On-Site Production of Hydrogen from Mineral Waste Oils by

On-site production of hydrogen and carbon nanofibers by thermocatalytic ... fleet of public urban vehicles powered by fuel cells or dedicated ICE, res...
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Environ. Sci. Technol. 2005, 39, 6871-6876

On-Site Production of Hydrogen from Mineral Waste Oils by Thermocatalytic Decomposition: An Aragon Case Study M A R IÄ A J . L AÄ Z A R O , I S A B E L S U E L V E S , A N D RAFAEL MOLINER* Instituto de Carboquı´mica CSIC, Miguel Luesma Casta´n, 4, 50.015-Zaragoza, Spain

On-site production of hydrogen and carbon nanofibers by thermocatalytic decomposition (TCD) of mineral oil waste (MWO) is analyzed. An experimental study carried out at lab scale to estimate the yields that can be expected from TCD of the MWO collected in the Aragon area is presented. Based on these results, mass and energy balance have been carried out to have a preliminary estimation on the products that could be obtained by processing the 10 000 tonnes/year of MWO that can be collected in the Aragon region. The process would consist of four steps: (1) drying, (2) vaporization, (3) primary decomposition, and (4) catalytic decomposition. After drying and vaporization, MWO is converted in step 3 into fuel grade carbon and a gas mixture that mainly contains hydrogen and methane. Methane is partially converted in step 4 into hydrogen and a carbon material that contains carbon nanofibers which could be used to manufacture utilities with high added value. The 10 000 tonnes/year of MWO would yield 705 t/y of H2, 4962 t/y of fuel grade carbon, and 1016 t/y of pure carbon. The mixture obtained (71% H2: 23% CH4) could be used as a hydrogen source to obtain pure hydrogen or hydrogen-natural gas mixtures to fuel a captive fleet of public urban vehicles powered by fuel cells or dedicated ICE, respectively.

Introduction Worldwide reduction of CO2 emission to decrease the risk of climate change requires a major restructuring of the energy system. The use of hydrogen as an energy carrier is a longterm option to reduce CO2 emissions. It is generally accepted that in the near-to-medium term, hydrogen production will continue to rely on fossil fuels, primarily natural gas (1). Nowadays, steam reforming of hydrocarbons, SR, is the most widely used technology to produce hydrogen. However, this process involves the emission of CO2 unless it is captured and stored, which will increase the cost of hydrogen production at least by 50% (2) if that is done in a large installation. In a hydrogen-based scenario, the issues concerning hydrogen transport and distribution would lead to a pattern of distributed on-site production centers, close to the consumption centers. If hydrogen is produced in distributed microreformers, carbon dioxide capture and transport to the final sink will be nearly impossible, and a significant potential * Corresponding author phone: (34) 976733977; fax: (34) 976733318; e-mail: [email protected]. 10.1021/es040479l CCC: $30.25 Published on Web 07/09/2005

 2005 American Chemical Society

benefit from using hydrogen will not be realized. In addition, it is worth noting that many questions, particularly, with regard to the duration and extent of CO2 retention still remain unanswered. An alternative process to SR is thermocatalytic decomposition, TCD, through the production of pure hydrogen and carbon. In this way, uncertainties about CO2 storage are overcome. A comprehensive analysis of the advantages of TCD against SR can be found in refs 3 and 4. The advantages of TCD over SR become more appreciable when applying to small scale hydrogen plants for decentralized production of hydrogen (5). In this scenario, capture of carbon as a solid, marketable product instead of CO2 is a very interesting alternative. In addition to natural gas, some wastes are one attractive source of hydrogen at a reasonable cost. In this way, two goals are reached: saving of primary energy resources and valorization of a waste. This is the case of mineral waste oils, MWO. Thermocatalytic decomposition of MWO produces H2 and carbon with low emission of CO2. Valorization of MWO by pyrolysis has been thoroughly studied at the institute of Carboquı´mica. In the first approach, R. Moliner et al. (6) showed that MWO coming from transport and industry is a good candidate for valorization via pyrolysis and subsequent utilization of gases and liquids produced. In the second stage, MWO was copyrolyzed with coal (7, 8) showing that copyrolysis of coal and MWO increases the quantity and quality of the products obtained as compared to the products obtained from coal pyrolysis. Special attention was paid to the fate of the metals contained in MWO during pyrolysis and their distribution into the pyrolysis products (9, 10). It was shown that chars trap most of V, Ni, Cd, and Cr in MWO. The experience gained in pyrolysis of hydrocarbons in the presence of chars has been used to implement the thermocatalytic decomposition of MWO for the production of hydrogen. MWO contains a high quantity of hydrogen that can be selectively separated from carbon in the presence of a catalyst that promotes carbon deposition. The reaction can be catalyzed by transition metals (11, 12) or by carbonaceous materials (13-15). Taking into account that MWO production concentrates in the urban areas, it represents a potential source of hydrogen for fueling a captive urban fleet. Recently, utilization of natural gas-hydrogen mixtures in internal combustion engines has been proposed (16-18). This type of engine is much cheaper than fuel cells and works with mature and well-proved technology. Using this type of engine would facilitate the progressive transit to fuel cells powered vehicles since the present grid of fuel stations that serves natural gas would be used to serve natural gas-hydrogen mixtures. In addition, these engines do not need the extremely restrictive specifications required for the hydrogen used in fuel cells. In this way, using wastes as hydrogen sources is favored. For instance, around 10 000 Tm of MWO are generated every year in the region of Aragon, Spain. Most of them are generated in the capital city, Zaragoza, where an important captive fleet of public vehicles is in service. The Environment Department of the Aragon Government funded ICB to carry out a study to evaluate the feasibility of on-site production of hydrogen by TCD of the MWO generated in the region. In the present work, the first results of this study concerning the mass and energy balance of the process are presented. An experimental study was carried out to evaluate at lab scale the yields that can be expected from TCD of the MWO collected in the Aragon area. Based on these results, mass and energy balance have been carried out in order to have VOL. 39, NO. 17, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 1. MWO by Type of Compounds GC/MS % mass linear and ramified chains < C20 linear and ramified chains > C20 cyclic hydrocarbons unsaturated hydrocarbons aromatics halogenates oxygenates not identified

1.2 72.1 0.92 3.25 0.40 0.78 4.80 16.45

a preliminary estimation on the products that would be obtained by this technology.

Experimental Section Materials. The sample of MWO used was provided by a local waste oil manager, GAUAR S.L. (Zaragoza, Spain). This oil was a mixture of automotive lubricating oil and other industrial waste oils, all of them considered as a residue. Before pyrolysis, the sample was filtered to >70 µm and heated at 110 °C to remove volatiles and water. It was shown by GC/MS that MWO was mostly composed of linear and ramified aliphatic compounds higher than C20, with a lower presence of cycles and unsaturated compounds. Table 1 shows a summary of the main type of compounds contained in the MWO. The elemental composition was as follows: C: 83.96; H: 13.71; N: 1.37; S: 0. 65; O: 0.31. A commercial activated carbon (PANREAC) was selected as adsorbent for the primary thermal decomposition (elemental composition: C: 88.10; H: 0.56 N: 0.17; O: 11.17; particle size of 0.5-3 mm. SN2: 981.6 m2/g, SCO2: 989.6 m2/ g). Two different catalysts were tested for the catalytic thermal decomposition: the same activated carbon used in the first step and a nickel catalyst. The nickel catalyst was a commercial catalyst supported in silica-alumina with a % 65 wt/wt of nickel and particle size < 100 µm (SN2: 190 m2/g). TCD Lab-Scale Installation. Figure 1 shows the TCD labscale installation. TCD was carried out in two steps: (1) primary decomposition and (2) catalytic decomposition. Primary decomposition was carried out in a tubular reactor (25 mm diameter and 500 mm long) filled with adsorbent (approximately 30 g) and externally heated by an electrical oven. The oil sample (around 13 g during each 1 h run) was fed from a graduated glass column and injected

FIGURE 1. Thermocatalytic decomposition lab-scale installation. 6872

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into the reactor by the upper side using a pump. An auxiliary nitrogen flow (100 mL/min) coaxial to the injection nozzle was passed in order to increase the spatial velocity of the decomposition products throughout the reactor. The gas stream leaving the reactor was cooled in a water-tap exchanger where the liquid compounds produced were collected. The cool gas was collected in a gas-meter in order to measure the total volume, and then the gases produced during a complete run were analyzed by GC. The weight of carbon and liquid produced were determined by the weight increase of the reactor and the trap respectively at the end of the run. When both steps were linked, the gas leaving the first reactor is fed to the second reactor for catalytic decomposition. The catalytic reactor is made of quartz (580 mm: height and 23 mm of internal diameter), and it is divided in two parts: the preheater section that is filled with ceramic rings and the reaction section. Gas products were collected, measured, and analyzed as mentioned above. The weight of carbon produced was determined by the weight increase of both the primary decomposition reactor and the catalytic decomposition reactor at the end of the run. The weight of liquids produced was determined by the weight increase of the trap at the end of the run. TCD Runs. All runs were carried out at atmospheric pressure. First, primary thermal decomposition was carried out at temperatures of 550, 600, 650, 700, and 750 °C before linking this step to the catalytic step in order to determine the best conditions for the primary decomposition. Finally, a temperature of 700 °C was selected for the primary decomposition when active carbon was used as a catalyst in the catalytic reactor. When Ni catalyst was used, two temperatures, 650 °C and 700 °C, were tested. The catalytic decomposition was carried out at 850 °C with active carbon and at 700 °C with Ni catalyst. Four grams of catalyst was used for the runs with active carbon and 2 g for the Ni catalyst runs. The Ni catalysts were subjected to a reduction treatment before the activity tests using a flow rate of 20 mL/min of pure H2 for 3 h at 550 °C.

Results and Discussion Primary Thermal Decomposition. Table 2 shows the yields obtained from thermal decomposition of MWO over the commercial activated carbon as a function of temperature. As the temperature increases from 550 °C to 700 °C, the solid yield increases from 44.53% to 54.22%, the gas yield decreases

TABLE 2. Yields from Primary Thermal Decomposition of MWO at Lab Scalea solids gases liquids

550 °C

600 °C

650 °C

700 °C

44.53 51.00 4.37

47.87 47.00 5.12

51.41 44.33 4.27

54.22 41.28 4.45

composition (% mole)

H2 CO CO2 CH4 H2S C2H6 C2H4 C3H8 C3H6 C4 HC g C4 a

yields (g)

550 °C

600 °C

650 °C

700 °C

550 °C

600 °C

650 °C

700 °C

28.1 1.41 1.53 40.7 0.28 7.98 1.84 8.44 3.58 4.97 27.68

40.2 1.10 1.22 43.43 0.32 5.15 1.63 3.15 2.56 1.03 13.64

45.52 1.11 2.15 43.49 0.04 3.58 1.92 0.64 0.25 0.24 6.66

53.8 1.0 0.4 40.1 0.1 1.60 2.82 0.08 0.53 0.13 5.28

1.43 1.01 1.73 16.64 0.25 6.12 1.32 9.48 3.84 7.25 29.69

2.72 1.04 1.81 23.51 0.37 5.22 1.55 4.69 3.63 2.01 17.43

3.17 1.08 1.31 24.23 0.25 3.75 1.87 0.98 1.82 0.47 8.97

4.58 1.10 1.70 27.38 0.34 2.05 3.36 0.15 0.94 0.26 7.07

Basis: 100 g of MWO.

from 51.00% to 41.28%, and the liquid yield, which is much lower, remains almost constant. Although the gas yields decreases as temperature increases, the hydrogen concentration increases, so that the hydrogen yield increases and the yield of C1-C6 hydrocarbons decreases. At 650 °C-700 °C, the gas phase is mainly composed of hydrogen and methane (accounting for approximately 90%) in detriment of saturated hydrocarbons C2-C6 that decrease from 27.68% at 550 °C to 5.28% at 700 °C. In previous works, where MWO was pyrolyzed alone (6), the solid yield did not significantly vary when increasing temperature, whereas liquid yield decreased and gas yield increased. In contrast, in the present work, increasing of temperature enhances solids production and depletes gas yield. It means that the adsorbent plays an important role in the thermal cracking of MWO. First of all, the presence of the active carbon increases the residence time of MWO in the reaction zone, which promotes the occurrence of a secondary reaction that enhances char and light gases formation. In addition, it has been reported in the literature (13, 15) that activated carbons present catalytic activity for the catalytic decomposition of methane and other hydrocarbons into carbon and hydrogen so the activated carbon bed also plays a catalytic role on MWO splitting into hydrogen and carbon. Catalytic Decomposition. The main components of the gas entering into the catalytic reactor are hydrogen and methane. So, the overall yield of hydrogen depends on the methane conversion in this reactor. Two different catalysts, nickel and carbon at 700 °C and 850 °C, respectively, have been used. The use of carbon materials as catalysts present some advantages over the use of Ni catalysts: (i) higher fuel flexibility and no sulfur poisoning, (ii) lower price, and (iii) the carbon formed can be used as catalyst precursor, so that the process is self-consistent. However, using metal-based catalysts allows producing high quality forms of carbon with a high selling price which could compensate for the cost of the catalyst (19, 20). Table 3 shows the results obtained when using activated carbon as catalyst. The concentration of methane and higher hydrocarbons significantly decreases, whereas the concentration of hydrogen increases (more than 60% of the gas phase). It shows that additional cracking of the light hydrocarbons formed during the primary decomposition takes place in this catalytic step. However, a not negligible quantity of liquid compounds still remains unconverted

TABLE 3. Lab Scale Yields of TCD of MWOa solids gases liquids

58.42 36.91 4.66 composition (% mole)

yields (g)

63.56 3.61 0.18 31.33 0.04 1.70

5.92 4.71 0.37 23.35 0.07 4.32

H2 CO CO2 CH4 H2S HC g C2

a First step: activated carbon at 700 °C and second step activated carbon, 850 °C. Basis: 100 g of MWO.

TABLE 4. Lab Scale Yields from DTC of MWOa temp of primary decomposition solids gases liquids

H2 CO CO2 CH4 H2S HC g C2

650 °C

700 °C

61.93 36.43 1.63

63.42 34.95 1.63

composition (% mole)

yields (g)

composition (% mole)

yields (g)

71.04 2.45 1.83 23.34 0.26 1.48

7.3 3.5 4.1 19.2 0.45 3.3

73.09 2.32 0.95 22.20. 0.19 1.66

7.96 3.5 2.28 19.3 0.35 3.61

a First step: activated carbon at 650 and 700 °C and second step: Ni catalyst, 700 °C. Basis: 100 g of MWO.

Table 4 shows the results obtained with the nickel catalyst. No significant differences are observed as a function of the temperature of the primary decomposition. In both cases, the concentration of methane and higher hydrocarbons decrease and the hydrogen concentration increases significantly. Compared with the behavior of the carbon catalyst, a significant increase of the hydrogen production is observed. At 700 °C the gas obtained is mainly composed by hydrogen and methane with higher hydrocarbons representing a small quantity, even lower than that occurring in natural gas. It is VOL. 39, NO. 17, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 2. Schematic diagram of the thermocatalytic process. worth noting the presence of not negligible amounts of carbon oxides, which is not desirable. These compounds are generated from oxygenated compounds in MWO and from Ni oxides present in the catalyst. Concerning the liquid compounds, the yield is much lower than using carbon catalyst, so that they near disappear.

An Aragon Case Study Based on the results presented above, mass and energy balance have been carried out to have a preliminary estimation on the products that could be obtained by processing the 10 000 tonnes/year of MWO that can be collected in the Aragon region. Description of the Process. The TCD process carried out at bench scale could be applied to MWO at commercial scale using well-proved technology. The process would consist of four steps: (1) drying, (2) vaporization, (3) primary decomposition, and (4) catalytic decomposition. Figure 2 shows the diagram of the process. Drying is necessary to eliminate the water contained in MWO. Water must be eliminated from the process stream in order to avoid formation of carbon oxides by gasification of the carbon deposited in steps 3 and 4. MWO is introduced at 25 °C in the heat exchanger IC-01 where the temperature rises to 105 °C. Two streams are formed: C-01, water vapor and C-02, dry MWO. The second step, vaporization, takes place in the heat exchanger IC-02, where the temperature of C-02 increases from 105 to 400 °C which is enough to evaporate MWO. It has been observed by GC/MS that no thermal cracking happens at the temperature and pressure used. In the third step MWO enters into the reactor R-01 where it suffers a primary cracking. The reactor R-01 consists of a moving bed of carbon particles. As shown at laboratory scale, this carbon particles act as a cracking catalyst that promotes splitting of hydrocarbons into carbon and hydrogen. In this way MWO splits into two streams: the carbon stream that is withdrawn from the reactor as a solid and the gas stream, C-03. A part of the carbon should be ground and reloaded to the reactor in order to keep constant the distribution of particle sizes along the reactor. According to the results obtained in a previous paper, most of the impurities contained in MWO, such as heavy metals, soot, metal nanoparticles, and ashes, will be trapped in the carbon produced in this step. This carbon could be used as a solid fuel for the cement industry 6874

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where it could be burnt in the clinker four without environmental damage. In this way, the most valuable element of MWO, hydrogen, is not burnt, whereas using MWO as an alternative fuel for cement industry, which contributes to save primary energy resources, is kept. Concerning the stream C-03, it will be assumed that its composition agrees with the results obtained at lab scale, Table 2. In this exercise, a temperature of 650 °C has been selected for step 3. In this way, a significant decomposition of hydrocarbons takes place, whereas most of the heavy metals and sulfur in MWO are retained in the carbon. The last step of the process is the catalytic decomposition of the compounds contained in C-03. Using a Ni catalyst at 700 °C has been assumed for this exercise. The stream C-03 coming from reactor R-01 is fed to the reactor R-02 where hydrocarbons are selectively decomposed into carbon and hydrogen. The reactor R-02 consists of a bed of a heat carrier and Ni-carbon particles. These particles are generated by deposition of carbon on the fresh Ni-catalyst. Different reactors to carry out decomposition of methane/hydrogen mixtures have been reported in the literature (21, 22). The selection of a specific technology does not fall into the scope of this paper. A material easy to separate from the Ni-carbon particle will be used as heat carrier. The input solid stream would consist of the fresh Ni catalyst and the heat carrier. The carbon produced would be continuously withdrawn from the reactor together with the catalyst. In this way the exit solid stream from this reactor would consist of Ni-carbon particles and the heat carrier. It has been shown (19, 20) that the carbon produced by this technology is structured as nano fibers that could be marketable as a premium carbon for carbon composites materials and other novel applications such as hydrogen storage (23). Concerning the gas stream, C-05, it is assumed that its composition agrees with the results obtained at lab scale, Table 4. At 700 °C conversion of methane is not complete due to thermodynamics limitations, so C-05 will contain unconverted methane. Heating Systems. The heat needed for the process could be supplied using different systems. In all of them, a part of the process products, carbon or gas, would be burnt. Concerning the reactor R-02, the carbon produced is a valuable product, even more than hydrogen, so a part of the gas produced (C-05) would be burnt and the flue gas used to heat the heat carrier and the fresh Ni-catalyst particles

TABLE 7. Mass Balance in Reactor R-02a

TABLE 5. Mass Balance in the Dryer C-01 f C-02 in (Kg/h) MWO H2O (steam) dry MWO(liquid)

out (Kg/h)

1142 40 1102

TABLE 6. Mass Balance in Reactor R-01a H2 CO CO2 CH4 H2S C2H6 C2H4 C3H8 C3H6 C4 HC > C4 solid C a

H2 CO CO2 CH4 H2S HC ) > C2 solid C a

Kg/h

Kmols/h

36.20 11.90 14.44 267.01 2.75 41.32 20.60 10.80 19.84 5.18 98.85 566.5

7.81 0.44 0.86 17.32 0.13 1.50 0.76 0.25 0.50 0.09 1.41

Basis: 10 000 tn/year of MWO.

before being loaded to the reactor. Heating would be carried out in an auxiliary fluidized bed (not shown in Figure 1) linked to the reactor. Concerning the reactor R-01, external heating is needed just for the initial not steady-state period. An auxiliary heater reactor system similar to the one depicted for reactor R-02 would be used. Concerning the heat needed to evaporate the MWO in the heat exchanger IC-02, a part would be obtained by heat exchange with the gas stream C-05 which is cooled from 700 °C to 150 °C. This sensible heat is lower than the total heat needed, so additional heating would be obtained burning a part of C-05 and the flue gases used as heat carrier. In the same way, the heat needed to dry MWO will be obtained by burning a part of C-05 and the flue gases used as heat carrier in C-01. Mass Balances. The mass balances were calculated on the basis of 10 000 tonnes/year of MWO. Heat Exchanger IC-01. The stream C-01 contains 3.5% water and 96.5% MWO which was assimilated to C20H42 for physical and thermal properties. Two streams are obtained: one in the gas phase, formed by steam and the other one in the liquid phase, C-02, formed by dry MWO (C20H42). Table 5 shows the mass flows in heat exchanger IC-01. Reactor R-01. This step is carried out at 650 °C on activated carbon. To calculate the mass balance in reactor R-01, the composition and yields of Table 2 have been used. MWO is decomposed into solid carbon (51.41%), gases (44.3%), and liquids (4.27%). The composition of the stream C-04 is assumed to be the same as at lab scale (Table 2). Table 6 shows the mass balance in reactor R-01. Reactor R-02. In this step, a nickel catalyst is used at 700 °C to crack the products formed in step 1 to hydrogen and carbon. To calculate the mass balance, the composition of C-05 has been taken from Table 4. Table 7 shows the mass balance in reactor R-02. Heating Requirements. Heat Exchanger IC-01. According to Table 1 the mass flows into IC-01 are as follows: C20H42 3.9 Kmol/h and H2O 2.22 Kmol/h. The heat, Q-01, to be supplied in IC-01 is required to heat the oil and the water from 25 to 105 °C and to evaporate the water, Q-01 ) 306 408 KJ/h.

Kg/h

Kmols/h

80.45 38.57 45.18 257.21 4.96 36.37 68.25

40.22 1.38 1.03 16.07 0.1458 0.6061

Basis: 10 000 tn/year of MWO.

Heat Exchanger IC-02. The heat, Q-02, to be supplied in IC-02 is (1) the one required to heat the oil from 105 to 400 °C, the temperature at which the oil is vaporized, and (2) the one required to vaporize the oil: (1) to heat the MWO from 105 to 400 °C, 998.553 kJ/h and (2) to vaporize the oil, 122.899 kJ/h, Q-02 ) 1 121 452 KJ/h. Reactor R-01. The temperature in the reactor is maintained at 650 °C and the pressure at 1 bar. The total heat needed is the sum of the heats corresponding to three partial processes: (1) to increase the temperature of vaporized MWO from 400 to 650 °C, Q1 (KJ/ h) ) 932 553, (2) to decompose MWO into carbon and hydrogen [For calculations, it is assumed that all MWO is decomposed into carbon and hydrogen and then recombined to form the products.], Q2 (KJ/h) ) -691 260, and (3) to form the compounds in stream C-03, Q3 (KJ/h) ) -241 293. So, in this reactor in stationary regime, it is not necessary to supply heat, Q-03 ) 0 kJ/h. Reactor R-02. The compounds obtained in the reactor R-01 are decomposed into hydrogen and carbon. The reaction takes place at 700 °C. The heat required is the sum of (1) the heat needed to increase the temperature of each component in the feed from 650 to 700 °C, Q1 ) 105 865 KJ/h and (2) the heat needed to decompose each component into hydrogen and carbon, Q2 ) 37 675 KJ/h. The total heat required in this step is Q-04 ) 143 540 kJ/h. Heating Production. The energetic requirements will be obtained by burning a part of the gas produced and by heatexchange with the hot gas stream produced. Reactor R-02. The energetic requirements will be obtained by burning a part of the gas produced in an auxiliary burner. The flue gases will be used to heat the particles in an auxiliary reheater reactor. To simplify the calculations, it has been considered that the gas stream is composed just of CH4 and H2 (40.22 Kmol/h; 16.07 kmol/h). The gas is introduced into the burner at 700 °C and the air at 25 °C.

nCH4CH4 + 2nCH4(O2 + 3.76 N2) f nCH4CO2 + 2nCH4H2O + 2(3.76)nCH4N2 nH2H2 + 1/2nH2(O2 + 3.76 N2) f nH2H2O + 1/2(3.76)nCH4N2 nH2 ) 2.50 nCH4 nCH4 ) 0.044 Kmol/h nH2 ) 0.11 Kmol/h ntotal ) nH2 + nCH4 ) 0.154 Kmol/h The heating requirements for the reactor R-02 will be obtained burning 0.924 Kg/h of the gas produced. Heat Exchanger IC-02. The gas of production will be cooled from 700 °C to 150 °C. The heat released in this process is Q ) 1 070 843 KJ/h which is not enough to cover the energy VOL. 39, NO. 17, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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required to heat and to evaporate the MWO processed. The rest of the energy will be supplied by burning a part of the gas produced. The equations for the combustion reactions are the same as shown above. The kmol/h of CH4 and H2 consumed are

nCH4 ) 0.090 Kmol/h nH2 ) 0.225 Kmol/h ntotal ) nH2 + nCH4 ) 0.315 Kmol/h The energy required for the heat exchanger IC-02 will be obtained by burning 2.07 Kg/h of the gas produced. Heat Exchanger IC-01. The heat needed to dry MWO will be obtained by burning a part of the gas produced. The kmol/h of CH4 and H2 consumed are

nCH4 ) 0.21 Kmol/h nH2 ) 0.525 Kmol/h ntotal ) nH2 + nCH4 ) 0.735 Kmol/h The energy required for the heat exchanger IC-01 will be obtained by burning 4.41 Kg/h of the gas produced. Overall Mass and Energy Balances. The overall mass and energy balance of the process shows that the 10 000 t/y of MWO would yield 705 t/y of H2, 4962 t/y of fuel carbon, 1016 t/y of premium carbon, and 2254 t/y of methane. The preliminary energy balance, which should to be depurated and revised in more detail, shows that just a very small part of the gas produced (around 2.4%) needs to be burnt to provide the energetic requirements of the process. In summary, the preliminary study carried out for the Aragon Government shows that the MWO collected in the Aragon region can be efficiently valorized by thermocatalytic decomposition since the energetic valorization of MWO, which contributes to save primary energy resources, is kept, whereas additional products with a high added value are obtained. The fuel carbon, which accounts for around half of the carbon in MWO, could be sold as a solid fuel to the cement industry which is at present the main user of MWO. An important quantity of high quality carbon containing carbon nanofibers is produced. Selling of this material would contribute to improve the economy of the process. The gas stream obtained could be used to produce pure hydrogen, which would be enough to supply the fuel of 60 fuel cell powered urban buses. As an alternative, it could be used to prepare 20-30% hydrogen-natural gas mixtures for ICE powered vehicles, so that the cost of separating methane from hydrogen and recycling methane would be avoided.

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Acknowledgments This work has been supported in part by the Aragon Government. I. Suelves wants to thank the Spanish Ministry of Science and Technology for the Ramo´n y Cajal Program Contract.

Literature Cited (1) ] Hart, D.; Freud, P.; Smith, A. Hydrogen Today and Tomorrow; IEA GHG Programme, April 1999, ISBN-1 898373248. (2) ] CO2 Abatement by the use of Carbon-Rejection Processes; IEA GHG Report PH3/36; February 2001. (3) ] Steinberg, M. Int. J. Hydrogen Energy 1998, 23, 419-425. (4) ] Steinberg, M. Int. J. Hydrogen Energy 1999, 24, 771-777. (5) ] Muradov, N. Int. J. Hydrogen Energy 2001, 26, 1165-75. (6) Moliner, R.; La´zaro M. J.; Suelves, I. Energy Fuels 1997, 11, 11651170. (7) La´zaro, M. J.; Moliner, R.; Suelves, I. Energy Fuels 1997, 13, 907-913. (8) Moliner, R.; La´zaro M. J.; Suelves, I.; Blesa M. J. Int. J. Power Energy Syst. 2004, 24, 186-193. (9) La´zaro, M. J.; Moliner, R.; Suelves, I.; Nerı´n, C.; Domen ˜ o, C. Environ. Sci. Technol. 2000, 34, 205-3210. (10) La´zaro, M. J.; Moliner, R.; Domen ˜ o, C.; Nerı´n C. J. Anal. Appl. Pyrolysis 2001, 57, 119-131. (11) Parmon, V. N.; Kuvshinov, G. G.; Sobyanin, V. A. Innovative processes for hydrogen production from natural gas and other hydrocarbons, Proceedings of the 11th World Hydrogen Energy Conference, Stuttgart, June 23-28, 1996. (12) ] Ermakova, M. A.; Ermakova, Yu, D.; Kuvshinov, G. G.; Plyasova, L. M. J. Catal. 1999, 187, 77-84. (13) ] Muradov, N. M. Energy Fuels 1998, 12, 41-48. (14) ] Kim, M. H.; Lee, E. K.; Jun, J. H.; Kong, S. J.; Han, G. Y.; Lee, B. K.; Lee, T.-J.; Yoon, K. J. Int. J. Hydrogen Energy 2004, 29, 189-193. (15) ] Moliner, R.; Suelves, I.; La´zaro, M. J.; Moreno, O. Int. J. Hydrogen Energy 2005, 30, 293-300. (16) Dahl, J. K.; Buechler, K.; Finley, R.; Stanislaus, T.; Weimer, A.; Lewandowski, A.; Bingham, C.; Smeets, A.; Schneider, A. Rapid Solar-thermal Dissociation of Natural Gas in an Aerosol Flow Reactor, Proceedings of the 2002 U.S. DOE Hydrogen Program Review, NREL/CP-610-32405. (17) Akansu S.; Dulger, Z.; Kahraman, N, Nejat, T. Int. J. Hydrogen Energy 2004, in press. (18) Collier K. Hydrogen & Natural Gas blends; converting light and heavy duty vehicles, 2004 DOE Hydrogen, Fuel Cells & Infrastructure Technologies Programme Review. (19) Li, Y.; Chen, J.; Quin, Y.; Chang, L. Energy Fuels 2000, 14, 11881194. (20) Suelves, I.; La´zaro, M. J.; Moliner, R. Hydrogen Production by Catalytic Decomposition of methane over Ni and Ni-Cu based catalysts, 16th International Symposium on Analytical and Applied Pyrolysis, May 23-27, 2004, Alicante, Spain. (21) Parmon, V. N.; Kuvshinov, G. G.; Sadykov, V. A.; Sobyanin, V. A. Natural Gas Conversion V. Studies in Surface Science and Catalysis; Parmaliana, A., et al., Eds.; Elsevier: 1998; Vol. 119, pp 677-684. (22) Weizhong, Q.; Tang, L.; Zhanven, W.; Fei, W.; Zhifei, L.; Guohua, L.; Yongdan, L. Appl. Catal., A 2004, 260, 223-228. (23) Atkinson, K.; Roth, S.; Hirscher, M.; Gru ¨ nwald, W. Fuel Cells Bull. 2001, 4, 9-12.

Received for review July 20, 2004. Revised manuscript received January 18, 2005. Accepted June 8, 2005. ES040479L