Application Notes
SO3 Measurement for Mercury Control
Background:
Federal regulations covering the capture and removal of mercury from generating station stack gas take effect in 2010. An increasing number of states are accelerating that deadline, some as early as 2008. Significant industry research suggests that injection of powdered activated carbon (ACI) into the flue gas prior to the particulate collection device (ESP/FF) will allow utilities to meet the near term regulations.
The level of mercury removal, while site specific, is generally related to the amount of carbon injected. The amount injected is usually defined in terms of pounds (of ACI) per million actual cubic feet of flue gas (Lb/MMacf).
Figure 1 is from an EPRI presentation on mercury capture in ESPs using both standard ACI and chemically pre-treated ACI. Today this generally means pre-treatment with bromine. Both ADA-ES and Sorbent Technologies provide this type of technology.
A review of the graph shows that mercury removal rates of 35% - 55% can be achieved with ACI and 55% - 75% with brominated ACI at an injection rate of 5 Lb/MMacf. These are fairly broad ranges, but give some insight into what is generally expected with activated carbon technology.
Figure 2 shows the results of ACI injection with high levels of SO3 (sulfuric.jpg)
acid) present in the gas stream. This data is from a published ADA-ES demonstration at Mississippi Power's Plant Daniel. While the source of the SO3 is an ESP Flue Gas Conditioning system, the results are typical of a gas stream with high SO3. Note that at 5 Lb/MMacf only a 22% removal was achieved and the max level seems to plateau at around 32%.
What is happening is this: the sulfuric acid (SO3) is highly reactive and competes with the mercury for the active sites on the carbon particle. The higher the SO3 level, the lower the effectiveness of activated carbon for the capture of mercury.
Application:
Placement of the AbSensor-SO3 probe downstream of the air heater, or the SO3 injection system, provides direct measurement of flue gas sulfuric acid level. Once the level of SO3 is known, a control process can be implemented to keep its level at a minimum.
High levels of native SO3 can be found in the following situations:
Flue Gas Conditioning is required to keep the ESP operational in low sulfur fuel environments. However, with an SCR in operation, the levels of native SO3 increase. Also, depending on flue gas temperature and ash resistivity, the level of injected SO3 required will vary.
Each site will required specific testing to determine exactly what level of FGC based SO3 injection is required for ESP performance after ACI is employed. Once that level is known, the AbSensor probe can provide feedback to the FGC skid to keep the sulfuric acid level in control.
In this application, the probe should be placed after the flue gas conditioning but before the activated carbon injection point.
Site testing:
Because the dynamics between SO3, activated carbon and mercury capture are site specific it is important that any plant considering activated carbon injection execute a thorough study of localized SO3 levels. Additionally, recent guarantee statements from the suppliers of ACI systems require that SO3 levels be maintained below a specified level. Currently, the AbSensor-SO3 system is the only available, reliable, method of on-line, automatic detection of flue gas SO3.
SO3 Measurement for Blue Plume Control
Background:
The phenomenon called "Blue Plume" results from light scattering through sub-micron droplets of condensed SO3, or sulfuric acid, at the stack exit and qualifies, in many states, as particulate emission. The extent of this emission is related to the concentration of sulfuric acid vapor and the concentration of sub-micron particles in the gas stream after the particulate collection system (ESP/FF).
Figure 1 is taken from a 2002 PowerGen International presentation by C. Erickson of Babcock Power and nicely captures the combined opacity effects of particulate and sulfuric acid vapor emissions.
The example shows that for a fine particle emission level of 5 mg/m3 (note: particle size of 0.15micron) and sulfuric acid vapor concentration of 16 ppm, the resulting opacity would be nearly 35%. Reducing this to the desired sub-20% would require a reduction in fine particle emission of over 60% (unlikely) or a reduction in
sulfuric acid vapor of 50% (see below).
Figure 2 is a well accepted curve that shows the relationship between sulfuric acid dew point, SO3 concentration and flue gas moisture level. To follow from the preceding example, and using 10% gas moisture, then a 16 ppm concentration would exhibit a dew point of roughly 272 deg F. To lower the opacity to below 20%, the dewpoint would need to drop to 260 deg F!
Currently there are several available chemicals that, when injected in the flue duct after the air heater and before the particulate collection device (ESP/FF), are highly effective in capturing and holding SO3, thus reducing the sulfuric acid condensable level at the stack outlet. The next page will explain the process for using the AbSensor-SO3 instrument for measurement and control of this SO3 mitigation process.
Application:
As of this writing the most cost effective, reliable method for mitigation of SO3 condensables is through the injection of an alkali based sorbent either above the furnace or between the air heater outlet and the particulate collection device input. While many options for mitigation chemicals exist, those most often mentioned include:
Commercially it is important to monitor the actual sulfuric acid dewpoint for two fundamental reasons:.jpg)
The graph to the right shows the change in untreated sulfuric acid dewpoint for a 24 hour period at a 450 MW plant. Notice that the dew point varies by nearly 40 degrees on it own. Clearly the amount of chemical required for SO3 control will also vary dramatically!
The graph below shows the change in dew point as a result of varying levels of chemical injection. The gap between the end of the red line and the start of the blue line is the time spent tuning the injection location and injection variables. Note that a drop of over 40 degrees in dew point was achieved with the first injection level and an additional 15 degrees with the incremental increase.
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At an average delivered cost of $125/ton of chemical, and if only a 20 degree reduction was needed (instead of 40), then 300pounds/duct or 600 pounds per hour of chemical would be saved. For this plant alone the savings of closed loop control would be $37.50/hour or an annual reduction of $300,000 for a 450 MW plant.
Finally, for a typical 450MW plant, an average ash load to the ESP would approach 7 tons/hr. The reduction in injection suggested above would reduce the loading on the ESP by 0.5% on average.
SO3 Measurement for Heat Rate Improvement
Background:
Air heater outlet temperature is a highly critical variable in the overall operation of an electric generating station. There are two simultaneous drivers at work requesting opposing goals for the AH outlet temperature. These are:
Most industry references suggest that ACET, or average cold end temperature (the average between #1 & #2) should be kept at or around, 200°F. This general number assumes a typical cold air inlet temperature of 100°F leaving 300°F for the AH outlet temperature.
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However, as the graph at the right shows, the actual sulfuric acid dew point can vary drastically over the course of a day. These variations are caused by changes in load (varying oxygen levels), and changes in fuel with varying sulfur and moisture levels.
Balancing these demands can only be accomplished with a feedback signal monitoring the real-time value of the sulfuric acid dew point. The AbSensor-SO3 instrument, in closed loop control mode can be used to control the air heater outlet temperature to a value just above the real time dew point, keeping both corrosion and heat rate at a minimum.
The two most common methods for boosting air heater outlet temperature are air bypass dampers and steam coils. This application note only deals with automating, and adding intelligence to, these mechanisms. For information on how to achieve a lower AH temperature through intelligent sootblowing and combustion control see other publications available from Breen Energy Solutions.
Application:
Rather than relying on ACET values, the air heater outlet will be controlled to a value just above the current actual dew point of the sulfuric acid vapor in the flue gas. To achieve this it is essential to know two variables:
Metal Matrix Determination
The AbSensor-AHC software, licensed from prior EPRI work and modified for real time operation by Breen, develops an instantaneous representation of maximum and minimum metal temperatures at any depth in the air heater. The software is site specific and constructs a unique air heater model for each application. A typical model output is shown at right.
As can be seen from the graph, the actual evaporation temperature of the sulfuric acid can be plotted against the actual air heater metal matrix to determine the location of acid deposition (if any). The air heater temperature can then be controlled so that the instantaneous max metal temperature at the cold end outlet just exceeds the measured sulfuric acid evaporation temperature.
Heat Rate Recovery Example
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If the air heater from the graph at left is controlled dynamically to an average of 280°F instead of the traditional 300°F a heat rate improvement of roughly 0.6% can be achieved. This represents a savings of over $220,000 for a 300MW plant, and a payback in less than six months!
It is important to understand that this application can be implemented in two phases. Initially the probe output can be used to control air heater outlet gas temperature only without benefit of the metal temperature curves. While this approach must allow some margin for assumed metal temperatures, it still can yield significant positive heat rate improvements. As plant management (and budget) allows for future enhancement, the metal temperature controller could be added for tighter control and additional savings.
Ammonium Bisulfate Measurement for SCR/SNCR Control
Background:
The process of converting harmful NOx compounds into nitrogen and water through chemical interaction with NH3 has been well documented and is well understood. It is also fairly well understood and documented that increased levels of un-reacted NH3 (ammonia slip) can lead to increased levels of ammonia salt formation, either as ammonium sulfate or ammonium bisulfate.
What is not so clearly documented is the inter-relationship between ammonia slip, flue gas moisture, SO3 levels and air heater fouling from sulfuric acid or ammonium bisulfate (AbS) deposition.
From a "non-chemical engineering" perspective, the process progresses something like this:
As can be seen, there are many variables influencing the formation and concentration of AbS. Because flue gas SO3, moisture and NH3 can all vary over the course of a few hours, it is not sufficient to simply know the level of free ammonia slip after the catalyst. The only way to predict and avoid air heater fouling is to directly measure the condensable end product, ammonium bisulfate. However, once you have identified and measured it, how do you control its formation?
The Double Peak Measurement
It is generally known that sulfuric acid has a dew point in the range of 250°F to 290°F and that AbS has a dew point in the range of 310°F to 350°F. Significant field experience with the AbSensor-AFP instrument in both SCR and SNCR applications has lead to the discovery of a measurable precursor to AbS formation. This point is only measurable using a controlled tip measurement process and has been termed the "double peak". It is the point where the formation temperature of the condensate corresponds to the sulfuric acid range, but the evaporation point lies in the AbS range.
The chart at left shows a progression from low ammonia feed, to higher ammonia feed, and then back to low. The waveforms (yellow/red) left of 0.7 hours show traditional sulfuric acid formation and evaporation temperatures. As NH3 is increased, the temperature curves diverge, slowly for one cycle, and then rapidly. At 1.8 - 2.5 hours classic AbS is present. Beyond 2.5 hours the curves converge back to sulfuric acid. From 0.7 - 1.5 hours, AbS is predicted but will not yet kinetically form in the air heater.
This temperature divergence signal indicates the potential for AbS propensity and can be effectively used as a feedback to the ammonia injection system to prevent full AbS formation.
Application:
The purpose of both SCR and SNCR processes is to minimize the amount of NOx emitted from the plant stack. The purpose of SCR/SNCR optimization is to control the ammonia compound injection so that NOx is minimized to the greatest extent possible, WITHOUT negatively impacting balance of plant processes. For the purpose of this application note this means 'without allowing ammonium bisulfate to form in, and foul, the air heater'.
The AbSensor injection control process consists of three steps:
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While this application note is based on closed loop control concepts, the process can be easily adapted, in the DCS, to advisory mode. Implementation of a "soft button" that allows the operator to "Accept/Ignore" the request-to-bias will allow operator intervention. "Ignore" keeps the injection rate at its current setting, while "Accept" makes the change suggested by the control algorithm.
Lastly, the Formation Temperature limit value should be established from a calculation based on the generally maintainable air heater outlet temperature. For more accurate control of this part of the process, or to automatically control air heater outlet temperature to maximize plant efficiency, please consult the experts at Breen Energy about the AbSensor-AHC Air Heater Controller system.
Sulfuric Acid Measurement for ESP Conditioning Control
Background:
It has long been known that the collection efficiency of an Electrostatic Precipitator (ESP) is related to the temperature, alkalinity and SO3 concentration of the flue gas/fly ash. The curve below (taken from the 2004 version of the EPA/ESP Training Manual) offers a summarized glimpse into the effects of these three variables. In a nutshell, depending on the level of SO3 and alkalinity, there can be as much as 2.5 orders of magnitude difference in the resistivity of the collected fly ash.
To offset some of the effects of lower sulfur fuels, the industry has historically embraced injection of SO3 gas, and potentially SO3 + NH3 gas to chemically condition the ash and lower the resistivity.
While this has proven to be highly effective for ESP collection, it has several negative side effects that are being aggravated by the installation of sophisticated SCR and SNCR systems for NOx reduction.
ESP operators (for collection efficiency reasons) and boiler operators (for heat rate reasons) want to operate at the lowest possible ESP inlet temperature. However, variations in instantaneous SO3 and moisture levels can often result in a flue gas temperature below the dewpoint of sulfuric acid leading to plate, duct and ESP shell corrosion.
Additionally, excess levels of SO3 can often result in increased levels of submicron sized acid mist at the stack outlet. This mist, known as Blue Plume in larger quantities, can appear as added particulate emissions in many states. This added particulate emission level can lead to opacity violations and potential unit derates when, in fact, the problem is simply operating at the wrong temperature for the current flue gas chemistry.
The key to successful operation is to know, in real time, the actual acid dewpoint of the flue gas taking into account all of the variables of fuel sulfur content, boiler SO2/SO3 conversion, SCR SO2/SO3 conversion, flue gas moisture, measured ESP conditioning injection and flue gas temperature at the ESP Inlet and Outlet.
Application:
SO3 Measurement for Mercury Control
Background:
Federal regulations covering the capture and removal of mercury from generating station stack gas take effect in 2010. An increasing number of states are accelerating that deadline, some as early as 2008. Significant industry research suggests that injection of powdered activated carbon (ACI) into the flue gas prior to the particulate collection device (ESP/FF) will allow utilities to meet the near term regulations.
The level of mercury removal, while site specific, is generally related to the amount of carbon injected. The amount injected is usually defined in terms of pounds (of ACI) per million actual cubic feet of flue gas (Lb/MMacf).Figure 1 is from an EPRI presentation on mercury capture in ESPs using both standard ACI and chemically pre-treated ACI. Today this generally means pre-treatment with bromine. Both ADA-ES and Sorbent Technologies provide this type of technology.
A review of the graph shows that mercury removal rates of 35% - 55% can be achieved with ACI and 55% - 75% with brominated ACI at an injection rate of 5 Lb/MMacf. These are fairly broad ranges, but give some insight into what is generally expected with activated carbon technology.
Figure 2 shows the results of ACI injection with high levels of SO3 (sulfuric
acid) present in the gas stream. This data is from a published ADA-ES demonstration at Mississippi Power's Plant Daniel. While the source of the SO3 is an ESP Flue Gas Conditioning system, the results are typical of a gas stream with high SO3. Note that at 5 Lb/MMacf only a 22% removal was achieved and the max level seems to plateau at around 32%.What is happening is this: the sulfuric acid (SO3) is highly reactive and competes with the mercury for the active sites on the carbon particle. The higher the SO3 level, the lower the effectiveness of activated carbon for the capture of mercury.
Application:
Placement of the AbSensor-SO3 probe downstream of the air heater, or the SO3 injection system, provides direct measurement of flue gas sulfuric acid level. Once the level of SO3 is known, a control process can be implemented to keep its level at a minimum.
High levels of native SO3 can be found in the following situations:
- Plants burning high sulfur coal
- Plants burning oil or petroleum coke
- Plants with SCRs using older catalyst
- Reduction of Sorbent usage costs (sorbents typically cost between $75 and $150/ton delivered. Proper control can cut the usage levels in half.
- Reduction of sorbent based ESP load. Marginal ESPs may not be able to handle the extra particulate loading. Proper control can keep the ESP viable.
- Reduction of sorbent based duct and perf plate fouling.
Flue Gas Conditioning is required to keep the ESP operational in low sulfur fuel environments. However, with an SCR in operation, the levels of native SO3 increase. Also, depending on flue gas temperature and ash resistivity, the level of injected SO3 required will vary.
Each site will required specific testing to determine exactly what level of FGC based SO3 injection is required for ESP performance after ACI is employed. Once that level is known, the AbSensor probe can provide feedback to the FGC skid to keep the sulfuric acid level in control.
In this application, the probe should be placed after the flue gas conditioning but before the activated carbon injection point.
Site testing:
Because the dynamics between SO3, activated carbon and mercury capture are site specific it is important that any plant considering activated carbon injection execute a thorough study of localized SO3 levels. Additionally, recent guarantee statements from the suppliers of ACI systems require that SO3 levels be maintained below a specified level. Currently, the AbSensor-SO3 system is the only available, reliable, method of on-line, automatic detection of flue gas SO3.
SO3 Measurement for Blue Plume Control
Background:
The phenomenon called "Blue Plume" results from light scattering through sub-micron droplets of condensed SO3, or sulfuric acid, at the stack exit and qualifies, in many states, as particulate emission. The extent of this emission is related to the concentration of sulfuric acid vapor and the concentration of sub-micron particles in the gas stream after the particulate collection system (ESP/FF).Figure 1 is taken from a 2002 PowerGen International presentation by C. Erickson of Babcock Power and nicely captures the combined opacity effects of particulate and sulfuric acid vapor emissions.
The example shows that for a fine particle emission level of 5 mg/m3 (note: particle size of 0.15micron) and sulfuric acid vapor concentration of 16 ppm, the resulting opacity would be nearly 35%. Reducing this to the desired sub-20% would require a reduction in fine particle emission of over 60% (unlikely) or a reduction in
sulfuric acid vapor of 50% (see below).Figure 2 is a well accepted curve that shows the relationship between sulfuric acid dew point, SO3 concentration and flue gas moisture level. To follow from the preceding example, and using 10% gas moisture, then a 16 ppm concentration would exhibit a dew point of roughly 272 deg F. To lower the opacity to below 20%, the dewpoint would need to drop to 260 deg F!
Currently there are several available chemicals that, when injected in the flue duct after the air heater and before the particulate collection device (ESP/FF), are highly effective in capturing and holding SO3, thus reducing the sulfuric acid condensable level at the stack outlet. The next page will explain the process for using the AbSensor-SO3 instrument for measurement and control of this SO3 mitigation process.
Application:
As of this writing the most cost effective, reliable method for mitigation of SO3 condensables is through the injection of an alkali based sorbent either above the furnace or between the air heater outlet and the particulate collection device input. While many options for mitigation chemicals exist, those most often mentioned include:
- Hydrated Lime
- Soda Ash
- Magnesium Hydroxide
Commercially it is important to monitor the actual sulfuric acid dewpoint for two fundamental reasons:
- Closed loop control of the injection rate of the chemical to the minimum level required to control SO3 has a significant cost benefit
- The chemical, along with the captured SO3, adds particulate loading to the ESP, and in high enough quantities can materially increase the resistivity of the overall ash
The graph to the right shows the change in untreated sulfuric acid dewpoint for a 24 hour period at a 450 MW plant. Notice that the dew point varies by nearly 40 degrees on it own. Clearly the amount of chemical required for SO3 control will also vary dramatically!The graph below shows the change in dew point as a result of varying levels of chemical injection. The gap between the end of the red line and the start of the blue line is the time spent tuning the injection location and injection variables. Note that a drop of over 40 degrees in dew point was achieved with the first injection level and an additional 15 degrees with the incremental increase.
At an average delivered cost of $125/ton of chemical, and if only a 20 degree reduction was needed (instead of 40), then 300pounds/duct or 600 pounds per hour of chemical would be saved. For this plant alone the savings of closed loop control would be $37.50/hour or an annual reduction of $300,000 for a 450 MW plant.Finally, for a typical 450MW plant, an average ash load to the ESP would approach 7 tons/hr. The reduction in injection suggested above would reduce the loading on the ESP by 0.5% on average.
SO3 Measurement for Heat Rate Improvement
Background:
Air heater outlet temperature is a highly critical variable in the overall operation of an electric generating station. There are two simultaneous drivers at work requesting opposing goals for the AH outlet temperature. These are:
- Possible sulfuric acid corrosion at the air heater outlet demands that the AH outlet temperature be kept as high as possible, while
- Overall plant efficiency (heat rate) demands that the AH outlet temperature be kept as low as possible.
Most industry references suggest that ACET, or average cold end temperature (the average between #1 & #2) should be kept at or around, 200°F. This general number assumes a typical cold air inlet temperature of 100°F leaving 300°F for the AH outlet temperature.However, as the graph at the right shows, the actual sulfuric acid dew point can vary drastically over the course of a day. These variations are caused by changes in load (varying oxygen levels), and changes in fuel with varying sulfur and moisture levels.Balancing these demands can only be accomplished with a feedback signal monitoring the real-time value of the sulfuric acid dew point. The AbSensor-SO3 instrument, in closed loop control mode can be used to control the air heater outlet temperature to a value just above the real time dew point, keeping both corrosion and heat rate at a minimum.
The two most common methods for boosting air heater outlet temperature are air bypass dampers and steam coils. This application note only deals with automating, and adding intelligence to, these mechanisms. For information on how to achieve a lower AH temperature through intelligent sootblowing and combustion control see other publications available from Breen Energy Solutions.
Application:
Rather than relying on ACET values, the air heater outlet will be controlled to a value just above the current actual dew point of the sulfuric acid vapor in the flue gas. To achieve this it is essential to know two variables:
- The actual current dew point of the sulfuric acid vapor, and
- The actual metal temperatures within the air heater itself
Metal Matrix Determination
The AbSensor-AHC software, licensed from prior EPRI work and modified for real time operation by Breen, develops an instantaneous representation of maximum and minimum metal temperatures at any depth in the air heater. The software is site specific and constructs a unique air heater model for each application. A typical model output is shown at right.As can be seen from the graph, the actual evaporation temperature of the sulfuric acid can be plotted against the actual air heater metal matrix to determine the location of acid deposition (if any). The air heater temperature can then be controlled so that the instantaneous max metal temperature at the cold end outlet just exceeds the measured sulfuric acid evaporation temperature.
Heat Rate Recovery Example
If the air heater from the graph at left is controlled dynamically to an average of 280°F instead of the traditional 300°F a heat rate improvement of roughly 0.6% can be achieved. This represents a savings of over $220,000 for a 300MW plant, and a payback in less than six months!It is important to understand that this application can be implemented in two phases. Initially the probe output can be used to control air heater outlet gas temperature only without benefit of the metal temperature curves. While this approach must allow some margin for assumed metal temperatures, it still can yield significant positive heat rate improvements. As plant management (and budget) allows for future enhancement, the metal temperature controller could be added for tighter control and additional savings.
Ammonium Bisulfate Measurement for SCR/SNCR Control
Background:
The process of converting harmful NOx compounds into nitrogen and water through chemical interaction with NH3 has been well documented and is well understood. It is also fairly well understood and documented that increased levels of un-reacted NH3 (ammonia slip) can lead to increased levels of ammonia salt formation, either as ammonium sulfate or ammonium bisulfate.
What is not so clearly documented is the inter-relationship between ammonia slip, flue gas moisture, SO3 levels and air heater fouling from sulfuric acid or ammonium bisulfate (AbS) deposition.
From a "non-chemical engineering" perspective, the process progresses something like this:
As can be seen, there are many variables influencing the formation and concentration of AbS. Because flue gas SO3, moisture and NH3 can all vary over the course of a few hours, it is not sufficient to simply know the level of free ammonia slip after the catalyst. The only way to predict and avoid air heater fouling is to directly measure the condensable end product, ammonium bisulfate. However, once you have identified and measured it, how do you control its formation?
The Double Peak Measurement
It is generally known that sulfuric acid has a dew point in the range of 250°F to 290°F and that AbS has a dew point in the range of 310°F to 350°F. Significant field experience with the AbSensor-AFP instrument in both SCR and SNCR applications has lead to the discovery of a measurable precursor to AbS formation. This point is only measurable using a controlled tip measurement process and has been termed the "double peak". It is the point where the formation temperature of the condensate corresponds to the sulfuric acid range, but the evaporation point lies in the AbS range.The chart at left shows a progression from low ammonia feed, to higher ammonia feed, and then back to low. The waveforms (yellow/red) left of 0.7 hours show traditional sulfuric acid formation and evaporation temperatures. As NH3 is increased, the temperature curves diverge, slowly for one cycle, and then rapidly. At 1.8 - 2.5 hours classic AbS is present. Beyond 2.5 hours the curves converge back to sulfuric acid. From 0.7 - 1.5 hours, AbS is predicted but will not yet kinetically form in the air heater.
This temperature divergence signal indicates the potential for AbS propensity and can be effectively used as a feedback to the ammonia injection system to prevent full AbS formation.
Application:
The purpose of both SCR and SNCR processes is to minimize the amount of NOx emitted from the plant stack. The purpose of SCR/SNCR optimization is to control the ammonia compound injection so that NOx is minimized to the greatest extent possible, WITHOUT negatively impacting balance of plant processes. For the purpose of this application note this means 'without allowing ammonium bisulfate to form in, and foul, the air heater'.
The AbSensor injection control process consists of three steps:
.jpg) |
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|
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| The AbSensor-AFP continuously measures condensable Formation and Evaporation temperatures and reports them to the plant DCS | The plant DCS receives the two values (per probe) and determines both the formation temperature and the spread between formation and evaporation. This identifies the critical double peak target point! |
If the formation temperature is above 300°F, or if the spread exceeds 40°F then a negative trim signal is sent to the NH3 Injection skid. If the Formation Temp is below 300°F and the spread is less than 40°F then a positive trim signal is sent to the NH3 injection skid. |
While this application note is based on closed loop control concepts, the process can be easily adapted, in the DCS, to advisory mode. Implementation of a "soft button" that allows the operator to "Accept/Ignore" the request-to-bias will allow operator intervention. "Ignore" keeps the injection rate at its current setting, while "Accept" makes the change suggested by the control algorithm.Lastly, the Formation Temperature limit value should be established from a calculation based on the generally maintainable air heater outlet temperature. For more accurate control of this part of the process, or to automatically control air heater outlet temperature to maximize plant efficiency, please consult the experts at Breen Energy about the AbSensor-AHC Air Heater Controller system.
Sulfuric Acid Measurement for ESP Conditioning Control
Background:
It has long been known that the collection efficiency of an Electrostatic Precipitator (ESP) is related to the temperature, alkalinity and SO3 concentration of the flue gas/fly ash. The curve below (taken from the 2004 version of the EPA/ESP Training Manual) offers a summarized glimpse into the effects of these three variables. In a nutshell, depending on the level of SO3 and alkalinity, there can be as much as 2.5 orders of magnitude difference in the resistivity of the collected fly ash.
To offset some of the effects of lower sulfur fuels, the industry has historically embraced injection of SO3 gas, and potentially SO3 + NH3 gas to chemically condition the ash and lower the resistivity.While this has proven to be highly effective for ESP collection, it has several negative side effects that are being aggravated by the installation of sophisticated SCR and SNCR systems for NOx reduction.
ESP operators (for collection efficiency reasons) and boiler operators (for heat rate reasons) want to operate at the lowest possible ESP inlet temperature. However, variations in instantaneous SO3 and moisture levels can often result in a flue gas temperature below the dewpoint of sulfuric acid leading to plate, duct and ESP shell corrosion.
Additionally, excess levels of SO3 can often result in increased levels of submicron sized acid mist at the stack outlet. This mist, known as Blue Plume in larger quantities, can appear as added particulate emissions in many states. This added particulate emission level can lead to opacity violations and potential unit derates when, in fact, the problem is simply operating at the wrong temperature for the current flue gas chemistry.
The key to successful operation is to know, in real time, the actual acid dewpoint of the flue gas taking into account all of the variables of fuel sulfur content, boiler SO2/SO3 conversion, SCR SO2/SO3 conversion, flue gas moisture, measured ESP conditioning injection and flue gas temperature at the ESP Inlet and Outlet.
- Real time control of the ESP Inlet temperature resulting in optimized heat rate and minimized opacity,
- Real time control of SO3 Injection rates to optimize collection without creation of acid mist
- Reduced/eliminated ESP Corrosion
Application:
- ESP Corrosion Control - Position the probe at the outlet of the ESP to monitor real-time acid dewpoint. Feed the information back to the air heater temperature control system and adjust ESP outlet temperature to 15 degrees above dewpoint,
- ESP Conditioning Control -
- Position one probe at the inlet of the ESP and after the SO3 injection system. Trim the injection rate to provide a net SO3 level inclusive of resident SO3 in the flue gas from SCR and Boiler generation,
- Position a second probe at the outlet of the ESP and monitor the level of SO3 absorbed by the ash layer (through dewpoint differential). When the dewpoint differential begins to decline (less acid absorbed by the ash blanket), trim the SO3 injection rate back.