Experimental Investigation of Oxygen Carrier Aided Combustion (OCAC) with Methane and PSA Off-Gas

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.

Oxygen Carrier Aided Combustion in Fluidized Bed Boilers in Sweden—Review and Future Outlook with Respect to Affordable Bed Materials

“…A laboratory-scale experimental study demonstrated that the conversion of methane to a larger extent takes place inside the bed when an oxygen carrier is used, compared to with inert silica sand. This shows that the heterogenous solid-gas reaction is significant [33]. The oxygen carrier is also acting as an oxygen buffer, meaning that the boiler could be better at handling a sudden increase in fuel feed.…”

Combustion of Biomass in Fluidized Beds: A Review of Key Phenomena and Future Perspectives

“…Switching from inert sand to an active material improves the oxygen distribution in the combustor and reduces the concentration of unburned components in the off-gas . Additionally, this approach leads to lower NO emissions because the active material promotes fuel conversion within the bed, minimizing flame combustion in the freeboard . Oxygen carrier-aided combustion can be readily implemented in the existing combustors if the hydrodynamics with the new bed material can match the design specification.…”

“…322 Additionally, this approach leads to lower NO emissions because the active material promotes fuel conversion within the bed, minimizing flame combustion in the freeboard. 323 Oxygen carrier-aided combustion can be readily implemented in the existing combustors if the hydrodynamics with the new bed material can match the design specification. The attractiveness of this approach also arises from the application of inexpensive carriers, such as ilmenite, 322,324 steel converter slag, 325 or natural ores.…”

An improved method for feeding ash model compounds to a bubbling fluidized bed – CLC experiments with ilmenite, methane, and K2CO3

“…Apart from creating an oxygen buffer in the bed, the oxygen carrier also enables solid‐gas reactions that are less temperature‐dependent than flame combustion. Laboratory‐scale experiments have shown that methane can be converted in the bottom bed, where the temperature is too low for proper flame combustion 4 …”

“…Laboratory-scale experiments have shown that methane can be converted in the bottom bed, where the temperature is too low for proper flame combustion. 4 OCAC has been implemented in several biomass-fired and waste-to-energy (WtE) plants in Sweden. The implementation has been done with an ilmenite-based bed material commercialized by Improbed AB (a subsidiary of the utility company E.ON.).…”