The past many years have seen the emergence of a growing desire worldwide to take actions to protect the environment from the degrading effects of all forms of pollution. The gaseous pollutant NOx is a primary air pollution composition from the combustion of coal for power generation. A broad range of NOx control technologies have been employed . The low NOx combustion and the flue gas de-nitrification technologies are the main methods to reduce nitrogen oxide emissions from coal-fired boilers. The fuel staging combustion technology is a promising technology at present . The fuel staging combustion, also known as re-combustion, has been widely concerned by scholars. This technology reduces NOx emissions by setting up a less oxygen-burning section in furnace to restore a part of NOx to N2  . There are several kinds of material that can be used as re-combustion fuel, such as ultra-fined coal, biomass raw materials and some combustible gases . Gaseous fuels are the first choice for re-combustion because they are easier to be fully mixed and burnt out . Zhang and others carried out the research of a biomass gasification gasifier coupled to a 330 MW coal-fired boiler, it was found that biomass gas has little impact on the normal boiler operation and can effectively reduce the emissions of NOx, SO2 and other pollutants . Glarborg et al. also carried out a study on the re-combustion characteristics of biomass gas, the results showed that, a higher methane concentration can lead to the greater NOx reduction rate at the excess air coefficient of 0.9 in re-combustion zone . Shi et al. studied the effect of methane on the mixing characteristics of gas re-combustion in a coal-fired boiler . Wu et al. studied the effect of biomass volatiles (CH4, CO, H2 and CO2) on NO reduction of re-combustion de-nitrification technology. The results showed that the main positive factors of NO reduction are CH4 and H2 . Su et al. performed the methane re-combustion de-nitrification test at four different levels of metamorphic coal species, the results showed that methane’s stay time in re-combustion area is at 0.7 - 0.9 s range, and the re-combustion de-nitrification efficiency is more than 50% . Lu et al. indicated that the excessive air coefficient, the ammonia nitrogen ratio and other parameters at the re-combustion area have a great impact on de-nitrification efficiency . Many early boilers had no re-combustion zone, causing high NOx emission. In this work, the re-combustion nozzles are added in the burner system to retrofit a 600 MW coal-fired boiler and methane is selected as the secondary fuel. The main objective is to investigate the influences of the methane co-firing ratio on boiler combustion characteristics and NOx emission. The results can provide a theoretical basis for retrofitting old units in service aiming NOx emission reduction.
2. Boiler Characteristics
2.1. Research Objects and Burner Renovation Program
A 600 MWe super-supercritical boiler is taken as the research object. This is a single furnace tangential firing boiler with a body of 63.55 m high, 18.816 m wide and 17.696 m deep.
As shown in Figure 1, the burner system before transformation is equipped with a total of 15-layer nozzles from bottom to top, including 6 layers of primary air nozzles (i.e., A, B, C, D, E and F), 7 layers of secondary air nozzles (i.e., AA, AB, BC, CD, DE, EF and FG), and 2 layers of separated over fire air nozzles (i.e., SOFA1 and SOFA2). The burner system is retrofitted by adding a layer of methane gas nozzles (i.e. G) and a layer of re-combustion air nozzles (i.e. GH). The combustion area of the furnace is divided into the primary combustion zone, the re-combustion zone and the burn-out zone after retrofit of burner system.
Figure 1. Boiler structure and burner layout before and after transformation.
2.2. NOx Emissions Reduction Mechanism
NOx generated in the furnace is divided into thermal type, fuel type and fast type. Its concentration is related to fuel characteristics and combustion conditions. NOx generated from coal combustion is due to high temperature at the primary combustion zone. As the re-combustion fuel is injected into furnace, a fuel-rich reductive atmosphere is formed. A part of NOx generated at the main combustion zone produces a reduction reaction in this area, being transformed to N2, hence the NOx emission is reduced.
The Methane as re-combustion fuel is broken down and produces a large amount of hydrocarbon root CHi to enhance the ability of reducing NOx. Therefore, it is considered one of most ideal re-combustion fuels.
2.3. Input Parameters
In order to study the effect of methane re-combustion ratio on combustion process and NOx emissions in furnace, four cases are considered. The total heat input to the boiler is constant. The basic case is coal combustion without re-combustion; the other three cases are the coal combustion with 7.5%, 10% and 12.5% methane re-combustion ratio (according to heat input) respectively. The input parameters at 4 cases are shown in Table 1.
3. Results and Discussions
3.1. The Temperature Distribution
The mean temperature distributions at horizontal cross-section in the furnace under different operating conditions are shown in Figure 2. The mean temperature gradually increases with furnace height and reaches its peak at the height of 24 m, and then it gradually decreases. With the increase of re-combustion ratio, the temperature at main combustion zone decreases, but increases at re-combustion zone. The peak temperature at case 1 is around 1900 K, 51 - 72 K higher than that of case 2 - 4. Temperature at the furnace outlet of case 1 is about 1300 K, 10 - 36 K less than that of case 2 - 4. This actually revealed the combustion condition and was related to flue gas concentration.
3.2. The Flue Gas Component Field Distribution
In this study, the main components in the flue gas are O2, CO, CO2 and NO. The volume fractions of O2, CO, CO2 and NOx mass concentration at each furnace horizontal cross-section under different operating conditions are shown in Figure 3.
As seen from Figure 3, the changing trend of O2 volume fraction is similar at different cases. The O2 volume fraction of case1 at the primary combustion zone
Table 1. Input parameters under different operating conditions.
Figure 2. Mean temperature distribution of horizontal cross-section (K).
Figure 3. Volume fractions of O2, CO, CO2 and NOx mass concentration.
is higher than that of case 2 - 4, but lower at the re-combustion zone. The CO and CO2 volume fraction have an inverse trend, being related to the concentration of O2. When O2 in flue gas decreases, CO increases, but CO2 decreases. This indicates that the simulation is reasonable. At the re-combustion area, the NO concentration decreases with the rise of CH4 volume fraction, in that a part of NO react with CO to generate N2 and CO2. At the re-combustion area, the NOx production decreases greatly with the rise of CH4 re-combustion ratio. Therefore, the productions of NOx at cases 2 - 4 are less than that at case 1. The mean mass concentrations of NOx at furnace outlet are 535.05 mg/m3, 497.48 mg/m3, 446.16 mg/m3, 398.00 mg/m3 respectively, where the NOx emissions decrease by 7.02%, 16.61% and 25.61%, respectively.
In this work, the numerical models are built to study the influence of different methane re-combustion ratio on the combustion process and the concentration distributions of the flue gas components in a retrofitted coal-fired boiler. The research work shows that methane gas re-combustion can lead to the temperature at primary zone decrease, reducing NOx formation in furnace. The re-combustion of methane gas can produce reduction atmosphere to further reduce NO to N2. The NOx concentration at the furnace outlet can be reduced by 7.02%, 16.61% and 25.61% respectively compared to strict coal combustion condition.
In order to ensure safe operation of boiler and effective reduction of NOx emissions from boiler, it is more reasonable to construct a re-combustion zone with 12.5% methane as re-combustion fuel. It is feasible to retrofit the old units in service to reduce NOx emission according to fuel staging combustion technology.
 Martin, M., Robert, D., Julian, W. and Andreas, H. (2017) Boiler Design with Solid-Gaseous Fuel Staging to Reduce NOx Emissions and Optimize Load Flexibility. Chemical Engineering and Technology, 40, 289-297.
 Kuang, M., Zhu, Q.Y., Li, Z.Q. and Zhang, X. (2013) Numerical Investigation on Combustion and NOx Emissions of a Down-Fired 350 MWe Utility Boiler with Multiple Injection and Multiple Staging: Effect of the Air Stoichiometric Ratio in the Primary Combustion Zone. Fuel Processing Technology, 109, 32-42.
 Yun, S. and Fan, Z. (2018) Catalytic Reduction of NOx by Biomass-Derived Activated Carbon supported Metals. Chinese Journal of Chemical Engineering, 10, 2077-2083.
 Zhang, X.T., Li, K.Y., Zhang, C. and Wang, A.J. (2020) Performance Analysis of Biomass Gasification Coupled with a Coal-Fired Boiler System at Various Loads. Waste Management, 105, 84-91.
 Glarborg, P., Kristensen, P.G. and Dam-Johansen, K. (2000) Nitric Oxide Reduction by Non-Hydrocarbon Fuels. Implications for Re-Burning with Gasification Gases. Energy & Fuels, 14, 828-838.