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Ammonia is expected as a potential fuel to substitute fossil fuels, because it does not discharge carbon dioxide and is easily handled by liquefaction. There are several ways for the direct use of ammonia as a fuel; for example, use in fuel cells and combustion devices. One of the possible application is the combustion use in thermal power plants. In particular, co-firing of ammonia in coal-fired power plants seems to have a relatively great advantage on the suppression of greenhouse gases, because coal is one of the main emission source of carbon dioxide. On the other hand, it is concerned that concentration of nitrogen oxides (NOx), which is one of the typical atmospheric pollutant, in the flue gas would considerably increase due to the oxidation of ammonia. To utilize ammonia as a co-firing fuel in existing pulverized coal-fired power plant, without causing additional costs for the modification of the denitration equipment, it is important to develop a combustion technology that can suppress the NOx concentration in the flue gas. Co-firing characteristics of pulverized coal and ammonia, however, had not been evaluated except in the case of very low co-firing rate for the purpose of denitration in the pulverized coal flame. In this study, basic co-firing characteristics of pulverized coal and ammonia were investigated using a bench-scale single burner test furnace.
A schematic diagram of the single burner test furnace is shown in Fig. 1. The rated thermal input of the furnace is 760 kW, which corresponds to approximately 100 kg/h of coal for common bituminous coals. The total air ratio (the inverse of the equivalence ratio) is 1.24, in which the oxygen concentration at the furnace exit is 4%. In addition, two-stage combustion is employed to reduce NOx emission by creating a reducing environment, and the staged air was fed from the ports located at a distance of 3.0 m from the burner. The rate of staged air is 30% of the total air flow. Ammonia was fed into the furnace in two different ways; one is the injection into the center of the pulverized coal burner, and the other is separate injection through the side port of the furnace wall.
First, we examined the effects of ammonia co-firing rate and injection method on NOx concentration in the flue gas. Fig. 2 shows the effect of ammonia co-firing rate on NOx concentration in the flue gas in the case of ammonia injection into the burner. There was no significant change in NOx concentration in the case of 5 and 10% co-firing rate compared to the case of single coal combustion. However, as increasing co-firing rate to 15 and 20%, NOx concentration also increased, and the concentration at co-firing rate of 20% was approximately 20% higher than that at single coal combustion. This indicates that fuel NOx was produced in the flame by the oxidation of ammonia, or combustion of pulverized coal was aggravated, and then it resulted in the increase of NOx emission. On the other hand, it was found that NOx concentration in the flue gas at 20% ammonia co-firing depended on the position of ammonia injection into the furnace. Fig. 3 shows the changes of NOx concentration against different locations of ammonia injection, where ammonia co-firing rate is constant at 20%. When ammonia was injected through the side port which is located at 1.0 m apart from the burner, NOx concentration decreased compared to the case of ammonia injection into the burner, and was almost equal to that for single coal combustion. When ammonia was injected through the port located at 1.4, 1.8 or 2.2 m apart from the burner, however, NOx concentration became higher as the injection position was shifted to downstream, and the NOx concentration exceeded that for single coal combustion. The reason that the NOx concentration did not increase when ammonia was injected through the port at 1.0 m from the burner is expected the denitration effect of ammonia on NOx generated in pulverized coal flame. Therefore, by injecting ammonia into the region of low-O2 and high-NOx concentration, it is expected that not only the generation of fuel-NOx is effectively suppressed, but also ammonia can work as a denitration agent against the existing NOx in the flame.
The influence of ammonia co-firing on unburned carbon concentration in the fly ash and concentration of ammonia and nitrous oxide (N2O) in the flue gas, where N2O is one of the strongest greenhouse gases, was also evaluated. When ammonia was injected into the center of the burner with the co-firing rate of 20%, unburned carbon concentration in the fly ash increased by approximately 20%. In this case, flame temperature near the burner was lower compared to the case of single coal combustion, and it presumably affected the increase of unburned carbon concentration in the fly ash. Concentration of unburned ammonia and N2O in the flue gas was also a little higher in this case, but these increases were less than a few ppm and 1 ppm, respectively. Since ammonia is also used in the denitration equipment with selective catalytic reduction process, increase of unburned ammonia by a few ppm would not cause significant problem on the actual coal-fired power plants. Also, increase of N2O by a few ppm is almost negligibly low to work as a greenhouse gas, because emission of carbon dioxide is decreased by almost 20% in the 20% ammonia co-firing condition. In the case ammonia was injected through the side port at 0.6 or 1.0 m from the burner, unburned carbon in the fly ash, unburned ammonia and N2O in the flue gas did not increase compared to the case of single coal combustion; therefore it is considered that the significant problem would not occur, either in this case. As a result, we could have demonstrated the possibility of ammonia co-firing up to 20% in the coal-fired thermal power plants.
This research was supported by a program of Energy Carrier in Strategic Innovation Promotion Program (SIP), led by Japan Science and Technology Agency (JST). We appreciate the support from all concerned.