Yuehua Liu, Lili Chen, Shoujun Liu*, Song Yang*, Ju Shangguan

1 State Key Laboratory of Clean and Efficient Coal Utilization, Taiyuan University of Technology, Taiyuan 030024, China

2 Shanxi Engineering Center of Civil Clean Fuel, Taiyuan University of Technology, Taiyuan 030024, China

3 College of Chemical Engineering and Technology, Taiyuan University of Technology, Taiyuan 030024, China

Keywords:

ABSTRACT

1. Introduction

China is the largest producer and consumer of coal in the world,it’s believed that coal will still play a dominant role in the energy consumption structure of China for decades [1,2]. According to incomplete statistics, the unregulated use of coal in the Beijing-Tianjin-Hebei region accounts for only 10% of the total coal consumption, but accounts for 50% of the primary pollutant emissions [3]. Coal combustion has been causing serious public concerns regarding pollution issues, especially the NOxemission problems. NOxhas been condemned for causing acid rain,ground-level ozone, and photochemical smog [4,5]. The main application of coal is combustion. The low cost of coal makes it especially popular despite its low combustion efficiency and intensive harmful pollutant emissions.In China,combustion coal is used every winter to meet people’s heating needs. Among them, about 200 million tons of coal are combustion each year for winter heating, accounting for 91% of total coal combustion. The pollutant emissions from coal combustion is an important cause of regional and city-level air pollution. Hence, it is imperative to find innovative ways to reduce the pollutant emissions from coal combustion.

The content of the nitrogen present in coal (coal-N) is typically in the range of 0.5%-2%(mass)(daf)[6,7]. NOxfrom coal combustion mainly comes from the reaction of nitrogen in coal and oxygen in air to produce fuel-type NOx[8].Among them,the production of fuel-type NOxexceeds 80%, and the proportion of fast-type and thermal-type NOxis relatively small [6,9].

Nitrogen oxide (NOx) is one of the main pollutants generated from coal combustion. One major technology to reduce NOxemissions from large scale coal-fired boilers is selective catalytic reduction(SCR)technology[10–12].Metal additives play important role in the catalytic reduction.Iron-based catalysts are commonly used for NOxremoval from flue gas due to their low price,high catalytic efficiency and easy availability. Researchers [13–15] pointed out that iron oxides can obviously promote the catalytic reduction of NOxand has high de-NOxactivity and selectivity of catalytic N2generation.And the effect of different forms of Fe2O3and the combination of iron and other additives on de-NOxperformance has been discussed [11,12,16,17]. Shu et al. [18] found that siderite was gradually oxidized and decomposed into alpha-Fe2O3with a nonporous structure and large surface area after calcination under air atmosphere, and the alpha-Fe2O3derived from siderite at 500 °C (H500) still favored the conversion of NO. The SCR of NOxat low temperatures was greatly improved by the Ce-doping of the iron oxide catalyst [15]. A series of bimetallic iron and manganese oxides were supported on the flyash-derived SBA-15 catalyst, and excellent NO conversion was found for SCR of NO by NH3at low temperatures. The denigration mechanism of the Fe-Mn/SBA-15 catalyst in the SCR reaction followed Langmuir-Hinshelwood, Eley-Rideal, and Mars—van Krevelen mechanisms[19].Iron also plays a role in the reduction and denigration of SNCR process [20,21]. Daood’s study [21] found evidence of an interaction between the Fe-based additive and SNCR. The interaction leads to greater ammonia utilization and an increased NOxreduction, and the interaction is theorized to be pseudo-catalytic with the fuel additive providing an active site for ammonia to reduce NO.The experimental results demonstrated that Fe2O3has an inhibitory effect on the SNCR de-NOxprocess that is more notable at low temperatures. The adsorption of NH3and its dissociation into-NH2on the surface of Fe2O3is the rate-controlling step in these processes that Fe2O3catalyzed NO reduction by NH3in the absence of O2and Fe2O3mainly catalyzed NH3oxidation in the presence of O2[22]. However, the above method is only suitable for large coal-fired boilers because of its high cost,complex desulfurization and de-NOxequipment,and cannot be installed on small coal-fired boilers. Therefore, a low-cost coal treatment for de-NOxhas been widely concerned.

In this paper,based on the present situation of coal and research findings on NOxreduction, a new strategy contained two steps toward the combustion of coal with remarkably reduced NOxemission is proposed. The first step is the production of coke with reduced nitrogen content by pyrolysis of coal loaded with Febased additives under high temperature conditions. The second step is the de-NOxprocess by a redox reaction between NOxand reducing agents (coke, HCN, NH3, etc.) during coke combustion.The effects of Fe-based additives and temperature on NOxemission during coke combustion were studied. Coke prepared by adding catalyst is not practical because of the need to add iron, and large-scale coal-fired boilers have flue gas desulfurization and de-NOxequipment. Therefore, the coke prepared by this method is only suitable for small-scale civil stoves.

2. Materials and Methods

2.1. Sample preparations

The coal was firstly mechanically crushed into a particle size of less than 0.2 mm,and then dried in a vacuum oven at the 105°C for 2 h. The prepared samples were cooled 24 h in the air for use.

The coke samples were prepared by pyrolysis of coal samples.The pyrolysis apparatus is shown in Fig. 1. The coal samples with or without the addition of Fe-based additives were heated to 800 °C in the furnace under argon atmosphere at a heating rate of 10 °C?min-1. Then the temperature was increased to 1050 °C at a heating rate of 5 °C?min-1and stayed for 1.5 h. After that,the coke samples were cooled to room temperature.

To confirm the effect of Fe on de-NOxperformance and eliminate the influence of inherent Fe-based minerals, demineralized coal (de-coal) was prepared by treating the coal samples with mixed acids(HCl+HF)in the water bath at 60°C for 4 h and then washed with deionized water until no Cl-was detected [26]. The dried de-coal samples were physically mixed with Fe-based additives. Then the mixture was dried at 105 °C and cooled for use.Coke, de-coke and special coke samples were prepared by respective pyrolysis of the coal,de-coal and de-coal loaded with Fe-based additives under the same pyrolysis condition as listed above. The coke prepared by pyrolysis of demineralized coal is defined as DC.The coke prepared by pyrolysis of demineralized coal with different proportions of iron-based catalysts is defined as FXDC (X refers to the addition amount of 1% (mass), 2% (mass), 3% (mass)and 4% (mass)). The proximate and ultimate analyses of the prepared samples were listed in Table 1.

2.2. Combustion experiments

The combustion apparatus (shown in Fig. 2) consist of a gas mixing section, a quartz fixed bed combustion reactor (I.D.10 mm) and an on-line gas analyzer. Combustion is conducted in the same quartz fixed bed reactor. Samples were heated to the specified temperature under argon atmosphere. Then the Ar was replaced with the O2/Ar (2:8) to start the combustion experiment.

2.3. Characterization and analysis

The composition of the exhaust gases (CO, O2, NO, NO2) was monitored by an on-line flue gas analyzer,KANE9506 and gas chromatography (GC7820). The crystal structure of samples was studied by X-ray diffraction (XRD) 7000S/L (40 kV and 30 mA). Data were collected between 5°and 85°in 2θ with a step of 4(°)?min-1.The surface chemical states of Fe in samples were characterized by X-ray photoelectron spectroscopic (XPS) measurements using a Thermo Scientific ESCALAB 250Xi spectrometer (15 kV, 10 mA)with Al Kα radiation (hυ = 1486.6 eV) as the photo source. The nitrogen adsorption and desorption of the prepared samples were measured using JW-BK222.Specific surface area and pore size distributions were calculated using both the BET method and the DFT method.

3. Results and Discussion

3.1.NOx emission during combustion of coal,special coke,coke and decoke

With the new strategy toward coal utilization, coke was obtained by pyrolysis of the coal with the addition of Fe-based additives. As shown in Fig. 3, the NOxemission behavior from the combustion of special coke was analyzed by comparing with that of the raw coal and the coke produced from pyrolyzed of raw coal respectively.

The results of NOxemission behavior of coal, special coke, coke and de-coke at the combustion temperature is 800°C are shown in Fig.3.It can be seen de-coke had the highest NOxemission concentrations, followed by the raw coal, coke and special coke.

Fig. 1. The schematic diagram of the experimental apparatus for coal pyrolysis.

Table 1 Proximate and ultimate analysis of samples

Fig. 2. The schematic diagram of the experimental apparatus for coke combustion.

Fig. 3. NOx emission of coal, special coke, coke and de-coke.

Compared with raw coal, the total amount of the NOxemission from the combustion of the special coke decreased to 46.1%,which means the new strategy can remarkably reduce the NOxpollutants and shows a better application prospect for mass production to reduce the air pollution than burning the coal. Because coke has gone through the pyrolysis stage,part of its nitrogen has migrated to the volatile, resulting in the nitrogen content of coke is lower than that of raw coal. This is also one reason why coke has lower NOxemissions than raw coal by direct combustion. While regardless of volatile nitrogenous components, the NOxemission of special coke still decreased compared with the coke generated from direct pyrolysis of the coal, which demonstrates that Fe-based additives promote the reduction of NOx. Therefore, the main reasons for lower NOxemissions following the two points: (1) when coal is pyrolyzed, a portion of the nitrogen in the coal migrates to volatile components in the form of NH3, HCN, N2and tar nitrogen,and a portion remains in the coke.As a result,nitrogen content in coke decreases[23];(2)during the coke combustion process,the catalyst catalyzes the conversion of some of the NOxto nonpolluting N2. Eventually, the total NOxemissions will go down.

The NOxemission behavior of the de-coke combustion was increased obviously in comparison to the coke,while there was little difference between the combustion of the coke and de-coke about Ndaf(shown in Table 1). This could be deduced to the effect of the Fe and Ca in the minerals of coal are helpful to reduce NOxemission under high temperature combustion[24].The analysis of ash (shown in Table 2) verifies the coal contains the amount of Fe2O3and CaO, which has a positive effect on the NOxemission during combustion.

3.2. Study on the effect of Fe-additives on NOx reduction

3.2.1. Study on the effect of iron compounds on NOxreduction

As mentioned earlier,to clarify the effect of Fe-additives on NOxreduction during combustion of the coal,the de-coal samples were prepared. The total NOxemissions and the NOxreduction ratio of the de-coke, de-coke with different iron compounds combustion at 800 °C were shown in Fig. 4.

The results revealed that loading iron compounds on de-coke could obviously reduce the NOxemission during combustion. In these experiments,the iron loading content was kept at 3%(mass).The NOxreduction rates of Fe(OH)3, Fe2O3and Fe3O4are 24%, 45%and 36%, respectively (the NOxreduction ratio is calculated as the ratio of NOxemissions from combustion of Fe-loaded compounds to direct combustion of coke). The catalytic effect of Fe2O3shows the highest de-NOxefficiency,followed by Fe3O4and Fe(OH)3.Mori et al. [25] studied the pyrolysis of coal loaded iron using aqueous solution of FeCl3, and the precipitated iron was reduced to Fe3C(cementite) and α-Fe under reducing gas atmosphere such as H2and CO released from coal during pyrolysis. Therefore, it can be concluded that active phase formation ability varies with different iron compounds, and the difference in anions also has a certain effect on the formation of intermediate active compounds. The metal cation of the transition metal compound is the active site of the catalyst, but usually changes the chemical environment of the active phase by regulating the type of anion [26,27]. The change of chemical environment around anions will affect the formation of intermediate active compounds[28],resulting in the difference of catalytic effects.So,through the new strategy of physical mixing, Fe2O3shows the best among those Fe-based compounds.Fe2O3which shows the best NOxreduction ratio was selected as the Fe-based additive in the following experiments.

3.2.2. Study on the effect of Fe-additive dosages on NOxreduction

The total NOxemissions and reduction ratio of de-coke and decoke with different Fe2O3dosages were shown in Fig. 5(a) after combustion at 800 °C. It is can be seen that with the increase of Fe2O3dosage, NOxemission of samples during combustion has decreased gradually, except for the de-coke with 4% (mass) Fe2O3which reflects a contrary tendency. De-coke with 3% (mass)Fe2O3shows the best de-NOxperformance,which could be related to the enough active phase generated during the sample preparation. It is generally accepted Fe has a positive effect on NOxemission reduction [13,19,24]. Phase transformation in samples was observed by XRD analysis (shown in Fig. 5(b)). Fe and Fe2O3were not detected in de-coke for most of the minerals contained in coal were removed from by mixed acid treatment process. From Fig. 5(b), it’s clearly observed that the characteristic peaks of Fe and Fe2O3appeared obviously with the increase of Fe2O3dosage.

Fig.4. NOx emission and reduction ratio of de-coke and de-coke with different iron compounds.

However, Fe analyzed by XPS showed a different tendency. It can be seen from Table 3 that F3DC has the highest Fe amount.The low Fe yield of F4DC may be related to the excessive aggregation of Fe2O3on the surface. The low Fe yield of F2DC is because there is not enough Fe2O3loaded on the active site. With the increase of catalyst addition, the catalyst will cover the surface of coke to form a protective film. The protective film prevents the rapid escape of small molecule intermediates.Finally,the reaction time of small molecules with coke and catalyst is increased,which leads to the reduction of NOxprecursor emission and the increase of N2yield. When the addition amount is too much, the catalytic effect will be poor based on two aspects. Firstly, the catalyst will aggregate on the surface of coke, resulting in poor dispersion and uneven catalytic effect. Secondly, the excessive catalyst will block the micropores of coke particle sand occupy the reaction site,leading to the occurrence of side reactions. Therefore, the catalytic effect of the catalyst and the amount show parabolic relationship.Appropriate Fe-additive dosage is better for the NOxreduction.

The surface area,pore volume and pore diameter are important factors on multiphase reaction. Through the method of BET and DFT,the surface area, pore volume and pore diameter characteristics were analyzed and the results showed that little difference could be found on the specific surface area of the different samples.While from Fig. 6, the pore size occurs obviously in samples with the increase of Fe2O3dosages, which results from the redox reaction between Fe2O3and C, reflecting that expanded pores were beneficial to the heterogeneous reaction of NOxand C.

In short, the appropriate number of Fe-additives would benefit from the increase of the Fe yield on the surface of the coke samples,which could help the expanding of the pore size, which could further promote the NOxreduction reaction.

3.3. Effect of combustion temperature on NOx reduction

Generally, the temperature of coal combustion will not exceed 1200 °C [29,30]. Thus, in this paper, the temperature of 800–1100°C was selected practically to perform the combustion experiments of the de-coke and the de-coke with 3% (mass) Fe2O3.The total NOxemissions and reduction ratio of the two samples were shown in Fig. 7(a). With the increase of combustion temperature,the emission of NOxgradually decreases and the highest NOxreduction ratio was obtained under the combustion temperature of 900 °C.

Table 2 Ash composition analysis of scattered coal

Fig.5. (a)NOx emissions and reduction ratio of de-coke and de-coke with different Fe-additive dosages.(b)Phase transformation in de-coke loaded with different Fe-additive dosages.

Table 3 The surface (XPS) compositions of iron

Fig. 6. Pore volume and pore size distribution of de-coke loaded with different Feadditive dosages.

One of the main reasons for NOxemission reduction is that the chemical reaction rate increases greatly with the increase of the combustion temperature.In addition,NOxemission can be reduced by intermediate gases such as HCN and NH3[18,20,31].And in situ redox reaction between NOxand coke finally converts NOxto N2to reduce the overall NOxemission[12,32].The reaction between NOxand coke can be confirmed by the following experiments.

In the combustion apparatus, the de-coke was heated to the specified temperature under argon atmosphere. Then the argon was replaced by high purity standard NOxto start the experiment and outlet gases were analyzed by the flue gas analyzer. Results shown in Fig. 7(b) revealed that the in-situ reduction reaction between NOxand coke happened,and the reduction ratio was proportional to the combustion temperature.

3.4. Nitrogen balance during the combustion process

Based on the proximate and ultimate analyses of the different samples,the nitrogen balance(shown in Fig.8)of the whole experiment was calculated by coke yield and exhaust gas yield. The exhaust gas of combustion was collected by air bag. Normally,NOxgenerated by direct combustion of coal is mainly fuel type NOx[10,17]. In our study, coke was produced by pyrolysis of the coal with Fe-based additives. On the one hand, part of fuel-N is released in the form of volatile nitrogen(NH3and HCN)during catalytic pyrolysis of the coal,which leads to the decrease of nitrogen content in coke nitrogen.At the same time,under the action of the catalyst,the volatile nitrogen and part of the coke nitrogen will be converted to N2.This is because the morphology of the catalyst will change with the increase of pyrolysis temperature.The sequence of chemical morphology transformation of iron-based catalysts is as follows: highly dispersed Fe—O/C complex, ultrafine FeOOH, crystalline Fe3O4, crystalline phase reduced α-Fe [33,34]. The formed α-Fe will react with char-N, NH3and HCN to form FexN, as shown in Eq.(1)[35].When FexN is generated,it will decompose and generate environmentally friendly N2, as shown in Eq. (2) [9,36–38].On the other hand, part of the NOxemitted during the coke combustion process was converted into N2for the catalytic effect of the Fe-based catalyst [39,40].

Fig.7. (a)NOx emissions of de-coke with 3%(mass)Fe2O3 and NOx reduction ratio compared with de-coke under different combustion temperatures.(b)NOx reduction under different temperatures.

Fig. 8. Nitrogen balance in experiments.

Nitrogen content in the coal was selected as the reference,which decreases to 63.5% in the special coke. After combustion at 900 °C, nitrogen existed in the form of NOxand N2, which were analyzed quantitatively and the result showed that about 43% of nitrogen in the special coke converts to NOx. All in all, the NOxemission through the combustion of the special coke prepared by the new strategy decreased to 27.3% compared with the direct combustion of the coal.

3.5. Mechanism of NOx reduction catalyzed by Fe-based additives

Fig. 9 shows the mechanism of NOxreduction catalyzed by Febased additives in coke combustion. The reduction of NOxby iron is mainly carried out by CO and coke.

Fe catalyst promotes the reduction of NOxby C/CO (Fig. S2 and Table S1). In this catalytic reaction process, Fe is oxidized to Fe oxide and loses its catalytic effect.However,the C/CO in the system reduces the Fe oxide, which makes the inactivated catalyst work again. In this cycle, NOxgenerated in the combustion process is continuously catalyzed to reduce, and the catalyst can continue to play a role. The main catalytic reduction reactions, such as Eqs. (3)–(7) [25,41]:

4. Conclusions

In this paper,a coke was produced by pyrolysis of the coal with Fe-based additives aims to achieve the goal of reducing NOxemissions and realize the clean utilization of coal. The effects of Febased additives dosages and the combustion temperature on NOxemission were systematically studied in a fixed bed.It is found that the NOxemission of de-coke with 3% (mass) Fe2O3load is 27.3%lower than that of coal when the combustion temperature is 900°C. The analyses found that the remarkably reduced NOxemission was mainly caused by the following two reasons: The first is the reduction of nitrogen content in coke due to the pyrolysis process.This is because during the pyrolysis of coal, the catalyst catalyzes the conversion of nitrogen in the coke to volatile nitrogen, which is released as NH3, HCN and N2; The second is that some of the NOxemitted during coke combustion are converted to environmentally friendly N2by the catalytic action of iron-based catalysts.This new strategy can effectively reduce the NOxpollution and has important practical significance for realizing efficient and clean utilization of coal.

Fig. 9. Mechanism diagram of iron-based additives for combustion and de-NOx of coke (a: reaction system without catalyst. b: reaction system after adding catalyst).

Data Availability

I have shared the link to my data at the attach file step.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by National Natural Science Foundation of China (21878210), Shanxi Provincial Science and Technology Achievement Transformation Guidance Special Program of China (202104021301052) and Shanxi Province Patent Transformation Special Program Project (202202054).

Supplementary Material

Supplementary material to this article can be found online at https://doi.org/10.1016/j.cjche.2022.11.017.