Processes | Free Full-Text | Numerical Simulation of Hydrogen–Coal Blending Combustion in a 660 MW Tangential Boiler

Figure 8 and Figure 9 demonstrate the H₂ distribution in the furnace and the average H₂ concentration at different heights. From Figure 8a, it can be seen that in the combustion zone, the combustion of pulverized coal generates a certain amount of H₂, which accumulates at the hopper. The center of the furnace also has a high concentration of H₂ due to the influence of the updraft. As the height rises, H₂ is basically burned out. The H₂ concentration has a more significant increase with the increase in the hydrogen blending ratio. It can be seen from Figure 8b and Figure 11a that the combustion of pulverized coal consumes the O₂ in the nearby zone. Due to the insufficient O₂ concentration, H₂ cannot be burned. A low concentration of H₂ exists after the combustion of pulverized coal. With the entry of hydrogen from the side, there is also a certain concentration of H₂ at the wall in Cases 2–4.

The variation in H₂ concentration along the furnace height is shown in Figure 9. In Cases 1–4, the H₂ concentration gradually increases with the increase in the hydrogen blending ratio. In conjunction with Figure 12a, the O₂ concentration at the bottom is low and insufficient to support hydrogen combustion. As the height increased, the air was replenished from the burners, and the H₂ concentration began to decrease. At the B-layer burners (at about 21.3 m), the addition of hydrogen gives a sudden increase in the H₂ concentration. In Case 4, the H₂ concentration reached a maximum of about 3.91%. In Cases 1–3, as the furnace height increased, the H₂ concentration fluctuated slightly due to the combustion. And the concentration shows an overall decreasing trend. In Case 4, the concentration of H₂ increases from 22.6 to 26.4m. The O₂ concentration is relatively low between the two groups of burners. The combustion of hydrogen produces a certain amount of H₂O. The reaction of H₂O with char produces a certain amount of H₂. From the upper burners to the top of the furnace, the large amount of O₂ replenishment makes the H₂ concentration decrease continuously. In the burnout zone, the H₂ concentrations in Cases 1–4 are all maintained at a stable value. The H₂ concentration in Case 4 is maintained at about 0.25% compared to the H₂ concentration in Case 1, which tended to be zero.

Figure 10a and Figure 11a show the O₂ distribution at the longitudinal and cross sections of the boiler for different hydrogen blending ratios, respectively. In the hopper and combustion zone, the combustion of pulverized coal and hydrogen consumes a large amount of O₂. In the combustion zone, the supplemental air gives a higher O₂ concentration at the burner outlet, as shown in Figure 11a. In the SOFA zone, the supplemental air brought a higher concentration of O₂. The amount of SOFA does not change, but the flue gas velocity is reduced. This affects the adequate combustion of pulverized coal, which leads to a lower amount of consumed O₂. From Cases 1 to 4, the O₂ concentration at the top gradually increased.

The specific O₂ concentration variation is shown in Figure 12a. From Case 1 to Case 4, the gradually decreasing total air volume attenuated the mixing between air and coal. Meanwhile, increasing amounts of H₂O and char are reacted with each other. All these affected the adequate combustion of pulverized coal. In Cases 1 to 3, the O₂ at the hopper is completely consumed. In Case 4, the reduction in the amount of pulverized coal in the B-layer burners resulted in a certain amount of O₂ concentration. The concentration is maintained near 0.5%. In the combustion zone, there is no obvious difference in the trend of O₂ concentration. A large amount of air is brought in by the auxiliary air in the BC, DE, and EF layers. The O₂ concentration increases significantly at the auxiliary air nozzles at 22.6 m, 29.9 m, and 32.6 m. The combustion basically ends after the SOFA is replenished. The O₂ concentration is basically stabilized in the upper part of the furnace. The increase in the hydrogen blending ratio affects the combustion of pulverized coal. More O₂ is not consumed. Compared with the O₂ concentration of 3.46% in Case 1, the O₂ concentration in Case 4 reaches 4.17%.

As can be seen in Figure 10b, CO (carbon monoxide) is mainly concentrated in the lower part. The O₂ concentration in this zone is insufficient for the pulverized coal to burn out, producing more CO. In the upper part, a sufficient amount of O₂ maintains the CO concentration at a very low level. From Figure 11b, it can be seen that there is a high concentration of CO at the walls in Case 4. In conjunction with Figure 11d, the combustion of hydrogen produces a large amount of H₂O, which in turn reacts with the char to produce CO.

The variations in CO concentration are shown more clearly in Figure 12b. From Case 1 to Case 3, the concentration of CO decreases with increasing hydrogen blending ratio from the hopper to the top of the combustion zone. From Figure 12d, the higher the hydrogen blending ratio, the higher the concentration of H₂O. The CO will react with H₂O, and the CO is consumed. In Case 4, which has the highest hydrogen blending ratio, the combustion of H₂ brings a large amount of H₂O. It greatly promotes the reaction between H₂O and char, and a large amount of CO is produced. In the burnout zone, the CO concentration gradually increases with the increase in the hydrogen blending ratio. Although there is a high O₂ concentration in the SOFA zone, the air–coal mixing is weak, and H₂O also affects the combustion. Eventually, the CO concentration is maintained around 0.12%, 0.33%, and 0.51% for Cases 2 to 4, respectively, compared to Case 1, where the CO concentration tends to zero.

The variations in CO₂ concentration at the longitudinal and transverse sections of the furnace can be seen in Figure 10c and Figure 11c. When the hydrogen blending ratio is gradually increased, the concentration of CO₂ decreases significantly. In the combustion zone, a large amount of CO₂ produced by pulverized coal combustion is affected by the rotating airflow. The CO₂ spreads to the walls of the boiler, and the concentration of CO₂ is higher near the walls. In the burnout zone, the large amount of O₂ brought by the SOFA supports the combustion of unburned pulverized coal. The CO₂ concentration increases. A certain amount of CO₂ can be observed at the inlet of the hydrogen burners in Cases 3 and 4, as shown in Figure 11c. The hydrogen burner is close to the pulverized coal burner. The amount of pulverized coal in the B-layer burners decreases after the increase in the blending ratio. The amount of primary air does not change. After the reaction between H₂O and char to form CO, the small amount of O₂ brought by the primary air in the B-layer allows CO to continue to be burned.

Figure 12c shows the average concentration of CO₂ at different furnace heights. At the bottom of the furnace, the reaction of CO with H₂O gives higher CO₂ concentrations in Cases 2 and 3 than in the non-hydrogen blended Case 1. From Figure 12b,d, the combined CO and H₂O concentrations in Cases 2 and 3 are closer to those at the hopper. This also puts the CO₂ concentrations close to each other in both cases, with both concentrations near 15.4% at the bottom. In Case 4, the CO₂ concentration has dropped significantly, with only 12.4%. In the combustion zone, there is a significant reduction in CO₂ concentration at the 21.3 m B-layer burners due to the influence of hydrogen blending. In Cases 1 to 3, the CO₂ concentration fluctuates considerably under the influence of combustion. In Case 4, the temperature distribution is more uniform (see Figure 7), and the fluctuation of CO₂ concentration is small. In the burnout zone, the CO₂ concentration gradually stabilized with the end of combustion. From Case 1 to Case 4, the CO₂ concentration stabilized at 15.6%, 15.4%, 14.5%, and 13.6%, respectively.

From Figure 10d, it can be seen that the distribution of H₂O is more uniform when hydrogen is not blended. After blending hydrogen, the H₂O concentration in the lower part has a significant increase. At the B-layer of burners, where hydrogen is injected, there is a large amount of H₂O production at the wall, as shown in Figure 11d. In Figure 12d, the average concentration of H₂O at different heights is shown. The H₂O produced from combustion is accumulated at the hopper by gravity. From Case 2 to Case 4, hydrogen blending increases the H₂O concentration with the increase in the hydrogen blending ratio. The H₂O concentration in Case 4 reached a maximum of 11.4%. Eventually, the H₂O concentrations in Cases 1 to 4 basically ceased to change at the top. The H₂O concentrations floated around 4.24%, 4.45%, 4.92%, and 5.73%, respectively.

The average concentrations of CO+CO₂ are displayed in Figure 13. From Case 1 to Case 4, the variation in CO+CO₂ concentrations is opposite to H₂O. The concentrations of CO + CO₂ gradually decrease as the proportion of blended hydrogen increases. The combustion of H₂ brings more H₂O. The proportion of CO + CO₂ produced by the combustion of pulverized coal is reduced. It is clear that H₂ blending favors the reduction in CO + CO₂ concentration.