Abstract
As known, dried sewage sludge, is a by-product produced from waste water treatment, contains significant amounts of organic content, and makes up to 60% with overall calorific value from 9 to 12 MJ/kg. Hence, it can be considered as material for thermal processing focusing on heat and power production. Among thermal conversion technologies, gasification is seen as the effective one because it can be easily combined with heat and power cogeneration units. On the other hand, due to high mineral content (40–50%) in the sludge, it is difficult to be gasified and obtain syngas with calorific value satisfactory enough for fueling the internal combustion engine. The dried sludge can be subjected to be gasified at temperature above 850 °C. However, large amounts of mineral content do not provide favorable conditions to obtain this required temperature. Thus, it is proposed to enrich the sewage sludge with biomass characterized with significantly higher calorific value. In the article, co-gasification of sewage sludge and Virginia Mallow—energetic crops was investigated. Results from experimental and numerical investigation have been presented. The dried sewage sludge enriched with Virginia Mallow at a mass ratio of 0/100%, 50/50% and 100/0% in tests and in the range from 0 to 100% for theoretical analysis was applied in order to achieve effective gasification process. As observed, lignocellulosic biomass like Virginia Mallow contains low amounts of mineral content below 2%, which makes it appropriate for thermal processing. It contributes to more stable and efficient gasification process. Additionally, Virginia Mallow caused that the process temperature possible to achieve, was 950 °C. Thus, sewage sludge was mixed with this high-energy component in order to improve the gasification parameters and obtain syngas with higher calorific value. A zero-dimensional, two-zone model was developed with aid of the POLIMI kinetics mechanism developed by CRECK Modeling Group to simulate gasification of low calorific substances enriched with high calorific biomass. Obtained results showed that sewage sludge can be completely gasified at presence of Virginia Mallow. Syngas calorific value of approximately 5 MJ/Nm3 was produced from this gasification process. The maximal percentage of Virginia Mallow in the mixture with the sewage sludge was set at 50% due to economic aspects of the technology. It was found, that satisfactory conditions for effective gasification were achieved at this 50/50% percentage of sewage sludge and Virginia Mallow. Potential intensity of gasification was predicted from this 0-D 2-zones model, which calculates area of reduction zone to area of combustion zone. This reduction-to-combustion area ratio for the sewage sludge-Virginia Mallow mixture was estimated at value of 2. Finally, the model was successfully verified with results from tests, hence it was proposed as a tool for preliminary investigation on poor fuels gasification.
Highlights
Sewage sludge, due to its physical-chemical properties and hazards that may pose for human health and the natural environment, have to be subjected to appropriate treatment and neutralized
Charge to a gasification reactor consisted of dried sewage sludge and Virginia Mallow premixed
The first substance was sewage sludge considered as burdensome material for gasification due to high mineral content
Summary
Due to its physical-chemical properties and hazards that may pose for human health and the natural environment, have to be subjected to appropriate treatment and neutralized. The final disposal of municipal sewage sludge is carried out through their storage, agricultural use, use for lands reclamation and thermal utilization. The use of sewage sludge in agriculture or for land reclamation is not always possible due to restrictions in law. The safest way to neutralize sewage sludge is thermal processing and the possibilities in this area are very wide. The most effective way to utilize sewage sludge for energy purposes seems to be gasification. 80%), wet sewage sludge is subjected to pre-drying process before gasification, what from the point of view of energy consumption, makes the process considerably costly. Gasification takes place in the atmosphere of air, or oxygen, and/or steam and at temperature regime usually above 850 ◦ C and not exceeding
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