Biomass energy

Solar Energy,Vol. 30, No. I, pp. 1-31,1983 Printed in Great Britain.

0038-092X/83/010001-31505.00/0 PergamonPress Ltd.

REVIEW ARTICLE BIOMASS ENERGY THE BIOMASS PANEL OF THE ENERGY RESEARCH ADVISORY BOARD

(Received 4 March 1982)

Abstract--The Energy Research Advisory Board (ERAB) was asked to prepare an analysis of the state of biomass systems developmentand use by the yr 2000.The study takes into account research funds and scientificmanpowerthat should be allocated to biomass energy investigations.The analysis and research recommendationsemphasize those programs that might make significantcontributions to the energy needs of the nation.

INTRODUCTION

The objectives of this report are to explore the potential of biomass energy systems as a renewable resource to contribute to the U.S. fuel supply, and to suggest research needed to accomplish this goal over the next 2 decades. This report analyzes energetics and economics of biomass use, and possible impacts on agriculture, forestry, land use, environment and society. The complex questions that relate to the many biomass sources and the numerous energy conversion technologies are identified. Prime attention is given to the research needed to develop biomass sources and improve conversion technologies to maximize the net additional energy available from biomass. The advantage of using biomass as a fuel is that, to the extent it can be grown and used without damage to the environment, biomass energy can be a renewable energy source. However, there are limits to our ability to produce biomass and simultaneously to conserve our agricultural and forest lands and water resources (GAP, 1977; SCS, 1977; CEQ, 1979; OTA, 1980; GAP, 1981). Proper resource management technologies should make it feasible both to add some solid, liquid and gaseous fuels from biomass, and to increase this amount in the future. In the United States in 1979, the fuel consumed from biomass was about 1.5 Q gross (1 Q = 10t5 Btu), about 2 per cent of the total U.S. energy consumption of 78 Q (USBC, 1979; OTA, 1980). In 1850, total U.S. energy consumption was only about 1.5 Q, and wood was the major fuel consumed (91 per cent). At that time, major cities were facing energy shortages due to their reliance on wood fuel, and the nation began to switch rapidly to fossil fuels. During the following century U.S. society moved to use primarily coal, gas and petroleum as fuel sources. SUMMARY AND FINDINGS

Biomass and total U.S. energy use The total annual energy consumption in the United States is about 78 Q,t which includes 38 Q of petroleum

Correspondenceshould be addressed to Dr. D. Pimentel, Cornell University, Coumstock Hall, Ithaca, NY 14853, U.S.A. tThe 78 Q is mostly gross energy.

liquids and 20Q of natural gas (Table 1). Liquid and gaseous fuels are heavily used because they are easily handled and transported, which makes them effective for a variety of purposes. Solid fuels such as biomass and coal, however, have more limited uses than liquid and gaseous fuels. In some cases, biomass can be efficiently substituted for coal, oil and gas when used as a source of heat for homes and industrial plants. However, for use as either a liquid or gas, it must be converted. If one of the major biomass-to-fuels objectives is to reduce U.S. dependence on the foreign importation of liquid fuels, then primary consideration should be given to the substitution of biomass for liquid fuels. Limitations of U.S. biomass production The total energy content of all plant biomass produced on all U.S. land and water areas is only about 54 Q per yr (Fig. 1). The total energy content of the annual harvest of agricultural crops and forest products that are included in the 54 Q is about 28 Q. High-grade energy production from biomass requires conversion of biomass to either liquid or gaseous fuels. The efficiency of available conversion processes ranges between 5 and 80 per cent. If the total of 54Q were converted to fuel, then no food and forest products would be available for other uses. Thus, biomass can contribute only a small fraction of the energy needed to meet current U.S. demands if society also meets its food and forest product needs. Gross energy vs net energy Biomass potentials are presented in most reports as "gross-energy" values. It is "net energy", however, that contributes to U.S. energy needs. For example, a tree that weighs 1 ton oven-dried contains 17 million Btu of gross energy. This tree, however, standing in a forest, does not provide 17 million Btu of either low-grade or high-grade energy for people in a distant metropolitan area. The biomass energy in the tree contributes net energy only after it is cut, harvested, transported and converted into a suitable fuel. When this is done, the resulting useful net energy, as either ethanol or heat, might range from only 10 to 60 per cent, respectively, of the gross energy.

Review article Table 1. Total energy use in the United States (USBC, 1979) UNITS 10g bbl oil

Total energy used

13.4

78

High grade fuels liquids

38. C

6.7

gas

19.8

3.4

subtotal

57.8

10.1

a_/ l O (quad) = 1015 BTU.

7(3 I00

O

::::::::::::::::::::: :.:.: : ..x.:,:<.:.;

::::::::::::::::::::::: 90

::::::::::::::::::::::

80

:4:: :X < .X.: +Z :::': ::::4 ::4:::;:::::::;.::: :::::X :::::::" ;:: :

70

:::::::::::::::::::::::

::.::::.:'.::':::: :::::::::::::::::::

.

.

...

::::::::::::::::::::::: :...:.:<.:./: : : :. : : : :C :.: :<<,:.: :::::::::::::::::::::: ........... .: : :...:.:,: ..:. : : ::::::::::::::::::::::: .......H..... .,..........

60

50

::::::::::::::::::::::: :.:.:.:.:.:...:.:.:.: : .......,.H..... ::::::::::::::::::::::::: :::::::::::::::::::::: ::::::::::::::::::::::: ::::::::::::::::::::::::: ::::::::::::::::::::: : : :.: :...:.;,: :.:.: ::::::::::::::::::::::: :::::::::::::::::::::::: :::::::::::::::::::::::: :::::::::::::::::::::::

40

:30

:i:i:i:!:i:!:i:i:!:i:i:i

20

:::::::::::::::::::::::

.:.: :.: :.:...:.:.:.:.:

: :.: : :.:.:.:,:..:.: ::::::::::::::::::::: .:.- ..:.:.: :.:.:.: : : ::::::::::::::::::::::: :::::::::::::::::::::::: :::::::::::::::::::::::

I0

Fossil energy consump'tion in U.S

54 Q :::::::::::::::::::::::: :.-:::.:.::.:.::

::::::::::::::::::::::::

i

:.:,:.::.::::.:.::. :::::...:.:.:.::.:,: :::::::::::::::::::::::: ::::::::::::::::::::::: :::::::::::::::::::::::::: :::::::::::::::::::::: ::::::::..N,:< ====================== :::::::::::::::::::::::::: ..... ::::::::::::::::::::: ====================== ::::::::::::::::::::::::: :::::::::::::::::::::::::: :::::::::::::::::::::::::

2B O

:::::::::::::::::::::::

::<::::::+:.:< ::::::::::::::::::::::::::

i:i:i:!:i:i:i:i:!:i:i:i:i: ,::.:.::::,:.:.:::: :::::::::::::::::::::::::: ======================= :.::,::.:::.:,:.:-:4, ========================= i!!i)i?iiiii)i!!i!i?!ii)i! i:i:i:i:i:i:i:i:?:i:i:i:i: iiiiii:?i??i?ili?iiiiiiiii

3Q E!:,:.:.:.;.;.;.;.;.;.;.;.:!

Total sotor energy fixed by oLL plant biomass

Total solar energy T o t a l solar energy conversion by ogricuL- conversion byo~her ture and forestry biomoss sources

Fig. 1. Biologicalsolar energy conversion compared with fossil energy consumption in the United States. All data are calculated for one year (Pimentel et al. 1978). Potential biomass energy

The total dry biomass potentially available for energy conversion about the yr 2000 is estimated to be about 700 MM (million) tons (Table 2). The gross-energy content is calculated to be about 11 Q, or about 11 per cent of the 100 Q of energy consumption projected for the year 2000 (USC, 1981). If the available biomass were used solely as a fuel in direct combustion (heat energy), a total net fuel energy of about 5 Q per annum would be produced for the nation (Table 2). However, if most of the biomass were converted into liquid or gaseous fuels, the potential net energy would be only 0.8--4.1 Q (140 to 700MM bbl of oil equivalents) (Table 2) with liquids at the low end and gaseous fuels at the high end of the range. Both the 5 Q and the 0.8--4.1 Q estimates are optimistic since they assume all biomass resources from sources such as farmers and other owners are readily available. This is not

the case. Also most of the analyses do not include transport, storage, and waste disposal energy inputs. In the United States, energy currently obtained from biomass is estimated to be as high as 1.3 Q net (Table 3). Thus, the maximum energy that the nation could obtain from biomass energy conversion is about four times the current net energy (mostly as heat energy) or about 2 MM bbl of oil equivalent per day of high-grade fuel (Tables 2 and 3). Biomass, therefore, assuming sound environmental management practices, eventually could provide the nation with as much as 5 per cent (a value of $20-$30 billion in oil equivalents) of its total projected energy needs of 100 Q. The projected actual contribution of biomass to the energy needs of the country for the yr 2000 is calculated to be 1.8 per cent gross (USC, 1981). Note that of all the biomass sources, tree biomass is projected to provide the major potential energy con-

Review article Table 2. Best estimates of potential sources of biomass available per year for energy conversion about the yr 2000 Total Btomass . (dr~ ~ t ~ / )

% Moisture (based on dry w t . }

Residues

200

50

~00

1,7

).00c/

Logging Residues

205

50

50

0.85

O.50dJ

Thinnlnq~ from Forest Stand

70

50

35

0,60

0.35dJ

Residentia] Fuel Wood

55

50

55

0.94

0.55(!/

310

50

30

0.St

0.3ud/

Binma~s Source Wood Mill

Available f o r Energy Conversion . (dr~ MMt)a.~/.

Potential Gross En~Fgy (dry Q-~'~

Potential ^al Net Energy (Q~/): D i r e c t Combustion

|mproven~nt

Mortallty and ~xce~s o er

I0

50

0

0.17

0.I0~1

Plantations Forage

750

15

Animal Wastes

175

85

Grains

440

15

4

50

Dagasse rood-Processing Wastes

80

70 iO - 2 2

1.4

1.2 0 . 1 7 - 0.37

15-45~ifn (Liquid) 55-75 ~J (Gas)

NA )5-4S~(Liquid)

I00 - 150

40- 60

23- 42

170 - 180

40 - 70

100

- 0.38

0.02 - 0.06

NA

35 (Liquid)

0.06 - 0.13

NA

NA

tlA

[ndustrial Wastes

0.08 - 0.73 0.28

0.04 - 0 . 4 5 0,52 - 0.90 0,11 - 0.23

0.29

HunicipaI SoJid

NA

3-33;~(Ltquid) 44-64'J(Gas)

0,77f~ I

0.03~/

17

]S-45~{C!quid)

- 0,38

0.47 o 0 . 6 4 0,09 - 0.27 0.33 - 0 . 4 5

0.08

0.06

70 - 90

NA 0.13

SO.70~-(Gas)

4

17

Potential ^a/ Net Energy (Q-~/); L i q u i d or Ga~

NA I5-45~ {Liquid) SS-lS~ (Gas)

55-75--'(0as)

¢larves L~J~

WoodEnergy

Not C o n v e r s i o n E f f i c i e n c y (S)~ / Ligyid ?r Gas~J

9-199/(glogas)

%o-2o~J(Biogas)

- 0.11

NA 0.03 - 0.05

0.Z8- 0.50~/

0.16 - 0.291-/

0.90~

0.45~

20-45~/(Liqoid) lO.20"J(Bioga$)

- 0.41 0,09 - 0.18

Wastes

NA

NA 0,18

Municipal Sewage

15

99

12

0.20

RA

|0-20°-/(Biogas)

0.02 - 0.04

Aquatic Plants

20

70- 95

3- 13

0,0S- 0.22

~A

7-i3PJ(Biogas)

0,004- 0.03

Crop Residues

600

50

80

1.4

TOTAL

3066 - 3126

677

-

718

10.72 - i l , 3 1

0.82~/ 5 . 1 1 - 5,24

a_/ Q = quads = 1015 BTU )~4t = m i l l i o n tons b/ These net percentages include fuel inputs f o r biomass production, harvest, and collection (discussed). The percentage conversions for current and future technologies are l i s t e d in the section of the text dealing with Biomass Conversion Technologies. c/ No collection and transport costs were included. One ton wood with 50% moisture has 8.5 million BTU minus I million BTU to evaporate water, minus 2.1 million BTU to account for losses in boilers and furnaces. Thus, the net heat return is 5.4 million BTU/t of wood. d/ Energy inputs for harvesting and collection of wood were calculated to be 535,000 BTU/t of wood containing 50% moisture. One ton of wood with 50% moisture has 8.5 million BTU. This was debited by 0.4 million BTU of liquid and gas used in collectionandnutrient manufacture, by I million BTU to evaporate water, and by a further 2.1 million BTU to account for energy loss in boilers and furnaces. Thus, the net heat return is 5.0 million BTU/t of wet wood. This value would be lower i f the liquid and gas consumedhad been obtained by a wood to liquid or gas conversion process. For conversion of wood to gas a 60 to 80% energy efficiency has been used. Therefore, f o r conversion to gas the 8.5 million BTU was debited f i r s t by 1.7 to 3.4 million BTU and then by 0.4 million BTU used in collection and nutrient manufacture f o r a net of 4.7 to 6.4 million BTU and a net conversion efficiency o f 55 to 75%. For conversion of wood to liquid a 20 to 50% energy efficiency has been used. Therefore, f o r conversion to l i q u i d , the 8.5 million BTU was debited by conversion losses of 4.7 to 6.8 million BTU and 0.4 million BTU used in collection and nutrient manufacture, f o r a net of 1.3 to 3.4 million BTU and a net conversion efficiency of %5 to 40%.

eJ Annual

fJ

tree mortality and excess production over harvest (see t e x t ) .

Energy inputs f o r production of forage (hayland) were calculated to be 2.4 mi]lion BTU/t of forages containing 159 moisture. One ton of forage with 15% moisture has 14.5 million BTU.. This was debited by 2.4 million BTU of liquid and gas used in the production of the ton of forage, by 0.3 million BTU to evaporate water, and by a further 3.6 million BTU to account for energy loss in boilers and furnaces. Thus, the net heat return is 8.2 million BTU/t of wet forage. This value would be lower i f the liquid and gas consumed had been obtained by a forage to liquid or gas conversion process.

16-45~(Lqquid) 5 6 - 7 ~ • (Gas)

0.22 - 0.64 0,79 - 1.05 0.82 - 2.58 2,8Z - 4,oe

(Liquid) (Gas I

4

Review article Footnotes to Table 2. For conversion of forage to gas a 60 to 80% energy efficiency has been used. Therefore, for conversion to gas, the 14.5 million BTD was debited f i r s t by 2.9 to 5.B million BTU and then by 2.4 million BTU used in forage production for a net of 6.3 to 9.2 million BTU, and a net conversion efficiency of 43 to 63%. For conversion of forage to liquid a 20 to 50% energy efficiency has been used. Therefore, for conversion to liquid, the 14.5 million BTU was debited by conversion losses of 7.3 to I I . 6 million BTU and by 2.4 million BTU used in forage production for a net of 0.5 to 4.8 million BTU and a net conversion efficiency of 3 to 33%.

~/ An energy efficiency of 10-20% was used f o r the conversion of manure to biogas. One ton of manure with 85% moisture contains 2.55 million BTU and was debited by conversion losses of 2.04 to 2.30 million BTU in biogas conversion, and 0.03 million BTU for transport and application of waste on land, thus net return is 0.22 to 0.48 million BTU/t of manure and the net conversion efficiency is 9 to 19%. h/ One million tons of dry grain containing 0.17 Q of potential gross energy is assumed to yield O.OOg Q of gross ethanol. The energy used in growing, harvesting, and transport of the grain has been calculated to be I/3rd of the gross or 0.003 Q. The net is therefore 0.006 Q and the net conversion efficiency is 35%. Energies used in converting grains to alcohol have been neglected on the expectation that they w i l l be supplied from coal. ~/ One ton of bagasse with 50% moisture has 7 million BTU. This was debited by 1 million BTU to evaporate water and 1.8 million BTU to account for energy losses in boilers and furnaces. Thus, the net heat return is 4.2 million BTU per ton of wet bagasse. The collection and transport energy have not been debited. ~/ An energy efficiency of ID to 20% has been used for the conversion of food processing wastes to biogas. NO collection, transport, or waste disposal costs have been debited. k/ One ton of industrial wastes with 50% moisture has 6 million BTU. This was debited by I million BTU to evaporate water and by 1.5 million BTU to account for energy losses in boilers and furnaces. Thus, the net heat return is 3.5 million BTU per ton of wet industrial wastes. The collection and transport energy have not been debited. ~/ No collection and transport costs were included. into heat energy.

Assume50% conversion

m/ Energy content was calculated to be 4,500 BTU/Ib. n/ No collection and transport costs were included. heat energy a 50% conversion rate was assumed.

For conversion into

2/ No collection and transport costs were included. ~/ An energy equivalent to 33% of the biogas produced was debited to account for energy consumed in agricultural harvesting, collection, and transport. ~/ Energy inputs forharvestingand collection of crop residues were calculated to be 0.3 million BTU/t of residue containing 505 moisture. One ton of crop residues with 50~ moisture has 8.5 million BTU. This was debited by 0.3 million BTU of liquid and gas used in collection and nutrient manufacture, by I million BTU to evaporate water, and by a further 2.1 million BTU to account for enerq¥ less in boilers and furnaces. Thus, the net heat return is 5.1 million BTU/t of wet crop residue. This value would be lower i f the liquid and gas consumed had been obtained by a crop residue to liquid or gas conversion process.

For conversion of crop residue to gas a 60 to BO% energy efficiency has been used. Therefore, for conversion to gas the 8.5 million BTU was debited f i r s t by 1.7 to 3.4 million BTU and then by 0.3 million BTU used in crop residue collection and nutrient manufacture for a net of 4.8 to 6.5 million BTU, and a net conversion efficiency of 56 to 76%.

For conversion of crop residue to liquid a 20 to 50% energy efficiency has been used. Therefore, for conversion to liquid, the 8.5 million BTU was debited by conversion losses of 4.7 to 6.8 million BTU and 0.3 million BTU used in collection and nutrient manufacture, for a net of 1.4 to 3.~ ~illion BTU and a net conversion efficiency of 16 to 41%.

Review article Table 3. Best estimates of yearly current and potential sources of biomass available and used for energy conversion Bioma~s Source

Total Biomass Idry MMta-/)

Potentially Available for Energy Conversion (dr~MMt a-J)

149

130

Wood Mil] Residues (forest) Logging Residues

Current Use A~unt Converted Net Energy _ (dry MMat~) Used {qa__/) 64 - 82

0,70-0.89 (Heat)

180

50

3.6

0.04 (Heat)

Tbinnings from Forest Stand Improvement

47

47

2.4

0.03 (Heat)

Residential Fuel Wood

30

30

30

0.20 (Heat)

310

30

.

.

.

.

.

.

Forage

750

130

.

.

.

.

.

.

Animal Wastes

175

50

Grains

353

lO- 22

Mortality and Excess over HarvestE/

Bagasse

l

0.001~/ (Biogas)

l.l

0.008~-d/ (Liquid)

4

4

15

]5

. . . . . .

Industrial Wastes

lO0

23

7.8

Municipal Solid Wastes

136

66

.

Municipal Sewage

13

iO

---

Aquatic Plants

20

3-13

Food-Processing Wastes

Crop Residues TOTAL

4

473

80

2755

67~ - 700

0.04 (Heat)

O.OSle-/(Heat) .

.

.

.

. 0~/ (Biogas)

.

.

.

.

.

.

.

.

.

.

]3T-Z--l~

. . 1.06 - 1 . 2 5 (lleat) 0.008 (Liquid) 0.001 (Biogas)

a/ Q = Quads = 1015 BTU. ~4t = million tons b/ Annual tree mortality and excess production over harvest (see t e x t ) . c/ Estimated. d/ I t is estimated that I00 million gal of ethanol is currently being produced. Since a large part of the alcohol being produced uses o i i and gas for d i s t i l l a t i o n , the net energy produced is really about zero. However, assuming that these plants convert to coal, then optimistically the net energy might be 7.6 x I012 BTU. e/ Estimated by Tillman (1977). f / About 19.2 x 1012 BTU of biogas is used to heat biogas generators, but this is not "net ener~y" (Cheremisinoff and Morresi, 1976).

tribution with some coming from forages and crop residues (Table 2).

Economics and alternatives Any evaluation of the feasibility and potential of biomass energy resources must include an economic comparison with existing energy alternatives. Some biomass, like wood which may be locally abundant, is useful as a direct fuel. As a solid fuel material, the value and limitations of wood are similar to those of coal. When considering the potential of biomass as a direct fuel, trade-off comparisons can be made with coal, including all of its advantages and disadvantages. The economic assessment of the potential of biomass as

either heat, liquid, or gaseous energy is highly complex because there are numerous kinds of biomass resources and these resources are only locally abundant in certain U.S. regions. In general, the potential biomass energy of 5 Q net corresponds to the maximum amount that could be made available or produced within the constraints imposed by agricultural and forest production, land and water resources, and environmental quality. It does not necessarily correspond to the amount that could be produced at costs that would be used in terms of alternatives. For example, the logging residues as potential biomass sources already exist, yet they are not currently collected and used to any large extent. The costs of collection considerably exceed most alternatives such as coal.

Review article

Biomass and the environment Sustainable production of the maximum potential net biomass energy of about 5 Q per yr is possible only when sound environmental management practices are employed. Even then, large-scale biomass energy conversion could result in an increase in environmental degradation. For example, soil erosion and water runoff are expected to increase unless all possible precautions are taken. Air and water pollution would also tend to increase. Natural biota and wildlife could also be adversely affected if forested areas were converted into more intensively managed monocultures. At the same time, the greater use of pesticides and fertilizers would likely increase chemical pollution. If efforts were made to double or triple the proposed maximum of 5Q, with current resource management technologies, the environment would be rapidly degraded. As a result, biomass energy resources would become nonrenewable and eventually would be depleted. Agricultural and forest productivity would also decline. In turn, the economy and society would suffer. Some current practices in agriculture and forestry might be changed to include environmentally sound resource management practices. Such changes might increase the potential biomass that could be harvested. Realistically, however, these alternative techniques will be adopted slowly. On the other hand, more widespread use of sound crop and forest management technologies could increase biomass energy resources and could at the same time improve agricultural and forest management practices and production.

Agriculture and Forestry: conflicting demands Agricultural production must increase about 25 per cent over the 1975 level to meet U.S. and world food demands by the yr 2000. Forest (roundwood) production must increase about 70 per cent over the next two decades to meet U.S. demand, not including fuel. Land and water resources will be major limiting factors in this expansion and these limited resources are also needed for biomass energy production. The need to use this limited land for fuel production, as well as for agricultural production, raises the question of what policies the United States ought to follow with respect to food and fiber export demand. Economic and ethical problems are associated with this question. The projected 1981 revenue from food exports is about $45 billion. In 1981, imported crude oil will cost the nation about $65 billion.

Beyond the yr 2000 Implicit in the present analysis is the assumption that current social, agricultural and silvicultural practices will evolve gradually and will retain to the yr 2000 the broad outlines of their present form. If environmentally sound resource management technologies were practiced more broadly in agriculture and forestry, more biomass (in some cases 2-fold) could be harvested for energy conversion. Three areas merit special attention: increasing biomass availability from privately-held lands, which constitute the majority of agricultural and forested areas;

increasing productivity of federal forest lands; and increasing the efficiency with which industry uses biomass resources. In addition, more agricultural and forest research is needed in some areas to improve insect pest, plant pathogen and weed control, as well as nutrient management. It is probable that these achievements in biomass production will be realized after the yr 2000. More visionary proposals have suggested greatly increased energy contributions from biomass; such results, however, could only be realized through dramatic changes in agricultural and forest practices. These changes, in addition, will require major alterations in other sectors of the U.S. society and economy. Extensive further analysis is necessary to determine whether the benefit/penalty trade-offs of such changes are sufficiently positive to justify serious consideration of these proposals. B1OMASS ENERGY PERSPECTIVE

To appraise the potential of biomass as a source of fuel energy for the U.S. economy, examination must be made of the solid, liquid and gaseous fuels needed, the potential biomass resources that are available, and the costs of supplying energy from biomass relative to existing alternative fuels.

Energy quality The use of fuel energy in society and technology is conveniently divided into different categories. Heat energy is one major source for use in homes and industry. Solid fuels, such as coal or biomass (without additional conversion), are suitable for use in supplying low temperature heat energy. Another major category involves producing heat at a high temperature for mechanical energy (work) use such as the internal combustion engine for vehicular transportation and for chemical transformations used in industrial processes (e.g. chemicals, fertilizers). In present technology, gas and liquids serve as the major fuels for this use category, and, therefore, are referred to as high-grade fuels. Aside from the high energy density for use in mechanical devices, the liquid fuels especially have the capabilities of ready transporf, control of feed rate, and easy storage. To some extent, gas and liquid fuels are interchangeable, with relatively low investment costs for interconversion. Liquid fuel needs (automotive power, high temperature heat generation) cannot usually be met by solid fuels such as coal and biomass. Currently, the "highgrade" fuel demands are supplied by petroleum and natural gas. The question of national energy independence is primarily concerned with the supply of liquid fuels. Large quantities of oil are imported by the United States to cover the demand. On the other hand, the nation has an abundance of a low-grade fuel resource in coal. The heat energy needs can be supplied by any fuel, of course, but solids are adequate, provided the technology and environment are suitable. On the other hand, if a solid resource (coal or biomass) is to displace liquid and gaseous fuels, these solids must be either thermochemically or biologically converted.

Review article

7

Table 4. Energy balance for ethanol production from corn (after ERAB, 1980)~ Thousand BTU/gallon~b/

Consumed

Best Available Technology High Quality Plant Fuel

Future Coal-Fueled Plant

FermentatiOn/c/ d i s t i l l a t i o n ='

69

Far~ing~-/

45

45

-114

-45

Total

0~d/

Produced Ethanol

76 (130)f-/

76 {130)~I

By-product My animal feed~'

II

II

N value 9~ crop residue~~

3 +90 (+144)~I

3 +go (+144)~1

!~et

-z4 (+ 30)fI

+4s (+ g9)fI

Refinery credit

+ 8 -16 {+ 30)~/

+ 8 +53 (+ 99) ~/

a/ Corn is the grain crop used for this example because i t is the most common food crop used to produce ethanol. Other grain and sugar crops could be used for ethanol production but, like corn, all require a significant energy input for culture (Pimentel, 1981) and similar energy inputs in the fermentation/distillation process (E.J. Honohan, 1979, personal communication, Pfizer, Inc.).

bJ

For consistency, a l l heating values are expressed as LHV (low heating values). Also we assume 2.5 gal of ethanol produced per bushel of corn.

cl

Energy inputs for fermentation/distillation vary depending on size of plant and technology employed and these range from 40,000 to 148,000 BTU (Scheller and Mohr, 1976; Scheller, 1979; Reilly, 1978; Katzen, 1978; David et a l . , 1978; ACR, 1978; DOE, 1979; Hertzmark, 1979; Weisz and Marshall, 1979; Chambers et a l . , 1979). For a modern 50 million gallon per year ethanol plant about a 69,000 BTU input is calculated per gallon of ethanol produced using vapor recompression evaporators (about IO0 BTU/Ib of water evaporated) (E.J. Honohan, 1979, personal communication, Pfizer, Inc.).

dJ Assumed to

be zero because coal is substituted f o r oil and gas.

e/ Energy inputs f o r raising corn vary depending on the technology employed, soil quality, r a i n f a l l , pest attack, and other factors. Reported energy inputs f o r corn production prorated p e r g a l l o n o f ethanol range from 35,000 to 74,000 BTU (Scheller and Mohr, 1976; ACR, I97B; Reilly, 1978; DOE, 1979; Hertzmark, 1979; Weisz and Marshall, 1979; Chambers et a l . , 1979). An average energy input for corn used to produce a gallon of ethanol is at least 45,000 BTU (Pimentel and Pimente], 197g).

f/ The value in brackets assu~es a mechanical equivalency, i . e . , that a gallon of gasohol will move an automobile as far as a gallon of gasoline. A gallon of gasoline has an equivalent of 115,000 BTU or as an equivalent of crude o i l is 130,000 BTU. A serious question exists concerning the assumption that a mechanical equivalence of 9asohol as gasoline exists. Energy credit is taken for d i s t i l l e r s ' grains, which are produced as a by-product and used for animal feed. Reports of credits range from 1,000 to 52,000 BTU per gallon produced (Scheller and Mohr, 1979; DOE, 1979; HertzmarK, JB7g; ~deisz and tlarshall, 1979; Chambers et a l . , 1979). For a 50 million gallon per year ethanol plant with-a well-designed drying f a c i l i t y , a credit of about ll,O00 BTU was calculated.

hJ

SE Vol,30, No.I--B

Crop residue contains about I% nitrogen, 0.1% phosphorus, ~.9% calcium, and 3.6% calcium ()~AS, 1978). Energy value as f e r t i l i z e r was calculated to be 3,000 BTU.

8

Review article

Btu comparison of different fuel categories Practical processes for converting solids to liquid fuels have an efficiency of about 20-50 per cent. Therefore, any application requiring 1 Btu of liquid fuel will require between 2 and 5 Btu of the initial solid resource (coal or biomass). Thus, I Q (low grade) is equivalent to only about 0,2-0.5 Q (liquid).

The "net energy" problem The translation of energy Btu's needed in solid fuels (biomass or coal) to create a Btu of liquid fuel, as mentioned, does not take into account any high-grade fuel that is consumed to produce the solid fuel. For example, when a liquid fuel is produced from coal by Fischer-Tropsch synthesis, 1 Btu of coal may produce 0.45 Btu of liquid. The mining operation to produce the coal does not, however, require much liquid fuel energy; otherwise it would have to be debited against the 0.45 Btu of liquid produced after conversion to determine the true net amount of liquid fuel produced, Biomass production in agriculture and forestry consumes appreciable amounts of high-grade fuel in cultivation, irrigation, fertilization and harvesting. This leads to a debit against any gross liquid or gas converted from this solid. The question of net energy production, of course, is applicable also to fossil fuel resources.

Alternatives and economics To have practical validity, any evaluation of the feasibility and potential of biomass energy resources must include an economic comparison of energy alternatives. Some biomass, like wood, is useful as a direct fuel. As a low-grade, solid fuel material, its utility and limitations in use are similar to those of coal. When considering the potential of biomass as a direct fuel, tradeoff comparisons must be made with coal, including all of its advantages and disadvantages. When liquid/gaseous fuels are to be replaced by solids--such as in home or commercial heating, or in steam boilers in industry or power generation--there is usually a need for hardware alterations, added space, solid storage and handling facilities. Coal clearly has certain advantages over biomass. It has an appreciably higher Btu content per unit volume and weight, which provides advantages in gathering, transportation, storage, handling and combustor technology costs; biomass, which is low in sulfur compared with coal, has the advantage of minimizing environmental impacts. When the potential of biomass as a direct fuel is examined, one must ask whether or not

tCurrent use data presented in this section are based on cited

literature. In the case of estimates for the yr 2000, a number of institutions and individuals were consulted, including several specialists in the U.S. Forest Service, WeyerhaeuserCompany, U.S. Department of Energy, and several colleges and universities. Advice on projected estimates was also solicited from the National Forest Products Associationand its Committee on Energy, which includes a broad representation of industrialists. In addition, advice was sought from 25 small forest product companies and the Western Associationof AgriculturalExperiment Station Directors.

biomass can be produced and delivered at lower cost than coal. To generate liquid or gaseous fuels, many existing conversion processes such as gasification, FischerTropsch synthesis of hydrocarbon fuels, or methanol synthesis, can be used. For example, the commercial production of methanol from wood or coal via gasification and synthesis is possible today, with a conversion efficiency of nearly 50 per cent. This will probably not be exceeded by either thermochemical or biological means in the near future. Thus, the difficulty in producing liquid fuels from biomass is not a lack of technology. The problems that arise in trying to meet this objective relate basically to availability of suffcient biomass raw material at the conversion site for an acceptable cost. "Acceptable" cost in this case can be gauged by comparison of the raw materials costs of biomass vs. coal at the point of use. Large-scale conversion of biomass requires a large, relatively nearby agricultural or forest production land base. The availability of contiguous areas large enough to support a large-scale operation constitutes a major question. On the other hand, the use of small parcels of land implies the need for small-scale conversion operations. Processing costs by thermochemical technologies rapidly rise as the size of the plant declines, Bioconversion methods (e.g. fermentation) may be cost effective for small-scale operations if labor and other services are assumed to be free. The questions concerning small-scale conversion that remain to be researched are: what total national contribution could result from small-scale plants, and how economic are small-scale plants? BIOMASS ENERGY SYSTEMS

Tree and wood biomasst Tree biomass, particularly wood, has become a popular source of fuel (Moslemi, 1980). Wood has about 8,500 Btu/lb (oven-dry). This compares with petroleum at about 21,000Btu/lb, and a pound of coal (although it varies substantially among the various grades) about 13,500 Btu. Wood biomass always contains some moisture, which reduces the value of wood as a fuel. Dry wood contains 8500Btu/lb, as noted earlier; however, it has only 5950 Btu with 30 per cent moisture and 4250 Btu with 50 per cent moisture. Although moisture has a direct and substantial influence on wood's unit energy content, technologies using solar and exhaust heat are developing that may reduce the moisture content of the wood biomass before combustion or gasification. The amounts of lignin and carbon in wood also influence Btu yield (Wenzel, 1970; Corder, 1973; Arola, 1976). Tree biomass for energy (energy fiber) could come from a variety of sources, from the standing forest to the wood-processing plant. The kinds of tree biomass include mill residues, logging residues, thinnings from timber-stand improvements, harvests of residential fuel wood, trees that have died, added growth in excess of harvest and trees on short-rotation plantations.

Review article

Mill residues. Mill residues are the by-products of processing operations that include the production of lumber, plywood, pulp and paper, and furniture. About 149 MMt (million tons) of dry mill residues from these operations are being produced annually (Table 5). These residues are the most readily available of all tree biomass sources. All but 20 MMt of this material is already being used for the manufacture of products and for energy. The 76.6MMt of residues from sawmills, veneer and plywood mills, and furniture industries are in a form that can be used for pulp and board manufacture. An undetermined portion of this 76.6 MMt is used for energy. The total energy produced from wood by the forest products industry is now estimated at I . l - l . 4 Q gross (Jamison et al., 1978; Buckman and Erickson, 1980; Muench, 1980; American Paper Institute, 1981). This amount of energy translates into 64--82 MMt of dry wood biomass based on its gross energy content (8500 Btu/lb) (Table 3). Mill residues are expected to remain a major source of fuel in the wood processing industry to the yr 2000. The projected quantity for the yr 2000 is 200MMt. This estimate takes into account the forest harvest projections made by the U.S. Forest Service (USDA, 1980a), and a 10 per cent reduction in the production of residues due to technological improvements. It is expected that approximately half of this amount will be used for product manufacture (pulp, board, etc.) and the remaining half will be converted to fuel (Table 2). Logging residues. Logging residues are the residues left after tree harvest. These residues include tree tops and branches, cull logs, standing live and dead trees, and stumps. For economic reasons, these residues are generally not being used today. Logging residues are the second most readily available energy fiber. It is estimated that of the total 180 MMt of logging residues, an annual 50 MMt is now available as potential fuel (Table 3). A small portion of this material is used as residential fuel. We have included this portion under residential fuel wood (p. 9). It should be noted, however, that if this biomass were collected, an undetermined proportion would be used for product manufacture. An estimated 0.85 Q (gross) of logging residues might be used for energy production after a decade or more (Table 2). Eventually these residues may become more valuable for product manufacture. By the yr 2000, 50MMt is projected to be used for energy production (Table 2).

9

Timber stand improvements. Thinning overstocked forests is a major element in timber stand improvement (TSI). The harvested trees are a viable source of fiber for products and fuel. Currently, TSI harvests are about 47 MMt each yr (OTA, 1980). An estimated 5 per cent (2.4 MMt) is used for fuel (Table 3). It is projected that TSI will increase with the need for more intensive management of the nation's forest stands. A 50 per cent increase in the volume of biomass harvested through TSI is anticipated by the yr 2000. About 70 MMt would then be available, half of which would be used for fuel (Table 2). Companies in the southeastern United States are overstocking the harvested forests so that two thinnings can be made prior to harvesting of sawlogs and peeler logs (veneer logs). Instead of planting 600 stems per acre, 1000 stems are planted per acre. This allows for an early "energy thinning", and a later "pulpwood thinning". The land can then produce wood for both energy and traditional products. Residential fuel wood. The use of wood for residential heating is increasing. In certain areas of the United States, such as the Northeast, this increase is especially rapid. Residential fuel wood comes from a variety of sources, such as thinnings from farm woodlots, logging residues, dead and downed timber, and industrial thinnings. In some areas, small tracts of forest lands are being harvested for specific use as residential fuel wood. It is estimated that some 30 MMt of wood per year is now used for residential fuel wood in addition to other categories noted in this report (Hornick, 1981). This material contains 0.51 Q (gross) of energy (Table 3). The use of wood for residential heating is expected to continue to grow and reach 55 Mt by the yr 2000 (Table 2). Tree mortality and growth in excess of harvest. Other forest biomass includes rough, rotten, and salvageable tree biomass. Three sources of such biomass are available. The first is the annual mortality of forest trees due to disease, insects, old age and other natural causes. The second is the excess growth over harvest that is substantial but not currently used. The third source is the accumulated dead and downed material in the standing forests. The annual tree mortality accounts for an estimated 95 MMt of tree biomass (Buckman and Erickson, 1980). The excess growth over harvest accounts for 215 MMt of biomass. Perhaps 5-10 per cent of dead trees might be

Table 5. Calculatedmill residues produced by major wood processingindustries Lu~er production Veneer and plywood Pulp and paper bark spent liquor solids Miscellaneous TOTAL

*NAS, 1976. **American Paper Institute, 1981.

Million dry tons 62* 13.4" 5.3** 67.1"* 1.2" 149.0

Review article Table 6. Forest residue accumulatedby region (nonrenewableresource) Gross

Region

Rough Trees

Rotten Trees

Salvageable Trees

Total

Energy Equivalent 1015 BTU

(in million oven-dry tons) North

165,6

111.9

3.7

281.2

4.8

South

265,8

106.1

3.4

375.3

6.4

Rocky Mountains

37.8

41.9

I00.5

180.2

3.1

Pacific Coast

39.6

35.7

72.9

148.2

2.5

Totals

508.0

295.7

180.7

984.9

16.8

harvested and about one-third of excess growth over harvest used as wood fuel (Tables 2 and 3). Both of these sources are renewable. However, the inventory of already accumulated dead and downed tree biomass is a nonrenewable source and is estimated to amount to nearly a billion tons (Zerbe, 1977). This source is scattered unevenly throughout the United States (Table 6) over millions of acres. The gross energy equivalent of this material is nearly 17 Q. Short-rotation wood energy plantations. There is a proposed agro-forestry practice in which fast-growing, short-rotation trees would be produced for energy conversion. These trees are selected for their rapid growth, ease of establishment, regeneration, and freedom from insect pest and disease problems. Such hardwoods as Alnus, Eucalyptus, Platanus and Populus have potential (Fege et al., 1979). Wood energy plantations would require intensive cultivation practices such as irrigation, fertilization and harvesting operations. There are several views concerning the feasibility of wood energy farming (Brown, 1976; Johnson et al., 1976; Tillman, 1978; Fege et al., 1979). Because of little experience, projections have to be made on several assumptions. The land area required to produce a significant amount of wood energy would be substantial. For example, it would take the harvests from 9600 acres of biomass feedstock (with 35 per cent moisture) to produce an annual gross of 0.001 Q (Tillman, 1978). Because of its needs for water, and because of similar demand by other energy developments (coal and oil shale) and by agriculture, wood energy farming might be in a poor position, particularly in the western United States. At present it appears that the return on investment is too small to attract private capital (TiIlman, 1978; Fege et al., 1979). It is conceivable, however, that this concept could be refined and made viable on scattered acreages on a small scale. The amount of tree biomass used for energy from tree plantations is currently small. High initial interest has declined in the United States because of the difficulty of locating large, contiguous areas of land. An estimated 10 MMt might be available from energy plantations by the year 2000 with 8 MMt used for fuel (Table 2). Tree biomass demands. Demand for lumber, plywood,

pulp and fiber/particle board products is projected to increase from 1976 to the yr 2000: lumber is to increase 40 per cent, plywood 45 per cent, board about 100 per cent, pulpwood 70 per cent, and other industrial wood products about 110 percent (USDA, 1980a, b). Most of this increase will have to be met by domestic forest resources. Improvements in utilization and in forest management, and increases in imports are expected to satisfy some of this demand. Increasing the use of tree biomass will increase competition for the resource. Increasing wood costs will probably lead to smaller amounts available for energy production as we enter the next century unless decisions are made to develop the nation's full potential in forest production. Mill residues, as noted earlier, are nearly fully exploited. The principal barrier to increased use of logging residues is economic. In many areas, current market prices for logging residues do not justify collecting, processing, and transporting. Most of the annual mortality, the growth in excess of harvest, and accumulated dead and downed material is currently unavailable because of lack of access, high costs of road construction, and lack of efficient retrieval technology. Another constraint on supply is the designation or proposed designation of large productive forests as parks or wilderness. Such withdrawals reduce the productive forest land, reduce the available tree biomass, and increase competition for the raw material. About 24 million acres are classified as parks, wilderness, or deferred lands (USDA, 1980a); some of these acres are comprised of commercial quality forest lands. An opportunity for increasing supplies of tree biomass lies in better use of "marginal" lands that are incapable of producing 20 f3 of wood per acre per yr. There are some 228 million acres of such forest lands. Direct combustion is now the primary mode of conversion for tree biomass. Cogeneration is the most efficient with conversion efficiencies of about 70 per cent (Moslemi, 1980) compared with 30 per cent for electricity production from dry biomass. The high conversion depends on whether the heat/steam energy can be used nearby. Most likely, the forest products industry will be the primary user of wood biomass for energy production over the next decade. This industry used about 6 per cent

Review article of the total industrial energy in 1978 (Zerbe et al., 1978). The industry now provides a little over half of its energy needs from wood biomass (primarily from wood residues), and the potential to produce an additional 1.31.5 Q is excellent. Larger plants are better positioned to take advantage of current technology, with their large requirements for raw material and capital. Because of large amounts of energy fiber in many localities, the forest-products industry in these localities could produce more energy than it needs. If institutional barriers between the industrial and utility sectors were removed, and proper incentives provided, this industry could supply its excess energy to the utility companies using existing lines; this is now gradually happening in a number of regions (Moslemi, 1981). Environmental impacts. There are major environmental concerns about the impacts of biomass removal on nutrient loss, soil erosion and compaction, and on water and air quality (Hewett et al., 1981). Logging residue collection will remove nutrients from the soil; about 80 per cent of the total nutrients present in the total wood resources is accounted for in slash 3 in. or smaller. Soil erosion, especially on steep slopes and decline in water quality are likely to increase when logging residues are removed. The burning of logging residues on site is generally practiced to reduce fire and pest hazards and prepare the site for regeneration. Overall air quality would probably improve if logging residues were transported and burned at a plant that used pollution control devices and had high efficiency of combustion. The air pollution, however, would then very likely shift from forest areas to areas near population centers. Already there are serious pollution problems in some regions of the nation from burning fuel wood in residences (DOE, 1980).

Forage biomass Forage grasses and legumes could be a large source of biomass that could be either converted to pellets for direct burning or gasification, or converted to alcohol. In general, hay and pasture lands in the eastern United States (including everything east of the Mississippi) are producing at only a fraction of their potential. The focus here is on the potential production from this region alone, because it has a favorable climate and adequate rainfall. Land might be brought into moderately intensive production of forage biomass as follows. First there would have to be an increase in the animal-carrying capacity of the existing grassland pasture, so that it could feed those animals now on cropland pasture and free the cropland pasture for hay production. The carrying capacity of the grassland pasture could be doubled by intensive management, especially by introducing legumes and by scheduling grazing times. Cropland pasture is expected to yield 3 dry tons of tThis is round trip transportationby light tractor trailer (diesel) over an average radius of 15 miles from the processingplant. :~A judgment based on a number of projections, including Doering, 1979and Schnittker Associates, 1980.

11

forage per acre. Two tons of this would be used to produce energy. The remaining ton would increase the stock of fodder. This would allow for future increased animal production, or for any dislocation as a result of turning this land into biomass production. At 2 tons an acre some 93 million tons of forage grass would be available. As an example, the premium fuel energy cost of raising 3 tons of forage on cropland pasture and delivering 2 tons to a processing plant might be as follows: Liquid fuels Production = 400,000 Btu/acre (USDA, 1977). Transportation to processor = 140,000 Btu/acre2 Fertilizer N @ 60 lb/acre = 1,866,000 Btu/acre. P205 @ 20 lb]acre = 111,000 Btu/acre. K20 @ 50 lb/acre = 214,000 Btu/acre. Total 2.7 million Btu/acre. Biomass could be obtained from the 18.6 million acres of existing harvested hayland (not producing alfalfa or alfalfa hay mixtures) by doubling the level of production on this land, which currently yields about 2 tons an acre. Then 37.2 million dry tons of forage would be available annually for conversion. Energy required to double the harvest would be in the form of additional fertilizer: 1501b N/acre, 301b P2Os/acre and 801b K20/acre. An increase of 50 per cent in liquid fuel would be charged against the extra forage produced for biomass, as would the charge for transportation, which would be similar to that for forage from grassland pasture. The premium energy cost of increasing production 2 tons per acre on existing hayland would be 5.5 million Btu per acre. The total energy value of the forage produced for biomass fuel would be 17 million Btu/ton (8500 Btu per pound of forage) or 34 million Btu/acre. The total gross forage biomass energy produced on the 65 million acres would be equal to 2.2 Q, The net energy produced from cropland pasture would be 31.3 million Btu per acre. The net energy produced from hayland would be 28.5 million Btu per acre. The total net energy would then be 1.2 Q, 60 per cent of the gross. By the yr 2000, 25 million acres might have been withdrawn from cropland pasture for use as cropland.:~ This would leave only 21.5 million acres of cropland pasture for forage biomass. The harvest from the remaining cropland plus the harvest from the hayland would produce a gross of 1.4 Q, and a net of as much as 0.46 Q of liquid fuel (Table 2). In this analysis of net energy, it is difficult to calculate the indirect energy in unbuilt plants for cellulose conversion or gasification, or the indirect energy in the farm machinery. As an example, on existing hayland the current farm machinery might be used more intensively, but yield increases will lower per unit machinery energy cost. On new hay production a farmer might or might not already have suitable machinery and/or complementary uses for the machinery that would greatly alter the machinery energy cost of forage production. However, these amounts are thought to be small.

Review article

12

Table 7. Major uses of U.S. land qmillionsof acres (USDA, 1979) Agricultural Land

1959

1969

1974

1977

391

384

382

3B5

Cropland used f o r crops

(358)

(333)

(361)

(377)

Idle cropland

(33)

(51)

(21)

(8)

944

890

860

857~/

Grassland grazed

(699)

(692)

(681)

(678)

Forestland grazed

(245)

(198)

(179)

(179)

1335

1274

1242

1242

483

525

539

539~/

146

172

1 B2

182a/

306

293

301

a/ 301~'

2271

2264

2264

Crops

Livestock

TOTAL Forest Land Private, Federal, State Urban and Other Lands

Used

Cities, roadways, recreation and public installations Unused Land

Deserts, rock areas • arshes, tundra TOTAL LANDAREA

2264

a_/ Estimated

Using forage for energy will compete with using forage for animals as well as introduce major seasonal labor requirements. Because of the high costs of producing increased amounts of forage, processors would have to pay at least $50 or more a dry ton to insure an adequate supply (Tyner et al., 1979). This would intensify the competitive demand for forage and increase costs in the livestock industry. If land is to be committed to it, forage biomass for energy will have to receive a price high enough to make it more profitable than other crops. This is most likely to occur on land already in hay production and on rough land where more valuable row crops are unsuitable. It is unlikely that hay can economically compete head-on with row crops. The use of perennial forages for energy has a number of attractive features. In addition to being perennial, these species can be grown on lands that are marginal or even unsuited to row crops. Adequate ground cover would remain after harvest of forage crops and would prevent serious erosion. The yields would he large compared to that for crop residues. High-yield management practices are known and are already applied in Europe, where the economic incentive is high, and higher yields are obtained. Specialized machinery for handling high yields is generally available. Harvesting forage for energy rather than feed can be spread over a season with more flexibility because animal feed quality will not be a factor, However, forage biomass use will still require

storage or the use of alternate feedstock outside of the growing season. Animal wastes The supply and availability of animal wastes, or manure, are related to animal production systems, in ways that severely limit the degree to which this resource might be expanded. In addition, there are competing uses for manure that may limit its availability for direct energy production: some of these save or replace energy and will thus respond to the same stimulus that might encourage energy production. Approximately 175 MMt (dry) of manure are produced in the United States annually by livestock and poultry (Miller and McCormac, 1978; Van Dyne and Gilbertson, 1978). Of the approximately 70MMt (dry) produced annually by confined animals and easily available for collection and use, only 50 MMt are currently being used as a fertilizer in land application. The current level of application contributes about 880,000 tons of nitrogen, 640,000 tons of phosphorus, and 1220 tons of potassium, which in 1977 represented 8 per cent of the N, 20 per cent of the P, and 20 per cent of the K applied to land in the United States (Miller and McCormac, 1978; Van Dyne and Gilbertson, 1978). The supply of manure for energy production is effectively limited to that available from confinement animal production; some 70 MMt (dry) should be readily available in the future (Table 2).

Review article Conversion to energy through methane production does not greatly reduce the value of this manure as a fertilizer; however, there may be higher nitrogen losses because during the production of methane much of the nitrogen is converted to a form that dissipates rapidly unless it is injected into the soil (Fischer and Ianotti, 1977). The other concern is manure's contribution to organic matter; manure has been shown to increase organic matter in low-organic soils and thus improve soil structure. Conversion to methane uses from one-quarter to three-quarters of the organic material in the manure, so this contribution would be reduced. However, the ultimate stable organic matter contribution would not be reduced by as great a proportion. In order to be practical or economically feasible, methane production must be viewed as a part of the total manure-handling, fertilizer-replacement system. Systems have been designed and operated for periods of at least one year at several levels of sophistication and capital cost. These systems have proved to be economic for specific applications, even though costs and returns vary widely (Coppinger et al., 1978; Jewellet al., 1980). Currently, each system has to be analyzed on its own in terms of its part in the existing or planned enterprise and in terms of the capacity of the enterprise to use the gas produced to its maximum economic benefit. In figuring the potential net contribution, we calculate the amount of methane that might be produced from either 50 or 70MMt (dry) of manure (Tables 2 and 3). The energy replacement value of the fertilizer is not applied as a credit, since this replacement takes place anyway. The conversion ratio to methane of 5-15 per cent currently and 10--20 per cent projected for the yr 2000, translates to a low of 0.03 Q today and a high of 0.23 Q in 2000 (Table 2). In addition, a certain amount of high-grade premium energy is required to run the conversion plant, mostly to keep the digester at the correct temperature, but also to run pumps and other machines. The amount of energy needed for temperature control varies depending on the scale of the operation, the season, and the climate. Jewellet al. (1980) indicated that in a northeastern climate, the expected energy available for the dairy will range from 40-71 per cent. AvailableEnergy as a Percent of Total Production Dairy Farm Size--Total Numbers Climate Conditions Summer Winter

25 head 59 per cent 40 per cent

100 head 66 per cent 52 per cent

500 head 71 per cent 60 per cent

In terms of an estimate of net energy, first the conversion loss takes place. Then, depending upon the climate and the scale of operation, only 40-71 per cent of the gas is available as high-quality energy.

Food and feed grains, starches and sugars Grains, starch crops, and sugar crops are suitable for energy production via fermentation to fuel alcohol. Corn, wheat, and sorghum (milo) are the principal sources.

13

Sweet sorghum is considered by some to have large potential for this use (Lipinsky and McClure, 1978; Salman et al., 1980). Sugar cane and sugar beets, excellent materials for alcohol, have a limited possibility due to total potential supply limitation and competing uses as well as economic constraints. Such starch materials (aside from grains) as Jerusalem artichokes and cattails have been mentioned for this use, but until there are important cultural and mechanical advances, they will probably remain minor contributors. U.S. grain production was about 353MMt in 1980 (USDA, 1981). Sugar beet production was 2.2MMt and sugarcane was 6.3MMt (USDA, 1979). The grains have a wide geographical and climatic range; the other starch and sugar crops are more regionally limited by soil and climate, Corn is the major source of fuel alcohol in the United States. In 1980 approximately 1.0MMt were used to produce 100 million of the 105 million gal. of fuel alcohol (NAFC, 1981). It is likely that corn will remain the major source over the next 5-7 yr or longer, and that milo will be a distant second. Corn will give way only when a commercial process for converting cellulose to ethanol is developed. Various expectations and targets have been expressed concerning the eventual gallons of fuel alcohol that could or should be produced from grains. Two billion gallons per yr was suggested by the President (Carter, 1980) and 10 billion gal. per yr by 1990 was suggested in PL 96-294, "The Energy Security Act". There is no technical reason to prevent either of these targets from being reached, but there are constraints due to variations in capital markets and availability of grains, and to potential soil erosion. The production of 2 billion gal. of fuel alcohol would require about 22MMt of corn grain (10 per cent of annual production) (OTA, 1980) (Table 2). Variations due to market and, more important, due to climate disturb even the most conservative forecasts of future grain production in the United States. For example, in 1980. corn production was 6.6 billion bushels, which was a 16 per cent drop from the 7.9 billion bushels produced the previous yr. Predictions for 1981 are for a record crop but futures prices should remain high. To estimate without qualification that a given amount of grain will be available for alcohol production is not only misleading, but dangerous. Estimates of amounts that can safely be reserved for alcohol production have varied between 10 and 44MMt (ERAB, 1980; OTA, 1980; Lockeretz, 1980; Schnittker Associates, 1980; NAFC, 1981). This extremely wide variation is caused by reliance on criteria such ~is worldwide needs, multi-year production averages, competition among users (food, feed, fuel), and rising costs of production that variably favor one use or another. The upper realistic limit appears to be 1022 MMt (Tables 2 and 3), above which the competition among uses may have a significant effect on the economy and on consumers (NAS, 1981). It is indisputable that people who have invested more than $100 million in an alcohol production unit will be forced to compete vigorously in the grain market in periods of shortages or high prices in order to protect that investment. Alleviation can only come through the use of new proces-

14

Review article

ses, probably cellulose conversion, which can use less costly raw materials. From 10--22MMt does convert to 1 to 2 billion gal. of fuel alcohol--a small but significant addition to U.S. fuel alternatives, equalling less than 1 per cent (net) of current (June 1981) gasoline consumption. The profitable conversion of grain to alcohol requires a high ratio of cost of raw materials to cost-of-goods-soldt in manufacture. The profit margin is thus very sensitive to even small variations in prices of raw materials. In terms of cost-of-goods-sold for 1981, the cost of grain in industrial dry milling alcohol plants at best conversion efficiencies is about 81 per cent gross (ERAB, 1980; corrected for June 1981 cash price). The effects of inflation on labor and processing costs are relatively small compared with the effect of increases in grain costs. Because market prices of grain are affected by general conditions and not by the precise final use of the grains, prices are not really a function of food-feed/fuel conflicts, unless the conditions described above come into play. A number of possibilities for alternative cropping for fuel have been suggested: rotating crops specifically for fuel use, using marginal lands, more intensively fertilizing and irrigating, and diverting exports to fuel use. Most of these entail higher energy uses for production, which would involve changes in policy and agricultural practices. The gains, however, would probably not equal the costs (ERAB, 1980). There are currently no lands set aside. It is estimated that an additional 20 MMt might be brought into the fuel market or an additional 0.3 Q of liquid fuel (gross). Since alcohol is currently used only in spark ignition engines, it can efficiently be used only as an additive to unleaded gasoline, or in automobiles appropriately modified for alcohol use. It cannot now be used as a fuel in diesel engines. Experiments using alcohol in various proportions of diesel fuel for diesel engines are being conducted, and some efforts are being directed toward engineering diesel uses for alcohol. One company offers a conversion kit for diesel engines. Nevertheless, use of alcohol, either pure or mixed with diesel fuel, is not recommended by any manufacturer because of uncertainties about the effects on engine parts and on performance (NAFC, 1981). According to surveys (DWR, 1980; NAFC, 1981)there are industrial plants on line with an annual capacity of about 105 million gal. of alcohol. By the end of 1982, planned construction, if completed, would add another 500 million gal. An additional 80-100 million gal. is planned with plants of less than 15 million gal. annual capacity each. Present indications are that this schedule will not be met due to the removal of certain federal assistance funds following a change in federal policy, and to the high relative price of corn and current easing in gasoline supplies. tCost-of-goods-soldis the accountingrepresentingthe cost of raw materials, cost of labor, plus cost of burden. These three represent the cost of making a product before adding general costs of management, service utilities, money, taxes and sales.

Best efficiencies for conversion are met by plants with a capacity to produce 20 million gal. or more annually. Plants that produce between 20 and 50 million gal. have little percentage increase in costs-of-goods-sold in terms of economy of scale, while the larger plants permit increased return on capital in an active market. Below the 10 million gal. capacity, operating and capital costs per gallon of capacity rise rapidly. The efficiency rates vary considerably (Katzen, 1980). The decision to bring new plants on line is based on capital availability, risk assessment, and availability of raw materials. Grain alcohol plants can be built anywhere, but appropriateness of a site will most probably be governed by its nearness to the least expensive supplies of raw materials, and the nearness to sufficient water. Thus, plants located in grain-producing areas will reduce transport costs. The increased production of grains for alcohol will increase soil erosion and competition for cropland (SCS, 1977; GAO, 1977; CEQ, 1980; Jackson, 1980). The production of ethanol from grains has other potential environmental impacts. For example, impacts of an accelerated gasohol program include increased air pollution from coal-fueled facilities, particularly sulfur oxides, nitrogen oxides and fly ash. Other increased air pollutants include CO: from the fermenters and dust from corn and dried stillage handling and processing. The plants may also vent some odors.

Bagasse Bagasse, the fibrous residue of sugarcane remaining after the juices have been drained from the ground cane, is used as boiler fuel, and under normal operating conditions no additional fuel is required during processing. One pound of bagasse (50 per cent water) contains 3500 Btu. Of this, about 500 Btu will be consumed in vaporizing the half pound of water. Assuming a 75 per cent boiler efficiency, the remaining 3000 Btu will yield a net heat energy of 2150Btu. Since bagasse waste normally is processed at the cane plant, collection and transport costs need not be considered. The estimated 4 MMt of bagasse available for combustion (Meade and Chen, 1977) has a gross energy value of 0.06 Q. Potential net recoverable heat energy is estimated to be 0.03Q (Table 3). The availability of bagasse for biomass energy is not expected to change much during the next 20 yr (Tables 2 and 3). No major environmental problems have been reported in using bagasse in sugar processing plants. Food processing wastes The estimated 15MMt of currently available food processing wastes has a gross energy value of 0.25 Q (Ware, 1976; Pimentel et al., 1980). These wastes are 70-90 per cent water. The amount of food processing wastes that is produced annually in the United States is projected to increase to about 17 MMt by the yr 2000. The gross energy would be about 0.29Q, and if the wastes were converted into biogas, the net energy potential would be about 0.03-0.06 Q (Table 2).

Review article In the United States in 1978, the total production of cheese whey, one of the food processing wastes, was about 35.8 billion lb, or 1.1 MMt dry (McKee, 1980). The yield of alcohol from whey is calculated to be about 3gal. per 10001b of whey (McKee, 1980). For most cheese factories the disposal of cheese whey is currently a problem. Increasingly, cheese whey is being converted to human food products and livestock feed. Use of some whey for the manufacturing of fuel alcohol would help reduce the whey disposal problem. Fermentation of cheese whey would be limited either to large cheesemaking facilities that produce at least one million pounds of whey per day or to areas where several cheeseproducing facilities are located so that trucking of whey or whey concentrates is practical (McKee, 1980). It is unknown exactly what portion of the 1.1 MMt of whey is not now being used and what portion of the remainder is located conveniently for alcohol production. Processing these wastes into biogas would probably reduce some of the normal environmental problems associated with disposing of them.

Industrial wastes An estimated 150MMt of industrial wastes currently are generated each year (Anderson, 1977). Subtracting the 20MMt of food-processing wastes, including bagasse, leaves 130 MMt of wastes from other industries, of which organic materials (paper, wood, plastic, organic chemicals, textiles and rags, petrochemicals and rubber) are estimated to comprise 76 per cent (Mantell, 1975), or about 100 MMt. About 23 MMt could be considered to be readily usable for conversion (Anderson, 1977). Assuming that this waste has an energy content of 6000 Btu/lb (Golar, 1975), its gross energy is then 0.30 Q. The potential net yield as heat produced by combustion would be about 0.15Q. Tillman (1977) estimated that 0.05Q is currently recovered as heat energy from industrial wastes, excluding the energy recovered from food-processing wastes. Processing industrial wastes into heat/steam energy would probably reduce some of the environmental problems normally associated with the disposal of these wastes. Waste paper, estimated to be as high as 59 per cent of industrial wastes (Mantell, 1977), could be recycled or burned. If all waste paper were recycled, the leftover would be 9.4 MMt or 0.11 Q. The potential net yield as heat produced by combustion would then be reduced to between 0.06 and 0.08 Q. The amount of industrial waste that would be available by the yr 2000 is highly uncertain because of the trend for industry to reduce losses from waste whenever possible. A conservative projection would be from 23 to 42 MMt, depending on how much waste paper would be recycled. (For some industries, such as the chemical industry, waste availability could grow at a higher rate, following the industry's historic growth rate of about 10 per cent per yr). The potential gross energy would then be from 0.28-0.50 Q, and heat produced by combustion would be 0.16-0.29 Q (Table 2).

15

Muncicipal solid wastes A total of 136 MMt of municipal solid waste (MSW) was estimated to be produced yearly in U.S. cities and towns (EPA, 1977). The fuel fraction comes primarily from paper, wood debris, and to a lesser extent, plastics. Paper waste ranges from 21.5-53.3 per cent, with a mean of 37.4 per cent. Subtracting about 70 MMt of recyclable or noncombustible materials (paper, glass, ferrous metal, and aluminum) leaves about 66MMt for energy conversion (Table 3), equating to a gross heat energy of 0.59 Q, assuming 4500 Btu/lb (OSWMP, 1975). If paper were removed, the unit heat content and gross heat energy would undoubtedly be considerably lower. Available MSW in the yr 2000 is projected to be 100 MMt, with a gross heat content of 0.90 Q (Table 2). The net energy as high-grade liquid fuel or gas would range from 0.09-0.41 Q (Table 2). The bulk of MSW makes transportation costly. Nearly 60 per cent of MSW, however, is generated in 150 major metropolitan areas (Franklin, 1975). The most efficient means of energy conversion of this municipal solid waste appears to be combustion for steam and electrical generation (Mantell, 1975), though conversion to ethanol is being considered as part of the Gulf/Emert plan. The conversion of biomass into electricity by combustion has some advantages (Reed, 1978; Slesser and Lewis, 1979; Pimentel et al., 1981): (1) biomass residues can be converted in an electrical power plant located close to the source of biomass, and electrical energy can be more efffciently transported to the consumer than raw biomass energy can be; (2) biomass is a clean-burning fuel compared with coal, and the residue ash is normally less than coal; and (3) the ash can be used as a fertilizer (P, K and Ca). The conversion of biomass into electrical energy is about 30 per cent efficient. The use of biomass for electrical generation may be economical in isolated instances (Slesser and Lewis, 1979; OTA, 1980), but coal and nuclear powered plants are generally more economical because of larger sizes, lower fuel costs and the ease with which electrical power can be distributed from central locations. Cogeneration of steam and electricity is a much more efficient process than generation of electricity alone (up to 70 per cent for producing low-grade heat and electrical energy, versus 35 per cent for only electrical energy). However, the major energy output is steam, which is not readily transportable. This type of energy conversion is economical when practiced adjacent to large industrial users of steam (e.g. chemical plants). It is therefore a promising way of using industrial wastes, and possibly some municipal waste. Municipal sewage Annual production of sewage sludge in the United States is projected to reach about 13 MMt (dry weight) by the yr 2000 (NAS, 1978). About 10 MMt of this sludge (gross energy content of about 0.15 Q) will be produced by municipal utilities (Hecht et al., 1975), and it will be accessible for energy conversion by anaerobic digestion (Cheremisinoff and Morresi, 1976) to methane gas (Table 2). Most of the gas that is now so produced (0.02 Q) is

16

Review article

used in the sewage treatment plants for heating the digesters. Some is flared in the summer. If all sewage were processed by anaerobic digestion, and the gas now flared were made available for use, then 0.02--0.04Q could be produced by the yr 2000, depending on the conversion efficiency achieved (Table 2). No known environmental benefits or risks would be associated with this increased production of methane gas.

Aquatic plants Certain plants such as water hyacinths and cattails may achieve exceptional rates of biomass production. Estimates of the potential of these biomass sources vary greatly. Estimated production rates range from 5-20 t/acre (dry) per yr. Yields of hyacinths averaging 25 t/acre/yr have been demonstrated at a Disney World project in Florida. Over 2 million acres of water hyacinths have been described as available for harvesting, and 2.9 million acres of marshland are said to be available for cattail growth in Minnesota alone (MLMI, 1979). Another 9 MMt of coastal aquatic vegetation is believed to be produced annually (Pimentel et al., 1978). Harvest of 10 MMt annually of hyacinths is said to be possible without harming water hyacinth stands (Wolverton and McKnown, 1976; Cornwell et al., 1977), but only 3.3 MMt of hyacinths and coastal weeds have been estimated as readily harvestable (Pimentel et al., 1978). Allowing for the uncertainties in production, the estimated yield by the yr 2000 is between 3 and 13 MMt, the estimated gross energy content is between 0.05 and 0.22 Q, and the estimated net production of biogas between 0.004 and 0.03 Q (Table 2). The high water content (about 95 per cent) of water hyacinths makes biogas production appear to be the most appropriate energy conversion method; the starch conten~'of cattails suggests ethanol production. However, sufficient large-scale experience, covering the range of operations from biomass growth through fuel generation, has not been obtained to permit reliable economic projections. A variety of pilot programs is underway. Among the more advanced is a combined project at Disney World, Florida, sponsored by EPA and the Gas Research Institute, in which water hyacinths are used to treat waste water and hyacinth/sewage sludge beds are used for methane production. It is assumed in some analyses that fertilizer requirements are negligible, but no experimental studies have yet been carried out. Aquatic plants can have beneficial environmental effects since they efficiently concentrate heavy metals and nutrients (phosphorus and nitrogen). Large quantities of waste water and solid residues rich in salts and in heavy metals must be disposed after biomass conversion. Until practical-scale projects have been carried out, the net environmental effects cannot be assessed. Because of the preliminary state of development of these energy systems, the contribution of aquatic plants to national energy supplies is expected to be limited over the next two decades. These plants might prove valuable in some regions (cat-tails in Minnesota, water hyacinths in the Southeast)•

Ocean farms of giant kelp have been visualized as vast potential sources of natural gas. It has been estimated that 8 Q of methane gas could be produced annually from an open ocean farm 235 miles on a side, if 20 dry, ash-free t per acre per yr could be obtained (GRI, 1980). This yield is significantly larger than yields from natural beds in coastal waters off southern California, where nutrients are provided by natural upwelling. To support dense populations of plant life, in the surface waters of the Pacific, nutrients must be pumped 1000-2000ft up from the ocean floor. For the system to produce energy, pumping energy must be very low; i.e. the concept depends on successful development of wave pumps. Tethered structures that can withstand the corrosive properties of seawater and survive extreme weather conditions must be developed to support the kelp. The feasibility of these concepts is under investigation as part of a several million dollar[year program sponsored by the Gas Research Institute; the program is directed to prototype farming off the Pacific coast. Other ocean locations with less demanding requirements are being sought. Even if the major technical challenges cited above can be overcome, production of substantial quantities of gas by this method will not be possible until well into the 21st century.

Crop residues An estimated 473 MMt of crop residues are produced annually (Stephens and Heichel, 1975). Most of these residues should remain in the field to maintain soil quality. Debates surround the question of exactly how much residue could be safely removed on a sustainable basis without causing serious land erosion or loss of nutrients. For present analysis, it is estimated that 80 MMt of crop residues per yr could be safely used (Larson et al., 1978; Larson, 1979; Tyner, 1980; Pimentel et al., 1981) and that the gross energy content of the 80MMt is 1.4 Q gross (Table 3). Although total crop residues will probably rise to nearly 600 MMt along with the projected agricultural increase, it is doubtful that more than 80MMt per yr could be safely used by the yr 2000 (Table 2). In Illinois and Indiana, the price of delivered corn residues ranged from about 27-$60 per dry ton (USC, 1978; Tyner and Bottum, 1979). This corresponds to 1.80-$4.00 per million Btu. This is more expensive than most coal (1-$2 per million Btu). The reason for the relatively high cost of crop residues is the large amount of labor, equipment and energy required to collect and transport residues. Crop residues may be converted into heat by direct combustion in boilers. If most of these residues are typical of corn stover, one ton of the residue will contain 8.5 million Btu with 50 per cent moisture. Taking into account the collection, nutrient loss, and conversion loss, the net energy per ton is 4.9 million Btu. Thus, by direct combustion the total energy is 0.82 Q (Table 2). The technology to convert crop residues into ethanol will presumably be commercially developed. If crop residues were converted into ethanol by such technology, the net energy would range from 1.2-3.3 million Btu per ton. Thus, the net energy of crop residue as high-

Review article grade liquid fuel will be a total of 0.22-0.64 Q (Table 2). Most of the corn, wheat, rye, oat and barley residues that might be available for conversion exist in regions producing large amounts of these crops on lands that are flat (0--2 per cent slope) and not susceptible to wind erosion. These requirements greatly restrict the regions where these residues might be used. Most other crop residues are for numerous reasons unsuitable for energy conversion (Pimentel et al., 1981). If it is assumed that 80MMt of residues could be removed annually from corn, wheat and three or four other crops in a few appropriate, fiat areas, what will prevent growers from using crop residues from areas where soil erosion is a serious problem? Agronomists agree that soil and crop management practices that decrease soil organic matter invariably reduce yields. This reduction can be measured in terms of the energy required to offset the yield depression (Black, 1957; Russell, 1961). For many agriculturally important soils in the United States, the question is not how much organic removal can be tolerated without yield losses, but rather how to add organic residues in order to recover productivity potentials that have already been lost (USDA, 1978). The long-term cost to agriculture of not returning residues to the soil is difficult to quantify precisely. However, it is clear that when all energy costs of transforming residues are considered, the net return is generally low (Table 2), and it is often negative (CEQ, 1980; Pimentel et al., 1981). The energy inputs required for collecting, transporting, and processing residues are large. In addition, there are energy costs of replacing the nutrients lost with the residue and of offsetting the reduced productivity of the land to the extent that this can be done. Many agronomists believe that, with present technologies for crop production, any short-term energy gains from the use of residues will likely be more than offset by the long-term degradation of the soil (Patterson, 1981). Grain production is closely linked to soil organic matter content. For example, a reduction in organic matter content from 3.8-1.8 per cent has been reported to reduce the calculated yield of corn about 25 per cent (Lucas et al., 1977). Another consequence of reduced soil organic matter content is that more fuel is needed to till the soil. For example, a reduction in soil organic matter content from 1.05-0.13 per cent resulted in a two-fold increase in energy input for deep plowing under moist conditions (Hadas et al., 1978). In general, the potentially adverse environmental consequences of current crop management practices, even under ideal cropping conditions, are so serious as to draw into question the use of residues for biomass energy production until crop production technologies are improved (Onstad et al., 1977; Pimentel et al., 1978; Lindstrom et al., 1979; Holt, 1979; CEQ, 1980; Pimentel, 1981). In addition, more definitive data are needed concerning the consequences that such practices would have on the natural biota. These environmental assessments must be developed before crop residues are committed to biomass energy programs.

17

Oil-producing crops Numerous plant species have been proposed as candidates for energy crops (OTA, 1980). For example, approximately 2000 species of Euphorbia produce a latex rich in hydrocarbons that can substitute for oil in practically all its uses (Calvin, 1979). Liquid hydrocarbons extracted from Euphorbia spp. may be processed into gasoline and similar fuels (Calvin, 1977; Johnson and Hinman, 1980). Several euphorbs are under mostly experimental cultivation. Under optimal conditions, these plants produce about 10 barrels of oil equivalents per acre per yr of gross raw material (Calvin, 1979). What the net yield in fuel will be when these euphorbs are grown on marginal land and the raw material is converted to high-grade fuel remains to be determined. Other interesting species produce oil that can be used to power a diesel engine: Copai[era langdorfii, Sim-

mondsia chinensis, Parthenium argentatum, Asclepias spp. (milkweed), squash, sunflower, safflower and others (Calvin, 1979; Johnson and Hinman, 1980; Anonymous, 1980; OTA, 1980; Peterson et al., 1980; Vietmeyer, 1980; Hall, 1981). When sunflower oil is burned directly in a diesel tractor, however, some problems arise from engine deposits. About 100 gallons of sunflower oil can be pressed from an acre of sunflower (Peterson et al., 1980). The total high-grade energy needed to produce the crop is about 90 gal. of fuel equivalents, so the energy balance for this species is barely favorable. Some added energy credits are included in the protein residue from the sunflower seeds. The cost of sunflower oil per gal. in 1980 was about $2.76. Some of the plants adapted to arid habitats will compete for rangeland now used for livestock production. A few of the plant species that have been suggested as row crops in these arid environments may cause serious soil erosion from wind. Under arid conditions, plant productivity is relatively low, and these crops would provide minimal protection of the soil for long periods of time. Irrigation is not possible because it typically increases the energy required for crop production by a factor of 3 to 5 (Pimentel, 1981).

Landfills Currently, most municipal solid waste (MSW) is disposed of in landfills. Methane-rich gases, naturally produced by anaerobic digestion of waste at these sites, can be tapped. If 79 per cent of MSW is put into landfills (Manteli, 1975) and the accumulation from the last 10 yr is considered, there should be approximately 1100 MMt of waste that can be considered a nonrenewable energy resource. Not all sites are suitable for methane recovery; Collins (1976) has estimated that 120 × 10 9 scf (standard cubic feet) of gas is recoverable per year from several of the suitable large landfills. This represents the yield from about 200 MMt of refuse (Ham et al., 1979), and has an energy value of 0.07Q (Wise et al., 1975). The few existing projects were recovering approximately 0.001 Q per yr in 1976 (Tillman, 1977). Because this is a nonrenewable resource, it was not included in Table 2.

Review article

18 BIOMASS RETRIEVAL AND STORAGE

The collection and transportation of certain kinds of biomass may require a significant proportion of the energetic and economic inputs in biomass energy conversion technologies. For example, the collection of corn residue, which is the most abundant residue per unit farm area, requires about 190,000 Btu/t (Pimentel et al., 1981). To this energy input must be added the fertilizer value of the removed residue, which has been calculated to be 556,000 Btu/t (Jenkins et al., 1979). If a ton of corn residue were processed into 80 gal. of ethanol, the energy input due to collection and to fertilizer energy inputs of the residues would be nearly l0 per cent of the ethanol energy produced. The collection of logging residues is even more energy intensive. About 535,000 Btu/t are required to harvest logging residues or slash (Cammack and Lambert, 1976; Smith and Corcoran, 1976; Pimentel et al., 1981). Again, to this must be added the fertilizer value of the logging residues, which is about one-half that of corn residues (Young et al., 1965; Dyer, 1967; Boyle, 1975). The energetic and economic inputs due to transportation may be high because of the moisture content of biomass, which varies from 10 to 95 per cent. For a moisture content of 50 per cent in forest biomass, for example, the transport costs are calculated to be 5600 Btu/dry weight ton per mile, including the return trip. In addition to the problem of the relatively high energy cost of transporting biomass, there is the problem of storage~ Biomass, such as wood chips or straw, can not be stored in piles over about 50 ft in height because of the danger of spontaneous combustion. Any storage leads to some deterioration of the biomass; however, the losses in storage are not large enough to warrant special structures or chemical treatments.

TECHNOLOGIES FOR THE CONVERSION OF BIOMASS

The contribution of biomass resources to the nation's energy needs depends on the form in which the energy is used by the consumer and related energy debits or credits in obtaining the biomass energy. The most desired forms of energy are liquid and gaseous fuels. To be transformed into these forms, biomass must be converted either thermally, chemically, or biologically. The liquid or gaseous product from conversion will always contain only a fraction of the energy of the natural biomass. Moreover, additional energy from another source, such as coal, natural gas, fuel oil, from combusring other biomass, or from electricity derived from any of the above or from nuclear reactors, usually must be supplied to the conversion process. Since the objective of conversion is to create highgrade liquids or gas, it is mandatory to consume minimum quantities of gas or fuel oil in the conversion. The conversion produces a gross yield of fuel energy (e.g. ethanol). Figure 2 illustrates the important parameters related to the actual net yield of fuel product, N, added to the market. The conversion process transforms biomass B (in

energy units) to gross high-grade fuel H, with a conversion efficiency defined by Y = H/B. The production as well as the conversion of the solid raw material (e.g. biomass or coal) may also require consumption of high-grade fuels from the market, H,,. The "net gain in high-grade fuel" created is logically N =H-H,.,or N = YB-H,,.

The "net energy efficiencies" listed in Table 2 correspond to the ratio of net fuel produced/biomass used, i.e. from formula (1). N/B = Y - H,,/B.

The above consideration is equally applicable to any resource to be converted to liquid fuel. For example, it applies to coal conversion as well as biomass conversion. However, for the production of the respective solid raw materials, a fundamentally different situation prevails: the coal production (mining) can be accomplished with a very small (about 2 per cent) input of high-grade fuel (Frabetti et al., 1975). In the case of agricultural and forest biomass production, large amounts of high-grade energy (for machinery, fertilizers, pesticides, and transportation) are consumed. For example, to produce corn, fuel consumption is estimated to be as much as 37 per cent of the energy content of corn (Pimentel et al., 1973). Thus, for B = 100 energy units of corn produced, fuel energy H,, consumed in its production alone would be 37. With a typical conversion efficiency of corn to alcohol of Y = 0.58, corresponding to 2.6 gal./bushel, the gross production of alcohol is 58 energy units, but the net contribution to the market may be only 58 minus 37, or 21 energy units. Thus, only 36 per cent of the visible, gross production at the plant represents a true contribution to the market. This assumes, furthermore, that no high-grade energy is used to support the conversion operation, such as the distillation or by-product processing. In distillery operations that are using gas or fuel oils, the net fuel contribution to the market is much less, and can be negative.

I orm I I rres' I

L Hc ~N'H-H¢

Fuet

market

Fig. 2. The parameters related to net fuel yield.

Review article In Tables 2 and 3, it has been assumed that process heat for distilleries is not supplied by high-grade fuel at all, but by coal (or biomass residues). Conclusive data, especially for the consumption of high-grade fuel in the production of biomass, are not available, and the values chosen for the tables are, in many cases, optimistic. A biomass conversion process can also produce valuable by-products. If through their sale and use these are able to substitute for commodities whose production requires liquid and gas, it is proper to credit that amount of liquid and gas to the conversion process. An example is the distillers' dried grain by-product from corn fermentation; it can substitute for soybean meal and replace the liquid and gas required to produce the soybeans. Heat The oldest and simplest use for biomass is heating or steam generation. In this application coal is a competing alternative. In some instances biomass, like coal, can replace more valuable liquids or gas and release them for premium applications. Residential and industrial fuel wood usage has been growing and is expected to grow further in response to increasing costs for conventional fossil fuels. About 1.11.4 Q (gross) is currently burned, principally by the forest products industry (Table 3). Relative to fuel oil, natural gas, or coal, fuel wood is a somewhat less efficient heat source. Its moisture content reduces its thermal output. For this reason fuel wood is often air-dried prior to use. Even with drying, however, it produces lower flame temperatures and requires larger fireboxes and heat transfer surfaces. The efficiency of transferring dry wood into heated air or steam varies with the furnace or boiler design and nature of the fuel wood. Conversion of biomass in large boilers to heat energy often is 70-75 per cent efficient, whereas in wood stoves in homes the efficiency may range from 50-70 per cent. Most fuel wood has a low sulfur content and a low ash production. Costs for control of emissions are lower than for coal and high-sulfur fuel oil. When a log is burned, the amount of C02 released into the atmosphere is the same as that removed from the atmosphere during photosynthesis that produced the log. Thus, there is no net increase of CO., that would lead to the frequently discussed greenhouse earthwarming effect. Because of fuel wood's relatively high moisture and low Btu contents, transporting it more than 50-100 miles is usually energy inefficient and uneconomical (Smith and Corcoran, 1976; Hewett, 1978; Bylinsky, 1979; OTA, 1980). Conversion of biomass to combustible gas Several gasifiers have been or are being developed to convert biomass as varied as hardwood chips or animal manure into a mixture of combustible gases. Most gasifiers are supplied with air and the product gases contain a nitrogen diluent (low Btu gas). The low Btu gas is usually used in nearby burners or engines. Conversion

19

efficiencies are generally good with 60--80 per cent of the Btu content of the dry biomass appearing in the gas. In some instances conversions higher than this have been reported but these must be debited by the energy to operate the gasifier. Use of biomass gasifiers offers flexibility in plant design and provides a way of coupling biomass into existing plants with interchangeability between biomass fuel and conventional fuel. Conversion o[ biomass to liquids Methanol. Existing technology for the production of methanol from wood and agricultural residues involves the initial gasification of the biomass by partial-oxidation methods to produce synthesis gas (a mixture of carbon monoxide and hydrogen, after appropriate purification steps). Production of methanol from the synthesis gas can be accomplished by one of several conventional, commercial processes. Processes for the initial biomass gasification step currently are being developed (SRI, 1979; Wan et al., 1979; Jones and Radding, 1980). Commercial designs are available (Rooker, 1980). Efficiencies for the overall conversion to methanol are in the range of 40-50 per cent. A process for the further conversion of methanol to gasoline is available, and improved versions may be possible (Meisel et al., 1976; Silvestri, 1981). In addition, there is the possibility that methanol might be converted efficiently to ethanol, should future fuel-substitution economics favor an ethanol product (Beuther, 1980). The relative values of gasoline, ethanol, and methanol are influenced by distribution economics and by future developments in engine technology. The gases resulting from biomass gasification are also suitable for conversion to other important chemicals by processes based on Fischer-Tropsch synthesis. Efficiencies and costs are estimated to be similar to those for methanol production from biomass (Schulz, 1980). Biomass conversion through gasification and synthesis of liquid products such as methanol, is directly analagous to the conversion of coal. However, the conversion costs that are derived from economics of the large-scale operation possible for coal, may not apply to biomass conversion. The problem is the potential availability of land on which sufficient biomass can be produced and supplied economically to a large-scale conversion plant. For example, present-day commercial industrial practices in methanol production suggest a significant size penalty for a wood-to-methanol plant having a capacity smaller than about 15,000 barrels per day of methanol product. About 4 tons of wood (dry basis) per acre per yr would be needed to support a plant of this size. The optimistic 4 t/A per yr (usually about 2.5 tons) of wood would require intensive silviculture on about 340,000 acres. It appears that improved technology for thermo-chemical conversion of biomass to methanol is not likely to change the economy of scale so that small-scale production will be favored. Ethanol. The conversion of sugars and starch by fermentation to ethanol is a well-established technology.

20

Review article

Although sugar from molasses and cheese whey, and starch from grains or tuberous plants (e,g. potato, cassava) are the most commonly used substrate, most of the recent interest for fuel ethanol production has been in the use of starch from corn. Corn, which is 80 per cent starch (dry basis), is prepared by either wet or dry milling to release the starch from the kernel. With wet milling, corn oil, gluten and fiber are isolated before fermentation as co-products. In the dry milling process, the primary co-product is dried distillers' grain (DDG), a useful protein feed that is prepared by drying the stillage remaining after'fermentation and distillation. The co-products in both cases are important to the overall economics of ethanol production. Starch derived from corn is hydrolyzed enzymatically to glucose syrup, which is fermented to ethanol. Ethanol fermentation has traditionally been done with yeast in an 18-48 hr batch process that produces 8-10 per cent ethanol by volume. The ethanol is recovered by continuous distillation. The theoretical mass conversion efficiencies of sugar and starch to ethanol are 0.51-0.57 g ethanol/g substrate, respectively; it is usual practice to achieve 90 per cent or better of the theoretical yields from carbohydrate (some of the suhstrate is consumed in the production of the yeast). Thus, the yield of ethanol is about 2.5gal./bushel of corn. (The range of net conversion used in the calculation is 20-50 per cent (Tables 2 and 3)), It is important to realize that about 81 per cent of the manufacturing cost of ethanol is the cost of the substrate. To reduce the cost of ethanol production, research has been on process improvements such as continuous fermentation, alternative organisms, and immobilized whole-cells; these and continued efforts in process and microbial (e.g. organism selection and genetic engineering) technology will increase the productivity and reduce the capital investment required for ethanol production. Some improvement is also realized in conversion yield; however, even if the fiber and protein in corn were converted to ethanol (at the expense of losing most of the feed value of residue), the maximum theoretical yield would be 3 gal./bushel. Ethanol recovery by distillation has been an intense area of development, In older beverage distilleries, more energy was used in distillation than was contained in the ethanol (Scheller and Mohr, 1976; Weisz and Marshall, 1979). By improvement in distillation technology and by efficient energy use in the whole process, energy requirements for ethanol production have been greatly reduced (Katzen, 1978). It is of interest to note that higher energy efficiency is achieved in distillation, as in many processes, by increasing capital equipment and cost. Energy consumption in ethanol manufacture in the dry milling process is split about evenly between: (i) substrate preparation and fermentation; (ii) distillation; and (iii) drying of the stillage. Most of the energy requiremerit is as heat. A net energy analysis for ethanol production is most useful when done on the basis of net high-grade energy, as shown in Table 4. Currently, about 65 per cent of fermentation ethanol is produced in plants

using gas or fuel oil (McKee, 1980) and the net energy balance is near zero or negative; when coal replaces high-grade fuels, the net high-grade energy balance becomes favorable (ERAB, 1980). Development of small (e.g. typically 100,000500,000 gal,/yr) on-farm units for ethanol has been under way. The objective is to provide secure fuel supplies for on-farm and rural energy needs through the replacement of gasoline, and potentially diesel fuel. So far, the ethanol production systems are relatively inefficient, usually achieving less than 2 gal. fuel grade (199 proof) alcohol per bushel of corn. However, they may make some fuel available locally where raw materials are found. In addition, in especially efficient operations, there might be advantages from using all the co-products such as the fuel for equipment, the wet stillage for animal feed, and even the CO: for green-house agriculture. Other liquid fuels such as butanol (more acceptable for addition to diesel than ethanol) and chemicals can be produced by fermentation from starch. Currently, about 5 per cent of petroleum used goes into the chemical industry. Biota.ass offers an alternative source of chemical feedstocks. In principle, a wide variety of chemicals can be derived from biomass feedstocks, but they are not currently economical (Tong, 1979). As shown in Tables 2 and 3, cellulosic biomass (e,g. wood, forages, municipal solid wastes, and crop residues) is the largest renewable resource available for energy conversion. Cellulosic biomass, however, is more complex in composition and structure than starch. For example, its main components are about 40 per cent cellulose (a polymer of glucose), 20 per cent hemicellulose (a polymer of mostly xylose), and 15 per cent lignin. On a dry mass basis, cellulosic biomass is much less expensive than starch; however, to realize the price advantage, it is essential to convert all the carbohydrates, both pentose and hexose sugars, to ethanol. The traditional yeast fermentation for ethanol uses only the hexose sugars. Several investigators are developing process technology for pentose conversion to ethanol and other fuels and chemicals (Cooney et al., 1978; Gordon et al., 1978; Wang et al., 1978; Gong et al., 1981). The theoretical conversion of carbohydrate in cellulosic biomass to ethanol is calculated to be 100gal. ethanol/t of biomass or a 40 per cent net energy conversion (the range used in the calculations is 20-45 per cent [Table 2]). This 100gai./t compared well to 120gal./t (0.56 of ethanol/g, of starch) for corn. The lignin that comprises about 15 per cent of the dry biomass has good heating value (heat of combustion is about 15,000 Btu/lb), and could provide a portion of the process energy. There is sufficient lignin present in most lignocellulosic biomass to provide all the heat energy required for distillation and stillage dehydration. Processes to convert wood to ethanol were developed and used during both world wars (OTA, 1980). Wood was acid hydrolyzed, the glucose was converted to ethanol in a yeast fermentation, and the pentoses were used to produce fodder yeast. More recently, a direct fermentation of cellulosic biomass by thermophilic bacteria has

Review article been developed (Cooney et al., 1978; Wang et al., 1979). Both the cellulose and hemicellulose are converted to ethanol. Lignin, although not degraded, is available as a process energy source. The yield of ethanol from corn stover is 0.25g. ethanol/g, dry biomass. In processing municipal solid waste by enzymatic hydrolysis and yeast fermentation of the resulting sugars to ethanol, Emert and Katzen (1980) observed a yield of 0.25 g. ethanoi/g. solid waste. Estimates of the cost of ethanol produced by fermentation of cellulosic biomass suggest that this technology will be competitive with ethanol production from grain and sugar crops (Jenkins and Reddy, 1979; McKee. 1980; Emert and Katzen, 1980). Development of lowcost, advanced technologies and improvement of acid and enzymatic hydrolysis to maximize yield of fermentable sugars for cellulosic biomass will further increase the ethanol yield and productivity. Considering the amount of cellulosic biomass available, its low cost, and the current and future promise of conversion technology, fermentation of cellulosic biomass is an important technology to develop. There is also the possibility of producing other liquid fuels such as butanol and chemical feedstocks from cellulosic biomass. Liquefaction processes. Efforts have been underway for a number of years to develop a commercial process for direct liquefaction of wood, including a pilot plant operated at Albany, Oregon. Energy yields from a synthetic oil between 40 and 56 per cent of the gross inputs have been achieved (Molton et al., 1978; SRI, 1979). The oils that are high in oxygen content and low in hydrogen content will require substantial hydrogenative upgrading to bring them to premium fuel specifications. The energy required for the latter will reduce the net energy yields. The low-grade oils that are produced, of course, can be used as heavy boiler fuel without further processing. Anaerobic Digestion to Methane This conversion technology can in principle use almost any biomass. The technology can be used in a plant of any size, from the small backyard fermenter that uses human wastes, to the large fermenters that use feedlot wastes. Manures are particularly amenable to this technology because of their abundant nutrients, their moisture content, and their lack of inhibitors of methanogenic bacteria. The gas produced during digestion is 50--70 per cent methane (with the balance CO2) and is best used locally for heating. In some instances, the gas may be purified and fed to a natural-gas pipeline (Stafford et al., 1980). In this present analysis it has been assumed that the fraction of biomass energy that is converted to methane varies from 10-20 per cent. This range takes into account both the yield of methane and the energy consumed in operating the biogas process. In practice, yields can vary widely with the nature of the biomass, the temperature, and the retention time in the fermenter. Moreover, the amount of energy needed for conversion can vary from almost nothing for small fermenters (except human effort) in warm climates to more than the fuel produced. The latter is the case for large fermenters that are stirred

and heated, and from which the gas is purified and compressed (Vergara and Pimentel, 1978; Slesser and Lewis, 1979). Research on the genetics of microbes and ecological studies of the interaction of microorganisms and various organic substrates might both improve the yield of methane and the stability of production. It should be noted that landfills where organic wastes have been buried contain valuable methane gas that could be removed (Collins, 1976). Pyrolysis This is an old technology that has been used for centuries to make charcoal and wood distillates (e.g. "wood alcohol" or methanol) (Hawley, 1923). Recently new technologies that use fluid beds and flash pyrolysis have been developed (Howard et al., 1978). Wood pyrolysis in general yields a char (charcoal) that comprises about 30 per cent by weight of the wood (dry basis) and contains about 42 per cent of its Btu content. It also yields a pyrolytic oil that carries about 32 per cent of the wood's Btu content. The pyrolysis gases that are also produced are usually burned to supply the processing plant energy. The oil is very low grade (low hydrogen, high oxygen content) (Knight et al., 1977; SRI, 1979) and would require extensive hydrogenation with accompanying energy losses to be used for any purpose except as boiler fuel (Sotter and Knowlton, 1974). INCREASING THE BIOMASS POTENTIAL AFTER THE YR 2000

Most current projections of future production of biomass suitable for generating energy, including projections in this report, assume that changes in agricultural and forest practices and in energy consumption will occur slowly. This seems a realistic assumption; however, we do not want to imply that this slow change is defensible. As the pace of technological innovation increases and as the pressure for international economic change increases, we hope that some known technologies such as soil conservation practices will be adopted more broadly in agriculture and forestry. Production of energy from biomass could exceed most current projections if a national commitment is made to do so. The capability of biomass to contribute to the national supply of energy on a sustained basis--as a truly renewable resource--will depend in part on the capability of owners of farms and forests. These owners will have to manage their land and water, and their production, in ways that provide a sustained flow of energyproducing biomass and that maintain productive agriculture and forestry (Pimentel et al., 1973; 1976; Carlson et al., 1979; Commoner, 1979; Jackson, 1980). These and other new management practices may be adopted as the values and priorities of society change. Clearly, major changes can be conceived but these will be influenced both by federal policy and by economic conditions that confront farm and forest owners as businessmen. Some of the problems that arise in increasing produc-

22

Review article

tion of biomass for energy must be solved at the level of national policy; others must be solved at the local level. Any trends toward more vegetarian diets or any development of techniques for protein production that are more efficient than conversion through livestock will affect the availability of land and other resources for the production of biomass for energy. For example, production of fish by using by-products from fuel ethanol production has been proposed as a more efficient way to grow animal protein than producing beef. The public, encouraged by price incentives, might become more willing to accept greater use of high protein grain byproducts as an alternative to meat. To avoid competition between food production and biomass energy resource production for limited land in a world facing growing food shortages, two conditions must be met: (1) food production must continue to increase in efficiency and quality; and (2) the production of biomass energy resources must contribute to food and fiber production. If these conditions can be met in the years ahead, then the food and biomass energy link can contribute to both the food and energy needs of the world. Many proposed energy crops may prove to have advantages over corn and sugar crops, but the demonstration of their feasibility awaits the private or public funding of demonstration projects and, in many cases, the refinement of technologies to convert cellulose. Future innovation and improvement in the production of energy from biomass lies, in part, under the power of those responsible for setting research priorities and for committing public and private resources. Even more fundamental issues in resource management are critical to the future of biomass energy resource production (Pimentel et al., 1973; 1976; Carlson et al., 1979; Commoner, 1979; Jackson, 1980). If wise soil conservation practices are not followed and present rates of topsoil erosion persist, yields of food, fiber, and biomass will decline. Erosion, however, is not the only issue in soil management. Many cultivation practices designed to increase yields over the short term are being found to decrease long-term productivity of soil (OTA, 1981). In many cases, maximization of short-term yields has taken precedence over wise principles of long-term soil management. Many farmers have argued the importance of maintaining adequate and reliably secure levels of farm income in the face of continued inflation in production inputs so they can pursue wise long-range goals rather than maximize short-term income. Income security, in and of itself, cannot insure that wise management practices will be followed, but the farmers' ability reliably to predict future income will perhaps encourage long-range planning. Farmers will likely be more willing to practice farsighted and conservation-minded management if they can rely on a stable market for their products. Science and technology could, in some cases, double biomass production in the future. For example, reducing forest and crop losses to insect pests, plant pathogens, and weeds through improved pest control could significantly increase biomass productivity. Also basic

genetic research might add the capacity of biological nitrogen fixation to some plants that do not have this ability. This would both reduce energy inputs for fertilizers while raising productivity levels. ENVIRONMENTAL, ECONOMIC, AND SOCIAL IMPACTS OF BIOMASS USE

Economic, environmental, and social impacts of converting biomass to energy are interrelated, and their e~tent depends on (1) how extensive conversion becomes; (2) what biomass resources are used; (3) what conversion technologies are employed; (4) how rapidly domestic and international demands for food and forest products grow in the future; (5) how extensively land and water resources are affected by the development of coal and other energy resources, and by urbanization and recreational development; and (6) what practices are employed in agriculture, in forestry, and in the production and harvest of biomass for energy conversion. From Table 2 we may determine the tonnages of the various kinds of biomass that are potentially available for energy conversion; the total potential contribution will be about 5 Q net by the yr 2000. The quantity that is "potentially available" is the amount deemed truly "renewable," the amount that, if properly managed, will have minimal impact economically, environmentally, or socially. Conversion of this available quantity can be seen as beneficial. For instance, converting wastes to energy from food processing, from industries, and from municipalities might save money, help preserve the environment (by reducing landfills), and provide employment in urban areas. Conversion of about 80 MMt of crop residues, 13-22MMt of grains, starches, and sugars, 80MMt of forages, and 278MMt of wood materials might be desirable, and have little undue negative impact. Converting two, three or more times these quantities would most likely have undesirable economic, environmental, and social consequences. Such conversion would not be sustainable and could be likened to "mining" the nation's soil and water resources. These larger quantities are commonly proposed, so the potential impacts deserve further discussion with particular emphasis on biomass from agriculture and from forestry. Economic impacts If biomass from agriculture and forestry is converted to energy in much higher quantities than those mentioned above, the result will be higher prices for both food and fiber. This will occur because underlying demand for food and fiber is now exceeding supply (notwithstanding the current economic recession). World production of grains and oilseeds is about 1700MMt, world trade is about 240MMt, and U.S, exports are about 120 MMt. A 1 per cent change in world production is 17MMt, which would be a 7 per cent change in demand for world trade and a 14 per cent change in demand for U.S. exports. It can be seen that under the current free market system, U.S. crop prices are tied to meteorological and political conditions worldwide. During the 1950s, 1960s, and most of the 1970s, U.S. crop production, spurred by ever increasing

Review article inputs of fossil energy in the form of fertilizer, kept pace with and sometimes exceeded demand. By 1980, however, stocks of grains and oilseeds neared all-time lows, cancelling the need for government acreage setasides, and leaving the United States with only enough stocks to fill its grain transportation "pipelines". The 1981 harvest is much improved from 1980 with relatively high stock levels, thus lowering prices paid to farmers; this change in outlook reminds us of the importance of good weather, and the narrow distance between "scarcity" and "surplus". In the much longer view, as the world continues population growth, continues its improvement of diets, and even continues the current period of favorable weather conditions, the demand for U.S. grain, both at home and abroad, will stay very strong. Diverting large quantities of grains to energy production simply adds to the demand for commodities that are already spoken for, which results in increased prices and inflation. The notion that "surplus" grain exists today that could be used for ethano! production is incorrect. Also incorrect is the notion that there is a usable supply of "spoiled" or "sample-grade" grain. This grain is in most years less than 2 per cent of U.S, production and can usually be used for feed when blended with higher grades. It is generally accepted that about 10-22 MMt of grain (primarily corn) can be converted to ethanol without unacceptable effects on grain price (given the continuance of favorable weather) (Tables 2 and 3). In addition, about 80MMt of forage crops could be converted to energy without unacceptable effects on economics (prices would increase about 10 per cent). The supply-demand balance for fiber is precarious internationally as well as domestically. In the next two decades, demand for lumber and certain other forest products wilt in some cases more than double (see p. 10). The increasing demand for fiber in the United States must be met by domestic forest resources, though this country is already a net importer of fiber (11 per cent imported). Improvements in domestic-forest management and use will help the supply, but increasingly, tree biomass will be desired as raw material for wood products. The added demand for converting tree biomass to energy will in general increase fiber prices. The forest products industry, however, is probably in good position to integrate its management and use, and thereby capture a significant amount of biomass energy to help power its own operation. At present 76-94MMt (0.6-0.7Q) are thus converted by the industry, and this could increase to 100 MMt (0.85 Q) over the next two decades (Tables 2 and 3). The discussion of biomass potentials and impacts does not address the question of economic competitiveness of the biomass fuel product relative to existing fuel alternatives. In the United States the most important alternative will be coal. More speculative is the availability of natural gas and the further discovery of crude oil. The costs of producing the biomass resources identified in this study are largely unexplored. At present there are a few instances where combustion of biomass can be attractive, i.e. for small-scale, local electrical SE VoL 30. No. I~C

23

generation in specific forested areas. On the other hand, the forest product industry does not harvest most of its logging residues because it has available alternative fuel (coal or gas) that is less expensive than the gathering costs.

Any research on biomass use must address the costs of production, collection, and transportation to points of use. These estimates must include costs of: land acquisition, land diversion from alternative crops, fuel, machinery, labor and maintenance of soil quality. These estimates are needed for each potential production region, and need to take into account the parameters of the conversion technology (e.g. locating required scale of operation). Environmental impacts The total land area in the United States is 2264 million acres (Table 7). Agriculture uses about 1240 million of this, and forestry uses 540 million, for a total of 1780 million, or nearly 80 per cent of U.S. land. Another 8 per cent is used for cities, roadways, recreation, and public installations. The remaining 12 per cent is deserts, rock areas, marshes, and tundra. No significant quantity of land is unused and suitable for additional biomass production. Some land is available for growing trees, but that is currently not economically feasible, and the net energy contribution would be minor. Significant additional biomass production would be from higher fertilization of existing land or at the expense of existing uses of land. Furthermore, such additional production will aggravate erosion, an already serious environmental problem. Increased erosion from biomass production would be due to the pressure to produce more from a given amount of land, or the removal of too much crop and forest biomass residues from the land. United States cropland and grazing lands are being rapidly degraded by a serious soil erosion problem (GAO, 1977; SCS, 1977; CEQ, 1979; OTA, 1981). During the past 200 yr, at least 100 million acres of cropland had to be abandoned because of soil erosion. In 1973-74, when the prices of some grains doubled, farmers intensified their production efforts, and soil erosion increased (22 per cent in Iowa alone) (Timmons, 1980). An estimated one-half of the topsoil of Iowa has already been lost (Risser, 1981). Further pressure on the land to produce large quantities of grains for both gasohol and export will probably result in increased soil erosion as documented in Iowa in the 1973-74 period. Cory and Timmons (1978) projected a 72 per cent increase by 1985 in soil erosion losses compared to the base period 1969-71 for the 12 corn belt states due to growing pressure on the land to raise productivity. Crop and logging residues are essential to productive U.S. agriculture and forestry from several standpoints. Residues protect land from erosion (Larson et al., 1978). Residues contain nutrients that would have to be replaced by commercial fertilizers (Boyle, 1975; NAS, 1978; Pimentel et al., 1981). Residues contribute organic matter to the soil, along with structure, water-holding capacity, increased cation exchange capacity, and stabil-

24

Review article

ized mineralization rates of nitrogen (Lucas et al., 1977). Amounts of crop-residue cover needed to prevent soil erosion depend on climate, soil types, rainfall, wind, slope, previous erosion and the cropping system (Flaim, 1979). As mentioned earlier, if soil quality is to be maintained, current agricultural technology allows removal of only about 17% of the crop residues (Tyner and Doering, 1979; Pimentel et al., 1981). Even on the relatively flat land (0-2 per cent slope), from which some crop residues might be removed for conversion to energy, great care will have to be exercised to maintain the organic matter and productivity of the cropland. Water is also a major limiting factor in U.S. agricultural production, Total water withdrawn from lakes, reservoirs and streams has more than doubled since 1950 (Murray and Reeves, 1977), which indicates another precarious supply-demand relationship. Scarce supplies of water mean that biomass fuel plantations cannot be irrigated. Ethanol production requires at least twice the quantity of water needed for processing shale oil and coal for liquid fuels. Producers of ethanol will have to compete for their share of water supply and afterwards must provide for waste-water disposal. Three other environmental impacts from biomass energy production are important. First, air pollution could increase because of the burning of coal or wood to power ethanol plants. Wood combustion for heating will also pollute the air (DOE, 1980). Second, increased levels of CO2 in the atmosphere could result from large-scale distillation of grain ethanol using coal or from removal of crop and forest residues, which allows a more rapid oxidation of both residue and exposed humus. Third, the conversion of some natural forest and grazing lands into intensively cultivated plantations reduces the diversity of natural vegetation, which threatens wildlife and other natural biota, and limits reaction. Social impacts Extensive production of energy from biomass could lead to larger labor forces in agriculture and forestry, to greater amounts of labor to produce a barrel of fuel, and to larger farms (OTA, 1980); smaller farms might be favored, however, depending on what policies are adopted. Biomass energy development will require additional labor to collect, transport, and process the biomass (Pimentel et al., 1978; OTA, 1980). The need for labor will be particularly strong in the operation of wood-fuel plantations and in the use of forest and crop residues. Harvesting crop residues for biomass will require extra hours of work by farmers or "custom" work in the fall. Unless he has extra help, a farmer may be required to forego other fall chores such as fall plowing (Tyner and Doering, 1979). Heavy demand in the fall for additional labor may have to be met by migrant workers. It is socially meaningful to examine the manpower requirements to produce a barrel of ethanol from corn compared with the requirements to produce a barrel of refined petroleum fuels. For example, about 47bbl of refined petroleum fuels can be produced per 8 hr worker day in the United States. This includes crude oil

exploration and production activity as well as refining. About 4-9 bbl of ethanol from corn may be produced per 8hr worker day. This includes agricultural effort plus conversion plant operation (API, 1975; Katzen, 1978; Pimentel and Pimentel, 1979; KAC, 1980; McKee, 1980). (The range corresponds to different estimates of the labor requirements to grow corn plus different labor requirements for conversion plants; the latter varies with plant size and type.) Thus, from 5 to 12 times more labor is required to produce the same quantity of fuel from this biomass than from petroleum. Harvesting crop residues for energy will directly benefit farmers in the midwestern corn belt, the location of most of the harvestable residue. As commodity prices increase, farm incomes will also rise (Tyner, 1980). According to Buttel (1979), the greatest effect that biomass technologies would have on the rural sector is accelerating the inflation of the price of land; land prices will increase as a result of increasing the demand for landed commodities. This increase in prices would be passed through the system in the form of higher rents, higher interest payments, and higher costs of production, and would significantly hurt the small farmer. The farmer (especially the small farmer) benefits from such inflation only by selling his land (Buttel, 1979). Food vs fuel The above sections on impacts describe the trade-offs between food and fuel in terms of economics. Environmental and social impacts are also subject to assessment by scientific means. Morality, however, is by definition beyond the purview of science (and thus strictly speaking beyond the scope of this Panel). But whether it is right or wrong to use food as fuel is of keen interest to the public, not only in this country but abroad. When at least 500 million humans are malnourished in the world (Brown, 1980), domestic and international opinion and politics could well be negatively affected by the use of large amounts of grain for fuel to drive U.S. automobiles. Policy makers should be well aware of the moral issues. They may perhaps be impossible to quantify, but nonetheless they are real. National security To the extent that biomass fuel can be produced without weakening the nation economically, environmentally, socially, or morally, it can contribute to our national security. If, however, biomass energy production relied on government subsidy, it could contribute to inflation, which weakens our security. If biomass energy production took place on a scale so large that food and fiber prices were to rise sharply, the income from exports were to drop, the environment were to suffer, and farms were to become large corporate organizations, then the relatively small net energy contributions could be difficult to justify on the grounds of national security. RESEARCH RECOMMENDATIONS

In the preceding sections an assessment was made of the relevant biomass energy resources and conversion technologies available now and for the next two decades.

Review article The efficiencies of available conversion technologies vary from 5-80 per cent but most range from 20-50 per cent. Major additions to net energy are more likely to come from increasing biomass resources and not from raising conversion efliciencies. Some of the constraints and limitations in increasing biomass resources are: (1) problems of land ownership, location, valuation, productivity, and current and projected needs for land use; (2) questions of fuel energy required to maintain land and water quality at current or improved productivity; (3) geographic and seasonal variations of the water available for both biomass production and conversion; (4) land and water degradation problems related to biomass production and use; (5) fluctuations in U.S. and world demand for food and forest products; (6) seasonable fluctuations in biomass and food yields and their economic impacts; and (7) moral issues raised by the public concerning food versus fuel. The following research recommendations are directed to help clarify and evaluate these constraints and help in reaching the biomass energy potential of up to 5 Q net by the yr 2000 (Table 2). The recommendations are intended to address and give priority to only those aspects of biomass energy among many considered by the Panel that have the potential for significant national impact. Underlying these recommendations are the following premises, which were developed in detail in the text of the report: • Net energy from biomass use could increase about four-fold over the current level of 1.1-1.3 Q net by the yr 2000. However, there is uncertainty concerning these projections because existing estimates of biomass availability have not included all possible constraints imposed by agriculture, forestry, technology, economics and the environment. • Further increases are possible beyond the yr 2000, but since these projections are even more uncertain than those to the yr 2000, they are not quantified in the report. • Biomass is most appropriately used as a regional resource. • Lignocellulosic materials constitute the majority of the biomass resources and offer the greatest potential for contribution to national energy needs. • Biomass energy research and development should be funded at a level consistent with limited potential of biomass to contribute to national energy needs. • Biomass energy potentials are primarily constrained by limits on agricultural and forest production and by the necessity to maintain a productive and high quality environment. • The value of biomass energy must be assessed in comparison with alternative fuels and technologies. • Food and other consumer goods compete for the available biomass resource. • Greater quantities of biomass resources might be available, if some changes in forest and agricultural practices were made; changes, however, occur slowly in forestry and agriculture. • Some of the research mentioned in the recommendations is underway. The recognition given the fol-

25

lowing research confirms its significant contribution to the biomass energy program. • Some of the applied research proposed here will help insure realization of the potential projected for the yr 2000. The proposed basic research can have a major impact, most likely after 2000, but this research must be carried out over the years ahead. The proposed research recommendations span the area of technical, as well as socioeconomic questions. The breadth of the recommendations also spans the boundaries of government agencies such as the Forest Service and Agricultural Research Service of the U.S. Department of Agriculture and the U.S. Environmental Protection Agency, as well as coincides with many of the interests of university and private industry researchers. Clearly research into biomass energy questions should cover broad community interests in a coordinated manner. Many of the recommendations focus on the technical problems that must be overcome, but there are policy issues that are equally important (e.g. private land ownership and the constraints it imposes on biomass availability). Research is described that would provide the data base for making informed policy decisions. If biomass energy is to achieve a significantly greater potential to serve U.S. needs, research and development programs must be implemented for long term, and be maintained even if short-term improvements or trends in the economics or fluctuations of petroleum should occur. Recommendations 1. Make an accurate determination of biomass resource availability. The assessment should focus on the present and future of biomass materials on a region-byregion basis, emphasizing forest and agricultural biomass. The "inventory" must be made in the context of all the essential social, economic, technical, and environmental constraints as they apply to each locale in contrast to national estimates of the past. Current assessments have not taken into account all major constraints including cost and availability of land, including questions of ownership and the environment. The assessments should analyze the resource base specially for a region rather than rely on average national estimations. A region-specific, realistic assessment is essential for attracting commercial interests and for effective siting of conversion facilities. Illustrative examples l-l. Using a common set of criteria and standard definitions conduct a nationwide biomass resource survey based on the USDA regions to determine specific biomass quantities and locations, fuel uses and employment needs that apply to these regions. 1-2. Determine the potential for using logging and forest residues and crop remains for biomass energy conversion, taking into account the potential environmental impacts with special attention to water runoff and soil erosion.

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Review article

I-3. Investigate the potential of various tree, food, and forage crops for biomass energy (oil, gas, alcohol) production with particular attention to land availability and effects on agricultural and forest products production. 1-4. Assess crop residues in the nation region by region. In addition, analyze the inputs required to maintain soil quality where it is environmentally practical to harvest some residues. 1-5. Obtain information on both quality and quantity of raw biomass material available, including tree biomass. Currently, quantities described as available are not always assessed in the context of agriculture, forestry, land and water resources, ownership, employment, use by society, and the environment. 1-6. Examine selected agricultural and forest operations to determine the fuel needs (quantity and type) for the entire system and for sustained annual production from the environment. 2. Investigate the potential of increasing biomass production. The productivity of the forest land base is now substantially below its potential. Productivity can be increased over present levels by using cultural practices to maximize biomass yields, while at the same time employing sound soil and water conservation practices. Forest productivity may be doubled in some locations by applying known cultural practices on a wide scale and developing new technologies to reduce losses from insects, disease, fire, and poor nutrient management. In addition, we need to examine the potential for a shift from the production of saw and veneer log production to the production of wood resources that are highly productive in supplying fiber needs. Clearly, new technologies will be needed to shift to the alternative fiber materials on a large scale. In agriculture, gains in productivity may be more difficult to achieve than in forestry; however, there are opportunities to increase biomass yields of current and new crops through improved insect, disease, and weed control as well as improved management of land and water resources.

Illustrative examples 2-1. Investigate the potential to reduce insect, disease, and weed losses in forestry and agriculture, e.g. the bark beetle in forestry and the corn rootworm in agriculture. 2-2. Investigate the limits of wider application of soil and water conservation practices in agriculture and forestry to increase long-term biomass productivity, e.g. reduction in soil erosion. 2-3. Investigate soil nutrient management especially biological nitrogen fixation to increase forest and crop biomass yields. 2-4. Investigate the integration of crop and livestock and forest product production with fuel energy production. 2-5. Investigate the genetics of crop and forest plants for increased productivity. 2-6. Investigate new plant species to determine their potential for biomass energy production. 2-7. Develop new collecting and harvesting tech-

nologies for forest and agricultural lignocellulosic materials. 3. Define the benefits and costs of major biomass energy programs to the economy, the environment and society. This will provide a better understanding of the interaction among the constraints and help determine appropriate biomass use strategies. A number of social, political, international, economic, technological, agricultural and environmental constraints determine the limits of biomass energy development. No energy resource development should take place without a careful analysis of its benefits and costs to society.

Illustrative examples 3-1. A detailed study is needed of the implications for agriculture and forestry if biomass energy production is to be included in the food and fiber systems. How much can appropriate scale production technologies contribute to biomass energy resource yields and economics? 3-2. An analysis should be conducted of biomass development strategies and policies to determine their impacts on society and the environment. Can tax policy be altered to raise biomass production and be harmonious with other societal needs? 3-3. Assess the trade-offs for various alternative energy development scenarios for meeting national energy requirements. Develop a methodology for measuring comparative societal and environmental impacts from various energy sources including biomass. Under what conditions should renewable fuels be favored? What are the trade-offs between solar photosynthetic energy (biomass) and other solar technologies? 3-4. Investigate combined technical and economic resource use policies. 3-5. Determine the effects of alternative governmental strategies as they effect biomass energy development. 4. Sponsor lignocellulose-to-fuel conversion research and development projects at a su~iciently high level to determine within the next 5 yr the practicality of using this energy source. The advantage of lignocellulosic biomass is its potential as a low-cost, abundant biomass resource. The effective use of lignocellulosic biomass for high-grade fuel production requires efficient conversion technology in units that process less than 1000 t per day. By comparison, a problem with thermo-chemical conversion is that it requires large-scale (5000t per day) operations to take advantage of economy of scale in reducing capital costs. On the other hand, biological conversion processes still fall short of maximum practical conversion efficiency. In addition, there are insufficient operating data to permit reliable scale-up of the biological conversion technologies. The research objective would be to provide further development of promising lignocellulosic conversion technologies. It is particularly important to extend the application of promising technologies to the most available biomass resources, e.g. agricultural and forest residues. Improvement in biological conversion efficiency is feasible. Development of small-scale thermochemical processes for production of gaseous fuels could make

Review article lignocellulose conversion suitable for placement in numerous locations in the United States. The development of both efficient large-scale biological and smallscale thermochemical processes could provide an incentive for commercial investments. It should be recognized, however, that the full potential of such lignocellulosic process development might not be achieved by the yr 2000. Illustrative examples 4-1. Improved biological conversion of lignocellulosics should include: (i) Development of more cost effective pretreatment processes to make more of the biomass available for conversion. (ii) A search for improved microorganisms for more efficient conversion of the cellulose and hemicellulose content of biomass. (iii) Development of improved techniques to operate high productivity processes, which will reduce the capital requirements. (iv) Investigation of efficiency and cost of recovery procedures for liquid fuels. (v) Development of means to use efficiently the lignin component of biomass, including its conversion to a liquid fuel. 4-2. Investigate thermochemical conversions focusing on the development of small volume, low-cost for either gasification or liquification systems for lignocellulosic materials. 5. Sponsor basic, exploratory research, which is essential for new leads in biomass resource development. Limited funding for basic research on biomass energy is justified to develop new leads and avenues for increased biomass resource production and conversion beyond the yr 2000. Less long-range research is needed concerning new routes to biomass energy conversion than the production of biomass resources, Research on new ideas should be given priority if it can be demonstrated that there is feasibility in principle and that the program can make a significant contribution to biomass energy. Illustrative examples 5-1. Analyze and establish the limits of photosynthetic productivity given by the constraints of solar radiation, CO2, water, and nutrients. What would be the upper theoretical limits of photosynthesis imposed by stoichiometry, thermodynamics, and other fundamental scientific constraints? How would the efficiencies of "new plants" compare to synthetic solar converters, e.g. solar cells producing electricity (at perhaps 10-20 per cent efficiency) and to photochemical production of gaseous or liquid fuels? 5-2. Determine whether genetic selection and genetic engineering can produce improved, less expensive biomass energy products. For example, can a large fraction of direct plant production be in the form of a high-grade fuel? Can such a plant be developed so that biomass product removed (fuel) does not remove appreciable nitrogen and minerals from the soil? Can the catalytic apparatus that generates the fuel be retained

27

and preserved for many years with minimal new inputs of external energy? 5-3. Determine whether nitrogen fixation can be incorporated in conventional plant systems, recognizing the limitations of chemical energetics and physiological plant structures. Is it possible to have a direct, solarpowered nitrogen-fixation organism or plant apparatus? 5-4. Investigate the possibility of redistributing the existing components of biomass (cellulose, digestible carbohydrate, proteins) among food requirements, industrial needs, and fuel use in a different manner in cropping systems that will be socially acceptable. If such a system were possible, what would be the impacts on health, environment, and the total society? REFERENCES

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Review article BIOMASS PANEL OF THE ENERGY RESEARCH ADVISORY BOARD Dr. David Pimentel (Chairman) Professor Department of Entomology and Section of Ecology & Systematics Cornell University Ithaca, NY 14853

Dr. All Moslemi Professor College of Forestry, Wildlife, and Range Sciences University of Idaho Moscow, ID 83844

Dr. Ivan L. Bennett Dean and Provost New York University Medical Center New York, NY 10016

Mr. Donald Patterson Agriculturalist The Plains, VA 22171

Dr. Charles Cooney Professor Department of Nutrition and Food Science Massachusetts Institute of Technology Cambridge, MA 02130 Dr. Otto Doering Professor Department of Agricultural Economics Purdue University West Lafayette, IN 47907 Dr. Richard L. Hinman Vice President Chemical Products R & D Pfiser, Inc. Groton, CT 06340 Mr. William Hudson Manager, Market Research The Anderson's Maumee, OH 43537

Dr. Robert Patterson Professor Department of Crop Science North Carolina State University Raleigh. NC 27514 Dr. Jack Spurlock Director Office of Interdisciplinary Programs Georgia Institute of Technology Atlanta, GA 30332 Dr. Paul Weisz Manager, Central Research Division Mobil Research & Development Corporation Princeton, NJ 08540 Consultant

Mr. Sanford Harris Washington, D.C. 20009

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