Halm som bränsle

The objectives of this project were a) to investigate and model the equilibrium moisture content of straw from cereals and oilseed crops, b) to investigate and quantify possible differences in straw equilibrium moisture content between different crops, varieties and threshing methods (shaker and axial flow combines), c) to investigate the moisture absorption rates in straw from different crops and threshing methods, d) to investigate the hysteresis characteristics of straw, i.e. the difference in moisture content between absorption and desorption conditions, e) to develop a model to simulate the moisture content in straw swathes on an hourly basis, and f) to estimate model parameters by monitoring the moisture content in straw swathes in field trials and to evaluate the general validity of the model. The equilibrium moisture content was investigated in climate chambers by keeping the isotherms +5°C, +15°C and +25°C constant while the relative humidity was increased stepwise from 35% to 53%, 72% and finally 90%. The samples comprised straw from winter wheat, spring wheat, triticale, rye, barley, oats, winter rape and spring rape. For winter wheat, there were samples from two varieties and from unthreshed, 'yellow' and 'grey' straw. For spring wheat, there were samples from two varieties and straw from shaker and axial flow combines. The results showed, for example, that a moisture content of 18% (wet basis) was reached at a relative humidity of 85-90% for winter wheat (see below). Five commonly used equations for modelling the equilibrium moisture contents (the Henderson, Chung-Pfost, Halsey, Oswin and GAB-equations) were adapted to the experimental data by non-linear regression. The Halsey equation (the version independent of temperature) had the best goodness of fit for most straw types investigated. In a comparison of straw from different crops, no statistically significant differences in the equilibrium moisture content were found, except for oat straw. This type of straw had somewhat higher equilibrium moisture content, especially at high relative humidity. At low relative humidity, straw from rape had a somewhat higher equilibrium moisture content, but this difference was not statistically significant. Furthermore, no statistically significant differences were found between 'yellow' and 'grey' straw, or between straw from shaker and axial flow combines. The moisture absorption rate was investigated at a temperature of +15°C when the relative humidity was increased from 53% to 72%. The results showed that oats had the highest rate initially, while straw from spring wheat had the lowest. However, all samples reached their equilibrium moisture content within about 10 hours. The climate chamber investigations also showed that straw has some absorption-desorption hysteresis. In these trials, the temperature was kept at +15°C, while the relative humidity was varied according to 53% → 72% → 90% → 72% → 53%. The hysteresis was estimated to be 1-2 percentage points. A computer model, which has earlier been applied to flax straw, was used to simulate the moisture content in straw swathes under field conditions. The model took into account factors such as evaporation of water and formation of dew. It used precipitation, global radiation, temperature, relative humidity, wind speed and cloudiness as input data to calculate the straw moisture content on an hourly basis. Three field trials were carried out during 2007-08, when the moisture content was monitored in swathes with winter wheat straw. The model parameters were then determined by regression. Comparisons between measured and predicted moisture contents showed that the model simulated the drying and rewetting processes with acceptable accuracy.