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Physical/mechanical test - moisture content and bulk density determination

Moisture content is a primary property for a successful utilisation of solid biofuels within the entire supply chain (e.g. for transportation costs, storage management, calorific value and conversion at end use). Strongly linked to the moisture content of solid biofuels is the bulk density, which is an important property with regard to space for storage and transportation and for volume based payment of biofuels.

Furthermore, bulk density is influencing rapid moisture content measurements. Thus, a wide scope of fuels from all over Europe was analysed to cover the broad range of biomass.

Basic results of methods for testing moisture content and bulk density of solid biofuel, which are identified to be the most promising for standardisation in terms of e.g. suitable performance and high reproducibility, are briefly summarised below.

Moisture content - reference method:
The applied moisture determination results gave overall comparable results. A statistical significant difference was found among the oven drying method between the standard method at 105 degrees Celsius compared to the temperatures 80 degrees Celsius and 130 degrees Celsius, respectively. According to this, a slightly increased amount of evaporated matter (volatiles and moisture) results for each temperature step.

The freeze-drying method determined significantly lower moisture content values compared to the standard method, whereas the results from the xylene method were deviating only for some of the fuels. This difference was partly attributed by the small sample size used for xylene distillation.

The GS-MS-method ("gas chromatography - mass spectrometry") has shown promising results as a standard reference method. So it was found that the amount of volatile matter was in order of 0.06 to 11.33% calculated as percentage of moisture content. Furthermore, it was revealed that the mass loss differences found among the different methods were primarily due to alpha- and beta-pinenes. Though, a more throughout optimisation of the method is required.

Moisture content ¿ rapid methods:
Among the seven tested rapid test devices about four types were identified to be particularly applicable to measure the moisture content of solid biofuels. These are:

- As on-site types: the thermogravimetric Mettler-Toledo HB45 as well as the capacitive devices Pandis FMG3000 and the Schaller FS2002-H.

- As on-line type: the optical MESA MM710. Although MESA was only tested in a reduced moisture range of 10 to 40% MC but the method is applicable to the full range of 0 to 100% MC.

However, the small nominal size and high time need (both Mettler-Toledo) as well as the need of fuel specific calibrations (Pandis, Schaller and MESA) should be considered when selecting the device. For the electrical devices including the bulk density in the calibration function can significantly increase measuring exactness. Moreover, reducing the scope of fuels increases the power of the calibration functions.

The Moist100, Wile25 and ACO estimated the moisture content with a higher variation and therefore cannot be recommended for moisture content determination in solid biofuels. Generally, further work is required to optimise and evaluate e.g. blank values, recovery, adsorption and extraction efficiency as well as response factors.

Bulk density:
The following was concluded for best practice in bulk density determination of solid biofuels:

- For all tested solid biofuels a measuring container size of 50 l is acceptable. A cylindrical container shape should be preferred for practical reasons due to higher stability and easier manageability.

- A standardised shock impact on the filled container significantly increases the measured bulk density (e.g. 6% for wood pellets, 10 to 12% for fuel chips and 18% for chopped miscanthus), while there was only found a minor improvement for the relative repeatability limits.

- The fuels moisture content during the measurement is of high importance, and consequently has to be recorded since an obvious increase in bulk density (wet basis) was observed with increasing moisture content. Thus, the comparability of bulk density data is only given if any inconsistency in moisture content between the samples is accounted for by the use of a correction factor of 0.712% for each 1% moisture difference. Moisture content effects are largely restricted to the moisture content range up to 25%. Beyond this point possible effects can be neglected.

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