|Application of Biomass-energy Technologies (Habitat, 1993)|
|VI. Conversion of biomass into electricity|
Usually, electricity from biomass is produced via the condensing steam turbine, in which the biomass is burned in a boiler to produce steam' which is expanded through a turbine driving a generator. The technology is well-established, robust and can accept a wide variety of feedstocks. However, it has a relatively high unit-capital cost and low operating efficiency with little prospect of improving either significantly in the future. A promising alternative is the gas turbine fuelled by gas produced from biomass by means of thermochemical decomposition in an atmosphere that has a restricted supply of air (Larson and Svenningson, 1991). Gas turbines have lower unit-capital costs, can be considerably more efficient and have good prospects for improvements of both parameters.
The basic principles of gasification have been under study and development since the early nineteenth century, and during the Second World War nearly a million biomass gasifier-powered vehicles were used in Europe. Interest in biomass gasification was revived during the "energy crisis" of the 1970s and slumped again with the subsequent decline of oil prices in the 1980s. The World Bank (1989) estimated that only 1000 - 3000 gasifiers have been installed globally, mostly small charcoal gasifiers in South America.
Biomass gasification systems generally have four principal components:
(a) Fuel preparation, handling and feed system;
(b) Gasification reactor vessel;
(c) Gas cleaning, cooling and mixing system;
(d) Energy conversion system (e.g., internal-combustion engine with generator or pump set, or gas burner coupled to a boiler and kiln).
When gas is used in an internal-combustion engine for electricity production (power gasifiers), it usually requires elaborate gas cleaning, cooling and mixing systems with strict quality and reactor design criteria making the technology quite complicated. Therefore, "Power gasifiers worldwide have had a historical record of sensitivity to changes in fuel characteristics, technical hitches, manpower capabilities and environmental conditions" (Sanday and Lloyd, 1991, p. 14).
Gasifiers used simply for heat generation do not have such complex requirements and are, therefore, easier to design and operate, less costly and more energy- efficient.. All types of gasifiers require feedstocks with low moisture and volatile contents. Therefore, goodquality charcoal is generally best, although it requires a separate production facility and gives a lower overall efficiency.
In the simplest, open-cycle gas turbine the hot exhaust of the turbine, is discharged directly to the atmosphere. Alternatively, it can be used to produce steam in a heatrecovery steam generator. The steam can then be used for heating in a cogeneration system; for injecting back into the gas turbine, thus improving power output and generating efficiency known as a steam-injected gas turbine (STIG) cycle; or for expanding through a steam turbine to boost power output and efficiency - a gas turbine/steam turbine combined cycle (GTCC) (Williams & Larson, 1992). While natural gas is the preferred fuel, limited future supplies have stimulated the expenditure of millions of dollars in research and development efforts on the thermo-chemical gasification of coal as a gas-turbine feedstock. Much of the work on coal-gasifier/gas-turbine systems is directly relevant to biomass integrated gasifier/gas turbines (BlG/GTs). Biomass is easier to gasify than coal and has a very low sulphur content. Also, BIG/GT technologies for cogeneration or stand-alone power applications have the promise of being able to produce electricity at a lower cost in many instances than most alternatives, including large centralized, coal-fired, steam-electric power plants with flue gas desulphurization, nuclear power plants, and hydroelectric power plants.
It appears that the BIG/GT technology could be available for commercial power generating applications before the turn of the century. According to Williams and Larson (1992), efficiencies of 40 per cent or more will be demonstrated in the mid-1990s, and by 2025 these could reach 57 per cent using fuel-cell technologies being developed for coal. Gasifiers using wood and charcoal (the only fuel adequately proved so far) are again becoming commercially available, and research is being carried out on ways of gasifying other biomass fuels (such as residues) in some parts of the world (Foley and Barnard, 1983). Problems to overcome include the sensitivity of power gasifiers to changes in fuel characteristics, technical problems and environmental conditions. Capital costs can still sometimes be limiting, but can be reduced considerably if systems are manufactured locally or use local materials. For example, a ferrocement gasifier developed at the Asian institute of Technology in Bangkok had a capital cost reduced by a factor of ten (Mendis, undated). For developing countries, the sugarcane industries that produce sugar and fuel ethanol are promising targets for near-term applications of BIG/GT technologies (Ogden et al, 1990).
Gasification has been the focus of attention in India because of its potential for largescale commercialization. Biomass gasification technology could meet a variety of energy needs, particularly in the agricultural and rural sectors. A detailed micro- and macroanalysis by Jain (1989) showed that the overall potential in terms of installed capacity could be as large as 10,000 to 20,000 MW by the year 2000, consisting of small-scale decentralized installations for irrigation pumping and village electrification, as well as captive industrial power generation and gridfed power from energy plantations. This results from a combination of favourable parameters in India which includes political commitment, prevailing power shortages and high costs, potential for specific applications such as irrigation pumping and rural electrification, and the existence of an infrastructure and technological base. Nonetheless, considerable efforts are still needed for large- scale commercialization.
In India, there has recently been increasing interest in large community-sized digesters, of which around 25 are now in operation nationwide. An example of one of the few successful community biogas plants can be found in Pura village, some 100 miles west of Bangalore. The Centre for Application of Science and Technology to Rural Areas (ASTRA) at the Indian Institute of Science at Bangalore helped to build the community plant. As explained earlier, despite careful planning and execution, the plant was initially subject to many problems. However, ASTRA learnt a lot from this first effort and used the experience and recommendations of the villagers to redesign the plant to meet different requirements (Ready et al, 1990).
As of April 1991, the population of Pura village was 463. Before the biogas system was installed, only 45 per cent of the homes were electrified from the grid (Rajabapaiah et al, 1992), and this often did not provide enough electricity to power their lights.
Although the team from ASTRA had assumed that gas for cooking would be a priority, the villagers of Pura actually put clean drinking water first. It had been calculated that the digester could supply enough gas to power a generator to supply electricity which could then be used to pump water to a reservoir. However, the villagers, understandably, wanted assurance that other people would supply dung before they handed over their own. At the same time, ASTRA wanted the assurance that people would supply dung before it set up the water pump. There seemed no way out of this impasse, and the biogas project stopped in 1984. Meanwhile, other developmental project work by the ASTRA team within the village continued, and it was eventually able to overcome many of the problems and revive the project.
It was possible to set up the biogas plant in the first place because the villagers had nothing to lose by participating in the project. Women already collected the dung for use as fertilizer, the project merely "borrowed" this dung and returned an equivalent quantity of better-quality slurry. However, to make it successful required the establishment of community involvement in the organization and running of the scheme. According to Rajabapaiah et al, (1992),
"The crucial administrative step in Pura was establishing a scheme for dung collection and sludge return based on a delivery fee (of Rs. 0.02 or 0.1586 US cents per kilogram), which goes primarily to the women. This ensures the involvement of women who are the principal beneficiaries of the water supply and the electric lights."
Once villagers experienced the benefits of the gas - clean drinking water from taps, a reliable source of electricity, improved fertilizer etc. - they were willing to take responsibility for the running of the digester, and ensuring that the benefits were distributed fairly. A village development society (grama vikasa sabha) was established involving the traditional community leaders. Pura had not had a village committee since before colonial days. They achieved an outstanding 93-per cent collection of dues from 1988 to 1991. Equity is maintained by keeping records of the weight of dung delivered and compost received for each family. These records are displayed publicly for all to see. The system appears to work well (Hall and Rosillo-Calle, 1991; Rajabapaiah et al, 1992).
The biogas system still consists of two plants with a common inlet tank connected to a dualfuel engine. Between September 1987 and April 1991, the engine ran for about 4521 hours, and an 80 per cent diesel replacement rate was achieved. The project is now managed by the villagers and employs two village youths to operate the biogas plant and the electricity and water distribution systems, while maintaining plant records and accounts. Their salaries are provided by the project sponsor, the Karnataka State Council for Science and Technology (Rajabapaiah et al, 1992).
In September 1987, the water-supply system began operating consisting of a 3 HP pump lifting water from a 50 m depth to an overhead tank from which it is distributed by gravity to nine public taps, the location of which was decided by the villagers. This saved the villagers travelling 1.6 km for water collection and caused an increase in per capita consumption of water, although the villagers have restricted access to the water supply to keep the consumption to a reasonable level. By September 1990, 29 private taps had been installed in households for which the owners pay a tariff. Excess electricity is used to power domestic lights (currently 103 X 20 W fluorescent tubes), for which the recipients also pay (Rajabapaiah et al, 1992).
The economics of production are highlighted in table 11. From September 1990 to April 1991, the revenues from lighting and private water taps covered 93 per cent of the expenditures apart from the workers' salaries. The biogas system is operated for about 4.2 hrs/day with a dung input of 291 kg/day and a unit cost of energy over $US0.25/kWh. However, the system is capable of handling 1250 kg dung/day (the amount of dung actually produced in the village) and operating for 18 hrs/day which would reduce the unit cost considerably. The income from lights and private water taps currently covers only about half of the recurring expenses, but as demand for electricity and supply of dung increases, and costs fall, this will become more economic. When the hours of operation reach 6 hrs/day, at current tariff charges, the system will cover all of its operating expenses and the surplus can be used to return the capital investment. At 15.1 hrs/day, the unit cost of electricity is lower than that from a central power station. Extra gas can be used, for example, for cooking or to provide power for local industries, thereby increasing living standards even further. There are plans under consideration for a dairy development scheme, selling milk and providing more dung.
Rajabapaiah et al, (1992) believe that
"The Pura biogas plant is held together and sustained by the convergence of individual and collective interests and that Non-cooperation with the community biogas plant results in a heavy individual price (access to water and light being cut off by the village), which is too great a personal loss to compensate for the minor advantages of non-cooperation to collective interests."
Residents now realise that biogas has raised their standard of living by making their lives more comfortable at a relatively low cost, so they will ensure that the system is maintained; this is a good illustration of the meaning of sustainable development. One resident is quoted as saying "The grid provides government power, but biogas provides people power, which is far more reliable".
People in Pura village are now thinking of building a wood gasifies to provide producer gas as a supplement to its supply of biogas. Their gasifier would be similar to the successful project in the nearby village of Hosahalli the next case study - which provides electricity for lighting and water pumping. They are establishing an energy forest to grow the feedstock and ASTRA is confident that the villagers will follow it through, having seen a nearby success of a gasification system and the benefits of electrification.
The ASTRA group in Bangalore (Ravindranath et al, 1990; Ravindranath and Mukunda, 1990) had carried out studies, at the micro-level, of biogas and producer-gas-driven electricity generating sets in the village of Hosahalli in Karnataka State. They believe that
"The field experiments at Hosahalli village have demonstrated so far the technical feasibility of a decentralized electricity generation system based on wood gasifier biogas technology for meeting various energy needs of remote rural settlements" (Ravindranath et al, 1990 p. 1975).
The wood gasifies provides gas to drive a 7 HP dual-fuel engine (80 per cent woodgas and 20 per cent diesel) and a 5 kVA 3-phase generator. The life of the engine has been taken to be 20,000 hours, after which the engine has to be replaced. The cost of the system is based on an operating time of 15 hours/day. Table 12 shows the operational and capital costs of a 3.7 kW wood system for generating electricity. The operational cost considering diesel and labour is Rs. 1.22 per kWh if wood is free, and Rs 1.54 if wood is to be purchased. The capital cost amounts to Rs. 63,600; in contrast, the capital cost of an equivalent engine running on diesel is estimated to be around Rs. 39,600.
According to Ravindranath and Mukunda (1990), at the current level of operation for lighting only for 4 hours/day, the wood-gasification system would be economic only if electricity is priced at Rs 3.5 per kWh. However, if the gasification system operates beyond 5 hours/day, the unit cost of energy becomes cheaper than the diesel system. The economic viability could be improved further by: (a) matching of the gasifier-diesel engine capacity (5 kW) since currently a 3.7 kW generator is connected; and (b) by diversifying the use of the gasifier system for meeting other energy needs such as pumping water which would lead to increased capacity utilization. At present, a subsidy is still required at this stage of development, but there is a strong possibility that the system will become economically viable in the near future. It should be noted that the centralized electricitygeneration and distribution systems are also subsidized in the district. For comparison, the present subsidized price of grid-based electricity is Rs. 0.65/kWh.
An important aspect of this project is that the villagers are prepared to pay over twice as much for their electricity (Rs. 1.3/kWh) because: (a) the supply is reliable; (b) it provides ancillary benefits (clean drinking water, flour mill etc.); (c) quality of supply; and (d) of emergence of self-reliance (the formation of a village management committee) (Ravindranath, in press).
Mauritius is dominated by the production of sugar, which represents around 88 per cent of the cultivable area Sugarcane accounts for nearly 20 per cent of the country's gross domestic product (GDP) and over 40 per cent of its export earnings. The economy is thus quite sensitive to fluctuations in domestic sugar production and world sugar markets. Almost 60 per cent of total energy
Notes: $1 -17.2 Rupees (Rs.) (January 1990).
The economic analysis of the wood gas-based electricity system was carried out using a discounted cash flow (DCF) technique, namely, net present value (NPV) method as follows:
NPV - Present value of life cycle benefits Present value of total life cycle costs. Total lifecycle benefits were calculated from the sale of electricity. The unit cost of sale of energy (Rs/kWh) was computed by setting NPV-O and solving for the cost of energy. The unit cost of energy was calculated taking a discount rate of 12 per cent, for the wood and gasbased system and then compared with a diesel-based system of similar capacity.
2 Life of gasifier 50,000 hours; annual maintenance cost 5 per cent, and operational level 20 hours/day.
Cost per month is calculated considering the current energy consumption of 10.74 kWh per day. Labour is priced at Rs. I 5 for 8 hours, and wood is freely available from the energy forest Its market price is Rs.0.25/kg of twigs.
Source: Revindranath et al, 1990; Ravindranath And Makunda, 1990 requirements in Mauritius (excluding wood fuels primarily used for cooking) are met by bagasse-fired generation of power and steam in the sugarcane industry. Bagasse is playing an increasing role in power supply and currently provides around 10 per cent of Mauritius' electricity requirements. Woody biomass supplied approximately 63 per cent (3.5x106 GJ) of all the energy required for household cooking in the country in 1988. There is a potential for producing 10.2x106 GJ, using the by-products of the sugar industry and to a lesser extent solar energy.
According to Baguant (1990) the Flacq United Estate Limited (FUEL) is the largest sugar estate in Mauritius with an annual average production of 700,000 tons of fresh cane, and 79,000 tons of sugar. FUEL was among the first sugar estates to produce excess steam for production of electricity for sale to the national grid in the mid-1950s. In 1982, the FUEL sugar estate installed a dual-fuel, bagasse and coal furnace to produce electricity all year-around and substantially increase its output. Bagasse is used at the rate of about 3540 t/hour and coal at about 14 l/hour. A boiler with capacity of 110 t steam/in, 42 bars pressure, 440°C and a condensing turbine coupled with a generator of 21.7 MW led to an average production of 75x106 kWh of excess electricity (30x106 kWh produced solely from bagasse during the crop season and 45X106 kWh from coal). This represents about 12-15 per cent of the total electricity requirements of Mauritius.
In 1989, the electricity output by FUEL increased to 94X106 kWh (26x106 from bagasse and 68X106 from coal) representing about 16 per cent of the country's total requirements and resulting in over 80 per cent of the electricity being sold to the grid. Unfortunately detailed economic costs of production are not yet available fdor the updated plant following negotiation with the electricity boards, but some available information is set out in table 13.
In August 1980, the Government of the Philippines raised the price of fuels, doubling the price of diesel within two years. This adversely affected the farmers who pumped water for irrigation and caused severe economic difficulties. The Government's response was to look for alternative energy sources and gasifiers were chosen due to the extensive R and D experience of the University of Philippines in this field (Foley and Barnard, 1983).
Attempts to introduce gasifiers on a large scale began in 1981 when the Government, with assistance from USAID, embarked on an ambitious programme which initially planned to retrofit 1150 diesel-pump-powered irrigation systems converting them to gas/diesel fuel operations with charcoal-fed gasifiers. This involved 495 irrigator's service associations (ISA) with a total membership of over 26,000 farmers and covered over 46 000 ha The Government's main agency in promoting the use of the gasifiers was the Farm Systems Development Corporation (FSDC). Bernardo and Kilayko (1990) carried out an analysis of the gasifier programme. The results were very disappointing, with just 1 per cent (three out of 248 plants) still being used in1987; and over 80 per cent in need of repair. The gasifier programme thus failed to achieve its objectives in reducing farmers' dependence on diesel fuel and in improving their financial position. The causes of this unsatisfactory outcome are claimed to arise more from institutional and management problems than from any inherent weakness in the technology itself.
According to Bernardo and Kilayko (1990), success required a "fit" between the technology, the users and the implementing agency. Many farmers did not view their irrigation systems as a means of improving their productivity and profitability, but largely as a type of insurance against inadequate rainfall. Therefore, they saw lime value in gasifier-powered pumps. The gasification programme was imposed from above with little understanding of the users' needs. The method of financing also failed to provide clear economic signals to the farmers, and failed to acknowledge the financial realities of the farmers' lives. Many farmers did not know how much the gasifier was costing them, and they frequently did not realize that its costs were covered by a loan rather than a grant. As projects failed, this clearly affected their ability to meet loan repayments.
The half-hearted support of the FSDC area officers and their more general financial difficulties created serious problems in implementation, and the FSDC were generally unable to enforce minimum requirements of its projects with many consequent failures. Additionally, the failure of the ISAs to observe proper maintenance practices ultimately resulted in engine failures, and even permanent damage to engines. Poor maintenance reduced the life expectancy of the gasifier which, in turn, raised the annual capital cost charge significantly. An additional problem was that inflation had a very negative effect on farmer's living standards forcing them to cut down on production inputs, one of which was irrigation - not a priority for many farmers despite FSDC's objectives.
As indicated in table 14, the use of gasifiers could have resulted in significant savings in fuel costs; however, this was not so. Solely on the basis of the cost of fuel, running a 50 HP diesel motor on 50 per cent diesel and 50 per cent charcoal produced only minor savings in 1982 and 1985, and losses in 1983 and 1984. This was partly because charcoal prices increased by 600 per cent from 1977 to 1985 and charcoal was, on occasion, more scarce and expensive than diesel due to increasing household and industrial demand. The greatest savings occurred in 1987 when charcoal prices fell faster than those of diesel. Charcoal gasifiers thus did not completely displace the use of diesel oil. The farmers found it inconvenient to procure two types of fuel without obtaining sufficient benefits for their extra efforts. The implementing agency was inadequately funded and was subject to unrealistic installation targets imposed by the political system. This case highlights the considerable difficulties in setting up and running an infrastructure necessary to carry out repairs and supply spare parts to support new technologies. It illustrates the even more difficult problem of ensuring an adequate supply of raw material (charcoal) at acceptable prices (Bernardo and Kilayko, 1990).
The island States of the South Pacific are generally dependent on imported fossil fuels. Due to the high oil prices in the early 1980s and plentiful indigenous biomass resources (on the larger islands), there was considerable interest in installing biomass gasification units for electricity production and crop drying. Available resources include residues from over 600,000 ha of copra plantations and almost 44.5 million ha of forested areas (Sanday and Lloyd, 1991). The main impetus for the introduction of power gasifiers into the South Pacific region was the European Community-funded Lome II Pacific Region Energy Programme (PREP) in 1983/X4. This proposed, and budgeted for, 17 gasified projects, but finally, only two were installed, both considerably reduced in scale, capacity and cost relative to the original proposals. Other gasifies units were also installed privately in the region. Sanday and Lloyd (1991) of the Energy Studies Unit (ESU) at the University of the South Pacific carried out a survey and monitoring programme of all power and heat gasifiers. They found that of the 16 power gasifiers installed altogether, only one was known to be still operating satisfactorily, the rest having ceased operation. Similarly, for the "Waterwide" heat gasifiers installed in Papua New Guinea, only 20 out of 80 were still in use in January 1990, and most of the other documented heat gasifiers in this region were also expected to have shut down.
The operational problems were thought mainly to be due to flaws in original designs resulting in shortened plant lifetime. The systems installed experienced severe operational and design problems that should have been solved prior to installation in remote sites. To Sanday and Lloyd (1991, p. 17) it seemed
"that the Pacific Islands have been used as experimental stations for technologies that have not been proven in industrial countries". (Furthermore,) "gasifiers have often quickly deteriorated resulting from mismanagement of operational and maintenance procedures, and the persisting hostile operational environment."
Most of the manufacturers were external to the region, some based as far away as Europe. Therefore, there was a lack of spare parts and skilled technicians to carry out maintenance and repair wore This situation was exacerbated by the fact that five of the six manufacturers who supplied systems to the region in the last decade went out of business. There was also a lack of infrastructure support within the region as personnel trained in gasifier technology were extremely scarce, so ordinary mechanics and technicians were often called on to carry out repair work with limited success. Since the gasifier locations were scattered amongst different islands it was difficult and costly to locate maintenance services and they could not be promptly available. Information on the technology was limited and usually in the form of papers for academics and other technical personnel rather than being designed for potential end-users.
The availability of biomass feedstocks may have been over-estimated originally, and the quality these feedstocks and their erratic supplies resulted in intermittent gasifier operation with some systems being periodically shut down. The shortages due to lack of fuelwood supplies were "compounded by domestic cooking receiving priority, difficulties associated with land availability and ownership, and soil salinity problems when replanting programmes were used" (Sanday and Lloyd, 1991, p. x). Also, lacking were schemes to collect scattered fuel and the failure to implement tree replanting programmes. Furthermore, the "Waterwide" heat gasifiers experienced problems with smoke contamination affecting the quality of dried agricultural products and causing heavy financial losses; this was mainly due to improper use.
Repetitive breakdowns and lack of maintenance support meant gasifier operators usually preferred to choose diesel systems which had been proved to be relatively successful and user-friendly in such situations. Furthermore, initial capital costs of gasifiers were high and unable to compete with equivalent diesel sets at current diesel fuel prices. All the problems experienced appear to have discouraged further developments towards implementation of gasifier technology in the region. Most success was found with small wood and husk-fuelled gasifiers installed in Papua New Guinea for agro-drying applications. The single power gasifier that was still operational, a BECK unit in Vanuatu, connected with a school, was successful due to "the availability of wood fuels, the commitment of the operators and the school management and the fortune to have a very gifted and enthusiastic support staff as one of the teachers at the school."
Rice husks are one of the most widely available agricultural residues in Indonesia, but they have few uses. They are a significant potential energy source if reliable conversion technologies can be developed. In Indonesia in 1986, milled rice production was over 26 Mt and 6.5 Mt of rice husks were subsequently produced - husks are estimated at 20 per cent of unmilled paddy weight. Use of 25 per cent of these husks (about 1.8 Mt) and a similar amount of available straw would yield about 3.6 Mt of energy feedstock which could produce an estimated 155 or 300 MW electricity depending of the amount of capital invested in the facility (USAID, 1988). Research and development work in gasifier technology has expanded considerably in Indonesia, and has been supported by the Government In 1987, the Government mandated that 10 gasifiers, manufactured in Indonesia, should be placed in field operation to help demonstrate their technical and economic viability.
This case study is based on a system designed by Manurung and Beenackers (1990). Their continuous, small-scale down-draft rice-husk gasification system appears to have overcome many of the previous problems related to the gasification of rice husks. Based on laboratory experience the first unit (10 kWh) was installed in a village, 100 km east of Bandung, West Java, in 1986. This was followed up by two scaled-up versions each with a capacity of 35-40 kWh. One powers a 1000 kg/hr rice mill and the other provides 10 kWh of electricity for 320 rural consumers.
Typical performance of these field units (called Gasifier I and II) are illustrated in table 15. Diesel fuel replacement up to 70 per cent was achieved; the rice husk to electricity conversion is about 2.4 and 2.0 kg/kWh for Gasifier I and II, respectively. The economic analysis shown in part C of the table is based on Gasifier II only, and demonstrates the pay-back period (PBP) and the net present value (NPV) of the investment. In addition, the economics of the gasification unit are compared with those of a conventional diesel-engine generating set of the same capacity. The costs are based on 1989 economic data and on the present actual performance of Gasifier II for that year.
Under present conditions, according to Manurung and Beenackers (1990), the operating costs of the dual-fuel plant were lower than the plant revenues, resulting in a positive income, with a pay-back period of 7 years compared with 8 years for the full diesel plant. Since the diesel costs for a full diesel plant are 66 per cent of its operating costs, and only 22 per cent for a dual-fuel plant, the economics are particularly sensitive to the price of diesel, and also to load capacity, total annual operating hours and the level of diesel substitution. If applied to rural electricity production, economic feasibility appears to be good with capacities of 30 to 50 kWh and upward, under Javan conditions of 1989.
Chinese-built rice-husk gasifies power plants (160 kWh) were installed in the early 1970s, according to Mahin (1989), at two rice mills at Dogofiri and N'Debougou. These have operated successfully since then with more than 55,000 hours operating experience, although economic analyses of these plants are not easily available. In 1986, with the assistance of GTZ (German Agency for Technical Cooperation), an additional Chinese-built gasifier was installed nearby at the rice mill in Molodo which processes about 20,000 t/yr of rice. The plant produces about 1 kWh for each 2.5 kg of rice husks used in the gasifier. It generates up to 160 kW of power. The total annual operating costs were DM146,877 (or DM0.261kWh) which is 54 per cent of that of a diesel- engine plant. The GTZ study (Mahin, 1989) indicates the difference in capital cost between the diesel and the gasifies plant. Investment costs of the gasifier power plant could be recovered in less than four years. Table 16 provides a summary of cost of the rice husk-fulled gasifier power plant at Molodo.
Although this paper is involved in the analysis of established bioenergy projects, it is also of value to examine the potential for electricity production in the north-east region of Brazil, as assessed by Carpentieri et al, (1992), since this has implications for other developing countries. The north-east region has a low population density, an economy heavily dependent on agriculture, and an energy consumption about half the national average. Over 90 per cent of all electricity produced in Brazil, and virtually all that is produced in the north-east is hydroelectric. To meet projected growth rates for electricity consumption in the north-east up to 2015 would require a capital investment in new power plants (all hydroelectric) in excess of £40 billion. It is planned to develop essentially all remaining hydroelectric potential in the Northeast by 2005, and costs will rise as less favourable sites are developed. However, the hydroelectric potential will inevitably be exhausted and alternative electricity sources must be found. One option under consideration is importing electricity from new hydroelectric projects to be located in the Amazon river basin. But this would be expensive, environmentally controversial and would involve little direct long-term investment or job creation in the north-east.
In 1982, the Division of Alternative Energy Sources of the Hydroelectric Company of Sao Francisco (CHESF), responsible for production and transmission of bulk energy in the north-east, initiated studies on alternative advanced technologies for converting biomass into electricity. There are three potential biomass resources in the north-east that could be utilized for electricity production: sugarcane residues, plantations. and the residues of other agricultural products. Of these, the first two show most promise for large-scale use as plantations are well established. In fact, both plantation industries in Brazil are recognized as world leaders.
In this region practically all woodfuel comes from natural forests with devastating environmental effects. Efforts to establish plantation have been quite successful in Brazil as a whole with over 40 per cent of all charcoal now being derived from this source (Abracave, 1992), and plantations are estimated to cover 4 to 6 million ha (mostly used by steel and paper and pulp producers). Large investments have been made in plantation technology and techniques resulting in a great improvement over the last 15 to 20 years. CHESF carried out a biogeoclimatic assessment to evaluate the potential for wood-plantation energy, considering only land area judged to be sub-optimal for agriculture (CHESF, 1990). It estimated that 50 million ha (a third of the land area of the north-east) was available for plantations with productivities ranging from 6 to 44 m³/ha/yr of wood, and that the total plantation production potential is about 1340 million m³/yr of wood which could produce 12.6 EJ/yr. This compares with a total energy use in the north-east of about I. I EJ. Cost estimates range from 7.3 c/kWh for condensing steam turbine technology (CST) down to 4.3 c/kWh for gas turbine/steam turbine combined cycle technology (GTCC). Over 86 per cent of the wood production would be at an average cost less than $1.35/GJ (Carpentieri et al, 1992).
Sugarcane, on the other hand, is already widely grown in Brazil and some sugarcane processing facilities are already selling small quantities of electricity produced from bagasse to utilities. The present biomass energy production potential in the north-east from the area of cane planted in 1989 is estimated to be 174 PJ/yr. Looking at a future scenario, the average cost of producing the electricity with STIG technology is around 4.04.4 c/kWh. This would be competitive with marginal costs of anticipated new hydroelectric supply. If tops and leaves are also used, the bioenergy available could be increased by up to 75 per cent, and off-season jobs baling and transporting the barbojo would be created (Carpentieri et al, 1992).
Carpentieri et al, (1992) constructed two alternative scenarios for the production of electricity in the Northeast to the year 2015, these are summarized in table 17. The "Hydro" scenario is based on CHESF plans for continued expansion of the hydroelectric system; while the "Biomass" scenario is "intended to be a plausible scenario of how biomass could be incorporated into the utility system." Both proposals include the initial installation of 4100 MW of hydroelectric power at a single site, Xingo 1. The Biomass scenario then assumes sugarcane CEST systems (including barbojo) begin to make a contribution in 1987 such that half of the total potential is installed by 2000 at 320 MW/yr. From 200O, STIG systems come on line at the rate of 280 MW/yr until 2010 when the full electricity-generating potential of sugarcane is realized. Plantation activity is assumed to begin in 1994 with stand-alone power stations first coming on line in 2000 based on GTCC technology with an installed capacity of 250 MW, and annual additions will increase up to 1000 MW of new supply in 2015 (this is supplied by only 4 per cent of CHESF's assumed potential fuelwood in the north-east).
Total new land required by the Biomass scenario would represent only 1.6 per cent of the total land area of the north-east. Since the biomass facilities are smaller and greater in quantity they offer more security and can follow demand more closely. The Hydro scenario would contribute 25 GW between 1990 and 2015 compared with 15 GW in the Biomass scenario. Average unit investment costs would be 25 per cent higher for the HYDRO case, the total required capital investment would be twice as much, average electricity production costs will be higher, and marginal production costs will be substantially higher. This scenario assumes "a reasonable commitment from government, utilities, industry and relevant R&D organizations, and the support of the population in general." (Carpentieri et al, 1992).
The energy potentially available from other agricultural residues in the north-east is estimated to be about 145 PJ/yr, which is equivalent to about 10 per cent of primary energy consumption in this region. Since these sources are widely dispersed and lack any infrastructure for energy use, they are of more importance for use locally, in a decentralized manner (Carpentieri et al, 1992). The Brazilian Government is promising to introduce a new policy across Brazil that will mandate the State-controlled electricity utilities to enter into long-term contracts to buy cogenerated power. This will encourage further the growth of biomass electricity production from residues.