The Medical College - Page 35

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56.

Which of the following items is/are not

addressed in the passage?

The Hoover Plan was more appropriate.

The Marshall Plan was more appropriate.

The Berlin Airlift solved a severe problem.

All of the above.

57.

The passage offers support for all but which of

the following observations?

Hoover competed with the Red Cross.

Churchill opposed Hoover's efforts.

Hoover opposed propaganda.

Hoover acted quickly when it came to

relief.

58.

Which statement applies to the passage as a

whole?

Hoover is pictured as a committed individ-

ual.

The author is sympathetic and fair.

It is a quick overview of a long record of

humanitarian service.

All of the above.

Passage IX (Questions 59–65)

Two ash samples received September 1989 and two

ash samples received November 1989 were ignited to

determine carbon content and then analyzed for min-

eral content. Data from the ash mineral analysis were

used as input to the Bickelhaupt resistivity prediction

model developed for the U.S.E.P.A. An additional com-

puter run was performed to predict the resistivity of an

ash that would result from burning 30% waste fuel

containing 0.5% magnesium oxide along with the coal

normally burned. The resistivity of the ash is an impor-

tant factor in the performance of the air pollution con-

trol equipment used to control particulate emission

from the combustion process.

The samples taken in September 1989 were taken

from the hoppers of Units 15 and 16. The samples

were taken while Unit 16 was having opacity prob-

lems to determine if resistivity problems related to

ash chemistry were responsible for the performance

problems. The analysis showed the ash mineral com-

position of the samples was virtually identical. The

primary difference between the two ash samples was

the loss on ignition, which was 23.33% for Unit 16

and 30.80% for Unit 15. The loss on ignition is pre-

sumed to be from unburned carbon.

With all of the carbon removed, the two samples are

predicted to have approximately a 10

11 ohm-cm resis-

tivity at 450°F. This resistivity would lead to high

sparking, and possible back corona in both units. Opti-

mum resistivity for these units at their 450°F gas tem-

perature is estimated to be about 7

9 ohm-cm. This

resistivity is lower than the 2

10 ohm-cm, which is

normally quoted as optimum for precipitators running

at lower temperatures. The lower resistivity is required

due to the lower dielectric strength of the flue gases at

450°. The lower dielectric strengths will promote back

corona in ash layers sooner than units running in the

200° to 250°F range.

The high resistivity predicted for the carbon free ash

will change dramatically with the carbon levels indi-

cated by the loss on ignition tests. Laboratory tests and

field data from other ashes have shown large reduc-

tions in resistivity from similar carbon levels. The

reductions are typically one to three orders of magni-

tude if the carbon is finely divided and well mixed with

the ash. The benefits of the carbon are usually observed

at higher gas temperatures, such as the 450°F

+ tempera-

tures at which Units 15 and 16 are normally run. The

higher unburned carbon levels on Unit 15 are believed

to be responsible in part for the better performance of

Unit 15 in September. A faulty spark sensing circuit on

the automatic voltage controllers for the outlet field of

Unit 16 also was later found to have contributed to

poor performance of the unit.

Ash samples from November 1989 were also ana-

lyzed. The higher sodium and iron content of the

November ash samples resulted in an ash with a resistiv-

ity that was about five to six times less resistive than the

September samples. ESP performance during this period

of lower predicted resistivities was reported to be good.

Increases in ash magnesium oxide were estimated

based on the following:

Fuel: 70% coal, 30% residue

Ash: 5.3%

0.5%

(5.3

70) + (0.5

.30) = total ash

3.71

0.15

= 3.86% total ash.

On a percentage basis, the coal contributes 3.71/3.86

or 96.1% of the total ash, and the residue contributes

0.15/3.86 or 3.9% of the total ash. The 3.9 ash from

residue was assumed to be mostly magnesium oxide.

This was added to the sample with the highest magne-

sium oxide content to create the analysis for hypothetical

Sample #5. The increase in magnesium oxide caused the

Adapted from a report by W. J. Borowy, VCU, 1990.

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