Experimental Investigation of NOx Reduction by Urea Solution Injection in Fuel-rich Zone of Coal Combustion

Experiments were carried out in an electrically heated vertical tandem-type drop-tube furnace system to evaluate the influence of stoichiometric ratio (SR), normalized stoichiometric ratio (NSR) and temperature (T) on NOX reduction by urea solution in coal combustion. The results indicated that, SR was the deciding factor for NOX reduction by ammonia under high temperatures. The reduction efficiency was inproved with the increase of NSR. There was a significant reduction in NOX emission with increase in temperature under reducing conditions. However, ammonia had the tendency to get oxidized to NO, in presence of excess O2 in the combustion zone at high temperature.


Introduction
NOx emitted from utility boilers makes contributions to environmental pollution problems. Many countries have lowered their NO X emission limits in flue gases. For example, in the European Union the permissible NO X emission has been reduced to below 200 mg of NO 2 /m 3 (6% O 2 ), after 2016 [1]. In China, by 2014, the permissible NO X emission was controlled to less than 100 mg/Nm 3 at 6%O 2 for all power plants [2]. There are numerous well-established methods, such as air staging, staged fuel combustion, selective non-catalytic reduction (SNCR), and selective catalytic reduction (SCR). SNCR is a conceptually simple process for reducing NO X emissions and has therefore been the most widely adopted method in coal-fired power plants [3][4]. However, in SNCR technology, a serious limitation of the SNCR for NO X reduction is the requirement of a narrow temperature window [5], developing new NOx control technologies is quite necessary, especially the technology of removing NOx in coal combustion process. NH 3 is formed during staged combustion of pulverized coal [6], which indicates itself that combustion modification could be an effective strategy for NOx abatement. Hence, Adding NH 3 reagent( such as ammonia, urea solution) into the fuel rich zone may be to furher reduce NO in staged combustion, which have received little attention in the present literature.Wang [7] reported the NO X reduction by urea solution,which showed that under hypoxic conditions(O 2 %=0), as the temperature increased, the NO X reduction efficiency increased rapidly. The maximum value of the optimal temperature was higher than in excess of O 2 (1~8%). In other words, NH 3 could react with NO X in fuel-rich conditions, even at the higher temperatures.  also reported this phenomenon in their recent experimental and modeling studies, which were carried out without the addition of any catalytic. According to Javed [10], the rate of NO destruction by ammonia would just be balanced by its formation by oxidation at the temperature of around 1230°C, in excess of O 2 . Therefore, it can be believed that the temperature balance is increased, if the oxygen concentration is  [11] conducted a study to investigate the index for modeling a NO X reaction mechanism of combustion of pulverized coal, under various burning conditions. Cremer et al [12][13][14][15] found that when ammonia reagent was introduced into the reduction region of the primary combustion zone in a cyclone-barrel-fired boiler, the NO X reduction efficiency improved significantly. Most of the researches focused on the features of NO X reduction at relatively lower temperatures, in excess of oxygen. However, there are only few reports, which explain the characteristics of NH 3 reaction with NO at high-temperatures, especially in a pulverized coal furnace.
In the present study, the characteristics of NH 3 reaction with NO in flue gas for typically high-volatile bituminous coal was investigated experimentally and concentrates in the evaluation of the NO X emission in reducing region from a novel vertical drop-tube furnace system.

Experamental Section
The experimental bench system is shown in Fig.1. The experimental setup consisted of an electric furnace reactor system, reducing agent injection device, pulverized coal feeder, flue gas analyzer, and other affiliated components. The reactor was an corundum tube which had an inner diameter of 50 mm and the length of 2100mm, and reaction zone is 1200 mm.. The pulverized coal was fed through a micro-screw pulverized coal feeder, and coal feed rate was0.2-0.5 kg/h for experiments. Pulverized coal and the air of supporting combustion was mixed in the nozzle which used a water-cooled probe to prevent pyrolysis of coal particles before injection.. Urea solution, with mass concentration of 5%, was injected into the reactor by a micro screw peristaltic pump. The coal used in the tests was high-volatile Shenhua bituminous coal from China. The characteristics of this coal are presented in Table 1. The coal sample was grounded and sieved to a size of 75-90 μm and air -dried at 101°C for 10 h. The concentrations of NO, O 2 , CO, and NO 2 were analyzed by a AFRISO-M60 flue gas analyzer, which maximum relative errors for the measured species were ±5% (>100ppm). The on-line data of NO concentration measured was in parts per million [ppm]. For the purpose of comparison, the NO X

Figure2. Influence of SR on NO X emission witout urea solution
Figure2 shows the influence of SR on NOX emission, when no urea solution was injected into the furnace. The results showes that the effect of the temperature on NOX generation exhibited different characteristics, depending upon the SR. The concentration of NOX emission decreased radically with SR at different temperatures. In an oxidizing atmosphere (SR>1.0), higher temperature favored the formation of a larger amount of NOX. However, under a reducing atmosphere (SR<1.0), the tendency was reversed. It could also be seen that at a higher temperature in the reducing zone, there was a sharper decline in the NOX emission as SR decreased.
In order to investigate the effect of SR on NO X reduction by urea solution, the experiments were carried out in the SR range of 0.65 to 1.2. Figure 3 shows the influence of the SR in the combustion zone at different NSR and temperatures.  As is shown, with decrease of SR in reaction zone, the atmosphere turned into a reducing atmosphere (SR<1.0). The concentration of NO X emission had a obvious decrease .The lower the SR is,The higher of the NO X reduction efficiency when the NSR and temperatures were constant. This was due to the fact that when SR of combustion zone decreased, the concentration of O radicals decreased and the concentration CO increased. A small number of O radicals could be prevented from getting oxidized to NO X . The regenerated OH radicals continued to promote the conversion of NH 3 to NH 2 , which reduce NO primarily The concentration of NO X emissions could be reduced to 50~100mg/m³in highly reduced atmosphere, with NSR=3 and SR=0.65. There are few studies that reported of SR on NO X reduction by ammonia at high temperature.

Influence of NSR on NOX Reduction with Injecting Urea Solution
In order to investigate the influence of NSR on NO X emission at high temperature, the experiments were carried out at different NSR values of 0, 1.0, 1.5, 2.0, 2.5, and 3.0.

Figure4.
Influence of NSR on NO X reduction by ammonia (t=1300°C.) Figure.4 illustrates the differences in NO X reduction efficiency with variations of NSR, in which baseline conditions for concentration of NO X emission corresponded to the different SR values when temperature is at 1300°C. . The efficiency of NO X reduction increased rapidly when NSR increased. However, when NSR>2, the rate of NO X reduction become slowed down. This was due to the fact that the concentrations of NO X and other reactive groups decreased as ammonia content was constantly increased. Thus, the reaction rate gradually slowed down. The effect of temperature on NO X reduction of efficiency while SR=0.65, 0.95 were shown in Figures5 and Figures6. It can be observedminimum NO X emission occurred at 1400°C, and the NO X emission value was lower at SR=0.65 than at SR=0.95. This could be attributed to the fact that the higher temperature in the reducing zone lowered the NO X emission with decrease in SR. Those results suggested when O 2 concentration was low in the fuel rich zone; temperature had great effect on the removal efficiency of NO X . When SR=0.95, O radicals were relatively high in number, which implied that reactions: NH 2 + O = NH + OH, NH + O = NO + H, HNO + OH = H 2 O +NO, which were the crucial steps that would decrease the reduction efficiency of NO X . At higher SR value, as shown in Figure 6(b), NOx concentration increased rapidly as temperature increased at different NSR.

Conclusion
An experimental investigation was conducted on a vertical drop-tube furnace system, and the conclusion is summarized as follows: SR had an obvious effect on the NO X emissions for pulverized coal combustion. The decrease of SR led to a highly reducing atmosphere and increased the reduction efficiency of NO X when urea solution participated in the reaction, and vice versa. NO X reduction efficiency increased more with increased in NSR, under reducing atmosphere, when the NSR>2, the rate of NO X reduction increased is not obvious. Increase of temperature was crucial factor for promoting the NO X reduction efficiency in a reducing atmosphere, and reversed results were obtained in a oxidizing atmosphere.