Effects of Design/Operating Parameters and Physical Properties on Slag Thickness and Heat Transfer during Coal Gasification

Insoo Ye,Junho Oh,Changkook Ryu

doi:10.3390/en8053370

Abstract

The behaviors of the slag layers formed by the deposition of molten ash onto the wall are important for the operation of entrained coal gasifiers. In this study, the effects of design/operation parameters and slag properties on the slag behaviors were assessed in a commercial coal gasifier using numerical modeling. The parameters influenced the slag behaviors through mechanisms interrelated to the heat transfer, temperature, velocity, and viscosity of the slag layers. The velocity profile of the liquid slag was less sensitive to the variations in the parameters. Therefore, the change in the liquid slag thickness was typically smaller than that of the solid slag. The gas temperature was the most influential factor, because of its dominant effect on the radiative heat transfer to the slag layer. The solid slag thickness exponentially increased with higher gas temperatures. The influence of the ash deposition rate was diminished by the high-velocity region developed near the liquid slag surface. The slag viscosity significantly influenced the solid slag thickness through the corresponding changes in the critical temperature and the temperature gradient (heat flux). For the bottom cone of the gasifier, steeper angles were favorable in reducing the thickness of the slag layers.

Highlights

IntroductionBecause the syngas temperature typically increases above the ash melting point, most of the coal ash is molten and deposits onto the gasifier wall to form slag layers
Many commercial coal gasification processes such as the Shell, Prenflo, Mitsubishi, Tsinghua and Siemens ones utilize entrained flow reactors feeding pulverized coal in dry or slurry forms [1,2,3].Because the syngas temperature typically increases above the ash melting point, most of the coal ash is molten and deposits onto the gasifier wall to form slag layers
The liquid slag rapidly built up at the top cone, and gradually increased in the vertical main body with the continuous deposition of the ash. Once it entered the bottom cone, δL suddenly increased because of the change in the wall angle (α) from 90° to 12°. This illustrates a large influence of the wall angle at the bottom cone, which is evaluated in detail later

Summary

Introduction

Because the syngas temperature typically increases above the ash melting point, most of the coal ash is molten and deposits onto the gasifier wall to form slag layers. At the bottom of the gasifier, the liquid slag is discharged through a slag tap to a water bath and quenched. These slag layers play an essential role in protecting the wall from excessive heat and chemical attack by the hot acidic gases. Because of its importance in ash discharge, wall protection, and heat recovery, controlling the slag behavior is a crucial issue for the design and operation of entrained flow gasifiers [3,4]. The ash fusion temperature and slag viscosity are important properties in relation to the flowability and thickness of the liquid slag, which are determined by inherent properties such as the ash composition and chemistry, as well as external conditions such as the temperature and reaction atmosphere [5,6,7,8,9]

Methods

Results

Conclusion