Heat Transfer and Thermal Efficiency in Oxy-Fuel Retrofit of 0.5 MW Fire Tube Gas Boiler
Abstract
:1. Introduction
2. Materials and Methods
3. Results and Discussion
3.1. Flame Image and Thermal Efficiency by Combustion Mode
3.2. Heat Transfer in Combustion Chamber
3.3. Heat Transfer in Fire Tube
3.4. Heat Transfer in Economizer
4. Conclusions
- When substituting the burner in a natural-gas-fueled fire tube industrial boiler with an oxy-fuel burner, a comparable level of heat absorption occurs within the combustion chamber, enabling retrofitting. With this modification, oxy-fuel with flue gas recirculation (FGR) achieved a thermal efficiency akin to that with air combustion, and oxy-fuel combustion exhibited a 3–4% enhancement compared to air combustion.
- Oxy-fuel combustion diminishes the exhaust gas flow rate, thereby reducing the fire tube’s contribution to heat transfer when applied to an existing fire tube boiler. Consequently, it was discerned that it would be cost-effective to devise a novel design featuring a smaller fire tube. While gas radiation in the fire tube accounts for less than 5% of the heat transfer in air combustion, its significance escalates in oxy-fuel and FGR combustion, necessitating consideration during design.
- Oxy-fuel combustion and FGR combustion augment the proportion of water vapor in the combustion exhaust gas, facilitating more efficient recovery of condensation heat as compared to air combustion. Furthermore, the efficacy of the sensible heat economizer in oxy-fuel combustion surpassed that in air combustion.
- The Dittus–Boelter equation, employed in fire tube design, accurately projected the heat transfer across all combustion methods under 100% load conditions. However, the predictions deviated slightly under 50% load conditions. Concerning fin–tube heat exchanger economizers, the Zukauskas equation aptly anticipated heat transfer solely for FGR combustion in the sensible heat section. In the latent heat section, a measured heat transfer coefficient 4 to 8 times higher than the value predicted by the Zukauskas equation was recorded.
5. Future Research
Funding
Data Availability Statement
Conflicts of Interest
Nomenclature
AF | surface area of flame [m2] |
AFT | surface area of flame tube [m2] |
AS | boiler external area [m2] |
AW | combustion chamber wall area [m2] |
cpg | specific heat of combustion gas [J/kgK] |
D | tube diameter [m] |
FGR | flue gas recirculation |
h | heat transfer coefficient [W/m2K] |
h0 | heat transfer coefficient outside boiler [W/m2K] |
hs | specific enthalpy of steam [J/kg] |
hw | specific enthalpy of feed water [J/kg] |
HHV | high heating value of fuel [J/kg] |
k | thermal conductivity [W/mK] |
mass flow rate of fuel [kg/s] | |
mass flow rate of fuel [kg/s] | |
mass flow rate of feed water [kg/s] | |
Nu | Nusselt number (=hD/k) |
Pr | Prandtl number (=ν/α) |
qf | heat rate absorbed per unit volume in combustion chamber [W/m3] |
r | radial coordinate at combustion chamber [m] |
R | radius of combustion chamber [m] |
Re | Reynolds number (=VD/ν) |
TFT | flame tube surface temperature [K] |
Tg | combustion gas temperature [K] |
Ts | boiler surface temperature [K] |
V | flow velocity [m/s] |
x | streamwise coordinate from burner [m] |
α | thermal diffusivity [m2/s] |
αg | absorptivity of combustion gas |
εg | emissivity of combustion gas |
η | thermal efficiency of boiler |
ν | kinematic viscosity [m2/s] |
σ | Stefan–Boltzmann constant [W/m2K4] |
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Ahn, J. Heat Transfer and Thermal Efficiency in Oxy-Fuel Retrofit of 0.5 MW Fire Tube Gas Boiler. Processes 2024, 12, 959. https://doi.org/10.3390/pr12050959
Ahn J. Heat Transfer and Thermal Efficiency in Oxy-Fuel Retrofit of 0.5 MW Fire Tube Gas Boiler. Processes. 2024; 12(5):959. https://doi.org/10.3390/pr12050959
Chicago/Turabian StyleAhn, Joon. 2024. "Heat Transfer and Thermal Efficiency in Oxy-Fuel Retrofit of 0.5 MW Fire Tube Gas Boiler" Processes 12, no. 5: 959. https://doi.org/10.3390/pr12050959
APA StyleAhn, J. (2024). Heat Transfer and Thermal Efficiency in Oxy-Fuel Retrofit of 0.5 MW Fire Tube Gas Boiler. Processes, 12(5), 959. https://doi.org/10.3390/pr12050959