Dual Fluidized Bed Biomass Gasifier Research Articles

Tar characteristics generated from a 10 kWth chemical-looping biomass gasifier using steel converter slag as an oxygen carrier

Tar management is one of the key components to achieve high energy efficiency and low operational costs connected to thermal gasification of biomass. Tars contain a significant amount of energy, and unconverted tars result in energy efficiency losses. Also, heavy tars can condense downstream processes, resulting in increased maintenance. Dual fluidized beds for indirect gasification operated with active bed material can be a way to better convert and control the tar generated in the process. Using an active material to transport oxygen in an indirect dual reactor gasification setup is referred to as chemical-looping gasification (CLG). A higher oxidative environment in the gas phase, in addition to possible catalytic sites, could mean lower yields in comparison to normal indirect gasification.This paper investigates the effect of using Steel converter slag (LD slag), a byproduct of steel manufacturing, as an oxygen-carrying bed material on tar species generated in a 10 kWth dual fluidized bed biomass gasifier. The results are compared to the benchmark oxygen carrier ilmenite and conventional silica sand. Three different solid biofuels were used in the reactor system: steam exploded pellets, pine forest residue and straw. Tar was absorbed from the raw syngas using a Solid Phase Adsorption (SPA) column and was analyzed using GC-FID. Bench-scale experiments were also performed to investigate benzene conversion of LD slag and ilmenite at different oxidation levels.The findings of this study suggest that oxygen carriers can be used to decrease the tars generated in a dual fluidized bed system during gasification. Phases in LD slag possess catalytic properties, resulting in a decreased ratio of heavy tar components compared to both ilmenite and sand. Temperature and fuel load showed a significant effect on the tar generation compared to the circulation and steam ratio in this reactor system. Increased temperature generated lower tar yields and lower ratios of heavy tar components for LD slag in contrast to sand.

Distribution and particle-scale thermochemical property of biomass in the gasifier of a dual fluidized bed

Steam-gasification of biomass feedstock via dual fluidized bed has been proven to be an efficient and promising approach for the thermochemical conversion of biogenic fuels into the high-quality nitrogen-free syngas. However, very little is known about the actual behavior of the biomass fuels in the bed. In this work, the reactive gas-solid flow in a full-loop pilot-scale (1MWth) dual fluidized bed biomass gasifier is numerically simulated with the multiphase particle-in-cell approach. The aim of this study is to explore the species segregation and particle-scale thermochemical property of biomass material. The results show that the species segregation induced by the size and density differences gives rise to a preferential distribution of biomass material close to the bed surface. The undesired segregation phenomenon inhibits the sufficient mixing of biomass and heat carrier, and results in the particular distribution of thermochemical property of biomass material, which therefore requires further optimization of geometrical design. Lagrangian tracking of solid phase provides insight in the particle-scale thermochemical properties (i.e., temperature, heat transfer coefficient, carbon content) of biomass material. The observation of lognormal distribution and large value in the splash zone of biomass heat transfer coefficient enhances the understanding of the heat transfer behavior of biomass material. Biomass locating near the bed surface has the largest temperature. The promoting effect of wider size distribution of heat carrier on biomass dispersion provides a guidance for the process operation. The results of this study provide a fundamental understanding of the high-fidelity microscopic biomass property in the gasifier of dual fluidized bed, which will be beneficial for the optimization of the process designs.

Applicability of a kinetic model for catalytic conversion of tar and light hydrocarbons using process-activated ilmenite

This work evaluates further the applicability of a previously formulated kinetic model that describes the conversion of tar and light hydrocarbons during the catalytic upgrading of a raw gas derived via biomass gasification. The model is based on the mechanism through which tar and light hydrocarbons are converted, and the distribution of CO/CO2 and tar/light hydrocarbons as the products of the destruction of the parent tar and light hydrocarbons. Furthermore, the model uses a pseudo tar to represent all the in situ-formed tar and light hydrocarbons. The experiments used an ilmenite catalyst that was activated in the Chalmers 12-MWth boiler, and a mature tar-containing raw gas that was produced in the Chalmers 2–4-MWth dual fluidized bed biomass gasifier. The temperature levels (800 °C and 850 °C) and ilmenite particle size ranges (45–90 µm and 125–180 µm) were investigated. Through fitting the model to the experimental data, the composition of the upgraded gas as a function of gas–solid contact time was derived for the operating conditions studied. The previously proposed conversion network for tar and light hydrocarbons, and the empirical model coefficients for ilmenite (125–180 µm) at 850 °C were validated. Overall, the obtained results confirm that the proposed model is able to describe the upgrading process. Further development of the proposed model and application of the results obtained for ilmenite in designing gas upgrading strategies are discussed.

Fate of Polycyclic Aromatic Hydrocarbons during Tertiary Tar Formation in Steam Gasification of Biomass

This work investigates the fate of polycyclic aromatic hydrocarbons (PAH) in relation to the process severity in the steam- and H2-containing reaction environment of steam gasification of biomass. The focus is on the regimen of tertiary tar formation during the gasification in a fluidized bed gasifier; the tertiary tar is tar that is predominantly aromatic compounds. The process severity reflects the following operating conditions: temperature, gas residence time in the reactor, and contact time between the gas phase and catalytic bed material. The conducted experiments employed a raw gas upgrading process downstream of the gasifier. A mature tar-containing raw gas produced in the Chalmers 2–4-MWth dual fluidized bed biomass gasifier was upgraded in a bench-scale, bubbling fluidized bed reactor, in which inert silica sand and a naturally occurring ilmenite catalyst were used as the bed materials. The obtained results show that, following the increased process severity, the growth of PAH can either enhance or suppress. For the growth of PAH being suppressed, it is required that the process severity is sufficient to convert steam and H2 into the reactive hydrogen intermediates that prevent the combination of the carbon-containing species. To ensure this, the application of silica sand as a bed material requires an operating temperature of 850 °C and a gas residence time of >11.5 s, while the use of ilmenite requires an operating temperature of 800 °C and a gas residence time of >3.4 s, together with a gas–solid contact time of about 0.7 s. In particular, the results obtained for ilmenite encourage the use of naturally occurring catalysts in fluidized bed gasifiers, despite the fact that their catalytic activities are lower than that of synthetic catalysts.

Performance evaluation at different process parameters of an innovative prototype of biomass gasification system aimed to hydrogen production

Gasification is currently considered one of the most effective technologies to produce power and hydrogen from biomass and the scope of this work is to determine performances of such an energy system in terms of production of pure hydrogen. The overall plant has been simulated by means of ChemCAD® software. It is composed of a dual fluidized bed biomass gasifier with Catalytic Filter candles (CF), innovatively integrated within the gasification reactor, Water Gas Shift reactor (WGS), equipped with a desulphurization reactor (DeS), and Pressure Swing Adsorber system (PSA), coupled with a micro gas-turbine system (mGT) as an auxiliary power generator aimed to supply inner needs of electricity. Research and pilot scale tests on gasifier, CF, WGS reactor and PSA unit allowed to validate the model. The components have been integrated in a relatively small size and innovative plant (1MWth as biomass input). This integration entails highly pure H2 and major efficiency. The model allowed a sensitivity analysis of basic parameters as WGS temperature, residence time and steam to biomass ratio (SB). Important results have been generated reaching a maximum hydrogen yield of 75.2gH2/kgbio and a maximum efficiency, HHV based, of 55.1%. Optimal compromise of results was obtained with SB equal to 2, WGS reactor temperature at 300°C and residence time at 0.8s. Finally, even the chance to generate hydrogen without consumption of auxiliary fuel (by exploiting off gas and waste heat recovery) has been investigated.

Compartment model for a dual fluidized bed biomass gasifier

A variety of novel processes are being proposed in order to face global challenges such as degradation of the environment and the efficient utilization of energy. Modeling and simulation tools play a crucial role in the understanding and enhancing of the execution, design and construction of these processes. Although different computational tools are available to quantify the process at different levels, they are normally utilized independently and on a stand-alone basis. This decoupled approach may undermine the true potential of these tools. The present study highlights the advantages of interlinked process modeling at different levels. This study focuses on a promising gasification technology, namely the dual fluidized bed gasifier. A computational fluid dynamics (CFD) model was used to understand the flow patterns inside a fluidized bed. This elevated the understanding of the hydrodynamics of the gasifier freeboard, which is neglected by the conventional two-phase methodology. The CFD simulation was utilized to perform a residence time distribution (RTD) analysis of the reactor. Four tracer approaches namely the frozen velocity approach, the snapshot approach, the data sampling approach and the transient approach, were compared. The RTD analysis formed the basis of a steady-state compartment model that was developed in ASPEN Plus simulation software. The ASPEN Plus gasifier model decoupled the pyrolysis, gasification, and combustion sections of the gasifier to affect a better comprehension of the process and results. The model predicated satisfactory results upon validation. Additionally, the model could also be used to predict the output for different biomass feedstocks.

Comparing Active Bed Materials in a Dual Fluidized Bed Biomass Gasifier: Olivine, Bauxite, Quartz-Sand, and Ilmenite

Active bed materials are in this work investigated for in situ gas upgrading of biomass-derived gas. Previous research on in situ gas upgrading has focused on assessing gas quality, in terms of the concentrations of tar and permanent gases. Other aspects of fuel conversion, such as char conversion and the impact of oxygen transport on the final gas, are not as well documented in the literature on gasification. In this paper, the overall biomass conversion in a dual fluidized bed biomass gasifier is investigated in the presence of the catalytic material olivine and the alkali-binding material bauxite. The impact of these materials on fuel conversion is described as the combination of four effects, which are induced by the bed material: thermal, catalytic, ash-enhanced catalytic effect, and oxygen transport. Quartz-sand and ilmenite are here used as the reference materials for the thermal and the oxygen transport effects, respectively. Olivine and bauxite show activity toward tar species compared to quartz-...

Analysis of Operation Parameters in a Dual Fluidized Bed Biomass Gasifier Integrated with a Biomass Rotary Dryer: Development and Application of a System Model

An integrated system model was developed in UniSim Design for a dual fluidized bed (DFB) biomass gasifier and a rotary biomass dryer using a combination of user-defined and built-in unit operations. A quasi-equilibrium model was used for modelling biomass steam gasification in the DFB gasifier. The biomass drying was simulated with consideration of mass and energy balances, heat transfer, and dryer’s configuration. After validation using experimental data, the developed system model was applied to investigate: (1) the effects of gasification temperature and steam to biomass (S/B) ratio on the gasification performance; (2) the effect of air supplied to the fast fluidized bed (FFB) reactor and feed biomass moisture content on the integrated system performance, energy and exergy efficiencies. It was found that gasification temperature and S/B ratio have positive effects on the gasification yields; a H2/CO ratio of 1.9 can be achieved at the gasification temperature of 850 °C with a S/B ratio of 1.2. Consumption of excessive fuel in the system at higher biomass feed moisture content can be compensated by the heat recovery such as steam generation while it has adverse impact on exergy efficiency of the system.

Model development and validation: Co-combustion of residual char, gases and volatile fuels in the fast fluidized combustion chamber of a dual fluidized bed biomass gasifier

A one-dimensional steady state model has been developed for the combustion reactor of a dual fluidized bed biomass steam gasification system. The combustion reactor is operated as fast fluidized bed (riser) with staged air introduction (bottom, primary and secondary air). The main fuel i.e., residual biomass char (from the gasifier), is introduced together with the circulating bed material at the bottom of the riser. The riser is divided into two zones: bottom zone (modelled according to modified two phase theory) and upper zone (modelled with core-annulus approach). The model consists of sub-model for bed hydrodynamic, conversion and conservation. Biomass char is assumed to be a homogeneous matrix of C, H and O and is modelled as partially volatile fuel. The exit gas composition and the temperature profile predicted by the model are in good agreement with the measured value.