Abstract
Nowadays the reduction of CO2 emissions is one of the main objectives in the field of energy production and using biomass as a renewable energy source is a promising means of achieving this aim. Biomass has several potential advantages: it is widely available, naturally distributed and carbon neutral.
Biomass can be transformed into several different energy products including heat for heating systems, heat for industrial facilities, electricity and fuels.
This dissertation focuses on the optimization of small-scale fixed-bed biomass gasification systems for the combined heat and power (CHP) production. Small-scale biomass gasification systems can be considered robust enough for practical and commercial application but they are often limited to very specific fuel properties and steady operation.
The main objectives of this research work focus on fuel flexibility and load modulation capability of biomass gasification systems. Fuel flexibility refers to the possibility of using different feedstocks (e.g. forest residual) or the same type of biomass but with different properties in terms of moisture and size distribution (e.g. chips and pellets). Load modulation capability refers to the control of the feeding load in order to produce energy meeting the energy demand. In both ways, the performance of the gasification system can be enhanced, producing energy when it is needed (load modulation) or using
several feedstocks (fuel flexibility).
The load modulation capability and the fuel flexibility of biomass gasification systems have been evaluated monitoring different gasification plants. In particular, three gasification systems are used for the experimental tests: a laboratory scale reactor and two CHP systems, one at laboratory scale and one at commercial scale. More than 70 experimental tests were carried out at different gasification conditions.
Besides, a thermodynamic equilibrium model was developed and applied to assess the gasification system behaviour. Unlike the classical equilibrium strategy that calculates the gasification products using the Gibbs energy minimization method at fixed temperature and pressure, the developed approach is based on the enthalpy balance between reactants and products using the concept of adiabatic gasification temperature. A correction factor has been calibrated versus the entire set of
experimental data in order to account for the differences between the theoretical hypothesis implemented in the model and the real system.
The model outcomes show a good agreement with the experimental data in terms of parameters of interest for gasification such as producer gas LHV, producer gas yield, char yield and cold gas efficiency.
Potential outcomes of this research are, firstly, the characterisation of the plant behaviour in terms of fuel flexibility and load modulation capability. Secondly, the development of a gasification model appropriate to be used in the plant control management and in general to predict the behaviour of fixed-bed gasifier system.