Oxygen-Carrier-Aided Combustion in a Bench-Scale Fluidized Bed
Journal article, 2017
In a circulating fluidized bed (CFB) boiler, one of the basic functions of the bed materials is to transport heat between different parts of the boiler to clecrease heat gradients and, thereby, smoothen the operation. A novel concept, oxygen Carrier-aided combustion (OCAC), replaces some or all of the bed materials with an oxygen carrier, which is a metal oxide that, through chemical redox reaction, not only transports heat but also transports oxygen between oxygen-lean and oxygen-rich areas of the boiler. The oxygen carrier also acts as internal oxygen storage, which increases the total amount of available oxygen in the boiler. The concept of OCAC was demonstrated by Thunman et al. at Chalmers University of Technology, where the bed material (sand) in a 12 MW boiler was replaced with iron titanium mineral (ilmenite). In this work, a small fluidized bed reactor is used to mimic the condition in OCAC with regard to fuel conversion as well as NO formation and to test other oxygen carrier, materials. Four oxygen carriers were tested, two ores (ilmenite and manganese ore), and two Oxide scales (denoted as AQS and LDst), which were byproducts from the steel production of the Swedish company SSAB. Sand was also used as a reference. Wood char was used as the fuel, and concentrations of CO2, CO, CH4, O-2, and NO in the exhaust gases were monitored. The effect of oxygen carriers on combustion efficiency and NO emission was investigated at different air/fuel ratios. The results Show that, utilization of an oxygen carrier reduces the level of CO emission, thereby increasing the efficiency of combustion. It is also Shown that the increase In combustion efficiency can be largely attributed to the reactivity of the oxygen carriers with CO. With OCAC, it was possible to reduce the level of excess air and, thereby, lower the level of NO emissions while maintaining the same level of CO emission.