Combustion and energy for the future

Plenary Lecture C O M B U S T I O N A N D E N E R G Y FOR THE FUTURE HOYT

C. HOTTEL

Massachusetts Instit~le of Technology, Cambridge, Massachusetts There are many interesting similarities in the energy problems of nations: the industrial revolution in England, followed--after long d e l a y - - b y pollution control, and the similar pattern, later and on a shorter timescale, in other nations; the finding of oil, the growth in its use and its displacement of coal from longstanding uses, occurring in the United States in the 1920's, in Europe a generation later, and now beginning in Australia; the early misuse of the perfect fuel, natural gas, after major discovery but before construction of long pipelines, occurring all over the world where gas is found (but modified as liquefaction has become feasible); the similarity, in fully industrialized nations, of the distribution pattern of energy use among transportation, power, industrial, and household and commercial users (after correcting for the effect of etimate on household heating). These similarities lead to the conclusion that the way energy crises in any industrialized nation are met hoIds lessons for other nations; that the energy stresses of a nation are destined to be repeated at other times in other places, and to other timescales. Because the United States is highly industrialized and a major user of world energy, and particularly because its energy problems have come under close scrutiny by many teams of engineers in the past several years, the U.S. energy scene will be used here as an example of what may properly be called world energy problems. Energy for the United States has, up to the present time, been supplied almost wholly by fossil-fuel combustion. In 1970, hydropower contributed less than 4%, even after weighting it threefold to allow for its fossil-fuel equivalent for power production. The good press that nuclear-power developments enjoy, and the enormous increase predicted in nuclear-power generation in the next decades, have created in many eircles--including some political ones-the impression that fossil fuel is too dirty, and

is on its way out. But the most-optimistic projections of nuclear proponents indicate that nuclear energy in the year 2000 will account for but a fourth to a fifth of our total energy consumption. The Combustion Institute has a long life-expectancy ! The objective here is to consider the factors affecting the future course of energy consumption, to examine the record of resources and reserves, briefly to look at nonfossil-energy sources, and finally to consider some of the technical problems associated with the projected gas and oil deficiency. Figure 1 is nothing more than a reminder of the factors to be taken into account in making an energy projection--past history and the difference between past and future. The difficulty is that those differences are themselves in the future and, therefore, presently difficult to identify; and the chief merit of the outline is to remind us how dimly we see ahead, how cloudy is the crystal ball. The equivalent of Figure 2 has appeared many times before, in one form or another, 1 but it merits detailed study. I t is a 120-year record of U.S. annual consumption of energy in the form of wood, coal, crude oil, natural gas, hydroenergy, nuetear energy, and their sum, from 1850 to 1970. A logarithmic scale is used for energy to minimize the tendency of all of us to say, no matter what our age, "the important changes in social organization and in i n d u s t r y - especially those changes related to g r o w t h - have occurred in my lifetime." The most significant feature of the plot is the relentless upward march of total energy consumption at a nearly constant annual growth rate averaging 3~% for the last 40 years (but 4½% for the last 9). The similarity of new-fuel growth is indicated by the times of attaining the figure 10%, then 20% of the total: for coal, in 1851 and 16 years later; for oil, 1918 and 9 years later; for gas, 1935 and 17 years later. Nuclear-energy consumption

PLENARY LECTURE L,qlAT IS TI~EHIX OF EIIERGYSUPPLYI;! THE FUTURE?I

IPROJECTION OF USES I

I PROJECTIONOF SOURCESI

\

I GROWTH

If FUEL COSTS1~PATTERNSI I STA?~DARDSU ~

6

7

/

8

9

lO

II

12

FIG. I. Factors involved in projections of energy supply and demand.

should be about equal to wood by 1975 (it was one-fourth in 1970). (Though that statement could be quite misleading in its implications.) Of maior concern to all, of course, is how the empty area from 1970 on is to be filled in. Let us postpone considering the separate fuel contributions, and look only at the total. I have perhaps watched this mounting curve longer than most readers. Off and on since around 1930, a graduate class in combustion has been led off with a brief look at the U.S. energy picture; and, in preparation, a bit of reading has been done on the latest prognostications. Always, in the literature, there has been the comment, "This upward m a r c h - this straight line on a semilog p l o t - - c a n ' t continue forever in a finite world; it must bend." But when? The consensus has most generally been "in about 15 years." M y present opinion is that those prognosticators who now say the bend will be very slight by the year 2000 are most nearly right. Continuation of the growth rate of the last 9 years would bring energy consumption to about 250 quintillion Btu's by the year 2000; the rate of the last 40 years would bring it to 200; 180 and 210 fairly well bracket a number of projections. We may properly ask, "Will such continued growth happen? For how long? Should it happen?" Before considering the answers, let us look at some more data. Figure 3 shows the per-capita energy consumption, expressed both as millions of Btu's per year and as a multiple of the average human caloric intake. In 1970, the consumption was equivalent to 6.7 gallons of petroleum per day, or 80 times the human-body caloric intake, which is a way of saying that our affluent way of life

is achieved by the equivalent of 80 slaves working for each of us. Increased energy consumption should produce an increase in goods and services. The former, measured by the per-capita gross national product (GNP) appears as a second curve in the lower part of the figure. If goods and services are produced in constant ratio, the quotient of G N P (goods) b y energy consumption should be a measure of the efficiency of energy use. This ratio, SGNP/106Btu, appears at the top line of the figure. We appear to have improved the efficiency of energy use since about 1920, although there is a recent sag (most probably due to our increasing use of energy for services ). Further evidence on the effect of per-capita energy consumption on G N P appears in Fig. 4, which contains two separate ideas. Look first at the heavy line, the position of which is supported by a cloud of 52 points, only five of which (the black dots) are shown. That curve shows the relation in one particular year, of the effect of per-capita energy consumption on GNP, as measured by 1969 data from 52 nations. (That 0.8-power on the curve-fitting relation, appearing on the figure, may suggest to some readers that we live in a turbulent world.) In addition to the effect of energy consumption on GNP, as measured by international comparisons, the march with time of both energy consumption and G N P is given for each of five nations. In France, for example, the energy and G N P are shown for 1958, 1961, 1963, 1966, 1969. For each nation, increased energy consumption has been associated with increased productivity. The slope is poorest in the U.S., which is using

i00 80 ~, qb

601

40-

b-.

"K o ©

q. % q.

i"

I ~ ' ~ ~ :"

~'-" "i ,, . . . . .

~ :

..,

,

,

~

~

j.,...L.

__~ ;

~

,,

.

~

. . . . .

. ~r~

121 Z

to ~Z

k.. .

.

.

.

.

.

.

.

~

.....

~i

~'';-v

: ' - ~

J~::L'.;

-:4

""

:

-:-

::'

:-£1

~:+:

....

b-J :Z bl

-:z

.>__ 2 ~'.

×~:_.::F~,~ -:i:

0

/

, r ~ : ....

, .....

'~'-ii

'k.::

j<

'--~,':-~<~-_-,~ ~: ; . . , ; # -

: : t i i : ' . ~ / . . . . . WT ! : ! i : ' ! t

I

;

1'1!i:~)1

.~w~' 1:| ~;;'

'

~i];

l!lllt'O,."~v",',ltJ

~ /I',

~q " :

i" I!l"

'~"~ I'E~ ,T~

4.,~},

' I'!lldq-/%J---~-t~:I-LH-.

~."-,~,;'...,2 ",-'

~ -j-

,~' ~ q I H ~,j+.

~

J~W'lTT-'~1T'!ffJffd

©

"i ~1 J

f

-40.8-

/

o [m H

m

~0.6C5

H C

0.2

:Z4 j.:t ":i<~Z:: 0 . / ,~i .... t+~,~,: ....

t50 '60

[ ~< .... ]:l:]

'70

LL!".;: .', ,,,,t~:)lfi,%ffil

/ ~'Sf(J

v.)~

!idf£~lJ|i!iiiiit;

'/900

il-iti~

+;j<',,':: '. . . v, ~ i, b f!l f l ~ :. ~ ,, ,. , , ;t,,~.., - t f f t r -,~ ¢ t M .~,~,~ , ~ ; ~ 1 .:.'.:-... , , , , :~, . . . . .,;:~"~¢i;t<4~< ,,

20yE A'20R 'aO

0

'50

'60 /'70

"80 "90 2000

Fro, 2. Annual energy consumption of the United St~tes, 1850 1970 (RcI. 1). ¢O

0c4d

k 2~

O ~d

,..j

1850

/860

/870

/880

/890

/900

/9/0 /920 [930 /g40 /950 /960 /970 /980 /g90 YEAR FIG. 3. Per-c~pit,~ energy consumption of the U.S., per-capita Gross National Product., and their ratio, in dependence on time (Ref. 1).

COMBUSTION AND ENERGY FOI~ TItE FUTURE energy increasingly for services (air conditioning is one example). Let us now return to the question, "Will growth in energy consumption continue as projected?" The remainder of this article could be spent discussing the point; there is time only to summarize: The clear evidence of the material gain associated with use of more energy, and man's appetite for material gain, imply only one realistic answer: "Growth will continue." And the time-constant of social and industrial change, measured by the life of industrial plants and corporations, and by man's slowness to change his values, suggests strongly that 30 years is a short time. The other question, "Should growth in energy consumption continue?" is harder to answer. There is no difficulty in convincing ourselves that the finiteness of world resources will prevent indefinitely continued growth. World energy growth at its present rate of about 5% per year--maintained now for some years--if continued for 200 years would increase out" energy consumption rate 17,000-fold! But when the question "Should it continue" refers to the near future (say, 30 years), my summary is: our substantial absence of knowledge of the energy-rate society can healthily support given infinite energy resources, the right of the many underprivileged in our society to more of the world's goods, the fact that it is more feasible and, to my mind, more effective in curing social ills to increase the world's wealth than to equalize its distribution--these all support the answer "Yes." What resources are available to support the projected energy growth to the year 2000, or even to 19857 An estimation of the energy resources and reserves of the U.S., based on numbers chosen from several sources purposely to indicate the lack of agreement which exists among experts, appears in Table I. For orientation on magnitudes:

Coal Consumption in 1970 was 0.49 (XI09 metric tons, the units used in the table). The U.S. coal supply is good by any standards (but the mining industry to supply it is quite inadequate).

Petroleum Consumption in 1970 was 5.iX109 bbls. The special term "discovered" implies high probability of oil in the field, but no knowledge of just where, or of percentage recoverability. "Proved reserves" is the basis for near-term planning. For many years, the ratio of proved

TABLE I Energy resources of the U.S. (Mixed sources and computation methods, not necessarily consistent) CO,L (units of 109 metric tons; 1 ton~27.8X 106 Btu) Known reserves Conservative estimate of mapped and explored coal < 1000 ft depth, 1400 1000-3000 ft, 1100 +3000 ft, 400 Estimated total remaining recoverable Proved and currently recoverable

4300

2900 1300 180

PETROLEU~I (109 barrels; 1 bbl~<-5.SN106Bin) Originally present onshore, 1400 off'shore, <200 m depth, 700 offshore, >200 m depth, 700 t~ecoverable with present, methods onshore, 246 offshore, 171 Discovered and not used To be discovered Proved reserves, incl. 9.6 in North Slope (For comparison, foreign proved reserves, 575 )

2800

417 329 388 39

SHALE On~ (109 bbls) Presently in oil shale deposits 1068 or 1500 or 2000 Deposits > 10 ft thick, >25 gM/ton federal, 480 private, 120 600 Proved and currently recoverable

77 or 130

(accessible, high grade) (10~ bbls) Estimated total recoverable Currently recoverable (For comparison, Canadian values are 500 and 80)

BITU~IX

NATUaALGAS (10I2art ft; 1 CUf i l l 0 0 0 Btu) Remaining recoverable Proved and currently recoverable NATURAL-GAs LIQUIDS Remaining recoverable

Proved and currently recoverable

2 1

1457 279 36

7

LTRANIUhI(units, 103 short tons oxide; 500X10 ~ Btu/ton oxide, if used in light-water reactors) At <$15/lb, proved and currently 975 or 1450 recoverable At $100/ib (A.E.C. projection) 25,000

PLENARY LECTURE

n;5

r!

>-

u!.

jJ

/

i

!

!

¢/)

//

0 0 04 '

£) d3 0 r~ 0_3

,

i

JE I [ FRANCE

z

i

?,

0 t-< Z 2

S A.

I

r

I

!

\

I

r

5 2-NATION AVERAGE, 196_9 : -~

Ul U9

¢t(9

leee

i

/

0

/

l

<

P"

--

GROSS'NATIONAL PRODUCT ~4 p e r c a p i t a ($U.S/YR.)= / /ENERGY CONSUMPTION,~0.81 0.54 [ KWH/YR. peccapita J |

UK. " WESTGERMANY

< 0 tit

Lfl

nO_ 0

~,PAN i ##A

Encircled p o i n t s - - d a t a f o r 1958, 1961, 1963, 196G, lC,f~t~ i~, :~q,e~.-.,~ f r o m lower l e f t , f o r f i v e nations I

I

I

I

J

10 20 30 40 50 60 70 80 90 100 PER CAPITA ENERGY CONSUMPTION, lO00's KWH/YR.

FIG. 4. World per-capita Gross National Product in 1969, in relation to energy consumption (heavy line) and similar relation over eleven-year period, for each. reserves to annual consumption was kept in the neighborhood of 12. As oil has become harder to find, it has dropped to 7. In comparison, Middle East proved reserves are enormous.

Shale Oil The last line indicates that a 16-26-year supply of oil (at the consumption rate of 1970) would be worth processing almost now, but for federal ownership and unfavorable leasing arrangements, and a cost somewhat above present oil prices; and that muck more is available at a price.

Bitumins The Athabasca tar sands of Canada are significant; those of the U.S. are not.

Natural Gas Consumption in 1970 was 22.5X1012 cu ft. The ratio of "currently recoverable gas" to

annual consumption is the most alarming number in the energy picture, when the phenomenal growth rate of gas consumption for the past 40 years is taken into account (see Fig. 2). A gas shortage is not only imminent, it is here. New customers are being refused gas in several parts of the country. Space has not permitted the presentation of the full basis for the projected near-term estimates of shortages in gas and oil. Figure 5 presents data from two separate sources on gas production and consumption, from 1955 to the present, together with projections to 1985; the two independent projections are placed backto-back, timewise, to minimize confusion. Only the right-hand set of curves shows, as a dotted line, the projected gas demand. These two studies support the AGA estimate of a gas deficiency of 13X1012 cu f t / y r by 1985, after allowance for Alaskan gas and for Canadian and L N G imports, and for a modest contribution from synthetic gas.

COMBUSTION AND ENERGY FOR THE FUTURE

S

30

folN! j ,%o :,

-15

/-/

(3

nY

re

>a-

--t0 0

2_o

t

.... FR!~ F

4B-STATE NET

U U

E

RESEavE~'I

PRODUCTION

#_

Z 0 J J

--

~

.

! ,'// I

\~"

i,i, RoM

,o

Z o3 _5-1 >__

o

i

t~ I--

I

U.S. G A S S U P P L I E S TWO PREDICTIONS COMPARED

~5

1960

196,5

1970

1975

1980

196.5

i965

f9@O

O

1975

1970

196,5

1960

'-0 {955

Fro. 5. Two independent projections, timewise back-to-back, of U.S. gas supply, to 1985. A similar comparison of recent-past and nearfuture proiections of oil supply and demand appears in Fig. 6. The two studies are in substantial agreement that, after allowance for Alaskan North Slope oil and for Canadian imports, a foreign, offshore, crude-oil importation of about 16 million barrels per day will be necessary to satisfy the U.S. demand. Figures 5 and 6 support the statement that, by 1985, the U.S. will be grossly deficient in energy supply from domestic gas and oil. A popular reaction to this energy crisis is to suggest turning to inexhaustible energy sources, like the sun, or to geothermal energy (which, in theory, is also substantially inexhaustible). Space prevents an adequate treatment of these two possibilities here. There is in the world, today, 700 MW of installed geothermal power, of which 82 MW is in the U.S., and an additional 330 is under construction. Prospecting for geothermal power has been inadequate to support projections, which range from 1000 to 5000 MW for the Geysers (in production), 20,000 for the Imperial Valley, 30,000-100,000 ultimately for the whole U.S. (installed U.S. capacity by 1985 will be about 700,000 MW). There are many problems, and one can only conclude that although geothermal power may grow, its contribution by 1985 wiIi be ahnost negligible.

A personal long-standing familiarity with solar-energy utilization, through chairmanship of a committee directing research in that area for many years, has convinced me that the prospects for thermally generated solar power are poor) A 24-hour-average 1000-MW solarpower plant, operating in the best U.S. solar climate at an efficiency of 10%, would require 11,000 acres of solar-beam intercepting surface plus at least an equal additional area to offset interference of adjacent elements. An optimization of the operating temperature, i.e., balancing of collector cost against power output, would, from past experience with computations on many systems, move the efficiency down rather than up. Power from silicon photovoltaie ceils for space research has cost $200,000 to $300,000 per kW for the cells alone. There is no consensus on the possible minimum cost of cadmium sulfide cells for terrestrial use; $9 per watt, $5 in a few years, $t to $1.50 later, 10/ ultimately are figures which have appeared. Such cells would have high economic significance at 50~ to $1 per watt, and stimulus to cheapen them is warranted; no one is presently able to evaluate the chance that this will happen. It is improbable that 1985 will see any significant use of the sun for power, although low-temperature heat for house-heating, domestic hot water, air conditioning, and food

PLENARY LECTURE 35,

35

U.S. PETROLEUM' SUPPL"I I

I <>d 3 3 0 - -

I TWO

PROJECTIONS

( 30

x"-~ T O T A

13:: Ld 0.

L

o EMA.O',,,

_J Z 5 L,.I

-q-

25

n-" < m LL 0

20

{.9 Z o 1

20 OFFSHORE 5

CRU DE

~

15

J ig

,

]0

NYMTFI -FTI(Z CRUDE

I0

i i

ff

-

ii 0~ 1960

1965

1970

[975

1980

1985

1985

i980

1975

1970

1965

1960

0

FIG. 6. Two independent projections, timewise back-to-back, of U.S. petroleum supply and demand, to 1985. dessication could by that time contribute a few percent of the U.S. total energy consumption. Forecasts were made in the 1960's that the capital cost of light-water nuclear power plants would come down to $100/kW, and that nuclear power would cost 4 mills per kilowatt-hour. Developments have not come close to meeting that promise. Rapid growth in nuclear power, however, is certain; and Table I I supports the statement that U.S. uranium resources are probably comparable to U.S. coal reserves, if the price of raw uranium concentrate is allowed to climb. According to the table, that price need only slightly more than double to meet the needs of the year 2000, even if the breeder has not been developed by that time (fast-breeder reactors would increase reserves about 130-fold in terms of power production). As already stated, however, the total U.S. energy requirement in 2000 will be supplied less than one-fourth by nuclear power. One thus arrives at the conclusion that, for at least the next two decades, the inevitable alternatives, not mutually exclusive, are: 1. Import large quantities of crude oil and LNG., 2. Encourage a more vigorous search for gas and oil, as by changing the federal position on gas-price control and federal land lease.

3. Increase the research effort on improved recovery of oil and gas from the ground. 4. Increase the efficiency of use of energy. 5. Burn coal cleanly. 6. Convert coal, shale oil and/or bitumins to clean gas or clean oil. In the broad sense in which an interest in combustion includes an interest in conversion of one fuel to another, the last three of the above alternatives are of vital interest to the members of The Combustion Institute. Comments will be made on these items.

Increasing

t h e E f f i c i e n c y of E n e r g y U s e

That the use of energy must have increased in efficiency over the years was indicated by the slopes of those lines of Fig. 4 which represent single-nation performance, a n d - - a t least from 1920 to 1966--by the top line of Fig. 3, the slope of which would be much steeper if allowance were made for the increasing diversion of energy to produce services rather than goods. Here are three examples of U.S. improvement in energy use:

In the 40-year period, 1925 to 1965, the average thermal energy required to produce a unit of

COMBUSTION AND ENERGY FOR THE FUTURE electrical energy--known as "heat rate," B t u / k W h - - h a s dropped from about 25,000 to about 10,000. In the 20-year period, 1950 to 1970, the coke needed to produce one ton of pig iron has dropped from 0.92 to 0.03. In the 19-year period, 1950 to 1969, the unit energy consumption by railroads--Btu per ton mile--has dropped phenomenally from 3500 to 750. Though the first two of these examples show evidence of having reached a temporary plateau, the possibility of a new thermodynamic cycle for power production (gas-steam, for example) or a major modification in blast-furnace practice (introduction of reducing gas near the top, for example) holds the promise of changes analogous to what happened when the Diesel engine took over on the railroads. The industrial scene is so full of far-younger and less-settled processes than the three examples given, and so full of gross entropy increase, as undoubtedly to offer prospects of further major increases in efficiency of energy use. These improvements include increased efficiency of combustion, lower At's in heat exchange, better thermodynamic cycles for power production and, especially, new ways of producing goods or converting materials. The combined efficiency of eonbustion and heat transfer in boiler furnaces is already so high that impetus to research is not thermal efficiency but pollution control; furnace effieiencies are already in the 90's. High-temperature operations--glass-making, smelting, roasting-are less efficient, but the cost of fuel has, up to now in most cases, been so low relative to the value of the product that engineering talent tends to be put on controlling product quality rather than on saving fuel. This will change as fuel prices rise during the coming decade; and many high-temperature heating operations will deserve and receive close study to improve them. I t is a challenge to solve this problem while simultaneously minimizing NO formation. A significant fraction of our energy--about 10%--goes into house heating. The ASHRAE Handbook lists efficiency percentages in household furnaces of 60-75% for hand-fired anthracite, 50-60% for hand-fired bituminous coal, 60-75% for stoker-fired bitmninous coal, and 70-80% for oil and gas. These high clean-furnace values, however, are somewhat misleading. Small furnaces tend, on the average, not to be in good adjustment, thereby reducing the above figures by 5% to 10% for continuous operation. In addition, space-temperature control by thermostat is conventionally based on O N - O F F burner operation. I)uring start-up and shut-down, oil atomiza-

TABLE Uranium

II

resources and requirements

Price of uranium concentrates, $/lb uranium oxide

Tons of uranium resources at this or lower price

Source: A.E.C., 1970 (Ref. 2) 8 594,000 10 940,000 15 1,450,000 30 2,240,000 50 10,000,000 100 25,000,000 Requirements for supplying projected nuclear-power needs with light-water reactors Electric-generating capacity (103 MW) Nuclear

Cumulative consumption of uranium ore concentrates, U~Os,

Year

Total

1970 1980 (1985) 2000

Source: Benedict, 1971 (Ref. 3) 300 6 523 145 200,000 (386,000) 1550 735 1,600,000

tons*

* 171 U.S. tons of U30s generates 1000 MW-years of electrical energy in light-water reactors (or 1 ton is approximately equivalent to 20,000 tons coal). tion is momentarily poor and the air-flow rate is quite variable because of stack-draft changes, and soot production can be high; during shutdown, air continues to be drawn through the furnace, cooling the firebrick setting, as well as the furnace-heat-transfer surfaces. Such intermittency can easily drop the over-all efficiency to as low as 50%, or, in hot-water heaters, even to 30%. If a national conscience is to be developed in the area of effective use of energy, increasing our home-heating furnace efficiency is an area worthy of attention. A guarantee of 80% average efficiency could be met by new equipment, and fuel prices to come will justify the capital investment necessary. H I G H - L O W rather than O F F - O N burners merit development, but the impetus to develop them is largely missing as long as the householder is not energy-conscious. The argument that the resulting fractional change in our national energy balance will be unimportant is a dangerous one; the totality of improvement is certain to be the sum of many small contributions. An improvement in the

PLENARY LECTURE

10 1,000 -

COMBUSTION

-

_

~

~NO,NE

SIEVE SODIUMZ[NI ~ i SULFUR

~N CIKEL100

o.,

EXTERNAL COMBUSTION ENGINE - LITHIUMw

40mph

~,,~ ;, '5 :' ci'.i.',. so~,,.,

,oo~,,.,

I~" 0 U

--~ . . . . . ..... \ 7~.k \ - --

.,;:;, U

tO 100 SPECIFIC ENERGY,watt-hrs/Ib

1,00°

FIG. 7. Motive-power source requirements for motor vehicles (superimposed scales of speed and range based on 2000-1b vehicle, 500-1b power source, and steady driving; power and energy taken at output of conversion device) (NAPCA, 1970). U.S. household furnace efficiency from 55% to 80% corresponds to a total saving equivalent to 400 million barrels of fuel oil per year.

Transportation Transportation consumes one-quarter of our energy supply, and the automobile most of that. The effect of imposing increasingly rigorous control of pollution has been to decrease the engine thermal efficiency, and the challenge here to the combustion scientist and engineer is to minimize the loss of efficiency associated with controlling CO and HC and NO emissions. A 1970 evaluation, by the Air Pollution Control Office of the Environmental Protection Agency, of nineteen approaches to solutions of the three types of pollution-control problems to meet 1975 standards gave a technical feasibility rating of "Poor" on sixteen and "Fair-to-Poor" on two, a probability of achieving the research-goal level (1975 standards) of 0 on sixteen, no rating on two, and 80-90% on one (the use of exhaust manifold reactor with exhaust recirculation to control CO and HC's). Research, development, and performance tests proceed in the following areas: CO and HC Reduction Secondary combustion (in thermal or catalytic reactors), modification of engine parameters (air-fuel ratio, ignition timing, manifold de-

pression, coolant temperature, compression ratio, volume-surface ratio). NO Reduction Exhaust-gas recirculation, optimization of airfuel ratio, and recireulation as affected by power and speed, catalytic converters, selective reactant addition (ammonia), use of low mean hydraulic radius of chamber, compression ratio, stratified charge, pre-combustion chamber. Alternatives to the Conventional Otto-Cycle Engine Figure 7 shows comparative specific energy (roughly corresponding to range) and specific power (roughly corresponding to speed) for several different power plants. A good engine is high on the extreme right of this figure--and not too expensive. A variety of engines are being explored as alternatives to the internal combustion engine. The APCO has chosen to concentrate on five systems: gas turbines, heat engine/electric hybrids, Rankine-cyele engines, heat engine/flywheel hybrids, and all-electric systems. Other systems receiving less attention include rotary combustion (Wankel) engines, fuel cells, stratified-charge engines, Stifling engines, and free-pistoI1 engines. Much work in this area is also being sponsored by private industry.

COMBUSTION AND ENERGY FOR THE FUTURE Clearly, research and development to consume cleanly one-quarter of the nation's energy in automotive transportation is proceeding along many fronts. To increase the chance that some of these projects will produce viable alternat i v e s - a l t h o u g h not by 1975--there is need for supporting research of a basic character. The automotive engineer is, typically, wary of basic research on gaseous reaction kinetics; perhaps he remembers how nearly valueless was the contribution of the kineticist to the knock problem of the 1920's and 1930's, compared to the contributions of hundreds of more-plebian measurers of engine performance, but he forgets that free radicals had hardly been mentioned in the days of ~£idgeley and Boyd. The impressive growth in our knowledge of gas reactions--much of it catalogued in the published volumes of this I n s t i t u t e - - h a s implications which must be exploited. We need to know how NO is formed, how combustion interacts with radiation and convection in engines, what limits the completion of exhaust-gas combustion. We need kineticists willing to test their knowledge of primary kinetic acts on systems more complex than one would choose to study if knowledge for its own sake were a sole objective, willing to force a complex kinetic system performance into its simpler approximate equivalent form, with careful statement of the limits of validity of the approximation, in order to produce results ready for engineers to use in their study of interaction of kinetics with flow and energy transfer. The kinetics of catalytic combustion must be examined, ineluding a continuous search for catalysts not poisoned by lead; and the kinetics of NO elimination by dissociation or by reduction must be formulated. In the area of fluid mechanics there is need for sealing and modeling studies to better understand cylinder-piston fluid mechanics and its interaction with chemical reaction kinetics. Control of burning and mixing patterns has been largely on a cut-and-try basis. With a clearer understanding of the physical processes involved in the generation of pollutants, the cuts would be better and the number of tries less. As an example of a possible contribution from fluid mechanics (for which I am indebted to Dr. Keck), a significant portion of the unburned gases comes from hydrocarbons in the quenched boundary layer, which tends to be rolled up along the walls by the piston on the exhaust stroke. If the vortex containing these hydrocarbons could be retained in the cylinder in the clearance volume, and thereby made to join the next cycle, hydrocarbon emissions should be reduced materially. More sophisticated studies of heat transfer

11

and its associated power requirement are needed, partieularly in application to the optimization of air-cooled condensers for Rankine-cycle engines. The search for storage-battery components of high specific power and specific energy should continue, coupled with an analysis of diffusional limitations. Fundamental studies, of which the above are typical, must be pursued to keep options open, but measurable movement toward a practical solution necessitates developmental effort by industrial laboratories, federally sponsored when the prospects of gain constitute an inadequate incentive. This is the present situation and, as already indicated, APCO has responded with a concrete program. Basic research of the type appropriate for sponsorship by N S F can constitute an effective support of the developmental effort.

Central-Station Power Cycles One-quarter of the energy consumed in the U.S. goes into electric-power production, 85% of that is based on the burning of fossil fuel to operate steam turbines, and 55% of the fuel for that combustion is coal. Although power cycles appear here as a part of the discussion of efficiency of energy use, another justification for their inclusion is that much of that U.S. coal which is availabIe in the East, where power is most needed, is high in sulfur; and one way around that problem is gasification to produce a clean gas-turbine fuel. Gas turbines are presently used in power plants almost exclusively for handling peak loads; they are low in thermal efficiency but also low in capital cost--important for low load-factor operation. A ease can be made for the assertion that, largely as a result of the great expenditures made on developing advanced-cycle gas turbines for military and civilian aviation use, the gas turbine is destined in the next two decades to replace the steam turbine as the primary power unit of fossi-fuel power plants. The efficiency of the gas turbine is limited primarily by intake gas temperature. Land-based turbines--often modified AC turbines--are operating on base load with 1800°F (1255°K) inlet; the engine for the 747 let has a top inlet temperature of 2400°F (1590°K) at take-off, 2250°F (1505°K) at cruise; military engines are running at 2500 °2800°F (1645°-1810°K). Differences between design philosophies for stationary power plants and for aviation propulsion systems may be expected to diminish as it becomes increasingly clear that, whereas the Rankine steam cycle has reached its peak of performance at about 40%, the gas turbine is still on the upward march.

PLENARY L E C T U R E

12 IMPROVEMENT

a.

IN C R E E P

STRENGTH

IN B L A D E

ALLOYS

2000

I,LI UJ r,,

u

1900

I,Z

"L,J' - -

1800

144 c~ a. o

o

o

1700

I.U

u

~ . 1600

,~ a.

~

1500

o ,=-

1400

ILl n~

=_z

~-

1300

144

~.

1200 1950

1946

m

I-.

1954

1958

1962

1966

YEAR

IMPROVEMENT

a..

IN C R E E P

STRENGTH

IN V A N E

ALLOYS

2000

tu I.U

,

U

1900 ~ - ~

Z

I

.

J

U~

,.., a. 1800

i

2 "°~ 1700

i

I,,Uo

~'~

~ SM'2 00DS ~

WI-52

I,u I..,.. 1

,-,~ 1600

X-40

0

I

0..~ 1500 oo

I,--o o I.g~

z

::::3-I-.

"" /.ILl

1400 - -

t

. _ -~ . . . . . . . .

1300

a.

tu 1-

1200 ....... 1946

i

t 1950

1954

1958

1962

YEAR FIG. 8. A d v a n c e s in aircraft-turbine materials (from Ref. 4).

1966

COMBUSTION AND ENERGY FOR THE FUTURE

13

IMPROVEMENT IN CREEP STRENGTH IN BLADE ALLOYS

2000

O= LM MJ e~

1900

U b=. Z ~M

1800 o

c~

o

tM

SM-20C '

1700 1600

U C~ 0n,,

IN-100

E

1

.I

1500

g ~J

1--

1400 1300

,,

,

I N C O - 7 ~3 ''' ~ ,1-" "

IASPAOY

..~_..SS~. _.

B

+

Zr ADDED

; ~ A " S PA L D Y NIMONIC ""~'x~'~" A I R M E ' __ 80/, ~'~'~)'~ , c.t

1200 1946

1950

I i i

1954

1962

1958

1966

YEAR

IMPROVEMENT IN CREEP STRENGTH IN VANE ALLOYS 13. '"

I

2000

t~

~Z

I

I

'

I

'

!p

i

U

1900 - -

U~ "u.I¢~ ' ~" 1 8 0 0

....

- 2 1700 °°

i

i

r' i i

Wl,,-

VITALLIUM

-

=

I

g___ 1 4 0 0 ~ iev e~

1300

I --- l

1200 I--

1946

I

I

!

!

!

i~

! I

~

. . . .

I 1950

i ¸k ii'i 1954

1958

1962

i

k¸ 1966

YEAR

FIG. 9. Creep strength for advanced turbine-blade materials (from Ref. 4). Some years ago, 42% was reached in an Ohio power-plant, but economic optimization has brought the figure back to 40%. Figure 8 presents the record of advance in aircraft-turbine materials, as measured by the temperature to produce a given creep in turbine blade and vane alloys.

Temperatures permitted in blade alloys have climbed steadily 600°F (333°C) in 20 years, vane alloys 300°F (167°C). The extent to which alloys needed for future turbine developments are presently available is indicated in Fig. 9. The gas-inlet temperature is related to, but

PLENARY LECTURE

--1 t i i I I

I I I I =I

l~I~l ]~]

I ~.I

-i

I

~

t I

i

I

]

I !

v I

u

[

l

I

~3

±

l h

1 r

%

I

.J-

\

I

I

\

\

=~

~

I

COMBUSTION AND ENERGY FOR THE FUTURE not equal to the permitte d blade temperature. For inlet temperatures of 1800°F (1255°K) and up, blade and vane cooling through several stages will be necessary. Progress in cooling has been impressive. Convective cooling by radial flow through the hollow blades will be replaced, as temperatures rise, by impingement cooling of the leading edge from inside the blade, permitting inlet temperatures as high as 2400°F (1590°K). Aircraft-engine studies of transpiration cooling provide a basis for further rise in inlet-gas temperature. These foreseeable improvements in stationary turbines, based largely on achieved improvements in advanced-design aircraft turbines, led to a UAC study 4 of the efficiency achievable from a simple advanced-cycle gas turbine operating, without heat exchange, on low-Btu gas from a coal-fired gas producer operating at about 20 atmospheres. The results are shown in Fig. 10, where the effect of compressor pressure ratio and turbine inlet temperature on the efficiency and the shaft power per unit air flow are shown, for nine sets of conditions. Roman numerals refer to first-, second-, and third-generation possibilities, corresponding to the early 1970's, 1980's, and 1990's. Consider the first generation: If the turbine-cooling air from the compressor is unprecooled and a turbine-inlet temperature of 2000°F (1365°K) and a compression ratio of about 20 are used, a thermal efficiency of 35.8% is possible; if the inlet temperature is raised to 2400°F (1590°K) and the compression ratio to 28, the cooling air must presently be preeooled to 250°F (394°K), and an efficiency of 37.7% is possible. The cycle is simple; when the fuel is low in heating value, and the temperature as high as proposed, regenerative cycles are found to provide no significant improvement in efficiency or specific output. More impressive than the simple cycle is the combined gas-steam cycle. With turbine-expansion ratio somewhat reduced, the gas-exit temperature is high enough to operate a wasteheat boiler to supply steam to a steam turbine. Again, on the basis of realistic coefficients of heat transfer and of compressor and turbine performance, the power-system efficiency has been computed, 4 with results as shown in Fig. 11. The striking values of 47%, 54%, and 58~o, for efficiencies possible in the first, second, and third generations, respectively, support the view that such a system holds great promise. After allowance is made for the efficiency of conversion of coal to gas in an advanced-design pressure gas-producer, a third-generation efficiency of about 50% is projected. But development of 300 megawatt advanced-cycle turbines, to replace the largest presently available "conventional" units (50 MW) will not be cheap.

15

Magnetohydrodynamic power generation is appealingly simple in principle but appallingly difficult to achieve, because of the materials problems related to the high-temperature necessary--2600°-3000°K--in the combustion gases. The general consensus is that the most-likely near-term hope for M H D is the open-cycle or once-through coal-fired system, using potassium sulfate as the seed material, with the M H D unit serving as a topping unit above a steam unit. The coal-combustion problem is serious. An adiabatic temperature of at least 2600°K must be achieved, using air and coal containing 10% ash; and the air preheat temperature must be limited to 1370°K until there is available either an effective technique of ash removal from the combustion products or preheaters operative with slag-laden gas. Highly efficient removal of ash or slag from the gases entering the M H D channel is desirable, but it must be achieved without an appreciable loss of seed in the slag. Two-stage combustion has been proposed, with slag removal between stages. This is the equivalent of operating a gas producer as a first step, but with the constraint that no temperature moderation by use of steam or CO~ be permitted because the second-stage temperature would thereby be dropped. The physical phenomena of ash removal at the temperatures anticipated-its vaporization and subsequent condensation, and its interaction with the seed material-present a combustion problem not previously encountered. Moreover, the complex composition of the gas, in association with the small residence time, may put it under chemical-kinetic rather than chemical-equilibrium control, complicated by interaction with condensed slag. The problem of NO control is not solved. Malty other problems not related to combustion remain. These obstacles leave little room for enthusiasm about the prospects for M H D development, especially since the contributio~ which M H D might eventually make--electricity from fossil fuel at a thermal efficiency exceeding 50%--appears now to be achievable by combined gas-turbine and steam-turbine plants. The Clean Burning of Coal It appears, to an increasing degree as domestic oil and gas supplies dwindle, that coal is destined to be the only fossil fuel plentifully available in the U.S., but there is no evidence of movement toward more extended use of it. Its potential use through

conversion to clean gaseous or liquid fuel is considered in the next section. The largest present use is in electric-power production, and the most important clean-burning problem is therefore

PLENARY

LECTURE

I,

oo

I

t, u.l 0

r~

~_o 0

z~ 0.

Z

0 Z

I-

o~

o I1.

L~ Z g

I,-

u

Z 0

~D

5_z

o

~ m

v

-.-.

~

i 0

E

_zw

m z

]° z

0 O0 wO

Rr-

/

o~ ,,J oo u,,

o

w a I --

-'~

~.\~-Y~

o

.I

o~

r~o

COMBUSTION AND ENERGY FOR THE FUTURE presented by the central-station boiler furnace. This is generally an enormous (ca. 75,000 fta; 6,500 m 3) pulverized-fuel-fired chamber. The problem, recognized in the last few years, is sulfur dioxide-pollution control; limitations have been placed on permitted concentrations of SO2 at ground level in the vicinity of sources, on the sulfur content of fuel, and on the SO2 content of stack gases. Although 83% of the low-sulfur-coal reserves are in the West, 95% of the U.S. coal is mined east of the Mississippi. But one-third of that is low-sulfur coal, of which half goes to metallurgical use and one-quarter to export. The remaining small supply of Eastern low-sulfur coal, far below the nation's power needs, brings $2/ton more than otherwise comparable high-sulfur coal; and this adds 0.8 mill/kWh to power costs. Stack-gas cleaning to remove SO2 has so far proven much more expensive than earlier estimates predicted, with capital costs in the range $17-$55/kW (adding 0.4-1.4 mills/kWh in new power plants, more in old ones) and operating costs in the range 0.6-1.6 mills/kWh. The total cost of 1-2.5 mills/kWh (equivalent to $2.80 to $7.00/U.S. ton coal) justifies a search for ways to burn high-S coal cleanly. Two approaches to burning coal directly, with sulfur capture before oxidation and dilution in air, have been investigated. Both require changes in boiler design, manufacturing procedures, and operation; and their acceptance by boiler manufacturers, utilities, and industry may be slow unless they receive sufficient support to insure their full consideration. One process burns the coal in a fluidized bed of limestone particles, which react with the sulfur. A portion of the bed is continuously removed and replaced with fresh limestone or regenerated line. In England, the National Coal Board has studied atmospheric and pressurized systems, and an oil company has been developing a two-stage system ill which high-sulfur residual fuel oil is burned in the first bed while its sulfur reacts with lime of form CaS; this goes to the second bed where it is air-blown to produce a gas high in S02. In the United States, the Office of Coal Research and APCO have supported work on a fluidized-bed coal-burning boiler with submerged heat-transfer tubes, and on studies of the applicability of fluidized combustion to industrial boiler systems. The proponents of fluidized-bed boilers claim, in addition to sulfur dioxide coi~trol, significant economic advantages, in moderate-capacity units, over conventional pulverized-coal systems. The fluidized-bed system may offer an additional advantage with respect to NO-pollution control, because of its ability to operate at lower tem-

17

peratures than those existing in pulverized-coal flames. But it is difficult to visualize fluidized-bed burning capacities high enough to compete with suspension combustion in large chambers. The second process prevents SO2 emission by the submerged partial combustion of coal in a bath of molten iron. The coal-carbon dissolves in the iron and diffuses to the surface of the air bubbles. The resulting carbon monoxide is burned above the melt, and the sulfur is removed with the ash slag, and subsequently recovered. This process should generate research on the intriguing problems of combustion controlled by turbulent diffusion in molten metal. The projected economics of the process are said to compare favorably with the lower estimates for conventional power plants employing stack-gas cleaning. The difficulty with comparisons of processes as different as a tangentially fired boiler, a fluidized-bed boiler, and submerged combustion under molten iron, is the absence of well-established performance coefficients on a single coal in the three systems. Small wonder, since pulverized-coat furnaces developed without benefit of such guidance, and the scientific fraternity is still not in agreement on how to predict the burnlag time of a single particle from first principles. Conversion of Coal, Shale Oil, and Bitumins to Gas or Oil

Natural-gas consumption has maintained an almost-constant annual growth rate of 6}% for 15 years, and a less-constant but identical longtime-average growth rate for 60 years; this corresponds to a doubling time of 11.5 years. The near impossibility of maintaining that figure many years longer has produced the generally accepted conclusion that the U.S. will be grossly deficieut in ability to supply the gas demand not many years hence, and has yielded projections like that already presented in Fig. 5--an unsatisfied demand, after allowance for Canadian imports, gas from Alaska, and LNG imports, of the order of 13X 10~2 cu ft/year by 1985. Such a demand cannot be met by gasifying crude oil; the latter will also be in short supply. To indicate tim magnitude of the problem, let it be assumed that the deficiency will be met by the construction of gas-from-coal plants, each having the capacity to produce 250 million cu ft/day of pipeline-quality gas (smaller plants would be uneconomic). Each plant would cost over $200 million and would consume about 15,000 tons of coal per day. To supply the anticipated 1985 annual deficiency of 13X1012 c u f t by a plantbuilding program starting in ~975, one new plant

18

PLENARY LECTURE

would need to go on stream every 26 days, continuously, for the ten-year period--a total of 142 plants, involving an investment of around $30 billion (30N 109), with each plant consuming more coal than the output of any U.S. coal mine today. The projection of need for gas may be off, but the importance of getting on with coal gasification is clear. Use of the present pipeline network to move the gas requires that the heating value of natural gas (about 1000 Btu/ft a) be matched, i.e., that the new gas be nearly pure methane. Although the reaction carbon plus steam going to half each of methane and carbon dioxide is almost thermally neutral (it absorbs only 1.3 kcal/ gram-atom C) and thermodynamically feasible at temperatures below about 500°F (ca. 530°K), it will not go at that temperature. The alternatives are: (1) pyrolysis; (2) use of the endothermic steam-carbon reaction to make CO and hydrogen for later reaction with carbon, in a different part of the reactor, to make methane; and (3) addition of hydrogen--made from char--to the steam-carbon reaction to increase the methane yield in the fuel bed; or a combination of these. Energy cannot be supplied to the bed by combustion with air; oxygen is generally used to prevent nitrogen dilution. Among many problems, a major one comes from the fact that the endothermic and exothermic reactions occur in different parts of the system. The only presently available process for making pipeline-quality gas is the German Lurgi process. The main reactor is essentially a fixed-bed gas producer, fed with steam and oxygen instead of air. It delivers a product gas which must be cleaned of soot, tar, H2S, and CO~, changed in H2: CO ratio to 2:1 by catalytic shift-conversion, and finally sent to a catalytic methanator, where the highly exothermic reaction of carbon monoxide with hydrogen yields methane and steam. The disadvantages of the Lurgi process are: that it has been developed for noneaking European coals and, although it contains a water-cooled bed-stirring device, it has an unproven performance on highly caking American coals; that its reactor, about 15 ft diam, is so small that some 25 units would be required in a gas plant of the size previously described; and that U.S. labor costs, on so many units, would be excessive. In consequence, the Office of Coal Research and the U.S. Bureau of Mines are developing some eight other processes. Space prevents presentation of more than the two now in pilot-plant stage. The Hygas-Eleetrothermal Process is in 80ton-per-day pilot-plant stage, just beginning to produce data. Figure 12 shows the flowsheet. One-eighth-inch coking coal is fed to an air-blown

fluidized preheater at 750°F (670°K), to destroy its agglomerating tendency, is then mixed with light oil to form a pumpable slurry which is fed, at 65-100 arm, to a fluidized drying bed where the light oil evaporates, thence in succession through two fluidized reactor stages to produce char which, in the present proposed scheme, goes to an electrothermal gasifier where, with steam and electrical energy, hydrogen is produced. In reverse flow, the hydrogen-rich steam enters the lower main reactor, maintained at about 1750°F (1250°K), where a CO:H2 mixture high in hydrogen is produced. This gas enters the staged reactor, where pyrolysis and some methanation occur. The raw top-gas is purified of H2S, CO2, and light oil, and sent to the catalytic methanator and thence into the high-pressure pipeline. It is probable that the electrothermal gasifier will ultimately be replaced, possibly by an oxygen-plus-steam-blown suspension char gasifier, which makes a CO:H2 mixture requiring shift-conversion and CO2 removal before it goes to stage 2 of the main gasifier. The CSG or CO2-Aeeeptor process, primarily for lignite gasification, is being studied in a 30ton/day pilot plant. Figure 13 shows the flowsheet. Dried .~/~-n. lignite passes through an atmospheric preheater to lock-hoppers, from which it is fed into a fluidized bed at 10 arm where, in the presence of steam, CO, H2, and dolomitie calcine (MgO-CaO), it is volatilized and partially methanated, with C02 being absorbed by the lime to aid the methanation step and also to supply heat. The temperature is kept at 1500°F (1090°K) by the addition of calcined dolomite at 1870°F (13000K) from the dolomite regenerator. Char and partially calcined dolomite, from the upper reactor, pass to the lower one, where gasification with steam occurs, the calcine again accepting C02 and shifting the water-gas reaction toward hydrogen production for use in the upper bed. The CaCO3-MgO is withdrawn from both beds near their bottom, where its concentration is high, and sent to an air-blown regenerator. The gas leaving the volatilizer undergoes purification and catalytic methanation. The low temperature (ca. 1500°F) maintained in the volatilizer is necessary to prevent melting of the product of the reaction of acceptor with steam; it is probable that only lignite and some sub-bituminous coals are reactive enough to meet this limitation. Clean low-Btu gas from coal, no substitute for natural gas in transportation, but important if the production of electric power in the U.S. follows the gas turbine-steam turbine route, can be made today with an air-plus-steam-blown Lurgi gas producer plus a gas-purification plant.

COMBUSTION AND ENERGY FOR THE FUTURE

H PURIFICATIOII

Raw Gas

HYDROGASF IE IR Drying Bed fluidized

19

CATALYTIC METHANATION p~1i ne RawCoal PiGas

Coal Slurry

I

Fuel

Gas

fluidized

Light Oil

]" fluidized), J300-1500 I "x,~TAGE

1 arm ~

SLURRY PREPARATIO[~

}lot Air Stack Hydrogen-

RiLr.h G

Gas

ELECTROTIERMAL GASIFIER

a

Power POWER ~].C. C A[ID

~

\(fluidized) \1/00-1800 F: 1000-1500psi

......

Steam y Steam

STEAM PLANT

aL~R rP S E ER DE US C TU O IR N C}~

SYSTEH

Char HYGAS-ELECTROTHEP34AL PIPELINE

PROCESS

GAS F R O M

F O R MAKI~qG

COAL

FIG. 12. Hygas-Electrothermal process for making pipeline gas from coal (Ref. 1). It could also be made by adaptation of one of the processes being developed for high-Btu gas, with omission of oxygen and of catalytic methanation. The energy balances on the plant components are so different, however, that major developmental effort will be required to redesign a process for low-Btu gas. As to oil, Fig. 6 indicated that, by 1985, in the absence of conversion of coal, shale, or bitumin to oil, the U.S. will need to import about 16 million barrels of crude oil per day. If giant 250,000-ton tankers are used, this oil supply will necessitate the docking and unloading, on the U.S. coast, continuously day and night, of one tanker every two hours.

Much of the information concerning research on oil from coal is proprietary. One process will be presented, the H-coal process, work on which was supported for a period by the Office of CoaI Research and now jointly by a group of five oil companies. Figure 14 shows the flowsheet. Crushed coal is mixed with a recycle aromatic oil, and the resulting slurry is pumped with hydrogen into a preheater operating at 180 atm. The slurry, plus preheated recycle gas from the main reactor, are pumped into the H-coal reactor, an ebullated-catalyst column operating at 850°F (730°K). The catalyst, cobalt molybdate, settles below a point in the bed at which liquid product is drawn off to a hot atmospheric

PLENARY LECTURE

20

( ~ (v~)) ~ ((

Boilet Feed Water

91~

S

Plant Steam

and spent

[

Ash

Dolomite DEVOLATILIZER

I FEEDBIN AND LOCK HOPPERS

I

~

psi a ] , \\ \\';, ~4C ~4o

\1500 F

|

~'"

PURIEICATION

mgO.CaO

~Xx

flutdized p,

CATALYTIC METHANATION

[~ P xRE ~HE ~ATER

MgO~CaC3 O |'(fluidized) !

f~IFIER Lignlte i Hot

Air"P Dried

Lignite

? Air

Pipeline Gas

X~\150 psia ' ' \ I ~ o , ~ \ ', i ~fluidized), \ "

i

Inert Gas ~

Hake-up

'

1------,Dolomite

HgO,CaCO[ Steam

I

Air

FIG. 13. CSG or CO2-Acceptor process for making pipeline gas from coal (Ref. 1). flash drum. The product there separates into an overhead stream--further separated by distillation--and a bottom stream, and the latter is split, part going to a vacuum flash drum and part to a return line to the slurrying operation. At the reactor, the overhead vapors are partly condensed, and the gas remainder (containing most of the fuel sulfur as H2S) is sent to a naphtha-recovery operation, not shown, thence to acid-gas removal, and finally to the hydrogen plant with other fuel gas. The products shown

are subject to further conventional refinery operations. The char-oil product, containing unconverted solids, can be used as a fuel or be subjected to carbonization to obtain more liquid product. Space prevents adequate consideration of shale oil and bitumins. A commercial plant for the latter is in operation in Canada, and sufficient pilot-scale work on oil shale has been completed to make operable processes available when oil prices justify action. When such oil does become

COMBUSTION

AND

ENERGY

FOR

THE

21

FUTURE

rg

~°'

l,.i

I~,¢i r.~l

0

e 2 Q

01 oo O

I,~

uI

\ ~

I o

22

PLENARY LECTURE

available, its high nitrogen content may present the serious problem either of removal before burning or of learning how to burn high-nitrogen fuels without putting NO into the stack gas. Should the U.S. energy shortage be supplied from gas or from oil synthesized from solids available? To a combustion engineer, it is axiomatic that any heating or almost any processing operation which uses clean oil or gas as a fuel can, with minor modification, use the other. The price differential on a Btu basis would need, in some applications, to compensate for the ease and convenience of using gas, but, in most industrial operations sensitive to fuel cost, oil would need to be no more than 5¢ or 10¢ cheaper per million Btu (29¢-58¢/bbl. On the other side of the ledger is the lower cost of transporting oil than gas by pipeline. The differential for a 1000mile run is about 8¢ per million Btu, or 47¢/bbl. These two factors offset each other, and one coneludes that the choice between gasifying coal and making syncrude from it will depend almost exclusively on the product cost per unit of energy contained. Gas from coal appears to have a slight edge today, but no one of the above-described processes, nor any of their competitors, has had its true cost established. Either gas from coal, or oil from coal, has a chance of winning the race; ahnost certainly the nation will need both. Bringing either of the gasification processes described above, or any one of the others now in bench-scale, to a stage of proven large-scale performance, is a Herculean task. One may say, eategorieally, that if the need for gas is as urgent as the nation's many energy-study groups, gas companies, and National Academy Panels have concluded it is, there is not in sight a program which will satisfy the need. The limited budgets of the Office of Coal Research and the Bureau of Mines have produced a large number of small groups doing big-group iobs-understaffed and lacking personnel for the correlating and planning functions that must go hand-in-hand with process development. The pilot-plant programs are proceeding at far too low a level of effort, and preparation for mining operations is negligible. As among contributions that can be made by fundamental studies of the various physical and chemical mechanisms involved in gasification, by bench-scale studies of parts of a process , and by pilot plants, there is no doubt that emphasis on the last of these is of the highest importance. But, as the new gas industry does begin to be established, it will undergo continuous change toward an economically optimum system, and some of the changes will be maior ones unless the early developmental effort is phenomenally sound. This contemplation

of an industry developing which will dwarf the sum-total of all present chemical industry eminently justifies a program of research on the better fundamental understanding of gasification, to guide the development engineer and help minimize expensive cut-and-try on a large scale. A few of the problems needing solution are:

(a) The evolution of gas from a heated coal particle is a complex phenomenon of interaction of heat transfer, mass transfer, and chemical kinetics, not really understood despite numerous papers in t h e area. The effects of temperature, particle size, ambient gas concentration, and total pressure on the kinetics of volatile evolution, char gasifieatio!~, and cracking and polymerizing reactions, exhib~d by primary volatile products, shot{ld be established quantitatively. (b) The tendency of coal particles to form cenospheres during heating is known to depend upon heating rate, final temperature attained, ambient gas eomposition, and coal type; particle size and total pressure are also important. At a certain stage, the particles agglomerate to a degree which depends upon the operating conditions. During gasification, ash particles are formed, with sizes ranging from that of the original elements of inorganic matter in the original coal to agglomerates which include ash from several coal particles. These phenomena bear on gasification behavior and on design features for removing ash and avoiding the need for pretreatment, but understanding is, at best, only qualitative. The history of ash constituents , during the gasification and ash-removal steps, should be determined on single particles and on clouds, and study should include both dry-ash and slagging conditions. (c) When chemical and/or physical reactions are carried out in a suspended-solid-gas system in vortex flow, knowledge of the distribution of residence times, as affected by mode of introduction of the feed streams, by axial momentum flux and angular momentum flux of the feed, by particle size and size distribution, and by density of the gas and particulate matter, is of high importance, especially when the reaction is fast. A thorough generalized study of this flow and transport process is warranted. (d) The behavior of fluidized beds, with regard to large-bubble formation, mixing, and heat transfer, is highly important in determining gasification behavior, but it is not well understood for high-pressure operation. Since this behavior, particularly local segregation or local surging, is very dependent on bed diameter, model research on this problem has its difficulties,

COMBUSTION AND ENERGY FOR THE FUTURE and large-scale studies may be necessary. The ability of high-pressure fluidized beds to use caking coals has not been established. Techniques being studied for overcoming problems associated with caking include pretreating and addition of grog. The problem of caking coals in a fluidized system is common to several proposed gasification systems, and deserves study on its Own.

(e) The thermodynamic tendency of the thermally near-neutral reaction C-t-H20--~0.5 CH4-t-0.5 C02 to go to the right eminently justifies an exhaustive search for a catalyst which will allow methane to be made more efficiently than by the present process of following a highly endothermic reaction, yielding synthesis gas, by a highly exothermic one yielding methane. (f) Organic-sulfur elimination during volatilizatiotl varies enough among coals to justify a more basic study of the elimination process, and possibly the study of what kind of pretreatment affects removal favorably. (g) Hydrogen production is so important in fossil-fuel conversion of all kinds that any improvement in its production would have wide

23

use. Its production from methane is today cheaper than from char. The solution of the above problems and proper progress on gas-from-coal pilot plants will not come until the U.S. realizes that nuclear power, though a clear necessity to develop fully, is for many purposes not a substitute for clean fossil fuel, which we are certain to be needing for many decades. BIBLIOGRAPHY 1. HOTTEL, H. C. AND HOWARD,J. B.: New Energy Technology--Some Facts and Assessments, 364 pp., MIT Press, Cambridge, Mass., 1971. 2. Atomic Energy Commission, Potential Nuclear Power Growth Patterns, Report Wash-1098, Dec. 1970. 3. BEXEmCT, M.: Electric Power from Nuclear Fission, Proc. Nat. Aead. Sci. 68, 1923 (1971). 4. ROBSON, F. L., Gn~AMOXTI,A. J., LEWIS, G. P., AXD GRU~ER, G.: Technological and Economic Feasibility of Advanced Power Cycles and Methods of Producing Nonpolluting Fuels for Utility Power Stations, Report by United Aircraft Research Labs. for N.A.P.C.A., U.S. Dept. of H.E.W., Dec. 1970.