Gasification reduces volume of sludge considerably but still the ash and product gas may contain the heavy metals and still it is as dangerous as direct use as fertilizer or landfill.
Heavy metals have normally density more than 5 or 6 gms/cc and most remain in solid form. Only a few metalloids like arsenic may become reduced in anaerobic conditions of the soil to arsine (ASH3)gas that may release during gassification . In absence of these ,probably, this process does not emit no other heavy metals in the atmosphere.
Dear Bijesh,
A global treatment of Step sludge by a single line currently does not seem feasible and it is therefore interesting to develop new processes to be able to best adapt to each purification unit. Gasification processes could be proposed as additional processes. Other advantages are regularly advanced in favor of gasification.
- production of a main by-product: a readily combustible synthesis gas
- significant reduction in sludge volume
- the use of high temperatures and / or a reducing environment associated with an inexpensive system keeps gas emissions subject to strict standards below the legal limits for dioxides and furans
- the thermal destruction of viruses and other pathogenic organisms. It requires a residence time of less than one second at 850 ° C. This duration is compatible with this type of process.
- the trapping of heavy metals in vitrified ashes and consequently more difficult to leach out than in the incineration traps
- an optimized energy balance. The possibility of upgrading a biological loop without transport, without special treatment, without storage contributes to the minimization of overall operating costs.
In the case of sewage sludge, it might be desirable to treat this waste with the help of a gasification unit based at the heart of the station, producing all or part of the energy required for the treatment of this waste.
Regards
Bachir ACHOUR
Phani Bhusan Ghosh sir, my doubt was regarding the ash remain after gasification still contain heavy metals. So the danger may still remain.
Dear Bachir Achour Sir, thank you so much for your reply. So, the trapping of heavy metals in ash makes it safer. And the high density makes it safer from being gasified and mixed with the product gas.
Dear Bijesh Raveendra ,
The gasification of feedstock contaminated with heavy metals is quite complicated in terms of the production of dangerous streams. The heavy metal concentration in the ashes is usually higher than in the feedstock, so that the resulting ash sometimes cannot be landfilled, while the original feedstock can. So far, the only way gasification seems to be a good remedy for heavy metal contaminated materials, is when plasma gasification is used. In this case, the ash is vitrified due to the high temperatures, and the heavy metals are kept trapped into a stone-like material which stops the leaching process. The product gas can also contain metals depending on the process temperature. At high temperatures, Pb, Cd, As, Hg and Zn vaporize. Therefore gasification is a solution for treating heavy metal contaminated materials, but in very specific cases.
Dear Yamid Gomez Rueda ,
Thanks for your reply. But, can you tell me what is the range of temperature required for the heavy metals to be trapped/locked in the ash?
Plasma gasifiers are not so commonly used. The widely used gasifiers like fluidised bed gasifiers deals with the teperature range 650 - 850 oC. Is that enough ?
Dear Bijesh Raveendra ,
The temperature of this process depends on the vitrification of the ash. The vitrification of the ash can be measured by the ash fusion temperature, which varies in function of the ash composition. Biomass ash complete fusion occurs at temperatures between 800-1600 C, (I do not have a reference for sewage sludge ashes). In an arc-plasma gasifier, temperatures of around 10.000K are reached. This sufficient enough for vitrification, but some heavy metals can vaporize at the same time. I recommend you to read the following articles for more specific info Article Treatment and Recycling of Incinerated Ash Using Thermal Pla...
Article Leaching properties of slag generated by a gasification/vitr...
As noted, sewage sludge gasification residues will fail testing for, "end of waste", so it will end up as a waste in its own right. I know from experience that the residues will fail leachate tests for several metals. There is not way to selectively treat the residues to remove hazardous metals.
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