Abgasreinigung mit einem radial durchströmten regenerativen Schüttschichtwärmeübertrager - Kombination von thermischer Nachverbrennung und integrierter Staubabscheidung

Reichenberger, Hans-Peter; Quicker, Peter (Thesis advisor); Faulstich, Martin (Thesis advisor)

Düren : Shaker Verlag (2022)
Book, Dissertation / PhD Thesis

In: Schriftenreihe zur Aufbereitung und Veredlung 84
Page(s)/Article-Nr.: XVIII, 273 Seiten : Illustrationen, Diagramme

Dissertation, RWTH Aachen University, 2022

Abstract

In order to clean exhaust gases that are contaminated with dust particles and volatile organic compounds, filters are mainly used for dedusting and thermal oxidation processes for converting the volatile pollutants into gaseous products such as water and carbon dioxide. So far, no system is available on the market that allows a combination of the two established process steps in one unit. A more compact but equally effective, quasione-stage purification system can reduce the effort required for multi-stage exhaust gas cleaning by saving some of the investment and operating costs and reducing the required space. Especially for small medium-sized companies such a solution can be economically advantageous. Therefore the present thesis deals with the development and testing of an alternative exhaust gas cleaning process that combines the two process steps in one unit. The functional principle of the regenerative packed bed heat exchanger with radial flow serves as the basis for this: It has already been used as a thermal post-combustion system for low dust exhaust gases and also used in a modified design as a packed bed filter for particle separation. Suitable bulk materials were examined for the new process and those parameters were experimentally determined that significantly influence the functionality of the overall system: This concerns the materials characteristics as well as the parameters determined in specially designed test facilities, such as the characteristic grain size properties as well as the voids fraction, the mechanical pressure, the pressure loss and the dust retention capacity of the packed bed. To design the new process principle, a transient quasi-two-dimensional calculation model was created in a calculation program for numerical flow simulation. Until now, a transient one-dimensional model was used, in which the temporal changes of the temperatures in the packed bed were determined via the radial spatial coordinate. With the extended calculation method, the packed bed - defined as a pipe system with parallel flow - can be represented axially in horizontal layers, whereby the thermo and fluid dynamic processes of the transient bed flow can be described as a function of height. The new model is only approximately two-dimensional in the spatial coordinates, as the energy and mass transfer between the individual horizontal layers is not taken into account. With sufficiently thin layers, the mean pressure ratios of the layers deviate so slightly from each other that hardly any cross-exchange of mass flows occurs. Compared to more complex simulation methods, this simplification offers the advantage that an approximate system design can be used for practical applications with relatively low calculation effort. In order to be able to test the suitability of the new process for the first time, a pilot plant for exhaust gas flowrates of up to 2,000 m³i.N./h was designed, installed and tested. Its special feature is the arrangement of the radial packed bed: it is divided vertically into two regenerator halves so that the heating and cooling phases for regenerative heat recovery can be realized compactly in one plant and simultaneously a specially designed bottom geometry enables easy removal of the bulk material loaded with separated dust. The test campaigns served to check the basic performance of the system, to optimise the system if necessary and to compare the measured values with the data of the newly created calculation model. To simulate real exhaust gases, ambient air was sucked in and model dust or as volatile pollutants ethanol and propanol or as a gaseous pollutant methane were dosed into it. Relatively high concentrations were generated in order to be able to derive a meaningful cleaning tendency of the plant within the time-limited operation of the pilot plant. During the first series of tests, localised false flows occurred and during the start-up and shut-down processes, thermal stresses caused instability of the plant structure at the points where high temperatures and high mechanical pressures were present at the same time. The plant was therefore partially improved structurally. The measured data could be simulated well with the new calculation model. However, the tests with volatile pollutants showed that the thermochemical influences not taken into account in the model led to a slight to sometimes strong difference between measured and calculated temperatures. Values between 88 % and 95 % were achieved as heat recovery efficiencies. They should be further improved in the future through optimised operation. Although up to 99.6 % of the model dust was separated in the preliminary tests, in the pilot plant the dust separation rates of the unregenerated packed bed dropped from 98 % at the beginning to values between 94.7 % and 89.5 %. This was mainly due to the dust deposits that were carried along when the flow direction was changed at the valves or were carried out of the packed bed again due to unfavourable flow conditions. The volatile pollutants could be thermally converted to about 90 % in each case, whereby at the beginning of the test campaigns the separation efficiency for methane, for example, was 99.5 % and that for ethanol 98 %. The cause of the deviations that occurred was pollutant slip due to flow short-circuits. Based on the findings to date, the new process is basically well-suited for exhaust gas cleaning. However, the functionality of the system technology still needs to be optimised and further studies need to be carried out so that more effective separation rates and a higher structural durability can be achieved. The optimised system must then be tested in industrial trial operation under real conditions and over a longer period of time.

Institutions

• Division of Mineral Resources and Raw Materials Engineering [510000]
• Unit of Technology of Fuels [512220]