by Larry Gallagher
AL
Lower Pollution Limits and Refinery Flare Monitoring Require Special Gas Control Equipment for Sampling
Implementation of Title 40 of the Clean Air Act has resulted in a dramatic reduction in critical air pollutants. Monitoring these pollutants at lower levels and tracking high concentrations of refinery flare hydrogen sulfide and total reduced sulfur (TRS) requires specialized gas control and sam- pling equipment that can rapidly deliver unaltered calibration standards and samples to the analyzers. Standards that were previously produced as hundreds of parts per million (ppm) with accuracy of +1% must now be provided at concentrations below 10 ppm to parts per billion (ppb) with the same level of accuracy.
Continuous emissions monitoring (CEM) and fuel gas analysis instru- mentation accuracy are determined by calibrating the readings against a known standard and properly zeroing the instrument. Factors to consider during calibration are the compatibility of the wetted materials in the pressure control system and the tubing used to transport the mixture to the detector. Analysis errors—such as diesel fuel that exceeds low-level sulfur limits and requires additional processing, and failure to report pol- lution levels—can cost hundreds of thousands of dollars.
The most accurate standards, referred to as primary standards, are produced gravimetrically by measuring the minor components and the balance gas against traceable weights from the National Institute of Standards and Technology (NIST). Theoretically, this method should produce mixtures with accuracy of +1%.
Environmental air pollutants such as carbon monoxide (CO), oxides of nitrogen (NOx), sulfur dioxide (SOx), reduced sulfur (e.g., hydrogen sulfide, H2S), carbonyl sulfide (COS) and carbon disulfide (CS2) must be produced according to U.S. EPA Method 600/R-12/531 guidelines (May 2012, “EPA Traceability Protocol for Assay and Certification of Gaseous Calibration Standards”) and their concentrations must be reported against traceable NIST Standard Reference Materials (SRMs), NIST Traceable Reference Materials (NTRMs) or Gas Manufacturers’ Intermediate Standards (GMIS).
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These methods dictate the relative accuracy of the reported results and must be within +1% of the stated concentration against those reference standards. For example, relative accuracy is +1.5 ppm when these con- centrations are at 150 ppm. With concentrations of 5 ppm or less, the same relative accuracy must be measured in the range of +0.05 ppm, or 5 ppb. Storing standards properly and choosing the correct materials and delivery process can help users attain this precision.
For refinery fuel gas combustion devices and refinery flares under Title 40, Part 60, Subpart Ja, this means a span value of 50 ppm for SO2 with a relative accuracy of 20% or 4 ppm and calibration drift of less than 5%, or total calibration accuracy of potentially less than 2.5 ppm for those who comply with SO2 emission limits for fuel gas combustion units. Refinery operators complying with H2S limits can monitor and report the level of H2S in the low sulfur fuel used in combustion devices at levels less than 30 ppm or, for flares, operate and monitor dual-span H2S and TRS analysis of the potential streams; in this case, feeding the flare that can be as high as 3000 ppm and as low as 30 ppm. Operators will need to ensure that the calibration gas controls deliver unaltered standards to those analyzers, and at the same time be robust enough to handle the corrosive effect of H2S concentrations at 3000 ppm.
When handling reactive or corrosive and potentially toxic minor compo- nents like H2S, the equipment should be resistant to corrosion and not be able to change the mixture’s concentration or composition, which would typically be a 316L stainless steel body with 316L stainless steel diaphragms, with either polytetrafluoroethylene (PTFE) or polychlorotri- fluoroethene (PCTFE) seats having leak integrity of 1 × 10–9
cc/sec helium.
This is generally suitable for most reactive or corrosive minor components at specific concentrations that are neither too low, where absorption of the minor component may be a concern, nor too high, where moisture can result in corrosion of even 316L stainless steel. The design should be such that the amount of polymeric materials, such as the seat, is minimized.
APRIL 2016
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