Fuel from straw : an in-field briquetting process
Disposal of large quantities of surplus straw, which lie in the fields after harvest, is a major annual problem to cereal farmers. The current preferred solution of burning the straw where it lies is environmentally unsatisfactory and appears to be a huge waste of a potentially valuable, renewable energy source. None of the currently available straw packaging systems provides an economically viable alternative. A process is proposed for producing industrial quality fuel briquettes using a tractor towed implement. The economic feasibility of such a system is investigated and comparisons are made with existing straw disposal methods. The projected cost of fuel, produced in this way, is compared with prevailing fossil fuel prices. A multistage continuous process machine concept is described and the various stages are proven workable both experimentally, in the laboratory, and analytically. Laboratory experiments determine the forces required to produce acceptable quality briquettes and comparisons are made between the power available from the tractor, the economical throughput rate and the energy consumed in the compaction process. The mechanism of bonding within the straw packages, under compression, is examined so that the parameters necessary to give the optimum machine design may be understood. The effect, on briquette quality, of variations in die shape within the constraints imposed by the machine concept is fully investigated. Experiments extend to compression at speeds representative of 'live' field operation and a die shape is developed which produces packages of consistently good durability. Many of the design ideas put forward in this thesis have now been incorporated in an original prototype machine, built and successfully field-tested by the company who has supported this project and now holds the relevant patents.