Advanced Solid Fuel Characterization for Reactivity and Physical Property Comparison

The main objective of this thesis was to formulate a method with which solid fuel combustion characteristics and physical properties could be accurately com- pared between different samples. The main study instrument used and further developed in the fuel reactivity tests during this work was a laminar drop-tube reactor (DTR). Five different solid fuel sample types were tested with the DTR. The samples were selected to represent a wide range of possible solid fuel types relevant to energy production in Finland. Fossil coal was selected as a reference fuel. Peat was chosen as it is a commonly co-fired with biomass in Finland. The three other samples, raw, torrefied, and steam-exploded woody biomasses, were chosen to find out how thermochemical pretreatment of biomass feedstock affects the fuel combustion characteristics.In addition to the DTR tests, the fine grinding energy requirement of the biomass samples was also examined. Moreover, collaboration work with other researchers was conducted to examine the effect of torrefaction on the fine grinding energy requirement, chlorine content, and heating value. Various domestic and foreign wood species were used in these studies. The torrefaction process was noted to reduce the energy required to fine grind the tested sample. It was also noted that during torrefaction the chlorine content of the solid matter was reduced and the specific heating value was slightly increased. Fine grinding the steam-exploded biomass produced more spherical particles compared to the raw and torrefied pellet samples.The combustion behavior of the five main samples was tested in the DTR. The samples were preground and the particles sieved with vibration sieves with an opening of 100-125 μm. The pyrolysis process was examined separately at a temperature range of 973-1173 K in pure N2. The combined pyrolysis and combustion tests were conducted at a reactor temperature of 1123 K. The O2 concentrations used in the combustion measurements were 3–21 vol-% in either N2 or CO2 atmospheres. The surface temperature of the combusting sample particles was measured with a two-color pyrometer. The initial size distribution of the sample particles as well as their size and geometry evolution as a function of conversion was studied by using optical techniques. The density, specific surface area, and mean pore diameter were measured from the samples with a mercury porosimeter. The reactivity parameters, which describe the pyrolysis and char oxidation rates of the samples, were determined by using the data from the measurements. Using discretized size distribution in the model calculations explained better the measured particle surface temperatures than using a mono sized single particle model. Moreover, combining the optical techniques with the DTR setup provided valuable data on the geometry evolution of the particles.Based on the reactivity parameters, the sample combustion characteristics could be compared with one another. The reactivity comparison method presented in this thesis relies on consistent DTR measurements, determining accurately the size distribution and porosity of the sample particles, and using multiobjective optimization in fitting the model parameters