John T. Riley

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

TG/FTIR/MS study of organic compounds evolved during the co-firing of coal and refuse-derived fuels

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.

Comparison of sulphur in HNO3-extracted and physically cleaned coals

Complementary thermal analytical techniques were used to analyze gaseous products evolved during the co-firing of coal with refuse derived fuels. The combined thermogravimetric . . . TG rFourier transform infrared FTIR rmass spectrometry MS techniques were employed to study the reaction pathways for the formation of gaseous products during combustion, as well as identify molecular chlorine, HCl, CO, CO , H O and various organic compounds. The discovery 22 of molecular chlorine has led us to look for the possible formation of chlorinated organic compounds in the combustion process. Chlorine and hydrocarbon species are released in the same temperature range and in higher concentrations during fast heating rates compared to relatively slow ones. These results indicate that there may be more opportunities to form chlorinated organic . compounds during the co-firing of coals with refuse-derived fuels RDF at the faster heating rates . that may occur in a fluidized-bed combustor FBC system. q 1999 Elsevier Science B.V. All rights reserved.

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

Abstract The sulphur contents of eight bituminous and subbituminous coals, after extraction with nitric acid, are compared with the sulphur contents of physically cleaned samples of the coals. Samples of −60 mesh (250 μm) coal were extracted with boiling 2 M HNO3, which removes essentially all mineral sulphur. After washing and drying, the extracted samples were analysed for moisture, ash, and total sulphur. The dry, ash-free (daf) sulphur values for the eight coals obtained by this method show excellent agreement with the daf sulphur values for physically cleaned samples of the coals. The physically cleaned samples were prepared by float/sink separation of −60 mesh coal in 1.30 specific gravity media, followed by milling the float coal to particle sizes less than 10 μm and subsequent float/sink-centrifugation cleaning. The daf sulphur values determined in the HNO3-extracted and physically cleaned samples were less than those obtained using ASTM Method D 2492 and differed by as much as 1.3%.

Co-firing high-sulfur coals with refuse-derived fuel☆

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.

Studies of fly ash using thermal analysis techniques

Abstract The fundamental thermal behavior of five materials (Illinois coal #6, Kentucky coal #9, polyvinyl chloride, cellulose and newspaper) has been investigated using the TGA/FTIR/MS system under different combustion conditions. At a fast heating rate, the decomposition temperatures shift towards higher temperatures and the maximum weight loss rates increase by 3–7 times those at the slow heating rate. The gases evolved from the decomposition have been analyzed kinetically. More organic compounds are identified at the faster heating rate. Also, molecular chlorine is observed in the oxidation of PVC. These two species may lead to the formation of chlorinated organic compounds. The results indicate that the TGA/FTIR/MS system can be used to evaluate the effect on the environment of co-firing high-sulfur coal with refuse-derived fuels.

The effect of limestone on the combustion of fuel blends

Improved thermoanalytical methods have been developed that are capable of quantitative identification of various components of fly ash from a laboratory-scale fluidized bed combustion system. The thermogravimetric procedure developed can determine quantities of H2O, Ca(OH)2, CaCO3, CaSO4 and carbonaceous matter in fly ash with accuracy comparable to more time-consuming ASTM methods. This procedure is a modification of the Mikhail-Turcotte methods that can accurately analyze bed ash, with higher accuracy regarding the greater amount of carbonaceous matter in fly ash. In addition, in conjunction with FTIR and SEM/EDS analyses, the reduction mechanism of CaSO4 as CaSO4+4H2 ↔ CaS + 4H2O has been confirmed in this study. This mechanism is important in analyzing and evaluating sulfur capture in fluidized-bed combustion systems.