Peak CO Concentration Research Articles

Enhanced Biomass Combustion via Bluff Body- and Impingement-Stabilized Natural Gas-Air Premixed Flames

ABSTRACT The preheating, flow recirculation and impingement effects of trapezoidal bluff bodies increased the biomass heating value-based combustion efficiency to 94% at a firing intensity of 78 MW/m3. A sawdust-laden air stream was combined to bluff body-stabilized natural gas-air premixed flames in three different configurations. A common impingement of opposing fuel-lean mixture jets with a sawdust-laden air cross-flow upstream of a solid bluff body provided a peak turbulent kinetic energy of 11.8 m2/s2. Using opposing jets with ports of multiple sharp corners minimized the flame length/combustor diameter ratio to 4.2. The combination of premixed flame impingement on the combustor base with the flow stagnation effect inside hollow bluff bodies provided a maximum blow-off velocity of 19.5 m/s at Ф = 0.8. Delaying the biomass interaction with the premixed flames decreased the peak exhaust NOx concentration to 0.17 g/kW.hr. Confronting the premixed flame jets by the sawdust-air laden jet provided a superior flow configuration in which the premixed flame high temperatures provided effective tar cracking, thus minimizing the exhaust total tar content to 50 mg/Nm3. A simultaneous peak exhaust soot volume fraction of 7 × 10−8 and a peak exhaust CH4 concentration of 0.1% were thus combined to a peak CO concentration of 14 mg/MJ.

Research on an in-situ synchronous CO elimination method in blasting operations and engineering experimental application based on nano-CO catalysts

The rapid and efficient elimination of CO by-products in mining and tunneling blasting operations poses a significant challenge. In this study, an in-situ synchronous elimination method (ISSE) for CO was proposed based on catalytic oxidation principles by deploying nano-scale catalysts in boreholes. Initially, Co3O4 catalyst was prepared and characterized. Subsequently, the catalyst was processed into a packing bag for in-situ CO elimination within boreholes. Field experiments were conducted at the Lujiatuo coal mine to verify the ISSE technology, and the effects of packaging materials, charging configuration, catalyst concentration, and length-to-diameter ratio of the packing bag on CO elimination efficiency were investigated. The experimental results revealed that when utilizing a nylon water cannon mud bag and charging 35 g of catalyst at the orifice, the peak CO concentration decreased from 735 ppm to 243 ppm, achieving an elimination efficiency of 66.9 % and reducing CO exceedance time by 28.5 %. These findings indicate that the ISSE technology surpasses existing techniques in terms of CO eliminating during mining and tunneling operations rapidly and effectively, that are expected to find widespread application in blasting production.

Experimental study on flame morphology, ceiling temperature and carbon monoxide generation characteristic of prismatic lithium iron phosphate battery fires with different states of charge in a tunnel

Lithium-ion battery (LIB) fire in a tunnel can generate a high-temperature environment, massive toxic and harmful smoke in a short period. This work carried out a series of thermal runaway (TR) experiments on large prismatic lithium cells in a model tunnel. Results showed that the flame height of LIBs with above 50 % SOC was above 40 cm for much time of the stage (Ⅰ) and (Ⅱ). The average flame height is further proposed to quantify the flame profile at different phases. The greater the SOC was, the higher the ceiling temperature and peak CO concentration were also relatively. For example, while decreasing the distance to the fire source from 0 m to -0.7 m, the peak ceiling temperature of 25 Ah batteries with 100 % SOC decreases from 309 °C to 118 °C, with a decay rate of 62 %. The CO concentration showed a comparable variation pattern to the tunnel ceiling temperature. Finally, a dimensionless relationship was employed to describe the ceiling temperature decay. Meanwhile, the ceiling maximum temperature rise for different capacity battery fires in a tunnel was integrated into a relational correlation. The study results are expected to further understand the battery fire hazards in tunnel.

A Novel Flame-Retardant, Smoke-Suppressing, and Superhydrophobic Transparent Bamboo.

Silica glass, known for its brittleness, weight, and non-biodegradable nature, faces challenges in finding suitable alternatives. Transparent wood, made by infusing polymers into wood, shows promise but is hindered by limited availability of wood in China and fire risks associated with its use. This study explores the potential of utilizing bamboo, which has a shorter growth cycle, as a valuable resource for developing flame-retardant, smoke-suppressing, and superhydrophobic transparent bamboo. A 3-layered flame-retardant barrier, composed of a top silane layer, an intermediate layer of SiO2 formed through hydrolysis-condensation of Na2SiO3 on the surface, and an inner layer of Na2SiO3, has been confirmed to be effective in reducing heat release, slowing flame spread, and inhibiting the release of combustible volatiles, toxic smoke, and CO. Compared to natural bamboo and other congeneric transparent products, the transparent bamboo displays remarkable superiority, with the majority of parameters being notably lower by an entire order of magnitude. It achieves a long ignition time of 116 s, low total heat release (0.7 MJ/m2), low total smoke production (0.063 m2), and low peak CO concentration (0.008 kg/kg). Moreover, when used as a substrate for perovskite solar cells, the transparent bamboo displays the potential to act as a light management layer, leading to a marked efficiency enhancement of 15.29%. The excellent features of transparent bamboo make it an enticing choice for future advancements in flame-retardant glasses and optical devices.

Influence of the parallel oil‐secondary air and F‐layer secondary air distribution on the flow, combustion, and NOx generation characteristics of FW down‐fired boilers retrofitted with a stable combustion technology

AbstractTo further reduce the NOx emissions of the down‐fired boiler while improving its low‐load stable combustion capacity, a new low NOx combustion organization mode that utilizes the high velocity parallel oil‐secondary air to improve the pulverized coal combustion process was proposed based on a novel stable combustion technology. We carried out cold airflow experiments under different oil‐secondary air and F‐layer secondary air distributions (mass flow rate ratios of 1:9, 1:3, 2:3) based on a 300 MWe Foster Wheeler down‐fired boiler to study its influence on the flow and mixing characteristics of the coal/airflow. Results show that the mixing between the oil‐secondary air and the fuel‐rich coal/airflow is gradually delayed with the increase of the secondary air distribution. When the secondary air distribution reaches more than 2:3, the oil‐secondary air starts to become the dominant airflow that influences the whole flow field in the furnace. Based on the result of the cold airflow experiment, we carried out industrial measurements under different parallel oil‐secondary air and F‐layer secondary air distributions (4:6, 5:5, 6:4) on a 600 MWe FW down‐fired boiler under the condition of using the high velocity oil‐secondary air. The ignition distance of the fuel‐rich coal/airflow, the flue gas temperature, the O2, CO, and NOx concentrations in the furnace were measured. At the secondary air distribution of 4:6, the ignition distance of the fuel‐rich coal/airflow was about 1.31 m. When the secondary air distribution was increased to 5:5 and 6:4, the ignition was put off, but the pulverized coal/airflow could still catch on fire in time. With the increase of the parallel oil‐secondary air proportion, the flame center in the furnace did not move downward substantially. In three cases, the lowest O2 concentrations in the inspection port area under the burner were all below 1% (peak CO concentration values were all above 28,000 ppm). The pulverized coal all burned in a strong reducing atmosphere. At secondary air distributions of 4:6, 5:5, and 6:4, NOx emissions of the boiler were, respectively, 825.83, 819.48, and 853.86 mg/Nm3 (O2 = 6%). The thermal efficiency was, respectively, 90.09%, 89.16%, and 90.98%. In actual operation, the recommended secondary air distribution is 6:4.

Global reaction mechanisms for MILD oxy-combustion of methane

This paper optimizes global reaction mechanisms under the MILD (Moderate and Intensive Low-oxygen Dilution) oxy-combustion combustion. Seven global mechanisms are compared and validated with two detailed mechanisms under the oxy-fuel combustion, MILD air-combustion, and MILD oxy-combustion conditions. The ability of these global models to capture the combustion process under different conditions is compared first using computational fluid dynamics (CFD) simulations of both the MILD oxy-combustion in a laboratory-scale furnace and non-premixed turbulent jet open flames, and later using a plug flow reactor (PFR) approach. Experiments of the MILD oxy-combustion are also carried out for the mechanism validation. The detailed comparison shows that the present optimized global mechanism significantly improves the prediction of temperatures, equilibrium concentrations of major species, and the peak CO concentration, relative to other global mechanisms, for the MILD oxy-combustion. The present refined mechanism is an appropriate global reaction mechanism for methane MILD oxy-combustion, if the computational cost of detailed reaction mechanism is unaffordable.

Characteristics of Char Gasification in Staged Oxygen-Enriched Combustion in a Down Flame Furnace

Staged oxygen-enriched combustion was carried out in a 20 kW down flame furnace (DFF). The experimental results were used to perform corresponding simulations. This paper proposes a refined char gasification model for accurately simulating the deep-staged combustion of pulverized coal at elevated oxygen concentrations. The following experimental conditions were used: burner stoichiometric ratio (SR) ranging from 0.696 (deep staged) to 1.200 (unstaged), O2 concentrations ranging from 21 to 30 vol %, and burnout stream injection ratio (Ms) ranging from 0.28 to 0.45. According to the experimental results, as SR decreased from 1.200 to 0.696, the peak CO concentration increased from low to very high levels (above 12 vol % under normal air conditions). The elevated oxygen concentration (30 vol % O2) further increased the peak CO concentration at SR = 0.696 to approximately 18 vol %. The reaction model in the simulation was refined on the basis of char gasification and then used to successfully reproduce the experimental results (especially the CO concentration profile). The influence of char gasification reactions on the staged oxygen-enriched combustion is investigated in detail. Furthermore, the pollutants formed are then presented by comparing the NO profiles between the experimental and simulation results.

Modeling study of oxygenated fuels on diesel combustion: Effects of oxygen concentration, cetane number and C/H ratio

The present modeling study aims to gain better insights on the effects of oxygenated fuels on the diesel oxidation and emission formation processes under realistic engine operating conditions. To do that, various blend fuels formulated from diesel, biodiesel, ethanol and DMC fuels were obtained with different oxygen concentrations, cetane numbers and C/H ratios. Simulations were conducted using the coupled KIVA–CHEMKIN code on a light duty diesel engine at a fixed engine speed of 2400rpm under full load conditions. Constructed numerical simulation models integrated with detailed chemical kinetics were validated against the experimental results with reliable accuracies. Simulation results revealed that as the overall oxygen concentration of the blend fuel increased, significant beneficial effects were shown with reduced NOx, CO and soot emissions. Particularly, with the increase of oxygen concentration, the peak CO concentration and its final emission level were found to be remarkably reduced due to the fuel borne oxygen, reduced carbon influx as well as the possibility accelerated CO oxidation rate. More tangible reductions were shown on the soot emissions probably because the C–O bond in the oxygenated blend fuels had played an important role in inhibiting the carbon atoms from soot formation. Furthermore, as oxygenated fuels were added, the peak concentration of the soot precursor C2H2 species and small hydrocarbon intermediates such as C2H4 and C2H6 were also significantly reduced. In general, it was found that compared to the effects of physical properties such as cetane number and C/H ratio of the blend fuel, the overall oxygen concentration seemed to be the major factor dominating the emissions and major intermediate species formation processes.

Kinetic analysis of co formation under oxy-combustion conditions

The paper reports the results of numerical computations concerning the formation of CO in the kinetic flame during natural gas oxy-combustion. The effect of temperature, reagent residence time and the composition of an atmosphere containing CO2 with 21 and 29 vol % O2 on the variation of CO concentration in the flame was examined. The oxy-combustion process was conducted with a 25 % excess oxygen. The analysis of reactions in flames at temperatures of 1500 K and 1800 K was performed within the Chemked II program using the combustion mechanism proposed by Mendiara & Glarborg, which includes 779 reactions. The computation results have confirmed that the rate of the key reactions responsible for the production of CO in the flame depends on the flame temperature and the oxy-combustion temperature. The peak CO concentrations are higher for the oxidizing mixture containing 29 vol % O2. After attaining a maximum, the CO flame concentration drops faster for an atmosphere richer in oxygen. The longer the time of reagent residence in the flame region, the lower the CO concentration. In different atmospheres and at different combustion temperatures, an identical CO level can be achieved in wet combustion gas. Irrespective of the temperature and atmosphere of oxy-combustion, most CO is produced as a result of the reaction OH + CO HCO2. The reduction of oxygen in the oxidizing atmosphere at flame temperatures of 1500 and 1800 K lowers the CO production in the dominant reactions responsible for CO formation. The contribution of individual reactions in the CO production for the identical atmospheres is different with varying temperature. In the case of the reaction HCO + O2 HO2 +CO, the temperature increase reduces the CO production. A reverse dependence of CO production on temperature characterizes the reaction H2 + CO + M CH2O + M. In addition, change in temperature changes the order in which the dominant reactions occur. Within the residence time equal to 100 ms, two periods of intensified CO production and consumption can be identified. The peak concentrations of H, OH and O radicals in the flame attain a maximum within the same time; as time goes by, the highest concentration is achieved by OH radicals. The presence of considerable levels of CO2 in the combustion substrates has an inhibiting effect on the natural gas oxidation process.

Absolute, spatially resolved, in situ CO profiles in atmospheric laminar counter-flow diffusion flames using 2.3 μm TDLAS

We developed a new, spatially traversing, direct tunable diode laser absorption spectrometer (TDLAS) for quantitative, calibration-free, and spatially resolved in situ measurements of CO profiles in atmospheric, laminar, non-premixed CH4/air model flames stabilized at a Tsuji counter-flow burner. The spectrometer employed a carefully characterized, room temperature distributed feedback diode laser to detect the R20 line of CO near 2,313 nm (4,324.4 cm−1), which allows to minimize spectral CH4 interference and detect CO even in very fuel-rich zones of the flame. The burner head was traversed through the 0.5 mm diameter laser beam in order to derive spatially resolved CO profiles in the only 60-mm wide CH4/air flame. Our multiple Voigt line Levenberg–Marquardt fitting algorithm and the use of highly efficient optical disturbance correction algorithms for treating transmission and background emission fluctuations as well as careful fringe interference suppression permitted to achieve a fractional optical resolution of up to 2.4 × 10−4 OD (1σ) in the flame (T up to 1,965 K). Highly accurate, spatially resolved, absolute gas temperature profiles, needed to compute mole fraction and correct for spectroscopic temperature dependencies, were determined with a spatial resolution of 65 μm using ro-vibrational N2-CARS (Coherent anti-Stokes Raman spectroscopy). With this setup we achieved temperature-dependent CO detection limits at the R20 line of 250–2,000 ppmv at peak CO concentrations of up to 4 vol.%. This permitted local CO detection with signal to noise ratios of more than 77. The CO TDLAS spectrometer was then used to determine absolute, spatially resolved in situ CO concentrations in the Tsuji flame, investigate the strain dependence of the CO Profiles and favorably compare the results to a new flame-chemistry model.

Transient experiments on a full-scale DOC—Methodology and techniques to support modelling

A methodology was developed, which enables transient experiments to be performed on a DOC connected to a live diesel engine, while the temperature of the DOC substrate is monitored at various locations. This was demonstrated on a full-scale Pt/γ-alumina DOC (o.d.=106mm; length=114mm), connected to a Ford 2.0L diesel engine. The engine was operated at a constant speed of 2000rpm, and by varying the engine torque, experiments were performed at fixed gas inlet temperatures to the DOC, e.g. 150, 320°C, and at each setting a constant background level of exhaust emissions was obtained. Then, a known quantity of pollutant (e.g. CO, propane) was injected into the exhaust, so as to create an approximated pulse input into the DOC (e.g. with peak CO concentration=3000ppm). The transient response of the system was monitored. So as to examine more closely local changes in response to a change in inlet conditions, experiments were also performed with a short 5mm thin-slice of DOC (o.d.=106mm).These techniques enable better control to be achieved over such experiments, and provide valuable data to support mathematical modelling. It was interesting to note, that in the examples studied, there was no evidence of any significant competition between CO and THCs for the active sites.The DOC was also characterised, and it was shown that key properties can have different values depending on the location from which the sample was taken – and this would also have important implications on modelling work.

Comparison of the structures of methane–air and propane–air partially premixed flames

A comparison of flame structures between methane–air and propane–air laminar partially premixed flames has been made through the centerline concentration distributions of selected species measured using gas chromatography. The concentrations of fuel, major species like O 2, CO and CO 2 and those of the intermediate hydrocarbons like C 2H 6, C 2H 4, C 2H 2 and CH 4 (for the propane flame only) have been compared. Distinct double flame structures are observed for the experimental conditions under study. With approximately the same equivalence ratio and jet velocity for the primary mixture, the height of the inner flame for propane is less than that of methane. The peak concentration of C 2H 6 in the propane flame is found to be only a little higher than that in the methane flame, while the peak concentrations of C 2H 4 and C 2H 2 are much greater in the propane flame than in the methane flame. In a methane partially premixed flame, the hydrocarbon concentrations drop from their peak values very rapidly at the inner flame tip, but in the propane flames it is more gradual. In a methane partially premixed flame, CO is formed at the inner flame and burns at the outer flame to CO 2. Similar distributions of CO and CO 2 are found in the propane flame also. However, the peak CO concentration in the propane flame is found to be higher than in methane flame. A radial measurement of species distribution for a particular case in the propane partially premixed flame is also done to ascertain the species distributions across the flame.

Air-pollution modelling in an urban area: Correlating turbulent diffusion coefficients by means of an artificial neural network approach

The vertical pollutant dispersion is quite sensitive to the eddy diffusivity, K V . Therefore, good estimations of K V are essential for improving the predictive performance of Eulerian dispersion models; especially in urban areas where literature based K V correlations are not always accurate. Here, we present a methodology to obtain a more accurate, but site-specific, K V correlation. It is based on using artificial neural networks (ANN) to find the best K V function for a particular urban area by minimizing, in a least-squares sense, the difference between ambient measurements of carbon monoxide and dispersion simulations of this tracer species. The resulting ANN- K V correlation is a function of three parameters namely, the stability parameter ( z / L ), the height within the mixing layer ( z / h ), and the scaled height ( z f C / u * )–hence the Monin–Obukhov ( L ), mixing ( h ) and Ekman ( u * / f C ) lengths are used to predict K V across the atmospheric boundary layer. We then assess how such an ANN- K V model improves the capability of a dispersion model (CAMx) to predict peak concentrations of ambient carbon monoxide in a large city. The evaluation has been performed with a set of eight air-quality meteorological stations evenly spread across the city of Santiago, Chile, during springtime. Results show that with the ANN- K V model, CAMx achieved better predictions of peak CO concentration levels than has been hitherto possible. Typically root-mean-square errors are reduced to half their original values. The resulting ANN- K V model—without any additional training—was then used to predict CO ambient concentrations at another period (summertime) and also to predict ambient concentrations of total carbon (PM 2.5) at both periods. A much-improved agreement was observed. Furthermore, the ANN formulation allowed for the quality of the urban emission inventory to be critically assessed indicating that the weekend emissions in Santiago are most likely underestimated.

A study of the effects of air preheat on the structure of methane/air counterflow diffusion flames

The effects of air preheat on flame structure are studied in counterflow methane–air diffusion flames, considering air temperatures in the range 300 to 560 K. Species concentrations for H 2, O 2, N 2, CH 4, CO, CO 2, C 2H 2, and C 2H 4 were measured using sampling and gas chromatography. Concentrations of NO were measured using sampling and chemiluminescence analysis. Results of numerical calculations using GRI-Mech 2.11 were compared with the measurements. The results of the numerical calculations and the measurements show excellent agreement for O 2, N 2, CH 4, and good agreement for CO 2, H 2, and CO. However, they show poor agreement for C 2H 2 and C 2H 4. Independent of the air temperature in the range 300 to 560 K, measured and predicted concentrations of CH 4, CO 2, O 2, and N 2 collapse reasonably well when plotted against the local equivalence ratio. The peak CO and H 2 concentrations increase with increasing air preheat. The peak CO concentrations increase because of enhanced dissociation of CO 2. The peak H 2 concentrations increase because of an increase in H atom concentrations causing enhanced rates of the reaction CH 4 + H → CH 3 + H 2. Both the measured and the predicted NO profiles showed approximately a 70% increase in the peak mole fractions with the increase in air temperature. The predictions of NO mole fractions in the fuel-lean region and near the peak are within 10% of the measurements. However, in the fuel-rich region, the predicted NO mole fractions are lower by up to 70% than the measured NO mole fractions. The increase in peak NO mole fractions with air preheat occurs primarily through the enhanced reaction rate of the prompt initiation reaction N 2 + CH → HCN + N. The NO production by the thermal mechanism increases significantly with air preheat, but still remains a very small portion of the total. The effects of air preheat on many species and reaction rates manifest through the increased H atom concentrations.

Analysis of signature patterns for discriminating fire detection with multiple sensors

Incorporating intelligence into a fire detector so that it can recognize signature patterns is intended to permit prompt fire detection while allowing the detector to discriminate between signatures from fire and nonfire sources. The primary purpose of this preliminary study is to investigate the patterns of signatures associated with fire and environmental sources using small-scale experiments. We generated products from a wide range of conditions, from flaming or pyrolyzing samples, to heated samples and samples obtained with an atomizer. We also measured gas concentrations, light obscuration, and temperature to characterize the products. By analyzing the data, we identified trends from which an elementary expert system can be formulated to identify the source of the airborne products. Several patterns are evident. The maximum CO2 concentrations achieved during experiments with flaming fires are significantly greater than the maximum CO2 concentrations achieved during experiments with nonflaming fires (pyrolyzing fires, heated liquids, and environmental odors). The nonflaming sources can be identified based on the CO and metal oxide sensor peak measurements. Except for three experiments using pyrolyzing solids, the peak CO concentration is greater—though the Taguchi detector response is less—for nonflaming fires than for environmental sources. Subsequent application of a neural network properly classifies all except one pyrolyzing fire.

A Perfectly-Stirred-Reaction Description of Chemistry in Turbulent Nonpremixed Combustion of Methane in Air

Explanation of the observed peak CO concentration in turbulent nonpremixed jet combustion of methane in air