Outlook for advanced biofuels
Introduction
Bioenergy is seen as one of the key options to mitigate greenhouse gas emissions and to substitute fossil fuels (Hall et al., 1993; Goldemberg, 2000). Large-scale introduction of biomass energy could contribute to sustainable development on several fronts, environmentally, socially, and economic (Ravindranath and Hall, 1995; Turkenburg, 2000; van den Broek, 2000).
Abundant biomass resources are available in most parts of the world (Rogner, 2000). The present bioenergy use covers 9–14% of the global demand (of about 400 EJ in 1998), most of which as traditional, low-tech and inefficient cooking and heating in developing countries (Hall et al., 1993; Turkenburg, 2000). Modern production of energy carriers from biomass (heat, electricity and fuels for transportation or ‘biofuels’)2 contributes a lower, but significant 7 EJ (Turkenburg, 2000). Different global energy scenario studies indicate that in this century biomass may contribute much more: up to 30% of the 2100 energy supply to biomass (Intergovernmental Panel on Climate Change, 2000), an average 50–250 EJ/yr in 2060 (2003); a global (technical) potential of primary biomass in 2050 of 33–1135 EJ/yr (Hoogwijk et al., 2003a), depending on population growth and food demand (diet), economic development, food production efficiency, energy crop productivity on various land types, competing biomaterial products, and land use choices. This range may be narrowed to 300–675 EJ or 40–60% of the energy demand in 2050, which could be produced on 4–10% of the terrestrial surface (Hoogwijk et al., 2003c). Large-scale bioenergy production also includes environmental, social and economic risks. They have not been evaluated here, but deserve attention in discussions about sustainability of bioenergy.
Biomass is currently almost exclusively used for the generation of heat and power. Only some 0.56 EJ of the biofuels are produced worldwide, largely ethanol from sugar/starch and a small amount of biodiesel from oil crops (His, 2004), of which 62 PJ in the European Union (Eur’Obser’ER, 2004). Also, the possibilities for electricity and heat production from biomass (esp. combustion) are well known (Faaij, 1997; van den Broek, 2000) and widely applied in various markets. But, while there are many other new or renewable technologies emerging for large-scale electricity production with low or no carbon emissions, the transportation sector, which is almost entirely based on fossil oil use, has fewer alternatives.
Transportation represents about 27% of the world's secondary energy consumption (21% of primary) and is almost exclusively fuelled by mineral oil. The share may increase to 29–32% in 2050 (Intergovernmental Panel on Climate Change, 2000; IMAGE-team, 2001; EIA, 2003). The rapidly increasing demand for transportation fuels is combined with rapidly decreasing mineral oil reserves of non-OPEC states. This increases dependency on a limited number of oil-providing countries (Rogner, 2000), with inherent risks for energy security and sudden price distortions.
Biofuels can play an important role in addressing both the greenhouse gas emissions of transport and the dependency on mineral oil.
A few main routes can be distinguished to produce biofuels: extraction of vegetable oils, fermentation of sugars to alcohol, gasification and chemical synthesis, and direct liquefaction. Many eventual fuels are conceivable: methanol, ethanol, hydrogen, synthetic diesel, biodiesel, and bio oil (Fig. 1). All have very different properties.
With the exception of sugar cane ethanol, the ‘short-term’ (van den Broek et al., 2003) or ‘traditional’ (Turkenburg, 2000) biofuels have a number of severe disadvantages that are related to the feedstock. The current costs of rapeseed biodiesel and ethanol from cereals or beets are much higher than the costs of gasoline and diesel. Substantial subsidies are needed to make them competitive. These high costs are a result of the low net energy yield of most annual crops (100–200 GJ/ha yr in the long-term), the high-quality (valuable) agricultural land required, and the intensive management. The lower productivity per hectare and high fertilizer requirement also limit the well-to-wheel reduction of fossil energy use, and the environmental benefits are also limited (Ranney and Mann, 1994; Van Zeijts et al., 1994; van den Broek, 2000; van Thuijl, 2002; Berndes et al., 2003).
The net energy yield of perennial crops (220–550 GJ/ha yr), grasses (220–260) and sugar cane (400–500) is much higher. These crops can be grown on less valuable land (Rogner, 2000). Compared with sugar, starch, and oil crops, the application of lignocellulosic biomass (e.g. wood and grasses) is more favourable and gives better economic prospects to the future of biofuels. Also, more types of feedstock are in principle suitable to produce a broader range of fuels than when applying traditional biofuels feedstock.
Higher overall (production, distribution and use) energy conversion efficiencies and lower overall costs are the key criteria for selecting biofuels for the longer term. Various options are considered/developed that have good potentials. Key examples are ethanol produced from lignocellulosic biomass, synthetic diesel via Fischer—Tropsch (FT), methanol, and hydrogen (Katofsky, 1993; Williams et al., 1995; Arthur, 1999; Turkenburg, 2000). But the research, development and demonstration status of all these fuels vary considerably. These fuels are selected for a detailed analysis of their long-term perspectives and RD&D needs.
Methanol, hydrogen and FT diesel can be produced from biomass via gasification. Several routes involving conventional, commercial, or advanced technologies under development are possible. Fig. 2 pictures a generic conversion flowsheet for this category of processes. A train of processes to convert biomass to required gas specifications precedes the methanol or FT reactor, or hydrogen separation. The gasifier produces syngas, a mixture of CO and H2, and few other compounds. The syngas then undergoes a series of chemical reactions. The equipment downstream of the gasifier for conversion to H2, methanol or FT diesel is the same as that used to make these products from natural gas (Williams et al., 1995), except for the gas cleaning train. A gas turbine or boiler, and a steam turbine optionally employ the unconverted gas for electricity co-production.
Ethanol, instead, is produced via (largely) biochemical processes (Fig. 3). Biomass is generally pretreated by mechanical and physical actions (steam) to clean and size the biomass, and destroy its cell structure to make it more accessible to further chemical or biological treatment. Also, the lignin part of the biomass is removed, and the hemicellulose is hydrolysed (saccharified) to monomeric and oligomeric sugars. The cellulose can then be hydrolysed to glucose. The sugars are fermented to ethanol, which is to be purified and dehydrated. Two pathways are possible towards future processes: a continuing consolidation of hydrolysis-fermentation reactions in fewer reactor vessels and with fewer micro-organisms, or an optimisation of separate reactions. As only the cellulose and hemicellulose can be used in the process, the lignin is used for power production.
Several studies exist that provide an overview of (part of) the biofuels field (Van Zeijts et al., 1994; Arthur, 1999; van den Broek et al., 2003). Other studies present the techno-economic performance of individual biofuels (Katofsky, 1993; Williams et al., 1995; De Jager et al., 1998; Wooley et al., 1999). However, often these studies incorporate only small biomass input scales (authors assume that large scale is a priori not feasible) and existing technologies. The potential for a better performance that could be obtained by applying improved or new (non-commercial) technologies, combined fuel and power production, and increasing scale giving higher efficiencies and lower unit capital costs, has not exhaustively been explored.
Another problem is the comparability of the results. The capital analysis for biofuels producing facilities has been done in different ways. The data quality is very variable. Also, the level of detail in analyses varies enormously: from very superficial, to thorough plant analysis. In either case the influence of individual parameters (e.g. feedstock costs) on the final product price is unclear.
This study is for a large part based on our earlier reported techno-economic studies on the production of methanol and hydrogen (Hamelinck and Faaij, 2002), FT diesel (Tijmensen et al., 2002; Hamelinck et al., 2004a) and ethanol (Hamelinck et al., 2005), and on the long-distance transport of biomass (Hamelinck et al., 2004b), which have resulted in a PhD thesis (Hamelinck, 2004).
The central questions are as follows: Which of the biofuel options have the better potential for the short-term and which have the best long-term (2030) prospects? And what developments are necessary to improve the performance of advanced biofuels production and use?
To answer these questions the short- and long-term technological and economic performance of biofuels are analysed and compared. This present study summarises and normalises results for the four selected fuels (methanol, ethanol, hydrogen, and synthetic diesel) from the named earlier studies, compares their well-to-wheel performance, and indicates the key factors influencing that performance. First, the key technologies for the production of these fuels, such as gasification, gas processing, synthesis, hydrolysis, and fermentation, and their improvement options are studied and modelled. Then, the production facility's technological and economic performance is analysed, applying variations in technology and scale. Finally, major biofuels chains (including distribution to cars, and end-use) are compared on an equal economic basis, such as costs per kilometre driven. The results are compared with the reviewed performance of classic biofuels such as rapeseed biodiesel and sugar/starch ethanol.
It is assessed which factors influence the fuel production and fuel chain's performance most, and which aspects are most uncertain. This gives insights both in the possible barriers to implementation that need to be overcome, and in the technological improvement options that should be stimulated by research development and demonstration.
Section snippets
Modelling mass and energy balances
For analysing the production of methanol, hydrogen and FT diesel, Aspen Plus (Aspen Technology Inc., 2003) flowsheet models were made and used for optimisation purposes. The gasifier, reformer and gas turbine deliver heat, whereas the dryer, gasifier, reformer, and water gas shift reactor require steam. The supply and demand of heat (taking into account steam conditions) is added to or drawn from the steam turbine, such that the surplus heat is turned into electricity.
Ethanol production was for
Gasification-based fuels
The findings of the previously published papers can be summarised as follows: gasification-based fuel production systems that apply pressurised gasifiers have higher joint fuel and electricity energy conversion efficiencies than atmospheric gasifier-based systems. The total efficiency is also higher for once-through configurations, than for recycling configurations that aim at maximising fuel output. This effect is strongest for FT production, where (costly) syngas recycling not only introduces
Main conclusions
Biomass could play a large and important role in a future sustainable energy supply as a source for modern energy carriers as electricity and transportation fuels. Especially the introduction of biofuels is attractive because it is one of very few options for low CO2 emission transport systems against (eventually) reasonable costs, and because it decreases or spreads fuel dependency. Of the many conceivable biofuels, fuels from lignocellulose biomass are the most attractive, because they allow
Acknowledgements
The PhD research (Hamelinck, 2004) underlying this article was made possible by financial help from the National Research Programme on Global Air Pollution and Climate Change (NOP-MLK), the Technology foundation (STW), the Cooperation for Sustainable Energy (SDE), Shell Global Solutions, Netherlands Agency for Energy and the Environment (Novem), and Essent. The author is much indebted to professor Wim Turkenburg (promoter) for detailed commenting on draft papers, and to many other colleagues
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