Agglomeration in fluidised bed gasification of biomass

Three promising biomass fuels for southern Mediterranean regions were tested for their agglomeration tendency in an atmospheric lab-scale fluidised bed (FB) gasifier using quartz and olivine as bed materials. The defluidisation temperatures of the energy crops Giant Reed (Arundo donax L.) and Sweet Sorghum bagasse were respectively approx. 790 °C and 810 °C, in both bed materials, while the agro industrial residue olive bagasse caused defluidisation of the quartz bed at 830 °C and olivine bed at > 850 °C. Agglomerates from these tests were analysed with SEM/EDS. Coatings and necks between bed particles were formed due to ash derived potassium silicate melt. For the first two fuels cluster-type agglomerates around remains of char particles were observed. Thermodynamic equilibrium simulations of each chemical system were performed to cross examine the predicted ash melting temperatures and chemistry with experimental findings. Predictions of potassium liquid compounds, like K₂O·SiO_2(l) were verified by EDS analyses on the particle coatings. FB gasification of olive bagasse resisted defluidisation up to higher temperatures because of its lower potassium and higher calcium content, especially in the case of olivine bed. The latter experimental finding coincided with thermodynamic predictions.     

Reactive bed materials for improved biomass gasification in a circulating fluidised bed reactor

Reactive bed materials for the optimisation of the biomass gasification in a fast internally circulating fluidised bed (FICB) reactor system were tested under sulphur-free (S-free) and H₂S enriched conditions in a micro-scale fluidised bed reactor. In the experiments, the bed materials (natural olivine, (Fe_xMg_1-x)₂SiO₄, perovskite-type oxides, Ba_0.3Sr_0.7Fe_0.9Mn_0.1O_3-δ and La_0.65Sr_0.35Cr_0.5Mn_0.5O_3-δ, Gd_0.1Ce_0.9O₂ and a natural calcite, CaCO₃) were examined under realistic redox-cycling conditions to study their oxygen capacity and release, their catalytic activity towards toluene reforming as well as their mechanical and chemical stability.It was found that the synthesised materials outperform the natural materials as reactive bed materials for the FICFB process under S-free atmosphere. Gd_0.1Ce_0.9O₂ has better catalytic properties, perovskites show a higher oxygen storage capacity. However, in the presence of H₂S, the perovskite loose their oxygen capacity, while calcite can form a sulphide/sulphate cycle which allows for significant oxygen capacity. Additionally, the catalytic activity goes up. Therefore, under real conditions, the two natural materials, calcite and to a lower extent olivine, have clear advantages with respect to price, catalytic activity and oxygen capacity.     

Effect of biomass particle size and air superficial velocity on the gasification process in a downdraft fixed bed gasifier. An experimental and modelling study

A one-dimensional stationary model of biomass gasification in a fixed bed downdraft gasifier is presented in this paper. The model is based on the mass and energy conservation equations and includes the energy exchange between solid and gaseous phases, and the heat transfer by radiation from the solid particles. Different gasification sub-processes are incorporated: biomass drying, pyrolysis, oxidation of char and volatile matter, chemical reduction of H₂, CO₂ and H₂O by char, and hydrocarbon reforming. The model was validated experimentally in a small-scale gasifier by comparing the experimental temperature fields, biomass burning rates and fuel/air equivalence ratios with predicted results. A good agreement between experimental and estimated results was achieved. The model can be used as a tool to study the influence of process parameters, such as biomass particle mean diameter, air flow velocity, gasifier geometry, composition and inlet temperature of the gasifying agent and biomass type, on the process propagation velocity (flame front velocity) and its efficiency. The maximum efficiency was obtained with the smaller particle size and lower air velocity. It was a consequence of the higher fuel/air ratio in the gasifier and so the production of a gas with a higher calorific value.     

Experimental test on a novel dual fluidised bed biomass gasifier for synthetic fuel production

This article presents a preliminary test on the 150 kW_th allothermal biomass gasifier at Mid Sweden University (MIUN) in Härnösand, Sweden. The MIUN gasifier is a combination of a fluidised bed gasifier and a CFB riser as a combustor with a design suitable for in-built tar/CH₄ catalytic reforming. The test was carried out by two steps: (1) fluid-dynamic study; (2) measurements of gas composition and tar. A novel solid circulation measurement system which works at high bed temperatures is developed in the presented work. The results show the dependency of bed material circulation rate on the superficial gas velocity in the combustor, the bed material inventory and the aeration of solids flow between the bottoms of the gasifier and the combustor. A strong influence of circulation rate on the temperature difference between the combustor and the gasifier was identified. The syngas analysis showed that, as steam/biomass (S/B) ratio increases, CH₄ content decreases and H₂/CO ratio increases. Furthermore the total tar content decreases with increasing steam/biomass ratio and increasing temperature. The biomass gasification technology at MIUN is simple, cheap, reliable, and can obtain a syngas of high CO + H₂ concentration with sufficient high ratio of H₂ to CO, which may be suitable for synthesis of methane, DME, FT-fuels or alcohol fuels. The measurement results of MIUN gasifier have been compared with other gasifiers. The main differences can be observed in the H₂ and the CO content, as well as the tar content. These can be explained by differences in the feed systems, operating temperature, S/B ratio or bed material catalytic effect, etc.     

Adiabatic fixed bed gasification of dairy biomass with air and steam

Including dairy biomass (DB) as feedstock in gasification processes for locally based power generation could mitigate the environmental impact from DB produced in large US farms (56 million dry tons per year) and fossil-fuels emissions, since biomass is a CO₂ neutral fuel. The current paper presents experimental results obtained from adiabatic, fixed bed gasification of DB using air and steam as oxidizers. The effect of equivalence ratio (ER) and steam to fuel ratio (S:F) ratio on temperature profile, gas composition (CO, CO₂, H₂, N₂, CH₄, and C₂H₆), gross heating value (HHV) and energy conversion efficiency (ECE) are discussed. The results show that the peak temperature (T_peak), ECE, and CO decrease and H₂ and CO₂ increase with increase in ER; the increase in S:F at same ER increases H₂, CO₂, CH₄, HHV, and ECE, and decreases CO.     

Characteristics of air-blown gasification in a pebble bed gasifier

High temperature air-blown gasification is a new concept to utilize the waste heat from gasifier that is called multi-staged enthalpy extraction technology. This process was developed to solve the economic problems due to air separation costs for the oxygen-blown as a gasifying agent. In this study, we have constructed a pebble bed gasifier and operated it by controlling the pebble size and bed height with three different types of coal (Kideco, Datong and Drayton coal). As a result, we can produce syngas with a calorific value of 700 kcal/Nm³ at an air temperature of 650 °C; the performance of high temperature air gasification was strong in the order of Kideco coal, Datong coal and Drayton coal. Also, from the data of the exterior analysis of slag that is attached to the surface of pebbles, we can know that the iron component is considerably high. This means the increase in restored metallic iron component seems to contribute to the solidification of slag.     

Interpretation of biomass gasification yields regarding temperature intervals under nitrogen–steam atmosphere

Gasification of some agricultural waste biomass samples (sunflower shell, pine cone, cotton refuse, and olive refuse) and colza seed was performed using a thermogravimetric analyzer at temperatures up to 1273 K with a constant heating rate of 20 K/min under a dynamic nitrogen–steam atmosphere. Derivative thermogravimetric analysis profiles of the samples were derived from the non-isothermal thermogravimetric analysis data. Gasification yields of the biomass samples at temperature intervals of 473–553 K, 553–653 K, 653–773 K, 773–973 K, and 973–1173 K were investigated considering the successive stages of “evolution of carbon oxides”, “start of hydrocarbon evolution”, “evolution of hydrocarbons”, “dissociation”, and “evolution of hydrogen”, respectively. Although, there were some interactions between these stages, some evident relations were observed between the gasification yields in a given stage and the chemical properties of the parent biomass materials.     

Axial gas profiles in a bubbling fluidised bed biomass gasifier

A 200 mm laboratory-scale atmospheric bubbling fluidised bed reactor has been used to obtain experimental data for the air/steam gasification of eucalyptus red gum wood chips and commercial wood pellets. The unique feature of this gasifier is the ability to examine the variations to axial gas composition along the bed height. At present no such data is available in the literature for biomass gasification. Gasification tests were performed using beds of; silica sand, char or clay to determine the effect of bed type on the gas composition. The behaviour of the major gas species (CO, H₂, CO₂) were observed to be strongly influenced by the water–gas shift reaction within the freeboard of the gasifier resulting in the exit gas being relatively similar in composition as compared to the in-bed variations. These small differences in gas composition for all bed types tested are the result of the achievement of equilibrium in the water–gas shift reaction. The influence of bed type exerted the most impact on the C₂–C₃ emissions (tar proxy) with the char bed found to best aid in their breakdown and to limit the amount of hydrocarbons surviving into the freeboard. The reduction of iron oxide (Fe₂O₃) content in the clay to a more reactive form of magnetite Fe₃O₄ by CO and H₂ in the product gas resulted in the clay bed to also exhibit a reduction in C₂–C₃ emissions compared to silica sand but less then char. The clay bed produced the highest calorific values for the producer gas. However, operation of the clay bed above 800 °C exhibits the potential for over reduction to form iron with subsequent agglomeration of the bed. Changing the fuel type to a biomass pellet resulted in higher emissions of C₁–C₃ hydrocarbons and in part its contribution is the result of primary particle fragmentation during screw feed conveying to the bed. Feeder location and bed design (conical or cylindrical) also exhibit an influence on hydrocarbon emissions.     

Modelling and simulation of coal gasification process in fluidised bed

A one-dimensional steady state mathematical model and a numerical algorithm have been developed to simulate the coal gasification process in fluidised bed. The model incorporates two phases, the solid and the gas. The gaseous phase participates in the emulsion (with the solid phase) and forms the bubble. The solid phase is composed of carbonaceous material, limestone and/or inert bed material. The model can predict temperature, converted fraction, and particle size distribution for the solid phase. For the gaseous phase, in both emulsion and bubble, it can predict profiles of temperature, gas composition, velocities, and other fluid-dynamic parameters. In the feed zone, a Gaussian distribution for the solid particle size is considered. This distribution changes due to attrition, elutriation, consumption and drag inside the reactor. A system of 29 differential and 10 non-linear equations, derived from the mass, energy and momentum balances for each phase, at any point along the bed height, are solved by the Gear and Adams Method. Experimental data from the Universidad de Antioquia and Universidad Nacional-Medellin have been used to validate the model. Finally, the model can be used to optimise the gasification process by varying several parameters, such as excess of air, particle size distribution, coal type, and geometry of the reactor.     

Feasibility study of a spouted bed gasification plant

We have studied the feasibility of building a biomass gasification plant with an innovative spouted bed reactor for distributed energy production. The process was simulated using a thermodynamic approach (concentrated parameter model) using LIBPF, a C++ process modelling library. A nominal size of about 100 kW total thermal power was chosen.     

The study of reactions influencing the biomass steam gasification process☆

Steam gasification studies were carried out in an atmospheric fluidised bed. The gasifier was operated over a temperature range of 700-900 °C whilst varying a steam/biomass ratio from 0.4 to 0.85 w/w. Three types of forestry biomass were studied: Pinus pinaster (softwood), Eucalyptus globulus and holm-oak (hardwood). The energy conversion, gas composition, higher heating value and gas yields were determined and correlated with temperature, steam/biomass ratio, and species of biomass used. The results obtained seemed to suggest that the operating conditions were optimised for a gasification temperature around 830 °C and a steam/biomass ratio of 0.6-0.7 w/w, because a gas richer in hydrogen and poorer in hydrocarbons and tars was produced. These conditions also favoured greater energy and carbon conversions, as well the gas yield. The main objective of the present work was to determine what reactions were dominant within the operation limits of experimental parameters studied and what was the effect of biomass type on the gasification process. As biomass wastes usually have a problem of availability because of seasonal variations, this work analysed the possibility of replacing one biomass species by another, without altering the gas quality obtained.     

生物质流化床气化炉气化过程的实验研究

在流化床生物质气化炉内 ,采用空气作气化剂 ,对七种农、林废弃物进行了气化实验研究。生成的燃气成分 :CO在 1 4 %～ 1 7%之间 ,H2 含量一般低于 1 0 % ,甲烷含量为 5%～ 1 0 %。燃气热值多数在 53 0 0～ 6 50 0 k J/ Nm3 ,气化效率 72 .6 %。实验结果表明 ,流化床生物质气化炉可用于生物质气化。     

Exposure of refractory materials during high-temperature gasification of a woody biomass and peat mixture

Finding resilient refractory materials for slagging gasification systems have the potential to reduce costs and improve the overall plant availability by extending the service life. In this study, different refractory materials were evaluated under slagging gasification conditions. Refractory probes were continuously exposed for up to 27 h in an atmospheric, oxygen blown, entrained flow gasifier fired with a mixture of bark and peat powder. Slag infiltration depth and microstructure were studied using SEM EDS. Crystalline phases were identified with powder XRD. Increased levels of Al, originating from refractory materials, were seen in all slags. The fused cast materials were least affected, even though dissolution and slag penetration could still be observed. Thermodynamic equilibrium calculations were done for mixtures of refractory and slag, from which phase assemblages were predicted and viscosities for the liquid parts were estimated.     

Ash behaviour of lignocellulosic biomass in bubbling fluidised bed combustion

The purpose of this work is to characterise biomass ash behaviour in a bubbling fluidised bed (BFB) combustion pilot plant (1 MWth) with silica as the bed material and using a woody biomass, poplar, and a herbaceous biomass, Brassica carinata. Ash characterisation and mass balances of the inorganic elements with relevance for bed agglomeration, fouling and emissions were performed in the BFB combustion pilot plant.The agglomerates formed in the silica bed material have not been found to be associated with chemical reaction sintering related to gas-solid reaction such as the formation of CaSO₄ and CaCO₃. Therefore this chemical reaction mechanism is neglected when it is compared to the bed agglomeration based on the partial melting of the alkaline compounds contained in the biomass ash. Similarities of crystalline phases and elemental content between ash deposited on the tubes and biomass ash obtained in laboratory at 550 °C enable to the laboratory tests of ash melting behaviour to predict the sintering in the ash deposited on the heat exchangers of the thermal plants.     

Hydrogen generation in a downdraft moving bed gasifier coupled to a molten carbonate fuel cell

This paper describes the steady-state simulation of a moving bed downdraft gasifier which allows the conversion of agricultural biomass into a hydrogen-rich gas mixture so that it has an adequate composition for being used as a feedstock in a molten carbonate fuel cell (MCFC). In order to emphasize the applicability of the results, fuel specifications for a 250 kW MCFC (HM-300, MTU, Friedrichshafen, Germany) was used as a reference. The final design makes possible to produce 350 Nm³/h of a biogas in a vessel of 0.8 m diameter × 2.5 m height capable of treating 50 kg/h of dry biomass using 45 Nm³/h of air at 800 K and 1 bar.     

Numerical simulation of coal gasification at circulating fluidised bed conditions

This paper presents modelling results for a new pressurised fluidised bed gasifier concept, called the Power High-Temperature Winkler gasifier (PHTW gasifier). The numerical simulation of the steam/oxygen blown and lignite-fuelled power plant gasifier is performed on the 4800 t/day (1000 MW) at a pressure of 33 bar. The formation of flow pattern, turbulence, product gas composition, temperature, and radiation heat transfer were investigated. Influence of diameter variation on the flow patterns at constant operating conditions is presented. A comparison between the calculation and literature data of similar fluidised bed systems shows good conformance. To anticipate the solid's behaviour, particle concentration, particle size change due to pyrolysis and surface reactions, and particle tracks were modelled using an Eulerian–Lagrangian approach. While varying the total particle mass flow, the pressure drop as a function of reactor height was observed.     

Gasification reaction kinetics on biomass char obtained as a by-product of gasification in an entrained-flow gasifier with steam and oxygen at 900-1000 °C

The purpose of this study was to investigate the gasification kinetics of biomass char, such as the wood portion of Japanese cedar char (JC), Japanese cedar bark char (JB), a mixture of hardwood char (MH) and Japanese lawngrass char (JL), each of which was obtained as a by-product of gasification in an entrained-flow type gasifier with steam and oxygen at 900-1000 °C. Biomass char was gasified in a drop tube furnace (DTF), in which gasification conditions such as temperature (T_g), gasifying agent (CO₂ or H₂O), and its partial pressure (P_g) were controlled over a wide range, with accompanying measurement of gasification properties such as gasification reaction ratio (X), gasification reaction rate (R_g), change of particle size and change of surface area. Surfaces were also observed with a scanning electric microscope (SEM). By analyzing various relationships, we concluded that the random pore model was the most suitable for the biomass char gasification reaction because of surface porosity, constant particle size and specific surface area profile, as well as the coincidence of R_g, as experimentally obtained from Arrhenius expression, and the value is calculated using the random pore model. The order of R_g was from 10⁻² to 10⁻¹ s⁻¹, when T_g = 1000 °C and P_g = 0.05 MPa, and was proportional to the power of P_g in the range of 0.2-0.22 regardless of gasifying agent. Reactivity order was MH > JC > (JB, JL) and was roughly dependent on the concentration of alkali metals in biomass feedstock ash and the O/C (the molar ratio of oxygen to carbon) in biomass char.     

Corrosion of refractory materials in fluidised bed gasification of alkali rich fuels

Gasification of renewable fuels is not common practice due to the high costs of technologies and the absence of reliably working refractories. Refractory degradation is of such high significance that improved refractory durability was ranked first by industry experts in a list of 20 research and development areas related to the economic viability of gasification. Therefore, for improvement of the reliability and durability of refractory linings, this work is dealing with the corrosion resistance of nine commercial refractories to a variety of emissions from potential fuels. The refractories were exposed to a gasifier-like, water vapour and alkali rich atmosphere. Exposures with a duration of 250 h produced corrosion effects that were investigated by scanning electron microscopy, energy dispersive X-ray spectroscopy and X-ray diffraction. Furthermore, thermodynamic calculations were included to further explain the equilibrium chemistry. The results show that extremely low silica refractories are promising candidates for gasifier utilisation.     

Gasification and co-gasification of biomass wastes: Effect of the biomass origin and the gasifier operating conditions

Air gasification of different biomass fuels, including forestry (pinus pinaster pruning) and agricultural (grapevine and olive tree pruning) wastes as well as industry wastes (sawdust and marc of grape), has been carried out in a circulating flow gasifier in order to evaluate the potential of using these types of biomass in the same equipment, thus providing higher operation flexibility and minimizing the effect of seasonal fuel supply variations. The potential of using biomass as an additional supporting fuel in coal fuelled power plants has also been evaluated through tests involving mixtures of biomass and coal–coke, the coke being a typical waste of oil companies. The effect of the main gasifier operating conditions, such as the relative biomass/air ratio and the reaction temperature, has been analysed to establish the conditions allowing higher gasification efficiency, carbon conversion and/or fuel constituents (CO, H₂ and CH₄) concentration and production. Results of the work encourage the combined use of the different biomass fuels without significant modifications in the installation, although agricultural wastes (grapevine and olive pruning) could to lead to more efficient gasification processes. These latter wastes appear as interesting fuels to generate a producer gas to be used in internal combustion engines or gas turbines (high gasification efficiency and gas yield), while sawdust could be a very adequate fuel to produce a H₂-rich gas (with interest for fuel cells) due to its highest reactivity. The influence of the reaction temperature on the gasification characteristics was not as significant as that of the biomass/air ratio, although the H₂ concentration increased with increasing temperature.