Kunlei Liu

Thermal characterization of organically modified montmorillonite

Abstract Polymer/organically modified layered silicate (PLS) nanocomposites are a new class of filled polymers with ultrafine phase dimensions. They offer an outstanding combination of stiffness, strength and weight that is difficult to attain separately from the individual components. Additionally, the nanoscopic phase distribution as well as synergism between polymer and the layered silicate result in additional properties, such as flame retardency, enhanced barrier properties and ablation resistance, which are not observed in either component. These nanocomposites are synthesized by blending the organically modified layered silicate (OLS) into the polymer melt. Thus, understanding the relationship between the molecular structure and the thermal stability (decomposition temperature, rate, and the degradation products) of the organic modification of the layered silicate is critical. During this study, modern thermal analysis techniques combined with infrared spectroscopy and mass spectrometry (TGA–FTIR–MS) were used to obtain information on the thermal stability and degradation products. The effect of chemical variation (alkyl chain length, number of alkyls, and unsaturation) of organic modifiers on the thermal stability of the organically exchanged montmorillonite are discussed. A range of interesting results is observed, however, not all are currently understandable.

Polycyclic aromatic hydrocarbon (PAH) emissions from a coal-fired pilot FBC system

Due to the extensive amount of data suggesting the hazards of these compounds, 16 polycyclic aromatic hydrocarbons (PAHs) are on the Environmental Protection Agency (EPA) Priority Pollutant List. Emissions of these PAHs in the flue gas from the combustion of four coals were measured during four 1000h combustion runs using the 0.1MW heat-input (MWth) bench-scale fluidized bed combustor (FBC). An on-line sampling system was designed for the 16 PAHs, which consisted of a glass wool filter, condenser, glass fiber filter, Teflon filter, and a Tenax trap. The filters and Tenax were extracted by methylene chloride and hexane, respectively, followed by GC/MS analysis using the selective ion monitoring (SIM) mode. In this project, the effects of operating parameters, limestone addition, chlorine content in the coal, and Ca/S molar ratio on the emissions of PAHs were studied. The results indicated that the emissions of PAHs in an FBC system are primarily dependent on the combustion temperature and excess air ratio. The injection of secondary air with high velocity in the freeboard effectively reduces PAH emissions. The addition of extra limestone can promote the formation of PAHs in the FBC system. Chlorine in the coal can possibly lead to large benzene ring PAH formation during combustion. The total PAH emission increases with an increase in the sulfur content of coal. Incomplete combustion results in PAHs with four or more benzene rings. High efficiency combustion results in PAHs with two or three benzene rings.

A study of chlorine behavior in a simulated fluidized bed combustion system

Fluidized bed combustion techniques have been widely used throughout the world in an effort to reduce sulfur oxide emissions, especially from burning high-sulfur coal. However, in the utilization of FBC systems for co-firing high chlorine coals with municipal solid waste (MSW) there are some concerns about the possible emission of polychlorinated dibenzodioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs). PCDD/Fs may be produced from the reaction of volatile organic compounds (VOCs) and molecular chlorine under relatively low combustion temperature conditions. In oxygen-rich conditions during combustion molecular chlorine can be formed through the Deacon Reaction when the temperature is around 600°C. It is also likely that chloride might affect the detailed chemistry of desulfurization in FBC processes. In order to better understand the behavior of chlorine in an FBC system during combustion processes, a comprehensive study was carried out in a simulated FBC system with an on-line feeder at Western Kentucky University. Conditions used simulated the flue gas and operating conditions of an FBC system. Optimum operating conditions to suppress HCl, Cl2 and SOx emissions from FBC systems were determined. A better understanding of the chlorine behavior during combustion will help in controlling possible PCDD and PCDF formation and reducing corrosion in FBC systems. The test results indicated that the formation of molecular chlorine is favored at temperatures above 600°C, in oxygen-rich atmospheres, and in relatively high HCl concentrations. The reaction temperature plays a key role in the capture of HCl. The optimum combustion conditions for controlling PCDD/Fs formation in FBC systems is to maintain combustion temperature around 850°C in the bed area, 600°C in the freeboard area, low oxygen concentrations in the flue gas and enough residence time for fine particles in the freeboard.

Interaction between emissions of SO2 and HCl in fluidized bed combustors

The ability to capture SO2 and halogens is one of the most important advantages of fluidized bed combustion (FBC). In order to clarify the affects of chlorine in the absorption of SO2 emission, experiments involving the addition of PVC to coals were carried out using the 0.3 m ID bench scale FBC system at Western Kentucky University. During the experiments, PVC was added to three coals in different percentages, and the mixtures fed into the FBC system. The Ca/S ratio was kept constant at 3:1. The experimental results indicate that chloride addition dramatically decreased the SO2 concentration in flue gases. The sulfur and chlorine contents in both fly ash and bed ash increased. At the same time, the utilization efficiency of calcium increased with an increase in the PVC weight percent. This reduction in SO2 is attributed to more voids on the sorbent particle surface (limestone and/or ash) through transient formation of a mobile halide ion-containing phase (CaCl2), and to the reactions: 4HCl 1 O2

Soluble polycyclic aromatic hydrocarbons in raw coals.

2Cl2 1 2H2O

A Study of Mercury Removal in FBC Systems Fired with High Chlorine Coals

Polycyclic aromatic hydrocarbons (PAHs) are considered to be a group of compounds that pose potential health hazards since some PAHs are known carcinogens. During coal utilization processes, such as coal combustion and pyrolysis, PAHs released may be divided into two categories according to their formation pathways. One category is derived from complex chemical reactions and the other is from free PAHs transferred from the original coals. PAHs released from complex chemical reactions during combustion and pyrolysis have received considerable attention in recent years. However, free PAHs contained in raw coals have not been seriously considered as a source of these materials to be released during the utilization of coal. The goal of this study was to observe the relation between the content of PAHs in different coals and the elemental composition of the coals. In this study, eight bituminous coals with dry, ash-free carbon values varying from 65% to 90% were selected. Each coal was extracted with dichloromethane in a Soxhlet extractor for 6 h. The extracts were quantitatively analyzed with a gas chromatograph/mass spectrometer (GC-MS). More than 20 kinds of PAHs were identified. The total amount of PAHs determined varied from 1.2 to 28.3 mg/kg from the various coal types. The maximum total PAHs extracted was reached when the carbon content exceeded 84% by weight.

An Atmospheric Pressure, Fluidized Bed Combustion System Burning High-Chlorine Coals in the Convection Section

The objective of this project is to study the reduction, at a low monetary cost, of mercury emissions from coal fired combustors by using HCl to convert elemental mercury to oxidized mercury species. The HCl atmosphere needed for the oxidation of elemental mercury is provided by the combustion of high chlorine coal in a utility boiler system at relatively low temperatures (500-600°C). By oxidizing elemental mercury inside the fluidized bed combustion (FBC) system, total mercury emissions can be reduced with high efficiency and low cost while maintaining low emissions of other pollutants. The results indicate that using high chlorine coal in an FBC system converted more than 99% of elemental mercury to an oxidized state — mainly HgCl2.

Characterization of Ash Deposition During Co-Combustion of Coal With Refuse-Derived Fuels in a Pilot FBC Facility

Abstract The possibility of fireside corrosion in power plant boiler components is always a major concern when the fuels include high-sulfur and high-chlorine coals (or refuse waste). Sulfur and ch...

The Effects of Ash Deposits on High-Temperature Corrosion of Alloys in a Fluidized Bed Combustion System

This paper presents data from a recent investigation of the character of ash deposition in the convective zone (547°C to 338°C) in a 0.1 MWth bench-scale FBC system at Western Kentucky University. The ash deposit samples were collected during co-firing experiments using two coals with various blends of a refuse-derived fuel (RDF). A low sulfur coal, a high sulfur coal, and commercially available RDF sample were selected to investigate the influence of sulfur and chlorine in the fuels on the formation of ash deposits. Limestone was added to the combustor as the bed material and desulfurization sorbent. The results showed that the formation of ash deposits had a close relationship to the active fine lime particles produced from the limestone. An increase in the concentration of SOx in the flue gas restricts the formation of the ash deposits because of the reaction between SOx and the fine lime particles, which drops the adhesive force of the fine lime particles by reducing the contact area among the particles. With an increase in the content of the RDF in the fuels, the rate of deposit of ash decreased because of the higher content of chlorine and aluminum, which also decreased the contact area among the particles, leading to a low deposition rate of the fly ash.Copyright

Investigation of Polycyclic Aromatic Hydrocarbons in Fly Ash from Fluidized Bed Combustion Systems

Abstract Five 1,000-h combustion tests firing 5 different coals were conducted in a 0.1-MWth fluidized bed combustion (FBC) facility at Western Kentucky University (WKU) with the operating conditions simulating those of the FBC unit at the Tennessee Valley Authoritys (TVA) Shawnee Power Station. Five coals were selected with various S and Cl contents ranging from high S (4.48%) and high Cl (0.47%) to low S (0.97%) and low Cl (0.012%). Three types of uncooled steel coupons (Types 304 [UNS S30400], 309 [UNS S30900], and 347 [UNS S34700] stainless steel [SS]) were prepared and installed, based on the advice of TVA, to simulate the evaporator tubes in the superheater region (550°C to 600°C [1,020°F to 1,100°F]). Metal wastage of each coupon was determined by measuring the thickness before and after each combustion test. In order to study the relationship between metal corrosion and ash deposits, a total of 170 ash deposits were collected from the test coupon surfaces during the 5,000-h testing and analyzed. ...