PRODUCT LAYER DEVELOPMENT DURING SULFATION AND SULFIDATION OF UNCALCINED LIMESTONE PARTICLES AT ELEVATED PRESSURES

Fluidized bed combustion or gasification allows for in-bed sulfur capt ure with a calcium-based sorbent such as limestone or dolomite. Sorben t particle size, porosity, internal surface, and their variation durin g conversion. have great influence on the conversion of the sorbent. T he uptake of SO2 and H2S by five physically different limestones is di scussed, for typical pressurized fluidized bed combustor or gasifier c onditions: 850/950 degrees C, 15/20 bar. Tests were done in a pressuri zed thermogravimetric apparatus (P-TGA), the size of the limestone par ticles was 250-300 mu m. It is stressed that the limestones remain unc alcined. A changing internal structure (CIS) model is presented in whi ch reaction kinetics and product layer diffusion are related to the in traparticle surface of reaction, instead of the outer particle surface as in unreacted shrinking core (USC)-type models. The random pore mod el was used for describing the changing internal pore and reaction sur faces. Rate parameters were extracted for all five limestones using th e CIS model and a USC model with variable effective diffusivity. Diffe rences in the sulfur capture performance of the limestones were evalua ted. Plots of the CaSO4 or CaS product layer thickness as a function o f conversion are given, and the relative importance of limestone poros ity and internal surface is discussed.