This work focuses on the application of plasma-assisted ignition and combustion in pulverised fuel burners. Plasma-assisted ignition of pulverised fuel as an alternative technology for the start-up of pulverised fuel furnaces is investigated in a 400 kWpilot-scale facility using various qualities of lignite as well as woody biomass with distinct ignition characteristics. Short-time plasma-supported ignition under the cold furnace condition was demonstrated with parametric studies regarding the required power of plasma torch, air swirling, torch positioning, particle size distribution, etc. to identify the influence of different burner design parameters. The knowledge and experience obtained during this work are expected to support the development of such systems under the condition that plasma is only used as an ignition source rather than a continuous flame stabilisation method. Experiments are also performed to investigate the potential of plasma-assisted combustion during part-load and low-load operation.
Analyses on the fuel ignition characteristics showed that the ignitability characteristic number and deflagration index represent the most pronounced influence regarding the fuel properties and could be related to the ignition performance of fuels observed in the pilot-scale experiments, to a good extent. Plasma-assisted ignition tests showed that lignites with a moisture content up to 15% form attached self-sustained flames under cold burner conditions with short-time plasma assistance and an increase in the moisture content up to around 20% does not hinder the formation of this self-sustained flame, though flame lift-off was observed and the operational range, e.g. the burner load and air ratio, was restricted. These lignite qualities all had a median particle size of below 450 μm and an ash content lower than 12%. Lignites with a median diameter above 500 μm were also ignited with the used 4.2 kW to 7 kW plasma torch but did not form a self-sustained flame in the absence of plasma.
Variations in the ignition boundary conditions, i.e. burner parameters, displayed more prominent effects on the fuels with lower ignitability. Increasing the plasma power expanded the ignition limit, mostly for the less ignitable fuels, where ignition was accomplished over a broader range of operational parameters, though no improvement was observed with respect to the formation of self-sustained flame. Moreover, comparisons between two plasma systems, differing mainly in the size of the plasma jet, suggested that the extent of plasma-particle contact plays a significant role in ignition initiation.
Parametric studies also highlighted the importance of the secondary air swirling not only for flame stabilisation but also the ignition initiation. Swirling of the primary air showed a detrimental effect on ignition and flame stability when the plasma torch was retracted from the burner head inside the primary air annulus. On the contrary, plasma retraction showed promising results in the case of non-swirling primary air owing to a reduced velocity at the burner outlet and improved plasma-particle contact.
Plasma-assisted combustion during part-load operation showed slight improvements in combustion performance up to 50% thermal load. Further reductions in the load deteriorated the combustion behaviour due to excessive heat loss and possibly improper burner aerodynamics, where plasma-assisted combustion could not contribute in a notable enhancement. Although an increase in CO and NOx concentrations was observed close to the burner zone at low thermal loads, e.g. thermal load of 38 %, flue gas NOx emission was even slightly reduced by decreasing the thermal load. Plasma-assisted combustion was also associated with higher NOx emissions compared with standard combustion, due to thermal NO formation by high-temperature plasma. This incremented NOx level was overcome successfully with the air-staging technique, where similar NOx levels were obtained for plasma-assisted combustion and normal combustion with a burner air ratio of 0.8 and 1.2, respectively.