BURNING A MIXTURE OF H2 AND NATURAL GAS

DOW’S PLAQUEMINE, LA, PLANT ADOPTS FUEL-BLENDING FOR GE 7FA TURBINES

GE 7FA gas turbines equipped with Dry Low NOx (DLN) combustion systems are often deployed in 100% pipeline natural gas applications. Although natural gas is available, industrial and petrochemical sites sometimes prefer to use process gases produced on-site to reduce plant operating costs. These facilities produce hydrogen-containing process gases (known as off-gas) as a byproduct of the main plant chemical processes. Some users desire to reduce natural gas consumption by blending a percentage of the process off-gas with their main natural gas supply.

In response to a request from The Dow Chemical Company, GE developed a fuelblending system allowing a hydrogen/natural gas fuel to be burned in a 7FA Gas Turbine with the DLN2.6 combustion system. Until its development, the fuel specification for this combustion system did not allow for hydrogen to be included in the fuel composition.

Fuel blending challenges

The Dow Chemical Company approached GE about the possibility of operating a set of gas turbines at one of its petrochemical plants on a blend of process gas and natural gas. The first step in this process was a series of feasibility tests in GE’s combustion test facility in Greenville, SC, to determine the capability and limitations of the current DLN2.6 combustion system to burn a fuel blend of natural gas and hydrogen. These tests were performed at full pressure, temperature and flow, simulating field conditions.

Parameters monitored during the tests included NOx and CO emissions, as well as combustion dynamics. These tests demonstrated that the DLN2.6 combustion system is capable of operating with up to 5% H2 blended with natural gas.

Based on these feasibility tests, the engineering team designed an off-base system, capable of blending the process gas with the natural gas.The systemis composed of instruments for measuring fuel flow and gas composition as well as valves for controlling flow.

Given that a top motivation for fuel blending is to reduce natural gas consumption, it is critical that the control system maintains operation on the blended fuel. Key design elements of the fuel blending system to enable this capability include:

• GE’s Mark VI turbine control algorithms, integrating process-gas fuel blending controls and turbine protection software

• Continuous monitoring of the fuel, including the hydrogen content of the blended fuel, to ensure it stays within the defined operating envelope.

These design elements allow the system and the gas turbine controls to adapt to varying amounts of hydrogen fuel, while protecting gas turbine components and enhancing performance. The controls can handle non-methane hydrocarbons in addition to hydrogen. This is important as some chemical processes contain small amounts of ethane or other higher hydrocarbons in addition to hydrogen.

Installation and start-up

The hydrogen fuel blending systems were delivered to Dow’s Plaquemine petrochemical facility in Plaquemine, LA. This site has four GE 7FA turbines operating in a combined cycle configuration. They provide internal power for the facility, and are available to the local grid during peak power periods. Until this upgrade, these units operated solely on natural gas.

The blending systems were installed on a platform specifically built for this equipment; the four blending systems are shown in Figure 1. This allows the natural gas header to be connected to the fuel blending systems, one for each turbine, resulting in four separate fuel streams.

First operation on blended fuel took place in May, 2010. The nominal blended fuel compositionwas 5.6%hydrogen equivalent (by volume), which includes hydrogen plus other non-methane hydrocarbons.

A performance validation performed on the turbine included acquisition of performance at base load, before and after conversion. Data collected included output, emissions, and combustion dynamics.

Combustion dynamics, as measured by instruments in the combustion chambers during operation on blended fuel, were lower than natural gas acoustics over most bands and within normal limits. NOx emissions, while operating on the blended fuel, showed no measurable change in output. These 7FA turbines, it was concluded, are operating like any others in the fleet.

All four 7FA Gas Turbines at the site are now capable of operating on blended fuel. To date, these units have accumulated more than 5,100 fired hours on blended fuel.

Authors

Jeffrey Goldmeer is F-platform Fuel Flexibility Manager, GE Power & Water.

Tommy Rozas is a Project Associate at Dow Energy Systems.