Twists and Turns around Biofuels

by Gerhard Kreysa

  Twists and Turns around Biofuels

… with its linked long-term reservoirs. The figure shows the carbon content of the reservoirs, reversible flows and uni-directional flows (italics), and the derived residence times for CO2 in each compartment and in the whole cycle (a: year).

 

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Twists and Turns around Biofuels

If we leave the superficially-correct economic delibera­tions out of consideration for the moment, then it would ac­tually be preferable if we were facing immediate exhaustion of our fossil raw materials. The greater the de facto reserves, the more irrational is their complete exploitation, since their combustion will shift the earth’s atmosphere back to a state which it once had, long before the appearance of human beings, and for which there is not the least evidence that it would be compatible with our continued existence. Figure 1 shows the carbon cycle in a somewhat unusual depiction, which is limited to those components that together can be termed the fast carbon cycle’. The numbers there for the carbon content of the reservoirs and its rate of flow be­tween them were adopted from the IPCC [6]. The net flows between the components atmosphere, land, and surface oceans (down to a depth of ca. 700 meters) are in the range of 1 Gt of C/a and demonstrate that equilibrium has not been established between these reservoirs. The most im­portant perturbing factor is the anthropogenic input of around 8 Gt C/a into the overall cycle due to the combus­tion of fossil raw materials.

In contrast to the unidirectional net flows, there is a very rapid, bidirectional CO2 exchange between the at­mosphere and the land as well as the surface oceans. Con­siderations similar to those applied to reaction kinetics in­dicate that the residence times in these coupled reservoirs are quite short and are in the range of 3.6 to 18.7 years.

It is much more enlightening, however, to look at the overall cycle instead of considering the individual reser­voirs. For the residence time of carbon in this cycle as a whole, the net drain of 1.8 Gt C into the deep oceans is rel­evant, and leads to a value of 2190 years. The significance of this statement becomes apparent through a comparison with nuclear power. For the direct final storage of spent nu­clear fuel elements, the lifetime of the radiotoxicity is over

100,0 years. If, however, the fuel elements are re­
processed and their long-lived plutonium is recycled back to the power reactors, this lifetime is reduced to around

1,0 years [7]. This comparison shows that waste dispos­al of anthropogenic CO2 confronts humanity with similar problems to those from the use of nuclear power. Wolf Hafele, a former chairman of the board of the Julich Re­search Center, often spoke even 20 years ago of the prob­lem of fossil waste disposal’, to point up this analogy. In re­cent years, several model calculations have been published [8-10], and they all confirm the extremely slow natural de­cay constant for CO2 which has been introduced into the cycle.

The anthropogenic fossil input amounts to only 7 % of the exchange flow between the atmosphere and the land. The value 120 Gt C/a corresponds to the annual photo­synthesis activity through assimilation. The equally large backflow is due to roughly equal parts of respiration and microbial decomposition [11]. The fact that photosynthe­sis exceeds the anthropogenic CO2 production 19-fold has for decades been a strong motivation for substituting fossil energy carriers by raw materials from the biomass, since the latter are produced by growing plants. Liquid biofuels seem especially attractive, offering a similarly high energy densi­ty to that of the fossil fuels, and requiring no essential mod­ifications of the infrastructure for motor transportation. Brazil took on the role of trailblazer in this area more than 20 years ago with its use of bioethanol. In 2007, the frac­tion of biofuels used in Germany as primary motor fuels al­ready amounted to 7.3 %, of which three-fourths consisted of biodiesel [1].

In Table 1, the various types of biofuels, their chemical constituents, their raw materials, conversion processes and their development status are shown comparatively. Only the fuels of the first generation are at present technically available on a large, industrial scale. Worldwide, bioethanol and biodiesel are quantitatively predominant. Now that the

Подпись:Подпись: FIG. 2 THE RELATIVE GREENHOUSE EFFECT OF BIOFUELS Подпись:initial euphoria has evaporated, the experts are largely in agreement [12-16] that the biofuels of the first generation can at best be seen as an interim solution, particularly in Eu­rope, and for a variety of reasons cannot be considered sus­tainable.

The destruction of rain forests to obtain land for culti­vating oil plants, which are for the most part then used to produce biofuels, is particularly absurd and reprehensible. This fuel is even exported to Europe! Here, again, a detailed examination of the carbon cycle makes the danger appar­ent: of the roughly 2,260 Gt of carbon stored on the land surface of the earth, only about one-fourth is in the plants; the remainder is mainly stored in humus near ground level [11]. When the forest is cleared, this humus is for the most part converted rapidly and irreversibly back to CO2. Since the later biomass-biofuel cycle is at best neutral with re­spect to CO2, the net result is an extensive shift of carbon from the land into the atmosphere and the oceans.

Hopes are thus currently focused on the second-gener­ation biofuels. The most technically advanced is the total gasification of biomass and subsequent synthesis of liquid fuels using the Fischer-Tropsch process. Recently, clear-cut progress has also been made in the fermentative production of bio-alcohols from cellulose and hemicellulose. The di­gestive pulping of lignocellulose is proving to be more dif­ficult, and it is the subject of intensive research.