Volatile Release Rate Research Articles

Adaptation of the Chemical Percolation Devolatilization Model for Low Temperature Pyrolysis in a Fluidized Bed Reactor

ABSTRACT In the present study, the CPD model originally developed based on predictions from heated grid (HGR) and entrained flow (EFR) experiments, has been adapted to analyze pyrolysis kinetics in a small-scale fluidized bed reactor. Impacts of particle feed, particle heat up as well as tar cracking reactions in the gas phase are considered. Furthermore, an optimized solver structure allows a time step independent solution and enables the use of implicit methods. A comparison with experimental results is undertaken for pulverized Rhenish lignite fuel particles in the temperature range from 673 to 973 K in N2 atmosphere. The comparison between simulated and experimentally derived volatile release rates reveals a good agreement, indicating that the high temperature derived kinetic parameters from HRG and EFR experiments can be extrapolated to lower temperatures. Nevertheless, discrepancies in the tar to light gas ratio occur with the proposed model implementation.

Numerical simulation on NOX emissions in a municipal solid waste incinerator

The simulations of combustion and NOX emissions process are conducted by computational fluid dynamics method with comprehensive physical and chemical models in a municipal solid waste incinerator. This study focused on the effects of primary NOX control methods (different loads, over fire air, the ratios of secondary air and over fire air) and secondary NOX control methods (selective non-catalytic reduction process) on NOX emission and combustion characteristic. Results showed that the moisture evaporation rate, volatile release rate, char burning rate, temperature and the strength of flow velocity decreased as decreasing the operating load. However, the NOX emission of furnace outlet increased from 273.18 mg/Nm3 to 602.10 mg/Nm3. 100% operating load was the best condition to obtain a lowest NOX emission and a best combustion characteristic among different loads. In addition, the combustion strategy combining secondary air and over fire air could improve the combustion performance and reduce NOX emission. Further, a higher over fire air ratio was conducive to the formation of strong swirl flow and the enhancement of flow velocity strength in the flue. The optimal over fire air ratio was 31%–35% with NOX emission of 267.05 mg/Nm3 to 273.18 mg/Nm3 and residence time of 2.75 s–3.03 s. Moreover, ammonia based selective non-catalytic reduction process, better than urea, achieved a NOX emission of 132.41 mg/Nm3, a NOX removal efficiency of 51.50% and NH3 slip of 11.08 mg/Nm3 or 6.71 ppm. The study indicated that the optimal NOX control method was a condition of 100% load, combining with over fire air and 31% and 35% over fire air ratio. Moreover, the best secondary NOX control method was the ammonia based selective non-catalytic reduction process.

Flood Basalts and Mass Extinctions

Flood basalts were Earth's largest volcanic episodes that, along with related intrusions, were often emplaced rapidly and coincided with environmental disruption: oceanic anoxic events, hyperthermals, and mass extinction events. Volatile emissions, both from magmatic degassing and vaporized from surrounding rock, triggered short-term cooling and longer-term warming, ocean acidification, and deoxygenation. The magnitude of biological extinction varied considerably, from small events affecting only select groups to the largest extinction of the Phanerozoic, with less-active organisms and those with less-developed respiratory physiology faring especially poorly. The disparate environmental and biological outcomes of different flood basalt events may at first order be explained by variations in the rate of volatile release modulated by longer trends in ocean carbon cycle buffering and the composition of marine ecosystems. Assessing volatile release, environmental change, and biological extinction at finer temporal resolution should be a top priority to refine ancient hyperthermals as analogs for anthropogenic climate change. ▪ Flood basalts, the largest volcanic events in Earth history, triggered dramatic environmental changes on land and in the oceans. ▪ Rapid volcanic carbon emissions led to ocean warming, acidification, and deoxygenation that often caused widespread animal extinctions. ▪ Animal physiology played a key role in survival during flood basalt extinctions, with reef builders such as corals being especially vulnerable. ▪ The rate and duration of volcanic carbon emission controlled the type of environmental disruption and the severity of biological extinction.

Effect of alkali and alkaline earth metals on co-pyrolysis characteristics of municipal solid waste and biomass briquettes

To investigate the influence of alkali and alkaline earth metals (AAEMs: K, Na, Ca, and Mg) on the co-pyrolysis of municipal solid waste (MSW) and biomass briquettes (CSBs), the pyrolysis properties and products distribution for the blends of pretreated CSBs and MSW were studied by thermogravimetric analysis with Fourier-transform infrared spectroscopy under N2 and CO2 atmospheres. Results showed that the AAEM salts were effective at reducing the initial temperature during pyrolysis process. The presence of AAEM salts inhibited the volatile release rate of the first pyrolysis stage of the blend, while the alkali and alkaline earth salts separately enhanced the volatile release rates of the second and third pyrolysis stages under both atmospheres. Replacement of N2 with CO2 caused lower residual masses of the blend, which were further reduced by the catalytic effect of the AAEM salts. The CO2 yields were enhanced by adding AAEM salts under the N2 atmosphere. Ca had the most obvious catalytic effect on the CO production in the third pyrolysis stage. Different AAEM salts added to the samples displayed different effects on the emissions of alkyl and carbonyl groups.

An Improved Comprehensive Model of Pyrolysis of Large Coal Particles to Predict Temperature Variation and Volatile Component Yields

In this work, an improved comprehensive model was developed for large coal particles to predict temperature variation and volatile component yields. The kinetics model of volatile component yields, where the volatile matters were assumed to comprise nine species, was combined with heat transfer model. The interaction between volatile yield and heat transfer during pyrolysis of large Maltby coal particles was investigated. An apparent temperature difference has been observed between the surface and core of particles at the initial heating stage. The non-uniform temperature distribution inside coal particles causes non-simultaneous volatile yields release from the surface and core area. The volatile release occurs after the coal temperature rises higher than 350 °C, and its yield steeply increases within the temperature range of 450–520 °C. The peak of volatile release rate corresponds to about 485 °C due to the rapid release of tar and H2O. The tar is almost completely released at around 550 °C. With the increasing particle size, the difference in temperature and volatile yield between the surface and core increases at the end of heating. The results are expected to provide insights into the interaction between heat transfer and volatile yields during pyrolysis of large coal particles.

Characteristics of volatile release rate and fuel nitrogen conversion for three typical coals under high temperature fast pyrolysis

Drop Tube Furnace experiments were carried out to investigate the fast pyrolysis characteristics of three typical coals. The process of fast pyrolysis was simulated by a one-step pyrolysis model as well as a Chemical Percolation Devolatilization (CPD) model. The results indicate that coal rank, reaction temperature and residence time have complex influences on volatile release and nitrogen conversion. Both activation energy and frequency factor, which decrease with increasing coal rank, show obvious differences for the three considered coals. The volatile release coefficient QR is relative to coal rank and heating rate which fluctuates from 1.02 to 1.72. Predictions of volatile release and coal nitrogen conversion obtained from CPD model agree well with experiments. In addition, the one-step pyrolysis model could predict the volatile release of low-rank coal, whose precision is related to coal rank.

Ignition sensitivity of different compositional wood pellets and particle size dependence

Partial or complete substitution of coal with renewable biomass like wood is a sustainable and effective solution to reduce the CO2 emissions in the atmosphere. Utilization of these woods in the form of compact pellets facilitates in its handling and transportation with higher energy density. However, for electric power generation the pellets are broken up into their constituent milled finer particles for burning as a pulverised biomass flame. There is a dust fire/explosibility hazards in the process handling facilities such as in storage, conveying and milling. In the present work, four commercial pellets were investigated, and each pellet sample was split into three size ranges <63 μm, 63–500 μm and <500 μm. The flame propagation characteristics and their lean flammability limit for each pulverised pellet was determined using modified Hartmann dust explosion tube. It was found that the fine particles, with lower lean flammability limits of 0.3–0.7 equivalence ratio, intensify the explosibility risk (dP/dt of 10–15 bar/s for most reactive concentration) of the dust due to fast volatile release rate. Comparison was also made between particle size distribution (PSD) of the finer fractions (<63 μm) of wood samples in comparison to crop residue samples that showed 70–80% larger size distribution for wood samples due to elongated particles as showed by Scanning Electron Microscope (SEM). Results showed that the ash + moisture content had a stronger effect on wood samples than on agricultural residue's samples. The results showed that the explosibility characteristics of the pellets industry's feed-stocks was variable and dependent on the pellet composition and this needs to be taken into account in explosion protection and in utilising the pellets in the main pulverised biomass combustion.

Devolatilization of millimeter-sized biomass particles at high temperatures and heating rates. Part 1: Experimental methods and results

In the framework of a general research on biomass combustion, this paper presents and discusses the experimental techniques developed to analyze the heating, drying and devolatilization of biomass particles of few millimeters, a size range scarcely studied in the past. Individual particles were suspended in a stream of hot gases generated by a Flat Flame Burner, with controlled temperature and composition. The evolution of their morphology was monitored by time-lapse photography; additionally, an estimate of the instantaneous volatile release rate was derived from the picture series based on the size of the volatile shell flame. The temperature inside the particles (3–15 mm in diameter) was measured with pairs of fine wire thermocouples located at the particle's center and near its surface. The systematic error associated to this technique (deviations up to 300 K in relation to the actual particle temperature), due to the intense heat transfer along the metal wires, was evaluated experimentally and modeled; the agreement found in the comparison allowed for establishing a procedure to correct those signals and thus adequately evaluate the thermal gradients within the particles. A subsequent comparison of the experimental results (evolution of size, shape, internal temperature and temperature gradients, volatile release rate in a variety of conditions) with simulations performed with models which alternatively consider or neglect internal gradients is presented in a separate paper [21].

Devolatilization of millimeter-sized biomass particles at high temperatures and heating rates. Part 2: Modeling and validation for thermally-thin and -thick regimes

This paper analyzes the relevance of intraparticle gradients in the devolatilization of millimeter-sized biomass particles under the high temperatures and heating rates typical of the industrial facilities. Experimental data (intraparticle temperature history, size evolution, devolatilization times and volatile release rates) were obtained in tests with isolated particles 3–15 mm in diameter in a flat flame reactor under controlled combustion conditions. These results were compared with the predictions of two different models that alternatively neglect or consider the internal gradients, after deriving model-dependent kinetics from the data; this allowed drawing conclusions regarding the range of validity of each model. Whereas the thick-particle model reasonably reproduces the behavior of the devolatilizing particles for the whole range of test conditions and particle sizes explored, the thin-particle model is unable to fit the trends observed in the data for different sizes. Neglecting the internal gradients leads to a delayed onset and a more abrupt release of the volatiles, which is relevant e.g. in the simulation of biomass-fired burners. Additionally, the experimental data set also allowed assessing the validity of two common assumptions regarding the char oxidation stage, i.e. oxidation limited by O2 external diffusion and sequential character of devolatilization and char oxidation; both are likely applicable in the conditions explored.

Predicting kinetic parameters for coal devolatilization by means of Artificial Neural Networks

The chemical percolation devolatilization (CPD) model has been shown to represent the devolatilization process of different coals and heating conditions with good accuracy. However, its use in computational fluid dynamics is limited because of its relatively high computational cost. Here, an Artificial Neural Network (ANN) based model for predicting coal devolatilization kinetics is developed based on a database constructed with the CPD model for a wide range of coals and heating rates. The heating rates and the information of ultimate and proximate analysis are chosen as inputs of the ANN model to consider the effects of coal types and heating conditions on coal devolatilization; the outputs are the kinetic parameters for the two-step kinetic model. The learning, validation, and application results show that the proposed ANN model has a competitive prediction capability on both the total volatile release and release rates when compared with the CPD model, but has obvious computational efficiency advantages. Furthermore, the relative impact of the coal type and heating rate on each kinetic parameter for coal devolatilization is quantitatively evaluated through the Garson equation. It is found that the heating rate has the strongest effect on the pre-exponential factor, while the coal types show significant influence on the activation energy and final yield of the two reactions in the two-step model.

The effects of particle size and reducing-to-oxidizing environment on coal stream ignition

Coal particles experience a transition from a reducing to oxidizing environment in the near-burner region of pulverized coal (pc) boilers. For the first time, we report a fundamental study of ignition of a coal-particle stream experiencing a flame environment that transitions from a reducing to an oxidizing environment (termed reducing-to-oxidizing environment). High-speed videography is used to observe the particles in situ, and scanning electron microscopy is used to characterize the sampled particles. The effects of particle size on ignition are presented for four size bins (63–74 µm, 75–89 µm, 90–124 µm and 125–149 µm) for PRB subbituminous coal at two nominal gas temperatures (1300 K and 1800 K). An oxidizing environment with 20% molar oxygen composition is used as base-case. In contradistinction to single particle studies where particles are reported to ignite heterogeneously at higher temperatures, this study shows that coal streams ignite homogeneously, irrespective of particle size, in the oxidizing environment. By changing nominal gas temperature from 1300 K to 1800 K, ignition time decreases, on average, by a factor of five for each of the particle size bins. For both gas temperatures, the trend in ignition delays as particle size changes is non-monotonic. However, at 1800 K nominal gas temperature, ignition delays are independent of particle size in the reducing-to-oxidizing environment and ignition delays are doubled on average when compared to those in the oxidizing environment. It is more noticeable at the lower gas temperature of 1300 K that homogeneous ignition of coal streams is oxygen-dependent below 90 µm particle size and temperature-dependent above 90 µm. In general, ignition delay is determined by volatile release rate (controlled by the particle temperature) and the local oxygen concentration. Micrographs of particles also confirm that ignition and char burnout times are longer in the reducing-to-oxidizing environments than those in the oxidizing environments.

Two Distinct Aerobic Methionine Salvage Pathways Generate Volatile Methanethiol in Rhodopseudomonas palustris.

ABSTRACT5′-Methyl-thioadenosine (MTA) is a dead-end, sulfur-containing metabolite and cellular inhibitor that arises from S-adenosyl-l-methionine-dependent reactions. Recent studies have indicated that there are diverse bacterial methionine salvage pathways (MSPs) for MTA detoxification and sulfur salvage. Here, via a combination of gene deletions and directed metabolite detection studies, we report that under aerobic conditions the facultatively anaerobic bacterium Rhodopseudomonas palustris employs both an MTA-isoprenoid shunt identical to that previously described in Rhodospirillum rubrum and a second novel MSP, both of which generate a methanethiol intermediate. The additional R. palustris aerobic MSP, a dihydroxyacetone phosphate (DHAP)-methanethiol shunt, initially converts MTA to 2-(methylthio)ethanol and DHAP. This is identical to the initial steps of the recently reported anaerobic ethylene-forming MSP, the DHAP-ethylene shunt. The aerobic DHAP-methanethiol shunt then further metabolizes 2-(methylthio)ethanol to methanethiol, which can be directly utilized by O-acetyl-l-homoserine sulfhydrylase to regenerate methionine. This is in contrast to the anaerobic DHAP-ethylene shunt, which metabolizes 2-(methylthio)ethanol to ethylene and an unknown organo-sulfur intermediate, revealing functional diversity in MSPs utilizing a 2-(methylthio)ethanol intermediate. When MTA was fed to aerobically growing cells, the rate of volatile methanethiol release was constant irrespective of the presence of sulfate, suggesting a general housekeeping function for these MSPs up through the methanethiol production step. Methanethiol and dimethyl sulfide (DMS), two of the most important compounds of the global sulfur cycle, appear to arise not only from marine ecosystems but from terrestrial ones as well. These results reveal a possible route by which methanethiol might be biologically produced in soil and freshwater environments.

Multi-point LIBS measurement and kinetics modeling of sodium release from a burning Zhundong coal particle

A multi-point Laser-Induced Breakdown Spectroscopy (LIBS) method for quantitative measurement of sodium concentrations in the gas phase, the surface temperature and the particle diameter during the combustion of a Zhundong coal particle is presented. To obtain multi-point LIBS data, the laser focusing and signal collection optics are mounted on a translational platform which is able to traverse cyclically. With this setup multi-point LIBS measurements above a burning particle can be performed and the time-resolved sodium release process can be obtained. The results show that 42.2% of the total sodium mass is released during the burning of the Zhundong coal sample. For a 4 mm particle, in the char burnout stage sodium is released most strongly, i.e., 87% of the total released sodium mass, while in the de-volatilization and ash reaction stages the percentages are 5% and 8%, respectively. The atomic sodium and NaOH are the most favored species at chemical equilibrium in the plume according to CHEMKIN. The sodium release is found to be closely related to the particle burning stages by analyzing the sodium release, particle surface temperature and its diameter. A linear relationship is found between the residual sodium mass in the particle and the volume of the particle. The volatile sodium release rate obeys a two-step Arrhenius expression. Predictions by the developed two-step kinetics model agree well with the measured sodium release profiles in all the three coal-burning stages.

Suppressing secondary reactions of coal pyrolysis by reducing pressure and mounting internals in fixed-bed reactor

Pyrolysis of Shenmu coal was performed in fixed-bed reactors indirectly heated by reducing operating pressure and mounting internals in the reactor to explore their synergetic effects on coal pyrolysis. Mounting internals particularly designed greatly improved the heat transfer inside coal bed and raised the yield of tar production. Reducing pressure further facilitated the production of tar through its suppression of secondary reactions occurring in the reactor. The absolute increase in tar yield reached 3.33wt% in comparison with the pyrolysis in the reactor without internals under atmospheric pressure. The obtained tar yield in the reactor with internals under reduced pressure was even higher than the yield of Gray–King assay. Through experiments in a laboratory fixed bed reactor, it was also clarified that the effect of reducing pressure is related to volatile release rate in pyrolysis. It did not obviously vary tar yield at pyrolysis temperatures below 600°C, while the effect was evident at 650 and 700°C but became limited again above 800°C. Under reduced pressure the produced tar contained more aliphatics and phenols but less aromatics.

Pyrolysis behavior of a typical Chinese sub-bituminous Zhundong coal from moderate to high temperatures

The pyrolysis mechanism of Zhundong sub-bituminous coal (ZD) was investigated by tube furnace, gas chromatography and GC/MS. Coal samples were pyrolyzed in a tube furnace at any of six temperatures (from 500 to 1000°C in 100°C increments) and the pyrolysis products were analyzed. The volatile release characteristics were investigated by gas chromatography and the tars generated during pyrolysis were analyzed by GC/MS. Additionally, relevant physicochemical properties of the obtained char were discussed by FTIR and BET. Results suggested that increasing pyrolysis temperature could improve the volatile release volume and rate as well as the calorific value of the resultant gas due to the substantial increase of H2 and an incremental increase of CH4. Pyrolysis also effectively increased the coal rank and reduced the O/C ratio of char. FTIR spectra indicated that during pyrolysis, functional groups on the organic macromolecule decomposed and released to form the gas volume. The average pore diameter of ZD char decreased with pyrolysis temperature to a minimum value at about 700°C after which the average pore diameter gradually increased. The trends of specific surface area and pore volume with pyrolysis temperature were opposite the trend for average pore diameter. As for the tar components, the aliphatic hydrocarbons decreased and aromatic hydrocarbons increased while the polar organics first increased to a high value and then decreased. Finally, the mass balance demonstrated the pyrolysis gas yield increased with the temperature while the char showed the opposite trend. Besides, the tar yield achieved its maximum value at about 600°C.

Global kinetics of the rate of volatile release from biomasses in comparison to coal

The chemistry involved in the propagation of pulverised biomass flames is not well understood. All biomass fuels release volatiles at a much lower temperature than coal and the proportion of volatiles is much greater than for coal, typically 80% compared to 30%. Thus, the rate of release of volatiles from biomass fuels is much more important in the pulverised fuel flame propagation than it is for coal, where the rate of char oxidation is more important. The rate of release of volatiles from dry biomass follows three stages: the first stage for typically 10% of the volatiles occurs over 200–300°C, the second stage of about 70% of the total volatile mass occurs over the temperature range 300–400°C. Finally, there is a slow loss of volatiles, accounting for remaining 20% of volatiles, over the temperature range 400–900°C. Stagg’s quick approximation method was used to determine the kinetics for the rate of volatile yield. Biomass samples were found to have lower activation energies and higher rate of release of volatiles in comparison with coal samples, up to 300–400°C. Similar release rates were found for the 3rd stage of volatile release. The release of volatiles at low temperatures potentially makes the biomass pulverised fuel more reactive and one consequence is shown in the measurement of minimum explosion concentration, MEC. A good correlation was found between activation energies and the MEC, determined on the Hartmann equipment. There is currently little understanding of the composition of the volatiles released at low temperatures from biomass, as most publications are for pyrolysis conditions at high temperature. It is possible that the volatiles are a mixture of mainly H2, CO and CH4 and the likely proportions of these were calculated from the elemental and thermo-gravimetric analysis. This was done for a range of biomasses and this showed that the most important volatile gas is likely to be CO and that CH4 yield is very low. This means that the conventional model used in coal combustion of char plus methane combustion is not applicable to biomass combustion.

Using ignition of coal dust produced by different types of mechanical treatment under conditions of rapid heating

Experiments aimed at studying the ignition of coal dust obtained by coal disintegration in high-energy mills are performed in a tubular furnace. Effective kinetic constants of ignition of coal dust ground in a vibrational–centrifugal mill and in a disintegration mill under conditions of rapid heating are determined for the first time. It is shown that the volatile release rate depends on the method of coal grinding.

The Influence of Host Plant Volatiles on the Attraction of Longhorn Beetles to Pheromones

Host plant volatiles have been shown to strongly synergize the attraction of some longhorn beetle species (Coleoptera: Cerambycidae) to their pheromones. This synergism is well documented among species that infest conifers, but less so for angiosperm-infesting species. To explore the extent of this phenomenon in the Cerambycidae, we first tested the responses of a cerambycid community to a generic pheromone blend in the presence or absence of chipped material from host plants as a source of host volatiles. In the second phase, blends of oak and conifer volatiles were reconstructed, and tested at low, medium, and high release rates with the pheromone blend. For conifer-infesting species in the subfamilies Spondylidinae and Lamiinae, conifer volatiles released at the high rate synergized attraction of some species to the pheromone blend. When comparing high-release rate conifer blend with high-release rate α-pinene as a single component, species responses varied, with Asemum nitidum LeConte being most attracted to pheromones plus α-pinene, whereas Neospondylis upiformis (Mannerheim) were most attracted to pheromones plus conifer blend and ethanol. For oak-infesting species in the subfamily Cerambycinae, with the exception of Phymatodes grandis Casey, which were most attracted to pheromones plus ethanol, neither synthetic oak blend nor ethanol increased attraction to pheromones. The results indicate that the responses to combinations of pheromones with host plant volatiles varied from synergistic to antagonistic, depending on beetle species. Release rates of host plant volatiles also were important, with some high release rates being antagonistic for oak-infesting species, but acting synergistically for conifer-infesting species.

The effect of process parameters on volatile release for a benzoxazine–epoxy RTM resin

Volatile release during cure is a potential cause of void formation during the resin transfer molding of complex thermosetting resins. In this study, a blended benzoxazine–epoxy resin system is analyzed to determine the rate at which volatiles are evolved, as well as the dependence of that rate on process parameters. The evolution of thermophysical and thermochemical resin properties is characterized using differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The identity and rate of evolution of the gaseous byproducts released during cure are determined at ambient pressure using a Fourier transform infrared spectrometer (FTIR) linked to a reaction cell. The results show that gas release during cure can be reduced but not eliminated by degassing at elevated temperature. Furthermore, the results indicate that the nature and rate of volatile release can be modified by judicious selection of cure cycle, as shown by a preliminary analysis of manufactured neat resin panels.

Effects of coking coal properties on coke pore structure: Prediction models

The porosity, pore size, and specific surface area of coke significantly affects its strength and thermal properties. To investigate the factors affecting the pore structure during the carbonization of coking coal, experimental studies were performed using 18 types of single and mixed coals. Based on the experimental results of the structure and properties of coke pores, regression equations were developed to predict the structural parameters. The structural parameters include porosity, average pore size, volume ratio of pores with diameters >150 ¯m, volume ratio of pores with diameters <30 ¯m, and specific surface area. The regression equations indicate that the maximum volatile release rate, Gieseler fluidity, and vitrinite maximum reflectance are the main factors affecting the formation of micron-size pores. The surface area of coke dominated by nanoscale micropores was significantly affected by volatiles released during 800−950 °C.