In Part III of this series on flare gas measurement, we consider the specific challenge of changing gas composition and hydrogen when measuring flare gas. Earlier in this series, we discussed other challenges thermal mass flow meters must overcome.
If you would like to see this series in full, visit Flare Gas Measurement Using Sage Thermal Mass Flow Meters.
Changes in Gas Composition and Hydrogen
Other unique challenges for engineering when selecting flow meters for flare gas applications involve changes in gas composition and the presence of hydrogen.
Gas Composition Impact
Likely, the most significant issue with measuring gas flow using a thermal mass flow meter is potential changes in gas composition. This is not as much of a problem when measuring biogas, landfill gas and natural gas. These gases may have some minor changes in overall gas composition, but the effect is negligible. Conversely, gas composition variation at a refinery and chemical plant applications can significantly affect flare gas flow measurement resulting in erratic performance.
A small amount of hydrogen within the flare mixture can cause erroneous results using TMFMs. A TMFM measures heat transfer caused by gases as they flow past a heated sensor, and gases have different heat transfer characteristics. With hydrogen (H2) having a high thermal conductivity combined with low density, H2 has a significantly higher cooling capacity than other gases. Therefore, a small amount of hydrogen can create a high level of heat transfer, which will result in a higher measurement of the gas flow.
Usually, a main flare gas line at refineries and chemical plants contain a mixture of gases depending on where in the facility or unit the gas is originating. At times, the gases have a relatively uniform composition; however, there can be significant variations most of the time. This change in composition can be further complicated when hydrogen is present or changes in hydrogen content. Additionally, when reporting environmental emissions or determining a mass balance, greater accuracy is required. All flow meters are affected by variations in gas composition; however, SAGE can evaluate changes in accuracy by knowing the gas mixture.
For this reason, in petroleum refineries or chemical plants with varying gas compositions, thermal mass flow meters are not traditionally suitable for accurate emission measurement or obtaining a mass balance for the main flare header. TMFMs, however, can be used in branch or feed lines in these facilities to discern which operation or unit is sending flow to the flare.
New Sage Metering Engineering Approach
By knowing the gas composition, even in refineries and chemical plants, SAGE can predict accuracy with knowledge of the typical base gas composition and gas composition variation. SAGE has developed a method for modeling heat transfer characteristics of a gas mixture. By analyzing the variations in gas composition, SAGE can use this model to predict variations in accuracy caused by gas composition deviations. Due to the complexity in convective heat transfer analysis, this determination of changes in convective heat transfer caused by varying gas composition cannot currently be done in real-time, thus requiring analysis by SAGE engineers.
In Part IV of this series, we will continue discussing flare flow measurement and the Mandatory Reporting of Greenhouse Gases Rule (40 CFR 98).
Perhaps you may find this video of interest.
Flare Gas Recovery, BBC Documentary