Hot-face refractories

In a gasifier utilizing gas or liquid as a carbon source, refractory failure is primarily caused by thermal, chemical, and structural wear of the refractory liner. The hot face refractory lining in the gasifier is typically a dense high-alumina material (low in Si02 and FeO), followed by a porous layer composed of a material such as... 

The standard ASTM PCE test is relative and used extensively only for alumina—siUca refractories and raw materials (see Table 5). However, the upper service limit is generally several hundred degrees below the nominal PCE temperature because some load is generally appHed to the refractory duriag service. In addition, chemical reactions may occur that alter the composition of the hot face and therefore the softening poiat. The relationship between PCE numbers and temperature is described ia ASTM C24. 

In North America, a special, high conductivity, low permeability, "hot-pressed" carbon brick is utilized almost exclusively for hearth walls. Because of their relatively small size and special, heat setting resin cement, and because the brick is installed tightly against the cooled jacket or stave, differential thermal expansion can be accommodated without refractory cracking and effective cooling can be maintained. Additionally, the wall thickness is generally smaller than 1 m, which promotes the easy formation of a protective skull of frozen materials on its hot face. Thus hearth wall problems and breakouts because of carbon wall refractory failure are virtually nonexistent. 

Used with appropriate refractory aggregates, calcium aluminate cements may be used to make refractory castables having applications in the steel and other heat-using industries. Cements made from bauxite may thus be used at hot face temperatures up to 1350 C. White calcium aluminate cements, which are low in iron and higher in alumina, can be used with appropriate aggregates to make castables similarly resistant up to 2000°C. 

Installed alone as a semi-refractory material, the foamed borosilicate glass block lining withstands hot face temperatures up to 960°F. It may also be used with refractory, chemical-resistant masonry or monolithic internal linings at temperatures above 960°F providing a unique combination of corrosion protection and heat conservation with little added weight and a lesser overall lining thickness. The foamed glass block may also be fabricated into nozzles, T-sec-tions, elbows, liner inserts and other custom shapes. 

Another form of alkali metal attack on the hot faces of refractory linings involves their high temperature reaction with various components of the brick to form expansive crystalline phases which cause brick to bloat on their hot faces and, subsequently, erode or spall. An example Is the case of alumina brick exposed to sodium at temperatures from about 1700°F to 3000°F. Although sodium does not form a low temperature melt with alumina, it reacts with the alpha phase of alumina, corundum, to form beta alumina, sodium aluminate. Beta alumina has a much greater volume than the very dense corundum and, therefore, disrupts the brick bonding matrix, causing eventual bond failure. 

When alkali metals are present in processes, reduced service life can be expected over inert atmosphere performance for any refractory material used on the hot face. However, lining performance can be optimized by judicious selection of the hot face brick composition and designing the thermal gradient in the lining to keep the hot face brick as cool as possible. 

Hot face wall Refractory Refractory Membrane wall Membrane wall... 

Rigby and Hutton [29] concluded that for refractories with a silica/alumina ratio greater than 1.5, shrinkage may occur at the hot face of the brick. The higher the silica content, the greater the shrinkage and the lower the liquidus temperature. On the other hand, expansion occurs for refractories with a ratio less than 1.0 which may be attributed to the formation of either sodium aluminate or beta-alumina. 

As a result, the overall heat transfer through the composite refractory wall is known. The hot face and cold face heat transfer coefficients can be calculated from known expressions for forced and free convection near a flat plate. These expressions have the same structure but different empirical constants and can be found in, for example, Reference 20. 

Selecting the most appropriate refractory lining for a lime kiln is important for the economic viability of the installation. The refractory at the hot face has to provide adequate performance with respect to ... 

Hame impingement on refractory often causes damage. When ultralow NO and the latest generation burners are being retrofitted, the larger burner diameter may result in the burners being spaced closer to the refractory. Unshielded refractory may require hot face protection. 

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