Solar gasification in dual fluidized bed using solids as heat carrier
Biomass steam gasification assisted by concentrated solar energy is an attractive technology for the production of storable renewable energy and the reduction of CO2 emissions. However, several challenges have stalled its deployment over the last decades: high temperature and/or large reactor volume...
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| Tipo de documento: | tese |
| Estado: | Versão publicada |
| Data de publicação: | 2022 |
| País: | España |
| Recursos: | Universidad de Sevilla (US) |
| Repositório: | idUS. Depósito de Investigación de la Universidad de Sevilla |
| OAI Identifier: | oai:idus.us.es:11441/138538 |
| Acesso em linha: | https://hdl.handle.net/11441/138538 |
| Access Level: | Acceso aberto |
| Palavra-chave: | Steam gasification solar energy dual fluidized bed gasifier fluid-dynamics loop seal |
| Resumo: | Biomass steam gasification assisted by concentrated solar energy is an attractive technology for the production of storable renewable energy and the reduction of CO2 emissions. However, several challenges have stalled its deployment over the last decades: high temperature and/or large reactor volume required for complete fuel (char) conversion, the achievement of a steady syngas generation independent of solar radiation variation, and accomplishing effective heat supply at high temperature in large-scale reactors. This thesis deals with the analysis and design of a new scalable process to carry out the solar steam gasification overcoming the mentioned technical challenges. This thesis proposes a new concept of biomass gasification assisted by solar thermal energy, as an extension of the state-of-the-art dual fluidized bed gasification (DFBG). In the process the gasification unit in the DFBG is not only heated by the solids coming from the combustion unit, but also by an external stream of solids that has been previously heated in a solar particle receiver. This configuration allows uncoupling the solar receiver and the reactor using a thermal energy storage (TES), while the thermal integration is highly efficient since carrier particles are directly used in the reactor. The reactor will operate with high share of solar external heat when available, while it will send more char to the combustor when the fraction of solar external heat decreases (at nights or during winter time). A model of the proposed solar DFBG (SDFBG) was developed and used to assess the performance of the system. The results show that char conversion of 70–80% is a compromise for having a significant solar share in the syngas (around 12 %, defined as the fraction of chemical energy in the product gas coming from the solar energy) while maintaining reasonable gasifier volume i.e., char residence time in the range of 20-30 min. This operation is far from those of existing DFBG, using a much smaller gasifier volume, with limited residence time (1-5 min) and extent of char conversion (10-30%). Different integrations for the external solids circulation into the DFBG system were studied and, two of them were identified as the most advantageous for implementation in the process. The required solids circulations for the operation at high solar share resulted similar to those reported from existing DFBGs but, the required solids inventory for that operation is typically ten times higher than that in conventional DFBGs. As a result, current DFBGs should be significantly modified to allow the operation both in solar mode and autothermal conditions. The new features imposed by the operation of the SDFBG require from a precise understanding of the fluid-dynamics of the system. A model of a conventional DFBG is developed to understand the hydrodynamic performance of current units, and then to extend the knowledge to solar conditions. The cold flow model (CFM) at TU-Wien, one of the pioneering CFM developed for studying the fluid-dynamics performance of DFBGs, is taken as reference and, experimental measurements from this unit are compared with the model results. The model was used to preliminary assess the fluid-dynamic characteristics of SDFBGs. The results show that typical solids inventories and solids fluxes required by an SDFBG can be achieved by increasing around three times the diameter of the gasification unit compared to that of a conventional DFBG. Moreover, the loop seal is identified as a key element for allowing the flexible operation of the SDFBG under different external heat loads. Motivated by the key role of loop seal unit in the SDFBG, experimental work was carried out in an isolated loop seal CFM, intended to shed light on the fluid-dynamics of these units and, looking for fundamental knowledge to improve the current semi-empirical models. Results from the study show that the resistance offered by the horizontal passage (opening) of the loop seal, leads to non-homogeneous gas-solids flow pattern which, ultimately, establishes the fluid-dynamic performance of the unit. The non-ideal gas-solids flow patterns occurring in actual loop seals were demonstrated not to be addressed by simple semi-empirical 1D models, needing more complex computational models to capture the 2D/3D effects. Although CFD is a useful tool to predict the behavior of these units, a comprehensive generalization is difficult; conclusions from this work can be useful to understand different designs. All the gained knowledge was gathered and used to design a flexible SDFBG able to operate both under autothermal and at high allothermal conditions. The previously identified most advantageous integration options of the external solids circulation into the DFBG were considered, leading to different design and operation requirements. Char conversions in the gasifier from autothermal to high allothermal operation ranges from 15 to 80%, respectively, while the maximum solar external heat load rounds 2.6 MJ/kgbio,daf (typically 14% of the heating value of the biomass). This contrasts with the maximum share calculated by equilibrium (≈5 MJ/kgbio,daf), indicating that there is still room for improvement if operating conditions in the gasifier are optimized. Overall, the work clearly demonstrates that the proposed SDFBG presents huge scale up potential, taking the most of current state-of-the-art technologies of DFBG after some modifications, mainly: a wider gasifier unit, a narrower riser-combustor and a dedicated lower loop seal for adapting the operation to changes in external heat loads. Moreover, reactor optimization measures such as the combination of the developed solar DFBG with catalytic gasification and addition of sorbent to capture CO2 seem to be promising extensions to provide even further benefits. |
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