ABSTRACT
In this doctoral thesis, a theoretical and experimental comparative study is carried out between two types of biomass gasification technologies, determining their technical, economic and environmental feasibility, for the production of electricity in low power applications (less than 100 kW). The kinds of technologies selected are among the most interesting ones in the field of biomass gasification: downdraft fixed bed and bubbling fluidized bed.
Most of the results presented in this work, comparing the two technologies, were obtained experimentally. Although the design experiments provide more reliable data than those which can be obtained through modeling or simulation, they are more expensive, without taking into account that the reactions that take place in the reactor are complex and difficult to model, especially in the conversion phase from solid to gas. In addition, most models focus on the production and gas composition, regardless not only of tar generation but also its separation from the gas, as essential as the process of gas generation. All these developments have forced the comparative study to focus on the experimental verification of the behavior of both types of gasifiers. To achieve this goal, a prototype with an electrical output of around 10 kW for each of the two configurations studied was designed, built and put into operation.
In this research work, the problem of gas cleaning and solids separation is also studied, especially the tars generated in the gasification process. For each of the two types of technologies, a different type of gas cleaning system (using both wet scrubbers and filtered by means of chips) have been defined and tested, but with different operating modes. The pilot study allowed us to determine the optimum configuration with which a gas is obtained suitable for burning in an internal combustion engine.  Gas cleaning is one of the most important considerations in a gasification plant, due to the cost and additional energy consumption associated with these systems.
From the perspective of economic viability, the prototype building cost was used to determinate that the plant generation power by means of biomass gasification with less than 50 kW power, are not economically profitable in Spain, except under restrictive conditions of low-cost biomass (less than 0.03 € / kg) and / or an important grant that would allow a sale price of over 0.2 € / kWh. Discarding the network connection, a low-power gasification plant was considered for its use in non-interconnected areas, comparing the costs of power generation in the gasification plant with a plant operating on natural gas or diesel. It has been determined that a gasification plant is more profitable and therefore, it would be an alternative method to generate electricity in rural areas, where using conventional methods is expensive and difficult.