It is estimated that biomass covers currently up to 15% of the world energy demand, almost 1/3 of all energy consumption in the Less-Developed Countries. This rate was over the last years rather constant, with increasing overall demand bioenergy consumption increased in absolute terms.
Table 1 : World Energy Consumption pattern 1997 (ref 1)
Total World 9.6 Bio TOE 1-1.5 Bio TOE 11-15%
Asia 2.3 Bio TOE 0.6-0.8 Bio TOE >30%
Africa 0.4 Bio TOE 0.2-0.27 Bio TOE >50%
Lat. America 0.4 Bio TOE
Table 1 makes it clear that the proportion of bioenergy is particularly high in Africa. In most sub-saharan countries biomass counts for over 80% of all energy needs. As it is mostly used for cooking, fuelwood is almost exclusively used. A large biomass resource of straw and agricultural residues remains untapped.
Biomass has a role to play in the current attempt to save the world’s climate and to find ways and means to implement the “Kyoto agreement”. The prospect that developing countries are going to increase substantially their greenhouse-gas (GHG) emissions sidelining current biomass consumption, is part of an intense bargaining between donor nations and LDC’s today.
Table 4 : Future Trends (Business as Usual) Population Growth (in Billion People)
World 5.2 7.9
EU 0.36 0.38
DCs 4 6.4
Table 5 : Future Trends (Business as Usual) Primary Energy Demand (in Billion TOE)
EU 1.3 1.6
DCs 2.5 7.3
Table 4 : Summarises the population growth scenarios. The energy consumption growth is shown in Table 5.
Highest growth rates are expected in Asia and Latin America. By 2020 the proportion of people living in cities in the LDC’s are expected to double. Accordingly, energy consumption in the cities will grow over proportionally. In the Sahel, cities and towns will see their energy quadruplicating over the next 20 years while it will grow by a “mere” 50% in the village.
Resource wise these “business-as-usual” projections over the next 20 years look straightforward and normal. Even for a tight situation like in Sahel the biomass resource required in such conventional scenarios will be available.
1. COMPARISON OF BIOENERGY WITH OTHER R.E.
• Sufficient competitiveness of biomass as energy resource in comparison with hydrocarbon
• High potentiality (large areas of crop-land ¬ marginal land - semiarid land)
• Possibility to penetrate all energy market (heat power - transport - chemicals)
• Possibility of bioenergy systems on very small scale (few KW) - or very large scale (hundred of MW)
• Significant environmental benefits as far as pollution concerns
• Positive effects on employment in rural areas for the biomass resource production
* Need of supplying expensive energy feedstock
* Difficulty in the identification of the most promising systems
* Optimisation of bioenergy activity requires very deep knowledge of wide sectorial competence (~100 sectors)
* Need to adopt horizontal and vertical integration of sub-systems to improve the economic basis of bioenergy complexes
* Water, soil, climatic, environmental constraints limiting the biomass productivity and the choice of plants
2. RESOURCES OF THE PLANET
On all continents the potential crop-land available for bioenergy is significant.
In the European Union, the potential crop-land is estimated to be 40 million ha, in the USA around 70 million ha, in Africa 700 million ha (also assuming that the land is used twice for the production of food). The figure for Latin America is estimated to be still higher.
Water is vital for biomass product. Increased human activity requires more and more water. Its availability is shrinking as shown in the figure below:
2.2. Living Species
Living Species (identified so far) are numerous.
Type Number of species
Monera (bacteria, blue-green algae) 4,760
Plantae (multicellular plants) 248,428
Platyhelminthes (flatworms) 12,200
Nematoda (roundworms) 12,000
Annelida (earthworms) 12,000
Non-insect arthropoda (mites, spiders, crustaceans) 123,161
Human food-industry activity is based only on a few hundreds types of crops. Therefore, there is wide scope to explore new biomass crops for energy. Good choice of crops becomes important for large scale biomass production play a leading role across the entire spectrum of energy needs of developing countries, while simultaneously achieving critical economic, social and environmental objectives. Sustainably produced biomass energy resources and products can include, among others:
(i) traditional woodfuels (fuelwood and charcoal);
(ii) briquettes from agricultural and woody-biomass residues;
(iv) bio-ethanol; and
Additionally, and of paramount importance, a wide variety of biomass resources can today be used in power generation through dendro-thermal and gasification processes. All of these biomass-based energy resources and products can be produced in a decentralized basis, can generate large number of employment in the rural areas, and can significantly contribute to conserve local ecosystems and to establish sustainable carbon sinks. What also has been learned from past experience, is that the success of getting substantial results will require on the one hand, the combined efforts of all involved, not only the communities, but the public and the private sector as well, and, on the other hand, continued efforts to facilitate the development of energy markets, so that biomass-based technologies can find their competitive edge along with other conventional or newer forms of renewable energy.
Within that context, the World Bank Group has elaborated a comprehensive energy sector policy platform which includes the development of the biomass energy sub-sector within an environmentally sustainable, economically viable and socially equitable framework. Hence, the World Bank Group is now increasingly positioned to supports its client countries to:
(i) formulate and implement an appropriate multi¬sectorial policy framework which promotes a rational and efficient production, transformation and use of biomass energy resources and products by the community and the public and private sectors;
(ii) establish natural resource management systems and schemes capable of sustainably producing traditional biomass fuels and modem and/or new biomass-based energy products, and capable of contributing towards the mitigation of desertification and climate change;
(iii) promote an active and equitable participation of the rural community in the production and marketing of biomass energy resources and products; and,
(iv) promote the participation of the private sector in the biomass energy sub-sector, with a special emphasis on the investment in modern and/or new biomass energy technologies and products.
In addition to its regular Regional Energy Units (Africa, Asia, LAC, etc.) The World Bank Group has several \ specialized energy programs, such as the World Bank/UNPD "Energy Sector Management Assistance Program - ESMAP", the "Africa Regional Program for the Traditional Energy Sector - RPTES" and the "Asia Alternative Energy Program - ASTAE", which assist client countries on biomass energy issues through:
(i) policy and institutional development support;
(ii) local capacity development programs;
(iii) knowledge dissemination activities;
(iv) identification and preparation of public investment programs and projects;
(v) mobilization of funding for public sector programs and project, through IDA, mRD, GEF, multi-lateral and bilateral co-financing and the newly established Prototype Carbon Fund (PCF); and,
(vi) identification of private sector projects, for follow-up by the International Finance / Corporation (IFC) of the World Bank Group.
Before closing, we would like to appeal to the industrialized countries and to the private sector for them to actively join in the efforts of the international development community to transfer modern biomass energy and other renewable energy technologies to the developing countries. Doing so will not only provide for significant development opportunities and economic growth in the recipient countries, but will open new markets and investment opportunities for the industrialized countries and private sector companies that participate in the process.
Finally, we would like to invite the participants of this "Ist World Conference and Exhibition on Biomass for Energy and Industry" to visit the World Bank Group exhibition stand located in the exhibition hall, were we are displaying a cross section of our Programs, products and services and thereafter to actively seek us a partners for the promotion of biomass energy as a potential instrument for environmentally sustainable development.
Food and Biofuel Production: New Challenges Ahead
Energy is a crucial and vital ingredient for the modern development of economic activities of our society. It provides services for cooking, space and water heating, lighting, health, food production and storage, education, mineral extraction, industrial production and transportation. It is so crucial that, throughout the world, the energy consumption patterns largely determines the way in which people live their lives.
A review of current and future overall energy scenarios shows three main critical issues. First, only 25 percent of the world's population have access to modern energy systems, using approximately 70 percent of the total world primary energy consumption, while more than two billion persons live in dire poverty without access to energy for basic needs and development. Second, it is expected that energy consumption in developing countries will increase considerably in the next decades. Third, global energy scenarios will remain fossil fuels¬based; hence the threat of global climate change related to atmospheric concentration of greenhouse gases (GHG) due to fossil fuels use.
Biofuel is a locally-available source of energy that not only can provide energy to meet the increased energy demand derived from the economic development of developing countries, but also contributes to climate change mitigation and rural development.
FAG, as an organization of the UN system with its mandate on agriculture, recognises fully the global energy-poverty perplexities and the bioenergy potentials. FAO's bioenergy programme bases its operations on the following concepts: a) bioenergy can stimulate diversification of agricultural and forestry activities; for example, through establishment of energy plantations with trees and crops; b) biofuels can provide locally the necessary energy to improve agriculture and forestry productivity; and c) bioenergy can attract investments to rural areas where most of the biofuels are produced.
However, several barriers must be properly addressed and removed for the full utilisation of bioenergy potential. One of the main concerns is the availability of land for food and biofuel production. This tissue is particularly important in developing countries where food security deserves the highest priority.
The analysis of land competition between food and biofuel production is not so simple and easy. There are many political, technical, economic, environmental and social implications that must be properly understood to have bioenergy systems integrated into agriculture and energy policies and strategies.
The following topics describe the characteristics of bioenergy systems, the interrelations between food and biofuel production chains and land requirements in order to identify new areas of challenges to be faced by bioenergy actors.
Biomass conversion into energy carriers (biofuels) consists of a network of several stages and operations regarding multidisciplinary aspects such as: biofuel supply sources (such as forests, industries, farming activities, etc.), trade and market issues and energy conversion devices. The combination of all these processes and operations is generically called "bioenergy system".
In general, the bioenergy systems are very site¬-specific and complex. At micro level, supply sources of a simple bioenergy system can include a single
Figure 2: Biofuel classification scheme
farm and the main biofuel produced is just fuelwood for charcoal-making to be sold in urban markets. In these cases, a' simplified charcoal making scheme includes, among others, the following main unit operations: growing the fuelwood, harvesting the wood; drying, and preparation of wood for carbonisation; carbonising the wood to charcoal, screening, storage and transport to warehouse and distribution to the market points. On the other hand, the layout of wood "energy systems at macro level (such as provincial and/or national level) can be generically represented by the woodfuel balance scheme represented in Figure 1. In practice, field studies of area-based biofuel flow show that bioenergy systems are even more complex where different supply sources for biofuel production and different biofuel types are converted into energy (Figure 2).
Figures from field studies show that forests and trees as well as agriculture activities provide a significant share of the world's energy use, accounting for about 14% of the total world primary energy used. For developing countries, wood energy is of considerably greater importance than in industrialised countries ¬about 15% of their energy needs come from woody biofuel, with about 80% of all their wood being used for this purpose (FAO, WEIS, 2000). Woodfuel production is overwhelmingly concentrated in the developing world with over three-quarters of annual production located there, while industrialised countries depend much more heavily on fossil fuels with only 2% of their energy demand coming from wood. Most bioenergy comes from natural or semi¬natural forests or woodlands, agricultural sources or other by-products, rather than from energy plantations.
This situation suggests that future bioenergy systems will continue to be based primarily on agricultural and forestry by-products (residues), as already done in several countries such as Sweden, Finland, Spain, USA and Malaysia, where energy policies have been adjusted for the use of their bioenergy potentialities. In the meantime, countries will develop more sophisticated bioenergy systems with the production of biofuels derived from energy (and/or multipurpose) plantations (FAO, Hall, 1994).
R&D NEEDS FOR BIOENERGY
l.l Status of Bioenergy
Some technologies, such as combustion, are already competitive in local economic environments but others, such as gasification and pyrolysis could become so within 3 to 6 years. Bioenergy is particularly suitable for regional or local applications and especially in countries with few indigenous fossil energy sources. The increase in supplies of equipment and services relating to the exploitation of biomass either from forestry operations, agricultural activities or dedicated crops, on a global scale, will have considerable impact on employment in several areas such as small and medium-sized enterprises. Well defined objectives are needed to provide a clear indication to consumers, producers and investors, and in general all stakeholders, that Bioenergy can make a real contribution to the quality of life for this, and future generations.
1.2 Bioenergy Areas
There has been a continuous development of Bioenergy technologies over the last three decades with various degrees of acceleration during certain periods in time as a reflection of the variations in the price of oil. Climate change offers the opportunity for long lasting policies for a constant support of Bioenergy. For this to be achieved, the Bioenergy technologies have to demonstrate that they have reached the degree of maturity and reliability needed for the local but also global economy. Thus, in order Bioenergy to successfully penetrate the energy markets, it must reach the same degree of development with that of fossil fuels so as to provide the same quality of services to the consumers.
All Bioenergy applications consists of four main technology related areas which will be examined individually in terms of the R&D needs necessary to intensify and accelerate the penetration of Bioenergy applications into the energy markets:
a) the resource production, supply, upgrading to a fuel and the storage of the fuel,
b) the feeding system and the conversion reactor,
c) the environmental protection measures, and,
d) the energy recovery for heat and/or electricity.
1. THE RESOURCE
The guaranteed supply of the fuel to a conversion facility is or primary importance and unless this can be contractually secured no project, irrespectively of its technology or other attractive elements, will be seriously considered by the bankers and other project stakeholders. The biomass and or waste recovered fuels form the basis of any Bioenergy application and often the physico-chemical characteristics of the fuel also define the type of technology to be used. The Bioenergy resource covers a very wide range of fuel types such as dedicated products (e.g. energy crops), residues (either agricultural such as straw or forestry such as thinnings), process waste (such as sawdust), waste recovered fuels (such as Refuse Derived Fuel) and unsorted municipal solid waste (MSW). In general, all operations such as collection, transport, size reduction, drying and storage for the residues, the process wastes and MSW have attained significant technical maturity and commercial solutions exist practically for most of these feedstocks. However, for all operations, the handling, and recovery has to be I improved in order to increase the yield although the chain from resource to fuel.
2.1 Enter Crops and Waste Recovered Fuels
The energy crops (with the exception of food related crops such as rapeseed) and the waste recovered fuels, however, need significant efforts in most operations before reliable systems with competitive economics can be developed. For the waste recovered fuels technologies have been developed to recover well calibrated fuels which can be used in energy plants, however, these technologies are now entering the demonstration stage and several plants are under construction or have recently been commissioned in countries such as Sweden, Finland, Germany and the Netherlands. These efforts have to be continued at the demonstration level since waste recovered fuels are indigenous, have low or even negative cost and in well managed facilities high quality fuels can be produced. The main critical element for market penetration of waste recovered fuels is the level of concentration of pollutants such as halogens, sulphur, nitrogen and heavy metals which may differentiate their utilisation between an incineration facility or an energy production facility.
For the energy crops the R&D needs are more general and for most operations the development work must continue and even be intensified. Critical areas are the quality of the various species used (such as willows, poplars and miscanthus) and their resistance to pests and diseases, the planting arrangements (e.g. several different species in one plantation), and the planting and harvesting machinery needed. So far, Short Rotation Forestry (SRF) has been successfully introduced in Sweden and lately in the UK while Spain and the US have recently undertaken careful steps for the introduction of SRF in the farming community. SRF offers several advantages whenever economic solutions can be found and these relate not only to employment in the farming community and related operations, the cultivation of set aside land and the supply of a "green" fuel but also to the regeneration and reclamation of derelict or contaminated land. On the other hand careful attention is needed to prove that SRF will not damage the local ecology and that the uptake of heavy metals by the crops from contaminated land can be controlled so as not to negatively affect the quality of the fuels to be produced.
2.2 New Crops
The above mentioned crops offer the possibility to satisfy the Bioenergy requirements in the short to medium term, however, there are uncertainties concerning the availability of all of the above resource for the long term and in few countries some of them are already used completely (e.g. biomass originating from process waste in Germany). In addition, there will come a point where competition with food crops will limit the availability of land for SRF or other energy crops. It is therefore advisable to look into new biomass resources such as aquatic plants and micro¬algae. These offer attractive options for the production of H2 for subsequent use in fuel cell applications. Furthermore, R&D work is needed to develop species for SRF which can increase the yields to >30 dry¬-t/ha.y.
In addition, the Bioenergy community must recognise that there will be increased competition for chemicals and polymers from biomass feedstocks as well as for several other traditional industrial products from biomass. It is therefore necessary to develop new concepts, such as bio-refineries for component fractionation, to satisfy the demand for food, industrial Bio-Products and Bio-Fuels for Bioenergy. Finally, for all feedstocks it is necessary to develop standards supported by a quality assurance system to provide for confidence between the fuel producers and the fuel users.
2. THE CONVERSION SYSTEM
Bioenergy is characterised by numerous conversion processes that can be generally classified as thermochemical or biological processes. The thermochemical processes include combustion (or incineration in the case of MSW), gasification and pyrolysis, while the biological processes include fermentation, anaerobic digestion and aerobic composting. Finally, liquid biofuels can be produced directly from crops such as rapeseed.
3.1 Thermochemical Processes
Combustion is a commercial technology with traditional fuels such as wood residues and MSW (incineration) and several successful applications can be identified worldwide. However, in order to increase the range of applications and thus the market for combustion technologies, new applications have to be developed for the more complex feedstocks such as straw, grasses, and mixtures of several different fuels. Grate systems are the preferred type of combustor due to their reliability and industrial experience especially for small to medium size applications (0.5 - 5 MWe). Fluidized bed combustors have been proven very successful due to the relative easy scale up and their capacity to operate with a variety of fuels so long as the bed sintering (due to the high ash content of some of the biomass fuels) can be effectively controlled. This has been demonstrated even with some difficult fuels such as poultry litter. The main range of applications for fluidized bed systems is for the medium to the large scale (3 - 20 MWe). A new market has started to develop and it concerns the incineration of RDF in fluidized bed systems with several plants already in operation. There is nevertheless need for long term operation of fluidized bed systems in order to prove their reliability.
Overall the main emphasis for future technical development should focus in using more difficult feedstocks and multi-fuel operation. In addition, the industry should follow the advances on materials made by the coal boiler technology in using supercritical steam conditions in view of increasing the conversion efficiency to electricity. This improves the overall process efficiency and the economics. The MSW incineration industry has initiated activities in this area aiming to increase the efficiency to electricity from the present level of 20-23% to above 30%. Combined Heat and Power (CHP) applications offer the best economic viability due to the sale of heat to district heating networks or industrial users.
Gasification only now, after about 3 decades of R&D work, appears to be ready to achieve commercialisation status. The main problem with gasification has always been the efficient removal of tar and there is general agreement in the industry that this can only be efficiently achieved with catalytic reforming of the tar. On the large scale applications, the successful operation of the Varnamo plant (6 MWe + 9 MWth, Foster Wheeler) in Sweden for over 3,800 hours on Integrated Gasification Combined Cycle provided credibility and the expected operation of the ARBRE plant (8MWe, TPS) in the UK during the first months of 2001 will be the first step towards commercialisation of the IGCC technology. Serious delays have been reported with the Brazilian project (32 MWe, TPS), however, it is expected that this project will also been implemented soon. The Battelle technology has also been successfully demonstrated at the Burlington, Vermont, project in the US.
The co-gasification applications have been very successful and the demonstration of the BioCoComb plant (10 MWth) in Zeltweg, Austria and the Lahti plant (40-70 MWth) in Finland have provided confidence in this relative simple application. The expected operation of the AMER plant (56 MWth) will also further increase the reliability of this type of application. The above three plants are based on different manufacturers (Austrian Energy, Foster Wheeler and Lurgi respectively) while all three gasifiers are circulating fluidized bed systems.
There is still no reliable technology for the small scale applications, <500 kWe, however, BTG plans to demonstrate a reverse flow catalytic bed which is claimed to eliminate tar in small to medium size plants. It should be noted that all small-scale plants are based on moving bed gasifiers. In the medium range applications there are efforts underway to demonstrate the effectiveness of Ni-based catalyst but these projects still have to be implemented.
¬Future R&D work should focus on the effective removal of tar and the various catalytic systems must prove their reliability and especially their economic viability based on an acceptable lifetime of the catalysts. In addition, the gasification technologies should aim to widen the range of fuels acceptable by the gasifiers such as fuels of high ash content and multi-fuel operation. For this to be achieved it is necessary to pay attention to feeding systems especially whenever difficult feedstocks such as bagasse, grasses and fluff RDF are to be used. RDF and waste recovered fuels will become more important in the near future for gasification applications. Thus the experiences gained by the Greve in Chianti plant (TPS, under modifications to increase the electricity output from 2.5 to about 6.0 MWe) will be valuable. Renewed interest has also been demonstrated for the production of synthesis gas and it's subsequent conversion to methanol as has been demonstrated by the novel methanol to gasoline process in New Zealand or Fischer- Tropsch synthesis products as demonstrated by SASOL in South Africa. Various Dutch research centres have proposed a once through configuration for methanol combined with an IGCC in order to improve the overall economics and process efficiency.
The main type of pyrolysis technology under development at present for biomass feedstocks is fast pyrolysis for liquids while slow pyrolysis technologies for RDF are starting to be demonstrated. The Canadian companies Ensyn, Pyrovac and Dynamotive are considered the technology leaders, however as there is not a reasonable market in North America all these companies are looking in the EU as the main market. Ensyn are operating several plants up to 84 t/d wood feed in the USA and Canada for products for the food industry and chemicals. ENEL's Bastardo plant (Ensyn technology, 10 t/d wood feed) in Perugia, Italy, although commissioned, has still to operate under full capacity effectively while the Pyrovac plant (84 t/d feed, mainly bark) in Jonquiere Canada has yet to achieve continuous operation for more than 24 hours. In the EU there are mainly three technologies under development, the rotating cone of BTG, Netherlands, the fluid bed of Wellman, UK, and the ablative type of fast pyrolysis at Aston University, UK. From these, the BTG process has been successfully operated at pilot scale at up to 6 t/d and a demonstration project to scale up the technology is underway in the Netherlands. The Wellman technology has only been operated at pilot scale while the Aston University process is at laboratory scale.
There are some attempts to demonstrate medium capacity fast pyrolysis plants, however, these continue to face technical problems which in some cases are related to the effective heat transfer at high heating rates between the heat carrier and the finely chopped biomass particles or to the rapid quenching of the pyrolysis vapours to freeze any further reactions. These problems result in variations in the bio-oil quality and its consistency, which present uncertainties in the marketing of the fuel.
Several studies are underway investigating the production of chemicals from fast pyrolysis oils but only Ensyn has reached commercial status with the Red Arrow plants in the USA, which focus on food flavourings, although there is considerable interest in adhesives and resins for the wood products industries. A significant amount of work has also been undertaken for testing the bio-oil in engines and boilers for power and or heat production. Since fast pyrolysis is a relative recent technology, it is understandable that technical problems still have to be overcome before commercial status can be achieved, however, there are reasonable expectations that within the next 5 - 8 years sufficient reliability and experience will have been gained to warranty commercial scale plants.
Slow pyrolysis for waste has been attempted for several years based on rotary kiln systems, however, these processes failed to convince the waste management industry that they were a serious competitor to incineration. Recent attempts have been undertaken based on pyrolysis of waste followed by combustion of the gas and vapour products with incineration of the char for power and/or heat generation. The most credible projects are those of Siemens at Furth and Thermoselect in Karlsruhe Germany but significantly longer performance is required before such processes will prove their techno-economic viability. In general the efficiency to power is lower for pyrolysis but it offers better control of contaminants, while on the other hand the environmental impact is higher for incineration.
Future work will have to concentrate on increasing the reliability of the various technologies and improving the performance of combustion equipment for bio-oil. In addition, attention must be paid in better characterisation and eventual standardisation of the bio-oil for wider market penetration.
3 2 Biological Processes
Ethanol, an anaerobic product of microbial etabolism can be recovered relatively easily through conventional distillation recovery methods. The production of industrial ethanol from integrated grain milling plants and sugar crops is a mature technology and has been successfully implemented in Brazil and the US. On the other hand, the absence of appropriate legislation has limited significantly the production of ethanol in the EU. The last decade, ethanol production from lignocellulosic materials has received increased attention by the research community and technical breakthroughs have been reported in the US and Canada.
3.2.2 Anaerobic Digestion
Liquid phase anaerobic digestion (dry solid matter in the digester in the range of 5 - 10 wt%) can be considered a commercial technology and several companies offer various technical solutions to a variety of effluents and substrates. Recent work has focused on multi-substrate operation with attention on the biodegradable traction of MSW especially in Denmark. Both for biogas and landfill gas, purification technologies have been developed to remove carbon dioxide and other corrosive gases (such as hydrogen sulphide) for utilisation of the rich methane gas as transport fuel. If the biogas or landfill gas can be purified extensively, then they can be used as a renewable substitute for natural gas in several applications.
Solid state anaerobic digestion (dry solid matter in the digester in the range of 30 - 35 wt%) is a technology approaching commercial status with Organic Waste Systems of Belgium and Valorga of France as the leaders and main competitors. Solid state operation results in significantly higher biogas productivity rates than liquid phase and a better quality of compost due to the mesophilic operation I (about 50 - 55 °C).
Aerobic composting is a commercial technology with demonstrated reliable performance is several projects. However, the economics of composting are strongly related to the quality of the compost and to a strong market for the compost that is not always evident. Composting source separated or "green" waste results in compost of higher quality due to the absence of plastics or other contamination. Except for the need of standards on the quality of the compost, there is no strong justification for extensive R&D work.
The R&D needs for the biological processes are related to pilot plant/industrial scale demonstration of ethanol form lignocellulosic feedstocks with increased yields, to the fine tuning of solid state digestion technologies and to the improved purification of biogas and landfill gas for various applications such as transport fuel or supply of methane in natural gas networks.
3. ENVIRONMENTAL PROTECTION
Bioenergy is considered as the resource with the highest environmental impact amongst the renewable energy sources and this does not refer to incineration or waste treatment only, but it even applies to wood combustion. This is due to the fact that there are always residues (such as ash and liquid effluents) to be disposed off and practically in all applications there is a stack or a chimney. This may induce the general public to relate bioenergy plants to coal fired plants, incinerators and in general emission of gaseous pollutants. It is therefore mandatory that all Bioenergy technologies must carefully and convincingly address the problem of emissions if they are ever going to be accepted by the general public, the political establishment and subsequently the industry.
Practically in all Bioenergy applications at the end the energy carrier (biogas, fuel gas, bio-oil, ethanol, solid fuel or hydrogen) will be used in a combustionprocess to release the chemical energy of the carrier. It is therefore of critical importance that emissions from the combustion of all energy carriers are minimised to whatever possible extend. It is preferable whenever possible to try to reduce the concentration of the pollutants in the energy carrier before it's combustion so as to minimise or eliminate emissions of sulphur oxides, heavy metals, halogenated compounds and other pollutants. This can (be achieved in gasification or pyrolysis processes, however, it is not possible for combustion unless extensive feedstock pretreatment takes place which is always relative expensive. In the medium term future this can prove an advantage for gasification and pyrolysis processes for contaminated biomass or waste recovered fuels in relation to combustion. Although there has been significant progress in the reduction of emissions for all the above pollutants, this is a field where the R&D work should be intensified. This is becoming even more critical as waste recovered fuels and contaminated biomass such as demolition wood are becoming an important cheap resource in countries where there is scarcity of other biomass feedstocks.
ETHANOL FUEL PROGRAM EVOLUTION
The fuel ethanol program in Brazil was formally established in 1975, aiming at reducing oil imports. Other important considerations were: the existing know-how in fuel ethanol production/utilization; land and labor availability; and low sugar prices in the world market. After 1980, with the ethanol-dedicate engines, local environmental benefits became evident (elimination of Pb in gasoline blends; lower CO emissions; less aggressive HC and NOx). The global environmental benefits, with a substantial reduction in CO2 net emissions, was well quantified in 1990. 
The program was established within the context of a strong government intervention in the sugar cane sector; before 1975 sugar was produced and commercialized within "production quotas", and all exports were made by the government. In the 90's all the production/commercialization of sugar and ethanol were totally de-regulated as follows:
From 1975 - 80's
- Level of guaranteed purchase, at controlled prices
- "Fixed" ratio of ethanol/gasoline selling prices:
0.59 (1975) 0.75 (1989)
- Low interest rate in loans for investment (1980-1985)
Sugar: Government issued "production quotas"
Exports: by the Government
* 1990 Sugar Exports Privatized
* 1994 Ethanol: 22% ethanol in gasoline blends
* 1995 Sugar: de-regulation (end of quotas)
* 1997 Anhydrous ethanol (blends) de-regulation (end of quotas and controlled prices)
* Ethanol: 24% ethanol in gasoline blends
* Hydrated ethanol (E 1 00): de-regulation
Those policies and the world market positions for sugar and oil resulted in productions as are summarized in Figure 1. It is important to see the sugar cane availability, its causes and consequences, to understand the possibilities today.
Figure 1: Production - Brazil 
Internal sugar consumption in Brazil grew in the expected way, somewhat above population growth in some periods. Sugar cane growth from 120 M ton to 240 Mt (from 1979 to 85) was almost entirely driven by the E-l00 (dedicated ethanol car) in the period. In the following years production stabilized; the government policy of paying the producer according to officially audited production costs was gradually abandoned (if not officially), and the "fixed" ratio of pump price ethanol/gasoline increased from 0.59 to 0.75. Sugar exports were privatized in 1990, and a shortage of ethanol (1989-90) led to consumer mistrust and very rapid decline in sales of ethanol dedicated vehicles.
In the 90's, the export privatization and the very competitive production costs led to an extraordinary increase in sugar exports (from 1.5 to 11 M ton sugar) pushing sugar cane production to the 300 MT level; this was stabilized again by the drop in sugar prices and the over supply of ethanol in the 97-2000 period.
In the same period, the relative utilization of sugar cane for ethanol or sugar is show in Figure 2, as well as the ethanol penetration in the Otto-cycle engine cars, in Brazil.
BIOMASS FOR ENERGY OR MATERIALS
Greenhouse gas (GHG) emission reduction is one of the most important environmental challenges for the next decades. Carbon dioxide (CO2) is the most important greenhouse gas, representing approximately three-quarters of the total GHG emissions. Biomass strategies pose an important option for CO2 emission reduction since CO2 is fixed during the biomass growth stage. Biomass can subsequently be used as a renewable resource, with zero net CO2 emissions. This is the basis for all biomass strategies (i.e. groups of activities with similar characteristics, concerning agriculture and forestry and aiming for GHG emission mitigation). The following biomass strategies can be discerned:
* Carbon storage above ground in new forests;
* Carbon storage below ground in soils;
* Carbon storage in wood materials and products;
* Substitution of energy carriers with biomass;
* Substitution of materials with biomass;
* Energy recovery from process waste and post¬ consumer waste.
The land availability and the biomass yield per hectare limit the total amount of biomass. Different biomass strategies compete for the limited amount of biomass and land available. Moreover, these strategies compete with other strategies for GHG emission reduction (e.g. other renewables, energy efficiency). A lot of attention has been paid to carbon storage strategies, to substitution of energy carriers and to materials substitution (e.g. 1-3). However, little attention has been paid to the interaction and the cost-effectiveness of these strategies.
The pervasiveness of GHG emissions complicates the analysis: many GHG emission reduction strategies influence each other's efficiency. For example emission reduction because of a switch to bioelectricity reduces the potential for emission reduction based on the increased efficiency of household equipment. The assessment of biomass strategies is further complicated by co-production and cascading: by-products from wood sawing and waste materials can be used for energy recovery. This illustrates why the assessment of biomass strategies is complicated, and why different studies result in different recommendations, depending on the scope of the study.
This study focuses on Western Europe. Current European policies with regard to biomass are aiming for bioenergy, especially electricity production (4). Transportation fuel research activities have been reduced in the last decade. As of yet, biomaterials strategies have received little attention from a GHG emission point of view. Some reforestation activities represent a continuation of a trend that started decades ago. It is unclear whether the current European biomass policy trends are optimal, given the conflicting study results and rapid technological change. For this reason, an energy and materials' systems engineering model has been developed in order to analyse the optimal use of biomass from agriculture and forestry for energy and/or materials. The selection of optimal biomass strategies has been investigated in the framework of the BRED (Biomass for greenhouse gas emission Reduction) project.
The MARKAL linear programming model was developed 20 years ago within the IEA/ETSAP (International Energy Agency/Energy Technology Systems Analysis programme) framework (5). MARKAL is an acronym for MARKet Allocation. A MARKAL model is a representation of (part ot) the economy of a region. The economy is modelled as a system, represented by processes with physical and monetary flows between these processes. The processes represent all activities that are necessary to provide products and services. Special emphasis is given to new emerging technologies and their potential integration in the energy and materials system. A MARKAL model calculates the least-cost system configuration that meets the fixed demand for products and services given a number of constraints. The selection of process alternatives is based on discounted full life cycle costs. Biofuels for example are characterised by higher costs than fossil fuels, but lower GHG emissions. Consequently, biofuels will only be selected if GHG emissions are endogenised in the costs. This is done on the basis of emission permit prices.
Figure 1: Biomass model structure
MARKAL is a dynamic model. The time span modelled is generally covering a period of decades. Within such a time horizon, technological change will be a major driving force for a changing systems configuration. Price elasticities of demand are endogenised in the MARKAL Elastic Demand algorithm (6). The least-cost system optimisation algorithm represents the market mechanism. The model user defines the database of processes and the constraints for individual processes and for the region as a whole. Constraints are for example determined by the availability of resources or environmental policy goals. Processes are characterised by their physical inputs and outputs of energy and materials, by their costs and by their environmental impacts. Many products and services can be generated through a number of alternative processes (e.g. electricity production can be based on coal, gas or biomass). The model database contains 1500 processes, covering the whole life cycle for both energy carriers and materials 'from cradle to grave'.
The MARKAL MATTER 4.2 model has been used for this analysis. Documentation on input data and analysis results can be found on the internet (7) and in a large number of publications (e.g. 8,9). The model covers more than 25 energy carriers and 150 materials. More than 100 products represent the applications of these materials. 30 categories of waste materials are modelled. The model has been developed especially for the analysis - of GHG emission reduction strategies. Four GHG emission permit prices have been analysed: 20, 50, 100 and 200 EUR/t CO2, The base case is the run without a permit price. In the emission reduction cases, the permit prices increase from zero in the year 2000 to their maximum level in 2020 and stabilise afterwards.
Figure 1 show a general overview of the model structure for biomass, showing the close relation between food, energy and materials crops. Europe is split into a northern region, a middle region and two southern regions in order to account for different climates and soil types. All important agricultural crop types are - covered (including energy crops like miscanthus and sweet sorghum). Afforestation (i.e. new forests on formerly agricultural soil, planted after 1990) is considered as a carbon storage strategy. Model input data for biomass (production and consumption processes) have been reported in five separate volumes (e.g. 10,11).
With the MARKAL MATTER 4.2 model, three scenarios have been analysed:
1. Globalisation, characterised by rapid technological progress, globalisation of economic activities and market liberalisation
2. Fortress Europe, characterised by moderate economic development and heavy reliance on transportation and building sectors
3. Sustain, characterised by environmental reorientation of society (initiated by lifestyle changes)
A more detailed discussion of the model and the results can be found in the final report (12).
Figure 2 shows the changes in agricultural land use for the 'Globalisation' scenario in - 2030. The figure shows the reference year 1990, the base case and permit price levels of 50,100 and 200 EUR/t CO2, In the base case and the 50 EURIt case, the use of biomass crops is negligible but some afforestation is introduced (based on a lower bound that represents current policy plans). Afforestation increases markedly in the 100 EURIt and 200 EUR/t cases and dominates biomass crops. The results show that the full land area is not yet used at lower, more realistic, permit price levels. This suggests that land availability should be no major issue in the biomass strategy discussion: the costs are the main driving force. The I preference for afforestation is a major difference with earlier modelling studies.
Figure 2: Agricultural land use, 'Globalisation', 2030
Western Europe has reached a status where its agricultural production potential exceeds food and fodder demand. This is largely accounted for by the steadily increasing agricultural productivity. If this trend continues, 10-20% of the agricultural land (both arable land and pastures) may become available for other purposes. If this land is used for high yield biomass crops, it can yield up to 500 Mt biomass per year. As a consequence, agricultural biomass crops can constitute an important option for GHG emission reduction. The results regarding biomass supply depend to a large extent on trends in agricultural productivity and trends in global markets for agricultural products. Extensification is not recommended from a GHG emission reduction point of view.
The use of biomass for energy and materials applications will increase by up to 200 Mt (compared to the case with no permit price) if significant greenhouse gas policies (i.e. emission permit price of 200 EUR/t CO2) are introduced. The growth mainly takes place in the energy market. Also the materials market grows up to a permit price level of 100 EUR/t. Figures 3 and 4 show the division of biomass use (both from forestry and from agriculture) into energy and materials applications at a permit price level of 100 EUR/t CO2 in 2030.
Figure 3: Bioenergy applications, 100 EUR/t CO2, 2030
Figure 3 shows significant changes in the biomass use for energy purposes between 1990 and the base case in 2030. The total biomass use increases significantly from 80 Mt to 200 Mt. While the biomass use for heat production nearly disappears, the energy recovery from waste biomass (mainly for electricity production) increases significantly and remains at a constant level up to the 200 EUR/t permit price. The energy recovery from lignin (via gasification and subsequent cogeneration) increases at 100 EUR/t, but declines again at 200 EUR/t. These changes are related to the ethanol production from wood from 100 EUR/t upward, which results in lignin by-products that are used for energy recovery. However in the 200 EUR/t case, part of the residual lignin is used for Hydro Thermal Upgrading (HTU) oil production. From 50 EUR/t upward, biomass use increases significantly, up to 390 Mt biomass in the 200 EUR/t case. The main increase can be attributed to the production of transportation fuels, especially ethanol and (at 200 EUR/t) HTU biodiesel. Figure 3 shows that the differences between the scenarios are limited, indicating that the GHG permit price level has more impact than the scenario characteristics.
Figure 4 shows an increase of the biomass used for materials applications from approximately 120 Mt in the reference year to 170 Mt in the 100 EUR/t case. The additional biomass is used as feedstock for petrochemicals, and a limited increase for construction materials. At higher permit price levels (200 EUR/t) feedstocks decline because HTU oil is applied in the transportation sector instead of for the production of biochemicals.
Figure 4: Biomaterials applications, 100 EUR/t CO2, 2030
4. COMPARISON WITH OTHER STUDIES
With regard to the biomass applications, the most remarkable differences with earlier studies are:
• The biofuels market will only develop at high emission permit values (100 EUR/t and higher). This development can be explained by new emerging production routes such as HTU (which are not considered in most preceding studies).
• The market prospects for bioelectricity have deteriorated due to the rapid technological progress of gas-fired power plants and the still improving supply prospects for gas. This progress is generally neglected in other studies.
• Western European biomass availability is no constraint at emission permit price levels up to 100 EUR/t CO2.
• At all permit price levels, considerable quantities of biomass (up to 175 Mt) are used for materials applications. Biomaterials applications constitute approximately one third of the total biomass use for energy and materials.
• Electricity production is limited to energy recovery from waste, lignin gasification and co-combustion in gas fired power plants. Energy recovery from waste is already introduced in a situation without GHG policies and dominates the bioenergy market up to permit price levels of 50 EUR/t.
• Substitution of petrochemical feedstocks is another important category that has received little attention as of yet. However, the relevance of this strategy depends on the biomass availability and the costs are comparatively high (especially relevant at emission permit prices of 100 EUR/t and higher). Consideration of these new market niches will result in a stronger penetration of biomass in the petrochemical market.
• The production of building and construction materials does not seem attractive at any permit price. The main reasons are the comparatively high costs and the limited potential of the building materials market (in physical terms when compared to the energy market). Moreover, the GHG intensity of competing materials (e.g. steel, cement) will decline significantly.
• Cascading of wood materials is of secondary importance, the main reason being the ample biomass availability. Increased cascading is introduced in the sense of increased energy recovery from waste materials and residues (a type of 'once through' cascade).
• The combination of biomaterials and bioenergy strategies results in additional biomass use for energy production from by-products of materials production. Especially lignin and by-products from pyrolysis processes can be used for energy recovery. Structural wood products with a long product life can contribute to energy recovery after a product life of decades. Increased recycling and energy recovery of biomaterials poses an important option that can simultaneously substitute fossil fuels and reduce methane emissions from disposal sites.
5. CONCLUSIONS AND RECOMMENDATIONS
This analysis provides some new insights. Regarding the relevance of biomass strategies, the total contribution to Western European GHG emission reduction can amount to 400 Mt CO2 equivalents in 2030. This contribution represents approximately 9% of 1990 emissions and requires roughly a doubling of biomass use. The model calculations show that a combination of biomass strategies with many other strategies is required in order to achieve a significant emission reduction.
Regarding the biomass supply side, considerable flexibility exists in agricultural production and in forestry in order to increase the biomass production (by increased yields and efficiency). This results in 20-30 million hectares available for energy and materials crops and for afforestations, equivalent to 15-20% of the current area used for agriculture. These figures are in the range of earlier estimates. The results regarding biomass supply depend to a large extent on trends in agricultural productivity and trends in global markets for agricultural products. Extensification is not recommended from a GHG emission reduction point of view.
Biomaterials applications constitute approximately one third of the total biomass use for energy and materials. On the materials side, especially the use of biomass as a feedstock for synthetic organic chemicals should receive more attention. Increased use of existing forestry products (building materials and paper) is of secondary importance. On the energy side, energy recovery from waste biomass increases significantly, even in a situation without GHG policies. Regarding bioenergy use, more attention should be paid to transportation fuels.
The model adds important insights regarding market mechanisms. Competing emission mitigation strategies will significantly inhibit the introduction of biomass. The competition of biomaterials and afforestations should be considered in development of bioenergy policies (and vice versa). It is recommended to take the insights from this study into consideration for European policy development and to apply this model or similar models in order to enhance the insight regarding future market responses to GHG policies.
COMPARATIVE EXTERNALITY ANALYSIS …….
• Biofuels offer clear advantages in terms of greenhouse gas emissions, but do they perform better when we look at all the environmental impacts from a life cycle perspective? To compare the environmental impacts and externalities of biodiesel and fossil diesel, these fuels and their impacts are assessed in a detailed way, combining Life Cycle Assessment (LCA) tools and externality assessment tools.
• Both environmental analyses require an objective basis for comparison, the so-called functional unit, which, reflects the function of the two fuels. According to Vito-measurements, it takes litre of biodiesel in relation to 0.95 litre of fossil diesel fuel to drive with an identical car and the same conditions (I). So both for the LCA and externality analysis litre of biodiesel is compared with 0.95 litre of fossil diesel fuel. This functional unit is consistent with the vehicle/km used in external cost analysis for the comparison of different fuels and technologies.
• Belgium was considered to be the geographical reference area for the biodiesel life cycle. With regard to fossil diesel fuel, West European conditions were taken into account. Both assessments start at the extraction of primary raw materials and conclude with the combustion of the fuels in the car engine.
• The paper uses interim results of this project to compare diesel and biodiesel.
• Figure I shows the life cycle trees for both fuel cycles. For all these steps, the most important emissions have been quantified.
2. COMPARISON BASED ON STANDARD LCA
• The analysis is based upon the LCA methodology described by ISO in its 14040 standard (2)
• The primary concern of the LCA is the question as to whether or not the production of biodiesel is comparable to the production of fossil diesel fuel, from an environmental point of view, taking into account all stages of the life cycle of these two products. The different environmental impacts are weighted based on traditional LCA techniques.
Figure 1: Life cycle tree for fossil diesel fuel and biodiesel
• One of the most important interim results from the impact assessment is that the agricultural processes of the biodiesellife cycle chain contribute significantly to most impact categories considered in the study. More specifically, the production and the use of chemical fertilisers have an important contribution.
• When comparing the two ecobalances, it is clear that the biodiesellife cycle only has a better effect score for the use of fossil fuels and for global warming. The better environmental score for the greenhouse effect is caused by the fact that rapeseed assimilates CO2 during its growth. Indeed, the CO2 balance has been closed in the life cycle inventory part of biodiesel; only the CO2 emissions with a 'fossil' origin have been taken into account. Considering the use of fossil fuels, it goes without saying that the biodiesel scenario consumes less fossil fuel in comparison with the fossil diesel scenario during its life cycle.
• As a result of the final valuation (3) the environmental index of biodiesel is a factor 2 higher than the one for fossil diesel (Fig. 2). Taking account of all the assumptions made at the moment, we could conclude that fossil diesel fuel is environmentally better than biodiesel. However, not all impact categories were weighted during valuation and moreover weighting factors, to a large extent, have a rather subjective nature.
Figure 2: Result of LCA-valuation
3. COMPARISON BASED ON EXTERNALITIES
3.1 The ExternE methodology
A very sophisticated method to weigh the different types of impact categories is to make a detailed assessment of the environmental damages caused by the emissions of the biodiesel and diesel fuel chain. To this purpose, Vito uses the ExternE (Externalities of Energy) accounting framework, developed under the Joule research project of the EC since 1992 (4). These days it is widely recognised as the most complete and up to date methodology for the quantification of external costs (damages) from energy and transport, as it integrates a large amount of European and US scientific data and knowledge. It applies the impact pathway approach for a detailed and systematic assessment of the long way from an emission or burden to an impact and damage (Fig. 3). To this purpose, site and technology dependent emissions are quantified; dispersion of these emissions is modelled using local and regional dispersion models. By means of dose¬response functions, the impacts on public health, agriculture, buildings and ecosystems are being quantified. For global warming, specific models are being used to quantify the physical impacts. In a last step, these impacts are valued based on market prices or results from 'willingness to pay' studies. To date, an accounting framework is available for the quantification of site and technology specific damages from the most important energy related emissions, including particles, SO2, NOx, CO, vot, benzene, and greenhouse gasses.
Figure 3: The ExtemE methodology - impact pathway damage function approach
3.2 Interim results: externalities for diesel and biodiesel
For both fossil and biodiesel, damages from particles on public health are the most important external cost category (Fig. 4). This reflects the growing concern over recent years about the impact from particles, sulphates and nitrates on health, especially with respect to chronic mortality. Its valuation takes the number of year lost into account. The emissions of particles come for 90 % from the use phase and because the impacts depend very much on population densities near to the roads. Table I shows a wide range for this pollutant. One has to take care for the comparisons of the fuels because potential differences in the nature and size of the particles from diesel and biodiesel are not fully reflected in these interim results and further research is needed. Impacts from SO2 and NOx are especially public health impacts from sulphates and are less location or technology specific. The evaluation of the contribution of VOC to photochemical oxidation (ozone) is based on a European single average value, which hides a large but unknown variation. The marginal contribution of NOx emissions in Belgium to ozone formation is considered to be zero, based on results for Belgium from ozone models. Comparing these results for Belgium with literature on air-borne emissions for the whole life cycle for biodiesel and diesel confirms our conclusions
Figure 4: External costs of diesel and biodiesel, following the ExtemE 1997 methodology
Table I: The following table shows the externalities for the different emissions for biodiesel and fossil diesel fuel
• The main conclusion is that, compared to the private production costs, external costs are high for both diesel and biodiesel. In comparison to fossil diesel, total external costs of biodiesel are 5% tot 20 % lower, depending on different assumptions. One has however to take into account that a number of indicators for which biodiesel performs worse (impacts on water, eutrophication, acidification and photochemical oxidant formation) have not or only partly been quantified and monetised. Figure 4 shows that the total social costs (private production costs + external environmental damage cost) of biodiesel are higher than for fossil diesel. Indeed; the private costs for biodiesel are substantially higher than for diesel, which is not completely compensated by somewhat lower environmental costs.
• Comparison with other fuels (petrol, LPG) will be elaborated.
1.1 Out of the total land area of India, measuring 3,29 million ha. (mha), 150 m ha. is uncultivated and 90 mha. is categorized as wasteland. The broad subdivision of the wasteland 1S categorized as saline and alkaline, wind and water eroded land forming considerable part of wasteland.
1.2 The process of regeneration of vegetation in degraded and denuded land, representing virtual sand dunes providing an insight into regeneration pattern and biodiversity of the region.
1.3 Complete vegetation pattern of Rajasthan has been studied (1, 2, 3, 4) and succession using hydrocarbon yielding plants has been established at energy plantation demonstration project centre (EPDPC) (5).
1.4 Present investigations were undertaken with an object to study colonizing wasteland under protected and natural condition.
2. MATERIAL AND METHODS:
2.6 Representative soil of the experimental area \ was analysed chemically. Table 1 (8).
3. RESULTS AND DISCUSSION:
3.1 The early colonizers :
Some of the early colonizers including small ephemerals include: Polygala erioptera DC.; Polycarpaea corymbosa (L.) Lamk. ; Gisekia phamacioides L ; Mollugo cerviana (L.) Ser. ; Side ovals Forsk. ; Corchorus tridens L. ; Triumfetta pentandra A. Rick. ; Indigofera essiliflora DC. ; I. linnaei Ali. These plant species have their value as initial colonizer and are not suitable as biomass resource because" their yield potential is very low. These early colonizers provide helpful association for any subsequent plant to come in the succession like Artemisia scoparia Waldst. ; Farsetia hamiltonii Royle. ; Tephrosia purpurea (L.) Pers ; Citrullus colocynthis (L.) Schrad.; Boerhavia diffuse L. and other herbs.
Among the shrub species which came in the next season Leptadenia pyrotechnica (Forsk.) Decene.; Calotropis procera (Ait.) RBr.; Side cordifolia L. ; Crotalaria burhia Buch. - Ham. ; Verbesina encelioides (Cav.) Benth. & Hook. and grass Saccharum munja L. were abundant. In the second year of growth the tree species became dominant and undergrowth diminished to some species.
The important tree species included Acacia torti/is (Forsk.) Hayne. ; Acacia nilotica (L.) Wind.ex. Del. ; Leucaena /eucocephala (Lam.) de WItt. ; Acacia senega/ (L.) Willd.; Prosopis chi/ensis Stuntz..
3.2 Initial association:
Initial plant formed association and appeared to benefit with each other. These association included Calotropis procera (Ait) RBr. with nitrogen fixing Crotalaria burhia Buch-Ham. Besides this at a later stage the nitrogen fixing Tephrosia purpurea (L.) Pers. was largely predominant with other plant like Verbesina encelioides (Cav.) Benth. & Hook. Artemisia scoparia, Waldst; Sericostoma pauciflorum stock; Sida corditolia L.; Crota/aria burhia Buch.-Ham.; Boerhavia diffuse L. The biomass productivity ranged from 0.5 tonnes per ha. (Citrullus colocynthis), to 52 tonnes dry matter per he. per annum (Saccharum munja). A combination of these plant could be used to form a three tier system to colonize the wasteland and get productive biomass as an alternative model to the hydrocarbon yielding plants (6).
3.3 Some other association:
Boemavia diffuse, Citrullus colocynthis, Artemisia scoparia largely cover the ground throughout the year due. to their xerophytic adaptation make good association with these plants R. purpurea, V. encelioides, S. pauciflorum, C. bilmia, C. bonplandium, S. cordifolia, H. marifolium, P. angustifolia, P. corymbosa, E. hirta.