| Application of biomass-energy technologies |
|IV. Conversion of biomass into ethanol|
Zimbabwe is an example of a relatively small country which has begun to tackle its energy import problem while fostering its own agro-industrial base. An independent and secure source of liquid fuel was seen as a sensible strategy because of Zimbabwe's geographical position, its politically vulnerable situation and foreign-exchange limitations, and for other economic considerations.
The consumption of liquid fuel in Zimbabwe is relatively modest (but crucial to the running of a modern economy), with diesel now accounting for 55 per cent and gasoline 32 per cent (the remainder being kerosene) of the country's total liquid fuel consumption. Zimbabwe's current annual consumption of motor gasoline is about 1,850,000 barrels. Zimbabwe has no oil resources and all petroleum products must be imported, accounting for nearly $120 million per annum on average in recent years (Steinglass et al, 1988) which amounted to 18 per cent of the country's foreign-exchange earnings in 1984. Because of its landlocked position Zimbabwe had to import petroleum fuels by means of a pipeline from Mozambique, or by road and rail through South Africa. Both means of import are subject to disruption.
Zimbabwe pioneered the production in Africa of fuel ethanol for blending with gasoline in 1980. Initially a 15-per cent alcohol/gasoline mix was used, but due to increased consumption, the blend is now about 12 per cent alcohol. This is the only fuel available in Zimbabwe for vehicles powered by spark-ignition engines (Scurlock et al, 1991). Annually, production of 40 million litres has been possible since 1983, although this has recently been severely constrained by the drought. Plans for a 35 million litres per year expansion have been finalized, but the expansion depends on the availability of water. Production costs in 1988 were approximately $0.75 per gallon (Steinglass et al, 1988) which at least break even with landed gasoline imports when compared with local molasses prices of approximately $25/ton.
Zimbabwe's sugar industry consists of two private sugar companies, Hippo Valley Estates Ltd and Triangle Ltd. both located in the south-east low-yield of the country. Together they operate two of the world's most efficient irrigated sugarcane estates and factories. Until recent droughts, each grew and processed approximately 2 million tons of cane per year. Zimbabwe was exporting some 240,000 tons of sugar in 1986 which constituted the country's ninth largest foreign-exchange earner.
Any estimates of cost must consider the volatility of the international sugar and oil prices, supply problems and transport difficulties. Alcohol production in Zimbabwe reduces the amount of sugar available for export, and so reduces foreign-exchange earnings. The sugar commodity market is notoriously prone to price fluctuations. In 1973 a ton of sugar could buy 76.6 barrels of oil but, only 2.8 and 9.5 barrels in 1984 and 1990, respectively. However, about half of Triangle's annual sugar production of 200,000 tons goes to the relatively unlucrative home market, and most of the rest is exported to the European Community under special trade agreements at around $450 per ton. Any remaining sugar has to be sold on the world market. Zimbabwe's sugar has to be transported through South Africa for export, which reduces the price obtained by around $100 per tonne. Scurlock et al (1991) discuss these economic factors in some detail.
At November 1989 sugar and oil prices, ethanol costs fractionally more than imported gasoline, but when the strategic advantage gained from greater liquid-fuel self-sufficiency is taken into consideration, the balance is firmly in favour of home alcohol production. In August 1990 the price of gasoline was increased by about 50 per cent due to the Gulf War and world sugar prices dropped by 40 per cent between 1989 and 1991. Now oil prices have fallen again and sugar prices have risen due to failing crops in the drought. This demonstrates the vulnerability of ethanol production to political factors and commodity prices.
Serious consideration is being given to the possibility of expanding both sugar and ethanol production. The area of land which would be needed to grow cane to provide enough alcohol to replace all imported gasoline and meet domestic sugar needs (but with none for export) is about 52,000 ha. This is less than double the total area now planted with sugar cane, and represents only 0.2 per cent of available agricultural land in Zimbabwe. An alcohol programme that would power all Zimbabwe's cars with pure alcohol would not, therefore, necessarily compete for land with food crops. However, water for irrigation is the key problem.
An integrated long-term plan has been drawn up allowing a flexible approach to changing variables. The expansion plan involves five phases, the first of which started with the opening of the Mushwe Dam in 1991 that would allow an extra 3000 ha of cane to be planted; the capacity of the ethanol plant could be extended to 50 million litres/year provided there is no appreciable increase in the demand for sugar. The second phase is also dependent upon the construction of the Tokwe-Mukorsi Dam (currently at the planning stage) which will substantially increase the water supply and allow for a significant increase in the sugar-growing area and eventually in ethanol production.
1. The Triangle Plant
The ethanol plant at Triangle is an example of a biomass-to-energy system which has successfully for almost a decade. In November 1978, Triangle Ltd., a company involved in producing sugar and cotton, received permission to build a distillery at Triangle in southeast Zimbabwe. Triangle farms 13,000 ha of irrigated sugarcane plantations, yielding, on average, 115 tons of cane (fresh weight) per ha The production of alcohol began in March 1980. The plant was designed to produce 120,000 litres ethanol per day, with, on average, I ton of sugarcane giving 125 kg of sugar and 7.5 litres of alcohol.
With a realistic 96 per cent time efficiency, and operating the distillery for 24 hours per day 50 weeks of the year, production can reach 40 million litres per year. After nine years of operating experience, the expected output was regularly achieved, or even exceeded as in 1986, when the plant produced 41.6 million litres of alcohol. However, the need to supply the increasing demand for domestic sugar can limit the output of ethanol when the cane harvest is low. Also, drought in 1987 reduced the output of ethanol to 37.4 million litres, and similar production levels were forecast for subsequent years for the same reason. In 1992, the droughts were severe and cane productivity fell to only 2 t/ha resulting in the loss of 3000 jobs in the agricultural sector (Nature Special, 1992). Since the cane will have to be replanted, and it takes a year to grow, there will not be a significant crop in 1993 either.
The plant was financed mainly by local capital (one strict government condition was that foreign capital had to be recouped within six months by savings in foreign exchange) and home-based technology was required rather than sophisticated equipment from abroad, whenever possible. All decisions concerning the construction of the plant were made locally. The plant was locally planned with local control over its running. There was considerable cooperation between the various parties involved with very few external constraints and the industry was able to select low-cost technology closely tailored to the industry's needs. By using this approach, Zimbabwe was able to build the plant at a capital cost of $6.4 million (at 1980 prices) - the lowest capital cost per litre for any ethanol plant in the world.
After considering a number of options, it was decided to build a standard batch-type fermentation plant. This process requires that tanks are emptied and sterilized after each fermentation, but the plant can be operated by existing staff at the sugar mill. The design was produced by foreign consultants, but the construction was carried out in Zimbabwe by a local project team. Instead of importing distillery components, the locally-available fabrication structure was exploited. The consultants provided technical assistance where necessary, but a remarkable 60 per cent of the plant was fabricated and constructed in Zimbabwe. Only specialist items such as plate heat exchangers, an air blower and instrumentation were imported. To ensure high standards, local welders were given spectral training. Few problems have been experienced so far; only the fermentation tanks have shown abnormal corrosion.
The sugar mill is capable of producing cane juice and molasses of varying purities and concentrations to suit the needs of both the sugar factory and the distillery. The ethanol plant was also designed to operate on a variety of feedstocks using different grades of molasses, cane juice, or even raw sugar itself. This flexibility means that the plant is fully integrated with the rest of the sugar production process and can respond rapidly to changes. Thus the fermentable sugar content, for example, of molasses entering the plant can be adjusted at the expense of sugar production, depending on relative market prices, in order to maximize the return on total investment in both sugar and ethanol production. Triangle also buys in cane from 150 local growers (small farmers and private companies) and molasses to supplement its own supplies.
The mill is powered from "free" sugarcane bagasse during the seven-month cane-crushing season, and coal for the remaining five months. The Triangle ethanol plant enjoys an advantage over the typical annex molasses-to-ethanol plants built in other locations, which can operate only during the harvest season. During the off-season at Triangle, it is more economical to generate electricity from coal-fired boilers than to purchase electricity from the grid to operate irrigation pumps. The ethanol plant thus serves as a condenser for the electrical turbines, thereby operating on what would otherwise be waste steam. This enables the plant to have an energy output:input ratio of 1.94 (Scurlock et al, 1992). This system permits year-round ethanol production (330 days average), reducing investment, operating costs and seasonal inventory accumulation costs. These factors also make it economically feasible to operate on purchased molasses during the off-season (Steinglass et al, 1988).
Triangle has overcome the stillage disposal problem by diluting the waste up to 200-fold with irrigation water. After cooling in ponds, the water-plus-stillage is applied as fertilizer to around 7500 ha, about half of the sugarcane plantation. Although returning stillage to the land increases crop yields by 7 per cent, care has to be taken not to damage the soil's nutrient balance. Therefore, stillage disposal at Triangle has become a carefully monitored recycling of nutrients. The stillage-rich irrigation water at present provides all the necessary phosphates, and an excess of potassium. The total value of potassium as fertilizer in Triangle's stillage is estimated at $1.1 million each year. Alternative methods of making use of stillage and wastes are also being investigated. One practical method of disposal, for example, is to use the liquid "wastes" to generate more energy by concentrating and then burning for heat and power generation. Alternatively, stillage could be anaerobically digested to make biogas.
Scurlock et al (1991) has recently completed an analysis of the plant. Unfortunately there is no detailed breakdown of costs due to security reasons. Over the first three years of operation, the ethanol production cost was around $0.35 to 0.40 per litre, compared with a "landed" cost of gasoline in Harare of $0.50 per litre. Ethanol therefore cost I I to 27 per cent more than gasoline in terms of energy content only, but this was paid for entirely in domestic currency once the initial foreign-exchange investment had been recouped. The entire production of ethanol has been sold to the State-controlled National Oil Corporation of Zimbabwe. Gasoline and ethanol prices, as well as profit margins for gasoline wholesalers and retailers, are fixed by the Government.
Zimbabwe has proved that a relatively small country can diversify its agro-industry, to become less dependent on the perturbations of the external oil and commodities markets. The country has now gained considerable experience in the building of fermentation and biotechnological industries. Zimbabwe has pioneered the production of fuel ethanol in Africa, and provided valuable experience for other countries wishing to diversify their sugar industry to include fuel production. It sets an example of technological initiative to increase biomass-energy use and achieve some degree of energy independence. From the outset Zimbabwe has maintained both local and national involvement in decision-making at all levels. It offers an example of good use of relatively simple technology and local infrastructure and political commitment. The very survival of this project till now demonstrates that it has fulfilled the important criterion of involvement with local security, industry and agriculture. Indeed, local motivation seems to underpin every aspect of biomass energy at Triangle (Scurlock et al, 1991).