Proving low NOx turbine technology in Santa Clara
23 April 1999Distributed generation will provide new opportunities for both users and manufacturers of small gas turbines. However, to capture this opportunity, users will have to meet new emission standards, at single-digit levels and falling. The Xonon combustion system, produced by Catalytica Combustion Systems, can enable turbines to achieve these standards. The first such system was installed at the Gianera Generating Station of Silicon Valley Power.
The Xonon (Xonon is simply no NOx spelt backwards) combustion system has been installed at the 60 MWe Gianera Generating Station of Silicon Valley Power, a municipally-owned utility serving the city of Santa Clara, California, USA. This was the first application of the Xonon system by a utility at a commercial installation. The Xonon system was installed on a 1.5 MWe Kawasaki M1A-13A gas turbine.
The turbine was started in October 1998, and early indications of the effectiveness of the Xonon system are said to be positive. It is planned to acquire at least 8000 hours of operating experience at this site. This will, it is hoped, give evidence of its reliability, durability and maintainability benefits. The Xonon system had previously acquired 1200 hours of experience in full-scale tests at a test facility in Tulsa, Oklahoma.
The Gianera installation was the first deployment of the Xonon technology in a commercial setting. Previously, the tests carried out on Xonon demonstrated its ability to control NOx emissions to below 3 ppm. The Xonon technology was described in the January 1998 issue of Modern Power Systems. It is now applied to small gas turbines.
Trends in air emissions regulations
In the US, the New Source Review (NSR) regulations pose the constraining emissions limitations for new gas turbines. These regulations require the use of Lowest Achievable Emission Rate (LAER) in areas that currently do not meet the ambient air quality standards, and Best Available Control Technology (BACT) in areas that do. Currently, new permits for gas turbines are in the range of about 3-15 ppm NOx for LAER requirements.
The change in emissions standards has been very noticeable over the last decade. Fifteen years ago, gas turbines producing emission levels of 100 ppm were shipped. Since then, lean pre-mix has become the standard, reducing emissions to typically 15-25 ppm, with 9 ppm being reached by some turbines. To achieve lower emission levels than this, there are two basic options. One is to prevent NOx formation in the first place, and the other is to clean it up in the exhaust.
Emissions levels will continue to be tightened. In part, this will be as a result of government regulations, in part from market demand, and in part as a result of the insistence of lending agencies. The World Bank, for example, is generally asking for 25 ppm as a lending requirement, and this level is clearly moving lower. It is not likely that there will be a great deal of change over the next year. Over the next 5 years, there will be a lot of changes. The EU, for example, has been discussing for some time what NOx emission level limits should be applied. The details of the various EU draft levels have varied widely, but it seems likely that 25 ppm will be adopted.
Tokyo, as another example, has adopted a limit of 5 ppm for new plant. This is unlikely to change in the foreseeable future.
Opportunities for small gas turbines
The new market for small gas turbines (typically under 20 MWe) is in the emerging area of distributed generation. Distributed generation means that the power is generated closer to the point of use. This typically means that distributed generation units have to meet stringent emission regulations.
These smaller projects are finding that costly emission control technology can destroy the economic feasibility of a project. In the USA,
Options for less than 5 ppm NOx
There are only two practical approaches to meeting the new LAER and BACT requirements of less than 5 ppm. One is to prevent NOx formation, and the other is to clean it up in the exhaust.
Selective Catalytic Reduction (SCR) is a clean up technology applied to gas turbines that already incorporate lean premix combustion. SCRs has been successfully used with gas turbines for years. However, there are environmental considerations. SCRs use ammonia, which requires special handling and permits, and produces ammonia slip as a toxic emission. The SCR catalyst contains toxic metals, which have to be disposed of as hazardous waste.
The Xonon combustion system is based on pollution prevention. This can achieve less than 3 ppm of NOx, while avoiding adverse environmental impacts. The Gianera plant is demonstrating this ability in practice.
Prevention of NOx formation
The Xonon system uses a combustor with a catalyst to enable lower combustion temperatures to be achieved. This has been known since the 1960s. Typically, noble metal catalysts and ceramic monoliths were used, but high combustion temperatures resulted in sintering and vapourisation of the catalytic metals and shattering of the monolith materials.
The Xonon system uses a two-stage process to overcome these problems. Combustion is initiated by the catalyst, but is completed by homogeneous combustion in the post-catalytic region where the highest temperatures are reached.
The technology involves a staged system in which a portion of the fuel is consumed within the catalyst region, but the final combustion that generates the highest temperatures takes place downstream from the catalyst. Typically, about half the fuel is combusted within the catalyst stages, and the remainder is burned via homogeneous combustion reactions after exiting the outlet stage catalyst. By isolating the highest temperatures downstream of the catalyst, many stability issues are resolved.
The system has a range of operating conditions over which it will provide the desired low emissions levels. This operating window can be described in terms of two factors - the inlet temperature and the adiabatic combustion temperature of the fuel-air mixture passing through the reactor. The window is constrained by three general features of the reactor's performance.
The inlet temperature must be high enough for the catalyst to become active for methane oxidation. Unless this 'minimum inlet' temperature is reached, the rate of the exothermic oxidation reactions occurring on the catalyst walls is too slow to generate the heat necessary to sustain system operation.
A second constraint requires that the gas temperature at the exit of the outlet stage is high enough to initiate homogeneous combustion and CO burnout downstream from the catalyst. This temperature is affected predominantly by the adiabatic combustion temperature in the reactor.
The third constraint requires that the wall temperatures do not exceed their design limits. This constraint will be exceeded if the combustion of catalyst inlet temperature and adiabatic combustion temperature places the operating point above the 'maximum catalyst wall' temperature boundary. Each catalyst stage has its own individual temperature characteristics, so the maximum catalyst wall limit may not be a simple single line.
The potential of catalytic combustion has been recognised for many years. However, the environment in a gas turbine combustor presents significant challenges for a catalyst. The gas temperature required at the combustor exit ranges from 1175°c to 1500°C, depending upon the particular turbine design. Such temperatures are well above the stability limits of most catalytic materials. Even ceramics that can survive the combustor temperatures are susceptible to thermal shock failure during the transients that accompany turbine operation. These durability issues have been a significant barrier to development of a viable catalytic combustion technology for gas turbines.
The Xonon system successfully addresses the unique challenges of gas turbine applications. This technology uses catalysts that are designed to limit the extent of fuel combustion that occurs within the catalyst structure itself. Limiting the reactions in this enables such systems to limit the maximum catalyst temperature and thus broaden the selection of catalyst components and extend catalyst life.
The Xonon combustor consists of four main sections:
Four sections
Preburner. The preburner carries the machine load at operating points where the conditions in the catalytic reactor are outside of the catalyst operating window. These are frequently the low load points where the fuel required for turbine operation is insufficient for the catalyst to generate the necessary minimum exit gas temperature. As the turbine load is increased, progressively more fuel is directed through the main injector and progressively less goes to the preburner. Ultimately, the preburner receives only enough fuel to maintain the catalyst above its minimum inlet temperature.
Main fuel injector. This unit is designed to deliver a fuel-air mixture to the catalyst that is uniform in composition, temperature, and velocity. A multi-venturi tube (MVT) fuel injection system was developed by GE specifically for this purpose. It comprises of 93 individual venturi tubes arrayed across the flow path, with 4 fuel injection orifices at the throat of each venturi.
Catalytic reactor. The catalyst must burn enough of the incoming fuel to generate an outlet gas temperature high enough to incite rapid homogeneous combustion just past the catalyst exit.
Downstream liner. This is the location of the final combustion reactions that complete the oxidation of the fuel and any remaining CO in order to achieve ultra low emissions. In general, the homogeneous reactions must be completed prior to injection of any dilution air into the hot gas path.
The catalytic reactor. The catalytic reactor consists of three individually supported stages, each 508 mm in diameter. Mechanical support was provided by large-cell honeycomb discs 13 mm thick made of Haynes Alloy 214 and attached to the walls of the container. The catalyst stages are formed by corrugating strips of oxidation-resistant metal foil 50 um thick and then depositing the active catalyst material as a coating on the strips. The strips are coiled to form channelled monolithic structures through which the fuel-air mixture can pass and react on the channel walls. The overall length of the catalyst container is 305 mm, with the catalyst occupying 230 mm.
Features of the combustion system
Three particularly important features of the combustion system are indicated in the diagram below. Prior tests had revealed a comparatively cooler region at the centre of the reactor, and the range of measured fuel/air inlet was broader than desired. Two changes were made in response to these non-uniformities.
The low-temperature in the centre correlated with a consistently low fuel/air ratio in that area, which was caused by higher than average air flow down the centreline of the combustor. The centre peak in velocity distribution was caused by detachment and consequent slowing of gas flow near the high angle diverging walls at the preburner diffuser section. Installation of a perforated plate smoothed the non-uniform velocity profile entering the main fuel injector.
Testing of the MVT fuel injection unit showed variations in fuel flows among the 93 venturis. The locations of the outliers in fuel flow could be correlated with the locations of temperature extremes. Tailoring specific injector orifices were shown to improve the uniformity of fuel flow. As a result, the catalyst module operates at a relatively low level, keeping NOx emission levels to a minimum.
Power output and heat rate
It is still important to maintain engine efficiency and power output. The Xonon system meets this by achieving the desired turbine inlet temperature profile and minimising the pressure drop.
The Xonon system is designed to fully burn fuel to produce a high temperature mixture. Dilution air is then added and adjusted to shape the temperature into the profile required at the turbine inlet. In addition, the Xonon module and combustor include low pressure drop preburners and fuel mixing systems to ensure minimal pressure loss. The tests in Tulsa demonstrated that the turbine inlet temperature profile could be made to be identical to that of a conventional diffusion flame combustor.
Starting and shutdown
The Xonon technology is new for gas turbines, and thus needs a specialised control strategy. Woodward Governor and Catalytica jointly developed a control system for the start-up, loading and shutdown of a turbine incorporating Xonon.
A turndown in load is accomplished by lowering fuel flow to the combustor. At reduced loads, Xonon emissions performance is maintained by increasing the fuel flow to the preburner to maintain the inlet temperature to the catalyst as the fuel flow is decreased. Fuel flows are automatically adjusted by the combustor control system.
Vibration
Many NOx reduction technologies can encounter flame instabilities that cause pressure pulsations and vibration within the engine. The Xonon technology has been demonstrated to give excellent stability with low dynamic pressure pulsations. The Tulsa test facility gave measured dynamic peak-peak pressures of less than 0.41 psi.
Pattern factor
Pattern factor is a term used to describe the inlet temperature profile. It measures the relationship between peak and average temperatures both along the blade and around the nozzle ring, and directly affects the following aspects of a gas turbine:
The Xonon system offers a potentially improved pattern factor. The system delivers a very uniform temperature profile for the exit gases leaving the combustor and moving into the turbine inlet. This uniform temperature profile can be easily tailored to the desired temperature profile through the use of cooling or dilution air introduced downstream of the combustor.
The flatter temperature profile also allows lower peak temperatures. As a result, fuel flow at maximum load can be increased to provide greater turbine power output.
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