.3 slip in addition to the NOx, the panels 19 include oxidation catalysts of a kind known in the art to minimize (or eliminate completely) the NOx sensor's exposure to NH.sub.3. Any NH.sub.3 slip flowing through the oxidation catalyst panels 19 is converted to NOx. The oxidation catalyst panels 19 perform this function by converting NH.sub.3 to NOx in a ratio of about one mole NH.sub.3 to one mole NOx. Therefore, the NOx sensor is continually reading NOx and is never exposed to any ammonia molecules.

(14) One oxidation catalyst panel 191 may be arranged toward the inlet side 21 of the linear box 17 and another of the panels 190 may be located toward the outlet side 23, with the NOx sensor 11 located in between. The inlet side oxidation catalyst panel 191 is doing most of the NH.sub.3 conversion, with the outlet side oxidation catalyst panel 190 providing an outlet 29 of the box 17. For example, depending on the main exhaust flow condition and flow dynamics in the stack 15, one of the panels 19 may be arranged as a bottom panel or face of the box 17 and the other as a top panel or face, with the NOx sensor 11 located in between and equidistant from each panel 19. In a low flow condition, the flow may be inverted downward.

(15) Because of this arrangement, the NOx sensor reading is:
NOx sensor reading=NOx real value+NH.sub.3 slip real value  (Eq. 1)
Because the NH.sub.3 slip sensor 13 is located outside of the box 17, the sensor 13 is reading a real (true) value for the NH.sub.3 slip in the exhaust gas stream with no bias to other molecules. In other words, the NH.sub.3 slip sensor 13 may be used to make an accurate and precise bias correction value in real time to the NOx sensor 11 reading. Therefore,
NOx real value=NOx sensor reading−NH.sub.3 sensor reading  (Eq. 2)
In embodiments, this real value is used by the injection control/dosing system for adjusting the injection of the reagent such as DEF.

(16) In embodiments, static mixers 41 may be located downstream of the SCR catalyst 43, see FIG. 2, but upstream of the stack 15 to mix the exhaust gas stream molecules out of the SCR NOx reduction process. This additional mixing process feeds the linear box 17 and stack 15 with an exhaust gas stream containing a low root-mean-square (“RMS”) deviation of NOx and NH.sub.3 content. Therefore, the NOx and NH.sub.3 slip sensors 11, 13 experience less variability. The mixing quality achieved, as measured by RMS deviation for the NOx and NH.sub.3 concentration, may be less than 3%, and may be in a range or 0.25% to 2.75%, there being discrete values and subranges within the broader range of 0.25% to 3%. In some embodiments, the RMS deviation is no greater than 1%. The low RMS deviation helps stabilize the proportional-integral-derivative (“PID”) loop feedback process and eliminates the need for sampling probes. The measured NOx and NH.sub.3 slip values are more representative of the whole exhaust gas stream in real time.

(17) In embodiments of a SCR closed loop control system of this disclosure a programmable logic controller (“PLC”), or its equivalent, including software residing in ladder logic with PID function may be used to perform the logic functions and mathematics required, and a human-machine interface may be provided for setup and operation of the control system. The software may incorporate features and functions such as, but not limited to, characteristic liquid-reductant agent curves, set point control at various engine loads, injection permissive logic, NOx value correction of NH.sub.3 bias, auxiliary functions (e.g. data logging, alarm management, remote access management). Other forms of electronic computation and control may be used. For example, one or more controllers of the system and method may use a microprocessor and associated software.

(18) In embodiments, the SCR closed loop control system receives signals from the NOx and NH.sub.3 slip sensors; an analog signal input from an engine indicating the engine load, the fuel flow, or some combination of the two; and an analog signal output from a PLC controlling the injection of the liquid-reductant agent. Injection may occur through one or more injection lances 54 configured to inject the urea or ammonia into a mixing duct of the SCR system. The box 17 may include a stack configuration that can be adapted for a round or a rectangular or square-shaped stack 15. The downstream mixers 41 can be adaptable to various SCR cross-sections and include a compact flow axis space requirement.

(19) In embodiments, the control system 10 may be used with a diesel emission reduction system 30 that include one or more of the following features: means 51 to reduce DPM such as a diesel particulate filter (“DPF”) of a kind known in the art, a diesel oxidation trap catalyst (“DOTC”) like that disclosed in PCT/US19/13433 to Catalytic Combustion Corp., or some combination of the two; a dosing system 53 to precisely meter a liquid-reductant agent such as diesel exhaust fluid (“DEF”) into an injection duct of a SCR system; and a mixing duct 55 and post-bed mixer 41 each including static mixers configured to create a homogeneous mixture of NH.sub.3 in the exhaust stream prior to an SCR catalyst bed 43. The closed-loop dosing control system 10 includes a controller 59 and the NO.sub.x and NH.sub.3 concentration sensors 11, 13 (which may include a controller area network (“CAN”) bus connector C) arranged to prevent cross-interference of the NOx sensor 11 by excess NH.sub.3 in the exhaust stream. Engine exhaust from a non-road diesel engine 60 is routed through the diesel emission control system 30 where the diesel emissions are converted to nitrogen, water vapor and CO.sub.2.

(20) The closed loop dosing control system 10 controls the amount of liquid-reductant agent or DEF that is metered into the exhaust stream via a targeted NO.sub.x value, utilizing the NO.sub.x concentration sensor 11 to sense the amount of NO.sub.x in the exhaust stream. Oxidation catalyst panels 19, see e.g. FIGS. 4 & 5, protect or isolate the NOx concentration sensor 11 to prevent NH.sub.3 cross interference. The NH.sub.3 sensor 13 is used to detect excess NH.sub.3 in the exhaust stream. The sensors 11, 13 are coupled to control logic which ensures extremely precise metering of the DEF into the exhaust stream. The engine control system 20 and the dosing controller 59 communicate so that the emission control system 30 will precisely inject the agent, such as DEF, appropriately based upon engine operation. Other agents of a kind known in the art may be used where appropriate.

(21) Embodiments of a method of controlling a dosing system (53) of a selective catalytic reduction system (30) configured for use with a non-road diesel engine (60) includes the following steps: obtaining a sensor reading (R) indicating a concentration of ammonia in an exhaust stream of the non-road diesel engine by way of an ammonia (“NH.sub.3”) slip sensor (13) located within an interior space (27) of an exhaust stack (15) of a selective catalytic reactor (31), toward an inlet end (25) of the stack (15); obtaining another sensor reading (R′) indicating a concentration of nitrogen oxides in the exhaust stream by way of a nitrogen oxides (“NOx”) sensor (11) contained within a housing (17) located within the interior space of the exhaust stack; the housing including face panels (19) defining an interior space (29) of the housing, at least two face panels (19.sub.I, 19.sub.O) containing an oxidation catalyst; sending said sensor readings to a dosing controller (59) in communication with the NH.sub.3 and NOx sensors, the dosing controller including a microprocessor with dosing logic embedded thereon; the dosing controller; receiving the sensor readings and calculating a NOx real value and a NH.sub.3 slip real value. wherein NOx sensor reading=NOx real value+NH.sub.3 slip real value; and wherein NOx real value=NOx sensor reading−NH.sub.3 sensor reading; and the dosing controller adjusting an amount of liquid reduction agent being injected into the exhaust stream in response to the NOx real value and the NH.sub.3 slip real value.

(22) While embodiments of a SCR closed loop control system have been described, the control system and method of its use are capable of modification by persons of ordinary skill in the art without departing from the scope of this disclosure. The claims include the full range of equivalents to which each recited element is entitled.