Study of Flax Shive Pellet Combustion in Cyclone-Bed Furnace with Bubbling Fluidised Bed

Flax shive is a by-product of flax processing enterprises. It can be used as fuel in municipal and industrial energy enterprises along with other types of agricultural waste (straw, sunflower husk, rice husk, etc.). The use of agricultural waste as fuel is very important for the sustainable development of municipal and industrial energy in small towns and villages, as it allows for the recycling of waste, a reduction in greenhouse gas emissions and the creation of new jobs in the production of pellets from waste. Flax shive has a low bulk density in its initial state. To achieve a significant reduction in transport and storage costs, flax shives should be produced as pellets. The pellets from flax shives can be combusted in a fluidised bed of inert material. To do this, an optimal hydrodynamic structure of the bed should be created, taking into account the rapid heating of pellets placed in the bed, as well as the efficient combustion of coke residue and volatile substances. Studies on a cold model showed that the hydrodynamic structure of the fluidised bed is influenced by the way the inlet air is distributed into the fluidised bed. Studies were carried out with flat inlet air distribution, with convex inlet air distribution (64% of the total amount of air for fluidisation is supplied under the central part of the air distribution grate) and with concave air distribution (80% of the air for fluidisation is supplied under the peripheral part of the air distribution grate). Concave air distribution creates optimum conditions for pellet combustion. Intensive particle circulation is noted in the lower part of the fluidised bed, which prevents the pellets entering the bed from settling on the air distribution grate. At the same time, in the upper part of the bed, conditions for uniform thermal destruction of pellets across the entire cross-section of the furnace are created. Experiments on a pilot furnace with vortex afterburning of volatile substances in the furnace’s disengaging space were carried out using a grid plate, forming a concave inlet air velocity profile. The combustion efficiency of pellets made of flax shive in such a furnace was 98.91–99.84%.