Co-combustion Of Municipal Sludge Research Articles

Heavy metals behavior of co-combustion ash from sewage sludge and coal slime: Temperature and mixing ratio dependence, interactions and speciation

Co-combustion of municipal sludge (SS) and coal slime (CS) is an effective route to achieve the co-disposal of solid wastes. This work addresses the problem of heavy metal pollution in co-combustion ash, aiming to reveal the effect of interactions in SS-CS co-combustion on the behavior of heavy metals and the control mechanism of minerals using the inductively coupled plasma mass spectrometry (ICP-MS) technique and the Community Bureau of Reference (BCR) method. The residual rate of heavy metals in CS ash monotonically decreased with increasing combustion temperature, while the residual rate of heavy metals in SS tended to increase when it was burned at high temperatures. Increased temperature enhanced the effect of SS-CS interaction on heavy metals. The interaction significantly inhibited the volatilization of As, and the As residue of the blend containing 50% CS (5SS5CS) at 1000 °C was 14.9% higher than the theoretical value. Typical minerals in SS had significant effects on heavy metal migration in CS, and the effects of CaO and Fe2O3 on heavy metals were mainly due to chemical interactions, in which CaO fixed As in the form of Ca3(AsO4). Density functional theory calculations show that the optimal adsorption sites for both As and As2O3 on the CaO(001) surface were O-top sites, and the presence of O defect on the surface led to enhanced interaction of As2O3 with the surface. Co-combustion led to an increase in the residual fractions of heavy metals, but the exchangeable and acid soluble fraction of As in 5SS5CS was still as high as 19.3%, so the ecological impact of As should be considered in further utilization of the co-combustion ash.

Experimental and ReaxFF molecular dynamic study of NO emission during municipal sludge/coal co-combustion

In this work, the emission behavior of NO during the co-combustion of municipal sludge (MS) and Meihuajing bituminous coal (MHJ) was studied using experiments and ReaxFF molecular dynamics methods. Both experimental and ReaxFF MD results indicated that there has negative synergistic effect on NO emission during MS/MHJ co-combustion. In addition, there has positive synergistic effect on N2 and NO2 emission during MS/MHJ co-combustion. Using the atomic labeling method, it was observed that the yield of MS-NO decreased, while the yield of MHJ-NO was almost unchanged during co-combustion. Moreover, the yields of MS-N2 and MS-NO2 during co-combustion were increased. It indicated that MHJ has a significant effect on the MS-NO emission during co-combustion, while MS has little effect on the MHJ-NO emission. It attributed to the fact that MHJ consumed more oxygen, which resulted in more OH radicals generated during co-combustion, accelerating the cleavage of pyrrolic and pyridinic structures in MS to form more nitrogen-containing compounds. As combustion proceeds, the nitrogen-containing compounds from MS were gradually converted to HCN. Subsequently, HCN was oxidized to NCO, which increased the yield of MS-NCO during co-combustion. Then, the increase in MS-NCO during co-combustion promoted the reduction reaction between MS-NCO and MS-NO, increasing MS-N2 yield. In addition, the increased yield of O radicals also promoted the oxidation of MS-NO to MS-NO2 during co-combustion.

The synergistic effect during co-combustion of municipal sludge and coal: Experimental and ReaxFF molecular dynamic study

In this work, the behaviors of municipal sludge/coal co-combustion were investigated by experiments and ReaxFF molecular dynamic simulations. The molecular models of coal and sludge were constructed based on experimental characterization. The characteristics of the separately combustion of sludge and coal from ReaxFF MD simulations were consistent with that from thermostatic thermogravimetric experiments. The synergistic effect on the co-combustion characteristics were investigated via performing sludge/coal co-combustion simulations under various conditions. The results showed that the sludge/coal co-combustion has positive synergistic effects on the co-combustion characteristics such as combustion rate, consumption of O2, and yield of CO2. By using the atomic labeling method, it was found that the combustion of sludge was inhibited while the combustion of coal was promoted during co-combustion process. The carbonaceous gas was found to be an important intermediate product in the generation of CO2. The synergistic effect promoted coal combustion because some ·OH radicals produced by the sludge attacked the aromatic ring structure in coal, which accelerated the formation of carbonaceous gas during sludge/coal co-combustion. This work provided new insight into the behaviors of municipal sludge/coal co-combustion.

Co-combustion of Municipal Sludge and Zhundong Coal: Ash Formation Characteristics

ABSTRACTZhundong coal (ZD), which has a high content of Na and low ash fusion temperature (AFT), causes a serious problem of slagging in boilers. P-rich municipal sludge has suitable properties for managing problems with alkali metals, but its insufficient utilization makes sludge management harder. Thus, co-combustion of municipal sludge and ZD may be a suitable approach to solve the problems. Different proportions of sludge were added to ZD at 900°C in different contents of O2 in a laboratory-scale tube furnace experimental system. The transformation of the minerals, the AFTs of residual ashes, and changes of the sodium retention ratio were investigated, respectively. The test results indicate that the ash fusibility of ZD and the sodium retention ratio can be improved because of the generated high-melting-point phosphates, such as Ca2.71Mg0.29(PO4)2, Ca5(PO4)3(OH), Ca9Fe(PO4)7, and Ca9MgNa(PO4)7. With increases in the proportion of sludge in blended samples, the AFTs increase, but the extent of the increase tends to be reduced. Modified sludge with a lower SiO2/Al2O3 ratio is more favorable for the improvement of the AFT. It is observed that 20% oxygen is the most suitable environment of those tested for co-combustion, avoiding the disadvantages of excessively high or low oxygen content.