Modeling and Simulation of an Electrified Drop-tube Calciner

Abstract

About 65% of the carbon dioxide emissions from a modern cement kiln system are generated through calcination (decarbonation). The calcium carbonate in the limestone is the primary source of CO₂, and the rest comes from fuel combustion. This gives a calciner exit gas consisting of N2, O2, CO₂, and H₂O, the CO₂ constituting up to 30 % of the mixture. In the future, electric power will have to come from renewable energy. Electrification of the calciner, i.e., replacing fuel combustion with electrically generated heat, will eliminate the fuel combustion exhaust gases. The calciner exit gas will then be pure CO₂ and removes the need for a separate CO₂ capture plant. Such a process may require a new type of calcination reactor, different from the currently used reactors in most cement kiln systems. In the current work, an electrically heated drop-tube reactor (DTR) is used to calcine the meal. The DTR may replace the traditional entrainment calciner. Essential characteristics in developing a DTR include the particle size distribution (PSD), particle settling velocity, operational temperature of the tube wall, and velocity of the product gas. A PSD ranging from 0.2 to 180 µm, where most particles have a diameter < 30 µm, was investigated. Also, to assess the effect of clustering, an effective particle diameter of 500 µm was evaluated. Two different DTR designs were compared, 1) cocurrent flow of gas and particles, 2) counter-current flow of gas and particles. The dimensions of a calcination reactor were calculated using simulations in Python 3.8. The tube diameter was selected as the key parameter to see how the overall design of the reactor was influenced.