Ash Performance Indices.

To evaluate fuel performance, MTI developed Coal Quality Management System (CQMS) software. This software calculates advanced indices relating coal characteristics to ash behavior in a coal-fired utility boiler. The development of the software began in June 1995, and since that time CQMS has been tested on numerous coals from all over the world. The software was developed because the prediction of ash behavior using conventional ASTM methods of analysis is inadequate to determine the chemical and physical characteristics of inorganic components in the coal. The software utilizes computer-controlled scanning electron microscopy (CCSEM) and chemical fractionation data as input as well as ash composition and ultimate analysis.

Fuel performance is estimated in terms of slag flow behavior, abrasion and erosion wear, wall slagging, high temperature silicate-based convective pass fouling, and low temperature sulfate-based convective pass fouling. Specific indices calculated by CQMS are listed below:

Wear Indices
Abrasion Index: This index indicates the potential for wear of fuel preparation and handling equipment, as related to the hardness of minerals in the coal. The primary minerals of concern are quartz and pyrite. Values range from 0.1-low to 10-severe.

Erosion Index: This index indicates the potential for wear of boiler parts due to the impaction of fly ash particles, particularly those containing hard minerals such as quartz. The erosion index is dependent upon particle size and velocity. Values range from 0.1-low to 1.0-severe.

Convective Pass Fouling Indices
Low-temperature convective pass fouling (sulfation) index: This index indicates the propensity of low-temperature fouling deposits to form in the convective pass of the utility boiler from 1000 to 1750°F. This index is based on the availability of alkali (Na and K) and alkaline earth (Ca and Mg) elements to react with SO2 and SO3 to form sulfate, which is the primary material that causes particle-to-particle bonding in high-calcium coals. Sulfates are thermodynamically stable at temperatures below about 1650°F. Index values range from 1-low to 10-severe.

High-temperature convective pass fouling (silication) index: This index indicates the propensity of deposits to form from 1600 to 2400°F. This index is related to the formation of high-temperature fouling deposits in which silicates are the primary accumulating materials and the primary bonding component. Information used to derive the index includes the size of minerals such as quartz and clay, availability of alkali and alkaline earth elements, and the viscosity of the silicate liquid phase. This index is used in combination with the strength index. Values range from 1-low to 200-severe.

Wall Slagging Index
This index indicates the propensity of deposits to accumulate on the radiant walls of a boiler, from 2000 to 3000°F. The slagging index is based on mineral size (especially illite, quartz, and pyrite), association of calcium (calcite can contribute to slagging), and viscosity of the silicate-based liquid phase. This index is used in combination with the strength index to assess slag deposit characteristics. Values range from 1-low to 20-severe.

Cyclone Slagging Index
This index provides information on the slag flow behavior in cyclones. This index is based upon partitioning of ash in the cyclone based on size and association of coal minerals. Standard partitioning criteria have been developed to provide the composition of the slag. Coal mineral composition is used to estimate the slag viscosity as a function of temperature. Index values range as follows: 1 (low viscosity), 1.5-2.5 (optimal viscosity), >3.0 (slag has the potential to freeze).

Learn more about additional slag flow behavior prediction methods.

Deposit Strength Index
This index predicts the strength of deposited material. It is used in combination with the slagging index to assess slag deposit characteristics. Index values of less than 0.25 indicate weak deposits. Values of 0.25 to 0.34 denote low to moderate strength, and values of 0.35 to 0.41 indicate strong deposits. Index values greater than 0.41 correspond to flowing slag.

Simplified Indices
Two simplified indices, the accumulation index and the accumulation and strength development (ASD) index, provide two single numbers to rank the potential of a coal to accumulate deposits and develop strength in coal-fired power systems. Both provide simplified outputs for easy interpretation and quick ranking of coals. They also show general relationships between coal quality and the overall potential to accumulate and develop strong deposits, without reference to location within the boiler. Both indices have values ranging from 1 (low) to 100 (severe).

The accumulation index provides an overall assessment of the potential to accumulate ash on heat transfer surfaces, regardless of location in the boiler. The accumulation index is derived from normalized values of the wall slagging, high-temperature fouling, and low-temperature fouling indices. The figure below illustrates the accumulation index for a range of coals in MTI’s database.

The ASD index incorporates the potential to develop strength with the potential to accumulate ash deposits on heat transfer surfaces. This index can be calculated for one temperature, such as furnace exit gas temperature (FEGT), or for a range of temperatures.

Applications
The graphs below illustrate the variability in the accumulation and ASD indices for U.S. coals. Lignites exhibit the greatest propensity for accumulation and strength development, followed by the Powder River Basin coals, Illinois Basin, and Appalachian bituminous coals. Greater internal variability exists within the lignite coals, compared to the Powder River Basin coals. Appalachian coals have low accumulation and ASD index values, due to their low potential to form low-temperature (sulfate-based) fouling deposits.

Slag Flow Behavior Prediction Methods
Slag flow behavior is influenced by slag composition and viscosity. Predictive methods for determining slag flow behavior include viscosity/temperature profiles, T₂₅₀, TCV, and cyclone slagging index.

T₂₅₀: This value represents the calculated temperature at which the slag viscosity will reach 250 poise, based on ash composition. Given that the upper limit of viscosity for slag fluidity is approximately 250 poise, the T₂₅₀ value is generally used as the maximum slag removal temperature. (The cyclone temperature must be greater than the T₂₅₀ in order for slag to flow from a cyclone boiler).

Notes on T₂₅₀: T₂₅₀ can be calculated by the Sage and McIlroy method (B&W method) or by the Urbain method (MTI method). The Sage and McIlroy method uses base-to-acid ratios to determine T₂₅₀ values. Values are affected by atmospheric conditions (reducing or oxidizing), with the slag viscosity profile being raised as the proportion of ferric iron decreases. The Sage and McIlroy method is considered valid for bituminous-type ash and for lignitic ash with acidic content over 60 percent.

The Urbain method categorizes slag constituents into three different groups: “glass formers,” such as silicon and phosphorous, which form the initial solid structures in molten slag; “modifiers,” such as sodium, potassium, calcium, iron, and titanium, which extend the solid structure over a larger range; and “amphoterics,” such as aluminum, boron, and iron, which influence the solid structure of slag. T₂₅₀ for a given slag composition can be calculated based on the interactions of these three types of slag constituents. The Urbain method has become an accepted technique for predicting viscosities for silicate-rich slags.

Calculation of Ash Resistivity
The resistivity of the ash has a significant impact on the collection efficiency of ash in electrostatic precipitators. High resistivity ash causes large voltage gradients across the ash layer on collector plates resulting in poor collection efficiency. The high resistivity ash significantly decreases the migration velocity of particles to the collection plate. Ashes with resistivities above 1010 ohm-cm cause poor performance. Bickelhaupt model is used to calculate resistivity as a function of temperature based on coal analysis and selected operational parameters.