Olive tree pruning was gasified with air in a laboratory fluidized bed (FB) reactor at 800, 850, and 900 °C and equivalence ratios (ERs) ranging from 0.12 to 0.35. A few additional tests were performed varying the fuel particle size, biomass feed rate, and oxygen enrichment in the air. The composition of the product gas was determined by measuring the light gas, water vapor, tar, and some inorganic contaminants. The solids produced were characterized by sampling from the cyclone and bed, providing approximate information about the char elutriation rate and residence time. The lower heating value of the gas, LHV (excluding benzene and tars), varied between 4.5 and 7.8 MJ/(Nm3) using air, whereas it increased to 9.3 MJ/(Nm3) using enriched air with 40% O2.. Carbon conversion increased with temperature (so did gasification efficiency), reaching 97% at 900 °C, indicating almost complete fuel conversion. Analysis of the results with the assistance of a previously developed FB gasification model indicated that most of the tests were carried out under allothermal conditions (with significant heat added to or removed from the gasifier) and only a few tests were representative of autothermal conditions, i.e., the mode of operation of industrial air-blown FB gasifiers (without heat addition and with small heat losses). The model was also used to scale-up the laboratory-scale results to predict the gas composition of industrial-scale FB gasifiers.

This work presents the development of suitable strategies focusing on greenhouse crop residues as energy and CO 2 sources for improved food production in greenhouses. The utilization of greenhouse crop residues in combustion processes for heating and carbonic enrichment in greenhouses has previously been developed and evaluated. Nevertheless, greenhouse crop residues present several problems that make it difficult to use them for these purposes. Among the characteristics that can impede their use are excessive moisture and ash contents as well as their low density. In this work, the relevant solid fuel properties for this type of biomass have been studied. In addition, three pre-treatment strategies are proposed and evaluated, which aim to enhance the fuel quality of this biomass. These strategies were: 1) first relates to the drying strategy employed for reducing greenhouse crop residue moisture. 2) the second one relates to a reduction in ash content by avoiding contact with greenhouse soil and 3) mixing with other biomass kinds with better quality as solid fuels. The assays performed showed that these strategies were successful, resulting in biomass with a high heating value, up to 26.9 MJ/kg, and a lower ash content than untreated residues, with values as low as 13.0% dry weight. This value is closer to that for the standard biomass most commonly employed in direct combustion applications. The biomass produced has been verified as suitable for conventional boilers with thermal efficiencies up to 70%. The methods developed allow to reuse greenhouse crop residues as greenhouse fuel, providing both heat and CO 2 ; thus enhancing production and sustainability.

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 (M ar = 12.9%, A r = 1.1%, V d = 82.2%, Q p,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 (M ar = 6.2%, A r = 5.5%, Vd = 83.4%, Q p,net,d =18,193 kJ/kg and Cl =15 mg/kg) and the holm oak prunings (M ar = 9.2%, A r = 4.1%, V d = 80.3%, Q p,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 (M ar = 82.6-29.6%, A r = 35.5-6.4%, V d = 75.1-59.1%, Q p,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 (M ar = 6.2%, A r = 5.5%, V d = 83.4%, Q p,net,d =18,193 kJ/kg and Cl =15 mg/kg)) and a reasonable cost (0.07€/kg).

Large amounts of crop residue are produced annually in areas such as Almeria (Spain). These residues have elevated moisture and ash contents, and are also very heterogeneous, which hinders their reutilization. With the aim of facilitating biomass utilization in energy recovery-related processes, a model for solar drying was developed. Experiments were performed inside a greenhouse with tomato and pepper residues, following two strategies (hung or stacked residues). The influence of temperature and relative humidity on the residues’ equilibrium moisture was also studied. The results were that a model allowed for determination of the equilibrium moisture as a function of ambient conditions (temperature and relative humidity), with the model’s characteristic parameters being different for each crop residue. Regarding the drying process, the results conform to first-order kinetics, with the values of the kinetic constants varying as a function of the crop residues and their arrangement. The variation in equilibrium moisture as a function of the annual variation in ambient conditions (temperature and relative humidity) in Almería means that it would only be possible to dry crop residues inside greenhouse below a moisture level of 0.43 kgwater/kgdrysolids (30% water content) from April to November.

This work presents an alternative for adding value to greenhouse crop residues, used for (1) heating and (2) as a CO2 source. Both options are focused on greenhouse agricultural production, but could be applied to other applications. The influence of factors, such as the air/fuel rate and turbulence inside the combustion chamber, is studied. Our results show that for pine pellets, olive pits, tomato-crop residues, and a blend of the latter mixed with almond prunings (75–25%), the thermal losses ranged from 19.5–53.1, 20.5–58.9, 39.9–95%, and 29.4–75.5%, respectively, while the NOX emissions were 30–247, 411–1792, and 361–2333 mg/Nm3, respectively. The above-mentioned blend was identified as the best set-up. The thermal losses were 39.2%, and the CO, NOX, and SO2 concentrations were 11,690, 906, and 1134 mg/Nm3, respectively (the gas concentration values were recalculated for 0% O2). Currently, no other work exists in the literature include a similar analysis performed using a boiler with a comparable thermal output (160.46 kW). The optimal configurations comply with the relevant local legislation. This optimization is important for future emission control strategies relating to using crop residues as a CO2 source. The work also highlights the importance of ensuring a proper boiler set-up for each case considered.