A density functional theory study on the decomposition of aliphatic hydrocarbons and cycloalkanes during coal pyrolysis in hydrogen plasma

To get deep understanding of the reaction mechanism of coal pyrolysis in hydrogen plasma, the decomposition reaction pathways of aliphatic hydrocarbons and cycloalkanes, which are two main components in volatiles from coal, were investigated. Methane and cyclohexane were chosen as the model compounds. Density functional theory was employed, and many reaction pathways were involved. Calculations were carried out in Gaussian 09 at the B3LYP/6-31G（d,p） level of the theory. The results indicate that the main pyrolysis products of methane and cyclohexane in hydrogen plasma are both hydrogen and acetylene, and the participation of active hydrogen atoms makes dehydrogenation reactions more favorable. H2 mainly comes from dehydrogenation process, while many reaction pathways are responsible for acetylene formation. During coal pyrolysis in hydrogen plasma, three main components in volatiles like aliphatic hydrocarbons, cycloalkanes and aromatic hydrocarbons lead to the formation of hydrogen and acetylene, but their contributions to products distribution are different.

CFD Simulation of a Hydrogen/Argon Plasma Jet Reactor for Coal Pyrolysis

A Computational Fluid Dynamics(CFD) model was formulated for DC arc hydrogen/argon plasma jet reactors used in the process of the thermal H_2/Ar plasma pyrolysis of coal to acetylene. In this model, fluid flow, convective heat transfer and conjugate heat conductivity are considered simultaneously. The error caused by estimating the inner-wall temperature of a reactor is avoided. The thermodynamic and transport properties of the hydrogen/argon mixture plasma system, which are usually expressed by a set of discrete data, are fitted into expressions that can be easily implemented in the program. The effects of the turbulence are modeled by two standard k-εequations. The temperature field and velocity field in the plasma jet reactor were calculated by employing SIMPLEST algorithm. The knowledge and insight obtained are useful for the design improvement and scale-up of plasma reactors.

Study of Pyrolysis Characteristics and Kinetic Analysis of Shenmu Coal at a High Heating Rate Using TG-FTIR

Coal pyrolysis is a fundamental reaction in the thermal processing and utilization of coal.Investigating the behavior and kinetics of coal pyrolysis is crucial for optimizing,designing,and developing a composite riser for the staged pyrolysis gasification process of pulverized coal.In this study,the non-isothermal pyrolysis behavior and kinetics of coal were examined at different heating rates(30,50,100,300,500,700,and 900℃/min)using thermogravimetry(TG)coupled with Fourier-transform infrared spectroscopy.Analysis of the TG/derivative TG(TG/DTG)curves indicated that coal pyrolysis mainly occurred between 300℃ and 700℃.Higher heating rates led to more volatiles being released from the coal,and a higher temperature was required to achieve rapid pyrolysis.Kinetic analysis showed that both the model-free methods(Friedman,Flynn-Wall-Ozawa,and Kissinger-Akahira-Sunose)and the model-based method(Coats-Redfern)effectively describe the coal pyrolysis process.The change in the Ea values between the two kinetic models was consistent throughout the pyrolysis process,and the most probable mechanism was the F2 model(secondary chemical reaction).In addition,the heating rate did not change the overall reaction order of the pyrolysis process;however,a higher heating rate resulted in a decrease in the Ea value during the initial pyrolysis stage.

Effect of demineralization on pyrolysis characteristics of LPS coal based on its chemical structure

The critical issue in developing mature Oxy-Coal Combustion Steam System technology could be the reactivity of deminer-alized coal which,is closely related to its chemical structure.The chemical structures of Liupanshui raw coal(LPS-R)and Liupanshui demineralized coal(LPS-D)were analyzed by FTIR and solid-state 13C-NMR.The pyrolysis experiments were carried out by TG,and the pyrolysis kinetics was analyzed by three iso-conversional methods.FTIR and 13C-NMR results suggested that the carbon structure of LPS coal was not altered greatly,while demineralization promoted the maturity of coal and the condensation degree of the aromatic ring,making the chemical structure of coal more stable.The oxygen-containing functional groups with low bond energy were reduced,and the ratio of aromatic carbon with high bond energy was increased,decreasing the pyrolysis reactivity.DTG curve-fitting results revealed that the thermal weight loss of LPS coal mainly came from the cleavage of aliphatic covalent bonds.By pyrolysis kinetics analysis of LPS-R and LPS-D,the apparent activation energies were 76±4 to 463±5 kJ/mol and 84±2 to 758±12 kJ/mol,respectively,under different conversion rates.The reactivity of the demineralized coal was inhibited to some extent,as the apparent activation energy of pyrolysis for LPS-D increased by acid treatment.

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 0.4 s, and that higher temperatures improve the pyrolysis efficiency. However, the yield of liquid products is decreased with increasing pyrolysis temperatures, especially above 670 ℃, because of additional cracking of the liquids.

ReaxFF molecular dynamic simulation of primary and secondary reactions involving in sub-bituminous coal pyrolysis for tar production

ReaxFF molecular dynamic simulation combined with experimental verification was performed to understand the overall reaction mechanism,especially the primary and secondary reactions involving in tar formation of sub-bituminous coal pyrolysis.Quantitative relationship at atomic level is clarified between bond breakage of functional groups and products generation,revealing that the amount and order in forming each product are subject to the number of corresponding functional groups and their bond energies respectively.The primary breakage of-C-O-and-C-C-bridge-bonds present in initial coal macromolecular generates molecular of heavy tar,whereas heavy tar can be converted into light tar through cracking side chain of aromatic rings and cyclic hydrocarbons at increased pyrolysis temperatures.At very high temperatures the cracking of short-chain hydrocarbons and residual atoms connecting to aromatic rings further occurs to generate light tar and gas.The remaining aromatic-ring fragments of heavy tar are likely cross-linked to form char.Furthermore,the simultaneous evolution tendency of tar yield and tar quality under different pyrolysis temperatures and heating rates is obtained at molecular level.For obtaining high yield and quality of tar,appropriately high temperature as well as suitable heating rate are needed to compromise the high yield of primary tar and high quality of secondarily upgraded products.

Investigation of kinetic and thermodynamic parameters of coal pyrolysis with model-free fitting methods

Kinetic analyses are important means for understanding the complicated coal pyrolysis process.This work conducted non-isothermal pyrolysis of four low-medium rank coals with thermogravimetric analyzer at different heating rates(5,10 and 20 K/min).Four model-free fitting approaches including KAS,OFW,Starink and Friedman methods were used to analyze pyrolysis kinetics of the coals.Thermodynamic parameters including enthalpy(ΔH)Gibbs free energy(ΔG)and entropy(ΔS)were calculated with the activation values(E)obtained with KAS method.For the kinetic analysis,the KAS,OFW and Starink methods give close E values over the whole conversion range,but the Friedman method shows more deviant results.The thermodynamic favorability of coal pyrolysis is discussed,bothΔH andΔG increase with conversion consistently with the variation of E,revealing that pyrolysis is endothermic and thermodynamic favorability is reduced with conversion rises.The minusΔS implies that coal structures evolve from disordered to organized ones as universally accepted.These results are expected to deepen our understanding and to optimize the conditions of coal pyrolysis.

Enhanced near-zero-CO2-emission chemicals-oriented oil production from coal with inherent CO2 recycling: Part I—PRB coal fast pyrolysis coupled with CO2/CH4 reforming

In this study,the Powder River Basin(PRB)coal fast pyrolysis was conducted at 700°C in the atmosphere of syngas produced by CH4-CO2 reforming in two different patterns,including the double reactors pattern(the first reactor is for syngas production and the second is for coal pyrolysis)and double layers pattern(catalyst was at upper layer and coal was at lower layer).Besides,pure gases atmosphere including N2,H2,CO,H2-CO were also tested to investigate the mechanism of the coal pyrolysis under different atmospheres.The pyrolysis products including gas,liquid and char were characterized,the result showed that,compared with the inert atmosphere,the tar yield is improved with the reducing atmospheres,as well as the tar quality.The hydrogen partial pressure is the key point for that improvement.In the atmosphere of H2,the tar yield was increased by 31.3%and the contained BTX(benzene,toluene and xylene)and naphthalene were increased by 27.1%and 133.4%.The double reactors pattern also performed outstandingly,with 25.4%increment of tar yield and 25.0%and 79.4%for the BTX and naphthalene.The double layers pattern is not effective enough due to the low temperature(700°C)in which the Ni-based catalyst was not fully activated.

Role of iron-based catalysts in reducing NO_(x) emissions from coal combustion

Nitrogen oxide(NO_(x))pollutants emitted from coal combustion are attracting growing public concern.While the traditional technologies of reducing NO_(x) were mainly focused on terminal treatment,and the research on source treatment is limited.This paper proposes a new coal combustion strategy that significantly reduces NO_(x) emissions during coal combustion.This strategy has two important advantages in reducing NO_(x) emissions.First,by introducing iron-based catalyst at the source,which will catalyze the conversion of coke nitrogen to volatile nitrogen during the pyrolysis process,thereby greatly reducing the coke nitrogen content.The second is de-NO_(x) process by a redox reaction between NO_(x) and reducing agents(coke,HCN,NH_(3),etc.)that occurred during coke combustion.Compared to direct combustion of coal,coke prepared by adding iron-based catalyst has 46.1% reduction in NO_(x) emissions.To determine the effect of iron-based additives on de-NO_(x) performance,demineralized coal(de-coal)was prepared to eliminate the effect of iron-based minerals in coal ash.The effects of iron compounds,additive dosages,and combustion temperatures on de-NO_(x) efficiency are systematically studied.The results revealed that the NO_(x) emission of the coke generated by pyrolysis of de-coal loaded with 3%(mass)Fe_(2)O_(3) decreases to 27.3% at combustion temperature of 900℃.Two main reasons for lower NO_(x) emissions were deduced:(1)During the catalytic coal pyrolysis stage,the nitrogen content in the coke decreases with the release of volatile nitrogen.(2)Part of the NO_(x) emitted during the coke combustion was converted into N_(2) for the catalytic effect of the Fe-based catalysts.It is of great practical value and scientific significance to the comprehensive treatment and the clean utilization process of coal.

Characterization of char from high temperature fluidized bed coal pyrolysis in complex atmospheres

The physiochemical properties of chars produced by coal pyrolysis in a laboratory-scale fluidized bed reactor with a continuous coal feed and char discharge at temperatures of 750 to 980 ~ C under N2-based atmospheres containing 02, H2, CO, CH4, and CO2 were studied. The specific surface area of the char was found to decrease with increasing pyrolysis temperature. The interlayer spacing of the char also decreased, while the average stacking height and carbon crystal size increased at higher temperatures, suggesting that the char generated at high temperatures had a highly ordered structure. The char obtained using an ER value of 0.064 exhibited the highest specific surface area and oxidation reactivity. Rela- tively high 02 concentrations degraded the pore structure of the char, decreasing the surface area. The char produced in an atmosphere incorporating H2 showed a more condensed crystalline structure and consequently had lower oxidation reactivity.

Research on coal staged conversion poly-generation system based on fluidized bed

A new coal staged conversion poly-generation system combined coal combustion and pyrolysis has been developed for clean and high efficient utilization of coal.Coal is the first pyrolysed in a fluidized pyrolyzer.The pyrolysis gas is then purified and used for chemical product or liquid fuel production.Tar is collected during purification and can be processed to extract high value product and to make liquid fuels by hydro-refining.Semi-coke from the pyrolysis reactor is burned in a circulating fluidized bed(CFB)combustor for heat or power generation.The system can realize coal multiproduct generation and has a great potential to increase coal utilization value.A 1 MW poly-generation system pilot plant and a 12 MW CFB gas,tar,heat and power poly-generation system was erected.The experimental study focused on the two fluidized bed operation and characterization of gas,tar and char yields and compositions.The results showed that the system could operate stable,and produce about 0.12 m^(3)/kg gas with 22 MJ/m^(3)heating value and about 10 wt%tar when using Huainan bituminous coal under pyrolysis temperature between 500 and 600℃.The produced gases were mainly H_(2),CH_(4),CO,CO_(2),C_(2)H_(4),C_(2)H_(6),C_(3)H_(6)and C_(3)H_(8).The CFB combustor can burn semi-coke steadily.The application prospect of the new system was discussed.

Benefaction and Pyrolysis of Sirnak Asphaltite and Lignite

Depending on advanced technological developments in energy production the low quality coals needed the most economical technologies and even in order to make it possible to produce coal-derived products. Compliance with environmental norms of coal pyrolysis or gasification of various type of coals, feasible combustion systems and energy production facilities are needed in today's modern technology, also enable the production of liquid and gaseous coal fuels. However, raw materials and chemical nature of them requires a variety of adaptation methods. This study examined the high sulfur and ash types of Kütahya, Denizli, Aydin, Soma lignite, Sirnak asphaltite and lignite. The representative samples were taken from local areas of the lignites. Fundamentally, the conditions regarding better desulfurization way, the high quality pyrolysis lignite oil production, high value light oil, coal tar and gas products were determined at the goal of high fuel producing yield.

Novel application of red mud as disposal catalyst for pyrolysis and gasification of coal

Red mud(RM)with the high alkalinity as a catalyst was evaluated for coal pyrolysis in a fixed bed as well as CO_(2) gasification of its resultant char in a thermogravimetric analyzer(TGA).The addition of RM into coal could improve the quality of tar during pyrolysis and enhance the reactivity of char during gasification.For catalytic pyrolysis with 12 wt%RM at 600℃,the light fraction in tar was 72.0 wt%,which increased by 20.0%,compared with coal pyrolysis alone.The role of metal oxides in RM on coal pyrolysis was further clarified as well.After catalytic pyrolysis with RM,the specific surface area of resultant char increased,especially for mesoporous surface area,and meanwhile the sodium in RM was proved to migrate to the char surface.These positive factors contributed to the CO_(2) gasification activity of char.RM with the high alkalinity showed a promising catalyst candidate for coal pyrolysis and gasification in terms of its catalytic effects and low cost.

Effects of iron compounds on pyrolysis behavior of coals and metallurgical properties of resultant cokes

The utilization of highly reactive and high-strength coke can enhance the efficiency of blast furnace by promoting indirect reduction of iron oxides.Iron compounds,as the main constituent in iron-bearing minerals,have aroused wide interest in preparation of highly reactive iron coke.However,the effects of iron compounds on pyrolysis behavior of coal and metallurgical properties of resultant cokes are still unclear.Thus,three iron compounds,i.e.,Fe;O;,Fe;O;and FeC;O;·2H;O,were adopted to investigate their effects on coal pyrolysis behavior and metallurgical properties of the resultant cokes.The results show that iron compounds have slight effects on the thermal behavior of coal blend originated from thermogravimetric and differential thermogravimetric curves.The apparent activation energy varies with different iron compounds ranging from 94.85 to 110.11 kJ/mol in the primary pyrolysis process,while lower apparent activation energy is required for the secondary pyrolysis process.Iron compounds have an adverse influence on the mechanical properties and carbon structure of cokes.Strong correlations exist among coke reactivity,coke strength after reaction,and the content of metallic iron in cokes or the values of crystallite stacking height,which reflect the dependency of thermal property on metallic iron content and carbon structure of cokes.

Structural optimization of baffle internals for fast particle pyrolysis in a downer reactor using the discrete element method

The structural optimization of baffle internals for fast pyrolysis of coal with particulate mixing and heat transfer in a downer reactor using the discrete element method(DEM)has been investigated in this research.The pyrolysis terminal temperature at the exit of the downer reactor is not only decided by the volume-feeding-rate ratio of the coal to the sand,but also is affected by the inner structural design of the baffle internals in the downer reactor.As presented in the previous publication of the author,the inhibition from the baffle internals in a downer reactor can improve the particulate-mixing degree and heat carrier,and increase the mean residence time of the coal and heat-carrier particles in the downer reactor.The structure of the baffle internals in the downer reactor mentioned in this research can be optimized by the independently developed 3D soft-sphere model of the DEM programme of a 40-mm baffle length,a 30°baffle-slope angle and at least four baffles designed in the downer reactor,which is beneficial for the process design of coal pyrolysis with a heat carrier in the downer reactor.