Catalytic combustor

Another potential solution is the use of catalytic combustors, which produce extremely low levels of emissions by the use of combustion catalysts such as platinum. The main disadvantage of catalytic combustors, however, is thek high cost. 

Three rapid oxidation methods are typically used to destroy combustible contaminants (1) flares (direct-fiame-combnstion), (2) thermal combustors, and (3) catalytic combustors. The thermal and flare methods are characterized by the presence of a flame during combustion. The combustion process is also commonly referred to as afterburning or incineration. ... 

New research in combustors such as catalytic combustion have great promise, and values of as low as 2 ppm can be attainable in the future. Catalytic combustors are already being used in some engines under the U.S. Department of Energy s (DOE), Advanced Gas Turbine Program, and have obtained very encouraging results. 

Figure 10-28. Schematic of a full scale catalytic combustor. Courtesy GE Power Systems and Catalytica Combustion Systems Inc.

The catalytic combustor has great potential in the application of gas turbines in new combined cycle power plants as the NO emissions in high attainment areas will have to be below 2 ppm. 

Schlatter, J.C., Dalla Betta, R.A., Nicholas, S.G., Cutrone, M.B., Beebe, K.W., and Tsuchiya, T., Single-Digit Emissions in a Full Scale Catalytic Combustor, ASME 97-GT-57. 

Catalytic combustor A device used to remove various solid, liquid, or gaseous pollutants from air or another gas, in which the gas is heated by an open burner to between 250 and 500 °C and passed through a catalyst bed in which the organic contaminants are oxidized into harmless by-products. 

A recent innovation includes use of a catalytic combustor. Several tests with large turbines indicate that this alternative can reduce NO, emission to less than 5 ppmv. The catalyst inside the combustion chamber actually causes a portion of the fuel to bum without the presence of a flame. This significantly reduces combustion temperature. 

Based on the results of the catalytic combustor, there is an etfori under way to develop this concept into a practical, field-proMTi technology. 

The authors developed a multi-layered microreactor system with a methanol reforma- to supply hydrogen for a small proton exchange membrane fiiel cell (PEMFC) to be used as a power source for portable electronic devices [6]. The microreactor consists of four units (a methanol reformer with catalytic combustor, a carbon monoxide remover, and two vaporizers), and was designed using thermal simulations to establish the rppropriate temperature distribution for each reaction, as shown in Fig. 3. 

The catalytic combustor provides heat for the endothermic reforming reaction and the vaporization of liquid fuel. The endothermic reforming reaction is carried out in a parallel flow-type micro-channel of the reformer unit. It is well known that the methanol steam reforming reaction for hydrogen production over the Cu/ZnO/AbOs catalyst involves the following reactions [10]. Eq. (1) is the algebraic summation of Eqs. (2) and (3). 

Catalyst monoliths may laos be employed as catalytic combustion chambers preceding aircraft and stationary gas turbines. As shown diagramatically in Fig. 16, a catalytic combustor comprises a preheat region, a catalyst monolith unit and a thermal region. In the preheat region, a small fuel-rich flame burner is employed to preheat the fuel-air mixture before the hot gases reach the monolith unit. Additional fuel is then injected into the hot gas stream prior to entry to the monolith where... 

Catalytic combustion has been commercially demonstrated to reduce NO.. emissions to below 3 ppm while keeping CO and UHC emissions below 10 ppm without the need for expensive exhaust clean-up systems. In addition, a catalytic combustor reduces typical DLN problems such as risk of blow-out and flame instability. Also, the economic advantage of primary methods including catalytic combustion as opposed to secondary clean-up measures (SCR and SCONOx) has recently been assessed [1]. 

Operating constraints of GT systems (Table 12.1) pose severe requirements on the catalytic combustor. Air is delivered by the compressor at temperatures which typically range from 300 to 450 °C depending on the load conditions and nominal pressure ratio of the machine. 

Different design concepts have been proposed to match the severe requirements of catalytic combustors. A main classification criterion is based on fuel/air stoichiometry in the catalyst section, which has a dominant effect on the selection of catalytic materials and on the operating characteristics of the combustor. In this section, only configurations based on lean catalytic combustion will be described. The peculiar characteristics of rich catalytic combustion will be described in a separate section. 

Figure 12.4 Emission performances of a GElO-1 (11 MWe) GT equipped with a catalytic combustor. Taken from [22].

On the other hand, following the development of hybrid combustor configurations that prevent operation of the catalyst module at temperatures above 900-1000 °C, the major drawback of metallic monoliths, namely the limited maximum operating temperature, has been overcome. Accordingly, honeycombs made of metal foils have been adopted in GT catalytic combustors in view of their excellent thermal shock resistance and thermal conductivity properties [9]. In addition, metallic substrates are a promising option for the fabrication of microcombustors. 

As a result, a rich catalytic combustor exhibits better light-off performance than a lean catalytic combustor. Veser et al. [136] also found that the ignition temperature correlates well with the C-H bond energy of the hydrocarbon, in line with the crucial role of activation of the first C-H bond proposed in the literature [140, 141[. 

Catalytic combustion for gas turbines has received much attention in recent years in view of its unique capability of simultaneous control of NOX) CO, and unbumed hydrocarbon emissions.1 One of the major challenges to be faced in the development of industrial devices is associated with the severe requirements on catalytic materials posed by extreme operating conditions of gas turbine combustors. The catalytic combustor has to ignite the mixture of fuel (typically natural gas) and air at low temperature, preferably at the compressor outlet temperature (about 350 °C), guarantee complete combustion in few milliseconds, and withstand strong thermal stresses arising from long-term operation at temperatures above 1000°C and rapid temperature transients. 

Finally, the use of hexaaluminates in gas turbine catalytic combustors is addressed. 

Different design concepts of catalytic combustors were proposed in order to meet the stringent requirements on emission control and on durability under the severe... 

Figure 11 Alternative configuration of catalytic combustors for gas turbines.

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