Effects of pyrolysis temperature, residence time on the reactivity of clean coals produced from poor quality coals

Combustion of poor quality coals is used today worldwide for energy production. The removal
of polluting compounds, i.e. S, N, Hg and Cl from them prior to combustion is important not
only for improving the coal quality, but also for meeting the strict environmental standards.
Thus, a preventive fuels pre-treatment technique, based on low temperature carbonisation,
has been developed that aims to convert ‘dirty’ fuels to cleaner ones. This study looks at the
effect of pyrolysis temperature and residence time on the reactivity of clean coals produced
by poor quality coals. Pyrolysis tests of three poor quality coals (Ptolemais, Bulgarian and
Australian) were performed in a lab-scale fixed bed reactor, under helium atmosphere and
ambient pressure. Ptolemais lignite is the main fuel for electricity production in Greece. The
effects of carbonisation temperature (200-900°C) and residence time (5-120 min) on the
properties of the obtained chars were investigated. Special attention was paid to the polluting
compounds (S, N, Hg, Cl) removal. The reactivity under combustion conditions of the
Ptolemais chars produced was also investigated. It was observed that low temperature
carbonisation could contribute to clean coal production by effectively removing the major part
of the existing polluting compounds in the coals. Thus, depending on coal type, nitrogen,
mercury and chlorine conversion continuously increase with temperature, while sulphur
removal seems to reach a plateau above 500 – 600°C. Furthermore, the prolongation of
carbonisation time above 20 min does not affect the elemental conversion of the pollutants.
Therefore, carbonisation at 500 – 600°C for ~20 min could be considered sufficient for clean
coal production from poor quality coals. The reactivity of the prepared clean coals was
evaluated by performing non-isothermal combustion tests in a TA Q600 thermobalance at
ambient pressure and 20°C min-1 heating rate. At increased pyrolysis temperatures higher
initial combustion temperatures were observed, due to the volatile reduction in char
production stage. The latter results to lower total conversions.