Drying of Coal

Coal is a raw material for many chemical syntheses as well as a fuel. De- pending on its initial moisture content, coal is dried to increase its calorific value and simplify loading, unloading, transport, and improve boiler-combus- tion efficiency. Although not commonly required in North America, drying of low-grade coals containing high levels of moisture is necessary in many parts of the world. Coal is also dried for briquetting, coking, gasification, car- bonization, liquid fuel synthesis, and so on. Coke-oven efficiency can increase 30% to 50% in preheating, and 10% to 15% in drying if coal is predried. Direct dryers (e.g., rotary, pneumatic, fluid bed, vibrating fluid bed, shaft dryers, etc.) are used commonly with hot air or combustion gas at 700 to 900°C before the dryer and 60 to 120°C after the dryer. Rotary dryers with indirect heating

are used for hard coals. These dryers have higher energy efficiencies, about 3100 kJ/kg water evaporated. For air fluidized bed dryers, the corresponding figure is 3100 to 4000 kJ/kg water evaporated. Frequently, fluid bed dryers are equipped with an internal heat exchanger (Figure 7.3). A commercial vibratory dryer for hard and brown coals (manufactured by Escher-Wyss of Switzerland) uses a vibrational frequency of 50 to 100 Hz and an amplitude of 0.5 to 3 mm, giving a conveying velocity of 0.01 to 0.3 m/s with an angle of inclination of 5° to the horizontal. Low gas velocities are needed since vibration suspends most of the pseudo-fluidized beds. The efficiency is better than in a conven- tional fluid bed employing high gas velocities. Attrition is reduced and gas cleaning requirements are minimized in a vibrated bed dryer.

In pilot trials, Potter et al. (1986) and Potter et al. (1988) have shown that extremely favorable heat transfer rates as well as drying efficiencies are obtained when drying brown coal in a steam-fluidized bed with internal heat exchanger tubes immersed within it. Typical processing conditions were re- ported as

Heating tube temperature: 140–170°C

Bed temperature: 110–127°C

Minimum fluidization velocities: 0.57 m/s (approximately) Steam temperature: 130–150°C

Coal feed rate: 40–70 kg/h Product: 16–28 kg/h

Using steam exhausted from one dryer stage as carrier steam for another stage, multiple-effect operation (similar to that common to evaporators) can be achieved yielding a steam economy of 1.9 for a triple-effect dryer. Potter et al. (1988) used a continuous fluid bed dryer for drying Victoria brown coal.

The rectangular 0.3⫻ 0.3 m fluid bed dryer was 3 m high with four bubble

caps to distribute steam. The disengaging region was 2.5 m. Both horizontal and vertical tube bundles were tried.

Faber et al. (1986) have compared drying rates in air and in steam- fluidized beds of pulverized coal. They found the inversion temperature of about 180°C above which the steam-drying rate in the constant-rate period in fluidized bed drying exceeds that in (dry) air drying. For a 2000-kg/h dryer for alumina, the capital cost was 20% lower for the steam dryer, while the total energy cost was lower by 15%. No credit was given to the steam produced in the steam dryer.

Faber et al. (1986) also reported on a successful industrial installation using a steam dryer for activated carbon pellets (2000 kg/h dry basis) from an initial moisture content of 50% to 2% (dry basis). The pellets were dried

authors reported smooth operation of the dryer since 1985. The installed cost of the steam-drying system was 40% lower than that for a conventional air dryer. Besides, the air dryer can operate at a maximum temperature of 125°C to avoid combustion in the dryer. The energy costs (1986 data) were estimated to be about $3.6 per ton of dry product in South Africa.

Woods et al. (1994) have reviewed steam-drying technologies for coal and presented interesting results on steam drying of 1.0 to 1.3 mm coal parti- cles and the evolution of volatiles during drying. It is noted that, in steam drying, the drying time (actual residence time in the dryer) does not affect the volatiles’ liberation, unlike air drying. Further, they found that under the

FIGURE7.3 Schematic of a fluid bed dryer for pulverized coal with immersed heat exchangers (German design).

conditions of their experiment, the constant-rate drying period is 6 to 7 times longer in steam drying and the heat transfer rate is 1.7 to 2.0 times that in air. They also report favorable industrial experience with steam-fluidized bed drying of brown coal with an evaporative capacity of 25 tons/h. No details are given about the use of steam produced by the dryer.

Black coal generally has low water content, while brown coal may have 60% to 80% (wet basis) moisture. Regardless of whether brown coal is burned, gasified, coked, or liquefied, the wet raw coal should be dried to 5% to 10% moisture for economic utilization. For some gasifiers, the additional require- ment of free flowability for a well-metered feed rate means that the pulverized coal must be low in moisture content. Coal drying in a steam-fluidized bed of coal containing 65% moisture (wb) is estimated to reduce energy wastage in the subsequent combustion step by two-thirds, resulting in some 15% in- crease in the overall powerplant efficiency, a similar level of reduction in CO2

emission, and a 30% reduction in flue gas generation for the same thermal output.

Steam drying is claimed in some reports to reduce power consumption for milling since the grindability index is increased due to steam drying. At elevated degrees of superheat and prolonged exposure times, some studies have reported a reduction in the sulfur content of coal. This is not necessarily an advantage since the sulfur will then enter the dryer exhaust steam. Addi- tional data are needed to evaluate this aspect in detail.

Following are some of the key advantages claimed for steam-fluidized bed drying of brown coal:

Better energy utilization by condensing steam generated in the dryer; an overall efficiency was increased from 36% (typical for a brown coal-fired powerplant) to 42%.

Ability to couple the dryer with powerplants to use latent heat of con- densed low-pressure steam (co-generation potential).

Coal moisture discharged as liquid water rather than as dusty vapor. No biological or chemical treatment is needed for the condensate water. Large-capacity dryers (up to 15 tons/h) are feasible.

The dryer is compact (e.g., heat transfer coefficients in the order of 200 W/(m2_{K ) versus 20 to 50 W/(m}2_{K ) in steam tube rotary dryers).}

The operation is safer (reduced insurance costs).

Product is dried to greater uniformity and better briquette strength. The coal dryer itself is not the most expensive component of the drying system; coal grinding prior to drying, milling after drying, and cleaning the