title: Impact of diversity in syngas/producer gas composition on stable and unstable laminar diffusion flames
Creators: name: Piemsinlapakunchon, Tananop
Publisher: name: University of Glasgow
Year: 2019
Subjects: subject: Combustion modelling, syngas, diffusion flame, producer gas, stable flame, unstable flame.

description: Renewable and sustainable energy are developed to address global energy demand which has increased dramatically. Among them, syngas/producer gas, defined as a gas fuel consisting of hydrogen (H2), carbon-monoxide (CO), carbon dioxide (CO2), water vapour (H2O), nitrogen (N2), and methane (CH4) as a major species, is one of the interesting options. The lower CO2 emission than fossil fuel and flexibility in the production process and feedstock are its advantages. However, these benefits have a side effect because of the diversity in fuel composition which affects the combustion characteristics directly [1]. The utilisation of syngas in the conventional gas fuel combustion systems would, therefore, require an in-depth investigation and microscopic understanding of the impact of each species of hybrid gas composition on combustion. This information is an essential step in the development of efficient combustion technologies. The research work presented in this thesis addresses some of these challenges with the identification of the knowledge gap in syngas combustion. The study focused on the numerical investigation of the impacts of syngas composition on the combustion and flame characteristics, with a particular focus on the role of syngas composition in heat generation capability, emission, and flame instability. The study firstly investigates the impact of the content of H2 in fuel stream on the diffusion flame characteristics including flame temperature, flame dimension, heat generation and emission. The studied fuel is H2/N2 mixed, and the first modelling on a stable co-flow diffusion flame is performed based on an axisymmetric burner presented in Toro et al. [2]. The content or volume of H2 in the fuel stream is controlled by the fuel rate as well as the concentration of H2 in fuel composition. The results generally show that an increase in the H2 content supplied into combustion escalate the flame size and heat generation. In particular, the flame width and flame temperature are dominated while the flame length is affected strongly by the fuel rate. The thermal nitrogen oxide (NOx) formation mechanism also plays a significant role since the flame with a higher flame temperature formulates NOx at a higher rate. Also, the heat generation from flame is affected strongly by the flame size and temperature, which compensate each other. Secondly, the impact of diversity in syngas/producer gas composition on the heat generation capability and emission are examined. The study reports that the total heat generation capability of the diffusion flame (Qtotal) depends strongly on the combustion heat release property of fuel (Qmax) and flame dimension. These parameters also compensate for the role of each other; for example, the higher value of Qtotal is obtained from a larger flame of fuel with a lower value of Qmax. CH4 encourages heat generation capability but decreases the consumption and emission per unit of heat generated. CO promotes heat generation and decreases fuel consumption at a weaker level than CH4. However, a higher production rate of CO2 emission per unit of heat generated is a side impact of a higher concentration of CO. H2 plays a significant role in the flame temperature but encourages heat generation at the weaker level than CO. Non-combustible species reduces the heat generation capability, and the weaker impact of CO2 causes it is more desirable than N2. Additionally, it is noticed that CO2 assists the radiative heat transfer property of the flame. Thirdly, an occurrence of oscillation in syngas laminar co-flow diffusion flame is studied. The phenomena occur due to the impact of Kelvin-Helmholtz instability on the mix of fuel and air at the flame front. Two types of flame oscillation ‘flickering and tip-cutting’ are found when the fuel and oxidiser are supplied into combustion at an improper ratio. The simulation model is formulated based on the burner geometry of Darabkhani et al. [3]. The oscillating flame is created without co-flow air to stabilise. The flame is stable at a low fuel rate then transforms into a flickering/tip-cutting flame at a higher fuel rate. The syngas with a lower H2:CO promotes a higher magnitude of flame oscillation. However, the value does not increase further when H2:CO &lt; 1; i.e. the syngas flame with H2:CO = 1 has a comparable impact of flame oscillation when considering H2:CO &lt; 1. Diluting CH4 in syngas composition significantly increases the magnitude of oscillation at a significantly stronger level than CO. Adding CO2 or N2 to syngas composition results in the opposite direction. Also, the flame oscillation directly affects the fluctuation in heat generation capability. Finally, further study is processed with an aim to minimise/eliminate the flame oscillation. The fuel rate is kept constant while the air rate supplied is escalated for this part of the study. The magnitude of oscillation of the flame reduces, then the flame becomes stable at a higher air rate since the vortices of the K-H instability are pushed outside the visible flame. Further, the impact of fuel composition on this phenomenon is analysed in order to derive a relation between the reduction of the magnitude of oscillation and air rate. Syngas with a relatively low ratio of H2:CO demands a less air rate to eliminate the instability. On the other hand, diluting CH4 to syngas composition significantly increases this demand, and the opposite result is observed when syngas is mixed with CO2 and N2.