Modelling and control of a small-scale fixed-bed biomass gasification system

Research output: ThesisDoctoral Thesis

Abstract

Sustainable and environmentally friendly energy and resource systems require renewable energy sources such as wind, solar thermal, or photovoltaic power. Due to their volatility, flexible technologies are needed that can compensate for intrinsic fluctuations. In addition to existing energy storage systems, flexible, renewable technologies that generate energy according to demand are especially needed. As such, biomass-based technologies are particularly relevant for generating heat, electricity, and bio-based products such as biochar. Currently, the most efficient use of solid biomass for electricity generation is through thermochemical conversion processes such as combustion or gasification. Due to their high efficiencies and low emissions, flexible biomass fixed-bed gasifiers, in particular, offer an up-and-coming solution for transitioning to more sustainable, environmentally friendly energy and resource systems.
However, fixed-bed biomass gasifiers are almost always operated at nominal loads to maximize their electricity production, as they are economically dependent on currently valid feed-in tariffs for the electricity generated. Consequently, their flexibility potential is not yet exhausted, and the control strategies currently applied are usually optimized for nominal operation. Any operation deviating from the nominal load usually requires manual intervention by the plant operators to overcome operational difficulties or lower efficiencies. Accordingly, currently applied control strategies must be improved to exploit the full flexibility potential of fixed-bed gasifiers. In this respect, model-based control strategies can offer a solution to this problem, as they allow for an explicit consideration of internal process interactions and can thus automatically deal with varying operating conditions.
For this reason, this work aims to make the operation of a biomass fixed-bed gasification system more flexible by means of a new model-based control strategy. A representative, small-scale industrial biomass gasification system for combined heat and power generation with a nominal thermal capacity of 300 kW and electrical capacity of 150 kW serves as the basis for this research. A detailed operational assessment supports identifying a new operating strategy to increase load modulation capability and fuel flexibility using only typically existing sensors and actuators. This process analysis lays the foundation for modelling the gasification plant and helps to determine the necessary changes in the currently applied control strategy. Derived from this, a new model-based control strategy consisting of two individual controllers is designed, implemented, and experimentally verified on this gasification system. The first controller ensures an appropriate discharge of hot char from the fixed-bed gasifier. The other simultaneously acts on the electrical power and gas pressure in the pipes connecting the gasifier with an internal combustion engine.
Experimental verification shows that the new model-based controllers contribute significantly to increasing flexibility compared to the currently applied state-of-the-art control strategy. For example, the new controllers perform a load change from 150 kW to 100 kW (−33 %) in less than 2 min, while maintaining a much more homogeneous gasification, especially in part-load operation. Furthermore, they reduce the fluctuation range of the relevant process parameters to less than 1 %. This controlled stabilization and homogenization of gasification under different operating conditions is also a prerequisite for future flexibilization measures, such as fuel variability or increasing product flexibility.
Due to the modular and model-based design, the new control strategy can also be transferred to other fixed-bed gasifiers of the same type by adapting the model parameters accordingly without requiring structural changes. In summary, the presented model-based control strategy makes an important contribution to flexible fixed-bed biomass gasification systems.
Original languageEnglish
QualificationDoctor of Technology
Awarding Institution
  • Graz University of Technology (90000)
Supervisors/Advisors
  • Horn, Martin, Supervisor
  • Gölles, Markus, Supervisor
Publication statusPublished - 2023

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