Social development has led to various issues related to fossil fuels such as resource shortages and environmental pollution [
1]. With economic development, the amount of sewage discharge is increasing; municipal sludge (MS), as a secondary pollutant, has not been treated over time, resulting in environmental pollution. Incineration can be used to produce energy from MS while it is converted into harmless materials, therefore, it could be stabilized, reduced, and recycled after incineration [
2]. China shows high potential for biomass energy utilization [
3]. The biomass produced every year is equivalent to hundreds of millions of tons of standard coal. Therefore, biomass energy is an important factor in diversifying a country’s energy structure [
4]. Corn straw (CS) has certain representativeness and rich yield. Biomass is not only an important renewable resource but is also carbon-neutral, which is of great significance for carbon neutralization [
5].
The development of biomass combustion technology is important for achieving the sustainable development of energy, the environment, and the economy. Single biomass combustion has various limitations, such as low calorific value, difficulty to burn, and ease of slag. Mixed burning may improve the combustion characteristics of biomass for energy production [
6]. Because biomass and MS have different basic characteristics, their mixed-combustion properties differ from those of the primary materials [
7].
At present, relevant scholars have conducted a great deal of research work on mixed-combustion behaviors, pollutant-releasing characteristics, and reaction kinetics of biomass, coal, and municipal solid waste [
8,
9]. Wang et al. [
10] investigated the combustion behaviors of biomass, coal, and their mixtures via the thermogravimetric (TG) method. The results showed that both the ignition and burnout temperatures of the blends decreased as the biomass mixing ratio increased, while the ignition and flammability indexes increased. Qian et al. [
11] explored the feasibility of electricity production with low emissions during the poultry litter and natural gas co-combustion process using a lab-scale advanced swirling fluidized bed combustion system. It was found that the emissions of SO
2 and NO
x could be reduced under appropriate conditions and also confirmed that poultry manure can be regarded as an energy source. Ryu et al. [
12] studied the fixed-bed combustion of biomass samples with different particle types and properties. The results showed that the fuel type, equivalence ratio, and particle size were all significant factors impacting the mixed-combustion process. Wang et al. [
13] conducted coal/biomass co-combustion experiments in a 0.1 MW oxy-fuel circulating fluidized-bed combustor and found that the mixed-fuel combustion of coal/biomass is able to realize negative CO
2 emission. Qin et al. [
14] conducted combustion experiments of oil shale sludge and semicoke on TG to study the mechanism of co-combustion involved. The synergistic effect of sludge and semicoke was assessed using the interaction coefficient and the relative error of the mean square root. The results show that, during combustion, the synergistic behavior is different according to the variation of heating rates, as well as the mixing proportion. Yin et al. [
15] studied the combustion kinetics of corn straw, papermaking sludge, and their mixtures via TG analysis. The comprehensive combustion characteristic index decreases with an increase in the mixing ratio of paper sludge in the blends and rises with an increased heating rate. Significant interactions occur in the mixed-combustion process. Wang et al. [
16] carried out a mixed-combustion test of biomass and coal char at different ratios in a high-temperature thermogravimetric analyzer. The results show that with the increase in the biomass mixing ratio, both the ignition and combustion temperatures of the mixed fuel decrease, and the maximum combustion rate, average combustion rate, and comprehensive combustion characteristic index increase. The activation energy of the low-temperature section rises, and the activation energy of the high-temperature section declines continuously. Tian and Liao [
17] examined a mixed fuel of coal, low-quality fuel oil shale, and high-sulfur fuel petroleum coke using the TG-differential thermal analysis method and differential subtraction methods. The mixed-combustion characteristic curve and mechanism were analyzed, and various burning parameters and combustion kinetic parameters of the sample were calculated. At an appropriate mixing ratio of coal, oil shale, and petroleum coke, the mixed-combustion characteristics were better than those of oil shale or petroleum coke alone. Dai et al. [
18] used a TG analyzer to evaluate the combustion performance and dynamic characteristics of mixed samples of coal gangue and coal in different mass ratios. As the proportion of coal in the mixed sample increased, the ignition temperature and burnout temperature were reduced, the comprehensive combustion characteristic index increased, and the ignition and combustion characteristics were improved.
The above results show that mixed combustion of some combustibles may improve fuel performance. However, the mixed-combustion characteristics and reaction kinetics of MS and CS have not been widely examined, and recent studies of mixed combustion have mostly used the TG analysis method. The reaction is greatly affected by heat and mass transfer, and the intrinsic reaction process cannot be realized, leading to deviations from the calculation results of kinetic parameters. Therefore, in this study, we investigated the mixed-combustion process of CS and MS at different mixing ratios using a micro-fluidized bed reactor coupled with a process mass spectrometer, which can greatly reduce the reaction delay caused by product diffusion during the reaction; that is, the reaction tends to be dynamic, and thus intrinsic kinetic parameters can be obtained [
19]. The effects of temperature and the blending ratio on carbon conversion and the reaction rate were analyzed. Isothermal kinetics were used to calculate the kinetic parameters, and the best blending ratio of CS and MS was also determined.