Patent Application
20170003246
The 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. In addition, flue gases from industrial processes exiting smokestacks are often regulated, and the amount of dangerous gases often must be minimized. Accordingly, one goal of the combustion process management industry is to maximize combustion efficiency of existing furnaces and boilers, which inherently also reduces the production of greenhouse and other regulated 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 process. 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 into proximity with the sensor. The sensor provides an electrical signal related to the amount of oxygen present in the gas.
A sensor system configured to detect oxygen in an exhaust stream of an industrial process is provided. In one embodiment, the sensor system comprises a probe with an oxygen-detecting sensor, wherein the oxygen-detecting sensor detects a concentration of oxygen in the exhaust stream. The system may also comprise a catalytic converter located on the probe near the sensor, wherein the catalytic converter is configured to convert carbon monoxide to carbon dioxide. The system may also comprise a signal detector configured to detect a change in oxygen concentration indicative of a carbon monoxide breakthrough. These and various other features and advantages that characterize the claimed embodiments will become apparent upon reading the following detailed description and upon reviewing the associated drawings.
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 cover 116 to make 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 the 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 to flange 120. Flange 120 is used to mount or otherwise secure the transmitter 100 to the sidewall of the duct. When so mounted, transmitter 100 may be completely supported by the coupling of flange 120 to the duct wall.
Electronics 106 provide heater control and signal conditioning, resulting in a linear 4-20 mA signal representing flue gas oxygen concentration. Preferably, electronics 106 also includes a microprocessor that is able to execute programmatic steps to provide the functions of diffuser diagnostics. 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 backlit display, signal conditioning and heater control within a NEMA 4X (IP 66) housing.
Ideally, combustion in an industrial process is perfect, and fuel and oxygen combust to create carbon dioxide and water, according to Equation 1 below, where the stoichiometric amount of carbon dioxide and water produced is dependent on the type of fuel used in a specific industrial process.
Fuel+O2→CO2+H2O Equation 1
Often, however, combustion in industrial processes is not perfect and, in addition to carbon dioxide and water, excess oxygen exists in the exhaust. In one embodiment, while the industrial process is in a regular operating mode, an oxygen sensor, for example, oxygen transmitter 100 with probe 104, measures the remaining oxygen gas in the exhaust of a the combustion process. Additionally, as is often the case, incomplete combustion occurs according to Equation 2 below.
Fuel+O2→CO2+H2O+(CO+NOx+SOx)incomplete products Equation 2
In an imperfect combustion, fuel comes into the industrial process with some contaminants and reacts to form, primarily carbon dioxide, CO2, and water H2O, with traces of other gases such as sulfur dioxide, nitrogen oxides, which come from the fuel impurities, as well as nitrogen oxidation. Additionally, when insufficient oxygen is provided to the industrial process, carbon monoxide forms as part of the incomplete combustion. The stoichiometric point, e.g. the ratio of fuel to oxygen with the highest efficiency and lowest emissions is very difficult to achieve in real combustion because of imperfect fuel to air uniformity, and of the fuel energy density and fuel to air flow variation.
Typically, flue gas oxygen excess concentration is 2-3 percent for gas burners and 2-6 percent for coal fired boilers and oil burners. The most efficient combustion may occur, in one embodiment, between 0.75 percent and 2 percent oxygen excess. While good combustion control can be accomplished with oxygen measurement alone, combustion efficiency and stability can be improved with the concurrent measurement of carbon monoxide, CO. As shown above in Equation 2, carbon monoxide is often a result of incomplete combustion of the fuel and oxygen supply and, therefore, a good first indicator that incomplete combustion is occurring in the process.
The development of carbon monoxide often occurs when the oxygen level is below a required amount for the industrial process to complete Equation 1 above. As carbon monoxide is a dangerous by-product of an incomplete combustion, its presence in exhaust gas may be regulated and an industrial process may be designed to include a catalytic converter to allow for conversion of carbon monoxide to carbon dioxide, according to Equation 3 below.
In one embodiment, the excess oxygen is sampled periodically by probe 104 throughout combustion. In one embodiment, the excess oxygen is sampled by probe 104 almost continually throughout a combustion process. The oxygen sensor output may be reported on an attached display, in one embodiment. The oxygen sensor output may also be transmitted to a database for storage. In another embodiment, the oxygen sensor output may be attached to an alarm system wherein certain maximum or minimum threshold oxygen concentrations may trigger a process alarm or an alert to a process engineer that a threshold has been surpassed. In one embodiment, the alert may be sent through text message, e-mail or another wireless-based delivery mechanism. In another embodiment, the alert may be an audiovisual alert within the industrial process, and may result in either a light being activated, a sound being emitted, or a combination of alert mechanisms that a threshold has been surpassed.
In another embodiment, oxygen transmitter 100 is coupled to controllers 24 and 26 such that readings from the oxygen transmitter can trigger automatic changes in the ratio of oxygen to fuel entering the industrial process. For example, upon a reading indicative of a rich exhaust mixture, indicating a high amount of unburned fuel and low remaining oxygen, the transmitter 100 may trigger controller 24, allowing more oxygen into the system, and/or may also trigger controller 26 inputting a lower amount of fuel into the system. The system may be calibrated, in one embodiment, to automatically adjust controllers 24 and 26 until a lean mixture is achieved. In one embodiment, a lean mixture is defined as a mixture of fuel and oxygen sufficient to convert the fuel into water and carbon dioxide without any incomplete combustion products.
In one embodiment, transmitter 100 is based on electrochemical zirconia-based cell technology. In one embodiment, the probe 104 is based on a solid-state electrochemical cell consisting of at least one zirconia ceramic located between an exhaust gas sample on one side, and a reference sample on the other side, wherein gas permeable electrodes are located on either side of the zirconia ceramic. The zirconia-based sensor 104 measures a concentration of remaining oxygen in the exhaust gas by measuring an output voltage across the zirconia ceramic, corresponding to a quantity of oxygen in the exhaust of the industrial process measured against a quantity of oxygen in a reference sample. The voltage measured corresponds to a concentration differential of oxygen between the two samples and, therefore, to an amount of oxygen consumed in a combustion reaction according to Equation 1 above. In one embodiment, the reference sample contains air of substantially atmospheric quality.
The oxygen sensor readings may depend logarithmically on the oxygen concentration according to the Nernst equation, Equation 4, below.
Zirconia based electrochemical oxygen sensors are widely used in industrial applications for oxygen measurements. In one embodiment, the sensor 104 works at temperatures in the 650-800 degree C. ranges and above, and measures the excess oxygen remaining after combustion. The response of the sensor to the differential oxygen concentration with a fixed oxygen partial pressure on the reference electrode, for example, fixed by using air can be calculated by using equation 4 above. In Equation 4, C is the constant related to the reference/process side temperature variation and thermal junctions in the oxygen probe, R is the universal gas constant, T is the process temperature, measured in degree Kelvin, and F is the Faraday constant.
In the combustion process, carbon monoxide is often the first indicator of an incomplete combustion. Operation at near trace CO levels of about 100 to 200 ppm and a slight amount of excess air would indicate the combustion conditions near the stoichiometric point with the highest efficiency. While there are many CO sensors available for applications ranging from workspace safety to exhaust gas analysis, the high temperature of typical industrial processes presents a difficulty in providing a reliable in-situ CO measurement for a combustion process.
A number of studies have been done on chemical gas sensors based on semiconducting oxides that are now used worldwide for combustible gas detection. This type of sensor, known as the Taguchi sensor, employs a solid state device made of sintered n-type metallic oxide (iron, zinc, and tin families), but poor selectivity and insufficient long term stability have been the major difficulties of these semiconducting sensors in the process environment.
Infrared absorption techniques relying on measuring the infrared light absorption would mostly require the flue gas conditioning system and thus add a relatively large expense to an industrial process. New, very sophisticated, and highly promoted tunable diode laser spectroscopy uses much more powerful laser light, is more reliable, and does not require a flue gas preconditioning. Unfortunately, fouling at a heavy particulate load, wide background radiation from the fireball, and required temperature and pressure compensation, as well as very high price, limit this technology to the applications in the chemical industry and for applications requiring high temperatures, such as combustion related processes. Currently, the only in-situ CO probe available on the market, and based on mixed potential zirconia technology was developed for very clean gas combustion application.
In one embodiment, the solid state potentiometric gas sensor for the oxygen measurements in the process comprises an oxide ion conducting ceramic in the form of a tube, disc or thimble, and two metallic or oxide catalytic electrodes that are exposed to the process and reference gases, respectively. In one embodiment, the ionic conducting ceramic is mostly a doped zirconia but could be stabilized cerium or bismuth oxide or any other oxide ion conducting solid electrolyte. The process reference electrodes are in one embodiment platinum, but any other electron conducting metal or metal oxide or mixed conducting pure or composite material could also be used. The oxygen sensor's process electrode is exposed to the flue gas, and the oxygen sensor is in an oxidizing environment in the regular potentiometric mode precisely measuring excess oxygen concentration in the combustion process flue gas. The highest peak of the derivative of the oxygen sensor signal is used as an additional carbon monoxide sensing output. The oxygen sensor, in one embodiment, is calibrated using fixed CO concentrations. This may correlate, for example, to the results shown in
Carbon monoxide is known to be one of the first products of an incomplete combustion to appear in a process. The presence of carbon monoxide results in a decrease in oxygen concentration as the carbon monoxide breaking through in the combustion process will, in one embodiment, be immediately converted to carbon dioxide, according to Equation 3 above, on a platinum electrode catalyst that is located on the sensor 112. In one embodiment, the platinum electrode catalyst is located very close to the oxygen-sensing portion of sensor 112. This will result in the oxygen concentration being significantly reduced near the oxygen-sensing electrochemical cell, as a result of the catalytic conversion of carbon monoxide to carbon dioxide, resulting in a sudden increase in the raw mV signal produced by sensor 112. This will result in the oxygen sensor output signal indicating an immediate reduction in oxygen concentration, especially in the milliseconds after carbon monoxide breakthrough in a combustion scenario. This may trigger, as indicated above, an alert provided to a process engineer, or it may trigger a change in the ratio of fuel and oxygen sources 20 and 18, respectively, through alteration of controls 26 and 24, respectively.
The detection of a drop in concentration of oxygen gas in the exhaust provides a quantitative indication of carbon monoxide that was present in the exhaust gas prior to the conversion and, therefore, an indication of the concentration of carbon monoxide produced as a result of the combustion. As can be seen from Table 1 below, CO presence is reducing the oxygen signal by almost 50 percent of the CO concentration, with CO conversion rate varying between 80 to 100% at 1000 ppm CO to 60 to 100% at 2% CO.
In Table 1 the change in sensor signal is theoretical ΔEt and calculated assuming 100% CO combustion by the platinum catalytic converter. The measured signal change ΔEm being close to theoretical change ΔEt, Table 1 does prove the effectiveness of an oxygen sensor, such as probe 112, to detect carbon monoxide in an industrial environment.
Examples of Carbon Monoxide Detection with an Oxygen Sensor
The sensor signal derivative over time, as shown in
In one embodiment, sensor 112 outputs the raw mV data graphically, as shown in
Thus,
Method 500 starts in block 502 with a combustion initiating in an industrial process. Method 500 continues with a carbon monoxide breakthrough occurring as shown in block 504. In one embodiment it may be minutes, hours, or longer between a combustion process starting and a carbon monoxide breakthrough occurring. Method 500 continues, in one embodiment, with the produced carbon monoxide being converted to carbon dioxide on a catalyst. In one embodiment, the catalyst is a platinum-based catalyst. In one embodiment, this occurs as described above with respect to Equation 3. As the carbon monoxide is converted to carbon dioxide, the measured oxygen concentration in the exhaust gas drops. This drop is detected in block 508 by the probe 104. In one embodiment, the detection is reported in block 510. In one embodiment, as illustrated by method 500, a concentration of carbon monoxide is not calculated as part of the detection process. In an optional embodiment, the method 500 moves on to block 512, where the fuel to oxygen input ratio is altered. This alteration may happen, in one embodiment, automatically, upon the detection of carbon monoxide. In one embodiment, this may result in additional air or oxygen being input to the system through source 18. In one embodiment, this may result in reduced fuel being input to the system through source 20.
The method then moves on to block 556 wherein a carbon monoxide concentration is calculated based at least in part on Equation 4, described above. The method may then continue, in one embodiment to block 558 where the detected carbon monoxide concentration is displayed, for example, on a connected computer or other display device. Additionally, in another embodiment, displaying the carbon monoxide concentration may comprise sending an alert to a process engineer, for example using wireless or other technology. This may trigger an indication to an operator of the industrial process that there has been a carbon monoxide breakthrough and that fuel to air ratios may need to be changed. The alert could be triggered visually, audibly, or through another means of notification. Additionally, in another embodiment, detection of a carbon monoxide breakthrough may result in an automatic change in the fuel to oxygen ratio, as indicated in block 562.
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.
