Abstract
Oxygen carrier aided combustion (OCAC) is utilized to promote the combustion of relatively stable fuels already in the dense bed of bubbling fluidized beds by adding a new mechanism of fuel conversion, i.e., direct gas–solid reaction between the metal oxide and the fuel. Methane and a fuel gas mixture (PSA off-gas) consisting of H2, CH4 and CO were used as fuel. Two oxygen carrier bed materials—ilmenite and synthetic particles of calcium manganate—were investigated and compared to silica sand, an in this context inert bed material. The results with methane show that the fuel conversion is significantly higher inside the bed when using oxygen carrier particles, where the calcium manganate material displayed the highest conversion. In total, 99.3–99.7% of the methane was converted at 900 °C with ilmenite and calcium manganate as a bed material at the measurement point 9 cm above the distribution plate, whereas the bed with sand resulted in a gas conversion of 86.7%. Operation with PSA off-gas as fuel showed an overall high gas conversion at moderate temperatures (600–750 °C) and only minor differences were observed for the different bed materials. NO emissions were generally low, apart from the cases where a significant part of the fuel conversion took place above the bed, essentially causing flame combustion. The NO concentration was low in the bed with both fuels and especially low with PSA off-gas as fuel. No more than 11 ppm was detected at any height in the reactor, with any of the bed materials, in the bed temperature range of 700–750 °C.
Highlights
In bubbling fluidized bed reactors with silica sand, it has been observed that most of the methane is converted in the freeboard [1,2] when fluidizing with air and methane/natural gas at moderate temperatures
If it is possible to display experimental results, showing that it is possible convert the methane inside the dense bubbling bed, it should be possible to convert other targeted gaseous fuels with most common oxygen carriers. This reasoning is in line with the results presented by Ortiz et al [25], for example, where it was displayed that it was more difficult to convert methane than PSA off-gas in a fluidized bed with oxygen carrier particles
The bed heights were estimated for the three bed materials at all bed temperatures
Summary
In bubbling fluidized bed reactors with silica sand, it has been observed that most of the methane is converted in the freeboard [1,2] when fluidizing with air and methane/natural gas at moderate temperatures. Methane is a very stable gaseous fuel. This is connected to the fact that it is sterically hindered, due to the symmetrical tetrahedral structure and the activation of the C-H bond with a dissociation energy of 439 kJ/mol [3]. In a bubbling fluidized bed, the solid particles effectively hinder the combustion of methane. Due to the presence of particles, heat is transferred to the surrounding particles in contact with the bubble [5], which otherwise could result in a local temperature increase, which could lead to ignition
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