In-bed Sulfur Capture Research Articles

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

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 NaCl, KC1, SO2. HCI, O2, and H2O in the exhaust from the bubbling bed in combination with ther-mochemical 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 SO2 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. SO2, 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, SO2, and HCI are the most important data requirements, together with the Na-, K-, and Cl-levels in the test coals and accurate measurement of the temperature and cooling rate at the sampling point.

Product Layer Development during Sulfation and Sulfidation of Uncalcined Limestone Particles at Elevated Pressures

Fluidized bed combustion or gasification allows for in-bed sulfur capture with a calcium-based sorbent such as limestone or dolomite. Sorbent particle size, porosity, internal surface, and their variation during conversion have great influence on the conversion of the sorbent. The uptake of SO2 and H2S by five physically different limestones is discussed, for typical pressurized fluidized bed combustor or gasifier conditions: 850/950 °C, 15/20 bar. Tests were done in a pressurized thermogravimetric apparatus (P-TGA), the size of the limestone particles was 250−300 μm. It is stressed that the limestones remain uncalcined. A changing internal structure (CIS) model is presented in which reaction kinetics and product layer diffusion are related to the intraparticle surface of reaction, instead of the outer particle surface as in unreacted shrinking core (USC)-type models. The random pore model was used for describing the changing internal pore and reaction surfaces. Rate parameters were extracted for all five limestones using the CIS model and a USC model with variable effective diffusivity. Differences in the sulfur capture performance of the limestones were evaluated. Plots of the CaSO4 or CaS product layer thickness as a function of conversion are given, and the relative importance of limestone porosity and internal surface is discussed.

Hydrogen Sulfide Capture by Limestone and Dolomite at Elevated Pressure. 1. Sorbent Performance

Sulfur emission control in fossil fuel gasification plants implies the removal of H{sub 2}S from the product gas either inside the furnace or in the gas clean-up system. In a fluidized-bed gasifier, in-bed sulfur capture can be accomplished by adding a calcium-based sorbent such as limestone or dolomite to the bend and removing the sulfur from the system with the bottom ash in the form of CaS. This work describes the H{sub 2}S uptake by a set of physically and chemically different limestones and dolomites under pressurized conditions, typically for those in a pressurized fluidized-bed gasifier (2 MPa, 950 C). The tests were done with a pressurized thermobalance at two p{sub CO{sub 2}} levels. Thus, the sulfidation of both calcined and uncalcined sorbents could be analyzed. The effect of p{sub H{sub 2}S} was also investigated for uncalcined limestones and half-calcined dolomites. The results are presented as conversion of CaCO{sub 3} or CaO to CaS vs time plots. The results are also compared with the sulfur capture performance of the same sorbents under pressurized combustion conditions.

Hydrogen Sulfide Capture by Limestone and Dolomite at Elevated Pressure. 2. Sorbent Particle Conversion Modeling

The physical structure of a limestone or dolomite to be used in in-bed sulfur capture in fluidized bed gasifiers has a great impact on the efficiency of sulfur capture and sorbent use. In this study an unreacted shrinking core model with variable effective diffusivity is applied to sulfidation test data from a pressurized thermogravimetric apparatus (P-TGA) for a set of physically and chemically different limestone and dolomite samples. The particle size was 250−300 μm for all sorbents, which were characterized by chemical composition analysis, particle density measurement, mercury porosimetry, and BET internal surface measurement. Tests were done under typical conditions for a pressurized fluidized-bed gasifier, i.e., 20% CO2, 950 °C, 20 bar. At these conditions the limestone remains uncalcined, while the dolomite is half-calcined. Additional tests were done at low CO2 partial pressures, yielding calcined limestone and fully calcined dolomite. The generalized model allows for determination of values for ...