Thanks to the increasing usage of biomass as a source of energy, the analysis of these materials in the context of R&D and quality control gains importance, too. Due to the complex properties of plant materials, adequate sample preparation can be rather a challenge.
Biomass is an umbrella term for vegetable-based raw materials which are used for the production of energy. Compared to solar energy or wind power biomass offers the great advantage that the energy is stored in the renewable raw materials, being available when needed. Thanks to the increasing usage of biomass as a source of energy, the analysis of these materials in the context of R&D and quality control gains importance, too. Due to the complex properties of plant materials, such as volatile components, fibrous structures or abrasive behavior, adequate sample preparation can be rather a challenge. Grinding vegetable materials as part of the sample preparation process requires some know-how and experience to ensure reliable and meaningful analysis results. Selecting a suitable laboratory mill does not only depend on the sample’s properties. Further processing and the relevant standards also have to be taken into account.
There are basically three different methods of producing energy from biomass: direct combustion, extraction and fermentation.
With direct combustion the raw material is burnt to produce energy, e. g. wood chips made from forest wood residues (crown, branches, parts of the trunk) which cannot be used for any other purpose. The wood is shredded and burnt in a furnace. The quality of wood chips is determined by their calorific value and ash content. The calorific value of dry wood is more or less identical for all types of wood (in relation to the weight). However, it decreases with growing water content; that is why the percentage of dry matter is decisive for its trade value. In this context, kiln-drying is the only standardized method (DIN 52 183). This method involves moisture determination by thermal gravimetric analysis, i.e. the sample is kept at a temperature of 103 °C until its weight remains constant. The ash content is determined in a similar way, though the sample is heated to 500 °C in an oxygen stream. To accelerate this process and obtain a representative sample, it is ground to a fineness of 2 to 3 mm before analysis. Depending on the feed size, a Retsch cutting mill for pre-grinding and an ultra centrifugal mill such as the ZM 200 for fine grinding are suitable for this process. The methods and specifications are stipulated in standards such as EN 14961. Another important feature is the size range of the wood chips, as smaller furnaces can only process chips of a defined size. Particles which are smaller than 0.5 mm impair the fuel quality because they can cause blockages in the screw conveyors and deflagrations in the furnace. For particle size analysis the standard EN 14780 specifies a horizontal sieve shaker (such as Retsch’s AS 400 control); vibratory sieve shakers which generate a throwing motion are not suited for fibrous particles.
Another common method to produce bioenergy is the refining of combustible parts of vegetable raw materials. In Europe, this is mostly oil which is extracted from rape seed. In the US soy bean oil is esterified to biodiesel. Biodiesel has the advantage of being carbon-neutral and practically not releasing any particles when being burnt. The European standard for biodiesel is DIN EN 14214. Quality control of the seeds comprises extraction of the raw materials after the seeds have been ground in an Ultra Centrifugal Mill (Retsch ZM 200) and quantification of seed and other vegetable components by horizontal sieving (Retsch AS 400 control) or vibratory sieving (Retsch AS 200 control). Figure 1 shows the grinding result of soy beans. For an effective extraction process, the sample needs to be reduced to a particle size of approximately 500 microns. If the particle size is substantially larger, the extraction takes too long; if it is much finer, the extraction thimble may be blocked. The soy beans were ground with Retsch’s Ultra Centrifugal Mill ZM 200 with a 1 mm distance sieve; the rule of thumb is that the final fineness is about half the mesh size of the ring sieve. The distance sieve prevents the sample from getting too warm and oil from escaping.
Biogas plants have strongly gained in importance for the production of energy. Basically, any type of biomass (e. g. algae or vegetable waste) can be used as raw materials in biogas plants. The plants are “digested” by microorganisms in the absence of oxygen and the resulting methane is combusted in a power station. Decisive factors for the methane yield are not only the type of plant but also the microorganisms used. To accelerate the degradation process in the laboratory experiment, the plant waste is first comminuted. A knife mill like Retsch’s Grindomix is perfectly suited for this step; this mill processes samples with a high moisture content, such as algae or silage, quickly and smoothly. The result is a homogeneous mash in which the material degrades much faster than in a heterogeneous sample.