ABSTRACT
We all have in our digestion system more than a pound of bacteria-we could not live without them. A professor at Collège de France in Paris explained to us the other day: “Take a teaspoon filled with simple ground from your garden; it contains a billion of bacteria and other creatures.” 2.2 Photosynthesis and Production YieldsProfessor Melvin Calvin from Harvard put it like this: “An ideal solar collector has already been designed. Requiring virtually no maintenance, it is economical and non-polluting; it uses an established technology and stores energy. It is called a plant.” Photosynthesis can be described by the following equation [3]: 6CO2 + 6H2O + Sunlight = C6H12O6 + 6O2 The actual complexity of photosynthesis in a simple leaf is such that even today it is not completely understood. A minimum of eight photons are required simultaneously for the process. The decomposition of water and the resulting liberation of O2 through photosynthesis were experimentally established by Ruben and Kamen in California only in 1941. If oxygen were not permanently reproduced by the living cover of the earth’s plants, in about 3000 years all of it in the atmosphere would have been consumed [4]. Globally photosynthesis produces an estimated 220 bn (1 billion = 1000 million) dry tons of biomass per year, equivalent in energy value to 10 times global energy use. The maximum photosynthetic efficiency is 6.7% [3]. In comparison to PV this is poor. Today a commercial PV panel has up to 25% light conversion efficiency. A fuel, hydrogen, can be generated by subsequent electrolysis at an overall efficiency of 20%. But hydrogen is not fit for feeding animals and us. For life on the earth nature has done the right choice, carbohydrates produced by photosynthesis! The maximum efficiency on the ground is achieved with “C4 plants.” Their name comes from the first product in the photosynthesis process that is a four-carbon sugar. Examples are maize, sorghum, and sugarcane. They are a lot more productive than
“C3 plants.” The latter come as trees, rice, wheat, and soybean and account for 95% of the global plant coverage. The maximum efficiency of C3 plants as compared to C4 plants is given as 3.3% only. This has to do with extra losses through photorespiration and light saturation at lower light intensities. In practice maximum biomass production rates in dry tons per hectare per year, taking into account locally available irradiation from the sun are estimated by the authors in [3] as follows: 136 tons/ha.year (tons per hectare.year) in England for C4 plants and half of this for C3 plants. For the US farm belt they calculate respectively 208 tons/ha.year for C4 plants and half of it for C3. Note that these are theoretical maxima efficiencies. In practice as we shall see later we will be talking of 10 to 30 tons/ha.year. 2.3 Biomass ResourcesFirst come to mind the residues of biomass. The energy content of residues from forestry and agricultural activity, including dung, amounted in 1985 globally to 111 EJ/year1, which corresponded to one-third of all commercial energy use at that time [3]. Residues amounting to approximately 30% to 50% of agricultural and forest production can be estimated globally for over 5000 million tons of that production. In Europe the amount of straw from cereal harvest comes in the range of 100 million tons/year. Part of it is of interest for energy production. Residues from the production of wood from forestry and its use are considerable. They are estimated at more than 60% of harvested raw wood. The Global BioEnergy Partnership (GBEP) in Rome estimates that 65% of the world’s RE is of woody origin. It is for most part derived from such residues. In the 1990s the potential of technically recoverable residues for energy purposes in the European Union (EU) was estimated at almost 10% of energy consumption [5]. The actual production of energy from solid biomass was identified for the EU in 2013 as 82.3 Mtoe [6]. That is in the range predicted in 1990. Most of that solid biomass comes from residues. There is also an EU directive on biowaste or municipal wastes (2008/98/CE). It concerns wastes such as organic wastes from gardens and parks, kitchen waste, 1EJ (Exa Joule) = 1000 Peta (10 to the 15th) Joule = 23.9 Mtoe = 278 TWhth
etc. The EU has produced 7.7 Mtoe from such wastes in 2009 [7]. Separately one has to take into account wastewater, sludge, etc. They are an important feedstock for biogas of which the EU has produced 8.3 Mtoe in 2009. In terms of biomass crops, 4100 million ha of forestry existed globally in 1987 compared to an agricultural cropland of 1480 million ha [3]. The forestry on stock was 417 000 million m3, or approximately 200,000 million tons of living material: an incredibly high amount. The annual increment is of 12,000 million m3. A closer look shows that globally the wood cover is shrinking. This phenomenon is badly affecting the LDCs, whilst it is typical for the Organisation for Economic Co-operation and Development (OECD) countries like the United States, Germany, France, etc., to harvest only less than half of the annual forest increment: in the industrialized countries a lot of CO2 is currently fixed in the growing forests, thus helping to alleviate climate change. From 1990 to 1995, 56 million ha of forests have disappeared worldwide but in the industrialized countries the forest coverage increased by 9 million ha [8]. Just in 2012, Germany has planted 350 million trees and Poland more than 1000 million: Europeans like their trees! Nevertheless, the conclusion is, conventional forestry is underexploited in the industrial countries and offers itself as a sizable resource for new energy supply. Moreover we got the large potential for energy plantations. Short-rotation forestry (SRF) and plantations of grasses, switch grass, Miscanthus, and the like and dedicated crops like sorghum and others are suitable contenders for woody feedstock generation. A lot of experience has been accumulated with them over recent years. A good example is Brazil. The country lacks hard coal reserves and produces since many years charcoal for its industrial needs, for instance, in the steel industry. Brazil produces 6.9 million tons of charcoal per year via SRF plantations employing eucalyptus trees [2]. The growing time for eucalyptus SRF in Brazil is two to three years. In temperate climates one employs growing times for SRF trees of up to seven years. Energy plantations raise in general terms the question of excessive extraction of nutrients from the ground. This should be avoided at least to some extent by leaving in the field during harvest the leaves and other plant material that is of not much interest
for energy use. It will also help to bring the ashes that contain the extracted minerals back to the fields after combustion of the biomass. With respect to the global energy crop potential on a large scale one has first to address the question of land availability for new plantations. A first opportunity is marginal land. Such land may have been discarded for the production of food or animal feed because of its lower productivity. But for energy plantations the quality criteria aren’t as high. Thanks to a new mobilization for energy crops such marginal land will gain in biological quality and economic value. A more difficult question is the conversion of high-quality agricultural land to the generation of energy instead of foodstuff or animal feed. Again, as in many cases when addressing the global biomass issue, one has to distinguish between the case of the industrialized countries and that of the developing world. They are most different from each other. In the OECD countries we notice-despite immigration of the poor, Latin American (LA) people to the US, Africans to the EU-that the number of inhabitants is by and large stable and food consumption stagnates. On the other hand, productivity of food and feed production on domestic land increased continuously over many years in the past by 1% to 2% per annum. And it keeps increasing. Europe had to deal with “milk lakes,” “meat mountains,” and other descriptions of the economic nonsense to produce an excess of high-value food stuff for which there is no market. Long ago already, the EU Commission as the main player of the common agricultural policy (CAP) had to address the problem. Policies changed over the years, but always well sticking to the principle that Europe must stay autonomous for its food supply and its farmers must be protected. Almost at any cost. EU directives introduced early on the principle “Set Aside” land, sometimes obligatory, sometimes on a voluntary basis, allowing farmers even to produce alternatives to food and feed such as biomass. In more recent years the trend went toward direct payouts to farmers for not using their land for agricultural crops with additional obligations to maintain its biological value. Already in 1992 an interservice group of the EU Commission bringing together nine of its directorate generals (DGs) competent on the matter met on my initiative and produced a report on its deliberations [9]. One of the findings was that the Set Aside regime
of 15% for all arable crop producers was available for nonfood purposes. On the horizon of 2025, a quarter of Europe’s agricultural land is becoming available for energy plantations with an even higher proportion beyond. In the United States one-third of the cropland was already kept idle in 1990 [3] for the same reasons described above. There is a wide variety of suitable species for the generation of woody material on marginal and converted arable land. In Ref. [10] 90 different species are reviewed. Choices depend on the local climate, the final market destination and the expected production cost, transformation, and logistical considerations. Harvesting should not exceed half of total production cost. Hauling over 40 km is considered uneconomical. The energy ratio, that is, the energy input for the establishment, fertilizers, herbicides, harvesting, and hauling as a fraction of harvested energy is highly positive as it lies typically between 10 and 15. One of the best yields of up to 65 dry tons/ha.year has been achieved with eucalyptus in Brazil. Sorghum can yield in temperate climates 30 dry tons/ha.year; hybrid poplar, and switch grass can yield 15 dry tons/ha.year; and salix up to 12 dry tons/ha.year. Ultimate production costs are dependent on the details of implementation. In general terms, when products come as pellets and chips on the market, they are currently considerably cheaper than the competing electricity and oil and gas from fossil sources. So far we have addressed energy plantations on land. Water is also an important potential supplier of biomass that was early recognized when interest in modern bioenergy was arising in the 1970s. A plant of choice are the algae. They have a high conversion efficiency of the sun’s light and come in a wide variety of types and sizes that grow in fresh or saline waters. Microalgae grow with some of the best efficiencies. Development of this route has, however, not led to commercial results. One of the reasons is that harvesting and drying of the material is cumbersome and eventual costs are prohibitively high. Energy plantations of algae in special ponds turned out to get infected from undesirable pollutants in the open air. Protection under glass cover generates material of too high cost for energy purposes, but of potential interest for the chemical of pharmaceutical industry.
