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Consideration has been given to problems of chemical activation of a solid product of hydrothermal carbonization (hydrochar) of biomass. Pellets from a mixture of poultry litter and straw subjected to carbonization in a fluidized bed in a medium of superheated water vapor at a temperature of 300°C were impregnated with water solution of ferric nitrate, dried in the air at a temperature of 100°C, and held for 2 h at a temperature of 300°C for degradation of iron(III) nitrate and its transformation into iron oxides. The pellets were activated at a temperature of 700–800°C in a horizontal tube furnace in an inert argon atmosphere. The obtained ferromagnetic sorbents have a rather developed system of micro- and mesopores. Furthermore, a twofold increase in the duration of activation results in an 18% increase of the pore specific surface, and the rise in the temperature of the activation process leads to an increase in this surface by 45%.

A study has been made of the hydrodynamic characteristics of the transition of a multicomponent bed consisting of coal sludge and straw pellets to a fluidized state. It has been shown that increase in the fraction of large particles (pellets) in the bed leads to a decrease in its relative height and in the amplitude of pulsations of the pressure difference in it; this contributes to the rise in the degree of conversion of chemical reations occurring in the bed. The possibility of assessing the minimum fluidization velocity of a multicomponent bed from the dependence of the standard deviation of a pulsation of the pressure difference in it on the velocity of air blown though the bed has been proved.

Fluidization of two types of a bidisperse bed with marked bimodality of particle size distribution has been investigated experimentally. The possibility of using the mixture equivalent diameter is shown provided the latter is calculated by the weighted-mean diameter of the particles composing this mixture.

Oxidation catalysts, e.g., those for biomass combustion in a fluidized bed (olivine sand and catalyst ShchKZ-1), were compared. Samples consisting of straw powder, straw powder and uncalcined olivine sand, straw powder and calcined olivine sand, and straw powder and crushed catalyst ShchKZ-1 were prepared. The calcined olivine sand and catalyst ShchKZ-1 had practically the same influence on the sample mass loss and heat generation from the straw by heating.

An experimental study has been made of the distributions of the coefficient of external heat exchange over the volume of the fluidized bed with different forms of inlet gas distribution. It has been shown that the grid giving a concave inlet profile of the air velocity ensures a required homogeneity of the bed′s structure at a high level of heat exchange.

Results of determination of effective heat capacity of untreated biomass granules are furnished. It is shown that evaporation of moisture and other volatile components present in the material affects heat absorption by the material. A mathematical model of the temperature field in reactor chambers during low-temperature biomass pyrolysis is given.

The possibility of determining the regime of fluidization of a bicomponent bed by the dependences of the statistical characteristics of the pulsations of the different pressure in this bed on the velocity of the gas blown through it is demonstrated. An algorithm of estimating the characteristic gas velocities corresponding to the transformation of the bed into the regimes of fluidization and turbulent fluidization at room temperature and at much higher temperatures has been developed. This algorithm allows one to determine the indicated velocities more exactly, as compared to the traditional methods, by the dependence of the differential pressure in the bed on the velocity of the gas blown through it.

A relationship is established between statistical characteristics of the random process of press-gradient pulsation in a fluidized bed and changes in the bed’s fractional composition. This relationship can be used to monitor the process of defluidization (transition of the bed from the fluidized to the non-fluidized state) based on the results of a comparative analysis of the above-mentioned statistical characteristics. The feasibility of performing such monitoring was confirmed experimentally from the combustion of lowmelting biogranules (straw granules) in a fluidized bed. It was found that the bed remains in the fluidized state as long as the dimensionless amplitude of the pressure-gradient pulsations does not exceed 0.1. The utility of the proposed monitoring method was also substantiated by comparative studies of changes in the fractional composition of a fluidized bed and the temperature and chemical composition of the flue gases.

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.

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.

1. A significantly higher uniformity of properties over the height of a lengthy component is achieved when it is quenched in a boiling layer with water injected onto its surface than after conventional quenching. In this case, the standard deviation of hardness and the warping of such a component are also appreciably lower. 2. The proposed method makes it possible to improve the quality of quenching, since no wetting of the boiling layer is required on the whole, the layer does not deteriorate in service, and no channels form in it. Use of liquid injected directly onto the component tends to reduce consumption of coolant, which is completely evaporated. The boiling layer remained dry on the whole, and its hydrodynamics is not disturbed. 3. The variation in the consumption of coolant over the height of the component and the subsequent reduction in aeration intensity make it possible to cool the component in precise conformity with its production thermograms; this improves the quality of the quench. Nonuniformity of component cooling with respect to its height is eliminated in this case.

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 © 2005 by ASME

According to our estimations, Russia can use up to 24 million tons of straw as fuel every year without harming either crop farming or agriculture. Straw as biofuel can be used for co-combustion with coal, including low-grade coal and coal waste (slurry). Taking into account the low bulk density of straw and granulometric composition of slurry (the particles are of size up to 0.2 mm) it makes sense to process this fuel into pellets, which are easy to transport and store. The feeding of pellets into a boiler furnace can be easily mechanized and automated. However, straw differs from wood and wood wastes by a high content of chlorine, silicon, phosphorus, and potassium. The presence of these compounds in straw contributes to the formation of slag agglomerates in the furnace, coherent deposits on the boiler heating surface and corrosion under these deposits. For the efficient combustion of biofuel with low-melting ash in boilers with furnaces with a small height the main combustion process should be carried out in the regime of a fixed, forced ventilated bed. Then, after a certain period of time, a bed must be transferred into a state of turbulent fluidization, and then returned to its original state. So the objective of the present work is to study the process of pellets combustion in such a bed (let's call it a pulsating bed).