The DT4Biomass project received a grant from the European Union within the call H2020-INNOSUP-2018-2019 (To better support innovation in SMEs). Thanks to this grant, we hired an expert person (PhD) in a field that could help nablaDot in the proposed project (DT4Biomass) during one year. Thus, nablaDot obtained resources to carry out the intended research while improving our PhD expert’s training and knowledge about European companies’ innovation needs.

The DT4Biomass project aimed to develop an accurate and affordable CFD model (in terms of computing resources) to simulate any biomass boilers. The main challenge in the simulation of biomass boilers is modelling the fixed bed or grate.In the grate, drying, devolatilization and heterogeneous combustion of the biomass particles take place; the chemical species released are carried into the furnace by the primary air introduced through the lower part of the grate. In the freeboard, the combustion of the volatiles released is completed. On the other hand, the fixed bed receives radiative heat from the furnace, which is essential for the processes previously enumerated to take place within it. Therefore, different but strongly coupled phenomena occur in the fixed bed. For this reason, simulation of the grate is paramount to represent the biomass boiler’s operation adequately.

Scheme of the physical and chemical phenomena that take place in the grate of a biomass boiler

The CFD model has been built to simulate a biomass spreader stoker boiler where the biomass is fed pneumatically on the grate. The fine particles complete the combustion in suspension mode, while the bigger particles are spread on the grate surface partially dried. These boilers can simultaneously burn different fuels (e.g. wood chips and straw) and are suitable for relatively large power or steam generation projects.

The modelling of these boilers must deal with the simulation of the fixed bed phenomena and calculate the particles’ trajectories and the processes occurring while the particles are in suspension (drying, devolatilization, combustion). Furthermore, it is needed to locate where the particles reach the grate and determine their state (moisture, volatile and char content) at this moment. The complexity of modelling these coupled phenomena and the boiler’s capacity to handle very different fuels demands the development of a comprehensive model (that can be easily adapted to other biomass boilers whose simulation is more straightforward). For this reason, this type of boiler has been chosen to elaborate the aimed model in this project.

The next Figure shows a scheme of one of the boilers on which the model has been validated. The biomass particles are injected from the front wall, falling on the grate, mainly close to the rear wall. The grate moves from the rear wall to the front wall. The primary air is introduced under the grate. The secondary air moves the flue gases towards the front wall, and, finally, the tertiary air provides the oxygen needed to complete the combustion. This Figure shows the complexity of simulating this type of boilers with multiple inlets and coupled phenomena.

 

Scheme of the modeled biomass boiler.

The Figure below shows some of the numerical results. Combustion occurs most intensively close to the rear wall, where most of the biomass particles have been spread. Higher flue gas temperatures and lower O2 concentrations are observed in this area. It can also be seen how the volatiles combustion is completed near the tertiary air inlet. The model developed has been successfully validated in two spreader stoker boilers.

Temperature contours in the biomass boiler.

Oxygen concentration contours in the biomass boiler.

Besides these results, this model allows calculating the boiler efficiency (unburned char), pollutant emissions (such as NOx) and other variables of interest for the boiler design or operation. Thus, examples of applications of this model are the optimization of the boiler operation, analysis of modifications in the biomass fuel (different moisture content, LHV, granulometry) or predictive maintenance.

DT4Biomass has enabled nablaDot to increase its portfolio of models related to the simulation of combustion facilities. nablaDot has a broad range of models to simulate every part of fossil fuel and biomass boilers: furnaces, heat recovery zones and flue gas cleaning (desulfurization, SCR) installations. Furthermore, the models developed in this project can be extended to other biomass processes (drying, pyrolysis, gasification).

The work developed in this project will be presented at the 29th European Biomass Congress (EUBCE 2021). Besides, more articles related to this project are expected to be published throughout this year.

This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 861842.