Thermodynamic parameters of chemical reactions in the system were carried out through thermodynamic analysis. According to the Gibbs free energy minimization principle of the system, equilibrium composition of the reactions of chemical-looping gasification (CLG) of biomass with natural hematite (Fe2O3 ) as oxygen carrier were analyzed using commercial software of HSC Chemistry 5.1. The feasibility of the CLG of biomass with hematite was experimental verified in a lab-scale bubbling fluidized bed reactor using argon as fluidizing gas. It was indicated the experimental results were consistent with the theoretical analysis. The presence of oxygen carrier gave a significant effect on the biomass conversion and improved the synthesis gas yield obviously. It was observed that the gas content of CO and H2 was over 70% in CLG of biomass. The reduced hematite particles mainly existed in form of FeO. It was showed that the reduction of natural hematite with biomass proceeds in a stepwise manner from Fe2 O3 →Fe3O4 → FeO. Reduction product of natural hematite can be restored the lattice oxygen by oxidation with air.
Chemical-looping gasification (CLG) is a novel gasification technique that involves the use of oxygen carrier, which transfers oxygen from air to the fuel, which is partially oxidized into H2 and CO, avoiding the direct contact between them. Therefore, the air to fuel ratio is kept low to prevent the fuel from becoming fully oxidized to CO2 and H2O. [
There are some works studying chemical looping gasification of solid fuels. He et al. [
The objective of this investigation was to explore the possibility of using natural hematite (Fe2O3) as the oxygen carrier in CLG of biomass. In the present work, the thermodynamics of biomass chemical looping gasification was analyzed, and the same time, CLG of biomass with natural hematite as oxygen carrier was experimen-
tally investigated in a lab-scale bubbling fluidized bed reactor using argon as fluidizing gas.
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 [
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
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 CH1.34O0.65, 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
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
The experiments were conducted with a bubbling fluidized bed reactor of quartz placed in a transparent furnace, as is illustrated in
*: by difference.
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.
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
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
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.
The process in CLG of biomass mixed with natural hematite as oxygen carriers was analyzed using the theory of thermodynamics, and then, the verification test was studied in a bubbling fluidized bed reactor with argon as fluidizing gas. It was found that the theory analysis was coincidence with the results of experiment. Consequently, it is feasibility natural hematite as oxygen carriers in CLG of biomass. It can obtain synthesis gas which mainly included CO and H2 by limiting the ratio of lattice oxygen to fuel between 0.2 and 0.7. The presence of oxygen carriers could obviously affect the process of biomass thermal conversion, which can significantly increase gas yield and carbon conversion rate. The main components of gaseous product were CO and H2 which reached about 70% amount of the total volume of the gaseous product. The concentration of CO was the highest and the concentration of CH4 was the lowest during the biomass gasification. In addition, the concentrations of H2, CO, and CH4 in the product gas slowly increased with the reaction proceeding, and the CO2 concentration showed an opposite trend. XRD analysis showed that the iron element in the reduced hematite particles mainly existed in form of FeO, with minor formation of Fe3O4. Further, the lattice oxygen released, corresponding to the transformation Fe2O3 ® Fe3O4 ® FeO, provided the oxygen element needed in biomass gasification. The capacity of lattice oxygen in reduced oxygen carriers can be recovered through air oxidization.
The financial support of National Natural Science Foundation of China (51076154) is gratefully acknowledged. This work was also supported by Science and Technology Project of Guangdong (2010B010900047), “12th Five Years” National Science and Technology Support Program (2011BAD15B05).