High Temperature Gasdynamics Laboratory

Thermosciences Group

Mechanical Engineering Department

Stanford University

Abstract

Coal is used as the energy source for about one-third of the electric power generated worldwide. Coal is the cheapest and most abundant source of energy on earth and because of this, it will continue to be the leading source of energy for generating electric power for the next several decades. When burned, each kilogram of carbon in coal produces about 3.7 kg of CO₂ that can end up in the atmosphere if nothing is done to control power plant emissions. Because of the role that CO₂ may play in contributing to global warming, it is prudent to develop advanced combustion and gasification schemes that limit CO₂ emissions. Evaluation of any of the proposed schemes in a timely and cost-effective manner will require predictive capability. To this end, fundamental studies on coal and biomass combustion and gasification phenomena were undertaken in order to gain the understanding needed to develop models that predict accurately coal and biomass conversion behaviors in environments likely to be established in advanced energy systems.

In this presentation, our latest research on the intrinsic chemical reactivity of coal and biomass chars to oxygen and carbon dioxide and our research on the mode of char particle burning at high temperatures and pressures will be discussed. The experimental procedures used to measure char reactivity properties and the models developed to describe the variations in the properties with char conversion will be briefly described. Of particular interest is our direct numerical simulation of a burning char particle from which we were able to characterize the reductions in size and apparent density and the variations in specific surface area that occur when char particles burn under conditions in which the combined effects of chemical reaction and pore diffusion control the overall char conversion rates. The model was used to establish a relationship between the effectiveness factor and Thiele modulus when a four-step heterogeneous reaction mechanism is employed to characterize the conversion of carbon to CO and CO₂ for particles having uniform internal structures. Relations were also derived for particles having cenospherical- and mixed-type morphologies so that the behaviors of the types of char particles produced at high pressures can be accurately described as they are converted to gaseous species.

The presentation will conclude with a brief overview of our research on development of an aquifer-based electric power generation scheme that combines coal conversion in supercritical water with CO₂ capture and sequestration. There are no gaseous emissions in this novel coal conversion technology. The supercritical water containing dissolved coal combustion products is returned to the aquifer after the sensible energy is transferred to a heat engine, and the precipitated solid matter is disposed of in conventional ways. Our research on development of a direct coal fuel cell will also be discussed.