Investigation of Sulfur Removal by Direct Limestone Injection

Abstract

Sulfur removal by direct injection of calcium-based sorbents is an attractive option for emerging coal utilization technologies. Recent experimental results indicate the feasibility of capture by direct injection into hot combustion products at temperatures that do not thermodynamically favor the absorption of sulfur by the limestone. The purpose of this study was to analytically investigate possible explanations of observed capture with the goal of evaluating the feasibility of direct limestone injection sulfur capture in emerging coal utilization systems. The method was to use current available data on the physical properties of limestone and the rates of the pertinent reactions, and to develop a mathematical model of the process experienced by the injected particles. The models were then used to predict extent of capture at the high-pressure, high-temperature, short residence time condition of interest. The goal was to first investigate capture in a single-pulse reactor (combustion bomb) and then to extrapolate these results to advanced coal-fired heat engine combustion environments. Model predictions were in good agreement with observed sulfur capture in cold wall combustion bomb studies and suggest that efficient sulfur capture (in excess of 80 percent calcium utilization) may be possible when limestone sorbents are injected into high-temperature combustion products, even when the gas temperatures exceed the thermodynamically favored temperature window by several hundred kelvins. This behavior is possible because particle temperatures are moderated and held at a level that thermodynamically favors sulfur capture due to the strongly endothermic calcinations reaction.