Influence of Environmentally Friendly and High-Efficiency Composite Additives on Pulverized Coal Combustion in Cement Industry

4 kinds of chemical reagents and 3 kinds of industrial wastes were selected as burning additives for 2 kinds of coals in cement industry. The work focused on the replacement of partial chemical reagents by industrial wastes, which not only reduced the cost and took full advantage of industrial wastes, but also guaranteed the high combustion efficiency and removed the NO X and SO 2 simultaneously. The experiments were carried out in DTF. The combustion residues were analyzed by SEM and XRD. The results showed that the burnout rate was increased after adding the additives; meanwhile, the NO X and SO 2 release concentration were reduced, but the degree of action varied for different additives and coals. The substitute of chemical reagents by industrial wastes was very effective; overall, the cold-rolled iron oxide worked better than others; the particles surface was tougher and the peaks of crystalline phase were lower than raw coal, which indicated that the additives played good roles in combustion process.


Introduction
Coal is widely used in the cement industry.Most of cement factories utilize high-quality coals as flues so that highquality coals are becoming fewer and fewer.Based on the situation, the cement industry has to utilize inferior coals.However, there are severe defects with inferior coals, such as high ash content, low-calorific value, and easy slagging [1].Those defects may influence the common cement production seriously, especially in rotary kilns or in precalcined kilns [2].Additionally, the new exhaust standards of air pollutant for cement industry (GB4915-2013) stipulate that the emission concentration of NO  and SO 2 in the current cement industry cannot surpass 400 mg/Nm 3 and 200 mg/Nm 3 which are half of the previous emission concentration.
In summary, the cement industry is faced with enormous challenge.In order to achieve the sustainable development of cement industry, inferior coals should be utilized effectively and sufficiently.Without changing the original cement industry devices, adding coal burning additives is the effective way to promote the combustion of inferior coals and meet the pollution emission requirement of GB4915-2013.In recent years, many investigators had used chemical reagents as coal burning additives [3][4][5][6][7][8].Although those chemical reagents could promote inferior coal combustion significantly, some of them were costly, such as CeO 2 and La 2 O 3 .Then, some investigators used industrial wastes coal as coal burning additives.Zhou and Li [9] and Wu et al. [10] found industrial wastes could promote the coal combustion, but the efficiency was less than chemical reagents.Though the investigators had made great achievements in coal burning additives, these findings were almost used in the boilers and the electricity generation.It was well known that the principle and the process of different industrial varied, so these additives were not completely suitable for the cement industry.He et al. [1] and Xie and He [11] had researched some chemical additives which were used in cement industry; however, they only researched the catalytic combustion effect of the additives and did not research the effect on reducing the release of the NO  and SO 2 simultaneously.Thus, in current background, it was significant to exploit the additives for cement industry which could promote the combustion and reduce the release of the NO  and SO 2 simultaneously.Therefore, in our work, chemical reagents and industrial wastes were composited as the coal burning additives, and the 3 kinds of industrial wastes were used to replace partial chemical reagents, respectively.The composite additives not only improved the combustion efficiency and saved the cost, but also reduced the release of the NO  and SO 2 and developed a high value-added way to dispose industrial wastes.

Coal Samples.
In current work, two different types of inferior coals were selected for investigation, code-named C1 and C2.The properties of coal samples were listed in Table 1.
It was concluded from Table 1 that the C2 was a high ash coal, and the volatile content of C2 was higher than C1.The coal samples were prepared by coning and quartering and were grounded to 150 m according to DTF system.

Composite Additives.
Based on the previous studies of coal burning additives [12,13], four kinds of chemical reagents and three kinds of industrial wastes were selected as additives in our work.Among them, the chemical reagents (BaCO 3 : MnO 2 : NaCl : NaClO 4 = 1 : 1 : 2 : 2) were a whole, code-named A. The three kinds of industrial wastes were steel slag (code name: A1), desulfurization slag (code name: A2), and cold-rolled iron oxide (code name: A3).The chemical compositions of industrial wastes were listed in Table 2. A1, A2, and A3 were used to replace partial A, respectively, so the composite additives were A + A1, A + A2, and A + A3.The amount of composite additives was 2.0% relative to the coal samples, and the additive amount of A1, A2, and A3 was 0%, 0.2%, 0.4%, 0.6%, 0.8%, and 1.0%, respectively; in other words, the amount of A was 2%, 1.8%, 1.6%, 1.4%, 1.2%, and 1.0%.The composite additives were added to the coals by drymixed method according to the above types and proportions.In order to mix equably, the catalytic coal samples were put into the rotary drum for 30 min, and the rotating speed was 20 r/min.

Experimental System.
In our experiment, a drop tube furnace (DTF) was used to simulate the coal catalytic combustion.The DTF experimental system was shown in Figure 1.
It included the power feeding, the cooling water, the gas distribution system, the combustion reactor, the temperature control cabinet, the sample collection system, and the gas analyzer.Among them, the amount of power feeding was 0.5 ± 0.02 g/min; the gas distribution system simulated air atmosphere, including O 2 : N 2 = 1 : 4; the combustion reactor was made of an alumina tube with a length of 2.3 m; the temperature was 950 ∘ C which approached the temperature of the cement decomposing furnace; the gas analyzer was used to detect the concentration of SO 2 and NO  ; the coal ash was collected by filter cartridge; then, it was sent to muffle furnace for secondary combustion.The burnout rate was calculated by this method, and the burnout rate formula was shown as follows: In the formula,  0 stands for ash contents in the raw coal and   stands for ash contents in the coal combustion residues.

Effect of Additives on the Burnout Rate.
The burnout rate is an important index during coal combustion.The higher the burnout rate, the better the coal burn.As shown in Figures 2 and 3, the burnout rate was all increased after adding additives, except in 1.0%A + 1.0% A1.It was well known that a lot of alkali metals, alkaline earth metals, transition metal, and their salts existed in the additives, and they acted as intermediates and promoted the diffusion of oxygen to the coal surface in the combustion process, especially MnO The reactions were shown as follows.Therefore, Fe 2 O 3 was in the circulating process of oxidation and reduction, which made the carbon surface to contact the oxygen, and the combustion of carbon particles was improved [10].Meanwhile, the NaClO 4 could decompose O 2 in high temperature, which also promoted the contact between the oxygen and carbon.Besides, the NaCl made the pulverized coal fluffy, promoting the combustion process further.However, when the addition of CaO was excessive, the CaO would sinter.So the pores of coal might be blocked because of the sintering, which would hinder the diffusion of oxygen to the coal surface.Table 2 indicated there was a large amount of CaO in A1.Thus, the additives of 1.0%A + 1.0% A1 might inhibit the coal combustion.In the view of the burnout rate, A + A3 showed a better catalytic effect, because a large amount of Fe 2 O 3 existed in it.The optimal additive amount of A3 was 0.2%.It could be seen from Figures 2 and 3 that the burnout rate of different coal varied.The burnout rate in the level of the order was C2 > C1, which was an evidence that the higher the volatile content, the bigger the burnout rate.As for C1, the biggest burnout rate was 71.03%, which was 7.15% bigger than the raw coal.As for C2, the biggest burnout rate was 84.5%, which was 3.44% bigger than the raw coal.It was found that the catalytic effect on C1 was better than C2.The possible reason was that the C2 was easier to burn than C1, and the catalytic effect for uneasy burned coal was obviously more.It was also concluded from Figures 2 and 3 that when only A was added to the raw coals C1 and C2, the burnout rate was 68.09% and 83.37%.While industrial wastes were added to replace partial A, the burnout rate improved comparing with pure A, so the replacement of chemical reagents by industrial wastes was feasible.

Effect of Additives on the 𝑁𝑂 𝑋 Release Concentration.
As shown in Figures 4 and 5, it was found that the release concentration of NO  after adding coal burning additives was lower than the raw coal, because of the existence of alkaline metals oxides, alkali metal salt, and transitional metal oxide in the additives, such as NaCl, MnO 2 , and Fe 2 O 3 .These substances suppressed the conversion of fuel-N to NO  [4].Among these three composite additives, A + A3 worked best.The reason was that a large amount of Fe 2 O 3 existed in A3, and Fe 2 O 3 could promote the selective catalytic reduction (SCR) activity; besides, it exhibited high thermal stability [15].It was found that the substitution amount of industrial wastes was not the more the better.As for A3, the optimal additive amount was 0.2%.It also could be seen from Figures 4 and 5 that the NO  release concentration of different coals varied.The raw coal of C1 and C2 was 778 ppm and 944 ppm.Though the nitrogen content of C2 was a litter lower than C1, the burnout rate of C2 was higher than C1 obviously, so the C2 might generate more NO  in the combustion process.It was known from Table 3 that the denitrification efficiency was 15.04% and 13.88% when only 2.0% A was added to the C1 and C2.However, when A1, A2, and A3 were used to replace partial A, the denitrification efficiency of C1 and C2 was increased under the certain amount and the highest denitrification efficiency was 19.28% and 25.32%.In the perspective of denitrification, the best substituendum was A3 and the best instead proportion was 0.2%.

Effect of Additives on the SO 2 Release Concentration.
A comparison of the C1 and the C2 with or without additives on the SO 2 release concentration was shown in Figures 6 and 7.These two figures showed that the additives could reduce the release concentration of SO 2 .From the overall trend, A + A1 showed the best effect on reducing the release of SO 2 .The reason was that 47.83% CaO existed in A1 (Table 2), and the reaction (4) happened as follows [16].Therefore, A + A1 worked best A + A2 worked better than A + A3.It was concluded that the SO 2 release concentration was lowest when the additive amount of A1 was 0.8%.It was also known from Figures 6  and 7 that the SO 2 release concentration of C2 raw coal was much higher than C1 raw coal, because the sulfur content of C2 was higher than C1 (Table 1).Besides, the burnout rate of C2 was higher than C1 too.In Table 4, when only A was added to the raw coal, the desulfurization efficiency of C1 and C2 was 4.42% and 8.17%.The possible reason was that reaction (5) happened [7].In addition, NaCl could enhance the sulfur retention capacity of coal ash and then decrease SO 2 release concentration [7].It also could be seen from Table 4 that the optimal additive was 1.2% A + 0.8% A1 and the highest desulfurization of C1  and C2 was 30.97% and 20.05%.With regard to the same additive and additive amount, the desulfurization efficiency of different coal varied.

The SEM of Combustion
Residue.The combustion residue of C1 raw coal and C1 with 1.8% A + 0.2% A3 was analyzed by the scanning electron microscopy (SEM) and the X-ray diffraction (XRD).SEM is often used to observe the surface morphology of the coal.Besides, the pore structure and the three-dimensional structure are observed intuitively.In our work, SEM was used to deduce the microstructure of different sample, and the micrographs were included in Figure 8.As shown in Figure 8, the surface morphology of coal surface was changed after adding the additive.From Figure 8(a), it could be seen that the particles surface was smooth relatively except for some small holes, and partial sintering phenomena existed.Figure 8(b) showed that the combustion of carbon particles was aggravated, and the particles surface was very tough and looked like a loose honeycomb, only leaving framework.Comparing Figures 8(a) and 8(b), it was concluded that the additive of 1.8% A + 0.2% A3 could promote the combustion process of pulverized coal commendably.It was because there were a lot of alkaline metals oxides, alkali metal salt, and transitional metal oxide existing in the additives, such as MnO 2 , NaCl, NaClO 4 , and Fe 2 O 3 .These substances promoted the contact between the oxygen and carbon.

The XRD of Combustion
Residue.X-ray diffraction (XRD) is an effective means to study the solid structure, and it offers particular advantages in identifying various kinds of mineral forms that relate to structural damage.In our work, XRD was used to reveal the structural changes of coals with or without additives.The XRD of combustion residues was represented in Figure 9.It was shown from Figure 9 that the diffraction pattern changed slightly after adding additives comparing to the raw coals.The main crystalline phases of raw coal were silicon dioxide and aluminum silicon oxide.When 1.8% A + 0.2% A3 were added to the raw coal, a new crystalline phase appeared.The possible reason was that the additives participated in the coal combustion reaction and generated a new compound.It also could be seen that the peaks of raw coal with additives were lower than the raw coal.As was known from Wang et al. [12], the lower peak of crystalline phase indicated that the samples burned more completely.Thus, from the X-ray diffraction, it was also concluded that the additives promoted the coal combustion.

Figure 1 :
Figure 1: Schematic of the DTF system.

Figure 2 :
Figure 2: The effect of additive on   of C1.

2 and Fe 2 O 3 [ 2 ,
14].Take Fe 2 O 3 as examples; at first, reduction reaction occurred between Fe 2 O 3 and carbon, and Fe 2 O 3 was reduced to FeO.Then FeO was oxidized into Fe 2 O 3 by oxygen.Next, the above reaction was proceeding once again.

Figure 3 :
Figure 3: The effect of additive on   of C2.

Figure 4 :
Figure 4: The effect of additive on NO  release concentration of C1.

Figure 5 :
Figure 5: The effect of additive on NO  release concentration of C2.

Figure 6 :
Figure 6: The effect of additive on SO 2 release concentration of C1.

Figure 7 :
Figure 7: The effect of additive on SO 2 release concentration of C2.

Table 3 :
The effect of different additives on denitrification efficiency of C1 and C2 (%).

Table 4 :
The effect of different additives on desulfurization efficiency of C1 and C2 (%).