Full Length ArticleEffect of staged combustion on low NOx emission from an industrial-scale fuel oil combustor in South Korea
Graphical abstract
Introduction
Fuel oil power plants were widely constructed in South Korea in the 1970s in response to the rapid industrialization and growth in power demand. Most of these fuel oil combustors use Bunker-C (BC) oil as fuel, which is the last fraction produced in the petroleum refining process. These fuel oil plants played a major role in power generation before the construction of numerous large capacity bituminous coal power plants in the 1990s. To date, more than ten fuel oil combustors remain operative for power generation and still account for 3% of domestic power generation in South Korea. However, the large amounts of NOx, SOx, and particulate matter (PM) emitted from these old facilities have caused environmental issues. NOx emission leads to the generation of photochemical smog upon exposure to sunlight and causes acid rain by reacting with moisture in the atmosphere. Therefore, South Korea has strictly regulated NOx emission since 1991 and the NOx emission standard has been progressively tightened from 250 ppm (1991) to 100 ppm (2015) for power plants having capacities higher than 100 MWe.
The fuel oil combustor considered in this study is a 400 MWe opposite-wall unit located in Ulsan, South Korea, utilizing high-sulfur fuel oil (Bunker-C with 2.5% sulfur content). The NOx emission from this power plant is around 150 ppm, which exceeds the emission standards of Korea. A selective catalytic reduction (SCR) reactor is used as a NOx reduction device. The reduction efficiency of the SCR reactor is quite low (approximately 50%); however, modification or reconstruction of the SCR reactor at this aged combustor is not planned as the lifespan of the combustor is expected to expire shortly. Among several auxiliary methods to the SCR, staged combustion causes the reduction of NO to form N2 by generating a fuel-rich zone upstream of the flue gas and results in combustion of unburned gases by supplying sufficient air downstream of the flue gas [1]. For the staged combustion, over-fire-air (OFA) has been widely used as air staging method. OFA applies a separated air supply port after the main combustion region, and supplies a portion of air through that port, which creates the air staged condition.
Operation tests are required to study NOx emission characteristics in combustor. However, power generation schedules are very constrained. Therefore, only specific and limited tests can be planned and executed, and the gathered data sets are usually incomplete and are subject to a large amount of uncertainty. To predict the response of the combustor, the use of numerical simulations currently seems to be more suitable, given that a large amount of complete and detailed data can be generated in a feasible way [2]. Several computational fluid dynamics (CFD) studies have been conducted to investigate the NOx emission characteristics. Tan et al. [3] investigated the effects of burner tilt angle on the combustion and NOx emission characteristics of a 700 MWe deep-air-staged tangentially pulverized-coal-fired boiler by using ANSYS FLUENT. As the burner tilt angle was increased from 0° to 22°, turbulent mixing from the furnace was intensified, heat transfer increased, and CO emission decreased by 69%. However, NO emission increased by 67% because fuel NOx formation increased and NO re-burning decreased. Yang et al. [4] investigated the effects of operating parameters on combustion in two identical 500-MWe coal-fired boilers using ANSYS FLUENT. The boiler with higher air staging ratio had a 79% lower thermal NOx formation rate. However, outlet NOx concentrations in both boilers were almost the same at 141 and 145 ppm because the fuel NOx formation rate is higher in boilers with a higher air staging ratio. Han et al. [5] conducted a numerical study on the re-burning (fuel staging) characteristics of biomass syngas in a 2 MW pilot scale heavy oil furnace. They observed a 47% reduction in NOx emission when fuel staging was applied, and ANSYS FLUENT was also used in their study. Liu et al. [6] numerically investigated flow, combustion characteristics, and NOx emissions for down-fired boilers with different arch-supplied OFA ratios using ANSYS FLUENT. As the OFA ratio increased from 0 to 25%, the outlet NOx concentration decreased by about 33%. Using ANSYS FLUENT, Kuang et al. [7] investigated the impact of OFA location on combustion improvement and NOx abatement of a down-fired 350 MWe utility boiler with multiple injection and multiple staging. By applying staged combustion at various positions, NOx emission was reduced by 45–57% compared to the absence of OFA. Zeng et al. [8] conducted experimental and numerical investigations of the combustion and NOx emissions characteristics of an OFA system in a 600 MWe boiler using ANSYS FLUENT for the simulation. When OFA was applied, the NOx concentration decreased by 60.4%. Choi et al. [9] investigated the characteristics of the flow, combustion, and NOx emission in a 500 MWe tangentially-fired pulverized-coal combustor using ANSYS FLUENT and compared the NOx emissions with and without OFA. The results showed that the NOx emission was 8.21% lower with OFA than without OFA. Using CFX, Diez et al. [2] investigated NOx emissions from a 600 MWe tangentially-pulverized coal-fired utility combustor under conventional and OFA operation and found the NOx emission was 13% lower with OFA operation. From numerical and experimental investigations of reduction of the NOx emissions in a 600 MWe pulverized coal-fired utility furnace by using OFA, Fan et al. [10] reported that applying OFA reduced NOx emission by 23% relative to conventional operation. Fan et al. [10] also utilized ANSYS FLUENT. Liu et al. [11] numerically investigated the effect of air-staged combustion by changing the OFA ratio emphasizing char gasification and gas temperature deviation in a large-scale, tangentially fired pulverized-coal boiler using commercial CFD code. The NOx emissions were lowered by 45% when the OFA ratio was increased from 0.17 to 0.42. Using ANSYS FLUENT, Ma et al. [12] investigated the effect of the separated OFA location on combustion optimization and NOx reduction of a 600 MWe FW down-fired utility boiler with a novel combustion system. By applying separated OFA in the novel combustion system, the calculated NOx emissions compared to the original combustion system was approximately 50% lower. Among the 3 possible locations for OFA ports, the upper OFA port had the lowest NOx emission. Zhang et al. [13] conducted a numerical investigation of low NOx combustion strategies in tangentially-fired coal boilers. They achieved approximately a 20% NOx reduction by applying HBC and OFA as predicted by ANSYS FLUENT. Using ANSYS FLUENT, Ma et al. [14] conducted a numerical study on reducing NOx emissions for a 600 MWe down-fired pulverized-coal utility boiler by applying a novel combustion system with separated OFA. At the maximum load of 600 MWe, they achieved a 50% NOx reduction compared to the original design without OFA. Liu et al. [15] investigated the effect of air staging conditions on the combustion and NOx emission characteristics in a 600 MW wall fired utility boiler using lean coal. By increasing the OFA ratio from 21.1 to 38.6%, they achieved a 17.6% reduction in NOx emissions as predicted by ANSYS FLUENT simulation. Zhou et al. [16] investigated the impact of OFA on combustion and NOx emissions of a large-scale laboratory furnace fired by a heavy-oil swirl burner. They achieved a 22.2% NOx reduction by applying a 20% OFA ratio as determined by ANSYS FLUENT.
All the previous CFD studies on staged combustion applied OFA as an air-staging method. However, many old boilers do not possess OFA ports and cannot be retrofitted. For these boilers, fuel and air staged combustion with multi layered burners is a feasible way to reduce NOx emission without additional modification of the device. This staged combustion method utilizes multi layered burners to create staged combustion conditions by applying different air/fuel ratios to each burner layer. To the best of our knowledge, no CFD study on staged combustion of the multi layered burners without OFA ports has been documented in the literature.
In this study, CFD simulation was performed for a 400 MWe fuel oil combustor to determine the effect of the degree of staged combustion on NOx emission by varying air/fuel ratios of multi layered burners. The simulation is validated by comparison with operational data.
Section snippets
The combustor and fuel oil
The combustor considered in this study has a height of 56 m and a cross-sectional area of 10 m × 12 m and a power capacity of 400 MWe. Water wall tubes (evaporator) are positioned on the wall of the lower part of the combustor and sixteen burners are located at Positions 1–4, as shown in Fig. 1. The system is comprised of superheater 1 (SH1), superheater 2 (SH2), reheater 1 (RH1), reheater 2 (RH2), and an economizer (EC) above the evaporator. Table 1 shows the data for ultimate and proximate analyses
Numerical modeling
The ANSYS 15 Design Modeler was used to model the geometry and grid. The number of grids was about 1,150,000. The grids were generated more densely near each burner because the flow and chemistry near the burner change more drastically than in other regions of the combustor.
ANSYS FLUENT 15 was used for the simulation. FLUENT 15 considers the flow, energy transport, and chemical reactions based on calculation with the continuity, momentum, energy, and species transport equations [12]. Combustion
Validation of the developed numerical model
The simulation was conducted by applying the reference operating conditions that were previously adopted in the Ulsan power plant (Table 2). The velocity contours in the central x-z plane and the velocity vectors in the x-y plane (position 2) are shown in Fig. 3(a). As the gas flowed upwards in the x-z plane, its velocity decreased and became uniform when the flow reached the tube bundles. At burner position 2, swirling flow occurred after the mixture of oil and air was injected from each
Conclusions
Under the initial conditions of staged combustion, the concentration of NOx at the exit of the combustor was calculated to be 362 ppm, which is in agreement with the measured value of 375 ppm. When more stringent staged combustion conditions were applied, the calculated concentration decreased to 309 ppm, which was also close to the measured value of 332 ppm. The total NOx concentration declined with stronger staged combustion because it resulted in a lower temperature and lower oxygen
Acknowledgements
This work was supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry & Energy (MOTIE) of the Republic of Korea (No. 20153010102030).
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