Tratamientos termoquímicos aplicados a biomasas leguminosas de rápido crecimiento

Abstract

The dependence on fossil fuels forces us to question the current energy model. Due to climate change and drifts in energy markets, promoting systems that use biomass for energy production is essential, hence the trends to promote biofuel production technologies and all kinds of biochemical products from residual lignocellulosic materials. Fast-growing leguminous species are interesting since they allow obtaining a large amount of lignocellulosic matter with soil improvements such as nitrogen uptake from symbiosis with bacteria, reforestation or treatment of soils contaminated with heavy metals. Leucaena leucocephala emerges as the most outstanding within this group, a species of great value due to its proven level of growth, climatic adaptation, and willingness to produce biofuels. Biomass thermochemical conversion technologies use heat to typically obtain solid, liquid, and gas fractions. Roasting is moderate heating, pyrolysis is carried out at a higher temperature and in the absence of oxygen to give a high-value liquid fraction known as bio-oil, gasification produces highly energetic gases with good hydrogen content, and combustion burns biomass producing heat that is used to produce electricity. All these processes have been perfected over time to selectively generate products with the lowest energy expenditure. The kinetic study of solid-state reactions of biomass is a fundamental step to understand thermochemical processes. With this purpose, models and methods of kinetic calculations arise to obtain activation energies, entropies, enthalpies, and other thermodynamic parameters of interest to design industrial equipment. In general, efforts are focused on finding out how the main structural components of biomass, namely: hemicellulose, cellulose and lignin, disintegrate in each of the conditions of destruction of the raw material at elevated temperatures. These methods arise from thermogravimetric analyzes that measure the mass loss of samples with increasing temperature at a constant rate of heating over time, generating curves. The wide variety of products that arise from the disintegration of biomass by heat treatment deserves a specific study. Hemicellulose breaks down into low molecular weight organic products such as furans, cellulose also generates anhydrous sugars such as levoglucosan, and lignin tends to form oxygenated aromatic compounds. Their study is a very useful tool to assess the potential of biomass in processes such as gasification, pyrolysis and combustion. Pilot plants make it possible to optimize the operating parameters to obtain a series of products and specific characteristics. By applying thermochemical treatments in pilot plants, the main fractions can be obtained: gas, bio-oil and biochar, allowing the characterization, optimization and improvement of these products with a view to treating them as future biofuels and bioproducts that substitute products from fossil sources. The kinetic analysis of the reactions of leguminous biomass has been one of the first stages of study to understand how they decompose, making it possible to optimize the process and provide essential information on the design, ultimately, of chemical reactors. In initial states, the reaction kinetics of conventional thermochemical processes of pyrolysis and combustion must be known through thermogravimetric studies and isoconversional methods. With these data and using previous studies, thermodynamic parameters of the reactions are obtained. In addition, through prior analyzes of the standardized raw materials and products by gas chromatography and mass spectrometry, the composition and possible recoverable products will be analyzed. Once these data are known, more complex processes, such as gasification, can be studied using more specific gas mixtures with substoichiometric amounts of oxygen. Furthermore, through gas analysis patterns are found that delimit different reactions with respect to the amount of oxygen used in the process. The optimization of the main parameters that affect these thermochemical processes is key when designing and scaling chemical plants, the main ones, as can be deduced from initial studies, are temperature, reactor design, reaction gas, type of biomass and the concentrations of different reaction gases in more complex processes such as gasification. All these aspects can be evaluated in the pilot plant with a fluidized bed reactor located in the facilities of the Superior Technical School of Engineering in Huelva, which has all the requirements to obtain quality data. By knowing the points of maximum degradation of the raw materials, the ideal temperatures and conditions in the pilot plant can be deduced. With the help of several experiments and choosing bibliographical references and previous studies as central points, designs such as the Box-Behnken are used, which allows optimizing the pyrolysis process of legumes, and, as a next goal, processes such as gasification using water to obtain gas from synthesis. In addition, the pilot plant also has an extensive collection system for products such as solids (biochar) that are separated from gas by cyclones, condensed liquids (bio-oil), and non-condensable pyrolysis gases. The analysis of these thermochemical treatment products is also an aspect of the investigation that should be highlighted. Bio-oil is the preferred product of pyrolysis and, after an improvement process through hydrogenation, it is used as biofuel or selectively separated to produce biochemical products of interest such as vanillin, levoglucosan or xylan. Biochar is a booming material that can be used for soil remediation, selective adsorbent, or catalyst support. The non-condensable pyrolysis gas is a fraction that is typically recovered for energy by burning it in the installation itself, but every day new methods of separating compounds such as ketones, aldehydes or alcohols appear for the pharmaceutical, cosmetic, or other industries.