Encyclopedia of Environmental Science and Engineering, Volume I and II

FOSSIL FUEL CLEANING PROCESSES 423

Ground coal (1/16) is introduced into the heated, tubu-

lar reactor. Gas fl ow is then adjusted to maintain bed fl uidity.

Reactor temperatures approach 950F.

The char is then removed through the bottom of the

reactor.

The factors most directly affecting desulfurization are par-

ticle size, reactor residence time, reaction temperature and fl u-

idizing velocity. Figure 13 summarizes relationships between

extent of desulfurization (expressed as the ratio of sulfur in

char to that in feed- ∆ S) and above mentioned parameters.

Other processes, in various stages of development

include causticized fl uidized bed desulfurization, gasifi ca-

tion to high and low BTU Gases and extr action of pyritic

sulfur from raw coal. The Bureau of Mines, bituminous coal

research and others have sought to remove sulfur (pyritic) by

washing, using various techniques including centrifugation,

fl otation and magnetic separation methods. None of these

has the potential to remove more than half the sulfur and

each leads to signifi cant product losses.

Bartok et al.^11 point out the advantages of systems which

combine advanced processing techniques with advanced

combustion recovery cycles. Gasifi cation combined cycle,

for example, couples the precombustion clean-up of coal

(via gasifi cation) with gas and steam turbines for power

generation. As the effi ciency of such plants improves their

economics will improve relative to conventional coal fi red

plants while having a superior environmental impact (much

lower nitrogen oxides, for example).

COAL ASH REMOVAL

Detroit Edison Company demonstrated the benefi ts of oper-

ating with low ash coals. In addition to the lower ash removal

requirements, they lead to reduced transportation costs,

higher heating values and improved boiler performance. One

possible disadvantage of water cleaning method is additional

drying which might be required prior to combustion.

Ash removal is considered primarily for coal production.

Ash contents of oils are relatively small and of natural gas

are negligible. The popular separation techniques usually

depend on specifi c gravity differences (physical) or froth

fl otation (chemical) methods. The dispersing medium for

specifi c gravity separation may be water, air or suspended

matter in water. Coal specifi c gravities may range from 1.1

to 1.8, whereas impurities typically have specifi c gravities

above 2.0 for carbonates and silicates and as high as about

5.0 for pyrites.

Bowling et al.^12 state that the levels of ash-forming min-

eral matter in most coals can be reduced by a combination

of physical and chemical methods, to yield ultraclean coals

with ash yields of 0.1–1%.

BLENDING WITH PETROLEUM COKE

To satisfy a worldwide power production growth rate of more

than 2.5% a year, with even higher rates in the developing

10 20 30 40 50 60

0.6 0.8 1.0 1.2

400 600 800 1000 1200

0.50

0.70

1.00

TEMPERATURE (OF)

FLUIDIZING VELOCITY (FT/SEC)

TIME (MINUTES)

FIGURE 13

C006_002_r03.indd 423C006_002_r03.indd 423 11/18/2005 10:27:12 AM11/18/2005 10:27:12 AM