Investigation of oxy-fuel combustion for methane and acid gas in a diffusion flame

Co-combustion of methane(CH4)and acid gas(AG)is required to sustain the temperature in Claus reaction furnace.In this study,oxy-fuel combustion of methane and acid gas has been experimentally studied in a diffusion flame.Three equivalence ratios(ER=1.0,1.5,2.0)and CH_(4)-addition ratios(CH_(4)/AG=0.3,0.5,0.7)were examined and the flame was interpreted by analyzing the distributions of the temperature and species concentration along central axial.CH_(4)-AG diffusion flame could be classified into three sections namely initial reaction,oxidation and complex reaction sections.Competitive oxidation of CH_(4)and H_(2)S was noted in the first section wherein H_(2)S was preferred and both were mainly proceeding decomposition and partial oxidation.SO_(2)was formed at oxidation section together with obvious presence of H2 and CO.However,H2 and CO were inclined to be sustained under fuel rich condition in the complex reaction section.Reducing ER and increasing CH4/AG contributed to higher temperature,H_(2)S and CH_(4)oxidation and CO_(2)reactivity.Hence a growing trend for CH_(4)and AG to convert into H_(2),CO and SO_(2)could be witnessed.And this factor enhanced the generation of CS2 and COS in the flame inner core by interactions of CH4 and CO_(2)with sulfur species.COS was formed through the interactions of CO and CO_(2)with sulfur species.The CS_(2)production directly relied on reaction of CH_(4)with sulfur species.The concentration of COS was greater than CS_(2)since CS_(2)was probably inhibited due to the presence of H_(2).COS and CS_(2)could be consumed by further oxidation or other complex reactions.

Model of limestone calcination / sulfation under oxy-fuel fluidized bed combustion

The characteristics of the simultaneous calcination/ sulfation of limestone under oxy-fuel fluidized bed combustion were studied and compared with those of the sulfation of precalcined CaO. During the calcination stage, SO2 can react with product CaO and slow down the CaCO3 decomposition rate by the covering effect of the CaSO4 product. The sulfation rate of simultaneous calcinatiort/sulfation is slower than that of precalcined CaO, but with a long enough sulfation time, the calcium conversion of simultaneous calcination/sulfation is higher than that of the precalcined CaO. A grain-micrograin model is established to describe the simultaneous calcination, sintering and sulfation of limestone. The graln-micrograln model can reflect the true reaction process of the calcination and sulfation of limestone in oxy-fuel fluidized bed combustion.

Comparative study on SO_2 release and removal under air and oxy-fuel combustion in a fluidized bed combustor

SO2 release and removal were studied under both the air and oxy-fuel combustion conditions using an anthracite coal from the Jincheng mine in China on a bench-scale fluidized bed combustor （FBC）. Special attention was paid to the effects of the combustion atmosphere, 02 concentration, bed temperature, and limestone addition. The released amount of SO2 was clearly higher under 30% 02/70% CO2 than that of the air atmosphere. As the O2 concentration in O2/CO2 mixture increased from 21% to 40%, the released amount of SO2 increased significantly, but then it decreased when the 02 concentration increased up to 50%. The bed temperature from 860 to 920 ℃ has no obvious influence on the the SO2 release but shows a strong influence on the desulfurization with limestone in both oxy-fuel and air conditions. The maximum SO2 removal efficiency appears to be at 880 to 900 ℃ for both the air and oxy-fuel combustion conditions.

Emission characteristics and combustion instabilities in an oxy-fuel swirl-stabilized combustor

This paper presents an experimental study on the emission characteristics and combustion instabilities of oxy-fuel combustions in a swirl-stabilized combustor. Different oxygen concentrations (Xoxy=25%～45%, where Xoxy is oxygen concentra- tion by volume), equivalence ratios (φ=0.75～1.15) and combustion powers (CP=1.08～2.02 kW) were investigated in the oxy-fuel (CH4/CO2/O2) combustions, and reference cases (Xoxy=25%～35%, CH4/N2/O2 flames) were covered. The results show that the oxygen concentration in the oxidant stream significantly affects the combustion delay in the oxy-fuel flames, and the equivalence ratio has a slight effect, whereas the combustion power shows no impact. The temperature levels of the oxy-fuel flames inside the combustion chamber are much higher (up to 38.7%) than those of the reference cases. Carbon monoxide was vastly produced when Xoxy>35% or φ>0.95 in the oxy-fuel flames, while no nitric oxide was found in the exhaust gases because no N2 participates in the combustion process. The combustion instability of the oxy-fuel combustion is very different from those of the reference cases with similar oxygen content. Oxy-fuel combustions excite strong oscillations in all cases studied Xoxy=25%～45%. However, no pressure fluctuations were detected in the reference cases when Xoxy>28.6% accomplished by heavily sooting flames which were not found in the oxy-fuel combustions. Spectrum analysis shows that the frequency of dynamic pressure oscillations exhibits randomness in the range of 50～250 Hz, therefore resulting in a very small resultant amplitude. Temporal oscillations are very strong with amplitudes larger than 200 Pa, even short time fast Fourier transform (FFT) analysis (0.08 s) shows that the pressure amplitude can be larger than 40 Pa.

Co-firing of coal and biomass in oxy-fuel fluidized bed for CO2 capture: A review of recent advances

The co-firing of coal and biomass in oxy-fuel fluidized beds is one of the most promising technologies for capturing CO2.This technology has attracted wide attention from academia and industry in recent years as a negative emission method to capture CO2 produced by carbon contained in biomass.In the past decades,many studies have been carried out regarding experiments and numerical simulations under oxy-fuel combustion conditions.This paper firstly briefly discusses the techno-economic viability of the biomass and coal co-firing with oxycombustion and then presents a review of recent advancements involving experimental research and computational fluid dynamics(CFD)simulations in this field.Experimental studies on mechanism research,such as thermogravimetric analysis and tube furnace experiments,and fluidized bed experiments based on oxy-fuel fluidized beds with different sizes as well as the main findings,are summarized as a part of this review.It has been recognized that CFD is a useful approach for understanding the behaviors of the co-firing of coal and biomass in oxyfuel fluidized beds.We summarize a recent survey of published CFD research on oxy-fuel fluidized bed combustion,which categorized into Eulerian and Lagrangian methods.Finally,we discuss the challenges and interests for future research.

Transformation of alkali and alkaline-earth metals during coal oxy-fuel combustion in the presence of SO_2 and H_2O

The occurrence modes of alkali and alkaline-earth metals（AAEMs） in coal relate to their release behavior and ash formation during combustion. To better understand the transformation of AAEMs,the release behavior of water-soluble,HCl-soluble,HCl-insoluble AAEMs during Shenmu coal（SM coal） oxy-fuel combustion in the presence of SO2 and H2O in a drop-tube reactor was investigated through serial dissolution using H2O and HCl solutions. The results show that the release rates of AAEMs increase with an increase in temperature under the three atmospheres studied. The high release rates of Mg and Ca from SM coal are dependent on the high content of soluble Mg and Ca in SM coal. SO2 inhibits the release rates of AAEMs,while H2O promotes them. The effects of SO2 and H2O on the Na and K species are more evident than those on Mg and Ca species. All three types of AAEMs in coal can volatilize in the gas phase during coal combustion. The W-type AAEMs release excessively,whereas the release rates of I-type AAEMs are relatively lower. Different types of AAEM may interconvert through different pathways under certain conditions. Both SO2 and H2O promote the transformation reactions. The effect of SO2 was related to sulfate formation and the promotion by H2O occurs because of a decrease in the melting point of the solid as well as the reaction of H2O.

Gas-phase oxidation of NO at high pressure relevant to sour gas compression purification process for oxy-fuel combustion flue gas

The removal of NO from oxy-fuel combustion is typically incorporated in sour gas compression purification process. This process involves the oxidation of NO to NO2 at a high pressure of 1–3 MPa, followed by absorption of NO2 by water. In this pressure range, the NO conversion rates calculated using the existing kinetic constants are often higher than those obtained experimentally. This study aimed to achieve the regression of kinetic parameters of NO oxidation based on the existing experimental results and theoretical models.Based on three existing NO oxidation mechanisms, first, the expressions for NO conversion against residence time were derived. By minimizing the mean-square errors of NO conversion ratio, the optimum kinetic rate constants were obtained. Without considering the reverse reaction for NO oxidation, similar mean-square errors for NO conversion ratio were calculated. Considering the reverse reaction for NO oxidation based on the termolecular reaction mechanism, the minimum mean-square error for NO conversion ratio was obtained. Thus, the optimum NO oxidation rate in the pressure range 0.1–3 MPa can be expressed as follows:-d[NO]/dt=d[NO2]/dt=0.0026[NO]2[O2]-0.0034[NO2]2 Detailed elementary reactions for N2/NO/NO2/O2 system were established to simulate the NO oxidation rate. A sensitivity analysis showed that the critical elementary reaction is 2 NO + O2? 2 NO2. However, the simulated NO conversions at a high pressure of 10–30 bar are still higher than the experimental values and similar to those obtained from the models without considering the reverse reaction for NO oxidation.

Molybdenum based amorphous and nanocrystalline coatings prepared by high velocity oxy-fuel spraying

The high velocity oxy-fuel(HVOF) based thermal spray process has developed as a potential advantageous approach for fabricating various kinds of functional coatings.In this article,the coatings of Mo-based alloy were synthesized using the HVOF process.The microstructure and the mechanical properties of the HVOF-processed coatings were investigated using SEM,TEM,XRD,and hardness and wear tests.Annealing treatment was applied to the as-sprayed coatings to develop the microstructure and its effect on the microstructure and mechanical properties of the coatings was examined.It is found that the HVOF-processed Mo-based alloy coatings are comprised of an amorphous splat matrix embedded with nano-sized crystalline particles.Annealing at temperatures over 950 ℃ results into crystallization of the amorphous matrix.The mechanical properties of the as-sprayed coatings are enhanced with annealing temperature up to 750 ℃ and from 950 to 1050 ℃,keeps constant between 750 and 950 ℃,and reduce over 1050 ℃.The change of the mechanical property with the microstructure was illustrated in the study.

Comparative investigation of microstructure and high-temperature oxidation resistance of high-velocity oxy-fuel sprayed CoNiCrAlY/nano-Al_(2)O_(3) composite coatings using satellited powders

Satellited CoNiCrAlY–Al_(2)O_(3)feedstocks with 2wt%, 4wt%, and 6wt% oxide nanoparticles and pure CoNiCrAlY powder were deposited by the high-velocity oxy fuel process on an Inconel738 superalloy substrate. The oxidation test was performed at 1050℃ for 5, 50, 100,150, 200, and 400 h. The microstructure and phase composition of powders and coatings were characterized by scanning electron microscopy and X-ray diffraction, respectively. The bonding strength of the coatings was also evaluated. The results proved that with the increase in the percentage of nanoparticles(from 2wt% to 6wt%), the amount of porosity(from 1vol% to 4.7vol%), unmelted particles, and roughness of the coatings(from 4.8 to 8.8 μm) increased, and the bonding strength decreased from 71 to 48 MPa. The thicknesses of the thermally grown oxide layer of pure and composite coatings(2wt%, 4wt%, and 6wt%) after 400 h oxidation were measured as 6.5, 5.5, 7.6, and 8.1 μm, respectively.The CoNiCrAlY–2wt% Al_(2)O_(3)coating showed the highest oxidation resistance due to the diffusion barrier effect of well-dispersed nanoparticles. The CoNiCrAlY–6wt% Al_(2)O_(3)coating had the lowest oxidation resistance due to its rough surface morphology and porous microstructure.

Comparisons of Fly Ash and Deposition Between Air and Oxy-Fuel Combustion in Bench-Scale Fluidized Bed with Limestone Addition

In Oxy-fuel circulating fluidized bed,the residual Ca O particles may react with high concentration of CO2 in flue gas to form bonded deposit on heat transfer surfaces in backpass when limestone is used as a sorbent to capture SO2.In this paper,experiments were designed on ash deposition in a bench-scale fluidized bed under oxy-fuel and air atmosphere. A novel ash deposit sampling probe was used to simulate the tubes of tail surfaces.The chemical composition of fly ash and ash deposit from both air-firing and oxy-fuel firing cases were analyzed by Inductively Coupled Plasma-Atomic Emission Spectrometry( ICP-AES) and Scanning Electron Microscopy( SEM),respectively. The degrees of carbonation reaction of ash deposits were measured by Thermo Gravimetric Analysis. The results showed that there are distinct differences in fly ash deposition rate between oxy-fuel and air firing cases,and oxy-fuel combustion with limestone addition can affect chemical composition of fly ash and ash deposit,especially for elements of Ca,Na,K,and S. However,the carbonation reaction degree of ash deposits is found weak,which is due to the relatively low Ca O content in ash deposit or not long enough of the sampling time.

Performance and Economic Study of Oxy-fuel Gas Turbine Power Plant Utilizing Nuclear Steam Generator

The authors propose a new closed cycle oxy-fuel gas turbine power plant that utilizes a nuclear heat generator. A pressurized water reactor （PWR） is designed to supply saturated steam to an oxy-fuel gas turbine for a specific power output increase The saturated steam from the reactor can have lower pressure and temperature than those of an existing PWR. In this study, the authors estimated plant performances from a heat balance model based on a conceptual design of a hybrid plant and calculated the generating costs of the proposed plant from the Japanese cost data of an existing PWR plant and an liquefied natural gas （LNG） combined cycle gas turbine plant. The generating efficiency of an oxy-fuel gas turbine plant without a nuclear steam generator is estimated to be less than 35%. Based on this efficiency, with a nuclear steam generator contributing to the power output of the proposed hybrid plant, the corresponding generating efficiency is estimated to be around 45%, even if the steam conditions are lower than in an existing PWR. The generating costs are 15-20% lower than those calculated from the weighted heat performances of both an oxy-fuel gas turbine plant without a nuclear steam generator and an existing PWR plant.

Microstructure and Performances of Glasses Melt under Oxy-fuel Combustion

We prepared a series of glass samples under the different simulated atmosphere.Systematic evaluation about the performances of the glasses fabricated under the different simulated atmosphere indicates that the increase of the H2O：CO2 ratio under the simulated atmosphere will decrease the softening point temperature,microhardness,viscosity,and chemical resistance,while increase the thermal expansion coefficient.Through the analysis of the hydroxyl content and network structure according to the IR transmitting spectra and NMR spectra,the structural origin of the evolution of the performances for the samples fabricated under different simulated atmosphere was elucidated.According to the feedback information from the customers,despite the decrease of some performances,the glass produced under oxy-fuel combustion can also fulfill the requirements of the engineering applications.Therefore,the technique of oxy-fuel combustion is worthy to be promoted in glass industry.

Effect of carbon dioxide on oxy-fuel combustion of hydrogen sulfide:An experimental and kinetic modeling

CO_(2) is an important component in the acid gas and it is necessary to study the effect of CO_(2) presence on the oxy-fuel combustion of H_(2)S with particular focus on the formation of carbonyl sulfide(COS).The oxyfuel combustion of acid gas was conducted in a coaxial jet double channel burner.The distribution of flame temperature and products under stoichiometric condition along axial(R=0.0)and radial at about 3.0 mm(R=0.75)were analyzed,respectively.The Chemkin-Pro software was used to analyze the rate of production(ROP)for gas products and the reaction pathway of acid gas combustion.Both experimental and simulation results showed that acid gas combustion experienced the H2S chemical decomposition,H_(2)S oxidation and accompanied by H_(2) oxidation.The CO_(2) presence reduced the peak flame temperature and triggered the formation of COS in the flame area.COS formation at R=0.0 was mainly through the reaction of CO_(2) and CO with sulfur species,whereas at R=0.75 it was through the reaction of CO with sulfur species.The ROP results indicated that H_(2) was mainly from H_(2)O decomposition in the H_(2)S oxidation stage,and COS was formed by the reaction of CO_(2) with H_(2)S.ROP and other detailed analysis further revealed the role of H,OH and SH radicals in each stage of H_(2)S conversion.This study revealed the COS formation mechanisms with CO_(2) presence in the oxy-fuel combustion of H_(2)S and could offer important insights for pollutant control.

Effect of Atmosphere on Volatile Emission Characteristic in Oxy-Fuel Combustion

A new type of power supply which was called oxy-fuel combustion power plant was introduced to reduce greenhouse gasses emission. In this paper the volatile emission characteristic of pulverized coal is studied under air atmosphere and oxy-fuel atmosphere. Combustion experiments of Datong bituminous coal were carried out in a wire mesh reactor at heating rates of 1 K/s, 10 K/s and 1000 K/s respectively under air and O2/CO2 atmosphere conditions in order to investigate the volatile emission characteristic. The concentrations of volatile (mainly CO and CH4) emission were on-line measured by infrared gas analyzer. It was indicated that the concentrations of CO and CH4 in O2/CO2 atmosphere were higher than those in air. The direct oxidation of carbon and gasification reaction between carbon and CO2 are the main causes of the increased amount of CO. The higher concentration of CO2 also results in the increased amount of CH4 in O2/CO2 conditions.

Low Steam Condition Heat Generator Combined with Advanced Oxy-Fuel Combustion LNG Gas Turbine for Power Generation

We propose a novel concept for power generation that involves the combination of a LSCHG （low-steam-condition heat generator）, such as a light water nuclear reactor or a biomass combustion boiler, with an advanced closed-cycle oxy-fuel combustion gas turbine-a type of complex and efficient oxy-fuel gas turbine. In this study, a LSCHG is designed to heat water to saturated steam of a few MPa, to assist in the generation of the main working fluids, instead of a compressor used in the advanced oxy-fuel gas turbine. This saturated steam can have a lower pressure and temperature than those of an existing nuclear power plant or biomass-fired power plant. We estimated plant performances in LHV （lower heating value） basis from a heat balance model based on a conceptual design of a plant for different gas turbine inlet pressures and temperatures of 1,300 ℃ and 1,500 ℃, taking into account the work to produce O2 and capture CO2. While the net power generating efficiencies of a reference plant are estimated to be about 52.0% and 56.0% at 1,300 ℃ and 1,500 ℃, respectively, and conventional LSCHG power plant is assumed to have an efficiency of about 35% or less for pressures of 2.5-6.5 MPa, the proposed hybrid plant achieved 42.8%-44.7% at 1,300 ℃ and 47.8%-49.2% at 1,500 ℃. In the proposed plant, even supposing that the generating efficiency of the LNG system in the proposed plant remains equal to that of the reference plant, the efficiency of LSCHG system can be estimated 37.4% for 6.5 MPa and 33.2% for 2.5 MPa, even though the LSHCG system may be regarded as consisting of fewer plant facilities than a conventional LSCHG power plant.

Experimental investigation on the NO formation of pulverized coal combustion under high-temperature and low-oxygen environments simulating MILD oxy-fuel combustion conditions

The NO formation experiments simulating moderate and intense low-oxygen dilution(MILD)oxy-coal combustion conditions were conducted on a laminar diffusion flame burner with the coflow temperatures of 1473-1873 K and the oxygen volume fractions of 5%-20%in O_(2)/CO_(2),O_(2)/Ar and O_(2)/N_(2)atmospheres.The flame images of pulverized coal combustion were captured to obtain the ignition delay distances,and the axial species concentrations were measured to obtain the variation of NO formation and reduction.The NO yield in O_(2)/Ar atmosphere decreased by nearly 0.2 when the oxygen volume fraction decreased from 20%to 5%and by about 0.05 when the coflow temperature decreased from 1873 K to 1473 K.The NO yield in O_(2)/CO_(2)atmosphere was 0.1-0.15 lower than that in O_(2)/Ar atmosphere.The optimal kinetic parameters of thermal NO and fuel NO formation rate were obtained by a nonlinear fit of nth-order Arrhenius expression.Finally,the relative contribution rates of thermal NO to total NO(Rth)and NO reduction to fuel NO(Rre)were quantitatively separated.Rth decreases with the increase of oxygen volume fraction,below 6%at 1800 K,25%at 2000 K.Rre is almost unaffected by the coflow temperature and affected by the oxygen volume fraction,reaching 30%at 5%O_(2).

Review on thermal-science fundamental research of pressurized oxy-fuel combustion technology

As the next-generation oxy-fuel combustion technology for controlling CO_(2)emissions,pressurized oxy-fuel combustion(POC)technology can further reduce system energy consumption and improve system efficiency compared with atmospheric oxy-fuel combustion.The oxy-fuel combustion causes high CO_(2)concentration,which has a series of effects on the combustion reaction process,making the radiation and reaction characteristics different from air-fuel conditions.Under the pressurized oxy-fuel condition,the combustion reaction characteristics are affected by the coupling effect of pressure and atmosphere.The radiation and heat transfer characteristics of the combustion medium are also affected by pressure.In recent years,there have been many studies on POC.This review pays attention to the thermalscience fundamental research.It summarizes several typical POC systems in the world from the perspective of system thermodynamic construction.Moreover,it reviews,in detail,the current research results of POC in terms of heat transfer characteristics(radiant heat transfer and convective heat transfer),combustion characteristics,and pollutant emissions,among which the radiation heat transfer and thermal radiation model are the focus of this paper.Furthermore,it discusses the development and research direction of POC technology.It aims to provide references for scientific research and industrial application of POC technology.

Development of Al356e-Al_2O_3 Nanocomposite Coatings by High Velocity Oxy-fuel Technique

In this research, development of AI356-AI203 nanocomposite coatings has been investigated. AI356-AI203 composite powders were prepared by mechanical milling of AI356 powder and 5 vol.% micro and nanoscaled alumina particles. The milled powders were used as feedstock to deposit composite coatings on A356-T6 aluminum alloy substrate using high velocity oxy-fuel （HVOF） process. X-ray diffractometry, optical and scanning electron microscopy, microhardness and wear tests were used to characterize the composite powders and coatings. The hardness of composite coatings containing micro and nanosized AI203 were 114.1 ± 5.9 HV and 138.4 ± 6.9 HV, respectively which were higher than those for substrate （79.2 ± 1.1 HV）. Nano and microcomposite coatings revealed low friction coefficients and wear rates, which were significantly lower than those obtained for AI356-T6 substrate. Addition of 5 vol.% micro and nanoscaled alumina particles improved the wear resistance by an average of 85% and 91%, respectively. This is mainly caused by the presence of AI203 in matrix and nanocrystalline structure of matrix. Scanning electron microscopy tests revealed different wear mechanisms on the surface of the wear test specimens.

Thermal Analysis and Kinetics of Coal during Oxy-Fuel Combustion

The pyrolysis and oxy-fuel combustion characteristics of Polish bituminous coal were studied us/ng non- isothermal thermogravimetric analysis. Pyrolysis tests showed that the mass loss profiles were almost similar up to 870℃ ha both N2 and CO2 atmospheres, while further mass loss occurred ha CO2 atmosphere at higher temper- atures due to char-CO2 gasification. Replacement of N2 ha the combustion environment by CO2 delayed the com- bustion of bituminous coal. At elevated oxygen levels, TG/DTG profiles shifted through lower temperature zone, ignition and burnout temperatures decreased and mass loss rate significantly increased and complete combustion was achieved at lower temperatures and shorter times. Kinetic analysis for the tested coal was performed using Kissinger-Akahira-Sunose （KAS） method. The activation energies of bituminous coal combustion at the similar oxygen content in oxy-fuel with that of air were higher than that in air atmosphere. The results indicated that, with O2 concentration increasing, the activation energies decreased.

Experimental Study on Oxy-Fuel Combustion and NO Emission in a Spouted-Fluidized Bed with under Bed Feeding

The spouted-fluidized bed is modified from the classical fluidized bed device,which combines the features of spouted and fluidized beds.In the present work,the performance of oxy-fuel spouted-fluidized bed combustion with under bed feeding and its effect on NO emission were systematically investigated.The results revealed that it was feasible to use a spouted-fluidized bed combustor for oxy-fuel combustion with real flue gas recycling.The transition from air combustion to oxy-fuel combustion was smooth and the concentration of CO_(2) in the flue gas could be as high as 90%steadily(dry base).Increasing the reaction temperature exhibited a negative effect on NO emission.Compared with that under the shallow bed,the concentration of NO in the flue gas was lower under the deep bed condition.Besides,the utilization of crush particles was favorable for suppressing NO emission because of the promoted mixing between coal particles and solid bed materials.Furthermore,the addition of limestone was proven to undesirably increase the NO emission during oxy-fuel spouted-fluidized bed combustion.