Versatile grinder technology for the production of wood biofuels

Highlights

• New grinding technology to obtain sawdust for pellets from wet chips.

• Versatile technology which can be used to obtain both wood sawdust and microchips.

• The prototype showed water removal capabilities.

Abstract

The exploitation of wood biomass for thermal energy production often represents an effective complementary source to petroleum, especially where there is the availability of extended forests. Focusing the attention on household plants, the wood pellet currently constitutes a widespread biofuel, which however is characterized by non-negligible production costs. Wood microchips constitute a recently developed alternative, which compensates its inferior characteristics by an easier production process. To obtain these biofuels, the particle size reduction is crucial, because it sensibly influences the power consumption of the drying processes, as well as the raw material supply strategies. In this context, this paper presents an innovative grinding technology, which can be exploited to produce wood particle sizes for both wood pellets and microchips production. In particular, the prototype of the grinder and the experimental plant are shown, which have been used for performing preliminary biofuel production tests. The main design characteristics of the prototype are provided, together with preliminary experimental results that provide first evidences about the potentialities of the proposed wood grinding technology.

Introduction

The woody solid biofuels represent a relevant energy source for heating and power generation. This is especially true for those territories characterized by the presence of vast wooded areas. Accordingly, Europe acknowledges high consumptions of wood pellets for thermal energy production in household plants (more than 18 million tonnes in 2014) [1], which is expected to further increase by 2020 [2]. The wood chips production follows the same trend of the pellet one, but the research of new biofuels is still an on-going activity that can provide hints for new fuel alternatives. For example, an additional bio-fuel type is currently under investigation, i.e. the “wood microchips”. Notwithstanding the lower energetic content (which can also be affected by the storage conditions [3]), this new biofuel is a real alternative to wood pellets, thanks to the limited moisture content of microchips (between 10% and 15%) and the specific particle size. Indeed, with minimal modifications to both feeding mechanisms and combustion controls, microchips can be used to feed conventional pellet stoves and boilers. Moreover, it can be easily manufactured by small enterprises using locally available raw materials and with low-investments [1].

The pellet production process starts with a primary grinding (chipping) of wood biomass to obtain chips having a variable size [[4], [5], [6]], whose moisture content is drastically reduced to 20% in weight by means of drying ovens. Then, a further grinding phase is required, to obtain finer particles, whose moisture content is further adjusted to 10–12% in weight. Then, pellets are obtained through a pressing and extrusion process. Differently, since wood chips with a length between 5 and 10 mm compose wood microchips, they can be obtained by particular settings of common wood chippers (i.e. reducing the clearances between chippers' blades and related discs or drums, and reducing the feeding speed). However, the standard ISO 17225-4 [7] prescribes that a mass unit of high-quality microchips should be composed by a 60% in mass of particles between 3.15 mm and 16 mm, no particles bigger than 31 mm, while smaller particles should be reduced under the 1% in mass. Therefore, a sieving process is required to obtain the desired quality.

It is not in the scope of the paper to discuss about pros and cons of the two biofuels, while it is worth to highlight the following issues.

Considering wood pellets, the required dewatering processes involve a non-negligible amount of thermal energy [8], e.g. for belt dryers it has been estimated that the thermal energy spent for drying wood chips is about the 25% of the whole pellet production process [9]. Although different types of drying oven can be used [10,11], also powered with renewable energy [12] or with a part of the wood biomass, this particular step is critical especially for small production plants [13]. It is acknowledged that the particle size heavily influences the process time in heating-based dewatering systems (the higher the particle size, the higher the time required for drying) [14,15]. However, smaller wood particles could lead to negative effects for certain drying models (e.g. the deep bed one [16]). Nevertheless, smaller particles also allow to exploit lower drying temperatures, and particular dryers have been developed accordingly [9], which can lead to lower the drying energy consumptions. Unfortunately, current mills available for refining wood chips into sawdust, encounter severe problems when they process wet biomass (i.e. green wood). One of the most diffused types of refining technology for woody biomass is that of hammer mill [9,[17], [18], [19]], which uses a static selective grid to obtain the desired particle size [20,21]. However, the wet sawdust produces a sort of extremely viscous pulp that rapidly obstructs the grid openings, leading the mill to jam [9]. Therefore, it is necessary to reduce the moisture content of wood chips to allow the use of the hammer mill, thus hindering the exploitation of the potential advantages coming from drying smaller particles.

Additionally, it has been observed in literature that powerful ultrasonic waves can further contribute to moisture reduction [22]. Accordingly, some devices can be found which claim to adopt high-speed mechanical energy for both comminution and dewatering of particles [[23], [24], [25]]. However, after more than 20 years from the first patent, this technology still not spread in small-scale pellet plants, as in Italian forestry sites. One possible reason could be that sizes, dimensions, complexity and design mass flow rates justify the costs of this system only for production plants characterized by high productivity (several tonnes per hour).

Concerning microchips production, the wood chippers' setting adjusted to obtain the required particle sizes involves the reduction of the productivity, while the percentage of smaller undesired particles increases. Moreover, the sieving process introduces an upper limit to the yield of microchips, i.e. the obtained quantity will be always a fraction of the total processed wood. A possible alternative is producing coarse wood chips with a chipper, set at its maximum productivity, and then to refine the produced chips with a refiner capable to transform almost the total mass of processed wood into the desired microchips. This would allow a continuous process with maximized output yield and production rates but it is unclear whether current refiners can produce the required particle size distribution without increasing the percentages of smaller particles.

In such a context, the paper presents a new grinding technology capable to process green wood chips to produce sawdust for pellet production, and microchips. In light of the state of the art shortly introduced in this section, the main requirements of the new grinding technology are:

• Throughput compatible with small pellet production plants (up to 1 t/h);

• Produce sawdust from wet woodchips to allow the reduction of drying energy consumptions.

• Produce microchips from wet wood chips, with reduced production of undesired smaller particles.

• Perform preliminary investigations about the adoption of “high-speed” mechanical energy to remove part of the moisture content.

Preliminary tests for the production of pellets and microchips have been performed with a proof of concept prototype [26] of the grinder, installed in a specific experimental plant.