Halm som energikälla

In Sweden, there are large quantities of straw that could be used for heating purposes. The amount of straw available for use as fuel has been estimated at 1 million tonnes or approx. 4 TWh. Today, only a fraction of this quantity is used. This can be compared to the approx. 1.5 million tonnes of straw that are used for fuel purposes in Denmark. The main purpose of this project was to present an overview of the existing techniques for harvesting, transport and combustion of straw in small- and large-scale systems, and to suggest research and development projects to increase the future use of straw for fuel purposes in Sweden. In the study, systems for harvesting, transport and storage of big bales (round bales and big square bales), and chaffing, gathering, transport and storage of loose chaffed straw are presented. Alternative systems with field-wafering and whole crop harvesting are also described. In addition, a study tour was made of fuel straw practitioners in southern Sweden and Denmark in order to review practical experiences. Costs were calculated for different systems including harvesting, transport to storage, storage, transport to heating plant and handling at heating plant for: big square bales, medium-sized square bales, round bales, chaffed loose straw and field-wafered straw. Both outdoor and indoor storage of the straw were studied. Systems with big square bales were cheapest, followed by systems with medium-sized square bales and field-wafered straw. Indoor storage had a considerable influence on the costs. Field-wafered straw had cost benefits in long transport and indoor storage due to its high bulk weight, which resulted in a high utilization of both transport vehicles and storage. When straw is burnt, several quality parameters are important, e.g. moisture content, bulk weight, ash content, ash melting point and lower heat value. Too high a moisture content results in operational disruption to the straw feeding equipment and might result in uneven combustion in the furnace. Ash with a low melting point may melt in the furnace and form clinker, which causes operational disruption. Straw should have a low content of alkali metals and chlorine to avoid the formation of clinkers. The content of these elements declines if the straw is left in the field, and, if possible, exposed to precipitation before harvesting. To obtain fuels with more well-defined properties that can be burnt in existing furnaces, the straw might be further processed into briquettes, pellets or powder. Techniques for handling and burning of straw are described on scales suitable for farms and district heating plants. New technology such as gasification, flash-pyrolysis, stirling engines, steam engines and use of straw as a raw material for the production of ethanol, methanol and Fischer-Tropsch diesel are reviewed. It is recommended in the study that future research and development projects should focus on the following fields: information and demonstration, handling costs and harvesting reliability, fuel quality, up-grading and ash handling. The techniques used today for handling and combustion of fuel straw work satisfactorily and at a competitive cost. Therefore, to quickly increase the use of fuel straw, there is a need for a variety of information work as well as demonstration plants that will serve as good practical examples. To increase the use of straw in regions with inferior harvesting conditions, there is a need for investigations about the variation in handling costs and harvesting security between different years. Furthermore, there is a need for studies of how various factors (rain, dew, soil type etc.) influence fuel quality, and for development of reliable and quick methods for measurement of various fuel properties. As straw is a comparatively cheap raw material, the possibilities to process it further and/or use it in new technical applications should be studied further. Finally, alternative ways to handle the ash, e.g. mixing it with various organic fertilizers, should be further investigated.