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Comparative analysis of hydrogen production systems from biomass based on different absorbent regeneration processes
International Journal of Hydrogen Energy 32 (2007) 80 – 85 www.elsevier.com/locate/ijhydene
Comparative analysis of hydrogen production systems from biomass based on different absorbent regeneration processes Chunzhen Qiaoa, b , Yunhan Xiaoa,∗ , Xiang Xua, b , Lifeng Zhaoa , Wendong Tiana a Institute of Engineering Thermophysics of Chinese Academy of Sciences, Beijing 100080, China b Graduate School of Chinese Academy of Sciences, Beijing 100081, China
Received 30 September 2005; received in revised form 21 February 2006; accepted 31 March 2006 Available online 27 July 2006
Abstract Hydrogen production through single-step process has been proposed for a long time. The process can produce H2 in gasifier, and simultaneously, achieve integration of exothermic and endothermic as well as gas production and separation in one reactor. The process consists of mainly twin fluidized beds—a gasifier and a CO2 absorbent regenerator. In this paper, two modes are suggested for absorbent regeneration. One case named PCC process is partial carbon conversion in the gasifier, and the carbon residue acts as a fuel for the regenerator; the other case named NO process is nickel oxidation in the regenerator to supply heat for absorbent regeneration. The mass and energy balances and cold gas efficiency were analyzed by thermodynamic calculation for the two different processes. When the feedstock is same, PCC process gives a higher cold gas efficiency of 0.74 than NO process of 0.67. 䉷 2006 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. Keywords: Biomass; Hydrogen production; Absorbent regeneration; System analysis; Aspen Plus
1. Introduction As a clean fuel, hydrogen is expected to be one of the primary energy carriers in the 21st century. In order to achieve the object of using hydrogen not only as raw material for chemical engineering but also as energy, it is important for producing hydrogen with low-cost and large scale, at the same time reducing CO2 emissions. If both sets of requirements are to be met without excessive economic disadvantage to the world economy, then new hydrogen production methods with low or zero CO2 emissions must be developed. The carbonation reaction, which was developed several decades ago, was employed in the CO2 acceptor process for coal gasification [1]. A summary of the process described the coal gasification by steam [2], but not pursued. In recent years, sorption-enhanced gasification process has received a great deal
of emphasis again [3–6]. On the basis of the above concept, Center for Coal Utilization of Japan proposed HyPr-RING hydrogen production process in 1999 [5], and the process was operated at supercritical condition, and feedstock was fed into by slurry. At the same time, Institute of Engineering Thermophysics of Chinese Academy of Sciences (IET) proposed the idea of “single-step process for H2 production and CO2 fixed by mineral” [6,7]; the concept is similar, but the dry materials were fed into and the pressure was lower than HyPr-RING. The single-step process proposed by IET includes two cycles: in one H2 O reacts with hydrocarbon to produce hydrogen, then hydrogen converts to H2 O; in the other CaO reacts with CO2 to form calcium carbonate, then calcium carbonate is calcined to CaO. In the single-step process, the raw materials are hydrocarbons, water and CaO. The process involves three main reactions in gasifier: C + H2 O → CO + H2 ,
∗ Corresponding author.
E-mail address: [email protected] (Y. Xiao).
0 H298 = 131.2 kJ/mol,
(1)
C + H2 O → CO2 + H2 ,
0 H298 = −41.2 kJ/mol,
(2)
CaO + CO2 → CaCO3 ,
0 H298 = −178.8 kJ/mol.
(3)
0360-3199/$ - see front matter 䉷 2006 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2006.03.009
C. Qiao et al. / International Journal of Hydrogen Energy 32 (2007) 80 – 85
The three reactions constitute overall reaction: C + CaO + 2H2 O → CaCO3 + 2H2 , 0 H298 = −88.8 kJ/mol.
(4)
The hydrogen produced by gasification reacts with carbon, and the byproduct, methane, is produced. C + 2H2 → CH4 ,
0 H298 = 74.85 kJ/mol.
0 H298 = 178.8 kJ/mol.
Given the input stream and operating conditions (temperature, pressure and flow rate) for every module, we can calculate the equilibrium composition of out stream using Aspen Plus, as well as the heat contained in the output. 2.1. Partial carbon conversion (PCC) for regeneration
(5)
The heat released by reaction (3) supplies to carbon gasification, at the same time, CO2 was separated by the carbonation process (reaction (3)) and high-purity concentration hydrogen was produced in the gasifier. In the regenerator, the follow reaction takes place: CaCO3 → CaO + CO2 ,
81
(6)
In the single-step process, the absorbent absorbs heat in the regenerator and transfers it into the gasifier, so regeneration process is the main expended energy part in the hydrogen production system; it has an important effect on the whole process. Shimizu et al. [4] investigated the process removed CO2 from flue gas using CaO as a CO2 absorbent in a twin fluidized bed, and analyzed the material balance, heat balance, power generation and power consumption for O2 production and CO2 compression. The heat of decomposition in the regenerator is supplied by the combustion of the feed-in coal and pure oxygen. Xiao et al. [7] constituted and analyzed the single-step system for hydrogen production from coal; when only part carbon was converted in the gasifier, the cold efficiency was above 90%. Lin et al. [8] analyzed components and their mass and energy balances of HyPr-RING process by thermodynamic calculations. The heat in the regenerator is supplied by burning the unconverted carbon in the gasifier, and the energy contained in the coal is distributed in the gasifier and the regenerator by a ratio of about 23 . But they did not investigate the effects of the different regeneration method on process, such as the changes of gas products and heat balance when the regeneration method was changed. In this paper, we compare and analyze the effect of different absorbent regeneration process on hydrogen production systems. Firstly, two different hydrogen production systems are constructed, in which the gasification part is same, namely single-step for hydrogen production; then thermodynamic calculation is employed to analyze the material and energy balances of the two systems; finally, we compare the cold gas efficiency and analyze the effect of different absorbent regeneration processes on the hydrogen production system, as well as the advantages and disadvantages of the two processes. 2. Options to produce hydrogen based on different regeneration process Single-step process for H2 production consists mainly of two reactors and some heat exchangers. The hydrocarbons gasify and react with absorbent and steam in the gasifier, which produce gases and solids. The other is a regenerator for absorbent (CaCO3 ), which is calcined to CaO. Several heat exchangers are used in the process for heat recovery.
Control the conversion of carbon in the gasifier in order to retain portion of the carbon burning in the regenerator to supply heat for regeneration; the principle figure of producing hydrogen is shown in Fig. 1 and the system is shown in Fig. 2. To produce hydrogen from biomass, biomass must be treated and mixed with absorbent in the mixer. Then the mixture was fed into the gasifier with steam which was preheated by the gases and solids vented from the cyclone. In the gasifier, biomass reacts with steam to produce mainly hydrogen, methane and carbon dioxide, and carbon dioxide is absorbed by absorbent and forms CaCO3 , which promotes the shift reaction (2), so CO concentration in the gas products is very low. The unreacted carbon is discharged with other solids (CaCO3 and ash) and then enters the regenerator, which burns there to supply the heat for regeneration. Finally, the CaCO3 was calcined to CaO and discharged with ash from regenerator. In order to produce pure CO2 , O2 is used as oxidizer in the regeneration process. The gases from cyclone contain mainly hydrogen and methane as well as a large quantity of steam, which exchange heat with water from pump in the heat exchanger and then enter into the condenser, where they are separated into gases and liquid (water). The separation of CaO (absorbent) and ash is not considered presently, so we dispose part of the mixture of CaO and ash and inject fresh CaCO3 into the regenerator considering the active absorbent declination. The biomass used in the calculation is shown in Table 1. 2.2. Nickel oxidation (NO) for regeneration Based on the conception of chemical-looping combustion (CLC) [9,10], taking into account the need for pure oxygen in the regenerator, as well as the catalysis of NiO on the steam reforming [11,12], we select NiO as the oxygen carrier to design
CO2
H2, CH4 C, Ash, CaCO3 Gasifier
Regenerator CaO
Hydrocarbon Steam
O2
Fig. 1. The principle of carbon conversion partial for hydrogen production.
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C. Qiao et al. / International Journal of Hydrogen Energy 32 (2007) 80 – 85
Feedstock
Steam Gas Cyclone Exchanger network
Flash
Mixer CO2
Gasifier
CaO and ash
Regenerator
Liquid Water
O2
CaCO3 Fig. 2. PCC for regeneration in the H2 production system.
Table 1 Properties of hydrocarbon Vol.
Mois.
FC
Ash
Proximate analysis (wt%) 79.55 3.76
14.83
1.86
C
N
S
H
tion. The other is CO2 absorbent (CaO) circulation. CaO reacts with CO2 from biomass gasification and forms CaCO3 , and simultaneously releases heat. Hereafter, CaCO3 is calcined in the regenerator using the heat from metal oxidation. Except reactions (1)–(6), the following reactions take place in the NO process: CO + NiO → CO2 + Ni,
Ultimate analysis (wt%) daf 46.21 5.56
1.2
H2, CH4
0.018
CO2
Ni, Ash,CaCO3 Gasifier
Regenerator CaO, NiO
Hydrocarbon Steam
O2
Fig. 3. The principle of chemic chain reaction for hydrogen production.
the system. The principle using NiO as the oxygen carrier to supply heat for regeneration is shown in Fig. 3. In the process, there are two chemic chain reactions. One is metal circulation, which is nickel oxidation reacted with CO in the gasifier and formation of nickel and carbon dioxide. Later, nickel reacts with oxygen in the regenerator and forms nickel oxide, and releases a great deal of heat for absorbent regenera-
Ni + 1/2O2 → NiO,
0 H298 = 43.34 kJ/mol,
0 H298 = −239.63 kJ/mol.
(7) (8)
Reaction (7) takes place in gasifier and reaction (8) in regenerator. Reaction (8) releases heat for CaCO3 calcination. Based on the principle, the system using chemical chain reaction to produce hydrogen is designed. The system is shown in Fig. 4. The flowchart is similar to the PCC process, except that nickel oxide is put into gasifier with biomass and absorbent. NiO is deoxidized by CO from carbon gasification and absorbs heat at the same time, and CO from reaction (1) is consumed in the process. Different from PCC process, the heat needed by absorbent regeneration comes from nickel oxidation. The purpose of this paper is to analyze the effect of a different regeneration process, so the complex exchange of water and exit gases and solids will not be taken into account in detail, which is the same as the PCC process. The biomass kind as well as feed quantity used in the NO calculation is same as the PCC process, and other parameters are shown in Tables 2 and 3. The calculation conditions (T and P) are chosen based on our earlier study [13]. 3. Modeling the processes ASPEN Plus was used to calculate the equilibrium compositions and enthalpy changes of the process, as well as establish the mass and energy balances of equipment selection and
C. Qiao et al. / International Journal of Hydrogen Energy 32 (2007) 80 – 85
83
Steam
Feedstock
Gas Cyclone NiO Exchanger network CO2
Gasifier
Water
CaO and ash
Regenerator
Mixer
Flash
Liquid
O2
CaCO3 Fig. 4. NO for regeneration in the H2 production system.
Table 2 Other calculation conditions and part results Supply hydrocarbon (woods): 1.00 × 105 (kg/d) High heating value of hydrocarbon: 18.72 × 108 (kJ/d) Supply absorbent: 1.4 × 105 (kg/d) Supply CaCO3 : 10 × 103 (kg/d) Supply H2 O: 1.8 × 105 (kg/d) Gasifier operating condition: 923 K, 3.0 MPa; Carbon conversion of hydrocarbon in gasifier: 68% Regenerator operating condition: 1300 K, 0.1 MPa Product of fuel gas (dry gas): 4.64 × 106 (mol/d) (97.6% H2 , 2.02% CH4 , 0.28% CO2 , 0.08% CO) High heating value of product gas: 13.78 × 108 (kJ/d)
Table 3 Other calculation conditions and part results Supply absorbent: 2.1 × 105 (kg/d) Supply nickel oxide: 2.28 × 105 (kg/d) Supply H2 O: 1.2 × 105 (kg/d) Gasifier operating condition: 923 K, 3.0 MPa Regenerator operating condition: 1300 K, 0.1 MPa Product of fuel gas (dry gas): 4.065 × 106 (mol/d) (906.06% H2 , 3.65% CH4 , 0.21% CO2 , 0.08% CO) High heating value of product gas: 12.47 × 108 (kJ/d)
design. Gasification section is composed of the RYIELD (yield) and RGibbs (minimize Gibbs free energy) reactor blocks. The gasification feedstock, wood, is first decomposed to its elemental components through a yield-basis reactor block. These species, with a stream representing the heat of decomposition, are then passed to a Gibbs reactor block with gasification steam. The Gibbs reactor block minimizes the Gibbs free energy to attain the equilibrium composition of the identified gasification products at the temperature specified for reaction (923 K). In the regenerator, unreacted carbon is combusted with pure oxygen for PCC process, while nickel is for NO process. Calcination of absorbent and combustion (oxidation for NO process) occur simultaneously in a Gibbs reactor block at a specified temperature of 1300 K. The temperature of the reactor
is controlled by balancing the heat required for calcination with the heat of combustion (oxidation). The balance is achieved by controlling the conversion of carbon in the woods for PCC process and varying the rate of nickel fed to the regenerator for NO process. The desired excess oxygen concentration in the flue gas is obtained by varying the oxygen feed rate to the regenerator. In the results, species whose mole fraction is less than 10−5 have been omitted from consideration. Further assumptions on the calculations include: the gasifier operates isothermally; the regenerator operates adiabatically; the heat rejected from gasifier is lost to the environment; the products of the gasifier (gases and solids) undergo a natural phase separation without the input of work; all heat exchange process are
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C. Qiao et al. / International Journal of Hydrogen Energy 32 (2007) 80 – 85
Table 4 Mass and heat balances of the gasifier Materials (t)
Input: Woods CaO Steam NiO Total heat: (108 kJ)
PCC 100 140 180
Output: H2 (103 kmol) CH4 (103 kmol) Steam (103 kmol) CaCO3 Ca(OH)2 Ash C Ni Total heat: (108 kJ)
PCC 4.531 0.094 7.930 243.5 4.7 1.8 14.8
Heats (108 kJ) Sensible and latent heats
Reaction heat
NO 100 210 120 228 PCC 27.48 NO 3.905 0.164 5.132 367 5.6 1.8
PCC
NO
6.05
4.18 0.59
NO 27.58 PCC
NO
6.58
4.31
1.93
3.13
PCC 18.72 2.71
NO 18.72 4.09
PCC
NO
13.78
12.47
4.85 179 PCC 27.15
7.32 NO 27.23
Table 5 Mass and heat balances of the regenerator Materials (t)
Input: CaCO3 Ca(OH)2 Ash C Ni O2 Total heat: (108 kJ)
PCC 253.5 4.7 1.8 14.8
Output: CO2 (103 kmol) H2 O (103 kmol) O2 (103 kmol) CaO Ash NiO
PCC 3.665 0.060 0.018 139.8 1.8
Heat recovery: (108 kJ) Total heat: (108 kJ)
40
Heats (108 kJ) Sensible and latent heats
Reaction heat
NO 367 5.6 1.8
PCC
NO
1.7
3.32
PCC
NO
4.85 17 40 PCC 7.15 NO 3.665 0.074 0.019 208 1.8 226 PCC 0.57 7.11
ideal; all reactions achieve chemical equilibrium; energy consumption, such as solids’ transport, has been omitted from consideration. 4. Comparison between the two different absorbent regeneration processes The flowcharts of different processes are shown in Figs. 2 and 4, and the complex exchange network is not analyzed amply. In the PCC process, about 68% carbon in the coal is suggested
7.32 0.53 NO 11.28 PCC
NO
1.88
1.85
1.34
1.34 3.55
PCC
NO
3.32
4.06
NO 1.29 10.75
to be gasified in the gasifier, and 32% carbon (char) remains in the solid residues as fuel for CaCO3 calcination in the regenerator. While 49% carbon in the coal is used to deoxidize nickel oxide in order to supply heat in the regenerator through nickel reacting with oxygen, only 51% carbon is gasified to form hydrogen. The equilibrium composition of the products and heat balance in the gasifier and regenerator is shown in Tables 4 and 5 for the PCC and NO processes, which is calculated on the basis of the input materials.
C. Qiao et al. / International Journal of Hydrogen Energy 32 (2007) 80 – 85
When gasification feed is same, which consists of 1 × 105 kg of woods, and operation condition is 923 K and 3.0 MPa, the equilibrium gas products are 4.53 × 103 kmol H2 for PCC process, while only 3.91 × 103 kmol for NO process. The dry gas contains 97.6 vol% H2 for PCC process and only 96.1 vol% for NO process. CO and CO2 comprise less than 0.3 vol%. The solid products of the gasifier in equilibrium are 2.435 × 105 kg CaCO3 and 0.047×105 kg Ca(OH)2 , with 0.148×105 kg unreacted carbon for PCC process, while 3.67 × 105 kg CaCO3 and 0.056 × 105 kg Ca(OH)2 , with 1.79 × 105 kg Ni for NO process. The heat balance of the gasifier in equilibrium is also shown in Table 4. As can be seen, the input heat is larger than the output for the gasifier, given that the allowable heat loss of the gasifier is 0.33 × 108 kJ for PCC process and 0.35 × 108 kJ for NO process. The input heat was designed to be larger than the output heat to allow a heat loss in the regenerator of about 0.04×108 kJ for PCC process and 0.53 × 108 kJ for NO process. The recovered heat is used to superheat steam for the gasifier and to generate power for oxygen separation and other power consuming parts of process. The cold gas efficiency is defined as cold gas efficiency = HHV of fuel gas/HHV of woods. From the heat value of the produced fuel gas and input biomass (woods), the cold gas efficiency (298 K, 0.1 MPa) was calculated for the two processes, which is 0.74 for PCC process and 0.67 for NO process. Based on the analysis, we can see that, at the same quantity of woods, the concentration of hydrogen in the gas products is only 96.1 vol% for NO process and is lower than 97.6 vol% for PCC process, moreover, the PCC process has a high cold gas efficiency of 0.74, while the NO process is only 0.67. In the NO process, the presence of NiO will promote the steam reforming methane reaction, but the reduction of NiO in the gasifier will absorb heat, which needs more CO2 absorbent than the PCC process for heat balance of NO process, which increased the material circulation quantity, and made the power for separating rise in practice. In the fuel-flexible gasificationcombustion technology [14], the metal oxide (NiO or Fe2 O3 ) is regarded as oxygen transfer material (OTM), which made the oxygen production section needless in the process, while the advantage is not been exhibited in the NO process. 5. Conclusion This paper analyzes the mass and energy balances of the process for hydrogen production from biomass with different regeneration methods by means of thermodynamic calculations, and the cold gas efficiency of different process is analyzed. The first H2 production option, which is called PCC process, regenerates absorbent through controlling carbon conversion in order to supply heat by carbon combustion. In this way, when all material and heat demands are satisfied, and carbon conversion is about 68% in the gasifier, the cold gas efficiency is 0.74, and H2 content in the fuel gas is 97.6 vol% in the dry basis.
85
Furthermore, the process needs no additional heat supply and the process operation is simple. The second H2 production option, which is called NO process, is to supply heat for regeneration by NiO chemical reaction release heat. With the same feedstock (woods), the cold gas efficiency is lower than PCC process, about 0.67, and H2 content is only 96.1 vol%. Moreover, oxygen carrier NiO and more absorbent increase the material circulation quantity, and make it more difficult and costly in practice. The ASPEN Plus process simulations for H2 production give reference for constructing and analyzing the scale-up H2 production system. More details need to be researched in the next step. Acknowledgments Financial support by the National High Technology Research and Development (863) Program (No. 2003AA529260) and the Special Funds for Major State Basic Research Projects of China (No. 2003CB214502 ) are gratefully acknowledged. References [1] Curran GP, Fink CE, Gorin E. CO2 acceptor gasification process: studies of acceptor properties. Adv Chem 1967;69:141–65. [2] Elliott MA. Chemistry of coal utilization. Second supplementary volume. Prepared under the guidance of the Committee on Chemistry of Coal Utilization. New York, 1981. p. 1642–8. [3] Han C, Harrison DP. Multicycle performance of a single-step process for H2 production. Sep Sci Tech 1997;32:681–97. [4] Shimizu T, Hirama T, Hosoda H. et al. A twin fluid-bed reactor for removal of CO2 from combustion process. Trans IChemE 1999;77(Part A). [5] Lin SY, Suzuki Y, Hatano H, et al. The concept of a new high efficiency and clean coal utilization process (HyPr-RING). The 10th international conference on coal, Taiyuan, China, 1999. [6] Xiao Y, Lin S, Suzuki Y, et al, Michiaki HARADA. Process analysis of hydrogen production by reaction integrated novel gasification (HyPrRING). Preprint of and presentation in the 65th annual meeting of society of chemical engineering, Japan, 2000. [7] Xiao Y-H. Hydrogen from coal with zero emission. J Eng Thermophys 2001;22:13–5. [8] Lin S, Harada M, Suzuki Y. Process analysis for hydrogen production by reaction integrated novel gasification (HyPr-RING). Energy Conver Manage 2005;46:869–80. [9] Mattisson T, Lyngfelt A. Applications of chemical-looping combustion with capture of CO2 . Second Nordic minisymposium on carbon dioxide capture and storage, Göteborg, October 26, 2001. Available at http://www.entek.chalmers.se/anly/symp/symp2001.html. [10] Naqvi R, Bolland O, Brandvoll ], et al. Chemical looping combustionanalysis of natural gas fired power cycles with inherent CO2 capture. Proceedings of ASME Turbo Expo 2004, power for land, sea, and air, Vienna, Austria, 2004. [11] Choudhary VR, Rajput AM. Simultaneous carbon dioxide and steam reforming of methane to syngas over NiO–CaO catalyst. Ind Eng Chem Res 1996;25:3934–9. [12] Courson C, Udron L, Petit C, et al. Grafted NiO on natural olivine for dry reforming of methane. Sci Technol Adv Mater 2002;3:271–82. [13] Qiao C, Xiao Y, Yuan K, et al. Impact of operation parameters on hydrogen from coal. J Chem Indus Eng 2004;55:34–8. [14] Rizeq G, West J, Frydman A, et al. Fuel-flexible gasification -combustion technology for production of hydrogen and sequestration -ready carbon dioxide http://www.osti.gov/energycitations/servlet-s /purl/835933-c9LoUe/native/835933.pdf.
Comparative analysis of hydrogen production systems from biomass based on different absorbent regeneration processes Chunzhen Qiaoa, b , Yunhan Xiaoa,∗ , Xiang Xua, b , Lifeng Zhaoa , Wendong Tiana a Institute of Engineering Thermophysics of Chinese Academy of Sciences, Beijing 100080, China b Graduate School of Chinese Academy of Sciences, Beijing 100081, China
Received 30 September 2005; received in revised form 21 February 2006; accepted 31 March 2006 Available online 27 July 2006
Abstract Hydrogen production through single-step process has been proposed for a long time. The process can produce H2 in gasifier, and simultaneously, achieve integration of exothermic and endothermic as well as gas production and separation in one reactor. The process consists of mainly twin fluidized beds—a gasifier and a CO2 absorbent regenerator. In this paper, two modes are suggested for absorbent regeneration. One case named PCC process is partial carbon conversion in the gasifier, and the carbon residue acts as a fuel for the regenerator; the other case named NO process is nickel oxidation in the regenerator to supply heat for absorbent regeneration. The mass and energy balances and cold gas efficiency were analyzed by thermodynamic calculation for the two different processes. When the feedstock is same, PCC process gives a higher cold gas efficiency of 0.74 than NO process of 0.67. 䉷 2006 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. Keywords: Biomass; Hydrogen production; Absorbent regeneration; System analysis; Aspen Plus
1. Introduction As a clean fuel, hydrogen is expected to be one of the primary energy carriers in the 21st century. In order to achieve the object of using hydrogen not only as raw material for chemical engineering but also as energy, it is important for producing hydrogen with low-cost and large scale, at the same time reducing CO2 emissions. If both sets of requirements are to be met without excessive economic disadvantage to the world economy, then new hydrogen production methods with low or zero CO2 emissions must be developed. The carbonation reaction, which was developed several decades ago, was employed in the CO2 acceptor process for coal gasification [1]. A summary of the process described the coal gasification by steam [2], but not pursued. In recent years, sorption-enhanced gasification process has received a great deal
of emphasis again [3–6]. On the basis of the above concept, Center for Coal Utilization of Japan proposed HyPr-RING hydrogen production process in 1999 [5], and the process was operated at supercritical condition, and feedstock was fed into by slurry. At the same time, Institute of Engineering Thermophysics of Chinese Academy of Sciences (IET) proposed the idea of “single-step process for H2 production and CO2 fixed by mineral” [6,7]; the concept is similar, but the dry materials were fed into and the pressure was lower than HyPr-RING. The single-step process proposed by IET includes two cycles: in one H2 O reacts with hydrocarbon to produce hydrogen, then hydrogen converts to H2 O; in the other CaO reacts with CO2 to form calcium carbonate, then calcium carbonate is calcined to CaO. In the single-step process, the raw materials are hydrocarbons, water and CaO. The process involves three main reactions in gasifier: C + H2 O → CO + H2 ,
∗ Corresponding author.
E-mail address: [email protected] (Y. Xiao).
0 H298 = 131.2 kJ/mol,
(1)
C + H2 O → CO2 + H2 ,
0 H298 = −41.2 kJ/mol,
(2)
CaO + CO2 → CaCO3 ,
0 H298 = −178.8 kJ/mol.
(3)
0360-3199/$ - see front matter 䉷 2006 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2006.03.009
C. Qiao et al. / International Journal of Hydrogen Energy 32 (2007) 80 – 85
The three reactions constitute overall reaction: C + CaO + 2H2 O → CaCO3 + 2H2 , 0 H298 = −88.8 kJ/mol.
(4)
The hydrogen produced by gasification reacts with carbon, and the byproduct, methane, is produced. C + 2H2 → CH4 ,
0 H298 = 74.85 kJ/mol.
0 H298 = 178.8 kJ/mol.
Given the input stream and operating conditions (temperature, pressure and flow rate) for every module, we can calculate the equilibrium composition of out stream using Aspen Plus, as well as the heat contained in the output. 2.1. Partial carbon conversion (PCC) for regeneration
(5)
The heat released by reaction (3) supplies to carbon gasification, at the same time, CO2 was separated by the carbonation process (reaction (3)) and high-purity concentration hydrogen was produced in the gasifier. In the regenerator, the follow reaction takes place: CaCO3 → CaO + CO2 ,
81
(6)
In the single-step process, the absorbent absorbs heat in the regenerator and transfers it into the gasifier, so regeneration process is the main expended energy part in the hydrogen production system; it has an important effect on the whole process. Shimizu et al. [4] investigated the process removed CO2 from flue gas using CaO as a CO2 absorbent in a twin fluidized bed, and analyzed the material balance, heat balance, power generation and power consumption for O2 production and CO2 compression. The heat of decomposition in the regenerator is supplied by the combustion of the feed-in coal and pure oxygen. Xiao et al. [7] constituted and analyzed the single-step system for hydrogen production from coal; when only part carbon was converted in the gasifier, the cold efficiency was above 90%. Lin et al. [8] analyzed components and their mass and energy balances of HyPr-RING process by thermodynamic calculations. The heat in the regenerator is supplied by burning the unconverted carbon in the gasifier, and the energy contained in the coal is distributed in the gasifier and the regenerator by a ratio of about 23 . But they did not investigate the effects of the different regeneration method on process, such as the changes of gas products and heat balance when the regeneration method was changed. In this paper, we compare and analyze the effect of different absorbent regeneration process on hydrogen production systems. Firstly, two different hydrogen production systems are constructed, in which the gasification part is same, namely single-step for hydrogen production; then thermodynamic calculation is employed to analyze the material and energy balances of the two systems; finally, we compare the cold gas efficiency and analyze the effect of different absorbent regeneration processes on the hydrogen production system, as well as the advantages and disadvantages of the two processes. 2. Options to produce hydrogen based on different regeneration process Single-step process for H2 production consists mainly of two reactors and some heat exchangers. The hydrocarbons gasify and react with absorbent and steam in the gasifier, which produce gases and solids. The other is a regenerator for absorbent (CaCO3 ), which is calcined to CaO. Several heat exchangers are used in the process for heat recovery.
Control the conversion of carbon in the gasifier in order to retain portion of the carbon burning in the regenerator to supply heat for regeneration; the principle figure of producing hydrogen is shown in Fig. 1 and the system is shown in Fig. 2. To produce hydrogen from biomass, biomass must be treated and mixed with absorbent in the mixer. Then the mixture was fed into the gasifier with steam which was preheated by the gases and solids vented from the cyclone. In the gasifier, biomass reacts with steam to produce mainly hydrogen, methane and carbon dioxide, and carbon dioxide is absorbed by absorbent and forms CaCO3 , which promotes the shift reaction (2), so CO concentration in the gas products is very low. The unreacted carbon is discharged with other solids (CaCO3 and ash) and then enters the regenerator, which burns there to supply the heat for regeneration. Finally, the CaCO3 was calcined to CaO and discharged with ash from regenerator. In order to produce pure CO2 , O2 is used as oxidizer in the regeneration process. The gases from cyclone contain mainly hydrogen and methane as well as a large quantity of steam, which exchange heat with water from pump in the heat exchanger and then enter into the condenser, where they are separated into gases and liquid (water). The separation of CaO (absorbent) and ash is not considered presently, so we dispose part of the mixture of CaO and ash and inject fresh CaCO3 into the regenerator considering the active absorbent declination. The biomass used in the calculation is shown in Table 1. 2.2. Nickel oxidation (NO) for regeneration Based on the conception of chemical-looping combustion (CLC) [9,10], taking into account the need for pure oxygen in the regenerator, as well as the catalysis of NiO on the steam reforming [11,12], we select NiO as the oxygen carrier to design
CO2
H2, CH4 C, Ash, CaCO3 Gasifier
Regenerator CaO
Hydrocarbon Steam
O2
Fig. 1. The principle of carbon conversion partial for hydrogen production.
82
C. Qiao et al. / International Journal of Hydrogen Energy 32 (2007) 80 – 85
Feedstock
Steam Gas Cyclone Exchanger network
Flash
Mixer CO2
Gasifier
CaO and ash
Regenerator
Liquid Water
O2
CaCO3 Fig. 2. PCC for regeneration in the H2 production system.
Table 1 Properties of hydrocarbon Vol.
Mois.
FC
Ash
Proximate analysis (wt%) 79.55 3.76
14.83
1.86
C
N
S
H
tion. The other is CO2 absorbent (CaO) circulation. CaO reacts with CO2 from biomass gasification and forms CaCO3 , and simultaneously releases heat. Hereafter, CaCO3 is calcined in the regenerator using the heat from metal oxidation. Except reactions (1)–(6), the following reactions take place in the NO process: CO + NiO → CO2 + Ni,
Ultimate analysis (wt%) daf 46.21 5.56
1.2
H2, CH4
0.018
CO2
Ni, Ash,CaCO3 Gasifier
Regenerator CaO, NiO
Hydrocarbon Steam
O2
Fig. 3. The principle of chemic chain reaction for hydrogen production.
the system. The principle using NiO as the oxygen carrier to supply heat for regeneration is shown in Fig. 3. In the process, there are two chemic chain reactions. One is metal circulation, which is nickel oxidation reacted with CO in the gasifier and formation of nickel and carbon dioxide. Later, nickel reacts with oxygen in the regenerator and forms nickel oxide, and releases a great deal of heat for absorbent regenera-
Ni + 1/2O2 → NiO,
0 H298 = 43.34 kJ/mol,
0 H298 = −239.63 kJ/mol.
(7) (8)
Reaction (7) takes place in gasifier and reaction (8) in regenerator. Reaction (8) releases heat for CaCO3 calcination. Based on the principle, the system using chemical chain reaction to produce hydrogen is designed. The system is shown in Fig. 4. The flowchart is similar to the PCC process, except that nickel oxide is put into gasifier with biomass and absorbent. NiO is deoxidized by CO from carbon gasification and absorbs heat at the same time, and CO from reaction (1) is consumed in the process. Different from PCC process, the heat needed by absorbent regeneration comes from nickel oxidation. The purpose of this paper is to analyze the effect of a different regeneration process, so the complex exchange of water and exit gases and solids will not be taken into account in detail, which is the same as the PCC process. The biomass kind as well as feed quantity used in the NO calculation is same as the PCC process, and other parameters are shown in Tables 2 and 3. The calculation conditions (T and P) are chosen based on our earlier study [13]. 3. Modeling the processes ASPEN Plus was used to calculate the equilibrium compositions and enthalpy changes of the process, as well as establish the mass and energy balances of equipment selection and
C. Qiao et al. / International Journal of Hydrogen Energy 32 (2007) 80 – 85
83
Steam
Feedstock
Gas Cyclone NiO Exchanger network CO2
Gasifier
Water
CaO and ash
Regenerator
Mixer
Flash
Liquid
O2
CaCO3 Fig. 4. NO for regeneration in the H2 production system.
Table 2 Other calculation conditions and part results Supply hydrocarbon (woods): 1.00 × 105 (kg/d) High heating value of hydrocarbon: 18.72 × 108 (kJ/d) Supply absorbent: 1.4 × 105 (kg/d) Supply CaCO3 : 10 × 103 (kg/d) Supply H2 O: 1.8 × 105 (kg/d) Gasifier operating condition: 923 K, 3.0 MPa; Carbon conversion of hydrocarbon in gasifier: 68% Regenerator operating condition: 1300 K, 0.1 MPa Product of fuel gas (dry gas): 4.64 × 106 (mol/d) (97.6% H2 , 2.02% CH4 , 0.28% CO2 , 0.08% CO) High heating value of product gas: 13.78 × 108 (kJ/d)
Table 3 Other calculation conditions and part results Supply absorbent: 2.1 × 105 (kg/d) Supply nickel oxide: 2.28 × 105 (kg/d) Supply H2 O: 1.2 × 105 (kg/d) Gasifier operating condition: 923 K, 3.0 MPa Regenerator operating condition: 1300 K, 0.1 MPa Product of fuel gas (dry gas): 4.065 × 106 (mol/d) (906.06% H2 , 3.65% CH4 , 0.21% CO2 , 0.08% CO) High heating value of product gas: 12.47 × 108 (kJ/d)
design. Gasification section is composed of the RYIELD (yield) and RGibbs (minimize Gibbs free energy) reactor blocks. The gasification feedstock, wood, is first decomposed to its elemental components through a yield-basis reactor block. These species, with a stream representing the heat of decomposition, are then passed to a Gibbs reactor block with gasification steam. The Gibbs reactor block minimizes the Gibbs free energy to attain the equilibrium composition of the identified gasification products at the temperature specified for reaction (923 K). In the regenerator, unreacted carbon is combusted with pure oxygen for PCC process, while nickel is for NO process. Calcination of absorbent and combustion (oxidation for NO process) occur simultaneously in a Gibbs reactor block at a specified temperature of 1300 K. The temperature of the reactor
is controlled by balancing the heat required for calcination with the heat of combustion (oxidation). The balance is achieved by controlling the conversion of carbon in the woods for PCC process and varying the rate of nickel fed to the regenerator for NO process. The desired excess oxygen concentration in the flue gas is obtained by varying the oxygen feed rate to the regenerator. In the results, species whose mole fraction is less than 10−5 have been omitted from consideration. Further assumptions on the calculations include: the gasifier operates isothermally; the regenerator operates adiabatically; the heat rejected from gasifier is lost to the environment; the products of the gasifier (gases and solids) undergo a natural phase separation without the input of work; all heat exchange process are
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C. Qiao et al. / International Journal of Hydrogen Energy 32 (2007) 80 – 85
Table 4 Mass and heat balances of the gasifier Materials (t)
Input: Woods CaO Steam NiO Total heat: (108 kJ)
PCC 100 140 180
Output: H2 (103 kmol) CH4 (103 kmol) Steam (103 kmol) CaCO3 Ca(OH)2 Ash C Ni Total heat: (108 kJ)
PCC 4.531 0.094 7.930 243.5 4.7 1.8 14.8
Heats (108 kJ) Sensible and latent heats
Reaction heat
NO 100 210 120 228 PCC 27.48 NO 3.905 0.164 5.132 367 5.6 1.8
PCC
NO
6.05
4.18 0.59
NO 27.58 PCC
NO
6.58
4.31
1.93
3.13
PCC 18.72 2.71
NO 18.72 4.09
PCC
NO
13.78
12.47
4.85 179 PCC 27.15
7.32 NO 27.23
Table 5 Mass and heat balances of the regenerator Materials (t)
Input: CaCO3 Ca(OH)2 Ash C Ni O2 Total heat: (108 kJ)
PCC 253.5 4.7 1.8 14.8
Output: CO2 (103 kmol) H2 O (103 kmol) O2 (103 kmol) CaO Ash NiO
PCC 3.665 0.060 0.018 139.8 1.8
Heat recovery: (108 kJ) Total heat: (108 kJ)
40
Heats (108 kJ) Sensible and latent heats
Reaction heat
NO 367 5.6 1.8
PCC
NO
1.7
3.32
PCC
NO
4.85 17 40 PCC 7.15 NO 3.665 0.074 0.019 208 1.8 226 PCC 0.57 7.11
ideal; all reactions achieve chemical equilibrium; energy consumption, such as solids’ transport, has been omitted from consideration. 4. Comparison between the two different absorbent regeneration processes The flowcharts of different processes are shown in Figs. 2 and 4, and the complex exchange network is not analyzed amply. In the PCC process, about 68% carbon in the coal is suggested
7.32 0.53 NO 11.28 PCC
NO
1.88
1.85
1.34
1.34 3.55
PCC
NO
3.32
4.06
NO 1.29 10.75
to be gasified in the gasifier, and 32% carbon (char) remains in the solid residues as fuel for CaCO3 calcination in the regenerator. While 49% carbon in the coal is used to deoxidize nickel oxide in order to supply heat in the regenerator through nickel reacting with oxygen, only 51% carbon is gasified to form hydrogen. The equilibrium composition of the products and heat balance in the gasifier and regenerator is shown in Tables 4 and 5 for the PCC and NO processes, which is calculated on the basis of the input materials.
C. Qiao et al. / International Journal of Hydrogen Energy 32 (2007) 80 – 85
When gasification feed is same, which consists of 1 × 105 kg of woods, and operation condition is 923 K and 3.0 MPa, the equilibrium gas products are 4.53 × 103 kmol H2 for PCC process, while only 3.91 × 103 kmol for NO process. The dry gas contains 97.6 vol% H2 for PCC process and only 96.1 vol% for NO process. CO and CO2 comprise less than 0.3 vol%. The solid products of the gasifier in equilibrium are 2.435 × 105 kg CaCO3 and 0.047×105 kg Ca(OH)2 , with 0.148×105 kg unreacted carbon for PCC process, while 3.67 × 105 kg CaCO3 and 0.056 × 105 kg Ca(OH)2 , with 1.79 × 105 kg Ni for NO process. The heat balance of the gasifier in equilibrium is also shown in Table 4. As can be seen, the input heat is larger than the output for the gasifier, given that the allowable heat loss of the gasifier is 0.33 × 108 kJ for PCC process and 0.35 × 108 kJ for NO process. The input heat was designed to be larger than the output heat to allow a heat loss in the regenerator of about 0.04×108 kJ for PCC process and 0.53 × 108 kJ for NO process. The recovered heat is used to superheat steam for the gasifier and to generate power for oxygen separation and other power consuming parts of process. The cold gas efficiency is defined as cold gas efficiency = HHV of fuel gas/HHV of woods. From the heat value of the produced fuel gas and input biomass (woods), the cold gas efficiency (298 K, 0.1 MPa) was calculated for the two processes, which is 0.74 for PCC process and 0.67 for NO process. Based on the analysis, we can see that, at the same quantity of woods, the concentration of hydrogen in the gas products is only 96.1 vol% for NO process and is lower than 97.6 vol% for PCC process, moreover, the PCC process has a high cold gas efficiency of 0.74, while the NO process is only 0.67. In the NO process, the presence of NiO will promote the steam reforming methane reaction, but the reduction of NiO in the gasifier will absorb heat, which needs more CO2 absorbent than the PCC process for heat balance of NO process, which increased the material circulation quantity, and made the power for separating rise in practice. In the fuel-flexible gasificationcombustion technology [14], the metal oxide (NiO or Fe2 O3 ) is regarded as oxygen transfer material (OTM), which made the oxygen production section needless in the process, while the advantage is not been exhibited in the NO process. 5. Conclusion This paper analyzes the mass and energy balances of the process for hydrogen production from biomass with different regeneration methods by means of thermodynamic calculations, and the cold gas efficiency of different process is analyzed. The first H2 production option, which is called PCC process, regenerates absorbent through controlling carbon conversion in order to supply heat by carbon combustion. In this way, when all material and heat demands are satisfied, and carbon conversion is about 68% in the gasifier, the cold gas efficiency is 0.74, and H2 content in the fuel gas is 97.6 vol% in the dry basis.
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Furthermore, the process needs no additional heat supply and the process operation is simple. The second H2 production option, which is called NO process, is to supply heat for regeneration by NiO chemical reaction release heat. With the same feedstock (woods), the cold gas efficiency is lower than PCC process, about 0.67, and H2 content is only 96.1 vol%. Moreover, oxygen carrier NiO and more absorbent increase the material circulation quantity, and make it more difficult and costly in practice. The ASPEN Plus process simulations for H2 production give reference for constructing and analyzing the scale-up H2 production system. More details need to be researched in the next step. Acknowledgments Financial support by the National High Technology Research and Development (863) Program (No. 2003AA529260) and the Special Funds for Major State Basic Research Projects of China (No. 2003CB214502 ) are gratefully acknowledged. References [1] Curran GP, Fink CE, Gorin E. CO2 acceptor gasification process: studies of acceptor properties. Adv Chem 1967;69:141–65. [2] Elliott MA. Chemistry of coal utilization. Second supplementary volume. Prepared under the guidance of the Committee on Chemistry of Coal Utilization. New York, 1981. p. 1642–8. [3] Han C, Harrison DP. Multicycle performance of a single-step process for H2 production. Sep Sci Tech 1997;32:681–97. [4] Shimizu T, Hirama T, Hosoda H. et al. A twin fluid-bed reactor for removal of CO2 from combustion process. Trans IChemE 1999;77(Part A). [5] Lin SY, Suzuki Y, Hatano H, et al. The concept of a new high efficiency and clean coal utilization process (HyPr-RING). The 10th international conference on coal, Taiyuan, China, 1999. [6] Xiao Y, Lin S, Suzuki Y, et al, Michiaki HARADA. Process analysis of hydrogen production by reaction integrated novel gasification (HyPrRING). Preprint of and presentation in the 65th annual meeting of society of chemical engineering, Japan, 2000. [7] Xiao Y-H. Hydrogen from coal with zero emission. J Eng Thermophys 2001;22:13–5. [8] Lin S, Harada M, Suzuki Y. Process analysis for hydrogen production by reaction integrated novel gasification (HyPr-RING). Energy Conver Manage 2005;46:869–80. [9] Mattisson T, Lyngfelt A. Applications of chemical-looping combustion with capture of CO2 . Second Nordic minisymposium on carbon dioxide capture and storage, Göteborg, October 26, 2001. Available at http://www.entek.chalmers.se/anly/symp/symp2001.html. [10] Naqvi R, Bolland O, Brandvoll ], et al. Chemical looping combustionanalysis of natural gas fired power cycles with inherent CO2 capture. Proceedings of ASME Turbo Expo 2004, power for land, sea, and air, Vienna, Austria, 2004. [11] Choudhary VR, Rajput AM. Simultaneous carbon dioxide and steam reforming of methane to syngas over NiO–CaO catalyst. Ind Eng Chem Res 1996;25:3934–9. [12] Courson C, Udron L, Petit C, et al. Grafted NiO on natural olivine for dry reforming of methane. Sci Technol Adv Mater 2002;3:271–82. [13] Qiao C, Xiao Y, Yuan K, et al. Impact of operation parameters on hydrogen from coal. J Chem Indus Eng 2004;55:34–8. [14] Rizeq G, West J, Frydman A, et al. Fuel-flexible gasification -combustion technology for production of hydrogen and sequestration -ready carbon dioxide http://www.osti.gov/energycitations/servlet-s /purl/835933-c9LoUe/native/835933.pdf.