Parametric studies in the gasification agent and fluidization velocity during oxygen-enriched gasification of biomass in a pilot-scale fluidized bed: Experimental and numerical assessment
Introduction
Upon the current society development, growth and populations’ long-lasting tendency, fossil fuels are becoming scarce once most of the commodities used nowadays are energy-based [1]. Therefore, novel ways of producing energy must be adopted in order to supply its high demand and to reduce the environmental impacts related to the use of natural resources. Gasification has been gaining notoriety as an established thermal technique [2], which converts solid residues as biomass into energy, producing syngas, a precursor for highly marketable energy assets [[3], [4], [5], [6]]. This technique consists in the thermal breakage of carbon-based materials into their constituent components, in the presence of air or oxygen at high temperatures [[7], [8], [9]]. For this, a sequence of steps and a multitude of reactions take place, as described elsewhere [[10], [11], [12]].
It is known that gasification has a strong capacity of reducing greenhouse gas emissions, depicting better environmental profile when compared to other methodologies [13,14]. Advantages such as higher feedstock flexibility, reduced auxiliary materials requirement, lower noxious emissions and higher syngas yields have also been reported [[15], [16], [17]]. Due to the reduced size restrictions in this technique, biomass is one of the most used feedstocks for gasification [18,19]. Also, it may be seen as a carbon-neutral feedstock [17,20,21] once it derives from photosynthesis and, if efficiently processed, may be used to produce new biomass or other intermediate products. In the case of gasification, the produced syngas is typically composed by H2, CO, CO2, CH4 and some minor contaminants as ash and tar [21,22], some of which are also required in photosynthesis. For all these reasons, biomass gasification is seen as promising technique in the view of a more sustainable energy production [7,23]. It may be said that gasification is a dual-benefit technique once, apart from offering the possibility of efficiently producing clean energy from carbon-based feedstocks, it also prevents these residues from being unsafely disposed of.
In order to maximize time and efforts for carrying experimental gasification runs to afford a high-quality syngas, the development of numerical methods that aid optimizing the operational conditions and predicting syngas composition is an important advance in this field [2]. Thus, computational fluid dynamic (CFD) models are being used worldwide to simplify such a complex combination of equations [24,25]. CFD entails conservation of mass, momentum, species and energy in a defined region and, when applied to gasification, it can provide relevant data for the process if combined with knowledge on the hydrodynamics of the reactor [26,27]. CFD affords temperature profiles of solid and gaseous phases in the reactor, predicting the concentration and behavior of the species on the produced syngas [9,28,29]. A reliable CFD method is a very useful tool to guarantee the most efficient results for a specific set of requirements, such as reactor type, feedstock characteristics and desired syngas features [12,30]. Operational conditions play a key role in achieving a high-quality syngas and its respective yield, each variable influencing the final result in a distinct way [10,[31], [32], [33]]. Parametrization studies have shown to largely contribute for the achievement of the ideal experimental conditions, as seen in Refs. [11,[34], [35], [36]]. A solid evidence for the time saved when using a CFD model to fit the experimental conditions is shown by Armstrong et al. [37], who were able to optimize the duration of the experimental runs when compared to literature data.
Fluidization is a characteristic of the particle movement, influenced by phenomena such as gravity, Archimedes push and drag force [27,38]. Therefore, this parameter may be varied and controlled in the simulation runs, balancing the forces so as to assess the optimal fluidization velocity, which helps to state the gas behavior within the gasifier.
Indeed, Xue and Fox [39] presented a CFD model of biomass gasification (wood) in a fluidized bed gasifier using air as fluidization agent, paying special attention to the interaction among the biomass particles and the reactive gas flow. To account for the evolution of the reacting species’ physical properties, the particle density was continuously varied and a time-step was implemented so that a strong chemical reaction and higher energy efficiency were achieved. Thankachan et al. [40] developed a numerical model for biomass gasification in a fluidized bed, considering the multiphase flow regime and modelling the particle motion through the kinetic theory of granular flow. Gas velocities, flow profiles and syngas characteristics were assessed, considering the heterogeneous and homogeneous reactions. Oevermann et al. [41] also simulated wood gasification in a fluidized bed, using a soft-sphere approach for assessing the particle dynamics. The influence of chemical reactions on the fluidization profile was assessed, wood feeding rate and temperature being varied in order to monitor syngas composition. Gerber et al. [42] reported on a multiphase approach for wood gasification using char as bed material in a fluidized gasifier. Besides being a low-cost option, potential benefits of using char when compared to traditional catalysts are the avoidance of catalyst regeneration need and the lower density, which enables lower pressure loss in the reactor. The authors investigated the effect of initial bed height and wood feeding rate as well as thermal boundary conditions. Couto et al. [9] developed a numerical model for the gasification of agro-industrial residues, evaluating the influence of the oxygen-enriched air atmosphere on gasification temperature, steam to biomass ratio and final syngas composition. They observed that the hydrogen and nitrogen molar fractions decrease as a function of the oxygen content and that the carbon dioxide shows the opposite trend. CGE increases with the oxygen content and decreases slightly with the steam to biomass ratio.
Equivalence ratio describes the relation between the actual air-to-biomass ratio and the stoichiometric air-to-biomass ratio [43] and may be used as a measure to compare the performance of different gasifying agents. Monteiro et al. [44] assessed the syngas produced from biomass (miscanthus) in a semi-industrial gasification plant describing the transport of mass exchange, momentum and energy for both the solid and gas phases. Higher temperatures were seen to improve syngas quality and conversion efficiency while reducing tar production, whereas higher equivalence ratios had the opposite effect. Ismail et al. [45] reported the negative influence of moisture content in the conversion efficiency of coffee husks through gasification in a fluidized bed, this effect decreasing for higher ER values. In the case of agro-industrial biomass samples, a subsequent work shows that higher temperatures enhance CO contents and reduce tar yields [38]. Conversely, Monteiro et al. [46] show that in the case of peach stone gasification, higher moisture contents seem to promote H2 and CO2 while decreasing CO, as confirmed by Mendiburu et al. [47]. Nevertheless, La Villetta et al. [48] state that the CO drop has a bolder effect than the H2 enhancement once the heating value is greater for CO than for H2.
Although there is a vast number of works reporting biomass gasification in fluidized beds, there is also a lack of modelling works on oxygen enriched air gasification.
Therefore, in the present work, a numerical CFD method was developed to model the gasification of agricultural residues in a fluidized bed under oxygen enriched air atmosphere. Parameters such as equivalent ratio, fluidization velocity and oxygen content were varied with the aim of optimizing the experimental conditions for the achievement of high-quality syngas. In order to achieve this a resumed number of numerical runs was held, as preconized by the implementation of mathematical models for gasification simulation.
Section snippets
Sample characterization and experimental conditions
The biomass used as feedstock in this study was a mixture of agricultural waste mainly containing straw. This biomass mixture was crushed into small particles with diameter size of 5–10 mm. The proximate analysis, ultimate analysis and heating value of the biomass are presented in Table 1. High-alumina bauxite was chosen as bed material, as its properties, high amount of Al2O3, can decompose tar, prevent slagging and improve gas quality [49]. Physical characteristics and chemical composition of
Model validation
Table 5 illustrate the comparison between numerical and experimental results. The deviation of the model results from experimental values is quantified by using the relative error also depicted in Table 5. The relative error is defined as the absolute difference between the simulated and experimental values divided by the experimental value.
The numerical results obtained by the developed model show a very good agreement with the experimental results providing relative errors less than 4.0%.
Temperature, gas and phase distributions
The
Conclusions
A comprehensive homemade two-dimensional CFD gasification model was proposed to study the effects of equivalence ratio, oxygen content in the gasifying agent and fluidization velocity on four process performances: syngas composition, LHV, CCE and CGE. Obtained results show excellent agreement between experimental and numerical results, which suggests that the proposed model is capable of predicting the syngas composition as well as the trends for the main conditions governing the gasification
Acknowledgements
This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 818012.
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