Sewage sludge is a large-scale resource available on our planet, part of which is used as an element of the circular economy to improve soil quality in agriculture. As this utilization is very limited, there is a significant need to utilize an increasing proportion of the resulting sludge for energy production purposes.

 

Sewage sludge is a significant source of renewable energy.

 

The energy utilization of sewage sludge as a wet biomass in the traditional way of incineration is not expedient, as due to the high moisture content of “dewatered” sludge (70 - 80%) the incineration efficiency is very low - the amount of energy obtained in this way, even in more modern processes also quite low. The energy balance of conventional combustion is hardly positive due to the energy demand of drying or support firing.

 

In modern processes, the sludge is fermented and the resulting biogas is combusted in a gas engine, electricity is generated and the heat of the gas engine's cooling water and exhaust gas is used to dry the sludge still containing significant organic matter from the fermentor and then dried sludge is burned for energy production. The energy balance of this process is much better, but its overall efficiency is still quite low.

 

In the case of wet biomass, including sewage sludge, the practical way of energy utilization is to gasify the wet biomass under supercritical conditions and utilize the resulting generator gases (methane, carbon monoxide and hydrogen). In this case, the energy efficiency of the process will be as high as possible.

 

The principle of gasification in supercritical water has long been known, but the structural materials previously available did not allow the implementation of industrial-scale continuous-operation reactors. Recently, high-alloy CrNi materials, such as Inconel 740 H, are now readily available in the mechanical engineering industry at reasonable prices, enabling the construction of high-performance, corrosion-resistant tube reactors that can also process wet biomasses in an energy-efficient manner.

 

The process of waste utilization by supercritical water gasification is as follows: the sewage sludge is properly prepared (shredded, preheated) and then fed into the supercritical tube reactor by a high-pressure feed pump. The gas-fired tubular reactor is powered by the produced generator gas. The length of the tube reactor is based on the reaction time required for the complete gasification of the sewage sludge. The mixture leaving the tubular reactor is separated by means of separators. Water, which is practically distilled water, can be used for industrial purposes or irrigation. The remaining inert solids can be used by the construction industry. The methane and carbon monoxide content of the generator gas is used in a cogeneration gas engine, where the produced electricity is supplied to the grid and the produced heat is utilized in the technology, thus increasing the processing efficiency. The hydrogen content of the generator gas, as a renewable “green” hydrogen, can be used in fuel cells and hydrogen gas networks for transportation, or can be filled into mobile tanks.

 

The above process, thanks to the pressure-energy recovery turbine used in the system, the heat exchangers supporting the operation of the supercritical tube reactor and the proper utilization of the heat energy of the gas engine in the sludge preparation process, has a much higher energy efficiency than current systems.

 

We also consider good efficiency to be important because sewage sludge, as a significant amount of available andpredictable renewable energy, can be the basis for a decentralized electricity generation with a predictable schedule and regulatory energy production, as well as the creation of a green hydrogen economy.

 

Advantages of the future process in comparison with the state of the art technology:

 

The above described method for the preparation of sewage sludge, with the main result of colloid formation, ensures a high degree of gasification of the sewage sludge (above 90%) and avoids the formation of larger coke pieces during gasification contrary to other similar technologies, which could lead to reactor tube blockages.

 

The above method of preparing the sewage sludge makes it unnecessary to add additional water to the “dewatered”, but still wet sludge with a moisture content of 70-80%, preventing unnecessary dilution to a moisture content of 91-93%. Thanks to the preheating, pre-shredding and then pressure cooking (release of the bound water), the resulting sewage sludge with a moisture content of 58-63% becomes excellently pumpable. A water / dry matter ratio of around 60% (compared to a water ratio of around 90%) contributes greatly to the energy efficiency of processing by gasifying about four times as much dry matter per cycle (with the same amount of thermal energy). The 4 screw auger countercurrent heat exchangers, the heat-insulated heating with a steam jacket, make the process of pressure cooking relatively inexpensive and energy efficient.

 

During the discharge of sewage sludge after boiling under pressure, the oxygen content of the mixture (boiled sludge and water) is removed together with the steam, which makes the amount of CO2 generated between the generator gases during gasification negligible - resulting in better quality generator gas. Due to the structure and design of the tube reactor it has an outstandingly high efficiency (above 90%), both in terms of energy efficiency and conversion (gasification) efficiency. The tube reactors have a high operational safety, which is mainly due to the ultrasonic power generators and the vibrating scrapers placed on the reactor tubes. The vibrating scrapers prevent the precipitation of inorganic salts in the mixture. Not only do they keep the tube reactor clean and avoid blockages, but they also help to increase the efficiency of gasification and achieve a high value of all the materials used.

 

The pressure energy of the supercritical mixture exiting the tube reactor is utilized in a disk type turbine, further increasing the overall energy efficiency of the equipment and the process. In addition to the separation of hydrogen and its sale as renewable hydrogen, the utilization of the remaining methane plus carbon monoxide in the gas engine, due to the usability of the waste heat of the gas engine in the process by heat exchangers, significantly improves the financial and economic parameters of the process.

 

The methane yield of our present wet-biomass gasification technology is more than twice the methane yield of biogas technologies.

 

Based on the first reference plant, the presented technology and its equipment can also form the basis for large-scale mechanical engineering exports and technology transfer. Furthermore the market introduction of the technology can have a significant job-creating effect.

 

Literature:

Gasification of Dutch sewage sludge in supercritical water.

https://publikationen.bibliothek.kit.edu/1000051638/6214421

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