Jeffrey Benoit, PSM, USA
Increasingly restrictive government regulations on emissions from gas turbines are driving key technology decisions across the US, Europe, the Middle East and Asia. In Texas in the US, for example, regulations are forcing permit holders in certain industrial areas to ratchet down oxides of nitrogen (NOx) emission levels by over 80 per cent by the end of 2008.
These restrictions are forcing power plant operators to evaluate their cost/performance strategies in an effort to ensure the continued profitability of their operations. Typically, they find themselves considering shutting down certain facilities, changing turbine operational profiles, replacing combustion systems with more modern technology or installing selective catalytic reduction (SCR) systems. Choosing the best solution from these strategies involves complex risk-based cost-benefit analysis.
Recent improvements in lean premix combustion technology for gas turbines have helped reduce NOx and carbon monoxide (CO) emissions to ultra-low levels, providing the end user with a cost-effective emissions reduction solution without recourse to SCRs or oxidation catalysts. The current field performance of dry, lean premixed combustion systems developed for use in Frame 6B, 7E and 9E industrial gas turbines, previously configured with either the OEM’s DLN-1 combustion system or converted from a standard diffusion combustion system, suggests that they will play an important role in helping plant operators meet new emissions regulations.
The LEC-III® combustion system, developed by Power Systems Manufacturing LLC (PSM) has demonstrated sub-4 ppm NOx and single-digit CO emission levels over the entire load range with natural gas fuel, from full baseload down to below 55 per cent of full load conditions, in customer machines. At the same time, rugged construction and performance optimization enable the hardware to meet extended service interval targets of more than 16 000 equivalent fired hours. With well over 250 000 fired hours of operation across several dozen machines and customers in three continents, the LEC-III system is a proven, validated alternative technology solution.
PSM provides new high capital cost replacements for E- and F-class hot-gas path components that meet or exceed current performance standards. A core competency of PSM is the design, development and delivery of B-, E- and F-class low-emissions combustion components, systems and diffusion combustion system conversions. These are aimed at operators of OEM gas turbine equipment looking to improve unit performance and fuel flexibility.
PSM’s LEC technology was first incorporated into GE 7E gas turbines in 1998. The LEC can-annular, reverse-flow combustion system, shown in Figure 1, was designed as a direct replacement for OEM DLN-1 systems in existing gas turbines. The lean premixed LEC system includes fuel nozzle assemblies, transition pieces, flow sleeves and combustion liners, which were initially designed to achieve less than 25 ppm NOx (corrected to 15 per cent O2) at baseload conditions
Figure 1: Cross-section of PSM’s MS7E/EA LEC combustion system
Noting the commercial need for a simple and cost-effective solution to ultra-low emissions requirements, PSM, working over the past decade, has developed and patented component features that allow customers to achieve sub-4 ppm NOx emissions over a large operating range with acceptably low CO and combustion dynamics. The currently available LEC-III system enables a plant to meet these stringent performance requirements.
Industry is beginning to seriously, if belatedly, address the environmental consequences of carbon dioxide (CO2) emissions and develop plans to capture it. However, gas turbines are significantly lower emitters of CO2 than typical coal fired power plants. In natural gas fired industrial turbines, today’s targeted and regulated emission pollutants are NOx and CO.
NOx formation in these turbines is a function of temperature and time. The process, known as thermal NOx production, occurs when high temperatures cause a decomposition of atmospheric nitrogen (N2) into single atoms that then react with oxygen to form NOx species. The decomposition of N2 is minor below about 1560 ºC, as is the corresponding NOx formation. But NOx production increases rapidly as the temperature increases beyond this point.
Average lean premix combustion system temperatures in a turbine are typically lower than the 1560 ºC threshold, but certain local regions within the chemical reaction zone may be significantly hotter than that. In these locations excess NOx will rapidly form. Minimizing these rich local areas by enhanced mixing is a key way of reducing NOx.
CO is generated by incomplete combustion. In an ideal combustor, CO is a short-lived, intermediate by-product that is allowed to fully oxidize to CO2 at the completion of combustion. However, operational combustion systems in gas turbines are not ideal, due to cost considerations, different design philosophies and system cooling techniques. Because of this, incomplete combustion and the resulting CO formation is a reality. As with NOx, enhanced mixing improves combustion efficiency and reduces CO emissions. Therefore, unmixed ‘cold’ stream in the reaction area needs to be eliminated to prevent CO becoming a problem.
A number of important technological advances enable the LEC-III system to perform as it does (see Figure 2).
Figure 2: LEC-III combustion system
Key mixing features of the LEC-III include: A reverse-flow cooled venturi; increased dilution air supply to the ‘head-end’ premixer by deploying the enhanced cooling efficiency of effusion cooling; and a fully premixed, ‘fin mixer’, secondary fuel nozzle. All these features, which will be discussed individually, contribute to the exceptional operating performance of the combined system.
Reverse-flow cooled venturi
The venturi plays a key role in the combustion process. The forward cone of the venturi, immediately downstream of the head-end premixer, converges and accelerates the premixing gas and air blend. At the venturi ‘throat’, premixed mixtures are discharged into the reaction zone at very high velocities. Immediately downstream of the throat is the venturi’s divergent cone, which creates a sudden expansion and strong recirculation region. This strong recirculation is essential for combustion stability. The high velocity of the premixed flow at the throat prevents flashback during the premix combustion mode, where all the reaction is contained in the liner mid-zone, just downstream of the venturi throat.
Throat velocities are sufficiently high in the bulk mainstream to be well in excess of reacting flame speed, even when coupled with the dynamic oscillations created from combustion noise. The minimum local velocity differential is the flashback margin. For the LEC-III system, this margin is significant.
In all operating modes, one or both of the venturi cones are exposed to high temperatures. The standard technique for preventing the venturi from overheating is convective cooling, in which the venturi supply air is channelled through an annular space between an inner and an outer wall. This process is shown in Figure 3, which illustrates the two different venturi design approaches.
Figure 3: Venturi cooling processes for conventional and LECIII designs
In the more conventional OEM design, the air enters the venturi cooling channel at its forward end, near the throat. The air flows aft, in the same direction as the combustion gas, eventually discharging into the combustor hot-flow stream on the liner walls near the aft end of the reaction zone. The cooling is effective in maintaining stable venturi metal temperatures.
However, a large cold stream of air is generated around the reacting gas volume. The stream represents a significant percentage of the entire liner flow, so it is a significant flow feature within the reaction zone. This cold air interacting with the hot gas exiting the venturi stops CO from fully oxidizing to CO2 and limits the combustor’s operational range.
In the LEC-III system, the cooling air enters the venturi at its aft end and flows forward, opposite to the hot gas flow, to cool its cylindrical section and throat area. The air picks up heat by convective heat transfer, with the assistance of ‘trip strips’, a commonly used feature in advanced turbine airfoil cooling. Metal temperatures are maintained at levels consistent with the traditional aft flowing venturi.
At the extreme forward end of the venturi, the cooling air is collected in an annular plenum outside, which then discharges the preheated cooling air into the head-end chamber for use in premixing.
This air has now been preheated, which increases its discharge jet velocities, enhancing mixing immediately at the point of head-end flow convergence. As result, the venturi cooling air is fully mixed, so that the cold stream is eliminated.
Increased premixer dilution air
In the head-end of the LEC-III liner, as shown in Figure 4, outer wall and dome plate cooling is accomplished using shallow-angled effusion cooling holes. Usually, the head-end premixer dome plate has cooling holes that provide impingement cooling to the dome heat shields. The liner walls are louvre cooled, which creates a film of cooling air on the wall surface. Louvre cooling is used extensively in combustion components. However, it is not nearly as effective as effusion cooling.
Figure 4: LEC-III combustion liner with head-end mixer effusion cooling holes
With effusion cooling, substantially less air is required for wall cooling, and the need for an industry-standard thermal barrier coating is eliminated. The additional cooling air is put into the head-end dilution jets to enhance mixing. Effusion holes are precisely controlled in terms of their size, angle, pattern and locations.
Airflow requirements are stringent. This is to ensure that liner-to-liner airflows within the system remain consistent. This airflow control, in combination with close fuel-flow control, maximizes the lean blow-out margin, minimizes exhaust temperature spreads, eliminates any ‘hot spot’ to improve component durability and reduces NOx emissions. Control of the mixing air and fuel flows is essential in establishing the target fuel-air ratios in the reaction zone, which is the source of pollutant generation.
Premixed secondary fuel nozzle
The secondary fuel nozzle is a key contributor to the stability of the OEM DLN-1 combustion system. The secondary fuel nozzle sets up a central ‘pilot’ zone of reaction and recirculation that acts as the ignition source for the surrounding reaction zones of premixed primary fuel. By design, this secondary reaction zone is a richer mixture, burning hotter to provide excellent combustion stability.
In a conventional DLN-1 system, this secondary fuel flow is channelled through two separate circuits. Most of the fuel is discharged from ‘pegs’ near the nozzle’s middle section. This fuel premixes with air as it travels along the length of the nozzle and then passes through ‘swirler’ vanes for final premixing prior to discharge into the reaction zone.
The second circuit within the nozzle has a small amount of fuel discharging at the tip (extreme aft end), which is not premixed. It burns in a ‘diffusion’ mode of combustion. This region has some areas of reaction temperatures above 1560 ºC, and the associated NOx formation is significant. It is only a small part of the total fuel flow, but its contribution to the system’s total NOx formation can be considerable.
Eliminating the diffusion burning that occurs with a conventional nozzle has been a key goal in LEC-III secondary fuel nozzle development. Much rig and engine development testing time has been devoted to the development of this nozzle. PSM’s latest secondary fuel nozzle offering, known as the Fin Mixer SFN, has now demonstrated its ability to significantly reduce NOx. This improvement is simple in concept and implementation, and it provides a step change in emission reduction in an already low-emission combustion system.
PSM has successfully installed combustion systems in customer machines that were already configured with OEMs’ DLN-1 combustion systems, immediately reducing NOx emissions by half. Figure 5 shows typical results for PSM’s combustion system: NOx emissions below 4 ppm and CO below 25 ppm across the desired premixed machine load range.
Figure 5: Performance data for an OEM 7E gas turbine equipped with PSM’s LEC-III combustion system
For a Frame 7E, a 1 ppm reduction in NOx is equal to approximately 1113 tonnes of NOx over a year of operation, which can be used to offset emissions on other customer plant equipment or sold as NOx credits, where an credit trading facility is in place. Another important advantage to the customer is that no changes are necessary to standard OEM installation procedures, and no additional gas turbine control complexity is required.
PSM has developed cost-effective combustion technologies with proven, field-validated, world class emission results for Frame 6B, 7E and 9E customers. The company’s ‘drop-in’ solution offers its customers an emissions reduction solution that meets the requirements demanded by regulators at a very competitive life-cycle cost.
PSM also offers customers looking to convert their diffusion combustion systems a turnkey solution by packaging the LEC-III system with all the necessary plant modifications and upgrades.
These upgrades have not only been completed on Frame 7E machines, but also on two 501D5 engines. They are currently being retrofitted into four 501B6 machines.
Jeff Benoit is VP-Business Development at PSM, an Alstom company, headquartered in Jupiter, Florida, USA. He has over 22 years of technical and leadership experience in the gas turbine and power generation business.
COMBUSTION TEST FACILITY
The emission results set out in this article were gathered using the PSM high-pressure combustion test rig during the LEC-III product line design and development process.
The full-scale test rig can deliver full F-class base load operating conditions, with the following maximum operating conditions:
- Air flow: 27 kg/sec
- Pressure: 24 bar
- Inlet air temperature: 650 ºC
- Exhaust temperature: 1925 ºC
The test rig provides optical access to the flame and allows the measurement of emissions, combustion dynamics, pressures and temperatures. Various fuel gases can be provided to the combustion system being evaluated through the facility’s alternative fuels infrastructure.
Click here to enlarge image
Air is provided to the test section in two locations from a single air feed pipe, as indicated by the blue arrows in the picture above. A series of baffles are installed in the test section to simulate the engine air-flow pattern to the combustor, as well as the machine-equivalent acoustic plenum. The latter allows the simulation of field machine combustion dynamics within the test rig.