Patent Application
20140260511
Industrial process industries often rely on energy sources that include one or more combustion processes. Such combustion processes include operation of a furnace or boiler to generate steam or to heat a feedstock liquid. While combustion provides relatively low-cost energy, combustion efficiency is sought to be maximized, and the resulting flue gasses exiting the smokestack are often regulated. Accordingly, one goal of the combustion process management industry is to maximizing combustion efficiency of existing furnaces and boilers, which inherently also reduces the production of greenhouse gases. Combustion efficiency can be optimized by maintaining the ideal level of oxygen in the exhaust or flue gases coming from such combustion processes.
In-situ or in-process analyzers are commonly used for the monitoring, optimization, and control of the combustion processes. Typically, these analyzers employ sensors that are heated to relatively high temperatures and are operated directly above, or near, the furnace or boiler combustion zone. Known process combustion oxygen analyzers typically employ a zirconium oxide sensor disposed at an end of a probe that is inserted directly into a flue gas stream. As the exhaust or flue gas flows into the sensor, it diffuses through a filter called a diffuser into proximity with the sensor. There are no pumps or other flow-inducing devices to direct a sample flow into the sensor; the gases diffuse passively through the diffuser filter. The sensor provides an electrical signal related to the amount of oxygen present in the gas. While the diffuser allows diffusion therethrough, it also protects the sensor from physical contact with airborne solids or particulates.
Some combustion applications can adversely affect the combustion analyzer. For example, combustion processes that generate a heavy particulate load in the flue gas stream can clog or otherwise reduce the efficacy of the diffuser. When a diffuser in an in-situ probe becomes plugged, either completely or partially, the response of the analyzer to process variable changes can be slowed due to reduced or ineffective diffusion from the process to the measuring cell. Moreover, calibration errors can be caused due to back pressure on the measuring cell during calibration. Finally, at the end of a calibration cycle, the process combustion gas measurement (such as oxygen level) may still be influenced by the calibration gas. Properly detecting a plugged diffuser in a combustion process gas analyzer would reduce the possibility and effects of the problems set forth above. Moreover, given that replacement of a diffuser of a combustion process gas analyzer may require the combustion process to be taken offline, it is also not desirable to replace the diffuser unless warranted. Providing an in-situ process combustion process gas analyzer and method that are able to effectively determine when diffuser replacement or reconditioning is warranted would represent an advance in the art of combustion process monitoring.
A process gas analysis system is provided. The system includes a probe insertable into a source of process gas and having a distal end and a chamber proximate the distal end. A gas sensor is mounted within the chamber and is configured to provide an electrical indication relative to a species of gas. A diffuser is mounted proximate the distal end of the probe and is configured to allow gas diffusion into the chamber. A source of calibration gas is operably coupled to the probe and is configured to supply calibration gas, having a known concentration of the gas species. Electronics are coupled to the sensor and configured to store a pre-calibration process gas concentration and to measure an amount of time (sensor return time) for the sensor response to return to the pre-calibration process gas concentration. The electronics are configured to compare a measured sensor return time with a known-good sensor return time to provide an indication relative to the diffuser.
Housing 102 has a chamber 114 that is sized to house electronics 106. Additionally, housing 102 includes internal threads that are adapted to receive and mate with external threads of end cap 116 to form a hermetic seal. Additionally, housing 102 includes a bore or aperture therethrough allowing electrical interconnection between electronics 106 and measuring cell or sensor 112 disposed within distal end 108 of probe 104.
Probe 104 is configured to extend within a flue, such as flue 14. Probe 104 includes a proximal end 118 that is adjacent flange 120. Flange 120 is used to mount or otherwise secure the transmitter 100 to the sidewall of the flue. When so mounted, transmitter 100 may be completely supported by the coupling of flange 120 to the flue wall.
Electronics 106 provide heater control and signal conditioning, resulting in a linear 4-20 mA signal representing flue gas oxygen. Preferably, electronics 106 also includes a microprocessor that is able to execute programmatic steps to provide the functions of diffuser diagnostics as will set forth in greater detail below. However, in some embodiments, transmitter 100 may simply be “a direct replacement” probe with no electronics and thus sending raw millivolt signals for the sensing cell and thermocouple providing indications representative of the oxygen concentration and cell temperature, respectively. In embodiments where a “direct replacement” probe is used, the probe is coupled to a suitable analyzer such as the known Xi Operator Interface available from Rosemount Analytical Inc. The Xi Operator Interface provides a back-lit display, signal conditioning and heater control within a NEMA 4X (IP 66) enclosure. The electronics of the Xi Operator Interface also provides features, such as automatic calibration, stoichiometer indications in reducing conditions, and programmable reference features for measuring at near-ambient levels. Accordingly, the Xi Operator Interface includes suitable processing abilities to perform diffuser diagnostics in accordance with embodiments of the present invention. Thus, in applications where the transmitter is a “direct replacement” probe embodiments of the present inventions can still be practiced.
Over time, it is periodically necessary to calibrate sensor 122. Embodiments of the present invention generally leverage the behavior of the oxygen sensor occurring between a calibration mode and a process monitoring mode. For reference, both modes are described with respect to
Embodiments of the present invention generally measure the temporal response of the oxygen sensor between the calibration mode and the process monitoring mode. The temporal response of the oxygen sensor can be analyzed to detect when diffuser 110 is plugged, either completely or partially. When the oxygen transmitter is new, either just manufactured, or newly installed, a sensor return time value is obtained for a known good configuration. For example, the analyzer can be installed into a new combustion installation, and can be operated to read a flue gas oxygen concentration. Preferably, just prior to calibration, the flue gas oxygen concentration is stored in memory, either the memory of electronics of the oxygen transmitter, or memory of the external device that is coupled to the direct replacement probe. Then, calibration is initiated wherein a calibration gas having a known oxygen concentration is flowed into the distal end of the probe between the measuring sensor and the diffuser. The calibration gas is flowed for a suitable length of time to ensure that all combustion gas is removed from the distal end. Then, a measurement of the calibration gas oxygen content is obtained from the sensor. A suitable amount of time can be a specific time, such as one minute, or can be based upon the sensor response, such that when the sensor response change level is below a certain threshold (indicating substantial steady state) then the calibration measurement can be made. Once the calibration measurement has been made, the calibration gas flow is ceased, a timer is initiated and the sensor output is monitored. The timer is used to measure the length of the time from the cessation of the calibration gas to the point in time where the sensor measures a combustion gas oxygen amount that matches the value that was stored just prior to calibration. Since, the measured time is obtained during a known good configuration, it is stored as a known-good sensor return time or threshold. Alternatively, the known-good threshold can simply be programmed into the transmitter at the time of manufacture. Further still, in some embodiments, the method may wait until the sensor is indicating substantial steady state. The objective is to have confidence that the sensor has returned to the combustion gas measurement, which may have changed during calibration.
Later, after the transmitter has operated for some time, such as months or years, each time a calibration cycle is performed, the time required for the combustion gas sensor to return to the process oxygen value, stored just prior to a calibration, is compared with the known-good configuration threshold. This comparison may be a simple comparison to determine if the later time measurement is equal to or less than the known-good threshold, thus indicating that the diffuser is operating effectively. Additionally, a small buffer can be added to the known-good time threshold such that a slight amount of obstruction can be tolerated. For example, the measured sensor return time can be compared to the known-good threshold and if the measured sensor return time is at or below 110% of the known-good threshold, the diffuser can be indicated as being effective. Conversely, if the measured sensor return time exceeds the known-good threshold with the optional buffer, then an indication can be provided that the diffuser has deteriorated to such an extent that it requires replacement or repair. This alert can be provided through a process communication loop, either using a known process communication protocol, such as the digital Highway Addressable Remote Transducer (HART®) communication standard, through a local operator interface, or both depending upon the application. Additionally, a local enunciator, such as an LCD or an audible alarm can be provided at the transmitter itself.
Embodiments of the present invention generally provide a method that is easily implemented in existing hardware to allow processors, such as the processor of the transmitter, or a processor of an operator interface to provide a diagnostic indication relative to the diffuser of the transmitter. This allows a technician to be alerted precisely when diffuser replacement or repair is required. Thus, accurate and timely measurements of combustion oxygen are provided, and technician time required to replace or refurbish the diffuser is minimized.
Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.
