Continuous Emissions Monitoring Systems (CEMS) determine a gas, matter concentration, or emission rate using pollutant analyser measurements and a conversion equation, graph, or computer program to produce results in units of the applicable emission limitation or standard. Government agencies provide strict guidelines for industries releasing pollutants into the air and require companies releasing these pollutants to provide quarterly reports in a specified format. Companies must continuously monitor output and hire external companies to perform quality assurance on measurement systems.
Nitrochem LLC needed to replace their DOS-based CEMS with a new, more flexible, system. The older system had a proprietary data logger without networking capabilities and could not be easily upgraded. Using a PC-based approach and commercial off-the-shelf (COTS) hardware, Data Science Automation developed a new CEMS for Nitrochem LLC that can be easily upgraded, has network capabilities for future use, and eliminates virtually all of the calibration paperwork that the older system required. The system can create detailed files, which is essential to comply with the requirements of the government agency.
National Instruments' FieldPoint provides rugged, cost effective data acquisition and process control for the CEMS. A network module provides connectivity to a PC-based system through RS232. Three other modules provide analog input, digital input, and digital output. With a durable enclosure to protect the modules from dirt and debris in the plant, these modules are hot swappable and their rugged design and ease of expandability make them well suited for the CEMS.
National Instruments' LabVIEW datalogging and supervisory control module was chosen for its features - such as input scaling, alarming, trending, security, and OPC server functionality. The software supports network protocols and can share resources with other CEMS. With driver software already available, users can quickly link FieldPoint networks to the LabVIEW datalogging and supervisory control module OPC server. The software uses an engine that runs separately to read and log data to a high performance historical database. The software driver makes the historical database ODBC compatible so that other applications can use the Structure Query Language (SQL) to query the database. With Windows NT as the operating system, users can easily upgrade programs and features as other technologies become available.
With a human machine interface (HMI), the user can easily control system functionality. As defined by the Pennsylvania Department of Environmental Protection, Nitrochem LLC must monitor and record nitrogen oxides (NOx) and stack flow data. To fulfil these requirements, a chart displays up-to-the-second trends from an analyser reading NOx and stack flow data. A message box displays indicators when automated events such as Daily NOx and mass flow calibrations and mass flow purging are in process. With buttons located on the HMI, the user can edit company configuration information, schedule automatic events, view data used by outside companies in quality control, and determine a calibration error number used in the final quarterly report.
The system integrators configured the LabVIEW datalogging and supervisory control module engine to log data with a deadband of one percent of full range for each input or every five minutes. The engine logs data every time data changes according to the deadband setting. If the data does not change, the engine does not log unless the user configures the engine otherwise.
During the NOx auto-calibration, the CEMS routes concentrated gasses to the analyser. With no user interaction required, the system records the automatic reading in a calibration file rather than in the hourly logging or the five-minute average logging. During the mass flow calibration, the time manager uses digital output signals to begin calibration and digital input signals to end calibration. As with NOx calibration, data is then stored in calibration files.
Two quarterly calibration functions for the NOx and the mass flow require user interaction. For the NOx calibration, the system operator selects the appropriate gas by controlling the gas valves with the CEMS. Three concentrated gasses read directly into the analyser compare values to the known concentration values of the gas. The system then generates a calibration error number and logs the error number in the calibration file. During the mass flow calibration, the operator can control each stage in the analyser. A table of values displays operator inputs as well as actual readings. Once the table is full of values, the system generates a calibration error and logs the error for future reference.
A time manager function in the CEMS processes scheduled events such as automatic calibration routines, report generation, and file naming algorithms. With user-defined intervals configured during set-up, the time manager feature of the CEMS uses air to clear pitot tubes of debris to ensure accurate stack flow measurements.
With an additional user interface, users can perform an annual relative accuracy test audit (RATA). During a RATA, an outside source uses portable equipment to compare stack readings to CEMS readings. To confirm that the reported data is accurate, the system generates reports such as daily calibration and purging reports, hourly and five minute logging reports, and quarterly reports in a table format and saves the report to a text file.
Using FieldPoint in conjunction with LabVIEW datalogging and supervisory control module gave DSA and Nitrochem LLC the flexibility to design and implement a customised, powerful, and reliable CEMS. As future need demands, additional signals can be networked into the system that may not be in the same geographical area as the original inputs. Using a PC-based approach with COTS hardware gave Nitrochem LLC an updated system that can be maintained by current engineering programming knowledge and can be upgraded as future technologies become available.
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