Use of Low Heating Value Natural Gas by Blending With Fuel Gas

Reza Mesgarian (National Iranian Gas Company- Head of Process Engineering) Mesgarian@nigc.ir

ABSTRACT

   The completion of a sweetening solvent change from Di ethanol Amine (DEA) into methyl Di ethanol Amine (MDEA) in sour gas sweetening units conducted off spec flash gas. Flash gas lined up directly from flash drums to incinerators in the sulfur recovery units (SRU), area was completely implemented in 2014. The flash drum gas is an off-spec gas with a high concentration of carbon dioxide. The fundamental objective is to use this low Btu, high CO₂ gas as a low cost fuel. Scrubbing the gas to remove the CO₂ makes it complicate and too costly to use as a fuel. However, if this gas could be blended with higher Btu spec gas typically supplied to incinerators, it would meet the objective of providing a lower cost fuel while volumetrically reducing the CO₂ content of the final mixture.

   Fuel gases with large percentages of an inert gas such as Carbon Dioxide will have a ratio of rich-to-lean flammability limits less than that of natural gas. Flammability ratios of less than 2.2 to 1 based on volume at standard conditions (14.7 psia and 15 ^oC), may experience problems maintaining stable combustion over the full operating range of the combustion incinerator burners.

  Burners can operate with fuel gases having a very wide range of heating values, but the amount of variation that a specific fuel system design can accommodate is limited, usually ± 5%. The fuel nozzles are designed to operate within a fixed range of pressure ratios and changes in heating values are accommodated by increasing or decreasing the fuel nozzle area or the fuel gas temperature. Since changing the fuel nozzle area is difficult, the temperature of the fuel gas is generally changed to accommodate significant changes in the heating values.

   The project objective was to design and install a blending stations utilizing either Btu or Wobbe Index as the basis to blend the gases such that the resulting fuel would be a lower cost alternative, and meet burner firing requirements for the incinerator and environmental emissions limits. A temporary analyzer that could determine the composition of the natural gas in near real time mode was employed. The analyzer outputs, as well as other instrumentation, were brought into the plant’s DCS for operator monitoring and control of the gas blending stations.

Our research of the Wobbe Index indicated that the preferred blending control mode should be based on the Wobbe Index. However, there did not appear to be any distinct advantage in using the Wobbe Index over the heating value or Btu. Since usually both Btu and Wobbe Index outputs are available from the gas analyzers, the logic configuration could developed such that the Operator select either Btu or Wobbe Index as the basis for controlling fuel gas blending.

The project evaluated two methods of blending, those being a single blending system at the plant gas yard or individual blending systems at each unit. However, the incinerators chose multiple blending systems because they intended to burn the blended gas only in the gas burners and felt the additional operability offset the cost of the additional blending stations.

    This would allow the plant to determine which method gave them the best control of the blending stations.

   We know that the maximum limit of the Low Btu fuel gas is a 50% ratio, but the minimum limit (low flow) is set by blending system characteristics. To ensure trouble free start-ups, plant Operations prefers to start the burners of incinerator on Hi Btu fuel gas. Once the unit is online and operating reliability; at a given load, the Operator places the blending station in service.

The success of the blending station depends on the quality of the temporary analyzer signals. If the blending station analyzer fails, the blending control is automatically, or by Operator action, placed on flow control and the Hi Btu gas remains on pressure control. If the gas yard analyzer fails, the station is automatically placed on flow control (Low Btu gas) and pressure control (Hi Btu gas).

   During the start-up phase of the project, both methods of controlling the blending stations were tested. There were no significant differences found between controlling based on Btu or Wobbe Index. Because the Wobbe Index is incremented in finer divisions, one would expect that finer blending control could be obtained, but for this application, very fine adjustments to the blending ratio are not necessary. The Low Btu gas on flow control is the default blending control method. This method was also tested, both as an Operator selected operational mode and as a default control mode when one of the analyzers fails.

   The plant is currently operating on flow control mode, selected by the Operator, because the Low Btu gas can yet provide enough gas to justify blending based on Btu or Wobbe Index.

KEY WORDS

Fuel Gas, Gas Blending, Btu, Wobbe Index, Natural Gas, Heating Value