Effect of Hydrogen Blending in Industrial Flames
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(Thesis)
Date
2024
Authors
Gee, Adam
Editors
Advisors
Medwell, Paul
Smith, Neil
Chinnici, Alfonso
Smith, Neil
Chinnici, Alfonso
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Abstract
The issue of climate change and depletion of non-renewable energy stores has become one of the defining challenges of the modern world. The research and development of alternative sources of renewable, low-carbon energy have become the focus of governments and industries all over the world — many of whom are looking toward hydrogen to play a crucial role in the future of our energy portfolio. For many industrial processes which rely on heat from the combustion of hydrocarbon fuels, such as natural gas, switching to electricity will not be a viable option. Among other benefits, hydrogen provides an alternative fuel for combustion that is free from carbon and potentially completely renewable. The partial or complete adoption of hydrogen is a promising solution to solve the issues associated with current fuels, however, the compatibility with existing burner designs and the overall performance of hydrogen as a fuel is still largely unknown. To address this gap in knowledge, four experimental investigations were completed, each of which has since been published or accepted for publication in archival journals. These journal papers quantify the effects of hydrogen addition under industry relevant conditions, each of which include the testing of a potential solution strategy to overcome one or more of the challenges associated with the use of hydrogen. The experimental methods, including selection of burners, diagnostic techniques and data analysis were chosen to provide a novel, yet industry relevant, perspective on the effects of hydrogen combustion. The first investigation considers the implications of blending hydrogen with natural gas in a scaled-industrial swirl burner are presented alongside the effect of varying a swirled coannular air stream. Two strategies for introducing hydrogen are considered, namely, conserving (i) heat input and (ii) velocity/volumetric flow of the original fuel. The addition of hydrogen was shown to reduce radiant heat transfer from flames and increase NOx emissions. The overall visibility of the flames decreased as hydrogen was added and the typical lifted flames became reattached to the burner. Compared with natural gas, the results with hydrogen showed a 33% reduction in the radiant fraction and up to a 380% increase in NOx emissions. The lift-off height was reduced by a maximum of 23% and 51% for addition of 10 and 30 vol% hydrogen addition, respectively, with 100% cases becoming completely attached to the burner. The influence of hydrogen-addition strategy and air adjustment was shown to be significant with respect to NOx emissions but less significant than the resulting changes in fuel composition and heat input with respect to flame appearance, stability and radiant heat transfer. In the second investigation, the addition of toluene as a means of improving soot loading and resulting radiant heat transfer and visibility was quantified. In this instance, bluff-body burners were used to study the effects of toluene addition to hydrogen-enriched flames while also characterising the influence of burner-induced recirculation. The results showed that toluene was effective at improving soot loading in the flames, with increased illuminance and heat flux measured — however, the quantity of toluene required to achieve this result far exceeded that which could reasonably be called a dopant and so it was concluded to be ineffective. Total heat flux and illuminance increased non-linearly with toluene concentration for all fuel blends and bluff-body diameters. By reducing the bluffbody diameter from 64 mm to 50 mm, a 20 vol% hydrogen mixture produced a more radiative flame than addition of 10 vol% hydrogen in the smaller bluff-body burner. Opposed-flow flame simulations of soot precursors indicate that as strain rate increases, although overall soot precursor concentration decreases, a 20 vol% hydrogen mixture will produce more soot than a 10 vol% mixture. This suggests the addition of hydrogen up to 20 vol% may be beneficial for soot production in high strain environments. Experimental work conducted on a commercial self-aspirating burner is presented in the third and fourth investigations. In the third investigation, the performance of the burner was characterised as hydrogen was added, including the upper limit of addition for normal operation with no changes to the burner. Hydrogen addition was shown to increase the entrainment of primary air while also decreasing the equivalence ratio of the mixture and increasing the burning velocity of the fuel, this led to flashback at >50 vol% hydrogen. To combat this challenge, the burner was modified by increasing the fuel injector size, such that the fuel stream momentum could be reduced — this strategy was shown to be effective at extending the flashback limit of the burner, with stable operation up to 100% hydrogen. A trade-off was found with this modification strategy where the decreased primary air entrainment caused a considerable increase in NOx emissions for a given fuel blend. In the fourth investigation, the use a hydrogen-biogas fuel blend as a mutually beneficial fuel mixture in a self-aspirating burner. The results showed that the high carbon dioxide content of the biogas mixture was effective at maintaining low NOx emissions as hydrogen content increased, and likewise the properties of hydrogen improved the stability and performance of the biogas mixtures. The added carbon dioxide and/or hydrogen content was, again, shown to increase the entrainment of primary air. This was shown to be the primary driver for impacts on NOx and radiant heat transfer, however, this was addressed by using the same modification of the fuel injector discussed in the third investigation. The diverse range of burner designs and experimental methods chosen provides both specific and general understanding of the implications of hydrogen addition to industrial burners, as well as valuable and novel insights to how hydrogen might be more smoothly integrated into the current gas infrastructure. Ultimately this work contributes to de-risking the adoption of hydrogen for use in industrial combustion applications and highlights the efficacy of several approaches to improve the combustion properties of hydrogen.
School/Discipline
School of Electrical and Mechanical Engineering
Dissertation Note
Thesis (Ph.D.) -- University of Adelaide, School of Electrical and Mechanical Engineering, 2024
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