Modeling of the oxy-combustion calciner in the post-combustion calcium looping process

Abstract

The calcium looping process is a fast-developing post-combustion CO 2 capture technology in which combustion flue gases are treated in two interconnected fluidized beds. CO 2 is absorbed from the flue gases with calcium oxide in the carbonator operating at 650 ºC. The resulting CaCO 3 product is regenerated into CaO and CO 2 in the calciner producing a pure stream of CO 2. In order to produce a suitable gas stream for CO 2 compression, oxy-combustion of a fuel, such as coal, is required to keep the temperature of the calciner within the optimal operation range of 880-920°C. Studies have shown that the calcium looping process CO 2 capture efficiencies are between 70 % and 97 %. The calciner reactor is a critical component in the calcium looping process. The operation of the calciner determines the purity of gases entering the CO 2 compression. The optimal design of the calciner will lower the expenses of the calcium looping process significantly. Achieving full calcination at the lowest possible temperature reduces the cost of oxygen and fuel consumption. In this work, a 1.7 MW pilot plant calciner was studied with two modeling approaches: 3-D calciner model and 1-D process model. The 3-D model solves fundamental balance equations for a fluidized bed reactor operating under steady-state condition by applying the control volume method. In addition to the balance equations, semiempirical models are used to describe chemical reactions, solid entrainment and heat transfer to reduce computation effort. The input values of the 3-D-model were adjusted based on the 1-D-model results, in order to model the behavior of the carbonator reactor realistically. Both models indicated that the calcination is very fast in oxy-fuel conditions when the appropriate temperature conditions are met. The 3-D model was used to study the sulfur capture mechanisms in the oxy-fired calciner. As expected, very high sulfur capture efficiency was achieved. After confirming that the 1-D model with simplified descriptions for the sorbent reactions produces similar results to the more detailed 3-D model, the 1-D model was used to simulate calcium looping process with different recirculation ratios to find an optimal area where the fuel consumption is low and the capture efficiency is sufficiently high. It was confirmed that a large fraction of the solids can be recirculated to both reactors to achieve savings in fuel and oxygen consumption before the capture efficiency is affected in the pilot unit. With low recirculation ratios the temperature difference between the reactors becomes too low for the cyclic carbonation and calcination. As a general observation, the small particle size creates high solid fluxes in the calcium looping process that should be taken into account in the design of the system.

Effect of dolomite decomposition under CO2 on its multicycle CO2 capture behaviour under calcium looping conditions

“…Firstly, the high temperature required to achieve a complete regeneration of the sorbent in short residence times since calcination has to be carried out under the high CO 2 partial pressure calciner environment [11][12][13][14]. Such high temperatures would be attained by oxy-combustion in order to avoid CO 2 dilution, which entails a marked energy penalty to the process due to fuel consumption and additional CO 2 production [9,10,[14][15][16][17][18].…”

A new model of the carbonator reactor in the calcium looping technology for post-combustion CO2 capture

“…The most feasible method to generate the heat 20 required in the calciner while at the same time avoiding CO 2 dilution is to burn coal by oxy-combustion [4,[11][12][13][14].…”

“…concentration of CO 2 at atmospheric pressure [11,23,[25][26][27]. Calcination kinetics becomes extremely slow under high CO 2 partial pressures nearby equilibrium (T ≈ 870-900ºC) [22].…”

“…CFBs are typically operated at atmospheric pressure under the fast fluidization regime, with gas velocities of the order of 5-10 m/s [9,10]. The partially carbonated particles are driven into a second fluidized bed reactor (calciner), where CaO is regenerated by calcination under high temperatures and necessarily high CO 2 concentration [11][12][13][14]. 20 Once CO 2 is captured in the carbonator and heat from the exothermic reaction is recovered, the flue gas almost free of CO 2 is released into the atmosphere.…”

Calcium looping process simulation based on an advanced thermodynamic model combined with CFD analysis

“…Results were compared with data from a 30 kW test rig of INCAR-CSIC [25]. In an updated study, the model was upgraded thus including the returning system of the CFB reactors and a more detailed description of the reactions taking place within the calciner [26]. In the University of Ottawa, the Endex CaL simulator has been recently developed, which is based on the thermal coupling between the two reactors [27].…”