Sulfur interactions with rock ilmenite during combustion of biomass in a 12MW_(th), CFB boiler

摘要

Fluidized bed combustion is recognized for its ability to convert complex fuels, such as fuels with high heterogeneity, varying ash amounts and moist concentrations. The fluidized bed technology plays an important role for the Swedish heat and power generation systems where some of the fuels used during the last decades have been biomass and waste. A common problem during conversion of biomass and waste is the formation of alkali chlorides (mainly potassium-chloride). As alkali-chlorides interact with the heat transferring surfaces in the combustor and convection pass, these can cause corrosion and fouling that can result in unplanned operational stops and increased maintenance costs. Common measure to avoid alkali deposition and subsequent corrosion is to add sulfur, a process known as alkali sulfation. By converting the formed chlorides into sulfates one can greatly reduce the risk of high-temperature corrosion on heat transferring equipment in the furnace and in the flue gas pass. Traditionally, silica-sand has been used as bed material in FB-units but today oxygen carrying materials are gaining interest as they increase the oxygen transfer in the furnace and thereby even out the temperature profile in the reactor and thereby decrease further the risk of corrosion. Several studies have been performed aiming to describe the interaction between silica sand and sulfur/formed sulfates. Still, to the knowledge of the authors, no such study has been published on sulfur interactions with oxygen carrier where more research is needed. In this study, sulfur/bed material interactions are addressed during Oxygen Carrier Aided Combustion (OCAC) conditions, where ilmenite, the oxygen carrying bed material (in this case a metal oxide) is used both as a heat carrier and media for increased oxygen transport. The study was performed in the Chalmers 12MW_(th) CFB boiler. The purpose was to evaluate the physicochemical properties of ilmenite during continuous addition of elemental sulfur in the boiler. The boiler was operated for 2 weeks with wood chips as fuel and with no bed material regeneration. A continuous sampling procedure allowed for a time resolved bed material collection and investigation of the physicochemical evolution throughout the experimental study. The aim has been to describe the interactions between sulfur and ilmenite at different time intervals. SEM/EDX method was used to follow the morphological and chemical changes in the bed material as a result of the sulfur addition.