Reaction Mechanisms of Magnesium Silicates With Carbon Dioxide in Microwave Fields

Abstract

The objective of the investigation was to determine whether microwave fields would enhance the reactions of CO 2 with silicates that are relevant to the sequestration of carbon dioxide. Three sets of experiments were conducted. (1) Serpentine and CO 2 were reacted directly at one atmosphere pressure in a microwave furnace. Little reaction was observed. (2) Serpentine was dehydroxylated in a microwave furnace. The reaction was rapid, reaching completion in less than 30 minutes. A detailed investigation of this reaction produced an S-shaped kinetics curve, similar to the kinetics from dehydroxylating serpentine in a resistance furnace, but offset to 100 ˚C lower temperature. This set of experiments clearly demonstrates the effect of microwaves for enhancing reaction kinetics. (3) Reactions of serpentine with alkaline carbonates and in acid solution were carried out in a microwave hydrothermal apparatus. There was a greatly enhanced decomposition of the serpentine in acid solution but, at the temperature and pressure of the reaction chamber (15 bars; 200 ˚C) the carbonates did not react. Overall, microwave fields, as expected, enhance silicate reaction kinetics, but higher CO 2 pressures are needed to accomplish the desired sequestration reactions. These reactions are thermodynamically favorable but suffer from exceeding slow reaction rates. Research by others has shown that reaction rates can be accelerated by use of aqueous carbonate solutions and high CO 2 pressures. The proposed research offers an alternative rate enhancement mechanism through the use of microwave fields. Scope of Work and Task ListThe Scope of Work and list of tasks are taken directly from the original proposal. Progress and accomplishments on the tasks are then summarized in the sections that follow.Two series of experiments were proposed. In series (1) a gas stream of CO 2 containing variable amounts of water vapor would be reacted directly with the magnesium silicates at modest temperatures in a microwave cavity. The objective would be to determine the rate of reaction as a function of temperature, water vapor partial pressure, magnesium silicate particle size, and microwave power. In series (2) experiments, aqueous solutions would be reacted with CO 2 under pressures up to 80 bars and temperatures up to 200 C in a Teflon high pressure reactor in a microwave field. One objective of this portion of the research would be to compare the efficacy of microwave processing with similar compositions, temperatures and pressures without microwaves. Task 1. Adapt existing microwave equipment for continuous gas stream reaction.Task 2. React magnesium silicates with wet CO 2 at a sequence of water contents, temperatures, and microwave power levels.Task 3. Characterize products from Task 2 experiments and determine rates of reaction.Task 4. Construct equipment for microwave hydrothermal experiments.Task 5. React silicates in aqueous alkali carbonate solutions at autogeneous steam pressures in the microwave field.Task 6. React silicates in aqueous...