Bioenergy covers about 17.2% of the gross energy consumption in Austria and is therefore the most important renewable energy source. In addition, the share of bioenergy should increase in the following years.
The combustion of woody biomass for heat generation is an established technology and one third of the total heat demand in Austria is covered in this way. Here, traditional wood log stoves are still of great importance. Austria's research and industry is an international leader in the field of bioenergy and, in particular, in biomass combustion. The Institute of Thermal Engineering at TU Graz plays a crucial role as key scientific partner of the Austrian biomass competence centre BIOENERGY 2020+. The two TU Graz professors Christoph Hochenauer and Robert Scharler are scientific leaders in the fields of combustion technology and modelling and simulation at BIOENERGY 2020+. Together with the competence centre, they are successfully researching in the field of innovative combustion technologies with high efficiencies and very low pollutant emissions, such as carbon monoxide (CO), nitrogen oxides (NOx) and fine dust.
Wood stoves are still very popular due to the low fuel costs, the ecological aspect of a renewable energy source and the visual appeal of the flame, which creates a pleasant feeling. However, currently available stoves are either expensive (highquality pellet stoves) or the pollutant emissions are too high (old and cheap wood-log stoves). In the "LowCostEmissionStove" project, which is funded by the Austrian Climate and Energy Fund, an innovative low-cost stove technology with very low pollutant emissions is currently under development. The project is coordinated by the engineering company HET (Salzburg), the implementation of the new technology is carried out by the stove manufacturer Justus, and the Institute of Thermal Engineering and BIOENERGY 2020+ are scientific partners. The new stove is based on an innovative and patented fuel supply system which leads to a very simple design. Three variants of the new stove technology are to be developed as part of the project:
The first variant, a world novelty, is a certified natural draft pellet stove characterized by a particularly low price, grid independency and a very nice flame pattern, and is clearly superior to current wood-log stoves with regard to comfort, ease of use and emission behaviour.
The second variant is a pellet stove with a low electricity demanding draft fan (see Figure 2). While the stove is considerably cheaper than current high-quality pellet stoves, it still fulfils the highest and most stringent relevant certificate for continuously operating room firing systems ("Blue Angel") in terms of emissions and efficiency.
Scheme of the low-energy demand pellet stove technology (left, middle) and photo of the first version of the pilot plant.
The third variant is a combi-stove, which can be fired with both wood logs and pellets. This should combine the advantages of both fuels. In this case, wood logs are the cheaper fuel and the flame image is superior to the pellet stove. The additional use of the pellet burner as a supporting burner for the combustion of the wood logs considerably reduces the high emissions of wood log combustion while increasing significantly the efficiency of the stove.
For the current-operated variants, a novel micro-heat storage concept with convection air ducts for the purpose of targeted discharge of the storage is being developed in order to achieve a better heat management and a prolonged heat dissipation.
In order to meet the objectives of the project, the technology of the furnace is to be newly developed. For this purpose, CFD (Computational Fluid Dynamics) simulations of the combustion processes in the furnace are carried out and systematic investigations with detailed emission measurements are performed on a test rig. Both the Institute of Thermal Engineering and BIOENERGY 2020+ have extensive experience in the field of CFD simulation of combustion plants. Together, a detailed CFD model for biomass firing systems has been developed, which can depict all processes from the simulation of the burning of wood chips and pellets in the grate to combustion in the combustion chamber and the formation of pollutant emissions (see Figure 3).
3D-CFD simulation of a 20 kW pellet burner: simulation of the combustion of the pellets on the grate.
As part of the project, a CFD-based pellet stove model was first developed and successfully used for the development of the first test rig of the new technology. This methodology allows for a detailed investigation of the interlinked processes of pellet and gas phase combustion in the stove (see Figure 4). Furthermore, a new CFD-based wood log stove model has been developed which can be used to simulate a complete combustion cycle for the first time, from the insertion of the wood logs to complete burnout.
Contours of release rates of different species during the thermal conversion of the pellets [mg/s] at a vertical cross section through the grate axis (left) and flue gas path lines coloured by CO concentrations in ppm at dry basis (right) under full load conditions of the new pellet stove technology.
Hence, the project „LowCostEmissionStove“ can be considered as a showcase project of applied science, where synergies of the partners involved are ideally utilized and the results of the project directly lead to a strengthening of the Austria’s economy.