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Ind. Eng. Chem. Res. 2007, 46, 2042-2046
PROCESS DESIGN AND CONTROL Ammonia Production via a Two-Step Al2O3/AlN Thermochemical Cycle. 1. Thermodynamic, Environmental, and Economic Analyses M. E. Ga´ lvez,† M. Halmann,‡ and A. Steinfeld*,†,§ Department of Mechanical and Process Engineering, ETH Zurich, 8092 Zurich, Switzerland, Department of EnVironmental Sciences and Energy Research, Weizmann Institute of Science, RehoVot 76100, Israel, and Solar Technology Laboratory, Paul Scherrer Institute, 5232 Villigen PSI, Switzerland
The production of ammonia via a two-step cyclic process is proposed as an alternative to its conventional production by the Haber-Bosch process. The first endothermic step is the production of AlN by carbothermal reduction of Al2O3 in a N2 atmosphere at above 1500 °C. The second exothermic step is the steam-hydrolysis of AlN to produce NH3 and reform Al2O3; the latter is recycled to the first step. Both reaction steps proceed at 1 bar, without added catalysts, and bypass the energy-intensive production of hydrogen, resulting in significant fuel and cost savings. Furthermore, the endothermic reduction step could be carried out using concentrated solar energy as the source of high-temperature process heat, eliminating concomitant CO2 emissions derived from fossil-fuelled processes. Introduction More than 90% of the world consumption of ammonia, the second largest synthetic chemical product, is manufactured from N2 and H2 via the catalytic Haber-Bosch process.1 Although 0 ) -46.2 kJ mol-1) and this reaction is exothermic (∆H25°C should occur spontaneously, significant energy input is needed for N2 to achieve the activated state because of its high dissociation energy (941 kJ mol-1). The use of catalysts lowers the activation energy and effects the reaction in the range of 250-400 °C, but even with added catalysts, the yield is low because of the unfavorable thermodynamic equilibrium. At 30 MPa, the yield after one pass usually does not surpass 25%, requiring separation by condensation and recycling of the unreacted H2-N2 mixture, about 4-6 kg per kg of NH3 synthesized.2 Furthermore, the overall process is characterized by the high-energy consumption associated with the production of the reactants. Usually, H2 is obtained by steam-reforming of natural gas, while N2 is obtained by cryogenic separation from air. Both of these processes require a major input of energy, either in the form of high-temperature process heat or in the form of electricity, and consequently cause a significant concomitant pollution derived from the combustion of fossil fuels for heat and electricity generation. In modern plants, the total energy requirement is estimated to be 28 GJ ton-1.2,3 In the present work, a novel cyclic process is proposed for the production of NH3 that involves two thermochemical noncatalytic steps. The first, high-temperature, endothermic step is the production of AlN and CO by carbothermal reduction of Al2O3 in a N2 atmosphere. Either C (e.g., charcoal, petcoke, etc.) as in reaction 1a or CH4 (e.g., natural gas) as in reaction * Author to whom correspondence should be addressed. Fax: +41 44 6321065. E-mail:
[email protected]. † ETH Zurich. ‡ Weizmann Institute of Science. § Paul Scherrer Institute.
1b can be used as reducing agents. The second exothermic step is the hydrolysis of AlN to produce NH3 and reform Al2O3; the latter is recycled to the first step. The overall reaction steps can be represented by (first endothermic step)
Al2O3 + 3C + N2 ) 2AlN + 3CO 0 ) 708.1 kJ mol-1) (1a) (∆H25°C
Al2O3 + 3CH4 + N2 ) 2AlN + 6H2 + 3CO 0 ) 931.9 kJ mol-1) (1b) (∆H25°C
and (second exothermic step)
2AlN + 3H2O ) Al2O3 + 2NH3 0 (∆H25°C ) -274.1 kJ mol-1) (2)
Figure 1 illustrates the proposed cycle. The CO produced in the first step when employing a source of fixed carbon as the reducing agent, reaction 1a, may be water-gas shifted to syngas and further used as a fuel or as an intermediate to methanol or Fischer-Tropsch products. Syngas with a H2/CO molar ratio of 2 suitable for CH3OH synthesis would be directly produced by employing CH4 as the reducing agent (reaction 1b). Relative to the conventional production of NH3 via the Haber-Bosch process, the proposed two-step process offers the following 3-fold advantages: (1) it eliminates the need for high pressure, minimizing costs and safety concerns; (2) it eliminates the need for catalysts; minimizing costs associated with their production and recycling; and (3) it eliminates the need for hydrogen as feedstock, reducing energy consumption and associated CO2 emissions. It does not, however, eliminate the need for nitrogen. Furthermore, the endothermic reduction of Al2O3, either by reaction 1a or by reaction 1b, is an attractive candidate for the use of concentrated solar energy as the source of hightemperature process heat, avoiding the discharge of greenhouse gases and other pollutants derived from the combustion of fossil
10.1021/ie061550u CCC: $37.00 © 2007 American Chemical Society Published on Web 02/21/2007
Ind. Eng. Chem. Res., Vol. 46, No. 7, 2007 2043
Figure 1. Scheme of the two-step cyclic solar thermochemical cyclic process for ammonia production. The first endothermic step is the solar production of AlN by carbothermic reduction of Al2O3 in a N2 atmosphere. The second exothermic step is the steam-hydrolysis of AlN to produce NH3 and to reform Al2O3; the latter is recycled to the first step.
Figure 2. Equilibrium composition of the system Al2O3 + 3C + N2 (reaction 1a) as a function of temperature at 1 bar.
Figure 3. Equilibrium composition of the system Al2O3 + 3CH4 + N2 (reaction 1b) as a function of temperature at 1 bar.
fuels. Previous relevant thermochemical processes effected in solar furnaces include the carbothermal reductions of Fe3O4, MgO, and ZnO with C(gr) and CH4 to produce Fe, Mg, and Zn, respectively;4-9 the carbothermal reductions of Al2O3, CaO, SiO2, and TiO2 with C(gr) in an inert atmosphere to produce Al3C4, CaC2, SiC, and TiC, respectively; and the carbothermal reductions of Al2O3, SiO2, TiO2, and ZrO2 with C (g) in a N2 atmosphere to produce AlN, Si3N4, TiN, and ZrN, respectively.10-12 Specifically, experimental studies on the carbothermal reduction of Al2O3 in the presence of N2 have been carried out at above 1500 °C using a graphite crucible reactor, directly exposed to high-flux solar irradiation.11,12 A review of the solar chemical process technology is found in ref 13. The chemical kinetics of the pertinent reactions has been investigated in the accompanying article.14
conversion of Al2O3 begins at about 1300 °C, is notably accelerated at 1500 °C, and reaches completion at 2000 °C. At this temperature, the gas phase consists of 94.9% CO, 2.2% N2, 1.3% Al (g), and 1.6% Al2O (g), while the solid phase consists of 97.4% AlN and 2.6% C. Figure 3 shows the equilibrium composition of the system Al2O3 + 3CH4 + N2 (reaction 1b) at 1 bar and as a function of temperature in the range of 1000-2000 °C. Methane cracking occurs at relatively low temperatures and is completed at 600 °C, leading to the formation of C (s), which in turn reacts with Al2O3 at higher temperatures. Al2O3 conversion reaches completion at 1900 °C. At 2000 °C, the gas phase consists of 31.3% CO, 63.4% H2, 1.5% N2, small amounts of Al (g), Al2O (g), H (g), C2H2 (g), AlH (g), and HCN (g), while the solid phase consists of 100% AlN. By applying either C or CH4 as reducing agents in a N2 atmosphere, no formation of Al4C3 or oxycarbides (e.g., Al2O2C, Al4O2C) is foreseen by the thermodynamic calculations in this temperature range. Since the presence of unreacted solids C or Al2O3 within the AlN produced is undesirable, the effect of adding O2 was evaluated in an effort to select the optimal conditions that minimize the inclusion of these residues in the final product. Results of thermodynamic equilibrium computations at 1800
Thermodynamic Analysis Thermochemical equilibrium calculations were performed using the FactSage,15 CET85,16 and HSC Outokumpu17 program codes. Product species with mol fractions less than 10-5 have been omitted. Figure 2 shows the equilibrium composition of the system Al2O3 + 3C + N2 (reaction 1a) at 1 bar and as a function of temperature in the range of 1000-2000 °C. The
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Ind. Eng. Chem. Res., Vol. 46, No. 7, 2007
Table 1. Thermochemical Equilibrium Composition at 1800 °C and 1 Bar, for an Initial Mixture of 1 mol of Al2O3 + 1.1 mol of N2 + 3.05 mol of Either C or CH4 as Reducing Agents in the Presence of Varying Amounts of O2a Initial
Equilibrium Mole Fraction
reducing agent
O2 (mol)
AlN (s)
Al2O3
C (g)
C C CH4 CH4 CH4
0 0.05 0 0.02 0.05
0.362 0.360 0.176 0.175 0.171
0.0128 0.0138 0 0 0.0016
0.048 0.032 0.0018 0 0
H2
CO
0.540 0.540 0.540
0.544 0.560 0.267 0.269 0.270
H2/CO molar ratio
yield of AlN(s) (%)
2.03 2.01 2.00
99.8 99.8 98.7 98.3 97.9
a Products are expressed in mol fractions. A mole fraction value of “0” means
