Coal Devolatilization Research Articles

Pyrolysis Characteristics and Char Reactivity of Oedogonium sp. and Loy Yang Coal

The proposition of combining the application of algae in wastewater treatment, CO2 fixation, and energy production maximizes the opportunity of algal bioremediation. In this paper, the freshwater macroalga Oedogonium, a key target species for the bioremediation of waste waters, was investigated for its utilization in energy production via gasification or co-gasification with coal. The pyrolysis characteristics of Oedogonium and the effects of pyrolysis conditions on char reactivity were investigated in this paper, and an Australian lignite was also studied for comparison purposes. The pyrolysis process of Oedogonium and coal can be divided into three stages: moisture evaporation, volatile release, and decomposition of the remaining carbonaceous compounds. However, the devolatilization of Loy Yang coal was slower and occurred over a wider temperature range compared to Oedogonium. As the heating rate increased, the pyrolysis curves for both Oedogonium and Loy Yang coal shifted to higher temperatures. To investigate the effects of pyrolysis conditions on char reactivity, a tube reactor was employed to generate char samples under a range of pyrolysis conditions. The produced char samples were characterized using a CHN analyzer and scanning electron microscopy technique. It was found that char samples almost maintain their original shape following pyrolysis at a low heating rate. However, evidence of char swelling was observed in the structure of chars prepared at a high heating rate. Pyrolysis conditions also had a significant influence on the reactivity of the derived chars. Char reactivity for both fuels was enhanced at a higher heating rate and lower pyrolysis temperature. Oedogonium char reactivity was around 2–5 times that of Loy Yang char prepared under the same pyrolysis conditions. However, the differences in reactivity became less significant as the pyrolysis temperature increased. Oedogonium has a higher devolatilization rate and higher char reactivity than Loy Yang lignite. It implies that it is easier to be gasified, and the co-gasification with lignite is advantageous.

Experimental research on mechanism and process of direct iron making reduction of mechanically milling scale with coal

This study delved into the fundamental mechanisms of a mill scale reduction. To achieve this objective, the DRI process was applied to facilitate the mechanical milling of coal-scale mixtures. Temperatures of 700–1000 °C were used to sinter the mechanically milled coal-scale mixtures. The powders were subjected to scanning electron microscopy (SEM) and X-ray diffraction (XRD). In order to identify the reduction of milled scale-coal mixture with the temperature, iron phase analytical techniques and the XRD were used. The differential thermal analysis (DTA) technique was used to determine the measurements of the thermal property. Based on the findings of the experiments, it was concluded that coal devolatilisation and scale reduction forms concurrently when the mixture is being heated. By 40 h milling, the milled coal-scale mixture's reduction start temperature decreased to 750 °C.

Influence of inherent moisture on the ignition and combustion of wet Victorian brown coal in air-firing and oxy-fuel modes: Part 1: The volatile ignition and flame propagation

This study for the first time reported the volatile ignition and oxidation for wet Victorian brown coal in both air-firing and oxy-fuel modes. The aim is to quantitatively understand the evaporation of inherent moisture and its overlapping with the subsequent volatile ignition and oxidation steps during pulverised coal combustion. A wet Victorian brown coal sample with the moisture content up to 30wt.% and a size range of 63–104μm has been tested. Its ignition, volatile flame propagation and char oxidation have been diagnosed optically in a laboratory-scale flat-flame burner reactor (FFBR) coupled with a high-speed camera. 1-D modelling was also conducted to predict coal ignition based on the classic coal drying theory and coal devolatilisation rate. As has been confirmed, the release of moisture on the initial drying stage is incomplete prior to the subsequent volatile ignition. The fraction of moisture released upon drying only accounted for 10% of its original amount in wet brown coal prior to ignition. This is due to a firm residence of the moisture within coal capillaries, and a rapid heat feedback from hot gas to the dried particle surface that was ignited instantaneously prior to the release of the remaining moisture. The unevaporated moisture was observed to be released with the volatiles together to form a thick cloud layer in the vicinity of the coal particle, thereby enlarging the sizes of the volatile flame remarkably, which are approximately twice the size of the dried coal flame. However, the intensity of the wet coal flame was much weaker than dried coal, due to the dilution effect of the inherent moisture on the volatile cloud. In the oxy-fuel combustion mode, the inherent moisture interrelated with CO2, a tri-atomic gas with a larger specific heat capacity than N2 to significantly delay the ignition of the wet coal particle and its flame intensity and propagation velocity.

Thermogravimetric analysis of the co-pyrolysis of a bituminous coal and pulp mill sludge

Thermal valorization of organic wastes by co-processing with coal may be a management option for such wastes. In this work, the separate pyrolysis of coal and primary and secondary pulp mill sludge was assessed by thermogravimetric (TG) analysis. Then, under the same experimental conditions, the pyrolysis TG curves of the coal blends with a 10 mass% of either primary or secondary pulp mill sludge were obtained. The corresponding differential thermogravimetry curves made evident that the devolatilization of coal starts at higher temperature and reaches higher char yields than that of primary or secondary paper mill sludge. Under blending, lower devolatilization temperatures and char yields, close to the weighted calculated ones, are observed. Non-isothermal kinetic analysis showed that the Arrhenius activation energy (E) corresponding to the pyrolysis of coal was lower than that corresponding to either L1 or L2. Moreover, lower E than the weighted calculated ones was determined for the co-pyrolysis of the blends.

Study of the high heating rate devolatilization of a pulverized bituminous coal under oxygen-containing atmospheres

This study deals with the analysis of pulverized coal devolatilization in oxygen-containing atmospheres. To do so, a newly developed flat flame reactor allowing coal jet flames to be stabilized with fuel heating rates of the order of 106Ks−1 has been used to mimic operating conditions close to those met in practical combustors. A high volatile bituminous coal milled in an industrial grinder has been fluidized by carrier gases containing various amounts of oxygen (air and pure oxygen) and then injected in hot gases (temperatures ranging up to ∼1240K) generated by propane/air flat flames. The thermal history of coal particles has been monitored by coupling particle image velocimetry (PIV) and pyrometric measurements to be integrated in constant-rate, single-kinetic-rate, two-competing-rate and DAEM devolatilization models. Devolatilization profiles derived from the analysis of the char collected at different residence times have then been compared with simulated data and a simplified fitting procedure has been implemented to find kinetic parameters leading to reproduce experimental data so as to evaluate the relative influence of the surrounding atmosphere on the devolatilization kinetics. Obtained results confirm that devolatilization is faster and more complete under oxygen enriched environments which has been related to an enhanced combustion of volatiles inducing a rise of the temperature of fuel particles. On the other hand, apparent devolatilization rates appeared to be only indirectly affected by the surrounding atmosphere through the way this one influences the heating of the coal. Furthermore, results obtained in this work tend to indicate the absence of overlapping between devolatilization and char oxidation stages. Measurements of gaseous species released during or formed after the devolatilization process finally confirm that the combustion of volatiles is more complete under oxygen enriched combustion (OEC). More CO2 is thus produced while CO concentrations significantly decrease. An enhanced fuel-N conversion has moreover been related to the higher temperatures and oxygen partial pressures met under OEC while SO2 concentrations ∼50% higher have been measured due the higher quantities of fuel-sulfur and oxygen available in the medium.

Depolymerization Model for Coal Devolatilization: Bridges and Side Chains as the Reaction Centers

A phenomenological extension is developed for coal devolatilization, based on FLASHCHAIN’s mechanism for tar and lumped noncondensables. It segregates the reaction center population further into bridges and side chains with dissimilar reactivities that vary with rank. At low and medium temperatures, most bridges dissociate to produce tar precursors and noncondensables, whereas only a minor portion of side chains decomposes into gases; a fraction of both types of reaction centers may also be shuttled away as an element in tar molecules. When devolatilization continues to sufficiently high temperatures, bridges are depleted and side chain decomposition takes over to dominate gas formation. In the evaluations against atmospheric reaction dynamics of 16 coals from lignite to anthracite, the extension predicts with accuracy the disparate time scales for the evolution of tar and noncondensables, as well as the rank-dependent devolatilization rates. Also, except for the earlier evolution history at low heating rates, the extension demonstrates extrapolations within experimental uncertainty over a broad domain of operating conditions (heating rates from 1 to 104 K/s and pressures from vacuum to 9 MPa) for coals across the entire rank spectrum.

K2CO3‐Catalyzed Steam Gasification of Indonesian Low‐Rank Coal for H2‐Rich Gas Production in a Fixed Bed Reactor

AbstractK2CO3‐catalyzed steam gasification of low‐rank coal in a fixed bed reactor was investigated in order to elucidate the effects of temperature and catalyst loading amounts on hydrogen yield. The optimum conditions for H2‐rich syngas were 102.08 g kg−1 coal at 1073 K with 20 wt % K2CO3; the carbon conversion (XC) reached 72.63 wt % and the cold gas efficiency (ηCGE) reached 87.05 %, simultaneously. However, at 1073 K the tar yield was too small to be ignored. Through Brunauer–Emmett–Teller (BET), Barrett–Joyner–Halenda (BJH), t‐plot, scanning electron microscopy (SEM), and X‐ray diffraction (XRD) analyses of the gasification process, the surface area and micro‐pore volume were found to reach a maximum value at 20 min, while the average pore size reached a maximum value at 10 min. XC was promoted by the enlargement of average pore size and surface area, mainly caused by coal devolatilization. Potassium compounds tightly adhered to the surface and interpore sites of coal, capturing sulfur compounds to form K2SO4.

Global Kinetic Modeling of Coal Devolatilization in a Thermogravimetric Balance and Drop-Tube Furnace

The kinetic analysis of pulverized coal devolatilization under an inert gaseous atmosphere has thus far been carried out under various thermal analytical methods at laboratory scale, by measuring the mass loss when the sample is heated or held at a constant temperature. However, results obtained in a slow thermogravimetric balance (TGA) may not be extendable to the situation in a fast drop-tube furnace (DTF). The difficulty of extrapolating slow TGA-derived kinetic data to fast thermal conditions has already been pointed out in the literature and vice versa. In this study, three pulverized coals are characterized from slow to fast thermal conditions using both complementary TGA and DTF devices. The experimental data obtained over a large range of thermal conditions are used for global kinetic modeling of pulverized coal devolatilization. For that purpose, the kinetic parameters are derived from a devolatilization model combined to an efficient optimization program. With regard to the kinetic scheme basically based on two competitive reactions, the differential reaction model has been refined to improve the quality of curve fitting and extrapolation issue. It was found that this assumption leads to a close agreement between the experimental and model results in slow TGA and fast DTF thermal conditions.

Weight Loss and Tar Evolution during Coal Devolatilization at Various Heating Rates

Temperature-resolved weight loss and tar yield during atmospheric devolatilization of pulverized coal have been obtained on a wire mesh reactor (WMR), which imposes prescribed thermal histories covering a wide range of heating rates on coals from lignite to anthracite. The accuracy of measurements has been improved by diminishing non-isothermality in the sample, ensuring independence of yields on loading density, and the development of a convenient tar collection method that inhibits secondary pyrolysis but also secures capture completeness. We reconfirm the continuous rank effects in terms of reaction dynamics and partitioning between tar and non-condensables but at disparate heating rates of 5 and 1000 K/s. In addition, we depict the constant variation in gas evolution histories among various coals before the cessation of tar release, whereas since the end of tar evolution, variations in gas formation kinetics for different coals gradually shrink with increasing temperatures. A larger fraction of total ...

The Influence of a Nature of Raw Materials and Applied Temperature on a Change in Make-Up and Characteristics of Fuel Shales in Baltic Area

In this paper the chemical composition and properties of briquettes from combustible slates is studied during the heat treatment, made of waste of a slate field of the Leningrad region. Formation of a large amount of ashes is the main problem connected with processing and use of combustible slates. The component structure of mineral part of slate is revealed. Change of mass of combustible slate at heat treatment in the range of temperatures 25- 950°C is defined. Reduction of mass of slates happens during five cycles about change of structure and physical and chemical properties. The analysis of allocations of gas components condensed in water solution during heat treatment showed that at increase in temperature A‘€AÂ environment decreases with 6,72 to 6,31 and P2O5, SO3 are allocated. Also in the form of a suspension at solution there are Al2O3, SiO2, CuO, ZnO, MgO. Studying of structure and physical and chemical properties of briquettes from combustible slates during heat treatment gives the chance to define the periods of devolatilization of coal and coking of samples.

Release and Transformation of Sodium during Pyrolysis of Zhundong Coals

Zhundong coalfield is one of the superhuge coalfields newly discovered in Xinjiang Autonomous Region of China. While many studies on alkali metals in coal/biomass have been reported previously, few efforts were conducted on Zhundong coals which are characterized by a high content of sodium. It is challenging to explore the ash deposition problems related to Zhundong coals directly using existing findings on the behavior of alkali metals. The occurrence mode and transformation of alkali metals during thermal conversion of Zhundong coals could be quite different. Specific investigation on sodium transformation during pyrolysis of Zhundong coals will make a contribution to our understanding on the fate and behavior of alkali metals involved in high-sodium coals. Here, the release and transformation fundamentals of sodium during pyrolysis of two Zhundong bituminous coals were investigated using an experimental approach of sequential chemical extraction. The sodium volatilization and evolution of occurrence modes in chars of Zhundong coal were probed in a laboratory-scale reactor. The occurrence of alkali metals in Zhundong coals greatly differs from those in other coals of China. Water-soluble sodium is the predominant chemical form in Zhundong coals. Most of the water-soluble sodium is released into gas phase as a volatile during pyrolysis and part of the remainder is converted into an insoluble form, but the release of sodium is not always synchronous with volatility. The release behavior of sodium demonstrates a nonmonotonic variation with the particle size of the coal while the effect of particle size includes many factors resulting in the challenge of demonstrating its individual contribution to sodium release. The duration time has an insignificant effect on volatile release and sodium transformation at low temperature, while it has an obvious influence at high temperature or in the initial stage of pyrolysis. With the extension of duration time at high temperature, no more sodium is obviously released from the residual chars of Zhundong coal but more water-soluble sodium will still transform into insoluble form. During the devolatilization of Zhundong coals under CO2 atmosphere, the release of sodium and its conversion into insoluble form will be inhibited compared to that under N2 condition.

Modeling coal pyrolysis in a cocurrent downer reactor

In this paper, a model for fast coal pyrolysis in a cocurrent downer reactor is developed, in which both hydrodynamics and coal pyrolysis kinetics are simultaneously considered. The results of simulations based on this model display reasonable agreement with experimental data obtained using Huolinhe coal as the feedstock, and this model is therefore suitable for predicting the fast pyrolysis of specific coal types. A series of simulations of fast coal pyrolysis in a cocurrent downer demonstrated that coal devolatilization is almost complete in the inlet region within a time span of 0.4s, and that higher temperatures improve the pyrolysis efficiency. However, the yield of liquid products is decreased with increasing pyrolysis temperatures, especially above 670°C, because of additional cracking of the liquids.

Effects of detailed operating parameters on combustion in two 500-MWe coal-fired boilers of an identical design

In a large coal-fired power plant, it is common for the multiple boilers to be of an identical design in order to reduce construction and operating costs. Despite the design and operating conditions being the same, the boilers often exhibit different combustion characteristics after being in operation for an extended period. Using computational fluid dynamics (CFD), this study diagnosed the combustion in two identical coal-fired boilers with a capacity of 500 MWe, but which exhibited different trends in their temperature and heat distribution. Based on a survey on the detailed operation parameters, individual windboxes with different damper settings were analyzed to determine the air flow distribution between the burners and the overfire air ports. Then, the swirl burners were investigated in an attempt to identify a correlation with the swirl strength for a range of vane angles and air flow rates. The results were introduced into the CFD simulations for the boilers, for which we adopted advanced models for the coal devolatilization and char conversion. The predicted temperatures and heat transfer rates were in reasonable agreement with the measured data. The main reason for the higher gas temperatures and heat transfer rates in boiler A was found to be the higher air flow rate in the burner zone. Individual burners produced different gas flow paths and, therefore, the low-O2 pockets downstream could be traced to particular burners. Those burners with narrower vane gaps produced stronger swirls in the secondary air, which led to wider-angled flames closer to the burners, together with strong internal recirculation zones. This increased the heat flux on the adjacent wall. This study shows that CFD simulations for existing boilers require careful consideration of the operating parameters that may lead to significant differences in the combustion characteristics.

On Predicting Char Burnout in Pulverized Coal Combustion

Abstract The objective of this paper is twofold: to summarise the basic knowledge on kinetics of heterogeneous reactions and to clarify several misconceptions concerning both derivation and usage of expressions for calculating char oxidation rates. This paper is concerned with char reactions only; coal devolatilization is not considered although it may affect char oxidation rates.

Particle‐scale modeling of coal devolatilization behaviors for coal pyrolysis in thermal plasma reactors

A generalized heat transfer and devolatilization model coupled with the thermal balance between the heating gas and particles was established to predict the complex coal pyrolysis behaviors in the practical plasma reactors. It was proved that this model could well describe the coal devolatilization behaviors in both the pilot‐scale and lab‐scale plasma reactors as the mechanisms of coal chemistry and particle‐scale physics were incorporated. The achieved understanding on the reactor energy balance demonstrated that the heat recovery of the quenching process was crucial to the thermal efficiency and economic benefit of the overall project. The in‐depth discussion of the influences of coal feed rate and particle size on the reactor performance revealed the dominant roles and presented the optimal values of these two factors. In particular, the simulation results of several coals could help to provide a simple, quick method of coal type selection for industrial plasma processes. © 2014 American Institute of Chemical Engineers AIChE J, 61: 913–921, 2015

Numerical investigation of low NOx combustion strategies in tangentially-fired coal boilers

A numerical model is developed to investigate the effects of horizontal bias combustion (HBC) and air staging combustion (over-fire air, OFA) technologies on the performance of a 200MWe tangentially-fired pulverized-coal boiler. The devolatilization rate and volatiles amount are determined by a kinetic devolatilization model, which predicts the coal devolatilization prior to the volatile matter and char combustion. The characteristics of the devolatilization, combustion, heat transfer and NOx emission are studied and compared to achieve a comprehensive understanding of the low NOx combustion. The prediction shows a good agreement with the on-site measurement results, which confirms that the model is capable of predicting the characteristics of the investigated boiler. The predicted results have shown that the OFA has a remarkable effect on the reduction of NOx emission. The HBC makes a significant NOx reduction in the primary combustion zone (PCZ) when there is no air staging. In terms of the NOx reduction, the air staging plays a dominant role in comparison with HBC burners. The application of OFA tends to lead to slagging in the PCZ, which can be avoided using HBC due to the higher stoichiometry close to the furnace wall. The details of this study improve the understanding of combustion and NOx emissions in tangentially-fired pulverized-coal boilers with low NOx combustion technologies, especially for boilers adopting HBC burners and OFA.

Structure characterization and metallurgical properties of the chars formed by devolatilization of lump coals

The thermal pyrolysis of two lump coals used in the Corex process was carried out, and the structure, strength and reactivity to CO2 of the resulting chars were comprehensively investigated. The results show that the characteristic of the resulting chars is greatly affected by coal properties and carbonization conditions. XLC char forms visible pores because of its good thermoplastic property, and the bigger pores could affect the yields of volatiles. However, fissures are produced in DLC char during pyrolysis due to the non-caking property and cracks appear to decrease the production of pores. With the increasing annealing time and temperature, pore size and pore volume of chars decrease, which can be related to the shrinkage of coal char. The mechanical strength of chars is enhanced with increasing carbonization time and temperature, but the crushing strength of DLC chars is significantly influenced by cracks. Chars produced with longer time and higher temperature primarily display a lower CO2 reactivity according to the gasification tests. The results further indicate that factors affecting the CO2 reactivity are different for the two series of chars. The char reactivity index appears to have a linear correlation with the char strength after reaction.

Numerical modelling of oxy fuel combustion, the effect of radiative and convective heat transfer and burnout

In this study, a numerical investigation on the radiative and convective performance of the combustion of pulverised Russian coal having higher Calorific value in a 0.5MWth combustion test facility (CTF) has been conducted for air and CO2-rich oxy-fired environment considering an IFRF Aerodynamically Air Staged Burner (AASB) configured with the furnace. This study is carried out considering a finite volume method (FVM) tool using AVL Fire version 2009.2 coupled with the user defined subroutines especially for the coal devolatilisation and char combustion modelling. This code is validated comparing the experimental radiative heat flux with the numerically predicted data and results show that reasonable agreement has been found with the measured radiative heat flux on the furnace wall. Different combustion environments were investigated including an air-fired as reference case and three recycled flue gas (RFG) fired combustion environments. The different cases are composed of varying recycled ratio (RR) between 65% and 75%. It was found that the flame temperature distribution for the reference case (air fired) and RR72% case were found to be similar. The flame temperature increased with O2 concentration and decreasing RR. With the decrease of RR, the length of the flame is also shortened. The ignition condition improved with enriched O2 concentration in the RR65% (O2 30.9%, CO2 69.2% by mass) case. The results show that radiative and convective heat flux is significantly manipulated by the RR. The position of peak radiative flux moves downstream with increasing RR due to increased mass flow rate and reduced O2 at higher RR. With the increase of normalised total flow (NTF), mean flame temperature and exit temperature decreased whereas with the increase of normalised O2 flow (NOF), mean flame temperature and exit temperature increase. The presented working range for the Russian coal, suggests that the air equivalent radiative heat flux can be obtained at a RR of ≈71% while air equivalent flame temperature were observed at RR of 72%. Reasonable agreement has been obtained for unburned carbon in ash (CIA). An improved burnout was observed in RR conditions.

Experimental Investigation on NOx Reduction Potential of Gas-Fired Coal Preheating Technology

A gas-fired coal preheating (GFCP) technology, offering a flexible method to reduce NOx, could be used with other de-NOx combustion technology such as air staging to seek maximum NOx reduction. The preheating chamber key apparatus of GFCP was investigated with the help of infrared camera. The results show that devolatilization and partial oxidation (combustion) of coal occurred in the preheating chamber, and this may prove the main heat source of preheating chamber is combustion of coal volatiles and gas is only used to prevent flameout. A self-sustaining combustion drop furnace was used to investigate the NOx reduction potential of GFCP with air staging. Gas species concentrations along furnace are plotted for several runs, offering details to further study and analyze the GFCP. With GFCP, much HCN, CiHj, and soot were produced in the preheating chamber, so under the similar air staging condition, the NO destructed by HCN and soot was stronger than that without. NOx reduction could archive up to 72% with...

Experimental study and kinetic modeling of pulverized coal devolatilization under air and oxycombustion conditions at a high heating rate

This work aims to study the devolatilization of pulverized coal particles under air and oxycombustion conditions. To do so, a newly developed experimental test bench has been used to stabilize coal jet flames with fuel heating rates similar to those found in industrial combustors. The thermal history of coal particles has been experimentally monitored by coupling pyrometric and particle image velocimetry (PIV) measurements. Char samples have been collected at different residence times to obtain devolatilization profiles that have been compared with data issued from 4 empirical models. New sets of kinetic parameters have been proposed to simulate coal devolatilization under high heating rate (>106K/s) with N2- and CO2-based atmospheres. The composition of the burnt gases has also been characterized at various heights above the burner (HAB). The analysis of the obtained results confirmed that an oxygen enrichment of the combustion atmosphere enhances the devolatilization process by favoring an increase of the coal particle temperature. From the comparison between measured and modeled data, it has been observed that apparent devolatilization rates and kinetics were influenced only by the thermal history of the fuel particles with no char-CO2 gasification or CO2-cross linking reaction at the surface of the char in the conditions investigated here. CO releases were found to depend on the devolatilization rates and on the temperatures of the flames. An increase of the SO2 emissions has been strongly correlated to the oxygen concentration in the medium for a given devolatilization yield. NO emissions were not significantly reduced during experiments conducted under oxycombustion conditions compared to those performed under air which is due to the fact that NO emissions have been mainly related to the formation of fuel-NO in this work.