一氧化碳在循环流化床锅炉中的燃烧分析

介绍了一氧化面料在循环流化床（CFB）锅炉中燃烧的研究状况,结合已有的试验研究结果和CFB锅炉运行状况,对实际测量的一氧化碳浓度场分布进行了分析。简述了一氧化碳的燃烧对CFB锅炉的一些影响,通过分析一氧化碳燃烧对氧气浓度场的影响,增加氧气的后期混合有利于提高CFB锅炉的燃烧效率,图4参9     

Analysis of in situ combustion of oil with pyrolysis and vaporization

We study one-dimensional flows, when air is injected into a porous medium filled with inert gas, medium or high viscosity oil and water, giving rise to a combustion wave in a process known as high-temperature oxidation (HTO). In the oil we distinguish three pseudo-components: asphaltenes, medium and light oil. At high temperatures, the heaviest components (“precoke”) are converted to coke, which undergoes combustion. Medium oil components are cracked at intermediate temperatures releasing gaseous oil. Light oil components and water are vaporized. The oxidation rate of gaseous oil components is negligible. Combustion regimes are described in the form of a sequence of waves. We develop a simple mathematical pathway based on Zeldovich’s approach to provide analytical formulae for parameters in these waves. It is shown that there is a combustion regime in which either coke or oxygen are partially consumed in the combustion as well as a regime in which both are consumed completely. Each of the regimes can be subdivided in two regimes, where the reaction is either trailing or leading with respect to the thermal wave. Explicit conditions for each combustion regime are given. The structure of the oil cracking layer is investigated. Stability of the solutions is studied. We analyse our formulae for typical in situ combustion data and compare the results with numerical simulations.     

Combustion rate of carbon: Combustion of spheres in flowing gas streams

Data are presented on the rate of combustion of spheres of carbon in various nitrogen-oxygen mixtures, under conditions in which both the rate of chemical reaction at the surface and the rate of diffusion of oxygen to the surface are factors. A quantitative formulation of combustion rate is presented, predicting effects of temperature, gas velocity, and gas composition in agreement with the experimental measurements. The combustion-rate equation is able to establish limits within which the absolute magnitude of the combustion rate must lie, but these limits are rather broad because of the lack of data on gas diffusivities at high temperatures and of an inexact knowledge of the net products of the primary reaction between oxygen and carbon. From similar data for the rate of combustion in carbon dioxide it is concluded that carbon, when burned in air, is consumed by direct oxidation with oxygen rather than by a mechanism in which oxygen is pictured as reaching the surface chiefly in the form of carbon dioxide, there forming carbon monoxide.     

Product formation mechanisms inside a burning cigarette

Inside a burning cigarette a large variety of chemical and physical processes are occurring in an oxygen-deficient, hydroge-rich environment, with temperatures up to 950°C. The physical processes occurring during combustion are rationalised and a picture is presented of the major mechanistic regions inside the cigarette during puffing and natural smouldering.There are two major regions inside the burning zone of the cigarette where products are released: a heat producing combustion zone, and a pyrolysis/distillation zone just downstream of the combustion zone. The vast majority of organic smoke products are formed in the pyrolysis/distillation region. The location of these zones has been determined by mapping internal gas concentrations, and by measuring internal density and temperature changes.Product formation routes have traditionally been unravelled using isolated pyrolysis experiments. Recent work with radioactively labelled nicotine has shown that the results from such experiments can be very different to those obtained from the dynamic conditions inside the cigarette. However, attempts have been made in recent years to design pyrolysis experiments which simulate more closely the real conditions inside the burning zone of the cigarette.The oxides of carbon are formed by both combustion and thermal decomposition of tobacco constituents. The major features of these two mechanisms are discussed. Many studies have also deduced that a significant proportion of carbon monoxide is formed by the carbonaceous reduction of carbon dioxide. The occurrence of this reaction is used to interpret the effect of ventilation on the ratio of carbon monoxide/carbon dioxide yields, and the relative values of this ratio in mainstream and sidestream smoke.The generation of mainstream and sidestream smoke is reviewed, together with factors that contribute to the relative deliveries for particular smoke components. The most critical factor is the mechanistic origins of the component.Finally, recent mathematical models of some of the processes contributing to smoke formation are discussed. It is likely that this technique will be used increasingly in the future to test our ideas on smoke formation mechanisms.     

Numerical studies on the combustion properties of char particle clusters

Particle clustering is an important phenomenon in dense particle–gas two-phase flow. One of the key problems worth studying is the reacting properties of particle clusters in coal particle combustion process in the dense particle region. In this paper, a two-dimensional mathematical model for the char cluster combustion in airflow field is established. This char cluster consists of several individual particles. The comprehensive model includes mass, momentum, and energy conservation equations for both gas and particle phases. Detailed results regarding velocity vector, mass component, and temperature distributions inside and around the cluster are obtained. The micro-scale mass and heat transfer occurred inside and around the char cluster are revealed. By contrastively studying the stable combustion of char particle clusters consisting of different particles, the combustion properties of char clusters in various particle concentrations are presented and discussed.     

Effect of CO2 and steam gasification reactions on the oxy-combustion of pulverized coal char

For oxy-combustion with flue gas recirculation, elevated levels of CO₂ and steam affect the heat capacity of the gas, radiant transport, and other gas transport properties. A topic of widespread speculation has concerned the effect of gasification reactions of coal char on the char burning rate. To asses the impact of these reactions on the oxy-fuel combustion of pulverized coal char, we computed the char consumption characteristics for a range of CO₂ and H₂O reaction rate coefficients for a 100 μm coal char particle reacting in environments of varying O₂, H₂O, and CO₂ concentrations using the kinetics code SKIPPY (Surface Kinetics in Porous Particles). Results indicate that gasification reactions reduce the char particle temperature significantly (because of the reaction endothermicity) and thereby reduce the rate of char oxidation and the radiant emission from burning char particles. However, the overall effect of the combined steam and CO₂ gasification reactions is to increase the carbon consumption rate by approximately 10% in typical oxy-fuel combustion environments. The gasification reactions have a greater influence on char combustion in oxygen-enriched environments, due to the higher char combustion temperature under these conditions. In addition, the gasification reactions have increasing influence as the gas temperature increases (for a given O₂ concentration) and as the particle size increases. Gasification reactions account for roughly 20% of the carbon consumption in low oxygen conditions, and for about 30% under oxygen-enriched conditions. An increase in the carbon consumption rate and a decrease in particle temperature are also evident under conventional air-blown combustion conditions when the gasification reactions are included in the model.     

Supercritical water oxidation of coal in power plants with low CO2 emissions

In this paper, the application of Super Critical Water Oxidation (SCWO) to direct combustion at low temperature of coal fine particles with pure oxygen for power generation is presented, including also a novel method for capturing and storing carbon dioxide as liquid. A detailed simulation model of a 100 MW_th coal-fired SWCO plant with low CO₂ emissions characterised by a steam cooled membraned SC reactor has been developed using Aspen Plus software. According to the well-known Semenov's thermal-ignition theory, the coal particle ignition temperature in SCW conditions has been also evaluated and the results have been integrated within the Aspen Plus model. This has been tested under different operating conditions. The simulation results are presented and the effects of the main plant operating conditions, such as ignition temperature, coal particle size and combustion pressure on the plant performances are discussed. The gross and net thermodynamic efficiencies of the power plant have been estimated to be around 44% and 28%, respectively. The pure oxygen production process results the main energy penalty.     

Experimental investigation of the two-phase theory in a fluidized-bed combustor

Though the two-phase theory of fluidization is well-accepted, no direct experimental measurements of the different gas concentrations predicted to occur in bubble and particulate phases could be found in the literature. For the first time, theoretical predictions of these different gas concentrations have been validated experimentally, using a combined oxygen/bubble probe. Based on the two-phase theory, a mathematical model was developed for the combustion of a batch of char particles in a fluidized-bed combustor. The experimental oxygen concentration in the particulate phase as a function of time was well predicted by the model. Slight discrepancies for the bubble phase values were eliminated when low-oxygen-concentration bubbles were excluded from the data, attributed to some char combustion occurring in bubbles being contrary to the model assumption. The temperature difference between char and bed particles (ΔT) was the only adjustable parameter in the model. A value of 20°C fitted the burnoff times measured by visual observation of the top of the bed, for both 5 and 10 g char batch masses. Model predictions of the oxygen concentrations were not sensitive to ΔT during the first half of burnoff, when mass transfer controlled the combustion rate, so the mass transfer processes were predicted correctly by the model effectively with no adjustable parameter. The ΔT value of 20°C was significantly lower than experimental measurements of maximum burning char particle temperatures, reported to be 70°C for the small-diameter bed particles used in this work. The discrepancy was attributed to two factors: (i) the decrease in char particle temperature towards the end of the burnoff, when kinetics significantly affected the combustion rate; and (ii) a lower char particle temperature in the particulate phase than in the bubble phase, with experimental char particle temperature measurements biased towards the higher bubble phase values. It was inferred: (i) that the maximum values of ΔT measured experimentally are too high for calculation of the char particle combustion rate during the kinetic-controlled latter stage of burnoff and (ii) that reported values of the heat transfer coefficient from burning char particles to the particulate phase deduced from these particle temperature measurements may have been underestimated.     

Effect of solid thermal conductivity and particle–particle contact on effective thermodiffusion coefficient in porous media

Transient mass transfer associated to a thermal gradient through a saturated porous medium is studied experimentally and theoretically to determine the effect of solid thermal conductivity and particle–particle contact on thermodiffusion processes. In this study, the theoretical volume averaging model developed in a previous study has been adopted to determine the effective transport coefficients in the case of particle–particle contact configurations. The theoretical results revealed that the effective thermodiffusion coefficient is independent of the thermal conductivity ratio for pure diffusive cases. In all cases, even if the effective thermal conductivity depends on the particle–particle contact, the effective thermodiffusion coefficient remains independent of the solid phase connectivity. We also found that the porosity can change the impact of dispersion effects on the thermodiffusion coefficients. For large values of the thermal conductivity contrast, dispersion effects are negligible and the effective thermal conductivity coefficients are the same as the ones for the pure diffusion case.Experimental results obtained for the purely diffusive case, using a special two-bulb apparatus, confirm the theoretical results. These results also show that, for non-consolidated porous media made of spheres, the thermal conductivity ratio has no significant influence on the thermodiffusion process for pure diffusion. Finally, the particle–particle contact also does not show a considerable influence on the thermodiffusion process.     

A model of the combustion of a single small coal particle using kinetic parameters based on thermogravimetric analysis

A mathematical model for the combustion in air of a single entrained spherical coal particle, 30 μm in diameter, has been developed incorporating thermogravimetric analysis data of Whitwick coal. The model is based on a set of ordinary differential equations, describing the reaction rates and the mass and heat transport processes. The system of equations was solved numerically. The combustion mechanism of the particle was described by locating the reaction zone at the solid surface, where gas-phase combustion of volatiles and heterogeneous reaction between gaseous oxygen and the carbon and hydrogen in the solid occurred in parallel. The combustion process was chemical-reaction-rate-controlled, with the oxygen partial pressure at the surface almost that of the surrounding bulk gas. The simulation results using this model, with the kinetic parameters for devolatilization and combustion derived from the experimental thermogravimetric data, are consistent with previously reported combustion lifetimes of approximately 1 s, for particles of this size and rank. They are also consistent with the anticipation that higher ambient gas temperatures should result in shorter burn-out times. The use of thermogravimetric data in the modelling of the combustion of small particles of these low-rank coals is a potentially valuable method for characterization of feedstocks for pulverized coal-fired boilers. © 1998 John Wiley & Sons, Ltd.     

Hydrogen and syngas production from propane and polyethylene

Hybrid filtration combustion of propane in a porous medium composed of aleatory polyethylene pellets and alumina spheres is studied to examine the suitability of the concept for hydrogen and syngas production. Temperature, velocity, and chemical products of the combustion waves were recorded experimentally in the range of equivalence ratios from stoichiometry (φ = 1.0) to φ = 1.65. The model predictions for combustion in inert porous media using GRI 3.0 reaction mechanism are in good qualitative agreement with experimental data. Hydrogen, carbon monoxide, methane and carbon dioxide are dominant combustion products for upstream sub-adiabatic temperatures recorder in all the range of equivalence ratios studied. The maximum hydrogen and carbon monoxide yields are close to 48% and 89%, respectively.     

Entrainment effects on combustion characteristics of various syngases using a perforated burner

This study concentrates on entrainment effects to achieve distributed regime for different syngas compositions (coke oven gas, biogas, and blast-furnace gas). For this purpose, numerical modeling has been performed through a computational fluid dynamics commercial code. In the modeling, The PDF/Mixture Fraction combustion model, standard k-? turbulence model, and P-1 radiation model have been selected to be used. Entrainment without decreasing oxygen concentration in air was provided from the flame holder wall of the newly generated burner at 10%, 20%, and 30% entrainment rates. For only coke oven gas, oxygen concentration was reduced to 15% O₂ with the entrainment in order to achieve ultra-low pollutant emissions level.The results showed that the temperature distributions were achieved more uniform for all cases. The flame zones became more thinner for all cases studied. This provided more invisible or colorless flame appearance, and as a result of this, more uniform thermal field was achieved without substantial decrease in combustion performance. Namely, the temperature values between the outlet temperatures for all entrainments have been close to each other (about 1200–1300 K). In terms of evaluation of distributed regime achievement on NO_X and CO pollutant emission levels, one can say that ultra-low emission levels (less than 1 ppm) were achieved with entrainment effects without decrease in oxygen concenration in air excluding coke oven gas. Distributed regime could be achieved at lower oxygen concentration with entrainment for coke oven gas combustion.     

Effect of air staging on fine particle,dust and gaseous emissions from masonry heaters

There is a need to decrease the detrimental particle and gaseous emissions from residential wood combustion appliances. One encouraging alternative is to stage the air supply which improves the combustion conditions in small appliances. In this study, two types of combustion technologies were studied in conventional masonry heaters (CMH) and modern masonry heaters (MMH). Air staging in the MMHs considerably reduced the particle and gas emissions. The greatest reduction was observed in gaseous and particulate organic emissions. Methane emissions were reduced by 74%–91% and carbon monoxide by 26%–81%. The reduction of fine particle mass (PM₁) was 14%–58%. Elemental carbon (EC, i.e. soot) emission increased in small combustion appliances but declined in large appliances. In addition, dust (TSP, Total Suspended Particulate matter i.e. Dust) emissions from hot flue gas were compared with the fine particle mass emissions from diluted sample. PM₁ emissions measured from diluted flue gas were 1.1–4.4-fold as compared to TSP collected from hot flue gas. This may be attributable to the fact that organic vapors partially had penetrated into the TSP filter in a gaseous form whereas when they were diluted, semivolatile species condensed on the particles. It can be concluded that air staging is an effective way to reduce gaseous and organic emissions from batch combustion appliances. Particle emission measured from diluted flue gas represents a more realistic results than TSP (hot sampling), because in dilution, also the organic fraction of the particle emissions is taken into account.     

Auto-gasification of a biofuel

A novel mechanism for gasifying a char is described. For thermally large particles (i.e., Bi > 0.1) the temperature distribution is non-uniform. Because different temperature regimes exist in the particle, the stages of drying, devolatilization (or pyrolysis), and reaction of the char may overlap. At some point the particle’s surface is fully devolatilized, while the particle’s interior is still undergoing drying and devolatilization. As H₂O and CO₂ flow out from the particle they pass through the hot surface layers of char. If the temperature is high enough, the char may be gasified. Black liquor was used here as a sample fuel. It has desirable properties for such auto-gasification; thermally large particles, a high initial water-content and a very porous and highly reactive char. Detailed numerical simulations suggest that 30 to 40% of the char may be converted simultaneously with devolatilization by auto-gasification. The larger the particle and the higher the temperature, the larger is the fraction of char gasified. For coal and peat, a typical particle size is too small for this mechanism to play any role when fired in pulverized fuel or fluidized bed furnaces. For burning wooden logs, the particle size is large and the pyrolysis time is ≈10 min., so then auto-gasification might be important.     

Comparison of carbon monoxide emissions and electricity consumption of modulating and non‐modulating pellet and solar heating systems

Emission and electricity consumption are important aspects of a pellet heating system. Low noxious emissions, particularly carbon monoxide, are a measure of a well‐performing system. High carbon monoxide emissions are often caused by unnecessary cycling of the burner, poor adjustment of the combustion air and insufficient maintenance. The carbon monoxide output, the thermal performance and the electricity consumption for modulating and non‐modulating operation mode have been investigated by simulations of four stoves/boilers as part of combined solar and pellet heating systems. The systems have been modelled with the simulation programme TRNSYS and simulated with the boundary conditions for space heating demand, hot water load and climate data as used in earlier research projects. The results from the simulations show that operating the pellet units with modulating combustion power reduces the number of starts and stops but does not necessarily reduce the carbon monoxide output. Whether the carbon monoxide output can be reduced or not depends very strongly on the reduction of starts and stops and how much the carbon monoxide emissions increase with decreased combustion power, which are in turn dependent on the particular settings of each pellet burner and how the heat is transferred to the building. However, for most systems the modulating operation mode has a positive impact on carbon monoxide emissions. Considering the total auxiliary energy demand, including the electricity demand of the pellet units, the modulating combustion control is advantageous for systems 1 and 4 for the used boundary conditions. The study also shows that an appropriate sizing of the stove or boiler has a huge potential for energy saving and carbon monoxide emission reduction. Copyright © 2006 John Wiley & Sons, Ltd.     

Hybrid filtration combustion of natural gas and coal

Rich and ultrarich combustion of natural gas in a porous medium composed of aleatory coal particles and alumina spheres was studied experimentally to evaluate the suitability of the concept for hydrogen and syngas production. Temperature, velocity and chemical products of the combustion waves were recorded experimentally in two stages: (1) natural gas in an inert porous medium at filtration velocities of 12, 15 and 19 cm/s for equivalence ratios (φ) from φ = 1.0 to φ = 3.8; (2) natural gas in a porous medium composed of coal and alumina particles for a range of volume coal fractions from 0 to 75% at φ = 2.3, and a filtration velocity of 15 cm/s. It was observed that the flame temperatures and hydrogen yields were increased with the increase of filtration velocity in inert porous media. In hybrid porous media the flame temperature decreased with an increase of coal fraction, and hydrogen and carbon monoxide were dominant partial oxidation products. Syngas yield in hybrid filtration combustion was found to be essentially higher than for the inert porous medium case. The maximum hydrogen conversion for the hybrid coal and alumina bed was ∼55% for a volumetric coal content of 75%.     

Oxyfuel concept development

Abstract

The reduction of greenhouse gas emissions and replacement of fossil fuels by renewable energy sources are important national and international targets. Oxyfuel (oxygen combustion technology) is one of the most promising technologies enabling carbon capture and storage from flue gases. The aim of oxyfuel concept development is to study different oxygen production technologies, combustion processes, CO₂ capture methods and integrate those to optimised concept. The goal is to create technical readiness for demonstration of oxygen combustion by using state of the art knowledge, experiments, modelling and simulation. Demonstration plan for oxygen combustion for an existing power plant(s) in Finland will be prepared. Main results will be an evaluation of oxygen combustion business potential for implementation in existing and new power plants, and improvement of competitiveness of Finnish companies in energy sector by developing CO₂ free power production technologies.

Before oxygen combustion can be demonstrated in full scale, small scale testing and model development must be done. Material exposure conditions in oxygen combustion will differ from any present day environment. Current high temperature steel grades have not been developed or tested for such aggressive conditions. VTT (Technical Research Centre of Finland) has in Jyväskylä unique small scale combustors applicable for oxygen combustion research.     

Combustion of isolated bubbles in an elevated-temperature fluidized bed

Combustion of isolated bubbles was investigated with a laboratory-scale fluidized-bed reactor. Two different combinations of oxygen and argon were employed as the fluidizing gas. Single bubbles of methane were injected into an incipiently fluidized bed maintained at elevated temperatures. Gas composition inside the bubbles was measured using a suction probe connected to an on-line mass spectrometer, and the temperature of the bubbles was monitored using a fast-response thermocouple. The effects of bed particle type, particle size, bubble size, bed temperature, and oxygen concentration in the emulsion phase were examined for bed temperatures between 923 and 1203 K. A theoretical model of homogeneous combustion within the bubble phase was developed for comparison to the experimental results. The model accounted for the heat and mass transfer between bubble and emulsion phases, but only considered combustion within the bubble. The results indicated that small bubble size and high oxygen concentrations in the emulsion phase enhanced bubble-phase combustion. The bed temperature also proved to be an important parameter, with higher temperatures promoting bubble combustion, but unlike some other investigations, no critical ignition temperatures were observed in either experiments or model results. The fluidized bed's particle size and particle composition influence the heat and mass-transfer coefficients, and therefore the bubble-phase combustion, but these have a smaller influence than bed temperature and bubble size. Model results for bubble-phase gas composition and temperature compared favorably with the experimental measurements.     

A soot particle surface reactivity model applied to a wide range of laminar ethylene/air flames

The effect of soot surface reactivity, in terms of the evolution of sites on the soot particles’ surface available for reaction with gas phase species, is investigated via modeling numerous ethylene/air flames, using a detailed combustion and sectional soot particle dynamics model. A new definition of a particles’ age is introduced. A methodology has been developed to study soot particle surface reactivity. Subsequently, it is investigated if the surface reactivity can be correlated with the particle age. An exponential function giving a smooth transition of surface activity with particle age is employed to model a variety of ethylene/air flames, which differ in fuel stream dilution levels, fuel stream premixing, and burner configurations. Excellent agreement with measured soot volume fractions of a variety of flames, burners, and datasets could be obtained with this approach. The newly developed function based on particle age eliminates the need to fit soot surface growth parameters to each experimental condition. Finally, the applicability and limitation of the new surface reactivity function for use in detailed soot formation models is discussed.