Modelling Biomass and Solid Recovered Fuel Combustion on Reciprocating Grates with CFD-application

摘要

The combustion of biomass for heat and electricity generation is understood to be a sustainable source of energy and to support the reduction of greenhouse gas emissions. However, the extended use of wood and other plants has already led to supply shortage and to increased prices. For power plants, this poses a challenge to reduce operating costs, especially by making use of cheap, low-quality fuels which are prepared from municipal or industrial waste materials. However, the combustion behavior of such solid recovered fuels differs significantly from that of genuine biomass. Therefore, the feasibility and application of low-grade fuels in industrial grate firings needs to be studied in small scale laboratory facilities. KIT has characterized the combustion properties and behavior of biomass and solid recovered fuels in a batch fixed bed reactor and on a reciprocating grate. Experimentally, key figures like reaction front velocity, ignition rate and mass conversion rate were determined in addition to the combustion temperature and the primary gas composition at the batch reactor KLEAA at ITC (～ 5 kg fuel). Moreover, a numerical model was developed which permits to simulate the combustion of wood chips, solid recovered fuels and mixtures thereof. In this way, available experimental results can be reproduced numerically and the understanding of most relevant fuel properties can be improved. The CFD extension of the model permits to simulate the composition of the combustion gases on their way through the furnace as well as the associated heat transfer. The model has been validated by comparison with batch, fixed bed reactor experiments and with results from continuously operating reciprocating grates (TAMARA at ITC, ～ 120 kg fuel/h, and MHKW Frankfurt, a solid waste to energy plant, ～ 2.3 · 10~4 kg fuel/h). In future developments, this model can also be used to track particle emissions from the fuel bed and particle deposition on important plant components in the boiler.