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1.
Reza Khatami Chris Stivers Kulbhushan Joshi Yiannis A. Levendis Adel F. Sarofim 《Combustion and Flame》2012,159(3):1253-1271
The combustion behavior of single fuel particles was assessed in O2/N2 and O2/CO2 background gases, with oxygen mole fractions in the range of 20–100%. Fuels included four pulverized coals from different ranks (a high-volatile bituminous, a sub-bituminous and two lignites) as well as pulverized sugarcane-bagasse, a biomass residue. Particles of 75–90 μm were injected under laminar flow in a bench-scale, transparent drop-tube furnace (DTF), electrically-heated to 1400 K where, upon experiencing high heating rates, they ignited and burned. The combustion of individual particles was observed with three-color optical pyrometry and high-speed high-resolution cinematography to obtain temperature and burnout time histories. Based on combined observations from these techniques, a comprehensive understanding of the behaviors of these fuels was developed under a variety of conditions, including simulated oxy-fuel combustion. The fuels exhibited distinct combustion behaviors. In air, the bituminous coal particles burned in two distinctive modes; the volatiles burned in bright envelope flames surrounding the devolatilizing char particles followed by heterogeneous char combustion. The volatile matter of sub-bituminous coal particles burned either in subdued envelope flames, surrounding devolatilizing and occasionally fragmenting chars, or heterogeneously at the char surface. Lignite particles typically burned with extensive fragmentation, and their volatiles burned simultaneously with the char fragments. The volatiles of bagasse particles burned in spherical and transparent envelope flames. Increasing the oxygen mole fraction in N2, increased flame and char surface temperatures, and decreased burnout times; particles of all fuels burned more intensely with an increasing tendency of the volatiles to burn closer to the char surface. When the background gas N2 was substituted with CO2, the combustion of all fuels was distinctly less intense; at moderate O2 mole fractions (<30%) most particles did not ignite under active flow conditions in the furnace (they did ignite under quiescent gas flow conditions in the DTF). Increasing the oxygen mole fraction in CO2 increased the likelihood of combustion and its intensity. Combustion of volatiles in envelope flames was suppressed in the presence of CO2, particularly under active gas flow in the DTF. 相似文献
2.
Combustion behavior in air of single particles from three different coal ranks and from sugarcane bagasse 总被引:1,自引:0,他引:1
Yiannis A. Levendis Kulbhushan Joshi Reza Khatami Adel F. Sarofim 《Combustion and Flame》2011,158(3):452-465
Comparative combustion studies were performed on particles of pulverized coal samples from three different ranks: a high-volatile bituminous coal, a sub-bituminous coal, and two lignite coals. The study was augmented to include observations on burning pulverized woody biomass residues, in the form of sugarcane bagasse. Fuel particles, in the range of 75–90 μm, were injected in a bench-scale, transparent drop-tube furnace, electrically-heated to 1400 K, where they experienced high-heating rates, ignited and burned. The combustion of individual particles in air was observed with three-color pyrometry and high-speed high-resolution cinematography to obtain temperature–time–size histories. Based on combined observations from these techniques, in conjunction to morphological examinations of particles, a comprehensive understanding of the combustion behaviors of these fuels was developed. Observed differences among the coals have been striking. Upon pyrolysis, the bituminous coal chars experienced the phenomena of softening, melting, swelling and formation of large blowholes through which volatile matter escaped. Combustion of the volatile matter was sooty and very luminous with large co-tails forming in the wake of the particle trajectories. Only after the volatile matter flames extinguished, the char combustion commenced and was also very luminous. In contrast, upon pyrolysis, lignite coals became fragile and experienced extensive fragmentation, immediately followed by ignition of the char fragments (numbering in the order of 10–100, depending on the origin of the lignite coal) spread apart into a relatively large volume. As no separate volatile matter combustion period was evident, it is likely that volatiles burned on the surface of the chars. The combustion of the sub-bituminous coal was also different. Most particles experienced limited fragmentation, upon pyrolysis, to several char fragments, with or without the presence of brief and low-luminosity volatile flames; other particles did not fragment and directly proceeded to char combustion. Finally combustion of bagasse was once again very distinctive. Upon pyrolysis, long-lasting, low-luminosity, nearly-transparent spherical flames formed around slowly-settling devolatilizing particles. They were followed by bright, short-lived combustion of the chars. Both volatiles and chars experienced shrinking core mode of burning. For all fuels, flame and char temperature profiles were deduced from pyrometric data and burnout times were measured. Combustion rates were calculated from luminous carbon disappearance measurements, and were compared with predictions based on published kinetic expressions. 相似文献
3.
Anthracite could be burnt efficiently at high temperature utilizing oxy-coal technology. To clarify the effects of temperature and atmosphere on char porosity characteristics, char morphology, fuel-N conversion, and reducing products release, rapid pyrolysis and CO2 gasification of anthracite was carried out in a high temperature entrained-flow reactor to simulate the condition in a pulverized coal furnace. Developed pore structure was formed in the gasification chars, which could be contributed to charCO2 reaction at high temperatures. More mesopores were formed in internal carbon skeleton and retained against collapse and coalescent for gasification chars than pyrolysis chars. Compared with pyrolysis char, smoother and denser surface was observed in gasification char with the irregular bulges disappeared due to the destruction of external carbon skeleton. Char-N could be oxidized to NO in CO2 atmosphere and then reduced to N2 by (CN) on the char surface. Char-N release was greatly promoted due to gasification reaction along with poly-condensation at high temperature; and the preact release of char-N would result in a larger portion of NOx reduction in the following reduction zone with the oxygen-staging combustion technology compared with that in air-staging combustion. Complementally, homogeneous reduction in NOx emission would play a minor effect for anthracite in oxy-coal combustion because of the deficiency of CH4 and HCN, especially at high temperature. 相似文献
4.
《Combustion and Flame》1985,62(1):21-30
The evolution of fuel nitrogen during devolatilization and the formation of NOx during combustion were studied for two Australian coals in crucible, thermobalance, and rapid heating (drop-tube furnace) experiments. The evolution of coal nitrogen during devolatilization was dependent on both temperature and mode of heating. Under near stoichiometric combustion, 20–30% of coal nitrogen was converted to NOx, Conversion increased markedly with increased fuel-lean conditions. The NOx formed from volatiles was proportional to the fraction of coal nitrogen evolved as HCN and NH3. The combustion of char at various temperatures and stoichiometries showed that the conversion of char nitrogen to NOx depended primarily on char burnout. The contribution of char nitrogen to NOx formation was greater than that of volatile nitrogen under fuel-rich conditions. 相似文献
5.
Three p.f. flames have been studied in a semi-industrial furnace, using different fuels: a bituminous coal, a lignite, and a biomass (oak sawdust). The operating conditions were exactly the same for the two coals, and very similar to those for the biomass flame. The objective of the study was to evaluate the impact of differences in fuel composition on flame characteristics, through measurement of the spatial distribution of the main parameters: temperature and concentrations of O2, CO, NOx, unburnt hydrocarbons, and N2O. The higher volatiles content in the lignite leads to higher temperatures and more intense combustion than the bituminous coal. Nevertheless, as might be expected, more marked differences are observed between the flames from the biomass and coals. The much higher volatiles content of the wood results in a more intense flame close to the burner, as indicated by visual observations and by concentrations of unburnt gases (CO and unburnt hydrocarbons) in that zone. It is remarkable that the combustion zone extends further for the biomass; while unburnt species were very low for the coals at an axial distance of 1 m, high values were detected for the pulverized oak. The measurements suggest that two stages can be distinguished in the biomass flame: a zone of intense combustion close to the burner, followed by a second region where the large biomass particles gradually devolatilize and are consumed. 相似文献
6.
《Combustion and Flame》2001,124(1-2):50-64
The effectiveness of the Precessing Jet nozzle at yielding low NOx levels from burning coal was examined at the pilot scale. Thus, a coal burner of nominal thermal load of 138 kW was sampled two dimensionally, and subsequently modeled. The one-dimensional, steady-state, semiempirical mathematical model considered the release and combustion of volatiles, and subsequent oxidation of char. A comprehensive reaction scheme was formulated to account for the oxidation of the resulting CO, the formation of NO from various sources (volatile, char, preheated air) and the oxidation of H2S to SO2. The agreement between the experimental and predicted profiles of coal burnout, [O2], [NO] and gas temperature was not good near the burner; however, the agreement improved in the postflame region. The model was also used to simulate the center line characteristics of the same flame, but with the secondary air preheated to 500°C. This flame was then scaled for constant velocity and constant residence time to 20 MW. It was deduced that constant residence-time scaling predicts ignition and combustion of the coal at the same axial location as the baseline flame. However, constant velocity scaling shifts combustion closer to the burner. Constant-velocity scaling was found to be more suitable for the theoretical scaling of pulverized coal flames. It was not possible to comment on the potential of the burner for low NOx in cement kilns, because the measured and computed gas temperatures were low. However, the model predicted low concentrations of fuel NOx. 相似文献
7.
Masayuki Taniguchi Yuki Kamikawa Tetsuma Tatsumi Kenji Yamamoto 《Combustion and Flame》2011,158(11):2261-2271
Staged combustion properties for pulverized coals have been investigated by using a new-concept drop-tube furnace. Two high-temperature electric furnaces were connected in series. Coal was burnt under fuel-rich conditions in the first furnace, then, staged air was supplied at the connection between the two furnaces. Reaction temperature (1800–2100 K) and time (1–2 s) were similar to those used in actual boilers. When coal was burnt at the same stoichiometric ratio as in actual boilers, similar combustion performance values as for actual boilers were obtained regarding NOx emission and carbon in ash. The most important factor for low NOx combustion was to raise the combustion temperature above the present range (1800–2100 K) in the fuel-rich zone. The NOx emission was significantly increased with decrease of burning temperature in the fuel-rich zone when the temperature was lower than 1800 K. But, NOx emission was cut to around 100–150 ppm, for sub-bituminous coal and hv-bituminous coal, in the latest commercial plants by forming this high-temperature fuel-rich region in the boilers. If the temperature and stoichiometric ratio could be set to the most suitable conditions, and, burning gas and air were mixed well, it would be possible to lower NOx emission to 30–60 ppm (6% O2). The most important NOx reduction reaction in the fuel-rich zone was the NOx reduction by hydrocarbons. The hydrocarbon formation rate in the flame was varied with coal properties and combustion conditions. The NOx was easily reduced when coals which easily formed hydrocarbons were used, or, when burning conditions which easily formed hydrocarbons were chosen. Effects of burning temperature and stoichiometric ratio on NOx emission were reproduced by the previously proposed reaction model. When solid fuel was used, plant performance values varied with fuel properties. The proposed drop-tube furnace system was also found to be a useful analysis technique to evaluate the difference in combustion performance due to the fuel properties. 相似文献
8.
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10.
Masayuki Taniguchi Yuki Kamikawa Teruyuki Okazaki Kenji Yamamoto Hisayuki Orita 《Combustion and Flame》2010,157(8):1456-1466
We have investigated an index for modeling a NOx reaction mechanism of pulverized coal combustion. The reaction mechanism of coal nitrogen was examined by drop-tube furnace experiments under various burning conditions. We proposed the gas phase stoichiometric ratio (SRgas) as a key index to evaluate NOx concentration in fuel-rich flames. The SRgas was defined as:
11.
Tao Song Laihong Shen Jun Xiao Dingqian Chen Haiming Gu Siwen Zhang 《Combustion and Flame》2012,159(3):1286-1295
Chemical-looping combustion (CLC) is a novel technique used for CO2 separation that has been investigated for gaseous fuel and solid fuel. The nitrogen transfer of fuel-N in the coal is experimentally investigated with a NiO/Al2O3 oxygen carrier under a continuous operation in a 1 kWth interconnected fluidized bed prototype. The effects of the fuel reactor temperature, coal type and operation conditions on the release of gaseous products of nitrogen species in the air reactor and the fuel reactor are carried out. Results show that the nitrogen transfer direction of fuel-N is toward N2 formation in the fuel reactor independent of fuel type. In the fuel reactor N2 is the sole product of nitrogen transfer of fuel-N. The concentration of N2 in the fuel reactor exit gas increases with the fuel reactor temperature. The NOx precursor of HCN can be oxidized by the oxygen carrier to form NO or N2 in the fuel reactor. However, in the fuel reactor NO from coal devolatilization and HCN oxidization by oxygen carrier is completely reduced to N2. The other NOx precursor of NH3 is completely converted to N2 due to oxidization by NiO and the catalytic effect of Ni on the decomposition of NH3. After coal devolatilization, char-N conversion in the fuel reactor is toward N2 formation according to the investigation of solid–solid reaction between char and oxygen carrier. The amount of residual char has a potential to cause formation of nitrogen contaminants in the air reactor. In the air reactor, NO is the only nitrogen contaminant, and there is no NO2 formation. The high fuel reactor temperature results in little residual char coming into the air reactor. The proportion of char-N converted to NO in the air reactor increases from 16.98% to 18.85% when the fuel reactor temperature changes from 850 to 950 °C. For the fuels containing more volatile matter, the possibility of NO formation in the air reactor is smaller than the fuels containing less volatile matter. For the fuels containing less volatile matter, char gasification rate is still a significant factor both for the carbon capture efficiency and NO formation. 相似文献
12.
Jin Jing Zhang Zhongxiao Li Ruiyang 《Frontiers of Energy and Power Engineering in China》2007,1(1):91-95
Numerical simulation and experimental study on NO
x
release along the boiler during pulverized coal combustion have been conducted. With the increase of temperature the NO
x
emission increased and the peak value of NO
x
release moved forward. But when the temperature increased to a certain degree, NO
x
emission began to reduce. NO
x
emission increased with the increase of nitrogen content of coal. The peak value of NO
x
release moved backwards with the increase of coal rank. NO
x
emission increased obviously with the increase of stoichiometric ratio. There existed a critical average diameter of the
pulverized coal (d
c
). If d ⩽ d
c
, NO
x
emission reduced with the decrease of pulverized coal size. If d > d
c
, NO
x
emission reduced with the increase of the pulverized coal size. The results showed that the simulation results are in agreement
with the experimental results for concentration distribution of NO
x
along the axis of the furnace.
Translated from Proceeding of the CSEE, 2006, 26(1): 35–39 [译自: 中国电机工程学报] 相似文献
13.
This paper is concerned with combustion of a refuse derived fuel in a small-scale flame. The objective is to provide a direct comparison of the RDF flame properties with properties of pulverized coal flames fired under similar boundary conditions. Measurements of temperature, gas composition (O2, CO2, CO, NO) and burnout have demonstrated fundamental differences between the coal flames and the RDF flames. The pulverized coals ignite in the close vicinity of the burner and most of the combustion is completed within the first 300 ms. Despite the high volatile content of the RDF, its combustion extends far into the furnace and after 1.8 s residence time only a 94% burnout has been achieved. This effect has been attributed not only to the larger particle size of fluffy RDF particles but also to differences in RDF volatiles if compared to coal volatiles. Substantial amounts of oily tars have been observed in the RDF flames even though the flame temperatures exceeded 1300 °C. The presence of these tars has enhanced the slagging propensity of RDF flames and rapidly growing deposits of high carbon content have been observed. 相似文献
14.
Alejandro Molina Eric G. Eddings David W. Pershing Adel F. Sarofim 《Combustion and Flame》2004,136(3):303-312
A modified drop-tube reactor that allows particle distribution over the reactor cross-sectional area, and oxidation of chars produced in situ, was used to study the conversion efficiency of char nitrogen to nitric oxide (αNO). The results confirm previous findings by other investigators that αNO decreases as the weight of char burned increases. αNO for coal was the same as (at 4% O2) or lower than (at 20% O2) that for an equal mass of char during oxidation. Since coal will yield approximately half its mass as fixed carbon, these results suggest that the local stoichiometry surrounding the particle is responsible for the observed reduction in αNO as sample size increases. The analysis of the exhaust gases showed increases in HCN concentration and a decrease in CO2/CO ratio as sample size increased, suggesting that local stoichiometry influences αNO. Additional experiments showed that αNO decreased as the background NO concentration was increased, at rates that diminished as the oxygen concentration increased, independent of particle size. The steep reduction in NO production as the background NO concentration increased was explained by the destruction of NO in the gas phase. 相似文献
15.
The formation of pollutant species in turbulent diffusion flames is strongly affected by turbulence/chemistry interactions. Unsteady counterflow diffusion flames can be conveniently used to address the unsteady effects of hydrodynamics on the pollutant chemistry, because they exhibit a larger range of combustion conditions than those observed in steady flames.In this paper, unsteady effects on the formation of soot (and its main precursors) and nitrogen oxides (NOx) are investigated by imposing harmonic oscillations on the strain rate of several counterflow diffusion flames fed with propane. Numerical results confirm that the dynamic response of each species is strongly affected by the strain rate oscillations and the characteristic time governing its chemistry. At low frequencies of imposed oscillations the soot and NOx profiles show strong deviations from the steady-state profile. At large frequencies a decoupling between the concentration and the velocity field is evident. In particular, the formation of soot and NOx is found less sensitive to velocity fluctuations for flames with large initial strain rate. The significant increase of soot and NOx concentrations in unsteady conditions appears to be a function of both forcing frequency and flame global strain rate. Moreover, the cut-off frequency, defined as the minimum frequency above which the strain rate oscillations have negligible effects on the formation of each species, was found to be strongly dependent on the chemical characteristic time and the flame global strain rate, but only marginally affected by the amplitude of imposed oscillations. 相似文献
16.
Combustion stability limits and NOx emissions of nonpremixed ammonia-substituted hydrogen–air flames
The combustion stability (extinction) limits and nitrogen oxide (NOx) emissions of nonpremixed ammonia (NH3)–hydrogen (H2)–air flames at normal temperature and pressure are studied to evaluate the potential of partial NH3 substitution for improving the safety of H2 use and to provide a database for the nonpremixed NH3-substituted H2–air flames. Considering coflow nonpremixed NH3–H2–air flames for a wide range of fuel and coflow air injection velocities (Vfuel and Vcoflow) and the extent of NH3 substitution, the effects of NH3 substitution on the stability limits and NOx emissions of the NH3–H2–air flames are experimentally determined, while the nonpremixed NH3–H2–air flame structure is computationally predicted using a detailed reaction mechanism. Results show significant reduction in the stability limits and unremarkable increase in the NOx emission index for enhanced NH3 substitution, supporting the potential of NH3 as an effective, carbon-free additive in nonpremixed H2–air flames. With increasing Vcoflow the NOx emission index decreases, while with increasing Vfuel it decreases and then increases due to the recirculation of burned gas and the reduced radiant heat losses, respectively. Given Vcoflow/Vfuel the flame length increases with enhanced NH3 substitution since more air is needed for reaction stoichiometry. The predicted flame structure shows that NH3 is consumed more upstream than H2 due to the difference between their diffusivities in air. 相似文献
17.
A numerical model for gas-particle flow dynamics has been combined with an NOx chemistry post-processor to predict the formation of nitric oxide in a three-dimensional, W-shaped boiler furnace burning pulverized fuel. The model includes complex interactions in gas-particle turbulent flow, heat transfer, gaseous chemical reaction, coal combustion, and NOx reaction chemistry. Because fuel nitrogen is released in proportion to burnout of pulverized coal particles, the particles are treated in a Lagrangian framework in order to track burning pulverized coal particles through the gas continuum. The results show capability of the model to describe NOx emissions under different operating conditions for full and partial loads. 相似文献
18.
An approximately spherical particle of coal (diam. 13–14 mm) was made by filing a larger piece. Next a hole (diam. 0.6 mm) was drilled through the centre of the sphere to end at 3 mm from the opposite face. Into this hole a fine thermocouple (o.d. 0.2 mm) was cemented. The coal was immersed into an electrically heated bed of sand, fluidised by nitrogen at 850 °C. During the subsequent pyrolysis, the temperature was measured at 3 mm inside the coal. Minima were found in the local internal rate of increase in temperature, when plotted against either time or this measured internal temperature. Here is new evidence for large coals thermally decomposing by a sequence of “endothermic waves” moving radially inwards through the coal to release volatile matter. These minima are each associated with a particular temperature and 21 of them were found (apart from that for drying) between 160 and 820 °C. Altogether four bituminous coals and one lignite were studied. Some “decomposition temperatures” were common to them all. Lower rank coals have major losses of volatile material at lower temperatures. These experiments thus support the modelling of pyrolysis using a suite of approximately 20 parallel reactions, each contributing to different extents and with different kinetic parameters and ΔH.Experiments in an identical bed, but fluidised by air, revealed endothermic waves in a coal. In this case, some of the volatiles burned in a counter-flow flame in the recently discovered cushion of air underneath such a relatively large coal particle. Also, towards the end of devolatilisation, the resulting char started to burn; different chars burned at different temperatures, all above that of the bed, even though burning was controlled by external mass transfer of O2 to the char particle. It appears that underneath a char particle, there is again a counter-flow flame, where CO (from burning the char) is oxidised by OH radicals to CO2 at ~900 °C. Most probably the carbon in the char is also oxidised by OH radicals to CO. The temperature at which a char burns is partly controlled by how much CO is oxidised by OH radicals close to the underside of the char particle. The oxidation of both CO and carbon in a char accordingly appears to be catalysed by hydrogen from the char. 相似文献
19.
Juan Riaza Reza Khatami Yiannis A. Levendis Lucía Álvarez María V. Gil Covadonga Pevida Fernando Rubiera José J. Pis 《Combustion and Flame》2014
A fundamental investigation has been conducted on the combustion behavior of single particles (75–150 μm) of four coals of different ranks: anthracite, semi-anthracite, medium-volatile bituminous and high-volatile bituminous. A laboratory-scale transparent laminar-flow drop-tube furnace, electrically-heated to 1400 K, was used to burn the coals. The experiments were performed in different combustion atmospheres: air (21%O2/79%N2) and four simulated dry oxy-fuel conditions: 21%O2/79%CO2, 30%O2/70%CO2, 35%O2/65%CO2 and 50%O2/50%CO2. The ignition and combustion of single particles was observed by means of three-color pyrometry and high-speed high-resolution cinematography to obtain temperature–time histories and record combustion behaviors. On the basis of the observations made with these techniques, a comprehensive examination of the ignition and combustion behaviors of these fuels was achieved. Higher rank coals (anthracite and semi-anthracite) ignited heterogeneously on the particle surface, whereas the bituminous coal particles ignited homogeneously in the gas phase. Moreover, deduced ignition temperatures increased with increasing coal rank and decreased with increasing oxygen concentrations. Strikingly disparate combustion behaviors were observed depending on the coal rank. The combustion of bituminous coal particles took place in two phases. First, volatiles evolved, ignited and burned in luminous enveloping flames. Upon extinction of these flames, the char residues ignited and burned. In contrast, the higher rank coal particles ignited and burned heterogeneously. The replacement of the background N2 gas of air with CO2 (i.e., changing from air to an oxy-fuel atmosphere) at the same oxygen mole fraction impaired the intensity of combustion. It reduced the combustion temperatures and lengthened the burnout times of the particles. Increasing the oxygen mole fraction in CO2 to 30–35% restored the intensity of combustion to that of air for all the coals studied. Volatile flame burnout times increased linearly with the volatile matter content in the coal in both air and all oxygen mole fractions in CO2. On the other hand, char burnout times increased linearly or quadratically versus carbon content in the coal, depending on the oxygen mole fraction in the background gas. 相似文献
20.
Formation of NOx in counterflow methane/air triple flames at atmospheric pressure was investigated by numerical simulation. Detailed chemistry and complex thermal and transport properties were employed. Results indicate that in a triple flame, the appearance of the diffusion flame branch and the interaction between the diffusion flame branch and the premixed flame branches can significantly affect the formation of NOx, compared to the corresponding premixed flames. A triple flame produces more NO and NO2 than the corresponding premixed flames due to the appearance of the diffusion flame branch where NO is mainly produced by the thermal mechanism. The contribution of the N2O intermediate route to the total NO production in a triple flame is much smaller than those of the thermal and prompt routes. The variation in the equivalence ratio of the lean or rich premixed mixture affects the amount of NO formation in a triple flame. The interaction between the diffusion and the premixed flame branches causes the NO and NO2 formation in a triple flame to be higher than in the corresponding premixed flames, not only in the diffusion flame branch region but also in the premixed flame branch regions. However, this interaction reduces the N2O formation in a triple flame to a certain extent. The interaction is caused by the heat transfer and the radical diffusion from the diffusion flame branch to the premixed flame branches. With the decrease in the distance between the diffusion flame branch and the premixed flame branches, the interaction is intensified. 相似文献