Clean and efficient wood stoves through improved batch combustion models and CFD modelling approaches

In modern wood stoves, produced after emission limits for particles from wood stoves was introduced in 1998, the emissions are relatively low compared to old wood stoves. The new stoves have two-stage air addition, which give better combustion conditions, and hence lower emissions. But this is not enough in the future, we must also improve today's modern wood stoves - they must become even better and smaller and/or release a lower and more stable effect to have a future in modern low energy and passive houses. To reduce emissions further and improve the heat comfort, better control of both the heat production and heat release to the room is needed. The ideal would be a continuous combustion process, without fluctuations leading to unstable combustion conditions and increased emissions. However, the combustion process in a wood stove is and always will be a batch process, i.e. the wood will change continuously from moist raw wood to charcoal, and finally ash. This is a challenge, with respect to combustion control. In WoodCFD we will especially focus on combustion chamber design for minimized emissions. CFD combines geometrical details and reactive flows. The result is a detailed picture of the combustion process and its quality. Various parameters can then be varied to improve the combustion process. For all modelling it is important with correct sub-models for the physical and chemical processes ongoing, together with correct input to the simulations that use these. Combined with experimental work this will contribute to wood stoves with emissions down to pellets stove level and 1 kW effect, which will be released considerably more stable than from today's wood stoves, i.e. well suited for low energy and passive houses. Results from this project can give considerable environmental benefits and more sustainable heating. In 2015 the scientific focus was on initial studies and establishment of sub-models for use in transient CFD simulations, as well as modelling of heat transfer in stoves and analysis of heat distribution to other rooms in a building. The work with the sub-models, also for stationary CFD simulations, continued in 2016. The employed PhD candidate in the project focused on development of a decomposition model for wood logs. In parallel work was ongoing regarding improvement of models and tools used for simulation of thermal comfort in energy effective buildings with wood stoves. In 2017 the work focused on testing and use of developed sub-models in CFD simulations, and the PhD candidate focused on further development of the decomposition model for wood logs, and published several works on this. The thermal comfort work in 2017 focused on experimental activity to provide detailed data for further development and validation of models. In 2018, det final year of WoodCFD, the focus was on transient CFD simulations with heat storage, wood stove optimization, NOx reduction and building integration via dynamic and CFD simulations. A handbok was made that sums up the project and gives recommendations.

Prosjektet har bidratt til betydelig kompetanseheving på flere fagområder knyttet til vedovner, inkludert forståelsen av forbrenningsprosessen i vedovner og hvordan denne kan påvirkes og forbedres, samt hvordan vedovner kan tilpasses fremtidens energieffektive bygninger. Modellene og verktøyene utviklet eller forbedret i prosjektet har lagt grunnlaget for det som kan bli et effektivt designverktøy for vedovner, til direkte nytte for vedovnsprodusentene ved design av fremtidens miljøvennlige vedovner med også forbedret varmekomfort. Prosjektresultatene har vært til stor nytte for prosjektdeltagerne og er av stor interesse for fagfeltet og industrien. Industrien vil ha utbytte av mer effektive design- og utviklingsprosesser for vedovner, brukerne vil ha utbytte av bedre vedovner tilpasset deres behov, og samfunnet for øvrig vil ha utbytte av mer miljøvennlige vedovner.

The Strategy for increased expansion of bioenergy states that Norway shall double the bioenergy production from 14 to 28 TWh by 2020 and that the major single contributor of new bioenergy production shall come from bioenergy use in small-scale heating appliances for space heating, meaning in practise the use of wood log combustion in wood stoves and fireplaces. New solutions and technologies that enable a more widespread and extensive use of wood log combustion are clearly necessary to reach these ambitious targets. New houses, as well as retrofit/upgrading of old houses, have increasingly focused on improved insulation, lowering the space-heating effect required to as low as 1 kW. Combining heat production, storage and distribution in an optimum way makes it possible to achieve a substantially more stable heat release and distribution; ultimately resulting in an increased number of operational hours per unit and also more units in active operation due to the increased heat comfort. Low load wood stoves and fireplaces in buildings with new insulation standards demand new technologies and solutions with an increased focus on the combustion process and its control, the combustion quality and optimum design to ensure low emissions and high energy efficiency to minimize the negative effects of the batch combustion process. There is a need for a more concentrated effort on the development of improved models and the use of advanced modelling approaches to be able to further improve wood stoves and especially low load wood stoves with respect to emissions and energy efficiency, as well as combustion stability and optimum room and building integration. The proposed project therefore focuses on clean and efficient wood stoves through improved batch combustion models and CFD modelling approaches. Improved models and modelling approaches, in combination with targeted experiments, are keys in the development of future's downscaled clean burning and energy efficient wood stoves.