Revisando y reconstruyendo “la tradición”

1. Volatile matter: Volatile matter is that portion of the coal driven off as gas or vapor when the coal is heated according to a standardized temperature test (ASTM D3175). It consists of a variety of organic

gases, generally resulting from distillation and decomposition. Vola-tile products given off by coals when heated have higher hydrogen/

carbon ratios than the remaining material.

2. Fixed carbon: Fixed carbon is the combustible residue left after the volatile matter is driven off. In general, the fixed carbon represents that portion of the fuel that must be burned in the solid state.

3. Moisture: Total moisture content of a sample is customarily deter-mined by adding the moisture loss obtained when air-drying the sam-ple and the measured moisture content of the dried samsam-ple. Moisture does not represent all the water present in coal; water of decomposi-tion (combined water) and of hydradecomposi-tion are not given off under stan-dardized test conditions.

4. Ash: Ash is the noncombustible residue remaining after complete coal combustion and is the final form of the mineral matter present in coal. Ash in coal is determined by ASTM D3174.

5. Sulfur: Sulfur is found in coal as iron pyrites, sulfates, and in or-ganic compounds. It is undesirable because the sulfur oxides formed when it burns contribute to air pollution, and sulfur compounds con-tribute to combustion system corrosion and deposits. Sulfur in coal is determined by ASTM D3177.

6. Nitrogen: Nitrogen is found in coal molecules and is also an unde-sirable coal constituent, because the nitrogen oxides (NOx) that are formed when coal burns contribute to air pollution. Nitrogen in coal is determined by ASTM D3179.

7. Ash—fusion temperatures: Ash-fusion temperatures are a set of temperatures that characterize the behavior of ash as it is heated.

These temperatures are determined by heating cones of ground, pressed ash in both oxidizing and reducing atmospheres according to ASTM D1857. When coal ash is heated to high temperatures it be-comes soft and sticky, and finally, fluid. These temperatures give an indication of where slagging may occur within a boiler.

The initial deformation temperature is related to the temperature at which coal begins to fuse and become soft.

The softening temperature is related to the temperature at which the coal ash shows an accelerated tendency to fuse and agglomerate in large masses.

The fluid temperature is related to the temperature at which the coal ash becomes fluid and flows in streams.

These temperatures are also affected by the furnace atmosphere; a reducing atmosphere generally gives lower ash-fusion temperatures than does an oxidizing atmosphere. The ash-fusion temperature

deter-134 Chapter 4

TABLE4.7 Types of Pulverizers for Various Materials Impact

and

Type of material Balltube attrition Ballrace Ringroll

Low-volatile anthracite ⫻ — — —

High-volatile anthracite ⫻ — ⫻ ⫻

Coke-breeze ⫻ — — —

Petroleum coke (fluid) ⫻ — ⫻ ⫻

Petroleum coke (delayed) ⫻ ⫻ ⫻ ⫻

Graphite ⫻ — ⫻ ⫻

Low-volatile bituminous coal ⫻ ⫻ ⫻ ⫻

Medium-volatile bituminous coal ⫻ ⫻ ⫻ ⫻

High-volatile A bituminous coal ⫻ ⫻ ⫻ ⫻

High-volatile B bituminous coal ⫻ ⫻ ⫻ ⫻

High-volatile C bituminous coal ⫻ — ⫻ ⫻

Subbituminous A coal ⫻ — ⫻ ⫻

mination is an empirical laboratory procedure, and thus it is some-times difficult to make general statements on its appropriateness for all types of applications.

8. Grindability index: The grindability index indicates the ease of pul-verizing a coal in comparison with a reference coal. This index is helpful in estimating mill capacity. The two most common methods for determining this index are the Hardgrove Grindability Method;

ASTM D409) and Ball Mill Grindability Method. Coals with a low index are more difficult to pulverize.

TABLE4.8

9. Free-swelling index: the free-swelling index (ASTM D720) gives a measure of the extent of swelling of a coal and its tendency to agglomerate when heated rapidly. Coals with a high free-swelling index are referred to as caking coals, whereas those with a low index are referred to as free-burning coals.

10. Burning profile: The burning profile of a coal, obtained by thermal

136 Chapter 4

TABLE4.8 Continued

TABLE4.8 Continued

gravimetric analysis, is a plot of the rate of weight loss when a coal sample is heated at a fixed rate. The burning profile of a solid fuel offers an indication of ignition and burning characteristics by compar-ing it with other burncompar-ing profiles of fuels with known performance.

11. Ash analysis Ash analyses give percentages of inorganic oxides present in an ash sample. Ash analyses are used for evaluation of the corrosion, slagging, and fouling potential of coal ash. The ash constituents of interest are silica (SiO2), alumina (Al2O3), titania (TiO2), ferric oxide (Fe2O3), lime (CaO), magnesia (MgO), potassium oxide (K2O), sodium oxide (Na2O), and sulfur trioxide (SO3). An indication of ash behavior can be estimated from the relative percent-ages of each constituent.

See Tables 4.7–4.13 for further details.

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TABLE4.9

TABLE4.10

140 Chapter 4

TABLE4.11

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TABLE4.13

V. MISCELLANEOUS FUEL A. Petroleum Coke [1,13]

Petroleum coke is a by-product of a process in which residual hydrocarbons are converted to lighter distillates. Two processes are employed: delayed coking and fluid coking.

1. Delayed Coking

In the delayed coking process, reduced crude oil is heated rapidly and sent to coking drums. The delayed coke resembles run-of-mine coal, except that it is dull black. Proximate analysis varies with the feed crude stock. The components range as follows:

Moisture 3–12%

Volatile matter 9.0–15.0%

Fixed carbon 71–88%

Ash 0.2–3.0%

Sulfur 2.9–9.0%

Btu/lb, dry 14,100–15,600

Some delayed cokes are easy to pulverize and burn whereas others are difficult.

2. Fluid Coking

The fluid coking process uses two vessels, a reactor and a burner. The coke formed in this process is a hard, dry, spherical solid resembling black sand. Again the proximate analysis varies with the crude feed stock. The range of analysis is

Fixed carbon 90–95%

Volatile matter 3–6.5%

Ash 0.2–0.5%

Sulfur 4.0–7.5%

Btu/lb, dry (HHV) 14,100–14,600

Fluid coke can be pulverized and burned, or it can be burned in a cyclone furnace or a fluidized bed. All three types of firing require supplemental fuel to aid ignition.

Note: Waterwall boilers that burn fuel on grates need a fuel with volatile content of 18% and higher, to sustain combustion. To burn petroleum coke as the only fuel, would be extremely difficult if not impossible.

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