Sulfation Behavior of Calcium-based Sorbents under Oxy-combustion Conditions at High Pressures

Open Access
Author:
Yang, Xiaojing
Graduate Program:
Energy and Mineral Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
August 10, 2015
Committee Members:
  • Sarma V Pisupati, Thesis Advisor
Keywords:
  • sulfation
  • limestone
  • dolomite
  • oxy-combustion
  • fluidized-bed
  • pressurized-systems
  • calcination
  • SEM
  • petrographic
Abstract:
Oxy-combustion, combustion in oxygen without nitrogen, has already been used to reduce fuel consumption and emissions to advance clean coal combustion technologies for electrical power generation. The same system can also use limestone and dolomite to capture SO2. Therefore, fluidized-bed oxy-combustion at high pressures is now being explored to produce and sequester a pure stream of CO2 as flue gas in an attempt to design zero-emission power plants. Sulfation of limestones typically occurs in two stages—calcination and then sulfation. Previous studies have shown that when the partial pressure of CO2 in a system is higher than the equilibrium pressure, direct sulfation of limestones without the calcination step occurs. However, the mechanism for this process is not clear. In oxy-pressurized fluidized-bed combustion (PFBC), the effects of CO2/O2 ratio (CO2 partial pressure), sorbent petrography, and MgCO3 are not well documented and need to be understood in order to properly design a PFBC combustor. The primary objective of this study was to compare the sulfation degrees of three different sorbents at pressurized and atmospheric conditions and to elucidate the mechanism for higher sulfation degrees under direct sulfation. The effect of CO2/O2 ratio on sulfation was also investigated. Sulfation tests on two limestones (Graymont and Michigan) and one dolomite (Ohio) were conducted in a fixed-bed reactor at 870 °C at higher pressure (8 bar) and at atmospheric pressure under typical oxy-combustion conditions. The reactor was equipped with a continuous gas analyser to monitor CO2, SO2, and O2. The decomposition study of the sorbents was carried out using hot-stage X-Ray Diffraction (XRD) for in-situ observation, simulating the phase changes during the heating process of each sorbent in the reactor. Further, a fixed-bed reactor was designed to use higher amounts of sample than a typical thermogravimetric analysis (TGA) so that subsequent analyses could be performed on the samples to understand the mechanism. Typically, a TGA uses 5-20 mg of sample. In this fixed-bed reactor, however, approximately one gram of sample was spread in a boat to reduce mass transfer effects. The degree of sulfation of the sulfated samples was determined using XRD technique, and the grain sizes of crystallites were also determined by XRD analysis. The cross-sections of the sulfated sorbents were then observed under a scanning electron microscope (SEM), and elemental maps were taken by coupling energy dispersive spectroscopy (EDS). The effects of petrographic characteristics, sulfation pattern, CO2 partial pressure, and total pressure were then analysed and correlated. Hot-Stage XRD analysis conducted at atmospheric pressure showed that under a pure CO2 environment, CaCO3 did not decompose between 200 and 900 °C, whereas CaMg(CO3)2 started to decompose to CaCO3, MgO, and MgCO3 at 650 °C. The sulfation results showed that the conversion increased with an increasing CO2/O2 ratio at any given pressure, and a higher conversion was observed at high pressure than that at atmospheric pressure for a given gas composition. Further, prediction of the sulfation patterns based on petrographic characteristics agreed with the SEM results. Sulfation patterns are categorized as network, core-shell, and uniform based on the way sulfur is distributed throughout the particles. For a given sorbent, the patterns were the same under both the pressurized system and the atmospheric system. However, under the same experimental conditions, the patterns were different for different sorbents. Therefore, sulfation pattern was correlated with the petrographic characteristics of each sorbent, independent of the chemical reaction. Graymont limestone had the lowest conversions among all three sorbents for a given condition. With a network-dominated sulfation pattern, increasing the CO2/O2 ratio improved the conversion of Graymont limestone more effectively than increasing the total pressure. Dolomite had a uniform dominated sulfation pattern and showed the highest conversions both at high pressure and at atmospheric pressure. The S/Ca ratios across the cross-sections were higher for the sorbents that have a higher conversion. The similar trend of S/Ca ratios across the cross-sections and conversions suggested that high pressures improved the conversions by increasing sulfur penetration depth in the particles with a core-shell sulfation pattern. Another interesting observation in this study was that the CaMg2(SO4)3 phase was found in Michigan limestone and in two sulphated dolomite samples at high pressure, indicating that Mg actually participated in the sulfation process at high pressure. Thermodynamic equilibrium simulations using FactSage™ confirmed that the formation of CaMg2(SO4)3 was controlled by the ratio between SO2 and Mg, which increased with the pressure. This phase has not been reported in previous sulfation literature; however, further experiments are needed to fully study the mechanisms of the formation of CaMg2(SO4)3