• Energy Research

Abstract MCM-22 zeolite samples, having different Si/Al ratios, have been studied for the fast-pyrolysis of acid-washed wheat straw at two catalytic pyrolysis temperatures aimed to the production of partially upgraded bio-oil. The best combination of bio-oil deoxygenation activity and energy yield is obtained when the catalytic bed was operated at 450 °C using the MCM-22 sample with the lowest Al content (Si/Al = 40). Interestingly, the increase in the reaction temperature results in a lower amount of coke deposited over the zeolite. On the other hand, reducing the zeolite Si/Al ratio had a negative effect as a higher concentration of acid sites promotes non-desired reactions: severe cracking of the bio-oil vapours, leading to the enhanced production of gaseous hydrocarbons, and coke formation. Coke produced over MCM-22 zeolite exhibits high oxygen content, whereas the bio-oil fraction presents a high concentration of oxygenated aromatics. These results denote the limited aromatization activity of MCM-22 zeolite for producing aromatic hydrocarbons, in particular when compared with ZSM-5, being of interest for the selective production of phenolic compounds by biomass catalytic pyrolysis.

Abstract New processes under development for producing hydrogen have been assessed using a life cycle methodology and compared to conventional ones. The aim of this paper is to determine the main obstacles to be beaten or the critical aspects to be addressed to ensure the feasibility of these processes. Water photosplitting, solar two-step thermochemical cycles and automaintained methane decomposition with different lay-outs were studied. They have been compared to methane steam reforming with CCS and electrolysis with different electricity sources. The results show the good behaviour of the automaintained methane decomposition. This process is one of the best options when the greenhouse effect emissions are evaluated. Nevertheless, the consumption of a great amount of a non-renewable resource, i.e., natural gas, as reagent can be negative. The two-step thermochemical cycles based on NiFe 2 O 4 is also an interesting option, but its behaviour depends largely on the infrastructure materials employed on the installations. The most promising option is photosplitting with CdS as catalysts. This process shows the best performance.

Abstract The potential of co-producing two different biofuels from a lignocellulosic substrate (wheat straw), according to a biorefinery concept, has been investigated. For such a purpose, simultaneous saccharification and fermentation (SSF) from the hemicellulosic and cellulosic fractions was performed for maximizing bioethanol production. The non-washed water-insoluble solid (WIS) fraction from the pretreated wheat straw totally inhibited the production of ethanol by Kluyveromyces marxianus independently of the inoculum size. In contrast, when using washed-WIS, higher ethanol productivities at 24 h of SSF were attained when increasing the inoculum size from 1 g/L to 3 g/L. The residual lignin from the bioethanol process was transformed by fast-pyrolysis into bio-oil that can be further converted into other biofuels or biochemicals. Thermal fast-pyrolysis of the residual lignin fraction produced 31.9 wt% of bio-oil ∗ (water free basis) mostly composed by oxygenated aromatics coming from the lignin monomers (guaiacol-, syringol- and phenol-derived compounds). On the other hand, catalytic fast-pyrolysis of the residual lignin fraction over HZSM-5 zeolite was used as preferentially promoted decarbonylation and cracking of the primary vapours. Coupling both processes significantly enhanced the production of liquid products from lignocellulose, improving the efficiency in the use of the raw material. In this way, compared to a simple process of bioethanol production, this approach allowed to increase the mass and the chemical energy yields 1.9 and 1.7-fold, respectively.

Abstract The Na–Mn thermochemical cycle is a three step process that has recently attracted renewed attention due to its potential for efficient hydrogen production. In this study, the two low temperature stages have been investigated in order to establish the factors determining the efficiency of both hydrogen production and recyclability of the different solid phases involved. The obtained result reveal that the influence of MnO particle size distribution is crucial for the solid–liquid reaction with NaOH and, therefore, for hydrogen production. Lower particle size and relatively high crystallinity causes a two-fold increment of the conversion, with respect to commercial MnO with very large particles. On the other hand, the influence of reaction conditions on the hydrolysis step has been analyzed in this study. Na extraction from the sodium manganese oxide is favored by performing the process at temperatures around 100 °C, in excess of water; during relatively longer periods and in inert gas. Nevertheless, it has been observed that the structure of the mixed oxide formed during the hydrogen production stage is the most relevant factor determining the efficiency of the Na–Mn oxide hydrolysis. This work reveals that α -NaMnO 2 presents the best ion exchange properties for the hydrolysis reaction, leading to more than 80% of Na recovery.

Abstract Used lubricating oils (ULOs) represent a serious problem for environment and human health due to the presence of highly harmful contaminants, being mandatory an adequate management based on efficient collection systems and treatment processes. Within this work, the environmental and energy performance of a re-refining process for ULOs upgrading is evaluated. The proposed regeneration process is based on the extraction of organic contaminants with liquid propane followed by a cascade of three consecutive distillation stages (two under atmospheric conditions and an additional one under vacuum). This process operates at plan scale in Spain recovering base oil for reuse. All the operations were simulated using Aspen Plus 8.6 and environmental issues and performance was determined by LCA, considering global warming potential, cumulative energy demand, acidification and toxicity as impacts categories. Results show that the whole upgrading process generates up to 363 kg-eq CO 2 /tonne base oil (mainly associated with distillations heating requirements) and it consumes 6144 MJ/tonne base oil. Vacuum distillation is the most important contributor to acidification and toxicity, due to heating and electricity requirements of the column. These parameters were compared for upgraded base oil and refinery lubricant oil, and results suggested that great environmental impacts can be reduced by recycling oil. Finally, different LCA scenarios were considered by partitioning impacts among base oil and plant by-products, using mass flow and economic criteria. Regardless the impacts allocation method, results clearly indicate that manufacturing base oil by ULOs recycling is a more environmental friendly option than the conventional refinery process.

Abstract Methane decomposition to yield hydrogen and carbon (CH 4 ⇆ 2H 2 + C) is one of the cleanest alternatives, free of CO 2 emissions, for producing hydrogen from fossil fuels. This reaction can be catalyzed by metals, although they suffer a fast deactivation process, or by carbonaceous materials, which present the advantage of producing the catalyst from the carbon obtained in the reaction. In this work, the environmental performance of methane decomposition catalyzed by carbonaceous catalysts has been evaluated through Life Cycle Assessment tools, comparing it to other decomposition processes and steam methane reforming coupled to carbon capture systems. The results obtained showed that the decomposition using the autogenerated carbonaceous as catalyst is the best option when reaction conversions higher than 65% are attained. These were confirmed by 2015 and 2030 forecastings. Moreover, its environmental performance is highly increased when the produced carbon is used in other commercial applications. Thus, for a methane conversion of 70%, the application of 50% of the produced carbon would lead to a virtually zero-emissions process.

Abstract Lamellar and pillared ZSM-5 materials modified with Mg and Zn oxides were synthesized and tested for in-situ catalytic upgrading of eucalyptus woodchips fast-pyrolysis vapors. The introduction of silica pillars into the lamellar ZSM-5 support led to a higher BET area, but also reduced the overall catalyst acidity. The incorporation of MgO and ZnO occurred with a high dispersion over the zeolitic supports, causing also a significant reduction in the value of their textural properties due to a partial blockage of the zeolite pores. Likewise, the acid features of the zeolitic supports underwent sharp changes by the addition of both MgO and ZnO with a strong decrease in the concentration of the Bronsted and Lewis acid sites present in the parent zeolite, as detected by pyridine adsorption followed FTIR spectroscopy. However, additional Lewis acid sites were created associated to the metal oxides deposited onto the zeolitic supports. Pyrolysis tests were accomplished using a lab-scale downdraft fixed-bed reactor working at atmospheric pressure and a temperature of 500 °C. The use of zeolitic catalysts increased the gas yield, mostly due to the formation of CO and CO 2 , to the detriment of bio-oil production. However, the so obtained bio-oils presented higher quality in terms of H/C and O/C ratios, and larger heating values. The incorporation of MgO and ZnO allowed tailoring the zeolite activity to avoid an excessive cracking of the bio-oil, which in turn resulted in a higher yield of the organic compounds present in the bio-oil, and decreasing the formation of undesired polyaromatic hydrocarbons and coke.

Abstract The effect of both indigenous (mineral components) and external (HZSM-5 zeolite) catalysts on bio-oil production by biomass fast-pyrolysis has being isolated and compared for two herbaceous and two woody biomass samples. Thereby, a variety of lignocellulosic biomasses (in both raw and de-ashed forms) have been subjected to fast-pyrolysis tests. Mineral components present in the raw biomasses were removed by an acid-washing treatment. The results obtained showed that both types of catalysts decreased the bio-oil* yield (water-free basis). However, whereas the indigenous catalysts almost did not affect the bio-oil* oxygen content, this parameter was significantly reduced when using the HZSM-5 zeolite. This finding denotes that mineral components are not really effective for bio-oil deoxygenation since they mainly promote the formation of additional char, which retains about 40% of the chemical energy contained in the raw biomass. In contrast, the external catalyst does favour oxygen removal from the bio-oil. Likewise, the deoxygenation route was strongly dependent on the type of catalyst. In the non-catalytic process dehydration was predominant, the indigenous catalysts favoured decarboxylation, whereas for the external HZSM-5 catalyst decarbonylation became the major deoxygenation pathway. Regarding the bio-oil* composition, both indigenous and external catalysts promoted the conversion of sugars and the formation of carboxylic acids, aldehydes and oxygenated aromatics. However, aromatic hydrocarbons were only produced over the external HZSM-5 catalyst, with a high proportion of alkyl-substituted benzenes and naphthalenes.

Nowadays, urban bio-wastes (food, park and garden residues) are mainly processed into compost and biogas for energy recovery, or even directly landfilled without exploiting their potential as feedstock for valuable products. As an alternative, this work reports a comprehensive experimental study for their potential valorization via catalytic co-pyrolysis. For that, different mixtures of fruit wastes (FW), a representative component of food refuse, and garden pruning residues (GP) were co-pyrolyzed in presence of a nano-ZSM-5 zeolite (and in absence of catalyst for comparison purposes) in an ex-situ fixed-bed reactor to evaluate the occurrence of different interactions that may affect the yield and properties of the pyrolysis bio-oil* (water-free basis). All pyrolysis fractions were characterized and the composition of bio-oil* was exhaustively identified and quantified, going further than the previous works devoted to the co-pyrolysis of feedstock comprising lignocellulose and fruit wastes usually performed at microgram scale (using micropyrolysis), with intrinsic limitations in terms of the product yields and properties determination. Thus, we found that the incorporation of higher FW amounts in the feed mixture led to lower oxygen contents in the bio-oil*, improving its quality and its potential use as bio-fuel. Moreover, the bio-oil* composition was notably narrowed to marketable chemicals, both phenolic derivatives (cresols and phenol) and especially monoaromatics (mainly xylenes and trimethylbenzenes).