• Energy Research

Olive tree pruning is an abundant and renewable lignocellulosic residue, which is generally burned in the fields, causing economic costs and environmental problems. This lignocellulosic residue can be considered a suitable raw material for the production of a wide range of byproducts in a biorefinery context due to its high content of potentially fermentable carbohydrates. To take advantage of its sugar content, pretreatment is necessary to enhance the accessibility of the enzymes to the cellulosic fraction. The aim of this work is to obtain sugars contained in olive tree pruning as a substrate for the production of bioethanol by fermentation. Specifically, the production of fermentable sugars by sequential pretreatment with sulfuric acid and sodium hydroxide is studied. A two-factor rotatable composite central design temperature and catalyst concentration (H2SO4 and NaOH) has been generated, and response surface methodology has been used to discuss and optimize the responses. This work shows that under optimal pretreatment conditions (130 °C, 1.90% w/v H2SO4 and 130 °C, 1.49% w/v NaOH) of 1 kg of olive tree pruning, a solution rich in sugars (102 g of glucose and 61 g of xylose) and a solid residue generating 99 g of glucose by enzymatic hydrolysis is obtained. Moreover, applying the combined severity to the acid pretreatment, it has been determined that 20% of the olive tree pruning is fast solubilization, and it was also found that the apparent activation energy of the acid hydrolysis reaction is 85.07 kJ/mol.

In Andalusia, residual biomass produced in the olive sector results from the large amount of olive groves and olive oil manufacturers that generate byproducts with a potentially high energy content, suitable for thermal and electrical energy production. The main residue, olive stone, is an important solid biofuel and is widely generated and consumed. Consequently, olive stone quality parameters must be studied in order to achieve an optimum energetic efficiency. Therefore, the main objective of this study is to describe olive stone energetic properties and to evaluate variability of these parameters before consumption. For this purpose, mean values, normal distributions, intervals and deviations of these parameters have been obtained and studied. Concerning to statistical results, climate and geographical variability of quality parameters has been described. Furthermore, variations between olive stone physicochemical parameters supplied by both olive oil factories and distribution companies have been calculated. Finally, a correlation between the ultimate analysis and higher heating values (HHV) of olive stone has been determined. Results obtained show that olive stone pretreatments developed by distribution companies have a significant effect on quality parameters such as moisture content and low heating value. Moreover, olive stone properties dependence on factors such as rainfall or soil type has not been confirmed. Lastly, the calculated correlation based on ultimate analysis (i.e. HHV(MJ/ kg) = 0.401C - 0.164H + 0.493N + 2.381S + 0.791) has been developed and validated with olive stone samples with HHV range from 20 to 21 MJ/kg (dry weight). Correlation has a mean absolute error (MAE) of 0.43% and a mean bias error (MBE) of -0.12% which indicate that it can be successfully used as a more economical and faster tool to accurately estimate olive stone HHV. The HHV prediction accuracies of 14 other correlations introduced by other researchers are also compared in this study.

Abstract In Andalusia (Southern Spanish region), olive industry presents a high potential of solid biofuel production because of residues generated from olive groves and those by olive oil industries. In this region, 25% of residual biomass is produced by olive sector and olive stone residues are among the most important since its production is over 450,000 tons/year. The objective of this research is to provide unambiguous and clear classification principles for olive stone residues based on their quality parameters, to serve as a tool to enable efficient trading of this biofuel and to achieve good understanding between seller and customer as well as to facilitate communication with equipment manufacturers. For this purpose, a total of 176 olive stone samples from 71 different places have been collected and analyzed in this research. Data obtained have been used to develop two quality systems. On the one hand, a classification of olive stone quality parameters has been developed and data are described in a similar way to standardized European quality label for wood pellets. On the other hand, a procedure to calculate a quality index has been designed. Both quality systems have obtained similar results. However, the first one has shown to be more strict. In conclusion, both of them could be used in order to provide a quality classification for olive stone residues.

Olive tree pruning is a widely available and low cost lignocellulosic residue generated every year, being a potential source for bioproducts and renewable fuels production. In this context, this work has as main objective to propose an efficient scheme for the production of second generation ethanol from olive tree pruning, focusing on the evaluation of different detoxification strategies (activated carbon, ammonium hydroxide and overliming) of the liquid fraction obtained after pretreatment, as well as different configurations of enzymatic hydrolysis and fermentation (including hydrolysis and sequential or simultaneous fermentation, pre-saccharification or co-fermentation) of the pretreated olive tree pruning solid. The best results of each fraction were tested in slurry, at varying initial solid loadings and aeration levels. The use of NH4OH 5N as a detoxification method and the pre-saccharification and co-fermentation configuration without aeration are proposed, resulting in 13.86 g ethanol/100 g raw material after 46 hours of process.

Biomass generated from pruning of olive trees is an abundant lignocellulosic residue that has been proposed for bioethanol production by means of a process including pretreatment, enzymatic hydrolysis, and fermentation. The pretreatment is critical step in the process, which allows the fractionation of the biomass. Lignin is the main byproduct of the process, and then its valorization is a key factor for the economical viability of the bioethanol production from olive tree pruning. In this work, two types of pretreatments have been compared steam explosion and organosolv in terms of the produced lignin suitability for further applications in the so-called new biorefinery concept. Starting from this fact, special emphasis was given to determine the physico-chemical properties of lignins produced by the two processes in order to determine their suitability for the development of added value products (e.g. materials, building blocks, additives). Overall, the lignin obtained from organosolv pretreatment had better properties for its further valorization as it presents more suitable qualities for higher-added value applications like fine phenolic chemicals production.

The production of fermentable sugars by biochemical conversion of pretreated sunflower stalks is studied in this work. Liquid hot water pre-treatment, in a temperature range from 180 to 230ºC was used. After pre-treatment, solid residue and liquid fractions are separated by filtration. Pretreated solids are further submitted to enzymatic hydrolysis for glucose release using a commercial enzyme preparation. Enzymatic hydrolysis yields show that 90% glucose conversion can be obtained after 72 h enzyme action on pretreated sunflower stalks at 220ºC. Nevertheless, under such pre-treatment conditions most of the hemicellulosic sugars are lost due to solubilisation and degradation reactions. Taking into account all sugars that can be obtained from this agricultural residue, the best results in terms of potential ethanol production is found when pretreating sunflower stalks at 190ºC. The characterization of hydrolysate composition is also discussed as inhibitor compounds are also found in liquid fractions issued from pre-treatment.

As a first step for ethanol production from alternative raw materials, rapeseed straw was studied for fermentable sugar production. Liquid hot water was used as a pre-treatment method and the influence of the main pre-treatment variables was assessed. Experimental design and response surface methodology were applied using pre-treatment temperature and process time as factors. The pretreated solids were further submitted to enzymatic hydrolysis and the corresponding yields were used as pre-treatment performance evaluation. Liquid fractions obtained from pre-treatment were also characterized in terms of sugars and no-sugar composition. A mathematical model describing pre-treatment effects is proposed. Results show that enzymatic hydrolysis yields near to 100% based on pretreated materials can be achieved at 210-220 degrees C for 30-50 min, equivalent to near 70% of glucose present in the raw material. According to the mathematical model, a softer pre-treatment at 193 degrees C for 27 min results in 65% of glucose and 39% of xylose available for fermentation.

The ethanolic fermentation of liquid fractions (hydrolysates) issued from dilute acid pre-treatment of olive tree biomass by Pichia stipitis is reported for the first time. On the one side, P. stipitis has been reported as the most promising naturally occurring C5 fermenting microorganism; on the other side, olive tree biomass is a renewable, low cost, and lacking of alternatives agricultural residue especially abundant in Mediterranean countries. The study was performed in two steps. First, the fermentation performance of P. stipitis was evaluated on a fermentation medium also containing the main inhibitors found in these hydrolysates (acetic acid, formic acid, and furfural), as well as glucose and xylose as carbon sources. The effect of inhibitors, individually or in a mixture, on kinetic and yield parameters was calculated. In a second step, hydrolysates obtained from 1% (w/w) sulfuric acid pre-treatment of olive tree biomass at 190ºC for 10 min were used as a real fermentation medium with the same microorganism. Due to inhibition, effective fermentation required dilution of the hydrolysate and either overliming or activated charcoal treatment. Results show that ethanol yields obtained from hydrolysates, ranging from 0.35 to 0.42 g/g, are similar to those from synthetic medium, although the process proceeds at lower rates. Inhibiting compounds affect the fermentation performance in a synergistic way. Furfural is rapidly assimilated by the yeast; acetic acid and formic acid concentrations decrease slowly during the process. Activated charcoal or overliming detoxification improve the fermentability of diluted hydrolysates.