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.     

Potential approaches to improve gasification of high water content biomass rich in cellulose in dual fluidized bed

Biomass containing water of 30-65 wt.% and rich in cellulose, such as various grounds of drinking materials and the lees of spirit and vinegar, is not suitable for biological digestion, and the thermal conversion approach has to be applied to its conversion into bioenergy. The authors have recently worked on converting such biomass into middle heating-value gas via dual fluidized bed gasification (DFBG) integrated with various process intensification technologies. This article is devoted to highlighting those technical ways, including the choice of the superior technical deployment for a DFBG system, the impregnation of Ca onto fuel in fuel drying, the integration of gas cleaning with fuel gasification via two-stage DFBG (T-DFBG), and the decoupling of fuel drying/pyrolysis and char gasification via the decoupled DFBG (D-DFBG). The attained results demonstrated that the superior deployment of bed combination for the DFBG should be a bubbling/turbulent fluidized bed gasifier integrated with a pneumatic riser combustor. In terms of improving efficiency of fuel conversion into combustible gas and suppressing tar generation during gasification, the impregnation of Ca onto fuel exhibited distinctively high upgrading effect, while both the T-DFBG and D-DFBG were also demonstrated to be effective to a certain degree.     

H2 rich syngas by selective CO2 removal from biomass gasification in a dual fluidized bed system — Process modelling approach

A process model of dual fluidized bed gasification is presented based on mass- and energy balances. The model further covers the evaluation of thermodynamic equilibrium states. The gasification is investigated for the special case that CaO/CaCO₃ is used as bed material allowing selective transport of CO₂ from the gasification reactor to the combustion reactor by repeated carbonation and calcination. Experimental data are used to determine the model parameters. An empirical approach towards the kinetics of fuel conversion allows prediction of process behaviour at varied fuel water content. The selective transport of CO₂ results in high H₂ contents in the produced syngas. The lower operating temperatures in the gasification reactor increase the efficiency of energy conversion. The results are in agreement with experimental data and show the thermodynamic limitations of the technology.     

Comparison of dual fluidized bed steam gasification of biomass with and without selective transport of CO2

A comparison of dual fluidized bed gasification of biomass with and without selective transport of CO₂ from the gasification to the combustion reactor is presented. The dual fluidized bed technology provides the necessary heat for steam gasification by circulating hot bed material that is heated in a separate fluidized bed reactor by combustion of residual biomass char. The hydrogen content in producer gas of gasifiers based on this concept is about 40 vol% (dry basis). Addition of carbonates to the bed material and adequate adjustment of operation temperatures in the reactors allow selective transport of CO₂ (absorption enhanced reforming—AER concept). Thus, hydrogen contents of up to 75 vol% (dry basis) can be achieved. Experimental data from a 120 kW_{Fuel input} pilot plant as well as thermodynamic data are used to determine the mass- and energy-balances. Carbon, hydrogen, oxygen, and energy balances for both concepts are presented and discussed.     

Modeling and simulation of biomass air-steam gasification in a fluidized bed

By considering the features of fluidized-bed reactors and the kinetic mechanism of biomass gasification, a steady-state, isothermal, one-dimensional and two-phase mathematical model of biomass gasification kinetics in bubbling fluidized beds was developed. The model assumes the existence of two phases — a bubble and an emulsion phase — with chemical reactions occurring in both phases. The axial gas dispersion in the two phases is accounted for and the pyrolysis of biomass is taken to be instantaneous. The char and gas species CO, CO₂, H₂, H₂O, CH₄ and 8 chemical reactions are included in the model. The mathematical model belongs to a typical boundary value problem of ordinary differential equations and its solution is obtained by a Matlab program. Utilizing wood powder as the feedstock, the calculated data show satisfactory agreement with experimental results and proves the effectiveness and reliability of the model. __________ Translated from Chemical Engineering (China), 2007, 35(10): 23–26 [译自: 化学工程]     

生物质流化床空气-水蒸气气化模型研究

根据流化床反应器特点,结合生物质气化动力学反应机理,建立了生物质在流化床内气化的等温稳态、一维二相动力学模型。该模型所做的主要假定如下:流化床分为气泡相和乳相,在气泡相和乳相内均存在化学反应,考虑二相内的轴向气体扩散,生物质热解过程瞬时完成,主要考虑焦碳以及CO,CO2,H2,H2O,CH4等在流化床内发生的8个主要化学反应。数学模型属于常微分方程组边值问题,利用数值计算软件M atlab7.0进行编程求解。以木粉为原料,将模型结果与实验结果进行了对比,模拟结果与试验数据符合良好,在一定程度上证明了模型的有效性和可靠性。     

Modeling and simulation of biomass air-steam gasification in a fluidized bed

By considering the features of fluidized-bed reactors and the kinetic mechanism of biomass gasification, a steady-state, isothermal, one-dimensional and two-phase mathematical model of biomass gasification kinetics in bubbling fluidized beds was developed. The model assumes the existence of two phases – a bubble and an emulsion phase – with chemical reactions occurring in both phases. The axial gas dispersion in the two phases is accounted for and the pyrolysis of biomass is taken to be instantaneous. The char and gas species CO, CO₂, H₂, H₂O, CH₄ and 8 chemical reactions are included in the model. The mathematical model belongs to a typical boundary value problem of ordinary differential equations and its solution is obtained by a Matlab program. Utilizing wood powder as the feedstock, the calculated data show satisfactory agreement with experimental results and proves the effectiveness and reliability of the model.     

Experimental study on catalytic steam gasification of natural coke in a fluidized bed

The gasification characteristics of natural coke from Peicheng mine with steam were investigated in a fluidized bed reactor. The effects of catalyst type, composition and dosage of catalyst on the yield, components and heating value of product gas, and carbon conversion rate were examined. The results show that fluidized bed gasification technology is an effective way to gasify natural coke. Also the results indicate that individual addition of K-, Ca-, Fe-, Ni-based catalyst effectively increases the gasification reaction rate of the natural coke samples. With the increase in catalyst dosage, the yield and heating value of product gas per hour increase obviously, and carbon conversion rate is improved substantially. Each of aforementioned catalysts has similar catalytic effect and trend, among which the effect of Ca-based catalyst is a little weaker. The optimum metal atom ratio of mixed catalyst is Fe/Ni/others = 35/55/10, and the mixed catalyst displays maximum catalytic performance when the catalyst dosage in the natural coke is about 4%. The experimental findings provide an interesting reference for large-scale development and utilization of natural coke.     

Production of H2 and medium heating value gas via steam gasification of lignins in fixed‐bed reactors

In this work, steam gasification of Alcell and Kraft lignins were carried out in a fixed‐bed reactor in order to produce H₂ and medium heating value gas. The conversion of lignins increased from a low of 64 wt% for Alceil lignin to a high of 88 wt% for Kraft lignin with increasing steam flow rate and temperature. Maximum H₂ production of 60.7 mol% was obtained at 800°C and at a steam flow rate of 15 g/h/g of Kraft lignin, whereas maximum heating value of 18000 kl/m³ of the product gas was obtained at 650°C and at 5 g/h/g of Alcell lignin. Also, the performance of a Ni‐based steam reforming catalyst for the production of H₂ was studied for both types of lignin in a dual fixed‐bed reaction system. A maximum H₂ production of 63 mol% was obtained at a catalyst bed temperature of 750°C and at a catalyst loading of 0.3 g for Alcell lignin. The sulfur present in Kraft lignin had detrimental effect on the catalyst performance.     

Steam hydration of sorbents from a dual fluidized bed CO2 looping cycle reactor

Results are presented on steam hydration of spent residues obtained from a 75 kW_th dual fluidized bed combustion (FBC) pilot plant unit operating in a CO₂ looping cycle mode. The samples were collected from the unit under various conditions, which included electrical heating of the reactor, as well as firing with coal, and biomass under oxy-fuel combustion conditions. In addition, different operating times, i.e., number of cycles (25 min–455 min/1–25 cycles) were examined, with the carbonator operating at temperatures of 600–700 °C and the calciner at 850–900 °C. The samples collected came from the calciner, carbonator and cyclone. Steam hydration itself was done under atmospheric pressure in saturated steam at 100 °C for periods of 15, 30 and 60 min. The original limestone sample, as well as the spent samples from the pilot plant and the hydrated samples were examined to determine their hydration and carbonation levels, as well as their unreacted CaO content using TGA and XRD analysis. In addition, samples were characterized for pore distribution (nitrogen adsorption/desorption: BET and BJH), skeleton characterization, with density by He pycnometry and particle surface area morphology (SEM/EDX), as well as changes in sample volume during hydration (sample swelling). The results obtained showed successful hydration (typically only ～10% unreacted CaO) even for hydration periods as short as 15 min, and very favorable sample properties. Their pore surface area, pore volume distribution and swelling during hydration are promising with regard to their use in additional CO₂ capture cycles or SO₂ retention. However, their predisposition to fracture is the main disadvantage observed with these samples. This may result in difficulties in terms of their handling in FBC systems, due to intensified attrition and consequent elutriation from the reactor.     

Factors affecting steam gasification rate of low rank coal char in a pressurized fluidized bed

A high-pressure bubbling fluidized bed reactor was used to study the steam gasification of coal char under pressure. Indonesian sub-bituminous coal char (Adaro) and Australian lignite char (Loy Yang) were gasified with steam in the reactor at temperatures below 1173 K and at total pressures ranging from 0.1 to 0.5 MPa. The steam gasification rates of the coal chars were determined by analysis of the gaseous products. Activation energies for the steam gasification of the chars were as high as about 250 kJ/mol, which suggests that the temperature dependence of the gasification was substantial. The apparent gasification rates under the study conditions were described by a Langmuir–Hinshelwood (L–H)-type equation. Analysis of the reaction kinetics on the basis of the L–H equation indicated that increasing steam pressure effectively increased the gasification rate.     

Inhibition of steam gasification of char by volatiles in a fluidized bed under continuous feeding of a brown coal

Steam gasification of a Victorian brown coal was performed in an atmospheric bubbling fluidized-bed reactor with continuous feeding of the coal. The gasification converted no more than 28, 51 and 71% of the nascent char (on a carbon basis) at 1120, 1173 and 1223 K, respectively. The char recovered from the fluidized bed was, nonetheless, gasified toward complete conversion when exposed to steam in another reactor, in which volatiles from the pyrolysis were absent while interaction between the char and products from the gasification was minimized. Atmosphere created in the fluidized bed thus prevented the char gasification from taking place beyond upper-limit conversion. In the absence of volatiles, nascent char underwent gasification catalyzed by inherent metallic species and non-catalytic gasification in parallel. The non-catalytic gasification was greatly decelerated by the presence of H₂ in the gas phase due to its dissociative chemisorption onto free carbon sites forming H-laden carbon. H₂ was, however, not a so strong inhibitor as to terminate the gasification. It was rather suggested that much more H-laden carbon was formed through dissociative chemisorption of volatiles and/or chemisorption of hydrogen radical from thermal cracking of volatiles in the gas-phase, which resulted in prevention of the non-catalytic gasification. It seemed that the char was converted in the fluidized-bed mainly by the catalytic gasification, while the conversion was limited due to deactivation of metallic species within the char matrix and their release from the char.     

Kinetics of K-catalysed steam and CO2 gasification in the presence of product gases

Kinetic studies have been carried out to describe the dependence of the K₂CO₃ catalysed steam and CO₂ gasification of Westerholt coal char on temperature, pressure and the composition of gasifying agents. By using a low ash char with two different catalyst loadings, it was possible to elucidate the burn-off-effect without taking into consideration the reactions between coal minerals and catalyst.     

Model for biomass char combustion in the riser of a dual fluidized bed gasification unit: Part II — Model validation and parameter variation

The two-phase combustion model for biomass char combustion in a riser of a dual fluidized bed gasification unit that has been presented in part I is validated using the data obtained from the 8 MWth dual fluidized bed reactor at Guessing/Austria. The model is capable of calculating the average temperatures in all zones, the gas phase composition, solid hold up, char feed rates and air ratio. The model predictions for the temperature profile along the riser and for the exiting gas composition are in good agreement with the measured values. The simulation results show that the residual char from the gasifier is only partly converted in the riser for char particles larger than 0.6 mm. Un-combusted char is circulated back into the gasification reactor. Parameter variations show that the exact location where additional liquid fuels are introduced in the middle zone of the riser does not affect the global behaviour of the combustion reactor. Based on the simulation results it is proposed that external supply of char (additional) may be a very effective method for reducing producer gas recycling to the riser, which is currently necessary to obtain the desired gasification temperatures.     

Simulation of a bubbling fluidized bed process for capturing CO2 from flue gas

We simulated a bubbling bed process capturing CO₂ from flue gas. It applied for a laboratory scale process to investigate effects of operating parameters on capture efficiency. The adsorber temperature had a stronger effect than the regenerator temperature. The effect of regenerator temperature was minor for high adsorber temperature. The effect of regenerator temperature decreased to level off for the temperature >250 °C. The capture efficiency was rather dominated by the adsorption reaction than the regeneration reaction. The effect of gas velocity was as appreciable as that of adsorber temperature. The capture efficiency increased with the solids circulation rate since it was ruled by the molar ratio of K to CO₂ for solids circulation smaller than the minimum required one (G _{s, min} ). However, it leveled off for solids circulation rate >G _{s, min} . As the ratio of adsorber solids inventory to the total solids inventory (x _w1) increased, the capture efficiency increased until x _w1=0.705, but decreased for x _w1>0.705 because the regeneration time decreased too small. It revealed that the regeneration reaction was faster than the adsorption reaction. Increase of total solids inventory is a good way to get further increase in capture efficiency.     

The characteristics of inorganic elements in ashes from a 1 MW CFB biomass gasification power generation plant

This paper investigated the characteristics of inorganic elements in ashes from biomass gasification power generation (BGPG) plant. The ash samples of the gasifier ash, separator ash and wet scrubber ash were collected in a 1 MW circulating fluidized bed (CFB) wood gasification power generation plant. Particle size distribution of ashes was determined by gravimetric measurement and super probe analyzer. The concentrations of trace elements and major ash-forming elements, such as As, Al, Ca, Cd, Cr, Cu, K, Mg, Na, Ni, Pb, Ti in different ashes as a function of particle size were determined by Inductive Coupled Plasma Spectrometer. The concentrations and distribution coefficient and enrichment factors of the inorganic elements in ashes were studied. X-ray fluorescence spectrometer and X-ray powder diffraction were used to provide information on the characteristics of the ashes. The results showed that most of the trace elements had an enrichment tendency in the finer size particles. A considerable amount of the ashes was residual carbon. Most of the volatile e.g. halogen elements and alkali elements existed mainly in wet scrubber ash and enriched in fly ash. Most of the Si, Ni, Pb, Zn, Cr, Cd were found in separator ash, indicating an enrichment of heavy metal elements in separator ash. K, S, Mn, Cu mainly existed in gasifier ash.     

Calculation of the conditions to get less than 2 g tar/mn3 in a fluidized bed biomass gasifier

The experimental conditions under which a fluidized bed biomass gasifier can generate a gas with a tar content below 2 g/m_n³ are analyzed by using and developing the model recently published for those gasifiers by Corella and Sanz [Fuel Process. Techn. 2005, 86, 1021–1053]. The analyzed experimental conditions were: the equivalence ratio, the partitioning of the air, between the primary and secondary flows, the location (height) of the inlet of the secondary air flow, the biomass moisture and the biomass flow rate. Results from the modelling work are presented for a given CFB biomass gasifier of commercial size. Some of these results are also being checked in a CFB biomass gasifier at small pilot plant scale. To obtain a gasification gas with a very low tar content the two most important experimental conditions are a high value for the equivalence ratio and a good in-gasifier material which determines the values of the kinetic constants of the reactions involved in the network at the gasifier.     

Effect of experimental conditions on co-gasification of coal,biomass and plastics wastes with air/steam mixtures in a fluidized bed system

The effect of temperature and of gasification medium was studied, using only air, only steam and mixtures of both as gasification medium, with the aim of optimising co-gasification of coal and wastes. The rise in gasification temperature promoted hydrocarbons further reactions, leading to a decrease in tars and hydrocarbons contents and an increase in H₂ release. Increasing temperature, from 750 to 890 °C, during gasification of a mixture with 60% (w/w) of coal, 20% of pine and 20% of PE wastes, led to a decrease in methane and other hydrocarbons concentration of about 30 and 63%, respectively, whilst hydrogen concentration increased around 70%. Hydrocarbons contents decrease was also achieved by increasing air flow rate, because partial combustion caused by oxygen decreased tars and gaseous hydrocarbons, with even a decrease in heating requirements. However, the presence of air is disadvantageous, because it decreases the higher heating value of the gasification gas, due to nitrogen diluting effect. The rise of steam flow rate has proven to be advantageous, because reforming reactions were favoured, thus hydrocarbons concentrations decreased and hydrogen release increased.     

Distribution of large biomass particles in a sand‐biomass fluidized bed: Experiments and modeling

The axial distribution of large biomass particles in bubbling fluidized beds comprised of sand and biomass is investigated in this study. The global and local pressure drop profiles are analyzed in mixtures fluidized at superficial gas velocities ranging from 0.2 to 1 m/s. In addition, the radioactive particle tracking technique is used to track the trajectory of a tracer mimicking the behavior of biomass particles in systems consisting of 2, 8, and 16% of biomass mass ratio. The effects of superficial gas velocity and the mixture composition on the mixing/segregation of the bed components are explored by analyzing the circulatory motion of the active tracer. Contrary to low fluidization velocity (U = 0.36 m/s), biomass circulation and distribution are enhanced at U = 0.64 m/s with increasing the load of biomass particles. The axial profile of volume fraction of biomass along the bed is modeled on the basis of the experimental findings. © 2014 American Institute of Chemical Engineers AIChE J, 60: 869–880, 2014