Abstract
The novel combustion concept Oxygen Carrier Aided Combustion (OCAC) is realized by addition of an active oxygen-carrying bed material to conventional fluidized bed boilers. The active bed material is meant to become reduced in fuel-rich parts of the boiler and oxidized in oxygen-rich parts, thus potentially providing advantages such as new mechanisms for oxygen transport in space and time. In this study, oxygen-carrier particles prepared from so called Linz-Donawitz (LD)-slag are examined as active bed material in a 12 MWth Circulating Fluidized Bed (CFB) boiler. LD-slag is the second largest by-product in steel making and is generated in the basic LD oxygen converter process. The experimental campaign lasted for two full weeks. The fuel was wood chips. LD-slag worked well from an operational point of view and no problems related to handling, agglomeration or sintering were experienced, albeit the production of fly ash increased. The boiler temperature profile suggested that fuel conversion in the main boiler body was facilitated, but the effect did not readily translate into reduced emissions from the stack. Spraying an aqueous solution of ammonium sulphate directly into the cyclone outlet with the aim of rejecting alkali metals as alkali suphates was found to solve the problems related to carbon monoxide emissions, suggesting that the problems could be due to the poor ability of LD-slag to absorb certain ash components. Use of a mixed bed consisting of 10–50 wt% LD-slag, with the remaining part being silica sand for ash absorption, also worked well. It is concluded that LD-slag could be a very cheap and readily available oxygen-carrying bed material for use in fluidized bed applications.
Highlights
The aims of this study are: (i) to examine LD-slag as bed material during Oxygen Carrier Aided Combustion (OCAC) of biomass in a circulating fluidized bed boiler with respect to operability and general behaviour, (ii) to establish whether LD-slag could provide benefits with respect to emissions compared to silica sand, (iii) to form a preliminary opinion about whether LD-slag could be commercially or environmentally attractive as bed material in fluidized bed boilers
There were no tendencies to form agglomerations or other problems commonly occurring during operation of biomass boilers
Since about one third of the bed material is replaced each day during normal operation, alkali metals and other ash components are prevented from accumulating in the bed
Summary
Fluidized bed (FB) boilers are commercially attractive as a technology used for combustion of solid fuels. For biomass and waste fuels, the fluidized bed usually consists of silica sand, in addition to char and ash, which are continuously added to the boiler via the fuel. The bed material is important for: (i) propagating heat transfer throughout the boiler; (ii) equalizing temperature in space and time;. (iii) the hydrodynamic performance of the boiler; and (iv) absorption of certain ash elements that otherwise would affect performance with respect to emissions, corrosion and sintering. While silica sand is currently the bed material of choice for biomass combustion, switching to other types of bed material is feasible and could potentially provide certain advantages. The use of a chemically active bed material is demonstrated in a large-scale combustion facility. The material used is Linz-Donawitz (LD)-slag, which is the second most abundant by-product in a typical integrated steel
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