| Gasification is technology that has seen increasing international interest through its ability to convert solid, carbonaceous feedstocks (i.e., coal or biomass) into energy, fuels, and chemicals. Additionally, gasification is considered a "clean" technology as harmful emissions and greenhouse gases are readily sequestered. The gasification of a carbonaceous material proceeds in two primary steps: the devolatilization of the feedstock followed by the gasification reactions of the remaining carbon (char). As the gasification reactions are significantly slower than combustion reactions or the devolatilization step, these reactions are rate limiting. While most modern commercial petrochemical processes are designed with a fundamental understanding of the reactions involved, the complex nature of gasification has prevented this for being true of gasification processes---the coupled heterogeneous and homogeneous kinetics, multi-mode heat transfer, fluid dynamics, and variability in feedstocks, among other factors, makes measuring or modeling these conversion rates challenging. In this study, we focus solely on char reactions with steam at high temperature and pressure (i.e., conditions that are applicable to entrained-flow gasification).;The feedstock used in this study was a char "surrogate" (a coal-derived activated carbon) which was thoroughly characterized to provide data regarding its composition, physical structure, and reactivity. Activated carbon was identified as a potential surrogate for char as it is physically consistent, has characteristics similar to that of traditional char, and is available for other researchers to acquire. Thorough characterization of the feedstock used in a study is critical as the gasification rate is heavily dependent on the feedstock properties (such as composition, surface area, pore size distribution, etc.) and it allows for the results from different studies to be compared. In order to perform detailed char gasification measurements, a novel experimental setup was designed to allow for single, macro char particles (nominally 2 mm) to be inserted into our reactor, exposed to a high temperature and high pressure steam environment, and then removed all while operating at elevated temperatures and pressures. This setup is unique as it allows for a single particle to be exposed to steam in a controlled environment with known boundary conditions that can be readily modeled. Additionally, the mass of the particles were measured before and after exposure to the steam environment using a microbalance and thermogravimetric analysis providing accurate measurement of the extent of carbon conversion. Experiments were conducted at temperatures of 1000 °C to 1400 °C and pressures between 1 bar and 15 bar. The resulting conversion data was analyzed using the random pore model, shrinking core model, and volumetric model, of which the random pore model provided the best fit. Apparent activation energy, frequency factor, and reaction order were calculated and agree well with literature values and trends. Finally, the conversion data from this study were compared against two studies in the literature that were performed under similar conditions and general agreement was found after considering the differences in experimental setup. |
