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Experimental parametric investigation of platinum catalysts using hydrogen fuel
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 3 ( 2 0 1 8 ) 2 1 3 0 7 e2 1 3 2 1
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Experimental parametric investigation of platinum catalysts using hydrogen fuel Annapurna Basavaraju a,*, Ashwini Bhadravati Ramesh a, Dalibor Jajcevic b, Franz Heitmeir a a b
Technical University of Graz, Inffeldgasse 25A, Graz, 8042, Austria Ses-Tec OG, Autal 55, Laßnitzh€ohe, 8301, Austria
article info
abstract
Article history:
The aviation organization is creating awareness for the overall reduction of NOx emissions
Received 28 July 2018
by up to 80% in the near future. This motivates to conduct research on the current state of
Received in revised form
art, catalytic stabilized combustion chamber using hydrogen. This was achieved by per-
22 September 2018
forming an experimental parametric investigation of Platinum catalysts in two phases.
Accepted 25 September 2018
Firstly, the design of three diverse configurations of mixers and was investigated experi-
Available online 23 October 2018
mentally and numerically. The chosen mixer was implemented in the parametric study of five different Pt catalysts varying in geometric and material properties. This was executed
Keywords:
at unpressurized and NOx emission solely due to the catalytic reaction was examined for
Mixer
varying thermal power and air/fuel ratios. Furthermore, temperatures were recorded.
Catalytic combustion
Additionally, CFD simulation was accomplished and compared with the measurement
Hydrogen
data. The overall least NOx achieved was 7.5 ppm at 5 kW for the metal catalyst. The result
Gas turbine engine
of this work proposed suitable catalyst for the development of a combined combustor
NOx emissions
configuration (including catalyst and combustion chamber) which will be intended for small aircraft engine applications. © 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction CO2 emissions have increased by about 80% and NOx emissions have doubled between 1990 and 2014. There are forecast to grow both emissions further by about 45% between 2014 and 2035 [17]. Therefore, organizations such as the International Civil Aviation Organization (ICAO) and the Advisory Council for Aeronautics Research in Europe (ACARE) are creating awareness on these issues. This is due to the fact that these emissions have significant impacts on health, environment as well as the performance of engines. Regarding the very first step towards solving these issues is to use replacement fuel. In
this work, hydrogen gas has been proposed because the sole pollutant during its oxidation is NOx. Another characteristic feature of hydrogen is the high heating value. Nevertheless, one of the biggest challenges is the storage of hydrogen, which can be solved by improving the materials of the tanks. Additionally, safety is a topic that cannot be ignored while dealing with hydrogen. NOx emissions are known to be dominant at very high temperatures and this is normally witnessed in conventional combustion mode [18]. Accordingly, it is important to reduce the temperature. One such method is to incorporate catalysts in the combustion chamber. This enables the reaction to take
* Corresponding author. E-mail address: [email protected] (A. Basavaraju). https://doi.org/10.1016/j.ijhydene.2018.09.170 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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Nomenclature f equivalence ratio 1D one dimensional 3D three dimensional aluminum oxide Al2O3 CFD computational fluid dynamics carbon di-oxide CO2 CPSI cells per square inch CPSM cells per square meter H radical hydrogen radical hydrogen H2 water H2O Le Lewis number NO nitric oxide nitrogen di-oxide NO2 NOx nitrogen oxides oxygen O2 Pd palladium ppm parts per million Pt platinum RCL rich catalytic lean burn RSM Reynolds stress model
place at lower temperatures (<1100 K) due to their lower activation energy and thus lowering NOx. Consequently, in the current work, the concept of heterogeneous catalytic combustion is implemented. This is defined as a chemical process in which a catalyst is used to oxidize the fuel at a lower temperature and thus leading to a Flameless combustion. The concept of catalytic combustion for gas turbine engines has been introduced by Dr. William C. Pfefferle in the early 1970s. Since then, the application of catalysts to reduce the emission of gas turbine engines is the state of the art in the research field. There are many successful findings in achieving lower than 5 ppm NOx for fuels such as methane and syngas over Pt/Pd catalysts using Rich Catalytic Lean-burn (RCL) [1e4,6]. In addition to lowering the NOx (i.e. ~2.5 ppmv corrected to 15% O2) and CO emissions (<2 ppm), Benjamin Baird et al. have also achieved lower acoustics in the catalytic combustor for natural gas [15]. Further, the benefit of catalytic reactions is not just restricted to limited fuels, but also for multiple alternative fuels: natural gas, biomass landfill gas, gasoline, refinery fuel gas etc [7]. The outcomes show that catalyst temperatures are stable and emissions are maintained at low values for all tested fuels [9]. Likewise, Alavandi et al. [8] have developed a catalytic combustor for methanehydrogen mixtures. In the case of hydrogen combustion, the NOx has been significantly reduced from 165 ppm to 23 ppm using Pt/Pd catalyst. These experiments have been conducted under the high pressure of up to 13 bar and a maximum inlet temperature of around 450 C [5]. Moreover, the NOx reduction potential is not only confined to RCL mode, but also the leanpremixed combustion has demonstrated the ability to achieve as low as 5e9 ppm corrected to 15% O2 during operation on natural gas [10e13]. The application of catalytic combustion is not only limited to gas turbines but also marked its
significance in the domestic kitchen applications. Recently the research has shown an ultra-low NOx of 0.09 ppmv [33]. Employing catalysts also improves the efficiency of the engine, the lifetime of gas turbine components, extended service interval time, fuel flexibility and combustion noise [3,8]. There are several analyses performed through computational simulations to get an insight of mechanisms involved in catalytic reactions. A numerical 1D Plug-flow model is developed by Yin et al. [19] and as a result, axial gradients of catalytic oxidation are attained. A similar transient 1D reactor model with an already validated multi-step surface reaction mechanism is developed by Zhu and Jackson [20]. Detailed two-dimensional simulations of catalysts are accomplished by Karagiannidis et al. [21]. An analytical model for the calculation of the catalysts is established by Joshi et al. [22]. Here, the detailed mechanism of catalytic reaction and the influence of geometric parameters of catalysts on the conversion of fuel has been very well explained. Besides the kinetics of catalytic reaction S. Pramanik et al. have observed that the homogeneous reaction rates are significant at higher pressures [34]. Even three-dimensional simulation has been performed by B.O. Arani et al. to investigate the turbulent combustion of fuel-lean hydrogen/air mixtures in a platinumcoated channel [35]. Schultze and Mantzaras numerically investigated both fuel rich and fuel lean catalytically stabilized combustion of hydrogen/air mixtures in Pt catalyst [14]. In the fuel-lean mode, the Lewis number of hydrogen is less than unity which led to super-adiabatic surface temperatures. On the contrary, in fuel-rich mode, the Lewis number of the deficient oxygen reactant is greater than unity which resulted in surface temperatures well below the adiabatic equilibrium temperature. Therefore, in this paper, the Lewis number is considered as a vital factor in the safe operation of catalysts. There are very few researches executed on the catalytically stabilized hydrogen fuelled gas turbine engine, especially for aircraft application. For the sake of achieving this objective, the entire study has been carried out in the following steps: a) Design and testing of a pre-mixer: The entrainment of fuel/ air mixtures into the combustion chamber is an important criterion. The pre-mixed combustion has an advantage of lowering the NOx over the diffusion, therefore in this work, a study on choosing the right pre-mixer has been performed. Generally, a pre-mixer has a risk of flashback; hence a novel configuration of micro-mixer is designed. In this research, the multipoint injection method is chosen due to the advantage of short mixing length and time scales which in turn decreases the possibility of flashback and autoignition. Additionally, this decreases the size and weight of the combustor particularly relevant on aircraft engines. Subsequently, the benefit of passive methods is being simple and works effectively during the normal conditions of the system, therefore the cross flow injection type is adopted in this paper and tested both experimentally and numerically. All the numerical simulations presented in this paper are conducted by the project partner Ses-Tec, Austria. b) Experimental parametric study of Pt catalysts: There are various catalysts available in the market, differing in their features such as size, shape, material, length, number of
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 3 ( 2 0 1 8 ) 2 1 3 0 7 e2 1 3 2 1
channels, loading, activation energy etc. Thereupon, selecting the right catalyst to fulfill the above-mentioned purpose is necessary. A parametric study has been performed for five Pt catalysts to investigate their influence on the NOx emissions. Platinum possesses high activation energy meaning it comparatively increases the rate of reaction; hence the combination of Pt and hydrogen would definitely yield a faster chemical reaction. Finally, the intention of such an investigation will be applicable in a small aero engine. Such a study is novel and hence the operating conditions were chosen to be as simple and safe as possible. Therefore the experiments are conducted to see the behavior of the catalyst by varying the thermal power from 3, 5 and 8 kW, as well as the equivalence ratio, were selected in both lean and rich ranges. This is executed both experimentally and numerically under atmospheric conditions.
Fig. 2 e Working principle of catalytic reactor [14]. Without catalyst: 2H2 þ O2 /2H2 O
Pt
The basic components in a gas turbine engine are a compressor, combustion chamber, and turbine. Fig. 1 shows the schematic representation of an adapted version of the gas turbine for catalytic combustion. Here, the fuel and air are combined in the pre-mixer zone. This mixture passes through the catalytic reactor and enters the combustion chamber. The working principle of a catalytic reactor is illustrated in the Fig. 2. The air/fuel mixtures are led into the catalyst part where partial oxidation of fuel takes place. Typically, bypass air is introduced around the combustor which helps in reducing the reactivity of hydrogen, by distributing part of its enthalpy via heat transfer. The maximum temperature that is allowed in the catalyst section is around 1100 K due to the material constraints. Additionally, secondary air is supplied to the combustion chamber to oxidize the unburnt hydrogen. Compared to conventional, less amount of energy is required to burn the rest of hydrogen in the lean zone. Thus, resulting in lower adiabatic flame temperature and hence lowers NOx emissions [18]. The above catalytic reaction takes place by altering the rate of chemical reaction, without itself being transformed or consumed. These chemical reactions with catalysts take place at lower activation energy than that of without catalysts. The chemical reactions shown below are with and without catalysts along with their respective adiabatic temperatures.
Fig. 1 e Schematic representation of catalytic reactor in a gas turbine engine [31].
(1)
And the adiabatic temperature is ~2273 K. With catalyst: 2H2 þ O2 /2H2 O
Theory
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(2)
And the adiabatic temperature is ~1473 K. The mechanisms involved during a catalytic reaction are represented in Fig. 3. The fig shows the cross section of catalyst that is divided into 2 different phases, namely bulk gas phase (homogenous: fuel/air mixture) and washcoat phase (heterogeneous: fuel/air and catalytic material ex: Platinum). The consecutive steps that determine the rate of reaction are: a) External diffusion: It is a process where the reactants diffuse onto the surface b) Adsorption: Here, the reactants adhere to the catalytic surface c) Surface reaction: In this, molecular rearrangement at the active surface sites takes place d) Desorption: where the disintegration of products from the surface occurs. e) External diffusion: Finally, the products are diffused away from the surface and entered back into the gas phase. In a catalytic reaction, the conversion efficiency of the fuel is greatly dependent on the reaction regime. Various characteristic regimes observed in a catalytic monolith are kinetic, washcoat diffusion and external mass transfer regime. The regime encountered during a reaction depends on distinct factors such as temperature, washcoat, catalyst loading and channel dimensions [22]. At low temperature, the reactants encounter kinetic regime where reaction resistance is present. As the temperature increases the reactants overcome this regime and enter the washcoat diffusion-controlled regime. Here, the resistance due to washcoat is dominant and hence time needed for internal diffusion becomes much higher than the reaction and external diffusion time. Finally, at higher temperatures, the reactants enter the external mass transfer regime where the characteristic reaction time approaches zero and highest conversion efficiency is attained. Therefore ideally, catalysts are expected to operate in this regime. Lewis number The catalysts are subjected to damage if the temperature exceeds about 1100 K. High temperatures are reached when
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Fig. 3 e Catalytic reaction mechanisms [32]. the mass diffusivity exceeds the thermal diffusivity, thus the lack of thermal distribution augments the local temperatures resulting in hot spots. This phenomenon can be explained by the term called “Lewis number”. This is defined as: Le ¼ a=D
(3)
where. a: Thermal diffusivity (m2/s) it is a measure of the capacity of the material to conduct thermal energy relative to its ability to store thermal energy. D: Mass diffusivity (m2/s) is a measure of diffusion of species within a substance. Generally, for rich hydrogen mixtures the Le 1, whereas Le < 1 for very lean hydrogen mixtures. It doesn't mean one cannot operate in lean conditions at all; however, care has to be taken to avoid the risk of reaching the limiting temperature of the catalyst (1173 K). At lower thermal power, it is possible to operate lean conditions, without damaging the catalyst.
XH2
c) Efficiency The thermal efficiency of the Brayton cycle is a ratio of total work done to the heat input, expressed as: h¼
a) Equivalence ratio is defined as the ratio of stoichiometric air/fuel ratio to the actual air/fuel ratio [16]: h¼ (4)
where, m_ f ¼ Mass flow rate of fuel [kg/s] m_ a ¼ Mass flow rate of air [kg/s] fstoichiometric ¼ is the stoichiometric air/fuel ratio For hydrogen fuel it is 34. b) Percentage of fuel conversion Consider a stream of hydrogen in air. This stream flowing through catalyst undergoes a chemical reaction represented in the Eq. (5). Considering the law of conservation of mass and assuming that the system is in an adiabatic steady state, one can say that the enthalpy change due to the reaction is absorbed by the reacting stream. Expressing enthalpy balance in terms of both fractional conversion and the heat capacity of the inlet stream as an average value, the rate of reaction of hydrogen is given by Ref. [8]:
(5)
where. Tin ¼ Reactant temperature [K] Tout ¼ Product temperature [K] MT ¼ Total mass fraction of oxygen and hydrogen MH2 ¼ Mass fraction of hydrogen Cp average ¼ Average heat capacity [J/mol K] DHR; Tout ¼ Standard Heat of reaction at Tout [J/mol] Here, Tout is substituted by wall temperature obtained from the experiment. This Eq. (4) is further applied to the comparison of the percentage of H2 conversion obtained from CFD results.
Calculations
m_ f fstoichiometric f¼ m_ a
Cp average MT ¼ ðTin Tout Þ MH2 DHR; Tout
Net work qout qin ¼ Heat input qin m_ T Cp air ðTout Tin Þ m_ f HV
(6)
(7)
where, qout ¼ Heat output [J] qin ¼ Heat input [J] m_ T ¼ Total mass flow rate [kg/s] Cp air ¼ Heat capacity of air [J/kg K] HV ¼ Heating value of hydrogen [J/kg] These results play a key role in the selection of catalyst and are presented in the upcoming sections.
Methods Design and testing of a pre-mixer Numerical simulation Simulation results were obtained using commercial CFD code ANSYS Fluent. A quasi-steady state condition was assumed
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and the transient behavior of the flow was neglected. The conditions applied in the CFD model were:
Steady-state solver Fluent RSM turbulence model Standard wall function Compressible ideal gas Species transport model (three species: O2, N2, and H2) Second order scheme (upwind) for momentum equation
A preliminary investigation of several configurations of fuel/air mixer was performed using CFD computation. This included a comparison of swirler and micro-mixer. The swirlers examined were with swirl angles of 25 , 50 , contrarotating 2 25 , and contra-rotating 2 50 . One example of the swirler design is depicted in the Fig. 4 (a). And the micromixer computed were of three configurations, namely mixer I, II and III. Each mixer consisted of 19 inlets for air with 4 subinlets for fuel. The fuel was distributed into these 19 inlets via 2 main inlets of 8 mm and 6 mm. These three mixers differed in their specifications which are shown in Table 1. The flow path of air and fuel were perpendicular to each other. The Fig. 4 (b) below shows the cross section of Mixer III. The outlet pressure was set to the atmosphere and the temperature at both inlet and outlet was set to 288.15 K. The solver conditions were: standard wall treatment, Fluent RSM turbulence model, compressible ideal gas and second order solver in the momentum equation. Homogenous mixing of fuel/air was necessary for efficient catalytic activity. Therefore, the velocity profile from CFD is an important factor to decide on the flow behavior after the mixer. Fig. 5 shows the correlation of axial velocity to the radial position at a distance of 70 mm from the mixer. The velocity profile of 25 , 50 , and 2 50 swirlers had suction behavior in the center of the stream Fig. 5(a),(b) and (c). Moreover, the swirler 2 25 (Fig. 5 (d)) had a non-uniform mixing along the diameter. On the other hand, uniform mixing and better velocity profile were achieved in the all the cases of micro-mixer Fig. 5 (e), (f) and (g). Additionally, a
Table 1 e Pre-mixer types. Micromixer
Fuel inlet Air inlet Air outlet diameter (mm) diameter (mm) diameter (mm)
I II III
1.0 0.5 0.5
4.5 4.5 4.5
3.0 3.0 4.5
uniform flow profile was already observed at a distance of 50 mm for micro-mixers. So, the minimum distance of catalyst from the mixer must be 50 mm. The pressure loss for all the mixers was calculated by using the formula: Pressure loss ¼
Pin Pout Pin
(8)
where, Pin ¼ Pressure before the mixer (bar) Pout ¼ Pressure after the mixer (bar) The pressure drop in micro-mixer was lower than swirler as indicated in Table 2. Therefore, micro-mixer of all 3 configurations was selected for the experimental investigation.
Experimental setup The velocity profile from CFD for all the three micro-mixer was identical. Consequently, velocimetry measurement was of no interest in the experimental study. Further to compare the fuel/air mixing behavior among these mixers, a test campaign for varying operating conditions was performed. In order to examine this visually an optical method of laser light sheet visualization technique was employed. Pressurized metered air at 3 bar was introduced from the inlet of the rig (Fig. 6 (1)). The air from the inlet is passed through a long pipe to ensure laminar flow. The oil from the seeding generator was fed through the fuel inlets at 1.225 bar (Fig. 6 (3)). The inlet air mixes up with seeding particles across the micromixer (Fig. 6 (4)). An optical window Fig. 6 (5) was provided to pass the laser sheet into the flow. Thus, the nature of mixing was captured by a camera (CANON-700D) through
Fig. 4 e a) Swirler; b) Cross section of mixer III.
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Fig. 5 e Axial velocity profile vs radial position at a distance of 70 mm from the mixer computed for all mixers from CFD. Swirler with swirl angle of: a) 25 ; b) 50 ; c) contra-rotating 2 £ 50 and d) contra-rotating 2 £ 25 . Micro-mixer: e) mixer I; f) mixer II and g) mixer III.
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Table 2 e Pressure loss computed from CFD for all mixers. Mixer Swirler 25 Swirler 50 Swirler 2 25 Swirler 2 50 Micro mixer I Micro mixer II Micro mixer III
Pressure loss (%) 7.32 6.03 5.52 5.92 4.55 0.90 1.10
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2. The air was let through fuel inlet and the seeding at the main inlet. This was performed to verify the results from step 1. 3. Helium along with seeding was let through the fuel inlets and air at the main inlet. Helium was chosen because it is inert and has a molecular size closer to hydrogen. These experiments were conducted for three operating conditions: constant mass flow through the fuel inlet, constant pressure through the fuel inlet and lastly constant velocity through the air inlet.
Fig. 6 e Schematic representation of experimental setup for the mixer test. 1) Inlet; 2) Orifice chamber; 3) Fuel inlet; 4) Mixer; 5) Optical window 2; 6) Optical window 1; 7) Outlet valve.
another optical window Fig. 6 (6) situated near the mixer. The pressure inside the rig was regulated by a valve at the outlet (Fig. 6 (7)). In order to ensure the uniform flow from the mixer, the experiment was carried out in the following three stages: 1. The air was let through the main inlet and seeding at the fuel inlet.
Instrumentation. The pressure of the inlet air was measured by a pressure transducer. The mass flow of the air was monitored across the orifice plate. An optical measurement technique called light sheet visualization had been used to analyze the flow pattern downstream of the mixer. Seeding was essential to obtain high-quality measurements; therefore, oil particles were added. A metered mass flow of these was introduced into the mixer through the seeding oil generator.
Fig. 7 e Light sheet visualization images from the mixer test. a) Test results obtained from all mixers. b) Test results for all three stages of experiment from mixer III.
40 40 32 32 5.3 10.6 5.3 5.3 5.3 a-Al2O3 a-Al2O3 a-Al2O3 e a-Al2O3 A B C D E
33 33 33 33 34
30 30 30 30 25
200 200 300 100 200
300 300 465 155 300
Metal Metal Metal Metal Ceramic
Platinum Platinum Platinum Platinum Platinum
Loading [kg/m3] Substrate CPSM CPSI Length [mm] Diameter [mm]
Table 3 e Specifications of selected catalysts.
Catalyst selection criteria: As discussed before, for an efficient conversion catalyst is anticipated to be in external mass transfer regime which is dependent on two factors: flow parameters such as velocity, pressure, equivalence ratio, residence time, etc. and geometric parameters such as length, diameter, cell density, loading, material, etc. It is studied that the length and diameter above 20 mm and 25 mm respectively help in reaching mass transfer regime during a reaction [22]. Additionally, cell density is referred to as the number of cells per unit area. The higher the cell density better is the conversion, but increases the pressure drop. Further, loading explains the amount of catalyst present per unit area, more is the value better is the durability and performance. However, excessive loading does not necessarily mean it is good for catalyst performance as it results in blocking the channels. Hence, deciding on the optimal loading of the respective application is essential. Catalysts are usually made up of either ceramic or metal. Ceramic is known for its low thermal expansion, high resistance to fracture due to thermal shock likewise metal has a lower pressure drop, more surface area, better thermal conductivity etc. Thus both metal and ceramic were considered in this work. Based on this guideline, five catalysts with varying parameters were chosen for the parametric study and are tabulated in Table 3. These catalysts were bought from a catalyst manufacturing company. In the interest to carry out the parametric investigation, experiments on different catalysts were conducted. The main aim of this study was to get an overview of the behavior of catalysts, their functionality with hydrogen fuel as well as to observe their impact on NOx emissions for gas turbine application.
Precious metal
Parametric investigation of catalysts
Washcoat
Mantle dimensions [mm]
An image captured at each operating point was subtracted with the respective reference image (flow from the main inlet only) to view just the seeding particles. All these images were processed uniformly using “Matlab” software and the intensity was maintained constant. The results presented here are from the 2nd category (i.e. seeding at the main inlet and air at the fuel inlet) at a constant mass flow through the fuel inlet. However, in all the three categories as well as in all the operating conditions similar results were found. The nature of the flow in mixers I and II was not well distributed. The red circled area for the mixer I in Fig. 7 (a) represents high turbulence and dense clouds of seeding particles that were spluttered and accumulated. Similarly, the red circled area for mixer II in Fig. 7 (a) depicts the area with fewer amounts of seeding particles which is again not desirable as an inlet flow to the catalyst. Contrarily, the green area for mixer III in Fig. 7 (a) showed a uniform mixing behavior. Additionally, the flow pattern in mixer III was similar to that of the ideal turbulence profile in a pipe. This nature was also evident in all the 3 categories as shown in Fig. 7 (b). Mixer III was chosen for the parametric study due to its uniform mixing behavior and turbulent like flow profile in a pipe for all operating conditions.
35 35 35 35 e
Foil thickness [mm]
Results
0.05 0.05 0.08 e e
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Catalyst
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The catalysts of different geometric and material characteristics were chosen for the parametric study as shown in Table 3. The channels of metal and ceramic catalysts were made up of sinusoidal and square shape respectively.
Numerical simulation Prior to the experiments, the catalytic combustion of the hydrogen/air mixture in a coated catalyst was investigated by means of 3D numerical simulations in Ansys Fluent software. This 3D model was coupled with the CHEMKIN-CFD Solver to enhance the accuracy of chemical reaction at the catalyst walls. The chemical reaction mechanism of H2/O2 over platinum was adopted from Bhatia et al. [29]. In the CFD model, the following setup was applied: 1. Steady-state solver 2. Turbulence model 3. Species transport model a. Reactions: Volumetric and Wall surface (Heat of surface reaction, diffusion energy source) b. Mixture properties imported from CHEMKIN c. Turbulence-Chemistry interaction: laminar finite-rate The velocity of the flow in the catalyst influenced the chemical reactions. Due to this, the channels at the outlet of the mixer was also included upstream of the catalyst. In the direction of reducing computational domain and time, only a portion of 60 (1/6) of the whole model for catalyst A was simulated.
Experimental setup The experimental parametric study for catalysts in Table 3 was conducted. The key objective of this investigation was to examine NOx emissions solely due to catalytic reaction. To accomplish this, the rig was designed as simple as possible. Accordingly, the entire test run was executed at the unpressurized condition. The schematic representation of the rig is displayed in Fig. 8 (a). A desired mass flow of air at 1 bar and room temperature was axially let through the mixer. Hydrogen was introduced through the mixer such that it was normal to the air flow. The detailed description of mixing air/fuel is presented in the section Design and testing of a pre-mixer. A catalyst was placed at a distance of 50 mm from the mixer. This air/fuel
Table 4 e Operating conditions for parametric study at atmospheric pressure. Thermal power (kW) 5 5 3, 5, 8 3, 5, 8 5 5
Equivalence ratio 0.3 0.5 0.7 1.4 2.0 3.0
mixture passed through the catalyst where catalytic reactions occurred and its exhaust was let into the atmosphere. The temperatures recorded during the experiments were: 1. Wall temperature: A thermocouple was placed in one of the channels at 70 mm from the mixer. This channel was blocked by an aluminum foil at both the ends. Thereby the temperature rise of the metal foil alone was measured. 2. Radial temperature: 5 thermocouples were placed radially at equal distance of 6.6 mm. These were positioned at a distance of 10 mm downstream of the catalyst. For few experiments, these thermocouples were traversed 360 at a step of 10 each whose results are presented in contour plots (section Radial temperature distribution). A probe was placed at the exit of the test set up to collect the samples of exhaust gases. This was then measured and analyzed by an electrochemical sensor (TESTO-350-S). The gases analyzed by TESTO were NOx and O2.
Results and discussion All the experiments were conducted for the operating points showed in Table 4 and the corresponding results are discussed in this section.
Influence of equivalence ratio on NOx and wall temperature for all catalysts NOx is composed of NO and NO2. However, a major contribution to the NOx is from NO. Formation of NOx is due to
Fig. 8 e Illustration of; a) experimental set up for parametric study and b) photo of reacting catalyst A at 5 kW, f ¼ 0.3.
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Fig. 9 e Results of NOx emissions for varying equivalence ratios.
diverse factors such as thermally-generated, flame-generated, or fuel-bound NOx. In hydrogen combustion, NOx is prominently caused due to the existence of H radicals. The mechanism involved in the oxidation of NNH radicals to form NO has been proposed by Bozzelli and Dean [25]. The nitrogen present in the air and the H radical from hydrogen produces NNH radical: N2 þ H / NNH
(9)
These NNH radicals oxidize in the consequent reaction with O atoms to form NO NNH þ O / NH þ NO
(10)
In catalytic combustion, H radicals are formed during the adsorption stage (Fig. 3 (b)). These radicals are enough to form NO from the above mechanism Eq. (9) &(10). The amount of formation of NO is thus dependent on the quantity of presence of H radicals. As a consequence, in the fuel rich case, the abundance of H2 molecules results in higher NO formation.
Fig. 10 e Results of wall temperature for varying equivalence ratios.
Fig. 9 represents the nature of NOx formation for varying equivalence ratio. The NOx was corrected to 15% volume of oxygen. As the mass of air was decreased the NOx level gradually increased and reached the maximum for f ¼ 3. This was due to the NNH radical (Eq. (9) & (10)). Fig. 10 illustrates the influence of varying equivalence ratio onto the wall temperatures for all catalysts. The curve of catalyst A depicts that as the mass flow of air was increased, the catalytic wall temperature increased from f ¼ 3 to f ¼ 1.4. Whereas a further increase in the mass flow of air resulted in the rapid removal of heat, that was released during the catalytic reaction. Thus, the temperature dropped between f ¼ 1.4 and f ¼ 0.7. However, the temperature elevated after f ¼ 0.7. Because in this case, the Lewis number is less than one which means, mass diffusivity is dominant compared to the heat transfer from the catalyst. For a very lean mixture say f < 0.3, the wall temperature would have crossed adiabatic and led to super-adiabatic. If this temperature rise was >1100 K then the catalyst would have definitely damaged. Therefore, for a given thermal power, determining the right equivalence ratios is one of the primary factors to ensure safe operation of a catalyst. A similar nature in temperature profile was seen for the catalyst B i.e. with different amount of platinum loading Fig. 10. Catalyst A had a loading of 5.3 kg/m3 and catalyst B had 10.6 kg/m3. An optimal amount of loading is sufficient for a catalytic reaction to take place. Any excessive loading would inhibit reactants’ diffusion into the pores of washcoat [23]. Therefore, even though the loading was high in the case of catalyst B, there was no increase in temperature achieved compared to that of catalyst A. Likewise; there was no difference in the NOx seen for Catalyst B (Fig. 9). The influence of cell density on NOx emission (Fig. 9) and temperature profile (Fig. 10) is observed between curves of catalyst A and C. Catalyst A had 300 CPSM and catalyst C 465 CPSM. Generally, an increase in cell density means higher availability of the catalytic surface area, which in turn leads to higher reactivity and therefore increase in temperature and decrease in NOx [24]. Whereas this was not witnessed for Catalyst C due to a thicker foil in comparison to catalyst A (Table 3). Thicker foil relates to the smaller open frontal area and less space for the reactants to flow, therefore, to get the benefit of higher cell density it is necessary to decrease the foil thickness. In Figs. 9 and 10 the comparison of catalyst A and D represents 2 varying parameters: washcoat and cell density. Catalyst D had a lower cell density and no washcoat. For the rich equivalence ratios the lower cell density decreased the pressure drop leading to higher residence time resulting in increase in temperature than catalyst A. For lean equivalence ratios the temperature of catalyst D was lower compared to catalyst A due to the absence of washcoat which decreased the availability of Pt surface area and hence lowered the reactivity. For the same reason, the NOx was lower in catalyst D than A for all equivalence ratios. In Fig. 10 the nature of temperature curves against equivalence ratio for metal (catalyst A) and ceramic catalyst (catalyst E) is displayed. The temperature increased up to f ¼ 2 and then lowered. This was due to the peculiar behavior observed during the process of reaching a steady state for f ¼ 1.4 which
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Fig. 11 e Influence of thermal power on all catalysts. NOx emission for a) f ¼ 1.4 and b) f ¼ 0.7; wall temperature for c) f ¼ 1.4 and d) f ¼ 0.7. is explained in section Steady-state behavior. Normally, ceramic catalysts have lower reactivity per unit volume and also higher pressure drop compared to metal catalyst [27]. Additionally, it can be seen from Table 3, that the length of
catalyst E is shorter than others; resultantly it leads to higher space velocity and lower residence time This is the reason for the decrease in temperatures and NOx formation between f ¼ 1.4 and f ¼ 0.5 of catalyst E in comparison to catalyst A.
Influence of thermal power on NOx and wall temperature for all catalysts
Fig. 12 e Rate of reaction curve a) stable behavior in catalyst A; b) unstable behavior in catalyst C and E.
The Fig. 11 (a) and (b) demonstrate the influence of the thermal power of 3, 5 and 8 kW on wall temperatures for an equivalence ratio of 0.7 and 1.4 respectively for all catalysts. The behavior of wall temperatures followed the similar trend in both cases i.e. as the thermal power increased, the temperature elevated. During the experiments, in order to maintain the same equivalence ratio, the mass flow rate of air was increased. Moreover, higher thermal power relates to a higher mass flow rate of hydrogen. Consequently, the number of reactants involved in the catalytic activity was comparatively more in higher thermal power and thus, augmented the temperature. In a conventional combustion, large thermal power leads to an increase in temperature and therefore produces more thermal NOx from Zeldovich mechanism [30]. Nonetheless, this is contrary in the case of catalytic combustion. This can be explained from Fig. 13 (c) and (d) for f ¼ 0.7 and f ¼ 1.4 respectively. The graphs exhibit the influence of thermal power on NOx emissions for all catalysts. In the Fig. 11 (c) and (d), for most of the cases it can be seen that at higher heat input, the NOx formation rate is reduced due to a lower residence time of the reactants [26]. Hence, the lowest NOx was achieved for 8 kW. In the rich case, catalyst B and D showed a poor performance at higher thermal power. For catalyst B, higher loading
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Fig. 13 e Contour plots of the radial temperature. a) Catalyst A; b) Catalyst D; c) Catalyst E for 5 kW and f ¼ 2.
resulted in the blockage of reactants into the catalyst surface. This effect was relatively dominant at 8 kW and accordingly resulted in maximum NOx formation. It can be concluded that NOx is a parabolic function of thermal power in this case [28]. Therefore, it is vital to focus on the upper and lower limit of heat input in order to gain lower NOx. Moreover, in catalyst C, higher cell density led to an increase in pressure drop and also back-end ignition [22]. Thus, there was no improvement in NOx emissions, i.e. it was
almost constant for varying thermal power see Fig. 11 (c) and (d). It can be inferred that residence time and kinetic pathway of production of NNH radicals have a prominent role for NOx formation in the catalytic combustion of hydrogen.
Steady-state behavior During the experiments, catalyst activity reached the steady state after a certain time. This duration lasted about 30e60 min.
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distribution in relation to sinusoidal channels of a metal catalyst. Similarly, the catalyst D with lower cell density possessed spacious channels relative to catalyst A, thus facilitated consistent velocity profile and hence better temperature distribution.
Catalyst thermal efficiency
Fig. 14 e Oxygen mass fraction for corresponding equivalence ratios from CFD results of catalyst A for 5 kW.
The rate of reaction for catalyst A is depicted in Fig. 12 (a). This nature was consistent in a majority of the catalysts. Interestingly, at f ¼ 1.4 for catalyst E at 5 kW and catalyst C at 3 kW, the steady state had a strange behavior as noticed in Fig. 12 (b). Here, the temperature surged, then suddenly declined and maintained the steady state at a lower value. According to Haruta et al., the temperatures attained by the catalysts are dependent on several factors such as heat input, type of catalyst body, location on the catalyst body, the amount of air supplied and operation duration [28]. Probably this could be one of the reasons for an unstable performance. Likewise, this was noticed at f ¼ 1.4 for both the cases. As it was nearing to the stoichiometric ratio, f ¼ 1.4 was a crucial stage for these catalysts to maintain the steady state.
Radial temperature distribution The result in the contour plots was obtained by traversing the radial thermocouples (Fig. 8) to 360 at a step of 10 each. Fig. 13 represents the temperature profiles for catalysts A, D, and E. On an overview, the temperature was higher in the center and decreased radially. This was mainly due to the convective effect that caused heat transfer from the catalyst mantel to the surrounding. However, catalysts D (Fig. 13 (b)) and E (Fig. 13 (c)) had a uniform temperature profile better than that of catalyst A. This was solely due to geometric dimensioning of the channels, which was further justified from the results acquired by rotating the catalyst. Whereas, the cross section of ceramic catalyst was made up of identical square channels. This enabled improved temperature
The maximum efficiency achievable by these catalysts was 76%, which was limited solely due to temperature constraint (1100 K) of the catalyst material irrespective of the catalyst's geometric parameters and equivalence ratio. The thermal efficiency calculated from the catalytic reaction yielded an average of 53% from all catalyst at f ¼ 0.3 for 5 kW. As discussed before this efficiency was achieved exclusively from the catalyst part. Moreover, the overall combustion efficiency could be 100% when the catalytic zone will be followed by a combustion chamber as a result of a complete conversion of hydrogen (Fig. 1).
Influence of varying length on wall temperature and NOx The length of the catalyst was increased by varying the number of catalyst from one to three, thence procuring a maximum length of 90 mm. Surprisingly, the temperature did not elevate by increasing the length. This was due to inadequate oxygen after the first catalyst, as justified by the CFD computation Fig. 14. As an outcome, no further reaction perceived despite the existence of catalyst surface and hydrogen. Additionally, catalyst channels acted like a radiator and ended up cooling the flow. Apparently, the increase in catalyst length did not benefit in minimizing the NOx. Owing to the remaining hydrogen fuel reacting over the abundant catalytic surface, produced H radicals thus magnifying the NOx levels. Increasing the length could be efficient if additional air is supplied ahead of each catalyst. This will ensure sufficient oxygen availability, thus enhancing the conversion of hydrogen.
Hydrogen conversion Validation of CFD results with experimental recordings is shown in Table 5 for catalyst A. The radial temperatures 1, 2 and 3 and the wall temperature values (Fig. 8) from the CFD correlated very well with the experiment. Also, the percentage conversion of hydrogen computed from numerical simulation and theoretical calculation (Eq. (5)) matched appreciably. As the mass of the air increased the conversion of hydrogen also
Table 5 e Comparison of experimental (EXP) and CFD (SIM) results for temperatures and hydrogen conversion. f
0.7 1.4 2.0 3.0
T1 [K]
T2 [K]
T3 [K]
TW 1 [K]
%H2 consumed
EXP
SIM
EXP
SIM
EXP
SIM
EXP
SIM
Analytical
SIM
725.8 722.0 662.7 624.7
805.6 812.2 732.8 642.2
837.9 848.4 784.0 733.9
859.5 873.7 794.8 677.4
875.7 891.7 821.0 764.2
869.5 881.1 801.7 680.3
801.0 833.3 761.4 709.3
804.6 812.6 734.3 645.2
100 55.5 44.0 31.0
98.19 62.87 44.88 26.96
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increased. Maximum conversion of 98% was achieved for f ¼ 0.7 Table 5.
Conclusion The present paper focused on hydrogen-fuelled catalytic combustion chamber as a concept for future aircraft applications. Preliminary steps towards this main goal were presented in this paper through 2 major phases. The first was the design and selection of appropriate mixer. Therefore 3 configurations of pre-mixer namely mixer I, II and III was selected over swirlers through numerical simulations. Additionally, these mixers were experimentally tested and mixer III was chosen due to its good mixing ability, uniform velocity profile and lower pressure drop over a wide operating range. In the second phase, a parametric investigation of various catalysts was conducted to examine the influence on NOx emissions. Five catalysts with different geometric and material properties were considered. The experiments were conducted for different thermal power 3, 5, and 8 kW for both rich and lean conditions. For all catalysts, the NOx emission decreased with increase in equivalence ratio and thermal power. In lean air mixture, the presence of high amount of H radicals resulted in higher NOx due to NNH mechanism. The residence time and kinetic pathway of production of NNH radicals have a prominent role for NOx formation in catalytic combustion of hydrogen. Catalyst A had a consistent behavior in all the operating conditions. Thereupon, the rest of the catalysts were compared with catalyst A. Catalyst B with higher loading did not improve the catalytic activity due to the blockage of the channels. Additionally, catalyst C with high cell density and thicker foil; reduced open frontal area and so resulted in augmentation of NOx levels for higher thermal power. Catalyst D without washcoat had an adverse influence on the NOx levels for higher thermal power due to a low catalytic surface area. Catalyst E made up of ceramic had an appreciable behavior in terms of thermal powers, radial temperature distribution and combustion efficiency. The residence time decreased due to the shorter length, which caused unstable behavior in few operating conditions. However, all the catalysts yielded an average thermal efficiency of about 53%. Increasing the length of catalyst had benefit neither in temperature increase nor in NOx reduction. This was due to the unavailability of adequate oxygen downstream of the first catalyst. The results from CFD computation had a good agreement with experimental results pertaining to temperature values. Also, the percentage of H2 conversion from the simulations matched substantially well with that of analytical calculations. Finally, catalyst A produced an overall least NOx of 7.5 ppm at 5 kW for f ¼ 0.3. Therefore, this catalyst is more desirable than others. However, for the future application, this catalyst is intended to be equipped in a combustion chamber, where an additional supply of air downstream of catalyst would lead to a complete conversion of the unburnt hydrogen. Ultimately, results in bringing down the emission level to ultra-low NOx.
Moreover, small turbo engines have higher fuel specific emissions compared to large engines due to their limited combustion chamber size. Therefore, the results of this study and outputs are preliminary steps towards lowering the emissions in small/model engines for aircraft industries.
Outlook The next step will be to focus on pressurized operating conditions for the above-selected catalyst. To accomplish this, an experimental set up for the same has been designed. Further, these results will be scaled up to the real micro-combustor case. And the same will be proposed for the application in aircraft engines.
Acknowledgment I would like to thank the Institute of Thermal Turbomachinery and Machine Dynamics, the Technical University of Graz for providing me an opportunity to perform the experiments. Also, I am thankful to the Austrian Research Promotion Agency for funding this research.
references
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Available online at www.sciencedirect.com
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Experimental parametric investigation of platinum catalysts using hydrogen fuel Annapurna Basavaraju a,*, Ashwini Bhadravati Ramesh a, Dalibor Jajcevic b, Franz Heitmeir a a b
Technical University of Graz, Inffeldgasse 25A, Graz, 8042, Austria Ses-Tec OG, Autal 55, Laßnitzh€ohe, 8301, Austria
article info
abstract
Article history:
The aviation organization is creating awareness for the overall reduction of NOx emissions
Received 28 July 2018
by up to 80% in the near future. This motivates to conduct research on the current state of
Received in revised form
art, catalytic stabilized combustion chamber using hydrogen. This was achieved by per-
22 September 2018
forming an experimental parametric investigation of Platinum catalysts in two phases.
Accepted 25 September 2018
Firstly, the design of three diverse configurations of mixers and was investigated experi-
Available online 23 October 2018
mentally and numerically. The chosen mixer was implemented in the parametric study of five different Pt catalysts varying in geometric and material properties. This was executed
Keywords:
at unpressurized and NOx emission solely due to the catalytic reaction was examined for
Mixer
varying thermal power and air/fuel ratios. Furthermore, temperatures were recorded.
Catalytic combustion
Additionally, CFD simulation was accomplished and compared with the measurement
Hydrogen
data. The overall least NOx achieved was 7.5 ppm at 5 kW for the metal catalyst. The result
Gas turbine engine
of this work proposed suitable catalyst for the development of a combined combustor
NOx emissions
configuration (including catalyst and combustion chamber) which will be intended for small aircraft engine applications. © 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction CO2 emissions have increased by about 80% and NOx emissions have doubled between 1990 and 2014. There are forecast to grow both emissions further by about 45% between 2014 and 2035 [17]. Therefore, organizations such as the International Civil Aviation Organization (ICAO) and the Advisory Council for Aeronautics Research in Europe (ACARE) are creating awareness on these issues. This is due to the fact that these emissions have significant impacts on health, environment as well as the performance of engines. Regarding the very first step towards solving these issues is to use replacement fuel. In
this work, hydrogen gas has been proposed because the sole pollutant during its oxidation is NOx. Another characteristic feature of hydrogen is the high heating value. Nevertheless, one of the biggest challenges is the storage of hydrogen, which can be solved by improving the materials of the tanks. Additionally, safety is a topic that cannot be ignored while dealing with hydrogen. NOx emissions are known to be dominant at very high temperatures and this is normally witnessed in conventional combustion mode [18]. Accordingly, it is important to reduce the temperature. One such method is to incorporate catalysts in the combustion chamber. This enables the reaction to take
* Corresponding author. E-mail address: [email protected] (A. Basavaraju). https://doi.org/10.1016/j.ijhydene.2018.09.170 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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Nomenclature f equivalence ratio 1D one dimensional 3D three dimensional aluminum oxide Al2O3 CFD computational fluid dynamics carbon di-oxide CO2 CPSI cells per square inch CPSM cells per square meter H radical hydrogen radical hydrogen H2 water H2O Le Lewis number NO nitric oxide nitrogen di-oxide NO2 NOx nitrogen oxides oxygen O2 Pd palladium ppm parts per million Pt platinum RCL rich catalytic lean burn RSM Reynolds stress model
place at lower temperatures (<1100 K) due to their lower activation energy and thus lowering NOx. Consequently, in the current work, the concept of heterogeneous catalytic combustion is implemented. This is defined as a chemical process in which a catalyst is used to oxidize the fuel at a lower temperature and thus leading to a Flameless combustion. The concept of catalytic combustion for gas turbine engines has been introduced by Dr. William C. Pfefferle in the early 1970s. Since then, the application of catalysts to reduce the emission of gas turbine engines is the state of the art in the research field. There are many successful findings in achieving lower than 5 ppm NOx for fuels such as methane and syngas over Pt/Pd catalysts using Rich Catalytic Lean-burn (RCL) [1e4,6]. In addition to lowering the NOx (i.e. ~2.5 ppmv corrected to 15% O2) and CO emissions (<2 ppm), Benjamin Baird et al. have also achieved lower acoustics in the catalytic combustor for natural gas [15]. Further, the benefit of catalytic reactions is not just restricted to limited fuels, but also for multiple alternative fuels: natural gas, biomass landfill gas, gasoline, refinery fuel gas etc [7]. The outcomes show that catalyst temperatures are stable and emissions are maintained at low values for all tested fuels [9]. Likewise, Alavandi et al. [8] have developed a catalytic combustor for methanehydrogen mixtures. In the case of hydrogen combustion, the NOx has been significantly reduced from 165 ppm to 23 ppm using Pt/Pd catalyst. These experiments have been conducted under the high pressure of up to 13 bar and a maximum inlet temperature of around 450 C [5]. Moreover, the NOx reduction potential is not only confined to RCL mode, but also the leanpremixed combustion has demonstrated the ability to achieve as low as 5e9 ppm corrected to 15% O2 during operation on natural gas [10e13]. The application of catalytic combustion is not only limited to gas turbines but also marked its
significance in the domestic kitchen applications. Recently the research has shown an ultra-low NOx of 0.09 ppmv [33]. Employing catalysts also improves the efficiency of the engine, the lifetime of gas turbine components, extended service interval time, fuel flexibility and combustion noise [3,8]. There are several analyses performed through computational simulations to get an insight of mechanisms involved in catalytic reactions. A numerical 1D Plug-flow model is developed by Yin et al. [19] and as a result, axial gradients of catalytic oxidation are attained. A similar transient 1D reactor model with an already validated multi-step surface reaction mechanism is developed by Zhu and Jackson [20]. Detailed two-dimensional simulations of catalysts are accomplished by Karagiannidis et al. [21]. An analytical model for the calculation of the catalysts is established by Joshi et al. [22]. Here, the detailed mechanism of catalytic reaction and the influence of geometric parameters of catalysts on the conversion of fuel has been very well explained. Besides the kinetics of catalytic reaction S. Pramanik et al. have observed that the homogeneous reaction rates are significant at higher pressures [34]. Even three-dimensional simulation has been performed by B.O. Arani et al. to investigate the turbulent combustion of fuel-lean hydrogen/air mixtures in a platinumcoated channel [35]. Schultze and Mantzaras numerically investigated both fuel rich and fuel lean catalytically stabilized combustion of hydrogen/air mixtures in Pt catalyst [14]. In the fuel-lean mode, the Lewis number of hydrogen is less than unity which led to super-adiabatic surface temperatures. On the contrary, in fuel-rich mode, the Lewis number of the deficient oxygen reactant is greater than unity which resulted in surface temperatures well below the adiabatic equilibrium temperature. Therefore, in this paper, the Lewis number is considered as a vital factor in the safe operation of catalysts. There are very few researches executed on the catalytically stabilized hydrogen fuelled gas turbine engine, especially for aircraft application. For the sake of achieving this objective, the entire study has been carried out in the following steps: a) Design and testing of a pre-mixer: The entrainment of fuel/ air mixtures into the combustion chamber is an important criterion. The pre-mixed combustion has an advantage of lowering the NOx over the diffusion, therefore in this work, a study on choosing the right pre-mixer has been performed. Generally, a pre-mixer has a risk of flashback; hence a novel configuration of micro-mixer is designed. In this research, the multipoint injection method is chosen due to the advantage of short mixing length and time scales which in turn decreases the possibility of flashback and autoignition. Additionally, this decreases the size and weight of the combustor particularly relevant on aircraft engines. Subsequently, the benefit of passive methods is being simple and works effectively during the normal conditions of the system, therefore the cross flow injection type is adopted in this paper and tested both experimentally and numerically. All the numerical simulations presented in this paper are conducted by the project partner Ses-Tec, Austria. b) Experimental parametric study of Pt catalysts: There are various catalysts available in the market, differing in their features such as size, shape, material, length, number of
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 3 ( 2 0 1 8 ) 2 1 3 0 7 e2 1 3 2 1
channels, loading, activation energy etc. Thereupon, selecting the right catalyst to fulfill the above-mentioned purpose is necessary. A parametric study has been performed for five Pt catalysts to investigate their influence on the NOx emissions. Platinum possesses high activation energy meaning it comparatively increases the rate of reaction; hence the combination of Pt and hydrogen would definitely yield a faster chemical reaction. Finally, the intention of such an investigation will be applicable in a small aero engine. Such a study is novel and hence the operating conditions were chosen to be as simple and safe as possible. Therefore the experiments are conducted to see the behavior of the catalyst by varying the thermal power from 3, 5 and 8 kW, as well as the equivalence ratio, were selected in both lean and rich ranges. This is executed both experimentally and numerically under atmospheric conditions.
Fig. 2 e Working principle of catalytic reactor [14]. Without catalyst: 2H2 þ O2 /2H2 O
Pt
The basic components in a gas turbine engine are a compressor, combustion chamber, and turbine. Fig. 1 shows the schematic representation of an adapted version of the gas turbine for catalytic combustion. Here, the fuel and air are combined in the pre-mixer zone. This mixture passes through the catalytic reactor and enters the combustion chamber. The working principle of a catalytic reactor is illustrated in the Fig. 2. The air/fuel mixtures are led into the catalyst part where partial oxidation of fuel takes place. Typically, bypass air is introduced around the combustor which helps in reducing the reactivity of hydrogen, by distributing part of its enthalpy via heat transfer. The maximum temperature that is allowed in the catalyst section is around 1100 K due to the material constraints. Additionally, secondary air is supplied to the combustion chamber to oxidize the unburnt hydrogen. Compared to conventional, less amount of energy is required to burn the rest of hydrogen in the lean zone. Thus, resulting in lower adiabatic flame temperature and hence lowers NOx emissions [18]. The above catalytic reaction takes place by altering the rate of chemical reaction, without itself being transformed or consumed. These chemical reactions with catalysts take place at lower activation energy than that of without catalysts. The chemical reactions shown below are with and without catalysts along with their respective adiabatic temperatures.
Fig. 1 e Schematic representation of catalytic reactor in a gas turbine engine [31].
(1)
And the adiabatic temperature is ~2273 K. With catalyst: 2H2 þ O2 /2H2 O
Theory
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(2)
And the adiabatic temperature is ~1473 K. The mechanisms involved during a catalytic reaction are represented in Fig. 3. The fig shows the cross section of catalyst that is divided into 2 different phases, namely bulk gas phase (homogenous: fuel/air mixture) and washcoat phase (heterogeneous: fuel/air and catalytic material ex: Platinum). The consecutive steps that determine the rate of reaction are: a) External diffusion: It is a process where the reactants diffuse onto the surface b) Adsorption: Here, the reactants adhere to the catalytic surface c) Surface reaction: In this, molecular rearrangement at the active surface sites takes place d) Desorption: where the disintegration of products from the surface occurs. e) External diffusion: Finally, the products are diffused away from the surface and entered back into the gas phase. In a catalytic reaction, the conversion efficiency of the fuel is greatly dependent on the reaction regime. Various characteristic regimes observed in a catalytic monolith are kinetic, washcoat diffusion and external mass transfer regime. The regime encountered during a reaction depends on distinct factors such as temperature, washcoat, catalyst loading and channel dimensions [22]. At low temperature, the reactants encounter kinetic regime where reaction resistance is present. As the temperature increases the reactants overcome this regime and enter the washcoat diffusion-controlled regime. Here, the resistance due to washcoat is dominant and hence time needed for internal diffusion becomes much higher than the reaction and external diffusion time. Finally, at higher temperatures, the reactants enter the external mass transfer regime where the characteristic reaction time approaches zero and highest conversion efficiency is attained. Therefore ideally, catalysts are expected to operate in this regime. Lewis number The catalysts are subjected to damage if the temperature exceeds about 1100 K. High temperatures are reached when
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Fig. 3 e Catalytic reaction mechanisms [32]. the mass diffusivity exceeds the thermal diffusivity, thus the lack of thermal distribution augments the local temperatures resulting in hot spots. This phenomenon can be explained by the term called “Lewis number”. This is defined as: Le ¼ a=D
(3)
where. a: Thermal diffusivity (m2/s) it is a measure of the capacity of the material to conduct thermal energy relative to its ability to store thermal energy. D: Mass diffusivity (m2/s) is a measure of diffusion of species within a substance. Generally, for rich hydrogen mixtures the Le 1, whereas Le < 1 for very lean hydrogen mixtures. It doesn't mean one cannot operate in lean conditions at all; however, care has to be taken to avoid the risk of reaching the limiting temperature of the catalyst (1173 K). At lower thermal power, it is possible to operate lean conditions, without damaging the catalyst.
XH2
c) Efficiency The thermal efficiency of the Brayton cycle is a ratio of total work done to the heat input, expressed as: h¼
a) Equivalence ratio is defined as the ratio of stoichiometric air/fuel ratio to the actual air/fuel ratio [16]: h¼ (4)
where, m_ f ¼ Mass flow rate of fuel [kg/s] m_ a ¼ Mass flow rate of air [kg/s] fstoichiometric ¼ is the stoichiometric air/fuel ratio For hydrogen fuel it is 34. b) Percentage of fuel conversion Consider a stream of hydrogen in air. This stream flowing through catalyst undergoes a chemical reaction represented in the Eq. (5). Considering the law of conservation of mass and assuming that the system is in an adiabatic steady state, one can say that the enthalpy change due to the reaction is absorbed by the reacting stream. Expressing enthalpy balance in terms of both fractional conversion and the heat capacity of the inlet stream as an average value, the rate of reaction of hydrogen is given by Ref. [8]:
(5)
where. Tin ¼ Reactant temperature [K] Tout ¼ Product temperature [K] MT ¼ Total mass fraction of oxygen and hydrogen MH2 ¼ Mass fraction of hydrogen Cp average ¼ Average heat capacity [J/mol K] DHR; Tout ¼ Standard Heat of reaction at Tout [J/mol] Here, Tout is substituted by wall temperature obtained from the experiment. This Eq. (4) is further applied to the comparison of the percentage of H2 conversion obtained from CFD results.
Calculations
m_ f fstoichiometric f¼ m_ a
Cp average MT ¼ ðTin Tout Þ MH2 DHR; Tout
Net work qout qin ¼ Heat input qin m_ T Cp air ðTout Tin Þ m_ f HV
(6)
(7)
where, qout ¼ Heat output [J] qin ¼ Heat input [J] m_ T ¼ Total mass flow rate [kg/s] Cp air ¼ Heat capacity of air [J/kg K] HV ¼ Heating value of hydrogen [J/kg] These results play a key role in the selection of catalyst and are presented in the upcoming sections.
Methods Design and testing of a pre-mixer Numerical simulation Simulation results were obtained using commercial CFD code ANSYS Fluent. A quasi-steady state condition was assumed
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and the transient behavior of the flow was neglected. The conditions applied in the CFD model were:
Steady-state solver Fluent RSM turbulence model Standard wall function Compressible ideal gas Species transport model (three species: O2, N2, and H2) Second order scheme (upwind) for momentum equation
A preliminary investigation of several configurations of fuel/air mixer was performed using CFD computation. This included a comparison of swirler and micro-mixer. The swirlers examined were with swirl angles of 25 , 50 , contrarotating 2 25 , and contra-rotating 2 50 . One example of the swirler design is depicted in the Fig. 4 (a). And the micromixer computed were of three configurations, namely mixer I, II and III. Each mixer consisted of 19 inlets for air with 4 subinlets for fuel. The fuel was distributed into these 19 inlets via 2 main inlets of 8 mm and 6 mm. These three mixers differed in their specifications which are shown in Table 1. The flow path of air and fuel were perpendicular to each other. The Fig. 4 (b) below shows the cross section of Mixer III. The outlet pressure was set to the atmosphere and the temperature at both inlet and outlet was set to 288.15 K. The solver conditions were: standard wall treatment, Fluent RSM turbulence model, compressible ideal gas and second order solver in the momentum equation. Homogenous mixing of fuel/air was necessary for efficient catalytic activity. Therefore, the velocity profile from CFD is an important factor to decide on the flow behavior after the mixer. Fig. 5 shows the correlation of axial velocity to the radial position at a distance of 70 mm from the mixer. The velocity profile of 25 , 50 , and 2 50 swirlers had suction behavior in the center of the stream Fig. 5(a),(b) and (c). Moreover, the swirler 2 25 (Fig. 5 (d)) had a non-uniform mixing along the diameter. On the other hand, uniform mixing and better velocity profile were achieved in the all the cases of micro-mixer Fig. 5 (e), (f) and (g). Additionally, a
Table 1 e Pre-mixer types. Micromixer
Fuel inlet Air inlet Air outlet diameter (mm) diameter (mm) diameter (mm)
I II III
1.0 0.5 0.5
4.5 4.5 4.5
3.0 3.0 4.5
uniform flow profile was already observed at a distance of 50 mm for micro-mixers. So, the minimum distance of catalyst from the mixer must be 50 mm. The pressure loss for all the mixers was calculated by using the formula: Pressure loss ¼
Pin Pout Pin
(8)
where, Pin ¼ Pressure before the mixer (bar) Pout ¼ Pressure after the mixer (bar) The pressure drop in micro-mixer was lower than swirler as indicated in Table 2. Therefore, micro-mixer of all 3 configurations was selected for the experimental investigation.
Experimental setup The velocity profile from CFD for all the three micro-mixer was identical. Consequently, velocimetry measurement was of no interest in the experimental study. Further to compare the fuel/air mixing behavior among these mixers, a test campaign for varying operating conditions was performed. In order to examine this visually an optical method of laser light sheet visualization technique was employed. Pressurized metered air at 3 bar was introduced from the inlet of the rig (Fig. 6 (1)). The air from the inlet is passed through a long pipe to ensure laminar flow. The oil from the seeding generator was fed through the fuel inlets at 1.225 bar (Fig. 6 (3)). The inlet air mixes up with seeding particles across the micromixer (Fig. 6 (4)). An optical window Fig. 6 (5) was provided to pass the laser sheet into the flow. Thus, the nature of mixing was captured by a camera (CANON-700D) through
Fig. 4 e a) Swirler; b) Cross section of mixer III.
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Fig. 5 e Axial velocity profile vs radial position at a distance of 70 mm from the mixer computed for all mixers from CFD. Swirler with swirl angle of: a) 25 ; b) 50 ; c) contra-rotating 2 £ 50 and d) contra-rotating 2 £ 25 . Micro-mixer: e) mixer I; f) mixer II and g) mixer III.
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Table 2 e Pressure loss computed from CFD for all mixers. Mixer Swirler 25 Swirler 50 Swirler 2 25 Swirler 2 50 Micro mixer I Micro mixer II Micro mixer III
Pressure loss (%) 7.32 6.03 5.52 5.92 4.55 0.90 1.10
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2. The air was let through fuel inlet and the seeding at the main inlet. This was performed to verify the results from step 1. 3. Helium along with seeding was let through the fuel inlets and air at the main inlet. Helium was chosen because it is inert and has a molecular size closer to hydrogen. These experiments were conducted for three operating conditions: constant mass flow through the fuel inlet, constant pressure through the fuel inlet and lastly constant velocity through the air inlet.
Fig. 6 e Schematic representation of experimental setup for the mixer test. 1) Inlet; 2) Orifice chamber; 3) Fuel inlet; 4) Mixer; 5) Optical window 2; 6) Optical window 1; 7) Outlet valve.
another optical window Fig. 6 (6) situated near the mixer. The pressure inside the rig was regulated by a valve at the outlet (Fig. 6 (7)). In order to ensure the uniform flow from the mixer, the experiment was carried out in the following three stages: 1. The air was let through the main inlet and seeding at the fuel inlet.
Instrumentation. The pressure of the inlet air was measured by a pressure transducer. The mass flow of the air was monitored across the orifice plate. An optical measurement technique called light sheet visualization had been used to analyze the flow pattern downstream of the mixer. Seeding was essential to obtain high-quality measurements; therefore, oil particles were added. A metered mass flow of these was introduced into the mixer through the seeding oil generator.
Fig. 7 e Light sheet visualization images from the mixer test. a) Test results obtained from all mixers. b) Test results for all three stages of experiment from mixer III.
40 40 32 32 5.3 10.6 5.3 5.3 5.3 a-Al2O3 a-Al2O3 a-Al2O3 e a-Al2O3 A B C D E
33 33 33 33 34
30 30 30 30 25
200 200 300 100 200
300 300 465 155 300
Metal Metal Metal Metal Ceramic
Platinum Platinum Platinum Platinum Platinum
Loading [kg/m3] Substrate CPSM CPSI Length [mm] Diameter [mm]
Table 3 e Specifications of selected catalysts.
Catalyst selection criteria: As discussed before, for an efficient conversion catalyst is anticipated to be in external mass transfer regime which is dependent on two factors: flow parameters such as velocity, pressure, equivalence ratio, residence time, etc. and geometric parameters such as length, diameter, cell density, loading, material, etc. It is studied that the length and diameter above 20 mm and 25 mm respectively help in reaching mass transfer regime during a reaction [22]. Additionally, cell density is referred to as the number of cells per unit area. The higher the cell density better is the conversion, but increases the pressure drop. Further, loading explains the amount of catalyst present per unit area, more is the value better is the durability and performance. However, excessive loading does not necessarily mean it is good for catalyst performance as it results in blocking the channels. Hence, deciding on the optimal loading of the respective application is essential. Catalysts are usually made up of either ceramic or metal. Ceramic is known for its low thermal expansion, high resistance to fracture due to thermal shock likewise metal has a lower pressure drop, more surface area, better thermal conductivity etc. Thus both metal and ceramic were considered in this work. Based on this guideline, five catalysts with varying parameters were chosen for the parametric study and are tabulated in Table 3. These catalysts were bought from a catalyst manufacturing company. In the interest to carry out the parametric investigation, experiments on different catalysts were conducted. The main aim of this study was to get an overview of the behavior of catalysts, their functionality with hydrogen fuel as well as to observe their impact on NOx emissions for gas turbine application.
Precious metal
Parametric investigation of catalysts
Washcoat
Mantle dimensions [mm]
An image captured at each operating point was subtracted with the respective reference image (flow from the main inlet only) to view just the seeding particles. All these images were processed uniformly using “Matlab” software and the intensity was maintained constant. The results presented here are from the 2nd category (i.e. seeding at the main inlet and air at the fuel inlet) at a constant mass flow through the fuel inlet. However, in all the three categories as well as in all the operating conditions similar results were found. The nature of the flow in mixers I and II was not well distributed. The red circled area for the mixer I in Fig. 7 (a) represents high turbulence and dense clouds of seeding particles that were spluttered and accumulated. Similarly, the red circled area for mixer II in Fig. 7 (a) depicts the area with fewer amounts of seeding particles which is again not desirable as an inlet flow to the catalyst. Contrarily, the green area for mixer III in Fig. 7 (a) showed a uniform mixing behavior. Additionally, the flow pattern in mixer III was similar to that of the ideal turbulence profile in a pipe. This nature was also evident in all the 3 categories as shown in Fig. 7 (b). Mixer III was chosen for the parametric study due to its uniform mixing behavior and turbulent like flow profile in a pipe for all operating conditions.
35 35 35 35 e
Foil thickness [mm]
Results
0.05 0.05 0.08 e e
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Catalyst
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The catalysts of different geometric and material characteristics were chosen for the parametric study as shown in Table 3. The channels of metal and ceramic catalysts were made up of sinusoidal and square shape respectively.
Numerical simulation Prior to the experiments, the catalytic combustion of the hydrogen/air mixture in a coated catalyst was investigated by means of 3D numerical simulations in Ansys Fluent software. This 3D model was coupled with the CHEMKIN-CFD Solver to enhance the accuracy of chemical reaction at the catalyst walls. The chemical reaction mechanism of H2/O2 over platinum was adopted from Bhatia et al. [29]. In the CFD model, the following setup was applied: 1. Steady-state solver 2. Turbulence model 3. Species transport model a. Reactions: Volumetric and Wall surface (Heat of surface reaction, diffusion energy source) b. Mixture properties imported from CHEMKIN c. Turbulence-Chemistry interaction: laminar finite-rate The velocity of the flow in the catalyst influenced the chemical reactions. Due to this, the channels at the outlet of the mixer was also included upstream of the catalyst. In the direction of reducing computational domain and time, only a portion of 60 (1/6) of the whole model for catalyst A was simulated.
Experimental setup The experimental parametric study for catalysts in Table 3 was conducted. The key objective of this investigation was to examine NOx emissions solely due to catalytic reaction. To accomplish this, the rig was designed as simple as possible. Accordingly, the entire test run was executed at the unpressurized condition. The schematic representation of the rig is displayed in Fig. 8 (a). A desired mass flow of air at 1 bar and room temperature was axially let through the mixer. Hydrogen was introduced through the mixer such that it was normal to the air flow. The detailed description of mixing air/fuel is presented in the section Design and testing of a pre-mixer. A catalyst was placed at a distance of 50 mm from the mixer. This air/fuel
Table 4 e Operating conditions for parametric study at atmospheric pressure. Thermal power (kW) 5 5 3, 5, 8 3, 5, 8 5 5
Equivalence ratio 0.3 0.5 0.7 1.4 2.0 3.0
mixture passed through the catalyst where catalytic reactions occurred and its exhaust was let into the atmosphere. The temperatures recorded during the experiments were: 1. Wall temperature: A thermocouple was placed in one of the channels at 70 mm from the mixer. This channel was blocked by an aluminum foil at both the ends. Thereby the temperature rise of the metal foil alone was measured. 2. Radial temperature: 5 thermocouples were placed radially at equal distance of 6.6 mm. These were positioned at a distance of 10 mm downstream of the catalyst. For few experiments, these thermocouples were traversed 360 at a step of 10 each whose results are presented in contour plots (section Radial temperature distribution). A probe was placed at the exit of the test set up to collect the samples of exhaust gases. This was then measured and analyzed by an electrochemical sensor (TESTO-350-S). The gases analyzed by TESTO were NOx and O2.
Results and discussion All the experiments were conducted for the operating points showed in Table 4 and the corresponding results are discussed in this section.
Influence of equivalence ratio on NOx and wall temperature for all catalysts NOx is composed of NO and NO2. However, a major contribution to the NOx is from NO. Formation of NOx is due to
Fig. 8 e Illustration of; a) experimental set up for parametric study and b) photo of reacting catalyst A at 5 kW, f ¼ 0.3.
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Fig. 9 e Results of NOx emissions for varying equivalence ratios.
diverse factors such as thermally-generated, flame-generated, or fuel-bound NOx. In hydrogen combustion, NOx is prominently caused due to the existence of H radicals. The mechanism involved in the oxidation of NNH radicals to form NO has been proposed by Bozzelli and Dean [25]. The nitrogen present in the air and the H radical from hydrogen produces NNH radical: N2 þ H / NNH
(9)
These NNH radicals oxidize in the consequent reaction with O atoms to form NO NNH þ O / NH þ NO
(10)
In catalytic combustion, H radicals are formed during the adsorption stage (Fig. 3 (b)). These radicals are enough to form NO from the above mechanism Eq. (9) &(10). The amount of formation of NO is thus dependent on the quantity of presence of H radicals. As a consequence, in the fuel rich case, the abundance of H2 molecules results in higher NO formation.
Fig. 10 e Results of wall temperature for varying equivalence ratios.
Fig. 9 represents the nature of NOx formation for varying equivalence ratio. The NOx was corrected to 15% volume of oxygen. As the mass of air was decreased the NOx level gradually increased and reached the maximum for f ¼ 3. This was due to the NNH radical (Eq. (9) & (10)). Fig. 10 illustrates the influence of varying equivalence ratio onto the wall temperatures for all catalysts. The curve of catalyst A depicts that as the mass flow of air was increased, the catalytic wall temperature increased from f ¼ 3 to f ¼ 1.4. Whereas a further increase in the mass flow of air resulted in the rapid removal of heat, that was released during the catalytic reaction. Thus, the temperature dropped between f ¼ 1.4 and f ¼ 0.7. However, the temperature elevated after f ¼ 0.7. Because in this case, the Lewis number is less than one which means, mass diffusivity is dominant compared to the heat transfer from the catalyst. For a very lean mixture say f < 0.3, the wall temperature would have crossed adiabatic and led to super-adiabatic. If this temperature rise was >1100 K then the catalyst would have definitely damaged. Therefore, for a given thermal power, determining the right equivalence ratios is one of the primary factors to ensure safe operation of a catalyst. A similar nature in temperature profile was seen for the catalyst B i.e. with different amount of platinum loading Fig. 10. Catalyst A had a loading of 5.3 kg/m3 and catalyst B had 10.6 kg/m3. An optimal amount of loading is sufficient for a catalytic reaction to take place. Any excessive loading would inhibit reactants’ diffusion into the pores of washcoat [23]. Therefore, even though the loading was high in the case of catalyst B, there was no increase in temperature achieved compared to that of catalyst A. Likewise; there was no difference in the NOx seen for Catalyst B (Fig. 9). The influence of cell density on NOx emission (Fig. 9) and temperature profile (Fig. 10) is observed between curves of catalyst A and C. Catalyst A had 300 CPSM and catalyst C 465 CPSM. Generally, an increase in cell density means higher availability of the catalytic surface area, which in turn leads to higher reactivity and therefore increase in temperature and decrease in NOx [24]. Whereas this was not witnessed for Catalyst C due to a thicker foil in comparison to catalyst A (Table 3). Thicker foil relates to the smaller open frontal area and less space for the reactants to flow, therefore, to get the benefit of higher cell density it is necessary to decrease the foil thickness. In Figs. 9 and 10 the comparison of catalyst A and D represents 2 varying parameters: washcoat and cell density. Catalyst D had a lower cell density and no washcoat. For the rich equivalence ratios the lower cell density decreased the pressure drop leading to higher residence time resulting in increase in temperature than catalyst A. For lean equivalence ratios the temperature of catalyst D was lower compared to catalyst A due to the absence of washcoat which decreased the availability of Pt surface area and hence lowered the reactivity. For the same reason, the NOx was lower in catalyst D than A for all equivalence ratios. In Fig. 10 the nature of temperature curves against equivalence ratio for metal (catalyst A) and ceramic catalyst (catalyst E) is displayed. The temperature increased up to f ¼ 2 and then lowered. This was due to the peculiar behavior observed during the process of reaching a steady state for f ¼ 1.4 which
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Fig. 11 e Influence of thermal power on all catalysts. NOx emission for a) f ¼ 1.4 and b) f ¼ 0.7; wall temperature for c) f ¼ 1.4 and d) f ¼ 0.7. is explained in section Steady-state behavior. Normally, ceramic catalysts have lower reactivity per unit volume and also higher pressure drop compared to metal catalyst [27]. Additionally, it can be seen from Table 3, that the length of
catalyst E is shorter than others; resultantly it leads to higher space velocity and lower residence time This is the reason for the decrease in temperatures and NOx formation between f ¼ 1.4 and f ¼ 0.5 of catalyst E in comparison to catalyst A.
Influence of thermal power on NOx and wall temperature for all catalysts
Fig. 12 e Rate of reaction curve a) stable behavior in catalyst A; b) unstable behavior in catalyst C and E.
The Fig. 11 (a) and (b) demonstrate the influence of the thermal power of 3, 5 and 8 kW on wall temperatures for an equivalence ratio of 0.7 and 1.4 respectively for all catalysts. The behavior of wall temperatures followed the similar trend in both cases i.e. as the thermal power increased, the temperature elevated. During the experiments, in order to maintain the same equivalence ratio, the mass flow rate of air was increased. Moreover, higher thermal power relates to a higher mass flow rate of hydrogen. Consequently, the number of reactants involved in the catalytic activity was comparatively more in higher thermal power and thus, augmented the temperature. In a conventional combustion, large thermal power leads to an increase in temperature and therefore produces more thermal NOx from Zeldovich mechanism [30]. Nonetheless, this is contrary in the case of catalytic combustion. This can be explained from Fig. 13 (c) and (d) for f ¼ 0.7 and f ¼ 1.4 respectively. The graphs exhibit the influence of thermal power on NOx emissions for all catalysts. In the Fig. 11 (c) and (d), for most of the cases it can be seen that at higher heat input, the NOx formation rate is reduced due to a lower residence time of the reactants [26]. Hence, the lowest NOx was achieved for 8 kW. In the rich case, catalyst B and D showed a poor performance at higher thermal power. For catalyst B, higher loading
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Fig. 13 e Contour plots of the radial temperature. a) Catalyst A; b) Catalyst D; c) Catalyst E for 5 kW and f ¼ 2.
resulted in the blockage of reactants into the catalyst surface. This effect was relatively dominant at 8 kW and accordingly resulted in maximum NOx formation. It can be concluded that NOx is a parabolic function of thermal power in this case [28]. Therefore, it is vital to focus on the upper and lower limit of heat input in order to gain lower NOx. Moreover, in catalyst C, higher cell density led to an increase in pressure drop and also back-end ignition [22]. Thus, there was no improvement in NOx emissions, i.e. it was
almost constant for varying thermal power see Fig. 11 (c) and (d). It can be inferred that residence time and kinetic pathway of production of NNH radicals have a prominent role for NOx formation in the catalytic combustion of hydrogen.
Steady-state behavior During the experiments, catalyst activity reached the steady state after a certain time. This duration lasted about 30e60 min.
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distribution in relation to sinusoidal channels of a metal catalyst. Similarly, the catalyst D with lower cell density possessed spacious channels relative to catalyst A, thus facilitated consistent velocity profile and hence better temperature distribution.
Catalyst thermal efficiency
Fig. 14 e Oxygen mass fraction for corresponding equivalence ratios from CFD results of catalyst A for 5 kW.
The rate of reaction for catalyst A is depicted in Fig. 12 (a). This nature was consistent in a majority of the catalysts. Interestingly, at f ¼ 1.4 for catalyst E at 5 kW and catalyst C at 3 kW, the steady state had a strange behavior as noticed in Fig. 12 (b). Here, the temperature surged, then suddenly declined and maintained the steady state at a lower value. According to Haruta et al., the temperatures attained by the catalysts are dependent on several factors such as heat input, type of catalyst body, location on the catalyst body, the amount of air supplied and operation duration [28]. Probably this could be one of the reasons for an unstable performance. Likewise, this was noticed at f ¼ 1.4 for both the cases. As it was nearing to the stoichiometric ratio, f ¼ 1.4 was a crucial stage for these catalysts to maintain the steady state.
Radial temperature distribution The result in the contour plots was obtained by traversing the radial thermocouples (Fig. 8) to 360 at a step of 10 each. Fig. 13 represents the temperature profiles for catalysts A, D, and E. On an overview, the temperature was higher in the center and decreased radially. This was mainly due to the convective effect that caused heat transfer from the catalyst mantel to the surrounding. However, catalysts D (Fig. 13 (b)) and E (Fig. 13 (c)) had a uniform temperature profile better than that of catalyst A. This was solely due to geometric dimensioning of the channels, which was further justified from the results acquired by rotating the catalyst. Whereas, the cross section of ceramic catalyst was made up of identical square channels. This enabled improved temperature
The maximum efficiency achievable by these catalysts was 76%, which was limited solely due to temperature constraint (1100 K) of the catalyst material irrespective of the catalyst's geometric parameters and equivalence ratio. The thermal efficiency calculated from the catalytic reaction yielded an average of 53% from all catalyst at f ¼ 0.3 for 5 kW. As discussed before this efficiency was achieved exclusively from the catalyst part. Moreover, the overall combustion efficiency could be 100% when the catalytic zone will be followed by a combustion chamber as a result of a complete conversion of hydrogen (Fig. 1).
Influence of varying length on wall temperature and NOx The length of the catalyst was increased by varying the number of catalyst from one to three, thence procuring a maximum length of 90 mm. Surprisingly, the temperature did not elevate by increasing the length. This was due to inadequate oxygen after the first catalyst, as justified by the CFD computation Fig. 14. As an outcome, no further reaction perceived despite the existence of catalyst surface and hydrogen. Additionally, catalyst channels acted like a radiator and ended up cooling the flow. Apparently, the increase in catalyst length did not benefit in minimizing the NOx. Owing to the remaining hydrogen fuel reacting over the abundant catalytic surface, produced H radicals thus magnifying the NOx levels. Increasing the length could be efficient if additional air is supplied ahead of each catalyst. This will ensure sufficient oxygen availability, thus enhancing the conversion of hydrogen.
Hydrogen conversion Validation of CFD results with experimental recordings is shown in Table 5 for catalyst A. The radial temperatures 1, 2 and 3 and the wall temperature values (Fig. 8) from the CFD correlated very well with the experiment. Also, the percentage conversion of hydrogen computed from numerical simulation and theoretical calculation (Eq. (5)) matched appreciably. As the mass of the air increased the conversion of hydrogen also
Table 5 e Comparison of experimental (EXP) and CFD (SIM) results for temperatures and hydrogen conversion. f
0.7 1.4 2.0 3.0
T1 [K]
T2 [K]
T3 [K]
TW 1 [K]
%H2 consumed
EXP
SIM
EXP
SIM
EXP
SIM
EXP
SIM
Analytical
SIM
725.8 722.0 662.7 624.7
805.6 812.2 732.8 642.2
837.9 848.4 784.0 733.9
859.5 873.7 794.8 677.4
875.7 891.7 821.0 764.2
869.5 881.1 801.7 680.3
801.0 833.3 761.4 709.3
804.6 812.6 734.3 645.2
100 55.5 44.0 31.0
98.19 62.87 44.88 26.96
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increased. Maximum conversion of 98% was achieved for f ¼ 0.7 Table 5.
Conclusion The present paper focused on hydrogen-fuelled catalytic combustion chamber as a concept for future aircraft applications. Preliminary steps towards this main goal were presented in this paper through 2 major phases. The first was the design and selection of appropriate mixer. Therefore 3 configurations of pre-mixer namely mixer I, II and III was selected over swirlers through numerical simulations. Additionally, these mixers were experimentally tested and mixer III was chosen due to its good mixing ability, uniform velocity profile and lower pressure drop over a wide operating range. In the second phase, a parametric investigation of various catalysts was conducted to examine the influence on NOx emissions. Five catalysts with different geometric and material properties were considered. The experiments were conducted for different thermal power 3, 5, and 8 kW for both rich and lean conditions. For all catalysts, the NOx emission decreased with increase in equivalence ratio and thermal power. In lean air mixture, the presence of high amount of H radicals resulted in higher NOx due to NNH mechanism. The residence time and kinetic pathway of production of NNH radicals have a prominent role for NOx formation in catalytic combustion of hydrogen. Catalyst A had a consistent behavior in all the operating conditions. Thereupon, the rest of the catalysts were compared with catalyst A. Catalyst B with higher loading did not improve the catalytic activity due to the blockage of the channels. Additionally, catalyst C with high cell density and thicker foil; reduced open frontal area and so resulted in augmentation of NOx levels for higher thermal power. Catalyst D without washcoat had an adverse influence on the NOx levels for higher thermal power due to a low catalytic surface area. Catalyst E made up of ceramic had an appreciable behavior in terms of thermal powers, radial temperature distribution and combustion efficiency. The residence time decreased due to the shorter length, which caused unstable behavior in few operating conditions. However, all the catalysts yielded an average thermal efficiency of about 53%. Increasing the length of catalyst had benefit neither in temperature increase nor in NOx reduction. This was due to the unavailability of adequate oxygen downstream of the first catalyst. The results from CFD computation had a good agreement with experimental results pertaining to temperature values. Also, the percentage of H2 conversion from the simulations matched substantially well with that of analytical calculations. Finally, catalyst A produced an overall least NOx of 7.5 ppm at 5 kW for f ¼ 0.3. Therefore, this catalyst is more desirable than others. However, for the future application, this catalyst is intended to be equipped in a combustion chamber, where an additional supply of air downstream of catalyst would lead to a complete conversion of the unburnt hydrogen. Ultimately, results in bringing down the emission level to ultra-low NOx.
Moreover, small turbo engines have higher fuel specific emissions compared to large engines due to their limited combustion chamber size. Therefore, the results of this study and outputs are preliminary steps towards lowering the emissions in small/model engines for aircraft industries.
Outlook The next step will be to focus on pressurized operating conditions for the above-selected catalyst. To accomplish this, an experimental set up for the same has been designed. Further, these results will be scaled up to the real micro-combustor case. And the same will be proposed for the application in aircraft engines.
Acknowledgment I would like to thank the Institute of Thermal Turbomachinery and Machine Dynamics, the Technical University of Graz for providing me an opportunity to perform the experiments. Also, I am thankful to the Austrian Research Promotion Agency for funding this research.
references
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