The technical and economic feasibility of Cynara cardunculus L. gasification
- Autores: Alberto Gómez García
- Directores de la Tesis: Domingo Santana Santana (dir. tes.)
- Lectura: En la Universidad Carlos III de Madrid ( España ) en 2012
- Idioma: inglés
- Tribunal Calificador de la Tesis: Bo Leckner (presid.), Javier Villa Briongos (secret.), José Antonio Almendros Ibáñez (voc.)
- Materias:
- Enlaces
- Tesis en acceso abierto en: e-Archivo
- Resumen
This PhD Thesis analyses the technical and economic feasibility of the gasification of one of the most promising energy crops in terms of biomass yield and plantation costs: Cynara cardunculus L. (cynara). The aim of this analysis is to assess the bioenergy production via fluidized bed gasification (FBG) and the ulterior treatment of the synthesis gas (syngas) produced in the FBG reactor to adequate it to end-use applications such as gas turbines and internal combustion engines. To achieve this objective, this thesis proposes a formulation model approach for evaluating the electricity generation costs (Chapter 2), the reactor performance (Chapter 3) and the syngas conditioning efficiency (Chapter 4). For this purpose, the Autonomous Community of Madrid (CAM) has been taken as study case. The analysis estimates that the cynara has the potential to provide 1708 GWh yr-1, that is, around 42% of national biomass-based electricity supply and exceeds 72% of total renewable-based electricity supply in CAM. Therefore, the implementation of cynara projects could help reducing the total energy consumption of CAM by 0.05%, what would suppose to avoid up to 66% of CO2 emissions from fossil fuels. The economic assessment performed in the present work evaluates the use of two thermochemical technologies for cynara conversion into electricity to be used for different applications or sold to the national grid. The technological solutions considered are: a Combined Cycle Gas Turbine (CCGT) plant and an Internal Combustion Engine (ICE) power generator. The CCGT solution was studied for an installed capacity range of 5-30 MW, while the ICE solution was analysed for a range of 1-30 MW. A sensitivity analysis was conducted to examine the effects of variables such as biomass yield, discount rate, transport cost, operation and maintenance. For a cynara yield of 17 t/ha in an 8 MW plant as base case, the economic analysis estimates a production costs of 21.60 c€/kWh and 24.32 c€/kW for the CCGT and ICE solutions, respectively. Accordingly, CCGT plants are the best choice for a plant size above 8 MW, while ICE plants constitute the most suitable technology below 8 MW. With regards to the discount rate, the results show that for the same base case (8 MW), for a discount rate of 10% the cost of electricity is estimated to be 16.69 c€/kWh for CCGT plants and 19.08 c€/kWh for ICE plants. On the contrary, the use of the lowest discount rate (1%) yields a cost of electricity of 12.70 and 15.13 c€/kWh for CCGT and ICE solutions, respectively. Concerning to the total capital investment, it grows with the plant size, representing up to 93 and 92% of the total CCGT and ICE plant cost, respectively. Such percentages correspond to 42.17M€ and 41.46 M€ for a CCGT and ICE plant for a base case of 8 MW. Nevertheless, the ICE plants show a stronger economy of scale in energy production than the CCGT solution. In addition to this, the total operating costs for an 8 MW CCGT scenario is estimated to be 2.94 M€ and around 3.65 M€ for an ICE plant. In relation to the thermochemical conversion route of cynara, the gasification of biomass in a FB reactor has been modelled to analyse such process for Cynara cardunculus L. taking into consideration the particular biomass behavior. It is well known that the FB reactor thermal state and the biomass volatiles generation are crucial in its operation and performance. Hence, the bubble flow pattern controls the FB temperature profile driving devolatilization and tars cracking kinetics. This underlies in the fact that alkali compounds of biomass fuels, which are featured by a low melting point, can transform into vapours and ash fly that are prone to deposit on heat surfaces in boilers and/or react with the particles of the inert bed material inside the FB. Thus, the formation of agglomerates (the so-called bed agglomeration) would start and then, the defluidization of FB leading to the shut-down of the FBG reactor. Therefore, a modelling approach focused on the bubble phase, which can act as “bypassing” hot spots inside the FB region influencing on ash-related problems, can help to monitor the location of ash sintering and bed agglomeration risk regions and predict undesired FBG reactor performance. A new formulation for biomass FBG reactor modelling that considers the instantaneous devolatilization and temperature peaks due to volatiles combustion inside the FB region is proposed in the present work. A bubble phase and a FB energy balance are used to monitor the gradual release of biomass volatiles along the FB and to check the performance of the FBG reactor. The one-dimensional, steady-state proposed model uses a two-phase (bubble and emulsion) and two zone (bottom dense bed and upper freeboard) modelling approach to account for the complex nature of FBG reactor dynamics. Furthermore, no catalytic effects of ash composition from biomass are taken into consideration. For further validation and tuning up of the model proposed, a sensitivity analysis of cynara gasification in FB, under bubbling regime, was performed considering the specification design of the pilot-plant scale FBG reactor in the Thermal and Fluid Engineering Department facilities at Carlos III University of Madrid. The simulation campaign yields a syngas composition (on dry basis) of 4.79-14.84% for CO, 19.77- 21.35% for CO2, 6.11-15.00% for H2 and 2.16-5.73% for CH4. Besides, the lower heating value and tar content of the syngas fall in the range of 2.25-6.25MJ/Nm3 and 60-180g/Nm3, respectively. These results correspond to a syngas-biomass flows ratio in the range of 1.309-2.392Nm3/kg, accounting for N2 in the raw syngas produced. The analysis of the results in comparison with previous experiments stands out: 1) the good predictive capability of the model proposed and 2) the discrepancies between simulations and experimental works are attributable to the data heterogeneity found in the literature, that is, different biomass compositions, operating conditions, (catalytic) bed material used, sampling methods for syngas and tar compositions, etc. Hence, further experimental research would help improving the predictive capability of the proposed model. Finally, the conditioning of the syngas produced from the FBG reactor is needed in order to achieve end-use requirements in ICE and gas turbines (GT) plants, since the lack or inefficiency of syngas clean-up could lead to operational problems in downstream equipment and then, unscheduled shut-down and extra maintenance and repair costs. For example, particulate material can cause clogging and fouling, while tars can condensate producing blockage and attrition in filters, exit pipes, heat exchangers, etc. Furthermore, the syngas treatment to reduce its pollutants would influence the performance, investment and operational costs of the gas cleaning devices. Nowadays, gas cleaning systems are aimed to reduce particulate and tars material levels below the allowable concentrations (mg/Nm3) for ICE and GT devices: 50-50 and 30-5, respectively. Thus, as a part of the present thesis, the modelling and analysis of a moving bed heat exchange filter (MBHEF) is proposed as hot gas clean-up equipment. The MBHEF stands out because its benefits: high temperature operation (700-800ºC the exhaust gas temperature from the FB reactor), no-clogging and non-pressure increase during operation, which can lead to unscheduled shut-down if using other typical hot gas cleaning devices such as ceramic filters, bag filters. Additionally, the MBHEF would provide a high contact area between gas and solids without entrainment nor elutriation of solids. This compact size equipment would allow saving costs. Eventually, the MBHEF solution for hot gas cleaning would also avoid extra costs derived from the reactor design modification and the use of additives/catalysts in order to remove tars. It is presented a modelling approach for simulating tars and particulate removal in a MBHEF. The two-dimension, adiabatic, steady-state proposed model accounts for twophase (gas and solid) and neglects conduction and mass diffusion. Tars condensation is modelled through representative tar class lumps: phenol (class 2), naphthalene (class 4), and pyrene (class 5) according to the literature. The model also considers tar concentration influence on tar dew point, while the filtration model is taken from literature. Furthermore, an exergy study was conducted in order to optimise the equipment size and help the choice of the less expensive operating conditions. A sensitivity analysis was performed varying the particle size and superficial gas velocity as key operating parameters. To accomplish this, a syngas composition from experiments reported in the literature has been taken as study case. Thus, maps of temperature, tars abatement and particulate removal efficiencies are presented, which show the MBHEF performance for reducing impurities content. The simulation results indicate the feasibility of use a MBHEF as tars removal equipment benefiting its advantages against other gas-cleaning methods with acceptable pollutant removal efficiencies, ranging 88-94%. As observed, the MBHEF yields efficiencies, at least, the same order of magnitude of the ones attainable with the use of catalytic crackers, venture scrubbers or sand filter at much lower temperatures and higher than the ones achieved by means of wash towers, wet electrostatic precipitators, fabric filters and fixed bed absorbers. In case of not reaching the reduction level for each end-use application, the MBHEF device can be used as effective secondary removal method for eliminating tars from the syngas, with the advantages stated above as opposed the rest of removal technologies. Results also point out that low gas velocities (0.5-1m/s) and high particle size (400- 700?m) for saving costs are the most suitable operating conditions. Nevertheless, the exergy optimization involves low or very low tar removal efficiency so that the pollutant reduction and exergy cannot be optimised simultaneously. The technical and economic feasibility of Cynara cardunculus L. via fluidized bed gasification carried out in the present PhD thesis has shown the cynara as a promising energy crop to meet energy demands in Mediterranean climate locations such the CAM (study case here). Besides, the modelling approach proposed for predicting the FBG reactors performance has been shown as a useful tool to help other diagnosis methods for the prevention of bed agglomeration and ash sintering in order to avoid operational problems and unscheduled shut-down of FBG reactors. Finally, the use MBHEF as hot gas clean-up method has been analysed by means of a modelling approach presented here. This study points out that the MBHEF is very effective equipment for removing particulate and tars from the syngas produced in FBG reactors. Thus, downstream tarsrelated problems such as fouling, blockage and attrition could be avoided. ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
