Some process fundamentals of biomass gasification in dual fluidized bed

The dual fluidised bed gasification technology is prospective because it produces high caloric product gas free of N₂ dilution even when air is used to generate the gasification-required endothermic heat via in situ combustion. This study is devoted to providing the necessary process fundamentals for development of a bubbling fluidized bed (BFB) biomass gasifier coupled to a pneumatic transported riser (PTR) char combustor. In a steam-blown fluidized bed of silica sand, gasification of 1.0 g biomass, a kind of dried coffee grounds containing about 10 wt.% water, in batch format clarified first the characteristics of fuel pyrolysis (at 1073 K) under the conditions simulating that prevailing in the gasifier intended to develop. The result shown that via pyrolysis more than 60% of fuel carbon and up to 75% of fuel mass could be converted into product gas, while the simultaneously formed char was about 22% of fuel mass. With all of these data as the known input, a process simulation using the software package ASPEN then revealed that the considered dual bed gasification plant, i.e. a BFB gasifier + a PTR combustor, is able to sustain its independent heat and mass balances to allow cold gas efficiencies higher than 75%, given that the fuel has suitable water contents and the heat carried with the product gas from the gasifier and with the flue gas from the char combustor is efficiently recovered inside the plant. In a dual fluidized bed pilot gasification facility simulating the gasification plant for development, the article finally demonstrated experimentally that the necessary reaction time for fuel, i.e. the explicit residence time of fuel particles inside the BFB gasifier computed according to a plug granular flow assumption, can be lower than 160 s. The results shown that varying the residence time from 160 to 1200 s only slightly increased the gasification efficiency, but the reaction time available in the PTR, say, about 3 s in our case, was too short to assure the finish even of fuel pyrolysis.     

Air blown partial gasification of coal in a pilot plant pressurized spout-fluid bed reactor

The advanced high efficiency cycles are all based on gas turbine technology, so coal gasification is the heart of the process. A 2 MW_th spout-fluid bed gasifier has been constructed to study the partial gasification performance of a high ash Chinese coal. This paper presents the results of pilot plant partial gasification tests carried out at 0.5 MPa pressure and temperatures within the range of 950–980 °C in order to assess the technical feasibility of the raw gas and residual char generated from the gasifiier for use in the gas turbine based power plant. The results indicate that the gasification process at a higher temperature is better as far as carbon conversion, gas yield and cold gas efficiency are concerned. Increasing steam to coal ratio from 0.32 to 0.45 favors the water–gas and water–gas shift reactions that causes hydrogen content in the raw gas to rise. Coal gasification at a higher bed height shows advantages in gas quality, carbon conversion, gas yield and cold gas efficiency. The gas heating value data obtained from the deep-bed-height displays only 6–12% lower than the calculated value on the basis of Gibbs free energy minimization. The char residue shows high combustion reactivity and more than 99% combustion efficiency can be achieved.     

Plastic waste elimination by co-gasification with coal and biomass in fluidized bed with air in pilot plant

Treatment of plastic waste by gasification in fluidized bed with air using dolomite as tar cracking catalyst has been studied. The gasifier has a 1 m high bed zone (diameter of 9.2 cm) followed by a 1 m high freeboard (diameter of 15.4 cm). The feedstock is composed of blends of plastic waste with pine wood sawdust and coal at flow rates of 1–4 kg/h. Operating variables studied were gasifier bed temperature (750–880 °C), equivalence ratio (0.30–0.46), feedstock composition and the influence of secondary air insertion in freeboard. Product distribution includes gas and char yields, gas composition (H₂, CO, CO₂, CH₄, light hydrocarbons), heating value and tar content in the flue gas. As a result, a gas with a medium hydrogen content (up to 15% dry basis) and low tar content (less than 0.5 g/m_n³) is obtained.     

Numerical modeling of a jetting fluidized bed gasifier and the comparison with the experimental data

A model for a jetting fluidized bed gasifier is developed, treating the grid, bubble and freeboard zones in series. Reactions including char combustion, steam gasification, CO₂ gasification and water–gas shift reaction are taken into account. The effects on model predictions of assumptions regarding the primary products of char combustion and char reactivity factor are analyzed by comparing the predictions with experimental data from a bench-scale jetting fluidized bed gasifier using different kinds of chars. Contributions of various reactions and different zones and phases to carbon conversion are analyzed.     

Catalytic gasification of woody biomass in an air-blown fluidized-bed reactor using Canadian limonite iron ore as the bed material

A Canadian limonite iron ore was tested for the first time as a catalytic bed material for air-blown gasification of pine sawdust at various equivalence ratios (ER, 0.20–0.35) on a pilot-scale fluidized bed gasifier, in comparison to a conventional olivine bed material. Effects of bed materials (iron ore and olivine) on tar formation and gasification efficiencies were comparatively investigated. The use of Canadian limonite iron ore as the bed material was found to be more active than olivine for tar reduction in the fluidized bed gasification of biomass at a small ER (?0.3), leading to a very low tar yield of 15–25 g/kg biomass at ER = 0.30. The yields of combustible gas (carbon monoxide hydrogen, methane and C₂ hydrocarbon gases) and cold gas efficiency were generally the highest at medium values of ER (0.25–0.30) for both bed materials. The iron ore was less active than olivine for producing combustible gases, leading to a lower cold gas efficiency (50% at ER = 0.30) compared to 75% for olivine. However, the use of the iron ore produced a higher yield of hydrogen than that of olivine in the gasification: 5.0 mol hydrogen per kg of biomass with the iron ore at ER = 0.30 which was about 25% higher than that with olivine.     

Effects of volatile–char interaction on the formation of HCN and NH3 during the gasification of Victorian brown coal in O2 at 500 °C

In a coal gasifier interactions between volatiles and char are significant. The partly reducing conditions in a gasifier would mean the presence of high concentration of partial oxidation products and radicals surrounding the char particles. Currently, little is known about the effects of in situ volatile–char interactions on the conversion of char-N. This study examines the effect of in situ volatile–char interactions on the formation of HCN and NH₃ during the low temperature (500 °C) gasification of Loy Yang brown coal in oxygen. Two novel reactor systems were used. The reactor configurations allowed the quantification of HCN and NH₃ from char-N gasification, volatile-N oxidation and volatile–char interactions separately. Our results indicate that volatile–char interactions can have drastic effects on coal-N conversion during gasification by providing an important source of the radicals for the formation of HCN and NH₃ from char-N during gasification in 4% or 8% O₂ at 500 °C. In the presence of radicals and O₂, N-containing structures in the nascent char can be easily broken down to give HCN and NH₃ during the gasification of the char. In the absence of O₂, some of the nascent char-N structures may stabilise into structures less favourable for the formation of HCN and NH₃ and more favourable for the formation of other N-containing species such as NO_x.     

Gasification reactivity of char from dried sewage sludge in a fluidized bed

The gasification reactivity of char from dried sewage sludge (DSS) applicable to fluidized bed gasification (FBG) was determined. The char was generated by devolatilizing the DSS with nitrogen at the selected bed temperature and was subsequently gasified by switching the fluidization agent to mixtures of CO₂ and N₂ (CO₂ reactivity tests) and steam and N₂ (H₂O reactivity tests)._. The tests were conducted in the temperature range of 800–900 °C at atmospheric pressure, using partial pressure of the main reactant in the mixture (CO₂ or H₂O) in the range of 0.10–0.30 bar. Expressions for the intrinsic reactivity (free of diffusion effects) as a function of temperature, partial pressure of gas reactant (CO₂ or H₂O) and degree of conversion were obtained for each reaction. For the whole range of conversion it was found that the char reactivity in an H₂O–N₂ mixture was roughly three times higher than that in a mixture with the corresponding partial pressure of CO₂. The reactivity was only influenced by particle size greater than 1.2 mm in the tests with steam at 900 °C. It was demonstrated that the method of char preparation greatly influences the reactivity, highlighting the importance of generating the char in conditions similar to that in FBG.     

Gasification of coal and PET in fluidized bed reactor

Blended fuel comprising 23 wt.% polyethyleneterephthalate (PET) and 77 wt.% brown coal was gasified in an atmospheric fluidized bed gasifier of laboratory-scale. The gasification agent was composed of 10 vol.% O₂ in bulk of nitrogen. Thermal and texture analyses were carried out to determine the basic properties of the fuel components. The influence of experimental conditions, such as the fluidized bed and freeboard temperatures on major and minor gas components and tar content, as well as features of the blended fuel gasification in comparison with the single coal gasification, were studied. In the case of coal with PET gasification, only the fluidized bed temperature showed significant influence on CO, CO₂, CH₄ and H₂ content in the producer gas, whereas the effect of the freeboard temperature was insignificant. In single coal gasification both temperatures had considerable and almost the same influence. The content of minor components, such as ethane, ethylene, acetylene and benzene, was found to be more dependent on the freeboard temperature than on the fluidized bed temperature. It was observed that the higher the freeboard temperatures get, the lower is the concentration of the minor components, with the exception of acetylene. The absolute contents of almost all minor and tar components were approximately three times higher in blended fuel gasification than that in single coal gasification. Finally, partition of carbon (char) and selected metals into bottom and cyclone ash in gasification of both fuels is discussed.     

Dynamic behaviour of sewage sludge incineration in a large-scale bubbling fluidised bed in relation to feeding-rate variations

In this paper, a mathematical model is developed for the simulation of a large-scale sewage sludge incineration plant. The model assumes the bed to consist of a fast gas phase, an emulsion phase and a fuel particle phase with specific consideration for thermally-thick fuel particles. The developed model is employed to predict the dynamic response of the bed combustion to fluctuations in sludge feeding-rate. Calculation results indicate that the bed combustion is sensitive to fluctuations with response times greater than 30 min, but severe delays exist for both outlet oxygen level and bed temperatures; from 6 to 13 min for O₂ and 22–45 min for temperatures. Depending on the fluctuation frequency, the corresponding phase shifts are 39–96° for outlet O₂, 138–336° for bed temperature and 80–336° for freeboard temperature.     

Mechanism of decomposition of aromatics over charcoal and necessary condition for maintaining its activity

Decomposition of mono- to tetra-aromatics over charcoal was investigated under conditions such as temperature; 700–900 °C, inlet concentrations of aromatics, steam and H₂; 7.5–15 g/Nm³, 0–15.5 vol% and 0–15.5 vol%, respectively, gas residence time within charcoal bed; 0.2 s, particle size of charcoal; 1.3–2.4 mm. The charcoal, with an initial surface area of 740 m²/g, was active enough to decompose naphthalene completely even at 750 °C. Aromatics with more rings per molecule were decomposed more rapidly. The aromatics were decomposed over the charcoal by coking rather than direct steam reforming irrespective of temperature and steam/H₂ concentrations. The coking, i.e., carbon deposition from the aromatics, caused loss of micropores and thereby activity of the charcoal, while steam gasification of the charcoal/coke formed or regenerated micropores. Relationship between the overall rate of carbon deposition by the coking and gas formation by the gasification within the charcoal bed showed that progress of the gasification at a rate equivalent with or greater than that of the carbon deposition was necessary for maintaining the activity of the charcoal.     

Numerical simulation of coal gasification in entrained flow coal gasifier

This paper presents modeling of a coal gasification reaction, and prediction of gasification performance for an entrained flow coal gasifier. The purposes of this study are to develop an evaluation technique for design and performance optimization of coal gasifiers using a numerical simulation technique, and to confirm the validity of the model. The coal gasification model suggested in this paper is composed of a pyrolysis model, char gasification model, and gas phase reaction model. A numerical simulation with the coal gasification model is performed on the CRIEPI 2 tons/day (T/D) research scale coal gasifier. Influence of the air ratio on gasification performance, such as a per pass carbon conversion efficiency, amount of product char, a heating value of the product gas, and cold gas efficiency is presented with regard to the 2 T/D gasifier. Gas temperature distribution and product gas composition are also presented. A comparison between the calculation and experimental data shows that most features of the gasification performance were identified accurately by the numerical simulation, confirming the validity of the current model.     

Combustion of olive cake and coal in a bubbling fluidized bed with secondary air injection

Combustion performances and emission characteristics of olive cake and coal are investigated in a bubbling fluidized bed. Flue gas concentrations of O₂, CO, SO₂, NO_x, and total hydrocarbons (C_mH_n) were measured during combustion experiments. Operational parameters (excess air ratio (λ), secondary air injection) were changed and variation of pollutant concentrations and combustion efficiency with these operational parameters were studied. The temperature profiles measured along the combustor column was found higher in the freeboard for olive cake than coal due to combustion of hydrocarbons mostly in the freeboard. Combustion efficiencies in the range of 83.6–90.1% were obtained for olive cake with λ of 1.12–2.30. For the setup used in this study, the optimum operating conditions with respect to NO_x and SO₂ emissions were found as 1.2 for λ, and 50 L/min for secondary air flowrate for the combustion of olive cake.     

Catalytic decomposition of biomass tars with iron oxide catalysts

Catalytic gasification of wood (Cedar) biomass was carried out using a specially designed flow-type double beds micro reactor in a two step process: temperature programmed non-catalytic steam gasification of biomass was performed in the first (top) bed at 200–850 °C followed by catalytic decomposition gasification of volatile matters (including tars) in the second (bottom) bed at a constant temperature, mainly 600 °C. Iron oxide catalysts, which transformed to Fe₃O₄ after use possessed catalytic activity in biomass tar decomposition. Above 90% of the volatile matters was gasified by the use of iron oxide catalyst (prepared from FeCl₃ and NH_3aq) at SV of 4.5 × 10³ h^?1. Tar was decomposed over the iron oxide catalysts followed by water gas shift reaction. Surface area of the iron oxide seemed to be an important factor for the catalytic tar decomposition. The activity of the iron oxide catalysts for tar decomposition seemed stable with cyclic use but the activity of the catalysts for the water gas shift reaction decreased with repeated use.     

CFB air-blown flash pyrolysis. Part II: Operation and experimental results

The main operational characteristics of a novel gasifier operating in the CFB mode are outlined in this paper, based on the experimental results from a total of 11 runs in the pyrolysis mode. The operation runs constituted the main experiments in the CFB reactor, carried out to derive meaningful mass balance and additional operational data for the CFB pyrolyzer. The experiments were conducted in varying operating conditions determined by the most important parameters, i.e., biomass flowrate, fluidizing gas flowrate, air factor, initial bed inventory), temperature in the CFB riser, vapor residence time and nominal air factor – or equivalence ratio, S_b.The results obtained showed that the reactor configuration successfully operated as a biomass fast pyrolysis system to maximize liquid yields reaching 61.50 wt% on a maf biomass basis, with the novel feature of providing for autothermal operation at 500 °C and with 0.46 s gas–vapor residence time, by utilizing the by-product char energy content in a single reactor. The reactor provides a very high specific throughput of 1.12–1.48 kg/h m² and the lowest gas-to-feed ratio of 1.3–1.9 kg gas/kg feed compared to other fast pyrolysis processes based on pneumatic reactors and has a good scale-up potential, providing significant capital cost reduction. Results to date suggest that the process is limited by the extent of char combustion. Future work should address resizing of the char combustor to increase overall system capacity, improve the solid separation and substantially increase liquid recovery.     

Synthetic coal chars for the elucidation of NO heterogeneous reduction mechanisms

In the present work, the mechanisms involved in NO–char heterogeneous reduction have been studied using a synthetic coal char (SC char) as carbon source. Another synthetic char (SN char) without nitrogen in its composition has also been employed in these studies. Isothermal reduction tests at different temperatures have been carried out. Two temperature regimes were considered: low temperature (T < 250 °C) where NO chemisorption takes place and high temperature (T > 250 °C) where NO–C reaction occurs. Step response experiments combining consecutive reaction stages with NO and ¹⁵NO were performed in order to determine the role of nitrogen surface complexes, C(N), in the reduction process. The results revealed N₂ and CO₂ to be the main reduction products under the experimental conditions employed in this work. NO chemisorption at lower temperatures results in N₂ emission and surface complexes (mainly oxygenated) formation, while char gasification by NO involves a direct NO attack on the char surface to form surface complexes. As a consequence of desorption of these complexes new sites of reaction are created.     

Simultaneous effect of temperature and pressure on catalytic hydrothermal gasification of glucose

Gasification of glucose in near- and supercritical water was investigated at temperature and pressure ranges from 400 to 600 °C and 20 to 42.5 MPa with a reaction time of 1 h. Hydrothermal gasification of glucose was performed in the absence and presence of catalyst (K₂CO₃) in a batch reactor. The influences of temperature and pressure in the supercritical regimes of water, catalyst were examined in relation to the yield and composition of the gases and aqueous products. The product gases were analyzed by gas chromatography, and the aqueous products were analyzed by high performance liquid chromatography. The gases produced were carbon dioxide, methane, hydrogen, carbon monoxide, and C₂–C₄ hydrocarbons and there was significant production of aqueous products and residue. The aqueous products composed of oxygenated compounds, including carboxylic acids (glycolic acid, formic acid, acetic acid), furfurals (furfural, 5-hydroxymethyl furfural, 5-methyl furfural), phenols (phenol, methyl phenols, hydroxy phenols, methoxy phenols), aldehydes (formaldehyde, acetaldehyde, acetone, propionaldehyde), ketones (3-methyl-2-cyclo-pentene-1-one, 2-cyclo-pentene-1-one) and their alkylated derivatives. Carbon gasification efficiencies were improved by addition of K₂CO₃ into the reacting system. Carbon gasification efficiency reached maximum (94%) at 600 °C and 20 MPa. The yield of hydrogen among gaseous products increased with increasing temperature and decreasing pressure.     

Speciation and distribution of alkali,alkali earth metals and major ash forming elements during gasification of fuel cane bagasse

Gasification of fuel cane bagasse, the waste residue from fuel cane, a hybrid of wild and commercial clones of sugar cane, was carried out in a novel 50 kWe air-blown autothermal downdraft gasifier. The speciation and distribution of alkali, alkali earth metals and major ash forming elements during gasification were investigated to evaluate the extent of volatilisation of these elements into the syngas and to determine the likely impact on syngas fuelled solid oxide fuel cell systems. Also assessed was the potential for defluidisation of the fuel bed due to agglomerate and deposit formation. Chemical fractionation studies showed that 30% of the potassium was captured by aluminosilicates and was retained in the ash, thereby reducing the alkali loading in the syngas and that more than 50% of the alkali earth metals were released to the syngas. In contrast, although the major ash forming elements were transformed from acid insoluble to acid soluble forms during gasification they remained hard bound in the ash and less than 30% of each one was released into the gas phase. The composition of clinkers and agglomerates produced during gasification was investigated by SEM-EDX and XRD which confirmed the presence of the eutectic systems KAlSi₂O₆–SiO₂, KAlSi₂O₆–CaMgSi₂O₆–SiO₂ and CaMgSi₂O₆–NaAlSi₃O₈. A preliminary model of the distribution behaviour during gasification of the ash forming elements has been developed.     

Metallic iron as a tar breakdown catalyst related to atmospheric,fluidised bed gasification of biomass

Tar formation is a major drawback when biomass is converted in a gasifier to obtain gas aimed for utilisation in power production plants or for production of chemicals. Catalytic cracking is an efficient method to diminish the tar content in the gas mixture. In this study, the capability of metallic iron and iron oxides to catalytically crack tars has been experimentally examined. To obtain metallic iron, small grains of hematite (Fe₂O₃) were placed in a secondary reactor downstream the gasifier and reduced in situ prior to catalytic operation. The fuel used in the atmospheric fluidised bed gasifier was Swedish birch with a moisture content of approximately 7 wt%.The influence of temperature in the range 700–900 °C and λ values (i.e. equivalence ratio, ER) between 0 and 0.20 have been investigated. In essence, the results show that raising the temperature in the catalytic bed to approximately 900 °C yields almost 100% tar breakdown. Moreover, increasing the λ value also improves the overall tar cracking activity. The iron oxides did not demonstrate any catalytic activity.     

Almond residues gasification plant for generation of electric power. Preliminary study

In this work, the results of the gasification process of almond residues (almond shell, almond tree pruning, and almond shell peel) generated by an industry (PASAT SAT) are presented. This study was performed in a laboratory fixed-bed reactor. The objective was the obtaining of low/medium heating value gases, which could be burnt in a gas engine to generate electric energy. The effects of varying the air flow rate (50–400 cm³ min^− 1) and the temperature (650–800 °C) in the gasification process of the almond shell peel were discussed. An air flow rate of 200 cm³ min^− 1 and temperature of 800 °C were the optimal conditions with respect to the quality of the gas. At these conditions, the gasification processes of almond shell and almond tree pruning were also studied. The molar fractions of the fuel components reached their maximum values at these conditions with an average gas composition of 2.9% O₂, 52.2% N₂, 13.3% H₂, 14.3% CO, 11.3% CO₂, 4.8% CH₄ and 1.2% C₂H₂, C₂H₄ and C₂H₆. The gas yield obtained was 1.66, 1.85 and 1.71 N m³/kg of residue for almond shell peel, almond shell and almond tree pruning, respectively. The higher heating value (HHV) of the gas obtained at these conditions (5.8, 6.5 and 6.4 MJ N m^− 3 for almond shell peel, almond shell and almond tree pruning, respectively) are comparable with the published data by other researchers. The carbon conversion was in the range between 81% and 90%. From the residues generated by the industry, with an average processing capacity of 1400 kg of residue/h, an energy potential of 3.99 MW thermal could be obtained. The design of a gasification plant for generation of electric energy with an alternator of 1.99 MW, considering a global efficiency of the process of 25%, could be performed.     

Reaction-diffusion model of TGA gasification experiments for estimating diffusional effects

A 2D non-isothermal and non-equimolar reaction-diffusion model is developed in order to assess the diffusional effects that may take place during the CO₂ gasification of a biomass char contained in the cylindrical crucible of a horizontal-arm thermogravimetric apparatus (TGA). The model takes into account the chemical reaction rate and the effective transport properties as a function of the local conversion and therefore as a function of time and position within the char bed. Intraparticle diffusion and structural changes of the char bed during the gasification are also included. The model results showed good agreement with the experimental ones obtained at different temperatures and CO₂ partial pressures, and points out the relevant role of diffusional effects in TGA experiments. The effectiveness factor defined as the ratio of the measured to the intrinsic reaction rates at the same gas conditions stronglly depends on temperature and CO₂ partial pressure and may be as low as 0.2 at 950 °C and 50% char conversion. The marked variation found for the effectiveness factor at different conversions suggests the necessity of a complementary analysis of the variation of the diffusional effects with conversion when TGA experiments for kinetic determination are carried out.