Dual Fluidized Bed Steam Gasification Research Articles

Minimum-Variance Model Predictive Control for Dual Fluidized Bed Circulation Control

Dual fluidized bed steam gasification enables the production of gaseous energy carriers from woody biomass or biogenic residues. The circulation of bed material in dual fluidized bed gasifiers strongly affects the process behavior. Therefore, precise control of the bed material circulation is desired. This paper presents a control algorithm addressing two aspects of the given problem setting: On the one hand, redundant control actuators are available. Typically, there are several air streams to the reactors influencing the bed material circulation. On the other hand, only black box models with uncertainties in their model parameters are available for model-based control design. The presented control algorithm uses a model predictive controller considering known uncertainties in the model parameters and drives the process in a region with the lowest model uncertainties. This results in an improvement of the closed-loop performance when the actual plant deviates from the internal model used for the model predictive control predictions. Simulations show 66 % less offset from the design trajectory with the presented algorithm when compared to a standard model predictive controller.

Dynamic modeling of dual fluidized bed steam gasification for control design

Dual fluidized bed steam gasification allows the production of high-value product gas from woody biomass or biogenic residuals. Advanced control concepts such as model predictive control are promising approaches to improve the process performance and efficiency. These control techniques require dynamic models of the process that can predict the plant’s behavior as a function of the manipulated variables. This paper presents a gray-box modeling approach based on mass and energy balances to obtain a mathematical description of the temperatures inside the two reactors and the total mass flows leaving the reactors. The model incorporates data-driven components where first-principle modeling is hardly possible with reasonable effort. An artificial neural network is utilized to model the bed material circulation between the two reactors. Experiments were carried out at a 100 kW pilot plant to generate measurement data both for system identification and model validation. Simulations verify that the model achieves reliable predictions for the dual fluidized bed gasification process.

Experimental investigation on the methanation of hydrogen-rich syngas in a bubbling fluidized bed reactor utilizing an optimized catalyst

Catalytic methanation processes allow the production of natural gas substitutes on a sustainable and renewable basis. This study investigates the catalytic methanation of syngas from dual fluidized bed steam gasification of biomass in an innovative bubbling fluidized bed methanation reactor with an optimized catalyst. Syngas from conventional gasification and a novel combination with syngas from sorption enhanced reforming were investigated. The applied fluidized bed reactor allowed an almost isothermal operation with optimal reaction temperatures between 320 °C–360 °C. Simultaneously, no chemical deactivation or mechanical attrition during 200 h of operation indicates a high long-term stability of the catalyst. The methane concentration downstream the methanation reactor increased from 43 to 74 vol.-%db through the methanation of a hydrogen-rich syngas produced via sorption enhanced reforming. Simultaneously, the methane yield is doubled to 95% and the hydrogen, carbon monoxide and carbon dioxide conversions are improved. Furthermore, it could be shown that a CO2 content below 1 vol.-%db is feasible in the (raw) synthetic natural gas, allowing grid injection without CO2 separation. The results indicate that sorption enhanced reforming in combination with an optimized fluidized bed methanation can lead to technical and economic improvements in sustainable synthetic natural gas production.

Integration of dual fluidized bed steam gasification into the pulp and paper industry

The pulp and paper industry represents an industry sector which is characterised by its already high degree of sustainability. Biomass is a renewable input material, and typically highly developed recovery cycles minimise the loss of chemicals used in the pulping process. However, certain parts of the recovery cycle are still operated on fossil fuels. This study deals with the substitution of the fossil-based gaseous fuel with product gas from biomass gasification.Gasification experiments have shown that bark available at pulp and paper mills is suitable to produce a product gas via dual fluidised bed steam gasification as a promising substitute for natural gas. Based on the comparison of process layouts regarding the separation of non-process elements, separation efficiency is derived for different setups. To ensure operational security of the chemical recovery cycle, comprehensive gas cleaning including heat exchangers, a particle filter, and a liquid scrubber unit is advised. The gas flow of fuel gas into the gas burner is increased as the heating value of the product gas is accordingly lower in comparison to natural gas. Furthermore, adaptions of the gas burner might be necessary to address the earlier ignition of the H2-rich product gas compared to natural gas.

Solubility of Tar Model Compounds in Various Solvents for Tar Removal in a Dual Fluidized Bed Biomass Gasification Process

Production of high quality product gas via biomass steam gasification is a promising technology. However, impurities in the product gas, namely tars, cause problems in the downstream gas processing operations and thus they need to be removed efficiently. Oil scrubbing is an effective solution for tar removal due to its non-polar characteristic which is similar to tar nature. In this research, solubility values of five simulated tar compounds were experimentally investigated for selecting the new scrubbing solvent. The simulated tar compounds investigated represent those found in the dual fluidized bed steam gasification of wood biomass, which are: naphthalene, biphenyl, anthracene, fluoranthene, and pyrene. The scrubbing solvents tested in this research are classified into biodiesels, vegetable oils, and diesel. Biodiesel used are rapeseed methyl ester (RME) and 2 different palm methyl esters (denoted as PME1 and PME2). Vegetable oils are sunflower oil, refined palm oil, Thai rice bran oil, and crude palm oil. All of the solubility tests were performed in the laboratory-scale test-rig at 30, 50, 70, and 80°C. Biodiesels are found to be the effective solvent in dissolving the tar compounds. PME1 shows the similar tar removal performance to RME but is more readily available; therefore, PME1 is chosen to be used as a scrubbing solvent at the Thailand 1 MWel prototype DFB gasifier at Nong Bua district in Nakhon Sawan province, Thailand.

Tar content and composition during a low-temperature steam gasification of rice husks

The research work extends our recent empirical knowledge on the rice husks, a carbonaceous solid material. This solid biofuel option was characterized with a potential net heating value of 16–17 MJ/kg, significant high ash deformation temperature recorded above 1450 °C, capable and considerable for thermochemical conversion systems. An experimental performance on a dual fluidized bed steam gasifier using rice husks pellets was carried out at a low temperature between 600 and 650 °C and fuel power of 100 kWth. Pure steam was used as a gasification agent in fluidization at a steam to fuel ratio of 1.2 kg/kg on dry basis. Calcite with mainly CaCO3 in composition was used as bed material for the reactor. Tar content and composition in the product gas stream were analyzed with a gas chromatograph coupled with mass spectrometer (GC–MS). Significant high amounts of total GC–MS tar components (without benzene, ethylbenzene, and xylene) and gravimetric tar (higher molecular tar) were detected at 29.44 and 17.82 g/m3 on dry basis. Benzene content was, respectively, presented on a value of 9 g/m3 on dry basis. Obtained products consisted of relevant amount of phenol and heterocyclic aromatic tars (class 2). The specific composition of common and additional analytes presented in GS–MS tar was summarized for better understanding of tar formation and reduction phenomena of rice husk gasification.

CO2 gasification of biogenic fuels in a dual fluidized bed reactor system

A 100 kWth dual fluidized bed steam gasification pilot plant has been developed at TU Wien to convert different types of biogenic fuels into a valuable product gas. In this paper, the conversion of different biogenic fuels in combination with the utilization of CO2 as alternative gasification agent was investigated in the mentioned pilot plant. For this purpose, five experimental campaigns were carried out aiming at the investigation of softwood as reference fuel, and rapeseed cake, bark and lignin as alternative fuels. Pure olivine as well as a mixture (90/10 wt%) of olivine and limestone were used as bed materials. The product gas compositions of the different biogenic fuels changed depending on the elemental composition of the biogenic fuels. Thus, a high amount of carbon in the fuel enhanced CO formation, whereas an increased content of oxygen led to higher CO2 contents. Additionally, the presence of alkali metals in the biomass ash favoured the production of CO. The addition of limestone enhanced the H2 and CO contents via the water gas shift reaction as well as steam and dry reforming reactions, but had no significant effect on tar contents. Overall, this paper presents the feasibility of the dual-fluidized bed gasification process of different biogenic fuels with CO2 as gasification agent.

Syngas for biorefineries from thermochemical gasification of lignocellulosic fuels and residues\u20145\xa0years\u2019 experience with an advanced dual fluidized bed gasifier design

In many processes proposed for biorefineries, recycling procedures, and industrial or agricultural production processes, residue is generated which could be further transformed by thermochemical conversion via gasification. The technology of dual fluidized bed steam gasification is capable of producing a valuable product gas out of such residue. The generated nitrogen-free product gas can be used for heat and power production and is suitable for separating gases (e.g. hydrogen). However, if the product gas is cleaned, its use as syngas is more beneficial for manufacturing renewable chemical substances, like synthetic natural gas, methanol, Fischer–Tropsch liquids, or mixed alcohols. This paper presents the results of experimental research from gasification test runs of different biogenic fuels, carried out with an advanced 100 kW pilot plant over the last 5 years at TU Wien. The focus is to provide an overview of measured results validated by mass and energy balances and to present key calculated performance indicating key figures of the test runs. In this way, the influence of various operational parameters and the composition of the product gas are evaluated. The presented results form the basis for the proper design of suitable gas-cleaning equipment. Subsequently, the clean syngas is available for several synthesis applications in future biorefineries.

A kinetic model of carbonation and calcination of limestone for sorption enhanced reforming of biomass

The dual fluidized bed steam gasification process aims for the production of a nitrogen-free product gas via a steam blown gasification reactor and an air blown combustion reactor. Limestone as bed material enables the in-situ removal of carbon dioxide from the gasification reactor and therefore hydrogen contents in the product gas up to 75 vol.-%db can be gained (sorption enhanced reforming). Thereby, the limestone experiences cycles of carbonation (CaO + CO2 ->CaCO3) and calcination (CaCO3 ->CaO + CO2). Typically, kinetics are of great relevance for heterogeneous gas-solid reactions. Thus, the influence of both, kinetics of carbonation and calcination, on the SER process is investigated and a model of the SER process is established. The theoretical results are compared to those experimentally gathered and show good accordance for low to medium cycle rates of the bed material. The calcination reaction is identified as limiting step during the process due to insufficient residence time and an unfavorable temperature profile in the combustion reactor.

Layer formation on K-feldspar in fluidized bed combustion and gasification of bark and chicken manure

Understanding layer formation on bed materials used in fluidized beds is a key step for advances in the application of alternative fuels. Layers can be responsible for agglomeration-caused shut-downs but they can also improve the gas composition in fluidized bed gasification. Layers were observed on K-feldspar (KAlSi3O8) impurities originating from the combined heat and power plant Senden which applies the dual fluidized bed (DFB) steam gasification technology. Pure K-feldspar was therefore considered as alternative bed material in DFB steam gasification. Focusing on the interactions between fuel ash and bed material, K-feldspar was tested in combustion and DFB steam gasification atmospheres using different fuels, namely Ca-rich bark, Ca- and P-rich chicken manure, and an admixture of chicken manure to bark. The bed particle layers formed on the bed material surface were characterized using combined scanning electron microscopy and energy-dispersive X-ray spectroscopy; area mappings and line scans were carried out for all samples. The obtained data show no essential influence of operational mode on the layer-formation process. During the combustion and DFB steam gasification of Ca-rich bark, a layer rich in Ca formed while K was diffusing out of the layer. The use of Ca- and P-rich chicken manure inhibited the diffusion of K, and a layer rich in Ca and P formed. The addition of P to bark via chicken manure also changed the underlying layer-formation processes to reflect the same processes as observed for pure chicken manure.

Dual fluidized bed steam gasification: Change of product gas quality along the reactor height

The impact of the counter-current column of the gasification reactor of a 100 kWth dual fluidized bed steam gasification pilot plant on the product gas quality was investigated. Through the advanced design of the gasification reactor by operating the lower part as bubbling bed and the upper part as counter-current column, the gas-solid interactions between downward flowing hot bed material particles with upwards flowing product gas could be enhanced. This was realized by equipping the counter-current column with constrictions, which increase the residence time and the bed material hold-up. Thus, the conversion efficiency of the fuel including the tar was improved. For the investigations three different experimental campaigns converting softwood pellets using a mixture of olivine and limestone (50/50 wt.-%), a mixture of feldspar and limestone (50/50 wt.-%), and 100 wt.-% quartz as bed materials were conducted. Higher H2 contents and lower contents of higher hydrocarbons could be detected along the height of the counter-current column. Especially heavy tar compounds could be reduced significantly. These two effects are explained by enhanced water gas shift and steam reforming reactions. In case of catalytically inactive quartz, only thermal effects are available and therefore lower effects on tar reduction could be obtained.

Assessment of correlations between tar and product gas composition in dual fluidized bed steam gasification for online tar prediction

Thermochemical conversion of biomass feedstock via dual fluidized bed steam gasification is a well-proven technology used to produce a medium calorific product gas for various applications in the energy or transportation sector or for chemical syntheses. At unfavorable gasification conditions, undesirable high amounts of tar, which are aromatic hydrocarbons, are present in the product gas. High tar contents are a major problem, and they lead to uneconomic operation due to sharply diminished quality of product gas or unexpected plant shut downs due to fouling of the product gas coolers. Currently, tar content is measured with a discontinuous wet-chemical analysis method, which needs several hours of sample preparation to receive the final tar content. The aim of this study is to establish valid correlations between online measured permanent gas components in the product gas and its tar content. The results show that hydrogen, methane, and ethene concentrations are strongly related to the tar content in the product gas, while the carbon monoxide and carbon dioxide content did not show a clear correlation. Using these correlations with online measured gas components provides the possibility of a direct and prompt response of a plant operator in case of unfavorable gasification conditions. Additionally, an optimization of the plant operation can be conducted and thereby, the operation hours and, consequently, the economic efficiency are improved.

A general method for the determination of the entrainment in fluidized beds

The measurement of the entrainment is complicated in a hot fluidized bed. Therefore, the entrainment or cycle rate of the total bed material inventory is typically estimated via mass and energy balance for e.g. the sorption enhanced reforming process. The sorption enhanced reforming process is an advancement of the conventional dual fluidized bed steam gasification process. It aims for the production of a hydrogen-rich product gas. The process is highly depends on bed material cycle rate, respectively bed material entrainment. Only limited knowledge about simple modelling methods of the entrainment of a fluidized bed are available to validate the cycle rate calculated by mass and energy balances. Therefore, measurements of the bed material entrainment at two different plants were conducted and a model based on easily accessible parameters for the entrainment of fluidized bed risers is presented. It is also verified, that the results from mass and energy balances fit well with the measured data for the entrainment.

Techno-economic assessment of biomass-based natural gas substitutes against the background of the EU 2018 renewable energy directive

This paper investigates biomethane and BioSNG production processes against the background of the 2018 renewable fuel directive of the European Union (EU). The investigated biomethane processes use manure, clover grass, and grass silage as feedstock, are based on membrane separation gas upgrading processes, and generate 1.0 and 4.8 MW of biomethane. The investigated BioSNG processes use wood chips as feedstock, are based on the dual fluidized bed steam gasification technology and the VESTA SNG process from Amec Foster Wheeler, and generate 6.1, 12.2, and 49.1 MW BioSNG. The techno-economic assessment shows that the biomethane processes have, in general, a lower break-even price for the generated natural gas substitute. However, their scalability is limited and at larger scale (49.1 MW BioSNG capacity), the BioSNG processes become competitive. The 1.0 MW biomethane and all BioSNG plants meet the 2018 renewable fuel directive of the EU. In contrast, the 4.8 MW biomethane process does not meet the directive as the feedstock, which is mainly based on energy crops, causes significant CH4 and CO2 emissions.

Fuel flexible gasification with an advanced 100 kW dual fluidized bed steam gasification pilot plant

Steam gasification enables the conversion of heterogeneous solid fuels into homogeneous gaseous energy carriers. The utilization of biogenic residues and waste fractions as fuel for this technology offers a sustainable waste management solution to produce heat and power, secondary fuels and valuable chemicals after several cleaning and upgrading steps of the product gas. However, residues and waste fuels show unfavorable properties for gasification and, therefore, cause technical challenges. This paper presents experimental results carried out at an advanced 100 kWth dual fluidized bed steam gasification pilot plant from nine single test runs. In the following the fuels that were gasified will be listed: (i) Five biogenic fuels, mainly residues: softwood, sugar cane bagasse, exhausted olive pomace, bark and rice husks; (ii) two different waste-derived fuels: a municipal solid waste fraction and a shredder light fraction; and (iii) a mixture of municipal solid waste fraction with a 25% blending of lignite based on lower heating value as well as pure lignite. Thereby, various product gas qualities were generated. The presented results offer the basis for a sustainable and promising waste management solution for the tested waste fuels.

Influence of different bed material mixtures on dual fluidized bed steam gasification

Within this paper, investigations to convert softwood with four different types of bed materials in the 100 kWth dual fluidized bed steam gasification pilot plant at TU Wien are presented and discussed. The results of ten different experiments were compared. Quartz, olivine and feldspar were mixed with limestone in mass ratios of 100/0, 90/10, 50/50 and 0/100. Limestone was used due to its catalytic activity at high temperatures as CaO and thus enhanced tar, char and water conversion of quartz, olivine and feldspar. The admixture of limestone to quartz, olivine and feldspar shifted the product gas compositions towards higher hydrogen and carbon dioxide and lower carbon monoxide contents. By using 100 wt.-% limestone as bed material a hydrogen content of 47.4 vol.-% could be generated. Additionally, the tar concentrations as well as the tar dew points decreased and especially the heavy tar compounds could be reduced. Already small amounts of limestone (<10 wt.-%) to the bed material mixture influenced tar reduction in a positive way. The low abrasion resistance of limestone resulted in increasing dust contents by increasing its amount. However, this could be balanced by the specific design of the separation system of the advanced pilot plant.

A review on dual fluidized bed steam gasification of pelletized solid biofuel

A review on dual fluidized bed steam gasification of pelletized solid biofuel

Experimental development of sorption enhanced reforming by the use of an advanced gasification test plant

Dual fluidized bed steam gasification of solid fuels enables the production of a nitrogen-free medium calorific product gas. Several commercial industrial sized plants are already operating the dual fluidized bed gasification process. The present work illustrates an improvement of the gasification process itself by the use of wood as fuel and calcite as bed material to produce a hydrogen-rich gas. Hereby, in the combustion reactor calcite (CaCO3) is calcined to calcium oxide (CaO) by the release of carbon dioxide (CO2) at high temperatures (∼850 °C). In the gasification reactor, CaO acts as a sorbent for CO2 from the produced calorific gas stream at lower temperatures (∼650 °C). Experimental results from two different test plants are compared by the use of a process simulation software. The results illustrate the observed improvements with respect to process performance by the application of an advanced reactor design operating this so called sorption enhanced reforming process. It is shown that the water conversion rate in the gasification reactor is raised up to 36% and the overall cold gas efficiency was improved from 58 to 71% with the advanced reactor design. Since the sorption enhanced reforming process is highly dependent on temperature, the influence of gasification temperature and further, bed material circulation rate are discussed as well. It is demonstrated, that a hydrogen (H2) to carbon monoxide (CO) ratio between 9 and 2 can be set by operating the gasification reactor with a certain temperature. Further, it is shown that a low bed material circulation rate is favorable to obtain high hydrogen (H2) contents in the product gas.

Advanced dual fluidized bed steam gasification of wood and lignite with calcite as bed material

This paper presents experimental results with a new generation of a 100 kW th dual fluidized bed steam gasification pilot plant with calcite as bed material, converting wood and lignite in separate test runs into product gas. The results are compared to experiments with the same fuels with olivine as bed material and the previous generation of the gasification pilot plant at TU Wien. The highly catalytic active calcium oxide shifted the product gas composition towards higher hydrogen and carbon dioxide and lower carbon monoxide content. The tar amount was decreased and the tar composition changed, resulting in lower tar dew points. The dust content in the product gas was reduced with the advanced pilot plant design with calcite in comparison to the classic design with olivine. Therefore, attrition of bed material was decreased by utilizing the advanced design and calcite with its benefits can be used without profuse continuous replacement of bed material.

CPFD simulations of an industrial-sized dual fluidized bed steam gasification system of biomass with 8 MW fuel input

Dual fluidized bed (DFB) systems for biomass gasification consist of two connected fluidized beds with a circulating bed material in between. Inside such reactor systems, rough conditions occur due to the high temperatures and the movement of the bed material. Computational fluid dynamics calculations are a useful tool for investigating fluid dynamics inside such a reactor system. In this study, an industrial-sized DFB system was simulated with the commercial code CPFD Barracuda. The DFB system is part of the combined heat and power (CHP) plant at Güssing, situated in Austria, and has a total fuel input of 8MWth. The model was set up according to geometry and operating data which allows a realistic description of the hot system in the simulation environment. Furthermore, a conversion model for the biomass particles was implemented which covers the drying and devolatilization processes. Homogeneous and heterogeneous reactions were considered. Since drag models have an important influence on fluidization behavior, four drag models were tested. It was found that the EMMS drag model fits best, with an error of below 20%, whereas the other drag models produced much larger errors. Based on this drag law, further simulations were conducted. The simulation model correctly predicts the different fluidization regimes and pressure drops in the reactor system. It is also able to predict the compositions of the product and flue gas, as well as the temperatures inside the reactor, with reasonable accuracy. Due to the results obtained, Barracuda seems suitable for further investigations regarding the fluid mechanics of such reactors.