Only a few years ago, gas turbine technology was thought to be the answer to the environmental problems of the power generation industry. But increasingly stringent emission controls are now jeopardizing this position; it is thought that even with current forms of emission technology, today’s advanced gas turbines will be unable to meet future regulatory targets, particularly of nitrogen oxides (NOx).
These regulatory trends have not gone unnoticed. Original equipment manufacturers (OEMs), among others, have developed advanced combustion systems and clean-up technology to reduce emissions from gas turbine machines. One company in particular, Catalytica Combustion Systems Inc., appears to have made a breakthrough with its ‘Xonon’ system.
Xonon is a flameless combustion system which prevents the formation of NOx and lowers emissions of carbon monoxide (CO) and unburned hydrocarbons (UHC) through the use of a catalyst. It can either be fitted to new gas turbines or retrofitted to existing machines. This technology has stimulated a considerable amount of interest from the market, with Catalytica breaking significant deals with industry giants such as Enron, GE and Woodward Governor Company.
Catalytica teamed up with Woodward Governor in September 1996, forming a new 50/50 joint venture company, Genxon Power Systems, to supply the Xonon product to the gas turbine retrofit market.
Shortly after, full scale engine testing of Xonon began on a Kawasaki M1A-13A gas turbine. In June last year, Genxon announced the successful operation of this unit at full load under field operation conditions; emissions of NOx, CO and UHC achieved were extremely low. Trials have continued over the past six months with similarly encouraging results which signify the impact that this technology could have on the power generation industry.
June 1997 also signified another landmark, when Genxon announced that a memorandum of understanding had been signed with GE for the application of Xonon to GE’s worldwide installed gas turbine fleet. This deal will be finalized by the end of the first quarter of 1998.
Minimizing emissions
The primary pollutant concern from natural gas fired gas turbines is NOx which is involved in the formation of secondary pollutants such as ground-level ozone. Today in the USA, emission regulations require new installations to meet NOx emission levels of between five and 25 ppm depending on the location and size of the installation. In southern California and Japan, NOx requirements are below 10 ppm. According to industry experts, the trend towards tighter restrictions is likely to continue, and current advanced technology such as selective catalytic reduction (SCR) and dry low NOx (DLN) will struggle to meet these requirements.
The amount of NOx formed in gas turbines depends on the maximum temperature attained in the combustor and the total time at this temperature. NOx formation can therefore be minimized by reducing the average temperature of the combustor and, in particular, eliminating hot spots, regions where a high fuel/air ratio leads to high local temperatures.
Currently, NOx emissions in gas turbines are controlled by three methods: water or steam injection, SCR and DLN. Injection of water or steam controls NOx by reducing the peak flame temperature, with NOx emissions of 40 ppm attainable. Operating costs for this technology are high due to the need for deionized water. When combined with water/steam injection or DLN technology, SCR can reduce NOx emissions to around 10 ppm. SCR involves treating NOx in the exhaust gases with ammonia, and therefore careful storage and handling are required, and so costs are high.
DLN technology is the latest development in NOx control with 15 to 25 ppm achievable. The next generation of this technology is likely to achieve around ten to 15 ppm. DLN promotes air and fuel mixing to lower the peak flame temperature, and while costs are low, higher CO and UHC emissions are produced.
Like DLN, Xonon is a NOx prevention process. Xonon flameless technology uses a combustor with a catalyst so that lower combustion temeratures can be achieved. Therefore, by reducing the combustion temperature to below 1500°C and limiting the residence time to 25 ms, minimal NOx is formed.
The potential for catalytic combustion to lower NOx emissions has been known since the 1960s. Typically noble metal catalysts and ceramic monoliths have been used but high combustion temperatures result in sintering and vaporization of the catalytic metals and shattering of the monolith materials.
The Catalytica Xonon system overcomes these problems through the use of a two-stage process. Combustion is initiated by the catalyst but is completed by homogeneous combustion in the post-catalyst region where the highest temperatures are attained.
The Xonon combustor consists of four main sections:
The preburner for start-up and acceleration of the engine
The fuel injection and fuel/air mixing system which supplies the catalyst with a uniform fuel/air mixture
The catalyst module, where a portion of the fuel is combusted without a flame to produce a high temperature gas. No NOx is produced here.
The homogeneous combustion region, where the remainder of the fuel is combusted. This is also a flameless process, producing less than 3 ppm NOx.
The combustor is able to produce the very high temperatures required of modern gas turbines while allowing the catalyst module to operate at a relatively low temperature. The catalyst module includes a chemical thermostat that limits catalyst temperature even at very high fuel/air ratios. Catalyst durability is therefore ensured.
Positive results
The most recent testing of the Xonon system has been carried out on a 1.5 MW Kawasaki M1A-13A gas turbine at a test facility in Oklahoma, USA. The test trials began in December 1996 and the machine has so far gained over 1000 operating hours with the Xonon combustor.
The test facility is owned by Tulsa-based Applied Global Cogeneration Inc. (AGC), a manufacturer of packaged cogeneration plants which has a license to build Kawasaki engines in the USA. All tests on the M1A-13A were carried out using natural gas.
The M1A-13A engine is a heavy duty industrial gas turbine often used in cogeneration applications. This single shaft machine consists of a two stage compressor and a three stage turbine. The turbine inlet temperature is 990°C and shaft output under baseload ISO conditions is 1550 kW. The centrifugal compressor has a pressure ratio of 9.4:1 and an air flow rate of 8.1 kg/s. Design speed is 22 000 r/min. The current available NOx controls for this machine are water or steam injection and a lean premix system. Both result in NOx output levels of around 25 ppm.
Prior to fitting the Xonon combustor, the baseline engine performance of the Kawasaki engine was measured on the test bed with the standard Kawasaki combustor. Instrumentation on the machine included a precision fuel flow measurement, continuous emissions monitoring, and numerous measurements of temperature and pressure in the combustor using thermocouples and pressure tabs. Average exhaust temperature was also measured and there was some calibration around the air inlet area. An infrared camera continually monitors the performance of the catalyst and the combustor, and the unit was also tested for dynamic pulsations.
A major part of Woodward’s involvement in this project was the development of the engine control system including fuel management and load step systems. For the initial start-up of the machine, Woodward developed a control scheme to enable correct fuelling of the catalyst relative to the preburner. Overfiring of the preburner during the initial start-up would damage the Catalytica combustor due to the temperature limitations of the hardware.
The tests on the M1A-13A gas turbine have shown extremely positive results. To date, the unit has accumulated over 1000 operating hours and over 200 start up cycles to full load in ambient conditions ranging from -12°C to 37°C (10°F to 98°F). The turbine has also successfully undergone load step changes of as much as 40 per cent and also numerous part load and full load trips, demonstrating the load step and trip capability of Xonon.
During these trials, the environmental performance of Xonon has been constant. Emissions of NOx have been maintained at less than 3 ppm, and emissions of less than 5 ppm for both CO and UHC have been achieved. Importantly, the efficiency of the Kawasaki machine was kept to within 0.5 per cent of the design efficiency of the standard Kawasaki combustor. Low levels of combustor dynamics, i.e. noise and vibration, were demonstrated, with negligible pressure pulsations matching those of current diffusion flame combustors.
Costs compared
According to Catalytica, the Xonon system is currently cost competitive compared to other conventional forms of NOx control such as SCR and DLN technology. For a retrofit, capital and operating costs of Xonon are comparable to DLN technology, depending on the engine. In addition, in an urban region with NOx requirements of around 5 ppm, Xonon can achieve the same emission levels as SCR and DLN put together. The technology is therefore currently cost competitive in the right regulatory environment, and as regulations become more stringent, Xonon will be able to expand its presence in the market.
Effective NOx control can also generate other benefits for utilities and power plant owners, particularly in the US under current federal regulations. Effective NOx control will help meet emission offset requirements, where the net emissions impact for a new project must be offset by reducing emissions at other facilities.
Lowering NOx emissions can also generate Emission Reduction Credits (ERC) and can speed up permitting, accelerating the process of bringing facilities on-line, an important factor in the deregulated market.
Good access to the gas turbine market is now what Catalytica is seeking, both at home and abroad, and its recent deals with GE and Enron have given the company probably the best start that it could have hoped for. According to Catalytica, the way forward will be to build on this position with similar deals with other OEMs in order to gain access to both new and existing capacity. With recent regulatory trends, and results from the Kawasaki tests in hand, this should not prove too tough a task.
TablesTable 1. Basic performance of the Kawasaki M1A-13A, ISO conditions, natural gas fuel
