The patented Dall Energy biomass furnace combines updraft gasification with gas combustion
Main functionality of the gasification furnace:
Biomass is dried and pyrolysed in the top of the fuel layer. Further down in the fuel layer the biomass is converted into a burnable gas and fine ash. The heat for the drying and pyrolysis process is a combination of convective heat from the gasification gases below and radiation heat from the gas combustion part above. Primary air injection is about half of what is used in traditional grate incineration, and, with a big furnace bottom area, the gas velocity in the bottom part is very low. Consequently, particles remain here and dust emission from the furnace is very low. Gas from the bottom part is combusted in the top section. The gas combustion (flow, temperatures, emissions etc.) is very stable and is controlled via flue gas recirculation.The control philosophy allows operation with lower excess air ratios than what is known from traditional incineration grate furnaces, and this is partly the reason why a higher overall efficiency is possible.
The history of updraft gasification:
The principle of updraft gasification has been known for centuries and was used in Europe for e.g. production of city gas on the basis of hard coal from the 1850s. In the 1980s several updraft gasifiers entered commercial operation in Scandinavia, but this required heavy maintenance because of the concept of external combustion ofthe gases. The Dall Energy furnace combines updraft gasification with gas combustion in the same chamber, which results in full performance andenvironmental benefits without the historical problems related to externalcombustion.
The gasification furnace as Thermal oxidizer:
Thermal oxidisers are used in the industry to destroy Hazardous Air Pollutants (HAPs) and Volatile Organic Compounds (VOCs) from industrial air streams. These pollutants are generally hydrocarbon-based and when they are destroyed via thermal combustion they are chemically changed to form CO2 and H2O. Historically, propane or natural gas was widely used as fuel for thermal oxidisers, but this is now proving to be inadequate for industries with goals for reducing their environmental footprint. The Dall Energy furnace can be used as a CO2-neutral thermal oxidiser for destruction of VOCs or other organic compounds in air streams. The first Dall Energy biomass-based thermal oxidiser was installed at Warwick Mills, New Hampshire, USA, where the plant was commissioned in 2014. The plant operated with a VOC destruction efficiency exceeding 99.8%, which is even higher than fossil fuel-based oxidisers.