Application Note 12: TrendView Recorders Continuous Emissions Monitoring

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1 Application Note 12: TrendView Recorders Continuous Emissions Monitoring Continuous Emissions Monitoring (often abbreviated to CEM) is the term commonly used to describe instantaneous or real-time environmental monitoring of gases and particulate matter that pass through flues, ducts and chimneystacks etc. CEM is also known as Stack Gas Monitoring or Flue Gas Monitoring. There are two main drivers for Continuous Emissions Monitoring. The first (and by far the most significant requirement) is to demonstrate compliance with environmental legislation. The Acid Rain Program and CEM use a market-based approach to reduce SO 2 emissions in a cost-effective manner, allowing for example, a utility to buy, sell, or hold allowances as part of its compliance strategy. An essential feature to implementing this market-based approach is to have complete and accurate emissions data that allows for the free trading of these allowances. The second driver is for process control (i.e. combustion efficiency) improvements to help to produce less total emissions. The owner or operator of a unit regulated under the Acid Rain Program must install CEM systems on the unit unless otherwise specified in the regulation. CEM systems include: An SO 2 pollutant concentration monitor. A NOx pollutant concentration monitor. A volumetric flow monitor. An opacity monitor. A diluent gas (O 2 or CO 2 ) monitor. A computer-based data acquisition and handling system (DAHS) for recording and performing calculations with the data. There are many types of systems available for CEM; some of the more common ones are: Extraction (Heated Line) - This is one of the oldest technologies for CEM and involves taking a sample from the stack with a probe and transporting the sample (in its supposed original state) to gas analyzers via a length of heated sample line. Although still widely used, there are inherent problems with this technique, mainly to do with the fact that it is very difficult, if not impossible, to maintain the sample of gas in its original state from the stack to the analyzers, especially with soluble acid gases such as HCl, SO 2 or NO 2. Extraction (Dilution Probe) - Extraction via dilution probe goes one stage further than the heated extraction method by ensuring that the composition of the sample gas is transported to the analyzers via the sample line in its original state. This is achieved by a number of means, including diluting the stack gas with zero air (by a factor of 20:1 for example) and utilizing two heated orifices in the probe tip to create a pressure drop from one side to the other. This will result in the dew point of the gas being lowered thereby removing the problems of transporting the gas to the analyzers. 43-TV Issue 2 Dec 14 Page 1

2 Cross Duct (or Cross-Stack) Monitoring - This is the most modern method of CEM and by far the most representative and accurate. Cross Duct gas and particulate analyzers measure stack gas emissions by a number of different technologies (i.e. IR/UV/DOAS/Laser). As the name suggests, the measurement is made across the duct, thereby eliminating the need for any probes or sample systems. Operators often favor these systems, as well as regulators, as they generally provide more reliable, accurate and representative measurements of gases and dust concentrations from one side of the stack to the other. In order to monitor SO 2 emissions in pounds per hour using a CEM system, a facility must use both an SO 2 pollutant concentration monitor and a volumetric flow monitor. For NOx, both a NOx pollutant concentration monitor and a diluent gas monitor are required to calculate an emissions rate in pounds per million British thermal units (lbs/mmbtu). Opacity monitoring, which measures the percentage of light that can be seen through flue gas, requires only an opacity monitor. The regulations do not require a utility to use a CEM system to measure CO 2. If a utility chooses to use a CEM system, however, a CO 2 or oxygen monitor plus a flow monitor would be used to compute emissions in tons per hour. The requirements for CEM in the USA are detailed in the Code of Federal Regulations, 40 CFR - Protection of Environment, which is overseen by the Environmental Protection Agency (EPA). Generally, Part 60 of 40 CFR provides the details concerning the Standards of Performance. The European Union began to regulate various air pollutants in the early 1980 s. This legislation covered SO 2 and suspended particulates (80/779/EEC amended by 89/427/EEC), lead (82/884/EEC), NO 2 (85/203/EEC) and ozone (92/72/EEC). In November 1996, a Framework Directive (96/62/EC) became effective, laying down, for the first time, common rules and principles for setting limit values as well as for the assessment and the management of air quality throughout the EU. It listed 13 pollutants for which legislation, including limit values and measurement and assessment requirements, had to be developed, and sets the timeframe for the development of these so-called "Daughter Directives". CEM monitoring applies to many industries and applications, not just to power generation, which is typically the application that first comes to mind when considering environmental and emissions monitoring. Of the many CEM applications, some of the most common are: - Aluminum Smelters - Large Power Plant (Coal and Oil fired). - Small-Medium Power Plant (Gas turbine) - Chemical Plants - Oil Refineries - Cement Plants - Brick Plants - Mineral Wool Plants - Waste Incineration (Municipal, Clinical, - Acid Plants Hazardous, Pharmaceutical) Calculations for Continuous Emissions Monitoring There are a number of calculations used for monitoring the emissions from various processes. Block and Rolling averages are widely used; the EPA requires Rolling Averages (RAV) for Opacity and particulate monitoring such as NOx, SOx. Rolling averages represent an average over a fixed time period that is continuously moving. For example, a 1-hour rolling average would represent the average of all the readings over a one hour time period that is continuously adding a new data point at the beginning of the hour time period while dropping off the data point at the end of the hour time period; thus the data point essentially rolls into the calculation of the average and eventually rolls off the end and out of the calculation at the end of an hour s time period. Rolling averages are used to smooth out the data and mitigate the effect of rapid data excursions. Block averages, on the other hand, are calculated by averaging the data readings over a fixed time period to provide a single reading that is the arithmetic average of all the data samples in that time period, which is then recorded at the end of that fixed time period. The process is repeated to determine the next block average. Looking at the data chart, we can see that the rolling average serves to smooth data, thus providing some built-in allowance for the emissions limit by allowing Page 2 43-TV Issue 2 Dec 14

3 higher instantaneous emissions as long as the averages do not exceed the daily limits. For example, if we refer to the figure below and suppose the emissions limit is 1.100, we can observe that point 8 exceeded the instantaneous limit, but the rolling average is still below the allowable limit, thus the operation would still be considered in compliance. Of course, determining compliance is dependent on the permit that has been issued by the governing authority, but for any CEM application, there are generally a number of levels that must be monitored and recorded in order to show compliance. The permit issued by the governmental agency for overseeing the emissions of particulates into the environment provides the specifics for the parameters needing to be monitored. These include not only the rolling averages which cannot exceed a prescribed limit but also the instantaneous emission limit which could exceed the rolling average limit as long as the limit is not exceeded more than a set number of times, it does not exceed a second higher maximum level and if it is between these two limits and it does not exceed a set amount of time (i.e. the time duration must be less than a certain amount). They also specify the minimum amount of total time that data must be collected for the facility in ordered to be considered compliant and be allowed to operate without being fined or shutdown. For example, when burning municipal solid waste (MSW), at a minimum, valid CEMS data shall be obtained for 75 percent of the hours per day for 75 percent of the days per month the affected facility is operated and combusting MSW. Rolling & Block Averages B1 B B R1 R2 R Values Rolling Block The TrendView recorders can be used as part of the CEM computer-based data acquisition and handling system (DAHS) for recording and performing calculations to prove compliance. To determine the block average, the best method is to set the logging rate for each pen being used for emissions monitoring to use the Average logging method as opposed to the Sample or Min-Max logging method. The log time would be set to the required time for the block average, whether it is a 6-minute average, in the case of Opacity, or a 1-hour average for the other parameters. The number of points averaged will be based on the scan rate selected for the inputs. For example, selecting a 6-minute average with a 500msec sample rate would provide an average made up of 720 samples; the average of all these samples is what is stored to the media for later review. The TrendView recorders allow each pen can have its own individual logging making it easy to set up one pen for a block average of say opacity and another pen for a block average for another parameter that requires a different block average time period. The time period of the block average is configured by setting the Rate units which can be milliseconds, seconds, minutes or hours and setting the required rate for the logged data. Another feature of the TrendView recorders is the Align function; this allows the user to initiate the data logging and align it to the next second, minute, 15 minute or hour mark based on the recorder s real time clock. So if you want to start logging data on the hour mark, the align feature would be set to Hour and logging would begin when the recorder s clock reaches the next hour point. The figure below shows how you would select the logging rate, the logging method and the Align function on the recorder. 43-TV Issue 2 Dec 14 Page 3

4 Selecting Logging Method Logging Method Set to Average In addition to block averages, the EPA also requires Rolling Averages (RAV) for Opacity and particulate monitoring such as NOx, SOx. Opacity is a 6-minute RAV while the other particulates use 15-minute averages and 3-hour Rolling Averages. To set up a rolling average in the TrendView recorders, no special Math option is required; it can be set up as part of the standard Pen configuration parameters. The Rolling Average calculation would be set up by going to the RAV parameter in the Pen configuration and enabling it. Once enabled, the screen that allows you to set the Rolling Average parameters appears, allowing you to set the number of samples you will use for the rolling average and the time interval for these samples. The set up screens are shown below. Set up of the RAV Samples & Interval Set up of the Pen to do a RAV To start, first determine the total time interval for the rolling average calculation; based on this, determine how you want to collect the data. For example, if you need a three hour rolling average which equates to seconds there are a number of different sample intervals and number of samples you can use to calculate the RAV and stay within the requirements for the number samples allowed and the sample interval allowed by the recorder. The recorder configuration boundaries for the RAV sample size is 1440 samples max and 3600 seconds max for the sample interval. So you could do any of the following depending on how smooth you want the RAV calculation to be: 8 Sec Interval 1350 samples 10 Sec Interval 1080 samples 15 Sec Interval 720 samples 30 Sec Interval 360 samples 1 Min Interval 180 samples 5 Min Interval 36 samples Page 4 43-TV Issue 2 Dec 14

5 The recorder also allows you to set up a condition called Prefill which sets how the first RAV average is handled. If you set the RAV to use Prefill, it takes the current pen value and fills the queue with that average, causing the "damping" effect to be base lined at the current reading, after that it does the normal RAV. With no Prefill, it is a progressive calculation for the first RAV; this will cause the first few samples to have a greater effect on the average, as they will not be diluted by 60 samples but will progressively average the samples together (1, 2, 3, 4, up to 60) for the first the 10 minutes. 3Hr RAV W Prefill 3Hr RAV No Prefill PV PV Replay mode - 3 Hour RAV with Prefill Replay mode - 3 Hour RAV with No Prefill In this example, you can see in the recorder chart on the left, which has Prefill turned on; the initial rolling average (the curve just to the right of the Setup Change) is more damped due to the Prefill than the graph on the right which has Prefill turned off. These graphs also show the effect of the RAV calculation and how it smoothes out the actual PV signal. (This is also shown on the Trend Manager Pro graph on the next page.) Because CEM requires monitoring both instantaneous, Block Averages and Rolling Averages in order to determine compliance, it is important to record these parameters to provide proof of compliance as well as to provide indications of how well the process is in control. All of the data can be used for process improvements as well as for proof of compliance. For example, if this graph represented the recorded emissions data from a smokestack, the instantaneous readings appear to be eradiate and may be outside the spec limits while the Rolling average is a smoother curve and could be within specification limits. If this is the case, this would be an indicator that even though the process may be in compliance, better control is required to improve the process to prevent an out of compliance condition based on the instantaneous process readings. The Trend Manager Pro software provides the capability to analyze the recorded data and to create graphs of the process data. In the case of the below diagram, the data was imported from the recorder s storage media into the Trend Manager Pro software. Tools within Trend Manager Pro allow the user to review and analyze the data and to create graphs which can be archived for proof of compliance purposes. In this case, the data was set up to show both the 3 hour Rolling Average with Prefill included and with No Prefill along with a 6 minute Block Average and the actual process variable. The graph can also be exported to Excel if the user is interested in doing additional analysis of the data. This Trend Manager Pro graph helps to show the different capabilities of the recorder for doing CEM calculations and how they affect the calculations. If you examine the data in Trend Manager, you will notice for the Block Average, a data point every 6 minutes, while data for the other points is collected at 30 second intervals. The independent data logging capability in the recorder allows you to set up the data logging for each pen to meet the needs of the application. It is this flexibility that makes it easy to set up multiple block and rolling averages to meet the requirements of the plant s license for monitoring emissions. 43-TV Issue 2 Dec 14 Page 5

6 The graph below also allows you to easily see how the first time period for the RAV with the Prefill or No Prefill setting is handled and after that first RAV the calculation is the same for each successive RAV calculation. 3Hr RAV with Prefill 3Hr RAV no Prefill PV Block Average data Trend Manager Pro Graph Showing a 3 Hour RAV with & without Prefill & Block Average The Event function in the TrendView Recorders provides additional capabilities to enhance the collection and documentation of the CEM data. Events can be used to document the starting and stopping of calibration runs, to mark the occurrence of a specific event, or to start/stop logging when specific conditions exist that could affect the averages. These could be conditions such as the start up or shut down of the process where the combustion process is not at the operating temperature, thus producing higher than normal emissions that if taken into account in the calculations could indicate erroneous results. To document total operational times, a totalizer could be established to monitor and record the total amount of time the facility is running along with the time the process is at the proper operating conditions. This would allow these times to be checked against the requirements specified in the permit. For example, if the minimum requirement when combusting municipal solid waste, for valid CEMS data is that the data must be obtained for 75 percent of the hours per day for 75 percent of the days per month the affected facility is being operated, this could be recorded as proof. In this case, two totalizers would be used, one to monitor total operational time and the other for monitoring the time for the collection of valid CEMS data; the difference would be used to verify that the requirements are being met. System Calibration A key part of any CEM system is the calibration process to prove that the system is working as designed; detecting each of the emission parameters (SO 2, NOx, etc.) and documenting their level to a known standard, thus insuring that the levels being recorded during normal operation are accurate. This process typically involves running known concentrations of sample gases through Page 6 43-TV Issue 2 Dec 14

7 the system, identifying start and stop times and calculation of the emissions levels to insure they match the expected levels. The Event system of the TrendView recorders allow for easy set up of messages to mark the chart for the start and end of the calibration test run, along with recording the levels of emission detected during the calibration cycle. The Events can be set up at the recorder or by using Trend Manager Pro. The set up screen in Trend Manager Pro would look similar to the screen shown below: The markers would be set to trigger at the start and end of the calibration process based on a digital input. This would be recorded at the recorder and can be recorded remotely over the Ethernet communications link for review and archiving of the data in order to show compliance. Product Requirements To use the TrendView recorders for CEM, the recorder requires the following features and functions: Analog Inputs Digital Inputs Events Totalization Data Storage TrendServer Pro Inputs for Opacity readings, SO 2 monitor, NOx monitor, Diluent Gas monitor (O 2 or CO 2 ), Volumetric Flow meters Inputs used to trigger the Event actions such as Start/Stop recording, Start/Stop Totalizer, Mark the Chart Firmware option used for setting up internal recorder actions based on how the Event Causes and Effects are set up. Firmware option used for Totalization of run times Select the required size internal memory to store data locally at the recorder provides the PC software for Data Analysis, Configuration, Communications and exporting of data to third party software packages References: 43-TV Issue 2 Dec 14 Page 7