A. General

(a) Introduction

This Section is devoted to a development, design and application of submerged-combustion devices/apparatuses as a part of the direct-contact heat exchangers. The objective is to compile and summarize the worldwide findings and experiences in this relatively new and fast advancing area of the research.

Submerged combustion (Collier, 1997), as its name implies, is the combustion of gas or fuel oil in such a manner that the hot combustion-product gases are released under the surface of liquid or melt. In this way, the energy released by the combustion process is transferred by direct contact with the liquid or melt. Therefore, two major classes of submerged combustion devices/apparatuses can be identified:

1. Submerged combustion devices (SCDs) (Collier, 1997) for a liquid heating and/or evaporation. In such devices, gas or fuel oil air mixture used for the combustion process. Most SCDs are arranged with a burner above the liquid level and a submerged exhaust system. Although it is possible for the burner itself to be submerged into the liquid (Pioro and Pioro, 2006). Submerged combustion is used in two classes of evaporators direct and indirect. In the first, it is used to concentrate corrosive or toxic materials. In the second, water is heated, which in turn, is then circulated over a tube bank containing the liquid to be evaporated. While the advantages of submerged combustion are: (a) achieving maximum heat transfer rates from combustion products to the liquid; (b) the absence of fouling or corrosion; and (c) the ability to handle highly viscous liquids or liquids containing up to 40% solids. However, the disadvantage is the contamination of the liquid by combustion products.
   
   
2. Submerged combustion melters (SCMs) (Pioro and Pioro, 2006) for a solid or powder-like charge heating and melting. In these apparatuses, gas air mixture or gas — air-oxygen mixture used for the combustion process. SCMs are always arranged with a burner submerged into or beneath the melt. The main goals of submerged combustion in this application are: (a) achieving maximum heat transfer rates from combustion products to the melt; (b) improving mixing (i.e., melt homogeneity); and (c) increasing rates of chemical reactions inside SCMs. Currently, various SCM designs have been developed for producing materials for the building industry from metallurgical slag, coal slag, and ash from coal-fired thermal power plants; fuming of slags of non-ferrous metals; melting silicate materials; producing mineral wool; producing molten defluorinated phosphates for agriculture; pyrohydrolysis of fluorine-containing wastes; vitrification of high-level radioactive wastes; and production of expanded-clay aggregate for lightweight concrete from non-selfbloating clays. Many of the developed technologies are intended for decreasing harmful effects of various wastes such as slags, ash, etc. on the environment by effectively reprocessing them into materials for the building industry or by safe infinite disposal of high-level radioactive wastes by including them into a glass matrix.