Improvement of Oxy-FBC Using Oxygen Carriers: Concept and Combustion Performance

Abstract

Fluidized bed combustors generally exhibit good mixing characteristics, but there can be localized regions where there is insufficient oxygen to fully combust the fuel. Through incorporation of an oxygen carrier into a high pressure, pulverized fuel based oxy-fluidized bed combustor it should be possible to reduce or eliminate regions with insufficient oxygen present to complete combustion. In this paper we investigate the use of a Canadian ilmenite ore based oxygen carrier to enhance combustion performance and sulfur capture for atmospheric and pressurized oxy-FBC systems. The paper includes a description of potential pressurized system configurations with a discussion of related benefits and potential challenges. CanmetENERGY's 50 kW th pilot scale atmospheric pressure oxy-fluidized bed combustor (oxy-FBC) was used to demonstrate the concept of oxygen carrier assisted combustion using two Canadian coals with under bed fuel and sorbent injection. The CO emissions were significantly reduced by replacing the inert bed material with the oxygen carrier (ilmenite ore); with CO concentrations in the flue gas reduced up to 30 vol % and 13 vol % when burning Highvale coal and Poplar River coal, respectively. The improvement in combustion was even more pronounced in the bed region, in particular, under conditions of low excess oxygen and/or low bed temperature. Analysis of solid samples (XRD) and flue gas condensates (acidity) has provided further evidence of the oxygen carrier improving combustion performance and reducing the risk of bed agglomeration.

Ilmenite as alternative bed material for the combustion of coal and biomass blends in a fluidised bed combustor to improve combustion performance and reduce agglomeration tendency

“…This moderate level of reduction is attributed to the potential of oxygen carriers to increase the carbon conversion, which depends on the reactivity of the oxygen carrier with CO [10]. Similar results have been reported by Hughes et al [19] and Wang et al [10] using ilmenite as bed material. As bed material, ilmenite also had a positive effect on the reduction of unburnt carbon, increasing the combustion efficiency, which will be further discussed below.…”

“…to be greater at low excess air levels [19,30]. This is due to the fact that the rate of reaction of CO with gaseous oxygen is much faster than with the oxygen carrier and therefore when oxygen is readily available, e.g.…”

“…Furthermore, there was a reduction in the accumulation of deposits on the heat exchange surfaces. Hughes et al [19] investigated the use of ilmenite as an additive and alternative bed material to improve the combustion performance and sulphur capture in a 50 kWth pilot-scale fluidised bed combustor under atmospheric oxy-fuel combustion conditions using two Canadian coals. Their results showed replacing the silica sand bed material with ilmenite led to a reduction of CO emissions by up to 30% and 13% corresponding to the two coals used.…”

Investigation of the Oxygen Supply and Distribution in a Bubbling Fluidized Bed by Using Natural Ilmenite for Oxygen Carrier Aided Combustion

“…Laboratory experiments with pulverized coal in a BFB revealed a CO emission reduction of about 30% and reduced agglomeration risk. 19 Additional operation with fluidization velocities u/u mf 80−110 indicated again an increase of NO x emissions. The reason for that is seen in the reduction of local CO and hydrocarbon partial pressure, which both can probably enhance the reduction of NO x .…”

“…Further investigations of CanmetENERGY focused on oxy-pressurized fluidized bed combustion (oxy-PFCB) with oxygen carriers, where the fuel is injected under the bed. Laboratory experiments with pulverized coal in a BFB revealed a CO emission reduction of about 30% and reduced agglomeration risk . Additional operation with fluidization velocities u / u mf 80–110 indicated again an increase of NO x emissions.…”