Devolatilization Time Research Articles

Devolatilization of oil sludge in a lab-scale bubbling fluidized bed

Devolatilization of oil sludge pellets was investigated in nitrogen and air atmosphere in a lab-scale bubbling fluidized bed (BFB). Devolatilization times were measured by the degree of completion of the evolution of the volatiles for individual oil sludge pellets in the 5–15 mm diameter range. The influences of pellet size, bed temperature and superficial fluidization velocity on devolatilization time were evaluated. The variation of devolatilization time with particle diameter was expressed by the correlation, τ d = A d p N . The devolatilization time to pellet diameter curve shows nearly a linear increase in nitrogen, whereas an exponential increase in air. No noticeable effect of superficial fluidization velocity on devolatilization time in air atmosphere was observed. The behavior of the sludge pellets in the BFB was also focused during combustion experiments, primary fragmentation (a micro-explosive combustion phenomenon) was observed for bigger pellets (>10 mm) at high bed temperatures (>700 °C), which occurred towards the end of combustion and remarkably reduce the devolatilization time of the oil sludge pellet. The size analysis of bed materials and fly ash showed that entire ash particle was entrained or elutriated out of the BFB furnace due to the fragile structure of oil sludge ash particles.

Transient Three-Dimensional Mathematical Model and Experimental Investigation of a Wet Devolatilizing Wood in a Hot Fluidized Bed

Devolatilization of a single wet wood particle in a hot atmospheric fluidized bed is studied through a mathematical model and experiments. A typical tropical woody biomass, “Casuarina Equisetifolia”, is used in the study. A three-dimensional (3D) transient model with detailed consideration of (i) fuel anisotropy, (ii) fuel moisture, (iii) shrinkage during drying and devolatilization, (iv) heat generation, and (v) variable properties with a suitable reaction scheme is developed to determine the devolatilization time (τd) and char yield (Yc) of cubic/cuboidal wood particles at various initial fuel moisture contents and bed temperatures. Experiments are conducted for 10, 15, 20, and 25 mm cube-shaped wood particles at bed temperatures of 1023, 1123, and 1223 K. The model predictions agree well with the measured data (present experiments and those reported in the literature) within ±10% for the devolatilization time and ±11% for the char yield, supporting the validity of the overall structure of the model. A ...

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 = 5 u mf and u = 10 u 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.

Devolatilization of coals of northeastern India in inert atmosphere and in air under fluidized bed conditions

Abstract Devolatilization of five coals having volatile matter in the range of 31 to 41% was studied in argon and in air under fluidized bed conditions. The diameter of the coal particles varied between 4 and 9.5 mm. The variation of devolatilization time with particle diameter was expressed by the correlation, t v = Ad v n . The superficial gas velocity was found to have a significant effect on the rate of devolatilization. The devolatilization rate increased with the increase in the oxygen concentration in the fluidizing gas. The correlations developed in this study fitted the mass versus time profiles of the coal particles satisfactorily. The same correlations were found to be appropriate for predicting devolatilization of a batch of coal particles. The correlations developed in the present study will be useful for the design of fluidized bed combustors.

Thermogravimetric characteristics of char obtained at high heat rate

Coal reactivity during combustion is determined by the thermogravimetric characteristics of char. The thermogravimetric characteristics of chars obtained in a droptube furnace were studied in this work. Chars from the devolatilisation of three bituminous coals were obtained at three times (100, 150 and 300 ms), three temperatures (900, 1,000 and 1,100ºC) and at high heat rate (104 K/s). The chars were burned using non-isothermal thermogravimetry (heated to 900ºC) and isothermal thermogravimetry at 700ºC, 800ºC and 900ºC to obtain their combustion profiles. Characteristic temperatures (ignition, peak and final temperatures) were determined by non-isothermal thermogravimetry; it was found that chars from La Yolanda coal gave the highest figures for the characteristic temperatures. Isothermal thermogravimetry revealed that the combustion rate for the three coals decreased with increased devolatilisation time and combustion temperature.

A new char combustion kinetic model 1. Formulation

This paper describes the development and formulation of a novel char combustion kinetic model for pulverised coal. The chemical reaction rate was considered as a function of coal intrinsic reactivity and fuel mass for a global reaction order n. The equation for char combustion reaction rate was devised by dimensional analysis using the Rayleigh method. It shows the dependency of chemical reactivity on coal parameters, such as maceral and mineral composition; char parameters, such as specify surface area and apparent density; devolatilisation time; temperature; and activation energy. The char combustion reaction rate equation developed, unlike most models, is dimensionally homogenous and is expressed by three dimensionless numbers which have physical chemical meaning. The model also exhibit similarities with transport phenomena models described by dimensionless numbers.

Numerical simulation of the devolatilization of a moving coal particle

The devolatilization of an isolated coal particle moving relative to the surrounding gas is numerically simulated using a competing reaction model of the pyrolysis and assuming that the released volatiles burn in an infinitely thin diffusion flame around the particle or not at all. The temperature of the particle is assumed to be uniform and the effects of the heat of pyrolysis, the intraparticle mass transfer resistance, and the variation of the particle radius are neglected. The effects of the size and velocity of the particle and of the temperature and oxygen mass fraction of the gas on the particle and flame temperature histories, the devolatilization time and the yield of light and heavy volatiles are investigated. The motion of the particle may have an important effect on the shape and position of the flame of volatiles, but it has only a mild effect on the devolatilization process for the particle sizes typical of pulverized coal combustion. This effect increases for large particles or in the absence of radiation. The relative motion enhances the heat transfer between the particle and the gas, causing the devolatilization time to decrease at high gas temperatures and to increase at low gas temperatures. The numerical results are compared with a blowing-corrected Nusselt number correlation often used in heat transfer models of the process.

Particle imaging of ignition and devolatilization of pulverized coal during oxy-fuel combustion

Oxy-fuel combustion of coal is a promising technology for cost-effective power production with carbon capture and sequestration that has ancillary benefits of emission reductions and lower flue gas cleanup costs. To fully understand the results of pilot-scale tests of oxy-fuel combustion and to accurately predict scale-up performance through CFD modeling, fundamental data are needed concerning coal and coal char combustion properties under these unconventional conditions. In the work reported here, the ignition and devolatilization characteristics of both a high-volatile bituminous coal and a Powder River Basin subbituminous coal were analyzed in detail through single-particle imaging at a gas temperature of 1700 K over a range of 12–36 vol % O 2 in both N 2 and CO 2 diluent gases. The bituminous coal images show large, hot soot cloud radiation whose size and shape vary with oxygen concentration and, to a lesser extent, with the use of N 2 versus CO 2 diluent gas. Subbituminous coal images show cooler, smaller emission signals during devolatilization that have the same characteristic size as the coal particles introduced into the flow (nominally 100 μm). The measurements also demonstrate that the use of CO 2 diluent retards the onset of ignition and increases the duration of devolatilization, once initiated. For a given diluent gas, a higher oxygen concentration yields shorter ignition delay and devolatilization times. The effect of CO 2 on coal particle ignition is explained by its higher molar specific heat and its tendency to reduce the local radical pool. The effect of O 2 on coal particle ignition results from its effect on the local mixture reactivity. CO 2 decreases the rate of devolatilization because of the lower mass diffusivity of volatiles in CO 2 mixtures, whereas higher O 2 concentrations increase the mass flux of oxygen to the volatiles flame and thereby increase the rate of devolatilization.

Devolatilization of coals of North-Eastern India under fluidized bed conditions in oxygen-enriched air

Oxygen-enriched air can increase the combustion efficiency, boiler efficiency, and sulfur absorption efficiency of atmospheric fluidized bed combustion (AFBC) boilers which use high-sulfur coal, and other combustion systems that use coal. Devolatilization is the first step in the gasification or combustion of coal. In this work, devolatilization characteristics of five run-of-mine (ROM) coals of North-Eastern India having particle-size between 4 mm and 9 mm are reported. The experiments were performed under fluidized bed conditions at 1123 K in enriched air containing 30% oxygen. The devolatilization time was correlated with the particle diameter by a power law correlation. The variation of mass with time was correlated by an exponential correlation. It was observed that the average ratio of yield of volatile matter to the proximate volatile matter decreased with the increase in volatile-content of the coals. A shrinking-core model was used to determine the role of film-diffusion, ash-diffusion and chemical reaction. The experimental results indicate the likelihood of film-diffusion to be the rate-controlling mechanism in presence of oxygen-enriched air. A cost-analysis was carried out to study the economy of the process.

Fluidized bed combustion of pelletized biomass and waste-derived fuels

The fluidized bed combustion of three pelletized biogenic fuels (sewage sludge, wood, and straw) has been investigated with a combination of experimental techniques. The fuels have been characterized from the standpoints of patterns and rates of fuel devolatilization and char burnout, extent of attrition and fragmentation, and their relevance to the fuel particle size distribution and the amount and size distribution of primary ash particles. Results highlight differences and similarities among the three fuels tested. The fuels were all characterized by limited primary fragmentation and relatively long devolatilization times, as compared with the time scale of particle dispersion away from the fuel feeding ports in practical FBC. Both features are favorable to effective lateral distribution of volatile matter across the combustor cross section. The three fuels exhibited distinctively different char conversion patterns. The high-ash pelletized sludge burned according to the shrinking core conversion pattern with negligible occurrence of secondary fragmentation. The low-ash pelletized wood burned according to the shrinking particle conversion pattern with extensive occurrence of secondary fragmentation. The medium-ash pelletized straw yielded char particles with a hollow structure, resembling big cenospheres, characterized by a coherent inorganic outer layer strong enough to prevent particle fragmentation. Inert bed particles were permanently attached to the hollow pellets as they were incorporated into ash melts. Carbon elutriation rates were very small for all the fuels tested. For pelletized sludge and straw, this was mostly due to the shielding effect of the coherent ash skeleton. For the wood pellet, carbon attrition was extensive, but was largely counterbalanced by effective afterburning due to the large intrinsic reactivity of attrited char fines. The impact of carbon attrition on combustion efficiency was negligible for all the fuels tested. The size distribution of primary ash particles liberated upon complete carbon burnoff largely reflected the combustion pattern of each fuel. Primary ash particles of size nearly equal to that of the parent fuel were generated upon complete burnoff of the pelletized sludge. Nonetheless, secondary attrition of primary ash from pelletized sludge is large, to the point where generation of fine ash would be extensive over the typical residence time of bed ash in fluidized bed combustors. Very few and relatively fine primary ash particles were released after complete burnoff of wood pellets. Primary ash particles remaining after complete burnoff of pelletized straw had sizes and shapes that were largely controlled by the occurrence of ash agglomeration phenomena.

Devolatilization of wood particles in a hot fluidized bed: Product yields and conversion rates

Abstract Spherical wood particles of various well-defined diameters were fed into a bed of fluidized olivine maintained at temperatures in the range of 700–900 °C. The fluidizing gas was either nitrogen or a steam–nitrogen mixture, and the wooden spheres were introduced in two different ways: on to the bed surface and deep into the bed itself. The resulting gas, tar and char yields were found to be influenced by the method of feed introduction as well as by the bed temperature, fluidizing gas composition and wood particle diameter. The total devolatilization time was correlated by means of a power law expression similar to that used for coal devolatilization and having the advantage of being able to fit well the experimental data over the full temperature range in both nitrogen and steam–nitrogen atmospheres.

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/(m 2 K). However particle shrinkage shows negligible effect on the devolatilization time but has a significant influence on the char yield.

Devolatilisation behaviour of shredded tyre chips in combusting environment

Tyres shredded into chips represent a practicable alternative fuel for cement kilns. The devolatilisation and final burnout of tyre chips in a combusting environment in a refractory lined, uniform temperature, radiative furnace have been investigated. The weight losses of tyre chips suspended in the furnace for temperatures of 700, 800, 900 and 1000uC were monitored. As expected, the devolatilisation time is greatly influenced by the furnace temperature and by the thickness of the tyre chip. The tyre chips burn with a vigorous flame during devolatilisation. The results also showed that, although the chips fragment on the surface during devolatilisation the cord fibres of the tyre structure tend to maintain the integrity of the chip until devolatilisation is complete when the chip disintegrate resulting in a wide range of char particle sizes. Upon burnout of the volatiles, the residual char burns to extinction in a low intensity flame. A simple heat transfer controlled model to simulate the devolatilisation of tyre chips is proposed.

The effect of pulverised coal’s devolatilisation conditions on char physical characteristics

This study examined the effect of pulverised coal fuel devolatilisation conditions on char’s physical characteristics. A devolatilisation system was thus designed and built where three bituminous coals were devolatilised at three different times (100, 150 and 300 ms) and at three temperatures (900°C, 1,000°C and 1,100°C). The resulting char was characterised regarding porosity, pore volume, surface area and particle size distribution. It was found that char porosity and pore volume increased with devolatilisation temperature and time. Devolatilisation operation conditions did not produce any effect on char’s specific surface area. However, char particle size was bigger than original coal size.

The devolatilisation of particles of a complex fuel (dried sewage sludge) in a fluidised bed

A complex fuel is understood here to be a solid whose pyrolysis or devolatilisation (i.e., thermal decomposition) involves many reactions. Sewage sludge is such a fuel and when dry can contain more than 80 wt% of volatile matter. Consequently, the combustion of a sewage sludge is mainly a question of burning the volatile matter. Sewage sludge particles in a fluidised bed of size 0.6–8 mm pyrolyse (in the absence of O 2) or devolatilise (in the presence of O 2, when the volatile material burns) at a rate determined by (i) chemical kinetics, or (ii) heat transfer, either by conduction within the pellet or from the fluidised bed to the pellet's exterior. A scaling analysis, in conjunction with kinetic information obtained from thermogravimetric experiments, showed that most of the volatile matter in our sludge was lost in a manner controlled by heat transfer, after the sludge was added to the fluidised bed. This observation is consistent with the times taken for the evolution of 60% or 80% of the volatile matter, when the fuel was pyrolysed in argon, varying approximately as: constant 1 × d s 2 + constant 2 × d s , d s being the diameter of the pellet. Devolatilisation time, t v , can be correlated with d s by t v = constant 2 × d s n , where n = 1.45 ± 0.15 . Previous workers obtained lower values of n. It is suggested that some differences in the value of n are related to the definition used for the devolatilisation time. The variation with d s for the evolution of 95% of the volatile material, for the smaller particles used, was much less clear than for the release of 60–80% of the volatiles. Given the scatter, a value of n = 0 could not be discounted. Such a value of n is consistent with the most tightly bound volatile material being released at a rate determined by chemical kinetics.

Devolatilization characteristics of large particles of tyre rubber under combustion conditions

A systematic experimental study has been performed in order to investigate the effect of particle size and temperature on the devolatilization rate of large tyre rubber particles. Cylindrical tyre particles with diameters between 7.5 and 22 mm were devolatilised in a macro-TGA reactor, at temperatures between 490 and 840 °C in an inert atmosphere. The effect of particle size and surrounding temperature on the rate of devolatilization was observed to be significant, i.e. larger particle diameters and lower temperatures increased the devolatilization time. A detailed mathematical model for the devolatilization process including internal and external heat transfer, three parallel independent devolatilization reactions and reaction enthalpy effects has been developed and solved using an implicit finite difference method. Comparison of the model predictions with experimental data, reveals that the devolatilization process of large tyre rubber particles at temperatures above 490 °C can be considered to be controlled mainly by heat transfer and reaction kinetics. The model analysis further shows that exothermic devolatilization reaction enthalpy effects cannot be neglected in the prediction of the intra particle temperature rise. A sensitivity analysis of the model parameters, demonstrates that the specific heat capacity of the virgin fuel and the activation energies of the devolatilization reactions is the most important model parameters in the prediction of devolatilization times of large tyre rubber particles.

A unified lumped approach in kinetic modeling of biomass pyrolysis

Thermogravimetry analysis and gas chromatography techniques are used at different heating rates (from 5 to 50 K/min) to map all the products and to develop suitable kinetic models of biomass pyrolysis. A three-independent-parallel-reactions model is used to model kinetic of total devolatilization. This part accounts for the total char yield and devolatilization time. The evolutions of condensable vapors (tar and H 2O) and non-condensable gases (H 2, CH 4, CO and CO 2) are also studied using gas chromatography technique. It is shown that the final total yield of gases increases by increasing the heating rate, whereas those of tar decrease by increasing heating rate. A kinetic model was then proposed and the parameters for that were calculated, which can predict the change of the gases yields at different heating rates. The performance of the kinetic models was evaluated for other experimental works available in the literature or by exposing the biomass to different heating program.

Thermochemical Treatment of Solid and Wastewater Effluents Originating from the Olive Oil Food Industry

Thermochemical treatment of waste effluents originating from the olive oil food industry was investigated at total pressure conditions slightly above atmospheric pressure (103−104 kPa) and temperatures of 700−855 °C. The experimental data were obtained in a laboratory-scale reactor. Wastewater that was concentrated by means of evaporation/drying (denoted as cww), with a biomass formula of CH1.3O0.5, mixed with olive husks (kernels) was used here as a sample fuel (i.e., a mixture of olive husks (kernels) and cww (95%/5%, as determined from the low heating value, LHV)). Pretreatment of the specific solid waste effluent by washing (leaching) and fractionation was studied. The particle size (dp) class of the olive husks (kernels) varied between 0.35 mm < dp < 0.50 mm (with a biomass formula of CH2.2O0.7) and 0.50 mm < dp < 1.0 mm (with a biomass formula of CH2.2O0.6). For H2O-vapor thermal treatment of the evolved gas obtained during pyrolysis of the biofuel used here, the composition products were analyzed. Gas compositions (viz., CO, CO2, O2, CH4, and C2+), along with the amounts of benzene and toluene, tar/liquid phase, and char total yields are given. In thermal treatment experiments in air, the major gas was CO2. The effect of the moisture content and the particle size class of the biofuel samples on the variation of the devolatilization rate, relative to devolatilization time, was determined at a given furnace temperature regime.

The Combustion of Solid Wastes as Studied in a Fluidized Bed

An electrically heated bed of silica sand fiuidized by air and at 900°C has been used to incinerate a wide variety of solids. These included vegetables, meat, plastics, wood, rubber, car-tyres and a Polish coal, as well as two commercially produced waste fuels. Combustion of such solids always involves two, almost sequential, stages: the first is when any moisture and volatile materials are expelled; during the second, the solid remaining from the first stage (i.e. char) is burnt. Some materials float on the hot fiuidized sand, occasionally when they are not expected to float; other materials sink, often unexpectedly Likewise some particles fragment, some produce a cloud of soot, but others do not. Devolatilization times ( t d ) for the bituminous coal varied as M 2/3 , the initial mass of the particle raised to the power 2/3. This is attributed to the rate-controlling step being provision of the enthalpy of thermal decomposition by heat conduction from a particle's exterior to a shrinking core of virgin coal. A simple model for such devolatilization is presented. The char's burn-out time also depended on M 2/3 , because control is by mass transfer of O 2 from the bed to a particle's exterior. By contrast a particle of fibre fuel (mainly compressed paper) has t d approximately proportional to M , but dRDF (densified refuse-derived fuel—actually domestic refuse, extruded after being screened) has t d independent of M . The char burn-out times for these two commercial fuels (with large volatile contents) were much less than for the coal and independent of M . Car tyres and rubbers produced large quantities of soot and CO. Plastics produced no char; they evaporated and rapidly produced clouds of hydrocarbon vapour. Such a laboratory-scale fiuidized bed, held in a transparent quartz cylinder, is excellent for observing a particle whilst it burns, as well as measuring gaseous emissions, the temperature of a particle whilst it burns and also the rates of devolatilization and char burn-out.

Effect of Moisture Content on Devolatilization Times of Pine Wood Particles in a Fluidized Bed

This work analyzes the effect of moisture content on devolatilization times of pine wood particles in a fluidized bed combustor. The devolatilization process was followed by measuring the CO2 and O2 concentrations obtained after the complete combustion of the volatiles. The devolatilization rate decreased and was more uniform along the devolatilization time as the moisture content of the wood particles increased. The devolatilization times increased almost linearly with moisture, and the slope slightly increased when the bed temperature decreased. The devolatilization times were correlated by a power-law relationship, which related the devolatilization time to the fuel particle diameter and shape factor [tv = a(dp,eqφ)n]. The values of exponent n were between 1.5 and 1.7 and were almost unaffected by the bed temperature or the moisture content. The values of the constant a decreased with increasing the bed temperature and with decreasing the moisture content of the wood particles. To predict the devolatil...