The direct solid-solid reaction between coal char and iron-based oxygen carrier and its contribution to solid-fueled chemical looping combustion

Abstract

Chemical looping combustion (CLC) is an advanced technology developed to achieve highly efficient fuel combustion with in-situ CO2 capture. In this process, metal oxide particles are used as an oxygen carrier (OC) to transport lattice oxygen for fuel combustion. In this process, a stream of CO2 and steam is produced by successful separation of atmospheric N2 and the gaseous product of combustion. In CLC of solid fuel, metal oxide particles are physically mixed and react with solid fuel at high temperature using gasification enhancer, such as steam, or CO2. A full understanding of the reaction mechanism between the OC and solid fuel is vital for OC development and the fuel reactor design. Several reactions may be involved in solid-fueled CLC when an iron-based OC is used, including (1) solid fuel devolatilization /gasification, (2) OC reduction with intermediate syngas, (3) the solid-solid reaction between OC and solid fuel via direct contact, and (4) the homogeneous water-gas shift reaction. The former two reactions have been extensively studied in recent years. This study focuses on the third reaction, the solid-solid reaction, which occurs thermodynamically at typical operational temperatures of CLC. The direct solid-solid reaction between coal char and two iron-based OCs via random particle collision in a fluidization bed regime was investigated and focuses on the reaction kinetics and the carbon conversion at different temperature. The contribution of the solid-solid reaction to the global carbon conversion was estimated for steam-gasified CLC at different temperature. The solid-solid reaction via static contact in a thermal-gravimetric analyzer (TGA) was also tested to evaluate the role of different OCs and to better understand the reaction mechanism between the two solid particles.

Activation of ilmenite as an oxygen carrier for solid-fueled chemical looping combustion

“…The experiment using SK 2 O(5)-I, FG-I-K 2 O(5), or SCuO(5)-I, experienced an appreciable amount of coal char oxidation in the N 2 stream, with only CO 2 present in the exit gas. This observation is a manifestation of the direct solid-solid reaction between OC and coal char [24]. As an example, Fig.…”

“…If SCuO (5)-I is considered as a physical mixture, containing 95% ilmenite and 5% CuO, the theoretical value of △X OC in the wet syngas is 4.23%, which very close to the experimental data (△X OC =4.14% for SCuO (5)-I). The oxygen uncoupling effect (CLOU) [24] was not seen during the heating of SCuO (5)-I in the TGA experiments. During the two stage-reductions, the fast reduction stage (the first 55 min) accounted for 97% of △X OC .…”

“…So the results revealed to what extent the wet syngas can be oxidized at different OC conversion. The schematic diagram and description of the fixed-bed facility can be found elsewhere [24]. This facility was also used in fluidized-bed mode for the coal char-fueled CLC experiments.…”

“…Compared to in-situ gasification in CLC, the solid-solid reaction between SK 2 O (5)-I, FG-I-K 2 O (5), or SCuO (5)-I and coal char was slow, but proceeds to a high level of carbon conversion. The relative reaction rate at a given carbon conversion (α -the ratio of the carbon conversion rate due to the solid-solid reaction to that of CLC) can be used to describe the role of the solid-solid reaction (the detailed definition can be found elsewhere [24]). Using this calculation, the average value of α, in the range of X C = 0.1-0.6, was 3.3% and 3.2% for SCuO(5)-I OC and FG-I-K 2 O (5), respectively.…”

Development and Particle Scale Modeling of Metal Oxide Oxygen Carriers in Chemical Looping Applications

“…The unreacted shrinking core model was first developed by Yagi and Kunii [43] refined by others [44] and used for CL applications [45] [46] [47]. The derivation is based on differential shell balances on a reacting sphere of which three characteristic resistances were considered for the generalized gas-solid reaction as displayed in (E3-1.…”