• Energy Research

The study compared the effect of chipper type on productivity, power demand, fuel consumption and product quality. Tests were conducted on two commercial chipper models, a disc and a drum chipper. Both chippers had the same diameter capacity, were applied to the same tractor and fed with the same feedstock types. Fifteen replications were conducted per machine and for each of four different feedstock types, reaching a total of 120 tests. The disc chipper had a higher energy efficiency and used 19% less fuel per unit product, possibly due to its simpler design, integrating comminuting and discharge system in one synergic device. In contrast, the drum chipper was 8% more productive, since it cut with the same energy all along the length of its knives. The drum chipper produced smaller chips, with a higher incidence of fines. Feedstock type had a strong effect on productivity, energy efficiency and product quality. The effect of feedstock type was mainly related to piece size, and may be stronger than the effect of chipper type. Further studies should determine the effect of blade wear on the relative performance of the two chipper types.

The need to mitigate climate change and the progressive fossil fuel exhaustion have engendered a great interest for renewable fuels. This boosted the demand for wood fuel in all industrialized Countries creating a considerable business opportunity for the different stakeholders of the forestry sector. To increase the efficiency of the entire wood fuel supply chain, it is necessary to reduce the cost of harvesting, processing and transportation. One possible way to cut the handling costs is the densification of the chips. This paper reports the results of a study conducted on the densification process of chips, with particular focus on the demand of energy required by the process and on the density achieved. The investigation has been carried out on chips from hybrid poplar (PC) chestnut (CC), and a mixture of spruce and eastern white pine (MC) at different pressures (20, 30, 50, 80 and 110 MPa). The study pointed out that average values for density ranges between 546-898, 559-1082 and 543-913 kg m-3 for PC, CC and MC respectively. Specific energy demand ranges between 22.5-58.1, 18.8-43.3 and 19.4-50.2 kJ kg-1 for PC, CC and MC respectively.

The excessive amount of pig slurry spread on soil has contributed to nitrate water pollution both in surface and in ground waters, especially in areas classified as vulnerable zones to nitrate in accordance with European Regulation (91/676/CEE). Several techniques have been developed to manage livestock slurries as cheaply and conveniently as possible and to reduce potential risks of environmental pollution. Among these techniques, solid-liquid separation of slurry is a common practice in Italy. The liquid fraction can be used for irrigation and the solid fraction, after aerobic stabilization, produces an organic compost rich in humic substances. However, compost derived from swine solid fraction is a low density material (bulk density less than 500 kg􀀀m–3). This makes it costly to transport composted swine solid fraction from production sites to areas where it could be effectively utilized for value-added applications such as in soil fertilization. Densification is one possible way to enhance the storage and transportation of the compost. This study therefore investigates the effect of pressure (20- 110 MPa) and pressure application time (5-120 s) on the compaction characteristics of compost derived from swine solid fraction. Two different types of material have been used: composted swine solid fraction derived from mechanical separation and compost obtained by mixing the first material with wood chips. Results obtained showed that both the pressure applied and the pressure application time significantly affect the density of the compacted samples; while the specific compression energy is significantly affected only by the pressure. Best predictor equations were developed to predict compact density and the specific compression energy required by the densification process. The specific compression energy values based on the results from this study (6-32 kJ􀀀kg–1) were significantly lower than the specific energy required to manufacture pellets from biomass feedstock (typically 19-90 kJ􀀀kg–1).

Comminution is a critical process in biomass supply chains and can be conducted with either chippers or grinders. These machines use different work tools(knives and hammers, respectively) and are applied to different feedstock types, but few studies are available on the inherent differences between the two technologies. A versatile grinder, alternatively equipped with hammers and knives, was studied to investigate the effect of tool type on machine productivity, fuel consumption, and product quality.The machine was powered by an agricultural tractor rigged with a flow meter, a torque meter, and shaft revolution meter for determining fuel consumption, power output, and engine speed. Three different feedstock types were processed: discarded pallets, solid logs, and residues from park maintenance, the latter consisting of branches from ornamental tree and shrub species. Depending on feedstock type, the knife configuration was 3080% (average 50%) more productive than the hammer configuration and required 1530% (average 24%) less energy per unit of product. Fuel consumption per dry ton was 3075% (average 52%) higher for the hammer configuration. That held true for all feedstock types. Product quality was better for the knife configuration, except for park maintenance residues. In general, hammers should be limited to product with severe contamination that prevents the efficient use of knives.

The experimental design consisted of ten repetitions per each combination of two wood species (locust or chestnut), two knife conditions (new or worn out) and two piece breaker options (with or a without). Each repetition consisted of a manually-fed log bunch, weighing about 30 kg. Average productivity with fresh material, wH2O = 36-42%, varied between 4 and 14 t h-1, or between 3 and 9 oven-dry t h-1. Productivity was 15-30% higher for locust than for chestnut, when working with new knives; it was 30-40% higher for chestnut than for locust, when working with worn knives. All else being equal, blade wear determined a 50% reduction of productivity. Installing a piece breaker resulted in a 30% decrease of productivity, but only when knives where new. Specific fuel consumption per unit of chipped biomass varied between 1.5 and 4.2 L t-1, of fresh matter, or 2.3 and 6.4 L t-1 of dry matter. Blade wear had a dominant effect, explaining between 70 and 75% of the total variability. Specific fuel consumption increased 1.4-2.8 times with blade wear, depending on species and piece breaker option. Knife wear and the use of a piece breaker determined a marked increase in the incidence of small chips (8-3 mm) and fine particles (<3 mm), and a parallel reduction in the incidence of large chips (45-9 mm). If the main goal of a chipper operator is increasing productivity and decreasing fuel consumption, managing knife wear should be a primary target.

The authors tested the same chipper under two alternative cut length settings (7 mm and 20 mm), with and without a piece breaker. The study included 10 repetitions per treatment, over 2 different feedstock types: chestnut logs and locust logs. The total number of repetitions was 80, each consisting of about 30 kg of logs. Cut length setting and piece breaker option are the main drivers of chip size, and they are manipulated with the main purpose of managing particle size distribution. Our study showed that the proportion of small chips increased dramatically with the shortest cut length setting (7 mm). Installing a piece breaker allowed maximizing the incidence of small chips, which reached 70% of the total mass when the piece breaker was used in combination with the shortest cut length setting. All else being equal, reducing cut length determined a substantial decrease of productivity (ca. 30%), and an even higher increase of specific fuel consumption (ca. 50%). All strategies to reduce chip size also resulted in increasing the incidence of fines. These results were obtained with new sharp blades. Blade wear may enhance or weaken the effect of cut length and piece breaker option.

The authors tested a chipper prototype adopting a new comminution device, designed to produce high quality chips when processing delimbed logs. The machine was fitted with innovative tubular blades, mounted on a flywheel. The prototype was powered by a 55 kW farm tractor through the standard power take-off. The machine appeared as efficient as most conventional disc or drum chippers in the same size class, but offered a much better chip quality. Chips were free from any particles longer than 45 mm, and with a very limited content of fine particles (max. 2.5%). Of course, this was achieved when using premium wood raw material, such as delimbed small logs. Performance varied with tree species: poplar was the softest and easiest to chip, whereas robinia was the hardest and required a much larger effort. Diesel fuel consumption varied between 3.4 and 4.3 dm3 per oven-dry tonne.

Chipper design is a main driver of production efficiency within forest fuel supply chains, but little scientific knowledge is available about the performance of different types. Two alternative drum chipper designs were tested on different feedstock types and under different knife wear conditions. The closed drum full-length knife design was more efficient than the open drum staggered-knife design, when negotiating branches, especially when knives were dull. Under these conditions, productivity was higher, fuel use lower and product quality better for the closed drum design. These differences were statistically significant. Therefore, a closed drum design is preferable when branches are the main feedstock, especially if product quality specifications are demanding. In general, the performance of both designs was significantly affected by feedstock type and knife wear.