Jurgen Maier & Reinhold Vetter

Institute for Land Management Compatible to Environmental Requirements, Mullheim Auf der Breite 7, D-79379 Mullheim, Germany

phone: 0(049) 7631-3684-0, fax: 0(049) 7631-3684-30, e-mail: poststelle@iful. bwl. de

In search of sensible alternatives for biomass production the cultivation of short — rotation coppices on agricultural crop land has to be tested. From an economic point of view this is a feasible long term option (i. e. , if the price for biomass increases according to the shortage of biomass by-products) due to the fact that in perennial cultivation and with low input it can produce a high yield of biomass within a short time.

These cultivation conditions and results may possibly have the ecological consequences wanted: high yield of biomass respectively energy as well as low input of energy result in a favorable energy balance and in a high energy output per unit area. The ecological assessment is improved by the renunciation of yield — increasing input such as fertiliser and pesticides; perennial cultivation is in general considered to be advantageous for soil protection, water and species. In the specific case of short-rotation coppice, research came to the conclusion that willow and poplar, for example, are suitable for the sanitation of soil and inshore waters. In comparison to other biomass — such as energy cereal and energy grass — another ecological advantage is the lower amount of emissions that can be expected in a technically optimised combustion process due to the favorable composition of fuel.

Three short-rotation coppices were tested on three locations in two different regions in the state of Baden-WOrttemberg (in the Southwest of Germany) with regard to an energetic use of biomass.

The two locations in the upper Rhine valley differ fundamentally in their yield potential (see Table 1). In MOllheim the soil is profound with a good water supply. With regard to the water supply, Grissheim is a marginal location where grain maize is irrigated. In the cooler location of Binsdorf, located in the foreland of the mountainous Alb, the temperature is the most limiting factor for thermophile plants.

The three short-rotation coppices, willow (Salix viminalis), poplar (Populus spec.) and empress tree (Paulownia tomentosa) were planted on large plots in the form of cuttings, originating from various tree nurseries. They were planted in different years (see Table 2). The standing crops were cultivated without any pesticides or fertilisers — with exception of the planting year, if necessary. In Grissheim the experimental plots with willows and poplars were irrigated as they were located within a field of grain maize which had to be irrigated regularly.

Table 1: Data of location Data of location Mullheim Grissheim Binsdorf Region Rhine valley Rhine valley mountains

	(early thrashing region, viticulture climate)	(early thrashing region, viticulture climate)	(late thrashing region, not good for grain maize)
Landscape	Southern upper Rhine valley	Southern upper Rhine valley	foreland of the mountainous Alb
Altitude (m)	232	230	600
Average temperature per year (°C)	9,5	9,8	6,8
Total annual precipitation (mm)	650	680	800
Soil type	para brown earth (profound)	para brown earth (shallow)	pelosol of black jura
Soil texture	uL	sL	hhstL
Valuation index of field	81	45	51

Table 2: Data of cultivation (until 2003) Data of cultivation Mullheim Grissheim Binsdorf Species willow empress tree willow poplar willow Planting year 1993 1996 1988 1988 1994 Planting density (plants/m2) 1 1 4 2 2 Irrigation — — + + — Rotation period (years) 2 3 5 2 3 4 3 Rotations 4 3 1 7 4 3 3 Useful life 10 10 5 15 15 15 9

The rotation period chosen for the short-rotation of willow used as coppice forest was 2 (U2) and 3 years (U3) at the two locations in the upper Rhine valley, and 3 years (U3) at the cooler location Binsdorf. Poplar was harvested in a 4-year-cyle, the empress tree was harvested once after a 5-year-rotation. The harvest was done by machine (maize chopper) or manually with a chain saw on the whole parcel, including the outer rows of plants. The 1996 yield of willow and poplar in Grissheim was not taken into consideration, as the outer rows of plants were not harvested.

Results

Yield

The three species of trees tested gave an average harvest of 11 tons of dry matter per hectare and year on the two locations in the Rhine valley and mountains, including the irrigated location Grissheim in the upper Rhine valley (see Table 3). By excluding the irrigated location, the average yield is 9 tons. The species obiviously differ in their yield potential. Willows show a lower yield level and seem to be limited mainly because of lower amount of water and not as much by temperature. Without irrigation willows achieved 8 tons. At the cooler location in Binsdorf they produced some more biomass in a 3-year-

Table 3: Long-term average yield of short-rotation coppice (until 2003) Yield (t dm/ha and a) willow poplar empress tree mean MQllheim 7,9 — 12,7 10,3 Grissheim (irrigated) 1 13,1 15,6 — 14,3 Binsdorf 8,4 — — 8,8 mean 9,7 15,6 12,7 11,0 without Grissheim 8,2 — 12,7 9,4 1 excluding 1996 harvest (harvest without outer rows of plants)

rotation than in MQllheim with a lower precipitation (on average of the two rotations U2 and U3). These yields correspond with the data of UNSELD (1999) who states the yield with a maximium of 7 to 8 tons of dry matter for willow and poplar on experimental locations on a dry soil in the summer. In Grissheim willows had not quite the level of poplars. In 1996 the appearance of the poplar leaf beetle (Melasoma populi) was discovered and caused defoliation of individual leaves. A yield reduction because of the beetle is not being assumed. Without irrigation, the thermophile empress tree produces at MQllheim in the Rhine valley nearly 13 tons of biomass, i. e. more than willow.

Fig. 1 shows that the 2- and 3-year-rotation obviously has no influence on the production of biomass on willow plantations. In MQllheim, an insignificant higher yield of biomass was ascertained; in Grissheim, the location with irrigation an insignificantly lower yield. FRIEDRICH (1999) did not find out any influence of the rotation time on the yield of dry matter either, that had been produced in a 5- or 10-year rotation. With regard to the biomass production of the individual rotations in Grissheim starting from the third respectively the first harvest, the biomass yield shows a significant reduction from one harvest to the next. After 10- or 12-year-cultivation the biomass yield is less than half of the starting yield. The rootstocks which did not sprout any more increased from one rotation to the next. Regarding the younger short-rotation coppice in MQllheim, the yield of the first harvest is lower than the following ones. This concerns both rotation periods. The reduction of the biomass production, beginning from the third rotation may originate in the loss of production because some rootstocks did not sprout. In Binsdorf’s 3-year-rotation the yield is the same as in MQllheim. In the 2003 harvest there were also less rootstocks sprouting than the harvest before.

20 18

Mullheim U2 Mullheim U3

14

0

1995 1996 1997 1998 1999 2000 2001 2002

8

4

20 18 16 14 12 10 8 6 4 2 0

t DM/ha and year

GriBheim U2 GriBheim U3

2

6

Mean

10

12

16

t DM/ha and year

20 18 16 14 12 10

t DM/ha and year

8

6

4

2

0

Binsdorf U3 I

8,4 I I

Fuel Characteristics

If the coppice biomass is used as solid fuel — for example in wood-fired heating (power) plants — the combustor operating company is mainly interested in the composition of the wood listed in Table 4.

Table 4: Fuel characteristics of short-rotation coppice biomass compared to timber wood Timber wood Willow Poplar Empress trunk wood1 Twigs2 Tree Number of samples 4 1 6 3 1 Water content % 75 61 53 49 48 Content in dry matter Heating value Ho3 MJ/kg 19,7 20,3 19,5 19,8 19,9 Ash % 0,4 3,3 2,0 2,2 0,5 C % 52,8 51,2 43,5 49,3 50,3 H % 5,7 5,9 5,5 5,6 10,6 O (calculated) % 40,9 38,3 41,2 42,2 38,4 S % 0,06 0,29 0,07 0,09 < 0,1 N % 0,1 1,1 0,6 0,5 0,2 Cl % 0,01 0,01 0,1 0,01 0,01 Volatile substances % 79,9 77,2 80,5 81,8 81,1 Fixed C % 17,3 19,5 17,5 15,9 18,4 1beech, oak, pine tree, spruce, 2spruce 3the upper heating value Ho differs from the lower heating value Hu, in the evaporation enthalpy of water. The water content in the flue gas derives from the fuel humidity and from the oxidation of the hydrogen in the fuel.

Compared to forest timber (trunk wood respectively twigs) coppice biomass contains more water, if it had not been left in the field to dry. The upper heat value H0 of the three different coppice biomass with about 20 Megajoule per kilogram dry matter is the same as of forest timber. The ash content of willow and poplar is about 4 times higher than of empress tree which has the same ash content as forest timber, i. e. 0,5%. With more than 3%, twigs of spruce have the highest ash content of all tested wood. For a low-emission combustor and its trouble-free operation, sulphur, nitrate and chloride are the relevant components. Mainly in a high-temperature range chloride is unwanted because of the danger of corrosion and dioxine emissions (MAIER et. al., 1998). The different coppice biomass has a little higher content of sulphur than trunk wood, but it is significantly lower than that of twigs of spruce. The nitrate concentration of the empress tree is comparable to trunk wood, whereas willow and poplar show a nitrate concentration which is 5 times higher than of trunk wood, but only half as high as of twigs of spruce. The chloride concentration of all coppice biomass is below 0,01% and therefore on a very low level, with exception of willow. Because of an individual value of more than 0,3% in the willows harvested in Grissheim in 1996, their chloride concentration is 10 times higher.

Summary

In two pedo-climatic different regions in the state of Baden-Wurttemberg three short — rotation coppices willow, poplar and empress tree were tested with regard to their biomass productivity on arable land and to their properties for energetic use. Between 8 and 13 tons of dry matter per hectare and year could be produced under extensive cultivation

conditions, over 15 tons with irrigation. Due to their composition, it can be assumed that their use as solid fuel in a biomass combustor is just as unproblematic as with forest timber.

Key words: short rotation, willow, poplar, empress tree, productivity, quality of fuel

Literature

E. Friedrich, 1999. Anbautechnische Untersuchungen in forstlichen Schnellwuchsplantagen und Demonstration des Leistungsvermogens schnellwachsender Baumarten. In: Bundesministerium fur Ernahrung, Landwirtschaft und Forsten, Bonn, Modellvorhaben „Schnellwachsende Baumarten“. Landwirtschaftsverlag Munster, Deutschland, 19 — 150.

J. Maier, R. Vetter, V. Siegle, H. Spliethoff, 1998. Anbau von Energiepflanzen — Ganzpflanzengewinnung mit verschiedenen Beerntungsmethoden (ein — und mehrjahrige Pflanzenarten); Schwachholzverwertung. Abschlussbericht Forschungsvorhaben Nr. 22-94.11, Ministerium Landlicher Raum Baden-Wurttemberg, Deutschland, 100 Seiten

R. Vetter & J. Maier, 1996. Production of energy crops in summerdry regions of Southwest — Germany for solid fuel. Biomass for energy and the environment, Proceedings of the 9th European Bioenergy Conference, Copenhagen, Denmark, Vol. 1: 528 — 530

R. Unseld, 1999. Kurzumtriebsbewirtschaftung auf landwirtschaftlichen Grenzertragsboden — Biomassenproduktion und bodenokologische Auswirkungen verschiedener Baumarten. Diss. Univ. Freiburg, Shaker Verlag, Deutschland, 193 Seiten.