CFD Analysis of Co-firing of Coke and Biomass in a Parallel Flow Regenerative Lime Kiln

Abstract

The lime industry is a highly energy intensive industry, with a huge presence worldwide. To reduce both production costs and pollutants emissions, some lime production plants are introducing more environmentally-friendly energy sources, such as local agro-industry residues. In this paper, a numerical tool is presented, which simulates a large-scale Parallel Flow Regenerative (PFR) kiln that currently uses coke as main fuel. The developed tool aims at investigating the combustion process under conditions of co-firing of coke and biomass and to assist the plant operators in the optimization of such operating conditions. To achieve this goal, a two-way coupling Euler–Lagrange approach is used to model the dynamics of the particulate phase and their interaction with the gas phase. Pyrolysis, volatiles oxidation and char oxidation are modelled by kinetics/diffusion-limited model (for heterogeneous reactions) and mixture fraction approach (for homogeneous reactions). Moreover, two methods are investigated for representing the limestone bed: a porous medium (PM) approach and a “solid blocks” (BM) tridimensional mesh. Comparison of the results for the case of 100% coke showed that the ideal “blocks” method is more accurate as it adequately simulates the scattering of fuel particles through the PFR kiln anchor, which is limited with the PM approach. Moreover, the temperature profile, maximum and minimum temperatures, as well as CO2 and O2 concentrations at outlet, are comprised in the expected range for this technology, according to available literature. Finally, the predicted results of a co-firing case with 60% biomass (in mass) were validated with measurements in an industrial facility, with production capacity of 440 calcium oxide tons per day. The results suggest that the model is fairly accurate to predict gas temperature, as well as O2 and NOX concentrations at the kiln outlet. Although some improvements are recommended to refine the CFD predictions, these promising results and the high computational efficiency laid the foundation for future modelling of co-firing of coke and biomass, as well as the modelling of the lime calcination process. It also paves the way for facilitating the reduction of pollutant emissions thus contributing to a more sustainable lime production. Graphical Abstract

Application of Flexible Tools in Magnesia Sector: The Case of Grecian Magnesite

“…A reduction of up to 50% can be obtained by sacrificing 400 • C degrees of temperature using pine, which is probably too much to keep the process inside the kiln. Great reductions of NO x have been reported for similar kilns when using a biomass [23,53]. Using olive kernel, it is possible to achieve substantial reductions in NO x emissions without affecting the temperatures greatly.…”

“…In addition, CFD simulation in industry already has a long history. Its main advantage is the capacity to realistically model the physical processes occurring in industrial assets, from furnaces to modern electrolyzers [22][23][24]. This approach provides rich information about the process at hand that can be used for the goal-oriented design, optimization or retrofitting of both the processes and equipment.…”