Effect of Al₂O₃ on NO_x and SO₂
of RDF combustion

1 Introduction

The total annual production of municipal solid waste in China increased from 31.3 million tons (1980) to 343 million t (2019) and could reach 480 million t by 2030 [1]. RDF (Refuse-derived fuel) is a fuel derived from the sorting, crushing, drying, and processing of refuse, which has high calorific value, stable combustion, and low dioxin emission [2-4]. Therefore, RDF can be used as a good alternative fuel for firing cement [5-7]. The utilization of MSW into RDF is not only conducive to the reuse of resources, but also reduces environmental pollution. Nowadays, the national and local governments actively encourage the use of cement kilns for collaborative disposal of RDF, which provides a new idea for the efficient disposal of municipal solid waste [8]. When RDF is used as an alternative fuel in cement kilns, it will inevitably be affected by complex raw material components. Studying the effects of raw material components on the RDF combustion process is of great significance for exploring the emission mechanism of NO_x, SO₂, and other pollutants when RDF is used as an alternative fuel.

Metal oxides have great influence on the formation of NO_x and SO₂ during RDF combustion. There have been some researches on the effect of metal oxides on the combustion process of solid fuels. Liu and Gao [9,10] analyzed the influence mechanism of metallurgical dust on NO emission in the coal combustion system, and the results showed that with the increase of the mass percentage of metallurgical dust, the peak value of NO emission decreased. Qi [11] studied the effects of mixtures (NH₄H₂PO₄, CaCO₃, and CaO) on the emission of NO and SO₂ in different coke combustion processes. The results showed that with the increase of the three mixtures, the emission of NO was enhanced to varying degrees, while the emission of SO₂ was significantly inhibited. This indicates that metal oxides have different action mechanisms on the peak value and total amount of NO produced in solid fuel. Shao and Sun [12,13] studied the catalytic pyrolysis of metal oxides on sludge and other samples, and the analysis showed that Al₂O₃ could shorten the total pyrolysis time of sludge, while CaO could extend the total pyrolysis time of sludge.

Al₂O₃ is an important raw material component in cement kilns, and it is important to study the mechanism of NO_x and SO₂ in the RDF combustion process. Fang and Liu [14-16] used thermogravimetric analysis (TGA) to study the influence of Al₂O₃ on the pyrolysis characteristics and activation energy of municipal solid waste (MSW) in N₂ atmosphere. The results showed that after adding Al₂O₃, residue quality decreased, pollutant emission decreased, aliphatic hydrocarbon ratio increased, oxygen-containing compound ratio decreased, and activation energy decreased.

However, studies on the mechanism of metal oxides in coal combustion are common at present, while studies on the co-combustion of metal oxides and RDF are rarely reported. Meanwhile, the interaction between gas components such as CO, NO_x, SO₂ in RDF combustion remains unclear. In this paper, the environment of the precalciner is simulated by the double-tube furnace reactor platform built in the laboratory. High-temperature combustion experiments were carried out on the mixed samples of RDF and Al₂O₃ of 0.00 g, 0.02 g, 0.04 g, and 0.08 g, and the influence of Al₂O₃ on the generation and transformation of CO, NO_x, SO₂ and other gases in the combustion process of RDF was quantitatively and qualitatively studied.

2 Materials and Methods

2.1 Raw materials

The experimental raw materials used in this study were RDF and Al₂O₃. The alternative fuel chosen for the RDF is the actual alternative fuel used by a cement company in Hubei, which comes from municipal household waste in Wuhan. The industrial analysis of RDF is carried out concerning the national standard (GB/T 212-2008). Elemental analysis by determination of the elemental content of C, H, O, N, and S in the fuel using a Vatio EL cube elemental analyzer from Elemental Analysis Systems, Germany. The calorific value was determined three times using an oxygen bomb calorimeter and the average value was taken. Table 1 shows the industrial analysis, elemental analysis and calorific values of the RDF used in the experiments. Al₂O₃ is produced by Tianjin Zhiyuan Chemical Reagent Co., Ltd. and the content of Al₂O₃ is ≥ 98.0%.

2.2 Experimental apparatus and methods

2.2.1 Tube furnace combustion experiments

The simulated combustion experimental system is shown in Figure 1, with a tube furnace model OTF-1200X and an adjustable temperature range: room temperature to 1200 °C. The combustion atmosphere is prepared by N₂/O₂ and the flue gas components of the specimen after combustion are collected online using an MRU NOVA PLUS flue gas analyzer from Germany.

Connect the experimental device as shown in Figure 1, turn on the flue gas analyzer, and preheat it for 30~40 min. After the flue gas analyzer has started up properly and entered the monitoring interface, the entire reaction system is ventilated for a gas tightness check. Set the high-temperature tube furnace to heat up to 900 °C at a rate of 10 °C/min and hold. Weigh 0.5 g of RDF and different masses of Al₂O₃, mix well and lay flat in a corundum porcelain boat of size 60 mm x30 mm x15 mm. Open the gas cylinder, adjust the gas distribution cabinet to set the flow rate, set the N₂ gas flow rate to 1000 ml/min, O₂ gas flow rate to 180 ml/min, and ventilate for 10~20 min. After the gas display on the flue gas analyzer has stabilized the porcelain boat with the sample in place is quickly pushed to the middle of the high-temperature tube furnace and continuous measurement begins until the results for CO and NO_x are 0 and the data on the flue gas analyzer returns to the set gas flow density, when the recording stops. Each set of experiments was repeated three times and the mean was taken as the final result. Where CO, NO_x, and SO₂ are measured in ppm. The yields of CO, NO_x, and SO₂ can be calculated separately from equation (1) by integration of the gas curves with time as follows.

where m_j is the yield of CO, NO_x, and SO₂ (mg), c_i is the concentration of CO, NO_x, and SO₂ (ppm), v is the volume flow rate 1000 (ml/min), M_j is the molar mass of CO, NO_x, and SO₂, t is the sampling interval 1 (s), n is the number of points measured, and j is CO, NO_x, and SO₂.

3 Results and Discussion

3.1 Analysis of RDF combustion at 900 °C

By analyzing the generation of gas products when RDF is burned in an atmosphere with O₂ concentration of 18%, it can be seen that the release curve of CO with time has two peaks, and the time of occurrence is 21 s and 262 s in turn. According to the carbon combustion mechanism, the two CO release peaks correspond to the peaks of volatile and fixed carbon precipitation in the fuel, so the two CO peaks are defined as volatile CO peak and fixed carbon CO peak. NO_x release curve over time forms two release peaks, which can be defined as volatile NO_x peak and fixed carbon NO_x peak according to the carbon combustion mechanism. The volatile NO_x peak appears earlier (t= 44 s) and has a larger peak; the fixed carbon NO_x peak appears later (t= 270 s) and has a smaller peak. Both peaks are in the form of a parabola. The release curve of SO₂ over time also takes the form of a “parabola”, and its peak time is 41 s. It can be seen from the analysis of Figure 2 that during RDF combustion, the peaks of volatile CO, volatile NO_x, and SO₂ peaks appear at similar times, and all peak within 60 s after the reaction begins. The peak times of fixed carbon CO peak and fixed carbon NO_x peak are also similar, and the peak time is about 270 s. At the same time, the peaks of volatile CO and volatile NO_x are also higher than those of fixed carbon CO and fixed carbon NO_x, which is because the volatile content of RDF accounts for 71.83% (Tab. 1). Under the condition of sufficient oxygen, volatile content in RDF can be rapidly burned to produce a large amount of volatile CO and volatile NO_x.

3.2 Effect of Al₂O₃ on the formation pattern of CO, NO_x and SO₂ of RDF combustion products

Figure 3 shows the release curve of CO, NO_x, and SO₂ over time when RDF is burned at 900 °C and mixed with different Al₂O₃ masses. Table 2 shows the peak concentration and formation time of CO, NO_x and SO₂ when RDF is burned at 900 °C with different Al₂O₃ masses mixed. Figure 4 shows the total amount of CO, NO_x and SO₂ generated when RDF is burned at 900 °C and mixed with different Al₂O₃ masses.

As can be seen from Figure 3 (a), two release peaks of CO are formed when RDF is burned at 900 °C under different Al₂O₃ mixtures, namely volatile CO peak and fixed carbon CO peak. The peak value of volatile CO decreases first and then increases with the increase of Al₂O₃ mixing amount. That is, when the mixing amount of Al₂O₃ is 0.00 g, the maximum peak value of volatile CO appears at 20 ppm, and the peak time is 21 s. At the same time, the peak time of volatile CO is 21 s when mixed with different quality Al₂O₃, so it can be concluded that mixed Al₂O₃ has little influence on the formation time of volatile CO peak. The peak value of fixed carbon-type CO showed a decreasing trend with the increase of Al₂O₃ mixing amount, and the maximum peak value of fixed carbon-type CO appeared when the mixing amount of Al₂O₃ was 0.00 g. The peak formation time of fixed carbon-type CO mixed with Al₂O₃ is significantly shortened (about 10 s in advance), but the peak value of fixed carbon-type CO is little affected by the mixing amount of Al₂O₃, and the peaks are small. Combined with Figure 4(a), the total amount of CO generated shows a decreasing trend with the increase of Al₂O₃ mixing mass. When the mixing amount of Al₂O₃ is 0.04 g and 0.08 g, the minimum value of the total amount of CO generated is 0.012 mg, which is because the mixing of Al₂O₃ will increase the contact area between RDF and O₂. As a result, more CO₂ is generated, reducing the total amount of CO produced and promoting combustion.

As can be seen from Figure 3(b), two release peaks are formed in the NO_x release curve over time when RDF is burned with different Al₂O₃ masses, namely volatile NO_x peak and fixed carbon NO_x peak. Both volatile NO_x peak and fixed carbon NO_x peak present a high and narrow “parabola” form. The peak of volatile NO_x is rapidly generated and released in the early stage of RDF combustion, and the peak time is 44 s, 42 s, 75 s, 61 s, and almost completely appears within 100 s after the start of combustion. The peak value of volatile NO_x decreases first and then increases with the increase of Al₂O₃ mixing amount. When the mixing amount of Al₂O₃ is 0.00 g, the maximum release peak of NO_x is 178 ppm, and when the mixing amount of Al₂O₃ is 0.04 g, the minimum release peak of NO_x is 148 ppm. The peak value of fixed carbon NO_x shows an irregular trend with the increase of Al₂O₃ mixing amount. Meanwhile, the peak formation time of fixed carbon NO_x is greatly affected by Al₂O₃, among which, the peak value is first reached when the mixing amount of Al₂O₃ is 0.04g, and the peak time is 243 s. By analyzing the total amount of NO_x generated in Figure 4(b), it can be concluded that when the mixing amount of Al₂O₃ is 0.04 g, the minimum amount of NO_x generated is 0.83 mg, and when the mixing amount of Al₂O₃ is 0.00 g, the maximum amount of NO_x generated is 0.93 mg. This may be the result of the combustion of Al₂O₃ as an amphoteric oxide against RDF. However, when the mixing amount of Al₂O₃ is 0.04g, the total amount of NO_x generated is at least 0.83mg. Combined with the total amount of Figure 4(a) CO generated, it can be seen that the total amount of CO generated at this time is the least. Therefore, when the mixing amount of Al₂O₃ is 0.04 g, the reduction reaction of NO_x is the most intense, resulting in the least total amount of CO and NO_x generated. At the same time, when the mixing amount of Al₂O₃ is 0.08 g, the total amount of NO_x generation increases to 0.86 mg, which is caused by the influence of coating.

In summary, the peak value of volatile NO_x decreases first and then increases with the increase of Al₂O₃ mixing amount, while the peak value of fixed carbon NO_x shows an irregular trend with the increase of Al₂O₃ mixing amount. When the mixing amount of Al₂O₃ is 0.04g, the total amount of NO_x generated is at least 0.83 mg, so mixing the right amount of Al₂O₃ can reduce the total amount of NO_x generated.

It can be seen from Figure 3(c) that the release curve of SO₂ over time when RDF burns under different Al₂O₃ masses is in the form of a “parabola”, and the release peak of SO₂ gradually decreases with the increase of the mixing amount of Al₂O₃, that is, when the mixing amount of Al₂O₃ is 0.00 g, the maximum release peak of SO₂ is 68 ppm. When the mixing amount of Al₂O₃ is 0.08 g, the minimum release peak of SO₂ is 48 ppm, which is because Al₂O₃, as a metal oxide, acts as an absorb-ent and forms aluminum sulfate, thus reducing the release amount of SO₂. At the same time, the peak time of SO₂ is 41 s, 41 s, 74 s, and 57 s successively, which indicates that the SO₂ generation rate in the RDF combustion process will be inhibited with the mixing of Al₂O₃, thus reducing the combustion speed of RDF.

In summary, the release curve of SO₂ from Al₂O₃ to RDF combustion shows a regular change, that is, the peak value of SO₂ and the total amount of SO₂ are constantly decreasing with the increase of Al₂O₃ mixing amount.

4. Conclusion

1. The mixing of Al₂O₃ will reduce the release peak value of volatile CO and fixed carbon CO, among which the release peak value of fixed carbon CO is small, and the total amount of CO generated is the least when the mixing amount of Al₂O₃ is 0.04 g and 0.08 g. Mixing Al₂O₃ will increase the contact area between RDF and O₂, thus generating more CO₂, reducing the total amount of CO generation and promoting combustion.

2. The mixing of Al₂O₃ will reduce the peak release of volatile NO_x. The maximum peak value of fixed carbon NO_x appears when the mixing amount of Al₂O₃ is 0.00 g and the minimum peak value of fixed carbon NO_x appears when the mixing amount of Al₂O₃ is 0.04 g. In the combustion process, mixed Al₂O₃ can inhibit the increase of the total amount of NO_x generation, and 0.04 g Al₂O₃ can best inhibit the increase of the total amount of NO_x generation.

3. The SO₂ release curve of Al₂O₃ for RDF cracking shows regular changes, that is, with the increase of Al₂O₃ mixing amount, the peak value of SO₂ and the total amount of SO₂ are constantly decreasing, which can provide certain guidance for SO₂ emission reduction of industrial cement furnace.