Biomass wastes provide an attractive and sustainable energy source. Moreover, biomass is considerer one of the key renewable resources of the future which already supplies 14% of the world’s primary energy consumption. In developing countries biomass is the most important source of energy and it is likely to remain an important global source into the next century [1]. Besides, biomass energy technologies can minimize the greenhouse gas emissions, because carbon dioxide released in biomass combustion is absorbed by photosynthesis during production of new biomass, therefore the net emission of carbon dioxide per closed carbon cycle is zero. Wood and furniture Industry generates a large amount of biomass wastes in processes, according to Statistics National Institute, INE 2014, 145.8 mile tons were produced in Spain in 2012. Sawdust and shavings (44%) along with wood chips (32%) account for 76% of the total volume of waste generated.

Spouted bed technology has been proven successfully for the treatment of coarse and sticky solids, of irregular texture [2], of a wide particle size distribution or density with low segregation [3], due to the vigorous cyclic movement of particles, obtaining good results in thermal processes, such as drying [4-11], combustion [12-16], gasification [17] and pyrolysis [18]. The clean technology based in the Spouted Bed gas-solid contact method in conical spouted beds is an alternative easier and more economic (regarding to design, construction and operation) that other solutions such as moving bed dryers, rotary, of trays or of fluidized beds, for energetic valorization, by combustion, of biomass wastes due to power consumption reduction, not requirement of previous milling of biomas and powder generation minimization. The exploitation of a renewable energy source as biomass in spouted beds contactors can be considered as an alternative that guarantee sustainable development.

In this paper, combustion spouted bed regime has been used to produce energy from biomass wastes. The behaviour of the conical combustor has been analyzed by means of hydrodynamic and thermal studies in beds consisting of granular materials. The combustion has been carried out in a conical reactor at operating conditions to improve the environment and the evolution of combustion gases with the time has been analyzed. From the composition of the gases, the effect of the temperature on combustion efficiency has been calculated. In addition, radial and axial temperature profiles have been measured inside the combustor.

The experimental unit designed for this purpose on a pilot scale, Figure, consists of a blower, the conical combustor, two high efficiency cyclones, an electrical resistance to heat the gas at the inlet of the contactor and thermocouples to measure temperature at different positions inside the reactor. The flow rate is measured by means of mass flowmeters controlled by computer and by rotameters.

Biomass wastes studied have been derivatives of wood Industry: sawdust of density, r_s, 560 kg/m³ with particle diameters between 0.8 and 5 mm and their mixtures, with moisture content of 12.5 wt % in dry base. Different fractions of biomass have been separated by means of meshes in Filtra FTI-0300 sieving machine. The low calorific value of the biomass wastes measured with the bomb calorimeter type PARR 1341 is 20.2 MJ/kg. The stagnant bed height values used are between 0.05 and 0.15 m.

In order to quantify the segregation, solid sampling has been carried out at different bed levels in the bed, by means of a probe connected to a suction pump. The probe is a vertical tube of 14 mm i.d., whose radial and longitudinal position can be established inside the contactor by means of a computer-coordinates displacement device, which established the coordinates of the sampling points following a given sequence. The optimum sampling duration was estimated between 3 and 5 s, as shorter times give way to errors inherent in withdrawal of small amounts of sample. Longer times, corresponding to considerable quantities of sample with respect to the inventory, alter the bed composition. Each sampling is repeated three times at each position in the bed and the solids are returned to the contactor after each sampling.

The segregation has been quantified by using the mixing index calculated from the experimental values of weight fraction of particles of greater diameter or density in the upper volume half of the bed,  and the weight fraction in the whole bed,  [19] as a function of the geometric factors of the contactor, of bed composition and of air velocity.

Stable operating conditions for the combustions of biomass wastes have been obtained in a conical spouted bed combustor. Minimum gas flow of the stable spouted bed regime overcomes largely the stoichiometric gas flow for the combustion, which ensures the complete process. The validity of the ranges of the geometric factors of the contactor and of the contactor-particle system for stable spouting has been established and minimum spouting and minimum dilute spouted bed velocities have been determined by means of an empirical correlation at combustion temperatures. The suitable design of the conical spouted bed reactor for the combustion of biomass wastes requires the previous knowledge of the ranges of the geometric factors of the contactor and of the contactor-particle system for stable spouting. The temperature profiles as well as the high combustion efficiency values obtained prove the good performance of the spouted bed combustor for combustion of biomass wastes.

 

 

 

 

 

 

 

 

 

Figure.       Experimental equipment conical spouted bed combustor

 

 

ACKNOWLEDGMENTS

This work was carried out with the financial support of the Spanish Ministry of Science and Innovation (Project CTQ2010-18697) and of the Government of the Basque Country (S-PE13UN117).

 

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