Environmental Engineering Reference
In-Depth Information
Figure 3.4. Example of heterogeneity in a single container.
It is difficult to compare the performance of commercially available measurement systems
since published results depend on a large number of factors. It is easy to achieve high precision
and accuracy for a homogenous material such as sawdust while it is far harder to measure moisture
content in complex mixtures of different types of biomass. Other factors that affect the outcome
of a comparison are the moisture content range itself, temperature differences or the presence of
frozen water. Finally, the performance of a certain instrumental technique depends on the cal-
ibration skills of the supplier. Rather than viewing a certain technology as superior or inferior,
evaluation should be made with the actual performance requirements of the intended user in mind.
It can also be noted that the sampling techniques in the below examples range from point wise
sampling to sampling along intersections of the bulk and analysis of a part-stream of the flow on
a conveyor belt. Therefore, a comparison of performance between methods should also consider
the impact of the respective sampling errors.
3.4.2.2
Near-infrared spectroscopy (NIR)
Near-infrared spectroscopy (NIR) uses the near-infrared region of the electromagnetic spectrum
in the range of approx. 800-2500 nm. The method uses information in molecular overtone and
combination vibrations. There is no immediate correlation between the elementary composition
and spectrum. Material properties are usually calculated using multivariate methods. Another
complication is that moisture peaks are temperature sensitive.
The penetration depth varies with the material but an indicative depth is 10mm for forest
residue. Therefore, analysis of bulk properties has to be made analyzing a part stream or an
intersection in the material. Figure 3.5 shows a system for on-line analysis on a conveyor belt
while Figure 3.6 displays equipment for bulk analysis of truck containers.
A clear advantage of NIR is that it provides information about the organic matrix. NIR can
e.g. be used in order to determine the mix of different fuels, estimate ash content or approximate
the energy content. Since various types of biomass differ in energy content such estimation of
the calorific value will be more reliable than if the moisture content is determined and a fixed
calorific value is used is used in order to estimate the energy content. A study by Lestander and
Rhén (2005) confirms that near-infrared spectroscopy might be used for estimation of ash content
as well as the gross calorific value in ground samples from stem and wood branches.
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