Fuel-rich Catalytic Combustion

Air from the compressor is split into two streams primary air is premixed with the fuel and then fed to the catalyst, which is operated under O2 defect conditions secondary air is used first as a catalyst cooling stream and then mixed with the partially converted stream from the catalyst in a downstream homogeneous section where ignition of gas-phase combustion occurs and complete fuel burnout is readily achieved. The control of the catalyst temperature below 1000 Cis achieved by means of O2 starvation to the catalyst surface, which leads to the release of reaction heat controlled by the mass transfer rate of O2 in the fuel-rich stream and of backside cooling of the catalyst with secondary air. To handle both processes, a catalyst/heat exchanger module has been developed, which consists of a bundle of small tubes externally coated with an active catalyst layer, with cooling air and fuel-rich stream flowing in the tube and in the shell side, respectively [24]. 

Premixer Catalytic Post-catalyst reactor mixing 

Two configurations of the RCL burn technology have been designed a catalytic pilot burner, which replaces the existing diffusion flame or partially premixed pilot of the DLN combustor [26], and a full catalytic burner [25]. 

The catalytic pilot burner processes only a fraction of the fuel and is targeted to retrofitting applications with minor combustor modifications. Test results indicate that to achieve effective stabilization of homogeneous combustion, 18-20% of the fuel-air must be processed in the catalytic pilot, which is a much higher fraction than the typical 2-5% processed in a conventional pilot burner. Under such conditions, test results demonstrated single-digit ( 5 ppm at 15% O2) emissions of NO and CO with low acoustics at 50 and 100% load conditions. 

In the full catalytic burner, all the fuel is processed within an RCL bum module which replaces a conventional premixer-swirler arrangement in the DLN combustor. 

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Eriksson S, WolfM, Schneider A, et al. Fuel rich catalytic combustion of methane in zero emissions power generation processes, Catal Today 117 447—453, 2006. 

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. 

Numerous studies have been published on catalyst material directly related to rich catalytic combustion for GTapplications [73]. However, most data are available on the catalytic partial oxidation of methane and light paraffins, which has been widely investigated as a novel route to H2 production for chemical and, mainly, energy-related applications (e.g. fuel cells). Two main types of catalysts have been studied and are reviewed below supported nickel, cobalt and iron catalysts and supported noble metal catalysts. 

Rich catalytic combustion will offer wide opportunities with respect to most of the above issues, including flexible integration in different machines, low-temperature ignition ability, tolerance to fuel concentration and temperature non-uniformities and fuel flexibility. Further, the production of syngas in short contact time catalytic reactors could be exploited in several energy-related applications such as fuel cell and oxy-fuel combustion. 

Figure 3.1 Hetero-Zhomogeneous combustion methodologies in power generation (A) catalytically stabilized thermal combustion (CST) and (B) fuel-rich catalytic/gaseous-lean combustion.

As mentioned in Section 1, the fuel-lean (CST) and fuel-rich catalytic/fuel-lean gaseous combustion concepts in Fig. 3.1 can also be used for hydrogen or hydrogen-rich fuels. The fuel-lean concept in Fig. 3.1 A is... 

Simulations for the fuel-rich combustion concept (Fig. 3.16A) are depicted in Fig. 3.19 for a total Uin = 10 m/s, three different total fuel-lean equivalence ratios tot and five fuel-rich catalytic equivalence ratios (Pcat-Additional results for a total UiN = 20m/s are shown for cat = 3.0. 

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... 

Results obtained in full pressure tests demonstrated the following advantages of rich fuel over lean fuel catalytic combustion ... 

Lyubovsky, M., Smith, L.L., Castaldi, M., Karim, H., Nentwick, B., Etemad, S., LaPierre, R., and Pfefferle, W.C. Catalytic combustion over platinum group catalysts Fuel-lean versus fuel-rich operation. Catalysis Today, 2003, 83, 71. 

This problem can be circumvented in a fuel-rich approach to catalytic combustion for gas turbines recently proposed. In this method 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 in a homogenous flame. Because the gases exiting the catalyst are fuel-rich, they cannot sustain combustion in the event of a homogenous flame backup. The promise of this method needs to be confirmed in full-scale turbine tests. 

Throughout this paper, the catalytic ignition behaviour will be discussed in terms of ignition temperature vs a corrected equivalence ratio. While the equivalence ratio is usually defined in the combustion literature as the ratio of the current fuel/air ratio divided by the fuel/air ratio at the stoichiometric composition for total oxidation to H2O and CO2. we prefer to depict the data vs a modified equivalence ratio wUich we define as >/(l 4>). This modification has the advantage that it puts equal weight on the fuel lean and fuel rich sides of the ignition curve, i.e. while the usual 4> maps fuel lean mixtures on a scale from 0 to 1 and fuel rich mixtures on a scale from 1 to infinity, the modified ratio maps... 

Catalytic combustion of hydrogen and hydrogen-rich fuels has attracted increased attention in numerous engineering applications, which include large thermal power plants, microreactors for portable power... 

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