Coal Quality Impacts on Alkali Vapor Emissions from Pressurized Fluidized Bed Coal Combustors

Abstract

This paper describes a modeling study that was undertaken to identify the major ways that coal quality affects alkali vapor emissions in PFBC exhaust systems. The central premise is that the impact of coal quality can be reliably evaluated from kinetically based predictions of the concentrations of NaCI, KCI. S02. HCI, 02. and H 2 0 in the exhaust from the bubbling bed in combination with thermochemical equilibrium relations for conversion of alkali chlorides into alkali sulfate fumes in the freeboard. This hypothesis was implemented in a PFBC simulator that accounts for the major aspects of coal combustion and sulfur capture, and includes several mechanisms for alkali transformations,The model predicts a huge variation in exhaust Na vapor levels for different coals, ranging from the ppm-level with low rank coals to tens-of-ppb with hv bituminous coals for realistic levels of in-bed sulfur capture. This range is consistent with measured Na vapor emissions from two laboratory-scale systems. Hence. this approach exhibits the potential to predict Na-vapor emissions within useful quantitative tolerances at temperatures above 900°C with any coal type that contains sufficient chlorine to shift all alkali vapors into their chloride forms. It also demonstrates that the impact of all the coal properties that affect alkali vapor emissions can be expressed in terms of only two primary variables, the S02 and HCI concentrations in the exhaust after the alkali sulfate fume has formed. Whereas the essential connections among alkali vapor emissions and the concentrations of alkali vapors. S02' and HCI are not widely recognized. this study identified a complete set of test conditions that are essential elements for stringently evaluating predicted alkali levels. Simultaneous measurements of alkali vapors, S02, and HCI are the most important data requirements, together with the Na-, K-, and Cl-tevets in the test coals and accurate measurement of the temperature and cooling rate at the sampling point.

On-line spectroscopic and spectrometric methods for the determination of metal species in industrial processes☆

“…The chemistry of alkali species in combustion processes has been considered in detail by Glarborg [113] and Glarborg and Marshall [114] Earlier investigations relating to combustion and gasification processes include Mojtahedi et al [115]; Niksa et al [116]; Thambimuthu [117]; Wall [118]; Wibberley and Wall [119]. Model simulations linked to thermodynamic databases predict the proportions of metal species formed during combustion or gasification as a function of temperature.…”

Investigation on Sodium Fate for High Alkali Coal during Circulating Fluidized Bed Combustion

“…The H 2 O-soluble sodium was released from the coal particle during the devolatilization stage, and one possible mechanism was the direct vaporization of NaCl from the coal particle . The acid-soluble sodium (NH 4 Ac-soluble and HCl-soluble) was found to release in the form of Na during the char combustion process. , …”

“…28 The acid-soluble sodium (NH 4 Ac-soluble and HCl-soluble) was found to release in the form of Na during the char combustion process. 28,29 Therefore, it is assumed that 26% of the H 2 O-soluble sodium is released in the form of NaCl during the devolatilization stage and 7% of the NH 4 Ac-soluble sodium is released in the form of Na during the char combustion stage, as shown in eqs 1−3. Mass fractions of the volatile components in eq 1 are determined by Loison & Chauvin's formula and adjusted by the mass/energy balance method.…”

“…37 Punjak, Uberoi, and Shadman 38−40 investigated the interactions of alkali-metal vapors with various solid sorbents, including bauxite, kaolinite, emathlite, and developed a mathematical model to describe the absorption process. On the basis of Punjak et al and Uberoi et al's work, Niksa 29 presented a simplified overall reaction (R1-7). This reaction is applied in this work.…”

“…On the basis of Punjak et al. and Uberoi et al’s work, Niksa presented a simplified overall reaction (R1-7). This reaction is applied in this work.…”