High-grade fuel from energy willow farming

Abstract. Energy willow farming was developed in Finland and in Sweden in the mid-seventies. Notable energy yields have been collected by selecting, breeding and raising fast-growing coppicing willow crops. At high altitudes with a severe wintertime, a one-year rotation is used; in central and southern parts of Nordic countries, a rotation period of 2-5 years seems to be the most productive. An energy system is described in which methanol is converted from wood produced on willow plantations. With moderate energy inputs and willow yield levels, the system works as an energy amplifier in terms of high-grade fuel. Net energy yields of 3000 - 4000 kg/ha/year methanol could be achieved with present-day technology. The sensitivity analysis of the system shows the importance of high-yielding willow clones in concentrating future R & D efforts.


Introduction

Energy farming is a discipline of cultivated plants and husbandry in which the solar radiation is collected and converted into biotic energy of phytomass. The aim is to produce high energy yields by selecting, breeding and raising fast-growing crops.

Promising results in northern energy farming have been achieved
with the use of selected willow clones. Energy willow farming was developed in Finland and in Sweden in the mid-seventies. It has its modern origin in the concept of minirotation forestry (1). Initially, the aim was merely to produce more raw material for the expanding wood industry, using methods as intensive as those used in agriculture.

The first field experiments were established in 1973. Promising results were obtained at once. A Danish willow clone, Salix cv. "Aquatica" produced in the latitude of the Arctic circle a dry matter (DM) yield of about 10 tons/ha already during the first summer (2).

More willow species have been screened in subsequent experiments. The annual yields have been maintained at their high level, between 10 and 20 tons/ha, in energy equivalents up to 350 gigajoules (GJ) per hectare (3). The largest dry matter yield so far reported (4) has been 32 tons/ ha, which includes the harvested stemwood only.

Aspects of willow husbandry

Energy willow farming can be carried out in two ways. If the clones used are completely frost resistant they can be grown like any other deciduous crop, the rotation period will vary between 2-5 years. If the shoots do not survive the winter, but the stumps and roots do, a one year rotation period is used.

So far it is not yet clear which one of the two practices will give the highest yields. With a rotation period of a number of years, dormancy reduces the level of photosynthesis in the autumn, but in early summer the foliage develops rapidly on the already existing shoots. With one year rotation, the growth early in the growing season is slower. However, as there is no need for winter hardening of the shoots, the leaves continue to fix carbon at a high rate right up until the onset of the autumn frosts. The active period of willow under one-year rotation lasts until October. The period is about one month longer than that for birch.

In the central and southern parts of the Nordic countries, 2-3 year rotation will probably give the highest yields. Moreover, perennial crops seem to produce more energy-rich shoots that also are structurally better suited to the cutting devices of future harvesting machines (5).

One advantage of energy willow is that it can be clonally propagated. The crop is established using cuttings. After the first harvest, the willow will coppice 5-20 shoots per cutting, depending on genetic differences between the clones.

The shoot density, which is greatly determined by the coppicing ability, has a clear effect on the yield. The main rule holds that the larger the number of vigorous shoots, the greater the yield will be (6). As a practical compromise, we aim at a shoot (coppice) number of 30 per square meter in one-year crops. With this density full coverage is established within one month in the spring.

Row cultivation can be carried out in such a way that ordinary tractors can later on operate without damaging the stools. Planting as well as harvesting can be mechanised at little cost.

Wood ash plays an important role in the recirculation of nutrients. In actual fact, if the energy crop is burned in a heating plant, most of the nutrients can be returned to the plantation in the ash. Nitrogen escapes to the atmosphere, minor potassium losses have also to be compensated for.

Nitrogen fertilizer is fixed from atmospheric reserves using a certain amount of energy. It has been estimated that the whole chain of nitrogen from the atmosphere through the factory to the field, requires an energy input of 77.0 MJ/kg nitrogen as fertilizer (7). About 100 kg/ha is annually needed to substitute the nitrogen removal (8). At this level, an energy input of 7.7 GJ/ha thus maintains the cycling of nitrogen between the atmosphere and the energy plantation.

Since willow plantations are still only on an experimental scale, the other inputs needed for the estimate can be obtained from information concerning similar husbandry with other crops. The total energy input in the energy willow may well be of the order 20 GJ/ha/year (9). This level corresponds, for instance with that for the intensive Populus "tristis" husbandry of Zavitkovski (7, without irrigation). With such an input a Salix cv "Aquatica" plantation is able to produce an annual stem yield of 12 tons/ha (DM) (8). In gross energy yield, it equals 197 GJ/ha/year (3).

High-grade fuel from energy willow

The energy willow system can work as an energy amplifier in terms of high-grade fuel. We choose methanol as an example since present-day technology converts methanol from wood more efficiently than the other possibility ethanol. Our calculation is based on a recent plan to convert Finnish peat fuel into methanol (10). With minor adjustments, the proposed plant could also accept forms of phytomass other than peat as feedstock. The approach and notations are from Weisz and Marshall (11).

The agricultural input required to produce the high-grade fuel is denoted by A and the gross energy yield by Yo. The input-output ratio f expresses the efficiency of the crop and of the type of husbandry practised (f =A/Yo). In the example, A= 20.0 GJ, Yo= 197 GJ, and f = 0.102. In other words, the energy input is 10 percent of the crop energy output.

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Pohjonen, V. & Kauppi, P. High-grade fuel from energy willow farming. Manuscript 31.12.1980. Web-txt 7.1.2018.