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Ultracompact biofuels catalytic reforming processes for distributed renewable hydrogen production
C H A P T E R
17 Ultracompact biofuels catalytic reforming processes for distributed renewable hydrogen production Vincenzo Palma*, Daniela Barba, Eugenio Meloni, Simona Renda, Concetta Ruocco University of Salerno, Department of Industrial Engineering, Via Giovanni Paolo II, Fisciano (SA), Italy * Corresponding author. e-mail address: [email protected]
1. Introduction Biofuel can be defined as any fuel deriving from biomass and, therefore, is considered a source of renewable energy. Today, the attention is focused on biofuels because they are a cost effective solution and constitute an environmentally clean alternative to petroleum and other fossil fuels, considering moreover the rising petroleum prices and the effects of the global warming by fossil fuels [1]. Anyway, there are apprehensions relatively to the utilization of biofuels because of the economic and environmental costs associated respectively to the refining process and the possible destruction of areas devoted to food production. Biofuels are usually classified as first-, second, and third-generation biofuels [1]. The firstgeneration biofuels are directly correlated to a generally edible biomass. Second-generation biofuels comprise fuels obtained from different
Catalysis, Green Chemistry and Sustainable Energy https://doi.org/10.1016/B978-0-444-64337-7.00017-3
feedstocks, from lignocellulosic to municipal solid wastes. Third-generation biofuels are related to algal biomass. Liquid biofuels are of particular interest because of the wide infrastructure already in place to use them, especially for transportation. The greatest production of liquid biofuel is represented by ethanol obtained by fermenting starch or sugar. The second most common liquid biofuel is biodiesel, which derives mainly from oily plants (soybean or oil palm) and to a lesser extent from other oily sources. Biodiesel is used especially in Europe in diesel engines and usually blended with petroleum diesel fuel in different percentages. The use of algae and cyanobacteria as a source of “third-generation” biodiesel, while representing a promising alternative, is characterized by many economic drawbacks, which limit the develop at an industrial level.
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17. Ultracompact biofuels catalytic reforming processes
Other types biofuels include methane and biogas, deriving from the decomposition of biomass in the absence of oxygen, and methanol, butanol, and dimethyl ether, which are in development. Biofuels are particularly suited for distributed hydrogen production because they allow the employment of a compact plant configuration. In general, central, semicentral, and distributed production are expected to play a key role in the evolution and long-term use of hydrogen as an energy carrier. Central production is the more diffuse way to produce hydrogen in large facilities that, to take advantage of the economy of scale, need a very high hydrogen demand. Semicentral productions are usually located in close proximity to the point of use of hydrogen; this configuration allows lower transport costs and infrastructure, even if they are still present. Distributed production means that hydrogen can be produced in small plants where it is needed: this can be applicable mainly to reforming of renewable gaseous or liquid fuels (such as bioethanol, bio-oil, biogas, and biodiesel) and for small-scale water electrolysis. As hydrogen is not currently employed as fuel yet, distributed production may be the most viable approach for introducing hydrogen in the fuels’ scenery, because the initial demand will be low and larger scale production could be not economically advantageous. Moreover, distributed hydrogen production can serve the hydrogen market at lower costs and environmental impact than the conventional production systems. The hydrogen production rate in distributed hydrogen generation (DHG) is about 1200 kg/day, so these systems can be sited near industrial users or fuel cell vehicle (FCV) filling stations. Small-scale hydrogen production has been attempted before, although the high cost due to economies of scale has limited its impact: the contemporaneous production of hydrogen together with power and heat solves the scale problem (Fig. 17.1).
2. Distributed hydrogen production from bioethanol In recent years, the potential use of bioethanol for distributed hydrogen production has attracted attention worldwide. Bioethanol, in fact, provides an efficient solution for H2 generation, especially due to its economic production, which enables bioethanol to compete with standard fossil-based fuels [2,3]. In this regard, different solutions have been proposed for distributed hydrogen production at or near the point of use, aiming at minimizing the safety issues related to H2 storage and delivery. However, suitable technologies for hydrogen production at a small scale need to meet specific requirements in terms of both compactness and rapid response. Therefore, reforming of renewable liquids, allowing for high storage energy density, offers potentials for efficient advances in the framework of distributed generation [4]. A schematic representation of an integrated system for distributed H2 production from bioethanol is presented in Fig. 17.2. The preheated bioethanol mixture is sent to the reforming unit, where external heat is supplied to sustain the endothermic reaction, and the reaction takes place, producing hydrogen and carbon oxides. The generation of highpurity hydrogen can involve different separation units (palladium-based membranes with relevant perm-selectivity toward hydrogen, preferential oxidation, as well as methanation of carbon monoxide). The as-produced stream can be fed to a fuel cell for electricity generation [5]. To assure a high level of integration of the various units and face the space optimization requirements typical of distributed hydrogen production systems, the employment of microreactors has been proposed as a potential solution [6,7]. Microreactors, in fact, are characterized by compact sizes and light weight; moreover, their employment assures reduced capital and operative costs. Their small linear
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2. Distributed hydrogen production from bioethanol
FIGURE 17.1
Central production compared to distributed hydrogen production.
dimensions increase heat and mass transfer rates, with very limited diffusional issues; however, due to the quite small passage size, pressure drops can become significant. Anyway, the very fast system response is particularly attractive in view of obtaining better process and high hydrogen yield. In addition, other strategies have been proposed for distributed hydrogen production from biofuels. In the following sections, some examples will be described. Benito et al. investigated the performance of a 1-kW bioethanol processor for the production of a high-purity hydrogen stream to be fed to a polymeric electrolyte membrane (PEM) fuel cell [8]. The downstream addition of purification
units (water gas shift and CO preferential oxidation) allowed generating a products mixture virtually free of carbon monoxide (CO concentration lower than 50 ppm). The processor operation, performed at 650e700 C and atmospheric pressure, involved zero CO2 emission and a notnegligible methane production that, however, has no interference on the activity of the PEM fuel cell. In the final prototype, the recovery of the energy produced by the exothermic reactions as well as the residual heat of the flue gas at the fuel cell anode resulted in an overall efficiency improvement higher than 25% for a steam-tocarbon ratio of 3 (and globally increased compared, for example, to the efficiency of internal combustion engines).
CO PrOx
WGS
Bioethanol Preheater
Reformer CO ~ 10%
External duty
FIGURE 17.2
CO ~ 1%
CO methanaon
Pd membrane
H2with < 50ppmCO
Fuel cell
Scheme of an integrated system for distributed hydrogen generation from bioethanol [5].
IV. Selected examples and case history
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The development of an innovative and compact steam reformer for pure hydrogen production from bioethanol was described in a work of Giaconia et al. [9]. The system has been designed to also assure the conversion of feedstocks different from bioethanol (i.e., biogas). To guarantee a rational exploitation of solar energy, heat is directly driven to the system by concentrating solar power plants using molten salts as heat transfer fluids; alternatively, a biomass combustor as well as an off-gas combustor can be used to supply heat. A high degree of compactness can be reached through the technology of membrane reactors [10]: the integration of reactor and separation in the same unit allows the production of highpurity hydrogen, reducing the volume or even avoiding further downstream operations. Moreover, due to the selective removal of the desired product from the reactor outlet, it is possible to obtain reaction conversions higher than those achieved under the thermodynamic equilibrium conditions (well-known “shift effect”) [11,12]. Compared to a conventional steam reformer, lower operative temperatures (400e550 C) are selected for this plant, thus assuring a relevant reduction in materials costs. At such temperatures, water gas shift pathway is also favored in the membrane reactor with very low outlet CO contents. A high degree of process intensification is also reached by selecting structured carriers as support for the reforming catalysts. The
choice of materials with high thermal conductivity assures an excellent heat management within the catalytic bed, with reduced thermal gradients and a better exploitation of the heat coming from the fluid medium. These systems also allow negligible pressure drops and concentration polarization phenomena (the high mass transfer rate assured by the particular geometry of the structured catalyst avoids reduced H2 permeation fluxes, which could be observed in the boundary layer near the membrane surface [13]), which is crucial for the good operation of the integrated reactor [14,15]. High performances were also reached through the development of a specific bimetallic catalyst with relevant activity and stability for the desired reaction [16] and the employment of composite palladium-based membranes. A schematic representation of the process, developed in the framework of the CoMETHy Project [17] (Compact Multifuel-Energy to Hydrogen converter) is reported in Fig. 17.3. Viviente et al. [18] have described the employment of bioethanol as feedstock for a microcogeneration PEM-based system in off-grid applications. They demonstrated that a distributed power generation system based on a micro-combined heat and power (m-CHP) technology can allow a reduction of primary energy consumption and energy costs compared to the conventional centralized generation. The optimization of thermal integration, in fact, can assure
FIGURE 17.3 Schematic representation of the CoMETHy concept [9].
IV. Selected examples and case history
3. Hydrogen production by catalytic steam reforming of bio-oil
an enhancement in overall plant efficiency higher than 90%. In addition, the integration of the fuel cell with the membrane reactor as well as the m-CHP system allows the reduction of the expenses related to both heat exchangers and auxiliary elements. Hydrogen production from bioethanol was studied in a fluidized bed membrane reactor, based on the integration of reaction and purification in a single unit. The reactor operates under autothermal conditions: air is co-fed with the ethanolewater mixture at proper ratios, selected to provide the heat supply for the endothermic steam reforming reaction. In addition, the heat duty needed for the plant operation comes from the retentate combustion, with no further fuels requirements [19]. The presence of air in the reaction system also contributes to maintain the catalyst surface clean from eventually deposited carbonaceous species, which could have a deleterious impact in terms of catalyst activity and stability [20]. An unwanted increase in membrane surface area, caused by concentration polarization phenomena, can be avoided thanks to the fluidized bed configuration of the catalytic reformer, which enhances external mass transfer rates from the catalyst toward the surface of the membrane. In addition, fluidized beds are characterized by very low pressure drops and practical isothermal operation, which is a critical issue especially for reactions involving high thermal duties. The catalyst was properly developed to assure adequate mechanical resistance and the absence of cohesive phenomena during fluidization [21]. At that end, a previously developed catalytic formulation was deposited on high surface area silica gel, and high performances during durability measurements for hundreds of hours were attained [20]. On the other hand, membranes with relevant hydrogen permeance have been developed by supporting ultrathin PdeAg layers on porous alumina tubes [22]; such a choice reduces the risks of membrane
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erosion during the fluidization. The operating temperatures of the reformer are lower than 500 C, to preserve membrane performances: by the way, high hydrogen yields were reached thanks to the membrane shift effect, which increase productivity at low temperatures. The optimal operating conditions of the reformer were selected by combining the reformer with a rated 5 Wel PEM fuel cell m-CHP system. The development of this technology was carried out in the framework of the Fluidcell Project [23] (advanced m-CHP fuel cell system based on a novel bioethanol fluidized bed membrane reformer), and a schematic representation of the final plant is reported in Fig. 17.4.
3. Hydrogen production by catalytic steam reforming of bio-oil Recently, researches regarding the steam reforming of bio-oil have been focused on how improve it to make an efficient candidate technology in the future energy scenario. Many efforts are now being proposed to find new catalysts with high coking resistance, good mechanical strength, and high activity at even lower temperature with easy regeneration. Improved reactor configurations and scale-up procedures together with enhanced process temperature control determine a cost efficiency and flexibility of the steam reforming of the bio-oil derivatives for hydrogen production. The bio-oil represents the liquid fraction deriving by biomass pyrolysis. It is multicomponent mixtures characterized by oxygenated compounds, such as acetic acid, ethanol, phenols, aldehydes, ketones, e.g. [24]. The main components are phenol and acetic acid, representing 38 wt% [25] and 30 wt%, respectively, of the water-soluble fraction of bio-oil [26]. Their noninflammable nature makes them safe hydrogen carriers [25].
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Membrane ATR reformer
PEM Fuel cell
Burner FIGURE 17.4
Layout of the fluidcell plant [18].
The steam reforming of oxygenates in the biooil aqueous fraction can be represented as follows (Eq. 17.1) [27]: CHxOy þ (2-y) H2O / CO2 þ (x/2 þ 2-y) H2 (17.1) The traditional steam reforming process was the object of study by many researchers [28,29]. However, the thermal conversion of acetic acid can lead to the side reactions and formation of intermediates on the catalyst surface [30]. These issues have involved the need for intensification and optimization of the operating conditions to overcome the drawbacks and the energy costs of the process. Basagiannis and Verykios [31] have observed that the steam reforming of acetic acid (AASR) at lower temperatures promotes the side reactions and also the coke formation depends strongly on the reforming temperature. A process intensification based on the chemical looping steam reforming (CLSR) has been promoted to overcome the difficulties observed in the steam reforming process; CLSR uses an
"oxygen transfer material" also known as "oxygen carrier" (OC) that drives the reactions in a cyclic process to supply the heat necessary by the oxidation reactions feeding air to the reactor, by avoiding the use of external burners to heat up the reformer and the costs associated with the use of highly corrosion resistant materials. In this way, the improvement of the heat transfer prevents the presence of great temperature gradients, so increasing the thermodynamic efficiency. Omoniyi et al. [28] have studied the redox cycling ability and process efficiency of chemical looping steam reforming of acetic acid (CLSR-HAc) in a packed bed reactor. CLSR is characterized by the reducing/reforming step (fuel-steam feed) and the oxidation step (air feed). The reducing/reforming step was carried out at temperatures of 600 and 650 C, because those were considered the optimal temperatures to carry out the steam reforming from the model calculation of the pyrolysis oils compounds [32,33].
IV. Selected examples and case history
3. Hydrogen production by catalytic steam reforming of bio-oil
The reactions that occur during the reducing/ reforming step are as follows (Eqs. 17.2 and 17.3): C2H4O2 þ 4 NiO / 2 CO2 þ 2 H2O þ 4 Ni (17.2) C2H4O2 þ 2 H2O / 2 CO2þ 4 H2
(17.3)
The first reaction is the auto-reduction of the catalyst by acetic acid; the second one is the acid acetic steam reforming. Obviously, the reaction system is more complex and other side reactions could occur. As already reported by Basagiannis and Verykios [31], the reactions that could take place in the system are steam reforming of acetic acid, steam reforming of CH4, thermal decomposition of acetic acid, ketonization, water gas shift, methanation, and Boudouard reaction. In fact, intermediate reactions could concern the catalyst reduction (NiO) by CO, H2, the acid acetic decomposition, methanation, and ketonization [33]. Anyway, the efficiency of the fuel conversion and hydrogen yield of CLSR is affected by the feedstock and operating conditions, and the worsening of the catalyst performance is generally due to carbon deposition, sintering, and thermal decomposition of the feedstock. Many authors have studied the steam reforming of acetic acid in conventional reactors, obtaining a quasi-total acid acetic conversion and high hydrogen yields (>97%). Pt-based ZrO2 catalysts have allowed reaching H2 yield very close to equilibrium, determining a total acetic acid conversion [26]. Basagiannis and Verykios [31] have investigated the performance of Ni-based catalysts supported on Al2O3 and La2O3; from the comparison of the two catalytic systems, it was obtained that the acetic acid interacts intensely with the Al2O3. In a recent paper focused on the study of the Rubased MgO/Al2O3 catalyst, these authors have concluded that the H2 formation is favored at high temperatures and low space velocities [34].
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These authors have studied different catalysts such Pt, Pd, Rh, Ru, and Ni, supported on Al2O3, La2O3eAl2O3, and CeO2eAl2O3 [35]; higher activity and H2 selectivity were observed for the Ni- and Ru-based catalysts. The hydrogen-rich gas deriving from the conventional reactors contains other products, such as CO2, CO, and CH4, and because the aim is to obtain a CO-free hydrogen stream, water gas shift (WGS) reactors, devoted to the CO conversion, are placed downstream of the reforming reactors. Based on these considerations, the membrane reactors (MRs), combining the reaction and separation step in the same unit, could represent a good alternative to the conventional reactors for the reaction of steam reforming of acetic acid, so reaching a high degree of process intensification. To the best of the knowledge, in the literature, there are not papers regarding the study of this reaction in MRs. Basile et al. [36] have investigated the reaction of steam reforming of acetic acid in a membrane reactor in the range of temperature 400e450 C in presence of two different catalysts. A Ni-based commercial catalyst has been placed inside the membrane lumen, while the double bed of Ru-based and Ni-based commercial catalyst has been located in the membrane lumen. The experimental results have demonstrated that is possible to produce H2 in considerable way (36%) in MRs at low temperatures (450 C) and at quite low pressures (2.5 bar). The only products obtained were CO2, H2, and CH4, even if a part of the hydrogen is lost as methane. The presence of a double catalytic bed characterized by an optimized load of Ni- and Rubased catalysts could allow overcoming this drawback, so the methane produced on the first catalyst is converted to hydrogen on the second catalyst.
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FIGURE 17.5
Scheme of laboratory plant: (A) pyrolysis (CSBR) and (B) fluidized bed reforming (FBR).
Arregi et al. [37] have developed a two-step process for the H2 production from biomass by combining the pyrolysis stage with the steam reforming. The pyrolysis step is performed in a conical spouted bed reactor (CSBR) connected in series with a fluidized bed reactor for the conduction of the steam reforming reaction. Two reactors operate in continuous regime and with mass and heat transfer rates similar to those of industrial reactors [37]. The scheme of the laboratory pyrolysisreforming apparatus is reported in Fig. 17.5. As it is possible to see from the scheme, the process is characterized by two sequential steps: pyrolysis in the CSBR and reforming of bio-oil and volatile compounds in the FBR. More in detail, the CSBR is provided from a gas preheater and a lateral outlet tube placed at the bottom of the bed for the removal of char particles from the bed. A cyclone is placed downstream from the pyrolysis reactor to retain the fine particles. The pyrolysis vapors formed in this stage are reformed in the FBR loaded with a Ni commercial catalyst supported on a-Al2O3. Different operating conditions were adopted for the conduction of the experimental tests (Table 17.1).
The best operating conditions able to reach a total conversion, with a hydrogen yield of 110 g H2 Kg1 biomass were identified: T ¼ 600 C, 1 space time ¼ 20 gcat$min$gvolatilies, steam/ biomass ¼ 4. The authors have noted that the steam atmosphere in the pyrolysis has not a significant effect on the product distribution, with results similar to those in which nitrogen is used as a fluidizing agent. This approach is a promising alternative to the bio-oil reforming process since that it allows overcoming the operational problems related to the bio-oil handling.
4. Distributed hydrogen production from biogas The increasing energy demand has determined a new interest in alternative sources for energy production. Nowadays, electrical energy is primarily produced through methane or other hydrocarbons combustion: in recent years, hydrogen acquired an increasing interest as alternative fuel, as it is carbon-free and so environmentally friendly. Despite its abundance in
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4. Distributed hydrogen production from biogas
TABLE 17.1
Operating conditions for the reforming step.
Temperature, C
550, 600, 650, 700 (space time [ 20,
Steam/biomass (fed in the CSBR) 1
Space time, (gcat$ min $g )
2, 3, 4, 5 (T
¼ 600 C, space time ¼ 20)
2.1, 4.2, 8.3, 12.5, 16.7, 25
the hydrosphere, hydrogen generation through water electrolysis is an expensive and noncompetitive process; production of hydrogen through reforming processes remains the better alternative. With the aim of reducing greenhouse gas emissions, recent studies report an interesting alternative to conventional hydrocarbon: because of its composition, principally made of CH4 and CO2, biogas is a high-potential raw material for reforming processes, which can be used as an alternative CH4 source [38]. The type of process that is more suitable for biogas conversion into hydrogen depends on several factors: principally, biogas composition, purity required for H2, and volume production of the desired H2. The most common processes are steam reforming (SR), partial oxidation reforming (POR), autothermal reforming (ATR), dry reforming, dry oxidation reforming, and tri-reforming. H2 bio-production can be easily adapted to on-site decentralized production of hydrogen, and this aspect allows avoiding the establishment of a large and costly distribution infrastructure [39]. One interesting approach to distributed hydrogen production for electrical energy generation is the employing of the produced hydrogen in fuel cells. Among all the type of fuel cells, the most suited for this application are PEM and SOFC, which are primarily different because of their dimension. The latter cell has high efficiency (50%e60%) and has no limitation on the size or weight of the cell because it is for fixed applications; moreover, it is a high-temperature cell, and for this reason, it has a higher tolerance to CO presence than other types of cells.
steam/biomass [ 4)
(T ¼ 600 C, steam/biomass ¼ 4)
Low-temperature fuel cells, such as PEM, require an extremely high degree of H2 purification, above 99.999%, because CO presence in more than 10 ppm strongly affects the performance of the system. For these applications, gas separation by pressure swing adsorption or palladium membranes is used to obtain highpurity-grade H2 [40]. For this reason, PEM fuel cells are not suitable for direct use of biogas, and they need a prereforming step for hydrogen production and a purification step of the reforming product, while the direct use of biogas in fuel cells is possible in SOFC [41], and it is known as “internal reforming.” A distributed PEM fuel cell power system is focused on the small scale, and the generated power is in the range of 50e250 kW for decentralized use (such as automotive) or <10 kW for households [42]. Methane reforming with carbon dioxide, using conventional catalytic processes as the ones described before, has two main problems: firstly, the reaction is highly energy requiring; secondly, the necessary high-temperature induces carbon deposition on the catalyst surface. Moreover, the conventional processes allow to obtain a syngas with a H2/CO ratio in the range of 2e3, depending on biogas composition, so with the aim of employing a PEMFC, a large amount of CO has to be converted or separated in subsequent steps. For these reasons, plasma technology is considered to perform the reforming process at lower temperature. Plasma is generally classified into two kinds: thermal plasma, that is equilibrium plasma, and nonthermal plasma that is nonequilibrium plasma. In the latter case, the electrical power
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is low and plasma not only provides energy to the system but also generates radical species that are able to initiate and enhance chemical reactions. The low energy consumption and minimum electrode erosion, together with small size and weight of nonthermal plasma reactors, made this technology attractive for mobile applications [43]. Chun et al. [43] evaluated the use of a nonthermal plasma reactor for biogas conversion into hydrogen with the gliding arc technique: gliding arc occurs when plasma is generated between two or more diverging electrodes placed in a gas flow (Fig. 17.6). A simulated biogas was fed to the reactor, varying the CH4/CO2 ratio from 6:4 to 4:6. The results evidenced that the change in composition affected the H2 and the CO concentration in the product mixture, but not the methane conversion, which was approximately 100% in each case. Moreover, H2 yield reached good values both for the CH4-rich feed gas and for the CO2rich feed gas, around 58% and 51%, respectively.
FIGURE 17.6 Scheme of a gliding arc plasma reactor.
Plasmatron-assisted CH4 reforming was studied by Chun et al. [44] with a single hightemperature plasma flame generated by air and arc discharge. The system worked with a flowrate of 5.1 L/min and an input electric power of 6.4 kW. The maximum H2/CO ratio obtained was 6.5, and this ratio decreases to 2.3 with the CO2 volumetric fraction increasing, but it is still higher than the conventional reforming processes. Rueangjitt et al. [45] evaluated the possibility of upgrading a simulated biogas stream for H2rich syngas production using a multistage AC gliding arc system. The authors studied a system made of a series of several plasma reactors that can provide, at the optimized conditions, a hydrogen-rich syngas with a H2/CO ratio of 6.9, which is extremely promising, with respect to the conventional reforming processes. As discussed, a possible approach to a CO-free mixture to be sent to a PEM fuel cell is syngas cleaning though dense membranes. In that case, the separation unit is downstream of a reforming unit, which can be either a conventional reformer or a plasma-reforming process. An even better alternative consists in MRs that are able to couple the reforming stage and the cleaning stages into one single unit. Considering that to obtain a H2-rich stream a conventional system requires five stages (Fig. 17.7), the employment of MRs can be considered a shortcut. The main issue about the coupled MR-FC configuration is about the separating efficiency of the membrane. Pd membranes are the most suitable for hydrogen separation: they have an excellent permeability and the ability of selfcatalyze the H2 dissociation and recombination reactions [46]. Iulianelli et al. [47] studied the SR of a simulated biogas mixture for generation of clean hydrogen in a membrane reactor, consisting of a H2-selective composite Pd/Al2O3 membrane. The authors evidenced that this system could be extremely selective toward hydrogen, with
IV. Selected examples and case history
5. Distributed hydrogen production from biodiesel
FIGURE 17.7
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Comparison between conventional hydrogen production and MR hydrogen production.
the purity of the obtained gas at 96% if it works at very low temperature (380 C); this condition, of course, is not favorable for the reforming reaction, and for this reason the overall hydrogen recovery is low. At higher temperature (450 C), hydrogen recovery is about 70%, but the characteristics of H2 perm-selectivity decrease, obtaining a purity lower than 70%. Gallucci et al. [48] investigated the production of pure hydrogen through the SR of biogas and SR of methane in a fluidized bed membrane reactor. The MR consisted of a shell-and-tube configuration, and the membrane was made of a thin Pd/Ag layer deposited onto an alumina porous tube. Experiments were carried out at temperature in the range of 430e540 C. The authors found a very low amount of CO in the permeate side, and a very high hydrogen purity, in the range 97.34%e99.88%, even if the methane conversion results were not very high compared to the methane SR. The main result of this work, therefore, is that the hydrogen separation was not affected by changing the feed gas from methane to biogas, so the latter can be considered a valid alternative to conventional feedstock. Anzelmo et al. [49] compared the reforming products obtained in a membrane reactor and a fixed bed reactor. The authors found a hydrogen
recovery of 82% with a permeate hydrogen purity of 100%; this result, together with the fact that the MR configuration compared to the fixed bed reactor produces double the amount of hydrogen, confirms the great applicability of the membrane technologies.
5. Distributed hydrogen production from biodiesel Today, hydrogen is predominantly produced by SR of natural gas in large-scale, central production plants. However, with an increasing share of FCVs in the market, central hydrogen production will suffer from additional costs associated with the distribution of gaseousphase hydrogen by trailer over long distances [50]. In contrast, DHG at fueling stations offers the advantage of using readily available liquid fuels such as diesel and biodiesel with high energy densities and existing infrastructure. DHG is widely seen as a promising alternative in the transition phase toward a fully renewable hydrogen production economy [51]. DHG is applicable but not limited to decentralized hydrogen production at fueling sites. There is an increasing demand for annealing applications, in particular for the steel industry and in
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the production of high-quality flat glass. According to Neumann et al. [52], conventional hydrogen generation processes up to 300 Nm3/ h H2 are being increasingly substituted with advanced SR technologies, in particular using biofuels as feedstocks. SR offers the advantage of high partial pressure of hydrogen in the product gas (70e80 vol.%, dry basis) compared to 40%e50% for ATR and partial oxidation (POX). Considering that compressing liquid fuels is less energy intensive than compressing gaseous feeds, SR of liquid fuels is considered to be the preferred option for stationary hydrogen generation [53]. Of all the biofuels, biodiesel is the most researched, and it is still work in progress despite the level of its development. This enormous interest by researchers, industrialists, and governments worldwide may not be unconnected with the inherent advantages. It is biodegradable, nontoxic, and renewable, has high cetane number, in-built oxygen content, higher combustion without or with low sulfur, aromatic components and other regulated emissions, complete carbon cycle, and availability of raw materials, and fits into existing engines with little or no modification and with a high flash point [54]. Furthermore, biodiesel is a nonpolluting resource with a low sulfur content (typically below 5 ppmw). This renders biodiesel a favorable feedstock for catalytic applications since sulfur is known to be a strong catalyst poison [54], and it also appears to be a promising feedstock for DHG by means of SR [55]. Biodiesel is a monoalkyl ester of fatty acids obtained from vegetable oil or animal fat through esterification or transesterification reaction with alcohol in the presence of catalyst. Specifically, it is a reversible reaction involving triglyceride with methanol (most commonly) or ethanol (less commonly) in the presence of NaOH, KOH, or H2SO4 as catalyst. The reaction is shown in Fig. 17.8. The use of sodium or potassium methoxide (CH3ONa or CH3OK) as catalyst has recently
become more preferred. This is to substantially minimize or avoid the moisture content associated with biodiesel production [54]. To complement the already-dwindling petroleum products, developed countries have responded to the production and use of biofuels, especially with the directive of the European Union (EU) that conventional fuels should have an addition of at least 5.75% biofuels by 2010, with the possibility of increasing it to 20% by 2020 [54]. The foregoing directive, in addition to the compliance with the Kyoto Agreement that member nations should reduce the level of CO2 emission to below 8% of 1990 level by 2012, has sharply increased the production of biodiesel [54]. Efforts by the EU can be seen from the available data from various sources. Biodiesel production in EU member countries was put at 1.93 million tons in 2004, and after 10 years, in 2013, it was put at 10.37 million tons, moving up to 11.58 million tons in 2016, with Germany and France taking the lead [54]. However, this directive has been reviewed more recently. EU member countries are to ensure that energy from renewable sources forms at least 10% of the transportation fuels by the year 2020 [54]. This is to improve energy efficiency and reduce greenhouse gas emission. It has also been reported in several works that biodiesel production will soar in the coming years, and this is evident from the series of diversified global research activities in boosting biodiesel production both in the areas of feedstock and catalysis. On feedstock, research is focused on the use of edible, nonedible, and waste oils [54]. Recent reviews have shown that more than 350 oil-bearing crops are potential feedstocks for biodiesel production [54]. Meanwhile, with regard to catalysis, research is now focused on the use of heterogeneous solid catalysts (acid and base) and enzymes [54]. Despite the diversification of research in biodiesel production, the cost is still not favorable when compared to conventional fuels. It was reported that the cost of biodiesel unit price is 1.5e3.0 times higher than
IV. Selected examples and case history
5. Distributed hydrogen production from biodiesel
FIGURE 17.8
329
Transesterification reaction for biodiesel production.
that of petroleum-derived diesel fuel, depending on the type of feedstock used [54]. In addition, the high production of biodiesel produces large volumes of waste, with glycerol as the major product. It has been reported in various literature that 10%e20% of the total volume of biodiesel produced is made up of glycerol. That is, for every 100 kg of biodiesel produced, 10 kg of glycerol is generated [54]. The growing biodiesel production will lead to an excess supply of glycerol, which has been described as having a low commercial value because of its low quality [54]. Biodiesel reforming can be represented by the following global reaction: C19H36O2 þ 17 H2O / 19 CO þ 35 H2 DH298 K þ 2645 kJ/mol 0 DH298K ¼ 2645 kJ=mol Even though biodiesel is well known as a renewable source of fuel for the future, biodiesel SR has not been investigated extensively. Reforming of biodiesel to generate hydrogen can occur without catalysts at around 1673 K in a fuel processor. Catalyst can be used to activation the reaction at a lower temperature and to achieve better control of reaction kinetics [56]. Due to the long carbon chain of biodiesel, the catalyst systems will be deactivated by rapid
carbon formation. Fuel cell complications were observed during soy-based biodiesel SR, causing a decrease in performance for both PEM fuel cell and processor [56]. In another study, canolabased biodiesel was converted into hydrogenrich gas by SR. The rapid carbon formation problem also occurred [56]. Some authors reported a thermodynamic simulation study of ATR and SR of various liquid hydrocarbon fuels. They found the highest theoretical conversion efficiency in gasoline, but biodiesel was in the same range (1% lower on average), depicting its feasibility for in-line reforming with fuel cells [57]. In a different study, biodiesel reforming has been simulated and tested in a heat-integrated fuel processor. A commercial precious metalbased catalyst was tested in the fuel processor [57]. These authors obtained 99% conversion in the ATR processor with a steam-to-carbon molar ratio of 2.5, added oxygen, pressure of 2.1 bar, and gas hourly space velocities (GHSV) of 30,000 h1. Other researchers performed an experimental study of ATR with platinum (Pt)- and rhodium (Rh)-based catalysts synthesized. Hydrogen was produced at temperatures higher than 510 C with a steam-to-carbon molar ratio of 2 and an oxygen to carbon molar ratio of 0.4.
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Coke formation was observed on the catalyst and reactor vessel walls. So far, no report has shown successful reforming of biodiesel with high efficiency and long-term stability. Further work should be conducted to develop catalysts with higher resistibility against deactivation for biodiesel fuel processor-fuel cell systems.
6. Conclusion and future trends Today there is growing interest in developing new technologies for hydrogen production from renewable sources, such as biomass and water, by high-energy-efficient, cost-competitive, and environmentally friendly means. In this way, several processes are currently being investigated for obtaining hydrogen from biomassbased compounds. Hydrogen can be produced starting from the biomass by biologic and thermochemical processes, as well as by the reforming of biomass-derived oxygenates. Biofuels are particularly suitable for distributed hydrogen production because they allow the employment of a compact plant configuration. In this chapter, hydrogen production from the most important biofuels, such as bioethanol, biogas, bio-oil, and biodiesel, was discussed. Bioethanol SR is carried out in different reactor configurations, such as the fluidized bed operating in autothermal conditions, MRs. A compact steam reformer for pure hydrogen generation from bioethanol has been designed to also assure the conversion of feedstocks different from bioethanol (i.e., biogas). High performances were obtained for bimetallic catalysts and Pd-based membranes. In almost all the configurations the coupling of the reaction and separation stage is realized in the same unit, so minimizing the overall reaction volume, promoting also the WGS reaction because of the selective removal of the desired product.
A high degree of process intensification is also obtained by employing structured carriers, the reforming catalysts having high thermal conductivity able to assure an excellent heat management within the catalytic bed and so reducing thermal gradients. Bioethanol is also used as feedstock for microcogeneration PEM-based system in off-grid applications. A micro-combined heat and power technology has allowed to reduce the primary energy consumption and energy costs with respect to the conventional centralized generation, obtaining overall plant efficiency higher than 90%. Different is the case represented by the hydrogen production from bio-oil that still represents a developing technology. The bio-oil SR reaction is performed in conventional reactors, in presence of Ni and noble metals-based catalysts supported on alumina and lanthana. Today, the attention is focused mainly on the intensification and optimization of the operating conditions to preserve the catalyst stability and avoiding side reactions that would lead to the intermediate products formation. In fact, many researchers have investigated new catalysts formulations having a high coking-resistance, good mechanical strength, easy regeneration, and high activity at lower temperature. Enhanced reactor configurations, as MRs and coupling of the pyrolysis and SR steps in one stage, together with scale-up procedures could determine a greater flexibility of the SR of the bio-oil derivatives for hydrogen production. Biogas can be considered a promising alternative to the conventional hydrocarbons, thanks to the high-potential raw material for reforming processes. It can be used for the decentralized production of hydrogen to feed to PEM and SOFC cells for electrical energy generation; PEMs are not suitable for the direct use of the biogas, a prereforming and purification step of the reforming products, while the SOFC does
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References
not require any treatment of the biogas. Biogas is also recently employed in a new technology based on plasma and nonthermal plasma thanks to the small energy consumption and minimum electrode erosion. Furthermore the small size makes this technology interesting for mobile applications. The SR of liquid fuels as diesel or biodiesel is considered the preferred alternative for stationary hydrogen generation, being less energy intensive than compressing gaseous feeds. Even though biodiesel is well known as a renewable source of fuel for the future, biodiesel SR has not been investigated extensively. Reforming of biodiesel can be performed without catalysts at very high temperature (w1673 K) in a fuel processor and in presence of Pt- or Rh-based catalysts at lower temperature. However, because of the long chain of biodiesel, the coke formation can determine the fast catalyst deactivation. Furthermore, to date, biodiesel reforming does not seem to be characterized by high efficiency. In fact, further work should be focused on the catalysts development with higher resistance against deactivation for biodiesel fuel processor-fuel cell systems.
List of abbreviations and acronyms AASR ATR CLSR CLSR-HAc CO PrOx CSBR DHG FBR FCVs GHSV HE m-CHP MR OC PEM
Steam reforming of acetic acid Autothermal reactor Chemical looping steam reforming Chemical looping steam reforming of acetic acid Preferential oxidation of carbon monoxide Conical spouted bed reactor Distributed hydrogen generation Fluidized bed reactor Fuel cell vehicles Gas hourly space velocity Heat exchanger Micro-combined heat and power system Membrane reactor Oxygen carrier Polymeric electrolyte membrane
POX SOFC SR Wel WGS
Partial oxidation Solid oxide fuel cell Steam reforming Electrical power Water gas shift
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IV. Selected examples and case history
17 Ultracompact biofuels catalytic reforming processes for distributed renewable hydrogen production Vincenzo Palma*, Daniela Barba, Eugenio Meloni, Simona Renda, Concetta Ruocco University of Salerno, Department of Industrial Engineering, Via Giovanni Paolo II, Fisciano (SA), Italy * Corresponding author. e-mail address: [email protected]
1. Introduction Biofuel can be defined as any fuel deriving from biomass and, therefore, is considered a source of renewable energy. Today, the attention is focused on biofuels because they are a cost effective solution and constitute an environmentally clean alternative to petroleum and other fossil fuels, considering moreover the rising petroleum prices and the effects of the global warming by fossil fuels [1]. Anyway, there are apprehensions relatively to the utilization of biofuels because of the economic and environmental costs associated respectively to the refining process and the possible destruction of areas devoted to food production. Biofuels are usually classified as first-, second, and third-generation biofuels [1]. The firstgeneration biofuels are directly correlated to a generally edible biomass. Second-generation biofuels comprise fuels obtained from different
Catalysis, Green Chemistry and Sustainable Energy https://doi.org/10.1016/B978-0-444-64337-7.00017-3
feedstocks, from lignocellulosic to municipal solid wastes. Third-generation biofuels are related to algal biomass. Liquid biofuels are of particular interest because of the wide infrastructure already in place to use them, especially for transportation. The greatest production of liquid biofuel is represented by ethanol obtained by fermenting starch or sugar. The second most common liquid biofuel is biodiesel, which derives mainly from oily plants (soybean or oil palm) and to a lesser extent from other oily sources. Biodiesel is used especially in Europe in diesel engines and usually blended with petroleum diesel fuel in different percentages. The use of algae and cyanobacteria as a source of “third-generation” biodiesel, while representing a promising alternative, is characterized by many economic drawbacks, which limit the develop at an industrial level.
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Other types biofuels include methane and biogas, deriving from the decomposition of biomass in the absence of oxygen, and methanol, butanol, and dimethyl ether, which are in development. Biofuels are particularly suited for distributed hydrogen production because they allow the employment of a compact plant configuration. In general, central, semicentral, and distributed production are expected to play a key role in the evolution and long-term use of hydrogen as an energy carrier. Central production is the more diffuse way to produce hydrogen in large facilities that, to take advantage of the economy of scale, need a very high hydrogen demand. Semicentral productions are usually located in close proximity to the point of use of hydrogen; this configuration allows lower transport costs and infrastructure, even if they are still present. Distributed production means that hydrogen can be produced in small plants where it is needed: this can be applicable mainly to reforming of renewable gaseous or liquid fuels (such as bioethanol, bio-oil, biogas, and biodiesel) and for small-scale water electrolysis. As hydrogen is not currently employed as fuel yet, distributed production may be the most viable approach for introducing hydrogen in the fuels’ scenery, because the initial demand will be low and larger scale production could be not economically advantageous. Moreover, distributed hydrogen production can serve the hydrogen market at lower costs and environmental impact than the conventional production systems. The hydrogen production rate in distributed hydrogen generation (DHG) is about 1200 kg/day, so these systems can be sited near industrial users or fuel cell vehicle (FCV) filling stations. Small-scale hydrogen production has been attempted before, although the high cost due to economies of scale has limited its impact: the contemporaneous production of hydrogen together with power and heat solves the scale problem (Fig. 17.1).
2. Distributed hydrogen production from bioethanol In recent years, the potential use of bioethanol for distributed hydrogen production has attracted attention worldwide. Bioethanol, in fact, provides an efficient solution for H2 generation, especially due to its economic production, which enables bioethanol to compete with standard fossil-based fuels [2,3]. In this regard, different solutions have been proposed for distributed hydrogen production at or near the point of use, aiming at minimizing the safety issues related to H2 storage and delivery. However, suitable technologies for hydrogen production at a small scale need to meet specific requirements in terms of both compactness and rapid response. Therefore, reforming of renewable liquids, allowing for high storage energy density, offers potentials for efficient advances in the framework of distributed generation [4]. A schematic representation of an integrated system for distributed H2 production from bioethanol is presented in Fig. 17.2. The preheated bioethanol mixture is sent to the reforming unit, where external heat is supplied to sustain the endothermic reaction, and the reaction takes place, producing hydrogen and carbon oxides. The generation of highpurity hydrogen can involve different separation units (palladium-based membranes with relevant perm-selectivity toward hydrogen, preferential oxidation, as well as methanation of carbon monoxide). The as-produced stream can be fed to a fuel cell for electricity generation [5]. To assure a high level of integration of the various units and face the space optimization requirements typical of distributed hydrogen production systems, the employment of microreactors has been proposed as a potential solution [6,7]. Microreactors, in fact, are characterized by compact sizes and light weight; moreover, their employment assures reduced capital and operative costs. Their small linear
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2. Distributed hydrogen production from bioethanol
FIGURE 17.1
Central production compared to distributed hydrogen production.
dimensions increase heat and mass transfer rates, with very limited diffusional issues; however, due to the quite small passage size, pressure drops can become significant. Anyway, the very fast system response is particularly attractive in view of obtaining better process and high hydrogen yield. In addition, other strategies have been proposed for distributed hydrogen production from biofuels. In the following sections, some examples will be described. Benito et al. investigated the performance of a 1-kW bioethanol processor for the production of a high-purity hydrogen stream to be fed to a polymeric electrolyte membrane (PEM) fuel cell [8]. The downstream addition of purification
units (water gas shift and CO preferential oxidation) allowed generating a products mixture virtually free of carbon monoxide (CO concentration lower than 50 ppm). The processor operation, performed at 650e700 C and atmospheric pressure, involved zero CO2 emission and a notnegligible methane production that, however, has no interference on the activity of the PEM fuel cell. In the final prototype, the recovery of the energy produced by the exothermic reactions as well as the residual heat of the flue gas at the fuel cell anode resulted in an overall efficiency improvement higher than 25% for a steam-tocarbon ratio of 3 (and globally increased compared, for example, to the efficiency of internal combustion engines).
CO PrOx
WGS
Bioethanol Preheater
Reformer CO ~ 10%
External duty
FIGURE 17.2
CO ~ 1%
CO methanaon
Pd membrane
H2with < 50ppmCO
Fuel cell
Scheme of an integrated system for distributed hydrogen generation from bioethanol [5].
IV. Selected examples and case history
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The development of an innovative and compact steam reformer for pure hydrogen production from bioethanol was described in a work of Giaconia et al. [9]. The system has been designed to also assure the conversion of feedstocks different from bioethanol (i.e., biogas). To guarantee a rational exploitation of solar energy, heat is directly driven to the system by concentrating solar power plants using molten salts as heat transfer fluids; alternatively, a biomass combustor as well as an off-gas combustor can be used to supply heat. A high degree of compactness can be reached through the technology of membrane reactors [10]: the integration of reactor and separation in the same unit allows the production of highpurity hydrogen, reducing the volume or even avoiding further downstream operations. Moreover, due to the selective removal of the desired product from the reactor outlet, it is possible to obtain reaction conversions higher than those achieved under the thermodynamic equilibrium conditions (well-known “shift effect”) [11,12]. Compared to a conventional steam reformer, lower operative temperatures (400e550 C) are selected for this plant, thus assuring a relevant reduction in materials costs. At such temperatures, water gas shift pathway is also favored in the membrane reactor with very low outlet CO contents. A high degree of process intensification is also reached by selecting structured carriers as support for the reforming catalysts. The
choice of materials with high thermal conductivity assures an excellent heat management within the catalytic bed, with reduced thermal gradients and a better exploitation of the heat coming from the fluid medium. These systems also allow negligible pressure drops and concentration polarization phenomena (the high mass transfer rate assured by the particular geometry of the structured catalyst avoids reduced H2 permeation fluxes, which could be observed in the boundary layer near the membrane surface [13]), which is crucial for the good operation of the integrated reactor [14,15]. High performances were also reached through the development of a specific bimetallic catalyst with relevant activity and stability for the desired reaction [16] and the employment of composite palladium-based membranes. A schematic representation of the process, developed in the framework of the CoMETHy Project [17] (Compact Multifuel-Energy to Hydrogen converter) is reported in Fig. 17.3. Viviente et al. [18] have described the employment of bioethanol as feedstock for a microcogeneration PEM-based system in off-grid applications. They demonstrated that a distributed power generation system based on a micro-combined heat and power (m-CHP) technology can allow a reduction of primary energy consumption and energy costs compared to the conventional centralized generation. The optimization of thermal integration, in fact, can assure
FIGURE 17.3 Schematic representation of the CoMETHy concept [9].
IV. Selected examples and case history
3. Hydrogen production by catalytic steam reforming of bio-oil
an enhancement in overall plant efficiency higher than 90%. In addition, the integration of the fuel cell with the membrane reactor as well as the m-CHP system allows the reduction of the expenses related to both heat exchangers and auxiliary elements. Hydrogen production from bioethanol was studied in a fluidized bed membrane reactor, based on the integration of reaction and purification in a single unit. The reactor operates under autothermal conditions: air is co-fed with the ethanolewater mixture at proper ratios, selected to provide the heat supply for the endothermic steam reforming reaction. In addition, the heat duty needed for the plant operation comes from the retentate combustion, with no further fuels requirements [19]. The presence of air in the reaction system also contributes to maintain the catalyst surface clean from eventually deposited carbonaceous species, which could have a deleterious impact in terms of catalyst activity and stability [20]. An unwanted increase in membrane surface area, caused by concentration polarization phenomena, can be avoided thanks to the fluidized bed configuration of the catalytic reformer, which enhances external mass transfer rates from the catalyst toward the surface of the membrane. In addition, fluidized beds are characterized by very low pressure drops and practical isothermal operation, which is a critical issue especially for reactions involving high thermal duties. The catalyst was properly developed to assure adequate mechanical resistance and the absence of cohesive phenomena during fluidization [21]. At that end, a previously developed catalytic formulation was deposited on high surface area silica gel, and high performances during durability measurements for hundreds of hours were attained [20]. On the other hand, membranes with relevant hydrogen permeance have been developed by supporting ultrathin PdeAg layers on porous alumina tubes [22]; such a choice reduces the risks of membrane
321
erosion during the fluidization. The operating temperatures of the reformer are lower than 500 C, to preserve membrane performances: by the way, high hydrogen yields were reached thanks to the membrane shift effect, which increase productivity at low temperatures. The optimal operating conditions of the reformer were selected by combining the reformer with a rated 5 Wel PEM fuel cell m-CHP system. The development of this technology was carried out in the framework of the Fluidcell Project [23] (advanced m-CHP fuel cell system based on a novel bioethanol fluidized bed membrane reformer), and a schematic representation of the final plant is reported in Fig. 17.4.
3. Hydrogen production by catalytic steam reforming of bio-oil Recently, researches regarding the steam reforming of bio-oil have been focused on how improve it to make an efficient candidate technology in the future energy scenario. Many efforts are now being proposed to find new catalysts with high coking resistance, good mechanical strength, and high activity at even lower temperature with easy regeneration. Improved reactor configurations and scale-up procedures together with enhanced process temperature control determine a cost efficiency and flexibility of the steam reforming of the bio-oil derivatives for hydrogen production. The bio-oil represents the liquid fraction deriving by biomass pyrolysis. It is multicomponent mixtures characterized by oxygenated compounds, such as acetic acid, ethanol, phenols, aldehydes, ketones, e.g. [24]. The main components are phenol and acetic acid, representing 38 wt% [25] and 30 wt%, respectively, of the water-soluble fraction of bio-oil [26]. Their noninflammable nature makes them safe hydrogen carriers [25].
IV. Selected examples and case history
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17. Ultracompact biofuels catalytic reforming processes
Membrane ATR reformer
PEM Fuel cell
Burner FIGURE 17.4
Layout of the fluidcell plant [18].
The steam reforming of oxygenates in the biooil aqueous fraction can be represented as follows (Eq. 17.1) [27]: CHxOy þ (2-y) H2O / CO2 þ (x/2 þ 2-y) H2 (17.1) The traditional steam reforming process was the object of study by many researchers [28,29]. However, the thermal conversion of acetic acid can lead to the side reactions and formation of intermediates on the catalyst surface [30]. These issues have involved the need for intensification and optimization of the operating conditions to overcome the drawbacks and the energy costs of the process. Basagiannis and Verykios [31] have observed that the steam reforming of acetic acid (AASR) at lower temperatures promotes the side reactions and also the coke formation depends strongly on the reforming temperature. A process intensification based on the chemical looping steam reforming (CLSR) has been promoted to overcome the difficulties observed in the steam reforming process; CLSR uses an
"oxygen transfer material" also known as "oxygen carrier" (OC) that drives the reactions in a cyclic process to supply the heat necessary by the oxidation reactions feeding air to the reactor, by avoiding the use of external burners to heat up the reformer and the costs associated with the use of highly corrosion resistant materials. In this way, the improvement of the heat transfer prevents the presence of great temperature gradients, so increasing the thermodynamic efficiency. Omoniyi et al. [28] have studied the redox cycling ability and process efficiency of chemical looping steam reforming of acetic acid (CLSR-HAc) in a packed bed reactor. CLSR is characterized by the reducing/reforming step (fuel-steam feed) and the oxidation step (air feed). The reducing/reforming step was carried out at temperatures of 600 and 650 C, because those were considered the optimal temperatures to carry out the steam reforming from the model calculation of the pyrolysis oils compounds [32,33].
IV. Selected examples and case history
3. Hydrogen production by catalytic steam reforming of bio-oil
The reactions that occur during the reducing/ reforming step are as follows (Eqs. 17.2 and 17.3): C2H4O2 þ 4 NiO / 2 CO2 þ 2 H2O þ 4 Ni (17.2) C2H4O2 þ 2 H2O / 2 CO2þ 4 H2
(17.3)
The first reaction is the auto-reduction of the catalyst by acetic acid; the second one is the acid acetic steam reforming. Obviously, the reaction system is more complex and other side reactions could occur. As already reported by Basagiannis and Verykios [31], the reactions that could take place in the system are steam reforming of acetic acid, steam reforming of CH4, thermal decomposition of acetic acid, ketonization, water gas shift, methanation, and Boudouard reaction. In fact, intermediate reactions could concern the catalyst reduction (NiO) by CO, H2, the acid acetic decomposition, methanation, and ketonization [33]. Anyway, the efficiency of the fuel conversion and hydrogen yield of CLSR is affected by the feedstock and operating conditions, and the worsening of the catalyst performance is generally due to carbon deposition, sintering, and thermal decomposition of the feedstock. Many authors have studied the steam reforming of acetic acid in conventional reactors, obtaining a quasi-total acid acetic conversion and high hydrogen yields (>97%). Pt-based ZrO2 catalysts have allowed reaching H2 yield very close to equilibrium, determining a total acetic acid conversion [26]. Basagiannis and Verykios [31] have investigated the performance of Ni-based catalysts supported on Al2O3 and La2O3; from the comparison of the two catalytic systems, it was obtained that the acetic acid interacts intensely with the Al2O3. In a recent paper focused on the study of the Rubased MgO/Al2O3 catalyst, these authors have concluded that the H2 formation is favored at high temperatures and low space velocities [34].
323
These authors have studied different catalysts such Pt, Pd, Rh, Ru, and Ni, supported on Al2O3, La2O3eAl2O3, and CeO2eAl2O3 [35]; higher activity and H2 selectivity were observed for the Ni- and Ru-based catalysts. The hydrogen-rich gas deriving from the conventional reactors contains other products, such as CO2, CO, and CH4, and because the aim is to obtain a CO-free hydrogen stream, water gas shift (WGS) reactors, devoted to the CO conversion, are placed downstream of the reforming reactors. Based on these considerations, the membrane reactors (MRs), combining the reaction and separation step in the same unit, could represent a good alternative to the conventional reactors for the reaction of steam reforming of acetic acid, so reaching a high degree of process intensification. To the best of the knowledge, in the literature, there are not papers regarding the study of this reaction in MRs. Basile et al. [36] have investigated the reaction of steam reforming of acetic acid in a membrane reactor in the range of temperature 400e450 C in presence of two different catalysts. A Ni-based commercial catalyst has been placed inside the membrane lumen, while the double bed of Ru-based and Ni-based commercial catalyst has been located in the membrane lumen. The experimental results have demonstrated that is possible to produce H2 in considerable way (36%) in MRs at low temperatures (450 C) and at quite low pressures (2.5 bar). The only products obtained were CO2, H2, and CH4, even if a part of the hydrogen is lost as methane. The presence of a double catalytic bed characterized by an optimized load of Ni- and Rubased catalysts could allow overcoming this drawback, so the methane produced on the first catalyst is converted to hydrogen on the second catalyst.
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17. Ultracompact biofuels catalytic reforming processes
FIGURE 17.5
Scheme of laboratory plant: (A) pyrolysis (CSBR) and (B) fluidized bed reforming (FBR).
Arregi et al. [37] have developed a two-step process for the H2 production from biomass by combining the pyrolysis stage with the steam reforming. The pyrolysis step is performed in a conical spouted bed reactor (CSBR) connected in series with a fluidized bed reactor for the conduction of the steam reforming reaction. Two reactors operate in continuous regime and with mass and heat transfer rates similar to those of industrial reactors [37]. The scheme of the laboratory pyrolysisreforming apparatus is reported in Fig. 17.5. As it is possible to see from the scheme, the process is characterized by two sequential steps: pyrolysis in the CSBR and reforming of bio-oil and volatile compounds in the FBR. More in detail, the CSBR is provided from a gas preheater and a lateral outlet tube placed at the bottom of the bed for the removal of char particles from the bed. A cyclone is placed downstream from the pyrolysis reactor to retain the fine particles. The pyrolysis vapors formed in this stage are reformed in the FBR loaded with a Ni commercial catalyst supported on a-Al2O3. Different operating conditions were adopted for the conduction of the experimental tests (Table 17.1).
The best operating conditions able to reach a total conversion, with a hydrogen yield of 110 g H2 Kg1 biomass were identified: T ¼ 600 C, 1 space time ¼ 20 gcat$min$gvolatilies, steam/ biomass ¼ 4. The authors have noted that the steam atmosphere in the pyrolysis has not a significant effect on the product distribution, with results similar to those in which nitrogen is used as a fluidizing agent. This approach is a promising alternative to the bio-oil reforming process since that it allows overcoming the operational problems related to the bio-oil handling.
4. Distributed hydrogen production from biogas The increasing energy demand has determined a new interest in alternative sources for energy production. Nowadays, electrical energy is primarily produced through methane or other hydrocarbons combustion: in recent years, hydrogen acquired an increasing interest as alternative fuel, as it is carbon-free and so environmentally friendly. Despite its abundance in
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4. Distributed hydrogen production from biogas
TABLE 17.1
Operating conditions for the reforming step.
Temperature, C
550, 600, 650, 700 (space time [ 20,
Steam/biomass (fed in the CSBR) 1
Space time, (gcat$ min $g )
2, 3, 4, 5 (T
¼ 600 C, space time ¼ 20)
2.1, 4.2, 8.3, 12.5, 16.7, 25
the hydrosphere, hydrogen generation through water electrolysis is an expensive and noncompetitive process; production of hydrogen through reforming processes remains the better alternative. With the aim of reducing greenhouse gas emissions, recent studies report an interesting alternative to conventional hydrocarbon: because of its composition, principally made of CH4 and CO2, biogas is a high-potential raw material for reforming processes, which can be used as an alternative CH4 source [38]. The type of process that is more suitable for biogas conversion into hydrogen depends on several factors: principally, biogas composition, purity required for H2, and volume production of the desired H2. The most common processes are steam reforming (SR), partial oxidation reforming (POR), autothermal reforming (ATR), dry reforming, dry oxidation reforming, and tri-reforming. H2 bio-production can be easily adapted to on-site decentralized production of hydrogen, and this aspect allows avoiding the establishment of a large and costly distribution infrastructure [39]. One interesting approach to distributed hydrogen production for electrical energy generation is the employing of the produced hydrogen in fuel cells. Among all the type of fuel cells, the most suited for this application are PEM and SOFC, which are primarily different because of their dimension. The latter cell has high efficiency (50%e60%) and has no limitation on the size or weight of the cell because it is for fixed applications; moreover, it is a high-temperature cell, and for this reason, it has a higher tolerance to CO presence than other types of cells.
steam/biomass [ 4)
(T ¼ 600 C, steam/biomass ¼ 4)
Low-temperature fuel cells, such as PEM, require an extremely high degree of H2 purification, above 99.999%, because CO presence in more than 10 ppm strongly affects the performance of the system. For these applications, gas separation by pressure swing adsorption or palladium membranes is used to obtain highpurity-grade H2 [40]. For this reason, PEM fuel cells are not suitable for direct use of biogas, and they need a prereforming step for hydrogen production and a purification step of the reforming product, while the direct use of biogas in fuel cells is possible in SOFC [41], and it is known as “internal reforming.” A distributed PEM fuel cell power system is focused on the small scale, and the generated power is in the range of 50e250 kW for decentralized use (such as automotive) or <10 kW for households [42]. Methane reforming with carbon dioxide, using conventional catalytic processes as the ones described before, has two main problems: firstly, the reaction is highly energy requiring; secondly, the necessary high-temperature induces carbon deposition on the catalyst surface. Moreover, the conventional processes allow to obtain a syngas with a H2/CO ratio in the range of 2e3, depending on biogas composition, so with the aim of employing a PEMFC, a large amount of CO has to be converted or separated in subsequent steps. For these reasons, plasma technology is considered to perform the reforming process at lower temperature. Plasma is generally classified into two kinds: thermal plasma, that is equilibrium plasma, and nonthermal plasma that is nonequilibrium plasma. In the latter case, the electrical power
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17. Ultracompact biofuels catalytic reforming processes
is low and plasma not only provides energy to the system but also generates radical species that are able to initiate and enhance chemical reactions. The low energy consumption and minimum electrode erosion, together with small size and weight of nonthermal plasma reactors, made this technology attractive for mobile applications [43]. Chun et al. [43] evaluated the use of a nonthermal plasma reactor for biogas conversion into hydrogen with the gliding arc technique: gliding arc occurs when plasma is generated between two or more diverging electrodes placed in a gas flow (Fig. 17.6). A simulated biogas was fed to the reactor, varying the CH4/CO2 ratio from 6:4 to 4:6. The results evidenced that the change in composition affected the H2 and the CO concentration in the product mixture, but not the methane conversion, which was approximately 100% in each case. Moreover, H2 yield reached good values both for the CH4-rich feed gas and for the CO2rich feed gas, around 58% and 51%, respectively.
FIGURE 17.6 Scheme of a gliding arc plasma reactor.
Plasmatron-assisted CH4 reforming was studied by Chun et al. [44] with a single hightemperature plasma flame generated by air and arc discharge. The system worked with a flowrate of 5.1 L/min and an input electric power of 6.4 kW. The maximum H2/CO ratio obtained was 6.5, and this ratio decreases to 2.3 with the CO2 volumetric fraction increasing, but it is still higher than the conventional reforming processes. Rueangjitt et al. [45] evaluated the possibility of upgrading a simulated biogas stream for H2rich syngas production using a multistage AC gliding arc system. The authors studied a system made of a series of several plasma reactors that can provide, at the optimized conditions, a hydrogen-rich syngas with a H2/CO ratio of 6.9, which is extremely promising, with respect to the conventional reforming processes. As discussed, a possible approach to a CO-free mixture to be sent to a PEM fuel cell is syngas cleaning though dense membranes. In that case, the separation unit is downstream of a reforming unit, which can be either a conventional reformer or a plasma-reforming process. An even better alternative consists in MRs that are able to couple the reforming stage and the cleaning stages into one single unit. Considering that to obtain a H2-rich stream a conventional system requires five stages (Fig. 17.7), the employment of MRs can be considered a shortcut. The main issue about the coupled MR-FC configuration is about the separating efficiency of the membrane. Pd membranes are the most suitable for hydrogen separation: they have an excellent permeability and the ability of selfcatalyze the H2 dissociation and recombination reactions [46]. Iulianelli et al. [47] studied the SR of a simulated biogas mixture for generation of clean hydrogen in a membrane reactor, consisting of a H2-selective composite Pd/Al2O3 membrane. The authors evidenced that this system could be extremely selective toward hydrogen, with
IV. Selected examples and case history
5. Distributed hydrogen production from biodiesel
FIGURE 17.7
327
Comparison between conventional hydrogen production and MR hydrogen production.
the purity of the obtained gas at 96% if it works at very low temperature (380 C); this condition, of course, is not favorable for the reforming reaction, and for this reason the overall hydrogen recovery is low. At higher temperature (450 C), hydrogen recovery is about 70%, but the characteristics of H2 perm-selectivity decrease, obtaining a purity lower than 70%. Gallucci et al. [48] investigated the production of pure hydrogen through the SR of biogas and SR of methane in a fluidized bed membrane reactor. The MR consisted of a shell-and-tube configuration, and the membrane was made of a thin Pd/Ag layer deposited onto an alumina porous tube. Experiments were carried out at temperature in the range of 430e540 C. The authors found a very low amount of CO in the permeate side, and a very high hydrogen purity, in the range 97.34%e99.88%, even if the methane conversion results were not very high compared to the methane SR. The main result of this work, therefore, is that the hydrogen separation was not affected by changing the feed gas from methane to biogas, so the latter can be considered a valid alternative to conventional feedstock. Anzelmo et al. [49] compared the reforming products obtained in a membrane reactor and a fixed bed reactor. The authors found a hydrogen
recovery of 82% with a permeate hydrogen purity of 100%; this result, together with the fact that the MR configuration compared to the fixed bed reactor produces double the amount of hydrogen, confirms the great applicability of the membrane technologies.
5. Distributed hydrogen production from biodiesel Today, hydrogen is predominantly produced by SR of natural gas in large-scale, central production plants. However, with an increasing share of FCVs in the market, central hydrogen production will suffer from additional costs associated with the distribution of gaseousphase hydrogen by trailer over long distances [50]. In contrast, DHG at fueling stations offers the advantage of using readily available liquid fuels such as diesel and biodiesel with high energy densities and existing infrastructure. DHG is widely seen as a promising alternative in the transition phase toward a fully renewable hydrogen production economy [51]. DHG is applicable but not limited to decentralized hydrogen production at fueling sites. There is an increasing demand for annealing applications, in particular for the steel industry and in
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the production of high-quality flat glass. According to Neumann et al. [52], conventional hydrogen generation processes up to 300 Nm3/ h H2 are being increasingly substituted with advanced SR technologies, in particular using biofuels as feedstocks. SR offers the advantage of high partial pressure of hydrogen in the product gas (70e80 vol.%, dry basis) compared to 40%e50% for ATR and partial oxidation (POX). Considering that compressing liquid fuels is less energy intensive than compressing gaseous feeds, SR of liquid fuels is considered to be the preferred option for stationary hydrogen generation [53]. Of all the biofuels, biodiesel is the most researched, and it is still work in progress despite the level of its development. This enormous interest by researchers, industrialists, and governments worldwide may not be unconnected with the inherent advantages. It is biodegradable, nontoxic, and renewable, has high cetane number, in-built oxygen content, higher combustion without or with low sulfur, aromatic components and other regulated emissions, complete carbon cycle, and availability of raw materials, and fits into existing engines with little or no modification and with a high flash point [54]. Furthermore, biodiesel is a nonpolluting resource with a low sulfur content (typically below 5 ppmw). This renders biodiesel a favorable feedstock for catalytic applications since sulfur is known to be a strong catalyst poison [54], and it also appears to be a promising feedstock for DHG by means of SR [55]. Biodiesel is a monoalkyl ester of fatty acids obtained from vegetable oil or animal fat through esterification or transesterification reaction with alcohol in the presence of catalyst. Specifically, it is a reversible reaction involving triglyceride with methanol (most commonly) or ethanol (less commonly) in the presence of NaOH, KOH, or H2SO4 as catalyst. The reaction is shown in Fig. 17.8. The use of sodium or potassium methoxide (CH3ONa or CH3OK) as catalyst has recently
become more preferred. This is to substantially minimize or avoid the moisture content associated with biodiesel production [54]. To complement the already-dwindling petroleum products, developed countries have responded to the production and use of biofuels, especially with the directive of the European Union (EU) that conventional fuels should have an addition of at least 5.75% biofuels by 2010, with the possibility of increasing it to 20% by 2020 [54]. The foregoing directive, in addition to the compliance with the Kyoto Agreement that member nations should reduce the level of CO2 emission to below 8% of 1990 level by 2012, has sharply increased the production of biodiesel [54]. Efforts by the EU can be seen from the available data from various sources. Biodiesel production in EU member countries was put at 1.93 million tons in 2004, and after 10 years, in 2013, it was put at 10.37 million tons, moving up to 11.58 million tons in 2016, with Germany and France taking the lead [54]. However, this directive has been reviewed more recently. EU member countries are to ensure that energy from renewable sources forms at least 10% of the transportation fuels by the year 2020 [54]. This is to improve energy efficiency and reduce greenhouse gas emission. It has also been reported in several works that biodiesel production will soar in the coming years, and this is evident from the series of diversified global research activities in boosting biodiesel production both in the areas of feedstock and catalysis. On feedstock, research is focused on the use of edible, nonedible, and waste oils [54]. Recent reviews have shown that more than 350 oil-bearing crops are potential feedstocks for biodiesel production [54]. Meanwhile, with regard to catalysis, research is now focused on the use of heterogeneous solid catalysts (acid and base) and enzymes [54]. Despite the diversification of research in biodiesel production, the cost is still not favorable when compared to conventional fuels. It was reported that the cost of biodiesel unit price is 1.5e3.0 times higher than
IV. Selected examples and case history
5. Distributed hydrogen production from biodiesel
FIGURE 17.8
329
Transesterification reaction for biodiesel production.
that of petroleum-derived diesel fuel, depending on the type of feedstock used [54]. In addition, the high production of biodiesel produces large volumes of waste, with glycerol as the major product. It has been reported in various literature that 10%e20% of the total volume of biodiesel produced is made up of glycerol. That is, for every 100 kg of biodiesel produced, 10 kg of glycerol is generated [54]. The growing biodiesel production will lead to an excess supply of glycerol, which has been described as having a low commercial value because of its low quality [54]. Biodiesel reforming can be represented by the following global reaction: C19H36O2 þ 17 H2O / 19 CO þ 35 H2 DH298 K þ 2645 kJ/mol 0 DH298K ¼ 2645 kJ=mol Even though biodiesel is well known as a renewable source of fuel for the future, biodiesel SR has not been investigated extensively. Reforming of biodiesel to generate hydrogen can occur without catalysts at around 1673 K in a fuel processor. Catalyst can be used to activation the reaction at a lower temperature and to achieve better control of reaction kinetics [56]. Due to the long carbon chain of biodiesel, the catalyst systems will be deactivated by rapid
carbon formation. Fuel cell complications were observed during soy-based biodiesel SR, causing a decrease in performance for both PEM fuel cell and processor [56]. In another study, canolabased biodiesel was converted into hydrogenrich gas by SR. The rapid carbon formation problem also occurred [56]. Some authors reported a thermodynamic simulation study of ATR and SR of various liquid hydrocarbon fuels. They found the highest theoretical conversion efficiency in gasoline, but biodiesel was in the same range (1% lower on average), depicting its feasibility for in-line reforming with fuel cells [57]. In a different study, biodiesel reforming has been simulated and tested in a heat-integrated fuel processor. A commercial precious metalbased catalyst was tested in the fuel processor [57]. These authors obtained 99% conversion in the ATR processor with a steam-to-carbon molar ratio of 2.5, added oxygen, pressure of 2.1 bar, and gas hourly space velocities (GHSV) of 30,000 h1. Other researchers performed an experimental study of ATR with platinum (Pt)- and rhodium (Rh)-based catalysts synthesized. Hydrogen was produced at temperatures higher than 510 C with a steam-to-carbon molar ratio of 2 and an oxygen to carbon molar ratio of 0.4.
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Coke formation was observed on the catalyst and reactor vessel walls. So far, no report has shown successful reforming of biodiesel with high efficiency and long-term stability. Further work should be conducted to develop catalysts with higher resistibility against deactivation for biodiesel fuel processor-fuel cell systems.
6. Conclusion and future trends Today there is growing interest in developing new technologies for hydrogen production from renewable sources, such as biomass and water, by high-energy-efficient, cost-competitive, and environmentally friendly means. In this way, several processes are currently being investigated for obtaining hydrogen from biomassbased compounds. Hydrogen can be produced starting from the biomass by biologic and thermochemical processes, as well as by the reforming of biomass-derived oxygenates. Biofuels are particularly suitable for distributed hydrogen production because they allow the employment of a compact plant configuration. In this chapter, hydrogen production from the most important biofuels, such as bioethanol, biogas, bio-oil, and biodiesel, was discussed. Bioethanol SR is carried out in different reactor configurations, such as the fluidized bed operating in autothermal conditions, MRs. A compact steam reformer for pure hydrogen generation from bioethanol has been designed to also assure the conversion of feedstocks different from bioethanol (i.e., biogas). High performances were obtained for bimetallic catalysts and Pd-based membranes. In almost all the configurations the coupling of the reaction and separation stage is realized in the same unit, so minimizing the overall reaction volume, promoting also the WGS reaction because of the selective removal of the desired product.
A high degree of process intensification is also obtained by employing structured carriers, the reforming catalysts having high thermal conductivity able to assure an excellent heat management within the catalytic bed and so reducing thermal gradients. Bioethanol is also used as feedstock for microcogeneration PEM-based system in off-grid applications. A micro-combined heat and power technology has allowed to reduce the primary energy consumption and energy costs with respect to the conventional centralized generation, obtaining overall plant efficiency higher than 90%. Different is the case represented by the hydrogen production from bio-oil that still represents a developing technology. The bio-oil SR reaction is performed in conventional reactors, in presence of Ni and noble metals-based catalysts supported on alumina and lanthana. Today, the attention is focused mainly on the intensification and optimization of the operating conditions to preserve the catalyst stability and avoiding side reactions that would lead to the intermediate products formation. In fact, many researchers have investigated new catalysts formulations having a high coking-resistance, good mechanical strength, easy regeneration, and high activity at lower temperature. Enhanced reactor configurations, as MRs and coupling of the pyrolysis and SR steps in one stage, together with scale-up procedures could determine a greater flexibility of the SR of the bio-oil derivatives for hydrogen production. Biogas can be considered a promising alternative to the conventional hydrocarbons, thanks to the high-potential raw material for reforming processes. It can be used for the decentralized production of hydrogen to feed to PEM and SOFC cells for electrical energy generation; PEMs are not suitable for the direct use of the biogas, a prereforming and purification step of the reforming products, while the SOFC does
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References
not require any treatment of the biogas. Biogas is also recently employed in a new technology based on plasma and nonthermal plasma thanks to the small energy consumption and minimum electrode erosion. Furthermore the small size makes this technology interesting for mobile applications. The SR of liquid fuels as diesel or biodiesel is considered the preferred alternative for stationary hydrogen generation, being less energy intensive than compressing gaseous feeds. Even though biodiesel is well known as a renewable source of fuel for the future, biodiesel SR has not been investigated extensively. Reforming of biodiesel can be performed without catalysts at very high temperature (w1673 K) in a fuel processor and in presence of Pt- or Rh-based catalysts at lower temperature. However, because of the long chain of biodiesel, the coke formation can determine the fast catalyst deactivation. Furthermore, to date, biodiesel reforming does not seem to be characterized by high efficiency. In fact, further work should be focused on the catalysts development with higher resistance against deactivation for biodiesel fuel processor-fuel cell systems.
List of abbreviations and acronyms AASR ATR CLSR CLSR-HAc CO PrOx CSBR DHG FBR FCVs GHSV HE m-CHP MR OC PEM
Steam reforming of acetic acid Autothermal reactor Chemical looping steam reforming Chemical looping steam reforming of acetic acid Preferential oxidation of carbon monoxide Conical spouted bed reactor Distributed hydrogen generation Fluidized bed reactor Fuel cell vehicles Gas hourly space velocity Heat exchanger Micro-combined heat and power system Membrane reactor Oxygen carrier Polymeric electrolyte membrane
POX SOFC SR Wel WGS
Partial oxidation Solid oxide fuel cell Steam reforming Electrical power Water gas shift
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