Properties of oxygen carriers based on natural and waste materials at high degrees of reduction

Licentiate thesis, 2022

Oxygen carriers play an important role in multiple energy conversion processes, such as chemical-looping technology and oxygen-carrier-aided combustion. This thesis particularly explores how a high reduction degree of an oxygen carrier can affect its performance. Oxygen carriers do not usually suffer from a high reduction degree in a normal operation of the widely known chemical-looping combustion. Still, high degrees of reduction may appear locally or during unoptimized conditions, as well as when partial oxidation of fuel is required, such as in chemical-looping gasification. This may lead to performance issues such as lowered reactivity, particle breakage, bed agglomeration, or even defluidization. The main experiments in this thesis were performed in a fluidized bed batch reactor, where it is possible to alternate the feeding of oxidizing and reducing gases, thus mimicking the situation of the air and fuel reactors in a real chemical-looping setup. All oxygen carriers studied here were non-synthesized, either from natural ores or industrial by-products, and are thus inexpensive.

 

The reactivity of the oxygen carriers toward relevant gaseous fuels is an important parameter. It was found that the syngas conversion decreased at higher reduction degrees of oxygen carriers. The by-products, which contain a lower iron content, saw a quicker decrease of syngas conversion compared to that of the ore-based materials. The same trend was seen on the methane conversion. The decrease seen toward different fuels was due to the exhaustion of the available oxygen in the oxygen carrier particles.

 

The fluidization performance of the bed particles is critical in a fluidized bed setup, a common arrangement for chemical-looping processes. Defluidization was observed on oxygen carriers based on iron ore materials at higher reduction degrees set by syngas, while the iron-based by-products and manganese ores did not defluidize at all. This was caused by the formation of wüstite and/or elemental iron under a highly reducing environment, which later migrated to the particles’ surface and triggered bed agglomeration. Manganese ores seem to be less prone to defluidization compared to the iron ores under highly reducing environment.

 

The conversion of pine forest residue char, a biomass-based fuel, was investigated at high reduction degrees using ilmenite and iron sand. The hydrogen partial pressure in the bed during the char conversion increased as the oxygen carrier reduction degree increased. The increasing hydrogen partial pressure caused a hydrogen inhibition effect, which subsequently slowed down the char conversion rate. The increasing hydrogen partial pressure was possibly caused by the equilibrium shifting in the water-gas shift reaction and/or the decreasing reactivity of the oxygen carriers themselves. The mass conversion degree thereby affected the char conversion indirectly. There was no significant difference between the reactivity of the char with ilmenite and that with iron sand at higher reduction degrees.

 

Given these findings, high reduction degrees of oxygen carriers during a chemical-looping operation should preferably be avoided to minimize the risk of performance issues. Despite this, the performance of several oxygen carriers would probably not be affected significantly by a highly reducing environment alone.