1 Introduction
When ethanol was first considered as a possible fuel, it was claimed that several problems would emerge. These included the availability of raw materials, the reduction of agricultural areas which might lead to an increase of hunger in the world, the high production costs, and, last but not least, the energy balance. Certainly, a com- parison between the 41.8 MJ net heat value of 1 kg of gasoline and the 26.7 MJ of 1 kg of ethanol would support these opinions.
Less biased, but not less interesting, data concerning the production of ethanol to be used as a fuel and its relevant energy balances were subsequently provided. As often happens, the balances furnished by those who were anxious to demonstrate the validity of their own theories did not reflect the real situation, or at least led to partial interpretations. The situation was further complicated by the difference of evaluation of single elements, the omission of some of them in the balances and, finally, the confusion coming from the use of different units (hectare and acre; kg and pound; bushel; joule, calorie and Btu; litre and gallon and maybe the further differences between American and British units and so on)1.
It would be impossible and unnecessary to the purpose here to quote the complete literature on the subject. Data are now numerous and reliable enough and ideas clear enough to allow both a precise definition of the question and the suggestion of some proposals for the problem of the energy balance of ethanol as a fuel.
The same proposals could be suitably adapted for the energy balances of butanol-1 and acetone (or isopropanol) produced biologically and to be used as fuel, whenever the condition for their production will be more interesting than now.
Table 1 supplies conversion factors between SI units and those of different systems most frequently used in the literature.
a valid substitute for gasoline — at least at first, independently from costs or any other element. Indeed the energy balance of methanol from coal (or possibly from natural gas) is negative by assumption. Generally 1.75 MJ are needed to produce a quantity of methanol whose net heat value is equal to 1.00 MJ. Following 1) the ratio methanol: coal for the heat values is 1:2. In order to obtain a better energy balance, vegetable sources should be used (e.g., wood or straw). Yet, the dimensions considered industrially valid for a methanol plant discourage such use at least in Europe.
However, attention must be paid to another fact: it is impossible to use a coal operated car without either replacing the Otto engine with a steam one or using an air gas generator. Besides, the use of natural gas as a fuel implies transportation of heavy gas cylinders on the car itself, difficulties may arise in filling, and its use is not free from risk. At present, gasoline can be partially or completely substituted only by a liquid fuel. Therefore, it is not proper to consider at the same level the energy produced from oil and the same amount of energy produced from coal, at least as far as road transport is concerned.
Another important consideration derives from this fact: for a realistic energy balance it is necessary to consider how much energy for fuel production is obtained from oil, how much generally from non-renewable energy sources (i.e., coal) and how much from a renewable source, such as a byproduct, either agricultural or industrial, of the main production (ethanol in this case).
Therefore, a merely thermic balance has only an academic meaning: a realistic view must take into account the quantity of non-renewable energy consumed and compare it with the energy produced (for automotive traction, not simply combustion energy, but mechanical energy produced).
Finally the energy produced will have to be compared with the non-renewable one resulting from the energy balance of the production of traditional fuels; it will be shown later how the total energy consumption for gasoline production and, of course, the thermal and volume efficiency of the engine will have to be taken into consideration.
2 Negative Factors in the Energy Balance 2.1 Raw Materials
A two-way distinction must be made for raw materials: those which are specifically grown for ethanol production (saccharines such as sugar beet, fodder beet, sugar cane, sweet sorghum and so on; starchy products such as potatoes, Jerusalem arti- chokes, cassava (manioc), grains; cellulosic materials such as wood), and those which do not require any energy, consumption before collection (agricultural residues, straw, municipal wastes, etc). It is clear that the energy consumption is different in each case, therefore the two categories are treated separately.
2.1.1 Raw Materials Specifically Grown for Ethanol Production2
As the different balance items quoted in the following, whether inputs or outputs, will subsequently be used as terms in formulas, a progressive alphabet letter reported in parentheses has been attributed to each.
2.1.1.1 Operation of Agricultural Machinery (A)
The item with one of the highest energy consumptions is the operation of agricultural machinery (fuel and lubricating oil consumed in soil preparation, planting, culti- vation, fertilizers and pesticides application, harvesting and, possibly, wood cutting).
These values related to the above mentioned operations may be quite different from one kind of cultivation to another and often are quite different from place to place and from author to author.
For instance, for sugar cane production the Battelle Institute of Columbus (Ohio) indicates an average consumption of gasoline + oil + lubricants equal to 1.67 MJ k g- 1
of ethanol2); a Brazilian report3) gives 1.87 MJ k g- 1, but the Instituto de Fisica de Sao Paulo brings this value to 2.84 M J k g- 1 4), while the same is reduced to
1.00 MJ kg- 1 for South Africa5).
For the ethanol production from sugar beet the Battelle Institute2) gives 2.89 MJ kg- 1, while the result of an Italian study indicates that 2.01 MJ k g- 1 are sufficient6). Brazilian data agree upon cassava: 1.38 MJ kg-1 3) and 1.26 MJ kg-1 4). The evaluations on ethanol from corn are rather different in the United States of America: 5.02 MJ kg- 1 according to Scheller 7) and 9.24 M J k g- 1 according to Chambers9) (even if this probably includes the energy consumption for irrigation).
Finally, energy consumption for sorghum, in Brazil4), is 1.32 MJ kg- 1. 2.1.1.2 Irrigation (B)
Irrigation accounts for a high percentage of the energy costs, although it varies from case to case as a function of the kind of cultivation and of climatic conditions of the agricultural area.
According to the Battelle Institute 2) the average value relative to the United States for ethanol from sugar cane is 1.91 M J k g- 1, while in South Africa5) it falls to 1.3 MJ kg- 1. As far as sugar beet is concerned, while the Battelle Institute 2) gives a value of 3.60 MJ kg- 1, the Italian report6) gives a value of 1.20 MJ k g- 1. For corn in the United States, Scheller 7) gives a very modest value: 0.22 MJ k g- 1. 2.1.1.3 Chemical Products (C)
A third quite high consumption of energy is the indirect one connected with the production of fertilizers, herbicides and pesticides.
For sugar cane the average value in the United States is, according to the Battelle Institute 2), 2.95 MJ k g- 1 of ethanol, while according to an official document10) it is 2.30 MJ k g- 1; for Brazil 0.22 MJ kg-1 3), 0.91 61) or 1.14 MJ kg"1 4), while in South Africa the value is 1.60 MJ k g- 1 of ethanol5).
2 In order to have homogeneous data, where necessary and possible it is here assumed that 1 kg of ethanol is obtained from 21.0 kg of sweet sorghum; 17.0 kg of sugar cane; 13.9 kg of sugar beet;
7.0 kg of cassava and 3.5 kg of corn.
For sugar beet the Battelle Institute2) — the values of which are generally quite high — indicates an average value of 4.20 MJ k g- 1 of ethanol in the United States, versus the 3.5 MJ k g- 1 of the quoted official document10). In Italy the value decreases to l.50 MJ kg-1 6).
In Brazil the average values relative to cassava are 0.14 MJ k g- 1 3) and 1.00 MJ kg-1 4); those relative to corn become very high, ranging from 4.6 MJ k g- 1
of ethanol 4) to 7.08 7) 7,23 8) and up to 11.16 MJ kg"1 9); for sorghum according to 4) the value is 1.50 MJ kg-1; for potatoes there is an energy consumption of 12.3 MJ kg -1 11.
2.1.1.4 Machinery Production and other Energy Consumption (D)
To the factors strictly connected with the agricultural production we must however add some other elements, even if they are less evident. For instance, we must take into account that energy which is generically wasted is difficult to quantify.
In sugar cane production, this kind of consumption could amount 2) to about 5 % of the total of A + B + C while according to others 3 , 4 ) it would be quite negligible.
And while the first author quotes 1 % of the above mentioned sum in sugar beet production, the Italian project gives a value of about 6 % 6). This percentage would be of 3.3 % for cassava and of 0.8 % for sorghum 4), but it would increase up to 10 % for corn7,8).
However, what is too often disregarded by every information source, is the quantity of energy spent for the production of agricultural machinery. As a matter of fact, the values relative to this production should not be neglected at all. For instance, the energy consumption related to steel production is equal to 70,000 MJ t-1 of steel. The cultivation of sugar beets requires 750 kg of steel per hectare, corresponding to an energy consumption of 53,250 MJ h a- 1. Since agricultural machinery has an expected life of 10 years, the consumption corresponds to 5,325 MJ h a- 1 y-1, which is equal to 1.71 MJ k g- 1 of produced ethanol. According to Austin 5) this value would be of 1.44MJ k g- 1 in the case of sugar cane and according to Chambers 9) of 2.34 MJ k g- 1 for corn. Following Bunger 8) the figure for corn should be 2.81. As far as sugar beet is concerned, the above mentioned value is about 17.7% of the sum of values indicated in A + B + C; about 36.9 % in the case of sugar cane cultivation in South Africa and about 11.5% or 21.7% in the case of corn. It is evident that the above factor is strongly influenced by the degree of mechanization in the cultivation, yet it seems correct for point 2.1.1.4 to add 20-22% to the sum, when no detailed information exists.
2.1.1.5 Transportation (E)
The agricultural product must be finally transported to the factory. This cost varies considerably as it is related to the average haul distance, and to the average capacity of the transportation vehicles (oil consumption per km per transported metric ton, possible use of rail transport). According to Battelle Institute 2) transportation has an incidence of 1 MJ k g- 1 of ethanol for sugar cane and 2.4 MJ k g- 1 for sugar beet.
The quoted reference specifies that transportation is calculated on the following parameters: 20 km by road for sugar cane on the basis of 2.5 MJ (t km)- 1 and for
sugar beet 16 km by road as above, and 240 km by rail on the basis of 0.44 MJ (t km) -1. For sugar cane other authors suggest 0.12 MJ k g- 1 3), 1.05 62) or 1.45 MJ k g- 1 5); for sugar beet 1.25 6), and 1.40 61) MJ k g- 1; for cassava, a value is given of 0.12 MJ kg -1
of ethanol 3); for potato culls 1.25 MJ kg-1 13).
Data for corn are extremely scattered; according to Scheller7' 0.45 MJ kg- 1 of ethanol is the appropriate figure. According to Chambers 9) it is 2.93 MJ kg-1
The incidence of energy used for the manufacture of trucks or any other transpor- tation vehicle on the energy consumption for transport may be considered modest, considering also the variability of other evaluations. Indeed, for amortization it has to be taken into consideration that transportation vehicles, once harvesting is completed, can be utilized for other products.
According to the Italian report 6), this consumption should not be over the 10%
of the direct energy consumption for transportation.
2.1.2 Raw Materials not Specifically Grown for Ethanol Production
It is evident that, in this case, many items quoted above must not be taken into account, particularly those included in 2.1.1.2 and 2.1.1.3. Items gathered in 2.1.1.1 and 2.1.1.4 must be deeply modified and item 2.1.1.5 itself shows basic variations. Actually, the only items having some weight — indeed a relevant one — are collection and transportation.
However, their value is practically zero if collection and transportation of municipal wastes is considered (these operations must be carried out in any case and it would be quite wrong to charge ethanol production with them) and if the utilization plant is near the assembling point. When the plant for the utilization of cellulosic materials from municipal wastes is far from such location, production of ethanol will be charged with the transportation of the raw materials. However, as the production requires a preliminary concentration of the cellulosic materials, it would be advisable to perform it before transportation.
Thus, the items to be counted for municipal wastes concern the costs of concentra- tion (separation, air classification, trommeling) and, possibly, additional transpor- tation.
For the transportation of agricultural wastes the consumption per carried unit is generally higher than that of agricultural products since the ratio mass per volume is usually lower (as in the classic case of straw). The drawback is counterbalanced by the greater yield in ethanol of products with high cellulosic content and by the utilization of processes capable of exploiting also hemicellulosic materials. When straw is transported along an average distance of 30 km, the energy consumption would be equal to about 2.5 MJ kg-1 14) or 1.2 MJ kg-1 of ethanol 15).
Some industrial byproducts, as molasses and whey, should be considered separately.
For neither of them is, evidently, appropriate to quote energy costs for production, since they are produced anyway and thus their energy consumption is to be attributed to the industry from which they originate (sugar, dairy industry).
However, the availability of molasses is relatively modest and their best use could consist in raising the strength of too diluted worts in order to reduce subsequent energy costs in ethanol recovery. Whey availability is low and one should also consider the low yield in ethanol (about 2.5% by vol.).