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NOx and N2O emissions from an ultra-supercritical circulating fluidized bed (CFB) boiler were predicted using a two dimensional (2-D) comprehensive computational fluid dynamics (CFD) combustion model. This model was developed from a three dimensional model for a supercritical CFB boiler previously constructed by our group. Based on an analysis of the NOx and N2O conversion processes in a CFB boiler, the primary formation and destruction reactions were introduced into the 2-D model and coupled. The resulting model was validated using data from the Baima 600 MW supercritical CFB boiler, and then applied to a 660 MW ultra-supercritical CFB boiler. The effects of excess air, the secondary air (SA) to (primary air (PA) plus SA) ratio and the SA injection height on NOx and N2O emissions were investigated. The results show that a higher excess air volume increases both NOx and N2O emissions, while increasing the SA/(PA + SA) ratio somewhat reduces both the NOx and N2O concentrations. On the basis of the results of this work, optimal locations for SA injection ports so as to lower NOx and N2O emissions are recommended.

Circulating fluidized bed (CFB) technology has been continuously developing with an increasing number of large-capacity and supercritical CFB boilers being put into operation. This article reviews the current developments in large capacity CFB boilers in China and presents a note on the most recent development trends. It includes discussions on (1) gas-solid hydrodynamics in large-size CFB furnaces, (2) heat transfer on suspended surfaces, (3) combustion atmosphere and emissions, (4) 3-D CFD numerical simulation for large-scale supercritical CFB boilers, (5) integrated energy saving technologies, (6) optimal operation of an integrated desulfurization system, (7) erosion and protection of heat transfer surfaces, (8) problems in operation and their solutions, (9) combustion of coal with high alkali content.

A combustion model of a large-scale supercritical circulating fluidized bed (CFB) boiler was developed for comprehensive computational-fluid-dynamics analysis. The model incorporates gas–solid hydrodynamics, coal combustion, heat transfer on heat exchange surfaces in the furnace, and heat transfer between furnace and working medium in the heat transfer tubes. In simulating the dense and dilute phases in the furnace, the gas–solid hydrodynamics is based on the Euler–Euler model and energy-minimization multiscale drag model. Coal combustion entails evaporation, devolatilization, char combustion, gas homogeneous reaction, and pollutant emission. The coefficient of heat transfer between gas–solid and the waterwall is estimated using the cluster renewal model, and for radiation, the discrete ordinate model is used. Moreover, thermohydraulic processes in the membrane wall are also included in the heat transfer process. The model was successfully applied in simulations of a 350-MW supercritical CFB boiler. Detailed distributions of solids concentration, oxygen, heat flux, and working medium temperature in the boiler furnace are presented.

High contents of alkali and alkali earth metals (AAEMs) induce severe ash slagging and fouling during coal combustion. In this regard, co-combustion is an effective method to mitigate ash deposition. The ash deposition behavior in co-combusting high-alkali Zhundong (ZD) coal and a bituminous coal, i.e. Shenhua (SH) coal was tested in a 30 kW circulating fluidized bed test system. The results revealed that the condensation of Na2SO4 triggered the slag formation of ZD coal in the furnace. The Na-Ca-Fe eutectics were responsible for the melting or partial melting of slags at high temperatures. After blending SH coal with ZD coal, the volatilization of Na was effectively suppressed, and compacted slags practically disappeared at 20% blending ratio of SH coal. In the convective backpass, fine particles (

For better understanding the release and transformation mechanisms of sodium compounds during high alkali coal combustion in circulating fluidized bed (CFB) boilers, a sodium migration model of coa...

High alkali and alkali earth metals (AAEMs) content in coal causes severe slagging and fouling during combustion in a boiler. In this study, the ash deposition behavior of a high-alkali coal at different bed temperatures and the effect of kaolin were investigated in a 30 kW circulating fluidized bed (CFB) test system using an ash slagging probe and deposition probe. The results show that the ash deposition tendency increases with the bed temperature. The condensation of Na2SO4 is an important inducement for slag formation in the furnace. The melting or partial melting of slags is attributed to Na–Fe–Ca eutectics. At 920 °C, Na2SO4 will react with CaSO4 to form the low-melting compound of Na2SO4–CaSO4. The deposited ash on the convection-heating surface consists of granular particles. On the windward side, the layered-structure ash deposits, i.e. the inner and outer layers, are formed at the bed temperature of 920 °C but are absent at lower temperatures (820 °C and 870 °C). The formation of the inner layer consists of fine particles (

Burning a coal containing a high content of alkali metals (Na, K) in utility boilers may result in ash-related problems such as slagging, fouling and corrosion on the heat transfer surfaces. This study investigated the transformation behavior of Na in four typical high-alkali coals including one Zhundong (ZD) coal and three Taer (TR) coals during combustion. The Na content in the coal is much higher than the K content, and different high-alkali coals have difference in the aspect of their existing forms. Measurement results of the tested coals showed that most of the Na in the coal is H 2 O-soluble Na. During combustion, the released alkali metals increase with the elevated temperature. The H 2 O-soluble Na and NH 4 Ac-soluble Na in the ash decrease after the coal is burnt. This indicates that the released Na is originated from the H 2 O-soluble and NH 4 Ac-soluble Na, with the H 2 O-soluble Na being the dominant constituent. The investigation also reveals that some of the H 2 O-soluble Na in the ash might be converted into HCl-soluble Na and insoluble Na through reacting with Si-Al minerals. Furthermore, the low volatility of Na for ZD and TR coals is closely associated with the content of Cl, Si and Al in the coal.

To install multi-stage anti-wear beams on waterwall is an effective method to protect against the erosion in a CFB boiler. However, it changes the gas–solid flow and heat transfer on the waterwall. For a clear understanding, this paper used a numerical method to analyze the effect of anti-wear beams on the wall heat transfer in a 330 MW CFB boiler. The gas–solid hydrodynamics in furnace was simulated based on the TFM and EMMS drag model, and the heat transfer characteristic was obtained by cluster renewal model. The results showed that the anti-wear beams would make the descending solid flow to be multi-staged with lower speed on waterwall. Not only the heat transfer area but also the heat transfer coefficient was decreased. Thus, the furnace heat absorption was reduced by about 5.5%, and the average temperature would rise by up to 30 °C after installing the beams. Moreover, the operation parameters have an effect on the wall heat transfer at the upper furnace, while minor effect on the middle and lower part. It can be explained by the protection of two adjacent anti-wear beams, which provide a relatively stable circumstance for solid descending flow and heat transfer.

The ash-related problems of fouling and slagging occur easily during high-alkali coal combustion in boilers. In this study, numerical calculations were performed to investigate the sodium migration mechanism and ash deposition on heating surfaces in a large-scale circulating fluidized bed (CFB) boiler during the burning of high-alkali coal. The calculation principle was based on a comprehensive three-dimensional CFB combustion model and a sodium migration model from our previous study. The computing object of this study was a 300 MW subcritical CFB boiler. By performing a simulation, the distributions of NaCl(g) and Na2SO4(g) along the furnace were displayed. The rates of sodium deposition via condensation, inertial impaction, and thermophoresis on the water walls and suspended panels were analyzed. Additionally, the net deposition rates of fly ash on the water walls and suspended panels were determined. Then, the tendency of fouling and slagging in different areas of the CFB furnace could be predicted and judged. Calculation results for the gas–solid flow, furnace temperature, and gaseous species agreed with the measured data.

Investigation of the release behaviors of alkali and alkaline earth metals is necessary to understand the mechanisms of ash-related problems such as fouling, slagging, and corrosion when burning fuel with high alkali contents. Using a 2-D circulating fluidized bed (CFB) combustion model combined with a sodium migration model, sodium transformation was predicted in a 30 kW CFB combustor. Simulation results from the 2-D model are in agreement with those from the 3-D model and experiments. Calculations and analysis of sodium transformation with different parameters, including furnace temperature, excess air, and secondary air ratio, were performed. The results show that a higher furnace temperature leads to a higher number of deposited sodium and ash particles on the probe surface. Increasing the amount of excess air or decreasing the secondary air ratio may reduce the amounts of deposited sodium and ash particles.