Hydrogen Production in Steam Gasification of Biomass with CaO as a

Energy & Fuels 2008, 22, 1997–2004

1997

Hydrogen Production in Steam Gasification of Biomass with CaO as a CO2 Absorbent Ligang Wei,†,‡ Shaoping Xu,*,† Jingang Liu,† Changhou Liu,† and Shuqin Liu† State Key Laboratory of Fine Chemcials, Department of Chemical Engineering, Dalian UniVersity of Technology, Zhongshan Road 145, Dalian 116012, China, and Department of Chemical Engineering and Materials, Dalian Polytechnic UniVersity, Qinggong Yuan 1, Dalian 116034, China ReceiVed December 7, 2007. ReVised Manuscript ReceiVed March 5, 2008

Characteristics of steam gasification of biomass for hydrogen production at ambient pressure in a laboratoryscale external circulating concurrent moving bed (ECCMB) system with CaO as a CO2 absorbent were investigated. In this ECCMB system, steam gasification of biomass, in situ CO2 capture, combustion of the produced char, and calcination of CaCO3 can occur simultaneously. The experimental results verified that the in situ CO2 capture was achieved with calcined limestone as a CO2 absorbent, which increased the extent of the water-gas shift reaction and enhanced the yield of H2. The H2 content of 60-70 mol % in dry gas can be obtained at a steam/biomass weight ratio (S/B ratio) of 0.38–0.59 and a CO2 absorbent/biomass weight ratio (A/B ratio) of 20 at the reactor temperature of 700–800 °C. The results showed that the addition of a CaObased CO2 absorbent is a good solution to increase hydrogen content in dry gas from the ECCMB process. An irreversible deactivation of the CO2 absorbent occurred in the cyclic carbonation/calcination because of the sinter of CO2 absorbent particles and the formation of inorganic adhesions on the surface of the particles from the solid-solid reactions between biomass ash and CO2 absorbent.

Introduction Hydrogen is an important raw material widely used in the chemical industry.1,2 Among processes for hydrogen production, steam gasification of biomass is a promising technology for its renewable and environmental benefits.3–5 However, there are still many obstacles required to resolve until the commercial breakthrough could be obtained. The basic requirements for a high-efficient gasification technology consist of a low tar production and a high H2 content of product gas. Catalysts were employed in biomass gasification processes to enhance the yield and quality of product gas and decrease tar production by the reforming/cracking of the high-molecular-weight organic components.6 Therefore, steam gasification of biomass [approximately expressed as the global reaction (eq 1) involving the pyrolysis of biomass and reforming of hydrocarbons and tars] with the addition of catalysts has gained attention from many researchers in the world.7–9 * To whom correspondence should be addressed. Telephone/Fax: 86411-88993837. E-mail: [email protected]. † Dalian University of Technology. ‡ Dalian Polytechnic University. (1) Norbeck, J. M.; Johnson, K. NRMRL-RTP-202, submitted to the U.S. Environmental Protection Agency (EPA) under Cooperative Agreement CR824308-01-0. (2) Bridgwater, A. V. Chem. Eng. J. 2003, 91, 87–102. (3) Turn, S.; Kinoshita, C.; Zhang, Z.; Ishimura, D.; Zhou, J. Int. J. Hydrogen Energy 1998, 23, 641–648. (4) Pfeifer, C.; Rauch, R.; Hofbauer, H. Ind. Eng. Chem. Res. 2004, 43, 1634–1640. (5) Aznar, M. P.; Caballero, M. A.; Corella, J.; Molina, G.; Toledo, J. M. Energy Fuels 2006, 22, 1305–1309. (6) Devi, L.; Ptasinki, K. J.; Janssen, F. J. J. G. Biomass Bioenergy 2003, 24, 125–140. (7) Delgado, J.; Azar, M. P.; Corella, J. Ind. Eng. Chem. Res. 1997, 36, 1535–1543. (8) Courson, C.; Makaga, E.; Prtit, C.; Kiennemann, A. Catal. Today 2000, 63, 427–437.

heat

biomass + H2O 98 tar + char + ash + H2 + CO + CO2 + CH4 ...

∆H0 > 0 (1)

CaO as an active catalyst for tar destruction and water-gas shift reaction (eq 2) in biomass gasification has been widely studied.10–12 Delgado et al. investigated the catalytic tar removal for hot gas cleaning with calcined limestone and inert silica sand for comparison, downstream of a fluidized bed biomass steam gasifier. The results indicated that the tar content decrease from 8 g N-1 m-3 above with silica sand to 1 g N-1 m-3 below with calcined limestone. The most important reactions for tar disappearance over the catalyst have been considered to be steam reforming and steam cracking in addition to thermal cracking.10 Another role of CaO in gasification is an in situ CO2 absorbent (also called CO2 acceptor).13–15 The in situ CO2 capture as shown in the reaction (eq 2) facilitates the reaction (eq 2) to produce hydrogen. In this case, the product gas with a low CO2 content and a high H2 content can be obtained. On the other hand, the energy released from the exothermic CaO carbonation reaction (eq 3) may compensate for the energy required by the endothermic gasification reaction (eq 1), which could increase the capacity and efficiency of a specific gasifier. (9) Garcia, L.; Benedicto, A.; Romeo, E.; Salvador, M. L.; Arauzo, J.; Bilbao, R. Energy Fuels 2002, 16, 1222–1230. (10) Delgado, J.; Aznar, M. P.; Corella, J. Ind. Eng. Chem. Res. 1996, 35, 3637–3643. (11) Garcia, X. A.; Alarcon, N. A.; Gordon, A. L. Fuel Process. Technol. 1999, 58, 83–102. (12) Sutton, D. Fuel Process. Technol. 2001, 73, 155–173. (13) Curran, G. P.; Clancey, J. T.; Scarpiello, D. A.; Fink, C. E.; Gorin, E. Chem. Eng. Process. 1966, 62, 80–86. (14) Mondal, K.; Piotrowski, K.; Dasgupta, D.; Hippo, E.; Wiltowski, T. Ind. Eng. Chem. Res. 2005, 44, 5508–5517. (15) Corella, J.; Toledo, J. M.; Molina, G. Ind. Eng. Chem. Res. 2007, 46, 6831–6839.

10.1021/ef700744a CCC: $40.75  2008 American Chemical Society Published on Web 05/01/2008

1998 Energy & Fuels, Vol. 22, No. 3, 2008

CO + H2O T H2 + CO2 CaO + CO2 f CaCO3

∆H0 < 0 ∆H0 < 0

Wei et al.

(2) (3)

However, during the gasification process with CaO as bed material, the CO2 capture decays continuously with the consumption of the CaO, so that fresh CaO should be added into the system. A better solution is the reuse of the CaO through a calcination loop, where a regenerator is coupled with the gasifier for the regeneration of CaO from calcination of the formed CaCO3 from the reaction (eq 3).16 Most of the processes with CaO additive operated at high pressure because of the volume-reducing CaO carbonation reaction (eq 3).17–21 At higher pressure (above 5 MPa) and lower temperature (about 650 °C), Lin et al. realized a nearly complete in situ CO2 absorption; the gas concentrations from the reaction of coal/CaO with high-pressure H2O in a fluidized bed reactor consisted of 76 mol % H2,20 while that in a microautoclave is up to 85 mol % H2.21 Therefore, a new method, HyPr-RING process, is proposed by Lin et al.,20 in which coal gasification and a CO2 separation reaction were integrated in a single reactor. In comparison to other ideas of the cyclic CaO/CaCO3 loop for hydrogen production, the specific characteristics of the HyPrRING process are the formation of reactive Ca(OH)2, from CaO and high-pressure H2O, which then absorbs CO2, giving CaCO3 and releasing heat. To our knowledge, only a few studies on steam gasification of biomass were conducted at ambient pressure,22,23 which favors the CaCO3 calcination reaction (eq 4) and simplifies the continuous calcination/carbonation cycle. Xu et al. examined gasification of a biomass fuel blended with CaO in an atmospheric bubbling fluidized bed gasifier.22 The results showed that CaO could be a substantial good CO2 acceptor for the atmospheric gasification of biomass. Hofbauer et al. proposed the absorption enhanced reforming (AER) process based on in situ CO2 absorption in a dual fluidized bed steam gasification system to produce a hydrogen-rich syngas.23 A 100 kW (fuel power) process development unit has been recently operated to investigate the potential of the selective CO2 transport, achieving a H2 content of up to 75 mol % (dry basis) in the produced gas. However, the operation experiences and data in literature are so limited that more detailed investigations on the cycle at ambient pressure are expected. CaCO3 f CaO + CO2

∆H0 > 0

(4)

In the ECCMB gasification process, as mentioned in our previous paper,24 a dry gas with H2 content of 53.3 mol % with calcined olivine as a catalyst at ambient pressure was obtained. To obtain product gas with higher H2 content and lower CO2 content in the ECCMB process, a CaO loop was coupled into the process in this work, where steam gasification of biomass, (16) Wang, Z. H.; Zhou, J. S.; Wang, Q. H.; Fan, J. R.; Cen, K. F. Int. J. Hydrogen Energy 2006, 31, 945–952. (17) Wang, J.; Takarada, T. Energy Fuels 2001, 15, 356–362. (18) Kinoshita, C. M.; Turn, S. Q. Int. J. Hydrogen Energy 2003, 28, 1065–1071. (19) Słowin´ski, G. Int. J. Hydrogen Energy 2006, 31, 1091–1102. (20) Lin, S. Y.; Harada, M.; Suzuki, Y.; Hatano, H. Fuel 2002, 81, 2079– 2085. (21) Lin, S. Y.; Suzuki, Y.; Hatano, H.; Harada, M. Energy Fuels 2002, 15, 339–345. (22) Xu, G. W.; Murakami, T.; Suda, T.; Kusama, S.; Fujimori, T. Ind. Eng. Chem. Res. 2005, 44, 5864–5868. (23) Marquard-Moellenstedt, T.; Sichler, P.; Specht, M.; Michel, M.; Berger, R.; Hein, K.; Ho¨ftberger, E.; Rauch, R.; Hofbauer, H. 2nd World Biomass Conference, Rome, Italy, 2004. (24) Wei, L. G.; Xu, S. P.; Liu, J. G.; Lu, C. L.; Liu, S. Q.; Liu, C. H. Energy Fuels 2006, 20, 2266–2273.

Figure 1. Basic idea of the ECCMB process with CaO as a CO2 absorbent.

in situ CO2 capture, combustion of the produced chars, and calcination of CaCO3 can occur simultaneously. Consequently, steam gasification of biomass with calcined limestone as a CO2 absorbent was conducted in a laboratory-scale ECCMB gasification system. Effects of the reactor temperature and bed height on the dry gas compositions were determined. Regeneration and deactivation of the CO2 absorbent were also investigated by X-ray diffraction (XRD) and local scanning electron microscopy combined with energy-dispersive X-ray (SEM/EDX) analyses. In this study, steam gasification of biomass with calcined limestone as a CO2 absorbent in a fixed-bed reactor was also performed to allow for a better understanding of the in situ CO2 capture. Basic Idea of the ECCMB Process with CaO as a CO2 Absorbent. The ECCMB gasification system is composed of two reactors, a gas-solid concurrent downdraft moving bed gasifier and a riser type combustor, as shown in Figure 1. During steam gasification of biomass, CaO-based CO2 absorbent, which may be combined with some other catalysts, circulates between the gasifier and combustor. The circulating CaO particles act not only as solid heat carriers from the combustor to the gasifier, which supplies the energy required by gasification reactions of biomass, but also as a catalyst for tar destruction and in situ CO2 absorbent as well. Xu et al. has verified the occurrence of the CaO carbonation reaction at temperatures below 805 °C at ambient pressure.22 In this work, equilibrium gas compositions were calculated for the reaction systems of C/H2O and C/CaO/H2O at ambient pressure and in the temperatures range from 500 to 1000 °C, and the results are shown in Figure 2. The molar ratios of H2O/C and CaO/C were set at 2.0 (equal to 3.0 weight ratio) and 1.0 (equal to 4.67 weight ratio), respectively. It is indicated that steam gasification of C (case a) produces a gaseous stream with a maximum H2 content of 57 mol %, while the addition of CaO (case b) could potentially produce a stream with more than 90 mol % H2. It is the in situ CO2 capture that increases the extent of the water-gas shift reaction (eq 2). However, in case b, the equilibrium H2 content in the dry gas is observed to decrease, while that of CO and CO2 increases with an increase of the temperature. It can be concluded that higher temperatures, for example above 850 °C, do not facilitate the CaO carbonation reaction to produce hydrogen but favor the decomposition of CaCO3 into CaO. According to the above results, the ECCMB process coupled with a continuous calcination/carbonation cycle is feasible when the gasifier temperature is below 850 °C, while the combustor temperature is above 850 °C. With this concept, it is possible to obtain a high-grade product gas with a high content of H2.

Hydrogen Production in Steam Gasification

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Figure 2. Dry gas composition of the C/H2O (a) and C/CaO/H2O (b) system at thermodynamic equilibrium. Table 1. Properties of the Pine Sawdust proximate analysis (wt %, ad basis)

ultimate analysis (wt %, daf basis)

volatile fixed moisture ash matter carbon carbon hydrogen oxygena nitrogen sulfur 5.01 a

0.34 77.71

16.94 50.26

6.72

42.66

0.16

0.20

By difference.

Table 2. Component Analyses of the Natural Olivine and Limestone composition (wt %)

CaO

MgO

SiO2

Fe2O3

Al2O3

loss by calcination

olivine limestone

0.37 52.00

51.80 1.50

36.50 1.50

9.14 1.16

0.88 2.71

1.31 41.13

At the same time, the deactivated CO2 absorbent could be regenerated without intervals. Experimental Section Biomass Feedstock. The biomass feedstock was pine sawdust, and its properties are presented in Table 1. The biomass samples were sieved, classified, and dried for 2 h at 100-105 °C before the test. The particle size of biomass used in the fixed-bed reactor experiments is 0.10-0.20 mm, while that used in the ECCMB process is 1.2-2.4 mm. Bed Materials. Limestone calcined at 900 °C for 4 h before the test was used as a CO2 absorbent in all experiments. Inert silica sand was also used in the fixed-bed experiments for a comparison. The particle size of the bed material in the fixed-bed reactor material is 0.6-0.9 mm. In the ECCMB gasification process, calcined limestone was mixed with calcined olivine (900 °C for 4 h) at a weight ratio of 1:1 as bed material. The chemical compositions of the natural occurring olivine and limestone are shown in Table 2. The particle size of the bed material in the ECCMB process is 1.0-1.2 mm. Experimental Apparatus and Procedure. The schematic diagrams and basic operation procedures of the fixed-bed reactor and the laboratory-scale ECCMB system used in this study are the same as described in our previous works.24,25 During the experiments in the fixed-bed reactor, the bed materials are located downstream of 1.0 g of biomass and the water flow rate was fixed at 0.8 mL/min. All of the results were based on the cumulative data within the reaction time of 10 min. Steam gasification of biomass with calcined limestone as a CO2 absorbent was performed in the laboratory-scale ECCMB system at the reactor temperature of 650-800 °C, bed height of 100-300 mm, circulating amount of bed material of 9600 g/h, and A/B ratio of 20. For a comparison to the mixed bed materials with the addition of calcined limestone, gasification experiments with calcined olivine alone as a bed material were also conducted.

During the ECCMB process, the gas was sampled after each 10 min interval for analyzing the gas compositions with time on streams, and the reactor temperature was measured at the mixed zone of bed materials and biomass particles as a reference. Characterization of CO2 Absorbent. In the ECCMB process, bed material sampling points are set along the gasifier and combustor to help analyze and control the process. The XRD analysis of the sampled CO2 absorbent was conducted to analyze the level of the calcination of the formed CaCO3. The SEM/EDX analysis of the CO2 absorbent was performed by computercontrolled scanning electron microscopy (JSM-5600LV), which can give information on the composition and structure of the particle surface.

Results and Discussion Fixed-Bed Reactor Studies. Effect of the Reactor Temperature. Dry gas compositions from gasification of pine sawdust mixed with silica sand and calcined limestone are given in Tables 3 and 4, respectively. As shown in Tables 3 and 4, the addition of calcined limestone significantly increases the H2 content and decreases the CO content in dry gas. It can be inferred that the in situ CO2 capture (CaO carbonation) occurs at ambient pressure in the reactor temperature range from 650 to 800 °C, which facilitates the water-gas shift reaction (eq 2) to hydrogen production. Thermogravimetric analysis (not shown here) revealed that the efficiency of the CO2 absorption [calculated from the molar amounts of CaCO3 divided by the total molar amounts of Ca(OH)2, CaCO3, and CaO in the sample] used in this study decreases with the increase of the reactor temperature according to the method reported by Lin et al.26 The efficiency of the CO2 absorption is 6.12 mol % at 700 °C, while that is 1.14 mol % at 800 °C. It can be inferred that a low reactor temperature favors CO2 capture by calcined limestone. According to the results in Table 4, it can be observed that, in the case of calcined limestone, H2 content in the dry gas increases from 46.6 to 67.5 mol % with the increase of the reactor temperature from 650 to 800 °C. It can be inferred that the reactor temperature affects the dry gas composition more importantly than the in situ CO2 absorption in the gasifier at ambient pressure. In other words, it implies that a relatively high temperature causes high kinetic rates of gasification of char and reforming/cracking of tar.27 (25) Hu, G.; Xu, S. P.; Li, S. G.; Xiao, C. R.; Liu, S. Q. Fuel Process. Technol. 2006, 87, 375–382. (26) Lin, S. Y.; Harada, M.; Suzuki, Y.; Hatano, H. Fuel 2006, 85, 1143– 1150.


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Table 3. Dry Gas Composition from the Pyrolysis and Gasification of Pine Sawdust on Silica Sand in a Fixed-Bed Reactor run biomass bed material steam feeding rate (mL/min) reactor temperature (°C) dry gas composition (mol %) H2 CH4 CO CO2

1

2

3

4

5

6

650

700

7

8

750

800

pine sawdust silica sand 0 650

700

19.9 10.2 38.5 31.3

0.8 750

20.3 10.4 36.9 32.5

800

28.9 8.6 36.1 26.4

28.6 8.2 31.6 31.6

31.8 9.1 28.3 30.8

41.5 8.0 27.5 23.1

43.4 8.4 29.9 18.4

47.6 6.9 25.4 20.1

Table 4. Dry Gas Composition from the Gasification of Pine Sawdust with Limestone as a CO2 Absorbent in a Fixed-Bed Reactor run biomass bed material steam feeding rate (mL/min) reactor temperature (°C) A/B ratio dry gas composition (mol %) H2 CH4 CO CO2

9

650 21 46.6 4.6 5.4 43.4

10

700 21 56.1 5.7 6.3 31.2

Effects of the A/B Ratio. As shown in Table 4, the H2 content in the dry gas increases from 53.8 to 67.5 mol % with the increase of the A/B ratio from 8 to 19. It is attributed to the in situ CO2 absorption, which enhances the water-gas shift reaction (eq 2) to hydrogen production. However, the H2 content increases slightly from 67.5 mol % at the A/B ratio of 19 to 68.2 mol % at the A/B ratio of 26 and even decreases slightly to 64.6 mol % at the A/B ratio of 39. The increase of the A/B ratio leads to the decrease in the gasification capacity for a specific gasifier and to an increase in energy supply in the regenerator and a higher cost of CaO-based absorbent. The choice of a suitable A/B ratio is dependent mainly upon the energy balance between the regenerator and the gasifier, the target product, the type of the biomass, the activity of the catalyst, and the efficiency of CaO-based absorbent. In this work, an A/B ratio of 20 is considered to be feasible. As the results indicated in Table 4, the CO and CH4 contents in the dry gas decrease with the increase of the A/B ratio. The decrease of CO content is due to the extension of the water-gas shift reaction (eq 2) with the absorption of CO2 by CaO. The decrease of CH4 is attributed to the steam reforming and dry reforming of CH4 catalyzed by CaO. Gasification in the Laboratory-Scale ECCMB Gasification System. Steam gasification of pine sawdust was performed in the laboratory-scale ECCMB system at reactor

11

750 20 60.8 4.7 5.3 28.7

12 pine sawdust calcined limestone 0.8 800 8 53.8 6.4 9.4 29.6

13

14

15

800 19

800 26

800 39

67.5 4.8 5.0 22.4

68.2 4.4 5.0 22.4

64.6 4.3 5.0 25.6

temperatures of 650 and 800 °C, a S/B ratio of 0.37, and a bed height of 200 mm with olivine as the bed material, and the results are shown in Figure 3. A dry gas with H2 content of 39.4-43.7 mol % and CO content of 45.2-47.6 mol % had been obtained at 800 °C. Effects of the reactor temperature on time series dry gas compositions from the ECCMB process with the addition of calcined limestone are presented in Figure 4. In comparison to the case with olivine as the bed material (shown in Figure 3), H2 content in the dry gas increases greatly with the addition of calcined limestone, whereas CO and CO2 contents decrease. It is mainly due to the in situ CO2 absorption by the CaO-based CO2 absorbent followed by the motivation of the water-gas shift reaction (eq 2). The H2 content in dry gas is higher at a reactor temperature of 800 °C than that at 650 °C, although thermodynamically lower temperatures favor CO2 absorption for CaO. However, it should be noted that the efficiency of the CO2 absorption is much less at 800 °C (only 1.14 mol % in our fixed-bed experiments). It could be inferred that the temperature of bed materials decrease to 800 °C below along the axis of the moving bed gasifier because of the endothermal gasification reactions, which favors the CO2 absorption. As shown in Figure 5, with the increase of the bed height from 100 to 300 mm, the H2 content in the dry gas increases

Figure 3. Time series dry gas composition from gasification of biomass in the ECCMB process with calcined olivine as the bed material.



Figure 4. Effect of the reactor temperature on time series dry gas composition from the gasification of biomass in the ECCMB process with calcined limestone as a CO2 absorbent.

Figure 5. Effect of the bed height on time series dry gas composition from the gasification of biomass in the ECCMB process with limestone as a CO2 absorbent.

from 52.6-69.7 to 59.7-73.6 mol %. It can be inferred that hydrogen production depends upon the bed height. However, in our early work,24 the H2 concentration increases from 23.5 to 46.2 mol % when the catalyst (calcined olivine) bed heights varied from 100 to 300 mm at the same operation conditions. It can be found that the growing percentage of hydrogen content with the addition of a CaO-based absorbent is less than that with calcined olivine only. One of the reasons may be that the CO2 absorption is so fast that the extent of carbonation of CaO reaches a limit in a short residence time, when a CaCO3 layer is formed on the surface of the CaO particle and the following CO2 absorption is controlled by its diffusion through the layer.28 Furthermore, a high bed height would result in a long residence time of the CO2

absorbent in the gasifier at a certain circulating amount of bed material, which causes the deactivation of the CO2 absorbent. As shown in Figures 4 and 5, with the increase of the reaction time, H2 content in the dry gas decreases and CO2 content increases, indicating an irreversible deactivation of the CO2 absorbent. Characterization of the CO2 Absorbent. XRD Analysis. In the ECCMB process coupled with CO2 absorption, the CaObased CO2 absorbent should be regenerated in the combustor by calcining the CaCO3 formed in the gasifier into CaO. After the eighth circles at a reactor temperature of 800 °C and bed height of 200 mm, the bed material at the bottom (corresponding to the absorbent before regeneration) and the top (corresponding


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Figure 6. XRD patterns of the CO2 absorbent under different conditions.

Figure 7. SEM paragraphs of the surface of the CO2 absorbent before the reaction (a) and after eight reaction circles (b).

to those after regeneration) of the combustor were sampled to analyze the regeneration level of the CO2 absorbent by XRD analysis. As shown in Figure 6, the XRD patterns of the calcined limestone before the gasification reaction show a strong CaO phase by the presence of the peaks at 32.2, 37.4, and 54.0 (2θ), while the XRD patterns of the CO2 absorbent after the reaction show a significant increase of the CaCO3 phase by the presence of the peaks at 29.4, 36.0, 39.5, 43.2, 47.5, and 48.5 (2θ). It is the result of the CaO carbonation that occurred in the gasifier. The XRD patterns of the absorbent after regeneration show a CaCO3 phase decrease and a nearly complete recovery of the CaO phase compared to that after the reaction. It can be inferred that the CaCO3 formed in the gasifier was decomposed into CaO in the combustor. In other words, the CO2 absorbent was well-regenerated without intervals in the ECCMB process.

SEM/EDX Analysis. Surface morphologies and local chemical compositions of the CO2 absorbent before the reaction (a) and after the eighth carbonation/calcination cycle (b) in the gasification at 800 °C were analyzed with SEM/EDX, and the results are shown in Figure 7. It can be observed that sintering of the CO2 absorbent occurred during the cyclic calcination/carbonation. The sintering causes a decrease in porosity and surface area of the absorbent and, therefore, results in the decrease of the CO2 absorption capacity and the irreversible deactivation of the CO2 absorbent. The sintering is favored by both high temperature and reaction time and is accelerated by the presence of CO2 and H2O during the process of steam gasification in the gasifier and char combustion in the combustor.29 As shown in Figure 8, many adhesions can be observed on the surface of CO2 absorbent, which seemed to be some club-shaped crystals (region 1) and amorphous residues (regions 2 and 3).



Figure 8. SEM/EDX analysis of the CO2 absorbent after eight reaction circles.

According to the results of EDX analysis in Figure 8, for all of these adhesions, the peaks corresponding to Ca, O, Si, etc. are commonly visible. It can be inferred that the club-shaped crystals (region 1) should be inorganic complex eutectics [such as calcium silicate oxides (CaO-SiO2), magnesium, and aluminates] of fine calcined limestone particles at high temperature over 800 °C during the cyclic of calcination/carbonation reactions. However, it should be well-noticed that the main element of biomass ash, such as K, is found in the residues (regions 2 and 3). It can be inferred that interaction between ash minerals derived from biomass and fine particles of caclined limestone may take place with the in situ CO2 adsorption and steam gasification of biomass. The same result was also found in the HyPr-RING process.30,31 During biomass gasification, the carbonaceous portion of char is consumed by steam and leads to the outcropping or releases contact between fine particles of CaO-based absorbent and minerals existing in char. These inactive inorganic adhesions deposited on the surface of particles will result in a decrease of pores and lead to irreversible deactivation of the CO2 absorbent. A long residence time of the CO2 absorbent favors the formation of the crystals and the residues, which will cause the CO2 absorbent deactivation.31 Therefore, a shorter residence time favors to keep carbonation capacity of CO2 absorbent during calcination/carbonation reactions. It should be considered in the scale-up and optimization of the ECCMB process. The trace element of sulfur is also found in both residues and crystals. It can be inferred that the occurrence of sulfur is captured by CaO in the gasification of biomass, which favors reducing the emission of sulfur oxides. For the CaO-based absorbent deactivation with the cyclic CaO/CaCO3, in the ECCMB process, fresh absorbents have to be inserted into the gasifier. Although natural limestone is very cheap and its deactivated residue can be seeded in the cement industry,32 the quantities of consumed limestone at the A/B ratio of 20 in this work are too much to be in practice for its costs (27) Corella, J.; Toledo, J. M.; Molina, G. Ind. Eng. Chem. Res. 2006, 45, 6137–6146. (28) Bhatia, S. K.; Perlmutter, D. D. AIChE J. 1983, 29, 79–86. (29) Stanmore, B. R.; Gilot, P. Fuel Process. Technol. 2005, 86, 1707– 1743. (30) Kuramoto, K.; Shibano, S.; Fujimoto, S.; Kimura, T.; Suzuki, Y.; Hatano, H.; Lin, S. Y.; Harada, M.; Morishita, K.; Takarada, T. Ind. Eng. Chem. Res. 2003, 42, 3566–3570. (31) Kuramoto, K.; Ohtomo, K.; Suziki, K.; Fujimoto, S.; Shibano, S.; Matsuoka, K.; Suzuki, Y.; Hatano, H.; Yamada, O.; Lin, S. Y.; Harada, M.; Morishita, K.; Takarada, T. Ind. Eng. Chem. Res. 2004, 43, 7989– 7995. (32) Abanades, J. C.; Anthony, E. J.; Lu, D. Y.; Salvador, C.; Alvarez, D. AIChE J. 2004, 50, 1614–1622.

and poor heat balance of the gasification system. Therefore, a cyclic stability of the CaO-based CO2 absorbent is required for improving the efficiency of the gasification system. There is much scope to improve the absorbent performance through reactivation, pretreatment, or the manufacture of completely synthetic CO2 absorbents.33–35 This is an obvious research subject in the future if the gasification system is to become of practical interest. At the same time, the operation parameters (such as bed height) and preparation of catalysts have to be optimized to minimize the amount of absorbents.36 Conclusions To improve the H2 content in dry gas and heat balance in the ECCMB process, the basic idea of the process coupled with CO2 absorption with CaO as an absorbent at ambient pressure was proposed. In this process, the in situ absorption of the produced CO2 in the gasifier increases the extent of the water-gas shift reaction and enhances the purity of H2. The main operator conditions in the ECCMB process are figured out according the thermodynamic calculations and the results of steam gasification in a fixed-bed reactor. The process should be operated in the reactor temperature range from 650 to 800 °C, and the regeneration temperature of the CaO-based absorbent should be above 850 °C. Higher temperatures favor an increase of H2 content in dry gas, although thermodynamically lower temperatures favor CO2 absorption for CaO. Consequently, steam gasification of biomass with calcined limestone as a CO2 absorbent was conducted in a laboratoryscale ECCMB system. During this process, in the case of the reactor temperature of 700–800 °C, S/B ratio of 0.38–0.59, bed height of 200–300 mm, and A/B ratio of 20, the gas of H2 content of 60-70 mol % can be obtained. An irreversible deactivation of the CO2 absorbent occurs in the cyclic carbonation/calcination reaction, which is mainly attributed to (1) the sintering of particles at high temperature and (2) the formation of the inert inorganic complex compounds on the surface of particles because of the solid-solid reaction between inorganic minerals in biomass and the CO2 absorbent. (33) Iyer, M. V.; Gupta, H.; Sakadjian, B. B.; Fan, L.-S. Ind. Eng. Chem. Res. 2004, 43, 3939–3947. (34) Li, Z. S.; Cai, N. S.; Huang, Y. Y.; Han, H. J. Energy Fuels 2005, 19, 1447–1452. (35) Ochoa-Fernandez, E.; Rusten, H. K.; Jakobsen, H. A. Catal. Today 2005, 106, 41–46. (36) Corella, J.; Toledo, J. M.; Molina, G. Ind. Eng. Chem. Res. 2008, 47, 1798–1811.


In this work, the consumption of limestone as a CO2 absorbent is very large because of the deactivation of the CaO-based absorbent. Therefore, a more efficient CaO-based CO2 absorbent has to be prepared, and the operating parameters and additional catalysts have to be further optimized to minimize the amount of absorbents for the scale up of the ECCMB process. However, according to the results, it can also be concluded that the addition of a CaO-based

Wei et al.

CO2 absorbent is a good solution to improve the content of hydrogen in dry gas from the ECCMB process. Acknowledgment. This study was performed under the Project 50776013 supported by NSFC. The authors thank Prof. Jose Corella from University Complutense of Madrid, Spain, for his kind discussions and suggestions. EF700744A

Hydrogen Production in Steam Gasification of Biomass with CaO as a

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