Comparison of the Combustion Reactivity of TGA and Drop Tube Furnace Chars from a Bituminous Coal

Abstract

Although thermogravimetric analysis (TGA) is a widely accepted technique for assessing coal combustion, conflicting trends have often been reported when relating apparent TGA reactivities to pulverized fuel (PF) burner conditions. Therefore, this paper compares the reactivity of chars generated in a drop tube furnace (DTF) to those from TGA. The implications of devolatilization temperature, heating rate and residence time are considered. For the smaller particle size ranges of the bituminous coal investigated (ATC), optimized devolatilization procedures were used to generate corresponding TGA burnout rates between the two char types. However, with fractions of >75 μm, the DTF chars showed an increased burnout propensity when moving from combustion regime II to combustion regime III. Scanning electron microscope (SEM) images and internal surface areas indicate that this is because of incompatible char morphologies. Thus, while chars produced under the conditions of TGA pyrolysis strongly resemble raw coal and display an undeveloped pore network; the DTF chars are highly porous, extensively swollen and possess considerably larger internal surface areas. Subsequently, char burnout variability was quantified, with the reactivity distribution for the DTF samples found to be up to an order of magnitude more significant than for the TGA chars. This is attributed to a fluctuating devolatilization environment on the DTF. Finally, a TGA study observed a robust particle size based compensation effect for the TGA chars, with the relative reaction rates and activation energies demonstrating the presence of internal diffusion control. However this phenomenon was partly alleviated for the DTF chars, since their higher porosities reduce mass transfer restrictions. Moreover, it should be realized that DTF char fractions of <38 μm, including those required to ensure true intrinsic control under the investigated burnout conditions, cannot be produced directly. This is because of bridging and sloughing in the DTF's screw-feeder. Instead, such samples must be created by grinding larger particles, which destroys the char's existing porosity.

Impact of CO2 on biomass pyrolysis, nitrogen partitioning, and char combustion in a drop tube furnace

“…N T volatiles = 100 − N T char (13) where N xchar is the mass fraction of nitrogen in the char; N T char and N T volatile are the total amount of nitrogen in the char and volatiles, respectively. Table 4 presents the calculated relative nitrogen distribution between volatiles and the residual chars at different pyrolysis temperatures.…”

“…The significance of CO 2 further highlighted by the different morphologies and significantly higher surface areas observed for biomass chars or combustion residues produced in various oxy-fuel conditions [12]. However, most of these early studies on oxy-biomass pyrolysis and combustion are carried out under TGA conditions that differ hugely from actual boiler conditions, and conflicting trends have often been reported when directly relating apparent TGA-derived combustion data to actual PF boiler conditions [13]. A drop tube furnace (DTF) is a much more suitable technique to study PF combustion reactivity key combustion parameters, particularly fast heating rates, high temperatures, short residence times and appropriate gas velocities can be replicated.…”

Kinetics investigation on the reduction of NO using straw char based on physicochemical characterization

“…Le Manquais et al (2009) compared the intrinsic reactivity of coal char prepared at 1173, 1373 and 1573 K using a TGA which showed that the reactivity decreased with the increasing in the char-preparation temperature. The presented TGA results are similar to those obtained by Sutcu andPiskin (2009) andAsadullah et al (2010a,b), who prepared the char from loquat stones and wood at 873, 973, 1073 and 1173 K. The contradictions between the previous results and the present work may be ascribed to the differences in fuel types.…”

Oxy-fuel combustion: The effect of coal rank and the role of char-CO2 reaction

“…This is particularly important at high temperature and high CO 2 partial pressure. There are many possible reasons for the scatter in results in literature, which include the furnace operating temperatures used by the authors, and method of char preparation [31]. Hence, the main objectives of this study are: 1.…”