Effect of Fe on NO release during char combustion in air and O₂/CO₂

Abstract

The chemistry of char was probed by studying nitrogen release under the reactions with air and oxy-fuel combustion. The experiments were conducted in a drop tube furnace and a fixed bed flow reactor. NO was observed during those experiments. The results show that the particle size of char generated at 1073 K in CO₂ is larger than that in N₂. However, at 1573 K, it is smaller in CO₂ atmosphere due to particle breaking by gasification of char and CO₂. The Fe addition increases the NO conversion ratio, and the effect of Fe rises steeply with the process going until it becomes stable in the end. The results also indicate that the release of NO increases more significantly with the Fe addition in oxy-fuel environment.

Keywords

NO / Fe / char / combustion / CO₂

Introduction

Coal is used all over the world as it is one of the abundant resources and is relatively inexpensive. However, the emission of carbon dioxide (CO₂), a cause of global warming, remains a serious problem which attracts lots of attention in the power industry. Therefore, technologies concerning capture and subsequent storage of CO₂ were gradually developed [1].

The concentration of CO₂ in flue gas in the conventional air combustion is approximately 16%, and the capture and storage of it is costly. After air separation and flue gas recirculation in oxy-fuel process [2,3], the concentration of CO₂ would increase to more than 95%, which makes it easier to be captured. The oxy-fuel combustion [4] is a recently developed promising technology to capture CO₂ from power plants. Researches on oxy-fuel combustion process have been conducted these years regarding heat and mass transfer effects, devolatilization and ignition, gaseous pollutants emissions like NO_x, SO_x and trace elements as well as ash and deposition chemistry [5-13]. Because recycled flue gas and separated O₂ have replaced air during combustion, the nitrogen chemistry is greatly influenced [14-16].

Three NO_x formation mechanisms are well accepted [3,4,16,17], which are thermal NO, prompt NO and fuel NO. Thermal NO is formed during the reaction of N₂ and O₂ at a high temperature (higher than 1500°C ). The amount of prompt NO is the smallest of all the NOs, and it is formed when hydrocarbon radicals in fuel-rich zones attack molecular nitrogen to form cyanide species, which subsequently form NO when oxidized. Fuel NO is derived from nitrogen in the fuel. Because there is almost no N₂ existed, fuel NO is the main formation of NO [17,18] under oxy-fuel combustion, which is closely related to the amount of nitrogen in coal, chemical forms of nitrogen, the distribution of minerals and many other factors [19-21], and is also greatly influenced by high concentration of CO₂.

The general conclusion is that the amount of NO_x from oxy-fuel combustion could be reduced to approximately one third or a half of that in air combustion [2,5,7,8,22] which are published in recent years. But for the adequate flame temperature, the oxygen concentration is approximately 27%-30%, which is higher than that in air combustion. And the high concentration of oxygen could cause an enhancement of fuel-NO_x formation. The mechanism for the reduction of NO_x during oxy-fuel combustion is studied in recent years. The fields include the effect of the increased NO concentrations, effect of the low N₂ concentration [23], effect of the increased CO₂ concentration [16,24-27], and the effect of changes to the flame and fuel/oxidizer mixing pattern. The effects of changes in the oxygen concentration, oxy excess flue gas recycling ratio and gas phase temperature on NO emission in oxy-fuel combustion have also been reported for different fuels [5,7,14,15,17,19,22]. Their overall conclusion is that faster destruction in oxy-fuel combustion is partly caused by the higher CO and NO concentration. And most laboratory-scale experiments on oxy-fuel combustion have been performed in once-through reactors, where the recirculated flue gas is simulated by CO₂, and there are significant differences between the results of once-through experiments and those with recycled flue gas. Effect of oxygen purity and air penetration are also studied. The data indicate that increased N₂ content may give rise to increased NO_x emission due to thermal NO formation. The effect of coal properties on the formation and reduction of NO_x in oxy-fuel combustion has been investigated by several researchers [17-22]. These investigations indicate that char with increasing reactivity in oxy-fuel combustion is caused by a greater sensitivity to char combustion temperature of NO_x formation mechanism.

The release of NO_x declines in oxy-fuel combustion due to the reduction of NO from flue gas recirculation. There are three types of reaction on NO reduction [4,16,17]. First, NO_x is reclined by reaction with hydrocarbon radicals formed from the released volatiles in the early flame zone [4]. Next, NO_x is converted to N₂ through reactions with other reactive nitrogen species, such as cyanides and amines [4]. Finally, NO_x is converted to N₂ by heterogeneous reactions on char [4]. In oxy-fuel combustion, the high concentration of CO₂ will have a great effect on the reduction of NO in the three types of reaction.

Park et al. [27] have performed experiments with char to investigate the nitrogen release during reaction with O₂, CO₂ and steam where the carried gas is He. The results show that the primary nitrogen products are NO and N₂ during the reaction of char and O₂. The release of N₂ rises much more quickly and greatly than that of NO when char reacts with CO₂. Zhao et al. [28] have studied the effect of minerals on NO release in char combustion. They find that the release of NO increases after demineralation of coal. The release of NO is declined much more dramatically by Fe under the help of CO.

Char structure also changes greatly due to the existence of Fe. Gong et al. [29] have conducted experiments with demineralised char and char with Fe₂O₃ addition. They have studied char with and without Fe₂O₃ addition by the measurements of SEM, FTIR, XRD and Raman. The results show that catalytic pyrolysis improves the formation of free radicals, and that Fe₂O₃ changes the structure of anthracite char and improves combustion reactivity.

Fe addition not only changes the nitrogen distribution, but also has a great effect on morphology of itself. Xu et al. [30] have invested the effect of Fe on nitrogen release in brown coal pyrolysis. The results show that Fe₂O₃ and Fe₃O₄ are found in chars under low temperature, however, under high temperature, α-Fe, Fe₃C and Fe₅C₂ are the major species.

The concentration of CO₂ could reach more than 95 percent after flue gas recirculation in oxy-fuel combustion. The high amount of CO₂ and its gas specialty has a great effect on Fe catalysis with char. In oxy-fuel combustion, CO₂ has a significant effect on nitrogen distribution in char combustion.

In this study, the chemistry of char N is further probed by studying its release and reaction in oxy-fuel combustion with Fe addition. Experiments are performed in a drop tube furnace and a fixed bed flow reactor. There results are also relevant with coal gasification under high concentration of CO₂, while CO₂ plays a significant role in nitrogen distribution.

Experimental

Char sample preparation

The experiments reported in this paper were performed with char prepared from a Chinese bituminous coal. The raw coal samples were pulverized in the laboratory to pass through a sieve of 200 μm. The properties of the pulverized coal are presented in Table 1. Additionally, XRF was used to analyze the amount of Fe in the coal.

The pulverized coal samples were burnt in a drop tube furnace (DTF). The description of DTF was detailed in Ref. [31]. The reactor is 2 m in length with an inner diameter of 56 cm. The furnace tube was electrically heated by three independently controlled furnace sections. The temperatures of the tube wall at the three sections were measured with thermocouples and were monitored at the same temperature during the experiment. The pulverized coal was introduced into the tube at the top of the reactor by a micro-feeder at a feeding rate of approximately 0.8 g/min. Before the experiments, the coal sample were dried at 45°C for several hours to ensure a continuous and stable feeding rate. At the bottom of the reactor, the reaction products were drawn out through a water-cooled high purity Ar quenched sampling probe. Under the probe, a glass fiber filter with a pore size of 0.3 μm was used to collect the char.

During the experiments, the furnace temperatures were set at 1073 , 1273 and 1573 K, respectively. The pyrolysis gas was Ar or CO₂. The chars which were generated from Ar or CO₂, were denoted as char-Ar or char-CO₂ respectively.

Analysis of char

Tables 2–4 present the analysis of char which were generated from Ar and CO₂ at three different temperatures, including the BET surface area, pore volume and particle size respectively.

From Table 2, it can be observed that the BET surface area of char-CO₂ is bigger than that of char-Ar at the temperatures of both 1073 K and 1573 K. At 1073 K, the BET surface area of char-Ar is slightly smaller than that of char-CO₂ which is 9.1 and 11.0 m²/g respectively. However, it drops dramatically at 1573 K which is standing at approximately 2.7 m²/g. The results indicate that at 1073 K, CO₂ only has a slight effect on the BET surface area of char during pyrolysis, but at 1573 K the BET surface area of char-CO_2, it increases due to gasfication of char and CO₂, which changes chars into smaller particles and increases the surface areas.

From Table 3, it can be noticed that the pore volume of char-Ar and char-CO₂ declines gradually with the temperature increasing. At 1073 and 1573 K, the pore volume of char-Ar is smaller than that of char-CO₂. The pore volumes of char-Ar and char-CO₂ are quite similar at 1073 K, which are both approximately 0.02 cm³/g. However, the pore volumn of char-Ar decreases sharply at 1573 K, which is standing at 0.0059 cm³/g. The results show that residual char-CO₂ has more pore volume due to the gasification of char and CO₂. CO₂ has little effect on char at the temperature of 1073 K. However, the effect increases rapidly at 1573 K where the pore volume of char-Ar reaching the bottom at 0.005968 cm³/g.

Figure 1 displays the size of char at difference temperatures. From Fig. 1, it can be seen that chars expand dramatically in higher temperatures, especially at the temperature of 1573 K. At 1573 and 1273 K, chars expand more sharply in Ar than that in CO₂. However, at the temperature of 1073 K, when there was little gasfication between char and CO₂, the expandability of chars is bigger in CO₂ than that in Ar. The results indicate that when there is little gasfication between char and CO₂, the lower coefficient of heat conductivity and higher specific heat of CO₂ cause different expandability of chars in different pyrolysis atmospheres. But there is gasfication between chars and CO₂ at the temperature of 1573 K, which changes chars into smaller particles and leads to smaller size distribution in the whole furnace.

As can be seen in Table 4, the amount of nitrogen in chars at different temperatures is similar at the temperature of 1073 and 1273 K. However, the amount of nitrogen declines in chars at 1573 K in pyrolysis atmosphere of CO₂. The results indicate that at the temperature of 1573 K, gasfication of CO₂ and char has caused nitrogen in the char to convert into other nitrous gases.

Reaction and analysis system

The reactor was a horizontal tube furnace, which was used for char combustion at a high temperature of 1273 K. The experiments were performed in air and oxy-fuel environments. 0.25 g char sample was spread in an alumina holder, which will be placed in the centre of the reactor after the reactor was heated to 1273 K and filled with air or O₂/CO₂ with a total gas volume of 1.5 L/min. The residence time was 10 minutes. NO was tested by KM950 from Kane.

Restults and discussion

Effect of atmosphere on NO release in char combustion

Figures 2 and 3 demonstrate NO release in char combustion in Ar and CO₂ at 1073 K. As can be seen from Fig. 2, there are two peaks in the curves of NO release. NO release from char combustion in air is quite similar to that in O₂/CO₂, where O₂ is 20%. However, when oxygen increases to 30% in CO₂, NO release rose more markedly and steeply than others and it only takes approximately 350 s for NO to release which is sooner than the other two.

As can be seen from Fig. 3, the release of NO is less in char-CO₂ combustion than that in char-Ar combustion. And it takes a longer time for char-CO₂ to combust in O₂/CO₂ where O₂ is 30%, standing at approximately 500 s, while it is approximately 350 s for that of char-Ar.

Figures 4 and 5 exhibit NO release in char combustion in Ar and CO₂ at 1573 K. As can be seen from Fig. 4, unlike the chars generated at 1073 K, the curves of NO release in char combustion where the chars generated at 1573 K in Ar, are quite similar. When oxygen increases to 30% in CO₂, it takes a shorter time for NO to release, which is standing approximately 350 s, but the peak of NO release does not appear at all from the one with 20% O₂ in CO₂.

As can be seen from Fig. 5, it can be observed that the produce of NO in char combustion is much less than that with the char which were generated at 1073 K. As in most cases, increased oxygen has shortened the NO release duration. And chars generated in CO₂ produce less NO than that of the chars generated in Ar at 1573 K. The results indicate that char-CO₂ which is generated at 1573 K has more BET surface area and pore volume, and produces less NO than that with char-Ar.

The gasification of char and CO₂ existing at 1573 K in drop tube furnace leads to a reduction of N in residuals. Figures 4 and 5 show the release of NO declines significantly in char combustion with the chars generated in 1573 K. And the effect of oxygen concentration on NO release falls rapidly.

When char is combusted in O₂/CO₂, the reaction of char and CO₂ increases the release of CO which is favorable for the reduction of NO. However, high oxygen concentration has a much stronger effect on the release of NO in char combustion.

Effect of Fe₂O₃ on NO conversion

The NO conversion was defined as follows, NO conversion=NO/char N (%). Figures 6 and 7 depict the effect of Fe on NO combustion in char combustion in Ar and CO₂ at 1073 K. From Fig. 6, it can be seen that with char-CO₂ generated at 1073 K, Fe addition increases NO conversion ratio in char combustion under oxy-fuel environments. When char-CO₂ is combusted with 20% O₂ in CO₂, NO conversion ratio begins to increase at approximately 100 s with Fe addition, and it rose steeply, reaching the peak at approximately 400 s. But with 30% O₂ in CO₂, Fe addition only works after 200 s, and the whole increase was smaller than that with 20% O₂. The results show that Fe addition has a much greater effect on NO release with 20% O₂ in CO₂.

From Fig. 7, it can be seen that Fe has less effect on NO conversion ratio with char-Ar, but the way in which Fe increases NO release is quite similar. Fe addition increases NO conversion ratio in char-Ar combustion with 20% O₂ in CO₂. And Fe also increases NO conversion ratio in char-Ar combustion with air.

Figures 8 and 9 illustrate the effect of Fe on NO combustion in char combustion in Ar and CO₂ at 1273 K. From Fig. 8, it can be noticed that NO release increases with the increase of oxygen concentration. And Fe addition also increases NO conversion ratio, which works at the very beginning. The effect of Fe rises steeply with the process going, and it remains stable till the end. Fe has a greater effect with 30% O₂ in CO₂.

From Fig. 9, it can be seen that Fe has a much greater effect on char-Ar from 1273 K than that from 1073 K. Fe increases the most NO release with 30% O₂ in CO₂, and the least with 20% O₂ in CO₂.

NO conversion ratio rises most rapidly with Fe addition with 30% O₂ in CO₂ of the three. The results indicate that the release of NO rises more significantly with Fe addition in oxy-fuel environment.

Conclusions

The following conclusions were drawn from the present work:

1) Liuzhi coal is expanded after pyrolysis. At higher temperature, the particle size is larger after coal pyrolysis. At 1073 K, the particle size of char in CO₂ is larger than that in Ar. However, at 1573 K, the size of char is smaller in CO₂ atmosphere. This is caused by gasification of char and CO₂ at 1573 K, which makes particles smaller after reaction. The nitrogen amounts are quite similar between char-CO₂ and char-Ar generated at 1073 and 1273 K, but it drops sharply from the chars generated at 1573 K, because of the gasification of CO₂ and char which introduces char-nitrogen into nitrous gases.

2) From chars generated at 1073 K, the NO produced by char-Ar is much more than that produced by char-CO₂. The amount of oxygen has a significant effect on NO release. The amount of NO released in char combustion with 30% O₂ in CO₂ rises markedly and steeply than others. However, NO release declines significantly in char combustion with the chars generated in 1573 K. Besides, the effect of oxygen concentration on NO release falls rapidly.

3) With the chars generated from drop tube furnace in Ar at 1273 K, NO release increases with the increase in oxygen concentration in char combustion. Moreover, Fe addition also increases NO conversion ratio, which works at the very beginning. The effect of Fe rises steeply with the process going, and it remains stable till the end. Fe has a greater effect with 30% O₂ in CO₂. The effect of Fe on NO release in char-CO₂ combustion has a similar tendency. The results show that the release of NO rises more significantly with Fe addition in oxy-fuel environment.

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