Dieter Stapf

Screw pyrolysis technology for sewage sludge treatment

Sewage sludge quantities have grown continuously since the introduction of the European Directive (UWWTD 91/271/EEC) relating to the treatment of urban wastewater. In the present, most of the sewage sludge is combusted in single fuels incineration plants or is co-fired in waste incineration or coal power plants. The combustion of sewage sludge is a proven technology. Other treatments, such as fluidized bed gasification, were successfully adopted to produce suitable syngas for power production. Besides, the number of large wastewater treatment plants is relatively small compared to the local rural ones. Moreover, alternative technologies are arising with the main target of nutrients recovery, with a special focus on phosphorus. The aforementioned issues, i.e. the small scale (below 1MW) and the nutrients recovery, suggest that pyrolysis in screw reactors may become an attractive alternative technology for sewage sludge conversion, recovery and recycling. In this work, about 100kg of dried sewage sludge from a plant in Germany were processed at the newly developed STYX Reactor, at KIT. The reactor combines the advantages of screw reactors with the high temperature filtration, in order to produce particle and ash free vapors and condensates, respectively. Experiments were carried out at temperatures between 350°C and 500°C. The yield of the char decreased from 66.7wt.% to 53.0wt.%. The same trend was obtained for the energy yield, while the maximum pyrolysis oil yield of 13.4wt.% was obtained at 500°C. Besides mercury, the metals and the other minerals were completely retained in the char. Nitrogen and sulfur migrated from the solid to the condensate and to the gas, respectively. Based on the energy balance, a new concept for the decentral production of char as well as heat and power in an externally fired micro gas turbine showed a cogeneration efficiency up to about 40%.

Flow reactor studies and testing of comprehensive mechanisms for NOx reburning

A plug flow reactor has been set up to study NOx reburning kinetics under conditions comparable to those in industrial boilers. The overall situation corresponds to low radical concentrations. Therefore, mixtures of natural gas and nitrogen were injected into a flow of flue gas containing 1000 ppm NOx and varying levels of residual oxygen. The reduction of nitrix oxide by attack of hydrocarbon radicals was investigated for equivalence ratios Φ between 1.1 and 1.6 and at temperatures of 1200, 1300, and 1400°C. Plug flow calculations were performed with three comprehensive mechanisms taken from recent literature A comparison of measured an computed concentration profiles showed that consumption of NO and production of HCN are overestimated by all models, especially when the initial oxygen content of the rich mixtures is increased. Analysis of the calculated NO production rates identified the attack of NO by the HCCOrradical with a temperature independent rate coefficient to be the dominating pathway of NO consumption. Reactions of nitric oxide with other hydrocarbon radicals exercise only a minor influence. However, recently measured data for the reactions of CH3 with NO underlines the importance of this pathway in reburning kinetics. If this reaction is emphasized in the calculations, the reaction of NO with HCCO has to become very slow in order to achieve good agreement with experimental data. One of the mechanisms from the literature was modified by means of sensitivity analysis and critical revisions of kinetic data for important reactions under rich conditions. Usage of this modified mechanism is leading to significantly improved agreement for NO profiles with experimental data. Nevertheless, there still remains some uncertainty in the description of HCN consumption and concerning the reaction pathways and rate coefficients of NO-consuming reactions under rich conditions with low levels of radicals.

Modeling of NOX reduction by reburning

Reburning is a powerful and relatively inexpensive technique to reduce the NO x emission from industrial incinerators or coal fired power plants. NO x reduction is achieved by means of secondary fuel injection. The aim of the reported theoretical and experimental investigations has been to develop modeling tools for combustor design and layout. Detailed kinetic models from the literature have been tested and modified to describe species interconversion under practical reburning conditions. Furthermore, a hybrid model has been developed which allows calculation of the NO reduction in practical flows. It separates the computation of the turbulent flow from the computation of NO reduction. The latter is performed by a postprocessor using detailed kinetics. Both models have been validated against experimental data from test facilities in semi-technical scale, and their predictions agree satisfactorily with the experiments.

Thermal Stability and Material Balance of Nanomaterials in Waste Incineration

Nanostructured materials are widely used to improve the properties of consumer products such as tires, cosmetics, light weight equipment etc. Due to their complex composition these products are hardly recycled and thermal treatment is preferred. In this study we investigated the thermal stability and material balance of nanostructured metal oxides in flames and in an industrial waste incinerator. We studied the size distribution of nanostructured metal oxides (CeO2, TiO2, SiO2) in a flame reactor and in a heated reaction tube. In the premixed ethylene/air flame, nano-structured CeO2 partly evaporates forming a new particle mode. This is probably due to chemical reactions in the flame. In addition sintering of agglomerates takes place in the flame. In the electrically heated reaction tube however only sintering of the agglomerated nanomaterials is observed. Ceria has a low background in waste incinerators and is therefore a suitable tracer for investigating the fate of nanostructured materials. Low concentrations of Ceria were introduced by a two-phase nozzle into the post-combustion zone of a waste incinerator. By the incineration of coal dust in a burning chamber the Ceria nanoparticles are mainly found in the size range of the fly ash (1 – 10 µm) because of agglomeration. With gas as a fuel less agglomeration was observed and the Ceria nanoparticles were in the particle size range below 1 µm.

Characterization of Biomass Fuels in Isothermal Plug Flow Reactor (IPFR)

Spalovani fosilnich paliv je jednim z nejdůležitějsich zdrojů energie. Nicmeně nizkouhlikova politika a environmentalni zavazky ovlivňuji vývoj spalovacich a spolu-spalovacich technologii. Využivani paliv z biomasy může být odpovědi na nove výzvy, i když je zapotřebi vice výzkumu o efektivnim využiti těchto paliv. V soucasne době spalovani paliv z biomasy, zejmena slamy, způsobuje řadu technických problemů, zejmena tvorbu strusky, zanaseni výměniků tepla uvnitř spalovaci komory a nedostatecne vyhořeni paliva. Tento přispěvek se zaměřuje na analýzu spalovanibiomasy. Lepsi znalost chovani procesů při spalovani biomasy může přispět k optimalizaci navrhu praskoveho kotle a vyhnout se tim technickým problemům. Výsledky výzkumu ukazuji, že teplota a koncentrace kysliku v reaktoru hraji významnou roli v procesu tvorby prchave hořlaviny a vyhořeni tuheho zbytku. Napřiklad během experimentů s vyhořelým tuhým zbytkem při teplotě 850 ° C při 14% koncentraci kysliku po 200 ms bylo dosaženo vice než 80% ubytku hmotnosti. Ve srovnani se 700 ° C při 14% koncentraci kysliku byla stejna uroveň ubytku hmotnosti dosažena po 500 ms. Experimenty provedene na izotermickem reaktoru (IPFR) v Institutu energetických procesů a technologii paliv (IEVB) na TU Clausthal byly soucasti projektu mezi IEVB a „Karlsruhe Institute of Technology (KIT)“ v Německu.