Using (MnxFe1-x)2SiO5 As Oxygen Carriers for Chemical-Looping With Oxygen Uncoupling (CLOU)

Using (Mn_xFe_1-x)₂SiO₅ as oxygen carriers for chemical-looping with oxygen uncoupling (CLOU)

Chemical-looping with oxygen uncoupling (CLOU) utilizes oxygen carrier materials which can release gas phase oxygen in the fuel reactor.  This could be beneficial for the fuel oxidation process, especially when considering solid fuels, as the fuel will be converted through normal combustion, which is not the case with normal chemical-looping combustion.  Copper oxide is the most widely investigated oxygen carrier for the CLOU technology, but recently a number of combined oxides have been proposed as feasible oxygen carriers.  One especially interesting mixed oxide system is the Fe-Mn-O combination, which releases oxygen at relevant temperatures and partial pressures of oxygen.  In earlier investigations by our research group at Chalmers, the behavior and functionality of this system has been shown to be highly dependent upon the molar ratio of Fe/Mn.  However, the stability during operation has not been satisfactory, with considerable attrition found when used in continuous operation.  Hence it is of interest to combine oxygen carriers of this system with active or inactive material in order to improve stability while keeping the reactivity intact.  In this work, the effect of SiO₂ addition to the Fe-Mn-O system has been investigated.  Three oxygen carriers based on the Fe-Mn-Si ternary were manufactured by spray-drying and evaluated with respect to parameters important for CLOU.  The general composition of the material was (Mn_xFe_1-x)₂SiO₅, with x=0.33, 0.5 and 0.67.  After the spray-drying procedure the material was calcined at 1100 and 1200°C for 4h and sieved to a size fraction of 125-180 mm.  The oxygen release properties and the reactivity with methane were investigated using a batch fluidized batch reactor made of quartz.  The uncoupling properties were investigated in the temperature interval 850 – 1050°C by exposing a sample of 15 g of particles to a flow of N₂, and then measuring the released oxygen as a function of time.  For the investigations with methane as the reducing gas, the oxygen carrier particles were exposed to a CH₄ flow of 450 Nml/min in the fluidized bed for 20 s.  The extent of reaction during the reduction was established by measurement of CO₂, CO and CH₄ in the outlet from the reactor. After reduction, the reduced particles were oxidized in 5% O₂.    All of the materials showed oxygen release in the investigated temperature range, although there was not a huge variation with temperature. The (Mn_0.67Fe_0.33)₂SiO₅ showed the highest release rate at 900°C, with 0.8% O₂ measured after 360 s reduction.  The reactivity with methane increased with temperature for the (Mn_0.67Fe_0.33)₂SiO₅  and (Mn_0.33Fe_0.67)₂SiO₅ materials, and the average conversion approached 98% for the Mn-rich oxygen carrier.  The oxygen carrier with a 1:1 molar ratio of Fe:Mn, i.e. (Mn_0.5Fe_0.5)₂SiO₅, had remarkably high reactivity already at 850°C, with an average methane conversion of 90%, which decreased somewhat at 900 and 950°C.   The investigated particles had a decent crushing strength, around 1 N, and little or no attrition was seen in the tests.  From the current investigation it can be established that the addition of SiO₂ to the Fe:Mn system could be feasible, as the oxygen uncoupling properties seem to remain intact and the reactivity with fuel gas high at certain conditions.  However, the mechanical stability of these type of materials need to be further investigated, to establish whether there is an advantage in comparison to pure Fe:Mn-oxides.