Flare Stack / Fuel Gas Optimization

Flare streams often contain waste gas from several process units with often rapidly changing compositions. Control of the net heating value is required to maintain destruction efficiency.

In some regions this is a mandated government requirement. In others, rapid detailed information is needed is to minimize the costs associated with steam and supplemental fuel gas. Expensive supplemental gas in the form of natural gas or propane is used to maintain the heating value. Steam may be required to eliminate smoking.

Flare control is a complex process and, in order to maintain a sufficient heating value, the refinery will need many vent gas composition updates. Slow analytical technologies, historically used for flare gas analysis, such us GCs (Gas Chromatographs) are increasingly insufficient for control, giving only one or two updates within a 15-minute block. Real-time data is necessary to avoid noncompliance penalties and wasteful over-application of supplemental gas and steam.

Application Challenges

High variability, sulfur content, and corrosivity can make these samples difficult for many analytical techniques. The fuel matrix is constantly changing as low quality fuel gas needs to be enriched with support gas/ syngas.

COSA's Solution – Calorimeter

The COSA 9610 BTU Analyzer’s measuring principle
is based on the analysis of the oxygen content in the flue gas after combustion of the sample. A continuous gas sample is mixed with dry air at a precisely maintained constant ratio, which depends on the BTU range of the gas to be measured.

The fuel air mixture is oxidized in a combustion furnace in the presence of a catalyst at 800°C, and the oxygen concentration of the combusted sample is measured by a zirconia oxide cell. The residual oxygen provides an accurate measurement for the Combustion Air Requirement of the sample gas, which can be correlated accurately to the Wobbe Index of the gas.

Real-time gas analyzers from CXC enable fast, efficient flare control. Root cause analysis is needed to identify which process units are the primary contributors to the flare stream at any given time; and often is part of the compliance regulations. For high-hydrogen flares, the speciated measurement of hydrogen is vital. With a net heating value of 274 Btu/scf, increasing hydrogen in the vent gas can drive a flare dangerously close to the allowable limits. Gas analyzers that speciate hydrogen will report a higher heating value than a calorimeter alone, keeping the refinery in compliance and saving
supplemental gas.

Cosa Xentaur manufactures the calorimeter and the hydrogen cell for reporting the hydrogen content from 400ppm to 100%. CXC offers an integrated hydrogen measurement package to allow for the reporting and obtaining credits for the percentage of hydrogen being combusted in a flare, thus lowering the required makeup/ assist gas used to reach heating value minimums. Rapid updates alert operations as the heating value changes. The operator has time to respond, remain in compliance and use a minimum of costly supplemental gas and steam. The COSA 9610 provides a direct measurement of the Combustion Air Requirement Index (CARI) of a fuel, which is ideally suited for the precise control of the fuel-air ratio of a combustion process.