Carbon stripping is critical to preventing char particles from reaching air reactors. In this study, a riser-based carbon stripper is proposed to separate the lighter char particles from the heavier oxygen carrier particles. The performance of the proposed riser-based carbon stripper in the separation of plastic beads from ilmenite particles was experimentally investigated, where plastic beads were used to simulate the behavior of char particles. The effects of gas velocity and solids circulation rate on the separation characteristics were studied. The results showed that 2−63 wt % mixtures can be entrained, containing 17−100 wt % plastic beads. An appropriate gas velocity range for separating plastic beads from ilmenite particles is 2−2.25 m/s. The appropriate mixture feeding rate is below 23.7 kg/m 2 •s. The power cost analysis shows that the riser-based carbon stripper is more efficient than the larger fuel reactor strategy in increasing CO 2 capture efficiency when using solid fuels that require long residence times.
In chemical looping combustion of solid fuels, carbon slip from the fuel reactor to the air reactor is a critical issue. This paper presents the design and experimental evaluation of a carbon stripper (CS) in a cold flow model of a 70 kW chemical looping combustor of solid fuels. In the dual fluidized bed system, stable long-term operation was achieved. Plexiglas beads simulating the behavior of fuel particles were mixed with ilmenite particles to study the separation characteristics of the carbon stripper. The factors affecting the carbon stripper performance, such as the gas velocity in the carbon stripper, the solids circulation rates, and the inner structure of the carbon stripper, were experimentally investigated. The results showed that the separation efficiency of the Plexiglas beads was in the range of 0 to 60%. The carbon stripper added 160% additional residence time in the reducing atmosphere for the fuel particles.
The carbon stripper (CS), which is a fluidization bed aimed at separating char particles from oxygen carriers during coal-fired chemical looping combustion (CLC), is vital for achieving high carbon capture efficiency of a CLC system. An effectively designed CS could transport most char particles back to the fuel reactor and simultaneously allow most oxygen carriers to reach the air reactor. An annular carbon stripper was designed, and a cold model apparatus was built for operation and optimization. The CS consists of an annular fluidized bed and a center riser. The riser was inserted into the annular fluidized bed, and the fluidized bed was divided into the annular zone and the cylindrical zone. Plastic beads were used to simulate char particles, and ilmenite was used as the oxygen carrier. The effect of operational parameters (solid feeding rate and gas velocities) and particle properties (average size of plastic beads and mass concentration of plastic beads) on the separation efficiencies of plastic beads and ilmenites was investigated in detail. The main parameters of the CS structure (the length of the annular zone and the diameters of the riser and annular fluidized bed) were studied and optimized. The axial distribution of the solid volume fraction and the mass concentration of light particles along the annular fluidized bed were measured, and the fluidization behavior in the CS was analyzed. The separation process in the annular CS and the important factors influencing the separation of binary particles were discussed. Under the optimized structure and operational conditions, the annular CS could be an effective apparatus to completely separate char particles from oxygen carriers, which could greatly improve carbon capture efficiency during the operation of a coal-fired CLC.
The oxidation of ammonia to NO x and N 2 was investigated under conditions that pertain to the fuel reactor during the chemical-lopping combustion of coal with ilmenite. The catalytic decomposition of NH 3 , the oxidation of NH 3 over ilmenite, and the reduction of NO by reduced ilmenite and NH 3 were studied experimentally. The catalytic decomposition of NH 3 , NO reduction by NH 3 , and reduced ilmenite were found to be important for N 2 formation. NH 3 oxidation over ilmenite was the only way in which NO could be formed in this system, with around 18% of the NH 3 being converted to NO at 850−950 °C. The oxidation of NH 3 was only slightly influenced by the reactor temperature but was strongly influenced by the concentrations of NH 3 and syngas. NO formation was promoted by high concentrations of NH 3 and decreased by high concentrations of syngas. The selectivity of the NH 3 toward NO formation was favored at low concentrations of NH 3 and syngas. The conversion of NH 3 was complete in most cases, although 15−25% of the NH 3 was not converted when the inlet syngas concentration increased to levels higher than 10−30%.
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