1.- Biomass Gasification,
Biomass is a hydrocarbon material mainly consisting of carbon, hydrogen, oxygen, nitrogen and minerals (2). Biomass is considered an ideal renewable resource given its abundance, its lower sulfur content and its CO2 neutral emissions (1). Biomass steam or biomass CO2 gasification (5) has become an area of growing interest because it produces a gaseous fuel (synthesis gas) with relatively high hydrogen content (H2/CO>1.5-2) (76,77) . This synthesis gas could be used for industrial applications, both for highly efficient electricity production and as a feedstock for chemical synthesis (3).
Furthermore, steam gasification has other advantages in that: (i) it produces a gas with higher heating value; (ii) it reduces the diluting effect of N2 from air and (iii) it eliminates the need for an expensive oxygen plant when both air and oxygen are used as gasification mediums (1). In biomass gasification, zero net emissions of carbon dioxide can be achieved because the carbon dioxide released from biomass is quantitatively recycled back into plants via photosynthesis (2). Biomass gasification produces very low levels of particulates as well as, very little amounts of NOx, and SOx when compared with the gasification of fossil fuels. Moreover, biomass can be used as a source to produce ethanol, di-methyl-ether, diesel and synthetic gasoline (6).
The production and consumption of fossil fuels have caused environmental concerns due to the increasing CO2 concentrations in the atmosphere. As the world's accessible oil reservoirs are gradually depleted, it is, important to developsuitable long-term strategies based on the use of renewable fuels from which utilization will gradually substitute fossil fuels. However, a serious issue for the broad implementation of the biomass gasification technology is the generation of the unwanted char and tars . Char or biochar is a solid carbonaceous residue. Tar is a complex mixture of condensable hydrocarbons, which includes single ring to 5-ring aromatic compounds along with other oxygen-containing hydrocarbons species (2).
Different approaches are currently used in biomass gasification (9): (i) direct synthesis gas treatment inside the gasifier (primary methods) and (ii) hot gas cleaning after the gasification process (secondary methods) . Primary treatment methods are the ones gaining much attention nowadays as they may eliminate the need for additional downstream equipment using hot-gas cleaning technology (7,8).
There are several factors to consider towards the development of an effective primary tar treatment method: (a) the proper selection of operating parameters, (b) the type of additive/catalyst used, and (c) the needed modifications in gasifiers to prevent tar build-up. Tar can be converted thermally. However, thermal conversion requires very high temperatures, greater than 1000oC. Catalytic reforming of tars into gaseous products offers, on the other hand, an effective method for tar removal and avoids the costly tar disposal. In this respect, Ni-based catalysts can contribute given their ability to convert tar as a result of their water-gas-shift activity. In addition, these catalysts have the capacity in reducing nitrogen containing compounds such as ammonia (4).
However, several deactivation mechanisms occur with nickel-based catalysts as follows: i) poisoning with sulphur, chlorine, and alkali metals, ii) sintering of Ni particlesand iii) coke formation (10). Under these conditions, Ni-based catalysts deactivate rapidly due to coke. While coke can be removed by combustion, coke removal can lead, if not carefully performed, to catalyst deactivation, poor selectivity and limitedcatalyst life (11).Coke deposition can also be minimized through the use of excess steam and with respect to the one required by gasification stoichiometry. In practice, this increases the overall energy costs for biomass gasification plant operation.
Synthesis gas produced from biomass, requires to be cleaned from trace amounts of H2S, COS, CS2, HCl, NH3 and HCN at the ppb level (9). To accomplish this without a negative impact on the process thermal efficiency, the synthesis gas has to be cleaned at high temperatures. In this regard, gas cleaning is considered a high temperature-high pressure process with the syngas produced being suitable for diesel synthesis via Fischer-Tropsch (6).
Catalytic biomass gasification is a complex reaction which includes numerous chemical reactions such as pyrolysis, steam gasification and water-gas shift reaction. Extensive research has been made to develop stable and highly active catalysts for biomass gasification producing high quality synthesis gas and /or hydrogen (38,39). However, designing a best catalyst for steam gasification requires additional insights into gasification thermodynamics, kinetics and reaction mechanismsin order to predict the end-reaction product composition distribution. Furthermore, long life catalysts for biomass steam gasification are required for large scale processes to operate in the 700-800°C range, yielding H2/CO ratios of about one or even higher, suitable for the alternative manufacturing of fuels such as ethanol and biodiesel.
Regarding biomass conversion, there are different processes which are utilized to produce heat and electricity, aswell as to convert biomass into various chemical species. However, there is little comprehensive work that we are aware of, on biomass catalytic gasificationwith emphasis on catalyst development and synthesis, thermodynamics, kinetics, and catalyst characterization as well as on feedstock characteristics. The work being developed at the CREC at the University of Western Ontario is certainly leading these efforts (38, 38,46).
Thermochemical transformation or gasification of cellulose, lignin or lignocellulose materials into synthesis gas (CO+ H2) is possible above 700oC in the presence of controlled amounts of oxygen. On the other hand, if lignocelluloses are heated in the absence of oxygen, then a mixture of gases, bio-oils, tars and char are generated. This pyrolysis process requires a high energy input (46)
Supercritical water gasification of biomass is a technology especially recommended for wet biomass where no biomass drying is required (72,73,74,75). In addition to the interesting prospects of forming less tar and char, supercritical water gasification can produce a significant fraction of extra hydrogen originating in the water rather than in the biomass. Supercritical water displays high acidity and thus the reactor materials are prone to corrode. As a result, supercritical gasification is penalized for its high capital costs, requiring a large monetary investment.
Theoretically, almost all kinds of biomasses with moisture content in the 5-30wt% range can be gasified. However, it is known that feedstock (biomass) properties such as: (i) specific surface area, (ii) size, (iii) shape, (iv) moisture content, (v) volatile matter and (vi) carbon content, all affect gasification. Other variables which also significantly influence gasification are: (a) the gasifier configuration, (b) the specific gasification process conditions used, and (c) the gasifying agent (37,38). Thus, it is critical to find an effective and efficient biomass conversion technology to utilize such renewable energy resource(2).
Cellulose and lignin are two major constituents of biomass. Glucose is a polymer, consisting of linear chains of glucopyranose units, with an average molecular weight of around 100,000 Kg/Kmole. Lignin is a complex polymer of aromatic alcohols known as mono-lignols.Lignin is most commonly derived from wood, and is an integral part of the secondary cell walls of plants.
Biomass gasification is a thermo-chemical conversion process of solid biomass into a gas-phase mixture of carbon monoxide (CO), hydrogen (H2), carbon dioxide (CO2), methane (CH4), organic vapors, tars (benzene and other aromatic hydrocarbons), water vapor, hydrogen sulfide (H2S), residual solids, and other trace species (HCN, NH3, and HCl). The specific fractions of the various species obtained may depend on process conditions andon the reaction environment (inert, steam) prevailing during gasification. Other inorganic materials contained in biomass such as Si, Al, Ti, Fe, Ca, Mg, Na, K, P, S, and Cl may also influence biomass gasification.
Upon heating, the biomass dries up, until it reaches 120oC. Volatiles are produced until temperature reaches 350oC and the resulting char is gasified above 350oC. Therefore, it is customary to classify the entire gasifier process into three steps: drying, devolatilization and gasification (2). Gasification itself is a combination of pyrolysis and oxidation reactions. Chemical species are heated up to 500-900°C in the presence of air, steam, CO2, or other components.Heat to drive the process is generated either outside the unit or in the same unit via exothermic biomass combustion (2).
During gasification, the inorganic components of the biomass are usually converted into ash, which is removed from the bottom of the gasifier (bottom ash), or into fly ash, which leaves with the product gas. The composition of the ash includes CaO, K2O, P2O5, MgO, SiO2, SO3, Na2O, and residual carbon. Volatile halogen elements and alkali elements are mainly found in wet scrubber ash and in fly ash while Si, Ni, Pb, Zn, Cr, Cd, K, S, Mn, Cu elements are typically contained in the ash separator exit, enriched with heavy metals.
Gasifiers can be divided into two principal types: fixed beds and fluidized beds, with variations within each type. A third type, the entrained suspension gasifier has been developed for the gasification of finely divided coal particles (<0.1–0.4 mm) only. Depending on the direction of airflow, fixed gasifiers are classified as updraft, downdraft, or cross-flow. Catalytic steam gasification of biomass in fluidized beds is a promising approach given its rapid biomass heating, its effective heat and mass transfer between reacting phases, and its uniform reaction temperature(49,50,51,52,53,54,55,56,57). Moreover, fluidized beds tolerate wide variations in fuel quality as well as broad particle-size distributions. Intense bed fluidization promoting solid circulation also favors the mixing of the hot bed material, the hot combustion gases and the biomass feed.Fluidized beds are used for the production of a broad variety of fuels. This flexibility with respect to different fuels is actually another critical advantage of fluidized beds.Furthermore, atypical intermediate tar level of 10 g/Nm3 is achieved in fluidized beds. Tar created is formed by a blend of secondary and tertiary tars.
To achieve a high carbon conversion of the biomass and a low tar content, a high operating temperature (above 800 °C) in the gasifier is recommended. With the increase in temperature, combustible gas content, gas yield,hydrogen, and heating value all increase significantly, while the tar content decreased sharply. Temperature not only affects the amount of tar formed, but also the composition of tar by influencing the chemical reactions involved in the gasification network. Several operational strategies are reported in the literature to produce a relatively clean gas (7,8,9).
Therefore, several factors including: i) tar content, ii) gas composition determining gas heating value and iii) char conversion, should all be taken into consideration and weighted carefully in selecting the gasifier operating temperature.
Catalytic reforming can be used to convert tar into gaseous products (29,30,31,32,33,34). The use of catalysts during biomass gasification promotes char gasification, changes the product gas composition and reduces the tar yield obtained when the gasifier is operated at lower temperatures. Moreover, the addition of dopants to the Ni based catalyst such as Fe, Mg, Mn, Ce, Pt, Pd, Rh, Ru not only influences the gas composition, but also the heating value of the product gas.
Catalysts have been employed directly in the gasifier and in these cases, they are referred to as primary catalysts. These primary catalysts such as dolomite, calcined dolomite, olivine (20), Ni/Al2O3 promote several important chemical reactions such as water-gas-shift and steam reforming.
The use of dolomite, a magnesium ore with the general formula MgCO3 CaCO3, as a primary and/or secondary catalyst in biomass gasification has attracted much attention since it is a cheap disposable catalyst that can significantly reduce the tar content of the product gas from a gasifier (7). The main issue with dolomite is its fragility, as it is soft and prone to attrition in fluidized beds under prevalent high turbulence conditions.
The highly beneficial use of activated alumina as a secondary catalyst for tar reduction comes from its high catalytic activity, comparable to dolomite, although it deactivates by coke faster than dolomite. Among the transition metals (group VIII), nickel is the most widely used in the industry for steam and dry reforming reactions. Commercially available nickel reforming catalysts have been used extensively for biomass gasification (15,18,21,22,23,24,25,26). According to Aznar et al (19), under the conditions of catalytic gasification, nickel catalysts, are more active for heavy hydrocarbon steam-reforming (i.e. CnHm +nH2O®n CO+ (n+m/2) H2) than for light hydrocarbon steam reforming (i.e. CH4 + H2O®CO+ 2 H2 ). These nickel catalysts also promote the water-gas-shift reaction (CO+H2O®CO2 + H2) (27,28), and are very effective in tar conversion. As a result, these nickel based catalysts reduce tars while increasing the H2/CO ratio, improving synthesis gas quality. According to Olivares et al, nickel reforming catalysts display 8-10 times more reactivity than calcined dolomite (2).
When using nickel-based catalysts,several deactivation mechanisms occur including: i) poisoning by sulphur, chlorine, and alkali metals, ii) sintering of Ni particlesand iii) coke formation. Ni-based catalysts deactivate rapidly due to coke formation and catalyst attrition. Coke formation is inherent to steam reforming processes. The high temperatures associated with reforming both promote higher hydrogen yields and undesirable coke formation. Regarding coke formation, it can be minimized through the use of excess steam vis-a-vis of the one required by gasification stochiometry. In this respect, itis possible to estimate a minimum steam/carbon ratio required to avoid coke formation (16).
Ni-based catalysts are also prone to deactivation by sulphur. The formulation of nickel catalysts may potentially involve the following components (i) an active component (i.e. Ni), (ii) a second added component (i.e. a dopant or promoter) and (iii) a support phase. Generally, higher nickel content results in lower tar yield and higher H2 and CO yields. On the other hand, the amount of nickel in the catalyst has a significant effect on the catalyst deactivation by coking (21). This suggests that a lower metal concentration results in stronger interaction with the support phase and higher metal dispersion. Thus, by controlling the metal addition, one can have a catalyst which is more resistant to deactivation by carbon fouling (76).
The support phase gives the catalyst mechanical strength and protection against severe conditions such as attrition and heat. The pore structure of the support, the metal-support interactions, and the acidity-basicity of the support all significantly influence the metal dispersion, the metal crystallite size and the carbon deposition on the catalyst surface; thus affecting the overall catalytic performance and catalyst coking resistance (76).
Alumina-based materials are considered to be the primary support materials for most reforming catalysts. In this respect, the performance of α-Al2O3 and γ-Al2O3 supported Ni catalysts for the reforming of methane with CO2 was investigated. It was reported that the Ni/α-Al2O3 catalyst provided lower methane conversion than the Ni/ γ-Al2O3, in spite of providing a more stable α-Al2O3 allotropic form, given the smaller surface area of α-Al2O3 (21)
Therefore, it is planned in the context of the present research to innovate with new nickel based catalysts for the production of synthesis gas from biomass with high hydrogen content and the reduction of tars. Furthermore, it is also planned to evaluate the new catalysts using the CREC Riser Simulator: a unique bench fluidized bed reactor unit. The CREC Riser Simulator is manufactured by Recat Technologies Inc. under exclusive license agreement with the University of Western Ontario. This unit is the only one available worldwide , which allows: a) the performance evaluation of fluidizable catalyst under carefully controlled gasification conditions (mixing, temperature and catalyst/feed ratio), b) the development of kinetic models for biomass gasification based on phenomenological relevant kinetic parameters (activation energies, heat of adsorption, adsorption constants) with the adequate statistical indicators (low cross-correlation coefficients, reduces spans for the 95% confidence intervals). We are not aware of a similar reactor performing all the above described catalyst test functions and as a result the CREC Riser Simualtor is a critical instrument for our research.
Dr. Benito Serrano Rosales from UAZ, the "Responsable Tecnico" (PI) in this proposal has led biomass catalytic gasification studies in collaboration with Dr. Hugo de Lasa from The University of Western Ontario. This has resulted in one PhD thesis completed in 2010 (46), other PhD theses in progress, two articles published in prestigious technical journals (37,38) and several communications in technical conferences (e.g AIChE-Minesotta conference) (78). While this research has been valuable to establish UAZ as a leader in catalytic biomass gasification reaction engineering in Mexico, the present project plans to develop a new phase of research on: i) a new meso-structured support in collaboration with Dr. Serge Kalaiguine, Universite Laval, a world expert on catalysis and catalytic processes, ii) gasification modeling with special emphasis given to Mexican available feedstocks.
2.- Hydrogen Production using Heterogenous Photocatalysis
En el presente proyecto se van a estimar las eficiencias cuánticas para la producción de hidrogeno a través de la disociación del agua, usando un foto catalizador de dióxido de titanio modificado con platino, en una suspensión acuosa, bajo irradiación UV. El foto catalizador se preparara usando el método de impregnación de humedad incipiente, con el objetivo de reducir la energía de banda prohibida, los experimentos serán efectuados en el reactor Photo CREC Water II modificado y adaptado para la producción de hidrogeno, el cual permite hacer balances macroscópicos de radiación, para determinar la velocidad volumétrica de absorción de energía. Con respecto a la formación de hidrogeno vía radicales hidrogeno, se usara etanol como consumidor de hoyos, para determinar la mejor cantidad a usar. Se pretende investigar el efecto del pH del agua, en la producción de hidrogeno en presencia de etanol. También se va a calcular la eficiencia cuántica durante la producción de hidrogeno, para verificar el grado de utilización del fotón, y la influencia del catalizador de TiO2 con platino en la producción de hidrogeno y la utilidad del reactor photo crec water II para la producción de hidrogeno vía ruptura del agua. Esta es una investigación de primera línea a nivel mundial y se sentaran las bases para generar una tecnología barata y que aprovechara los recursos naturales (luz solar, agua y biomasa), para la producción de hidrogeno.
3.- Descontaminación del agua usando Fotocatalisis Heterogenea.
This study reports the efficiency profiles for the Photo CREC Water II reactor during the photo catalytic degradation of phenol using the following TiO2-based catalysts: DP25, Anatase 1, Hombikat UV – 100, Anatase 2, DP25+Fe and Anatase 2 + Fe. Quantum Yield (QY) and the Photochemical and Thermodynamic Efficiency Factor (PTEF) were used to measure reactor efficiencies. In order to obtain these rector efficiencies a unified kinetic model (UKM) was used, phenol and its intermediates were detected and quantified allowing determining the reaction rate of the formation of hydroxyl radicals at every step of the degradation. With the reaction scheme adopted, it was then possible to obtain the total reaction rate of OH radical consumption. During phenol degradation it is shown that the iron ions have an important effect and enhance the efficiency of the reactor during the experiments, and also it is shown that the maximum quantum yields obtained are above 60 %, indicating the suitability of the reactor Photo CREC Water II to achieve the photodegradation and to be scaled up.
4.- Laboratorios Remotos para realizar prácticas de laboratorio.
The internet was used to do remote experiments in Universidad Autonoma de Zacatecas Mexico,
using the laboratories in the University of Tennessee at Chattanooga, specifically, the system of
two non-interacting tanks. In the first part, step and pulse perturbations were applied, and in the
second part, a proportional controller was used and step perturbation on the set point was
applied. The mathematical models were deducted in all the cases, linearizing the expression for
the flow between tank 1 and 2.
In all the cases, the agreement between experimental and predicted results was very good,
indicating the models were conceptually correct, and the most important, the use of remote
laboratory was possible and allowed a relationship between two universities.