A review of the mechanical behaviour of straw when compressed to high densities

Abstract

The present state of knowledge is reviewed concerning the high density compression of straw in relation to the production of briquettes for fuel or other purposes. Consideration is given to the extensive body of experimental data and theoretical analyses of the mechanics of the compression of fibrous materials. The available information on the formation of high density briquettes in closed and tapered dies is discussed. The information is classified under four broad aspects, namely: (1) the physical properties of straw, (2) the compression characteristics, (3) the formation of briquettes, and (4) theoretical considerations of compression. The many variables involved are discussed in relation to die pressure, specific energy required and the relaxed density and durability of the straw briquette. Particular attention is given to factors of briquette formation such as die size, die charge, maximum pressure, dwell time, straw chop length and moisture content, and type of straw. Some consideration is given to relevant information from work on the compression of grass and lucerne. Work on the effects of temperature and additives is considered and the mechanism of briquette formation discussed. The theoretical work on the compression of fibrous materials in dies is examined in relation to empirical formulae relating pressure and density. These include studies of pressure distributions within the material and rheological models based on spring and damper elements. The value of approaches using dimensional analysis and multivariate analysis is also discussed. Areas of work are identified where further research is required to advance the design of machines for producing high density straw briquettes.