WSP07510

• File Number: 8200.760
• Description: Yampa River General
• State: CO
• Basin: Yampa/White
• Water Division: 6
• Date: 1/1/1981
• Author: USGS
• Title: Assessments of Impacts of Proposed Coal-Resource and Related Economic Development on Water Resources - Yampa River Basin - Colorado and Wyoming - A Summary - 1981
• Water Supply Pro - Doc Type: Report/Study

WATER-USE AND RESIDUALS
IMPLICA TIONS OF
COAL-CONVERSION AND
TRANSPORTATION ALTERNATIVES

By ROBERT M. HIRSCH

Each coal. use alternative has a set of implica.
tions regarding discharge of residuals into the en.
vironment and use of water (Gold and others,
1977). In this chapter, a comparative analysis is
made of alternative cooling systems in power-
plants. Then, assuming a fixed amount of coal
production, relative water-use implications are
compared in use of this coal for electric-power
generation, coal gasification, or a coal-slurry
pipeline. Finally, a comparison of total residuals
discharged as a result of the seven assumed coal.
development plans gives some insight into the pos-
sible environmental problems confronting resource
managers in the Yampa River basin.
A coal.burning powerplant generating electricity
must reject a vast amount of waste heat into the
environment. For every megawatt-hour (MWh) of
electricity produced, approximately 6.2 million
Btu (6.5 billion joules) of heat is produced. There
are five major methods available for dealing with
this residual. Four of these methoda-{)nce-through
cooling, cooling ponds, wet-cooling towers, and
dry-cooling towers-were investigated to determine
their implications for the use of water and energy as
well as for the residuals produced in the heat.
production process. A fifth method not examined
here is the beneficial use of waste heat.
The relative amounts of the heat load rejected by
each mechanism are dependent upon: (1) the type
of cooling device used; (2) climatic conditions
(temperature, humidity, and wind); (3) hydrologic
conditions (quantity and temperature of available
water); and (4) certain plant.operating conditions.
The first three of these methods rely, to varying
degrees, on evaporation as a cooling mechanism.
For each kilogram of water evaporated. a water
body loses 2,400 Btu (2.5 million joules) of heat. As
a consequence of this evaporation, there is not just
an addition of moisture to the atmosphere and loss
of surface water available for other uses, but there
also will be an increase in the concentration of dis-
solved and suspended solids in the remaining

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water. Thus, the consequences of heat-rejection
methods involve changes in atmospheric condi.
tions, water availability, and water quality.
Schematics of the four cooling systems con-
sidered in this analysis are shown in figure 3. In the
comparative analysis, the results of which are sum.
marized in table 6, it was assumed that the cooling
systems would be installed at a complex of
powerplants using 12.5 million tons (11.3 million t)
of coal annually and that the total plant-generating
capacity would be about 4,000 MW, with operating
efficiency of 39 percent and a load factor of 83 per-
cent (I. C. James II, E. D. Attanasi, T. Maddock
III, S. H. Chiang, B. T. Bower, and N. C. Matalas,
written commun., 1978).
For once-through cooling, the annual flow
diverted from the river would average 3,580 ft'/s
(101 m'/s) or total 2.l5 million acre-ft (2.65 billion
m') (table 6). This water would be pumped from
the river, passed through the powerplant con-
densers where it would be heated, and then
returned to the river (fig. 3A). Because the three
physical mechanisms of heat loss (evaporation,
conduction, and long-wave radiation) proceed at
rates which are increasing functions of water
temperature. the river would begin to cool and
gradually return to its natural temperature. In the
reach of the river affected by this elevated
temperature, there would likely be changes in the
number and variety of aquatic species.
Because the evaporation rate of a free-water sur.
face is an increasing function of water temperature,
the evaporation rate for the river will be larger than
it would be if no waste heat had been discharged to
the river. The rate of water evaporation at-
tributable to the powerplant is this difference
between the actual rate and the natural rate of
evaporation, integrated over the surface area of the
river. The rate of water evaporation is an increasing
function of wind speed, natural water temperature,
and altitude.
By computing the rate of water evaporation for
the various climatic and hydrologic conditions as-
sumed to exist throughout the year. the annual
evaporation by a hypothetical 4.000-MW power-
plant located on the Yampa River would be 27.800
acre-ft (34.3 million m3), or 1 percent of the annual
withdrawal rate (table 6). This would result in a
2,7-percent decrease in the mean annual flow of the

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