Experimental Investigations of the Ignitability of Several Coal Dust Qualities

Reyhane Youssefi,Tom Segers,Jörg Maier,Frederik Norman,Günter Scheffknecht

doi:10.3390/en14196323

Abstract

The ignition characteristics of coal dust is of high importance for the flame stability in coal-fired power plants. We investigate the ignitability of six lignite dust qualities and one hard coal using dust explosion tests and an ignitability characteristic number. The paper aims to identify the degree of impact of the properties of coals, such as the moisture content, the ash content etc., on the ignition characteristics and ultimately to compare the identified relevant ignition parameters to the ignition performance of the dust qualities in an industrially relevant environment. The minimum cloud ignition temperature (MCIT), the maximum rate of pressure rise ((dp/dt)max), the maximum explosion pressure (pmax), the deflagration index (Kst-value) and the modified ignitability characteristic number (ZWZmod.) were determined and were attributed to the moisture content, the ash content and the median particle size. The MCIT was largely influenced by the volatile content, whereas the variations of moisture and ash contents within the range of 10% to 20% did not have a significant impact on the MCIT. The maximum explosion pressure did not differ considerably and stayed in a narrow range among the tested dust qualities. The deflagration index showed a higher sensitivity to the dust properties. The deflagration index and the modified ignitability characteristics number dropped as the moisture content increased and the volatile content reduced. The Kst and ZWZmod. values showed the highest susceptibility to the coal dust properties. Hence, they were used as representative parameters for further comparison with the ignition performance of coal dust in a pilot-scale testing. The results showed that both parameters predicted the ignition performance relatively well and can be used as indicators for the prediction of the ignition performance.

Highlights

In pulverized fuel systems, the ignitability of the fuel plays an important role in the combustion efficiency and the flame stability
In coal-fired power plants, the flame stability and, the plant efficiency deteriorate when the temperature inside the furnace drops during part-load operation or when the fuel quality varies notably from the standard fuel designed for that firing system
The minimum cloud ignition temperature (MCIT) for four lignite qualities with 10% to 20% moisture content, 5% to 15% ash content and median diameters of 83 to 150 μm changed in a narrow range of 410 to 440 °C and did not show a clear correlation to these properties within this range

Summary

Introduction

The ignitability of the fuel plays an important role in the combustion efficiency and the flame stability. In coal-fired power plants, the flame stability and, the plant efficiency deteriorate when the temperature inside the furnace drops during part-load operation or when the fuel quality varies notably from the standard fuel designed for that firing system. In industrial burners during the plant start-up, solid fuel is ignited via supporting burners, e.g., natural gas burners, where the surrounding temperature is notably above the temperature required for dust ignition. The solid dust particles ignite once they enter the combustion chamber. During cold power plant start-up, the solid fuel particles are ignited with a plasma torch in an ambient-temperature-environment. The plasma torch ignites a portion of the solid fuel stream and the resulting flame propagates through the dust cloud [1,2]

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