Pyrolysis characteristics of pulverized coal in circulating fluidized bed with a multi-section variable-diameter riser

Abstract

An innovative circulating fluidized bed (CFB) structure with a multi-section variable-diameter riser was proposed to pyrolyze pulverized coal for tar production in this study. Experimental investigations on pulverized coal pyrolysis were conducted to explore the operational characteristics and pyrolysis product distribution of this specially designed CFB reactor, as well as the interconversion mechanisms among distinct pyrolysis products. Results show that the riser’s constricted structure enables the spatially partitional reaction regime: a high-temperature dilute-phase char partial gasification occurs below the constricted section, while a low-temperature dense-phase coal pyrolysis above the constricted section. By adjusting the oxygen concentration and equivalence ratio in gasification zone, the sensible heat and composition of gas-solid heat carrier (GSHC) can be effectively controlled, thereby changing the temperature of the pyrolysis zone. Whether it is the increase of oxygen concentration or the increase of equivalence ratio would lead to the increase of syngas content and the decrease of char content. Under the same pyrolysis temperature, the increase of oxygen concentration in gasification zone led to the increase of pyrolysis tar content. A simple molecular model of pulverized coal was established by the FTIR and Raman analysis to reveal transformation of functional groups. The aromatic ether and carboxylic acid, situating at the periphery of molecular model, served as primary sources of oxygen-containing substances. Aliphatic hydrocarbons underwent condensation reaction around 600 ˚C, which enhanced the concentration of larger polycyclic aromatic hydrocarbons in pyrolysis tar.