Jan Erik Mattsson

Particle and handling characteristics of wood fuel powder: effects of different mills

Wood fuel powder cut in various types of impact mills and knife mills was characterised using sieve analysis, laser diffraction, image analysis and a funnel method for measuring the tendency to bridge. In addition, this study aims to find the relationship between particle characteristics, tendency to bridge and the type of mill. The different types of mills produced wood powder with different properties. Both sieve analysis and laser diffraction showed that impact mills produced wood powder with more fine particles than wood powder produced by knife mills. Image analysis showed that particle sizes and shapes differed between the two types of mills. The bridging tendency, measured as the size of a funnel opening needed for the powder to pass freely, was higher for powder produced by impact mills than by knife mills. The bridging tendency was found to be a combined effect of particle size and shape.

Tendency of wood fuels from whole trees, logging residues and roundwood to bridge over openings

Abstract Solid biofuels such as wood chips are common fuels in many heating plants. The fuels are by nature heterogeneous and the fuel particles can form a stable structure across openings, which may stop the fuel flow in the feeding systems of heating plants. This phenomenon is known as bridging and in the present paper the bridging properties of several common wood fuels are studied and the influence of particle size and proportion of long particles, bed depth, raw material, species and moisture content is described. The fuels tendency to bridge was defined as the maximum slot opening over which a fuel will create a bridge. Measurements were done for wood fuels made from whole trees, logging residues and roundwood from coniferous species (Norway spruce, Sitka spruce and Contorta pine) and various deciduous trees and cut by two chippers to four different nominal particle sizes (30 and 35 mm chip and 50 and 100 mm chunk). The most important variable was the proportion of particles longer than 100 mm , which was a result of chipper type, knife setting and raw material. For most fuels, a higher moisture content increased the tendency to bridge. Wood fuels made from roundwood showed no significant correlation between tendency to bridge and moisture content. The study shows that the bridging problems can be reduced by adaptation of the fuel preparation to the given opening.

Basic handling characteristics of wood fuels: angle of repose, friction against surfaces and tendency to bridge for different assortments

Basic handling characteristics were measured for sawdust, fuel pellets, fuel chips, hog fuel and chunkwood. The aim was to find relations between the basic handling characteristics and easily measurable fuel properties to facilitate the design of feeding systems in heating plants. The angle of repose was 25–55° and increased with the increasing ratio of particle length to thickness and with the increasing content of hooked or long particles. The angle of static friction, 10–40°, was affected more by the kind of surface than the fuel and followed the ascending order: coated plywood, urethane rubber, particle board, stainless steel, concrete and rubber belt conveyor. The tendency to bridge varied considerably and increased with greater content of hooked or long particles, a deeper fuel bed depth over the opening and a higher moisture content. The results indicate that more attention should be paid to the particle shape.

Method and apparatus for measuring the tendency of solid biofuels to bridge over openings.

Abstract The paper describes the development of a method to measure the relative bridging properties of solid biofuels over openings. The system consists of a silo, with a bottom made of rubber mats, which can be rolled on a drum, so that a gap is created. Over the gap, a “bridge” of fuel particles is formed. When the gap is wide enough, the bridge collapses and the distance between the drums is measured. This number is a measure of the relative bridging property of that specific fuel. General results are given for a variety of solid biofuels. The bridging property of solid biofuels is mainly controlled by the shape and the size of the particles. If the fuel contains many hooked or long and thin particles, the bridging tendency is high. The influence of the moisture content is not as pronounced, but exists. In further articles, specific results will be discussed.

Influence of particle size and moisture content on tendency to bridge in biofuels made from willow shoots

Abstract Different systems for harvesting and storage of wood fuel from willow shoots give fuels with different particle size distribution, particle shape and moisture content. These factors influence the tendency for wood fuel particles to form a stable bridge over openings, which prevents the feeding of the fuel. The influence of cutting and storage method on the tendency to bridge was studied for chips and chunks made from 3 to 5 year old willow shoots harvested in January and December. Shoots were cut with four different machines to produce five fuel assortments with nominal particle length from 28 to 200 mm , and stored outdoors, in central Jutland, Denmark, in 160 m 3 loose volume piles. Some piles were uncovered, some covered with plastic and two were sealed in an airtight silage plastic film enclosure. The bridging tendency was measured at the end of May and September by determination of how wide a “bridge” of fuel over a slot opening could be before it collapsed. With a 500 mm thick layer of fuel above the slot opening, the bridge width varied between 58 mm for the small chips and 977 mm for the large chunks. Most of the variation was due to two fuel properties, the proportion of particles longer than 100 mm and the moisture content of the fuel.

Tendency to bridge over openings for chopped Phalaris and straw of Triticum mixed in different proportions with wood chips

Solid biofuels with poor flow properties can cause problems in heating and power plants. Bridging over openings is one problem that was studied for chopped reed canary grass (Phalaris arundinacea) and wheat straw (Triticum aestivum) mixed in different proportions with wood chips. The aim was to determine whether the bridging tendency in chopped reed canary grass and in chopped wheat straw depends on the particle size, and if it would be reduced if grass and straw were mixed with dry wood chips. The bridging tendency was defined as the slot opening when a “bridge” of material formed over the opening breaks, and was measured with equipment developed and tested for wood fuels. Earlier studies showed that long and thin particles have a high tendency to bridge. This was confirmed for the straw and grass studied. Their tendency to bridge was 6–8 times higher than that of wood chips. Mixing wood chips in grass or straw decreased the bridging tendency, but only marginally. Even when 80% of the dry matter was wood chips the mixtures had a bridging tendency that was 3–5 times higher than that of pure wood chips. The results indicate that mixing wood chips into chopped reed canary grass or straw is not a promising concept to decrease their bridging tendencies. The lack of suitable methods to classify particle shape in mixtures of wood chips and straw or grass was an obstacle to a closer analysis of how the particle shape affects the bridging tendency.

IEA Bioenergy Agreement Task IX: harvesting and supply of woody biomass for energy 1992-1994

Abstract The achievements of Task IX Harvesting and Supply of Woody Biomass for Energy are discussed. The main achievements are an increased industrial involvement in the Task, an improved contact between researchers dealing with ecological issues and those working with R,D&D of harvesting and supply systems, development of a spread-sheet decision support system to evaluate integrated biomass to heat or electricity systems, and a greater awareness of the importance to control and improve fuel feedstock quality in all stages of harvesting and supply systems.