Char Surface Research Articles

Microscopic mechanism and kinetics of NO heterogeneous reduction on char surface: A density functional theory study

A quantum chemistry calculation with density functional theory at M06-2X/6-31G(d,p)//def-TZVP level is conducted to gain insights into the microscopic mechanism of NO heterogeneous reduction on char surface. Electronic properties of reactants based on Mulliken atomic charge and electrostatic potential reveal the attack sites and electron donation from char to NO during the chemisorption, which is consistent with HOMO and LUMO analysis. Two stages of NO secondary chemisorption, N2 and N2O formation are included in the evolution pathway of NO reduction, and the second NO chemisorption is determined as the rate-limiting step of NO reduction. Meanwhile, the migration of the oxygen atom is favorable for NO reduction, resulting in the preferred pathway of N2 formation than N2O in thermodynamics. According to the kinetic calculation, the reaction rate constant of NO reduction to release N2 is 1.438E+01 s−1 at 900 K, and the corresponding activation energies of N2 and N2O formation are 37.97 and 78.02 kcal/mol respectively, which is in coincidence with previous experiments. Mayer bond order analysis exhibits the C–N, N–O and N–N bond evolution of two rate-limiting steps, together with the bond strength between atoms.

Sorption of Cd2+ on Bone Chars with or without Hydrogen Peroxide Treatment under Various Pyrolysis Temperatures: Comparison of Mechanisms and Performance

In this study, bone char pretreated with hydrogen peroxide and traditional pyrolysis was applied to remove Cd2+ from aqueous solutions. After hydrogen peroxide pretreatment, the organic matter content of the bone char significantly decreased, while the surface area, the negative charge and the number of oxygen-containing functional groups on the bone char surface increased. After being pyrolyzed, the specific surface area and the negative charge of the material were further improved. The adsorption kinetics and isotherms of Cd2+ adsorption were studied, and the influence of solution pH and the presence of ionic species were investigated. The experimental results showed that the samples with lower crystallinity exhibited less organic matter content and more surface oxygen-containing functional groups, resulting in stronger adsorption capacity. After being treated with hydrogen peroxide and pyrolyzed at 300 °C, the maximum adsorption capacity of bone char was 228.73 mg/g. The bone char sample with the lowest adsorption capacity(47.71 mg/g) was pyrolyzed at 900 °C without hydrogen peroxide pretreatment. Ion exchange, surface complexation, and electrostatic interactions were responsible for the elimination of Cd2+ by the bone char samples. Overall, this work indicates that hydrogen peroxide-treated pyrolytic bone char is a promising material for the immobilization of Cd2+.

An experimental and theoretical study of NO heterogeneous reduction in the reduction zone of ammonia co-firing in a coal-fired boiler: Influence of CO

The co-firing of ammonia (NH3) and coal in boilers and furnaces has attracted increasing attention worldwide. However, only a few studies have been reported on the mechanism of NO reduction in the reduction zone of coal and ammonia co-firing. This work adopted theoretical calculations and experiments to explore the mechanism of NO reduction by char and NH3 and the role of CO. Theoretical results indicated that CO exhibits a synergistic effect with the amino agent on NO reduction. In char/NH/CO/NO system, the hydrogen-rich environment created by H atoms weakens the energy of N-C bond, thereby promoting NO heterogeneous reduction. However, CO occupies the active sites on the char surface and slightly inhibits the NO heterogeneous reduction in char/NH/CO/NO system. Experimental results revealed that CO improves the NO reduction efficiency in the NH3/char/NO system. CO plays different roles in the heterogeneous reduction of NO by NH on the char surface in high-temperature furnace experiments as well as in theoretical calculations. These results are attributed to the fact that the homogeneous reduction rate of NO by the amino and CO in the ammonia and coal co-firing is greater than the heterogeneous reduction rate of NO by CO-coordinated char.

Release characteristics of AAEMs and physicochemical structural evolution of char during rapid coal pyrolysis

Rapid pyrolysis experiment of Zhundong coal was conducted in a high-frequency furnace to investigate release characteristics of alkali and alkaline earth metals (AAEMs) and its correlation with changes of physical and chemical properties of char. Residence time and atmosphere during pyrolysis were important process parameters that affected the migration characteristics of AAEMs and physicochemical structural evolution of char. Experimental results show that the release of AAEMs in coal char increases over time. The maximum release rate of Na, Mg and Ca is 61.05%, 64.47%, and 44.01%, respectively. Promoting effect of CO2 on release of Na mainly occurs in the initial stage of rapid pyrolysis, nevertheless the promoting effect on release of Mg and Ca mainly is in the middle and late stage. CO2 accelerates release of AAEMs by promoting release of volatiles, facilitating decomposition of oxygen-containing functional groups and aliphatic functional groups, and promoting formation of cracks on surface of coal char.

Organic-inorganic hybrid filler for improved thermal conductivity and anti-dripping performance of polybutylene succinate composite

While polybutylene succinate (PBS) has gained increasing interest following the recent shift from conventional polymers to biobased and biodegradable polymers, its high cost, high flammability, and low thermal conductivity have limited its practical applications. In this study, PBS hybrid composites with improved heat dissipation capability and anti-dripping performance were fabricated by incorporating a novel organic-inorganic hybrid filler. The hybrid filler was fabricated by attaching silicon carbide particles to the surface of coffee husks treated with a flame retardant agent. Spectroscopic and morphological analyses confirmed that the hybrid filler was successfully fabricated. PBS composites filled with the hybrid filler exhibited better filler/matrix interfacial interaction, which resulted in a 250% enhancement on the thermal conductivity of the hybrid composite with the incorporation of small vol% of conductive SiC. They also showed superior mechanical properties: the tensile strength and Young's modulus were enhanced by 40% and 70%. During a burning test, the composite attained the UL-94 V-0 rating without any dripping and with surface char formation. Overall, the PBS hybrid composite showed promising properties that could potentially broaden the applications of PBS-based materials.

Dancing and recrystallizing of Na2CO3 particles during catalytic gasification: Instructing the industrial catalysts loading procedures

The migrating of alkali at high temperature is closely related to catalytic gasification, activated carbon preparation and volatilization of alkali. In this work, a simple but effective and visible method was used to reveal the newfound migrating modes between Na2CO3 particles and coal char by situ heating stage microscope. Moreover, the effects of Na2CO3 migration on the gasification and the catalyst-loaded procedure were also evaluated. The video results recorded by microscope visually shows that Brownian motion and recrystallization of Na2CO3 particles contribute to the migration between Na2CO3 and coal during catalytic gasification. Brownian motion acts on the small Na2CO3 particles (<20 μm) and recrystallization acts on the big particles. The results show that Na2CO3 particles (<20 μm) actively moves at temperatures range of 660–730 °C, and when the moving Na2CO3 closes to the coal char particles, the coal char draws the jumping Na2CO3 particles, and the moving Na2CO3 particles adheres to the coal char surface. For the big Na2CO3 particles, lots of tiny particles can migrate to coal char by recrystallization in the surrounding of Na2CO3 particles, and the coal char has priority to attract the recrystallization of the tiny particle on the coal char surface. The Brownian motion and recrystallization complete Na2CO3 migration from Na2CO3 to coal, and Na2CO3 loaded by mechanical mixing can complete even Na dispersion as impregnation method does.

Experimental Study to Replicate Wood Fuel Conversion in a Downdraft Gasifier: Features and Mechanism of Single Particle Combustion in an Inert Channel

Downdraft gasification is a promising process of energy conversion of wood biomass. There are such fuel conversion conditions that differ favorably from conventional conditions. In such conditions, there is no pyrolysis zone in the fuel bed, which precedes the oxidation zone. Fuel is supplied into the oxidizing zone without charring, where it reacts with the intensive cold air flow from tuyeres. The study aims to replicate the conversion of particles in a gasifier close to tuyeres. For this purpose, the individual particles are burned in the muffle furnace space and the quartz channel replicating presence of other bed particles at a first approximation. In the experiment, the furnace temperature was varied, as well as the velocity of air supplied to the particle. Two-stage and single-stage mechanisms of particle combustion were identified. A two-stage process is observed in the range of tuyere velocities below 20 m s−1. The two-stage mechanism is characterized by a stage of devolatilization and volatiles combustion, followed by a stage of char residue combustion. The stages are predominantly separate from each other, and their degree of overlapping is low, amounting to 24%. At the tuyere velocities above 125 m s−1 combustion of particles is realized primarily as a single-stage process. The intensive air flow reaches the fuel particle surface and initiates combustion of the surface char layer. In this case, the stages of devolatilization and char residue combustion run concurrently for the most part. In the single-stage mechanism, the degree of stage overlapping is significantly higher and amounts to 60–95%. For the two-stage combustion mechanism, the effect of cyclic movement of the flame across the particle surface is evident. The number of cycles can reach eight. This effect is due to the change of conversion stages. At air velocity above 95 m s−1, fragmentation of fuel particles commences. A layer of char formed at an initial stage of burning heats up in the intensive air flow and is separated from the particle surface. The heated walls of the quartz channel contribute to the intensification of particle combustion. This effect is probably due to the swirling of the flame between the wall and the particle surface.

Effect of K-decoration on the generation and reduction of N[formula omitted]O onto a biochar surface

Density functional theory calculations, with corrections for London dispersion interactions, have here been used in studying the generation and decomposition of N2O through the heterogeneous reduction of NO on a biochar surface. The effect of K-decoration on this specific mechanism has also been investigated.The results show that there are two possible pathways for the generation and decomposition of N2O on the pristine char surface, depending on type of reaction site. In the formation stage of N2O, the reactions rates of the two pathways are remarkably similar. However, the activation energies of those pathways are different for the decomposition stage of N2O (166.75 kJ/mol and 117.51 kJ/mol respectively).Decoration by K does not change these original reaction pathways. However, it decreases the activation energy for the rate-determining step and, thus, increases the total reaction rate. The activation energy is reduced by 13 kJ/mol for the generation of N2O and the effect is even larger for the process of N2O decomposition. For this latter type of process, the K-induced activation energies for the two pathways are reduced by 97.69 kJ/mol and 48.45 kJ/mol in comparison to pristine char.Moreover, the reaction rates of the different reaction pathways were observed to gradually increase with an increase in temperature. In addition, the difference between the reaction rates for the pathway of the pristine char, and that of the K-decorated char, was found to decrease, which is most probably due to the sublimation of K atoms at higher temperatures.

RETALT_TPS design and manufacturing

The present document discusses the development of a new trowelable Thermal Protection System (TPS), able of being mixed, applied and cured directly onto the vehicle structure, with the aim to fulfill the requirements of the thermal properties for the re-usable launch vehicle studied in the Retro Propulsion Landing Technology (RETALT) project. During the development of this TPS, several formula optimizations were made to improve or eliminate cracks in the char surface, increase char stiffness, rheological adjustments, and adhesion improvement to different substrates. The most promising material developed is composed by cork and epoxy resin, together with a set of rheological and thermal resistance additives, that makes it possible to be applied with a spatula, while at the same time it is able to withstand the demanding environmental conditions during atmospheric reentry. In terms of thermal properties, the developed material has a higher thermal conductivity than the current P50 TPS commercialized by Amorim Cork Composites (ACC), but it has a better behavior when exposed to flame conditions. It is expected that the absence of cracks improves its structure and resistance to demanding conditions. The development work included a detailed study of the composition and processes required for the development of a TPS material, which were evaluated by several types of flame characterization tests and thermal properties analysis.

A Theoretical Insight into the Mechanism of No Heterogeneous Reduction on Char Surface: The Catalytic Effect of Potassium

A Theoretical Insight into the Mechanism of No Heterogeneous Reduction on Char Surface: The Catalytic Effect of Potassium

The effect of a phosphorus-based FR on the fire performance and flammability properties of basalt fiber-reinforced acrylonitrile-butadiene-styrene composites.

This study focuses on the improvement in fire performance, and flame retardancy (FR) properties of chopped basalt fiber-reinforced acrylonitrile-butadiene-styrene (ABS) composites. For this purpose, different amounts of aluminum diethyl phosphinate (AlPi) compound (5, 10, and 15 wt%) were incorporated in the composites. The FR properties of the composites were examined via limiting oxygen index (LOI), UL-94 standard, and mass loss calorimeter tests. Thermogravimetric analysis was carried out to analyze the decomposition behavior of the composites. SEM inspection was also performed to examine the char surfaces of the composites. The results and findings showed that the introduction of AlPi compound into the composite structure leads to promotion in the char yield and improves the fire performance of the ABS matrix. As the added amount of AlPi into the composite increased, the LOI value of the composite increased. The addition of 15 wt% AlPi resulted in a UL-94 rating of V1 and the LOI value of 31.4%.

Mechanisms of the N2O formation and decomposition over coal char surface

N2O is an important gaseous pollutant in fluidized bed combustion. This work first presents a comprehensive thermodynamics study of the formation of N2O and its simultaneous destruction using the density functional theory (DFT). In addition, the kinetics is reported in a 0-D batch reactor using the rate parameters computed based on the transition state theory (TST). The armchair configuration with pyridine nitrogen is selected as the char(N) model. This work emphasizes the effect of chemisorption mode, temperature, and oxygen form on N2O evolution during the NO+char(N) reaction. There are two modes (NN-OC and NO-NC) for the chemisorption of NO on char surface leading to the generation of N2O. By comparison, N2O formation is more favorable under the NO-NC mode, which can avoid the largest barrier caused by the NO antibonding node. The involvement of ether oxygen promotes low-temperature generations of N2O (<673 K), while that of ketone oxygen facilitates moderate-temperature generations of N2O (673–873 K). In oxygen-free conditions, N2O is released at relatively high temperatures (973–1073 K). Once the temperature is higher than 1073 K, a large amount of N2O will be decomposed to gaseous N2 and CO. The results can explain why the available N2O formation mechanisms from char combustion considering the involvement of oxygen species, how different types of oxygen affect the N2O formation, and the well-known temperature inflexion point in N2O evolution. These findings will advance our understanding of the reaction and help develop a more accurate kinetics model to predict N2O emission.

One new channel for the reduction of NO during gasification condition: An insight from DFT calculations

ABSTRACT NCO is an important nitrogen-containing intermediate in coal combustion systems. This work reports the heterogeneous reaction of NCO+NO under gasification conditions using density functional theory (DFT) calculations at the M06–2X/6–311 G(d) level. The initial carbonaceous surface is represented by an armchair model. Under gasification conditions, NCO can be incorporated into the char surface and H2O has greater reactivity as compared to CO2. The optimal paths with five products (Pro1-5) are obtained by analysis of the reaction energy and the highest energy in the transition state (HETS). The results demonstrate that, compared with Pro 3 and Pro 4, the reaction paths of Pro 1 and Pro 2 possess lower HETS, which means that these reactions are more likely to occur. As for Pro 5, its highest value of HETS among the five products and the most unstable configuration signify that it is far from the expected product. Subsequently, we propose the reaction mechanism based on the Chemkin calculations. NCO+NO reaction can not only occur in the gas phase, but also can heterogeneously at the char surface. It can occur without external energy input for Pro 1 and Pro 2, indicating it will be one of the NO reduction channel under gasification conditions. The release of CO2 is more favorable than the release of CO. CO2 can be eliminated during the reactions and consequently provides good environment for N2 removal with low energies. This work will provide some new understanding of the NO reduction under gasification condition.

CO2 Gasification Reactivity of Char from High-Ash Biomass

Biomass char produced from pyrolysis processes is of great interest to be utilized as renewable solid fuels or materials. Forest byproducts and agricultural wastes are low-cost and sustainable biomass feedstocks. These biomasses generally contain high amounts of ash-forming elements, generally leading to high char reactivity. This study elaborates in detail how chemical and physical properties affect CO2 gasification rates of high-ash biomass char, and it also targets the interactions between these properties. Char produced from pine bark, forest residue, and corncobs (particle size 4–30 mm) were included, and all contained different relative compositions of ash-forming elements. Acid leaching was applied to further investigate the influence of inorganic elements in these biomasses. The char properties relevant to the gasification rate were analyzed, that is, elemental composition, specific surface area, and carbon structure. Gasification rates were measured at an isothermal condition of 800 °C with 20% (vol.) of CO2 in N2. The results showed that the inorganic content, particularly K, had a stronger effect on gasification reactivity than specific surface area and aromatic cluster size of the char. At the gasification condition utilized in this study, K could volatilize and mobilize through the char surface, resulting in high gasification reactivity. Meanwhile, the mobilization of Ca did not occur at the low temperature applied, thus resulting in its low catalytic effect. This implies that the dispersion of these inorganic elements through char particles is an important reason behind their catalytic activity. Upon leaching by diluted acetic acid, the K content of these biomasses substantially decreased, while most of the Ca remained in the biomasses. With a low K content in leached biomass char, char reactivity was determined by the active carbon surface area.

The effect of ammonia co-firing on NO heterogeneous reduction in the high-temperature reduction zone of coal air-staging combustion: Experimental and quantum chemistry study

Ammonia is a carbon-free renewable energy and is considered as a potential alternative fuel. And the co-firing of coal and NH3 in boiler has attracted more and more attention. Notably, NH3 is a kind of efficient reductant. The homogeneous reduction of NO by NH3 and the heterogeneous reduction of NO by char occur in the reduction zone of high temperature and oxygen-lean during air-staged combustion. It is necessary to study the heterogeneous reaction process of NH3/char/NO in the high-temperature reduction zone. Experimental and density functional theory are employed to systematically explore the heterogeneous reduction mechanism of NO in high temperature reduction zone under NH3 co-firing in coal-fired boiler. Experimental results show that NH3 co-firing promotes NO reduction in the reduction zone, and is synergistically promoting NO heterogeneous reduction with char. In the heterogeneous reaction, two radical forms of NH and NH2, respectively, are considered in the theoretical calculation. The findings show that the energy barrier for the rate-determining step in the heterogeneous reaction of NHNO on the char surface is less than that for the rate-determining step in the homogeneous reduction of NO by NH3, which clarifies the synergistic promotion mechanism of char and NH on NO reduction. The energy barrier for the rate-determining step in the heterogeneous reaction of NH2-NO on the char surface is greater than that for the rate-determining step in the homogeneous reduction of NO by NH3. Bond order and electron localization function (ELF) analysis reveals that the NNH free radical formed by the B2 structure (intermediate structure formed by NH2 adsorbing on the char in N-down form and NO in parallel form) enhances the binding energy with the C atom and inhibits the reduction of NH2 and NO. The kinetic analysis shows that the rate constants of the rate-determining step in the heterogeneous reaction of NHNO are higher than those in the homogeneous reduction of NH3-NO and heterogeneous reaction of NH2-NO. The result not only strengthens the leading role of NH in ammonia denitrification, but also clarifies the role of char in promoting NO reduction by NH. This study enriched the mechanism of N migration and transformation under the co-firing condition of the pulverized coal and ammonia.

CO2 based synergistic reaction effects with energy and exergy (2E) analysis of high density polyethylene with high ash bituminous coal for syngas production

A larger accumulation of medical waste is one of the major environmental issues due to the COVID-19 pandemic situation. The segregation and use of waste plastics from medical waste as a source of fuel for syngas production are highly beneficial. In packaging plastics, high density polyethylene (HDPE) occupies 20% of the total waste plastics. Hence, the conversion of HDPE into syngas is essential for waste reduction and energy production. Further, Indian coals possess high ash content (20–40%) and hence they produce a low calorific value gas on gasification. Hence, the co-gasification of these coals with suitable fuels such as HDPE can raise the heating value of syngas. In this context, HDPE is co-gasified with a high ash Indian coal using CO2 as the gasification agent for syngas production. Co-pyrolysis and co-gasification behavior of HDPE with high ash coal (HAC) under CO2 atmosphere are analyzed by using a fixed bed reactor and a thermogravimetric analyzer. Unlike reported in the recent literature, the present study showed that a high quality syngas can be obtained from the co-gasification process (1000 °C) with the heating value of 22 MJ/m3 using CO2 as the gasification agent. The chemical reactivity between CO2 gas, coal volatiles, and ethylene molecules significantly altered the surface morphology of char, which becomes highly porous making suitable for Boudouard reaction. On the surface of char, ester, and acid functional groups are formed. The co-feeding conditions raised the cold gas efficiency by 51% and the specific exergy of syngas by seven times that of HAC.

A comprehensive study of char reaction kinetics: Does CO always promote NO reduction on char surface?

This investigation focuses on the NO reduction process over chars and quantifies the effects of CO on char-NO reactivity under different conditions. Kinetic data were measured in a lab-scale fixed-bed reactor at temperatures between 1023 K and 1223 K, involving three coal chars with coal ranks ranging from lignite to anthracite. Experimental results show that CO can promote the reduction of NO over chars at low temperatures. However, there is a significant decrease in NO conversion rate at high temperatures compared to the case without CO present in the gas. The promotion or inhibition effects of CO on char-NO reactivity will become more obvious if the CO concentration is higher. Moreover, the consumption rate of NO is about the same as the generating rate of CO2 under high-level CO conditions, while a considerable amount of CO is generated during the direct reduction of NO by carbon. Further analysis based on Langmuir adsorption theory also indicates that char plays a catalytic role in NO reduction rather than acting as a reactant when CO is added. In addition, under the same temperature and gas conditions, the reactivity of SC lignite char is roughly five times higher than that of HP bituminous char and ten times higher than that of WH anthracite char, namely, the increase of coal rank leads to the decline in char-NO reactivity. Besides, the char made from higher rank coal has a broader range of temperature at which the CO presents a positive effect on NO reduction. Finally, a simple expression was proposed to calculate the NO reduction rate on char surface based on the experimental data, which can be easily applied in the description of nitrogen chemistry for fluidized bed combustion.

Functionalization of Graphene Oxide with Polysilicone: Synthesis, Characterization, and Its Flame Retardancy in Epoxy Resin

A novel polysilicone flame retardant (PMDA) has been synthesized and covalently grafted onto the surfaces of graphene oxide (GO) to obtain GO-PMDA. The chemical structure and morphology of GO-PMDA was characterized and confirmed by the Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectrometer (XPS), atomic force microscope (AFM), and thermogravimetric analysis (TGA). The results of dynamic mechanical analysis (DMA) indicated that the grafting of PMDA improved the dispersion and solubility of GO sheets in the epoxy resin (EP) matrix. The TGA and cone calorimeter measurements showed that compared with the GO, GO-PMDA could significantly improve the thermal stability and flame retardancy of EP. In comparison to pure EP, the peak heat release rate (pHRR) and total heat release (THR) of EP/GO-PMDA were reduced by 30.5% and 10.0% respectively. This greatly enhanced the flame retardancy of EP which was mainly attributed to the synergistic effect of GO-PMDA. Polysilicone can create a stable silica layer on the char surface of EP, which reinforces the barrier effect of graphene.

Insights into the interaction between NO and char(N) containing different functional forms: Mechanistic, thermodynamic and kinetic studies

Char-bound nitrogen [char(N)] is confirmed to be the real intermediate in heterogeneous reduction during coal combustion. Aiming to have a deep insight into the interaction mechanism between NO and char(N) containing different functional forms of nitrogen, a comprehensive theoretical exploration with density functional theory at M06–2X/6–31G(d,p)//def-TZVP level is performed. The detailed electronic descriptions of char(N) surface based on spin density, Mulliken atomic charge and electrostatic potential exhibit the attack sites and electron donating capability during NO chemisorption in the following order: char(N-5) < char(N-X) < char(N-6). Oxygen surface complex, an intermediate for N2O formation and a catalyst for NO reduction, can weaken the connected CN bond and promote N2 separation. The migration of oxygen atoms to adjacent active sites is thermodynamically conducive to reduction reactions, leading to the preferred pathway of N2 release on char(N-5) and char(N-X) surfaces. Contiguous active sites remaining along char edge are unfavorable for N2O detaching, while beneficial to NO further chemisorption for consecutive reactions, resulting in the great contributions of char(N-5) in reducing NO. The lower energy barriers for NO reduction follow the sequence coincident with the results of electronic property analysis. The kinetically favorable temperatures and activation energies of different char(N) for reducing NO are determined, which is consistent with previous experiments. The theoretical results provide evidences to explain microscopic mechanism of NO-char(N) interaction, making contributions to effectively minimize NOx emissions in the future.

Mechanism of the Heterogeneous Reduction of NO on a Sodium-Doped Char Surface: A First-Principles Study

Mechanism of the Heterogeneous Reduction of NO on a Sodium-Doped Char Surface: A First-Principles Study