Char Combustion Stage Research Articles

Fuel Characteristics of Briquettes Manufactured by Natural Stacking Bamboo/Chinese Fir Mixtures.

Bamboo wastes were naturally stacked for 1 month and were uniformly mixed with Chinese fir. Briquettes were manufactured by a briquette extruder at different process temperatures and mixing ratios. The physical, mechanical, pyrolysis, and combustion characteristics of briquettes were determined. The results showed that the mixing ratios and process temperature had a significant impact on the fuel properties of briquettes. The optimum briquettes were manufactured by 70% bamboo/30% Chinese fir blends at a process temperature of 520 °C. The fuel properties of optimum briquettes met the standard requirement of LY/T 2552-2015 and GB/T 28669-2012. The lower heating rate at the primary pyrolysis stage increased the yield of charcoal during the carbonization process of briquettes. The combustion process of briquettes added a char combustion stage, compared with the pyrolysis process. There were no synergistic interactions of bamboo and Chinese fir during pyrolysis and combustion process. The results of this research are helpful to develop large-scale production of bamboo briquettes or charcoal.

Thermal behavior and kinetic analysis of co-combustion of wheat straw and polyvinyl chloride under different oxygen concentrations through thermogravimetry

Co-combustion characteristics of wheat straw with polyvinyl chloride under different oxygen concentration atmospheres were investigated using a thermogravimetric analyzer. Experimental samples included wheat straw, polyvinyl chloride, and their blends. The results showed that when the wheat straw was blended with polyvinyl chloride, there was an inhibition for devolatilization in the volatile combustion stage. However, during the char combustion stage, a promotion was observed. As the oxygen concentration increased from 10% to 40%, the ignition temperatures, burnout temperatures, and maximum reaction rate temperatures of all samples decreased, and the combustion characteristic factors ( SN) increased. The combustion of wheat straw was significantly affected by oxygen concentration, whereas the polyvinyl chloride and blends were affected relatively slightly. Especially, as the oxygen concentration increased from 30% to 40%, the combustion of polyvinyl chloride and blends improve slightly. The reason for this behavior is attributed to the diffusion effects of the oxygen and the microstructure changes of the samples. In addition, the activation energies of the co-combustion process under the four atmospheres, for all samples, were calculated by Flynn–Wall–Ozawa method. The kinetic results show that the diffusion effect of the oxygen influenced the apparent activation energies of combustion reaction and the compensation effect of activation energies exists.

Insight into the calcium carboxylate release behavior during Zhundong coal pyrolysis and combustion

In this paper, the dynamic behavior of calcium carboxylate release during Zhundong coal pyrolysis and combustion is studied via reactive molecular dynamics (ReaxFF MD) simulation. The molecular structure model of Zhundong coal is constructed based on the combination of the classic Hatcher coal model and experimental characterizations. Pyrolysis simulations on the coal model are performed at different temperatures ranging from 2000 K to 2800 K. The pyrolysis experiments are also carried out to validate the ReaxFF simulation. The results show that most of the calcium are released into the volatiles by the thermal decomposition of CM-Ca (coal/char matrix with calcium bonded) after releasing CO2. The distributions of the calcium bonded to gas, tar and inorganics as well as the atomic calcium in the volatiles are quantitatively classified. The thermal cracking of tar fragments are significant at high temperatures leading to the conversion of calcium from tar into the organic gas. Furthermore, the nascent char model is constructed to study the release behavior of calcium in char combustion stage. The calcium is initially released in the form of oxidized calcium and atomic calcium. With increasing temperature, the oxidized calcium trends to convert to the organically bonded calcium. By using the Arrhenius expression, the kinetic parameters for the release of calcium into various species during pyrolysis and char combustion stages are quantitatively determined.

Combustion behaviour of biochars thermally pretreated via torrefaction, slow pyrolysis, or hydrothermal carbonisation and co-fired with pulverised coal

The combustion characteristics and kinetics of straw biochars thermally pretreated via torrefaction, slow pyrolysis, or hydrothermal carbonisation (HTC) and co-combusted with bituminous coal were examined in this study to explore the effects of various pretreatment methods and blending ratios on the combustion characteristics of biochars to assess potential practical applications. Non-isothermal experiments were performed using a thermogravimetry analyser. The results showed that the thermogravimetric curve of biochar obtained after HTC was similar to that of raw straw, thereby indicating similar combustion characteristics and reactivity. The average reactivity ( R m ) index of hydrochar and straw was 1.648 and 2.082, respectively. By contrast, the biochars produced after torrefaction or pyrolysis were more similar to bituminous coal, with the R m index of 0.696, 0.78 and 0.773, respectively. A positive interaction was observed during the char combustion stage of co-firing, with the activation energy of the blends tending towards the lower side. The catalytic effect of biomass ash was likely the predominant reason for the better interaction of pyrolytic biochar with coal. • Combustion behaviors and kinetic of straw biochars and their co-firing were studied. • Pre-treatments include torrefaction, slow pyrolysis, or hydrothermal carbonisation. • Combustion characteristics and reactivity of hydrochar are close to those of straw. • Combustion of biochar after torrefaction or pyrolysis is similar to bituminous coal. • Positive interaction was observed during the char combustion stage of co-firing.

Catalytic combustions of two bamboo residues with sludge ash, CaO, and Fe2O3: Bioenergy, emission and ash deposition improvements

The catalytic combustions of bamboo leaves (BL) and branches (BB) with textile dyeing sludge ash (SA), Fe2O3, and CaO were qualitatively analyzed using thermogravimetric and Fourier transform infrared spectroscopy analyses, and thermodynamic equilibrium simulations. The catalysts (Fe2O3 > SA > CaO) exerted a more pronounced effect in the char combustion (third) stage and enhanced the volatiles and comprehensive combustion indices with 40 °C/min. The catalysts (CaO > SA > Fe2O3) reduced C- and N-containing gas emissions in the devolatilization (second) stage. CaO elevated the N-containing gas emission in the third stage, whereas Fe2O3 and SA inhibited the formation of NO precursors. BB presented a higher risk of slagging than did BL, while the improved empirical indices of the ash deposition pointed to CaO as the optimal catalyst. Our simulations showed the final ash components of BL and BB were mainly as SiO2 and K2Si4O9. The addition of CaO alone helped to form a high-melting point Ca-silicate. Although the addition of Fe2O3 had no effect on the ash conversion, SA reduced the formation of K-silicate in the ash. The catalysts (CaO > SA > Fe2O3) reduced the activation energy. Overall, the catalytic combustions improved the bioenergy and the N-containing gas emissions. SA as a Fe and Ca-rich industrial waste enhanced the combustion performance in terms of reductions in waste streams, gas emissions, and ash deposition. Our results supplied new insights into the efficient and clean bioenergy production of bamboo residues, and the waste utilization of SA.

Combustion kinetics and mechanism of biomass pellet

The effect mechanism of biomass pelletization on the property of bio-char, combustion kinetics and mechanism of pellet remain unclear. Therefore, in this work combustion behavior and kinetics of typical biomass pellets were investigated using TGA and Distributed Activation Energy Model. The bio-chars, from the pellets and raw biomasses, were also characterized in detail, to deeply understand the combustion mechanism of biomass pellet. The results showed that the activation energies of devolatilization stage of the pellet combustion were 118.1–230.2 kJ/mol, higher than that of raw biomasses, however, the activation energy decreased to 201.4–222.4 kJ/mol, for char combustion stage of pellet combustion, lower than the raw biomass. The compact physical structure of the biomass pellet caused by pelletization suppressed the mass and heat diffusion and thus enhanced the activation energy of devolatilization stage. The pelletization also suppressed the deoxygention, polymerization and aromatization reactions during the devolatilization stage, which caused higher reactivity of the pellet char than raw biomass char. This was the main reason for the lower activation energy of the pellet char combustion stage. Thus, the controlling factor of biomass pellet combustion is mass and heat diffusion for devolatilization stage, on the contrary, it was chemical reactions for char combustion stage.

Preparation and characterization of nano silver immobilized hydrochar derived from hydrothermal carbonization of tobacco stem

In this present study, silver nanoparticles (AgNPs) immobilized hydrochar was prepared through a facile one-pot co-hydrothermal carbonization of tobacco stem and silver nitrate. The effect of concentration of silver precursor, hydrothermal temperature and reaction time on the chemical compositions, structural property, combustion performance, as well as gaseous products release behavior of the resultant AgNPs immobilized hydrochar were evaluated. The co-hydrothermal treatment decreased the volatile matter while increased the ash content of hydrochar. Single-phase AgNPs were successfully generated and evenly immobilized onto the hydrochar during the hydrothermal process. The immobilized AgNPs played a notable catalytic role during the char combustion stage, thereby decreased the ignition temperature, burnout temperature and maximum rate temperature for char combustion. Meanwhile, TG-FTIR analysis suggested that the emission amount of harmful gas CO during hydrochar combustion was reduced due to the catalytic effect of AgNPs. This phenomenon may indicate the appropriate amount of nanoparticles immobilization could enhance the combustion behavior of hydrochar fuel.

Investigation of combustion reactivity and NO emission characteristics of chars obtained from the devolatilization of raw and partially dried lignite

AbstractIn an attempt to achieve the clean and efficient utilization of lignite, drying pre‐treatment was performed in this study before lignite combustion. The combustion reactivity and NO emission characteristics of the partially dried lignite samples in the char combustion stage were investigated by means of TG and isothermal combustion tests, and the reactivity could be summarized as the following order: L1>L0.5>raw>L3>LT>L5 (chars obtained from the devolatilization of the raw and partially dried coals at 378 K for 0.5, 1, 3, 5, and 120 minutes) and the NO conversion ratio of L1 was the lowest. When the moisture content in the coal particles was relatively high (19.68%‐35.84%), the drying treatment could increase the combustion reactivity and inhibit NO emission in the char combustion stage; When the moisture content was within a relatively low range (0.17%‐19.68%), the moisture removal had negative effects on the reactivity and NO emission in the char combustion stage. The surface behaviour and microstructure of the raw coal char and chars derived from the partially dried coals were clarified by temperature programmed desorption/reduction (TPD/TPR) and Raman spectroscopy. The results illustrated that L1 derived from Lc1 (19.68%) was the most reactive sample with the largest amount of C(O) on the particle surface. There were also more reactive aromatic structures in L1 than other samples. Compared with direct combustion or excessive drying treatment, lignite should be dried to a certain degree (19.68%) for optimized lignite combustion.

Study on the Influence of the Activation Energy on the Simulation of Char Combustion

The numerical simulation of coal char combustion process can quickly obtain the combustion characteristics of char. It has guiding significance for the design and operation industrial boilers. But during the numerical simulation, how to choose the appropriate kinetic parameters is a difficult problem. The values of combustion activation energy in previous literatures were different. This paper focused on the influence of activation energy on the simulation results of char combustion. The activation energies of 20 Chinese typical coals were measured by a two-step in-situ reaction analyzer. CBK (Carbon burnout kinetics) model was used to simulate char combustion process. Results showed that when activation energy varied greatly, the the predicted combustion rate and particle temperature were very different, especially in the later stage of char combustion.

Characterization of moxa floss combustion by TG/DSC, TG-FTIR and IR

The combustion of moxa floss was investigated by thermogravimetric/differential scanning calorimeter (TG/DSC), thermogravimetric/Fourier transform infrared spectroscopy (TG-FTIR) and an infrared radiation spectrum-detecting system. The average mass loss rate at the major stage of combustion was 1.08%/min with a peak at 7.86%/min. Moxa floss burning released the integral heat of 9188 mJ/mg from the temperature of 200 °C to 500 °C. The peak heat flow during char combustion stage was higher than those of other stages. The results indicated the combustion of moxa floss was a mild and slow process, releasing heat continuously and steadily. The infrared radiation spectra of burning moxa stick were in a range of 2.5–13.0 µm and showed double peaks at 3.75 µm and 8.75 µm. Thus, this study provided insight into why moxa floss biomass is the optimal material for thermal therapy and is beneficial for the development of electric thermal stimulation alternative.

Experimental investigation of combustion characteristics and NOx formation of coal particles using laser induced breakdown spectroscopy

To study the mechanism of coal combustion and NOx formation, the combustion of coal particles in different atmospheres (O2/N2, O2/CO2) with different O2 concentrations was investigated using the CO2 laser as a heat source. The spatial distribution of atoms and groups (e.g., H at 656.2 nm, O at 777.3 nm, CN at 388.3 nm) relating to the combustion flame were measured simultaneously using laser induced breakdown spectroscopy (LIBS). The residual energy was measured during the collection of LIBS spectra in the combustion process, which could be characterized the temperature profiles of combustion flame due to the positive correlation with temperature. The combustion stage could be clearly discriminated by the emission of H and CN, along with the flame temperature. The residual energy obtained in different atmospheres indicated that the impact of combustion atmosphere on flame temperature is greater in the char combustion stage rather than volatile combustion stage. It was determined from the temporal and spatial distribution of residual energy and CN intensity that a higher flame temperature leads to a higher concentration of CN. The correlation between the generation of CN and the NOx formation was also investigated to show that the formation approaches of NOx are similar in the O2/CO2 and O2/N2 atmospheres, while the fuel-N conversion paths are different between volatile combustion and char combustion stages. The measurement of temporal and spatial distributions of LIBS spectra with varying flame temperatures is significant in revealing the mechanism of coal-particle combustion and NOx formation.

Pyrolysis and combustion kinetics of lignocellulosic biomass pellets with calcium-rich wastes from agro-forestry residues

The pyrolysis and combustion kinetics of biomass pellets (i.e., rice husk, herb residue, and wood residue) with the calcium-rich wastes (i.e., CaO, CaCO3, and eggshell) from agro-forestry residues were comparatively studied. During pyrolysis or combustion of biomass, the Ca-rich wastes could slightly influence the decomposition rate in the stage of devolatilization at relatively lower temperatures (e.g., <400 °C). However, the lignin decomposition and the char combustion were obviously influenced by the calcium-based catalysis at higher temperatures (>700 °C). Particularly, the eggshell had a lowest activation energy in the stage of char combustion. The presence of alkali and alkaline-earth metals (AAEMs) in the eggshells might have positive effects on volatile and char combustion. During the combustion, the decomposition temperatures of CaCO3 and eggshell were decreased, thereby favoring to uptake CO2. Furthermore, by identifying the small molecular products, it was found that both CaCO3 and CaO can improve the pyrolysis of RH, but CaCO3 showed better performances, especially on CO2 capture at lower temperatures and on the enhancement of CO production.

Bottom ash characteristics and pollutant emission during the co-combustion of pulverized coal with high mass-percentage sewage sludge

A municipal and a coal-coking industrial sludge (MS and CS) were separately co-combusted with bituminous coal at a sludge mass ratio (SMR) of 10%–50%. Quartz, larnite, gehlenite, mayenite, and calcium sulfate were formed during co-combustion. Slight or medium slagging was predicted to occur when slagging tendency was assessed based on the mineral components and fusion temperature of the ash. The SO2 emission peak at the devolatilization stage increased with increasing SMR of MS but only slightly changed with increasing SMR of CS. SO2 emission at the char combustion stage was suppressed due to the sulfur retention effects of sludge ash. NO, HCN, and N2O were the main emission forms of N-containing gases. With increasing SMRs of MS and CS, HCN emissions became increasingly prominent, whereas NO showed only a slight increase and even decreased. In addition, the volatilization percentages (VP) of Cd, Zn, Cr, and Ni decreased. By contrast, the VP of Pb increased when coal was combusted with CS because Pb was considerably influenced by high Cl. The concentrations of Cd, Cr, Ni, Pb, and Zn in the bottom ashes were determined and satisfied China's threshold limit value for landscaping land application or soil amendment.

High-temperature pyrolysis behavior of a bituminous coal in a drop tube furnace and further characterization of the resultant char

In order to study the high-temperature pyrolysis behavior of bituminous coal in the drop tube furnace (DTF) and better utilization of the obtained char, a typical Shenhua bituminous coal (SH) was pyrolyzed in a DTF at the temperatures of 1000–1300 °C (100 °C increments). The obtained char was then characterized using BET and FTIR methods, while its combustion, gasification and pollutant emission characteristics were explored by a thermogravimetric analyzer and online flue gas analyzer. Results show that higher pyrolysis temperature leads to more CO and less CH4 for the increased free radical rearrangement by high heating rate and carbon reduction reactions at high temperatures. Pyrolysis generally decomposes the functional groups, increases the volume of char pores, and upgrades the rank of coal. The combustion activation energy increases from 22.99 kJ/mol to 27.42 kJ/mol and the gasification is hindered with the higher pyrolysis temperature. It is notable that the NOx and SO2 emissions of the obtained char combustion are both below 30 mg/s-MJ. High-temperature pyrolysis can enhance the evolution of NH3 and HCN as volatiles, thus decreasing the NOx formation in further char combustion stages while partly decomposing the organic and inorganic sulfur compounds to impact the SO2 emission.

Influence of H2O on NO formation during char oxidation of biomass

The present study investigates conversion of char-N to NO in mixtures of O2/N2 and in O2/H2O/N2. Biomass particles of spruce bark were combusted in an electrically heated single particle reactor at 900 °C at various O2/H2O/N2 concentrations. NO concentrations of the product gases were measured during the char combustion stage. The conversion of char-N to NO was significantly higher with H2O as compared to without H2O in the gas. Additional fixed bed experiments were conducted to investigate the products of the reaction between H2O and spruce bark char. The results showed that NH3 is the primary product in the reaction between char-N and steam. These results explain the observation that more NO is formed during char combustion in the presence of steam: the char-N reacts partly with H2O to form NH3, which reacts further to NO.

Experimental Investigation of Combustion Kinetics of Wood Powder and Pellet

The combustion kinetic characteristics of wood powder and pellet were investigated within thermogravimetric analyser (TGA) and tube furnace system. The kinetic parameters of these two different forms of woody fuel were measured and derived by double-step-and-double-equal and isothermal method, respectively. The results showed that the combustion mechanisms of wood powder kept consistent through the whole process, while the combustion mechanisms of wood pellet differed significantly between the volatile and char combustion stages. The most probable mechanism functions of the two different forms of woody fuel were not the same due to the differences in internal heat and mass transfer properties. In addition, activation energy values varied from 92.33 kJ·mol−1 for wood powder to 71.20 kJ·mol−1 for wood pellet, while the preexponential factor value of wood powder (2.55×108 s−1) was far greater than the one of the wood pellet (78.55 s−1) by seven orders of magnitude.

The effect of carbon dioxide on flame propagation speed of wood combustion in a fixed bed under oxy-fuel conditions

This paper presents an investigation of wood combustion in a laboratory-scale fixed bed with an aim of establishing the effect of CO2 environment on flame propagation speed and flame structures. Different oxy-fuel combustion atmospheres in which the composition of O2 in CO2 was varied from 21% to 50% by volume were tested and compared to air-fuel condition. Euler-Lagrange (Computational Fluid Dynamics - Discrete Element Method, CFD-DEM) approach which captures information of individual particle processes is used to model wood conversion in a packed bed. Results show that flame front propagation speed in oxy-fuel atmosphere reduced to 78% of that of the air-fuel condition with similar O2 concentration. For oxy-fuel conditions, propagation speed increased with increase in O2 concentration. The CFD-DEM model agrees very well with experimental values for mass loss, propagation speed and flame front positions. However, peak temperatures are poorly predicted at lower oxygen concentrations. The accuracy of temperature prediction improves at higher oxygen concentrations. During initial and devolatilization stage, mass fraction of tar predicted in CO2 environment are smaller than in N2 environment, while the amount of CO predicted is almost equal in both environments. However, during char combustion stage a high amount of CO is observed in oxy-fuel conditions.

Spontaneous Emission Measurements of Selected Alkali Radicals during the Combustion of a Single Biomass Pellet

Spontaneous emission intensities of Na*, Ca*, and K* during the combustion of single wood and straw pellets doped with known concentrations of Na, Ca, and K were measured using optical bandpass filters mounted on an intensified charge-coupled device camera. The impregnated biomass pellets were suspended in a natural gas flat flame at 750 and 1000 °C. Before the pellets were made, the biomass samples were washed and soaked to demineralize and doped with different concentrations of Na, Ca, and K (0.5, 2, and 5 wt %). During the experiments, the temperature at the center of the pellets was measured with a thermocouple and the combustion stages were identified with the help of the temperature derivative. The results reveal that, at the lower gas temperature, the emission of the selected alkalis is marginal, in agreement with previous studies. At the higher gas temperature, the emission profiles reveal that K and Na are released distinctively in the volatile combustion and char combustion stages. The presence of large amounts of silica and alumina may trap the alkalis in the solid phase, leading to a reduced emission of these species during the char combustion. Calcium does not evaporate at the tested temperature conditions, but the temperature is high enough to promote the decomposition of calcium oxalate in the outer layer of the pellet, leading to a flat emission profile during the combustion stages. Finally, the total integrated emission increased proportionally with the increase of the doping concentration for all species.

The combustion performances and thermo-oxidative degradation kinetics of plane tree seeds (PTS) (Platanus orientalis L.)

Combustion characteristics (reactivity and reactivity evaluation, ignition index (Di), burn-out index (Df), combustion performance index (S) and combustion stability index (RW)) and thermo-oxidative degradation kinetics of plane tree seeds (PTS) were investigated. Oxidation analysis shows that PTS exhibits stable and high combustion activity, where comparison of ignition and burn-out index values showed that a more developed pore structures of PTS particles exist, and higher volatile matter content is present. Results are also confirmed through derived experience constant value (ζ) and which were compared with other energy sources. It was established that different maximal reaction rate values at various heating rates point out the PTS complex nature, which is stimulated with carbon oxidation reactions and volatile matter release. Kinetic analysis has shown that process can be divided into two stages: devolatilization stage (280–380 °C) and char combustion stage (380–600 °C). In order to obtain a stable solid product, rich in carbon, the PTS shows good performance but with application of high-temperature processing conditions.

Conversions of fuel-N, volatile-N, and char-N to NO and N2O during combustion of a single coal particle in O2/N2 and O2/H2O at low temperature

Abstract Oxy-steam combustion is a promising next-generation combustion technology. Conversions of fuel-N, volatile-N, and char-N to NO and N2O during combustion of a single coal particle in O2/N2 and O2/H2O were studied in a tube reactor at low temperature. In O2/N2, NO reaches the maximum value in the devolatilization stage and N2O reaches the maximum value in the char combustion stage. In O2/H2O, both NO and N2O reach the maximum values in the char combustion stage. The total conversion ratios of fuel-N to NO and N2O in O2/N2 are obviously higher than those in O2/H2O, due to the reduction of H2O on NO and N2O. Temperature changes the trade-off between NO and N2O. In O2/N2 and O2/H2O, the conversion ratios of fuel-N, volatile-N, and char-N to NO increase with increasing temperature, and those to N2O show the opposite trends. The conversion ratios of fuel-N, volatile-N, and char-N to NO reach the maximum values at = 30 vol% in O2/N2. In O2/H2O, the conversion ratios of fuel-N and char-N to NO reach the maximum values at = 30 vol%, and the conversion ratio of volatile-N to NO shows a slightly increasing trend with increasing oxygen concentration. The conversion ratios of fuel-N, volatile-N, and char-N to N2O decrease with increasing oxygen concentration in both atmospheres. A higher coal rank has higher conversion ratios of fuel-N to NO and N2O. Anthracite coal exhibits the highest conversion ratios of fuel-N, volatile-N, and char-N to NO and N2O in both atmospheres. This work is to develop efficient ways to understand and control NO and N2O emissions for a clean and sustainable atmosphere.