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High-pressure catalytic combustion of gasified biomass in a hybrid combustor
Applied Catalysis A: General 293 (2005) 129–136 www.elsevier.com/locate/apcata
High-pressure catalytic combustion of gasified biomass in a hybrid combustor J.C.G. Andrae a,*, D. Johansson a, M. Bursell a, R. Fakhrai b, J. Jayasuriya b, A. Manrique Carrera b a
Department of Chemical Engineering and Technology, Chemical Reaction Engineering, Royal Institute of Technology, Teknikringen 42, SE-100 44 Stockholm, Sweden b Department of Energy Technology, Heat and Power Technology, Royal Institute of Technology, SE-100 44 Stockholm, Sweden Received 10 February 2005; received in revised form 5 July 2005; accepted 6 July 2005 Available online 19 August 2005
Abstract Catalytic combustion of synthetic gasified biomass was conducted in a high-pressure facility at pressures ranging from 5 to 16 bars. The catalytic combustor design considered was a hybrid monolith (400 cpsi, diameter 3.5 cm, length 3.6 cm and every other channel coated). The active phase consisted of 1 wt.% Pt/g-Al2O3 with wash coat loading of total monolith 15 wt.%. In the interpretation of the experiments, a twodimensional boundary layer model was applied successfully to model a single channel of the monolith. At constant inlet velocity to the monolith the combustion efficiency decreased with increasing pressure. A multi-step surface mechanism predicted that the flux of carbon dioxide and water from the surface increased with pressure. However, as the pressure (i.e. the Reynolds number) was increased, unreacted gas near the center of the channel penetrated significantly longer into the channel compared to lower pressures. For the conditions studied (l = 4– 6, Tin = 218–257 8C and residence time 5 ms), conversion of hydrogen and carbon monoxide were diffusion limited after ignition, while methane never ignited and was kinetically controlled. According to the kinetic model surface coverage of major species changed from CO, H and CO2 before ignition to O, OH, CO2 and free surface sites after ignition. The model predicted further that for constant mass flow combustion efficiency increased with pressure, and was more pronounced at lower pressures (2.5–10 bar) than at higher pressures (>10 bar). # 2005 Elsevier B.V. All rights reserved. Keywords: Catalytic combustion; Gasified biomass; Platinum; High pressure; Catalytically stabilized combustion; CHEMKIN
1. Introduction Catalytic combustion has proven to be a suitable alternative to conventional flame combustion in gas turbines for achieving ultra-low emission levels [1,2]. Of the different design possibilities for the operation of a catalytic combustor for gas turbine applications, catalytically stabilized combustion seems to be the most promising alternative [3–6]. In catalytically stabilized combustion, the supplied fuel is partly burnt in a catalyst bed where a mixture of gaseous emissions and unburned fuel is formed, and then the subsequent combustion of the mixture is completed in a * Corresponding author. Tel.: +46 8 790 82 51; fax: +46 8 696 00 07. E-mail address: [email protected] (J.C.G. Andrae). 0926-860X/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2005.07.003
thermal combustor. Introduction of the hybrid catalytic combustor concept has also been able to overcome the material constraints associated with fully catalytic combustors to meet the higher turbine inlet temperatures. As a result of the growing concern for the effects of increasing emissions of carbon dioxide caused by the extensive use of fossil fuels, the use of biomass for power production is becoming more important [7]. Fuels from gasification of biomass are complex mixtures, consisting of carbon monoxide, hydrogen, methane, and to a minor extent ethane as combustible components along with gasification by products, carbon dioxide and steam, and nitrogen. Also present are minor amounts of N- and S-containing compounds (ammonia and hydrogen sulphide), tars, and aerosols. Ammonia may be oxidized to nitrogen oxides,
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J.C.G. Andrae et al. / Applied Catalysis A: General 293 (2005) 129–136 Table 1 Composition of gasified biomass fuel (mole fractions)
leading to increased NOx emissions, while hydrogen sulphide gives rise to SOx during the combustion and also have a poisoning effect on the catalyst by blocking active sites. One possibility to utilize gasified biomass is in gas turbine catalytic combustors, where the hybrid design may be the preferable choice. A pilot flame would not be needed as gasified biomass contains considerable amounts of hydrogen, which ignites easily on noble metals and facilitates the ignition of other fuel components [8,9]. Catalytic combustion of gasified biomass on platinum at atmospheric pressure has been studied both experimentally and numerically [9–11]. However, investigations at high pressures have not been reported much. Recently, it was shown theoretically that significant fuel–NOx reduction (>95%) could be gained in catalytically stabilized combustion of a synthetic gasified biomass (fuel–N represented by 1000 ppm ammonia) by feeding a rich mixture (l = 0.8) to the monolith where the gas ignited [12]. The major adiabatic temperature rise and NO reduction took place in the subsequent homogeneous zone where all oxygen and methane were consumed. To lower the combustion temperature and burn out remaining carbon monoxide and hydrogen, secondary air was added (to l = 2.5) downstream the reduction zone. For proper design of catalytic combustors using gasified biomass, more understanding of the influence of pressure in combustion with multi-fuel mixtures is required. In the present paper, we have conducted high pressure (5–16 bar) catalytic combustion experiments in a monolith reactor with a synthetic gasified biomass (low heating value gas with no ammonia or hydrogen sulphide added). Every other channel in the monolith is coated and the linear inlet velocity is kept constant. The experiments are interpreted with CHEMKIN simulations and the model is finally used to examine the influence of pressure for constant mass flow.
CO H2 CH4 CO2 LHV (kJ/mol) LHV (MJ/kg)
Fuel
LHV (kJ/mol)
0.332 0.240 0.106 0.322 237 9.24
283 242 802 – – –
monolith were diameter 3.5 cm and length 3.6 cm. The catalyst with platinum as active phase was prepared by the incipient wetness technique and was supported on alumina (Puralox HP-14/150, Condea). First, an aqueous solution of the metal ion was prepared, in this case from H2PtCl6 salt. The alumina powder was then impregnated with the aqueous solution, dried for 2 h at 150 8C, and reduced in hydrogen at 375 8C. The catalyst powder was mixed with ethanol, ballmilled and coated onto the monolith. Finally the monolith was calcinated at 900 8C for 1 h. The catalyst powder had a loading of 1 wt.% Pt/g-Al2O3 and the wash coat loading of the total monolith weight was 15 wt.%. 2.2. Fuel The fuel had a predefined composition according to Table 1. Mixing with nitrogen further decreased the lower heating value of the fuel (see Table 2). In order to simplify the interpretation of the results, neither steam nor minor components such as ammonia, hydrogen sulphide and tars, which normally would constitute a gasified biomass, are included in the gas. Steam in the feed has been found to moderately inhibit conversion of methane on fresh platinum catalysts supported on alumina [13]. Ammonia would ignite together with hydrogen and carbon monoxide [11] with small further influence on the combustion. Obviously the oxidation of ammonia on platinum under fuel lean conditions would form significant amounts of fuel–NOx, one of the main hurdles with gasified biomass as fuel. Sulphur compounds (e.g. hydrogen sulphide) would give rise to SOx during the combustion and generally act as a poison for a platinum catalyst by blocking active sites. However, in opposite to hydrogen and carbon monoxide for which the oxidation is sensitive to sulphur in the feed [14,15], the conversion of methane seems to be improved by sulphur, and could be attributed to the coexistence of sulphates on alumina near adsorbed O-atoms on platinum,
2. Experimental 2.1. Catalyst preparation To represent a monolith segment in a catalytically stabilized combustor, a cordierite monolith (400 cpsi, Corning) was designed to have half of the amount of channels coated. In order to achieve this, all channels were first blocked in one end of the monolith by paraffinic dipping, where after an opening of every other channel was conducted by mechanical aids. The dimensions of the Table 2 Experimental conditions for the hybrid catalytic combustor Case
m ˙ Fuel (g/s)
m ˙ N2 (g/s)
m ˙ Air (g/s)
l
p (bar)
Tin (8C)
uin (cm/s)
Red
1 2 3 4 5
0.809 1.09 1.62 2.42 2.40
0.411 0.554 0.821 1.22 1.22
13.0 13.0 26.0 25.9 38.9
6.04 4.47 6.02 4.02 6.07
5.19 5.39 10.7 11.6 16.2
231 227 232 218 257
565 558 545 515 567
821 854 1634 1745 2361
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pressure air compressor (piston type, 40 bar and 100 g/s), compressed air storage tank (1 m3) and air filtering system. The pressure and the mass flow rate into the combustor are set by a pressure regulator and a motorized flow control valve installed at the inlet of the pressure vessel. The pressure inside the combustor is adjusted by the back pressure valve situated at the exit of the combustor. A group of electrical heaters are placed in the pressure vessel to preheat the compressed air uniformly. Programmable temperature controllers control pre-heating air temperature and that is set as an independent variable of the test facility. The fuel is supplied in gas bottles and fuel compositions and quantities are controlled by set of mass flow controllers. Overall operations are monitored and controlled through a computer-based software control system. Measurements are acquired by the software and saved in the computer at 1 Hz frequency. More information of the high-pressure facility can be found in [21]. 2.4. Experimental procedure
Fig. 1. Catalytic combustion high-pressure test facility.
which acts as new sites for methane oxidation [16,17]. For the small amounts of hydrogen sulphide present in gasified biomass [7], zinc oxide sorbents [18] or adsorption and catalytic decomposition over active carbon impregnated with iron [19] may be used as gas cleaning methods before the catalytic combustor. For tars and particles, resent results show that the heavy tar naphtalene may be removed to a significant extent by a Ni/CaO catalyst in the pores of a catalytic candle filter in the presence of hydrogen sulphide up to 200 ppm [20].
Five different experimental cases with pressure range of 5–16 bars were considered (see Table 2). The compressed air flow, pressure and inlet temperature were set as independent variables of the experiment. Based on the air flow rates set into the combustor, the fuel requirement was calculated for required excess air–fuel ratio (l). The calculated fuel rate was executed through mass flow controllers. Temperature at varies locations along the combustor including surface temperatures in the active and passive channels of the monolith were recorded. Temperatures inside the test facility were measured by thermocouples. The choice of thermo-
2.3. Test facility Fig. 1 shows a schematic design of the high-pressure combustion test facility. The test facility is designed to operate at any required pressure within the range of 1– 35 bars. The catalytic combustor, fuel injection, mixing system and electrical heaters are located inside a cylindrical pressure vessel of diameter 300 mm and height 3 m. The size of the catalytic combustor is diameter 35 mm and 500 mm height that represents the ‘‘canular’’ combustor shape. The combustor section is well insulated in order to achieve nearly adiabatic conditions. The test facility is supplied with compressed air from an external air supply system. The air supply system has a high-
Fig. 2. Example of temperature curves from the experiments. Case 3 and Case 4 in Table 2.
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couples for the temperature measurements was N type. Thermocouples with two different sizes in diameter (1.5 mm and 0.8 mm) were mounted. The bigger diameter thermocouples were placed for the measurements of bulk temperatures at inlet and outlet of the monolith while the smaller once were for the surface temperature measurements. The placement of smaller thermocouples (at one centimeter from the downstream end of the monolith) was done by cementing the thermocouple into the channel and the particular channel was blocked with cement at both ends. Fig. 2 gives an example of measured temperature profiles as a function of time. As it is seen, in Case 3 (first temperature rise) and Case 4 (second temperature rise) stepwise temperature rise was observed with the change of fuel concentration. The wall temperature is higher in the active channels than in the passive channels and heat is transferred from the former to the latter. Table 3 shows the results of the temperature measurements (average values) for Case 1–5. Tout and Tin is the bulk temperature at the inlet and outlet, respectively. The combustion efficiency, hcomb, is defined as the ratio of the actual temperature difference over the monolith section (Tout Tin) and the theoretical adiabatic temperature rise (Tad Tin), where Tad is the adiabatic flame temperature. Tact and Tpas is the measured temperature in the active and passive channels, respectively. A trend in the data is that the temperature difference between the active and passive channels increases with pressure and is higher at lower lvalues. This would be explained by the amount of fuel that has to be heated increases with pressure, thereby lowering the temperature in the passive channels. However, the reaction rate and temperature in the active channels would increase with pressure. This is clearly seen for Case 2 and 4 (l = 4) in Table 3. 2.5. Modeling Fig. 3 shows a sketch of single channel in the monolith. To model the flow in the single channel in the monolith, a parabolic, two-dimensional, steady state model was used. This was based on the high but laminar Reynolds numbers for Case 1–5 (see Table 2). In the boundary layer assumption, at high flow rates (i.e. high Reynolds number) the axial diffusive transport is diminished in comparison to the radial diffusion and the convective transport. Results gained by a boundary layer model can be as accurate as the results from a full Navier–Stokes model at high but laminar flow rates [22]. The radial mesh from symmetry line to Table 3 Measured temperatures in the monolith (8C) Case
Tin
Tout
Tad
hcomb
Tact
Tpas
(Tact + Tpas)/2
1 2 3 4 5
231 227 232 218 257
439 522 384 485 392
702 833 705 880 725
0.44 0.49 0.32 0.40 0.29
491 597 485 688 532
485 584 450 575 462
488 590 468 631 497
Fig. 3. Dimension of an individual channel in the monolith.
channel wall was discretized into 50 mesh points (enough to get mesh independent solutions) with the mesh points concentrated near the wall. The numerical solution was accomplished with CHEMKIN [23] assuming a cylindrical shape of the channel. For interpretation of experiments in terms of surface chemistry, a detailed multi-step mechanism was used. There are some developed models in the literature for the oxidation of multi-fuels on platinum [24–28]. The most recent is the scheme from Vlachos and co-workers [27,28], which we employed in this work. The site density was set to 2.49 10 9 mol/cm2, resembling a Pt (1 1 1) crystal. Although developed for polycrystalline platinum, the high reactivity of this mechanism was recognized for catalytically stabilized combustion of lean methane/air mixtures on polycrystalline platinum walls [29]. However, in this work a multi-component fuel (including hydrogen) was combusted on a supported platinum catalyst for which a higher reactivity could be expected compared to polycrystalline surfaces. Conducting simulations with and without gas phase reactions from [30] included revealed that gas phase reactions had no impact on the conversion of the fuel. Moreover, adsorption of CH2, CH and C on the platinum surface was neglected as the concentration of these species in the gas phase was very low.Other important model assumptions were The temperature at the channel wall was taken as the average of the measured temperature in the active and passive channel (see Table 3). This semi-empirical approach means that the resulting heat transfer between the active and passive channels would be embedded in the calculated wall temperature. For each Case, the average wall temperature above was reached in 1 mm through a temperature ramp from the inlet temperature (surface ignition near the entrance of the channel). A flat velocity profile at inlet to monolith was used. Backward heat transfer by wall conduction was neglected for a ceramic monolith [31]. For governing equations and boundary conditions (see [12,32]). The inlet gas composition to an individual channel was calculated with the gas mixer model in CHEMKIN, and the inlet velocity by dividing the volume flow with the flow area (blockage for monolith is 24%).
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Fig. 4. Experimental and modeled combustion efficiency for Case 1–5 in Table 2. The prediction by the boundary layer model agrees well with the experimental results.
3. Results and discussion 3.1. Model validation Fig. 4 shows the experimental combustion efficiencies as a function of pressure from Table 3 together with the predictions made by the boundary-layer model. As can be seen the model predicts with good agreement the trends found in the experimental results, which implies that the boundary-layer equations are a very good approximation to the full Navier–Stokes equations for the conditions studied. Also, the other assumptions for the model seem well taken. Fig. 5 shows predicted conversion of individual fuel components and bulk temperature for Case 1–5 as a function of the axial distance in the monolith. Realistic conversion of each fuel component is achieved with hydrogen being the most
Fig. 5. Model predictions of conversion of individual fuel components and bulk temperature for Case 1–5 in Table 2.
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Fig. 6. Calculated residence time for Case 1–5 in Table 2.
active fuel component. Methane would not react much under the current conditions [33], correctly predicted by the model. The fact that the combustion efficiency decreases with pressure is in some sense what could be expected. The species and thermal boundary layers inhibit mass transfer between the gas and the surface and the diffusion coefficient is a function of both temperature and pressure. Diffusion increases with increasing temperature as molecules move more rapidly, and decreases with increasing pressure (which packs more molecules in a given volume), making it harder for them to move. For the conditions studied the linear inlet velocity is held constant, which means that the mean residence time is not affected significantly (see Fig. 6). For higher flow rates (i.e. Reynolds numbers) unreacted gas near the centerline (i.e. the symmetry axis in Fig. 3) may penetrate far into the
Fig. 7. Calculated mole fraction of hydrogen (panel a) and carbon monoxide (panel b) at the wall and centerline of channel for Case 1 (solid line), Case 3 (dotted line) and Case 5 (dashed line). The conversion of hydrogen and carbon monoxide is diffusion limited after ignition.
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Fig. 10. Site fractions for major surface species. Case 3.
Fig. 8. Calculated flux (mol cm 2 s 1) of carbon dioxide (panel a) and water (panel b) from the platinum surface for Case 1, 3 and 5 as a function of axial position in the monolith.
channel. This is illustrated in Fig. 7, which shows the mole fraction of hydrogen and carbon monoxide for Case 1, 3 and 5 at both the centerline and at the wall as a function of axial position in the monolith. At the centerline there is significant difference in concentration, with more unreacted gas at the centerline at higher pressure. However, at the wall the concentration drops quickly to zero for all cases at the ignition point at an axial distance around 1 mm, indicating a mass transfer limited process after ignition. Although favoured at higher pressure, decreased combustion efficiency would still be negative for the homogeneous reactions after the monolith as the inlet temperature to the homogeneous section decreases. A decreased conversion when increasing the pressure (constant linear velocity) has also been seen in catalytic combustion studies with methane as fuel [26,34,35]. Fig. 8 shows the wall flux of carbon dioxide and water for Case 1, 3, and 5. As the pressure increase the concentration and thereby the reaction rate increase. However, the
increased flux with increased pressure is not enough to compensate for the increased mass transfer limitation. The model further predicts that Case 5 ignites earlier than Case 3 as both inlet and average wall temperature is higher. Figs. 9–11 show the surface coverage for major surface species (defined here as a site fraction larger that 0.01) for Case 1, 3, and 5. In the first regime before ignition, the surface reactions are kinetically controlled, and the surface reaction rate is monotonically increasing with the availability of free surface sites, PT(s). Before the ignition point, the surface is to a large extent covered by carbon monoxide, CO(s), hydrogen atoms, H(s), and carbon dioxide, CO2(s). After ignition of hydrogen and carbon monoxide, the surface is covered by atomic oxygen, O(s), CO2(s), hydroxyl, OH(s), and free surface sites. When the averaged wall temperature is reached at 1 mm, the process is under the influence of mass transport. It should be noted that the fraction of free sites is much higher for Case 1 compared to Case 3 and 5. As the pressure increase the partial pressure and adsorption rate of hydrogen and carbon monoxide increase. This will lead to a higher coverage of CO2(s), and OH(s) but also lower coverage of free sites after ignition. The negative pressure dependence for the availability of free surface sites would restrain the overall reaction rate [29] and the kinetic model used in this work [27,28] is able to predict this trend. 3.2. Effect of pressure for constant mass flow
Fig. 9. Site fractions for major surface species. Case 1.
It has been shown that for methane/air mixtures (i.e. simulated natural gas) that while keeping the mass flow constant, the conversion increases with pressure [6,29]. Therefore the effect of pressure at constant mass flow on the combustion efficiency was also investigated with the synthetic gasified biomass fuel used in this work. No experiments were done for constant mass flow. However, a calculation with the assumption that every channel is coated may be conducted to estimate the wall temperature profile in a hybrid monolith [12]. Doing these calculations for Case 1 and Case 3 and keeping the mass flow
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Fig. 11. Site fractions for major surface species. Case 5.
constant (by decreasing the inlet velocity) super-adiabatic wall temperatures were achieved and the super-adiabatic temperature increased somewhat with pressure. The temperature increase was more significant for lower pressures (2.5– 7.5 bar). In catalytic combustion of fuel–lean mixtures of hydrogen, the catalyst may reach super-adiabatic temperatures because of the low Lewis number of the deficient fuel [36]. Based on these findings and the temperatures gained in the experiments above for constant inlet velocities, wall temperatures were estimated for each pressure. The conditions for the calculations are shown in Table 4 and the predicted combustion efficiencies are shown Fig. 12. The combustion efficiency increase with pressure, the increment more significant up to around 10 bar. The results are in agreement with the results for methane as fuel [6,29]. An increased concentration with pressure leads to a higher reaction rate and wall temperature. The increased surface reaction rate is balanced by the negative dependence in surface free site coverage with increasing pressure. Moreover, increased residence times at higher pressures compensates for the decreasing diffusion velocities (increasing mass transfer limitations). For pressures over 10 bar the combustion efficiency seems to be more independent of pressure. This has to be confirmed by experiments but may be important in combustor design. If the combustion efficiency would be independent of pressure, mass transfer limitations cannot be compensated for by increased residence time in the monolith, and a maximum power for a certain inlet temperature exists.
Fig. 12. Predicted combustion efficiency at constant mass flow for Case 1 and Case 3. See Table 4 for calculation conditions.
4. Conclusions High-pressure catalytic combustion experiments with synthetic biomass were conducted in a monolith reactor with every other channel coated and constant linear inlet velocity. Interpretation of the experiments with a two-dimensional boundary layer model coupled to a multi-step surface mechanism could predict with good accuracy that the combustion efficiency decreased with increasing pressure. Increased surface reaction rate with increased pressure cannot compensate for the increased mass transfer limitations. The results for constant mass flow indicate that a decrease in combustion efficiency can be compensated for by increased residence time in the monolith. However, that effect is only seen at lower pressures.
Acknowledgments The authors thank Professor Pehr Bjo¨rnbom and Katarina Persson for their advice. They also wish to thank Professor Torsten Fransson and Jan Fredriksson for making the highpressure catalytic combustion facility available for this research. The work was financially supported by the Swedish Energy Agency under Contract No. 21529-1.
References Table 4 Conditions for constant mass flow calculations p (bar)
uin, Case 1 (cm/s)
uin, Case 3 (cm/s)
Twall (8C)
2.5 5.0 7.5 10.0 12.5 15.0
1173 587 391 293 235 196
2343 1171 781 586 469 390
479.0 486.7 492.0 495.0 497.0 498.5
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High-pressure catalytic combustion of gasified biomass in a hybrid combustor J.C.G. Andrae a,*, D. Johansson a, M. Bursell a, R. Fakhrai b, J. Jayasuriya b, A. Manrique Carrera b a
Department of Chemical Engineering and Technology, Chemical Reaction Engineering, Royal Institute of Technology, Teknikringen 42, SE-100 44 Stockholm, Sweden b Department of Energy Technology, Heat and Power Technology, Royal Institute of Technology, SE-100 44 Stockholm, Sweden Received 10 February 2005; received in revised form 5 July 2005; accepted 6 July 2005 Available online 19 August 2005
Abstract Catalytic combustion of synthetic gasified biomass was conducted in a high-pressure facility at pressures ranging from 5 to 16 bars. The catalytic combustor design considered was a hybrid monolith (400 cpsi, diameter 3.5 cm, length 3.6 cm and every other channel coated). The active phase consisted of 1 wt.% Pt/g-Al2O3 with wash coat loading of total monolith 15 wt.%. In the interpretation of the experiments, a twodimensional boundary layer model was applied successfully to model a single channel of the monolith. At constant inlet velocity to the monolith the combustion efficiency decreased with increasing pressure. A multi-step surface mechanism predicted that the flux of carbon dioxide and water from the surface increased with pressure. However, as the pressure (i.e. the Reynolds number) was increased, unreacted gas near the center of the channel penetrated significantly longer into the channel compared to lower pressures. For the conditions studied (l = 4– 6, Tin = 218–257 8C and residence time 5 ms), conversion of hydrogen and carbon monoxide were diffusion limited after ignition, while methane never ignited and was kinetically controlled. According to the kinetic model surface coverage of major species changed from CO, H and CO2 before ignition to O, OH, CO2 and free surface sites after ignition. The model predicted further that for constant mass flow combustion efficiency increased with pressure, and was more pronounced at lower pressures (2.5–10 bar) than at higher pressures (>10 bar). # 2005 Elsevier B.V. All rights reserved. Keywords: Catalytic combustion; Gasified biomass; Platinum; High pressure; Catalytically stabilized combustion; CHEMKIN
1. Introduction Catalytic combustion has proven to be a suitable alternative to conventional flame combustion in gas turbines for achieving ultra-low emission levels [1,2]. Of the different design possibilities for the operation of a catalytic combustor for gas turbine applications, catalytically stabilized combustion seems to be the most promising alternative [3–6]. In catalytically stabilized combustion, the supplied fuel is partly burnt in a catalyst bed where a mixture of gaseous emissions and unburned fuel is formed, and then the subsequent combustion of the mixture is completed in a * Corresponding author. Tel.: +46 8 790 82 51; fax: +46 8 696 00 07. E-mail address: [email protected] (J.C.G. Andrae). 0926-860X/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2005.07.003
thermal combustor. Introduction of the hybrid catalytic combustor concept has also been able to overcome the material constraints associated with fully catalytic combustors to meet the higher turbine inlet temperatures. As a result of the growing concern for the effects of increasing emissions of carbon dioxide caused by the extensive use of fossil fuels, the use of biomass for power production is becoming more important [7]. Fuels from gasification of biomass are complex mixtures, consisting of carbon monoxide, hydrogen, methane, and to a minor extent ethane as combustible components along with gasification by products, carbon dioxide and steam, and nitrogen. Also present are minor amounts of N- and S-containing compounds (ammonia and hydrogen sulphide), tars, and aerosols. Ammonia may be oxidized to nitrogen oxides,
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leading to increased NOx emissions, while hydrogen sulphide gives rise to SOx during the combustion and also have a poisoning effect on the catalyst by blocking active sites. One possibility to utilize gasified biomass is in gas turbine catalytic combustors, where the hybrid design may be the preferable choice. A pilot flame would not be needed as gasified biomass contains considerable amounts of hydrogen, which ignites easily on noble metals and facilitates the ignition of other fuel components [8,9]. Catalytic combustion of gasified biomass on platinum at atmospheric pressure has been studied both experimentally and numerically [9–11]. However, investigations at high pressures have not been reported much. Recently, it was shown theoretically that significant fuel–NOx reduction (>95%) could be gained in catalytically stabilized combustion of a synthetic gasified biomass (fuel–N represented by 1000 ppm ammonia) by feeding a rich mixture (l = 0.8) to the monolith where the gas ignited [12]. The major adiabatic temperature rise and NO reduction took place in the subsequent homogeneous zone where all oxygen and methane were consumed. To lower the combustion temperature and burn out remaining carbon monoxide and hydrogen, secondary air was added (to l = 2.5) downstream the reduction zone. For proper design of catalytic combustors using gasified biomass, more understanding of the influence of pressure in combustion with multi-fuel mixtures is required. In the present paper, we have conducted high pressure (5–16 bar) catalytic combustion experiments in a monolith reactor with a synthetic gasified biomass (low heating value gas with no ammonia or hydrogen sulphide added). Every other channel in the monolith is coated and the linear inlet velocity is kept constant. The experiments are interpreted with CHEMKIN simulations and the model is finally used to examine the influence of pressure for constant mass flow.
CO H2 CH4 CO2 LHV (kJ/mol) LHV (MJ/kg)
Fuel
LHV (kJ/mol)
0.332 0.240 0.106 0.322 237 9.24
283 242 802 – – –
monolith were diameter 3.5 cm and length 3.6 cm. The catalyst with platinum as active phase was prepared by the incipient wetness technique and was supported on alumina (Puralox HP-14/150, Condea). First, an aqueous solution of the metal ion was prepared, in this case from H2PtCl6 salt. The alumina powder was then impregnated with the aqueous solution, dried for 2 h at 150 8C, and reduced in hydrogen at 375 8C. The catalyst powder was mixed with ethanol, ballmilled and coated onto the monolith. Finally the monolith was calcinated at 900 8C for 1 h. The catalyst powder had a loading of 1 wt.% Pt/g-Al2O3 and the wash coat loading of the total monolith weight was 15 wt.%. 2.2. Fuel The fuel had a predefined composition according to Table 1. Mixing with nitrogen further decreased the lower heating value of the fuel (see Table 2). In order to simplify the interpretation of the results, neither steam nor minor components such as ammonia, hydrogen sulphide and tars, which normally would constitute a gasified biomass, are included in the gas. Steam in the feed has been found to moderately inhibit conversion of methane on fresh platinum catalysts supported on alumina [13]. Ammonia would ignite together with hydrogen and carbon monoxide [11] with small further influence on the combustion. Obviously the oxidation of ammonia on platinum under fuel lean conditions would form significant amounts of fuel–NOx, one of the main hurdles with gasified biomass as fuel. Sulphur compounds (e.g. hydrogen sulphide) would give rise to SOx during the combustion and generally act as a poison for a platinum catalyst by blocking active sites. However, in opposite to hydrogen and carbon monoxide for which the oxidation is sensitive to sulphur in the feed [14,15], the conversion of methane seems to be improved by sulphur, and could be attributed to the coexistence of sulphates on alumina near adsorbed O-atoms on platinum,
2. Experimental 2.1. Catalyst preparation To represent a monolith segment in a catalytically stabilized combustor, a cordierite monolith (400 cpsi, Corning) was designed to have half of the amount of channels coated. In order to achieve this, all channels were first blocked in one end of the monolith by paraffinic dipping, where after an opening of every other channel was conducted by mechanical aids. The dimensions of the Table 2 Experimental conditions for the hybrid catalytic combustor Case
m ˙ Fuel (g/s)
m ˙ N2 (g/s)
m ˙ Air (g/s)
l
p (bar)
Tin (8C)
uin (cm/s)
Red
1 2 3 4 5
0.809 1.09 1.62 2.42 2.40
0.411 0.554 0.821 1.22 1.22
13.0 13.0 26.0 25.9 38.9
6.04 4.47 6.02 4.02 6.07
5.19 5.39 10.7 11.6 16.2
231 227 232 218 257
565 558 545 515 567
821 854 1634 1745 2361
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pressure air compressor (piston type, 40 bar and 100 g/s), compressed air storage tank (1 m3) and air filtering system. The pressure and the mass flow rate into the combustor are set by a pressure regulator and a motorized flow control valve installed at the inlet of the pressure vessel. The pressure inside the combustor is adjusted by the back pressure valve situated at the exit of the combustor. A group of electrical heaters are placed in the pressure vessel to preheat the compressed air uniformly. Programmable temperature controllers control pre-heating air temperature and that is set as an independent variable of the test facility. The fuel is supplied in gas bottles and fuel compositions and quantities are controlled by set of mass flow controllers. Overall operations are monitored and controlled through a computer-based software control system. Measurements are acquired by the software and saved in the computer at 1 Hz frequency. More information of the high-pressure facility can be found in [21]. 2.4. Experimental procedure
Fig. 1. Catalytic combustion high-pressure test facility.
which acts as new sites for methane oxidation [16,17]. For the small amounts of hydrogen sulphide present in gasified biomass [7], zinc oxide sorbents [18] or adsorption and catalytic decomposition over active carbon impregnated with iron [19] may be used as gas cleaning methods before the catalytic combustor. For tars and particles, resent results show that the heavy tar naphtalene may be removed to a significant extent by a Ni/CaO catalyst in the pores of a catalytic candle filter in the presence of hydrogen sulphide up to 200 ppm [20].
Five different experimental cases with pressure range of 5–16 bars were considered (see Table 2). The compressed air flow, pressure and inlet temperature were set as independent variables of the experiment. Based on the air flow rates set into the combustor, the fuel requirement was calculated for required excess air–fuel ratio (l). The calculated fuel rate was executed through mass flow controllers. Temperature at varies locations along the combustor including surface temperatures in the active and passive channels of the monolith were recorded. Temperatures inside the test facility were measured by thermocouples. The choice of thermo-
2.3. Test facility Fig. 1 shows a schematic design of the high-pressure combustion test facility. The test facility is designed to operate at any required pressure within the range of 1– 35 bars. The catalytic combustor, fuel injection, mixing system and electrical heaters are located inside a cylindrical pressure vessel of diameter 300 mm and height 3 m. The size of the catalytic combustor is diameter 35 mm and 500 mm height that represents the ‘‘canular’’ combustor shape. The combustor section is well insulated in order to achieve nearly adiabatic conditions. The test facility is supplied with compressed air from an external air supply system. The air supply system has a high-
Fig. 2. Example of temperature curves from the experiments. Case 3 and Case 4 in Table 2.
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couples for the temperature measurements was N type. Thermocouples with two different sizes in diameter (1.5 mm and 0.8 mm) were mounted. The bigger diameter thermocouples were placed for the measurements of bulk temperatures at inlet and outlet of the monolith while the smaller once were for the surface temperature measurements. The placement of smaller thermocouples (at one centimeter from the downstream end of the monolith) was done by cementing the thermocouple into the channel and the particular channel was blocked with cement at both ends. Fig. 2 gives an example of measured temperature profiles as a function of time. As it is seen, in Case 3 (first temperature rise) and Case 4 (second temperature rise) stepwise temperature rise was observed with the change of fuel concentration. The wall temperature is higher in the active channels than in the passive channels and heat is transferred from the former to the latter. Table 3 shows the results of the temperature measurements (average values) for Case 1–5. Tout and Tin is the bulk temperature at the inlet and outlet, respectively. The combustion efficiency, hcomb, is defined as the ratio of the actual temperature difference over the monolith section (Tout Tin) and the theoretical adiabatic temperature rise (Tad Tin), where Tad is the adiabatic flame temperature. Tact and Tpas is the measured temperature in the active and passive channels, respectively. A trend in the data is that the temperature difference between the active and passive channels increases with pressure and is higher at lower lvalues. This would be explained by the amount of fuel that has to be heated increases with pressure, thereby lowering the temperature in the passive channels. However, the reaction rate and temperature in the active channels would increase with pressure. This is clearly seen for Case 2 and 4 (l = 4) in Table 3. 2.5. Modeling Fig. 3 shows a sketch of single channel in the monolith. To model the flow in the single channel in the monolith, a parabolic, two-dimensional, steady state model was used. This was based on the high but laminar Reynolds numbers for Case 1–5 (see Table 2). In the boundary layer assumption, at high flow rates (i.e. high Reynolds number) the axial diffusive transport is diminished in comparison to the radial diffusion and the convective transport. Results gained by a boundary layer model can be as accurate as the results from a full Navier–Stokes model at high but laminar flow rates [22]. The radial mesh from symmetry line to Table 3 Measured temperatures in the monolith (8C) Case
Tin
Tout
Tad
hcomb
Tact
Tpas
(Tact + Tpas)/2
1 2 3 4 5
231 227 232 218 257
439 522 384 485 392
702 833 705 880 725
0.44 0.49 0.32 0.40 0.29
491 597 485 688 532
485 584 450 575 462
488 590 468 631 497
Fig. 3. Dimension of an individual channel in the monolith.
channel wall was discretized into 50 mesh points (enough to get mesh independent solutions) with the mesh points concentrated near the wall. The numerical solution was accomplished with CHEMKIN [23] assuming a cylindrical shape of the channel. For interpretation of experiments in terms of surface chemistry, a detailed multi-step mechanism was used. There are some developed models in the literature for the oxidation of multi-fuels on platinum [24–28]. The most recent is the scheme from Vlachos and co-workers [27,28], which we employed in this work. The site density was set to 2.49 10 9 mol/cm2, resembling a Pt (1 1 1) crystal. Although developed for polycrystalline platinum, the high reactivity of this mechanism was recognized for catalytically stabilized combustion of lean methane/air mixtures on polycrystalline platinum walls [29]. However, in this work a multi-component fuel (including hydrogen) was combusted on a supported platinum catalyst for which a higher reactivity could be expected compared to polycrystalline surfaces. Conducting simulations with and without gas phase reactions from [30] included revealed that gas phase reactions had no impact on the conversion of the fuel. Moreover, adsorption of CH2, CH and C on the platinum surface was neglected as the concentration of these species in the gas phase was very low.Other important model assumptions were The temperature at the channel wall was taken as the average of the measured temperature in the active and passive channel (see Table 3). This semi-empirical approach means that the resulting heat transfer between the active and passive channels would be embedded in the calculated wall temperature. For each Case, the average wall temperature above was reached in 1 mm through a temperature ramp from the inlet temperature (surface ignition near the entrance of the channel). A flat velocity profile at inlet to monolith was used. Backward heat transfer by wall conduction was neglected for a ceramic monolith [31]. For governing equations and boundary conditions (see [12,32]). The inlet gas composition to an individual channel was calculated with the gas mixer model in CHEMKIN, and the inlet velocity by dividing the volume flow with the flow area (blockage for monolith is 24%).
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Fig. 4. Experimental and modeled combustion efficiency for Case 1–5 in Table 2. The prediction by the boundary layer model agrees well with the experimental results.
3. Results and discussion 3.1. Model validation Fig. 4 shows the experimental combustion efficiencies as a function of pressure from Table 3 together with the predictions made by the boundary-layer model. As can be seen the model predicts with good agreement the trends found in the experimental results, which implies that the boundary-layer equations are a very good approximation to the full Navier–Stokes equations for the conditions studied. Also, the other assumptions for the model seem well taken. Fig. 5 shows predicted conversion of individual fuel components and bulk temperature for Case 1–5 as a function of the axial distance in the monolith. Realistic conversion of each fuel component is achieved with hydrogen being the most
Fig. 5. Model predictions of conversion of individual fuel components and bulk temperature for Case 1–5 in Table 2.
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Fig. 6. Calculated residence time for Case 1–5 in Table 2.
active fuel component. Methane would not react much under the current conditions [33], correctly predicted by the model. The fact that the combustion efficiency decreases with pressure is in some sense what could be expected. The species and thermal boundary layers inhibit mass transfer between the gas and the surface and the diffusion coefficient is a function of both temperature and pressure. Diffusion increases with increasing temperature as molecules move more rapidly, and decreases with increasing pressure (which packs more molecules in a given volume), making it harder for them to move. For the conditions studied the linear inlet velocity is held constant, which means that the mean residence time is not affected significantly (see Fig. 6). For higher flow rates (i.e. Reynolds numbers) unreacted gas near the centerline (i.e. the symmetry axis in Fig. 3) may penetrate far into the
Fig. 7. Calculated mole fraction of hydrogen (panel a) and carbon monoxide (panel b) at the wall and centerline of channel for Case 1 (solid line), Case 3 (dotted line) and Case 5 (dashed line). The conversion of hydrogen and carbon monoxide is diffusion limited after ignition.
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Fig. 10. Site fractions for major surface species. Case 3.
Fig. 8. Calculated flux (mol cm 2 s 1) of carbon dioxide (panel a) and water (panel b) from the platinum surface for Case 1, 3 and 5 as a function of axial position in the monolith.
channel. This is illustrated in Fig. 7, which shows the mole fraction of hydrogen and carbon monoxide for Case 1, 3 and 5 at both the centerline and at the wall as a function of axial position in the monolith. At the centerline there is significant difference in concentration, with more unreacted gas at the centerline at higher pressure. However, at the wall the concentration drops quickly to zero for all cases at the ignition point at an axial distance around 1 mm, indicating a mass transfer limited process after ignition. Although favoured at higher pressure, decreased combustion efficiency would still be negative for the homogeneous reactions after the monolith as the inlet temperature to the homogeneous section decreases. A decreased conversion when increasing the pressure (constant linear velocity) has also been seen in catalytic combustion studies with methane as fuel [26,34,35]. Fig. 8 shows the wall flux of carbon dioxide and water for Case 1, 3, and 5. As the pressure increase the concentration and thereby the reaction rate increase. However, the
increased flux with increased pressure is not enough to compensate for the increased mass transfer limitation. The model further predicts that Case 5 ignites earlier than Case 3 as both inlet and average wall temperature is higher. Figs. 9–11 show the surface coverage for major surface species (defined here as a site fraction larger that 0.01) for Case 1, 3, and 5. In the first regime before ignition, the surface reactions are kinetically controlled, and the surface reaction rate is monotonically increasing with the availability of free surface sites, PT(s). Before the ignition point, the surface is to a large extent covered by carbon monoxide, CO(s), hydrogen atoms, H(s), and carbon dioxide, CO2(s). After ignition of hydrogen and carbon monoxide, the surface is covered by atomic oxygen, O(s), CO2(s), hydroxyl, OH(s), and free surface sites. When the averaged wall temperature is reached at 1 mm, the process is under the influence of mass transport. It should be noted that the fraction of free sites is much higher for Case 1 compared to Case 3 and 5. As the pressure increase the partial pressure and adsorption rate of hydrogen and carbon monoxide increase. This will lead to a higher coverage of CO2(s), and OH(s) but also lower coverage of free sites after ignition. The negative pressure dependence for the availability of free surface sites would restrain the overall reaction rate [29] and the kinetic model used in this work [27,28] is able to predict this trend. 3.2. Effect of pressure for constant mass flow
Fig. 9. Site fractions for major surface species. Case 1.
It has been shown that for methane/air mixtures (i.e. simulated natural gas) that while keeping the mass flow constant, the conversion increases with pressure [6,29]. Therefore the effect of pressure at constant mass flow on the combustion efficiency was also investigated with the synthetic gasified biomass fuel used in this work. No experiments were done for constant mass flow. However, a calculation with the assumption that every channel is coated may be conducted to estimate the wall temperature profile in a hybrid monolith [12]. Doing these calculations for Case 1 and Case 3 and keeping the mass flow
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Fig. 11. Site fractions for major surface species. Case 5.
constant (by decreasing the inlet velocity) super-adiabatic wall temperatures were achieved and the super-adiabatic temperature increased somewhat with pressure. The temperature increase was more significant for lower pressures (2.5– 7.5 bar). In catalytic combustion of fuel–lean mixtures of hydrogen, the catalyst may reach super-adiabatic temperatures because of the low Lewis number of the deficient fuel [36]. Based on these findings and the temperatures gained in the experiments above for constant inlet velocities, wall temperatures were estimated for each pressure. The conditions for the calculations are shown in Table 4 and the predicted combustion efficiencies are shown Fig. 12. The combustion efficiency increase with pressure, the increment more significant up to around 10 bar. The results are in agreement with the results for methane as fuel [6,29]. An increased concentration with pressure leads to a higher reaction rate and wall temperature. The increased surface reaction rate is balanced by the negative dependence in surface free site coverage with increasing pressure. Moreover, increased residence times at higher pressures compensates for the decreasing diffusion velocities (increasing mass transfer limitations). For pressures over 10 bar the combustion efficiency seems to be more independent of pressure. This has to be confirmed by experiments but may be important in combustor design. If the combustion efficiency would be independent of pressure, mass transfer limitations cannot be compensated for by increased residence time in the monolith, and a maximum power for a certain inlet temperature exists.
Fig. 12. Predicted combustion efficiency at constant mass flow for Case 1 and Case 3. See Table 4 for calculation conditions.
4. Conclusions High-pressure catalytic combustion experiments with synthetic biomass were conducted in a monolith reactor with every other channel coated and constant linear inlet velocity. Interpretation of the experiments with a two-dimensional boundary layer model coupled to a multi-step surface mechanism could predict with good accuracy that the combustion efficiency decreased with increasing pressure. Increased surface reaction rate with increased pressure cannot compensate for the increased mass transfer limitations. The results for constant mass flow indicate that a decrease in combustion efficiency can be compensated for by increased residence time in the monolith. However, that effect is only seen at lower pressures.
Acknowledgments The authors thank Professor Pehr Bjo¨rnbom and Katarina Persson for their advice. They also wish to thank Professor Torsten Fransson and Jan Fredriksson for making the highpressure catalytic combustion facility available for this research. The work was financially supported by the Swedish Energy Agency under Contract No. 21529-1.
References Table 4 Conditions for constant mass flow calculations p (bar)
uin, Case 1 (cm/s)
uin, Case 3 (cm/s)
Twall (8C)
2.5 5.0 7.5 10.0 12.5 15.0
1173 587 391 293 235 196
2343 1171 781 586 469 390
479.0 486.7 492.0 495.0 497.0 498.5
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