In what reportedly is the first application of its kind, refinery burns RFG to run turbines and generate electricity while cutting NOx emissions and using no water.
The Compañia Española de Petróleos (CEPSA) Gibraltar-San Roque refinery in southern Spain may be nearly 50 years old, but the approximately 240,000-barrel per day facility is applying a new approach to meet stricter curbs on emissions of nitrogen oxides (NOx).
In order to meet a European Union requirement to lower the refinery's NOx releases from 42 to 25 parts per million (ppm), CEPSA had GE Power & Water's Power Generation organization overhaul the facility's gas turbine systems and install new combustion technology. According to GE, Gibraltar-San Roque can now use refinery fuel gas (RFG) – a by-product of various oil refining processes also known as "off-gas" – to generate additional on-site power.
Refineries can use off-gas to run various processes, but their inability to store it often necessitates flaring off excess RFG – and increasing NOx emissions, explained Predrag Popovic technical leader for fuel flexibility with GE. "Gas turbines can incinerate RFG, but the challenge is you have to pressurize it to use it in turbines," he said. "It's not typical for refineries to have a compressor for fuel gas."
In addition to leveraging a fuel source that would otherwise be wasted into the atmosphere, the CEPSA refinery's revamped power plant eliminates the need to use steam to manage gas turbine emissions and neither sacrifices efficiency nor increases operating costs, GE contends.
"Overall they reduced the NOx plus freed steam for use in the refinery," said Popovic.
Deleting diluent costs
Using gas turbines – rather than boilers – to generate electricity at a refinery is not a new concept. However, in the past if the fuel gas that was used by the gas turbine contains greater than 5 percent hydrogen, the combustion system required a "diluent" such as steam, water or nitrogen injected into the turbine's combustion system in order to manage emissions.
The Dry Low NOx (DLN) combustion system installed at Gibraltar-San Roque features a "premix combustion mode" where NOx emissions are very low and need no diluent fluid, stated GE. "It reduces the cost of the plant," said Manuel Cardenas, principal engineer for engineering systems and studies with GE's Power & Water unit. "There's no need to produce or consume high-quality steam to manage production of NOx."
GE's DLN combustion system. SOURCE: GE
Extended inspection intervals
According to GE, the gas turbine has operated on RFG for more than 9,000 hours. The company stated that a fleet-leader inspection of the gas turbine components at 4,000 and 8,000 hours found all components in excellent condition. The new combustion system was designed to operate without a required maintenance event for 24,000 hours, representing a maintenance interval three times longer than that of the older combustion design, GE added. "The elimination of the steam usage and extended inspection intervals of the turbine components will have a large reduction in operation cost," Cardenas explained.
The combustion system is also designed to burn fuel gas containing a high level of hydrogen, GE noted. If the hydrogen content in RFG fed into a combustor reaches approximately 32 percent, the RFG must be blended with natural gas to adhere to the combustion system's hydrogen limit, said Cardenas. Moreover, the advanced fuel air mixing of the DLN combustion system reduces the maximum temperature that lowers NOx emissions, he added.
"The DLN combustion system has two fuel circuits – primary and secondary – where the flow split between the two circuits is used to manage emissions and operability characteristics of the combustion system," continued Cardenas. "The primary and secondary fuel circuits can operate at different maximum hydrogen levels. So the controls for both fuel blending and split is a critical element for this new capability of the 'B/E' DLN combustion system to achieve low emissions while consuming fuels with high hydrogen." (Note that a "B/E" class heavy-duty gas turbine produces up to 150 megawatts of power.)
Although DLN technology is installed at hundreds of sites worldwide, what is most unique about the Gibraltar-San Roque application is the fuel-blending process that it employs, said Jay Bryant, product marketing manager with GE Power & Water. "That's new," he said. "We've designed it so that it can be used elsewhere. It's flexible."
In fact, GE is installing a larger version of the combustion system in Canada for a customer in the oil sands sector. "In the oil sands of Alberta, steam is even more valuable because every pound of steam they don't have to push into a gas turbine for emissions control is used to produce bitumen," said Cardenas, referring to the steam-assisted gravity drainage process that employs a pair of steam wells to help bitumen flow to the surface. "That's huge for those guys."
Knocking down NOx levels
The revamped gas turbines and combustion system have operated at Gibraltar-San Roque since early 2014, logging more than 10,000 hours in service. GE reports that the results have been significant.
"There are three value stories with this application," said Bryant. "First, it reduced NOx emissions by 9 percent. Second, it reduced the amount of off-gas flared. Third, it eliminated the need for steam injection into the turbine's combustion system and allowed CEPSA to redirect the steam to other processes. They can potentially increase process output because they're not diverting steam to emissions control. The steam can be used for the refinery itself."
Also, the upgrades allow CEPSA to add time between combustion system inspections, said Cardenas. "Inspection intervals have gone from every 8,000 hours to every 16,000 to 24,000 hours," he said. "That means fewer outages and less resources needed to pull assets out of service."
The "solution helped us to increase plant efficiency and reduce our environmental footprint, supporting our goals to produce cleaner energy and meet the region's increasingly stringent emissions requirements," concluded Antonio Berlango, operations manager with CEPSA.
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