Chemical-looping combustion of solid fuel in a 100 kW unit using sintered manganese ore as oxygen carrier

Artikel i vetenskaplig tidskrift, 2017

Carbon capture and storage (CCS) offers the opportunity to avoid CO2 emissions from for example power plants and cement factories. Chemical-looping combustion (CLC) is one of the most promising capture technologies with potentially very low cost of CO2 capture. In this study we present findings from a solid-fuel 100 kW chemical-looping combustor. A new oxygen carrier - a sintered manganese ore called Sinaus - has been studied in the Chalmers 100 kW unit. The material has been investigated for an operational time of 51.5 h using five fuels: two bituminous coals, two types of wood char, and petcoke. The operational results clearly demonstrate the viability of the CLC process. In comparison to previously used iron-based oxygen carriers, the Sinaus material showed higher gas conversion - up to 88% - and lower loss of char to the air reactor, with carbon capture reaching as high as 100%. Furthermore, the solid-fuel conversion was higher, which is mainly an effect of the choice of fuel size. It was found that the choice of fuel has a crucial impact on performance. Previous experience has shown that the use of large fuel particles gives low carbon capture, whereas pulverized fuel leads to low solid-fuel conversion. By choosing the appropriate - intermediate - size of fuel, it is possible to combine high carbon capture with high solid-fuel conversion. Previous studies indicate that the drawback of many manganese ores is the mechanical stability. Hence, a lot of emphasis was put on an in-depth study of the lifetime of the Sinaus material. Analyzing the production rate of fines, it was found the expected lifetime of the Sinaus particles was 100-400 h. This is lower than what has been found for iron-based material, but most likely sufficient for operation in full-scale chemical-looping applications. Whilst the production of fines was highest during operation with fuel, a lot of fines were produced also during operation without fuel. Seven experiments without fuel, i.e when the observed mechanical degradation was only due to high-velocity impacts and not chemical stress caused by phase transformations, gave a lifetime in the interval 220-1230 h. In conclusion, this first-of-its-kind investigation shows that the lifetime of the oxygen carrier is related to both the change in oxygen-carrier conversion and high-velocity impacts.