Pure Hydrogen Production from Pyrolysis Oil Using ... - ACS Publications

Ind. Eng. Chem. Res. 2010, 49, 53–64

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Pure Hydrogen Production from Pyrolysis Oil Using the Steam-Iron Process: Effects of Temperature and Iron Oxide Conversion in the Reduction M. F. Bleeker, H. J. Veringa, and S. R. A. Kersten* Department of Science and Technology, Research Institute IMPACT, UniVersity of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands

In the steam-iron process, relatively pure hydrogen can be produced from pyrolysis oil in a redox cycle with iron oxides. Experiments in a fluidized bed showed that the hydrogen production from pyrolysis oil increases with increasing temperature during reduction. The experimental hydrogen production at nearly 1000 °C with noncatalytic (blast furnace) and catalytic (ammonia synthesis) iron oxide was found to be 1.39 and 1.82 Nm3 of H2/kg of dry oil, respectively. However, this high hydrogen production could be achieved only when a low relative conversion (R) of the iron oxide in the reduction was maintained (about 7%). It was found in all experiments that the reduction rate decreased strongly with increasing relative conversion of the iron oxide [at 800 °C, the relative conversion rate (dR/dt) decreased from 3.0 × 10-4 s-1 at R ) 0.6% to 8.8 × 10-6 s-1 at R ) 10.0%]. The gasification of pyrolysis oil over an iron oxide bed results in an increased carbon-to-gas conversion compared to gasification over a sand bed. Near-complete gasification of oil is achieved when temperatures above 900 °C are applied in a fluidized-bed setup containing iron oxide. A lumped reaction path scheme is proposed for char formation in pyrolysis oil gasification. 1. Introduction Hydrogen is currently used mainly in the production of ammonia and in the operation of petroleum refineries.1 The need for hydrogen could increase in the near future not only because of an increase in hydrogen demand in existing industries, but possibly also because of the expected increase in fuel-cell-based power-generation devices. Currently, hydrogen is mainly produced from fossil fuels. In line with the current debate on sustainability, hydrogen should be produced from renewable sources in the future. Hydrogen can be obtained from different renewable energy sources such as solar, wind, and biomass. Biomass can be converted by both thermal and biological processes into hydrogen.2 The biological processes are not discussed in this work. In the thermal route, biomass is gasified to a mixture of gases that subsequently needs to be further converted and purified to obtain (pure) hydrogen.2 There are currently no commercial biomass gasification processes for direct hydrogen production. The steam-iron process, as discussed in this article, can produce pure hydrogen in a two-step redox cycle using iron oxides, without the need for additional purification steps. For the reduction of the iron oxide biomass, oil and coal from which reducing gases such as CO and H2 and solid C are obtained can be used as feedstock. The oxidation of the reduced iron oxide is conducted with steam, resulting in the formation of hydrogen (see Figure 1). As a result, 100% pure hydrogen can be obtained in the oxidation only if the reduced iron oxide is not contaminated with carbon-containing compounds. The advantage of this process is that the hydrogen production (oxidation) is performed separately from the feedstock gasification (reduction). Therefore, removal of contaminants, such as tar, CH4, CO, and CO2, is minimal compared to other proposed biomass-to-hydrogen routes.2 Bleeker et al.4 reported a process design study of the steam-iron process using pyrolysis oil, also discussing advantages and disadvantages. This * To whom correspondence should be addressed. E-mail: s.r.a.kersten@ utwente.nl. Tel.: +31-(0)53-4894430. Fax: +31-(0)53-4894738.

study revealed that, under optimal conditions, the efficiency toward hydrogen is 53% on the basis of lower heating value (LHV). In our previous work, gasification of pyrolysis oil over an inert fluidized sand bed was described.3 It was concluded that, with increasing temperatures, the oil-to-gas conversion increases and more CO and H2 are produced at the expense of hydrocarbons and H2O. Both the increased oil-to-gas conversion and the changed gas composition improve the reducing capacity of the oil, especially at temperatures above 850 °C.3 From the gas composition obtained, an estimation of the hydrogen production from pyrolysis oil in the steam-iron process was made. A hydrogen production of 0.37 Nm3 per kilogram of dry oil input was expected at 810 °C and increased to an expected hydrogen production of 0.86 Nm3 per kilogram of dry oil at 950-977 °C (the highest temperatures at which measurements have been made). Therefore, with increasing temperature, more pure hydrogen is expected to be produced from pyrolysis oil in a redox cycle. Previously, redox experiments with noncatalytic and catalytic iron oxide were performed at temperatures of about 800 °C in a fluidized bed.3 In these experiments, it was observed that carbon deposition took place on the iron oxide surface during oil injection in the fluidized bed. A period between oil injection and oxidation was required to finalize the reaction of these carbon deposits with the iron oxide. Therefore, to obtain sufficiently pure hydrogen, a complete redox cycle consists of

Figure 1. Concept of the steam-iron process with pyrolysis oil.

10.1021/ie900530d CCC: $40.75  2010 American Chemical Society Published on Web 11/23/2009

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Ind. Eng. Chem. Res., Vol. 49, No. 1, 2010

Table 1. Reactions in the Steam-Iron Process with Pyrolysis Oil no.

reaction

1 2 3 4 5 6

oil f CO + CO2 + H2O + H2 + CH4 + CxHy + C + tar 1.2Fe3O4 + H2 T 3.8Fe0.945O + H2O 1.2Fe3O4 + CO T 3.8Fe0.945O + CO2 1.2Fe3O4 + C T 3.8Fe0.945O + CO CO + H2O f CO2 + H2 CxHy + xH2O f xCO + (x + y/2)H2 C2H6 + 2H2O f 2CO + 5H2

three steps instead of two in the experimental setup. The second step can be minimized or omitted in the process if the purity of hydrogen can be relaxed. Both the endothermic gasification of pyrolysis oil and iron oxide reduction are expected to be more effective with increasing temperature (based on thermodynamics). On the other hand, the steam conversion in oxidation will decrease with increasing temperature, as a result of a decrease of the equilibrium steam conversion with increasing temperature. The increased steam requirements for the oxidation with increasing temperature will have a negative effect on the total energy balance of the process, when the hydrogen is separated from the product stream by condensation of the steam fraction.4 In this work, the expected higher hydrogen production from pyrolysis oil at higher temperatures was tested by conducting several redox cycles at different temperatures between 800-1000 °C in a fluidized-bed setup. In addition to temperature, the effect of the degree of conversion of iron oxide on the gaseous products in both reduction and oxidation was also measured. To minimize the effect of structural changes of the iron oxide on the experimental results, a deactivated material that had already been used in several redox cycles was applied. It was found in a our previous work that strong deactivation of the catalytic iron oxide takes place in the first redox cycles, after which it reaches a more steady performance.5 1.1. Reactions. When pyrolysis oil is gasified, it decomposes into gaseous, liquid, and solid products. With increasing temperature, more gaseous products can be expected at the expense of the solid and liquid products (reaction 1, Table 1). The gasification products CO, H2, and C will reduce the iron oxide (reactions 2-4, Table 1), basically from magnetite to wustite.3 Further reduction to iron is possible, but it is slow and should be prevented to minimize deactivation of the iron oxide.5 Reduction of iron oxides with hydrocarbons can take place, especially Fe2O3, but it is slow for Fe3O4 and lower oxides.6 However, CH4 and C2+ can both contribute significantly to the reduction of Fe3O4 if they are converted to CO and H2 by steam reforming reactions, for example (reaction 6, Table 1).7 Another gas-phase reaction that influences the gas composition during oil gasification and reduction is the water-gas shift (reaction 5, Table 1). The hydrogen product is obtained after reoxidizing the iron oxide with steam in a separate step (reverse of reaction 2). Oxidation of Fe3O4/Fe1-δO with steam to Fe2O3 is thermodynamically impossible, and oxidation to Fe2O3 can be performed only with oxygen [∆G°r for the oxidation of Fe3O4 to Fe2O3 with steam is 58 kJ/(mol of H2O) at T ) 327 °C and 108 kJ/ (mol of H2O) at T ) 927 °C]. 1.2. Model for the Interpretation of the Reduction. The reduction reaction that takes place in the experimental setup was analyzed by a simplified gas-solid reaction model. The reduction was performed with pyrolysis oil, from which different reducing compounds can be obtained (see section 1.1). To simplify the reduction reaction in the model, it was assumed

∆Hr(T)827°C) (kJ/mol)

∆Gr(T)827°C) (kJ/mol)

61 27 197 -34

-5.2 -5.1 -27 0.1 -176

378

that the reduction takes place with only one gaseous component, called A. The quantity of this gaseous compound A in the model is the sum of the experimentally produced CO and H2 from the gasification of pyrolysis oil Fe3O4 + A(g) T 3Fe1-δO + P(g) The amount of product P formed in this way represents the total amount of CO2 and H2O formed after reduction. The gas-solid reaction can be described by many different gas-solid particle models, which can be found in the literature.8-10 The appropriate particle model depends on the material properties, such as the porosity of the solid and the external conditions applied. In general, the overall reaction rate includes the mass transport of the gaseous reactants and products to and from the reaction surface and the chemical reaction itself, which takes place at the reaction surface. The rate of these processes depends on the degree of conversion of the material applied. For example, a solid product layer can grow upon reduction, resulting in an increased boundary layer through which diffusion of the reactants/products to the reaction site needs to take place. Also, the available surface at which the reaction can take place can decrease with increasing degree of conversion of the particle/ grain (shrinking core model/grainy pellet model). These phenomena can all be included in the overall relative conversion rate, which can be described as dR ) k(T) f1(R) f2(gas) dt

(1)

in which k(T) is the temperature-dependent rate constant, f1(R) is a function that describes the effect of the relative conversion (R) on the reduction rate, and f2(gas) is a function describing the effect of the gas concentration on the conversion rate. In most gas-solid reactions, the order of the gas concentration is 1. In general, the reversible reduction can be described by R < Rmax R g Rmax

(

dR R ) k(T) 1 dt Rmax dR )0 dt

)( n

CA -

CP Ke

)

(2)

The value of n determines the extent to which the conversion rate decreases with increasing conversion of the material. For example, when the core reaction, according to a simple shrinking core reaction, is limiting the overall reaction rate, the value is n ) 2/3.11 From previous experiments, it was observed that the maximum conversion (Rmax) of the material depends on the H2/ H2O ratio in the reactant gas and the temperature applied.12,5The relative conversion rate becomes very low when approaching Rmax. The reaction rate constant, k(T), is dependent on the temperature and the material properties. For example, in the grainy pellet model, k(T) is directly related to the grain size. The value of k(T) in the model is derived from packed-bed experiments with hydrogen as reduction gas.13 The conversion rate with CO is lower,14 and therefore, the value of k(T) in this

Ind. Eng. Chem. Res., Vol. 49, No. 1, 2010 Table 2. Elemental Composition and Water Content of the Pyrolysis Oil Used component C H Oa H2O a

oil wt % (wet) 34 8 58 32

Determined by difference.

calculation is too high, but it can still be applied to estimate trends and understand observations described in this work. Equation 2 is used to interpret the increase in iron oxide conversion in the fed batch reactor as a function of the amount of oil fed in time. For the gas-phase components (A and P), continuous ideally stirred tank reactor (CISTR) behavior is assumed. The inlet concentration of reduction gases (CA) is estimated from experimental results, and the equilibrium constant Ke is taken for the reduction of magnetite to wustite with hydrogen. The conversion of the gas phase is used to calculate the conversion of the iron oxide in the reduction. From this conversion, the amount of hydrogen produced in the oxidation is calculated by assuming that full oxidation back to Fe3O4 is achieved. The calculated hydrogen production per kilogram of oil in a redox cycle is compared with the experimental results. Rmax and n are used as fit parameters in this model.

To have a minimal effect of deactivation on the experimental results presented in this work, two additional high-temperature redox cycles were performed (temperature reduction is 970-950 °C) with the previously deactivated BIC iron oxide material. After these two high-temperature redox experiments, the measured BET (Brunauer-Emmett-Teller) surface area was in the range of 0.05-0.1 m2/g for the BIC iron oxide. This confirms that a strongly deactivated material was used and that structural changes and accompanying deactivation of the material had a minimum effect on the results presented in this work. Also, the BF iron oxide, which is a nonporous iron oxide, was previously used (20 cycles),3 and deactivation of this material was minimal, because the surface area of the fresh material (BET ) 0.19 m2/g) was already low. 2.3. Definitions. A summary of the indicators used to interpret the experimental data is presented in this section. The gas products measured during reduction and oxidation were used to determine the mass balance of the process. This method does not result in a complete balance, as components such as tar, char, and water are not included. Two integral carbon balances were calculated. The carbon conversion to the gas phase, Cgas, is the amount of carbon transferred from the pyrolysis oil to gaseous products and is calculated from the gases formed during the pyrolysis oil injection period ζC to gas )

2. Experimental Section 2.1. Setup. The setup is described in detail in a previous article.3 Here, a short summary is given for convenience. The redox cycle experiments were performed in a fluidized-bed reactor (i.d. ) 0.078 m, length ) 1 m). Hot fluidization gases (nitrogen and/or steam) entered the reactor in the lower conically shaped part of the reactor by four entrances. Pyrolysis oil was fed with a dual-head hose pump to the reactor through a cooled atomizer, which sprayed a cold nitrogen/pyrolysis oil mixture into the lower conical mixing zone of the reactor. The atomizer temperature was maintained below 100 °C, to prevent boiling of the water in the pyrolysis oil and polymerization of the oil. Gases from the reactor passed through a small cyclone, to separate small particles that were entrained with the product gas. The gas product was cooled in two water coolers to separate and collect the water and other organic liquids that were formed during reaction. Eventually, a gas sample was pumped through the gas analysis unit. Gases were analyzed by online analyzers for CO, CO2, CH4 (infrared), and H2 (thermal conductivity detection). A gas chromatograph was used to analyze hydrocarbons (C2 and C3). 2.2. Materials. Pyrolysis oil from the pyrolysis of pine wood was obtained from the Biomass Technology Group (Enschede, The Netherlands). The composition of the pyrolysis oil is given in Table 2. Two types of iron oxides were used: (i) a catalytic iron oxide, normally used in the ammonia industry, obtained from the Boreskov Institute of Catalysis (Novosibirsk, Russia), and (ii) grinded sintered pellets, used for steel production in blast furnaces from CORUS (IJmuiden, The Netherlands). The iron oxides are referred to as BIC iron oxide and BF iron oxide, respectively, and details are given in ref 3. The BIC iron oxide initially had a high surface area of 31.2 m2/g. However, after repeated cycling (14 cycles) at 800 °C, the surface area diminished, and ultimately, a surface area of 0.37 m2/g was obtained in our previous work.3

55

{∫

tend oil injection+τd

tstart oil injection+τd

[ΦCO(t) + ΦCO2(t) + ΦCH4(t) +

}

2ΦC2H4(t) + 2ΦC2H6(t) + 3ΦC3H6(t) + 3ΦC3H8(t)] dt /

[∫

tend oil injection

tstart oil injection

]

Φm,oil(t)fc dt Mc

(3)

The total amount of carbon measured (ζC total) is ζC to gas plus the amount of carbon compounds deposited on the bed material, which react with the iron oxide or steam (during oxidation) to CO and CO2 after oil injection. The experiments were conducted in such a way (section 2.4) that the major part of the carbon deposits reacted with the iron oxide before the oxidation started. Therefore, the amount of carbon deposits is defined as

ζC deposit )

∫

tend N2 injection+τd

tstart N2 injection+τd

∫

[ΦCO(t) + ΦCO2(t)] dt

tend oil injection


Φm,oil(t)fc dt Mc

(4)

The gas products produced during oil injection are defined as (for H2, as an example) H2 )

∫

tend reduction+τd

tend oil injection

∫

tend oil injection


ΦH2(t) dt (RgT/P)

Φm,oil(t)(1 - Xm,H2O) dt

(Nm3 /kg of dry oil)

(5)

The gas production defined here represents the integral (cumulative) gas production during reduction, starting from Fe3O4 to a reduced iron oxide material with a certain conversion depending on the amount of oil injected and the temperature applied. The final conversion of the iron oxide obtained during reduction is difficult to control, as it depends on both the temperature and the amount of oil injected. However, the amount of oil injected per amount of iron oxide present in the fluidized bed is known in each experiment and defined as

56


R)

Fe3O4 initially present

∫

tend oil injection


(6)

Φm,oil(t) dt

This quantity is used in this study as an experimental parameter to influence the conversion of the iron oxide at a specific temperature. Specifically, experimental results obtained at a low and high R values (high and low conversions, respectively) are compared. The conversion of the iron oxide can be determined only from the amount of hydrogen produced during the oxidation. The amount of oxygen from the iron oxide that reacted during reduction and oxidation is ∆Oiron oxide )

∫

tend steam injection+τd

tstart steam injection+τd

[ΦH2,ox(t) - ΦCO,ox(t) 2ΦCO2,ox(t)] dt (mol)

(7)

which can be used to determine the relative conversion of the iron oxide material in the redox cycle as R(Fe3O4 - Fe1-δO) )

∆Oiron oxide × 100% (%) S0

(8)

The value of S0 is defined in such a way that the relative conversion (R) equals 1 if full reduction of magnetite to wustite is achieved. This definition for the relative conversion was chosen because it was observed in a previous work that the conversion to iron is difficult, because of the presence of water in the pyrolysis oil.3 The amount of hydrogen produced in a redox cycle during the oxidation of each kilogram of dry oil used in the reduction is defined as H2,ox )

∫ ∫

tend oxidation+τd

tstart oxidation+τd

tend oil injection


ΦH2,ox(t) dt (RgT/P)

Φm,oil(t)(1 - Xm,H2O) dt

(Nm3 /kg of dry oil)

(9)

2.4. Procedure. The redox cycles were performed in three steps in the fluidized bed (see Figure 2A). First, the reduction was performed by injecting pyrolysis oil into the fluidized bed, which contained iron oxide particles (reduction I). The temperature of the fluidized bed decreased during oil injection as a result of the endothermic reduction reaction, cracking, and evaporation of the pyrolysis oil. This temperature decrease depends on the oil injection time and amount of pyrolysis oil injected. To minimize this temperature decrease as much as possible, a low flow of pyrolysis oil was applied. The lowest temperature obtained during reduction is defined as the reduction temperature. In this work, the oil injection time was varied. Because the same start temperature was chosen, this resulted in a slight fluctuation of the final reduction temperature for these experiments. During the second phase (reduction II), only hot nitrogen was fed to the reactor to continue the reduction of the iron oxide with the previously deposited carbon from the oil. This reduction step was needed to finalize the reduction by converting the carbon deposits on the iron oxide to the gas phase and resulted in the formation of CO and CO2. The temperature steadily increased to the initial start temperature during this step. When the CO and CO2 levels were close to zero, oxidation was started (third phase). In the oxidation, nitrogen and steam were fed to the fluidized-bed reactor, resulting in the formation of hydrogen. The temperature slightly increased to above the start

Figure 2. Redox cycle at (A) 800 and (B) 920 °C with BIC iron oxide (R ) 17). Table 3. Experimental Settings for the Redox Experiments with Fluidized Iron Oxides

Φm pyrolysis oil Φv N2_atomizer Φv N2_hot Φm H2O_hot ureactor (T ) 800 °C) Mbed (Fe3O4)

kg/h NL/min NL/min kg/h m/s kg

reduction I

reduction II

oxidation

0.25 5.5 8.0 0.23 1.3-1.4

5.5 8.0 0.15 1.3-1.4

2.4 4.0 0.47 0.18 1.3-1.4

temperature (oven temperature) because of the exothermic character of the oxidation reaction. The gas and liquid flow rates used in the redox cycle are listed in Table 3. 3. Results 3.1. Typical Experiment for R ) 17 at 800 and 920 °C. The gas products released over the entire redox cycle, comprising the three different steps, are shown in Figure 2A and B at two temperatures. In both plots, it can be seen that pyrolysis oil gasification and iron oxide reduction took place in the time interval of t ) 0-20 min (reduction I). During oil injection, a changing gas composition was observed. In the first 1-2 min of the experiment, the CO2 level was high, whereas at the end of oil injection, mainly H2 and CO were produced. This effect was found to be stronger at higher temperatures. Figure 2 shows that the gas composition changed with time of oil injection or progressive increase of the conversion of the material. In Figure 2A, it can be seen that CO and CO2 were formed in the second phase (reduction II), which was not the case when the reduction was performed at 920 °C (Figure 2B). Apparently, no carbon deposits were present on the iron oxide after oil injection at 920 °C. The final step, the oxidation, started at t ) 55 min (Figure 2A) or 26 min (Figure 2B) and resulted in the production of hydrogen. The oxidation rate was high at the start but decreased with time, which can be seen from the tailing off

Ind. Eng. Chem. Res., Vol. 49, No. 1, 2010 Table 4. Carbon Balance of a Redox Cycle at 800 and 950 °C, for Gas Production Shown in Figure 3 T (°C) 800 ζC to gas (%) ζC deposit (%) ζC total (%)

58.3 9.9 68.2

Table 5. Experimental Conditions Applied for the Experimental Data Points and Fit Parameters and Constants Used in the Model (Figure 3) Experimental Conditions

950 98.0 0.5 98.5

of the initial high hydrogen production. The CO and/or CO2 production was zero (920 °C) or close to zero (800 °C) in the oxidation stage. The total carbon-to-gas conversion (ζC total) and the carbonto-gas conversion during oil injection (ζC to gas) (see Table 4) clearly increased with temperature. At a high temperature, less carbon deposition was measured on the iron oxide after oil injection (ζC deposit). The amount of carbon compounds reacting with steam during oxidation was close to zero. An increase in the total carbon conversion with temperature was not observed when a sand bed was used,3 as discussed in more depth in section 3.5. 3.2. Application of the Reaction Model. The measurement conditions applied to vary the ratio of iron oxide to oil (R) were all due to changes in the oil injection time. The fluidization regime was, in all cases, the same except for a minor difference in the gas velocity due to a change in temperature. Obviously, the same variations in ratio could be obtained by keeping the oil injection time constant and varying the amount of iron oxide present in the bed. In the latter case, the contact time between the iron oxide and the gasified oil would also change, which could have an effect on the gas and solid conversions during reduction and oxidation. In the previous section, the gas production over time in the redox cycle is discussed (Figure 2). The observed decrease in

Figure 3. (A) Hydrogen production in a redox cycle with BIC iron oxide at a temperature of about 800 °C. The fit lines were obtained using eq 2, and the fit parameters and constants are given in Table 5. (B) Predicted decrease in the conversion rate and increase of conversion with oil injection time.

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T (°C) N2 flow (NL/min) oil flow (kg/h) Miron oxide (kg)

800 13.6 0.25 1.3

Fit Parameters n Rmax

3 0.12 Constants

k(T) [m3/(mol s)] Ke

2.8 1.55

the CO2 production clearly shows that the rate of reduction decreased with increasing oil injection time or conversion of the iron oxide. The final conversion of the iron oxide in the reduction could be determined only by the measured amount of hydrogen produced in the oxidation. Therefore, the effect of the conversion of the iron oxide on the reduction rate was measured by performing several redox experiments with different oil injection times. Figure 3A shows the measured results of the hydrogen production per kilogram of dry oil for different oil injection times at 800 °C. The experimentally obtained data were fitted with eq 2. The strong decrease in the hydrogen production with increasing oil injection time could best be predicted with a value of n between 2 and 3 in eq 2. A much stronger decrease in the conversion rate (Figure 3B) was measured than can be predicted on the basis of a simple core reaction limitation (n ) 2/3).11 However, for any value of n, a strong decrease in the relative conversion rate is expected when approaching Rmax. The predicted conversion rate became very low (Figure 3B) at oil injection times higher than 500 s. In Figure 2A, for t > 500 s, a stable gas production was observed. This observation also shows that the reduction was restricted already at low conversions of the material, and at the applied temperature (800 °C), Rmax was fitted to be 0.12. The low value of Rmax can be attributed to the formation of (carbon-based) contaminants on the solid surface, restricting the diffusion of reactants and products. Scanning electron microscopy and X-ray diffraction (XRD) tests were performed on the iron oxide at low temperatures (800 °C). A dense carbon layer could be clearly distinguished. Another explanation is the high H2O and CO2 concentrations in the gas phase, which could result in poisoning of the iron oxide surface.15-17 3.3. Gas Production and Composition during Reduction. The effect of temperature on the average gas production during reduction (reduction I) is shown in Figure 4 for high and low conversions of iron oxide. A low R value of 17 resulted in a relative conversion of the iron oxide in the range of 7-40%, depending on the temperature applied, and a high R value of 150 resulted in a low conversion (1.4-7.8%). In Figure 4, the main compounds during reduction using a BIC or BF iron oxide are compared with the gas production obtained over a sand bed and with equilibrium predictions. The gas production changed with increasing oil injection time. The gas production shown in Figure 4 represents the integral gas production during oil injection, starting with Fe3O4. Therefore, the gas production at a high iron oxide conversion (R ) 17) includes the gas production at different degrees of conversion of the material. The extent to which the gas production changed can be seen in Figure 2, where the gas

58


Figure 4. Average gas production during reduction/gasification with pyrolysis oil over and BIC iron and BF oxide bed with Fe3O4 /oil ratios of 17 and 150.

production with increasing oil injection time (decreasing Fe3O4/ oil ratio) for two temperatures is shown. The total dry gas production measured in the experiments with both iron oxides was about 1.2 Nm3/kg of dry oil when a high conversion of the iron oxide was obtained (R ) 17). A lower dry gas production (average 1.0 Nm3/kg of dry oil) obtained at a low conversion (R ) 150) can be explained by the fact that the reduction rate was higher and, therefore, more H2O was produced at the expense of hydrogen. Both iron oxides favored the formation of H2 compared to the sand bed, when a high conversion (R ) 17) of the iron oxide was achieved and at a temperature lower than 870 °C. A lower hydrogen production would be expected if reduction reactions with hydrogen were taking place. Both iron oxides worked as shift catalysts, resulting in more H2 (with the BIC iron oxide being the more active catalyst) and, thus, the highest hydrogen production under these conditions. Although the shift reaction

does not influence the total reduction potential of the gas phase, it does influence the concentration of H2 and CO. As the reduction with H2 is faster than that with CO, the shift reaction might ultimately influence the reduction rate of the iron oxide with the gas phase.12 The change in temperature hardly had an effect on the hydrogen production when the conversion of the iron oxide was relatively high, probably because of the dominating water-gas shift equilibrium. The hydrogen production decreased with increasing temperature especially for the BIC iron oxide when the conversion was low (R ) 150), indicating that the reduction reaction benefitted. A lower hydrocarbon production was obtained with iron oxide compared to sand, but the differences were small, especially at high temperatures (see Table 6). This small difference was mainly caused by the increased C2+ conversion with iron oxide as the bed material. The methane production was hardly affected by the temperature or the presence of the iron oxide. Reforming


59

Table 6. Hydrocarbon Compounds Produced from Pyrolysis Oil by Gasification, Using Iron Oxide and an Inert Sand As Bed Material BIC Fe3O4/oil (g/g): T (°C): CH4 [Nm3/(kg of dry oil)] C2H4 [Nm3/(kg of dry oil)] C2H6 [Nm3/(kg of dry oil)]

BF

sand

17 800

17 950

17 789

17 950

810

950

0.10 3.3 × 10-2 2.0 × 10-3

0.10 0.4 × 10-2 -

0.10 3.6 × 10-2 2.0 × 10-3

0.09 1.8 × 10-2 -

0.12 5.0 × 10-2 1.7 × 10-3

0.11 1.5 × 10-2 0.4 × 10-3

of the methane would require a reforming catalyst, preferably nickel. Addition of such a catalyst to the fluid bed containing iron oxide as well would therefore benefit the overall process efficiency. The amount of C2+ hydrocarbons produced during reduction was influenced by the temperature and did not appear to depend on the conversion or type of iron oxide. In our previous work,3 we stated that BIC iron oxide has a beneficial effect on the steam reforming of C2+ components compared to BF iron oxide, which was observed in the present experiments only for the results obtained at 950 °C (see conversion of C2H4 in Table 6). The low surface area of the deactivated BIC iron oxide particles probably reduces the catalytic activity of the iron oxide. The CO + CO2 production and the CO/CO2 ratio compared with the equilibrium ratio (based on the reduction of Fe3O4 to Fe0.945O with CO) are also given in Figure 4. The CO + CO2 production clearly increased with increasing temperature and did not seem to be dependent on the conversion of the iron oxide. The increased production was mainly caused by the enhanced carbon-to-gas conversion of oil (see section 3.5) with increasing temperature. The CO/CO2 ratio was much lower with an iron oxide bed than with the sand bed under all experimental conditions applied. The CO/CO2 ratio with the sand bed was in the range of 6-7, as only small amounts of CO2 were produced. The lower CO/ CO2 ratio with the iron oxide is an indication that reduction reactions were taking place. The equilibrium CO/CO2 ratio was obtained with BIC iron oxide when the conversion of the iron oxide was low (R ) 150). This was also the case for the BF iron oxide, but only if a high temperature of 975 °C was applied. When the conversion of the iron oxide increased, the equilibrium CO/CO2 ratio was not achieved (R ) 17), which indicates that the full reduction capacity of the oil was not being used. Increasing the temperature was beneficial for the reduction (lower CO/CO2 ratio) with BF iron oxide, whereas it seemed to have no effect on the CO/CO2 ratio for the BIC iron oxide. A better reduction of the iron oxide with CO could be obtained with the BF iron oxide than with the BIC iron oxide at temperatures higher than 825 °C and high conversions of the iron oxide (R ) 17). When a relatively high conversion of the iron oxide was obtained, the reduction rate decreased (section 3.2), so that other reactions, such as the water-gas shift reaction, determined the final gas composition after reduction. This could result in a more constant CO/CO2 ratio for the BIC iron oxide with temperature. 3.4. Hydrogen Production. The hydrogen produced in the oxidation provides the only means of determining the oxygen transfer of the iron oxide per unit of oil. This hydrogen production per kilogram of dry oil is shown in Figure 5 for different relative conversions of the iron oxide obtained in the redox cycle. This figure shows that the hydrogen production per kilogram of dry oil decreased with increasing relative conversion of the iron oxide. This means that the efficiency of the reduction with oil was high at low conversion of the iron oxide, but decreased strongly when the relative conversion of the iron oxide increased during reduction. This is also discussed in section 3.2, where a decrease in the relative

conversion rate is described with increasing conversion of the iron oxide. The relative conversion rate decreased strongly when the relative conversion of the iron oxide approached a certain maximum value (Rmax). For both iron oxides, the experimental data points have a linear appearance for the tests performed at temperatures less than about 900 °C, with an intercept of the x axis near the maximum obtainable conversion (Rmax). For higher temperatures, the hydrogen production leveled off with increasing conversion, which was caused by the low conversion rate. A limitation of the reduction of magnetite to wustite based on thermodynamics is not expected, as the reduction potential of the gas phase obtained when pyrolysis oil is gasified is high enough to enable the full reduction of magnetite to wustite.3,4 It can therefore be concluded that the full reduction to wustite was kinetically restricted; transport of reactant/product gases or Fe2+ ions to the reaction site through the product layer (Fe1-δO) must have become slow or impossible with increasing conversion. In our previous work,5 it was shown that the conversion of iron oxide decreases with increasing water content in the gas phase. In this case as well, a lower conversion than expected (based on thermodynamics) was obtained, which could only be attributed to kinetic limitations, such as diffusion, in the material. The BIC iron oxide appeared to approach Rmax at a lower conversion than the BF iron oxide. Apparently, the reduction

Figure 5. Hydrogen production obtained during oxidation for different temperatures and relative conversions of iron oxide in the redox cycle. Relative conversion (R) as defined in equation 8.

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Figure 6. Carbon-to-gas conversion (ζC to gas) and carbon deposition (ζC deposit ) ζC total - ζC to gas) obtained after oil injection using BIC and BF iron oxide as bed materials for different temperatures. The data shown were obtained from experiments with an Fe3O4/oil ratio of 900 °C) and a low relative conversion of the iron oxide (see Figure 5). This could be expected at these temperatures, as the experimental oil-to-gas conversion was near completion at these temperatures (see Figure 6) and, therefore, the whole oil contributed to the reduction (see section 3.5). For lower temperatures, only part of the oil was converted to the gas phase, and therefore, the equilibrium hydrogen production could not be reached in the experimental setup at these temperatures. The higher hydrogen production with BIC iron oxide compared to BF iron oxide at low conversion could be a result of the presence of additives in the BIC iron oxide that enable a better reduction. Furthermore, the enhanced water-gas shift reaction, especially with BIC iron oxide, resulted in a better reduction at low temperatures. 3.5. Carbon Conversion. The carbon-to-gas conversion (ζC to gas) is shown in Figure 6. Only data for R < 60 are included in this figure, because these data points were less influenced by experimental errors. It was previously observed, from the CO

and CO2 gas production in Figure 4, that the carbon-to-gas conversion was mainly influenced by temperature and hardly by the conversion of the iron oxide. The scatter observed in Figure 6 is therefore not expected to be caused by a difference in conversion of the iron oxide obtained in the experiments. The scatter might be due to agglomeration of the iron oxide in the fluidized bed or fluctuations in the atomization of the oil. Strong agglomeration of the iron oxide (no fluidization of the bed material measured) occasionally occurred at the highest temperatures applied (T > 950 °C) and resulted in a decrease of the carbon-to-gas conversion and an increase in the formation of carbonaceous compounds in the reactor. The experiments in which strong agglomeration occurred are not included in Figure 6. This, however, shows that, if some agglomeration of iron oxide took place, it might influence the carbon-to-gas conversion during oil injection. Scatter might also be a result of small differences in atomization of the pyrolysis oil with each experiment. If carbonaceous compounds formed on the atomizer during oil injection and were not removed (during oxidation or by the attrition of the iron oxide bed), it would result in a slightly worse atomization of the oil in the following experiment. Near-complete conversion of oil to the gas phase was possible in a BIC iron oxide bed; at a temperature of 1030-960 °C, an oil-to-gas conversion of 99.6% (R ) 17) could be obtained. Another experiment at a temperature of 965-906 °C showed an oil-to-gas conversion of 98.0% (R ) 17). Thus, complete gasification at high temperatures is possible, although the experiments were afflicted with some technical difficulties. Furthermore, the high oil-to-gas conversion at R ) 17 shows that the reaction of the carbon deposits with the iron oxide was less influenced by the conversion of the iron oxide. Apparently, the presence of carbon on the surface resulted in a high local reduction potential, which resulted in a better reduction even when a product layer had been formed. The gas production during reduction and oxidation is shown at 800 and 950 °C (R ) 17) in Figure 2. It can clearly be seen from this figure that the reaction of carbon with the iron oxide continued after oil injection at a temperature of 800 °C, which did not occur at 950 °C. This shows that the reaction of iron oxide with carbonaceous deposits was much slower when a low temperature was applied. The reaction of deposited carbon with the iron oxide clearly had an effect on the carbon-to-gas conversion during oil injection. This can be seen in Figure 6, where it is shown that the carbon-to-gas conversion was higher when an iron oxide bed was used instead of a sand bed at temperatures above 850 °C. Apparently, gasifying the pyrolysis oil over a fluidized bed containing an active solid oxygen source (the iron oxide) improved the gasification of the oil. At temperatures below 800 °C, the carbon-to-gas conversion was hardly influenced by the presence of the iron oxide. In Figure 6, the amount of carbon from the oil that reacted with the iron oxide in the period between oil injection and oxidation (ζC deposit) is also shown. There is no distinction in the amount of carbon reacting in the second reduction phase between the BIC and BF iron oxides. Figure 6 also shows that hardly any carbonaceous deposits remained on the surface after oil injection at temperatures above 850 °C. The reaction of the carbonaceous deposits with Fe3O4 was tested by thermogravimetric analysis (TGA) and is discussed in more detail in section 3.6. The carbonaceous compounds that reacted with the iron oxide probably consisted of secondary char (carbon content > 70-73 wt %).18 The decrease in the measured amount of carbon deposition on the iron oxide (ζC deposit) in the second reduction


Figure 7. TGA results of (A) an oil sample and (B-D) three different BIC iron oxide/oil samples [1.0-3.3 (g/g)]. Tramp ) 20 °C/min, N2 flow ) 30 mL/min (STP).

step at high temperatures could be caused by two mechanisms: (I) less char being formed on the iron oxide during oil injection with increasing temperature or/and (II) the reaction rate of iron oxide with char becoming higher with increasing temperature. In a previous publication,3 we found that the amount of carbon deposits is independent of temperature in a sand bed. Therefore, the first mechanism is unlikely. Rather, mechanism II can explain the decrease in the amount of carbon deposits measured in reduction step II and can also explain the improved carbonto-gas conversion during reduction step I when higher temperatures were applied in the reduction. 3.6. Iron Oxide Oil Reaction by TGA. To study the reaction of the carbonaceous deposits formed during the gasification of oil with iron oxide, thermogravimetric analysis (TGA) was performed. In the TGA experiments, oil evaporation from oil and oil/BIC iron oxide mixture samples was studied. The experiments were carried out with a temperature ramp of 20 °C/min under a nitrogen flow of 30 mL/min (STP). In these experiments, the iron oxide was present in the form of Fe2O3 at the start of the reaction, in contrast to the fluidized-bed experiments, where the BIC iron oxide consisted mainly of Fe3O4 at the start of the reaction. Figure 7A shows the weight loss rate when the oil was heated. At the start of the ramping up of the temperature, light compounds and water clearly evaporated from the oil, resulting in a peak at about 100 °C. The evaporation of the oil sample was almost finished at a temperature of 550 °C, with an oilto-gas conversion (mass-based) of 87.9%. At this temperature, most of the components in the oil had evaporated, and the residue consisted mainly of secondary char.18 The weight loss rate of the oil sample was close to zero when the temperature ramp was continued from 550 to 900 °C in the TGA apparatus. In Figure 7B-D, the weight loss rates for different oil/BIC iron oxide mixture samples (in different weight ratios) are given. Again, a high rate is visible at 100 °C, because of the evaporation of light components and water from the oil. Between 200 and 550 °C, a steady decrease in the weight is observed because of continued oil evaporation, and for oil/BIC iron oxide mixtures, the reductions of CuO to Cu and Fe2O3 to Fe3O4 are also expected to take place here (based on thermodynamics).

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The most important observation is the decrease of the sample weight at temperatures above 550 °C when iron oxide was present in the sample. This can only be a result of the reaction of char with the iron oxide (reduction of magnetite to iron), which slowly started to take place at T J 550 °C. The reaction became fast at temperatures above 650 °C and was completed at about 850 °C (see Figure 7B-D). Decreasing the BIC iron oxide/oil ratio in the sample resulted in a higher temperature or longer time (as a temperature ramp was applied) for the reaction to be completed, as well as a higher conversion of the iron oxide. The total relative conversion of oil and BIC iron oxide in a TGA experiment with a BIC iron oxide/oil ratio of 1.0 was 95% [) ∆M/(Moil + Mreactive O in sample) × 100%, where M represents mass]. This means that the amount of oil added in this sample was nearly sufficient to obtain a complete reduction of the iron oxide in the sample to iron. In all further TGA experiments, the BIC iron oxide/oil ratio was kept above 1.0, in which case the amount of iron oxide was sufficient to convert all of the deposited char (based on a stoichiometric reduction reaction to CO2). XRD analysis of some TGA samples after reduction confirmed that all of the char reacted with the iron oxide. Decreasing the BIC iron oxide/oil ratio below 1.0 would not result in a much higher conversion of the iron oxide and would actually result in a lower total relative conversion of the sample, as a part of the char would not react. In the fluidized-bed reactor, an iron oxide/oil ratio well above 1.0 was used (R > 17). Figure 7 also shows that two peaks appeared with decreasing BIC iron oxide/oil ratio. This might be a result of the two-step reduction reaction (Fe3O4 f Fe1-δO f Fe), which apparently became more pronounced with an increased amount of char on the iron oxide. In Figure 6, the amount of char on the iron oxide (28% C of the total carbon input) expected at 550 °C is shown. This is based on the amount of char formed from an oil sample in the TGA apparatus at this particular temperature. According to TGA with the BIC iron oxide/oil samples, char does not react with magnetite at this temperature, and it can therefore be related to the amount of carbonaceous deposits on iron oxide in Figure 6. The change in the amount of char on the iron oxide with increasing temperature (above 550 °C) was calculated from the TGA results (see Figure 6 for BIC iron oxide/oil ratios of 3.3 and 1.4). In this calculation, the measured weight loss in the TGA experiment was related to the weight of the gaseous products formed. In the reaction of char with iron oxide, the products could be CO, CO2, and some H2O. From calculations, the measured weight loss could only be attributed to the formation of H2O and CO2. In the case of H2O and CO formation, not enough char was present on the surface to explain the measured weight loss. The amount of carbon remaining on the iron oxide (ζdeposit) in the TGA experiment was derived from the amount of CO2 produced in the reaction of char with iron oxide. The amount of char on the iron oxide in the TGA samples showed a trend similar to that found for the amount of char on the iron oxide surface in reduction step II in the fluidized bed. In both the TGA and fluidized-bed experiments, the reaction of char with the magnetite hardly occurred below 650 °C, and a temperature of 850 °C was high enough to convert almost all carbon deposits to the gas phase (see Figure 6). Based on thermodynamics, the reaction of magnetite with solid carbon can only occur for temperatures above 700 °C (∆Greaction < 0). The reaction of char with magnetite starts at a slightly lower temperature (∼650 °C), probably because of the presence of oxygen in the char.

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Figure 8. Proposed mechanism for pyrolysis oil gasification over an iron oxide bed.

The TGA confirmed that the use of iron oxide at temperatures above 650 °C can increase the carbon-to-gas conversion in the fluidized bed, as the reaction of char with iron oxide is enabled above this temperature. However, the rate of reaction of char with iron oxide is high enough to enhance the carbon-to-gas conversion during oil injection in the fluidized bed only at temperatures above 850 °C. 3.7. Oil Gasification Mechanism. When oil is atomized in an inert hot sand bed, small oil droplets are formed and can either hit the solid particles and evaporate on the solid particle surface or, depending on the size, evaporate before hitting a particle. In the first case char will deposit onto the solid particle, while part of the oil is evaporated. In the second case, light char particles are formed in the gas phase that can be entrained from the fluidized bed or can deposit on the solid surface. For the latter, contacting has to be efficient, and the solid carrier and char should be cohesive. The char that deposits on the particles can be removed from the surface by the movement of the particles in the fluidized bed. In the sand-bed experiments, an average of about 5% of the total carbon input was found on the sand for all temperatures after oil injection.3 This amount is likely related to the specific surface area of the particles, as no temperature influence was found. With a higher surface area, more carbon would be expected to remain on the surface and be less easily removed by attrition. About 70% of the carbon3 was converted to the gas phase at the highest temperature measured in the sand bed, and therefore, about 25% of the carbon was not recovered when sand was used. The unrecovered carbon is attributed to the formation of low-density char particles in the gas phase, which could not be separated or analyzed and left the setup with the product gas. The reaction of carbon with iron oxide can take place only if direct contact between the two phases is established. Because, in some cases, a nearcomplete carbon conversion to the gas phase was found at high temperatures, it is concluded that, in principle, it is possible that all formed char eventually contacts the iron oxide particles and reacts to the gas phase. The gasification data result in the proposed mechanism as shown in Figure 8. This mechanism describes the gasification of char (from the oil) to the gas phase when iron oxides are used in the fluidized bed. The char particles in the gas phase can deposit again on the iron oxide before being entrained from the reactor, increasing the possibility for the char to react to CO and CO2. The amount of char initially formed on the surface of the iron oxide depends

on the contacting pattern between the iron oxide particles and oil in the fluidized bed. The rate of attrition is largely determined by the porosity of the particles, the possibility of char penetrating the pores, and the fluidization conditions. The reaction of char with iron oxide is enhanced by increasing the temperature and the reactivity of the bed material. Complete gasification of oil to the gas phase during oil injection is therefore possible, if the overall reaction rate of iron oxide with char is higher than the overall rate of char removal by attrition and the overall rate of char formation. The char formed during gasification of the oil should be converted to CO and CO2 before being entrained from the reactor to obtain a full oil-to-gas conversion and to optimally use the reduction potential of the oil. The final amount of char on the particle leaving the reactor should be zero to obtain a pure hydrogen product in the redox cycle, without using an intermediate stripping section. 3.8. Steam Conversion. The steam conversion in the oxidation was found to be dependent on the relative conversion of the iron oxide, the temperature applied, the contact time, and the type of iron oxide used. In general, a high temperature resulted in a higher reaction rate but decreased the equilibrium steam conversion during oxidation of the iron oxide.4 The reaction rate was also influenced by the conversion achieved during reduction, as the oxidation became very slow when the oxidation to Fe3O4 was near completion. A higher relative conversion of the iron oxide therefore resulted in a higher oxidation rate at the start of the oxidation, but eventually also led to a low oxidation rate (see Figure 9). In practice, the steam conversion in the oxidation needs to be at equilibrium in order to make the overall process efficient.4 To achieve this equilibrium conversion in the setup, the contact time of the steam with the iron oxide should be sufficient. The oxidation rate of the BIC iron oxide is higher than that of the BF iron oxide,3,5 and therefore, the equilibrium steam conversion can be obtained with the BIC iron oxide for high R (see Figure 9 and ref 3). This is not the case for the BF iron oxide, and therefore, experiments were performed to check whether the contact time of the steam with the BF iron oxide and the concentration of steam in the gas phase could influence the steam conversion. In these experiments, the equilibrium steam conversion was not reached, although increasing the concentration or contact time resulted in a higher steam conversion. A long contact time required to obtain an equilibrium conversion of steam in the oxidation will have a negative impact on the reactor size of the


63

Acknowledgment The authors gratefully acknowledge the funding support within the Sustainable Hydrogen Program of ACTS/NWO in The Netherlands. Nomenclature

Figure 9. Steam conversion (Tox ) 970 °C) during oxidation for different relative conversions (R) obtained in the reduction. Relative conversion (R) as defined in equation 8.

oxidator. This will especially be the case when a low conversion of the iron oxide is used in the redox cycle. On the other hand, a low conversion of the iron oxide in the redox cycle is beneficial for the iron oxide reduction with oil. 4. Conclusions This study found that both the temperature and conversion of iron oxide strongly affect the efficiency of hydrogen production from pyrolysis oil in the steam-iron process. According to existing particle models, the relative conversion rate of iron oxide in both reduction and oxidation is expected to decrease to some extent with increasing relative conversion of the iron oxide. However, the observed decrease in the relative conversion rate in the reduction when pyrolysis oil was used was much higher. Also, the final relative conversion that could be obtained with pyrolysis oil was lower than expected (based on thermodynamics), and a full reduction to wustite could not be achieved at the applied temperatures. The presence of water in the pyrolysis oil most probably restricts the conversion during reduction. This results in a pore utilization of the iron oxide, especially at temperatures of around 800 °C, and in an enhanced decrease in the relative conversion rate with increasing conversion. The maximum obtainable relative conversion of the iron oxide can be increased by increasing the temperature. Complete gasification of pyrolysis oil (>98%) could be achieved in the fluidized bed employed in this work at temperatures above 850 °C with BIC iron oxide (catalytic iron oxide for ammonia synthesis) as the bed material. The improved gasification can be attributed to the fast reaction of deposited char with the iron oxide. In the experimental setup, the maximum possible (according to equilibrium) hydrogen production in the redox cycle was nearly achieved at a low conversion of the BIC iron oxide (R ) 7%) and a high temperature (998 °C). The hydrogen productions under these conditions with BF (blast furnace) iron oxide (which does not reach equilibrium) and BIC iron oxide were, respectively, 1.39 and 1.82 Nm3 of H2/kg of dry oil. In these two cases, near-complete gasification of the oil and a low conversion of the iron oxide were maintained during reduction, as required to achieve an equilibrium hydrogen production in the process. The relative conversion rate of the iron oxide during oxidation in our setup was low when the oxidation was near completion. This shows that a low conversion of the iron oxide in the reduction has a negative effect on the oxidation. Very long contact times will be required to achieve an equilibrium steam conversion in the oxidation with a low conversion of the iron oxide.

CA ) concentration of reduction gases (H2 + CO) (mol/m3) CP ) concentration of reduction products (H2O + CO2) (mol/m3) f ) weight fraction f1(R) ) function which describes the effect of the relative conversion on the reduction rate f2(gas) ) function describing the effect of the gas concentration on the conversion rate (mol/m3)* [The asterisk means when the reaction is first-order] Ke ) equilibrium constant k(T) ) temperature dependent rate constant (m3/mol s) M ) molecular weight (g/mol) Mbed ) mass of the bed material (kg) n ) fit parameter P ) pressure (Pa) R ) mass ratio of iron oxide and oil Rg ) gas constant (J/mol K) S0 ) amount of oxygen in iron oxide (mol) T ) temperature (°C) t ) time (s) τd ) delay time (between reactor and analyzer) (s) ureactor ) superficial gas velocity in the fluidized bed reactor (m/s) XH2O ) weight fraction of water R ) relative conversion R(Fe3O4-Fe1-δO) ) reduction degree of iron oxides, where Fe3O4 is the start material (R ) 0) and Fe1-δO is the final product (R ) 1) Rmax ) maximum conversion of the iron oxide obtained in the reduction ∆Hr ) reaction enthalpy (kJ/mol) ∆Gr ) reaction Gibbs energy (kJ/mol) ζC · to · gas ) conversion of carbon from the oil to the gas phase ζC · total ) conversion of carbon from the oil to the gas phase and deposited on the bed material ζC · deposit ) amount of carbon from the oil that is deposited on the bed material ζH2O ) conversion of steam during oxidation ∆Oiron · oxide ) amount of oxygen in the iron oxide that reacts in the redox cycle (mol) F ) density (kg/m3) Φm ) mass flow (kg/s) Φv ) volumetric flow (NL/s) Φ ) molar flow (mol/s) * ) when reaction is first order

Literature Cited (1) Ramage, P. R.; Agrawal, R., The Hydrogen Economy: Opportunities, Costs, Barriers and R&D Needs; The National Academies, National Academies Press: Washington, DC, 2004. (2) Kreith, F.; Yogi Goswani, D., Handbook of Energy Efficiency and Renewable Energy; CRC Press, Boca Raton, FL, 2007. (3) Bleeker, M. F.; Kersten, S. R. A.; Veringa, H. J. Pure hydrogen from pyrolysis oil using the steam-iron process. Catal. Today 2007, 127, 278. (4) Bleeker, M. F.; Gorter, S.; Kersten, S. R. A.; van der Ham, A. G. J.; van den Berg, H.; Veringa, H. J. Hydrogen production from pyrolysis oil using the steam-iron process. A process design study. Clean Technol. EnViron. Policy, published online Jul 8, 2009,http://dx.doi.org/10.1007/ s10098-009-0237-0.

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(5) Bleeker, M. F.; Veringa, H. J.; Kersten, S. R. A. Deactivation of iron oxide used in the steam-iron process to produce hydrogen. Appl. Catal. A: Gen. 2009, 357, 5. (6) Takenaka, S.; Hanaizumi, N.; Son, V.; Otsuka, K. Production of pure hydrogen from methane mediated by the redox of Ni- and Cr-added iron oxides. J. Catal. 2004, 228, 405. (7) Hacker, V. A novel process for stationary hydrogen production: The reformer sponge iron cycle (RESC). J. Power Sources 2003, 118, 311. (8) Szekely, J.; Evans, J. W.; Sohn, H. Y. Gas-Solid Reactions; Academic Press: New York, 1976. (9) Levenspiel, O. The Chemical Reactor Omnibook; Oregon State University Bookstores: Corvallis, OR, 1979. (10) Doraiswamy, L. K.; Sharma, M. M. Heterogeneous Reactions: Analysis, Examples, and Reactor Design; John Wiley & Sons Inc.: New York, 1984; Vol. I: Gas-Solid and Solid-Solid Reactions. (11) Heesink, A. B. M. High Temperature Coal Gas Desulphurization; Offest-drukkerij CopyPrint: Enschede, The Netherlands, 1994. (12) Gasior, S. J. Production of Synthesis Gas and Hydrogen by the Steam-Iron Process: Pilot Plant Study of Fluidized and Free-Falling Beds, Report of Investigations 5911; Bureau of Mines, U.S. Department of the Interior: Washington, DC, 1961.

(13) Bleeker, M. F. Pure Hydrogen from Pyrolysis Oil by the SteamIron Process; Ipskamp Drukkers: Enschede, The Netherlands, 2009. (14) Tokuda, M.; Yoshikoshi, H.; Ohtani, M. Kinetics of the reduction of iron oxide. ISIJ Int. 1973, 13, 350. (15) McKewan, M. C. Reduction kinetics of magnetite in H2/H2O/N2 mixtures. Trans. Metal. Soc. AIME 1961, 221, 140. (16) McKewan, M. C. Reduction kinetics of hematite in hydrogenwater vapor-nitrogen mixtures. Trans. Metal. Soc. AIME 1962, 224, 2. (17) Wimmers, O. J.; Arnoldy, P.; Moulijn, J. A. Determination of the Reduction Mechanism by TPR: Application to Small Fe2O3 Particles. J. Phys. Chem. 1986, 90, 1331. (18) Branca, C.; Di Blasi, C.; Elefante, R. Devolatization and heterogeneous combustion of wood fast pyrolysis oils. Ind. Eng. Chem. Res. 2005, 44, 799.

ReceiVed for reView April 2, 2009 ReVised manuscript receiVed October 8, 2009 Accepted October 13, 2009 IE900530D

Pure Hydrogen Production from Pyrolysis Oil Using ... - ACS Publications

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