The use of biomass for energy purposes is related to its moisture content, availability, and pre-treatments such as the drying process.
The moisture content of the biomass used for energy production is a key parameter for the proper management of the power plant or in the densification process.
Generally, the wet wood biomass has a moisture content, on a wet basis, higher than 50 percent and the natural drying process hardly lowers the moisture contents under 35 percent in 3–4 months of storage.
High moisture content of fuels increases the cost of transport, reduces the combustion efficiency, and decreases the potential energy input for steam generation.
Consequently, a reduction in the calorific value of the fuel gas produced in gasification is experienced, with a negative effect on the efficiency of power generation in combustion, gasification systems, and pyrolysis processes. Concerning human health, higher biomass moisture content causes an increase of CO and VOC emission as well as the formation of carcinogenic compounds from wood combustion.
The fine particles may be responsible for severe diseases, like invasive pulmonary infections or broncho-pulmonary allergies, whereas the larger particles one may have a role in air and soil contamination.
Forced hot air drying is a process for the conditioning of biomass (firewood and/or wood chips) which allows increasing the efficiency and flexibility of combustion, transportation, and storage process. It may increase the calorific value, lower the emissions and save fuel.
The principles of biomass drying can also be applied to increase the time to preserve food.
However, the choice of a suitable drying system and drying conditions is critical to achieve the required final moisture content.
Although the forced drying process is a suitable alternative to natural drying, it presents higher production costs. The drying process consumes a significant amount of energy, so it would be very important to implement energy saving strategies to reduce energy consumption during the drying process.
Rotary Dryer and Prototype
The most common industrial systems for drying biomass are conveyor dryers, rotary dryers of single or multiple passes, fixed and mobile bed dryers, perforated floor bin dryers, direct and indirect fired rotary dryers, cascade dryers, superheated steam dryers, microwave dryers, fluidised bed dryers, screw conveyor dryers, and flash or pneumatic dryers.
However, it is known that rotary dryers have a low cost of maintenance and consume 15 percent and 30 percent less in terms of specific energy that the pneumatic and cascade types, respectively. An exhaustive description of the drying systems is present in Mujumdar.
The rotary dryer is the most diffuse system for drying small-sized woody biomass. Considering the method of heat transfer, rotary dryers, can be classified as direct, indirect-direct, indirect, and special types.
The direct rotary dryers consist of a slightly inclined metal hollow cylinder, rotating around its axis. The internal space is designed to ensure direct contact between the biomass and the drying fluid, usually hot air.
Rotating dryers have the advantage of being less sensitive to particle size and can accept the hottest exhaust gases of any type of dryer. They have lower maintenance costs and greater capacity than any type of dryer. The drying process of the wet biomass in a rotary dryer can be challenging owing to the prolonged time for the uniform drying of the biomass, this can also increase the fire hazard inside the dryer.
The prototype of rotary dryers was a cocurrent rotary dryer with drum designed for wood chips composed of a metallic rotating cylinder of 5 m length and 0.8 m diameter and a volume of 2.5 m3.
The cylinder was provided with four openings: two for the loading and unloading of the product and two for the inlet and outlet of the drying fluid (hot air). The dryer was placed on a mobile floor and was equipped with a system of ventilation of the MZ aspirator (Italy) and dust recovery with two bag filters connected in parallel with a 50 cm diameter, a height of 1 m, and a total dust collection efficiency of 98 percent.
The wet biomass was loaded into a hopper equipped with a 150 mm diameter screw conveyor of Pelltech (Germany) for the transport inside the cylinder. Here, the advancement and the subsequent unloading of the biomass was favored by the rotation at 5 revolutions per minute and by an angle of the cylinder of 2° to the horizontal axis.
The rotation of the cylinder was regulated trough an electrical board and a gear reducer. The adjustment of the height occurred trough the setting of two support legs powered by two electrics motors.
The internal metal structure was provided with flights (48) differently shaped which favored the mixing of the mass. The set of wings was fixed on a supporting structure, removable from the cylindrical body, which rotates with the drum.
The prototype was connected to a commercial hot air generator of 80 kW (Company D’Alessandro Termomeccanica mod. GSA) through a 250 mm insulated pipe. The volume of the hopper for the supply of the hot air generator was 0.19 cubic metres.
The internal temperature of the cylinder was monitored by two k-type termocouples with a thermal range from −60 to 350 degree C and a resolution of 0.2 degree C; the first positioned at the entry point of the fresh biomass, the second placed at the exit.
Both values were displayed on the electric panel. The cylinder was closed inside an insulating structure to limit thermal dispersion and to reduce the risk of contact with hot surfaces.
Experimental Procedure and Characterisation
The feedstock used in the study was woodchip of poplar, black locust, and grapevine pruning. The drying cycle had a constant air flow and lasted 8 h for poplar, 6 h for black locust, and 6 h for pruning of grapevine.
Poplar plants were processed with a chipper FARMI mod. The black locust was processed with a drum chipper of the Pezzolato mod. The quantities of processed biomass were as follows: 250 kg for both the poplar tests, 207 kg for the black locust, and 144 kg for the grapevine. It should be noted that the pruning of vineyard underwent a preliminary drying in open field for ten days before chipping.
During the tests, biomass sampling was carried out every two hours. Along the system, temperature, rate, and speed of the airflow were measured at three control points (checkpoint, ChP).
The first (ChP1) was positioned on the duct conveying the hot air from the boiler to the rotary dryer. It was chosen a point far from the boiler five-fold the diameter of the duct, to avoid the influence of turbulence when reading the data.
The second (ChP2) was placed in the same duct immediately before the entrance of the drying fluid in the dryer, while the third (ChP3) corresponded to the output of exhausted fluid from the system. The values of temperature, speed, and rate of the airflow were detected at each ChP with a wire thread anemometer (TSI mod. 9535-A).
The temperature inside the cylinder was measured by two probes (Probe 1 and 2) and displayed by the control panel. During the drying process, to optimise economic and environmental performance, the dryer generator was fed with the same dried biomass (Poplar) as was obtained from the dryer.
On average, the airflow temperature, rate, and speed were higher in poplar than in grapevine and black locust. The reader must be aware about the differences of the biomass in terms of initial moisture content (higher in poplar) as well as their particle size distribution and mean particle size (higher for grapevine).
As a general behaviour, there was an abatement of all the variables going through ChP1 and ChP2 (before the entry) to ChP3 (at the exit), showing a clear interaction between the energy provided during the drying process and the resident biomass.
Overall, the data showed as drying required different patterns of heat transfer depending. Each pattern was influenced by the characteristic of the biomass which driven the energy demand during the exsiccation process as well as the time needed to dry.
To this aim, it should be noted that the initial moisture contents of the biomass was greatly different, those of Poplar 1 and Poplar 2 being higher than 50 percent and those of grapevine and black locust slightly above 30 percent.
Moreover, even if the moisture was comparable, grapevine and black locust differed for the bulk density and the particle size distribution. High values of moisture content appeared to influence more the heat content of the airflow rather than the rate or its speed.
When using drier biomass, the flow of the air led to a higher loss of both the rate and the speed of the airflow. Viewing the process as a chain (recovery of waste energy at small scale for drying residual biomass), the best results can be obtained through the optimisation of each step involved.
This means that a trade-off must be sought between the improvement of the residue’s properties which influence the drying efficiency (moisture content, bulk density, particle size distribution) and the variables affecting the drying rate which are determined by the characteristics of the device (in this case, temperature and rate of the drying fluid, thermal insulation of the dryer, systems of recirculation of the drying air).
Characterisation of the Biomass
The characterisation of the biomass was carried out at the LAS-ER-B laboratory of CREA-IT in Monterotondo (RM).
The moisture content was monitored at regular intervals of two hours. In all tests, three samples of approximately 300g each were collected in plastic bags, sealed, labeled and transported to the laboratory where they were dried in an oven with forced ventilation for 24 h at 105 ± 2 degree C.
The bulk density of the biomass was determined before and after drying, making five weighing with a normalised cylinder of 0.026 m3. Before the drying tests the analysis of particle size distribution was performed on the incoming biomass.
As reported by the cited authors, if the mean particle size can be calculated using geometric means, which partly compensate for such skewness towards the lower size classes. The mean particle size is therefore obtained by a weighted average of all particle classes.
During the experiment, except for grapevine pruning, the amount of dried biomass was above 200 kg. The reduced amount used for the grapevine pruning (143.7 kg) was imposed by the physical characteristics of the biomass, because a higher amount would have increased the risk of clogging at the flight system during the movement of the biomass inside the cylinder.
In the drying process, the deflectors cyclically moved the solid fluid by lifting and dropping it through the drying fluid. In this way the hot air flow blended directly with the wood chips inside the cylinder, and then released its heat.
The heat was mostly conveyed during the drop of the product. However, the system presented some losses caused by the contact of the biomass with the cylinder walls, when the heat was transferred by conduction and irradiation.
The type of biomass and the characteristics of the drying fluid influenced each other leading to different conditions inside the cylinder and, in turn, to a different pattern of drying.
In eight hours, the moisture content of the Poplar 1 decreased from 54.4 percent to 43.3 percent.
On the other side, after a drying cycle of 6 h the moisture content of the biomass dropped to 15.3 percent for the grapevine and 11.8 percent for the black locust.
The two dehydration curves showed a similar trend although in all point of data recording the difference between black locust and the vine pruning residues value was significant.
However, these values were obtained with a trend of the temperature inside the cylinder that was completely different with respect to the poplar and between the grapevine and the black locust too. During drying of the grapevine, the temperature inside the cylinder rose from 40 degree C to 61 degree C while for the black locust it increased from 39 degree C to 45 degree C.
These results confirm the observations of Kocsis about the influence of the temperature of the drum on the drying rates of biomass. With increasing temperature, the rate and the time required to lower the moisture content decline.
The heat provided at the beginning increases the temperature of particles. In our case, this increment was affected by initial moisture content (higher in poplar than in black locust and grapevine) and a higher amount of heat was spent to warm up the particles of poplar.
In the following phase, the drying rate of particles was affected only by drying conditions involved in transferring the water layered on the surface of particles to gas flow. In the case of vineyard and black locust a role in increasing the level of the temperature inside the cylinder was played by the different air permeability of the biomass.
In fact, as described previously, the higher mean particle size of the grapevine with respect to the black locust lowered the pressure resistance to air leading to a more pronounced increase of the temperature into the drier.
Although, with the increase of temperature inside the cylinder, the time required to achieve the same moisture content may decline in the present study the higher temperature during the grapevine drying did not bring an improvement of the drying rate of the grapevine (see below). Other characteristics of the woody particles like the thickness and weight may have affected such a result.
Viewing Rotary Drier as an Excellent Device
The characteristics of the biomass have shown to influence the technical parameters of the drying process. The moisture content of the biomass as well as the particle size distribution and the bulk density determined a difference in the intensity of airflow temperature, rate, and speed, and this in turn affected the energy demand of the rotary drier.
In the present study, the drying process allowed a reduction of the moisture content of 35 percent, 53 percent, and 63 percent respectively for poplar, grapevine, and black locust, with a corresponding increase in the energy content of the biomass of the 52.1 percent, 33.1 percent, and 43.0 percent.
On the other hand, at the same operating thermodynamic conditions, the data indicate a thermal efficiency for the grapevine of 12 percent compared to 37 percent of poplar and 27 percent of black locust.
Based on the results, in our opinion the rotary drier presented and assessed in the present study may be viewed as an interesting device for the small farms equipped with energy plants (biogas, gasifiers, and cogeneration). The main strengths of the prototype are the the simplicity of the design, the small size, and its easy handling and transportability.
In agricultural contexts where the environmental awareness favours the adoption of energy approaches of self-consumption, the prototype may provide the opportunity to dry residual biomass at low cost through the recovery of waste heat from the energy plant.
This choice may also entitle to access at incentive rates for the recovery of residual heat. Being a prototype, the drier is susceptible of further improvements increasing its efficiency: these should concern the recirculation of the drying air, the thermal insulation of the dryer, and the increase in the temperature of the drying fluid.