Determining the impacts of fermentative bacteria on wollastonite dissolution kinetics.

Silicate minerals can be a source of calcium and alkalinity, enabling CO2 sequestration in the form of carbonates. For this to occur, the mineral needs to be first dissolved in an acidifying process such as the biological process of anaerobic fermentation. In the present study, the main factors which govern the dissolution process of an alkaline silicate mineral (wollastonite, CaSiO3) in an anaerobic fermentation process were determined. Wollastonite dissolution kinetics was measured in a series of chemical batch experiments in order to be able to estimate the required amount of alkaline silicate that can neutralize the acidifying fermentation process. An anaerobic fermentation of glucose with wollastonite as the neutralizing agent was consequently performed in a fed-batch reactor. Results of this experiment were compared with an abiotic (control) fed-batch reactor in which the fermentation products (i.e. organic acids and alcohols) were externally supplied to the system at comparable rates and proportions, in order to provide chemical conditions similar to those during the biotic (fermentation) experiment. This procedure enabled us to determine whether dissolution of wollastonite was solely enhanced by production of organic acids or whether there were other impacts that fermentative bacteria could have on the mineral dissolution rate. The established pH profiles, which were the direct indicator of the dissolution rate, were comparable in both experiments suggesting that the mineral dissolution rate was mostly influenced by the quantity of the organic acids produced.

Thermal behaviour of ESP ash from municipal solid waste incinerators.

Stricter environmental regulations demand safer treatment and disposal of incinerator fly ashes. So far no sound technology or a process is available for a sustainable and ecological treatment of the waste incineration ashes, and only partial treatment is practised for temporary and short-term solutions. New processes and technology need to be developed for comprehensive utilization and detoxification of the municipal solid waste (MSW) incinerator residues. To explore the efficiency of thermal stabilisation and controlled vitrification, the thermal behaviour of electrostatic precipitator (ESP) ash was investigated under controlled conditions. The reaction stages are identified with the initial moisture removal, volatilization, melting and slag formation. At the temperature higher than 1100 degrees C, the ESP ashes have a quicker weight loss, and the total weight loss reaches up to 52%, higher than the boiler ash. At 1400 degrees C a salt layer and a homogeneous glassy slag were formed. The effect of thermal treatment on the leaching characteristics of various elements in the ESP ash was evaluated with the availability-leaching test. The leaching values of the vitrified slag are significantly lowered than that of the original ash.

Thermal treatment and vitrification of boiler ash from a municipal solid waste incinerator.

Boiler ash generated from municipal solid waste (MSW) incinerators is usually classified as hazardous materials and requires special disposal. In the present study, the boiler ash was characterized for the chemical compositions, morphology and microstructure. The thermal chemical behavior during ash heating was investigated with thermal balance. Vitrification of the ash was conducted at a temperature of 1400 degrees C in order to generate a stable silicate slag, and the formed slag was examined with chemical and mineralogical analyses. The effect of vitrification on the leaching characteristics of various elements in the ash was evaluated with acid leaching. The study shows that the boiler ash as a heterogeneous fine powder contains mainly silicate, carbonate, sulfates, chlorides, and residues of organic materials and heavy metal compounds. At elevated temperatures, the boiler ash goes through the initial moisture removal, volatilization, decomposition, sintering, melting, and slag formation. At 1400 degrees C a thin layer of salt melt and a homogeneous glassy slag was formed. The experimental results indicate that leaching values of the vitrified slag are significantly reduced compared to the original boiler ash, and the vitrification could be an interesting alternative for a safer disposal of the boiler ash. Ash compacting, e.g., pelletizing can reduce volatilization and weight loss by about 50%, and would be a good option for the feed preparation before vitrification.

Vitrification of bottom ash from a municipal solid waste incinerator.

During incineration of municipal solid waste (MSW), various environmentally harmful elements and heavy metals are liberated either into bottom ash, or carried away with the off-gases and subsequently trapped in fly-ash. If these minor but harmful elements are not properly isolated and immobilized, it can lead to secondary environmental pollution to the air, soil and water. The stricter environmental regulations to be implemented in the near future in The Netherlands require a higher immobilization efficiency of the bottom ash treatment. In the present study, MSW incinerator bottom ash was vitrified at higher temperatures and the slag formed and metal recovered were examined. The behaviour of soluble elements that remain in the slag is evaluated by standard leaching test. The results obtained can provide a valuable route to treat the ashes from incinerators, and to make recycling and more efficient utilization of the bottom ash possible.

Understanding of hazardous waste incineration through computational fluid-dynamics simulation.

Rotary kiln incinerators are widely used in the incineration of hazardous wastes of various types. However, the complex transport and chemical processes within the kiln system are still not well understood. The complete destruction of hazardous compounds depends very much on gas mixing behavior of different air and waste streams, the distribution of gas temperature and residence time within the kiln and the secondary combustion chamber (SCC). Due to large variations of waste types and difficulties in feed characterization (physical, chemical and thermal properties), the incineration process meets great challenges in a smooth operation, with substantial fluctuations of gas temperatures within the system. The temperature fluctuations lead to uncertainties in the process chemistry and difficulties in emission control. The newly enforced regulations from the European Union with stricter emission levels require a better understanding of the incineration process and improved process control for lower emissions and a better environmental impact. In order to get better understanding of the incineration process within the rotary kiln system, research was carried out to study the kiln behavior in relation to better process control. One of the focuses was on the process simulation by using Computational Fluid-dynamics (CFD) to characterize gas flow, temperature distribution and waste combustion in the rotary kiln incinerator. Temperature measurement of the operating rotary kiln incinerator at AVR-Chemie, located at the Rotterdam harbor in The Netherlands, was conducted to validate the CFD model and to provide the information to kiln operators at AVR. This paper will address the environmental issues related to the hazardous waste incineration, and summarize the results from the current research project for the simulation of gas flow and mixing, combustion heat transfer, and new ideas to use CFD simulation results for process control of an incineration plant.