Gasificación catalítica de pulpa de café: desarrollo y caracterización de catalizador, modelado cinético
Fecha
2020-12-04
Tipo
tesis doctoral
Autores
Torres Quirós, Cindy
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Resumen
El sector agropecuario en Costa Rica conforma una importante parte del Producto Interno Bruto del país. El cambio climático y las nuevas políticas en torno a la descarbonización de la economía, demandan estrategias y procesos productivos más eficientes, resilientes, así como sostenibles. Por tanto, el presente trabajo de investigación aborda aspectos relacionados al avance del conocimiento de la ingeniería de las reacciones para la conversión de pulpa de café utilizando procesos de gasificación para la generación de gas de síntesis, así como el tratamiento de los alquitranes que se forman producto de su termoconversión. Se estructuró en tres secciones: a) análisis y modelo termodinámico, b) propuesta, elaboración y caracterización de catalizador para tratamiento de alquitranes, y c) modelado cinético.
En relación con el análisis termodinámico se estudió el sistema considerando un modelo en el equilibrio y se validó experimentalmente. Como principal resultado, se desarrolló una metodología numérica para incluir al biochar dentro del análisis termodinámico y para el reactor de lecho descendente, se determinaron dos zonas de equilibrio, heterogénea y una homogénea. Esto permite no sólo predecir satisfactoriamente la composición del gas de síntesis obtenido, sino también la masa de biochar, y constituye una herramienta para escalar este tipo de procesos con un error relativo menor al 13%.
Seguidamente, se desarrolló una propuesta de catalizador con baja toxicidad, alta área superficial y fluidizable. Se utilizó γ-alúmina como soporte, 10 % de CaO como promotor, así como 4 % de hierro reducido parcialmente como fase activa. Se caracterizó el material utilizando diferentes técnicas analíticas adecuadas, mostrando interesantes ventajas en la reducción de la acidez fuerte del soporte y el hierro como catalizador. Así mismo, el desempeño de dicho catalizador se evaluó en un reactor prototipo de lecho fluidizado para establecer la efectividad en el craqueo de alquitranes. Los resultados obtenidos confirman el alto rendimiento del catalizador desarrollado, con una conversión del 2-metoxi-4-metilfenol (MC) cercana al 100% y con selectividades de hasta el 98,6 % en fase gaseosa para especies que contienen carbono de bajo peso molecular, a 500 °C, con un tiempo de reacción de 7,5 s y 1,5 g de vapor / g de MC.
Así mismo, se desarrolló el modelo cinético térmico y catalítico para el craqueo con vapor de un modelo compuesto de lignina (MC). Se realizaron experiencias a tres temperaturas (500 °C, 535 °C y 550 °C) y dos razones de vapor/biomasa (1 y 1.5). Se propuso el mecanismo de reacción, el conjunto de reacciones potencialmente involucradas, las expresiones de velocidad de reacción, y se determinaron los parámetros cinéticos intrínsecos asociados a cada reacción, así también su intervalo de confianza. Se realizó la validación del modelo utilizando un set independiente de resultados experimentales y se concluye que las predicciones describen satisfactoriamente las observaciones experimentales.
Finalmente, se puede concluir que los objetivos planteados en el presente trabajo de investigación fueron alcanzados satisfactoriamente, generando conocimiento cuya divulgación se ha llevado a cabo en revistas arbitradas y ponencias en actividades académicas de primer nivel. Esto potencia el desarrollo tecnológico que puede acompañar iniciativas de economía circular en Costa Rica.
In Costa Rica, an essential part of the Gross Domestic Product comes from the agricultural sector activities. Thus, climate change and the new economic policies about decarbonization demand more efficient, resilient, and sustainable production strategies and processes. Therefore, this research deals with the advancement of biomass conversion using gasification processes, specifically the focus on coffee pulp transformation to syngas and the tar treatment as a by-product formed during this biomass waste thermal conversion. This study was structured in three sections: a) thermodynamic modeling, b) proposal, preparation, and characterization of catalyst for tar cracking, and c) kinetic modeling. In the first place, a chemical equilibrium thermodynamic-based model was successfully established for the simulation of a demonstration-scale downdraft biomass gasifier unit operated at the ICAFE Costa Rica facilities. A minimum set of independent reactions was properly selected via a Gaussian Elimination Algorithm to achieve this goal. The chemical equilibrium thermodynamic model solution numerical method was validated by developing an equivalent numerical simulation. This comparison was carried out with Aspen-Hysys software and employed graphite as the solid gasification residue. The chemical equilibrium thermodynamic model also accounted for the biochar using both elemental composition and biochar thermodynamic properties to provide completeness. The biochar Heat of Formation was determined using a calorimeter, while the Gibbs Free Energy of Formation was calculated via an iterative non-linear regression method. The established gasifier model was further enhanced by including a homogeneous “upflow” exit section, following the heterogeneous “downflow” section. The proposed model was found effective in predicting various product molar fractions at the downdraft gasifier unit's outlet showing a relative error of less than 13%. Subsequently, an eco-friendly catalyst was prepared considering critical aspects of the industrial operation, such as fluidization, toxicity, and raw material cost. The incipient impregnation technique was used for the preparation. The γ-alumina was used as support. This material had a high surface area, which was not substantially modified when adding the promoter in a 10% CaO load and 4% iron oxides partially reduced. The catalyst characterization was led using different analytical techniques in a satisfactory way (XRD, TPD, FT-IT, TPR, XPS). Likewise, this catalyst's performance was evaluated in a bench-scale fluidized bed reactor to establish its effectiveness in cracking tars, with positive results. The obtained results confirm the high performance of the developed catalyst, with 2-methoxy-4-methylphenol (MC), conversion being close to 100% and with a selectivity of up to 98.6% for C1-C2 carbon-containing species, at 500°C, with a 7.5 s reaction time and 1.5 g steam/g of the MC. Finally, thermal and catalytic kinetic models were developed to describe the steam cracking of a tar model compound, MC. Experiments were carried out at three different temperatures (500 ° C, 535 ° C and 550 ° C) and two steam/biomass ratios (1 and 1.5). Mass balances closure at (90±2)%. The analysis was conducted considering novel aspects, such as coke inclusion as solid phase and incorporating other carbon species in the reactions set. Furthermore, the reaction constants parameters and the activation energy were determined employing non-linear numerical methods, and their confidence interval was established. The model predictions describe the experimental observations satisfactorily.
In Costa Rica, an essential part of the Gross Domestic Product comes from the agricultural sector activities. Thus, climate change and the new economic policies about decarbonization demand more efficient, resilient, and sustainable production strategies and processes. Therefore, this research deals with the advancement of biomass conversion using gasification processes, specifically the focus on coffee pulp transformation to syngas and the tar treatment as a by-product formed during this biomass waste thermal conversion. This study was structured in three sections: a) thermodynamic modeling, b) proposal, preparation, and characterization of catalyst for tar cracking, and c) kinetic modeling. In the first place, a chemical equilibrium thermodynamic-based model was successfully established for the simulation of a demonstration-scale downdraft biomass gasifier unit operated at the ICAFE Costa Rica facilities. A minimum set of independent reactions was properly selected via a Gaussian Elimination Algorithm to achieve this goal. The chemical equilibrium thermodynamic model solution numerical method was validated by developing an equivalent numerical simulation. This comparison was carried out with Aspen-Hysys software and employed graphite as the solid gasification residue. The chemical equilibrium thermodynamic model also accounted for the biochar using both elemental composition and biochar thermodynamic properties to provide completeness. The biochar Heat of Formation was determined using a calorimeter, while the Gibbs Free Energy of Formation was calculated via an iterative non-linear regression method. The established gasifier model was further enhanced by including a homogeneous “upflow” exit section, following the heterogeneous “downflow” section. The proposed model was found effective in predicting various product molar fractions at the downdraft gasifier unit's outlet showing a relative error of less than 13%. Subsequently, an eco-friendly catalyst was prepared considering critical aspects of the industrial operation, such as fluidization, toxicity, and raw material cost. The incipient impregnation technique was used for the preparation. The γ-alumina was used as support. This material had a high surface area, which was not substantially modified when adding the promoter in a 10% CaO load and 4% iron oxides partially reduced. The catalyst characterization was led using different analytical techniques in a satisfactory way (XRD, TPD, FT-IT, TPR, XPS). Likewise, this catalyst's performance was evaluated in a bench-scale fluidized bed reactor to establish its effectiveness in cracking tars, with positive results. The obtained results confirm the high performance of the developed catalyst, with 2-methoxy-4-methylphenol (MC), conversion being close to 100% and with a selectivity of up to 98.6% for C1-C2 carbon-containing species, at 500°C, with a 7.5 s reaction time and 1.5 g steam/g of the MC. Finally, thermal and catalytic kinetic models were developed to describe the steam cracking of a tar model compound, MC. Experiments were carried out at three different temperatures (500 ° C, 535 ° C and 550 ° C) and two steam/biomass ratios (1 and 1.5). Mass balances closure at (90±2)%. The analysis was conducted considering novel aspects, such as coke inclusion as solid phase and incorporating other carbon species in the reactions set. Furthermore, the reaction constants parameters and the activation energy were determined employing non-linear numerical methods, and their confidence interval was established. The model predictions describe the experimental observations satisfactorily.
Descripción
El presente trabajo de investigación resume aspectos relacionados al avance del conocimiento de la ingeniería de las reacciones para la conversión de pulpa de café utilizando procesos de gasificación para la generación de gas de síntesis, así como el tratamiento de los alquitranes que se forman producto de su termoconversión. El detalle científico se desarrolla en cada una de las cinco publicaciones relacionadas a este trabajo. Al final del documento se encuentran las referencias respectivas.
Palabras clave
gasificación de biomasa, análisis termodinámico, modelo de equilibrio, craqueo catalítico, catálisis heterogénea, modelo cinético, catalizador óxidos de hierro