Study on the Influence of mixed burning Sludge on slagging characteristics of easily slagging Coal

. In this paper, combined with experiments and theoretical calculations, the effect of urban low-calorific value sludge on the slagging characteristics of easy-slagging coals is studied. The results show that when the melting point of sludge ash is lower than that of coal ash, the ash melting point of the mixture after the two blends will be lower than the melting point of raw coal ash. When SiO 2 and Al 2 O 3 , which are relatively high in sludge, are used as single additives, the ash melting point of coal will first decrease and then increase. As the mixing ratio of sludge increases, coal ash slagging characteristics show a trend of gradual relief. For coals that are prone to slagging and rich in alkali metals, they can be appropriately mixed with sludge rich in silicon and aluminum compounds or additives to improve Slagging characteristics.


Introduction
All manuscripts must be in English, also the table and figure texts, otherwise we cannot publish your paper. Please keep a second copy of your manuscript in your office. When receiving the paper, we assume that the corresponding authors grant us the copyright to use the paper for the book or journal in question. Should authors use tables or figures from other Publications, they must ask the corresponding publishers to grant them the right to publish this material in their paper. The reduction, harmlessness, stabilization and resource utilization of municipal sludge have become one of the major problems in the rapid development of urbanization [1] , the moisture content of the wet sludge treated by the sewage treatment plant is generally above 80%, and the calorific value is about 0.1 ~ 3MJ/kg, and the calorific value of the dried sludge is about 6 ~ 12MJ/kg. The main components of the sludge include heavy metals, refractory trace organics, viruses, bacteria and other pathogenic microorganisms. In addition, some minerals are mainly silicate, aluminate and oxides [2] .
As complex mineral combination system, coal ash is composed of acidic oxides such as SiO 2 , TiO 2 and Al 2 O 3 , basic oxides such as Fe 2 O 3 , Cao, MgO, Na 2 O, K 2 O, and SO 3 . It is generally believed that more acidic oxides such as SiO 2 , TiO 2 and Al 2 O 3 can increase the fusion temperature of coal ash, while the content of basic oxides such as Fe 2 O 3 , Cao, MgO, Na 2 O and K 2 O will reduce the fusion of coal ash. but the fusion temperature of coal ash is ultimately determined by the minerals formed under high temperature [3] ,When a large amount of foreign substances are added, the fusion characteristics of coal ash will be affected and the slagging characteristics of coal ash will be changed.LI Ming et al. [4] found that with the increase of the proportion of sludge, the ash melting point decreased significantly , because of low temperature co-melting of minerals in coal ash sludge ash. Gao et al [5] found that the high content of hematite and anorthite in the sludge ash is the main reason for the decrease of the melting characteristic temperature of sludge/coal ash. Deng et al's [6] results show that Mullite with high temperature resistance and anorthite which can reduce the ash melting temperature can make the melting characteristics of sludge/coal ash change nonlinearly. In this paper, the effect of sludge on slagging characteristics of alkali rich coal was studied by combining experiment with theoretical calculation.

Introduction to the experimen 2.1 Equipment and Method
In this paper, the fusion characteristics of coal ash samples were measured by hr-4a microcomputer ash melting point tester which met the requirements of GB/T219-2008 "determination method of coal ash fusibility". The preparation method of ash sample is slow ashing method: firstly, the wet sludge is dried at 105 ℃ for 3 h, then the dried sludge or sludge pulverized coal mixture is slowly raised to 500 ℃ at normal temperature, then it is kept for 30 min, then the target temperature is reached at the heating rate of 10 ℃/min, and the ash sample is taken out and cooled to normal temperature in a drying dish, Finally, the ash melting characteristics are analyzed.

Ash Composition of Sludge and Coal
The ash composition analysis of selected coal and sludge is shown in Table 1. It can be seen from table 1 that the main components of ash in the selected sludge are Al 2 O 3 and SiO 2 .

Effect of sludge on coal ash fusion characteristics
The influence of sludge mixing ratio on coal ash fusion characteristics is shown in Fig. 1. It can be seen from table 1 and figure 1 that the deformation temperature(DT), softening temperature(ST) and flow temperature(FT) of coal ash decrease with the increase of sludge mixing proportion, which indicates that mixing sludge with low melting point can significantly reduce the fusion characteristic temperature of coal ash.It can be seen from Figure 1 that with the increase of sludge mixing ratio, the temperature difference of DT, ST and FT of coal ash also increases, but it is still in the short slag area.For the above phenomenon, some studies [7,8] think that the reason is that SiO 2 in coal ash mainly exists in the form of amorphous, which is easy to form vitreous material with no fixed melting point with other metal oxides and non-metallic oxides in coal ash. This kind of material gradually softens with the increase of temperature until it begins to flow. The higher the content of SiO 2 , the more amorphous vitreous body will be formed, which will lead to the temperature of ST and ft of coal ash However, SiO 2 accounts for nearly 50% of the sludge ash selected in this paper, so it will lead to this phenomenon.

Effect of main components of sludge (Al 2 O 3 and SiO 2 ) on coal ash fusion characteristics
The influence of sludge on the ash melting point will change due to the different composition of sludge and coal ash. Through literature review, it is found that most of the sludge components are SiO 2 and Al 2 O 3 . Therefore, this paper discusses the influence of single additive on coal ash fusion characteristics. The single additive is SiO 2 and kaolin (Al 2 O 3 ), and the proportion of raw coal: additive = (97:3, 94:6, 91:9). The influence of additives on the fusion characteristics of coal ash is shown in Fig. 2 and Fig. 3. It can be seen from Fig. 2 (a): with the increase of SiO 2 mixing proportion, DT, st and FT temperatures of coal ash decrease first and then rise, which indicates that mixing a small proportion of SiO 2 will reduce the fusion characteristic temperature of coal ash; when the proportion of SiO 2 reaches 9%, the temperature of ST and ft of coal ash will exceed 1500 ℃.It can be seen from Fig. 2 (b): the change trend of ash fusion characteristic temperature after adding Al 2 O 3 is similar to that after adding SiO 2 . The temperature of DT, st and ft of coal ash decreases first and then increases. The difference with SiO2 is that when the addition ratio of Al 2 O 3 reaches 6%, the fusion temperature of ash sample is close to the characteristic temperature value of raw coal ash sample The ash melting temperature can be improved when the addition ratio of Al 2 O 3 is more than 6%.In addition, the difference of St temperature between 6% and 9% Al 2 O 3 is only 25 ℃, which is different from that of SiO 2 . This is because when the addition ratio of Al 2 O 3 is 3%, the phase in the ash is mainly composed of anorthosite, Calc Huang feldspar and forsterite at high temperature, and the low-temperature eutectic formed by the three will lead to a sharp decrease in the fusion temperature of coal ash [9]. When the addition ratio of Al 2 O 3 reaches 6%, the Calc Huang feldspar in the ash will be transformed into anorthite, becoming the main material in the ash and playing a "skeleton" role in high-temperature melting [9] .Based on the results of this paper and related studies, it is suggested that the additive rich in Al 2 O 3 should be preferred.

Effect of sludge on slagging characteristics of coal ash
Many researchers [10,11] have studied the evaluation index of coal slagging characteristics to ensure the normal and safe operation of boilers. The commonly used indexes for predicting coal ash slagging at home and abroad mainly include slagging index "Sc", silica alumina ratio"SiO 2 /Al 2 O 3 ", alkali acid ratio "B/A", silicon ratio "G" and comprehensive discrimination index"Rz". Indexes for judging slagging tendency of coal ash are shown in Table 2. The indexes for predicting the slagging characteristics of coal ash listed in Table 2 are obtained by Chinese researchers according to the ash composition of a large number of coals and their practical application in power plant boilers. He et al. [10] used the optimal segmentation model to calculate the ash and slag characteristics of more than 250 kinds of coal in China, and obtained the slagging grade boundary value of each index.Although the E3S Web of Conferences 204, 01006 (2020) ICRTEG 2020 https://doi.org/10.1051/e3sconf/202020401006 slagging characteristics of coal have not been predicted completely and correctly by a single index, each of the above-mentioned items has considerable accuracy, especially the comprehensive judgment index R Z . The accuracy of R Z can reach 90%, which is obtained by the weighted average method based on the statistics of ash and slag characteristics of more than 90 kinds of coal by Chinese scholar Li Yongxing et al. [11] .
According to the ash composition analysis results of sludge and raw coal and referring to table 2, the slagging index calculation results of sludge and raw coal can be calculated, as shown in Table 3. It can be seen from table 3 that although the st temperature of the selected coal is higher, many slagging indexes are serious slagging, which is consistent with the results in the actual application process. For sludge, although the st temperature is less than 1300 ℃, the calculation results of many slagging indexes are slight slagging, which shows that it is onesided to judge the slagging characteristics of sludge and raw coal only by the st temperature of ash fusion.
Based on the analysis results of ash formation rate and ash composition of sludge, the components of coal ash mixed with different proportions of sludge are calculated, as shown in Table 4.  table 2 and table 4, the ash slagging index of the mixture can be calculated, as shown in Table 5.   3 and table 5 that the slagging characteristics will be improved after mixing sludge except for the ash melting temperature. Therefore, the slagging characteristics of this kind of coal can be improved by blending sludge or additives rich in silicon and aluminum compounds.