The expected growth of the world’s population and the consequent increase in the global energy demand, together with the volatility of the energy market, results in specific uncertainties in the future world economy, which are reflected in the range of oil price projections both in the short and long term. Cuba is an island that lacks sufficient proven fossil energy resources to be able to have an advance in long-term sustainable economic and social development. However, the largest source of domestic energy production comes from imported oil. Hence the importance of the promotion and development of renewable sources of energy. In this sense, sugar cane bagasse (SCB) and trash (SCT) are the main residues derived from the production of sugar from cane which accounted for 3.3% of the total electricity production in the country, albeit the trash not being valorized to a significant extent.
Fast pyrolysis is an attractive alternative technology to process sugar cane residues, whereby solid biomass is converted mainly to bio-oil. The bio-oil has the potential for use as a fuel or as a feedstock for the extraction and/or synthesis of more valuable chemical compounds. However, the high heterogeneity and poor fuel quality of crude bio-oil (i.e. high oxygen and water fraction, high acidity, and high viscosity) impose the need for upgrading processes. Nowadays, research efforts have been conducted in pretreatment to address problems associated with the presence of naturally occurring alkali and alkaline earth metals (AAEMs) in biomass. In terms of specific elements, Si, Na, K, Mg and Ca are generally the major inorganic components in sugar cane. These elements have been demonstrated to act as catalysts during fast pyrolysis. Also, they possess large potential to reduce the yield and the bio-oil’s stability, and to alter the resulting bio-oil composition in a negative way. Most notably, AAEMs appear to suppress and/or suppress the pyrolytic production of levoglucosan out of cellulose and favor the production of lighter oxygenates instead.
This thesis focuses on the pretreatment of sugar cane residues prior to fast pyrolysis (500 °C) to produce bio-oils to be considered as a fuel or as a chemical platform. First study is centered on the effect of citric acid (CA) pretreatment for removing AAEMs from SCB and SCT and to compare its effectiveness to that of other well-known leaching agents such as demineralized water, HCl and H2SO4. Both raw and pretreated materials structure, thermal behavior and chemical composition are analyzed based on compositional analyses, Fourier transform infrared spectroscopy (FTIR-ATR) and thermogravimetric analysis (TGA). The influence of leaching temperature (T = 25 – 50 °C) and contact time (t = 1 – 12 h) on the removed ash fraction was assessed by using the nonparametric bifactorial analysis of variance proposed by Sokal and Rohlf. It was found that the mass fraction of ash removed (i.e. by leaching with CA and the reference leaching solutions) was in a narrow range (between 38.9 and 54.1%) regardless of biomass type and leaching conditions tested. The nonparametric bifactorial analysis of the results revealed that the nature of the leaching solution and its interaction with contact time are of major significance when demineralizing both SCT and SCB but no significance of temperature and its interaction with contact time was found. The FTIR-ATR revealed a modification of the xylan fraction and the reduction of CO by OH groups, caused by hydrolysis, demonstrating to be important for subsequent thermal degradation.
The biomass samples derived from the previous experimental design were then used to evaluate the effect of SCT and SCB leaching by CA (compared with well-known leaching agents including H2SO4, HCl and water) on the chemical composition of the pyrolysis vapors, viz. by applying micro-pyrolysis (Py-GC/MS) at 500 °C. As a result, the yields of levoglucosan in the pyrolysis vapors increased by 5–8 fold when sugar cane biomasses were leached with acids (i.e. CA or well-known inorganic acids). Differences in the range of leaching conditions tested had only minor influence on the composition of the pyrolysis vapors derived from CA pretreated sugar cane residues. CA treatment generally favored the reduction in the total production of ketones and furans independently of temperature and leaching time.
Experiments in a bench-scale pyrolysis reactor were conducted to see whether the results obtained in Py-GC/MS can be scaled to an actual fast pyrolysis reactor. The effects of leaching (25 °C and 1 h) sugar cane trash and sugar cane bagasse with CA on the yields and quality of fast pyrolysis bio-oils were studied, a comparison was made with commonly used leaching agents such as water or solutions of HCl and H2SO4. The quality of the obtained bio-oils was assessed by elemental analysis, total acid number (TAN) and water content determinations, combustion calorimetry and gas chromatography-mass spectrometry (GC/MS) analysis. Results from the fast pyrolysis of SCT or SCB pretreated with acids revealed higher yields on raw-feedstock basis (38–45 wt.%) of the organic bio-oil fraction than those from raw and water–leached feedstock, but lower yields of water and char. The most important observations related to bio-oils chemical composition from leaching with CA are a significant increase of the relative abundance of sugars from 15.1% in raw SCT to 44.7% in CA–leached SCT, as well as a decrease in carboxylic acids, ketones, furans and phenols with respect to the raw biomasses. Also, the bio-oils from the pyrolysis of CA–leached SCT and SCB have slightly higher HHVs than those obtained from reference leaching solutions (HCl and H2SO4).
The economic viability of pretreatment will depend on the minimization of CA consumption. In this sense, the effect of CA concentration used in pretreatment on the demineralization of SCT and SCB was studied. A comparison was made with H2SO4 as well-known leaching agent. An additional aim was to identify the optimal acid pretreatment concentration and its influence on the chemical characteristics of leached biomass and on the chemical composition of pyrolysis vapors, viz. by applying micro-pyrolysis. In general, the ash removal was found to decrease in both sugar cane residues upon a decrease in the concentration of the leaching solution. Small differences in total mass loss were associated with the type of biomass and the leaching agent used. The HHV of all pretreated samples had negligible differences, although at higher acid concentrations, a small reduction was observed. The proximate and ultimate analysis of leached SCT and SCB showed similar results for all H2SO4 or CA concentrations tested. Py-GC/MS studies at 500 °C of pretreated SCT and SCB showed an increase in the production of levoglucosan with respect to the raw feedstock. However, ketones production decreased by half or more compared to the raw materials irrespective of the acid concentration in pretreatment and the phenols didn’t follow any trend. The nonparametric analysis of results revealed that the pretreatment with lowest concentrations for CA (0.096 kgdm−3) and 0.251 kgdm−3 for H2SO4 have similar influence on the production of levoglucosan than higher concentrations tested. When comparing both biomasses, the nonparametric analysis demonstrated better results in SCB than in SCT with respect to the leaching solution concentration as dependent variable.
Raw sugar cane bagasse and trash were pretreated with CA having a concentration of 0.096 kgdm−3. The pretreatments were performed on a 10 dm3 scale and compared to the previous results at 0.25 dm3. The suitability of the pretreatment parameters established for scale up at 10 dm3 were verified by comparing the compositional characteristics of the treated biomasses and its products distribution in the vapor phase (by Py-GC/MS analysis) with previous results at 0.25 dm3 scale. Bio-oil samples were then obtained on a continuous auger pyrolysis reactor. Physicochemical characterization of the bio-oil included GC/MS, elemental composition, higher heating value, water and solids content and pH. Additionally, dynamic viscosity and stability (ageing) of the bio-oils were assessed. The average bio-oil yield on raw-feedstock basis revealed differences below 5% between SCT and SCB for both raw and leached with CA. The most important observations related to the effect of leaching with CA on the bio-oil’s chemical composition are a significant increase of the sugars, as well as a decrease of the carboxylic acids and phenols. From the analysis of the properties of bio-oils from pyrolysis of sugar cane residues, it is concluded that bio-oil samples from both raw or CA–leached SCT and SCB may not be suitable for direct use as a fuel but could be valuable as a chemical platform. The poor quality of bio-oils obtained from the pyrolysis of raw SCT or SCB makes them difficult to be considered as fuel nor for the production of chemical platform molecules.
A preliminary economic and environmental analysis associated with the installation and operation of a 2.5 t∙h−1 pyrolysis plant was carried out. The plant was designed for processing sugar cane residues raw (SCT and SCB) or pretreated (L-SCT and L-SCB). The analyses are based on comprehensive mathematical models and experimental data derived from a bench scale pyrolysis reactor using the auger technology. The models were implemented in Aspen One v10.0 and describe the heat, momentum and mass balances involved in both the pretreatment and pyrolysis stages presented as four scenarios. These models allow gathering basic engineering data; and provide the possibility to identify major issues and bottlenecks during the pyrolysis of raw and pretreated feedstock. Additionally, a preliminary cost analysis on the process to foresee its potential application at industrial scale is presented. Finally, an environmental impact study was performed using Life Cycle Assessment (LCA) methodology and extending the system boundaries to agriculture and sugar cane industry stages. The scenarios that use raw biomass are preferred to those that use pretreatment when economic (as well as environmental) aspects are taken into account. Although, the lower quality of bio-oils from raw SCT and SCB allows only using it as a low quality fuel. On the other hand, the investment costs for installing the L-SCT plant are in the same order, with the remarkable difference in the quality of the bio-oil. This last option could attract the attention as a way to valorize the non-edible and usually wasted SCT.