Computationally Inexpensive CFD Approach for the Combustion of Sewage Sludge Powder, Including the Consideration of Water Content and Limestone Additive Variations

Abstract

As a result of growing interest in the thermal treatment of sewage sludge with methods suchas combustion, gasification or pyrolysis, and also in processes that aim to recover precious componentssuch as phosphorus from this waste, a growing demand has been observed for Computational FluidDynamics (CFD) models that provide solutions rapidly and accurately for efficient application inresearch and development. This study was carried out to develop a computationally inexpensivemodelling approach for the combustion of pulverized sewage sludge in entrained flow furnaces.Sewage sludge is a very volatile-rich fuel. Therefore, the Steady Diffusion Flamelet model (SFM), incombination with a validated skeletal reaction mechanism, was applied to consider the pulverizedfiring of sewage sludge. It was possible to represent the complex composition of volatiles emittedfrom the sludge particles by releasing surrogate fuels. In addition, the influence of limestoneadditive (calcination reaction) and varying water content (water–gas shift reaction) was investigatedexperimentally and modelled via CFD. The simulation results confirm that the surrogate fuel approachis valid and can be used to describe pulverized sewage sludge effectively. The temperature andspecies concentration results, including the influence of the additive and different levels of watercontent, were confirmed by experimental data, which is usually hard to obtain due to the tendency ofPSS to form agglomerates in entrained flow combustion furnaces. The model yields plausible andexperimentally validated results for the combustion of sewage sludge powder across a wide range ofoperating conditions.