Physical Property Measurment Of Rice Straw
Dear Simon,
You have chosen an awkward material to quantify some of the properties described. For many, the measurements are dependent on both the prevailing stress and the stress history, so the defining conditions of sample preparation is most important.
Dealing with some of these: -
Size. Products of irregular size, dendretic shape, with high aspect ratio or uneven surface texture can be difficult to define with precision, so the purpose of seeking quantification is important. For reference purposes, samples may be selected to reflect the extreme range of the product, with a statement of how this is likely to vary with season, source, process or other factor.
Bulk density. There is no unique bulk density value for any particulate product although some will vary considerably more than others, especially fine powders and bulk materials that have soft and flexible members. In general, loose-poured, fully-settled material in shallow beds and lightly tapped samples will highlight basic states, but controlled uniaxial compaction tests are needed to establish the 'compactability' and determine porosity in differing density conditions. The process condition of interest should provide a guide to the means of preparing and testing a sample. Note that the condition of bulk density is a key, defining characteristic of the potential strength of a bulk solid.
Solid density This usually has little relevance to flow situations and members with hollow stalks or inclusions can deprive the value of significance.
Static angle of friction. Actually, this is the value normally estalished by the sliding plate test. To determine the dynamic angle of friction it is necessary to stimulate sliding and than find the minimun angle of slope at which the sliding will continue.
Angle of repose. This is a much over-valued measurement as it relates only to the slope formed by bulk materials prepared in a specific manner. In some cases there is no meaningful angle or repose for materials such as straw, that can adopt any surface sope and overhang to be inverted in certain circumstances. The 'poured' angle of repose is usually formed by a dilated flow strem of product onto a single point fill of a pile. The main use of this value is to establish the degree of ullage in a hopper or the volume of a stockpile. The 'drained' angle of repose is usually a conical surface developed during core flow to an internal flow channel. This is invariably steeper than the poured repose angle because the surface flow stream is converging, rather than diverging, so there is particle interferance instead of separation, meaning that both values are to some extent radially dependent. It also depends on the storage stresses and time consolidation, so may be highly variable to the extent of being meaninless. Plane flow repose surfaces differ again, and whilst being less common in practice do provide a more fundamental value of surface behaviour of free flowing solids.
Shear strength This has not been mentioned and is most awkward to evaluate for overlapping compnents, such as straw-like materials where the isotrophic structure has great significance. Shear cells are generally not equipped to deal with such materials and an unconfined failure test is usually the best way to secure guidance of to flow prospects, (which are generally pretty lean !). Hopper design and feeders for interlocking bulk materials are a specialists task.
For your interest, I am preparing a 'Paupers guide to powder testing' that includes an unconfined failure method, if you care to contact me at lyn@ajax.co.uk for a copy. ■
Camel's Back
This is a complex quest. Further to Lynn's comprehensive advice there is the "...defining conditions of sample preparation is most important." Indeed.
What will be the longest straw? If fibres protrude after the pressing then they will get nipped between adjacent briquettes.
Degree of nip will also be influenced by the edge radii and mould draught.
In this day & age why not consider optimising the lump geometry from Discrete Element Modelling. It ought to be a cost effective & accurate exercise since you only have one particle size, at a time, to run except for variable densities. Methinks. ■
Physical Properties of Rice Straw Briquettes
Determination of physical properties of rice straw briquettes
Dear bulk-online community,
We’re doing a project on briquetting of agricultural residues for energetic use, namely of rice straw.
I am investigating on the effect of different applied pressures and composition of the raw material (particle size and moisture content) on physical properties of the briquettes. The briquettes will be cylindrically shaped, with a length of approximately 50 mm and a diameter of 65 mm.
Physical properties measured will be size, volume, bulk density, solid density, porosity, the briquette durability, the coefficient of static friction on different materials (plywood, aluminum, stainless steel, rubber), the angle of repose and of course the heating value. Chemical properties will also be measured, but since methods to determine them are very well explained in European standards (CEN/TS 335) these are not part of my questions here.
My first question is about your opinion concerning these parameters. Are there from a practical point of view other physical properties that should be investigated on?
The other question is a little bit more complicated. For most of the measurement I’m able to use European standards. However, this is not the case for the coefficient of static friction and the angle of repose.
Static friction should not be a problem, since I’m using the inclined plane method which is very well described in literature and can be adapted to different scales.
For the static angle of repose, I am using an adaption of ISO 4324:1977 “Surface active agents -- Powders and granules -- Measurement of the angle of repose” for larger material with a baseplate of 0.8 m in diameter and a falling height of 0.7 m. The material is transported to the baseplate via conveyor belt.
The problem is the emptying angle of repose. I know, the dynamic angle of repose is often referred to the angle, a material forms when in movement, such as on a conveyor. I read now through literature for several days and it was quite confusing. Some people refer to the emptying angle as another static angle of repose, others always call it the dynamic angle of repose. Do you have any hints, what is correct here?
I found several methods, like the revolving cylinder method, the tilting surface and the emptying box method that could be used for determination of the emptying angle. Most of them are not applicable for me, be it because of the size of my bulk or because of external restrictions (lack of measurement devices).
I now thought about using the emptying box method with some adaptions.
Either as described in this patent (http://www.freepatentsonline.com/3940997.pdf ), I would use a box with an open top then and slowly rotate it 90° so the briquettes can flow. The resulting angle on the inside of the box would then be my emptying (dynamic?) angle.
Another way would be again to use a box with a removable front panel. The box would be situated 0.5 m above ground, so the material could float freely. The resulting angle on the inside, when the material comes to rest again would be my emptying angle.
What is your opinion about these methods? Which one would you prefer? Are there any other suggestions or experiences on measurement of this angle?
The data I will collect in fact shall be utilizable for usage in practice, so any advices from professionals are welcome!
Kind regards and many thanks in advance
Simon ■