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Thesis for the Degree of Master of Science

 

Year 2010
Alexandre Boriouchkine

Dynamic Modelling of a BioGrate Boiler


Increasing utilization of renewable energy has created new energy efficiency challenges for industry. Biomass is one of the most important raw material for renewable energy. All the available biomass sources have to be considered for energy production. Fuel properties of biomass vary a lot depending on its origin, on processing and handling for fuel. Variable properties cause fluctuations in combustion and set challenges to develop new combustion control strategies.

One of the latest successful processes developed, which use wood waste as a fuel, is a BioGrate-boiler technology developed by MWBiopower. The combustion of wood waste is, however, a very complex process involving several highly coupled chemical reactions. Furthermore, operational conditions of the furnace greatly affect the yields of chemicals produced during the combustion process, i.e., fractions of tars, gases and char. Moreover, not only the yields of chemicals differ under different combustion conditions, but also their reactivity in succeeding reactions. As a result of such complexity, optimization of a boiler control strategy requires a detailed process model.

The aim of this Master’s thesis is to develop a dynamical model for a BioGrate boiler. The purpose for the modeling work is to construct a dynamic model that provides an insight into the chemical and physical phenomena occurring inside the process. The literature part of the thesis discusses the chemical and thermodynamic phenomena involved in the boiler process, including fuel combustion, flue gas convection and heat exchange between the flue gas, water and steam. Each process part is then broken down into chemical reactions and physical processes. Each chemical reaction and physical process is then discussed in detail, and an appropriate model is presented for each chemical reaction and physical phenomenon.

In the experimental part, the dynamical simulator of the furnace of a BioGrate boiler is developed and implemented in the MATLAB environment. In addition, a GUI is developed in order to provide user-friendly operability, and then attached to the simulator. After the implementation of the simulator, several simulation studies were conducted with various process parameters, including fuel moisture content, fuel quality and combustion air flow. The simulation results were investigated in order to find the most significant parameters affecting the combustion process inside the furnace.

The results of the sensitivity analysis showed that the combustion time increased linearly with an increase in the moisture content. The study on the particle diameter indicated that the larger the particle diameter the shorter is the combustion time, because heat conduction improves significantly with an increased particle diameter. Simulations with different wall thicknesses revealed insignificant dependence of the combustion time on the particle wall thickness: the combustion time increased only slightly with an increase in wall thickness. A study on the effect of the air flow indicated that oxygen deficiency slowed down the combustion process. However, excess air, on the other hand, increased the combustion time by cooling the bed with consisting of small particles. The results obtained from the simulator were found to be in agreement with those reported in the literature.


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