Study On The Transformation Behavior Of Pyrite During Oxy- Fuel Combustion

Oxy-fuel combustion (O2/CO2 combustion) technology is a promising approach for CO2 capture from coal-fired power plants. Therefore, it is considered to be a competitive technology for capturing CO2. Compared with traditional air combustion, oxy-fuel combustion brings a lot of changes which lead to many new problems for the design, alteration and operation of oxy-fuel coal-fired boilers. Ash deposition is still an unresolved problem in the development of the oxy-fuel combustion technology. However, pyrite in coal is known as one of the major contributors to ash slagging. Therefore, the investigation of the kinetics and models of pyrite transformation under the conditions of oxy-fuel combustion is a critical component in prediction of ash formation and slagging, and mitigation of slagging problems. To date, the relevant work has rarely been reported. This thesis works on the kinetics and models of pyrite transformation under the conditions of oxy-fuel combustion. The primary emphasis of this work is to reveal the role of CO2 and H2O on the decomposition and oxidation of pyrite. Consequently, the kinetic mechanisms of pyrite transformation in oxy-fuel combustion can be deduced. The main research contents of this thesis are as follow:In order to reveal the role of CO2 on the decomposition process of pyrite, pure pyrites are heated on a thermo-gravimetric reactor in CO2 and N2. CO2 plays a chemical role in pyrite decomposition, pyrite reacts with CO2 to form pyrrhotite, CO and SO2. The kinetic process of pyrite decomposition in CO2 can be divided into three stages:the first stage, CO2 does not participate the decomposition of pyrite, pyrite decomposes to form pyrrhotite via self-decomposition reaction; at the second and third stage, the formed high sulfur pyrrhotite react with CO2 to form low sulfur pyrrhotite, CO and SO2. The first and second stage can be described by a shrinking core model and a three-dimensional diffusion model, respectively. The activation energies are 70KJ/mol and 61 KJ/mol, respectively. Accordingly, the decompose reaction rate of pyrite in CO2 is faster than that in N2.In order to reveal the role of CO2 on the oxidation process of pyrrhotite, pure pyrrhotites are heated on a thermo-gravimetric reactor in CO2. The transformation of pyrrhotite in CO2 consists of three stages, i.e.. the fast weight loss stage (the first stage), the slow weight loss stage (the second stage) and the slow weight gain stage (the third stage). The mechanisms involving CO2 at these three distinct stages are quite different. At the first stage, ferrous sulfide is formed by the decomposition reaction of pyrrhotite with CO2. At the second stage, the oxidation reactions of ferrous sulfide with CO2 are responsible for the formation of magnetite or hematite, CO and SO2. The third stage is dominated by the oxidation of magnetite to form hematite with CO as the only gas product. For the oxidation of ferrous sulfide in CO2, the kinetic process can be described by three-dimensional diffusion model (n=l/2). and the activation energies are 86.6KJ/mol.The kinetics of pyrrhotite oxidation in CO2, H2O, and the mixtures of CO2 and H2O are explored on a thermos-gravimetric reactor. It is found that the oxidation of pyrrhotite (FeSi+x) is the oxidation of ferrous sulfide (FeS) in nature. Similar to char gasification, the adsorption and desorption elemental reactions of CO2 or H2O on ferrous sulfide oxidation are essentially the same. In all cases, the three-dimensional diffusion model is applicable for the oxidation of ferrous sulfide. The oxidation ability of H2O on the oxidation of ferrous sulfide is stronger than CO2, and the reaction rate constants increase with the increase of the partial pressure of CO2 or H2O. The relations between the rate constant and the partial pressure of CO2 or H2O can be well expressed by Langmuir-Hinshelwood rate forms. The FeS-CO2 reaction and the FeS-H2O reaction in the mixtures of CO2 and H2O can be well described by a modified Langmuir-Hinshelwood kinetic model, it suggests that the relation of FeS-CO2 oxidation reaction and FeS-ItO oxidation reaction is mutually independent on the separate active sites while mutually competitive on common active sites.Pure pyrite is blended with a demineralized coal in the ratio of 1:9, which is used to simulate the excluded pyrite in coal. The raw sample is combusted on a drop tube furnace. It is found that, the transformation behaviors of pyrite during O2/CO2 and O2/CO2/H2O are similar to that in O2/N2. Pyrite decomposes to pyrrhotite first, and then be oxidized to magnetite and hematite, magnetite is oxidized to hematite finally. It is different that, the presence of CO2 and H2O can react with pyrite and accelerate the realese of sulfur. The modeling data show that the presence of CO2 can accelerate the decomposition of pyrite, and also the oxidation of pyrrhotite or magnetite. On the other hand, it will drop the paricle temperature. From O2/N2 combustion to O2/CO2 combustion, the propotion of the duration time of liquid phases increases slightly.