Influence of operating parameters on N₂O emission in O₂/CO₂ combustion with high oxygen concentration in circulating fluidized bed

Highlights

• Relationships between N₂O and operating parameters in oxy-CFBC are analyzed.

• Higher temperature and higher overall oxygen concentration lead to lower N₂O emission.

• Higher excess oxygen ratio causes the increase of both N₂O and NO emissions.

• Gas and oxygen staging has slightly negative effects on N₂O emission.

Abstract

Oxy-fuel circulating fluidized bed (CFB) combustion combining the advantages of oxy-fuel combustion and CFB technology is one of the most promising technologies for capturing CO₂. However, the emission of N₂O in oxy-fuel CFB combustion can be high enough to raise some related environmental issues. This study investigated N₂O emission in O₂/CO₂ CFB combustion with a high oxygen concentration based on the experimental results using a the 0.1 MW_th oxy-fuel CFB. This study focused on the operating parameters. It was found that increasing the combustion temperature and overall oxygen concentration can lead to lower N₂O emission, while increasing the excess oxygen ratio can lead to higher N₂O emission levels. Both gas staging and oxygen staging have slight adverse influences on controlling N₂O owing to the higher oxygen concentration in the secondary gas. Finally, the excess oxygen ratio in the primary zone was determined in order to assess the actual effects of gas and oxygen staging on both N₂O and NO emissions.

Introduction

As is well known, the consumption of fossil fuels, such as coals, increases significantly with the rapid development of the global economy, which results in surging emissions of pollutants. Among these pollutants, carbon dioxide (CO₂) is considered to be one of the most destructive. CO₂ can intensify greenhouse effects and contribute to global warming. Currently, many researchers and relevant academic institutions are engaged in studies on CO₂ capture and storage (CCS). To date, some of the main options of CCS for coal combustion and gasification have been scaled up to industrial levels successfully, including pre-combustion, post-combustion and oxy-fuel combustion technologies [1], [2].

Oxy-fuel combustion is conducted with oxygen diluted with recycled flue gas (RFG) that is free of N₂; as a result, the concentration of CO₂ in the flue gas can be adequate for purification, compression, and storage. There are several suitable combustion systems for oxy-fuel retrofitting, one of which is the circulating fluidized bed (CFB) system [3]. In oxy-fuel CFB combustion, oxygen can be mixed with RFG, and the mixture serves as the inlet gas instead of air. Because of the flexibility of the heat transfer surfaces arranged inside the combustor and additional heat recovery from the recycled solid materials outside the combustor, it is easier to increase the inlet oxygen concentration than through oxy-pulverized coal (oxy-PC) combustion. As a result, the ratio of RFG can be reduced as well as the size of the CFB boiler. However, one of the most prominent issues of CFB combustion is the high concentration of N₂O in flue gas [4]. Some researchers have stated that the emission of N₂O can range from 20 to 400 ppm in CFB combustion, much higher than that in PC combustion, which is merely 10 ppm [5].

N₂O plays an important role in the depletion of the ozone layer and aggravates greenhouse effects owing to its stronger greenhouse gas effect compared to CO₂. Thus far, many scholars have conducted numerous investigations on N₂O formation and destruction mechanisms in CFB combustion. The paths of conversion from fuel–nitrogen to N₂O can be reviewed following with the coal combustion process. Combustion of coal can be separated into two stages: the first is devolatilization and the second is char burning. During devolatilization, which occurs as soon as coal is fed into the combustor, tars and volatiles are released initially as well as fuel nitrogen in the forms of HCN, NH₃, tar-N and N₂ [6]. The second stage, char combustion which can last much longer, takes place following devolatilization. During the second stage, char-N conversion begins. In addition, the fuel nitrogen released in the first stage reacts with char-N. This means that N₂O can be formed through complex homogenous and heterogeneous reactions during both of these stages [7].

For N₂O homogeneous formation reactions, the most important source appearing to be oxidation in gas-phase is HCN produced mostly during coal pyrolysis, as followed [8]:

$$\text{HCN}+\text{O}\to \text{NCO}+\text{H}$$

$$\text{NCO}+\text{NO}\to {\text{N}}_2\text{O}+\text{CO}$$

Many researchers have verified this theory based on their respective investigations [9]. In addition, Ren has studied the effects of temperature on N₂O formation from NCO. N₂O could be generated from NCO only at lower temperatures, while the main product at high temperatures was NO [10].

Meanwhile, in addition to homogeneous formation, there still exist homogeneous destruction reactions of N₂O by radicals which can be expressed as follows [11]:

$${\text{N}}_2\text{O}+\text{H}\to {\text{N}}_2+\text{OH}$$

$${\text{N}}_2\text{O}+\text{OH}\to {\text{N}}_2+{\text{HO}}_2$$

Similarly, destruction of N₂O through thermal dissociation in collisions with any molecule M via the following reaction is also important under flame conditions or at high temperatures (>900 °C) [12]:

$${\text{N}}_2\text{O}+\text{M}\to {\text{N}}_2+\text{O}+\text{M}$$

Moreover because of the solid–gas interaction in CFB combustion, the heterogeneous reactions can occur together with the homogeneous reactions. Char nitrogen is the main source in the heterogeneous formation of N₂O. Many researchers are studying the mechanism of N₂O formation from char nitrogen. To date, three main theories have been established [13], [14], [15]. The first states that N₂O is the product of direct transformation of char-nitrogen through the following reactions [16]:

$$(-\text{CN})+{\text{O}}_2+(-\text{C})\to (-\text{CO})+(-\text{CNO})$$

$$(-\text{CNO})+(-\mathit{CN})\to {\text{N}}_2\text{O}+2(-\text{C})$$

However, some scholars consider that homogeneous reactions of HCN, which came from the char nitrogen, generate N₂O, similar to the transformation of volatile nitrogen [17]. From another point of view, other researchers have found that N₂O was formed through heterogeneous reactions of char nitrogen and established the following conversion paths [18], [19]:

$$(-\text{CN})+{\text{O}}_2+(-\text{C})\to (-\text{CO})+(-\text{CNO})$$

$$(-\text{CNO})\to \text{NO}+(-\text{C})$$

$$(-\text{CNO})+\text{NO}\to {\text{N}}_2\text{O}+(-\text{CO})$$

Likewise, along with the formation, N₂O can decompose heterogeneously in the meantime. Char can stimulate the reductions of N₂O with CO by itself, which can be described as follows [20]:

$${\text{N}}_2\text{O}+\text{CO}\stackrel{\text{chars}}{\to }{\text{CO}}_2{\text{N}}_2$$

Liu et al. [21] examined the effects of char on N₂O reduction in a bench-scale CFB combustor. By comparing the results of Knudsen numbers with microscopic characteristics, it was found that char with a greater pore volume and specific surface area had stronger N₂O reduction ability because of the molecular diffusion and Knudsen diffusion. Moreover, the conversion ratio of fuel-N into N₂O or NO_x increased with the nitrogen content of coal. Barišić et al. [22] reported that the catalytic activity of the bed material samples increased as the amount of the catalytically active oxides (CaO, MgO, Fe₂O₃, Al₂O₃) increased. Xiangsong et al. [23] investigated the roles of different metal oxides in the catalytic decomposition of N₂O with a fixed bed reactor. Experimental results showed that CaO and Fe₃O₄ were very active while Al₂O₃ contributed less to N₂O destruction. Wu et al. [24] studied the effects of in situ SO₂ removal on N₂O reduction in CFB. They stated that the removal of the surface-adsorbed O atom was the rate-determining step of the decomposition of N₂O. As a result of the diversity of the catalysts, the main N₂O reduction reaction is simplified as follows [25]:

$${\text{N}}_2\text{O}\stackrel{\text{catalysts}}{\to }1/2{\text{O}}_2+{\text{N}}_2$$

For oxy-fuel combustion research, although there are many reports on the nitrogen pollutants, most of them are about the emission of NO_x [26], [27]. Studies on the characteristics of N₂O emission in oxy-fuel CFB combustion thus far are rare, and are summarized in the following paragraph:

Wang et al. studied the conversion of pyridine-N to N₂ by using a flow reactor under an O₂/CO₂ atmosphere. They found that NO can also be consumed through reactions that involve NCO and NO in oxy-fuel combustion, as shown in R2, which means that it can also increase N₂O production [28]. de Diego et al. have conducted some experiments on pollutant emissions in a 3 kWth continuous bubbling fluidized bed combustor under oxy-firing conditions. The results implied that approximately 60–70% of the recycled NO was reduced to N₂ and N₂O [29]. Wang et al. designed some experiments to deal with N₂O reduction with CO in oxy-fuel combustion via synthetic coal samples. It was stated that as the temperature increased, an increase in N₂O reduction could be observed regardless of the presence of CO [30]. Yoshiie et al. estimated the N₂O emission using drop-tube furnace equipment. The results showed that the N₂O emission increased slightly under a CO₂–O₂ combustion atmosphere as a result of the higher concentration of CO compared to that in air combustion [31]. Recently, Loscertables [32] investigated the NO and N₂O emissions in oxy-fuel coal combustion in a bubbling fluidized bed combustor. This revealed that the influence of temperature on N₂O emission was the same that under conventional conditions.

However, because of the different styles and scales of equipment, the emission results of N₂O may be different from those of actual CFB combustion as a result of asymmetrical distribution of bed materials, which have great influences on the formation and destruction of N₂O. Meanwhile, according to a recent publication based on the experiments conducted using CIUDEN’s 30 MWth oxy-CFB [33], when petcoke was adopted as the fuel, N₂O could act comparable concentration (ppmv) as NO, especially at 850–900 °C, which happens to be the general operating temperature for CFB combustion. When anthracite was used, the emission of N₂O (ppmv) was even higher than the emission of NO at temperatures ranging from 840 °C to 880 °C. Similarly, Pikkarainen et al. [34] studied the emissions of pollutants during oxy-fuel combustion via their pilot-scale CFB. The results regarding N₂O were also less than positive. In tests of an anthracite/petcoke fuel blend at a 42% oxygen concentration, although N₂O decreased as the temperature increased, its emission was still higher than that of NO.

Based on the above results, for oxy-fuel CFB combustion, especially with a high oxygen concentration, N₂O should not be neglected, unlike in the situation of traditional air-firing. Once the oxy-fuel technology is commercialized, N₂O will be one of the most important pollutants on which attention must be focused. Since the emission of N₂O depends highly on the coupling effects of different operating conditions, investigations on the relationships between N₂O release and operating parameters are required. Furthermore, when a high oxygen concentration is adopted in oxy-fuel, the intensity of reaction and combustion will typically change. The traditional operating principle may not be suitable for the new technology. However, there are few reports about the effects of fundamental operation strategies on N₂O production in oxy-fuel combustion with a high oxygen concentration. In addition, most of the results reported about N₂O emissions were from lab-scale combustors, and may not fully represent for the full-scale CFB combustors. Therefore, there is a clear need to explore the actual-scale oxy-fuel CFB combustors, especially with high oxygen concentrations, to reveal the connection between N₂O emissions and operation parameters.

The aim of the present study was to provide detailed knowledge about the effects of operation parameters on N₂O emission during O₂/CO₂ CFB combustion processes with high oxygen concentrations using the 0.1 MW_th oxy-fuel CFB combustion systems.