Selection of biomass supply for a gasification process in a solar thermal hybrid plant for the production of electricity

Abstract

The great potential for bioenergy in Spain is undeniable given our country´s enormous biomass supply. This fact contrasts with the limited evolution in the biomass sector for thermal and electricity generation over recent years. In this paper, we consider the utilization of fluidized bed gasification (FBG) as a biomass utilization technology incorporated into a thermal electric system to improve power plant production both thermally and electrically. Firstly, we studied the biomass resources available within a 100 km radius of the plant’s location in Almería province (Spain). This biomass included almond shells, olive tree prunings, holm oak prunings and vegetable residues from greenhouse tomato and pepper plants. Technical criteria were applied to determine the most appropriate biomass to use in the gasification process; this included the physical-chemical characterization, the cost and the logistic-agronomic profile. The physical-chemical characterization included humidity, ash, calorific value, an elemental analysis, sulfur and chlorine, etc. On the basis of this characterization, almond shells were found to be the optimal biomass (Mar = 12.9%, Ar = 1.1%, Vd = 82.2%, Qp,net,d =18,470 kJ/kg and Cl = 60 mg/kg), and depending on certain parameters, could be classified as A1 or A2. Both the olive tree prunings (Mar = 6.2%, Ar = 5.5%, Vd = 83.4%, Qp,net,d =18,193 kJ/kg and Cl =15 mg/kg) and the holm oak prunings (Mar = 9.2%, Ar = 4.1%, Vd = 80.3%, Qp,net,d =16,335 kJ/kg and Cl =12 mg/kg) were also considered to be good biomass resources, and were given an A2 or B1 classification. However, greenhouse vegetable residues (tomato and pepper) did not have suitable technical parameters (Mar = 82.6-29.6%, Ar = 35.5-6.4%, Vd = 75.1-59.1%, Qp,net,d = 17277-11529 kJ/kg and Cl = 1196-751 mg/kg) for use in the gasification process. Concerning the economic criteria, the best cost per kilogram (0.01€/kg) was found for the greenhouse vegetable residues, followed by the olive tree prunings (0.04€/kg); the highest cost corresponded to almond shells (0.07€/kg). With regard to the logistic-agronomic criteria, the theoretical hours of production in the power station are determined by the total availability of the resource in the particular location. The results indicate that the amount of almond shells available in the area was not sufficient (3854 h) to ensure the operation of the power station at full load (8760 h) but it would be possible in conjunction with other biomass types. The final decision regarding the optimal biomass to use was made on the basis of a multivariable analysis using the Visual Preference Ranking Organization Methods for Enrichment Evaluations (PROMETHEE) tool. From this analysis, olive tree prunings were selected as the optimum biomass to use because of their extensive local availability (58,080 t/year), in addition to them having suitable physical-chemical characteristics (Mar = 6.2%, Ar = 5.5%, Vd = 83.4%, Qp,net,d =18,193 kJ/kg and Cl =15 mg/kg)) and a reasonable cost (0.07€/kg).