Melting of municipal solid waste incinerator fly ash by waste-derived thermite reaction

This work describes a novel approach for melting municipal solid waste incinerator (MSWI) fly ash, based on self-propagating reactions, by using energy-efficient simulated waste-derived thermite. The self-propagating characteristics, the properties of the recycled alloy and slag and the partitioning of heavy metals during the process are also studied. Experimental results demonstrate that the mix ratio of fly ash to the starting mixture of less than 30% supports the development of the self-propagating reaction with a melting temperature of 1350-2200 degrees C. Furthermore, metallic iron (or alloy) and the slag were retrieved after activation of the thermite reactions among the starting mixtures. It was noted that more than 91wt.% of iron was retrieved as alloy and the rest of non-reductive oxides as slag. During the thermite reactions, the partition of heavy metals to the SFA and flue gas varied with the characteristics of the target metals: Cd was mainly partitioned to flue gas (75-82%), and partition slightly increased with the increasing fly ash ratio; Pb and Zn, were mainly partitioned to the SFA, and the partition increased with increasing fly ash ratio; Cu was partitioned to the SFA (18-31%) and was not found in the flue gas; and moreover stable Cr and Ni were not identified in both the SFA and flue gas. On the other hand, the determined TCLP leaching concentrations were all well within the current regulatory thresholds, despite the various FA ratios. This suggests that the vitrified fly ash samples were environmental safe in heavy metal leaching. The results of this study suggested that melting of municipal solid waste incinerator fly ash by waste-derived thermite reactions was a feasible approach not only energy-beneficial but also environmental-safe.     

Emissions investigation for a novel medical waste incinerator

Medical waste constitutes one of the waste streams that should be dealt with special priority due to its potential negative impact on public health and on the environment. Incineration is a process that is widely used for the treatment of medical waste. However, self-supporting combustion of medical waste cannot avoid releasing many hazardous pollutants into our environment. The most favored solutions are firing additional fuels of high calorific value and direct purification by air pollution control devices (APCD). This process entails not only large first time investment but also an increase in the operation cost. A novel incinerator is proposed for better utilization of energy of the incineration process. Its originality is essentially due to combining a feeder, a rotary grate, a cylindrical gasifier and a “coaxial” secondary combustion chamber into a unique unit. The structure of the incinerator as well as the principle of the incineration process is presented in this paper. A full-scale trial of the novel incinerator with APCD was carried out from March to May 2008 to investigate how the distinct configuration influenced the incineration process. Data on PM, CO, NO_X, O₂ were recorded by a continuous emission monitoring system during the study period. Heavy metals and PCCD/Fs were also sampled and measured. Measuring results were compared with the China and U.S. EPA guidelines. The concentrations of contaminants were below their respective limits in emission control standards. Results from testing the novel medical waste incinerator confirmed that this technology has a good suitability for neutralization of medical wastes and purification of flue gases.     

Influence of flue gas SO2 on the toxicity of heavy metals in municipal solid waste incinerator fly ash after accelerated carbonation stabilization

The influence of CO₂ content and SO₂ presence on the leaching toxicity of heavy metals in municipal solid waste incinerator (MSWI) fly ash was studied by examining the carbonation reaction of MSWI fly ash with different combinations of simulated incineration flue gases. Compared with raw ash, the leaching solution pH of carbonated ash decreased by almost 1 unit and the leaching concentrations of heavy metals were generally lower, with that of Pb decreasing from 19.45 mg/L (raw ash) to 4.08 mg/L (1# carbonated ash). The presence of SO₂ in the incineration flue gas increased the leaching concentrations of heavy metals from the fly ash to different extents after the carbonation stabilization reaction. The pH of the leaching solution was the main factor influencing the leaching concentrations of heavy metals. The increase in buffer capacity with the pH of carbonated ash caused an increase in heavy metal stability after the carbonation reaction. Accelerated carbonation stabilization of MSWI fly ash could reduce its long-term leaching concentrations (toxicity) of Cu, Pb, Se, and Zn. The leaching concentrations of heavy metals from carbonated ash also likely had better long-term stability than those from raw ash. The presence of SO₂ in the incineration flue gas increased the proportion of exchangeable state species of heavy metals; slightly increased the long-term leaching toxicity of Cu, Pb, Se, and Zn; and reduced the long-term stability of these metals in the fly ash after the carbonation reaction.