Numerical and experimental analysis of municipal solid wastes gasification process

As the quantity of municipal solid waste (MSW) increases with economic growth, problems arise in regard to sustainable management solutions. Thermal treatment presents a valid option for reducing the amounts of post-recycling waste to be landfilled. Incineration technology, besides reducing the total volume of waste and making use of the chemical energy in MSW for power generation, has negative environmental impact from high emission of pollutants. Recent policy to tackle climate change and resources conservation stimulated the development of renewable energy and landfill diversion technology, thereby giving gasification technology development renewed importance. In this work a two-dimensional CFD model for MSW gasification was developed and an Eulerian-Eulerian approach was used to describe the transport of mass, momentum and energy for the solid and gas phases. This model is validated using experimental data from the literature. The numerical results obtained are in good agreement with the reported experimental results

Investigating the Thermochemical Conversion of Biomass in a Downdraft Gasifier With a Volatile Break-Up Approach

An affordable, reliable and clean energy supply is the major challenge facing by the modern world. Biomass energy is playing a promising role to that, but gasification technology able to convert biomass efficiently to valuable gases for power and heat generation is a vital need. The aim of this study is to develop a robust computational fluid dynamics (CFD) model to better understand the gasification thermochemical processes of a selected biomass (rubber wood) in a 20 kW downdraft gasifier, which includes all the four zones, drying, pyrolysis, oxidation and reduction. A step-by-step approach is proposed to evaluate the composition of different species as a result of volatile break-up during gasification. Effect of the equivalence ratio on the synthesis gas composition is studied with results validated against a kinetic model

Modeling Biomass Substrates for Syngas Generation by Using CFD Approaches

Recent reports from top universities state that in spite of having great national importance, there are dozens of fields of study that are suffering due to a lack of funding. Perhaps the greatest tool available to assist researchers with this regard is numerical simulation. This tool allows cutting costs, decreasing the necessary design cycle and allows an enormous amount of physical insight on the process itself). Numerical model’s ability to correctly predict a complex system was tested in this chapter by drawing from a previously developed computational fluid dynamics model for biomass gasification. Numerical results were compared with both experimental results (pilot scale plant) and available literature. Results from common Portuguese biomass substrates were found to be within a satisfactory margin of error of 20%. Influence of all major operational conditions was then investigated and the model was once again able to predict all the expected trends. All the relevant process products were also analyzed. Finally, the numerical model was coupled with an optimization model. Maximum efficiency value was found at 900°C with a SBR of 1.5 for MSW and 1 for forest residues. Results showed that numerical models could have a preponderant impact on biomass gasification field

Method of Identifying an Operating Regime in a Bubbling Fluidized Bed Gasification Reactor

This work presents a new method for identifying the bubbling regime of a fluidized bed gasification reactor. The method has been developed using experimental measurements and a computational model. Pressure drops are measured in experiments, and pressure drop as well as solid volume fraction fluctuations are calculated by implementing the model. Experiments are carried out with sand and limestone particles of mean diameter 346 μm and 672 μm, respectively. A computational particle fluid dynamics (CPFD) model has been developed for the reactor and implemented using a commercial CPFD software Barracuda VR. The model is validated against experimental measurements. The validated model is used to analyse the fluctuation of pressure drop and solid volume fraction as a function of superficial air velocity. The change in standard deviation of pressure drop and solid volume fraction fluctuation is used to predict the transition from one regime to another. The method can be used in the design and operation of a bubbling fluidized bed gasification reactor. The results show that the minimum fluidization velocity for sand and limestone are 0.135 m/s and 0.36 m/s, respectively and are independent of the particle aspect ratio. Both types of particle beds make the transition into bubbling regime as soon as they get fluidized. The bed aspect ratios have almost no effect on the onset of bubbling f uidization regime. The slugging velocity decreases with increasing aspect ratio for both types of particles. The operating range of the bubbling fluidized bed for sand particle is 0.2–0.4 m/s and 0.5–0.8 m/s for the limestone particles

Analysis of the effect of steam-to-biomass ratio in fluidized bed gasification with multiphase particle-in-cell CFD Simulation

Biomass has been identified as a key renewable energy source to cope with upcoming environmental challenges. Gasification of biomass is becoming interested in large scale operation, especially in synthesis of liquid fuels. Bubbling and circulating fluidized bed gasification technology has overrun the interest over fixed bed systems. CFD studies of such reactor systems have become realistic and reliable with the modern computer power. Gasifying agent, temperature and steam or air to biomass ratio are the key parameters, which are responsible for the synthesis gas composition. Therefore, multiphase particle-in-cell CFD modeling was used in this study to analyze the steam to biomass, S/B, ratio in fluidized bed gasification. Due to the complexity of the full loop simulation of dual circulating fluidized bed reactor system, only the gasification reactor was considered in this study. Predicted boundary conditions were implemented for the particle flow from the combustion reactor. The fluidization model was validated against experimental data in beforehand where Wen-Yu-Ergun drag model was found to be the best. The effect of the S/B ratio was analyzed at a constant steam temperature of 1073K and a steam velocity of 0.47 m/s. Four different S/B of 0.45, 0.38, 0.28 and 0.20 were analyzed. The biomass was considered to be in complete dry condition where single step pyrolysis reaction kinetics was used. Each gasification simulation was carried out for 100 seconds. 8% reduction of hydrogen content from 57% to 49% and 17% increment of carbon monoxide from 13% to 30% were observed when the S/B was reduced from 0.45 to 0.20. Countable amounts of methane were observed at S/B of 0.28 and 0.20. The lower heating value of the product gas increased from 10.1 MJ/kg to 12.37 MJ/kg and the cold gas efficiency decreased from 73.2% to 64.6% when the S/B was changed from 0.45 to 0.20. The specific gas production rate varied between 1.64 and 1.04 Nm3/kg of biomass

A review on the modeling and validation of biomass pyrolysis with a focus on product yield and composition

Article reviewing the modeling and verification of products derived from biomass pyrolysis and discussing the possible solutions towards more accurate modeling of biomass pyrolysis

Development of an Equilibrium-based Model of Gasification of Biomass by Aspen Plus

Abstract Agricultural and forestry residues are usually processed as wastes; otherwise, they can be recovered to produce electrical and thermal energy through processes of thermochemical conversion, such us torrefaction, pyrolysis and gasification. Currently, the gasification of residual biomass for producing neutral CO 2 fuel for energy production is in development stage. In this context, this study proposes anequilibrium-based model, developed by the commercial software Aspen Plus, of a co-current gasifier fueled with agriculture residual, which allows estimating the chemical composition and theheating value of the syngas produced. The prediction of such model includes the main gaseous species, the yields of char and tar and describes the gasification process through the mass and energy balances, the water-gas shift (WGS) and the methanation reaction. The model validation was carried out through the comparison with experimental data, concerning two biomass with different moisture content and different gasification conditions, for sixteen cases compared. Overall, the comparison between the results of the simulations and the experimental data have shown a good agreement

A novel approach for integrating concentrated solar energy with biomass thermochemical conversion processes

Concentrated solar energy provides thermal energy that can be utilised for thermochemical conversion of biomass to produce liquid fuel and gases. This creates an efficient and a carbon-free process. The fast pyrolysis of biomass is an endothermic thermal process that occurs within 400-550oC at fast heating rates of >300 oC/second in the absence of oxygen. This temperature is within the range produced in a parabolic trough arrangement. The process of biomass gasification is the conversion of biomass fuels to non-condensable gases usually for chemical feedstock or as fuel using a fluidising medium. Solar intermittence is a major issue; this can be resolved by proposing a continuous process from concentrated solar energy to fuels or chemical feedstock. Computational fluid dynamics has proven to be a tool for design and optimisation of reactors. The Eulerian-Eulerian multiphase model using ANSYS Fluent has shown to be cost-effective at describing the characteristics of complex processes. The project entails using parabolic trough for fast pyrolysis of biomass; it is integrated with a gasification process with utilities produced entirely from solar energy. The scope of the project are: (i) A Computational fluid dynamic (CFD) model analysis of the novel reactor is to be developed to model biomass pyrolysis (ii) Investigate the potentials of integrating the proposed solar reactor with a conventional circulating fluidised bed (CFB) gasifier to create a highly efficient and sustainable closed loop thermo-solar process (iii) Validate the circulating fluidised bed model with an experimental scale Circulating fluidised bed (CFB) gasifier at Aston University’s European Bioenergy Research Institute. The report studied the use of CFD modelling to investigate fast pyrolysis of switch grass biomass using a solar parabolic trough receiver/reactor equipped with a novel gas-separation system. The separator controls the effect of tar-cracking reactions and achieves high separation efficiency compared to other gas-solid separation methods. The study assumes an average heat flux concentrated along the receiver/reactor. Pyrolysis reaction was represented as a single global first order Arrhenius type reaction with volatiles separated into condensable (bio-oil) and non-condensable products. The drying of moisture of the switch grass was represented as a mass transfer process. The separation efficiency achieved by the conical deflector was about 99%. The proposed reactor at the considered operating conditions can achieve overall energy efficiency of 42%; the product yield consist of 51.5% bio-oil, 43.7% char and 4.8% non-condensable gases. The average reactor temperature, gas residence time, and maximum devolatilisation efficiency were 450 °C, 1.5 s, and 60% respectively. There was good agreement in comparison with experimental findings from literature. A sensitivity analysis was conducted to study the effect of heat flux conditions, heat transfer, sweeping gas temperature, and particle size. The heat flux distribution showed that non-homogeneous provides a greater heating rate and temperature compared to the homogeneous flux. Radiation negligibly affects the final product composition; the radiation heats the biomass mainly rather than cause devolatilisation. The larger the biomass diameter the more bio-oil is produced, when a uniform particle temperature is assumed. An experimental study was conducted for the validation of the hydrodynamic model of a circulating fluidised bed. The experiment measured the pressure profiles and the solid recirculation rate. The experiment result showed that particle size has a negative correlation to the ease of fluidisation. High fluidising gas flowrate has a positive impact on the fluidising regime and pressure in the riser. The following parameters were compared with experimental results: grid size, turbulence model, drag laws, wall treatment, and wall shear properties (specularity coefficient and restitution coefficient). The results proved the optimum hydrodynamic model through comparison of pressure profiles of the model with experimental results. The gasification of char in a circulating fluidised was studied using the optimum hydrodynamic model validated from experiment. The model considered the effect of turbulence on the species evolution and tar reforming with char. Over the range of operating conditions, the results looked into the hydrodynamics and product yield of the gasifier. The product yields obtained for the base case was CO (12%), CO2 (19%), H2 (6%), CH4 (0.7%), and N2 (63%). The results proved that for smaller particles the evolution of species are dominated by kinetics. The catalytic effect of char showed improvement in tar yield and CGE to 15.12g/Nm3 and 67.74%. The product yields showed improvement with the compositions of CO2 and H2 due to reforming reactions. The yields and efficiency were in qualitative agreement with results from literature. The proposed models described will provide details on the procedures for future design of integrated solar biomass thermochemical conversion systems

CFD modelling of particle shrinkage in a fluidized bed for biomass fast pyrolysis with quadrature method of moment

An Eulerian-Eulerian multi-phase CFD model was set up to simulate a lab-scale fluidized bed reactor for the fast pyrolysis of biomass. Biomass particles and the bed material (sand) were considered to be particulate phases and modelled using the kinetic theory of granular flow. A global, multi-stage chemical kinetic mechanism was integrated into the main framework of the CFD model and employed to account for the process of biomass devolatilization. A 3-parameter shrinkage model was used to describe the variation in particle size due to biomass decomposition. This particle shrinkage model was then used in combination with a quadrature method of moment (QMOM) to solve the particle population balance equation (PBE). The evolution of biomass particle size in the fluidized bed was obtained for several different patterns of particle shrinkage, which were represented by different values of shrinkage factors. In addition, pore formation inside the biomass particle was simulated for these shrinkage patterns, and thus, the density variation of biomass particles is taken into account