INFLUENCE OF DENSITY OF WOOD BIOMASS FILLING ON THE CHARACTERISTICS OF ITS DRYING UNDER HIGH TEMPERATURE HEATING

Abstract

Relevance. The analysis of the forecasts of specialists shows that despite the active attempts of many states to introduce renewable energy sources into national energy systems by 2040, the bulk of the production of thermal and electric energy will still fall on thermal power plants that burn coal. For this reason, at present, active development of new technologies for fuel combustion is underway, which will completely (or at least significantly) reduce the use of coal at thermal power plants. One of the most promising areas is the combustion of woody biomass in the furnaces of boiler units. But the full-scale introduction of biomass as a fuel at thermal power plants is hampered by the lack of knowledge of heat and mass transfer that occur during wet wood drying. This is due to the fact that, as a rule, freshly harvested wood is significantly saturated with moisture. Combustion of such a highly watered biomass in combustion devices is ineffective. With a high initial humidity, a lot of heat will be spent on evaporation and, accordingly, the efficiency of the steam generation cycle may decrease. Therefore, the initial moisture-saturated raw material must be pre-dried. In this regard, one of the open problems of the energy sector at the present time is the lack of study of the main regularities of heat and mass transfer, occurring in a layer of finely dispersed moist woody biomass during its drying. The latter is necessary for development of the fundamental foundations for conducting experimental design work to substantiate the design characteristics of modern installations for biomass thermal preparation for combustion. In this case, as a rule, drying of large volumes of wood is carried out in large-sized equipment of the bunker type. In such bunkers, the height of the biomass layer can reach several tens of meters (up to 20 m). Obviously, under these conditions, wood density gradients (due to caking) will also be significant. It has been reliably established that the characteristics of moisture removal from a layer of moist woody biomass are influenced not only by the ambient temperature, but also by the filtration characteristics (density, pore size, permeability, porosity) of the dried material. But until now in the world scientific periodicals there are no publications describing the results of assessing the influence of moist finely dispersed wood filling density on the characteristics and conditions of its dehydration. The aim: experimental studies of the influence of moist woody biomass layer backfill density on the integral characteristics of its drying. Object: wet woody biomass (pine sawdust). Experimental studies were carried out at the following values of the degree of compaction of biomass γ: 1; 1.5; 2 (where γ=ρ/ρ0). Method: experimental studies of moisture removal rate and the time of moist woody biomass dehydration at high-temperature heating (Tg=333–393 K) in the SNOL-3.5-M2U42 drying chamber. Results.The paper introduces the results of experimental studies of moisture removal from a layer of moist woody biomass (based on typical woodworking waste – sawdust) under intense radiation-convective heating. The authors have analysed the influence of the wood layer bulk density (ρ) on the dehumidification characteristics. It was found that the increase in the backfill (ρ) density leads to a significant change in the dynamics of moisture removal (an increase in γ from 1 to 2 leads to a twofold increase in the drying time td). It was shown that such increase in td is characteristic for all values ​​of Tg. Also, according to the results of experiments, the characteristic values ​​of the mass rate of moisture removal (Weva) were established depending on time at various degrees of compaction of the biomass layer. It is shown that under conditions of relatively low ambient temperatures, the function Weva(t) changes non-monotonically, but has a wave character. A hypothesis is formulated describing such a dependence of the Weva values ​​on time. The results of the studies carried out significantly expand modern understanding of drying finely dispersed woody biomass under conditions of intense radiation-convective heating.

Keywords

• woody biomass
• woodworking waste
• backfill density
• drying
• heat and mass transfer
• moisture removal rate