Bioenergy is expected to become one of the key energy-resources for global sustainable development. Biomass maintained adequately is renewable and free from net CO₂ emissions. However, the annual amount of available bioenergy cannot be infinite, since the land area available for biomass production is limited and a certain amount of biomass production must be reserved for the required food and materials. Bioenergy production will be limited more strongly when the growths of the population and economy in the world cause the growth of biomass demand for food and materials in the future. On the other hand, bioenergy can be produced not only from bioenergy plantations, which occupy land but also from biomass residues (such as straw, animal dung and wood scrap) which do not occupy land directly. These biomass residues are discharged from various processes in the biomass flow from harvest to consumption.

The purpose of this study is to evaluate the global bioenergy potential comprehensively, considering energy potentials of both bioenergy plantations and biomass residues. For this purpose, we have developed a global land-use and energy model (GLUE) including the mechanisms of land-use competition and overall biomass flow.

The details of the model are explained in Ref. [1].

1. Modeling technique: The model is described by the SD (system dynamics) technique, which is adequate to describe the stock and flow of biomass explicitly.

2. Simulation period: The time scope of GLUE is 125 years from 1975 to 2100, with 1-year time steps.

3. Regions in the model: The world is divided into two regions: a developed region and a developing region. The developed region comprises OECD countries (excluding Turkey, Mexico, and Korea), former USSR, Eastern Europe, Israel, and South Africa; the developing region includes all the other countries.

4. Structure of the model The model (GLUE) consists of a land-use sub-model and an energy sub-model.

5. Land-use sub-model: This, modified from the sub-model reported in Ref. [8], considers a food sector and a wood sector [1, 2]. The sub-model includes land use competition among various uses of biomass applications such as paper, timber, food, feed, and energy. The sub-model covers a wide range of land uses and biomass flow including food chains from feed to meat, paper recycling, and discharge of biomass residues.

6. Energy sub-model: This was developed following the basic structure of the Edmonds–Reilly model. The sub-model includes a module chemical flow, in order to evaluate the energy potential of chemical-products scrap. The latter is considered to have a worthwhile energy potential in municipal wastes.

7. Relationship between these two sub-models: The supply of modern bioenergy calculated in the land-use sub-model is substituted for demand of coal in the energy sub-model.

8. Non-commercial energy: The energy sub-model handles only commercial energy. The land use sub-model handles not only commercial energy including modern bioenergy, but also non-commercial energy including traditional bioenergy.

We determined a reference case for the GLUE based on data of the FAO (Food and Agriculture Organization of the United Nations) and base scenarios of the World Bank, IPCC, and Ref. [15]. The area of arable land in the developing region will double during the period from 1990 to 2100, which is based on the RIGES (Renewable-Intensive Global Energy Scenario) in Ref. [16]. The data set of the reference case is shown in Table 1, and discharge rates of biomass residues and practical energy-usable rates are shown in Table 2.

In this study, we use a low heat value (LHV) for the biomass. The weight of the biomass means the air-dry biomass weight (20% water content) unless specified otherwise; heat value of biomass is 15 GJ/air-dry ton of biomass; and carbon (C) content of biomass is 0.50 ton per ‘bone’-dry ton of biomass.

We consider the simulation results of the reference case-study as calculated via GLUE.

1. Biomass balance-table: We arranged the simulation results in the form of a biomass-balance table (BBT). The BBT is a unified framework of various biomass statistics showing the biomass flows quantitatively and analyzing bioenergy potential.

Because of the limitations of space, we show only two BBTs (namely for wood biomass and food biomass) for the developing region in 2100. In the BBT, horizontal items mean kinds of biomass. Vertical items mean biomass-utilization processes, and the two vertical items from the bottom mean ‘ultimate bioenergy potential’ and ‘practical bioenergy potential’ respectively Positive values mean the production or import of biomass; negative values mean the consumption or export of biomass; and hatched values are subtotals in the Table.

As shown in Table 3 and Table 4, the developing region in A.D. 2100 will harvest 407 EJ/yr of primary biomass (148 EJ/yr of primary wood and 259 EJ/yr of primary food) and consume 133 EJ/yr of secondary biomass (37 EJ/yr of secondary wood, 44 EJ/yr of secondary food, and 52 EJ/yr of traditional bioenergy) The amount of total primary biomass supply (407 EJ/yr) exceeds the total primary energy supply (330 EJ/yr) in the world in 1995. In addition, the primary biomass demand for wood, food, and the total amount will increase five times, twice, and twice respectively between 1990 and 2100 in the developing region, because both the biomass demand per capita and the population increase.

2. Ultimate bioenergy-potential: We show ‘ultimate bioenergy potential’ in the developed region and the developing region in Fig. 4 and Fig. 5 respectively; the information being summarised in Table 5.

Fig. 4 shows that, in the developed region, the potential of energy crops produced on surplus arable land will be large (100 EJ/yr) in 2100. This is because it is assumed that the food demand will be stable and the productivity of arable land will increase. The potential for energy crops in the world will reach 154 EJ/yr in 2100. The potential is sensitive to parameters concerning food supply and demand. For example, the potential in 2100 will reduce by half if animal food demand per capita in the developing regions is 25% larger than that in the reference case.

On the other hand, there will be a large energy potential for biomass residues in the developing regions. The ultimate energy potential of biomass residues (223 EJ/yr) will account for 80% of all the bioenergy potential (277 EJ/yr) in the developing regions in 2100.