Rafail L. Isemin

Investigation on Agropellet Combustion in the Fluidized Bed

Agricultural wastes (straw, sunflower or millet husk, etc.) are difficult to use as fuel because of low bulk density and relatively big ash content with a low melting point. It is possible to produce agropellets of agricultural wastes which are suggested to combust in a fluidized bed of pellets alone, their char particles and ash. The characteristics of the process of fluidization of agropellets are investigated at room temperature. The experiments on agropellet combustion in a fluidized bed are carried out in an experimental set-up. The results of the experiments have shown that in such a bed the pellets produced of straw and millet husk combust with the same rate as those of wood though the latter contain 8.76 – 19.4 times less ash. The duration of combustion of the same portion of straw pellets in a fluidized bed is 3.74 – 7.01 times less than the duration of combustion of cut straw in a fixed bed. Besides, the movement of agropellets prevents agglomeration and slagging of a boiler furnace.

Co-Fluidization of Fine Particles and Straw Pellets at Room and Elevated Temperatures

The results of the research on fluidization of multi-solid materials in cold and hot models have been introduced. It turned out that the change graphs of the statistical characteristic of pressure fluctuation can be used for evaluate values of fluidization velocity. Index Terms—Multi-solid bed, Pressure fluctuation, Straw pellets combustion, Turbulent fluidization. I. I NTRODUCTION The technology of dedicated biomass combustion and coal and biomass co-combustion in a fluidized bed is increasingly applied aiming to achieve a better control over a combustion process with a simultaneous decrease in greenhouse gases, ash and sulfur oxides emissions into the atmosphere. Straw is one of the most easily available biomass resources and its energy utilization is also constantly expanding requiring longer transport distances of the material. This stimulates the production of straw pellets, with a bulk density of 650-750 kg/m³ over 100-150 kg/m³ of baled straw. Fuel combustion can be carried out in a co-fluidized or multi-solid fluidized bed of fine coal ash particles or straw ash and straw char particles loaded with straw pellets. A multi - solid fluidized bed is a circulating fluidized bed in which an entrained bed consisting of fine particles is transiting through a fluidized dense bed of coarse particles, i.e. a bubbling bed in the bottom of the riser (1)-(4). The value of the minimum fluidization velocity of particles forming the bed is required for the design of fluidized bed furnaces. However, as has been proven (5)-(14) it is not possible to apply the known method of a minimum fluidization velocity evaluation from the pressure drop through a bed versus gas flow rate curve for a confined, multi solid or binary particle mixtures fluidized bed. The purpose of the present study is to define a method for the experimental evaluation of the minimum fluidization velocity of a bed compounded of fine and coarse particles mixtures at room and elevated temperatures.

On Effect of Air-Separating Grid on Low-Grade Solid Fuels Combustion in a Fluidized Bed

In furnaces and other types of fluidized bed reactors one can easily observe gas bubbles flow and fluid jet preferable rise zones where solid particles in form of fountains are carried away from the layer. Let us call these layers as zones of local flowing. We assume that in these zones of local flowing occurs the removal of unburned fuel particles from fluidized bed. It results in heat leakage up to 20–40% due to mechanical imperfection of fuel combustion in types of vessels with fluidized bed furnaces. We investigated particles with diameter of 2,5–3,0 mm motion and local flowing zones formation in a model column of 172 mm in diameter at room temperature. Experimental application of air-separating grid enabled to form at the layer input “flat” and “convex” air velocity line. At the “flat” line the air distributed uniformly on the input section of the device. When the line was “convex” the air velocity at the layer input in the device center was twice higher than at the device periphery. It was found out that at “flat” line when air velocity increases there appeared zones of local flowing in the layer. To eliminate these zones one should increase the height of stationary particles layer. At “convex” line of input air velocity elimination of local flowing zones is possible by stable increase of air velocity or by lowering the height of stationary particles layer. It was concluded that for small fluidized bed fireboxes it is better to apply air-separating grids forming “convex” input air velocity line. The grid under discussion was manufactured to be applied in experimental fluidized bed firebox. There was carried out a detailed study of ordinary anthracite combustion in a firebox. It was established that at “convex” input air velocity line fuel loss due to entrainment amounted 2,0–2,5%. According to experimental results there were designed water boilers with heating efficiency from 300 up to 800 kilowatt. In these water boilers there was applied the grid forming “convex” input air velocity line. During tests there was burned culm (particle size from 0 up to 6 mm, heating efficiency of 20,6 MJ/ kg, output of volatile matter up to 6%, zoning 26,2–31,1%). Efficiency factor of these water boilers is 83–85% (without entrainment return to the firebox).Copyright