Fe₂O₃ as oxygen carriers in the air reactor, the main reactions at 750˚C probably are:

In the fuel reactor, the main reactions probably are: pyrolysis of biomass:

The reduction reactions of oxygen carrier particle with pyrolytic products of biomass at 650˚C [9]

Therefore, in the presence of oxygen carriers, these reactions occur sequentially and simultaneously during biomass pyrolysis and gasification. The final products of biomass gasification are determined by the interaction of a couple of above mentioned reactions.

Effect of the temperature on the Gibbs free energy (∆rG) and chemical equilibrium constant (Logk) of reactions (4)-(11) was shown in Figure 2. In Figure 2, it found that the ∆rG is less than zero and the Logk is more than zero, which means that the reactions (4)-(11) can react in the thermodynamics. If dividing the reactions (4)-(11) into four groups, that is (4)-(5); (6)-(7); (8)-(9); (10)-(11), it can be seen that the former is less than the later for ∆rG, but the trend of Logk is opposite from the Figure 2, further, the trends of change increased with the increase of temperature. As we known, if the smaller ∆rG and the greater Logk, the reaction will be easier to occur. So, it can conclude that oxygen carriers were reduced gradually in the atmosphere of biomass pyrolysis. It meant the oxygen carriers changed as follows: Fe₂O₃ ® Fe₃O₄ ® FeO.

There were a number of reactions in the CLG according to the theory analysis. However, some reactions were predominant and the other reactions were secondary in the actual process. If the secondary reactions were ignored, it can help us a better understanding of the CLG. According to the principle of Gibbs free energy minimization, equilibrium components of biomass with Fe₂O₃

were investigated through HSC Chemistry 5.1 software, which is a produced by Outokumpu company in Finland. The approximate formula of biomass is CH_1.34O_0.65, regardless the S and N.

In order to illustrate the influence of the biomass to Fe₂O₃ ratio on equilibrium composition, the equilibrium composition was calculated by changing Fe₂O₃ content at the same temperature. The result was shown in Figure 3. The temperature is set at 750˚C and the initial value of CH_1.34O_0.65 is 1kmol in the process. In Figure 3(a), the Fe₂O₃ content was gradually increased from 0.02 kmol to 6 kmol. As seen in figure, the content of incomplete oxidation products (CO + H₂) gradually decreased and the content of complete oxidation products (CO₂ + H₂) gradually increased with the amount of Fe₂O₃ gradually increased, respectively. As the amount of Fe₂O₃ is nearly 5 kmol, the amount of H₂ + CO is almost zero, and the content of H₂O + CO₂ is almost constantly. At the same time, the amount of carbon which produced by carbon deposit reaction decreased with increase the amount of Fe₂O₃, the carbon is almost disappeared as the Fe₂O₃ content near 1kmol. It can be observed the reduction products of Fe₂O₃ are mainly FeO and Fe₃O₄. As the amount of Fe₂O₃ is less than 3.5 kmol, the main reduction product is FeO, however, the main reduction product is Fe₃O₄ when the amount of Fe₂O₃ is more than 3.5 kmol. In this paper, it concerned to produce syngas which is mainly composed of H₂ and CO. Thus, in order to obtain syngas instead of CO₂ and water, it is necessary to keep the ratio of lattice oxygen to fuel as low as to prevent the fuel from being fully oxidized to CO₂ and H₂O. The effect of the amount of Fe₂O₃ from 0.02 to 1.2 kmol on the equilibrium composition is shown in detail in Figure 3(b). As shown in the figure, the amount of CO reaches maximum when the Fe₂O₃ content is about 0.2 kmol, and then which is declined gradually with the amount of Fe₂O₃ increasing. It is worth to note that the amount of CO₂ is more than CO as the Fe₂O₃ content is more than 0.7 kmol, which means that biomass is completely oxidized due to the lattice oxygen increasing. So, it can produce more CO₂ rather than CO. Therefore, the amount of Fe₂O₃ must be kept between 0.2 kmol and 0.7 kmol to obtain the syngas. The reduction product of Fe₂O₃ is mainly FeO, and almost nonexistent Fe₃O₄. At the same time, it can infer that carbon deposition was caused by material excessive from the Figure 3.

According to the analysis of the thermodynamic theories, it can obtain the synthesis gas which the main component is CO and H₂ through the ratio of fuel to lattice oxygen is kept low level. Meanwhile, the main reduction product of Fe₂O₃ is FeO.

In the next work, CLG of biomass with natural hematite as oxygen carrier was experimentally studied in a lab-scale bubbling fluidized bed reactor using argon as

fluidized gas.

The sawdust of pine with particle sizes between 250 - 425 μm was dried in the oven which kept 105˚C. The dry-basis proximate analysis and ultimate analysis of the sawdust are showed in Table 1. The hematite with particle sizes between 180 - 250 μm which was supplied by Guangdong Iron & Steel Group Co. Ltd. The elements composition analysis of the hematite is presented in Table 2. According to the Fe³⁺ fraction, it is calculated that the Fe₂O₃ content is about 81.66% in the hematite.

The experiments were conducted with a bubbling fluidized bed reactor of quartz placed in a transparent furnace, as is illustrated in Figure 4. The reactor is in length of 1000 mm and an inner diameter of 60 mm. The bed tem-

Table 1. Proximate and ultimate analysis of pine.

^*: by difference.

Table 2. Elements composition analysis of iron ore (%).

perature was measured 5 mm above the porous quartz plate using a K-type thermocouple. Oxygen carrier particles were placed on the porous plate and were then heated in air flow to set temperature. During the reducing period, the fluidizing gas was argon (800 L/hr), which was introduced from the bottom of the reactor. When the temperature comes to the set value, biomass was continuously fed (by a screw feeder) in a hopper at the top of the reactor. At the same time, argon (200 L/hr) was introduced from the top of the hopper. Therefore, the biomass sample was pushed by argon flow into the fluidized bed through a drop tube. The flue gases are passed through the cold trap to collect solid particles, water, and tar then introduced into the sampling bag. The composition of the gas products is measured using a gas chromatograph (SHIMADZUA Gas Chromatograph, GC-20B). All the oxygen carrier samples were performed with an X-Ray Diffraction (X’Pert PRO XRD).

Based on the thermodynamic analysis, the mass of oxygen carriers and biomass material were set to 150 g and 44 g in a single test, respectively. The ratio of lattice oxygen to fuel is 0.42.

The effect of the presence of hematite particles on the CLG of biomass was investigated at temperatures of 750˚C. Figure 5 shows the comparison of the gas yields

of each gaseous component and the sum of the yields of H₂, CH₄, CO and CO₂ in the process of biomass CLG with those of blank tests during testing period of 30 min. In the blank tests, the hematite particles were replaced by silica sand. Therefore, the process taking place in the blank tests is only pyrolysis rather than CLG of biomass. As shown in Figure 5, the volume yields of each gaseous constituent were obviously enlarged in the presence of hematite particles. The total gas yields were increased from 25.17 L up to 42.39 L at 750˚C. The yields of the generated gases (H₂, CH₄, CO, and CO₂) obtained with hematite particles were apparently larger than with silica sand, which suggest that the lattice oxygen in the hematite particles was used as the gasifying agent during biomass gasification. The main components of gaseous product are CO and H₂ which reached about 70% amount of the total volume of the gaseous product. The promoter action of lattice oxygen to carbon conversion was more obvious than to hydrogen conversion. So, the carbon can convert into carbon dioxide and carbon monoxide more completely, it can cause the volume of gaseous product which contained carbon element going up obviously, especially the volume of carbon dioxide. Figure 5 also illustrates gas concentrations (on the basis of argon free) of biomass gasification with oxygen carrier and pyrolysis with silica sand during 30 min at 750˚C. Clearly, it was found that the concentrations of H₂, CH₄, CO and CO₂ were kept stable when biomass was pyrolyzed with inert silica sand. Whereas, there were significant changes for the gaseous concentration of biomass with hematite particles as reaction time increasing, especially the concentration of CO₂ and H₂. The CO and CH₄ concentrations of biomass with hematite particles were both lower than those of biomass with silica sand. H₂ concentration increased with the reaction time going on in the CLG of biomass probably owing to a part of biomass was totally oxidized into CO₂ and H₂O which subsequently accelerated the hydrogen generation reactions taking place rightward. In addition, in the initial stage of biomass gasification with hematite particles, the CO₂ fraction was more than 30% in the generated gas due to the easily availability of active lattice oxygen in the oxygen carrier particles. As the reactions proceeded, the concentration of CO₂ decreased gradually to 10% approximately. These results may be contributed to the amount of valid hematite particles decreasing with the reactions proceeding. So, portion of biomass taken out by fluidized gas after only pyrolytic reaction, which can cause the proportion of pyrolysis gas increasing in the gaseous products. Through the contrast test, it experimental verified the feasibility of biomass chemical-looping gasification with hematite particles as oxygen carriers.

Figure 6 shows the XRD patterns of fresh oxygen carrier, the oxidized oxygen carrier with the air, and the reduced oxygen carrier of the fuel reactor under different temperature, respectively. In Figure 6(b), it can be seen that the mainly existed in form of Fe₂O₃ and SiO₂ for fresh oxygen carriers. Comparison to fresh oxygen carrier, there was no chemical changes occurred in the regeneration of reacted oxygen carrier, and Fe₂O₃ was found the major phase with minor amounts of Fe₃O₄. The forms of oxygen carriers after the reduction reaction changed as temperature rising, as seen in Figure 6(a), at lower temperature (650˚C), the variation of oxygen carriers was from Fe₂O₃ to Fe₃O₄, the main existence form of reduced oxygen carriers was FeO at higher temperature (≥750˚C).

However, the variation of hematite particles as oxygen carriers was from Fe₂O₃ to Fe₃O₄ in the CLC of biomass [10-12].

The elements content of Fe in hematite particles was analyzed as shown in Figure 7, it found that the ferric

iron was reduced to ferrous iron in the CLG of biomass. The tendency of conversion was more and more obviously with increase of temperature. The capacity of lattice oxygen in oxygen carriers was recovered by air oxidization, which was affected slightly on the reduction temperature. So, it is feasible that the hematite particles can be used circularly as oxygen carriers in the CLG of biomass, which have higher oxygen transfer ability.

According to the analysis of existence form of reduced oxygen carriers, it found that hematite particles undergone the process of biomass gasification as oxygen carriers, and the mainly existence form of hematite particles were FeO after reduction as the temperature higher than 750˚C. Further, the reduction of hematite particles with biomass proceeded in a stepwise manner from Fe₂O₃ ® Fe₃O₄ ® FeO. So, the main reactions were (2), (5), (7), (9), and (11) in the CLG of biomass with the hematite particles as oxygen carriers.