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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.

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.

CFD modelling and experiments are carried out to study the influences of flow field on the measurement accuracy of ultrasonic water meters. The swirl generator and flow disturber are selected to change the flow field distributions inside the pipe. The calculation results show that the installation of left-handed swirl generator induces a specific flow pattern of the fluid inside the pipe, while the installation of flow disturber leads to the distortion of the axial velocity distributions. The critical distance of the fluid reaching the steady flow is 10D for left-handed swirl generator and flow disturber. In the experiment part, six different ultrasonic water meters are selected for the intrinsic errors tests. The results show that the installations of swirl generator and flow disturber induce the intrinsic errors shift to negative values. Compared with the flow disturber, the swirl generator has greater impact on the measurement accuracy.

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.