1. Introduction
The treatment and disposal of large amounts of sewage sludge are a major social problem in China today [
1,
2]. The total annual sludge production is increasing year by year with the acceleration of urbanization process. It is well-known that the incineration of sewage sludge has been a widely used solution for its treatment and disposal, especially in Europe, America, Japan, and other developed countries [
3,
4]. In 2016, the incineration disposal of municipal sludge in Germany accounted for 64.4%, in which the co-combustion of large coal power plants accounted for 26%, and the rest accounted for 10% [
5]. It makes sludge harmless, stable, and reduced. However, sludge in China has the characteristics of low volatile content and low heating value compared with other developed countries [
6,
7,
8]. The organic content of sludge in developed countries is approximately 60–70%, while the organic content of sludge from Chinese sewage plants is only 30–60% [
9]. In general, the self-sustaining combustion of sludge cannot be achieved when the moisture content is high, so it often needs to be co-burned with coal, natural gas, or other auxiliary fuels. In Germany, sludge is generally dried to 75–90% TS and then co-incinerated with circulating fluidized bed boiler or pulverized coal boiler [
10].
Sludge co-combustion disposal refers to the technology where sludge is mixed with coal, domestic garbage, biomass, and other high calorific value fuels in a certain proportion and then sent to the corresponding incineration equipment for combustion [
11]. Many researchers have conducted a lot of work on the co-combustion characteristics of sludge and coal by thermogravimetric analysis [
12,
13,
14,
15,
16,
17]. It has been reported that there is no obvious difference between the results of coal and mixtures with a low sludge mixing ratio (<10%). In this range of sludge mixing ratio, coal combustion is absolutely dominant during the co-combustion process. Zhang et al. [
18] concluded that the performance of the co-combustion of sewage sludge with coal was sometimes better than the single burning of coal by drop tube furnace experiments. Leckner, B et al. [
19] indicated that when sludge energy fractions were less than 25%, co-combustion could be carried out in a circulating fluidized bed plant without exceeding the EU or German emission limits, except for chlorine emissions, which that may have to be reduced by flue gas treatment.
Table 1 shows the discharge standards for the incinerator of a sludge incineration plant in China [
20]. The high content of N, S, Cl, P, and ash in sludge and the flue gas generated during its incineration contain a large amount of pollutants such as SO
X, NO
X, HCl, and fly ash particles, which may affect the existing incineration equipment and flue gas cleaning system in long-term co-combustion. Therefore many scholars have studied the pollutant emission characteristics of sludge combustion.
Shimazu et al. [
21] found that dried sludge combustion produced higher concentrations of N
2O than normal coal combustion, and that sludge with a higher water content produced lower concentrations of NO after combustion. It was assumed that the FeCl
3 added during sludge pretreatment oxidized to Fe
2O
3 during incineration, thereby catalyzing the production of NO
X. Zhang et al. [
22] concluded that NO and SO
2 were mainly produced at the initial stage of combustion and that the SO
2 emissions were reduced when the auxiliary fuel was anthracite. Sun et al. [
23] stated that NO
X emissions were due to the decomposition of proteins and aliphatic compounds, while SO
2 emissions were attributed to the decomposition of aromatic sulfur, FeS
2, and FeSO
4. Zhang et al. [
24] studied the co-combustion of dried wood sludge and anthracite and found that SO
2 and NO emissions decreased with increasing sludge content, and increased slightly with increasing temperature. Generally speaking, chlorine in sewage sludge is mainly released in the form of HCl and temperature corresponding to the HCl release rate, which is related to the temperature of the maximum CO
2 release rate. Oxygen has a positive effect on HCl release [
25].
Zhundong coal has the advantages of high volatile matter and low ash content, but the content of alkaline earth metals in the coal seams of Zhundong coal far exceeds that of other coal types [
26]. Guo et al. believed that the higher the temperature, the more active the chemical properties of the heavy metals Pb and Ni, and the higher the mole fraction in the gas phase. They also found that there was competition between the alkali metal elements and heavy metal elements in the fuel, of which Na was the most significant [
27].
Our study aimed at this problem by carrying out combustion experiments using a lab-scale fluidized bed furnace. The blended fuel was made of dried coal and sludge with a different sludge mixing ratio (≤20%). The temperature distribution of the furnace was observed and the emissions of the main gaseous pollutants (SO2, NOx, and dioxins) were measured during the combustion process. The heavy metal contents of fly ash and bottom ash were detected to later analyze the migration characteristics.
4. Conclusions
In this study, the co-combustion of coal and a small amount of sludge was conducted. (1) When the sludge mixing ratio was 0~20%, the furnace temperature distribution and combustion efficiency changed little. (2) When the sludge mixing ratio was lower than 20%, the emission of gaseous pollutants such as SO2, NOX and dioxin increased linearly with the sludge mixing ratio, but it could also meet the national standard for the pollution control of municipal solid waste incineration in China. (3) When the sludge mixing ratio reached 20%, due to the high chlorine content in the sludge, the discharge of PCDD/Fs was 0.3925 ng I-TEQ/Nm3, which was more than four times that of the pure coal test conditions. (4) Among the main heavy metals contained in sludge and coal, chromium and nickel were more likely to be enriched in fly ash; zinc content in the bottom ash was higher; and the concentration of Cu and Pb was not different to that of fly ash. (5) This research was based on the co-combustion of coal and sludge under the condition of coal-fired boilers, and the original emission control facilities in the flue of coal-fired boilers can help the pollutants meet the emission standards while saving on the costs of pollutant treatment. Therefore, according to the situation studied in this paper, the co-combustion of a small amount of sludge in coal-fired boilers is a feasible approach to sludge treatment and disposal in China.