Additional fouling and slagging parameters have been developed; however, many of these serve as guides to be used in conjunction with other parameters and the operating experience of the various coals and boiler units. Some of these factors include:

1. The silica/alumina ratio (SiO2/Al2 O3) can provide additional information relating to ash fusibility [8]. The general range of values is between 0.8 and 4.0; for two coals having similar base/acid ratios, the one with a higher silica/alumina ratio should have lower fusibility temperatures.

2. The iron/calcium ratio (Fe2O3/CaO) indicates the fluxing (i.e., lowering of the ash fusion temperature) potential of the iron and calcium in the ash. Iron/calcium ratios between 1.0 and 0.2 have a marked effect on lowering the fusibility temperatures of coal ash, and extreme effects are evident between ratios of 3.0 and 0.3 [8].

3. A dolomite percentage (DP) is defined as:

which is used primarily for coal ashes with a basic oxide content over 40% (i.e., western U.S. coals). It has been empirically related to the viscosity of coal ash slags and, at a given basic concentration, a higher DP usually results in higher fusion temperatures and higher slag viscosities [8].

4. A silica percentage (SP) is defined as:

which has been empirically correlated with the viscosity of coal ash slags. As SP increases, the slag viscosity increases [8].

The development of a reliable coal-screening tool has long been a goal of the utility industry. The indices based on ASTM coal and ash analysis have provided useful information to boiler designers and operators but are not refined enough to be applied to all coals and all boilers. Also, because many of these indices were developed for bituminous coals, they are poor indicators of performance when applied to low-rank coals; consequently, different analytical techniques are being applied and new indices are continually being developed. Many of these new indices rely heavily on computer-controlled scanning electron microscopy coupled with coal and coal ash analyses. The technique of automated or computer-controlled scanning electron microscopy has enabled the identification and sizing of coal mineral matter in situ and is also used to determine the inherent or extraneous nature of the mineral matter in coal. The direct analysis of mineral matter in coal, size determinations, and observation of the association of mineral matter with coal particles are essential to determining the behavior of mineral matter during combustion of pulverized coal. Examples of indices include wall slagging, convective pass fouling based on either sulfates or silicates being the primary bonding component, cyclone slagging, and deposit strength [31]. Many of the indices developed by industry are proprietary.

As emphasized in this section, a major factor affecting furnace performance in large coal-fired utility boilers is the inorganic matter of the fuel. Most problems with the ash are associated with its effect on heat transfer by thermally insulating furnace wall tubes and convective pass tube banks. Accumulation of ash deposits can decrease the heat transfer rate to the tube surface, resulting in high flue gas temperatures.

Boiler manufacturers have developed their own empirical relationships to predict furnace performance and the effect of ash slagging and fouling. Also, testing coals in pilot-scale slagging and fouling combustors or boilers is routinely performed by boiler manufacturers, universities, and other test facilities to assess deposition performance. Each boiler manufacturer has its own criteria to allow for the effects of the ash characteristics; these are based on sound engineering judgment and years of experience with boilers of similar size.

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