Hydrogen Production from Methanol and Ethanol Partial Oxidation

Article pubs.acs.org/EF

Hydrogen Production from Methanol and Ethanol Partial Oxidation Mario Toledo,*,† Freddy González,† and Janet Ellzey‡ †

Department of Mechanical Engineering, Universidad Técnica Federico Santa María, Avenida España 1680, 2340000 Valparaiso, Chile ‡ Department of Mechanical Engineering, University of Texas at Austin, Austin, Texas 78712, United States ABSTRACT: This paper examines rich and ultrarich combustion of methanol and ethanol in a porous media reactor with an external heat exchanger to vaporize liquid fuels and recover combustion energy. Temperature, velocities, and chemical products of the combustion waves were recorded experimentally at a range of equivalence ratios from ϕ = 1.6 to 8.2 and oxygen content in the oxidizer from 10 to 23%. Experimental results show that the reactor can handle ultrarich combustion up to an equivalence ratio of 8.2 and obtained maximum conversion of hydrogen of 43 and 30% for methanol and ethanol, respectively. A high oxygen content in the oxidizer results in higher hydrogen and carbon monoxide concentrations. Conclusions show that the reactor for filtration combustion can be used to reform liquid fuels into hydrogen and syngas. complete vaporization was achieved upstream of the flame front. The results also showed that, at the entrance region, evaporation is dominated by the heat transfer between phases. Kayal and Chakravarty15,16 presented a numerical analysis of combustion of liquid fuel droplets suspended in air inside an inert porous media. A one-dimensional heat-transfer model15 was developed assuming complete vaporization of oil droplets prior to their entry into the flame. They concluded that the inert porous medium with a low absorption coefficient produced high downstream, radiative output with large oil droplet sizes. Jugjai and Polmart17 constructed a porous burner with a second porous section downstream of the combustor. This second section, a packed bed, acted as a radiative emitter and enhanced droplet evaporation rates, thus eliminating the need for finely atomized fuel droplets. Reforming of liquid fuels, with the purpose of producing syngas, has also been conducted in porous reactors. Dixon et al.6 showed that heptane was successfully converted to syngas in a packed-bed reactor. Pedersen-Mjaanes and Mastorakos18 examined rich combustion of methanol, methane, octane, and automotive-grade petrol inside inert porous media. The maximum mole fraction of hydrogen was 28% from methanol, 13% from methane, and 11% from octane inside the porous foam burner. Smith et al.19 studied the combustion of rich mixtures of ethanol with different fractions of water and demonstrated that high conversion efficiencies could be obtained even with significant amounts of water. In this paper, we report the results of a study on an inert porous media burner with an external heat exchanger to vaporize the fuel. Experiments are conducted over a wide range of equivalence ratios and with mixtures that have either oxygenenriched or oxygen-depleted air. Hydrogen and carbon monoxide, the primary components of syngas, are presented for various operating conditions.

1. INTRODUCTION Hydrogen is a promising fuel for applications, such as fuel cells and combustors. Because it is not a carbon-based compound, it produces no CO or CO2 upon reaction. Hydrogen in the form of H2, however, does not appear in abundant quantities in nature and, therefore, must be obtained by processing another molecule, such as a hydrocarbon. This process, called reforming, is often carried out using a catalyst that accelerates the reactions. A robust alternative to a catalytic technique is a combustor filled with inert porous media (IPM), in which heat is recirculated through the solid to the incoming reactants, such that the local peak temperatures are increased. Extensive previous research has demonstrated the effectiveness of converting rich mixtures of gaseous fuels to syngas, a mixture of H2, CO, and other species, in IPM.1−7 Reforming of methane has also been demonstrated in other non-catalytic designs, such as a spouted bed reactor8 or a ceramic burner.9 Significant research has also been conducted using liquid fuels in IPM, and much of this work has focused on the delivery of the fuel and the evaporation of the droplets. Kamal and Mohamad10 wrote a review of combustion in porous media and concluded that porous media offer an advantage over free flames by promoting evaporation rates and stability of spray flames. They suggested that combustion of liquid fuels can be improved by preheating the solid media through a simultaneously non-premixed/premixed flame operation. Howell et al.11 discussed a porous burner consisting of two sections: an upstream small pore section followed by a larger pore downstream section. They concluded that an upstream region of about 3 cm is sufficient for complete evaporation of droplets having 25 μm initial diameters. A similar burner operating on kerosene was studied by Vijaykant and Agrawal,12 who investigated the effect of two types of fuel injectors on the combustion process. Martynenko et al.13 modeled the heat transfer and evaporation processes for a liquid fuel in a porous burner. Haack14 also studied evaporation of fuel droplets in a porous reactor and determined that the effectiveness of vaporization depended upon the distances traveled by the droplet and its absorptivity. For droplets of 15, 20, and 25 μm, © 2014 American Chemical Society

Received: January 25, 2014 Revised: April 7, 2014 Published: April 7, 2014 3453

dx.doi.org/10.1021/ef500352v | Energy Fuels 2014, 28, 3453−3459

Energy & Fuels

Article

about ∼3 L to ensure a constant supply. The injection flow of the liquid fuel is regulated by a rotameter calibrated to work with ethanol and methanol. When the flow of fuel is regulated, it passes through the heat exchanger, where it is vaporized to be mixed with air and injected into the burner. 2.3. Acquisition System. The temperature and gas chromatography acquisition data systems measure and record the results of the partial oxidation of the fuels. The temperature is measured through five S-type thermocouples with 0.88 mm of diameter, fabricated by OMEGA. Thermocouples are distributed axially and are located at 8− 20 cm measured from the burner exhaust, and their measure points are separated 3 cm between each other. They are protected by a ceramic tube to avoid direct contact with the alumina spheres and recorded temperatures very close to the temperatures of the solid phase. The voltage measured by the thermocouples is recorded by an OMB-DAQ54 acquisition data module and is converted by the Personal DaqView software. To measure the exhaust gases, a PerkinElmer gas chromatograph, model CLARUS 500, is used. The gas sampling is made through a pyrex tube that is introduced 3 cm inside of the porous media, and its other extreme is connected to a nylon tube with a inner diameter of 5 mm, outer diameter of 9 mm, and length of 2 m, which is connected to the chromatograph. The suction sampling is made through a vacuum pump connected to the chromatograph exhaust. The gas chromatograph is equipped with a thermal conductivity detector (TCD), a packed column (or stationary phase), HayeSep N of 5 m length and 1/8 in. diameter. To detect H2, CO, CH4, and CO2, helium with a flow of 26 mL/min is used as the carrier gas (mobile phase). Chromatography results are saved and analyzed by TotalChrom Navigator software. The experimental error in the temperature measurements was estimated as 20 K; the error in the wave velocity measurements performed on the basis of displacement of thermal profiles along the reactor length was ∼10%. The accuracy of chemical sampling was close to 10%. 2.4. Experimental Procedure. The experimental procedure consists of four stages: starting, operation with liquid fuel, data acquisition, and shutting down the equipment. The burner is started with LPG, a gaseous fuel that is easy to deliver and has known behavior. The flame front is established in the reactor and allowed to come to temperature that ensures the evaporation and ignition of the liquid fuel. After the start up with LPG, the flame front is taken to the injection position. The gas supply is shut off, and methanol or ethanol is injected with an equivalence ratio of 1.6. Sampling of exhaust gases is made through a pyrex graduated cylinder. The gases inside of the sampling line are purged, and the gas sample is taken. After the flame front has reached its last thermocouple (1 or 5 depending upon the front direction), the test is completed and the reactor is shut down.

2. EXPERIMENTAL DESIGN The experiment was designed to demonstrate the reforming of methanol and ethanol. The experimental equipment has three principal parts: a porous media reactor, a supply and flow control system (fuel and oxidizer), and the measuring and acquisition system for temperatures and gas chromatography. 2.1. Porous Media Reactor. The inert porous media reactor (Figure 1) consists of a quartz tube (length of 270 mm, an external

Figure 1. Schematic of the experimental setup. diameter of 76 mm, and an internal diameter of 72 mm) filled with solid spheres of alumina (Al2O3) with a diameter of 5.5 mm, which results in a porosity of 40%. The quartz tube is wrapped with 4.2 m of copper tube with a diameter of 3/16 in., which acts as a heat exchanger. The fuel flows through this tube and is evaporated by the heat from the reactor. The quartz tube is covered with a thin layer of ceramic fiber “Fiberfrax” to absorb the thermal expansion of the porous media and to protect the quartz from direct contact with the heat exchanger. Outside of the heat exchanger, two layers of fiber glass insulation minimize heat losses to the surroundings. Through 11 holes of 1.5 mm in the aluminum base, the fuel−oxidizer mixture is supplied to the porous reactor. A water heat exchanger at the base maintains the temperature at ∼80 °C, which prevents condensation of the vaporized fuel. 2.2. Flow Control System. The fluid supply and control system provides the mixture of the air, oxygen, nitrogen, liquefied petroleum gas (LPG), and liquid fuels. The air is supplied by the piston compressor, fabricated by Bauker, and consists of an accumulation tank of 50 L and a pressure regulator. The compressor impulses the air to the Aalborg air flow controllers and can be pure or mixed with nitrogen or oxygen for a depleted or an enriched mixture with air. LPG is supplied from its own tank, regulated by an Aalborg flow controller, and then mixed with the oxidizer to the burner before entering. The air supplied by another compressor is connected with the air−fuel tank; the liquid fuel is displaced by the air with enough pressure to be injected into the burner. The tank is made of a high-pressure polyvinyl chloride (PVC) tube with a diameter of 150 mm and a capacity of

3. EQUILIBRIUM CALCULATIONS Thermodynamic equilibrium calculations provide insight into the thermodynamic characteristics of methanol−air and ethanol−air systems at minimal computational cost and have proven to be predictors of trends in species production as a function of reactant composition.19 Calculations were performed using the Chemical Equilibrium with Applications (CEA) code.20

4. RESULTS AND DISCUSSION Experimental data were collected for a filtration velocity of 5.6 cm/s in the range of equivalence ratios of 1.6 < ϕ < 8.2 for fuel mixtures with air and ϕ = 4.0 and 5.0 for fuel mixtures with depleted (10 and 15%) and enriched (23%) oxygen in air, respectively. For these experiments, the inlet velocity was held constant. The percentage of fuel, oxygen, and nitrogen was changed by reducing the fuel and air flows proportionately and adding nitrogen for fuel mixtures with depleted oxygen. For cases with enriched oxygen, oxygen was added to the air while maintaining the overall equivalence ratio at the specified value. 3454


Energy & Fuels

Article

The experiments were similar for methanol and ethanol mixtures in procedure and apparatus. Combustion temperatures as well as product compositions were measured, and the propagation rates were obtained from thermocouple traces. The combustion temperature is the maximum temperature measured during a specific test. 4.1. Combustion Wave Temperature and Propagation Rate. Dependent upon the experimental conditions, the reaction front may propagate either upstream or downstream or be stationary. The velocity of the front is typically several orders of magnitude less than the inlet velocity. Figure 2 shows

Figure 3. Combustion temperature and velocity of methanol−air filtration combustion waves for equivalence ratios of 4.0 and 5.0 as a function of the oxygen content of the oxidizer.

Figure 2. Combustion temperature (Tc) and velocity (u) of methanol−air and ethanol−air filtration combustion waves. The adiabatic combustion temperature (Tad) as a function of the equivalence ratio is presented for reference.

the variation of the combustion temperature and adiabatic equilibrium temperature as a function of the equivalence ratio for operation on methanol and ethanol. For methanol, the temperature (Tc) shows only a modest decrease of about 100 K from 1030 K at ϕ = 1.6 to 933 K at ϕ = 8.2, while for ethanol, the decrease is somewhat more substantial from 1143 K at ϕ = 2.4 to 987 K at ϕ = 8.2. Both fuels exhibit superadiabatic temperatures, i.e., temperatures that are greater than those predicted by equilibrium, beyond about ϕ = 4.0 and 3.0 for methanol and ethanol, respectively. Consistent with the findings by Smith et al.,19 this division between super- and subadiabatic operation is also reflected in the direction of propagation of the front. In the subadiabatic regime, the wave propagates upstream, while in the superadiabatic region, it propagates downstream. All propagation speeds are less than ∼0.0025 cm/s. In summary, for two fuel−air mixtures, the regimes of sub- and superadiabatic combustion also manifest themselves in the directions of the wave propagation.21 Figures 3 and 4 show combustion temperature and wave velocity for different oxygen contents in the air for the cases above with equivalence ratios of 4.0 and 5.0 for the two fuels. For methanol (Figure 3), the decrease in the temperature with an increasing percentage of oxygen is within experimental uncertainty. For ethanol (Figure 4), however, there is a modest increase of about 50−100 K, depending upon the equivalence ratio. For the experimental results, upstream (subadiabatic) or downstream (superadiabatic) wave propagation relative to the solid lead to positive or negative enthalpy fluxes to the

Figure 4. Combustion temperature and velocity of ethanol−air filtration combustion waves for equivalence ratios of 4.0 and 5.0 as a function of the oxygen content of the oxidizer.

combustion zone. The adiabatic temperature of the mixture no longer defines the combustion temperature. It is mainly controlled by the kinetics mechanism of combustion. 4.2. Combustion Products. At equivalence ratios above 1.0, complete combustion cannot be achieved because of the insufficient oxygen content of the mixture.21 The experimental results for concentrations of major combustion products are presented in Figure 5 over a wide range of equivalence ratios for both methanol and ethanol. Hydrogen generation for both fuels increases with the equivalence ratio up until ∼ϕ = 6. For methanol, the maximum is ∼40% at ϕ = 6.3. Ethanol exhibits similar behavior18 but with lower concentrations, as expected from equilibrium considerations, and has a maximum value of 22% at ϕ = 6.3. Figures 6 and 7 show the hydrogen concentration for methanol and ethanol, respectively, for different percentages of oxygen. For both fuels, the depletion of the oxygen produces a decrease in hydrogen generation. All experimental values are significantly less than those predicted by equilibrium, but the trend of increasing hydrogen with an increasing oxygen concentration is consistent. 3455


Energy & Fuels

Article

Figure 5. Hydrogen concentration of methanol−air and ethanol−air filtration combustion.

Figure 7. Hydrogen concentration of ethanol−air filtration combustion waves for equivalence ratios of 4.0 and 5.0 as a function of the percentage of oxygen in the oxidizer.

Figure 6. Hydrogen concentration of methanol−air filtration combustion waves for equivalence ratios of 4.0 and 5.0 as a function of the percentage of oxygen in the oxidizer.

Figure 8. Carbon monoxide concentration of methanol−air and ethanol−air filtration combustion.

Carbon monoxide concentration in the exhaust gases for methanol and ethanol is shown in Figure 8. In both cases, carbon monoxide increases to a value of ∼23% at ϕ = 5.0 and then levels off. Experimental values compare well to equilibrium at the lowest equivalence ratio of ∼1.5. As the equivalence ratio increases, the measured percentage of CO is much greater than the equilibrium value. The behavior of carbon monoxide as a function of oxygen in the oxidizer is shown in Figures 9 and 10 for two different equivalence ratios. For both fuels, the concentration of carbon monoxide increases with the enrichment of the oxidizer reaching maximum values of 24.4% for methanol and 24.2% for ethanol with ϕ = 5.0 and 23% of oxygen in the oxidizer. The equilibrium results indicate that carbon monoxide is expected to increase with an increasing oxygen concentration in the oxidizer. Although there are significant differences between the experimental and equilibrium results, the overall trend of a

higher carbon monoxide concentration with an increasing oxygen concentration in the oxidizer is consistent. Under ideal circumstances, the original fuel would be completely converted to hydrogen and carbon monoxide with a minimum of other hydrocarbon compounds in the exhaust. Figure 11 reveals that, particularly for ethanol, there is significant methane in the product with a maximum value of ∼12% at ϕ = 5.0. This indicates that the process has the potential to be optimized further. 4.3. Hydrogen and Carbon Monoxide Yields. One important way of examining the performance of the reactor is through conversion efficiency or yield, where the hydrogen yield is the percent of hydrogen bound in the original fuel that appears as diatomic hydrogen in the exhaust. The corresponding definition for the carbon monoxide yield is the percent of C in the original fuel that is in the form of CO in the exhaust. These two values are presented in Figure 12. Both yields follow the same trends for the two fuels with a peak hydrogen yield at 3456


Energy & Fuels

Article

Figure 11. Methane concentration of methanol−air and ethanol−air filtration combustion. Figure 9. Carbon monoxide concentration of methanol−air filtration combustion waves for equivalence ratios of 4.0 and 5.0 as a function of the percentage of oxygen in the oxidizer.

Figure 10. Carbon monoxide concentration of ethanol−air filtration combustion waves for equivalence ratios of 4.0 and 5.0 as a function of the percentage of oxygen in the oxidizer.

∼ϕ = 6.0 of 43 and 24% for methanol and ethanol, respectively. Equilibrium predicts a maximum yield at a leaner equivalence ratio of 3 or 4. These results indicate that the process could be optimized further to produce greater yields of hydrogen. 4.4. Effect of the Filtration Velocity for Ethanol−Air. Additional experimental data were collected for a filtration velocity of 9.3 cm/s in the range of equivalence ratios of 3.0 < ϕ < 6.0 for ethanol mixtures with air with the aim of optimization of hydrogen yields. For these experiments, the wave was almost stationary with a velocity of ∼0 cm/s and the

Figure 12. Conversion percentage of (A) H2 and (B) CO for methanol−air and ethanol−air filtration combustion. 3457


Energy & Fuels

Article

The temperature profiles establish that methanol and ethanol combustion with superadiabatic temperatures can be obtained for ultrarich mixtures at a filtration velocity of 5.6 cm/s; maximum temperatures are 1030 K (ϕ = 1.6) for methanol and 1143 K (ϕ = 2.4) for ethanol. Both up- and downstream propagation was observed depending upon the equivalence ratio and the percent oxygen in the oxidizer. Successful conversion of both fuels was observed, although measurements of methane indicated that, particularly for ethanol, the conversion was incomplete for filtration velocity of 5.6 cm/s. The increase of the filtration velocity from 5.6 to 9.3 shows better conversion for the ethanol−air cases. Important contributions are that the porous media reactor with an external heat exchanger ensures recover combustion energy and the high oxygen content in the oxidizer shows higher hydrogen and carbon monoxide concentrations.

methane concentration was negligible in the product gases. These experiments also showed that the heat exchanger was capable of vaporizing large quantities of liquid fuel. Figure 13 shows that the increase of filtration velocity increases the combustion temperature and hydrogen concen-

■

AUTHOR INFORMATION

Corresponding Author

*Telephone: +56-32-2654162. Fax: +56-32-2797472. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

Figure 13. Combustion temperature (Tc) and hydrogen and carbon monoxide concentrations of ethanol−air for filtration velocities of 5.6 and 9.3 cm/s.

ACKNOWLEDGMENTS The authors acknowledge the support by the CONICYT-Chile (FONDECYT 11080106 and 1121188).

trations and decreases the carbon monoxide concentration. Figure 14 shows hydrogen and carbon monoxide yields. These

■

REFERENCES

(1) Kennedy, L. A.; Bingue, J. P.; Saveliev, A. V.; Fridman, A. A.; Futko, S. I. Chemical structure of methane/air filtration combustion waves for fuel-lean and fuel-rich conditions. Symp. (Int.) Combust., [Proc.] 2000, 1431−1438. (2) Xia, Y.; Shi, J.; Li, B.; Yu, C.; Xu, Y.; Xue, Z. Experimental investigation of filtration combustion instability with lean premixed hydrogen/air in a packed bed. Energy Fuels 2012, 26 (8), 4749−4755. (3) Zhang, J.; Cheng, L.; Zheng, C.; Luo, Z.; Ni, M. Numerical studies on the inclined flame front break of filtration combustion in porous media. Energy Fuels 2013, 27 (8), 4969−4976. (4) Bingue, J. P.; Saveliev, A. V.; Fridman, A. A.; Kennedy, L. A. Hydrogen production in ultra-rich filtration combustion of methane and hydrogen sulfide. Int. J. Hydrogen Energy 2002, 27, 643−649. (5) Dhamrat, R. S.; Ellzey, J. L. Numerical and experimental study of the conversion of methane to hydrogen in a porous media reactor. Combust. Flame 2006, 144, 698−709. (6) Dixon, M. J.; Schoegl, I.; Hull, C. B.; Ellzey, J. L. Experimental and numerical conversion of liquid heptane to syngas through combustion in porous media. Combust. Flame 2008, 154, 217−231. (7) Schoegl, I.; Newcomb, S. R.; Ellzey, J. L. Ultra-rich cokmbustion in parallel channels to produce hydrogen-rich syngas from propane. Int. J. Hydrogen Energy 2009, 34, 5152−5163. (8) Weinberg, F. J.; Bartleet, T. G.; Carleton, F. B.; Rimbotti, P.; Brophy, J. H.; Manning, R. P. Partial oxidation of fuel rich mixtures in a spouted bed combustor. Combust. Flame 1988, 72, 235−239. (9) Ytaya, Y.; Oyashiki, T.; Hasatani, M. Hydrogen production by methane-rich combustion in a ceramic burner. J. Chem. Eng. Jpn. 2002, 35 (1), 46−56. (10) Kamal, M. M.; Mohamad, A. A. Combustion in Porous Media. Proc. Inst. Mech. Eng., Part A 2006, 220 (5), 487−508. (11) Howell, J. R.; Hall, M. J.; Ellzey, J. L. Combustion of hydrocarbon fuels within porous medium. Prog. Energy Combust. Sci. 1996, 22, 121−145. (12) Vijaykant, S.; Agrawal, A. K. Liquid fuel combustion within silicon-carbide coated carbon foam. Exp. Therm. Fluid Sci. 2007, 32, 117−125.

Figure 14. Conversion percentage of H2 and CO for ethanol−air filtration combustion at velocities of 5.6 and 9.3 cm/s.

results show the sensitivity of the conversion to the filtration velocity and suggest that the process can be optimized by controlling this parameter.

5. CONCLUSION This work shows that methanol and ethanol can be converted to hydrogen and syngas through a porous reactor at the equivalence ratio range of 1.6 < ϕ < 8.2. Temperature profiles, combustion wave velocities, and products of methanol and ethanol partial oxidation were studied. 3458


Energy & Fuels

Article

(13) Martynenko, V. V.; Echigo, R.; Yoshida, H. Mathematical model of self-sustaining combustion in inert porous medium with phase change under complex heat transfer. Int. J. Heat Mass Transfer 1998, 41 (1), 117−126. (14) Haack, D. P. Mathematical analysis of radiatively enhanced liquid droplet vaporization and liquid fuel combustion within a porous inert medium. M.Sc. Thesis, University of Texas at Austin, Austin, TX, 1993. (15) Kayal, T. K.; Chakravarty, M. Combustion of liquid fuel inside inert porous media: An analytical approach. Int. J. Heat Mass Transfer 2005, 48, 331−339. (16) Kayal, T. K.; Chakravarty, M. Modeling of a conceptual selfsustained liquid fuel vaporization−combustion system with radiative output using inert porous media. Int. J. Heat Mass Transfer 2007, 50, 1715−1722. (17) Jugjai, S.; Polmart, N. Enhancement of evaporation and combustion of liquid fuels through porous media. Exp. Therm. Fluid Sci. 2003, 27, 901−909. (18) Pedersen-Njaanes, H.; Chan, L.; Matorakos, E. Hydrogen production from rich combustion in porous media. Int. J. Hydrogen Energy 2005, 30, 579−592. (19) Smith, C. H.; Leahey, D. M.; Miller, L. E.; Ellzey, J. L. Conversion of wet ethanol to syngas via filtration combustion: An experimental and computational investigation. Proc. Combust. Inst. 2010, 33 (2), 3317−3324. (20) http://www.grc.nasa.gov/WWW/CEAWeb/. (21) Toledo, M.; Utria, K.; Saveliev, A. V. Ultrarich filtration combustion of ethane. Energy Fuels 2014, 28 (2), 1536−1540.

3459


Hydrogen Production from Methanol and Ethanol Partial Oxidation

Recommend Documents