Kinetics of volatile product evolution in coal pyrolysis: experiment and theory

Abstract

Relatively few studies have addressed the kinetics of individual volatile species evolution, particularly at high-temperature, high-heating-rate conditions. In addition to the sparsity of species evolution data, substantial controversy surrounds the wide variation (factors of 1000) in reported kinetic rates for both overall weight loss and species evolution. The aim of this study was to usé data from three types of reactors, each with different heating characteristics, to develop a more accurate reactorindependent, heating-rate-independent, and coal-independent set of kinetic parameters. Toward this end, several steps were taken to obtain better measurements of the pyrolysis rates and heat-transfer rates for coal. In addition to improvements to the experiments, improvements were also made to a previously described functional group (FG) model for coal pyrolysis. Two submodels were added to describe (a) the cracking of hydrocarbon species released in primary pyrolysis and (b) the equilibration of oxygén-, hydrogen-, and carbon-containing species at high temperatures. Comparisons of data obtained in the three reactors with the predictions of the improved FG model are presented for six coals. In general, the agreement of the FG model and the data is quite good for all the pyrolysis products at temperatures below 1100 °C. As the temperature increases above 1100 °C, secondary reactions, including soot formation and gasification, begin to play an important role. This léads to overprediction of olefins, CH4, H20, C02, and tar and underprediction of CO, H2, C2H2, and benzene. The results for weight loss during primary pyrolysis are in reasonable agreement with predictions of a single first-order model for primary pyrolysis weight loss that uses a rate constant k = 4.28 X 1014 exp(-54570/fiT) s'1 11. This indicates that the rate of primary pyrolysis is much higher at elevated temperatures (>700 °C) than predicted by commonly used rate expressions.

Kinetic Parameters for the Thermal Decomposition of Forest Waste Using Distributed Activation Energy Model (DAEM)

“…Pitt [15] used it for coal devolatilisation problems. Later on adopted by Hanbaba and his co-workers [16] and Anthony [17], [18].…”

“…(14) converge between range of (0,1). This is approximated with help of step function U as previously approximated [14], [15], [24], [25] can be given by: …”

The dual role of oxygen functions in coal pretreatment and liquefaction: Crosslinking and cleavage reactions

“…These studies will also be needed to establish the baseline for considering the effect of polymeric environment. Carboxyl functions have been implicated in the crosslinking of coals during at relatively low temperatures [30,31], and Solomon and coworkers [32] have been able to model the pyrolytic loss of solvent swelling by including one additional crosslink in the network for every CO 2 evolved. Moreover, pretreatments that have been found to be effective in promoting liquefaction have also shown a corresponding decrease in the early CO 2 evolution [33].…”

Measurement and modeling of advanced coal conversion processes, Volume I, Part 2. Final report, September 1986--September 1993

“…To improve the knowledge of particle temperatures, a new method was developed which employs a Fourier Transform Infrared (FT-IR) spectrometer to measure the emission and transmission (UT) in a reacting particle stream (Solomon and Hamblen, 1985;Serio et al, 1987). This technique has been applied to measure the temperature of coal particles in an entrained flow reactor (EFR) ) and a heated tube reactor (HTR) ).…”

“…The ultimate analyses are approximately those reported in Serio et al (1987). Both samples were sized to 200 x 325 mesh.…”