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. 2023 May 18;8(21):18530-18542.
doi: 10.1021/acsomega.3c00264. eCollection 2023 May 30.

Numerical Optimization on Char Conversion and NOx Emission under Various Operating Conditions in a Retrofit Biomass Boiler

Viet Thieu Trinh  1   2 , Byoung-Hwa Lee  3 , Seung-Mo Kim  3 , Chung-Hwan Jeon  1   3
Affiliations

Numerical Optimization on Char Conversion and NOx Emission under Various Operating Conditions in a Retrofit Biomass Boiler

Viet Thieu Trinh et al. ACS Omega. .

Abstract

Retrofitting retirement or existing fossil boiler with biomass is an important method of curbing electricity shortage and lowering the cost of modern power plants. However, the use of biomass combustion is hampered by operational problems, such as the resulting high unburned carbon, amount of bottom ash, and nitrogen oxide (NOx) release. In this study, we investigated the burning of pulverized biomass in a retrofitting boiler power plant using computational fluid dynamics of commercial software fluent ANSYS to determine the optimal combustion conditions. The objective of this study was to investigate a 125 MWe pulverized biomass boiler that was retrofitted from an anthracite down-fired boiler. The air distribution, including the influence of the secondary air ratio and the location of the burner standby, was evaluated. Key factors such as biomass ash mass at the hopper, char conversion, and high zone temperature relating to NOx formation/reduction were calculated. The adjustment of the secondary air ratio from 30 to 50% of the total air and the mass ash at the hopper significantly decreased to a low value at 247 kg/h and a high value of char conversion at 97.33% in case R (SA40%). The standard deviation temperature was 240 K at the BNR B-A level for case R, which was significantly lower than in other cases. This implies that the best mixing of air and biomass occurs in case R at 40%. Comparative analysis of the burner standby conditions showed that the NOx emission was similar at the boiler outlet (approximately 94-116 ppm). Burner A on standby, with a secondary air ratio of 40%, was used as the optimal case with the highest value of char conversion at 98.43%, the lowest bottom ash release of 204 kg/h, and a low-NOx emission of 106 ppm.

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Conflict of interest statement

The authors declare no competing financial interest.

Figures

Figure 1
Figure 1
Schematic configuration of the biomass boiler and burner geometry.
Figure 2
Figure 2
Swirl burners at the furnace wall.
Figure 3
Figure 3
Derived weight loss (DTG) profiles from thermal decomposition.
Figure 4
Figure 4
Combustion process of biomass in CFD.
Figure 5
Figure 5
High-temperature contour and average and deviation temperatures in the horizontal cross section for (a) case 1-SA30%, (b) case R-SA40%, (c) case 2-SA50%, and (d) the volatile and char burnout rates.
Figure 6
Figure 6
Char conversion at the exits of each particle size depending on the SAR change.
Figure 7
Figure 7
(a) Contour of the thermal NOx formation rate, fuel NOx formation, fuel NOx reduction, HCN, NH3 formation, and NH3 reduction at the reference case. (b) Average volume rate of NOx below and above the OFA port depending on the change in the SAR.
Figure 8
Figure 8
(a) Char conversion at the exits of each particle size based on the burner standby and (b) average volume rate of NOx below and above the OFA port based on the burners in standby.
Figure 9
Figure 9
Overview of the results under the effect of the burner standby based on the key parameters at the exits of the biomass boiler.
See this image and copyright information in PMC

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