Theoretical modelling of a grate-combustion plant for sludge

In the framework of the PerFORM WATER 2030 Project, the development/experimentation of the new grate-combustion technology for pelletized dried sewage sludge and its modelling are carried out in parallel, with continuous exchange of information.

Which phenomena are modelled?

"Modelling of the phenomenon": what does it mean?

A strong connection between modelling and experimentation: why is it so important?

Which is the innovative contribution that modelling of sludge thermal treatment can provide within PerFORM WATER 2030?

Which phenomena are modelled?

A schematic representation of the processes involved in the technology that is being tested in PerFORM WATER 2030 can be found in the following Figure 1.

Up to now, the modelling has focused on the processes experienced by the bed of solid material (pelletized dried sludge) over the grate. As the material is introduced in the combustor, it undergoes further drying, hence pyrolyzes by releasing combustible gases and evolving to char (a mix of solid carbon and ash). In the end, char reacts directly with the oxygen in the combustion air, leaving on the grate only ash.

Figure 1 missing
Figure 1 missing

Figure 1. Schematic of the processes involved in the combustion of dried and pelletized sludge on the grid

The description of these processes involves modelling no less than seven chemical-physical phenomena:

  • fluid dynamics of air / flue gas though the bed of solid material;

  • heat exchange between air / flue and the solid matrix;

  • transport of chemical species in the gaseous phase;

  • evaporation of the moisture of the solid material;

  • pyrolysis of the volatile part of the solid material;

  • oxidation (i.e. combustion) of the gaseous combustible species that have been released by the pyrolysis process;

  • oxidation of char.

"Modelling of the phenomenon": what does it mean?

The modelling of the several chemical-physical phenomena that take place in the grate-combustion of pelletized dried sludge is carried out by means of a thermo-fluidynamics simulation program. Among the available tools, the COMSOL Multiphysics® software package has been selected as the optimal trade-off between simulation capability and ease of use.

The modelling that has been carried out is based on the application of the finite element method. It entails dividing the volume of the bed in many small tetrahedral elements, within each of which the description of the different properties (temperature, velocities, concentration of chemical species, etc.) is based on simple curves (i.e. mathematical functions). The shape of such curves is tuned in order to satisfy the mathematical equations describing the various phenomena (e.g. the mass balance, the energy balance).

The different elements exchange each other mass, energy, chemical species, etc. Therefore, the solution of the various equations within each element must be coordinated with those within the surrounding elements. An iterative process is thus required, with subsequent approximations until a "convergence" situation is not reached.

The final solution is a representation of what happens in reality. It is as much realistic, as good the starting information is and as accurate and complete the description of the involved physics are.

Therefore, the use of the model allow predicting how much rapidly the bed of solid material is consumed and, as a consequence, how much material can be loaded per unit time over the grate so that its complete combustion is warranted. Similarly, it is possible to study whether a different air distribution underneath the grate can favor a better combustion process, in terms of rapidity, better temperature distribution and formation of possible polluting species.

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Figure 2. Preliminary results of the CFD simulation of the grate combustion of dried and pelletized sludge. The different colors represent the solid carbon concentration and show its complete consumption during the process.

A strong connection between modelling and experimentation: why is it so important?

Theoretical modeling and practical experimentation of a new technology can only be strictly interconnected, if optimal results are to be obtained.

Modelling supports experimentation since it can provide useful indications on which operating conditions are the most promising in terms of system setup and other controllable parameters. Thus, it can lead the experimentation faster toward the identification of the optimal operating conditions, which allow determining the maximum achievable performances and, therefore, the potential of the technology.

On the other hand, experimentation is of fundamental relevance to obtain really representative modelling. In fact, several experimental data are needed to properly tune physical-chemical models, so that they can be reliably used to test also yet uninvestigated configurations.

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Figure 3 missing

Figure 3. Connections between pilot plant testing, thermo-fluid dynamics simulations and the design of full-scale plants

Which is the innovative contribution that modelling of sludge thermal treatment can provide within PerFORM WATER 2030?

PerFORM WATER 2030 represents an exceptional occasion for the development of models able to reliably describe the complex variety of physical-chemical phenomena involved in the grate-combustion of pelletized dried sludge. The objective was therefore pursued within a research activity specifically dedicated to the theoretical modeling of the sludge combustion phenomena.

Measurements and modelling for sludge thermal valorization

Project details

The availability of the pilot plant, properly equipment with the required instrumentation (for more details see Sludge thermal valorization) and the continuous interaction with the technology developers allow testing several models, selecting the most promising ones and tune them properly. In a near future, based on the results of such an activity, even better performing and environmentally friendly plants will be designed and built.

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