Solid Fuels
2.2 Renewable Solid Fuels
2.2.1 Potential and Current Utilisation
2.2.1.1 Biomass from Farming and Forestry By-Products of Farming
Residuals and by-products from farming can be used as fuels for power production.
Straw is obtained as a by-product in the production of cereals. In sugar production from sugar cane, bagasse is a by-product which is widely used as a fuel, as are pressing residues arising in the production of vegetable oils, if they do not have a use as food supplement in the feeding of livestock.
In Germany, in terms of farming residuals as fuels, straw is essentially the only one. The straw yield can be estimated from the data on the area under cultivation and the straw obtained from the respective cereal type. The amount is about 46 million tonnes/year (Schneider et al. 2007). Of the gross yield of straw, however, only a fraction can be exploited for energy purposes – the fraction that remains after farming uses has been exhausted. These uses include the ploughing of the straw into the soil to improve the soil structure and/or for the formation of humus and using it as litter or forage for livestock (Kaltschmitt et al. 2006). Based on the assumption that only about a fifth of the straw is usable as an energy carrier, the result is an energy potential of about 130 PJ/year or 4.4 million TCE/year, corresponding to a fraction of the primary energy consumption of 0.9%.
2.2 Renewable Solid Fuels 31 By compiling worldwide data on the fractions of herbaceous residual matter and by-products which can be used as fuels, and taking into account the relevant restric- tions, it can be estimated that there is a global technical potential of about 17,000 PJ/year (580 million TCE). The biggest energy potentials in this context are found in Asia. In Europe, straws from cereals, rape and maize arise in farming. Cereals, with a cultivated area of about 33 million ha, are the most significant of these. Assuming a 20% utilisation of the straw produced, the technical potential of straw amounts to about 485 PJ/year in the EU 15 and to 721 PJ/year for the EU 30 (EU 27, Norway, Switzerland, Turkey) (Schneider et al. 2007). Including other herbaceous biomass fractions such as grass, the potential amounts to 1,000 PJ/year in the EU 15 and 1,500 PJ/year in the EU 30.
Residual Wood
In Germany, completely naturally grown forests hardly exist nowadays, apart from a few exceptions such as the Bavarian Forest National Park. Instead, forests are cultivated to obtain wood for industrial use. Besides trunk wood as the main product, the processes of thinning out and trunk wood harvesting produce residual material which today remains in the forest, to a large extent unused. This material consists of trunk wood sections and thick branches which are not suitable for industrial pur- poses but can be used as fuel. The additional biomass in the forest, such as withered branches and twigs, bark and leaves cannot be utilised as fuel in an industrially reasonable way and should remain in the forest to conserve the humus and nutri- ent cycle.
For the regional distribution of the yield, the points of reference are the woodland areas. In Germany, the well-wooded southern federal states are characterised by higher and the sparsely wooded northern states by lower potentials (Kaltschmitt et al. 2006).
In trunk wood processing, residual matter is produced in particular in sawmills and in the processing of the timber. These residues, however, are for the most part utilised as feedstocks for the paper industry and in chipboard manufacturing or as a fuel in the wood-processing industry. Wood biomass is also sourced from waste wood, i.e. wood no longer used for its original purpose (Kaltschmitt 2001; Fr¨uhwald 1990; Wegener and Fr¨uhwald 1994).
The technical potential of residual wood in Germany amounts to about 424 PJ/year of forestry residues (waste timber, bark, etc.), 57 PJ/year from the wood- processing industry and 78 PJ/year of waste wood. The total potential is 570 PJ/year, corresponding to a fraction of the primary energy consumption of 4%.
The worldwide potential can be calculated on the basis of existing wooded areas and the average of the different wood yields. The result of such a calculation is a potential of approximately 42,000 PJ/year or 1,400 million TCE. Broken down, this amount is composed half by the wood yield theoretically exploitable as a fuel, 13 and 17% by the production residuals from timber cutting and further industrial processing, respectively, 7% by the waste wood produced each year and 8% by other kinds of wood residues. The biggest potential for the exploitation of wood
32 2 Solid Fuels as an energy source is found in North America due to the currently unused large yield of wood. In the countries of the European Union, the potential yield of woody biomass, including waste wood, amounts to some 3,200 PJ/year in the EU 15 and to almost 5,000 PJ/year in the EU 30.
Energy Crops
For areas of arable land no longer needed for food production, one potential use under discussion is the plantation of energy crops. The biomass types in question are the following (Kaltschmitt et al. 2006; Kaltschmitt 2001; Lewandowski 1996):
• Conventional cereals (barley, rye, triticale, maize). Cereals, besides producing grain for food and forage, can also be grown for use in power production. In this process, the above-ground parts of the cereal plant (the straw and the grain) are used as a solid fuel. The advantage of the plantation of cereals to produce a solid energy carrier is the known, mature technology for cultivation and harvest.
Depending on the local conditions, the resulting average annual yields of dry mat- ter (straw and grain) for cereal crops such as triticale, winter wheat, winter barley and rye range between 9 and 13 tonnes of dry matter/ha. Arguments against the combustion of these crops for power production, which could also serve as food, are the ethical and moral concerns which arise from the context of the continued, widespread hunger around the world.
• Fast-growing reed and grass plants. Fast-growing reed and grass plants are C4 plants, which in the process of photosynthesis, consuming carbon dioxide from the atmosphere, build up a compound with four carbon atoms as a first building block. The group of these plants includes maize, millet and sugar cane. In con- trast, most of the plants on Earth, and almost all European plants, are C3plants (Borsch 1992). Due to their more efficient photosynthetic mechanism, C4grasses consume less water per kilogram of produced dry matter while also providing a higher yield per acre (Lewandowski 1996). The plant, dry after the growth period, can be used as a solid fuel.
The advantage of C4 plants is their high yield of biomass; the drawback is the scant experience of large-scale cultivation and harvesting. Among the plants suitable for cultivation for energy purposes in Germany, those most suitable are those characterised not only by high yields but also by relatively low requirements for soil, climate and care.
Due to its high yields, Miscanthus sinensis in particular has become known as a potential energy carrier. Miscanthus, also called the Chinese reed, is a C4 plant native to East Asia, which belongs to the Poaceae family. In contrast to annual grass plants such as cereals or maize, Miscanthus is a perennial plant which has subterranean perennial organs (rhizomes) from which new shoots develop in spring (Lewandowski 1996). Miscanthus is grown for a period of about 10 years, producing full yields from the third year or so. The anticipated high yields, of up to 40–50 tonnes of dry matter/ha, have in practice, in Europe, not met expectations. Instead, yearly maximum yields of 20–25 tonnes dry matter/ha from the third year seem to be
2.2 Renewable Solid Fuels 33 realistic from fields in central Europe (Hartmann and Strehler 1995). Depending on local and climatic conditions, the yield may also be a lot lower (Kaltschmitt 1993).
In central Europe, frost in winter may damage the rhizomes and hence diminish the yield.
Other C4 plants are the reed and the giant reed, types of millet also belonging to the Poaceae family. Compared to Miscanthus, however, they are expected to pro- duce lower yields under central European conditions. The millet types which can be cultivated in Germany are C4 plants of tropical origin too. In conditions of heavy precipitation and mild climate, the achievable dry matter yields range between 15 and 22 tonnes/ha yearly.
• Fast-growing trees (willow, poplar). Biomass can also be produced through fast- growing tree types, such as willow or poplar, which are grown as short rotation crops. After a breeding phase, the above-ground biomass is mechanically har- vested after 4–20 years. In the form of woodchips, it can be used as a solid fuel.
The tree stumps sprout again. The biomass can be harvested again after 2–12 years, respectively, depending on the site, climate and the tree type. In Germany, the respective yearly yields are 12–15 tonnes of dry matter/ha (Kaltschmitt 1993;
Hartmann and Strehler 1995).
The fundamental parameter for the technical potential of energy crops is the area available for cultivation. Worldwide, this area is estimated to be between 350 and 950 million ha. In industrial nations, the area of the existing arable land which can be assumed to be available for the cultivation of energy crop averages around 7%.
In developing countries, the area theoretically available and suitable for energy crop growing is on average considerably higher. Supposing that a mix of plants suited to the given location was cultivated on these areas, a technical energy potential can be calculated. The calculated potentials vary between 37 and 82 EJ/year. The highest potential in this respect is in Africa. The potential in Europe is limited. The countries of the European Union offer a potential in the range of 1.8–3.8 EJ/year. In Germany, in the medium term, a maximum area of 2 million ha will probably be useable for energy crop cultivation, so a potential of about 365 PJ/year has to be assumed.
Summary of Potentials and Current Utilisation
Table 2.6 compiles the above-discussed potentials for Germany and shows the extent of current use. At present, almost all residual wood from forestry and industry, as well as all waste wood, is exploited. Other sorts of wood and straw remain unused, so there is a potential to increase the share of biomass in primary energy consump- tion from the current 2% or so up to about 8%. Other authors state a potential use of solid biomass of between 2 and 15% of the primary energy consumption.
The dominant renewable energy source in Europe is biomass, with a share of 4.5% of the primary energy consumption in 2005 and 68% of total renewables.
Biomass provides 30% of the PEC in Latvia and nearly 20% in Finland. Most of this is wood. Sweden is not far behind with 17.5% (Eurostat 2007). The specific differences between the countries result from differing boundary conditions, such
34 2 Solid Fuels Table 2.6 Biomass potential and utilisation in Germany (Schneider 2007)
Potential Utilisation Potential/PEC Utilisation/PEC
in PJ/yr Share in %
Residual forest wood 169 147–165 3.0 1.0–1.1
Small wood 123
Additional forestry wood
132 Wood industry
residuals
57 51 0.4 0.4
Waste wood 78 62 0.5 0.4
Other woody biomass 10 1 0.1 0
Straw 130 3 0.9 0
Grass, other 48–77 0 0.4–0.6 0
Energy crops 365 0 2.6 0
Total 1,112–1,141 261–279 7.8–8.0 1.8–2.0
PEC: Primary energy consumption
as the fraction of forest area, the fraction of agriculturally productive land, the cli- matic conditions or national policies. Furthermore, in countries which, compared to Germany, have a higher use of biomass, the potentials are higher than the current utilisation.
Worldwide, though, the share of biomass in primary energy consumption is sig- nificantly higher than in Europe. Table 2.7 shows the worldwide potentials of wood Table 2.7 Biomass potential, current utilisation and share of PEC in different regions of the world (Schneider 2007; Van Loo 2008; Kaltschmitt et al. 2009)
North Latin Middle Former
America America Asia Africa Europe East SU Total
Potential [EJ/a]
Wood 12.8 5.9 7.7 5.4 4.0 0.4 5.4 41.6
Herbaceous biomass
2.2 1.7 9.9 0.9 1.6 0.2 0.7 17.2
Dung 0.8 1.8 2.7 1.2 0.7 0.1 0.3 7.6
Biogas (0.3) (0.6) (0.9) (0.4) (0.3) (0.0) (0.1) (2.6)
Energy crops 4.1 12.1 1.1 13.9 2.6 0.0 3.6 37.4
Total 19.9 21.5 21.4 21.4 8.9 0.7 10.0 103.8
Current utilisation [EJ/a]
Trad.
biomass
1.2 22.5 9.7 33.4
Modern biomass
4.1 2.4 3.6 2.3 3.4 0.7 16.8
Total 4.1 3.6 26.1 12 3.4 0 0.7 50.2
PEC [EJ/a] 120.4 21.8 154.8 25 78.9 19.5 46.5 473
Utilisation/
PEC [%]
3 17 17 48 4 0 2 10.6
Potential/
PEC [%]
17 98 14 86 11 1 22 22
2.2 Renewable Solid Fuels 35 and herbaceous residual matter and energy crops differentiated by region and related to the primary energy consumption. Globally, a technical potential of biomass of about 100 EJ/year can be surmised, which corresponds to a share of 22% of the total primary energy consumption in 2006. The current utilisation of biomass, as a per- centage of the primary energy consumption, is 10.6%. This high share comes about from traditional biomass use in fast-developing and developing countries, for exam- ple as firewood. Table 2.7 distinguishes between modern and traditional biomass utilisation. Modern refers to modern technologies, such as biomass combustion for combined heat and power production. The share of modern biomass in PEC is around 3.5% worldwide (Van Loo and Koppejan 2008; Schneider et al. 2007).