New Development in EfW Boiler Process Modeling: Fully Integrated CFD Model

Abstract

This paper focuses on a Computational Fluid Dynamics (CFD) application to Energy-from-Waste (EfW) boiler systems, which are much smaller than utility boilers, but typically have more complicated design and performance issues due to the nature of their fuel: municipal solid waste (MSW). The majority of the commercially available CFD software packages have impressive capabilities in homogeneous flow modeling, which make them very suitable for gas fired boilers and with some reasonable simplifications to oil and pulverized coal fired boilers. But this is not the case for EfW boilers, especially mass-burn technology, where MSW combustion on the grate is the “heart” of the process. These boilers have two interacting phases: MSW burning on the grate and combustion products with entrained fly ash particles above the grate. This challenge has been recognized from the very first applications of CFD modeling in the EfW industry. This paper describes several approaches to numeric modeling of MSW boilers. In the most successful cases, two different models have been built: one for the grate combustion and another for the homogeneous gas flow, with back-and-forth iterations between these two models. Such an approach has given Covanta Energy (Covanta) a good start in its CFD modeling program. A number of models have been built, tested and validated, resulting in several successful project executions. However, some serious limitations have been found in this approach as explained in this paper. Recognizing these deficiencies, Covanta has recently made a new significant step in its CFD development program by creating an innovative, fully integrated CFD model that comprises solid fuel combustion on the grate, gaseous phase interactions above the grate, and the rest of the boiler. While this integrated model and its validation are still in progress, Covanta is moving forward on its application to on-going projects.