Article pubs.acs.org/EF
Energy Assessment of Ethanol-Enhanced Steam Reforming by Means of Li4SiO4 Carbon Capture Xiaopeng Qiao,† Pilar Lisbona,*,‡,§ Xin Guo,*,† Yolanda Lara,§ and Luis M. Romeo§ †
State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, 1037 Luoyu Street, Hongshan, Wuhan, Hubei 430074, People’s Republic of China ‡ Escuela Universitaria de Ingenierías Agrarias de Soria, Universidad de Valladolid, Campus Universitario Duques de Soria, s/n, 42004 Soria, Spain § Research Centre for Energy Resources and Consumption, Universidad de Zaragoza, Mariano Esquillor Gómez, 15, 50018 Zaragoza, Spain ABSTRACT: High-temperature solid looping cycles for carbon capture provide a number of benefits when coupled with fuel reforming because they may combine the fuel reactor where the fuel is decarbonized with the CO2 sorption step in the capture cycle. The heat released in the carbonation reaction may be used to run the endothermic steam reforming occurring in the same reactor, leading to an overall autothermic reaction. Another benefit derived from the presence of a CO2 sorbent is the shifting of the equilibrium to greater hydrogen yields. A very promising approach is the utilization of Li4SiO4 because this sorbent does not present a cyclic degradation as pronounced as the traditional Ca-based sorbents. The extremely high cost of Li4SiO4 may be overcome through the production of the sorbent from rice husk, a kind of agriculture waste, which provides the silica source. Furthermore, rice husk Li4SiO4 exhibits better sorption properties compared to that of pure Li4SiO4 because of the alkaline element content from rice husks. In the State Key Laboratory of Coal Combustion, the kinetics of this enhanced process has been characterized. Different configurations may be adopted to integrate this capture process in a precombustion process for enhanced hydrogen production and minimize the energy penalty associated with sorbent regeneration. A fixed bed system has been modeled to assess the energy requirements of the system and the available energy for integration. By application of the experimental results obtained for Li4SiO4 kinetics, the developed model allows for the estimation of syngas compositions, production rate, and energy flows. This model represents an interesting tool for the assessment of further applications of the enhanced reforming of gaseous fuels through in situ carbon capture with Li4SiO4.
1. INTRODUCTION Together with modern development, the utilization of fossil fuels has brought serious environmental problems. Besides, the depletion of these fuels has promoted the pursuit of new energy sources that should ideally be cheap, widely available, and sustainable. To face the issues above, the concept of H2 produced from biomass-derived oxygenates was proposed by several researchers.1−4 Hydrogen presents a number of advantages, such as its clean oxidation, its high energy density, and the relatively mature utilization technology, while ethanol is one of the most favorable biomass-derivable fuels with a relatively high power density.5 Among the different production techniques, hydrogen may be generated from cyclic reforming of ethanol.6 The carbon dioxide released in the conversion process would be compensated by the carbon dioxide intake by the photosynthesis of plants, which are used as feedstock to produce further ethanol through fermentation. The net zero carbon emission process seems promising when compared to the traditional methods, where fossil fuels, such as methane, were used as reforming materials. An effective way to improve steam reforming of ethanol (SRE) is to enhance the process by CO2 removal, which will displace the thermochemical equilibrium beyond the limits of the original reaction, yielding higher hydrogen concentrations. The ethanolreforming process under the presence of a carbon dioxide sorbent is known as sorption-enhanced steam reforming of © XXXX American Chemical Society
ethanol (SE-SRE). The reforming process under real conditions involves quite a complex system of reactions. Apart from carbon dioxide, various byproducts, such as ethylene and acetaldehyde,7 will appear as a result of reaction pathways. These reaction pathways are affected by catalyst types and operating conditions.8 A nickel-based catalyst is one of the most studied materials for the catalytic SRE process as a result of its relatively low costs and good activity compared to other metals.9 A kinetic mechanism based on a nickel-based catalyst is reported in equations I−IV10 in Table 1. The sorbent must also be carefully selected because the sorbent properties will strongly affect the effectiveness of the Table 1. SE-SRE Reactions ethanol dehydrogenation acetaldehyde decomposition methane steam reforming water-gas shift reaction CO2 sorption
C2H5OH = C2H4O + H2 C2H4O = CH4 + CO CH4 + H2O = CO + 3H2 CO + H2O = CO2 + H2 Li4SiO4 + CO2 = Li2CO3 + Li2SiO3
(I) (II) (III) (IV) (V)
Special Issue: 5th Sino-Australian Symposium on Advanced Coal and Biomass Utilisation Technologies Received: September 30, 2015 Revised: December 20, 2015
A
DOI: 10.1021/acs.energyfuels.5b02272 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
reactor. The conversion ratio of the sorbent calculated from experimental data obtained in TGA tests can be expressed as shown in eq 1
sorption-enhanced process. Several materials, such as hydrotalcite-like compounds,6 calcium-containing sorbents,11 and lithium-based ceramics,12 have been proposed as a CO2 acceptor for their large capacity, high selectivity, fast kinetics, and good cycle performance at relatively high temperatures. Among them, a novel lithium-based ceramic, rice husk lithium orthosilicate (RH−Li4SiO4), seems to be a promising material with interesting sorption behavior and a much lower price (with an agriculture waste, rice husk, as its silica precursor) compared to a pure Li4SiO4 pellet.13 The reaction for CO2 sorption is shown in equation V in Table 1. In a previous research, Wang et al.13 derived SiO2 from rice husk powders. SiO2 and Li2CO3 were mechanically mixed and calcined at a high temperature to produce RH−Li4SiO4. Experiments showed that RH−Li4SiO4 exhibited even better sorption properties when compared to pure Li4SiO4 because of the alkaline element content from rice husks. Wu et al.14 performed a SE-SRE experiment based on hydrotalcite-like compound sorbent (HTLc) and proposed a numerical simulation method toward the process. Li and Cai15 built a numerical model to analyze the multiple cycle performance of the sorption-enhanced system based on Ca-based sorbents. Essaki and his group4 succeeded in removing the byproduct CO2 selectively from the SE-SRE process with Li4SiO4 as the sorbent, and a strong equilibrium shift phenomenon was discovered. However, the performance of the SE-SRE system is still unknown when the novel RH−Li4SiO4 sorbent acts as a CO2 acceptor in the ethanol reformer. The purpose of this work is to derive sorption kinetics for RH−Li4SiO4 and then use it as sorbent in a SE-SRE process to assess system performance and energy consumption.
CaO
Fe2O3
SiO2
Al2O3
wt %
3.42
0.03
2.98
0.09
92.45
1.03
(2)
where t is the time (s), m is the reaction order (m = 1 when the chemical reaction is the rate-determining step, and m = 2 when diffusion controls the reaction), and k is the kinetic constant (s−1), following Arrhenius’ law (eq 3) k = k 0e−Ea / RT
(3)
where k0 is the pre-exponential constant (s−1), Ea is the energy of activation (kJ/mol), R is the gas constant (kJ mol−1 K−1), and T is the temperature (K). The carbonation rate of RH−Li4SiO4 has been derived from experimental results. After differentiation of eq 2, the conversion rate of variation may be expressed as presented in eq 4. (1 − X )2/3 dX 3 = k 0e−Ea / RT dt m [1 − (1 − X )1/3 ]m − 1
(4)
The sorption rate constant, kV (mol kg−1 of sorbent s−1), may be expressed as presented in eq 5. To solve eq 5, the Arrhenius pre-exponential constant, k0, and the active energy, Ea, must be defined from the experimental results.
Table 2. RH−Li4SiO4 Chemical Components Na2O
(1)
kt = (1 − (1 − X )1/3 )m
A Rigaku RIX 300 X-ray fluorescence (XRF) spectrometer was used to determine the chemical composition of RH−Li4SiO4 to define its active ratio, which is defined as the mass ratio of the CO2 acceptor in RH−Li4SiO4. RH−Li4SiO4 chemical composition is presented in Table 2,
K2O
wsηreactive MWCO2
where ηreactive is the mass ratio of the active sorbent, ws is the weight of the fresh sorbent (kg), Δw(t) is the weight increase at time t (kg), and X(t) is the conversion ratio, which means the molar fraction of the sorbent that has been converted. Jander’s equation is frequently used in gas−solid reactions,18−22 and it has been considered for the sorption model. The general form of this equation is shown in eq 2
2. EXPERIMENTAL SECTION
sample
Δw(t )MWLi4SiO4
X (t ) =
kV =
ηreactive
dX MWLixM4−xSiO4 dt
(5)
The variation of the RH−Li4SiO4 conversion ratio in a time interval could be expressed as in eq 6.
which shows SiO2 as the major component. Trace elements (such as potassium, sodium, calcium, iron, and aluminum) will improve the microstructure of RH−Li4SiO4 associated with the better sorption properties when compared to pure Li4SiO4.16,17 The origin of the rice husk used in this investigation was Wuhan, China. The procedure to prepare the final sorbent (RH−Li4SiO4) is described by the following. First, rice husks were soaked in water, and impurities were washed out. Then, rice husks were calcined at 900 °C for 6 h and ground into fine powders (
