Experimental study on Egyptian biomass combustion in circulating fluidized bed

The present study investigates the combustion of four kinds of biomass in a circulating fluidized bed. The combustion chamber is a steel cylinder with 145 mm inner diameter and 2 m height. Tests were conducted on wheat straw, sawdust-wood, cottonseed burs, and corncobs. Excess air was varied for each fuel. Temperature, heat flux and gas emissions were measured along the combustion chamber and at the chimney inlet. Results showed that sawdust-wood produces the highest values of CO emissions (about 3000 mg/Nm³). On the other hand, cottonseed burs produce the lowest values of CO emissions (about 250 mg/Nm³). The SO₂ emissions were very low in all tests (less than 20 mg/Nm³). The lowest emission value occurred at an excess air ratio (EA) of 1.24 except for cottonseed burs where it was 1.4.     

Experimental study on syngas production by co-gasification of coal and biomass in a fluidized bed

The main results of an experimental work on co-gasification of a Chinese bituminous coal and two types of biomass in a bench-scale fluidized bed are reported in the present study. Experiments were performed at different oxygen equivalence ratio, steam/carbon ratio and biomass/coal ratio. In addition, stabilization of co-gasification process was investigated. It was found that a relatively low oxygen equivalence ratio favors the increase of syngas yield (CO + H₂). There is a maximum value in the curve of syngas yield versus steam/carbon ratio. Moreover, the content of H₂ in gas increases with the increase of biomass ratio while that of CO and syngas yield decrease. A continuous stable operation can be gained.     

A model of the dynamics of a fluidized bed combustor burning biomass

A dynamical model of an atmospheric, bubbling, fluidized bed combustor of biomass is presented. The model, based on one previously developed for the steady combustion of high-volatile solids, accounts for the fragmentation and attrition of fuel particles, the segregation and postcombustion of volatile matter above the bed, as well as thermal feedback from the splashing region to the bed. The model was used to assess how the dynamic behavior of the combustor varies with some of the operating parameters. To this end, a bifurcation analysis was first used to study the influence of selected parameters on the number and quality of steady state solutions. Moreover, direct integration of the governing equations provided a simulation of the dynamic behavior of the combustor after perturbing the parameters. Results of the bifurcation analysis indicated that extinction may take place through limit point bifurcations when varying the moisture content of the biomass and the flow rates of feed or air. Dynamic simulations showed that the bed temperature changes slowly when a stepwise change is imposed on one of the parameters. Either a new steady state or extinction eventually results, depending on the stepwise change. While relaxation of the bed temperature occurs rather slowly, the dynamics of the splashing region and of the freeboard are much faster, due to the shorter time-scales associated with homogeneous oxidation reactions. The relaxation time of the bed is determined by the heat capacity of the fluidized solids and by the fraction of the heat released recycling to the bed as thermal feedback.     

Techno-economic analysis and life cycle assessment of hydrogen production from different biomass gasification processes

The paper presents techno-economic analyses and life cycle assessments (LCA) of the two major gasification processes for producing hydrogen from biomass: fluidized bed (FB) gasification, and entrained flow (EF) gasification. Results indicate that the thermal efficiency of the EF-based option (56%, LHV) is 11% higher than that of the FB-based option (45%), and the minimum hydrogen selling price of the FB-based option is $0.3 per kg H₂ lower than that of the EF-based option. When a carbon capture and liquefaction system is incorporated, the efficiencies of the EF- and FB-based processes decrease to 50% and 41%, respectively. The techno-economic analysis shows that at a biomass price of $100 per tonne, either a minimum price of $115/tonne CO_2e or a minimum natural gas price of $5/GJ is required to make the minimum hydrogen selling price of biomass-based plants equivalent to that of commercial natural gas-based steam methane reforming plants. Furthermore, the LCA shows that, biomass as a carbon-neutral feedstock, negative life cycle GHG emissions are achievable in all biomass-based options.     

Ash deposition behaviors during combustion of raw and water washed biomass fuels

Biomass-fired boilers have the tendency to suffer from severe problems of fouling and slagging due to the high potassium content of biomass fuel. The troublesome potassium, however, can be removed efficiently by water washing pretreatment. In this study, the ash deposition behaviors during combustion of raw and water washed biomass fuels were investigated by a one-dimensional furnace and a deposition probe. Two biomass fuels (corn stalk and wheat straw) were used, and deposition mass, deposition efficiency, composition and morphology of the deposit were studied. The ash deposition while firing raw biomass exhibits a “fast?slow?fast?slow” trend with the sampling time. After water washing, the deposition mass decreases dramatically, and the deposition efficiency reduces gradually as the sampling time increases. The analyses of elemental composition, morphology and chemical composition on the deposit from raw biomass imply that the condensation/thermophoresis is quite significant in the earlier deposition stage, whereas the chemical reaction is remarkable in the later stage. After water washing, the potassium content of the deposit decreases significantly. Morphology and chemical composition analyses indicate that the deposit from water washed biomass ascribes to the physical accumulation of non-viscous fly ash particles. The deposition mass can easily approach a maximum value. The ash fusion temperatures of deposits increase remarkably after water washing. In addition, ash deposition mechanisms during biomass combustion are discussed.     

A 7 year long measurement period investigating the correlation of corrosion,deposit and fuel in a biomass fired circulated fluidized bed boiler

The present investigation involves a unique, 7 year (2001–2007) long study of corrosion and deposits on superheater tubes in a biomass fired circulated fluidized bed boiler. These measurements are correlated against the different fuels used over this period. In the earlier years, the boiler was run with a mixture of different biomass fuels and peat. In later years, recycled wood was introduced into the fuel mix. The deposit growth rate approximately doubled when the recycled wood content of the fuel was increased to 10–20%. Small amounts of chlorine and zinc were found both in the recycled wood and in the deposit layer. These elements together with alkali metals from the biomass, have the potential to form sticky compounds that increase the deposit growth rate. The corrosion rate of the superheater tubes varied over the study period. A number of possible explanations for this phenomenon are discussed.     

Particle-metal interactions during combustion of pulp and paper biomass in a fluidized bed combustor

We compare interactions between metals and solid particles during the classic fluidized bed combustion (FBC) and a new low-high-low temperature (LHL) combustion of selected biomass. The biomass was a mixture of bark and pine wood residues typically used by a paper mill as a source of energy. Experiments, conducted on a pilot scale, reveal a clear pattern of surface predominance of light metals (Ca, Na, K) and core predominance of heavy metals (Cd, Cr) within the LHL-generated particles. No such behavior was induced by the FBC. Metal migration is linked to the evolution of inorganic particles. A composite picture of the metal rearrangements in the particles was obtained by a combination of independent analytical techniques including electron probe microanalysis, field emission scanning electron microscopy, inductively coupled plasma spectrometry, and X-ray diffractometry. It is suggested that the combination of (1) the high-temperature region in the LHL and (2) changes in the surface free energy of the particles is the driving force for the metal-particle behavior. Important practical implications of the observed phenomena are proposed, including removal of hazardous submicron particulate and reduction in fouling/slagging during biomass combustion. These findings may contribute to redesigning of currently operating FBC units to generate nonhazardous, nonleachable, reusable particles where heavy metals are immobilized while environmental and technological problems reduced.     

Modeling the effects of the operational parameters on H2 composition in a biomass fluidized bed gasifier

In this study, effects of the operational parameters such as gasifier temperature, bed operational velocity, equivalence ratio, biomass particle size and biomass-to-steam ratio on hydrogen production from an atmospheric biomass FB gasifier is simulated by presently developed model. The model is one-dimensional, isothermal and steady state, and the fluid-dynamics are based on the two-phase theory of fluidization. Tar conversion is taken into account in the model. The model simulation results are also compared with and validated against experimental data given in the literature. As a result of this study, it is observed that H₂ composition increased remarkably with the rise of the gasifier temperature. Small biomass particles improves H₂ composition. It is unfeasible to apply too small or too large ER in biomass air-steam gasification. The increases in the mole fractions of H₂ with increases in the steam flow rate indicated that the gas shift reaction has a substantial effect in air-steam gasification.     

Ash effects during combustion of lignite/biomass blends in fluidized bed

Aiming at investigating the role of minerals in evaluating co-firing applications of low rank coals and biomass materials, agricultural residues characteristic of the Mediterranean countries, one lignite and their blends with biomass proportions up to 20% wt, were burned in a lab-scale fluidized bed facility. Fly ashes and bed material were characterized in terms of mineralogical, chemical and morphological analyses and the slagging/fouling and agglomeration propensities were determined.The results showed that combustion of each fuel alone could provoke medium or high deposition problems. Combustion of raw fuels produced fly ashes rich in Ca, Si and Fe minerals, as well as K and Na minerals in the case of biomass samples. However, blending of the fuels resulted in a reduction of Ca, Fe, K and Na, while an increase of Si and Al elements in the fly ashes as compared to lignite combustion, suggesting lower deposition and corrosion problems in boilers firing these mixtures. The use of bauxite as an additive enriched bottom ash in calcium compounds. Under the conditions of the combustion tests, no signs of ash deposition or bed agglomeration were noticed.     

Numerical simulation of hydrogen production by gasification of large biomass particles in high temperature fluidized bed reactor

Biomass as a renewable fuel compared to fossil fuels usually contains high moisture content and volatile release. Hydrogen production by large particle biomass gasification is a promising technology for utilizing high moisture content biomass particle in the high temperature fluidized bed reactor. In the present work, simulation of large particles biomass gasification investigated at high temperature by using the discrete phase model (DPM). Combustible gases with homogeneous gas phase reactions, drying process with a heterogeneous reaction, primary and secondary pyrolysis with independent parallel-reaction by using two-competing-rate model to control a high and low temperature were used. During the thermochemical process of biomass, gaseous products containing of H₂, H₂O, CH₄, CO and CO₂ was obtained. The effects of concentration, mole and mass fraction and hydrodynamics effects on gaseous production during gasification were studied. The results showed that hydrodynamic effect of hot bed is different from cold bed. Concentration and molar fraction of CO and H₂ production by continually and stably state and small amount of CO₂, H₂O, and CH₄ was obtained. The hydrodynamic of bed plays the significant role on the rate of gaseous products.     

Production of hydrogen-rich gas from plant biomass by catalytic pyrolysis at low temperature

The catalytic pyrolysis of plant biomass was investigated in a dual-particle powder fluidized-bed (PPFB), where the primary decompositions and secondary reactions occurred simultaneously under ambient pressure. The yields and distributions of the pyrolysis products were studied under various operating conditions. In the absence of catalyst, the amount of volatile released from woody biomass depended on the pyrolysis temperature, and only 13.8 g H₂/kg biomass (def: dry ash-free basis) was produced at 1173 K. NiMo/Al₂O₃ catalyst promoted the decomposition of tar and light aromatic hydrocarbon compounds from the primary decomposition reaction, and significantly reduced the temperature required for the secondary phase reaction. With NiMo/Al₂O₃ catalyst at 723 K, clean combustion gas accounted for 91.25 vol% of the total gas products, which was composed of 49.73 vol% of H₂, 34.50 vol% of CO, and 7.03 vol% of low molecular weight hydrocarbon gases. The contents of H₂ and CO were 33.6 g H₂/kg biomass (def) and 326.3 g CO/kg biomass (def), respectively. Therefore, it is critical to control the secondary phase reaction conditions during the catalytic pyrolysis in order to produce hydrogen-rich gas.     

A two phase model of high temperature steam-only gasification of biomass char in bubbling fluidized bed reactors using nuclear heat

A two phase biomass char steam gasification kinetic model is developed in a bubbling fluidized bed with nuclear heat as source of energy. The model is capable of predicting the temperature and concentration profiles of gases in the bubble, emulsion gas and solid phases. The robust model calculates the dynamic and steady state profiles, as well as the complex parameters of fluidized bed. Three pilot scale gasifiers were simulated in order to see the effect of the H/D ratio and the bed heating dynamics in the gasification kinetics, these parameters are found to be really important in order to enhance the water-gas shift reaction, and consequently, the hydrogen production. For the system modeled, hydrogen is the principal product of the steam-only gasification, as reported in the literature data. The carbon dioxide yield seems to be smaller than the ones in other works, but these differences are due principally to the energy source (no combustion is conducted) and that char (no oxygen in the solids) was used as the carbon source.     

Experimental investigation of the effect of physical pre-treatment on air-blown fluidized bed biomass gasification

The effect of comminution, drying, and densification on bubbling fluidized bed gasification was investigated by fractionating a forestry residue into a feedstock consisting of different particle sizes, moisture levels, and by densifying to pellets. The gasification performance was evaluated at nominal average bed temperatures of 725°, 800° and 875 °C at a constant fluidizing velocity (0.91 m s⁻¹) with feed input rates between 9 and 24 kg h⁻¹.The gas composition was observed to be influenced by both the particle size and form. Smaller particles led to a gas richer in carbon monoxide and depleted in hydrogen. The gasification of pellets led to a gas with the greatest hydrogen to carbon monoxide ratio. The smallest particles tested resulted in the worst gasification performance, as defined by cold gas efficiency, carbon conversion, and tar production. Despite differences in the gas composition among the larger particles and the pellets, similar carbon conversion and cold gas efficiency was observed.Relative to comparable test conditions with dry feed fractions (having a moisture mass fraction of 7–12%), an average 11% increase in carbon conversion was observed for the wetter feed fractions containing a moisture mass fraction of 24–31%. This increase in carbon conversion offset much of the expected decrease in cold gas efficiency by using a wetter feed material. A slight increase in hydrogen production and negligible change in tar production was observed for the wetter feed fractions relative to the dry feed fraction.     

Process analysis of hydrogen production from biomass gasification in fluidized bed reactor with different separation systems

Gasification is one of the most effective and studied methods for producing energy and fuels from biomass as different biomass feedstock can be handled, with the generation of syngas consisting of H₂, CO, and CH₄, which can be used for several applications. In this study, the gasification of hazelnut shells (biomass) within a circulating bubbling fluidized bed gasifier was analyzed for the first time through a quasi-equilibrium approach developed in the Aspen Plus environment and used to validate and improve an existing bubbling fluidized bed gasifier model. The gasification unit was integrated with a water-gas shift (WGS) reactor to increase the hydrogen content in the outlet stream and with a pressure swing adsorption (PSA) unit for hydrogen separation. The amount of dry H₂ obtained out of the gasifier was 31.3 mol%, and this value increased to 47.5 mol% after the WGS reaction. The simulation results were compared and validated against experimental data reported in the literature. The process model was then modified by replacing the PSA unit with a palladium membrane separation module. The final results of the present work allowed comparison of the effects of the two conditioning systems, PSA and palladium membrane, indicating a comparative increase in the hydrogen recovery ratio of 28.9% with the palladium membrane relative to the PSA configuration.     

Co-firing of biomass waste-derived syngas in coal power boiler

The paper deals with waste gasification as a technology allowing for indirect co-firing of large quantities of biodegradable wastes in coal-fired power boilers. In contrast to common landfilling and direct co-firing, gasification of wastes presents a number of advantages. Problematic species in original feedstocks can be partly safely incinerated in the furnace and partly retained in the gasification residues.     

Evaluation of different biomass gasification modeling approaches for fluidized bed gasifiers

To develop a model for biomass gasification in fluidized bed gasifiers with high accuracy and generality that could be used under various operating conditions, the equilibrium model (EM) is chosen as a general and case-independent modeling method. However, EM lacks sufficient accuracy in predicting the content (volume fraction) of four major components (H₂, CO, CO₂ and CH₄) in product gas. In this paper, three approaches—MODEL I, which restricts equilibrium to a specific temperature (QET method); MODEL II, which uses empirical correlations for carbon, CH₄, C₂H₂, C₂H₄, C₂H₆ and NH₃ conversion; and MODEL III, which includes kinetic and hydrodynamic equations—have been studied and compared to map the barriers and complexities involved in developing an accurate and generic model for the gasification of biomass.This study indicates that existing empirical correlations can be further improved by considering more experimental data. The updated model features better accuracy in the prediction of product gas composition in a larger range of operating conditions. Additionally, combining the QET method with a kinetic and hydrodynamic approach results in a model that features less overall error than the original model based on a kinetic and hydrodynamic approach.     

Research on the characteristics of biomass gasification in a fluidized bed

The depletion of fossil fuels and the increasing environmental problems, make biomass energy a serious alternative resource of energy. Biomass gasification is one of the major biomass utilization technologies to produce high quality gas. In this paper, biomass gasification was performed in a self-designed fluidized bed. The main factors (equivalence ratio, bed temperature, added catalyst, steam) influenced the gasification process were studied in detail. The results showed that the combustible gas content and the heating value increased with the increase of the temperature, while the CO₂ content decreased. The combustible gas content decreased with the increase of the equivalence ratio (ER), but CO₂ content increased. At the same temperature and at different ratios of CaO (from 0 to 20%), H₂ content was increased significantly, CO content was also increased, CH₄ content increased slightly, but CO₂ content was decreased. With the addition of steam at different temperature, the gas in combustible components increased, the content of H₂ increased obviously. The growth rate was 50% increased. As the bed temperature increased, gas reforming reaction increased, the CO and CH₄ content decreased, but CO₂ and H₂ content increased.     

Simulation of air-steam gasification of woody biomass in a bubbling fluidized bed using Aspen Plus: A comprehensive model including pyrolysis,hydrodynamics and tar production

A comprehensive model was developed to simulate gasification of pine sawdust in the presence of both air and steam. The proposed model improved upon the premise of an existing ASPEN PLUS-based biomass gasification model. These enhancements include the addition of a temperature-dependent pyrolysis model, an updated hydrodynamic model, more extensive gasification kinetics and the inclusion of tar formation and reaction kinetics. ASPEN PLUS was similarly used to simulate this process; however, a more extensive FORTRAN subroutine was applied to appropriately model the complexities of a Bubbling Fluidized Bed (“BFB”) gasifier. To confirm validity, the accuracy of the model's predictions was compared with actual experimental results. In addition, the relative accuracy of the comprehensive model was compared to the original base-model to see if any improvement had been made.Results show that the model predicts H₂, CO, CO₂, and CH₄ composition with reasonable accuracy in varying temperature, steam-to-biomass, and equivalence ratio conditions. Mean error between predicted and experimental results is calculated to range from 6.1% to 37.6%. Highest relative accuracy was obtained in CO composition prediction while the results with the least accuracy were for CH₄ and CO₂ estimation at changing steam-to-biomass ratios and equivalence ratios. When compared to the original model, the comprehensive model predictions of H₂ and CO molar fractions are more accurate than those of CO₂ and CH₄. For CO₂ and CH₄, the original model predicted with comparable or better accuracy when varying steam-to-biomass ratio and equivalence ratios but the comprehensive model performed better at varying temperatures.     

Gasification of biomass to hydrogen-rich gas in fluidized beds using porous medium as bed material

Experiments were carried out to study the characteristics of biomass gasification in a fluidized bed using industrial sand and porous medium as bed materials. Analysis was conducted to investigate the effects of different operation parameters, including bed material, gasification temperature (600 °C–900 °C), oxygen enrichment in the gasifying agent (21 vol.% to 50 vol.%), and steam flow rate (1.08 kg/h to 2.10 kg/h), on product yields and gas composition. The results of gas chromatography show that the main generated gas species were H₂, CO, CO₂, CH₄, and C₂H₄. Compared with industrial sand as bed material, porous medium as bed material was more suitable for gasifying biomass to hydrogen-rich gas. The physical characteristics of porous structure are more favorable to heat transfer, producing the secondary crack of heavy hydrocarbons and generating more hydrogen and other permanent gases. The product yields of hydrogen-rich gas increased with increasing gasification temperature. The hydrogen concentration improved from 22.52 vol.% to 36.06 vol.%, but the CO concentration decreased from 37.53 vol.% to 28.37 vol.% with increasing temperature from 600 °C to 900 °C under the operation parameters of porous bed material at a steam flow rate of 1.56 kg/h. With increasing oxygen concentration, H₂ concentration increased from 12.36% to 20.21%. Over the ranges of the examined experimental conditions, the actual steam flux value (e.g., 1.56 kg/h) was found to be the optimum value for gasification.     

Gasification of biomass: comparison of fixed bed and fluidized bed gasifier

Gasification as a thermochemical process is defined and limited to combustion and pyrolysis. A systematic overview of reactor designs categorizes fixed bed and fluidized bed reactors. Criteria for a comparison of these reactors are worked out, i.e. technology, use of material, use of energy, environment and economy. A utility analysis for thermochemical processes is suggested. It shows that the advantages of one of the reactor types are marginal. An advantage mainly depends on the physical consistency of the input. As a result there is no significant advantage for the fixed bed or the fluidized bed reactor.