Translated Abstract
With the development of sewage treatment system, disposal of sewage sludge is becoming an increasingly problem in China these years. As the by product of sewage treatment, sludge contains most of the toxic substances in sewage. Therefore, safe disposal of sludge has become a critical impact of China’s sustainable development strategy. Supercritical water treatment of pollutants is a powerful and environment friendly technology which can solve this challenge. Due to the unique nature of supercritical fluid, it can achieve the goal of
harmless disposal and resource utilization. Based on the former research, we designed a continuous multi-functional system which can conduct supercritical water oxidation, supercritical water gasification and supercritical water partial oxidation. The capacity of the system is 4.5L·h-1. And with this test system, some in-depth research work, aimed at
promoting the harmless treatment of sludge and resource utilization, can be carried. The main contents and conclusions are as follows:
The reaction path and mechanism of key intermediate product, formaldehyde, are studied. The key reactions of formaldehyde supercritical water gasification are: HCHO+HCOOH → CH3OH+CO2, 2HCHO+H2O → CH3OH+HCOOH , CO+H2O → CO2+H2, and
2CH3OH→CH3OOCH+2H2 . Experimental results proved that higher gas efficiency can be achieved by promoting these reactions. We obtained the chemical reaction equation: 17HCHO+10H2O ⇒ 20H2+9CO2 +4CO+3CH3OH+HCOOH , which can stand for the whole reaction. Experiments also supported the following conclusions: higher temperature and higher pressure are more favorable in supercritical water gasification than pressure. H2 content of volume percentage will increase with the condition mentioned before. Thermodynamic analysis of formaldehyde supercritical water gasification is carried out. The results showed that the constraints of supercritical water gasification are the kinetic factors.
Sewage sludge from Xi’an Beishiqiao water treatment plant is used as experiment material. The main conclusion is that supercritical water partial oxidation of sewage sludge can achieve the goal of harmless disposal and resource utilization. Heavy metal is oxidized into a steady state, which is safe to the environment. Gas product contains a high volume amount of hydrogen and methane, which is of great use for energy recovery. The following are some other results. Alkaline catalyst NaOH plays a key role in supercritical water partial oxidation reaction. It can prove the hydrogen product efficiency significantly. Only with the catalyst,
can supercritical water partial oxidation enhance the rate of hydrogen production. If there is no catalyst, the partial oxidation does not promote the efficiency. At the same time, the amount of oxidant will also affect the rate of H2 production with the presence of catalyst. When the added oxidant was 20%, the H2 production rate reached a maximum value. Besides, alkaline catalyst has no significant effluence of COD degradation rate. As for heavy metals in sludge, supercritical water treatment can effectively transfer the three harmful unstable states, which are exchangeable, carbonate salt-bound and iron-manganese oxide bound, into harmless residue state. This will effectively reduce the heavy metal pollution. Of three kinds of unstable state, iron-manganese oxide bound is the most difficult to convert into residue state, while the other two kinds can achieve almost 100% conversion. Oxidant plays an important role is to promote the exchange of state. Pressure has more impact than the temperature. Under subcritical pressure, the state conversion rate is very small. When reaching the supercritical state, the conversion rate has improved significantly.
Neural network is established to simulate the supercritical water oxidation of organic pollutants. For the organic pollutants, only the main elements of content and parameters of experimental conditions are needed. The model can predict the results very accurately. This can greatly reduce a large number of repeated experiments to determine the optimal working conditions.
The reaction heat released from supercritical water treatment of sludge is calculated based on the enthalpy change. Calculations showed that the supercritical water gasification reaction is an endothermic reaction. When the adding amount of oxidant reached 25%, due to partial oxidation which will release a lot of heat, the entire reaction is self-sustained. And with the increase in the amount of oxidant, organic matter was completely oxidized to release more heat. When the oxidant concentration was 100%, the heat value of dry sludge is 19513kJ·kg-1. This value is higher than the heat value of lignite. This shows that the sludge can be used as an energy source.
Exergic efficiency of the system is analyzed. The results showed that the whole exergic efficiency of SCWG system can reach 45.2%, while the SCWPO has the efficiency of 37.4% and the SCWO is 28.7%, which is the lowest of the three systems. Exergic efficiency of each unit is also calculated. Pre-heater and the back-pressure valve have the least exergic efficiency. For future system design, increasing the exergic efficiency of these two units should be considered first to improve the whole system’s efficiency
Translated Keyword
[Supercritical waterPartial oxidationIntermediate productExergic analysis]
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