Biomass is a rather simple term for all organic material that stems from plants (including algae), trees and crops. Biomass sources are therefore diverse. Biomass has always been a major source of energy for mankind. Also nowadays biomass contributes significantly to the world's energy supply. In the literature a figure ranging from 10 to 14% is mentioned.¹⁴ According to Hall et al.¹⁷, in 1985 biomass accounted for 14% of the entire world's energy supply. The largest proportion was used in developing countries (33% of the total energy supply). In the industrialised countries the contribution was about 3%. Fuelwood accounted for most of the biomass utilisation in developing countries.

The potential of biomass energy from forest and agricultural residues worldwide can be estimated at about 30 EJ/yr¹⁷ (compared to a worldwide energy demand of over 400 EJ). Clearly, if biomass is to contribute to a larger extent to the world's energy supply, then energy farming, cultivation of dedicated crops for energy purposes, will be required. Either fallow land and marginal lands, the latter being largely unsuited for food crops, could be used for this purpose. When energy crops are considered as a source of biomass, the total energy potential of biomass for energy production may be considerably larger than the energy potential of biomass residues. Up to 200-300 EJ/yr is mentioned in various energy scenarios for the world's energy supply in the future.^{14,25,34,35,37}

Numerous crops have been proposed or are being tested for commercial energy farming. Potential energy crops can be divided roughly into woody crops and grasses (all perennial crops), starch and sugar crops and oilseeds. In general, the characteristics of the ideal energy crop are: high yield (maximum production of dry matter per hectare), low in (energy) input, low cost and composition with the least contaminants and nutrients. Desired characteristics will also depend on local climate and soil conditions. Water consumption can be a major constraint in many areas of the world and make drought resistance of the crop an important factor. Other important characteristics are pest resistance and fertiliser requirements.

In the past 10 years there has been renewed interest worldwide in biomass as an energy source. There are several reasons for this:

Firstly, technological developments relating to the conversion, crop production, etc. promise the application of biomass at lower cost and with higher conversion efficiency than was possible previously. For example, when low cost biomass residues are used for fuel, the cost of electricity is already now often competitive with fossil fuel based power generation. More advanced options to produce electricity are looking promising and allow a cost-effective use of energy crops. Production of methanol and hydrogen by means of gasification processes may be another promising application route.²⁴

The second main stimulus is the agricultural sector in Western Europe and in the United States, which is producing food surpluses. This situation has led to a policy in which land is set aside in order to reduce surpluses.^13,15 Related problems like depopulation of rural areas and payment of significant subsidies to keep land fallow make the introduction of alternative non-food crops desirable. Demand for energy will provide an almost infinite market for energy crops grown on this (potential) surplus land.

Thirdly, the potential threat of climate change caused by high emission levels of greenhouse gases (CO₂ being the most important one) has become a major stimulus for renewable energy sources in general. Biomass, when produced sustainably, emits roughly the same amount of carbon during conversion as is taken up during plant growth. The use of biomass therefore does not contribute to a build up of CO₂ in the atmosphere.

But these three main issues are not the only stimuli: biomass is also an indigenous energy source available in most countries and application may diversify the fuel supply in many situations which in turn may lead to a more secure energy supply. Biomass can generate employment and if intensive agriculture is replaced by less intensively managed energy crops there might be environmental benefits, such as reduced leaching of fertilisers and reduced pesticide use.³⁰ Moreover, if appropriate crops are selected, restoration of degraded lands may be possible as well. Depending on the crops used and the way the biomass is cultivated an increase in biodiversity may be obtained compared to current agriculture.^30,33

When speaking about the supply of energy from biomass, or abbreviated: bio-energy, one should realise that there is a large diversity of biomass sources, conversion options, end-use, applications and infrastructures involved. In general any bio-energy system consists of the following main components (figure 1):

- Biomass has to be produced either by the cultivation of dedicated crops (such as wood by means of short rotation forestry, perennial grasses), by harvesting forest and other residues (thinnings, straw, etc.) or by collecting biomass waste (such as sludge, organic industrial waste and organic domestic waste).

- Next, the biomass has to be harvested or collected, then transported and if necessary it may have to stored and transferred.

- Biomass can be converted by means of numerous processes. The actual choice of a process will depend on the type and quantity of available biomass feedstock, the desired energy carrier(s) (end-use), environmental standards, economic conditions and other factors.

In the following section (2.1) biomass conversion routes will be discussed. Section 2.2 will deal with the overall performance of complete bioenergy systems.

Most biomass energy conversion processes can be divided into thermochemical conversion and biochemical conversion routes. With respect to thermochemical conversion options, a distinction can be made between combustion, gasification and pyrolysis. Biochemical conversion options can be divided into digestion (production of biogas, a mixture of mainly methane and carbon dioxide) and fermentation (production of ethanol). Extraction is another, mainly mechanical, process for producing an energy carrier from biomass (e.g. rapeseed oil from rapeseed). With regard to the energy carriers produced from biomass a distinction can be made between the production of heat, electricity and fuels. Major conversion technologies and processes are outlined schematically in figure 2. The various options are at different stages of development.

The major biomass conversion routes that will be briefly discussed here are shown in figure 2. It should be noted that there are numerous ongoing technological developments in the field of biomass energy conversion. A detailed overview is however bevond the scope of this introduction. Such an overview can be found elsewhere, for example in a report by van den Heuvel.¹⁸

* Combustion is widely used on various scales to convert biomass energy to heat and/or electricity with the help of a steam cycle (stoves, boilers, power plants). Production of heat, power and (process) steam by means of combustion is applied for a wide variety of fuels, and from very small scale (for domestic heating) up to a scale in the range of 100 MWe. Co-combustion of biomass in (large and efficient) coal fired power plants is an especially attractive option as well because of the high conversion efficiency of these plants. It is a proven technology, although further improvements in performance are still possible.

Net electrical efficiencies for biomass combustion power plants range from 20 to 40%. The higher efficiencies are obtained with systems over 100 MW_e or when the biomass is co-combusted in coal-fired power plants.⁴

* Gasification is the thermochemical conversion of biomass into gaseous fuels by means of partial oxidation of the biomass at high temperatures. The low calorific gas (typically 4-6 MJ/Nm³) that is produced can be fired directly or be used for firing engines and gas turbine cycles and can also serve as syngas in the production of chemicals (e.g. methanol). A promising concept is the integrated biomass gasification integrated gasification/combined cycle (BIG/CC). Gas turbines can convert gaseous fuel to electricity with a high efficiency. An important advantage of BIG/CC systems is that the gas is cleaned before being combusted in the turbine which means that more compact and less costly gas cleaning equipment is required. The integration ensures a high conversion efficiency; on a scale of 30-60 MW_e, net efficiencies of 40-50% (LHV basis of the incoming fuel) can be expected.^7,8,11 BIG/CC systems offer a relatively high efficiency on a modest scale. This is important to limit transportation distances for the biomass delivered. A variety of biomass gasification processes are commercially applied.¹⁸ The integrated gasification/combined cycle technology is however currently at the demonstration stage.

Production of synthesis gas from biomass by means of gasification also allows for the production of methanol or hydrogen, each of which may have a good future as transportation fuels. For the production of methanol or hydrogen indirect or oxygen blown gasification processes are favoured because of the medium calorific (typically 9-11 MJ/Nm³) gas that is produced by such processes.²⁴ Such schemes are currently close to demonstration.

* Pyrolysis converts biomass to liquid (bio-oil or bio-crude), solid and gaseous fractions by heating the biomass to about 500^oC in the absence of air. Pyrolysis can be used for the production of bio-oil if flash pyrolysis processes are used and is currently at pilot stage.¹² The conversion of biomass to crude oil can have an efficiency of up to 70% for flash pyrolysis processes. The so-called biocrude can be used in engines and turbines. Its use as feedstock for refineries is also being considered. Some problems in the conversion process and use of the oil need to be overcome; these include poor thermal stability and corrosivity of the oil. Upgrading by lowering the oxygen content and removing alkalis by means of hydrogenation and catalytic cracking of the oil may be required for certain applications.

Other ways to produce biocrudes are the Hydro Thermal Upgrading process (HTU) and liquefaction. HTU converts biomass in a wet environment at high pressure to partly oxygenated hydrocarbons. The process is almost at the pilot stage.¹⁶ Liquefaction takes place at moderate temperatures at, high pressure with the addition of hydrogen. The interest in liquefaction is low because the reactors and fuel feeding systems are more complex and more expensive than for pyrolysis processes.¹²

* Digestion is the biochemical conversion of organic material to biogas, a mixture of mainly methane and carbon dioxide. The biomass is converted by bacteria in an anaerobic environment. The energy of the biogas produced amounts 20 to 40% of the lower heating value of the feedstock. Anaerobic digestion is a commercially proven technology and is widely used for treating wet organic wastes.¹² Biogas can for example be used for firing engines. It can also be upgraded to natural gas quality and applied in grids. When used in an engine the overall electrical conversion efficiency is about 10-16%.

* Fermentation, a biochemical conversion process, is used commercially on a large scale in various countries to produce ethanol from sugar crops (sugar cane, beet) and starch crops (maize, wheat).³⁹ The biomass is ground down and the starch is converted by enzymes to sugars. Yeast then converts the sugars to ethanol. Pure ethanol is produced by distillation which is a relatively energy intensive step. About 450 litres of ethanol can be produced per tonne of dry corn. The remaining solids can be used as cattle feed. In the case of sugar cane the remaining bagasse can also be used as fuel for boilers or gasification processes.

Conversion of lignocellulosic biomass (such as wood and grasses) requires acid or enzymatic hydrolysis before the resulting sugars can be fermented to ethanol. Such hydrolysis techniques are currently at the pre-pilot stage.^12,39

* Extraction is a mechanical conversion process which can be used to derive rapeseed oil from rapeseed. In this case, the process produces not only oil but also rapeseedcake, which is suitable for fodder. About 3 tonnes of rapeseed are required per tonne of oil. Rapeseed oil can then be esterified to obtain Rapeseed Methyl Ester (RME) or bio-diesel. This process is being used commercially on a substantial scale, especially in Europe.¹¹

To evaluate the performance of a bioenergy system, the entire chain from biomass production up to the end-use should be considered. A major criterion for comparing total bioenergy systems is the net energy yield per hectare. If this net yield is low, the amount of land needed for the net production of a certain amount of energy is high and vice versa. Since land is a scarce commodity, high net energy yields per hectare are favoured. Also the environmental impacts of a specific energy crop are a criterion for selection. Logically, another major criterion is the cost of the biomass per GJ produced (or per GJ of fossil energy replaced).

The International Energy Agency has made a comprehensive comparison between the following bioenergy chains: production of ethanol from maize and sugarbeet, RME from rapeseed, and methanol and electricity production from wood. Both the state-of-the-art as well as future projections for the overall performance of such systems are discussed.²¹

Energy ratios of bio-energy chains are given as ratios of the energy output versus the energy input, compared to the conventional fuel life cycle. Thus, a figure below 1 implies that the energy input is higher than the energy output. Ethanol production via maize, wheat and beet yields ratios of 1.5 to 0.9 in the current context, indicating that there may be no net energy production in certain situations if the required energy input is covered by fossil fuels. Projections based on improved technologies and low input, high yield crops, indicate ratios of about 2 to 3. Production of RME from rapeseed gives a ratio of about 1.5 in the current context and 3 in an improved system.^21,30

By comparison: electricity from wood can currently obtain an energy ratio of 6-7 and values of 10-15 are feasible. This ratio has been confirmed by Ranney et al.³⁰ For methanol a ratio of 6-12 is foreseen in the somewhat longer term.²¹

Net energy yield per hectare is another important criterion for comparison. Table 1 compares the net energy yield per hectare of wheat and beet for ethanol production, RME production and electricity from wood of, taking into account the energy use for planting, fertiliser, transport, etc., expressed as amounts of fossil fuel use avoided. Electricity from wood is favoured in the IEA study, although in the longer term ethanol from beet may give a relatively high energy production per ha as well. Biewinga et al.² give a different ranking. For the Netherlands they obtain high net energy yields for ethanol from wheat and from RME. In the longer term the net energy yields of the selected crops are of the same order of magnitude. The main explanations are the rather pessimistic yield estimates for wood production by means of Short Rotation Coppice (like Willow) and the optimistic assumptions by Biewinga et al. with respect to energy production from straw of wheat and rapeseed (IEA assumes 10 oven dry tonne (odt)/ha.yr for SRC in the current context and 12 odt for the somewhat longer term.²¹ Biewinga et al. mention 8 odt/ha.yr actual yield and 10 odt/ha.yr attainable yield.² The large difference between RME and wheat energy yields is caused especially by the difference in the assumed energy production by utilization of straw mentioned in the two studies. The energy production from straw is assumed to be a factor 2-3 higher in Biewinga et al.²).

Ranney et al. give an overview of the environmental performance of energy crops. General conclusions are that perennial crops show lower environmental impact than annual crops, in terms of nitrogen leaching, use of biocides and N₂O emissions and higher energy ratios.³⁰

Kaltschmitt et al. confirm this general view in a comprehensive LCA study that compares the full life-cycle of a number of crops. Perennials have the highest energy yields and lowest environmental impact. Especially the traditional biofuel options: RME and ethanol production, are less favourable in this respect.²³ Other sources readily confirm the general conclusions that woody crops (SRC) and perennial grasses show in general a better environmental performance than annual crops. Inputs in terms of (fossil) energy, fertilisers and pesticides are on average lower, although the difference in performance between perennial and annual crops varies with the locations selected and the time frame considered.^27,30 Biewinga et al. are in particular less optimistic about Poplar, Willow and Miscanthus, compared to e.g. the IEA²¹, Ranney et al.³⁰, and Kaltschmitt et al.^23,2 The outcomes of this study are less pronounced than the ones discussed before. Hemp (an annual crop) is also identified as a promising energy crop for the more northern European countries.

Furthermore, it should be noted that experience with relatively 'new' crops like Willow and Miscanthus for energy purposes is limited compared to conventional food crops. The improvement potential of SRC and perennials might therefore be larger.

The costs of the biomass are strongly influenced by the location where it is produced. Especially the costs of land and the required (or desired) farmers' incomes are determining cost factors. Another dominating parameter for the costs per tonne of biomass or per GJ of bio-energy is the yield.

Hughes et al. estimate the costs of biomass produced by short rotation forestry delivered on site (without subsidies) to be at 1.6 - 3 ECU/GJ for yields between 12.5-22.5 dry tonne/ha.yr for US conditions. The higher value is given for Eucalyptus on irrigated soil.²⁰ According to Hughes et al. improvements are feasible in every main aspect affecting the costs. Not only will plant material become cheaper but development of clones with higher yields and improved pest resistance, development of planting and harvest techniques seem possible too. Significant cost reductions are therefore considered to be possible. Hughes et al. project costs of 1-1.3 ECU/GJ on the longer term (e.g. 2010) with doubled yields and improved production systems.²⁰

For a country like the Netherlands with higher land costs (rent) and higher farmer incomes than the USA, the projected costs for poplar and Miscanthus for 2000 as presented by Rijk³¹ are 3.6 - 3.9 ECU/GJ (yields of 12-15 ton dry matter/ha.yr). For SRC Willow with a projected yield ranging from 9 to 15 tonne dry matter/ha.yr and no subsidies included, Rijk estimates the cost to be 4.7 - 12.3 ECU/GJ.³¹ The higher figures in the range describe the current costs. These costs are based on very intensive management, high fertiliser and pesticide use and the high cost of the planting material (For comparison: In 1992 the costs of oil, gas and coal amounted 2.9 - 5.4 ECU/GJ, 2.6 ECU/GJ and 1.3 - 1.6 ECU/GJ respectively. The world energy market prices varied between 1.5 - 3.6 ECU/GJ from 1980-1990 for natural gas and between 2 - 4.5 ECU/GJ for oil in the same period.¹¹).

The International Energy Agency has also made a comparison of the costs of bioenergy from various systems. Prices are expressed as ratio compared to conventional fuel costs that they replace. In case of RME this is diesel. Ethanol and methanol replace gasoline (applied in internal combustion engines) and electricity replaces grid electricity (US and EU cases). The results are summarised in table 2. Note that a number of options are excluded from this overview (such as use of pyrolysis for the production of bio-oil and digestion for the production of biogas). IEA calculated these results for the EU and for part of the US situation on the basis of numerous assumptions about yields, costs, etc. The outcomes are determined logically by these assumptions and are therefore more indicative than absolute. The results are however largely confirmed by other studies that compare bioenergy chains in similar ways.^27,40

According to IEA short rotation coppice (wood) gives better results for electricity and methanol production both in terms of net energy ratio and in costs per unit of energy produced, than for RME and ethanol production. Methanol production is not considered a realistic option in the short term since further technology development is still required before methanol can replace petrol as transportation fuel on large scale. It should be noted that when methanol (or hydrogen) is used in fuel-cell powered vehicles, the overall chain efficiency is substantially better than with other biomass-derived liquid fuels used in internal combustion engines.²⁴

It can be concluded that in addition to low cost biomass residues, woody crops (e.g. Short Rotation Forestry) and perennial grasses are particularly suited energy crops because of their good net energy yields, non-intensive production and (expected) minimal environmental impact. Also production costs per GJ may be relatively low compared to annual crops. However, at present biomass as a fuel is in most cases still more expensive than fossil fuels. Therefore, efficient conversion to high value energy carriers (i.e. electricity and transportation fuels) are desired conversion routes. Considering the conversion efficiency, costs and environmental performance it would seem that BIG/CC systems (for electricity production or for combined heat and power production) and systems that produce methanol or hydrogen by means of gasification of woody biomass are likely to be the most favourable ones in the longer term.

At present biomass contributes 10-14% to the world's energy supply.¹⁴ The largest part is used in developing countries (on average one third of the total energy supply). In the industrialised countries the contribution is about 3%. In developing countries most biomass is used as fuelwood. This is often not sustainable since it may contribute to deforestation, erosion and desertification.¹⁷

The present use of fuelwood in developing countries can be categorised as non-commercial use of biomass. Commercial use of biomass for conversion to heat, steam and electricity, is based largely on the utilisation of agricultural and forestry residues. An example is bagasse, a left-over from sugar production from sugar cane, which is used in the sugar industry to produce process steam and electricity. Other examples are wood residues from commercial forestry activities, such as thinnings, straw, waste wood and numerous organic wastes produced in the food & beverage industry.

Of the industrialised countries, particularly the United States, Sweden, Austria and Denmark are currently utilising biomass as a commercial energy carrier. There are also programs to convert surpluses of grain to ethanol in the US. Also in the US, about 7,000 MW_e of biomass fired generation capacity has been installed.²² A considerable land area is used for the production of rapeseed for rapeseed oil or RME (Rapeseed Methyl Ester) in the EU.^12,13 The production of RME and of ethanol from maize and wheat is however subsidised to allow competition with petrol and diesel.

Brazil is probably the world's leader in biomass utilisation. The implementation of the ProAlcool programme has led to the large scale use of sugar cane for ethanol production for automotive applications.³⁹ A brief overview of the extent to which some countries use biomass for energy purposes is given in table 3.

Biomass is expected to play an important role in the world's energy supply in the future according to various energy scenarios. A variety of conversion technologies, such as combustion and digestion, are readily available and are being applied commercially. Substantial experience has been obtained in the handling of biomass as a fuel in various contexts. The resource is largely understood and already available in significant quantities in many countries in the form of biomass residues. The use of low cost residues is in many situations already economically attractive. Further technological developments in crop growing and conversion technology could make bioenergy from dedicated crops economically competitive with fossil fuels in the relatively short term. In many scenarios, therefore, bioenergy is considered to make a large contribution to the world energy system (see table 4). The absolute contribution varies between 94 - 325 EJ (both figures are given for 2050), which is the quivalent of 14% to 46% of the total expected world's energy requirements.

If biomass is to play a major role in the world's energy supply, energy farming is required. However, it is uncertain whether fallow land will be available to produce biomass for energy instead of food. The main stimulus in the EU and US for introducing non-food crops in agriculture is the surplus production of food. World wide food demand is expected to increase considerably. How much food will be required depends on many factors. Population growth, as well as increasing incomes that may result in shifts to more protein-rich diets, both lead to increased demand for food crops. Loss of agricultural land caused by erosion, limited water resources and the increased demand for land for urbanisation, infrastructure and also preservation of nature all limit the possibilities for energy crops.

The World Watch Institute (WWI) is pessimistic about the world's future food supply. The decreasing amount of cropland per capita, non-sustainable use of freshwater reserves and stagnation in the development of crop productivity give rise to this pessimistic view.^5,6,14 Consequently, according to this view a large contribution of cultivated biomass to the world's energy supply must be seen as unrealistic.

However, other studies take the opposite view. A study by Luyten²⁶ on sustainable world food production and environment, indicates that the productivity of the world's agriculture is currently not at its limit. Depending on the type of agriculture used, the world population growth and the demand for food products, the world's potential food production exceeds the potential demand for food by a factor 2-20 (expressed in grain equivalents). Regional shortages may occur however, especially in parts of West and South Africa and in West Asian regions.²⁶ Dyson⁹ roughly confirms the conclusions that the worlds food supply is not in jeopardy. Furthermore, he stresses that it is very difficult to predict the future developments in the world's food supply and demand due to numerous uncertain trends. An example is the development of food production in potentially major cereal exporting regions (such as the former Soviet Union and in Latin America). This development is an uncertain factor but will play a key role in the global supply of food in the future.¹⁵

A study by the Netherlands Scientific Council for Governmental Policy (WRR)³⁸ on the subject of land-use and developments in agriculture in Europe shows that the land area required to meet the demand for food crops in the European Union (EU-12), including exports, can lie between 31-92 Mha, depending on underlying assumptions like productivity increments, environmental standards, economic context and shifts in diet, whereas the current demand is 127 Mha. In this study additional demands for land for preservation and development of nature have been taken into account. The figure of 31-92 Mha also includes the impact of a possible shift towards a more protein-rich diet.²⁸ (When focusing on the potential surplus of agricultural land with respect to energy production, this land surface could represent an annual energy production of 10-26 EJ (assuming an average biomass yield of 15 tonnes dry matter per hectare per year with an energy content of 18 GJ/tonne dm).)

The study by the WRR recognizes that other developments (such as expanding land-use due to shifts to less intensive agricultural production methods) are possible as well. These would reduce the surplus of land. However, developments like modernisation of the main agricultural regions in Eastern Europe (such as Poland, Romania, Bulgaria and the Ukraine) may lead to even larger surpluses in the European Union than at present, since the productivity of agricultural land in those countries is at present much lower than in NW Europe.¹⁵ We conclude that the large-scale introduction of energy farming in the longer term is strongly linked to further modernization of food production.

The availability of land for energy farming however does not depend only on the availability of agricultural land. Degraded land, for instance, which is not really suitable for food crops can represent a large potential for energy farming. At the same time energy crops may help restore these damaged soils. Yields, however, will generally be lower than from good quality land. More experience in this area is needed.

Depending on developments in agriculture and land-use, the potential of biomass as an energy source is believed to be capable of covering at least one third of the world's energy demand by the middle of the next century.

Although the prospects for biomass are good and a variety of factors are encouraging further development of the use of biomass for energy purposes, there are at present a number of barriers that hamper implementation. When a substantial resource base is available, the degree to which biomass is (commercially) utilised for energy purposes differs widely between countries. A number of factors can be distinguished:

Costs: Probably the main barrier to large-scale utilisation of biomass is the fact that in most cases the energy carriers produced are not competitive. The costs are directly related to the performance of both the energy conversion technology and the yield of energy crops. Technology development could reduce the costs of bioenergy, as explained before. In many situations where cheap or negative cost biomass wastes and residues are available, the utilisation of biomass is or could already be competitive. In Sweden and Denmark, where a carbon and energy tax has been introduced, more expensive wood fuels and straw are now utilised on a large scale. However, on a worldwide basis, the commercial production of energy crops on a large scale is almost non-existent. Brazil is a major exception; there subsidies have been introduced to make ethanol from sugar cane competitive with petrol.

The need for efficient, cheap and environmentally sound energy conversion technologies: Strongly related to the cost issue are the availability and the full-scale demonstration of advanced conversion technology that combines a high efficiency with low investment costs. This is essential for competition with fossil fuels when relatively high-cost energy crops are used an energy source. Advances in the combustion and co-combustion of biomass can considerably increase the attractiveness of combustion as a conversion technology. However, the development and the application of the Biomass Integrated Gasifier/Combined Cycle technology has the potential to attain higher conversion efficiency at lower costs. Demonstration and commercialisation of this technology are therefore important. Gasification technology is also of major importance for the production of methanol and hydrogen from biomass. First, commercial units are most likely to be fuelled with cheap biomass wastes and residues to compensate for the initial high investment costs. The availability and costs of these biomass streams are therefore important parameters for the further development of this new technology. The further development is expected to lead to higher efficiencies and lower investment and maintenance costs which will allow for the economically feasible conversion of more expensive (specifically cultivated) biomass fuels.

Required development of dedicated fuel supply systems: Numerous field trails exist for Short Rotation Coppice and perennial grasses. Also much experience has been gained with so-called fibre farms for pulpwood production. Experience with dedicated fuel supply systems based on 'new' energy crops like perennial grasses and SRC, is however very limited compared to man's long experience of cultivating traditional food crops and developing forestry techniques.

Improvement of yields, increased pest resistance, management techniques, reduction of inputs and further development of machinery are all necessary to lower costs and raise productivity. The same is true for harvesting, storage and logistics. Furthermore, varying climatic, soil and socio-economic conditions will make different demands on biomass production systems on the local scale. Specific crop production systems will have to be developed further for various local conditions.

Specific biomass characteristics: Biomass has a low energy density. For comparison: coal has an energy density of 28 GJ/ton, mineral oil of 42 GJ/ton, liquified natural gas of 52 GJ/ton and biomass of 8 GJ per ton of wood (50% moisture content). Because of the relatively large land surfaces required to produce a substantial amount of energy, transportation distances can become a limiting factor, both from an economic and energetic point of view.

Another complicating factor is that biomass production is usually bound to seasons, which complicates the supply and logistics of a total system. Varying weather conditions will affect year-to-year production, as well as the quality of the biomass produced, e.g. the moisture content. The required logistics to fuel a larger biomass conversion system is therefore a complex matter. Storage is required to compensate for variations in supply.

Socio-economic and organizational barriers: The production of crops based on Short Rotation Forestry or perennial grasses is substantially different from the production of conventional food crops. Annual crops provide farmers with a constant cash flow for each hectare of land. In the case of SRC, however, the intervals between harvests can range from 2 to 10 years. Consequently, the flexibility of farmers to shift from one crop to another is restricted since the production of SRC or perennial grasses is more economical when it continues over a longer period. Furthermore, pests and water stress might be a disturbing factor in the desired continuous fuel supply, although this may be partly overcome by diversifying the fuel supply.

Bioenergy systems are complex in terms of organization and the number of actors that can be involved in a total energy system. The biomass is most likely to be produced by farmers or foresters. Transport and storage are likely to be the responsibility of another party, whereas utilities may be responsible for the energy production. The combination of the utilities on the one hand and the agricultural system on the other will create a number of non-technical barriers.

Public acceptability: Large-scale introduction of bio-energy will influence land-use, landscape and energy systems. Since biomass energy systems require substantial land areas if they are to make a significant contribution to the total energy supply, the related changes in land-use, crops and landscape might lead to public resistance. Increased local transport in biomass production areas might also be experienced as a negative aspect.¹

Ecological aspects: Potential ecological constraints will also be important. Insight into the effects on landscape and biodiversity of large-scale energy farming is very limited. Energy crops will have to fit into the landscape both ecologically as well as aesthetically. A good understanding of the ecological and environmental impacts of large-scale production and utilisation of energy crops is essential for public acceptability. Besides minimising the environmental impact of biomass energy systems, major attention should be paid to fitting biomass production into existing agricultural systems. In this context the effects on biodiversity of large-scale biomass production systems are important points to watch.

Competition for land-use: As discussed in section 3.3, competition for land or various land claims may turn out to be a limitation in various regions in the world. Whether or not land availability will become a serious bottleneck for the production of energy crops will strongly depend on developments and rationalisation in agriculture worldwide. Opinions differ regarding the extent to which (agricultural) land will become available for energy crops. However, it is an accepted principle that biomass production for energy purposes should not conflict with food production.

This thesis focuses on a number of aspects relating to the utilisation of cultivated biomass as well as biomass wastes and residues for energy purposes. The general objective of this work can be described as follows:

In the short term biomass residues and wastes are the only available biomass resources since energy farming is not yet a commerical activity. The potential of biomass wastes, residues and for energy crops should therefore be known.

As explained, BIG/CC technology may allow for cheap and efficient power generation from biomass. However, since the technology is not commercially applied yet, first plants are expected to be expensive. Biomass residues and wastes may be available at low or even negative costs and can compensate for the expectedly high investment costs. In this case however, the performance of BIG/CC systems which are fired with biomass materials with strongly varying properties needs to be evaluated.

Application of BIG/CC technology to waste treatment may be an interesting option. However, other technologies are emerging or are being improved. Therefore, a more detailed analysis is desired of which waste treatment systems will be best in the longer term. Such an analysis is also useful for assessing the potential energy production from waste and its related costs.

In the somewhat longer term, energy farming can be considered. However, the use of land for energy crops may conflict with land-use for agriculture, nature and urban development. The land potential for biomass production therefore needs to be evaluated.

Energy production from cultivated biomass is generally more expensive than from fossil fuels. However, various costs and benefits are not expressed in the costs of bioenergy and fossil fuels (external costs). An evaluation of the external effects of bioenergy systems is therefore required. This may create a more level playing field for energy from biomass. The following more specific research objectives are therefore formulated for this thesis:

Chapter II deals with the characteristics and current availability of biomass residues and waste streams in the Dutch context and evaluates to what extent they are suited for conversion to energy, especially by means of gasification.

The approach starts with an analysis of all potentially suitable streams for gasification. Costs are evaluated on the basis of current waste treatment costs or observed market prices for residues. Results of characterisation experiments will be used to determine the composition and degree of contamination of the waste. Conclusions are drawn with respect to the energy potential, supply, costs and composition of the analysed wastes and residues. In addition, conclusions are drawn about the technical limitations of gasification for electricity production of biomass wastes and residues in relation to their characteristics.

In Chapter III in the technical and economic aspects of gasification of both wastes and clean biomass for electricity production are investigated. Focus is on the potential of this option in the short term. Therefore the analysis will be restricted to Atmospheric Circulating Fluidized Bed gasifier technology, integrated with a Combined Cycle. The technical and economic feasibility of this option will be discussed, as well as environmental aspects involved in the Dutch context.

The performance of the system is evaluated by means of ASPEN^plus modelling. Performance is simulated for a wide range of potential biofuels to assess the sensitivity of the system to the fuel composition. An economic evaluation is made based on component data and on a chain analysis that includes the costs of the biofuels and logistics.

Chapter IV evaluates the final waste treatment system in the Netherlands. It investigates to what extent changes in waste production and the implementation of new waste treatment technologies can affect the energy production and final waste treatment costs. An assessment of advanced present and new waste treatment technologies will be made. On the basis of a future projection of the waste supply, the potential of different waste treatment systems will be evaluated. The possibilities to improve energy recovery and to reduce the waste treatment costs are given particular attention in this evaluation. On the basis of the results, conclusions are drawn about the potential role of biomass wastes and residues in the Dutch energy system and about the technologies that are most attractive for realising this potential.

Chapter V focuses on longer term developments in land-use in the Netherlands. Expected future demand for various purposes (like housing, infrastructure and nature) is evaluated as well as the possible development in land supply (especially from agriculture). Conclusions will be drawn regarding the potential land-base for energy farming in the Netherlands as well as regarding the production of biomass residues that are related to the type of land-use. Thereafter, the potential role of biomass residues and cultivated biomass for the Dutch energy system is discussed.

Chapter VI addresses the question of costs and benefits, of the biomass fuel cycle and focuses especially on the external costs of biomass-based electricity production. In addition a comparison is made with coal-based electricity production. Various methods are used to quantify these costs. Both environmental externalities (such as emissions) and indirect socio-economic effects are analysed. Attention will be given to uncertainties in the outcomes and the implications of the results for the economic feasibility of the production of electricity from biomass in the Dutch context.

The results are summarised at the end of this thesis and some general conclusions are drawn.

Last update: August 20, 1997
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