1. INTRODUCTION
Since the early 1980s, the need to diversify the cane sugar industry has become progressively acute (Alexander, 1984a,b, 1985 and 1986a). Substitution of cane with alternative farm commodities is an obvious answer in some circumstances. However, in a large majority of sugar-planting countries, the alternative of internal diversification to multiple products remains a viable option. This alternative is an assertion of the plant's lignocellulose (biomass) components that historically have been both underrecognized and underutilized by sugar interest (Alexander, 1985; Van Dillewijn, 1952; Deere, 1911).
To a large measure, the international effort to diversify sugarcane since the 1974 petroleum “crisis” has been traditional. Greater attention to the usage of bagasse boiler fuel and alcohol from cane fermentable solids has centred on traditional sugarcane, managed with its historic emphasis on sugar (Lipinsky, 1978; Moreira, 1980; Perleman, 1982). A reorientation of cane management to harness its lignocellulose attributes has not generally occurred. Essentially three “total biomass” conditions must be recognized in order to accomplish the biomass reorientation: (a) utilization of the whole cane plant, including tops and trash components formerly discarded; (b) growth-oriented management that aims from the onset to maximize whole cane production; and (c) integration with cane of related tropical grasses to assure a year-round, stabilized supply of lignocellulose to the new biomass-consuming industry. These features are discussed in this paper.
2. TOTAL BIOMASS THEORY
2.1 Total biomass concept
Few, if any, agricultural commodities have the enormous latent potentials to produce biomass that characterize Saccharum species (Alexander, 1973 and 1985; Van Dillewijn, 1952; Alexander et al., 1982). However, even the comparatively constrained biomass yield of conventional, sugar-oriented cane is frequently underappreciated and hence underutilized. Illustrated schematically in Figure 1, the total plant as a botanic entity consists of four major components, all of which contribute materially to total yield potential. Only one of these, the “millable stem”, has seriously concerned the historic sugar planter, the refinery manager, and the mill engineers responsible for bagasse disposal. Studies in Puerto Rico have confirmed that the three non-sugar bearing components are both quantitatively significant and technically harvestable (Alexander, 1985, Alexander et al., 1982).
Once the whole cane plant is recognized as an entity of value, it follows that the increased yield of whole cane components becomes a matter of justifiable concern to plantation managers. In essence, the Puerto Rico studies found that growth-oriented management can increase whole cane yield in the order of 250–300 percent, at cost increases in the order of 35–40 percent. Hence, from this plant, the tradeoffs in yield over costs are weighted very heavily in favour of the crop planter, once the inherent value of lignocellulose is established. It is important to add here that at no time does the millable stem component, of historic interest to sugar planters, become any less important than in the past. Hence, sugar and molasses remain as important products of total biomass management. Only now are they seen as contributors to a multiple-products package from an authentic multiple-products commodity (Alexander, 1984a, b and 1985)
2.2 Sugar partitioning for growth
Such favourable tradeoffs in yield over cost are neither artifacts nor an automatic response to be activated by decree. Rather, they are the fruits of conscious effort by field managers to assist the cane plant to do what it is designed to do best. Even within the cane sugar industry, very few people thoroughly understand the botany of the cane, or recognize more than superficially that growth rather than sugar storage is the age-old objective of Saccharum evolution (Alexander, 1973 and 1985; Van Dillewijn, 1952).
What the cane plant has accomplished botanically over an almost incomprehensible expanse of time is a superior capacity for sucrose production and storage - well appreciated by man - and an equally superior expertise to utilize sugar in support of its own growth processes (Alexander, 1973 and 1985). The latter is not so clearly appreciated but this does not make the botanic fact any less true. Even the most acclaimed “sweet” hybrids of commerce will preferentially utilize their stored sucrose for bud and shoot induction, tissue expansion, and crown expansion when suitable water and nutrient supplies are provided. Assessment of the Puerto Rico studies indicates that sugarcane will apportion roughly 35–40 percent of its stored sucrose for lignocellulose production. In this “partitioning” of its sugar product, the cane plant is something more than a novice. It gives back several-fold increases of lignocellulose, that is, it produces exceedingly well the product of greatest concern to the plant itself (Alexander, 1985; Alexander et al., 1982). Ironically, the mere intervention of human error and the intangibles of a harvest campaign can cause 30 or 40 percent sugar loss with no gain at all of biomass. For truly reliable sugar expenditure we must learn to let this plant “do its thing,” and to encourage and assist its natural inclination for growth.
2.3 Allied grasses integration
Whether managed for sugar or as a total biomass resource, sugarcane has the twin characteristics of year-round growth but a harvest capability that is often seasonally constrained. Because lignocellulose industrial consumers must base their operations on feedstock supplies that are both abundant and reliably secure on a 365-day basis, the seasonal bagasse must be supplemented with some form of stored biomass. Storage of humid bagasse is possible but not very desirable for technical reasons. A better alternative is the stockpiling of solar-dried grasses, particularly certain high-yielding tropical grasses that are generic relatives of Saccharum (Alexander, 1985; Alexander et al., 1982).
In addition to stabilizing the cane bagasse supply, the entry of such grasses offers a series of new benefits not historically enjoyed by cane sugar enterprises. These include:
The solar-drying of such grasses as a means of water removal is perhaps the happiest feature of their contribution. The issue of hauling water-laden stalks to a distant mill, itself a comparatively poor drying mechanism, is avoided entirely. Dehydrated by the sun to less than 20 percent moisture, these materials are readily compacted into bales that are more economic to haul, more efficiently stored, and more safely stored without hazard of spontaneous combustion.
3. WHOLE CANE PRODUCTION
3.1 Agronomic modifications
From the cane agronomists' point of view, essentially three new features will enter the management routines for total biomass: (a) greater attention to the conditions of supply and utilization of growth-sustaining resources; (b) involvement of new machinery and practices dictated by greater tonnages of biomass and the needs of integrated grasses; and (c), greater attention to detail in the management of all production resources. Indeed, some phases of biomass cropping, particularly seed and seedbed management, will shift perceptibly in the direction of horticultural-like attention to detail (Alexander, 1985).
In maximizing cane biomass, field managers will give greater attention to the selection of sites, installation of drainage and irrigation systems, and preplant levelling operations than is normally done with conventional sugarcane (Alexander, 1985). Certain soil series and land contours that were marginal for sugar planting might be disqualified altogether for high cane biomass. However, they could still retain excellent suitability for allied grasses biomass in support of the same cane enterprise (Alexander, 1984b 1986a). Specific seedbed preparetion tasks will routinely include thorough ploughing and aeration, subsoiling, surface rotavation, and frequently precision grading with a landplane. Such operations are designed to accommodate the very elaborate upper- and lower-root zone profiles that ideally will proliferate during the first two years of energy cane establishment.
Planting and postplanting tasks essential for maximizing cane biomass are detailed elsewhere (Alexander, 1985). In essence, important emphasis is placed on creating early a “continuous stool” complex, offering much greater numbers of stalks per acre, with attendant early closure of the canopy and efficient utilization of incremental fertilizer and water. There is also an efficient suppression of weed growth through exclusion of light. Having created these conditions for growth and growth-resources utilization, it is in the late-juvenile and adult phases of cane development that the vast growth potentials of Saccharum are finally realized. Particularly in the ratoon crops that produce new top growth almost from the day of harvest, enormous yields are returned for six years or longer, well compensating the added costs and attention to detail expended in the original crop establishment (Alexander, 1985, 1986a and b).
Harvest of the high-tonnage cane involves several technical issues that are resolved already for the most part (Alexander, 1985 and 1986b; Alexander et al., 1982): (a) harvest of vastly higher tonnages, commonly in excess of 100 tons per acre; (b) harvest of whole cane, including tops, attached trash, and detached trash; (c) clean cane harvest (that minimizes contaminating soil and inorganic extraneous matter); and (d) synchronization of cane and grasses operations for efficient commitment of machinery and human resources. At this moment, effective harvest of high tonnage cane is possible with cutting/windrowing machinery and loading machinery applied in separate operations. Performed on a flat, precision-graded seedbed (Alexander et al., 1982; Alexander, 1986b) with avoidance of mechanical bunching, harvest is completed with minimal incorporation of soil and damage to the cane crown. Alternatively, even cleaner harvests with less seedbed damage are possible where inexpensive labour is still available and employment of labour is itself an important objective of cane production (Alexander, 1985).
Correct synchronization of cane and grasses harvest operations is crucial. It is largely a function of correct planning and execution of the production/harvest campaign. This must begin at the conference table before a given year's planting and ratoon-crops operations are begun. One harvest feature (actualy post-harvest) common to grasses but entirely new for cane producers is the recovery of leafy trash left on the seedbed surface. This component is remarkable and can exceed 20 tons per acre for some varieties (Alexander, 1985; Alexander et al., 1982). Its “harvest” is accomplished on the seedbeds by the same equipment used for grasses conditioning, solar-drying and compacting, and the removal and storage of large bales.
3.2 Potential cane yields
Well within the botanic capability of sugarcane, the potential yields of Saccharum intentionally managed for its total biomass productivity are strikingly greater than those obtained from the traditional sugar commodity. For example, the world average for commercial cane was barely 23 tons per acre/year in 1983 (Table 1), while typical sugar varieties were yielding over 80 tons per acre when managed as a biomass commodity. Varieties noted for their high tonnage as a sugarcanes yielded over 120 tons per acre as biomass resources (“second generation” energy cane, Table 1). Further to this, a significantly greater yield potential remains unfulfilled in sugarcane's tropical habitat. The upper practical limit, that is, under practical farming circumstances given favourable climate, soils and water resources, is thought to be in the neighbourhood of 150–160 tons whole cane per acre year (Table 1). Such productivity in commercial enterprises will require conceptual reorientation in growth management, enterprises will require combination with new varietal genotypes from previously untapped Saccharumgermplasm (Chu, 1979 and 1982; Stevenson, 1985; Irvine and Benda, 1979).
3.3 New uses and yield issues for grasses and cane
The above yield figures for sugarcane, managed as “energy cane”, assume that growth processes will operate to accrue lignocellulose on a diurnal basis 365 days per annum. The entry of livestock feed consideration offers an added scenario where such maximum yields are less likely to be attained. For example, dairymen in Puerto Rico prefer to harvest cane at about five or six months of age for use as greenfeed. Mature cane is also used for this purpose and is well accepted by dairy cows, but younger cane is preferred. Experience with energy cane has shown that the combined tonnage from two six-month crops will amount to roughly 70–80 percent of a single 12-month crop (Alexander et al., 1982). Similarly, the combined tonnage from three four-months crops offers further tonnage reduction, in the order of 50–70 percent of a single 12-month crop.
The attendant tradeoffs in biomass enterprise security, economic flexibility, and social/cultural acceptance are nonetheless favourable. This feature is most clearly seen in the concept of allied grasses integration with sugarcane on behalf of biomass consumers, in addition to the traditional sugar consumers. The immediate impact of such species is an added flexibility and stabilization of lignocellulose supply in what still remains as a predominantly 12-month crop of biomass and sugars.
However, since the early 1980s, an unwelcome dimension has superimposed it self on the sugarcane world which has increased the importance and attractiveness of the supplemental grasses. It is international in scope and includes the following features: (a) a persistent deterioration of sugar-planting economics, that is, of low sugar values and inroads of cane sugar substitutes combined with rising costs of production; (b) general recognition that sugar planters must grow something in addition to conventional sugarcane, as a matter of survival rather than choice; (c) recognition of livestock production as a valid alternative to sugar planting in most of the developing tropical world; and, (d) persistently rising costs of commercial protein, fibre, and fats used in cattle feeds and feed blends.
In Puerto Rico's case, the values of imported dry feed components had risen to about US$ 150.00 per ton by 1984 (Table 2). Freight charges alone amounted to US$24.00 per ton (Alexander, 1986a). Ironically, while some US$ 115 million are expendedannually for this purpose in Puerto Rico, the same materials or suitable alternative feed sources can be grown there domestically at less than one-fourth the cost of imported dry feeds.
The supplemental grasses discussed above as integrated lignocellulose backups for cane could figure prominently as livestock feeds in carefully integrated cane and livestock operations. For example, solar-dried grasses could be available or stockpiled as feeds many months before a lignocellulose-products factory could be constructed and made operational (Alexander, 1986a). Routinely, excess grasses stemming from overproduction or factory down-time would find alternative markets in the livestock industry. Their long-term stability in storage also enables them to wait for favourable market conditions. In essence, such species correctly managed would offer both flexibility and security and added sources of revenue to a biomass-consuming industry operating primarily on an energy cane base (Alexander, 1985 and 1986a).
Two additional benefits of the supplemental grasses deserve mention at this point. First, because such species do not accumulate dry trash and remain non-combustible before harvest, they offer effective fire-breaks for the concurrent energy cane crops. In suitably-planned field design, napier grass can materially limit the spread of fire, even when started maliciously. A second added benefit is a kind of psychological support or assurance for farmers who are encouraged to plant such grasses as industrial lignocellulose feedstocks. Such growers might never wholly understand what products manufacturing from lignocellulose is all about, yet they will always perceive the age-old use of grasses as livestock feed.
4. CANE BIOMASS UTILIZATION
After the petroleum crisis of the mid-1970s, sugarcane often has been depicted as a source of bagasse for boiler fuel and alcohol for motor fuel. This view is perhaps correct in circumstances where sugar and conventional sugarcane management continue to dominate the cane industry thinking. However, it is an overly simplistic view of the potential contributions of cane as a multiple-products commodity. This particularly true where its lignocellulose values are fully appreciated, and where agricultural management is reoriented to maximize total biomass production.
4.1 Biomass hierarchy of values
As a primarily biomass system, sugarcane must be viewed in the context of critical new roles that biomass in general must play as a renewable, domestic resource. Fermentable solids are an important by-product of some biomass systems, including energy cane, and their long-term role in fuel alcohol production seems assured. Opportunities for lignocellulose itself are considerably more complex and less clearly preceived.
A convenient illustration of lignocellulose potentials and opportunities is found in the “hierarchy of values”. As presented in Figure 2, the plotted logarithms of lignocellulose sales volume versus value give a straight-line relationship, a hierarchy of industrial utilization opportunities. It is seen that the direct combustion of biomass as a humid boiler fuel is the least-valuable option for a potentially high-value resource. Figure 2 is itself an over simplification. For exampale, entire sub-hierarchies could be plotted for items such as synthesis gas, and little application is seen in the near-term for perfume production from energy cane. However, very real opportunities already exist for the upgrading of humid bagasse to higher-value fuels. Proprietary fuels might include free-flowing powders (for co-combustion withoil), or pelletized products offering chemicals and combustible gases by-products. (Beningson, 1982a and b; Lijinsky and Jenkins, 1982; Anon., 1982a and b; Bouvet and Suzor, 1980; Hasselriis and Bellac, 1980).
An appropriate example of still higher-value opportunities from existing industrial technology is found in the synthesis gas option (Pruett, 1981; Parker, 1978; Wishart, 1978). Consisting of a mixture of hydrogen (H2) and carbon monoxide (CO), it comprises a kind of chemists' soup from which numerous organic products needed by a modern society can be formed. A product of immediate concern to sugarcane-planting countries is nitrogenous fertilizer (Alexander, 1984 a and 1985).
During most of the present century, the feedstocks for synthesis gas production have been petroleum and natural gas. However, coal and biomass are equally suitable substrates, a fact of enormous future consequence for both coal-rich temperate countries and “energy poor” tropical societies with established cane industries. By the summary reactions,
CH4 + H2O → CO + 3H2 (endothermic)
n(C-H) + O2 or H2 O → n(CO) + n(H2)
Biomass
Coal
the methane component of biogas can be reformed to carbon monoxide plus hydrogen (equation no. 1), or whole biomass can be reformed to the same products in a somewhat more complicated sequence (equation no. 2). Because natural gas bears almost pure methane and is easily managed and inexpensive, it is the preferred feedstock of the current chemical industry. However, the coal and biomass options must evetually assume this role (Pruett, 1981; Parker, 1978).
It is worthwhile mentioning a few of the valuable products that in future will derive from cane biomass. For example, both pure and oxygenated hydrocarbons can be formed from synthesis gas via the Fischer-Tropsch reaction. To future agricultural planners, a great comfort will be found in the fact that nitrogenous fertilizers can be manufactured from domestic biomass feedstocks (Wishart, 1978). Hence, the combination of synthesis gas H2, with the virtually free N2 of the atmosphere, provides the essential ammonia feedstock for future N-fertilizers manufacture. Additional products from the synthesis gas resource can include urea, methanol, glycol, glycerol, acetates, ethylene, aromatic compounds, lactides, and various polymers and copolymers. From the aromatics alone, whole families of critical downstream products can be manufactured from the original cane biomass feedstocks.
4.2 Feeds vs industrial materials feedstocks
Future biomass usage opportunities as depicted in Figure 2 are a convenient background for plant components just beginning to be recognized as industrial raw materials in their own right - not merely as residues from an older farming or forestry commodity. Only a tentative placement of the livestock feeds option on the same hierarchy of values is possible for purposes of this communication. Individual cane-growing regions will preceive the livestock application differently. For some, cattle production is a prime existing reality while others are only beginning to assess this option. Through economic “sensitivity analyses” will be needed in most circumstances. For exampale, the question of cane usage as feed must evaluate the tradeoffs of lower tonnage and loss of sugar versus the very real benefits of no longer having to transport and dewater mature cane in a sugar mill. Similarly, will the cane feed derive from plantings managed as feed from the onset or does feed usage apply merely to the bagasse and molasses byproducts of conventional sugarcane? For Puerto Rican circumstances we are confident that cane to be used as feed ought to be managed as feed from preplanting onward.
For present purposes, it is estimated that energy cane utilized as green-feed for diary cattle would have a final value intermediate between boiler fuel and feedstock for a proprietary industrial fuel (Figure 2). Energy cane management for maximum biomass is assumed, together with harvest intervals of approximately 3–4 and 5–6 months, i.e. three or two greenfeed crops per acre year. Sugarcane cannot express fully its maximum biomass growth capabilities in such shortened time-spains; however, high-growth management is probably justified by the great untapped growth potential of this plant.
5. CONCLUSIONS
Sugarcane is a unique botanic resource whose growth potentials were long underrecognized and underutilized as a monolithic sugar crop. More than ever before, sugarcane is needed today as a multiple-products commodity in the developing tropics. Its management as energy cane is not a substituation or even de-emphasis of sugar. Rather, the plant is encouraged to utilize some of its lignocellulose. contributes to a series of new saleable products from lignocellulose. The accured series value exceeds that of historic sugar plus its molasses and bagasse by-products. Cane utilization as a livestock feed is consistent with energy cane management. Some growth potential is sacrificed but highly favourable tradeoffs emerge. There is added agricultural base flexibility and cash flow to an energy cane enterprise, and elimination of costly transport and milling operations. Equally favourable feed contributions derive from the allied tropical grasses already incorporated into energy cane agriculture.
REFERENCES
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Alexander, A.G. 1984b 1–3 Nov. Energy cane as a multiple products alternative. Proceedings Pacific Basin Biofuels Workshop, Honolulu,
Alexander, A.G. 1985 The energy cane alternative. Amsterdam, Elsevier Science Publishers B.V. 509 pp.
Alexander, A.G. 1986a Rapid implementation options and opportunities for energy cane biomass producers. Energy from biomass and wastes (preprint) Washington, D.C. 7–10 April.
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Alexander, A.G., 1982 Allison, W., Chu, T.L., Velez-Santiago, J., and Smith, L. Production of sugarcane and tropical grasses as a renewable energy source. Final Report to the US Department of Energy, Contract No. DE-AS05-78ET20071.
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Figure 1
Schematic illustration of the four above-ground components of “whole” sugarcane, as produced for its total biomass lignocellulose in a multiple-products commodity. Reproduced by permission of Elsevier Science Publishers B.V., Amsterdam (Alexander, 1985).
Figure 2
Hierarchy of utilization opportunities for plant Lignocellulose. Adapted from Alexander, 1985.
| Parameter | Green yield1 (Tons/acre year) |
|---|---|
| World commercial average (1983) | 22.6 |
| Puerto Rico commercial average (1985) | 24.0 |
| Energy cane, first generation | 83.0 |
| Energy cane, second generation | 125.0 |
| Approximate theoretical maximum: | |
| Subtropics | 112.0 |
| Tropics | 160.0 |
| - Materials: Corn, grains, cotton and soybean meal | ||
| - Costs: approximately 115 million (US$) total: | ||
| - Materials | 126.00 | US$/ton |
| - Shipping | 24.00 | " |
| - Total cost/ton | 150.00 | " |
| - Cost of local napier grass, solar-dried, delivered, and size-reduced | 28.502 | US$/ton |
2 Estimated from 1982 production data (Alexander et al., 1982).
La plantación de caña de azúcar es una reatidad que està intimamente entrelazada con el legado historico de numerosas comunidades rurales del tropico. Las poderosas fuerzas para acelerar la diversificación pueden ser absorbidas e integradas dentro de la infraestructura dedicada a la producción de caña. Cabe señalar, sin embargo, que el enorme potencial de la caña para la producción de biomasa tiene que ser apreciado adecuadamente para poder aportar el nivel de manejo requerido por un cultivo a fines múltiples. Este trabajo incluye análisis y comentarios sobre los requerimientos para la utilización integral de la planta de caña, para reorganizar la producción de caña de modo de maximizar económicamente la total idad de la biomasa producida, y para integrar junto a la caña algunas especies tropicales de alta productividad que le están emparentadas. La producción de biomasa lignocelulósica se propone, a titulo propio, como una meta escencial de manejc. Se estima que el azúcar y la melaza se mantendrán como los principales subproductos comerciales. El estudio identifica nuevos usuarios industriales, como también productos lignocelulósicos de major valor económico; ambos temas son analizados.
