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Continuous Fuel Gas Quality Analysis For Combustion Control

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Furnace utilization improvement

The composition and energy content of fuel gas can fluctuate heavily. When burning fuel gas in a furnace, control over the energy flow is key to optimize the product quality, throughput and efficiency.

Still, many industrial furnaces are equipped with traditional flow and density meters in combination with a stack analyzer for feedback control. These control loops are too slow to handle fast fluctuations in fuel gas composition, and density meters are sensitive to matrix effects when correlating the density to heating value. As a result, these furnaces cannot be operated with maximum efficiency.

The fast-responding WIM COMPASTM F is installed in the fuel gas line to allow feed-forward combustion control. By providing information on the energy flow and/or air-fuel ratio, the combustion process can be optimized, even when high H2 fluctuations are expected. This results in increased efficiency, product quality and throughput, while emissions are reduced.

Payback time on this application is typically measured in months.

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Turbine availability / efficiency improvement

Gas turbines are used to generate energy for the power network or industrial processes. In either case, high availability and efficiency is critical. Power plants can be fined in case of non-deliveries, and industrial plants can suffer from huge collateral damages due to a failing turbine.

A change in fuel gas quality can result in a stage down (a safety mode with less efficiency) or the complete shutdown of a turbine. Rapid fluctuations in fuel gas composition become more common and require attention to guarantee on-spec performance of the turbine.

By controlling the incoming (modified) Wobbe Index or heating value, maintenance, downtime and emissions are reduced and efficiency is improved.

Special attention should be given to sample handling. As process pressures are high, the lag time of the overall loop has to be taken into account to allow for feed-forward control. With the Hobré Flow Impact probe, the response time of the overall system can be reduced to seconds.

Hundreds of WIM COMPASTM F units are supplied globally for this application, as a result of its superb performance and simple installation in harsh environments.

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Feed-forward control air-fuel ratio

Besides the energy flow, the amount of air required for complete combustion of a fuel gas is an important parameter to optimize the efficiency of the combustion process.

Too little air results in unburned hydrocarbons being vented, wasting energy. Too much air does not work either. Air contains approximately 80% nitrogen, which cools down the process and forms NOx at higher temperatures. In both cases efficiency is lost and unwanted emissions are vented into the atmosphere. Besides the combustion efficiency, the furnace atmosphere is often important to the end-user (e.g. glass and steel producers), to guarantee a certain product quality.

Stack O2 measurements are slow-responding, and are therefore only allowed to control the air supply within a narrow bandwidth.

The Hobré WIM COMPASTM F can be used to provide highly accurate information on Wobbe Index or Heating Value, and Air Demand or CARI (Combustion Air Requirement Index) for use in a feed-forward control loop. These parameters allow end-users to have maximum control over their oven atmosphere and combustion efficiency.

Payback time depends on the size of the furnace and application: on average, it is between six months and one year. In cases where product being lost as a result of a process setup, payback time can be much faster.

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CO2, SO2 emission reporting

CO2 and/or SO2 emission monitoring on industrial combustion applications is typically done by installing online analyzers in the stack. In addition to the stack analyzers, a Hobré WIM COMPASTM F is often installed in the fuel gas supply line to optimize the combustion efficiency and air-fuel ratio to prevent unnecessary emissions.
Inside the WIM COMPASTM F, a small slipstream of gas is combusted. The air required is measured and used to determine the energy content and air-fuel ratio. The combustion of this sample also forms products like CO2 and SO2. This provides a representative measure for the amount of CO2 and SO2 formed during the actual combustion process. By measuring the combustion products in the WIM, stack analyzers can be avoided.

The main advantages of this approach include:
• Maintenance can be performed on ground level (no need for scaffolding).
• Only one common analyzer is required on the main fuel gas line feeding into multiple combustion processes.
• Fast feed-forward signal.
• High accuracy as the measurement is not affected by external influences (in-leak of air or flow patterns in a stack).

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Manual burner adjustment (portable)

When burners are started-up, or periodically checked, new parameters are needed to set the ratio between gas and air required for optimal combustion. To calculate these settings the calorific value and/or Wobbe Index of the gas is often requested. Most of the time people use old data for this or rely on assumptions. This in turn leads to relatively high safety margins, and inefficient combustion as more air is added to the mix than required.

The key to achieving the optimal combustion settings is knowing the actual calorific value or Wobbe Index at the time you make each adjustment. With this data, the air-fuel ratio can be optimized, leading to safer, more economical burner operation and greater maximum capacity.

To address this issue, Hobré Instruments has development the HIGAS, a portable Caloric Value / Wobbe Index analyzer which can be used each time new settings are required.

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