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Über dieses Buch

This book focuses on pulverized coal particle devolatilization, ignition, alkali metal release behavior, and burnout temperature using several novel optic diagnostic methods on a Hencken multi-flat flame burner. Firstly, it presents a novel multi-filter technique to detect the CH* signal during coal ignition, which can be used to characterize the volatile release and reaction process. It then offers observations on the prevalent transition from heterogeneous ignition to hetero-homogeneous ignition due to ambient temperature based on visible light signal diagnostics. By utilizing the gap between the excitation energies of the gas and particle phases, a new low-intensity laser-induced breakdown spectroscopy (PS-LIBS) is developed to identify the presence of sodium in the particle or gas phase along the combustion process. For the first time, the in-situ verification of the gas phase Na release accompanying coal devolatilization is fulfilled when the ambient temperature is high enough. In fact, particle temperature plays a vital role in the coal burnout process and ash particle formation. The last part of the book uses RGB color pyrometry and the CBK model to study the char particle temperature on a Hencken burner. It offers readers valuable information on the technique of coal ignition and combustion diagnostics as well as coal combustion characteristics.

Inhaltsverzeichnis

Frontmatter

Chapter 1. Introduction

Abstract
Coal is a complicated solid fuel, containing C, H, O, N, S, etc. When the particle is heated, moisture, volatile and tars will be released and involved into multistep chemical reactions. The char after devolatilization also undergoes heterogeneous reaction. Generally, numerous elements, complicated structures and multiple stages of combustion are the main reasons for the complexity of coal combustion. The coal combustion process mainly includes devolatilization, ignition and char oxidation, also accompanying with metal release and partitioning behavior. The combustion efficiency and pollutant formation of coal combustion are interrelated with the above four processes. Therefore, in order to understand the coal combustion process, this thesis focuses on the above four aspects. Traditional experimental methods always change the combustion environment and the sudden change will cause target variation, collecting mirror and other problems. Therefore, in order to illustrate the detailed combustion process of pulverized coal particles and the dynamic behavior of metal precipitation process, this thesis aims to apply in situ optic diagnostic technology into the coal combustion research. Focusing on combustion and fine particulate matter formation, optic methods about molecule, radical, emission intensity, metal atom and surface temperature are mainly investigated.
Ye Yuan

Chapter 2. Experimental and Theoretical Investigation of Coal Devolatilization

Abstract
This chapter describes the experimental set up, from the Hencken mulit non-premixed flat flame burner to work condition design and five kinds of coal used in this work. The consecutive images through multiple filters near CH* emission band, centered at 420, 430 and 440 nm, are exquisitely processed to subtract the interferences from continuum blackbody radiation of particles and soots in the CH* band. Then, the signal of CH* chemiluminescence is capable of providing a good indicator of the coal devolatilization process. The results suggest that devolatilization occurs in 15 and 18 ms in 1800 and 1500 K and no distinct volatile released into surrounding gas occurs before the ignition in 1200 K. In higher ambient temperature, the devolatilization process begins earlier and the devolatilization time is relatively shorter. The devolatilization process almost occurs at the same time in different oxygen concentrations and the devolatilization time is a little longer in 10 and 30% oxygen mole fraction. The temperature effect of the devolatilization process is also examined using theoretical method.
Ye Yuan

Chapter 3. Ignition Mechanism Research on Dispersed Pulverized Coal Particles

Abstract
This chapter is assessing a study of the collective ignition behaviors of dispersed coal particle streams, with ambient temperature from 1200 to 1800 K and oxygen mole fractions in the range of 10–30%. The dispersed coal particles of 65–74 μm are injected into an optical Hencken flat-flame burner by a novel deagglomeration feeder. The normalized visible light signal intensity, deleting the background noise, is established to characterize the ignition delay of coal particle streams. Firstly, the prevalent transition from heterogeneous ignition to hetero–homogeneous ignition due to ambient temperature is observed. The pure homogeneous ignition rarely occurs, with an exception under high temperature and low oxygen for high-volatile coal. By comparing time scales between pyrolysis and heating processes, the competition of the volatile evolution and heterogeneous surface reaction are discussed. Then, the effects of ambient temperature, oxygen mole fraction and coal rank on the characteristic ignition delay are examined. Besides that, the experimental investigation is also conducted on the combustion of pulverized coal particle streams in either conventional or oxy-fuel conditions. Finally, the transient mode is developed, which not only well interprets the observed ignition transition phenomena, but also approximately predicts a variation of heterogeneous ignition time as a function of oxygen fraction.
Ye Yuan

Chapter 4. Dynamic Behavior of Na Release During Coal Combustion

Abstract
In this chapter, we examine the dynamic behavior of sodium (Na) release during the pulverized coal combustion of ZDL lignite using a laminar, Hencken flat-flame burner technique. By utilizing the gap between the excitation energies of the gas and particle phases, a new low-intensity laser-induced breakdown spectroscopy (LIBS) is developed to distinguish the existence of sodium in the particle or gas phase along the combustion process. Within the coal flame domain, Na atomic spectra in the particle phase are clearly detected that consistently agree with the NIST database. For the first time, the in situ verification of the gas phase Na release accompanying coal devolatilization is fulfilled when the ambient temperature is high enough. The residence time, indicating Na release from particle to gas phases, is determined from the signal. By using a theoretical analysis, the Na release time approximately coincides with the characteristic pyrolysis time of lignite, further confirming the observation above. The effects of ambient temperature, coal rank and oxygen concentration are further discussed. This work may provide a basis for exploring the formation mechanism of submicron fine particulates during coal combustion.
Ye Yuan

Chapter 5. Experimental and Theoretical Research on Coal Surface Temperature

Abstract
An optic pyrometry is adopted to detect particle surface temperature. Besides that, the char temperature history, char burnout time and physical properties changing behavior in different ambient temperatures are investigated. The pyrometry is RGB three color pyrometry (RGB pyrometry) and the theoretical model is a carbon burnout kinetics model based on reaction sites (CBK model). The dispersed coal particles of 65–74 μm are injected into an optical Hencken flat-flame burner by the de-agglomeration feeder. The HBL is used in this chapter to study the particle temperature in heterogeneous ignition process and BC for surface temperature in char burnout. The particle emissivity is a key factor in RGB color pyrometry measurement. In combination with the spectral radiation in the visible light region (390–710 nm) detected by a spectrometer, a calibrated spectral emissivity model is established for the dispersed chars in the combustion system. Further, this model is applied to modify the lookup table of RGB pyrometry. Better predictions on char particle temperatures under different ambients are obtained in RGB pyrometry when referring to the modified lookup table.
Ye Yuan

Chapter 6. Conclusions

Abstract
This thesis uses Hencken multi-flame flat flame burner as the platform and establishes an advanced optic diagnostic system to study the key issues during coal combustion, such as devolatilization, ignition, dynamic behavior of alkali metals, char burnout, particle temperature, etc. The research focuses on the effect of ambient temperature and oxygen concentration. The main research contents include that novel multi filter array and visible light signal methods are separately adopted to investigate the devolatilization and ignition process, low-intensity selective laser induced breakdown spectroscopy and RGB color pyrometry are also used to study the alkali metal dynamic behavior and char surface burnout temperature. In the end, transient ignition model and carbon burnout kinetic (CBK) model offer theoretical analyses for the devolatilization, ignition and burnout process.
Ye Yuan

Backmatter

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