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Energy produced from poplars and other plants – how efficiently can this be done?

Renewable energies do not come out of nowhere. The sustainable cultivation of firewood requires land for plantation. The main criticism of the use of land for the production of renewable energies is that the production of renewable resources is in strong opposition to the production of human food. The forestry scientist Janine Fischbach and her colleagues at the University of Freiburg are investigating ways to avoid a “plate – petrol tank” dilemma.

A five-month old short rotation coppice (SRC) area. © Janine Fischbach

Poplars and willows are characterised by rapid growth. Over the last few years, large industrial companies such as RWE have realised the potential of rapidly growing plants. These companies are buying up more and more land in Germany for short rotation coppice (SRC) plantations. Conservationists are against this because they believe that the same land could be used for the production of corn and vegetables. The debate on the "plate - petrol tank" dilemma is often highly emotive, and also reflects a social problem: How can available land be equitably divided among energy and food producers?

One solution might be to circumvent the competition, for example by cultivating wood for energy use on soils that are too meagre to be suitable for the production of food. "This is one of the reasons why our project is focusing on the biological, ecological and economic aspects of such marginal sites for short rotation coppice plantations," said Janine Fischbach, scientist and doctoral student at the Institute of Forest Utilisation and Work Science at the University of Freiburg.

Which clone is best?

The project entitled “Sustainable production of biomass using SRC poplar plantations at marginal sites” (ProBioPa; Nachhaltige PROduktion von BIOmasse mit Kurzumtriebsplantagen der PAppel auf Mariginalstandorten) is part of the German Federal Ministry of Education and Research’s (BMBF) Bioenergy 2021 programme which is providing funding for a period of five years. The project involves the Institute of Forest Utilisation and Work Science, researchers from the Freiburg-based Institute of Forest Botany and Tree Physiology, the Karlsruhe Institute of Technology (KIT), the Forest Research Institute of Baden-Württemberg (FVA) and Netafin GmbH as industrial partner. The goal of the project is to improve the ability of rapidly growing poplar lines to use water and nutrients; this means the optimisation of forests for the resource-conserving production of biomass and a sustainability analysis in which all greenhouse gases and volatile organic compounds are determined and quantified.

On the one hand, the researchers are faced with purely biological problems: how can the production of combustible biomass be maximised and the discharge of harmful emissions be kept at a minimum? Can biotechnology contribute to the optimum adaptation of the physiology of the plants to nutrient-poor areas with a low water content.

The biologists involved in the project are looking at a number of different poplar clones. They are cultivating seedlings from ten different clones on various fields to assess different conditions: how do the plants react to artificial irrigation and additional fertiliser? Does the yield increase without intervention? The researchers hope that they will be able to find a poplar clone that is perfectly adapted to marginal locations and cultivation methods that are optimal.

Laboratory scientists are focusing on genetically modified plants, for example the selection of poplar mutants that can take up water and nutrients efficiently to produce high biomass yields. The researchers are also looking at the characteristics of roots that enable the efficient uptake of water and nutrients. This work also focuses on the genes that are responsible for this.

Input and output in the spotlight

Seedlings are used to establish SRCs © Hansjörg Reitermayer

Fischbach is particularly interested in ecological and economic issues. Which cultivation and harvest procedures and applications have the least impact on the environment? How can costs be reduced and energy gain be increased? "Most ecobalances go no further than the combustion of the wood products," said the researcher going on to add that the project partners are working to "take into account all inputs and outputs, from land reclamation to cultivation, the treatment of the plants, their growth and harvest up to application and the waste arising from the combustion of wooden products in industrial and private ovens".

Typical "inputs" of such analyses are for example the use of fertiliser and pesticides, the hours of work needed to cultivate plants with and without roots, shoots and seedlings, the petrol consumption of machines used for planting and harvesting, the water content and the capacity of containers or the length of transportation paths to the final destinations. "Output" refers to the gain of energy from biomass, the discharge of exhaust gas of machines used (for example for planting, care, harvest and transport), the emissions arising from the combustion of wood in different ovens, the alteration of carbon and nitrogen supplies in the soil and the ashes that need to be disposed of (or be reused on the fields).

Harvest of an SRC plantation. This is a one-tier process in which the wood chips are fed directly into a container. © Janine Fischbach

"Initially I read publications and attended conferences to identify typical process chains in SRC plantations," said Fischbach. The process chains differ in the selection of the type of planting used (manually or using machines), the use of a particular harvest system (choppers or chuff cutters) and type of machines used, the growing time of plants, the processing of wood trimmings and offcuts into different final products such as wood chips or pellets, and the type of oven used for combustion.

"I then examine each individual step in terms of input and output. This means that I spend many hours in the field with a stopwatch in order to collect information about the length of time it takes for each individual work step in certain harvest machines," said Fischbach. How long does the machine take to start, to grip and cut trees? How long does a chuff cutter need to harvest a row of trees? Does the machine have to wait for the container to arrive in order to deliver the wood chips? Can one or several of these steps be optimised? The project also focuses on the cost-benefit ratio of typical process chains.

Energy source for the future?

The German government is also extremely interested in finding ways to enable renewable energies to replace fossil fuels. However, the production of energy from alternative energy carriers requires efficient and innovative methods. The results of the ProBioPa project will not only benefit industrial giants. “Many small farmers have turned their nutrient-poor agricultural areas into SRCs,” said Fischbach. “It is especially important for small farmers to find out the type of method that is best suited to a certain type of soil.”

Fischbach also calls on farmers to think well in advance about the future use of their wood: is the wood more suitable for small heating plants that do not run efficiently with moist wood, and are limited storage and drying facilities available? This would require the farmers to put two-tier processes in place where the cut shoots are left on the edges of fields to dry for four to six weeks. Fischbach’s work is likely to shed light on the advantages and disadvantages of different provision chains, and to improve the ecological and economic balance of energy recovery from sustainable sources.

Further information:
Janine Fischbach
Institute of Forest Utilisation and Work Science
University of Freiburg
Werthmannstraße 6
D-79085 Freiburg

Tel. +49 761-203 3754
Fax +49 761-203 3763
E-mail: janine.fischbach(at)fobawi.uni-freiburg.de

Website address: https://www.biooekonomie-bw.de/en/articles/news/energy-produced-from-poplars-and-other-plants-how-efficiently-can-this-be-done