Effect of Metal Oxides on Combustion Characteristics of Excess Sludge

. In this paper, it was studied that the effects of heating rate and four metal oxide on the combustion characteristics of excess sludge, such as CaO, Fe 2 O 3 , MgO and Al 2 O 3 , and the combustion kinetic parameters of sludge were obtained based on Coats-Redfern model. This result indicated that the ignition of sludge can be restricted with the increase of heating rate, but it was conducive to burnout, and the increasing about the flammability, stability and comprehensive combustion characteristic index of sludge. The addition of Fe 2 O 3 and Al 2 O 3 could improve the ignition performance of sludge to a certain extent, which was most obvious at the heating rate of 10 and 20 °C/min, respectively. Furthermore, the existence of Fe 2 O 3 , MgO and Al 2 O 3 can improve the burnout performance of sludge, represented by MgO. Under the condition of low heating rate, the presence of Fe 2 O 3 and MgO was beneficial to improve the comprehensive combustion characteristics of sludge. According to the distribution of combustion kinetics parameters, it was showed an increasing trend about the activation energy of sludge with the increase of heating rate at the range of 374.88-575.61 °C, which was also unfavorable to the combustion of fixed carbon. At the same heating rate, the activation energy decreased by adding Fe 2 O 3 and Al 2 O 3 at the range of 148.40-374.88 °C, which was beneficial to the release of volatiles and the combustion reaction, while the activation energy at the range of 374.88-575.61 °C decreased after adding CaO, which promoted the combustion of fixed carbon.


Introduction
With the expansion of sewage treatment volume and treatment scale, the annual production of excess sludge in China has also increased significantly. The composition of sludge is complex, containing a large number of heavy metals, pathogenic bacteria and pathogenic bacteria [1] . It can lead to serious secondary pollution if the sludge was not treated rationally. In recent years, sludge combustion has been widely used in China and abroad. In 2015, the amount of sludge combustion reached 1.2882 million tons in the EU, and it accounted for 41.3 %, while this value was as high as 66.9 % in Japan [2] . Although the proportion of sludge combustion in China was still low, numerous engineering practices about sludge combustion, coal-fired power plants and domestic waste incineration plants have been gradually carried out in various parts of China, which shows that sludge combustion has a wide application prospect.
Based upon the increasingly serious environmental problems such as water pollution and air pollution in China, as long as sludge combustion was involved, the concentration of polluting gases produced by combustion must be controlled within the prescribed limit. Numerous studies have shown that the instantaneous and average emissions of NO, N 2 O and SO 2 from sludge combustion in all atmospheres are much higher than those from bituminous coal combustion [3] . Consequently, many scholars have carried out research in this area to look for ways to reduce sludge combustion pollutant emissions. Chen et al. found that kaolin can greatly reduce the release of NO X , SO 2 can be fixed in CaSO 4 after reacting with CaO, and sulfur can be effectively fixed in the bottom ash at 900 °C [4] . Hao et al. investigated that sludge bio-drying can reduce pollutant emissions in subsequent combustion [5] .
Ascribe to the abundant source of excess sludge, the calorific value distribution of sludge was different significantly, different methods should be adopted in terms of different conditions in sludge combustion, which can be divided into separate and mixed combustion. In most cases, sludge can be directly combustion after pressure filtration and drying process. Nevertheless, there were still many unknown details about the combustion process of sludge samples after different treatment process. Many researchers have carried out a lot of research on improving the combustion characteristics of sludge and obtained many breakthroughs. Gao et al. proposed that the increasing of the heating rate can reduce the activation energy of the sludge combustion, thus promoted this process [6] . Liu et al. showed that the addition of FeCl 3 and CaO can improve the combustion performance of sludge, and the combustion performance of sludge was improved more significantly with the increase of the amounts about exogenous additives [7] . Thereby, thermogravimetric analysis was used to explore the effects of metal oxides and heating rate on sludge combustion. The combustion characteristics and combustion kinetics of sludge were evaluated based on the data of each sample, which provided a theoretical basis for improving the sludge combustion characteristics.

Materials
Excess sludge was collected from a municipal wastewater treatment plant in Changsha, China, and storied at 4 o C before the experiments. This plant treated 120,000 m 3 /d of municipal wastewater using the Anaerobic-Anoxic-Oxic process. The proximate analysis of the sludge was illustrated in Table 1, which was included moisture (M), volatile (V), ash (A), fixed carbon (FC) and obtained by using the muffle furnace. The specific steps of this analysis mainly refer to the research of Tan et al [8] .

Sludge combustion experiments
Before the combustion process, sludge samples were crushed into >100 mesh particles after completely dried at 105 o C in electric drying oven. In this study, the excess sludge was mixed with CaO, Fe 2 O 3 , MgO and Al 2 O 3 respectively, and the details of which were listed in Table  2.
Combustion experiments of all samples were carried out in a thermal gravimetric analyzer (HCT-2, Hengjiu, China) at the atmosphere of N 2 . Among of them, the five samples were controlled at 10.0 ± 0.5 mg, placed in alumina crucibles and heated from 35 o C to 800 o C at the heating rate of 5, 15 and 20 o C /min separately. Additionally, the variation of weight loss of samples and temperature were recorded in thermogravimetric (TG) and derivative thermogravimetric (DTG) formats. In order to further investigating the mechanism characteristics of five samples in combustion, some parameters (such as flammability, combustion stability and integrated combustion characteristic) and the Coats-Redfern (CR) model was used to evaluate the combustion performance and calculate the kinetic parameters, respectively. The calculation steps of combustion parameters were referred to Deng et al [9] .

Statistical method
The generation of plots and statistical analysis was performed with OriginPro 9.0 (Originlab, USA), and Excel 2019 (Microsoft, USA) was used to determine the distribution of combustion data about each sample.

Combustion curve of sludge sample
The TG-DTG characteristic curve of excess sludge at the heating rate of 15 °C/min were shown in Figure 1, which mainly included the evaporation of moisture (Ⅰ), the releasing of volatile (Ⅱ), the discomposing of fixed carbon (Ⅲ) and the burnout of residual component (Ⅳ) [10] .Taking the sample of S group as an example, the four stages of sludge combustion were located in the temperature ranges of 100-148.40 °C, 148.40-374.88 °C, 374.88-575.61 °C and 575.61-800 °C, respectively. Among of them, there was a small weight loss peak in the stage of Ⅰ, and the weight loss rate is 7.51%. In the stage of Ⅱ, there was a significant peak, the weight loss range is large and the combustion rate (dw/dt) reaches the maximum at 280.42 °C, which indicated that the volatile content of excess sludge was at a relatively high level. The weight loss rate of this stage was 33.30%, and the proportion of weight loss reached 65.03%. There was also an obvious weight loss peak in the stage of Ⅲ . and the total weight loss of stage III and stage II can reach 85.45% of the total weight loss of sludge, indicating that these two stages were the main stages of sludge combustion. With the gradual progress of sludge combustion, the TG curve of sludge in stage IV is almost at a level, illustrating that the sludge was burned out. The weight loss rate of the whole combustion of sludge was 62.76 %, which was consistent with the results of proximate analysis.

Effect of metal oxides on sludge combustion
As illustrated in Figure 2, the TG curves about the sample of S1, S2, S3 and S4 were located above the sample of S, which was attributed to the addition of different metal oxides. The existence of these materials can result in an increase in the content of ash and a reduction in the combustible components inner the sludge samples. In terms of the distribution of the DTG characteristic curves, the sample of S, S2, S3 and S4 had no significant weight loss peak at stage. Nevertheless, there was a great difference in the sample of S1. Two weight loss peaks appeared in the stage IV of S1, which was closely related to the improvement effect of 10% CaO addition on the inorganic composition of sludge. Moreover, the addition of CaO can form CaCO 3 when combined with CO 2 , which can be decomposed into CaO and CO 2 at 600 °C, thus resulting in weight loss peak [11] . The TG curves of S2 almost did not fluctuate in stage IV, and there was no obvious weight loss peak. But a small peak appeared in the sample S2 at this stage when the heating rate was 20 °C/min, which may be owing to the decomposition of Fe 3 O 4 .

Effect of heating rate on sludge combustion
The TG and DTG characteristic curves of different samples were exhibited in Figure 3 under different heating rates. With the increase of heating rate, the weight loss of sludge decreased slightly and the TG curve shifts to the zone at high temperature, thus leading to the emergence of combustion lag, which was mainly due to the fact that the heating rate was at a higher level, affecting the heat transfer of sludge sample and the decomposing of organic matter inner sludge adversely [12] . At the same time, it can be seen from the DTG curves that there were three weight loss peaks in the sludge combustion, among which the second peak was the most obvious. As the heating rate increased, the last two weight loss peaks become more significant, which indicated that the reaction becomes more and more intense with the increase of the heating rate.
As for the samples mixed with metal oxides, the weight loss rate of S1 was the largest at the heating rate of 15 °C/min, which illustrated that the combustion process of combustibles inner S1 was the most sufficient at the 15 °C/min. From the DTG curves, it can be observed that the S1 had five weight loss peaks during the combustion process. Among of them, the four groups of peaks in the stage Ⅰ-Ⅲ were more obvious, with the second peak as a significant representative. Similar to the sample of S, the weight loss peak of S1 became more and more obvious with the increase of heating rate. In particular, the TG characteristic curves of sample S2 almost coincided at different heating rates, and the combustion lag was not obvious. Contrary to the variation trend of S1, the weight loss rate of S3 was the lowest at 15 °C/min, while the weight loss rates at 10 °C/min and 20 °C/min were similar. The overall trend of the TG curves of S4 was similar to that of S1. As the heating rate increasing, the weight loss of the sample can decrease and the value of it was at a high level. This was owing to the he adhesion of Al 2 O 3 on the fuel particles under high heating rate, which limited the sludge combustion process. It can be seen from the DTG characteristic curve that the rules of S2, S3 and S4 at different heating rates were similar to those of the sample of S.

Combustion characteristics of sludge samples
In the sludge combustion process, the ignition temperature (T i ) was an important index of the ignition performance of the sample. The lower the Ti, the better the combustion performance of the sludge. In addition, the burnout temperature (T h ), the maximum combustion rate ((dw/dt) max ), and Corresponding temperature on the maximum combustion rate (T max ) were also the important parameters for the evaluation of sludge combustion, which can be obtained by the methods involved in the study of Namkung et al [13] . As can be seen in Figure 4, the T i of the S1 was the lowest at the heating rate is 15 °C/min, which had intimate relation with the effect from the CaO. At the same heating rate, the T i of S2 at the heating rate of 10 and 20 °C/min was 2.06 °C and 10.77 °C lower than that of S, respectively, which indicated that the addition of Fe 2 O 3 improved the ignition performance of the sludge. Similarly, the T i of S4 was lower than that of S, and the heating rate of 10 °C/min was the most obvious, which decreased by 4.73 °C, reflecting the positive effect of Al 2 O 3 on the ignition performance of excess sludge.
In addition, the (dw/dt) max of the five samples improved with the increase of heating rate, which indicated that the enhancement of sludge combustion reaction. Among the three heating rates, the T max of the S, S3 and S4 were the lowest at 10 °C/min, indicating that the sample was easier to reach the maximum combustion rate at low heating rate. The T max of S1 was the largest, indicating that the addition of CaO was not conducive to combustion after ignition. At the same heating rate, the maximum combustion rate of sludge with metal oxides was smaller than that of sludge, and the corresponding temperature was higher than that of sludge, which further reflected that these metal oxides were not conducive to the subsequent combustion after ignition.
As for about combustion performance parameters of five samples, the increase of the heating rate can produce positive effect on sludge combustion process in accordance with the distribution of the improvement of three index of sludge. Under the same heating rate, the flammability index and combustion stability index of S were the largest. However, the addition of metal oxides cannot have a positive effect on the both, which may be due to the excessive addition of metal oxides. It should be noted that the comprehensive combustion characteristic index of S2 and S3 was slightly larger than that of S at the condition of heating rate of 10 °C/min, indicating that the addition of Fe 2 O 3 and MgO at low heating rate can improve the comprehensive combustion characteristics of sludge to a certain extent.

Combustion kinetics analysis
According to the higher combustion rate in stage II and III of the DTG curves in all samples, CR model was further used to analyze the mechanism of sludge combustion in the temperature range corresponded to the sludge samples at the two stages of combustion. As shown in Table 3, the correlation coefficient R 2 of linear fitting was between 0.9070 and 0.9944when the reaction order n=1 was taken in the second stage of combustion of each sample and the reaction order n=2 is taken in the third stage, which indicated that the reasonable of the determined reaction orders. Based upon the determined reaction order, the fitting equation and the combustion activation energy (E) can be further obtained. Comparing the E about stage II and III of the samples with different heating rates, the E of the second stage of S and S1 was the lowest at 15 °C/min, which indicated that the ignition performance of sample S and S1 was the best at this heating rate. Meanwhile, the E of S2, S3 and S4 in the stage II was the lowest under the condition 10 °C/min, which was consistent with the distribution of ignition temperature. Among them, the E of S in stage III enhanced with the increase of heating rate. This was because the high heating rate, which lead to the decrease of temperature gradient between the inner and outer layers of carbon particles inner sample, thus slowing down the heat diffusion rate. The variation of E from S4 at the stage III was basically the same as that of sample S. At high heating rate, the E was higher than that at low heating rate. For the E of different samples, the E of S1 in stage III was lower than that of S under the same heating rate. This was because the decompose of some organic matter resulted by CaO addition [14] , which can promote the combustion reaction of volatile matter and fixed carbon in stage III. The E of the S2 and S4 at the stage II was lower than that of the S, which indicated that the addition of Fe 2 O 3 and Al 2 O 3 was beneficial to the releasing of volatiles and the combustion reaction in the stage II. At the heating rate of 20 °C/min, the E of sample S2 at the stage III was lower than that of sample S, which further reflected the improvement of the combustion reaction of fixed carbon by the addition of Fe 2 O 3 at this heating rate.

Conclusion
In this study, CaO, Fe 2 O 3 , MgO and Al 2 O 3 were mixed with the excess sludge. And thermogravimetric experiments were carried out at heating rates of 10, 15 and 20 °C/min to further determine the combustion characteristic parameters and kinetic parameters of each sludge sample. The main conclusions were as follows: (1) With the increase of heating rate, the weight loss peak became more obvious. Although it was not conducive to sludge ignition, it promoted the flammability, combustion stability and integrated combustion characteristic index of sludge combustion. In addition, the TG characteristic curve shifts to the high temperature zone, and the weight loss of the sample decreases significantly. This was because the adhesion of Al 2 O 3 on sludge surface at high heating rate, which restricted the sludge combustion.
(2) The weight loss rate of sludge decreased after adding metal oxides, which was ascribe to the increasing of the ash content and the reduction about the combustible components inner sludge sample. Under the heating rate of 10 and 20 °C/min, the T i of S2 was 2.06 °C and 10.77 °C lower than that of S, respectively, which indicated that the addition of Fe 2 O 3 improved the ignition performance of sludge. Furthermore, the intervention of Al 2 O 3 improved the ignition performance of sludge, especially at the 10 °C/min.
(3) Under the same heating rate, the (dw/dt) max of sludge after the addition of metal oxides was smaller than that of the sample of S, and its corresponding temperature was also at a higher level, which indicated that the presence of metal oxides was not conducive to the subsequent combustion after ignition. Additionally, adding CaO was not conducive to burnout from the increasing of the T h . But the presence of Fe 2 O 3 , MgO and Al 2 O 3 was beneficial to the reduction of T h , and the sludge with MgO was the most significant, which indicated that the addition of MgO can improve the burn-out of excess sludge.
(4) The sample of S had the best flammability and stability index, but the addition of metal oxides cannot have a significant positive effect on the both. The integrated combustion characteristic index of S2 and S3 was slightly larger than that of sample S at the 10 °C/min, which illustrated that the addition of Fe 2 O 3 and MgO at low heating rate was beneficial to improve the comprehensive combustion characteristics of sludge. the E of S2 and S4 at the stage II was lower than that of sample S, which indicated that the addition of Fe 2 O 3 and Al 2 O 3 was conducive to the releasing of volatiles in the stage II.