Pyrolysis of a lignite briquette - Experimental investigation and 1-dimensional modelling approach

A laboratory-scale shaft furnace was used for the pyrolysis of the lignite briquette together with thermogravimetric analysis to study the intrinsic pyrolysis kinetics under two different heating rates, 10 degrees C/min and 100 degrees C/min. Apart from coal conversion rate, the tar yield, quality and radical concentration in char were also measured to explore the difference between lignite briquette and the respective pulverised powder. Additionally, a 1-D model coupled with the chemical percolation devolatilisation (CPD) code was developed to quantitatively understand heat transfer using temperature dependent parameters; product distribution and yields, and the pyrolysis mechanism. It was discovered that heat transfer was the limiting factor for pyrolysis of the lignite briquette, which subsequently lowered the heating rate and led to increased cross-linking and decreased tar production compared to a coal particle. Simultaneously, the primary tar also underwent internal cracking and even deoxygenation to decompose into light aromatics and gases inside the briquette char matrix. Providing a hot gas environment was found to facilitate the cracking of tar species compared to a slow heating rate where tar is released at a lower temperature. The changes in radical concentration in the solid material were linked to the structural changes predicted by the CPD model including bridge-breaking, tar release and cross-linking phenomena.