Status and perspectives of catalytic combustion for gas turbines
Introduction
The NOx emission regulations in United States (US) are based on the National Ambient Air Quality Standards (NAAQS): the ozone standard is not attained in several areas (85% of the population in US), whereas the NOx standard is usually attained but NOx is deemed as ozone precursor. Attainment areas require the use of the Best Available Current Technology (BACT), which is now 9 ppm NOx for natural gas-fired turbines. The non-attainment areas apply the Lowest Achievable Emission Rate (LAER) technology, irrespective of the economic implications. Today LAER is considered to be 2 ppm NOx, with most permits considered at 3.5–5 ppm.
The technology currently practiced to control the NOx emissions from industrial gas turbines involves either water/steam injection or lean premixed combustion. To meet the stringent emission regulations many installations include a selective catalytic reduction (SCR) unit to remove NOx in the exhaust gases. Dry low NOx (DLN) systems deliver NOx emission levels of 15–20 ppm, the latest advanced version of these systems being designed for 9 ppm. Further significant reduction in NOx emissions via the DLN approach may be precluded by flame stability problems.
Catalytic combustion has the potential to reduce the emissions of NOx to near zero and to achieve ultra-low emissions of CO and unburned hydrocarbons (UHC) in natural gas-fired turbines. Besides catalytic combustion reduces the risk of blow-out or instability and dynamics during combustion, does not require the use of expensive cleanup systems and implies negligible efficiency loss compared to gas turbines with conventional flame combustion systems [1], [2], [3], [4].
The costs of various NOx control technologies have been compared in a recent report by Onsite Energy Corporation commissioned by DOE [5]. Gas turbines with small (5 MW), medium (25 MW) and large (150 MW) size have been considered and the following technologies have been analysed: water/steam injection; DLN; conventional, low-T and high-T SCR; catalytic combustion; SCONOX. Although the cost values for the various technologies are constantly being reduced, the obvious advantage of primary measure, catalytic combustion, as opposed to secondary cleanup measures (SCONOX and SCR) in both reduction levels and cost are evident. The disparity in the cost impact between primary and secondary measures is particularly large for small gas turbines, that are deemed for the distributed generation market which in turn is threatened by strict environmental regulations.
The potential of catalytic combustion has been recognised since more than 30 years, but only recently this technology has been proven to be viable in commercial gas turbine service.
This paper provides a review of the status and of the perspectives of this technology. First the development activities of catalytic combustion systems carried out in the last few years are reported. Then the relevant characteristics of PdO supported catalysts and of transition metal-substituted hexaaluminates (HAs), that have been most extensively considered for this application, are addressed. Next the use of mathematical modelling as a tool for the design and analysis of catalytic combustors is discussed. Finally a novel fuel-rich approach to catalytic combustion is illustrated and the perspectives for this technology are briefly outlined.
Section snippets
Development of catalytic combustor systems
In a traditional flame combustion system compressed air and fuel are mixed, then combusted in a flame and the hot gases expand and drive the turbine. Cooling with bypassed compressed air of the hot gases is required to reduce the temperature for delivery to the turbine inlet in the range from 1200 to 1500 °C. In order to have a stable flame the fuel must be concentrated in the flame zone which results in a localised high temperature (around 1800 °C) with associated NOx formation. In a catalytic
Combustion catalysts
The combustion catalysts that have been most extensively investigated for gas turbine application are Pd-supported materials and metal-substituted HAs.
Modelling of catalytic combustors
Mathematical models represent a powerful tool for the design and analysis of catalytic combustors for gas turbines. Single channel models have been customarily used to describe catalytic combustors, which appears reasonable considering that global adiabatic conditions and uniform distribution of the variables at the catalyst inlet section are approached in reality.
In principle many physical and chemical phenomena that occur within the reactor need to be considered, including: (i) heterogeneous
Novel fuel-rich approach to catalytic combustion
The conventional approach to catalytic combustion involves the complete oxidation of lean methane/air mixture to CO2 and H2O with parallel release of heat. A fuel-rich approach to catalytic combustion for gas turbines has been recently proposed [64]. In this approach fuel is mixed with air to form a fuel-rich mixture that is reacted over the catalyst to produce both partial and total oxidation products. The reaction products are then mixed with excess air and burned to completion downstream in
Conclusions
It has now been demonstrated that catalytic combustion is a viable technology for natural gas-fired gas turbine commercial service: a service life of more than 10,000 h has been demonstrated for 1.5 MW Kawasaki M1A-13A gas turbine [63]. For both economic and technical reasons catalytic combustion appears today particularly attractive for small size gas turbine with single-can engine and not too high firing temperature. Catalytic combustion has demonstrated several advantages over DLN, including
Acknowledgements
This work was supported by MIUR, Rome (Italy).
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