Brick or refractory failure is a complex problem to solve. When brick or refractory linings fail it is usually the result of a combination of factors, not just one. Finding the root cause of a failure requires: 1) having a brick or refractory material sample for testing; 2) knowing how and where the material was stored (and for how long); 3) knowing when the brick or refractory was manufactured; 4) knowing exactly when the brick or refractory material was installed to compare ambient air conditions; and 5) having information on the curing and dry-out procedures that were followed (required for refractory only).
Brick or refractory failure can also occur due to chemical or corrosion attack from alkali, sulfur, hydrocarbons, vanadium, steam, as well as from ash and slag. Ash and slag, in particular, should be completely understood especially when selecting brick or refractory material.
First, it is important to define some important terms relating to ash and slag:
Oil Ash is the residual product that is left over after burning of the fuel oil. Of the many elements that may appear in oil ash deposits, the most important are vanadium, sodium, and sulfur. Even the smallest amounts of this type of ash can cause severe problems to refractory materials.
Coal ash is a slagging and fouling ash. Coal ash that has a low fusion point and high basic-oxide content is very corrosive to refractory materials.
Slagging is the formation of molten, partially fused or re-solidified deposits of ash on the furnace walls. Ash that forms on the inside of a furnace can raise the furnace gas temperatures.
Stoichiometry is the ratio of air to fuel at the burners. If the stoichiometry is not right, one of four things can happen: 1) the burner flame can get longer; 2) there is excessive turbulence in the flame; 3) variance of temperatures; and 4) an increase in ash content.
Reducing air applies to the combustion process in a furnace whereby the amount of air required for combustion and going directly into the burners is reduced, and added into the combustion process in another location (i.e. under-fire or over-fire air.)
Spalling is the loss of fragments or spalls from the face of a brick or refractory structure through cracking and rupture with exposure of inner portions of the original mass (brick or refractory.)
Thermal spalling is caused by stresses resulting from unequal rates of expansion or contraction. It’s usually associated with rapid changes in temperature.
Mechanical spalling is caused by changes in the physical structure due to expansion or some external force or object, such as a large piece of slag called a “clinker.”
Chemical spalling is caused by a reaction of one or more of the chemical or mineral components of the refractory material to one or more of the chemical or mineral components of the fuel oil, fuel oil ash, or to the coal ash, such as vanadium.
Ash and slag
Slag is the formation of molten, partially fused, or re-solidified deposits (ash). Slag is a function of deposit temperature and deposit composition. Deposit composition is a function of the local atmosphere particularly for ash with significant iron content. Slag deposits can reduce heat absorption, raise exit-gas temperature, and interfere with ash removal. Air and fuel imbalance (stoichiometry) can also cause slagging, especially when there exists high iron content in the fuel or materials being burned or heated.
For slag to adhere to a clean surface (brick or refractory face) and form deposits the particles must have a viscosity low enough to wet the surface. Iron raises all four values of ash fusion temperatures (initial deformation, softening, hemispherical, and fluid). Therefore, the greater the iron-content in the ash, the greater is the difference in ash fusibility between the oxidizing and the reducing condition.
Selecting brick or refractory
When choosing a brick or refractory material it is necessary to take into account the area of usage and the environment (vanadium, sulfur, potassium, moisture content in the fuel or raw material, stoichiometry, ash, slag, reducing atmosphere, etc.) to which the brick or refractory will be exposed . For refractory, it’s also necessary to take into account the method of application (e.g., gunning, ramming, trowel, pouring). For brick, the type of mortar and the size of the mortar joint must be considered.
Here are the basic steps to follow for selecting a brick or refractory material: 1) Examine the existing brick or refractory (or the lack thereof); 2) Calculate the base to acid ratio of the slag and ash and, 3) Select the right materials based on the application, usage, and environment.
Step 1: Examine the existing brick or refractory. When replacing old brick or refractory material, don’t automatically use the same material as the original design called for, or what “you always use.” It is better to examine the reasons for failure and adjust the selection accordingly. Ask yourself: Did the material spall due to thermal shock? Has it shrunk due to temperatures above its use limit? Does that gouge in the refractory lining indicate mechanical abuse? If the surface appears “glassy” is it due to operation at temperatures above the use limit? Does the lack of material indicate improper installation? The old brick or refractory lining will offer several good clues.
Step 2: Calculate the base-to-acid ratio when ash and slag are present. This will give you a starting point on what type of material to choose.
When the base to acid ratio is less than or equal to 0.25 this indicates an acid condition. An acid condition would indicate that a SiO2 type brick or refractory should be considered.
When the base to acid ratio is greater than 0.25 but less than 0.75 this indicates a neutral condition. A neutral condition would indicate that an Al2O3, SiC, or chrome-type brick or refractory material should be considered.
When the base-to-acid ratio is greater than or equal to 0.75 a basic condition is indicated. A basic condition further indicates that an MgO, or dolomite-type brick or refractory material should be considered.
Step 3: Select the right materials. Look at all service conditions (fuel or raw materials being used, ash content, air temperature, stoichiometry, etc.) before choosing a material. Brick and refractory materials vary widely in temperature-use limits, thermal shock resistance, and abrasion resistance. Pick a material with the best combination of properties for the application. Also, consider the type of fuel or raw materials being used and the environment (i.e. reducing atmosphere) that the brick or refractory will be exposed to. For example:
- When using silicon carbide base material the moisture content in the fuel or raw materials can affect the brick or refractory materials. A fuel or raw material that has high moisture content or the combined moisture content of the fuel (or raw material) with a reducing atmosphere can cause a separation of the silicon-carbide base material (grain) and could result in total brick or refractory failure. This separation of the base material occurs when the total percentage of the moisture content found in the fuel (or raw material) is greater than 15%. This separation also may occur when the combined total percentage of the moisture content in the fuel (or raw materials) plus the reducing atmosphere percentage are greater than 15%.
- When using a calcium-aluminate cement bonded material certain amounts of chemicals found in the fuel, raw materials, slag, or in the coal ash, can react with the calcium-aluminate (cement) and cause total material failure. This is especially true when there is a reducing atmosphere present. The most damaging of these chemicals to a calcium-aluminate base material are ferrous oxide, when the levels exceed 23%; potassium, when the levels exceed 2%; sulfur, when the levels exceed 2%; and vanadium, when levels exceed 1%.
Brick or refractory failure is a major contributor for foundry shut downs. Brick or refractory that is properly designed (and installed) will last longer and help minimize the amount of shutdowns required and could lead to savings in annual fuel cost. Eliminating brick and refractory failure begins with understanding ash and slag. That is why experts say “Brick and refractory designed and installed to save energy also saves money at a rate that is essential for efficient plant operation.”