Influences of Waste Iron Residue on Combustion Efficiency and Polycyclic Aromatic Hydrocarbons Release during Coal Catalytic Combustion

Effects of adding waste iron residue on the combustion efficiency and the removal of Polycyclic Aromatic Hydrocarbons (PAHs) during coal combustion were investigated in a laboratory scaled drop tube furnace at a temperature range of 950–1250°C. The experimental results indicated that adding 0.5% waste iron residue had a good performance on enhancing combustion efficiency during coal combustion. Meanwhile, the addition of 0.5% waste iron residue was also favor for reducing PAHs emission as well as PAHs TEQ concentrations during coal combustion at a temperatures range of 950–1250°C. With adding 0.5% waste iron residue (WIR), the coal combustion efficiency was enhanced by 6.15%, and the total PAHs concentration in flue gas was decreased by 44.94% at 1150°C. The maximum removal of the total PAHs TEQ concentration was as high as 45.24% at 1250°C.


INTRODUCTION
Coal has been and will continue to be a main primary energy source, especially with the increase of population and the rapid economic development in China.It was reported that coal occupies about 70% of the primary energy in China, of which over 80% is directly burnt in pulverized form, e.g., in the metallurgical, chemical and energy industries (Gong et al., 2010a).Due to old boilers and poor coal property, the coal combustion efficiency is not enough high.In order to improve coal combustion efficiency, especially the high rank coal with low reactivity, many methods were applied during coal conversion process, including advanced reactor (Loscertales et al., 2015), oxygen enriched technology (Toftegaard et al., 2010) and catalytic combustion (Ouyang et al., 2014).
Catalytic coal combustion is one of the most promising approaches of improving energy utilization efficiency, which has been mainly applied in power plant boiler (Li et al., 2007), entrained-flow reactor (Molina et al., 2005) and blast furnace (Zou et al., 2014).Catalytic coal combustion can not only increase the combustion efficiency, but also decrease the contaminant emission such as SO 2 and NO x (Yao et al., 2007;Qiu et al., 2010).At present, catalysts used for coal combustion are mainly alkali and alkaline earth metals (AAEM) and some transition metal compounds such as iron or its oxides.Unfortunately, AAEM compounds used as catalysts for coal catalytic combustion would result in the negative effects including fouling, slagging and corrosion (Zhao et al., 2006).Alternately, Fe based compounds such as Fe 2 O 3 are environment-friendly catalysts, which have less shortcomings and low cost (Gong et al., 2010b).In recent years, the influences of Fe based catalysts on combustion behavior and pollutants emission have been widely investigated.Gong et al. (2010a); Mendiara et al. (2014); Zou et al. (2014) and Zhang et al. (2014) reported that Fe 2 O 3 had a better catalytic effect during pulverized coals combustion.Daood et al. (2014a) found Fe based catalysts could enhance thermal cracking of heavier hydrocarbon due to its better heat transfer properties.Reddy et al. (2004) and Tsubouchi et al. (2008) claimed that iron oxides could reduce NO to N 2 .Daood et al. (2014b); Zhao et al. (2002); Wu et al. (2014) also proved that Fe based catalysts (Fe 2 O 3 or Fe based waste) can not only increase the combustion reactivity, but also decrease the NO x emission during coal combustion.In addition, Liu et al. (2002) and Zhang et al. (2013) reported that Fe 2 O 3 could inhibit SO 2 emission during coal combustion.However, the effects of Fe based catalysts on the volatile organic compounds emissions such as polycyclic aromatic hydrocarbons during coal combustion were few studied.
Polycyclic aromatic hydrocarbons are a class of organic matters composed by more than two benzene rings, which are considered to be a potential health hazardous substance due to its immunotoxicity, genotoxicity, carcinogenicity, reproductive toxicity properties (Dimosthenis et al., 2015).Hence, United States Environmental Protection Agency (US EPA) defined 16 PAHs as priority pollutants (Ribeiro et al., 2012).Ambient air quality standard for the PAHs were summarized in Table 1 (Rubailo et al., 2008).It was reported that the global total annual atmospheric emission of 16 PAHs in 2007 was 504 Gg.Among them, South (87 Gg), East (111 Gg), and Southeast Asia (52 Gg) were the regions with the highest PAHs emission densities (Shen et al., 2013).In China, the total emission of 16 polycyclic aromatic hydrocarbons species was around 25300 tons in 2003, the related contributions of the 5 sources (coal combustion; coking coal; oil burning for transportation; nontransportation oil burning and natural gas combustion) were in the sequence as 66.6, 30.6, 0.9, 1.8 and 0.1% (Xu et al., 2006).
In this paper, the effects of the waste iron residue on combustion efficiency and polycyclic aromatic hydrocarbons removal efficiency during coal combustion were investigated in a laboratory scale drop tube furnace at a temperatures range of 950-1250°C.

Raw Materials
A Chinese coal and a waste iron residue (WIR) were used in this study.Before the experiment, the coal and waste iron residue (WIR) were dried, milled and sieved to less than 74 µm.Then the waste iron residue and coal were uniformly mixed with a ratio of 0.5% (waste iron residue/coal).The fractions of C, H, O, N, and S in the coal were 91.43%, 2.83%, 3.40%, 1.34% and 1.00%, respectively.Proximate analysis revealed that the fraction of fixed carbon, moisture, ash, and volatile matter were in the sequence as 65.30%, 0.98%, 27.79%, and 5.93%.The properties of the coal and the waste iron residue were listed in Table 2.The inorganic materials of the waste iron residue were analyzed by X-ray fluorescence spectrometry (Mode EAGLE III, EDAX Co, America).It can be seen that the main compositions of the waste iron residue were Fe 2 O 3 (63.21%),SiO 2 (16.44%),CaO (7.84%) and Al 2 O 3 (8.38%).The morphologies of the waste iron residue were investigated by powder X-ray diffraction (PXRD), as shown in Fig. 1.It can be seen that the main compositions of waste iron residue were Fe compounds (such as Fe 2 O 3 , Fe 3 O 4 and Fe 2 SiO 4 ).

Experimental Apparatus
The laboratory scale drop tube furnace (DTF) used in this experiment was described detailly in our previous paper (Han et al., 2012;Chen et al., 2014).In brief, the DTF consisted of a feeding system, a high temperature furnace and a sampling system.Coal entrained in a primary airflow was fed into the DTF by a Sankyo Piotech Micro Feeder (Model MFEV 10; Sankyo Piotech Co., Osaka, Japan).The feeding rate was 0.3 g min -1 .The furnace has a length of 2 m and an inner diameter of 56 mm.It was electrically heated by three sections and the temperatures were automatically controlled by thermocouples.The experimental temperatures were 950-1250°C.The primary airflow was mixed with the feeding coal prior to enter the furnace.The primary and secondary airflow feed rate was 1 and 4 L min -1 , respectively.The residence time of coal in the furnace was about 2 s.The concentrations of CO, CO 2 , SO 2 , NO x and O 2 in the flue gas were recorded by a gas analyzer (PG 250, Horiba Corp, Japan).The repeatability and the linear for CO, CO 2 , SO 2 , NO x and O 2 are ≤ 1% full scan and ≤ 2% full scan, respectively.In addition, the ash was collected by filter.The ash tracer method was used to calculate the combustion efficiency (Wu et al., 2014).The experimental conditions, the combustion efficiency and the flue gas composition were presented in Table 3.The combustion efficiency (η) was given as follows (Wu et al., 2014): where, A 0 is the ash content in the raw coal.A i is ash content in the coal combustion residue.
In the experiments, polycyclic aromatic hydrocarbons were also isokinetically sampled by a sampling system according to US EPA Method 23, as described in detail in our previous paper (Qin et al., 2014).Before the PAHs sampling system, a cyclone separator was used to remove the particles above 10 µm.In the sampling system, a cooling tube was applied to condense polycyclic aromatic hydrocarbons of the light molecular weight, followed by XAD 2 resin used to capture polycyclic aromatic hydrocarbons.Other gaseous polycyclic aromatic hydrocarbons are absorbed by dichloromethane solution.

Polycyclic Aromatic Hydrocarbons Analysis Procedure
All samples containing polycyclic aromatic hydrocarbons were treated by Soxhlet extraction, Kuderna Danish concentrator, purification with the activated silica gel, and analyzed by a high performance liquid chromatography (HPLC) (Agilent Corp., USA).The analysis procedure of polycyclic aromatic hydrocarbons was also described in our previous paper (Qin et al., 2014).In this study, each run was performed three times, and the average data was used.
The Removal Efficiency (RE) was defined as follows.
where, Q 1 is the concentration of polycyclic aromatic hydrocarbons or polycyclic aromatic hydrocarbons TEQ concentration in flue gas without catalyst.Q 2 is the concentration of polycyclic aromatic hydrocarbons or polycyclic aromatic hydrocarbons TEQ concentration in flue gas with catalyst.
The toxic equivalent factor (TEF) is generally used to assess the toxic of polycyclic aromatic hydrocarbons on the basis of the most toxic polycyclic aromatic hydrocarbons (BaP).US EPA specified the TEF of 16 polycyclic aromatic hydrocarbons, as Table 4 (Tsai et al., 2014).In brief, polycyclic aromatic hydrocarbons TEQ concentration can be calculated by Eqs. ( 3) and (4).
TEQ i is the TEQ concentration of the i th polycyclic aromatic hydrocarbons (µg TEQ Nm -3 ), C PAHi is the content of the i th polycyclic aromatic hydrocarbons (µg TEQ Nm -3 ), and TEF PAHi is the TEF of the i th PAHs.The total TEQ concentration of 16 polycyclic aromatic hydrocarbons specified by US EPA is calculated as Eq. ( 4).
TEQ is the total TEQ concentration of the 16 specified polycyclic aromatic hydrocarbons (µg TEQ Nm -3 ).

Effect of Temperature on Combustion Efficiency and PAHs Emission
The effect of combustion temperature on the combustion efficiency during coal combustion at 950-1250°C is presented in Table 3 and Indeno[1,2,3cd]pyrene (InP) were firstly increased and then decreased when the temperature was increased from 950 to 1250°C, and the maximum amount of PAHs in flue gas occurred at about 1150°C.Fig. 6 presented the effect of combustion temperature on the grouped PAHs concentration during coal combustion.The experimental results showed the four rings and five rings PAHs were dominant in PAHs at 950-1250°C.Meanwhile, the contents of five rings and six rings PAHs increased with temperature at 950-1250°C, while the two-rings, three rings and the total PAHs emission in flue gas were increased with temperature at 950-1150°C,    PAHs formation mechanisms during coal combustion mainly including (Mastral et al., 2001): (1) Thermal decomposition Coal contains many organic materials which have complex chemical structures that are able to be thermal cracking during combustion processes.Radicals and smaller fragments will release during thermal cracking process that can undergo a series of reactions such as thermal breaking reaction, dehydrogenation reactions and oxidation reactions.PAHs will be formed by a series of reactions such as thermal breaking reaction and dehydrogenation reactions.However, oxidation reactions will result in the breakdown of large organic molecules to smaller hydrocarbons and in efficient combustion the only products should be CO 2 and H 2 O. (2) Radicals pyrosynthesis As a result of the pyrolytic process, the radicals released, due to their high reactivity and their short average lifetime, undergo different reactions that compete among themselves as a result of the combustion conditions.The reaction of radicals includes two opposite types: On one hand, condensation reactions, which imply the association between radicals and generate compounds of higher molecular weight, PAHs; on the other hand, oxidation reactions, which imply the elimination of radicals leading to CO and H 2 O formation.
It was reported that PAHs are mainly formed by the thermal decomposition in the low temperature zone, while the pyrosynthesis is the main route for PAHs formation in the high temperature zone (Cao et al., 2005;Qin et al., 2014;Chen et al., 2014).Hence, the rate of thermal breaking reaction and dehydrogenation reactions may be increased which result in the increase of PAHs concentration when the temperature increased from 950 to 1150°C.When the temperature was 1250°C, the rate of thermal breaking reaction and dehydrogenation reaction are dramatically decreased and the oxidation reaction rate are increased, which led to the decrease of the PAHs concentration.The pyrosynthesis formation (condensation reaction) was the main contributor for the PAHs formation at temperature of 1250°C during coal combustion.

Effect of Adding 0.5% Waste Iron Residue on the Combustion Efficiency and PAHs Emission
The effect of adding 0.5% waste iron residue on the combustion efficiency during coal combustion at 950-1250°C was also presented in Table 4. Result showed that adding 0.5% waste iron residue could obviously enhance the combustion efficiency.With adding 0.5% waste iron residue, the combustion efficiencies under 950, 1050, 1150 and 1250°C were increased by 3.10%, 5.89%, 6.15%, and 2.73%, respectively.Figs.2-6 also represented the effect of adding waste iron residue on PAHs emission during coal combustion at 950-1250°C.It can be seen in Figs.2-5, the individual PAHs emissions concentration during coal combustion in presence of waste iron residue was markedly lower than that without waste iron residue.For example, the emissions concentrations of Naphthalene (Nap), Fluoranthene (FluA), Pyrene (Pyr) and Benzo[a]pyrene (BaP) during coal combustion at 1150°C were 0.676, 0.754, 0.454 and 0.363 mg Nm -3 , respectively.Whereas, with adding 0.5% waste iron residue, Naphthalene (Nap), Fluoranthene (FluA), Pyrene (Pyr) and Benzo[a]pyrene (BaP) were decreased to 0.207, 0.619, 0.000 and 0.241 mg Nm -3 , respectively.Fig. 6 also compared the grouped PAHs emission during coal combustion with and without catalyst under different temperature.The contents of the three ring, four ring, five ring, and the total PAHs were reduced obviously during coal combustion with adding 0.5% waste iron residue.The contents of the two ring, three ring, four ring, five ring and the total PAHs during coal combustion at 1150°C were reduced by 0.469, 0.390, 0.751, 0.221 and 1.829 mg Nm -3 .When the combustion temperature was 1250°C, the contents of the three ring, four ring, five ring, six ring and the total PAHs during coal combustion were reduced by 0.390, 0.751, 0.221, 0.115 and 1.829 mg Nm -3 .Fig. 7 demonstrated that the removal efficiencies (RE) of the two ring, three ring, four ring, five ring and six ring PAHs under 1150°C were 69.38%, 46.69%, 47.35%, 25.57% and -1.51%, respectively.When the temperature was 1250°C, the removal efficiencies (RE) of the two ring, three ring, four ring, five ring and six ring PAHs were -1.46%, 39.15%, 47.12%, 54.29% and 76.15%, respectively.The above value indicated that the two ring, three ring and four ring were the main contributor for PAHs removal at 1150°C, whereas the four ring, five ring and six ring were the main contributor for PAHs removal at 1250°C.As for the total PAHs, the removal efficiencies (RE) under 950, 1050, 1150 and 1250°C were 24.81%, 15.41%, 44.94% and 44.43%, respectively.The morphology and the crystal compositions of the ash produced from coal combustion with catalyst or without catalyst were analyized by Powder X-ray diffraction (PXRD) and scanning electron microcopy (SEM), as shown in Figs. 8 and 9. From Fig. 8, it can be seen that the crystal composition of catalytic residue was a little different from raw coal combustion residue.The main composition of raw coal combustion residue was SiO 2 , a small amount of NaFeO 2 and KAlSiO 3 were also existed, when adding 0.5% waste iron residue, the peak of NaFeO 2 was increased, the peak of KAlSi 3 O 8 almost disappeared.However, the main peaks did not change significantly, indicated that the microscopic composition of coal did not change much by adding waste iron residue.Fig. 9 presented that the particles appearance became extremely irregular with the adding 0.5% waste iron residue.In Fig. 9(a), the particles surface of raw coal combustion residue was smooth, and partial sintering phenomena existed; when combustion with adding 0.5% waste iron residue, the particle surface presented like loose honeycomb as shown in Fig. 9(b), a lot of small pores appeared and the surface was very rough.
Therefore, PAHs reduction pathway could be governed by the following reaction mechanisms: (Kim et al., 2008): The direct oxidation leads to the consideration of catalytic PAHs precursors oxidation by iron species through reactions such as the following processes: C solid + FeO → CO + Fe (6) C solid + OH + Fe → CO + H + Fe (8) In addition to oxygen, the role of OH as a PAHs precursors oxidizer may be important in the presence of iron catalysts, where, for example, the following surface reactions may be responsible for the creation of OH: where, Fe(s) denotes a free iron-surface site, and O(s) and H(s) indicate surface species.Finally, OH radicals are desorbed from the surface of the iron and are available to oxidize the solid carbon via Reaction (8).In addition to catalysis by elemental iron, it is perhaps more reasonable to assume oxidation of elemental iron and the subsequent catalysis by iron oxide species: Alkaline earth metals (CaO) have been also proven to be able to reduce PAHs emission during combustion.As compounds of alkaline earth metals are added to the flames, alkaline earth hydroxide and hydroxyl free radical are formed via the reaction of metal oxide with water vapor.It has been reported that the mechanism of the formation of alkaline earth metal hydroxide ions at combustion temperature is as follows: CaO + H → MOH + + e - ( 15) Wei and Lee (1998) reported that a considerable extent of oxidation of the PAHs precursors, rather than the already- formed PAHs, with the hydroxyl free radical occurred; this caused the major suppression of PAHs formation.By an analogy to PAHs, part of the PAHs or their precursors formed during coal catalytic combustion would be subject to oxidation by the hydroxyl free radical, which resulted in the PAHs decrease.
In addition, Fe catalysts with increase content of iron, increased pore structure and surface area facilitated the thermal degradation of heavier hydrocarbon into lighter hydrocarbons (Daood et al., 2014).

Effects of Temperature and Adding 0.5% Waste Iron Residue on PAHs TEQ Concentration
PAHs TEQ concentration during coal combustion at 950-1250°C was listed in Table 5. BaP, DbA and BkF were the three dominant PAHs TEQ concentrations during coal combustion, followed by the BaA, BbF and BghiP.The five rings PAHs TEQ concentration during coal combustion under 950, 1050, 1150 and 1250°C were 427.9, 427.3, 576.9 and 795.5 µg Nm -3 , respectively, which accounted for 99.28%, 97.25%, 96.00% and 98.09% of the total PAHs TEQ when the reaction temperature was increased from 950 to 1250°C.Table 5 also listed the effect of adding 0.5% waste iron residue on PAHs TEQ concentration during coal combustion.It can be seen that the five ring PAHs (such as BaP and DbA) also was the main contribution for the toxic equivalent (TEQ) concentration.With adding 0.5% waste iron residue, the five ring TEQ concentrations under 950, 1050, 1150 and 1250°C were decreased to 232.4, 354.9, 413.6 and 444.4 µg Nm -3 , and the total PAHs TEQ concentrations under 950, 1050, 1150 and 1250°C were 236.0, 364.0, 431.0 and 452.0 µg Nm -3 , respectively.The reduction efficiencies of the total PAHs TEQ concentration under 950, 1050, 1150 and 1250°C were in the sequence as 45.24%, 17.27%, 28.29% and 45.27%.

CONCLUSIONS
Effects of adding 0.5% waste iron residue on the combustion efficiency and PAHs removal efficiency during coal catalytic combustion were investigated in a laboratory scale drop tube furnace at a temperatures range of 950-1250°C.The following conclusions have been drawn: Adding 0.5% waste iron residue can obviously enhance the coal combustion efficiencies and have a good performance of decreasing PAHs or PAHs TEQ concentration during coal combustion.With adding 0.5% waste iron residue, the coal combustion efficiencies under 950, 1050, 1150 and 1250°C were enhanced by 3.10%, 5.89%, 6.15%, and 2.73%, respectively, and the removal efficiencies of the total PAHs under 950, 1050, 1150 and 1250°C were in the sequence as 24.81%, 15.41%, 44.94% and 44.43%.Moreover, the five rings PAHs were the main contribution for PAHs TEQ concentrations, and the maximum reduction efficiency of 45.27% for the total PAHs TEQ concentration was obtained at 1250°C.

Fig. 2 .
Fig. 2. The individual PAHs produced from coal combustion with and without catalyst at 950°C.

Fig. 3 .
Fig. 3.The individual PAHs produced from coal combustion with and without catalyst at 1050°C.

Fig. 6 .
Fig. 6.The grouped PAHs produced from coal combustion with and without catalyst at 950-1250°C.

Table 1 .
Ambient Air Quality Standard for the PAHs.

Table 2 .
The properties of coal and the waste iron residue (W%).

Table 3 .
The experimental condition.

Table 4 .
TEF of 16 kinds of PAHs.