The influence of raw material characteristics on the industrial pelletizing process and pellet quality
Introduction
Fuel pellets production in Sweden is mainly from sawdust and planar shavings of Scots pine and Norway spruce. These saw mill residues are low in density and it is therefore beneficial to compress them into dense and high energy content pellets for domestic or industrial use. In the pelletizing process the raw material moisture content is reduced from approximately 50% to around 8–12% in a dryer. After that the dried raw material is ground in a hammer or ball mill, forced by rollers through the holes of a hot die to form dense 6 mm or 8 mm diameter and 20–40 mm long pellets and finally allowed to cool. During the process the moisture content is further reduced. The moisture content of the raw material is an important parameter for the pelletizing (compaction) process because moisture facilitates the heat transfer and friction [1]. If the moisture content is too low the friction between the raw material and the die will become too high resulting in an elevating power consumption and eventually the die holes might become clogged [2]. The moisture content is also important for the thermal softening and self-bonding of individual particles in the pellet [3], [4].
It is generally assumed that a mixture of fresh and stored (matured) sawdust will improve the durability of pellets but also lower the power consumption of the pelletizing machine [5]. The origin of the raw material also influences the pelletizing performance. Fresh Norway spruce sawdust is more difficult to pelletize than pine sawdust probably due to differences in moisture distribution and extractive content [5]. Sawdust from beech is reported to be much harder to pelletize than pine sawdust due to structural differences between hardwoods and softwoods [6].
The mechanical characteristics of pellets depend much on the size and shape of individual particles and it is beneficial to use a mixture of particle sizes because this increases the durability [7]. The particle size distribution also influences the pelletizing performance. Large particles have a lower packing density than small ones and are less prone to block the dies [6], [8].
The basic properties of pellets such as moisture content, bulk and single pellet densities, compression strength and durability (abrasion resistance) depend on both the raw material composition and the pelletizing process variables [3], [5].
Currently bulk density of wood pellets is according to the Swedish standard SS 18 71 20 [9] specified at ≥ 600 kg m− 3 but the pellet industries recommend 650 kg m− 3 [10].
The European CEN/TS standard (in preparation) will recommend 650 ± 20 kg m− 3 as limits for wood pellet bulk density [11].
The durability, reported as percentage of the total weight of a pellet sample after tumbling and removal of fine fraction, should according to Swedish standard SS 18 71 20 [9] not be below 97.7 wt.% and moisture content according to the same standard should not exceed 10 wt.%.
Next to raw material drying, densification (compaction) is the most energy demanding step in the pelletizing process and a reduction in energy consumption could much lessen the process costs for the pellet industries.
The specific energy consumption of densification in the pelletizing machine can almost be halved by increasing the pelletizing temperature from 100 °C to 200 °C [12]. However, it has been reported that high pelletizing temperatures will initiate auto-oxidation of wood extractives in pellets and cause emission of volatile organic compounds in pellet storage [13].
In order to produce pellets with a high and even quality at low energy consumption in the densification step, more research is needed to understand the important relationship between raw material composition and pelletizing parameters. This could also make it possible to design pellet qualities for different end users.
The aim of this study was to investigate the influence of raw material composition and moisture content on the energy consumption in industrial pelletizing and to evaluate if it is possible to design the pelletizing process for low energy consumption with even and high pellets quality.
The study was carried out in a large industrial production plant using an experimental design where each experimental run produced from 15 to 22 tons/h. Multivariate data analysis was used to model the relationships between the factors, process parameters and quality parameters.
Section snippets
Wood pellets
Pellets were made from fresh and matured sawdust of Norway spruce (Picea abies Karst) and Scots pine (Pinus sylvestris L) in a large scale pellet plant (production capacity 130,000 tons/year) in northern Sweden. The pelletizing plant uses ring matrix presses with a press length (die length) of 65 mm. Before pelletizing the sawdust was dried to a moisture content varying between 8.2 and 11.7% of dry weight, ground and sieved. For each run the raw materials were mixed in separate stacks and the
Results and discussion
The pelletizing runs were carried out according to an experimental design in the factors (1) raw material moisture content before pelletizing, (2) raw material composition in three ingredients: fresh and stored pine and fresh spruce. For the raw materials a mixture design was made allowing 100% for the fresh respectively stored pine and up to 20% for the spruce. The eleven runs are shown in Table 1. The runs were not randomized for practical and logistic reasons. An extra run was added, making
Conclusions
The results from this investigation showed that it is possible to obtain fairly reliable estimate of pellet quality (bulk density, durability and moisture content) as well as for energy consumption for the process using the composition of ingoing raw material as factors in an experimentally designed pelletizing process. It is shown that is possible to carry out designed experiments on an industrial scale as screening of raw material and process parameters. Relationships between process
Acknowledgements
Financial support from the Swedish Energy Agency, the Swedish Pellet Production Industry, the Swedish University of Agricultural Sciences, are gratefully acknowledged.
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