Estimation of wood char size at the end of devolatilization in a bubbling fluidized bed combustor

Two processes, namely shrinkage and primary fragmentation are known to be the major causes of size reduction of wood during its devolatilization in a fluidized bed combustor. A simple phenomenological model incorporating these effects to compute the average char size at the end of devolatilization is proposed. Experiments are conducted in a bubbling fluidized bed combustor using wood having three different shapes namely, cylinder, cuboid and sphere, to measure the average char size at the end of devolatilization. The model prediction of average char size agrees with the measured values within a deviation of 15%. An experimental correlation is derived to determine the number of fragments and is used to estimate the mean char size.     

Transient thermal behaviour of a cylindrical wood particle during devolatilization in a bubbling fluidized bed

This work proposes a transient heat transfer model to predict the thermal behaviour of wood in a heated bed of sand fluidized with nitrogen. The 2-D model in cylindrical coordinates considers wood anisotropy, variable fuel properties, fuel particle shrinkage, and heat generation due to drying and devolatilization. The influence of initial fuel moisture content, thermal diffusivity, particle geometry, shrinkage, external heat transfer coefficient, chemical reaction kinetics and heats of reaction on temperature rise is presented. The cylindrical wood particles chosen for the study have length (l) = 20 mm, diameter (d) = 4 mm and l = 50 mm and d = 10 mm, both having an aspect ratio (l/d) of 5. The bed temperature is 1123 K. The model prediction is validated using measurements obtained from literature. The temperature rise in the wood particle is found to be sensitive to changes in the moisture content and thermal diffusivity and heat of reaction (in larger particles) while it is less sensitive to the external heat transfer coefficient and chemical kinetics. Also shrinkage is found to have a compensating effect and it does not have any significant influence on the temperature rise. Beyond an aspect ratio of three, the wood particle behaves as a 1-D cylinder.     

Mass loss and apparent kinetics of a thermally thick wood particle devolatilizing in a bubbling fluidized bed combustor

This work presents the results of experiments conducted to determine the mass loss characteristics of a cylindrical wood particle undergoing devolatilization under oxidation conditions in a bubbling fluidized bed combustor. Cylindrical wood particles having five different sizes ranging from 10 to 30 mm and aspect ratio (l/d = 1) have been used for the study. Experiments were conducted in a lab scale bubbling fluidized bed combustor having silica sand as the inert bed material and air as the fluidizing medium. Total devolatilization time and mass of wood/char at different stages of devolatilization have been measured. Studies have been carried out at three different bed temperatures (T_bed = 750, 850 and 950 °C), two inert bed material sizes (mean size d_p = 375 and 550 μm) and two fluidizing velocities (u = 5u_mf and u = 10u_mf). Devolatilization time is most influenced by the initial wood size and bed temperature. Most of the mass is lost during the first half of the devolatilization process. There was no clear influence of the fluidization velocity and bed particle size on the various parameters studied. The apparent kinetics estimated from the measured mass history show that the activation energy varied narrowly between 15 and 27 kJ/mol and the pre-exponential factor from 0.11 and 0.45 s⁻¹ for the wood sizes considered.     

Modeling the temperature in coal char particle during fluidized bed combustion

The temperatures of a coal char particle in hot bubbling fluidized bed (FB) were analyzed by a model of combustion. The unsteady model includes phenomena of heat and mass transfer through a porous char particle, as well as heterogeneous reaction at the interior char surface and homogeneous reaction in the pores. The parametric analysis of the model has shown that above 550 °C combustion occurs under the regime limited by diffusion. The experimental results of temperature measurements by thermocouple in the particle center during FB combustion at temperatures in the range 590-710 °C were compared with the model predictions. Two coals of different rank were used: lignite and brown coal, with particle size in the range 5-10 mm. The comparisons have shown that the model can adequately predict the histories of temperatures in char particles during combustion in FB. In the first order, the model predicts the influence of the particle size, coal rank (via porosity), and oxygen concentration in its surroundings.     

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.     

典型尺寸燃煤颗粒富氧燃烧特性及燃烧本征动力学研究

利用微型流化床加热速度快、温度分布均匀以及气体近平推流等优势,在直径20 mm自动控温的微型流化床反应分析仪中研究了粒度分布为1.7~3.35 mm和0.12~0.23 mm两种典型尺寸燃煤颗粒在790~900℃温度范围内的富氧燃烧行为。通过快速响应过程质谱对燃烧产生的烟气进行实时监测,成功地识别和记录了粗颗粒燃烧过程中经历的挥发分燃烧和原位新生半焦燃烧两个主要阶段。挥发分析出速度最快,然后快速燃烧,而半焦燃烧速度较慢。相比之下,细颗粒燃烧的这两个阶段具有几乎相同的速率,因而相互耦合而难以区分。根据实验结果,挥发分析出和燃烧为快速反应,煤颗粒燃烧过程速率受原位新生半焦燃烧过程控制。进一步研究了挥发分和原位新生半焦燃烧动力学行为,获得其本征动力学的活化能分别为107.2和143.9 kJ/mol。     

NO emission on pulverized coal combustion in high temperature air from circulating fluidized bed – An experimental study

High temperature air was adopted by combustion in high excess air ratio in a circulating fluidized bed. Experiments on pulverized coal combustion in high temperature air from the circulating fluidized bed were carried out in a down-fired combustor with the diameter of 220 mm and the height of 3000 mm. The NO emission decreases with increasing the residence time of pulverized coal in the reducing zone, and the NO emission increases with excess air ratio, furnace temperature, coal mean size and oxygen concentration in high temperature air. The results also revealed that the co-existing of air-staging combustion with high temperature air is very effective to reduce nitrogen oxide emission for pulverized coal combustion in the down-fired combustor.     

Modelling and experimental investigation of devolatilizing wood in a fluidized bed combustor

A two-dimensional model is developed for the determination of devolatilization time and char yield of cylindrical wood particles in a bubbling fluidized bed combustor. By using the concept of shape factor, the model is extended to particles of cuboid shape. The model prediction of the devolatilization time agrees with the measured data (present and those reported in the literature) for cylindrical and cuboidal shaped particles within ±20% while the char yield is predicted within ±17%. Influence of some important parameters namely, thermal diffusivity, external heat transfer coefficient and shrinkage, on the devolatilization time and char yield are studied. Thermal diffusivity shows noticeable influence on devolatilization time. The external heat transfer coefficient shows little influence beyond a value of 300 W/(m² K). However particle shrinkage shows negligible effect on the devolatilization time but has a significant influence on the char yield.     

Hydration products of FBC wastes as SO2 sorbents: comparison between ettringite and calcium hydroxide

Fluidized bed combustion (FBC) enables the in situ capture of SO₂, but generates large amounts of wastes whose composition and physico-chemical properties make both landfilling and reuse in traditional fields of application (e.g., cement and concrete industries) problematic. Reactivation by water hydration of the desulphurizing ability of these residues is considered a viable mean for their recycling: besides Ca(OH)₂, this process can generate other hydration products, such as ettringite. This paper is devoted to a comparison between the behaviour of Ca(OH)₂ and ettringite as SO₂ sorbents. To this end, synthetic preparations (in the particle size range 0.4–0.6 mm) of the two materials were dehydrated and then sulphated in a lab-scale fluidized bed reactor. Sulphation tests were carried out at 850 °C by fluidizing the bed with an SO₂–N₂–O₂ mixture (1800 ppm SO₂) at 0.8 m/s. Calcium conversion degree and fines elutriation rate were evaluated as a function of sulphation time. The propensity of the sorbents to undergo fragmentation was also estimated by particle sizing of in-bed exhausted sorbent particles, with the aid of laser granulometry. Mercury intrusion porosimetry of samples was directed to the assessment of the influence of sorbent dehydration and subsequent sulphation on pore size distribution and porosimetric texture. X-ray diffraction and differential thermal analysis on the synthetic sorbents complemented the characterization. Results showed that dehydration/thermal decomposition brought about a significant increase of the overall porosity for both sorbents, more extensive than it is commonly observed with calcined commercial limestones. Upon sulphation, the two sorbents showed satisfactory degrees of calcium conversion, larger than those usually observed with limestones. Sulphation resulted into a decrease of particle voidage (that of the Ca(OH)₂-based sorbent was negligible after the process). Ettringite was more prone to attrition/fragmentation than calcium hydroxide. Results are discussed with a focus on differences between calcium hydroxide and ettringite and on key-parameters affecting the performance of the two materials as sorbents.     

Carbon conversion of solid fuels in the freeboard of a laboratory-scale fluidized bed combustor—application of in situ laser spectroscopy

The carbon conversion of different solid fuels (i.e. beech wood, fir wood, bituminous coal) was investigated in the freeboard of a laboratory-scale fluidized bed combustor by in situ tunable diode laser absorption spectroscopy. A room temperature continuous wave InGaAsSb/AlGaAsSb quantum well ridge diode laser emitting at 2.3-2.35 μm was wavelength tuned at 300 Hz to determine simultaneously CH₄ and CO during devolatilization and char combustion in situ 10 mm above the fuel particles. The lower detection limit was 0.2 vol% (5000 ppm m) for both species. In addition, CO, CO₂ and O₂ were determined ex situ by conventional methods.The experimental results obtained for the bituminous coal were compared to a detailed chemical kinetic model.The in situ measurements proved to be advantageous compared to conventional ex situ concentration measurements. The calculations confirm the determination of the primary products of solid fuel combustion during devolatilization and char combustion. A rather simple model for the devolatilization products was proven to describe well the release rates of CH₄ and CO for the bituminous coal.     

Effects of particle shape and size on devolatilization of biomass particle

Experimental and theoretical investigations indicate particle shape and size influence biomass particle dynamics, including drying, heating rate, and reaction rate. Experimental samples include disc/flake-like, cylindrical/cylinder-like, and equant (nearly spherical) shapes of wood particles with similar particle masses and volumes but different surface areas. Small samples (320 μm) passed through a laboratory entrained-flow reactor in a nitrogen atmosphere and a maximum reactor wall temperature of 1600 K. Large samples were suspended in the center of a single-particle reactor. Experimental data indicate that equant particles react more slowly than the other shapes, with the difference becoming more significant as particle mass or aspect ratio increases and reaching a factor of two or more for particles with sizes over 10 mm. A one-dimensional, time-dependent particle model simulates the rapid pyrolysis process of particles with different shapes. The model characterizes particles in three basic shapes (sphere, cylinder, and flat plate). With the particle geometric information (particle aspect ratio, volume, and surface area) included, this model simulates the devolatilization process of biomass particles of any shape. Model simulations of the three shapes show satisfactory agreement with the experimental data. Model predictions show that both particle shape and size affect the product yield distribution. Near-spherical particles exhibit lower volatile and higher tar yields relative to aspherical particles with the same mass under similar conditions. Volatile yields decrease with increasing particle size for particles of all shapes. Assuming spherical or isothermal conditions for biomass particles leads to large errors at most biomass particle sizes of practical interest.     

Experimental study of NO reduction over biomass char

Some biomass fuels produce more NO_x than coal on the basis of heating value, giving rise to the necessity and importance of controlling NO_x emission in biomass combustion. The present study investigated the NO reduction over biomass char in a fixed bed quartz reactor in the temperature range of 973–1173 K. The reaction rates of three biomass chars (sawdust, rice husk and corn straw) with NO were compared with Datong bituminous coal char. The results show that the reaction orders of biomass chars for NO are of fractional order and independent of temperature. Biomass chars are more active in reducing NO than coal char. The characteristics of biomass char affect NO conversion. Biomass char formed at high pyrolysis temperature, especially large in particle size, is less active in reducing NO. To some extent, increase of reaction temperature and char loading enhance NO conversion. There exists an optimum bed height for the highest NO conversion. Moreover, NO reduction over biomass char is also enhanced in the presence of CO, O₂ and SO₂.     

Near-wall porosity characteristics of fixed beds packed with wood chips

Packed beds of fuel wood chips are commonly found in thermal conversion processes such as combustion or gasification. Wood chips in particular are mostly used as fuel for small-scale domestic heating boilers but also for commercial-scale combustion units. The characterization of spatial voidage distribution inside the wood chip beds is of great importance for flow and reactor modelling. This study focuses on the radial porosity variations of cylindrical beds of three different types of commercially available wood chips including chips classified as G30 size class. The conventional technique of consolidating packed beds with a resin was chosen as the experimental procedure. The radial voidage distribution in different cylindrical beds is determined by image analysis of sections of the solidified packings. Additionally, a packing of monosized spheres was investigated in order to assess the selected procedure in comparison with widely available literature data for spheres. The results are discussed and summarized in a mathematical expression correlating the radial voidage distribution depending on average wood chip size, packing core porosity and dimensionless distance from the tube wall.     

Combustion of liquid petroleum gas in a fluidized bed furnace with a jetting-mixing distributor

Liquid petroleum gas (LPG) fluidized beds have potential applications in metal heating or workpiece heat treatments. The combustion of LPG and the controls of the atmosphere inside the bed and the bed temperature are very concerned. The combustion of LPG has been investigated in a pilot-scale bubbling fluidized bed with a jetting-mixing nozzle distributor and hollow corundum sphere particles of 0.867-1.212 mm in diameter and 386-870 kg/m³ in bulk density at 800-1100°C. Experiments were carried out for fuel-rich mixtures to explore the possibility to obtain mild oxidizing, non-oxidizing or reducing atmosphere in the bed. Air factor (the ratio of the volume of air actually fed into the bed to that in a stoichiometric mixture) is in between 0.3 and 1.0 and U/U_mf 1.3-3.0. The distributor brings LPG and air into an intense contact sufficient to permit in-bed combustion without backfire problems. The experimental results show that the fluidized bed furnace offers excellent thermal uniformity and temperature control. The size of the combustion zone is usually larger than that of the temperature variation zone. Particle properties, initial bed height, air factor and U/U_mf all affect the bed temperature profile, whereas only the air factor and U/U_mf have significant effects on the combustion in the bed. The bed temperature can be adjusted by separate or combined adjusting of air factor and U/U_mf.     

Fuel and emissions properties of Stirling engine operated with wood powder

From viewpoints of the environment and fuel cost reduction, small-scale biomass combined heat and power (CHP) plants are in demand, especially wood-waste fueled system, which are simple to operate and maintenance-free with high thermal efficiency similar to oil fired units. These are requested by wood and other industries located in mountainous region. To meet these requirements, a Stirling engine CHP system combined with simplified biomass combustion process with pulverized wood powder was developed.In an R&D project started in 2004 considering wood powder properties as a fuel, combustion performance and emissions in combustion flue gas were tested using combustion test apparatus with commercial size units. The wood powder combustion system was modified and optimized during the combustion test results, and the design of the demonstration plant combined with 55 kW_e Stirling engine power unit was considered. The demonstration plant was finally completed in March of 2006, and test operation has been progressed for the future commercial CHP system.In the wood powder combustion test, wood powder of less than 500 μm is mainly used, and a combustion chamber length of 3 m is applied. In these conditions, the air ratio can be reduced to 1.1 without increasing CO emission of less than 10 ppm and combustion efficiency of 99.9%. In the same conditions, NO_x emission is estimated to be less than 120 ppm (6% O₂ basis). Wood powder was confirmed to have excellent properties as a fuel for Stirling engine CHP system. This paper summarizes the wood powder combustion test, and presents the evaluation of the burner design parameters for the biomass Stirling engine system.     

Pyrolysis and gasification of pellets from sugar cane bagasse and wood

Wood pellets have become a popular form of biomass for power generation and residential heating due to easier handling both for transportation and for feeders in the treatment units, improved conversion and storage possibilities. The research on wood pellets as fuel has also been intensified during the past decade. However, other biomass sorts in pellet form, such as sugar cane bagasse, have not yet been extensively studied, especially not physical effects on the pellets during thermal treatment. Bagasse and wood pellets of different origin and sizes, shredded bagasse and wood chips have been studied in a thermogravimetric equipment to compare the effects of sort, origin, size and form of biomass during slow pyrolysis and steam gasification. Physical parameters such as decrease of volume and mass during treatment, as well as pyrolysis and gasification rates are of primary interest in the study. An important observation from the study is that for pellets the char density decreased during pyrolysis to a minimum around 450 °C, but thereafter increased with continued heating. The wood chips behaved differently with a continuous char density decrease during pyrolysis. Another conclusion from the work is that the size of the pellet has larger impact on the shrinkage behaviour throughout the conversion than the raw material, which the pellet is made of.     

Fluidized bed pyrolysis of distilled spirits lees for adapting to its circulating fluidized bed decoupling combustion

Distilled spirits Lees, rich in cellulose, water and N element, are difficult to burn efficiently and cleanly in grate chain stock boiler. The circulating fluidized bed decoupling combustion (CFBDC) was therefore proposed to burn the distilled spirits lees efficiently and with low-NOx emission. The pyrolysis behavior of the distilled spirits lees was investigated in a fluidized bed reactor for optimizing the pyrolysis conditions of the pyrolyzer in CFBDC. The results showed that the distilled spirits lees began to devolatize at 250 °C and at 350–450 °C the tar yield reached its maximum of about 16.3 wt.% (dry base). The chemical oxygen demand (COD) value of the condensed liquid reached its maximum of about 50,000 mg/L at 450 °C. With raising temperature the pyrolysis gas tended to contain more CO and H₂ and less CO₂. The functional groups H-O, aliphatic C-H, aromatic ring, C=O and C-O were all presented in the char generated at low-temperatures, while only the C-O group was identified for the char from the pyrolysis at 650 °C. The article suggested that the pyrolysis for the CFBDC was better around 500 °C so that certain volatiles could remain in the char to sustain stable combustion.     

Pulverized coal combustion and NOx emissions in high temperature air from circulating fluidized bed

A new technique of achieving high temperature air was adopted by combustion in high excess air ratio in a circulating fluidized bed (CFB). Experiments on pulverized coal combustion in high temperature air from the CFB were made in a down-fired combustor with the diameter of 220 mm and the height of 3000 mm. High temperature air with lower oxygen concentrations can be achieved steadily and continuously by combustion in the circulating fluidized bed. Pulverized coal combustion in high temperature air shows a uniform temperature profile along the axis of the down-fired combustor and the combustion efficiency is 99.8%. The NO_x emission is 390 mg/m³, 13% lower than the regulation for thermal power plants in China. The HCN and NH₃ emissions, as well as N₂O, are about zero in the exhaust.     

Modelling of circulating fluidized bed combustion of a char

The combustion of a char in the 41 mm ID riser of a laboratory circulating fluidized bed combustor has been investigated at different air excesses and rates of solids (char and sand) circulating in the loop. Riser performance was characterized by an axial oxygen concentration profile as well as by the overall carbon content and particle size distribution. The proposed model accounts for carbon surface reaction, intraparticle and external diffusion, and attrition. External diffusion effects were relevant in the riser dense region where char was potentially entrapped in large clusters of inert solids. Experimental data and results of the model calculations are in satisfactory agreement.     

Characteristics and mechanism of catalytic effect of inner minerals on combustion of oil shale coke

The micro fluidized bed reaction analyzer (MFBRA) was used to study the combustion characteristics of oil shale mineral catalytic semi-coke. This study compared the effect of minerals inside char and bed material (oil shale ash) outside char on char combustion, and the process and mechanism of char combustion in the fluidized bed were further revealed. Both of minerals inside char and bed material outside char had a marked catalysis for char combustion and their combined catalysis was most notable. It is found that the CaO and Fe₂O₃ were the major active components in oil shale minerals for catalytic combustion of char, and the catalysis of CaO was stronger than that of Fe₂O₃. The activation energy of char combustion ranged from 60.41 kJ/mol to 78.97 kJ/mol, and it would significantly decrease with presence of the catalysis by minerals in oil shale. For char combustion in a fluidized bed, the contribution of minerals to catalytic combustion was mainly reflected in four reactions, such as volatiles cracking and combustion, surface carbon combustion, internal carbon combustion and CO combustion.