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Process gases: flaring alternatives

12 January 2016

Torsten Haug discusses a high-performance measuring system with fast response time which enables problem-free utilisation of process gases in many industry sectors.

Many process-related operations produce process gases that, traditionally, have been regarded as by-products and have been flared at the expense of the environment. With a growing interest in greater energy efficiency, this practice is changing.

Today the process gases produced in many process-related operations, the steel industry in particular, are being collected in gasometers and used as combustion gas in place of natural gas in thermal processes. However, the fluctuating composition of the process gas - especially the CO content - and the related fluctuating energy content, requires greater consideration.
The energy content of the gas composition is stabilised through the controlled addition of natural gas, which is essential for meeting the requirement for a uniform furnace atmosphere in each thermal process. A high-performance measuring system with fast response time, consisting of calorimeter and gas analysis, provides the controlled variable for conditioning of process gases for energy content.

In steel production, with hot-rolling mills, the process gases are utilised as combustion gas in the burners of the downstream hot-rolling mill. The properties of the combustion gas must meet two very different requirements. A defined excess of air must be ensured in the various zones of the hot-rolling furnace to achieve the desired steel quality. Simultaneously, the CO concentration in the off-gas from the flue must not exceed a defined limit because the plant will otherwise be automatically shut down in line with requirements of the Emissions Directive. The plant operator must quickly and accurately detect the changes in process gas composition and, on this basis, add natural gas in a controlled manner to continuously meet the two requirements. This is only possible using gas measuring technology with corresponding performance capability.

Gas analysis 
A measuring system configured specifically for this task is the CWD2005 direct calorimeter from UNION Instruments with additional integrated gas analysis and measured value processing. The system is able to directly determine the Wobbe index and gas density values as well as the concentrations of CH4 and C2+. The heating value and air requirement are also calculated from this. The combined information collected from the calorimetry and gas analysis provides a basis for sufficiently accurate control of the gas addition. Of particular importance is the higher air requirement for combustion of alkanes (CH4, C2+, etc.) compared to CO, which the gas analysis takes into account by determining the concentration of these two components.

It is also important that the measuring device reacts quickly enough to ensure the control process. This requires special measures owing to the size of the plant and the mixing of gases in the pipe system. 

A redundant analysis system consisting of calorimeters with fully-integrated gas analysis for CO, CH4, C2+ ensures efficient, environmentally sound combustion of process gases in hot-rolling furnaces.
A redundant analysis system consisting of calorimeters with fully-integrated gas analysis for CO, CH4, C2+ ensures efficient, environmentally sound combustion of process gases in hot-rolling furnaces.

UNION Instruments has created a computational model based on delay elements that can be adapted to different plant dimensions by assigning parameters. To increase plant availability, the measuring system can be designed redundantly. Each system supplies measured and converted values to the controller, which then determines the controlled variable for the gas mixing device.

This measuring technology contributes to the more energy-efficient use of process gases as well as greater plant safety.

Torsten Haug is director of UNION Instruments.

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