1. Field
The disclosure pertains to extractive sampling in continuous emission monitoring systems and, in particular, filtration of the extracted samples in such emission monitoring systems.
2. Related Art
Extractive sampling systems are typically used for continuously monitoring emissions of off-gases for combustion processes such as, for example, electric arc furnaces (EAF), rotary furnaces, and waste incinerators. In the extractive sampling systems, the extracted sample is often conditioned or treated before analysis. For example, for conventional analyzers, the extracted sample must first be conditioned by removing particulate materials of certain sizes via filtration and by separating water vapor from the gaseous stream (e.g., via a condenser, chiller and/or membrane) prior to being delivered to the analyzer.
An extracted gas sample measurement is conducted by inserting a probe into the process environment at a monitoring or measuring point of interest. Depending upon the process conditions (e.g., high temperature conditions), the probe can be water or gas cooled prior to and/or during insertion or, alternatively, the probe can be constructed of a high temperature resistant material. The extracted gas sample is transported through a sampling line to a particle filter and then to a water/gas separation system. When conditioning of the gas is complete, analysis of the conditioned gas is performed using conventional techniques such as non-dispersive infrared (NDIR) detectors for analysis of CO, CO2 and NOx, paramagnetic resonance detectors for analysis of O2, and thermal conductivity detectors for analysis of H2.
Extractive sampling of high density processes (e.g., EAF, glass melters, aluminum furnaces, etc.) often require periodic maintenance due to restriction or plugging that can occur within the sampling line, particularly at the filters, which in turn reduces or completely restricts the flow of gases to the analyzer.
Filter plugging can be overcome by utilizing a purge or “blowback” technique in which a high-pressure gas is blown through the sampling line and filter in the opposite direction as the gas-sample stream that is provided to the analyzer. Depending upon the particulate material characteristics in a gaseous sample stream for a particular process, the frequency of blowback or purging that is required can range from about every 15 minutes to about every 8 hours or more, with the blowback process requiring a duration of about 5-10 seconds or more.
During a blowback step, any other gas conditioning devices (e.g., condensers or chillers) as well as the analyzer must be isolated from the high-pressure gas to prevent damage to such devices. The blowback process further interrupts the continuous gas monitoring process, which can become frequent for high density sample gases which require a shorter time interval between blowback steps to prevent clogging of the sample line. In addition, after the blowback process, the sampled process gas in the sample line can become diluted for a time period that is controlled by the size or volume of the sampling line and the sampling rate. For example, a high volume sampling line with a low sample rate will require more time for the blowback gas to be removed from the sampling line so that the actual process gas can be processed by the analyzer.
Interruptions and delays in the sample gas measurements by the analyzer (or analyzers), as well as the above-noted dilutions of the sample gas that can occur, due to blowback processing can become problematic particularly for dynamic processes that require continuous and real-time monitoring. In addition, for sampling streams containing a high density of particulate materials, other potential problems in the sampling process are possible. In particular, when the filter starts to collect and build up particles during gas sampling, the pressure differential within the sampling line will increase and the sampling flow rate will decrease. This results in sampling rate variations as well as delays in sampling measurements, which can be detrimental to the sampling process.
For example, in a dynamic process such as an EAF batch mode process, the particulate material density in the sampling stream can be as high as 150 g/Nm3. The EAF process requires continuous off-gas monitoring to control O2 injection into the EAF, which improves energy efficiency. In the EAF process, filter blowback can occur during charging cycles every 30-60 minutes without interrupting the process measurement. However, the particle density of this process is so high that sampling rate variations caused by pressure drop variations across the filter are very likely to occur between blowback processing steps. When the sampling measurements are coupled to a process control scheme for O2 injection, the variation in sampling can result in undershooting or overshooting the desired O2 concentration in the EAF, which in turn reduces the potential for energy recovery through CO/H2 combustion in the process.